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U.S. GEOLOGICAL SURVEY BULLETIN 2136

Kinematics of the Short Hill Fault-Late Paleozoic Contractional Reactivation of an Early Paleozoic Extensional Fault, Blue Ridge-South Mountain Anticlinorium, Northern Virginia and Southern Maryland

By Scott Southworth

ABSTRACT

The west limb of the Blue Ridge-South Mountain anticlinorium (BR-SMA), in northern Virginia and southern Maryland, is repeated by the Short Hill fault (SHF). Field relations, fabric diagrams of structural elements, drill core data, and microstructures show that the SHF is a younger-over-older thrust fault that has been folded with the rocks of the northeast-plunging BR-SMA. The SHF can be traced for over 60 km from a shear zone in Middle Proterozoic basement rocks in the core of the BR-SMA to the west limb where the fault places younger Lower Cambrian rocks over older Lower Cambrian rocks. Hanging-wall rocks (Swift Run, Catoctin, Weverton, Harpers, Antietam, and Tomstown Formations) lie in a regional syncline that is interpreted to be genetically related to contractional motion on the SHF. The hanging-wall syncline was cut by the SHF during contractional reactivation. The SHF is transected by pressure solution crenulation cleavage that is axial planar to the BR-SMA.

The SHF is an early structure that is parallel to cleavage in greenschist-facies rocks. When the contractional structures are restored, the SHF can be interpreted as an extensional fault. Contractional reactivation of the extensional fault is inferred to have occurred during the Alleghanian orogeny. The extensional fault formed after deposition of the Lower Cambrian Tomstown Formation, but before late Paleozoic metamorphism (Mississippian?), that obscured evidence of the extensional structures. The SHF may be part of a late Proterozoic rift zone in the axial region of the BR-SMA that was subjected to post-Cambrian(?) extension. Similar faults in the Blue Ridge province from Virginia to Pennsylvania may mark the Late Proterozoic to Early Cambrian rifted continental margin of North America that was thrust and uplifted during the Alleghanian orogeny.

INTRODUCTION

Interpretation of seismic reflection, aeromagnetic, and gravity data in the southern Appalachians suggests that Late Proterozoic extension affected a larger part of Laurentia than previously has been documented (Favret and Williams, 1988; Johnson and others, 1994). Normal faults in the Middle and Late Proterozoic rocks in the core of the central Appalachian Blue Ridge anticlinorium (Espenshade, 1986; Simpson and Kalaghan, 1989; Kline and others, 1991; Bailey and Simpson, 1993) provide evidence for extensional tectonics interpreted to be the result of Late Proterozoic rifting of Laurentia to form the Iapetus Ocean (Rankin and others, 1989). Field evidence is presented here to support the presence of a regional-scale, fault of younger rock over older rock that involves rocks as young as Lower Cambrian, or possibly Middle Cambrian, that have been folded and cut by cleavage. Thrust faults that place younger rock on older rock line been mapped in the Blue Ridge province (Hadley and Goldsmith, 1963; King, 1964); some have been identified by restoration of cross sections (Boyer and Mitra, 1988; Connelly and Woodward, 1992), and some have interpreted to be extensional faults that were later reactivated by thrust motion (Robinson and others 19921 These data suggest an early Paleozoic extensional event that post-dated Late Proterozoic rifting and predated the contractional deformation during the late Paleozoic Alleghanian orogeny.

STUDY AREA

The study area is in the west-central part of the BR-SMA (Cloos, 1951) (fig. 1), in the Appalachian Blue Ridge tectonic province. The Appalachian orogenic belt resulted from continental and (or) island arc collisions that took place during the closing of one or more oceans that existed in the Late Proterozoic and Paleozoic (Rankin and others, 1989). From Alabama to central Virginia, the Blue Ridge tectonic province is dominated by emergent thrust faults that affected Middle Proterozoic basement. From northern Virginia to Pennsylvania, the structure is a large anticlinorium within which few thrust faults have been recognized.

Figure 1. Geologic map and cross sections of the Blue Ridge-South Mountain anticlinorium (BR-SMA). Inset map shows the regional of the study area keyed to the Short Hill fault (SHF). Cross sections modified from Demicco (1985) (A–A') and Mitra (1987) (B–B'). [Download a high-quality PDF file.]

The BR-SMA records a diverse tectonic history of orogenies followed by episodes of rifting (table 1). Stratigraphy, structure, and metamorphism of the rocks record several Wilson cycles of opening and closing ocean basins (Wilson, 1966). The alternation of contractional and extensional events facilitated the reactivation of inherited structures and inversion tectonics.

The core of the BR-SMA is composed of Middle Proterozoic crystalline rocks (Burton and others, 1992a; Aleinikoff and others, 1993; Southworth and others, 2005) that were folded along with their Late Proterozoic and Lower Cambrian cover during the Paleozoic (Cloos, 1951; Nickelsen, 1956; Southworth, 1991a). The BR-SMA is flanked on the west by a conformable sequence of lower Paleozoic carbonate platform rocks that underlie the Great Valley of the Valley and Ridge province.

The BR-SMA is an allochthonous fold complex above one or more blind thrust faults (Harris, 1979; Mitra and Elliott, 1980; Elliott and others, 1982; Mitra, 1987; Evans, 1988) (fig. 1). A cleavage fan in the BR-SMA (Cloos, 1951), suggested to Mitra and Elliott (198Q that thrusting, folding, and cleavage were coeval. These structures were interpreted to have formed during a greenschist-facies metamorphic event in the late Paleozoic Alleghanian orogeny. The fault-bend fold geometry of the BR-SMA along the Potomac River has been interpreted to result from a ramp, in the footwall of a blind thrust fault (Elliott and others, 1982; Mitra, 1987; Evans, 1988).

The east limb of the BR-SMA is overlain by Triassic and Jurassic sedimentary and igneous rocks of the Culpeper and Gettysburg basins. The extensional border faults of the Mesozoic basins are reactivated thrust faults along the east limb of the BR-SMA (Root, 1989). The plunging nose of the BR-SMA in Maryland and Pennsylvania is truncated by the western border fault of the Gettysburg basin, and numerous faults in the core of the anticlinorium have been interpreted to be Triassic normal faults (Root, 1989).

Along the Potomac River, Blue Ridge-Elk Ridge, Short Hill-South Mountain, and Catoctin Mountain (fig. 1) are underlain by Late Proterozoic and Lower Cambrian rocks that make up the limbs of the BR-SMA; the limbs are flanked by valleys underlain by Middle Proterozoic crystalline rocks. Blue Ridge-Elk Ridge is the overturned west limb and Catoctin Mountain is the upright east limb of the anticlinorium. Short Hill-South Mountain is the central ridge that has the same stratigraphy as the overturned west limb of the BR-SMA.

Short Hill-South Mountain is bounded on its lower west slope by a southeast-dipping fault, herein called the Short Hill fault (SHF), that has brought younger rocks above older rocks. The SHF and its regional relations are the main topic of this investigation. Other faults similar to the SHF in the BR-SMA have been interpreted as (1)thrust faults that cut folds (Bloomer and Werner, 1955; Werner, 1966), (2) Early Cambrian normal faults (Spencer and others, 1989; Simpson and Eriksson, 1989), and (3) Mesozoic normal faults (Fauth, 1977; Root, 1989; Bartholomew and others, 1991). The SHF was previously interpreted as a Triassic normal fault (Cloos, 1951; Nickelsen, 1956; Parker, 196y, but work reported herein suggests otherwise.

In this study, the SHF was remapped for a distance of 60 km to determine its geometry and structural significance. Geologic mapping was conducted from the winter of 1989 to the spring of 1991 as part of a cooperative study between the U.S. Geological Survey (USGS) and the Loudoun County Department of Environmental Resources (Burton and others, 1992a; Southworth and others, 2005). Geologic maps and cross sections at a scale of 1:24,000 (Southworth, 1991a, 1993a, in press; Southworth and Brezinski, 1996) provide data of the central, southern, and northern regions of the study area (fig. 1).

GEOLOGIC SETTING

The oldest rocks in the study area are a sequence of Middle Proterozoic paragneisses (Burton and Southworth, 1993) that were largely assimilated by granitoid plutonism between 1145 and 1058 Ma (table 1) (Aleinikoff and others, 1993). Rod and bleb perthite, almandine, hypersthene, hornblende, red biotite, and blue quartz in the gneisses record granulite-facies metamorphism that has a hornblende cooling age of 920 Ma (Kunk and others, 1993). Compositional layering and foliation in these granulite-facies rocks strike northwest, parallel to unit contacts. Sparse exposures of migmatite contain dome and basin fold interference patterns that are interpreted to have formed during the Grenville orogeny.

The Grenville orogeny was followed by a long period of Late Proterozoic extension. The Robertson River Igneous Suite (Tollo and others, 1991), the largest sake of anorogenic plutons in the central Appalachian Blue Ridge province, indicates a period of incipient continental rifting that lasted from 730 to 700 Ma (Tollo and Aleinikoff, 1992). These rocks occur in a linear belt that probably marks an ancient crustal fracture (R.P. Tollo, George Washington University, oral commun., 1991) Granite plutonism was followed by rift-related sedimentation and peralkaline volcanism (Hutson and Tollo, 1992).

Late Proterozoic metasedimentary rocks of the Swift Run Formation were deposited unconformably on an irregular surface of basement granitoid rocks. Rocks of the Swift Run Formation are correlative with rocks of the Fauquier Formation (Espenshade, 1986), Mechum River Formation (Gooch, 1958), and the Lynchburg Group (Wehr, 1985). Exposures of the Fauquier Formation and Lynchburg Group are restricted to the east limb of the BR-SMA and were probably deposited in a restricted shallow marine basin (Conley, 1989). The Mechum River Formation occupies a synclinorium, interpreted to be a Late Proterozoic graben by Gooch (1958) and Schwab (1986), along the axial region of the anticlinorium. Fault-bounded outliers of the Mechum River Formation also are found on the east margin of the Robertson River Igneous Suite (Conley, 1989).

Late Proterozoic high-angle normal faults that strike northwest were active during sedimentation of the Fauquier Formation (Espenshade, 1986; Kline and others, 1991). Cataclastic breccia (Espenshade, 1986) and extensional shear fabrics (Kline and others, 1991) formed by these faults are found in the footwall of the Marshall Metagranite. Elsewhere, clasts of granitoids in the basal Fauquier Formation have a greenschist-facies fabric that Kline and others (1991) and Kline (1991) interpret to be the result of Late Proterozoic extension. Microstructural indicators of normal motion along the Rock Fish Valley, Lawhorne Mill, and White Oak Run fault zones in Virginia also are interpreted to be the result of Late Proterozoic extension under greenschist-facies conditions (Simpson and Kalaghan, 1989; Bailey and Simpson, 1993).

The Middle Proterozoic gneisses and Late Proterozoic granitoids and metasedimentary rocks are intruded by hundreds of metadiabase dikes as well as some metarhyolite dikes. The density of the dike swarm increases to the northeast and is evidence for a highly extended continental crust. These dikes are part of a major dike swarm that extends the length of the Appalachian orogenic belt and are related to the extensional event that preceded Iapetus rifting (Ratcliffe, 1987). They may be feeder dikes to the bimodal metavolcanic rocks of the Catoctin Formation (Stose and Stose, 1946; Nickelsen, 1956; Parker, 1968) that have ages of 570 to 565 Ma (table 1), which date the main phase of continental rifting (Rankin and others, 1989).

The metavolcanic rocks of the continental (Iapetus) rifting are overlain by a fining-upward sequence of fluvial rocks that record a marine transgression of the newly formed Iapetus Ocean. Rocks of the Lower Cambrian Chilhowee Group were deposited during the transition from rift to passive continental margin. Deposition began with quartzite of the Weverton Formation and ended with carbonate rocks of the Tomstown Formation. Rocks of the Weverton Formation are interpreted to have resulted from alluvial sedimentation (Schwab, 1986) from a westward source (Whitaker, 1955). Skolithos burrows (trace fossil) in the Harpers and Antietam Formations support the interpretation that these rocks were deposited in a deltaic to shallow marine environment (Simpson and Sundberg,1987).

The Paleozoic tectonic history of the Appalachian orogen is complex (Hatcher, 1989), and the timing of events affecting the BR-SMA remains an enigma (Rankin and others, 1989). The Taconic orogeny is thought to be the first major compressive event resulting from the closing of the Iapetus Ocean, but, although strong evidence of late Early and Middle Ordovician (480 to 458 Ma) deformation and metamorphism of the Taconic orogeny is well known in the Appalachian internides, there is no evidence for these in the immediate study area (Drake and others, 1989). Taconic deformation is documented 90 km to the northeast by the emplacement of the Hamburg klippe (Drake and others, 1989). The Devonian Acadian orogeny of the northern Appalachians was another compressional event that occurred during closing of the Iapetus Ocean (Hatcher, 1989), but no evidence for this orogeny has been documented in this area.

The Iapetus Ocean finally closed with the continental collision of Africa and Laurentia during the late Paleozoic Alleghanian orogeny. The Alleghanian orogeny began in the Late Mississippian and ended in the Late Permian (340 to 250 Ma) (Hatcher, 1989). At least two noncoaxial phases of folding have been identified in the central Appalachians (Geiser and Engelder, 1983; Dean and others, 1988; Gray and Mitra, 1993).

Previous workers interpreted the BR-SMA as an Alleghanian structure because the South Mountain cleavage, which is geometrically associated with it (Cloos, 1951), can be traced westward through rocks as young as Devonian age (Mitra and Elliott, 1980). Because cleavage and folds of only one phase of deformation were recognized, Mitra and Elliott (1980) concluded that thrusting, folding, and cleavage formation were coeval with greenschist-facies metamorphism at about 350°C temperature and 3.5 kbar pressure. In general, cleavage an the east limb of the BR-SMA is steeper than on the west limb, so it forms a fan across the anticlinorium (Cloos, 1951; Mitra and Elliott, 1980). Local and regional variations in the orientation of the South Mountain cleavage have been interpreted to result from later folding (Cloos, 1951; Nickelsen, 1956; Mitra and Elliott, 1980; Onasch, 1986; Mitra, 1987), from the position of different order structures (Mitra, 1987), or from movement along the North Mountain fault above its footwall ramp (Mitra, 1987).

The timing of deformation in the BR-SMA can only be determined by its relation to stratigraphy and metamorphism (Drake and others, 1989). For example, isotopic data on greenschist-facies minerals from cleavage in central Virginia (20 km to the southwest) strongly suggest Taconic deformation (Stromberg, 1978; Bartholomew and others, 1991). Evans (1991) and Bartholomew and others (1991) suggest that the Taconic foliation was simply transported during Alleghanian deformation and is parallel with Alleghanian cleavage that affected rocks of the foreland. Muscovite from cleavage on the east limb of the BR-SMA in this region has a 340-Ma cooling age and complex spectra (Burton and others, 1992b; Kunk and others, 1993).

The Alleghanian orogeny was followed by Mesozoic extension that reactivated Paleozoic thrust faults as normal faults (Root, 1989) that produced isolated half grabens filled by terrigenous sediments along the east limb of the BR-SMA. Sedimentary and igneous rocks that dip west suggest that the border faults become listric at depth. Later extension allowed swarms of diabase dikes to cut across the basins into the Valley and Ridge province. Northwest-trending diabase dikes intruded the BR-SMA at 200 Ma (Kunk and others, 1992).

PREVIOUS WORK ON THE SHORT HILL FAULT

Evidence of the SHF has been reported by numerous workers whose efforts are summarized an figure 2. The SHF was first recognized by Cloos (1951) along the Potomac River (fig. 2), where he mapped the Cambrian Harpers Formation in contact with Middle Proterozoic granitic gneiss. This and other observations led him to portray the geology at the northern end of the SHF to be a series of high-angle normal faults that bounded horsts and grabens of Triassic extension. Following Keith (1894), Stose and Stose (1946) interpreted the northern segment of the SHF to be the “Harpers Ferry overthrust,” that placed the Catoctin and Weverton Formations on the Tomstown Formation.

Figure 2. Index map of the study area showing A, the trace of the Short Hill fault (SHF) and B, a composite of previously mapped segments of the fault. [Download a high-quality PDF file.]

Nickelsen (1956) mapped the SHF south of the Potomac River for 10 km to a point within granodiorite gneiss 2 km south of Short Hill Mountain where it juxtaposed basement rocks. Nickelsen (1956) recognized that (1) the fault truncated folds of the Weverton Formation, (2) footwall rocks included some Late Proterozoic rocks, and (3) a syncline of Late Proterozoic rocks within its hanging wall extended south of the mapped fault termination. He also interpreted the SHF as a Triassic normal fault, by analogy with the border fault of the Culpeper basin farther east.

Parker (1968) mapped Nickelsen's (1956) syncline to the south and placed a fault along the west limb of the syncline as an extension of Nickelsen's (1956) Triassic normal fault. Parker (1968) first recognized that the petrology of the basement rocks west of the fault differed from that of basement rocks east of the fault. Howard (1991) mapped the SHF in the basement rocks south of Black Oak Ridge (BOR), but west of Parkers' fault trace, and called it the Goose Creek cataclasite belt (fig. 2). The 1-km-wide cataclasite belt marks the contact between garnet and biotite-bearing quartzofeldspathic gneiss on the west and quartzofeldspathic gneiss on the east (Howard, 1991). He suggested that the Goose Creek cataclasite belt probably marks a significant tectonic boundary between basement rocks from different crustal levels. Howard (1991) was the first to show the SHF as a continuous structure from the Goose Creek cataclasite belt in the south to the northern terminus in Washington County, Md.

Wojtal (1989) interpreted a down-plunge projection of the northern region of the SHF that had been constructed by David Elliott (John Hopkins University, deceased). Elliott recognized that the fault was folded (Wojtal, 1989; Howard, J.L., oral commun. 1991, Wayne State University) and that the rocks on South Mountain had been overridden by a thrust fault. Wojtal's (1989) schematic reconstruction of Elliott's down-plunge projection portrays the structure as an Early Cambrian graben that was folded in the late Paleozoic. Brezinski and others (1991) and Campbell and others (1992) interpreted the SHF at the Potomac River to be the “South Mountain fault,” which is part of a late-stage, linear, en echelon fault system that they had traced to Pennsylvania. Brezinski (1992) considers the South Mountain fault to be coincident with the Rohrersville fault that he interpreted to be an early Paleozoic normal fault.

ACKNOWLEDGMENTS

This paper is the result of a thesis at University of Maryland prepared under the direction of Eileen McLellan, Nicholas Woodward, Robert Ridky, and David Harding. Reviews by David Prowell and Avery Ala Drake, Jr. (USGS) improved the report. Thanks to David Daniels and Kevin Bond (USGS) for geophysical data. Financial support for the core drilling is gratefully acknowledged from USGS and Maryland Geological Survey; special thanks to property owners of the drill sites: Mr. Palmer, Mrs. Moss, Mr. and Mrs. Newman, and Mrs. Furr. Discussions with my colleagues in the USGS, as well as with David Brezinski (Maryland Geological Survey), Dick Nickelsen (Bucknell University), Jeff Howard (Wayne State University), and Steve Wojtal (Oberlin College), were beneficial to the study.

STRATIGRAPHY

The stratigraphy of the Late Proterozoic and Lower Cambrian rocks used herein was established by Keith (1894) and revised by Jonas and Stose (1939), Stose and Stose (1946), Bloomer and Bloomer (1947), King (1950), and Nickelsen (1956). The stratigraphy of the Middle Proterozoic rocks was established during the mapping of Loudoun County, Va., and on the basis of crosscutting relations (Burton and others, 1992a; Burton and Southworth, 1993; Southworth and others, 2005) and U-Pb isotopic data (Aleinikoff and others, 1993).

MIDDLE PROTEROZOIC BASEMENT ROCKS

Garnet-graphite gneiss, graphitic phyllonite, sericite schist (Yp) and quartzite (Yq) constitute a sequence of paragneiss (Southworth, 1993b) of probable metasedimentary origin (Burton and Southworth, 1993) that was assimilated by plutons during the Grenville orogeny. Abraded zircons from correlative rocks have central Virginia range in age from 1800 to 1400 Ma (Herz and Force, 1984; Sinha and Bartholomew, 1984). These rocks have a northwest-striking compositional layering that parallels the trend of contacts. Norite (Yn), composing principally plagioclase, hornblende, and hypersthene, is interpreted to be a metabasite of mafic volcanic omen (Southworth, 1995).

Porphyroblastic granite gneiss (Ypg) is megacrystic monzogranite that has an upper intercept U-Pb age of 1144±2 Ma (Aleinikoff and others, 1993; Southworth, 1993b. It is restricted to three northwest-trending belts in the footwall where the contacts and foliation are truncated by the SHF. Granitite gneiss (Ylg) is well layered and locally migmatitic and is interpreted to have intruded and assimilated the suite of paragneiss because of the spatial association of the two units (Southworth, 1993a). Granite gneiss has preliminary 207Pb-206Pb ages that range from 109 to 1092 Ma (Aleinikoff and others, 1993). Granite gneiss is restricted to the footwall where its contacts, layering, and foliation are truncated by the SHF.

Biotite granite gneiss (Ybg) has the composition of monzogranite and contains porphyroblastic augen of perthite. It has an upper intercept U-Pb zircon age of 1127±7 Ma (Aleinikoff and others, 1993) and is restricted to the hanging wall of the SHF (Southworth, 1993a). Hornblende gneiss (Yhg) (Southworth, 1991a, 1995) is lithologically correlative with the granulite gneiss of the Pedlar massif (Evans, 1991). It has a preliminary upper intercept U-Pb age of about 1110 Ma (Aleinikoff and others, 1993) and is almost exclusively restricted to the footwall of the SHF.

Garnet monzogranite (Ygt) is intrusive into the porphyroblastic granite, the granitic gneiss, and the hornblende gneiss. It has an upper intercept U-Pb zircon age of about 1070 Ma (Aleinikoff and others, 1993) and it crops out continuously along the Virginia side of the Potomac River on both sides of the SHF. Charnockite (Yc), composed primarily of hornblende, hypersthene, almandine, and potassium feldspar, is found in both northwest-trending linear belts and elliptical bodies in the footwall of the SHF (Southworth, 1993a). Leucocratic metagranite (Ym) has an upper intercept U-Pb age of 1058±3 Ma (Aleinikoff and others, 1993) and it is restricted to the immediate hanging wall of the SHF in the southern region (Southworth, 1993a).

LATE PROTEROZOIC AND PALEOZOIC ROCKS

The Swift Run Formation (Zs) consists of sericitic metasandstone, quartz sericite schist, phyllite, slate, and marble. The metasedimentary rocks generally fine upward, and crossbedding suggests that they are of fluvial origin. The Swift Run Formation unconformably overlies basement and grades up into the overlying metavolcanic rocks of the Catoctin Formation.

Hundreds of metadiabase dikes (Zmd) and a few metarhyolite dikes (Zrd) intrude the basement rocks, and some cut rocks of the Swift Run Formation. Metadiabase dikes constitute as much as 50 percent of the volume of the core of the BR-SMA as exposed along the Potomac River (Southworth, 1991a). The chemical similarity of the dikes to metabasalt and metarhyolite of the Catoctin Formation (Southworth, 1991a) suggests that they are feeder dikes for the flows.

The Catoctin Formation (CZc) consists of metabasalt, agglomeratic breccia, tuffaceous phyllite and schist, and metarhyolite. Metabasalt of the Catoctin Formation has a Rb-Sr whole-rock age of 570±36 Ma (Badger 1988), and a metarhyolite dike, immediately east of the study area has a U-Pb upper intercept age of 571.5±5 Ma (Aleinikoff and others, 1995).

Chilhowee Group.—The Chilhowee Group consists of the Loudoun (CZI), Weverton (Cw), Harpers (Ch), and Antietam (Ca) Formations (Nickelsen, 1956). The contact between the Chilhowee Group and the underlying Catoctin Formation was considered by previous works to be either transitional (Nickelsen, 1956) or unconformable (King, 1950; Reed, 1955). In the study area, the contact is sharp and probably unconformable.

The Weverton Formation (Cw) consists of a lower member and an upper member both of which fine upward. The upper member, however, is coarser and less well sorted the lower member. The lower member is a well-sorted, mature quartzite that grades upward into quartzite interbedded with metasiltstone and was deposited unconformably on hornblende gneiss on Blue Ridge. The upper member is a diagnostic “gun-metal blue” (Nickelsen, 1956) to greenish-gray pebbly quartzite. The base of the unit is a poorly exposed, dark-colored metasiltstone that resembles rocks of the Harpers Formation.

The Harpers Formation (Ch) is predominantly phyllitic metasiltstone. Pebble metaconglomerate magnetite-bearing meta-arkose are found locally in the lower part of the unit, and thin sandstone beds are found locally in the upper part. The sandstone (Chs) is correlative with the Mount Alto Member of the Harpers Formation in Pennsylvania and Maryland (Stose, 1906; Brezinski, 1992).

The poorly exposed Antietam Formation (Ca) consists of thin bedded metasandstones interbedded with metasiltstone and is gradational with the underlying Harpers Formation. The uppermost strata are fine- to coarse-grained, crossbedded metasandstone and quartzite.

The Tomstown Formation (Ct) (Stose, 1906) in Maryland has been subdivided by Brezinski (1992) into (1) a lower sheared dolomitic marble member, (2) a middle member of burrowed dolomite having a mottled appearance, and (3) an upper member of massive saccaroidal dolomite. Rocks of the Tomstown Formation were not differentiated during the study area because of intense deformation.

MESOZOIC ROCKS

Near-vertical, north- and northwest-trending diabase dikes (Jd) of Early Jurassic age intrude the BR-SMA. The dikes are discontinuous and are en echelon. Two dikes cut the SHF near the Potomac River, and one has an 40Ar-34Ar age of about 200 Ma (Kunk and others, 1992).

STRUCTURAL ELEMENTS

The description and crosscutting relations of structural elements provide a framework for further discussions of field relations and kinematics of the SHF. Structural elements studied in connection with the SHF include faults, foliations, lineations, and folds. Table 2 summarizes the types and distinctive features of structural elements, and table 3 summarizes the relative chronology of deformation events and structural elements along the SHF. Structures are best developed in the area portrayed on figure 3. See figure 4 for equal-area projections of poles to foliation, figure 5 for photographs of selected elements, and figure 6 for equal-area projections of lineations and fold axes.

Figure 3. A, Geology (for reference), B, faults and folds, and C, dominant foliation and lineation orientations of the Short Hill fault (SHF), and vicinity. [Download a high-quality PDF file.]

Figure 4. Equal-area projections (lower hemisphere) of poles to foliation in the footwall and hanging wall blocks of the Short Hill fault (SHF). [Download a high-quality PDF file.]		Figure 5. Photographs of A, cleavage (Sc) cut by crenulation cleavage (Scc) B, shear band cleavage (Ssb) cutting phyllonitic foliation Sp, C, Scc that is axial planar (AP) to F2 folds, and slickenlines (Ls). [Download a high-quality PDF file.]

FAULTS

Faults were delineated on the basis of stratigraphic omission. The SHF is a younger-over-older fault that has cut upsection northward from Middle Proterozoic to Lower Cambrian rocks. The Turners Gap fault (TGF, northern region, fig. 3B) has thrust Catoctin Formation onto Weverton Formation; the stratigraphic separation is about 1 km. The Lambs Knoll fault (LKF, northern, fig. 3B) has thrust Loudoun Formation onto Weverton Formation. Locally it duplicates the lower member of the Weverton Formation on the east limb of the Lambs Knoll anticline (LKA). Map relations suggest that the northern end of the Lambs Knoll fault is cut by the Turner Gap fault. The Locust Grove fault (LGF, northern, fig. 3B) is interpreted to have thrust Harpers Formation on Tomstown Formation because rocks of the Antietam Formation crop out discontinuously. Map relations suggest that the north end of the LGF is cut by the TGF. Interpretation of the southern end of the LGF is controversial because no strata are missing and the fault does not offset the trace of the SHF nor lithologic units in the footwall (FW) of the SHF.

The White Rock fault (WRF, central region, fig. 3B) is an intraformational, bedding-parallel, thrust fault in the lower member of the Weverton Formation. Shearing resulted in meter-sized blocks of quartzite that float in a matrix of foliated vein quartz that constitutes as much as 80 percent of outcrops (Southworth and Brezinski, 1996). Intraformational thrust faults in the lower member of the Weverton Formation north of the Potomac River (Weverton Cliffs) truncate 1-m-wavelength isoclinal folds (WC, fig. 3B) (Southworth and Brezinski, 1996).

Rocks of the Late Proterozoic Swift Run Formation are in fault contact with Middle Proterozoic garnet monzogranite along the Gapland fault (GF, northern, fig. 3B). The field relations suggest a normal fault because phyllitic metasiltstone is in contact with gneiss. Phyllite, schist, and metasandstone in the hanging wall (HW) cannot be traced to the southwest, but they were mapped to the northeast where they are cut by the SHF.

FOLIATIONS

Foliation along the SHF includes gneissic foliation (Sg), bedding (So), cleavage (So), mylonitic foliation (Sm), phyllonitic foliation (Sp), shear band cleavage (Ssb,), and crenulation cleavage (Ssc). The time relations of the S surfaces were determined from metamorphic mineral assemblage, the rock units in which they were found, and crosscutting relations seen in outcrops and thin sections.

GNEISSIC FOLIATION

Gneissic foliation (Sg) in the Middle Proterozoic basement rocks is expressed by the planar arrangement of dark and light colored minerals that are parallel to leucocratic and melanocratic layers that range from 1 cm to 0.5 m thick. The dark minerals include hornblende, pyroxene, biotite, almandine, and opaque minerals. The light minerals are quartz, plagioclase, and alkali feldspar. These minerals formed during amphibolite to granulite-facies metamorphism during the Grenville orogeny (Kunk and others, 1993). Sg (figs. 3C and 4) is well shown in the basement rocks in the footwall of the SHF (southern region), where it consistently strikes northwest and dips northeast. Sg was seldom observed in hanging-wall basement rocks of the SHF. The different orientation of Sg (fig. 4) across the SHF suggests a fault.

BEDDING

Bedding (So) was recognized in most of the Late Proterozoic and Lower Cambrian cover rocks, and beds strike parallel to the SHF. Graded and crossbeds are common in the Weverton Formation and are less common in rocks of the Swift Run Formation. So was consistently traced only the quartzites of the Weverton Formation (fig. 4). In other units, So attitude varies over short distances reflecting folding and faulting. The grain size and color of phyllite schist of the Swift Run and Catoctin Formations were seen only in sawed hand specimens and thin sections. So in the Harpers Formation is marked by the refraction of Sc cleavage. So in the Tomstown Formation (hanging wall) can seen locally, but stratigraphic tops were difficult to determine. Locally, the Swift Run, Catoctin, and Tomstown Formations (hanging wall) contain intrafolial folds and lineations. These rocks are L-S tectonites.

Cleavage.—The cleavage (Sc) is penetrative and is characterized by the parallel planar arrangement of chlorite, quartz, sericite, magnetite, epidote, and actinolite. The SHF is generally parallel to the regional southeast-dipping Sc. Sc is axial planar to F1 isoclinal folds. This is the regional southeast-dipping Sc mapped as roughly axial planar to the BR-SMA by Cloos (1951), Nickelsen (1956), Mitra and Elliott (1980), and Mitra (1987).

Sc that cuts gneissic foliation is marked by millimeter-wide, anastomosing, ductile-deformation zones (Mitra, 1978). Sc is strongly developed in the leucocratic metagranite along the SHF (hanging wall, southern region). Sc in the Late Proterozoic dikes is parallel to the dike contacts and the Sc in the adjacent basement rocks. The Sc in the Late Proterozoic and Lower Cambrian cover rocks varies from smooth and planar to rough and anastomosing. It is best developed in the fine-grained phyllites of the Swift Run, Catoctin, and Harpers Formations where it obliterates So. The Sc in the basement, dikes, and cover rocks is coplanar because these rocks were deformed together (Southworth, 1991a). As seen in figure 1, the stereonets of Sc in different rocks are virtually interchangeable. Sc is locally folded, and these folds have an axial planar crenulation cleavage (Scc) (fig. 5A). Most folds of Sc are in the footwall of the SHF (fig. 4). The scatter of poles to Sc is due in part to late folds that were superposed on the BR-SMA.

Mylonitic Foliation.—Mylonitic foliation (Sm) is marked by planar aggregates of sericite, chlorite, and fine-grained recrystallized quartz in layers 5 cm to 1 m thick in the otherwise gneissic foliated basement. Microscopically, Sm shows evidence of grain-size reduction and recrystallization. Sm is abundant in the leucocratic metagranite (Sm, hanging wall, southern region) (figs. 3C and 4) where discrete, discontinuous, and en echelon zones resemble phyllites. Sm is sparse in the footwall rocks of the SHF. Sm is coplanar to Sc (hanging wall, southern region) and is interpreted to have formed synchronously with Sc. Sm is equivalent to phyllonitic foliation (Sp); the difference reflects the competence of the rock.

Phyllonitic Foliation.—Phyllonitic foliation (Sp) is a lustrous, wavy surface in phyllosilicate-rich rocks resulting from grain-size reduction, recrystallization, and the interlacing of multiple foliations. Sp is only found along the SHF in outcrops of Harpers Formation (fig. 3C) and in drill core of the metabasalt dike (footwall, central region) and the Catoctin Formation (footwall, northern region). Sp contains microscopic enclaves of asymmetric mica (fish), and asymmetric buttons are men in hand specimens. Sp is coplanar with Sp, but it contains microscopic recrystallized quartz grains and the chlorite is of retrograde origin. Sp is cut by shear band cleavage (Ssb, hanging wall and footwall, central region) and crenulation cleavage (Scc, footwall, northern,fig. 3C).

Shear Band Cleavage.—Shear band cleavage (Ssb) (White and others, 1986) is restricted to rocks along the SHF where it cuts Sp (hanging wall and footwall, central, fig. 3C). Ssb contains opaque residues and locally realigned the phyllosilicates of Sp. The shear bands are nearly horizontal (Ssb, fig. 4), and the asymmetry with Sp is consistent with an east over west sense of shear. Dextral kinks fold the Ssb (fig. 5B). Where strongly developed near the Potomac River, Sp and Ssb anastomose to produce a lustrous button phyllonite. Elsewhere, Ssb locally cut L-S tectonite in the Tomstown Formation (Ssb, hanging wall, northern region). Ssb is geometrically and kinematically equivalent to extensional (Platt and Vissers, 1980) and normal sense (Dennis and Secor, 1990) crenulation cleavage.

Crenulation Cleavage.—Crenulation cleavage (Scc) is discontinuous and spaced with both zonal and discrete types (Gray, 1979). Scc transects the SHF in the northern region. Scc is axial planar to folds of Sc (figs. 3C and 5C). Scc is strong in the immediate footwall rocks, but it becomes weak away from the SHF for 2.5 km to the southwest. The Scc is seen in the Tomstown Formation within 10 m of the SHF (Scc, hanging wall, northern region). Elsewhere, Scc is found only in areas of complex folding where it is axial planar to F2 folds (fig. 3B and C). Scc separates microlithons that have conflicting sense of offset in the Catoctin (footwall) and Tomstown (hanging wall) Formations (Scc, fig. 5A). Insoluble opaque residues concentrated in the laminae suggest that it is a pressure solution cleavage. The apparent offset is the result of volume loss (Gray, 1979). As seen in figure 4, Scc is coplanar with the regional Sc.

LINEATIONS

Slickenlines.—Slickenlines (Ls) are best developed in the Tomstown Formation (hanging wall, northern) (fig. 3C and 5D). The Ls are grooved lineations on “beds” that contain intrafolial folds. Where the Ls are locally penetrative, the rocks are L-S tectonites. Some of the lineations identified as Ls. may be stretching lineations related to the intrafolial folds. The Ls. have a consistent plunge direction of N. 65° W. (fig. 6) that is roughly parallel with the regional mineral lineations of Cloos (1971). Ls have been folded by open and inclined, west-verging F2 folds.

Figure 6. Equal-area projections (lower hemisphere) lineations (L) and axis folds (F) in the footwall and hanging-wall blocks of the Short Hill fault (SHF). Crenulation is the intersection of Sc and Sc cleavage. F1 folds bedding, and F2 folds Sc. [Download a high-quality PDF file.]

Crenulations.—Crenulations (Lc∧cc) of the intersection of Sc and Scc are abundant in the Catoctin Formation (footwall, northern, fig. 3C). The Lc∧cc plunge gently to the northeast or southwest and are presumed to be F2 (fig. 6).

FOLDS

The BR-SMA A a first-order fold with three limbs that underlie Blue Ridge-Elk Ridge (BR-ER), Short Hill-South Mountain (SH-SM), and Catoctin Mountain. The limbs of the first-order fold contain many second- and third-order folds that verge northwestward up the limbs. The second-and third-order folds mostly have S and Z shapes on the normal and overturned limbs, respectively, as viewed down-plunge to the northeast (fig. 7).

Figure 7. Schematic cross section showing the geometry and order of folds and foliations on the limbs of the Blue Ridge-South Mountain anticlinorium (BR-SMA). Cross sections and poles to bedding (So) of the Weverton Formation (Cw ) show the S- and Z-shaped geometry of the Hillsboro syncline (HS) and Lambs Knoll anticline (LKA). SHF, Short Hill fault; Sc, cleavage; Sb, bedding; Scc, crenulation cleavage; F1 and F2, folds. [Download a high-quality PDF file.]

BR-ER and SH-SM consist of the limbs of the second-and third-order recumbent folds of the Weverton Formation. BR-ER consists of a complex of tight, recumbent folds that plunge both northeast and southwest (figs. 6 and 7). The outcrop width of the complexly folded rocks that underlie BR-ER is 0.8 km. In contrast, SH-SM comprises a syncline-anticline pair of isoclinal folds that plunge to the northeast. These folds, the Hillsboro syncline (Southworth, 1991a, 1995) (HS, central and southern regions) and the Lambs Knoll anticline (LKA, northern, fig. 3B), have an outcrop width of 0.5 km. In figure 4, the cluster of poles to So are overturned beds on the homoclinal ridge that underlies SH-SM.

At least two different fold styles were recognized in the study area: (1) F1 isoclinal folds having axial planar Sc, and (2) F2 inclined folds having axial planar Scc. The F1 and F2 folds are roughly coaxial, and both plunge gently to the northeast and southwest. F1 are of bedding and F2 are of Sc. F2 was recognized in outcrop by folded Sc and Scc cleavage. The fold phases were established north of Purcell Knob. First mapped by Nickelsen (1956), the Purcell Knob folds (PKF) involve rocks from Middle Proterozoic hornblende gneiss through the Weverton Formation (footwall, central region, fig. 3B) (Southworth, 1991a). An antiformal syncline, a synformal anticline, and several inverted parasitic folds deform both basement and cover rocks. F2 antiforms and synforms are superimposed on the F1 and Sc. A syncline of Swift Run Formation east of the SHF has a complex map pattern suggestive of refolded folds (Mt. Olivet syncline, hanging wall, central region, fig. 3B). The Red Hill anticline is capped by sandstone of the Antietam Formation and plunges out adjacent to the end of Elk Ridge (RHA, footwall, northern, fig. 3B).

Mesoscopic folds of Sc along west-verging F2 (5-cm to 0.5-m wavelength) in the Catoctin (footwall) and Tomstown (hanging wall) Formations (Rohrersville fault, northern, fig. 3B). Microscopic folds include intrafolial folds, microkinks, and kink bands. Northwest-verging intrafolial isoclinal folds having horizontal fold axes were seen in the Tomstown Formation. Microkinks in the Tomstown Formation plunge gently northeast or southwest. Northwest-dipping dextral kinks in the Harpers Formation (hanging wall, central region) fold Sc, Sp, and Ssb.

FIELD RELATIONS OF THE SHORT HILL FAULT

CENTRAL REGION

HANGING WALL

The immediate hanging-wall rocks of the SHF are the Late Proterozoic and Lower Cambrian cover rocks of the Hillsboro syncline. The SHF cuts the Catoctin and Weverton Formations on the normal limb. It then cuts upsection into the Harpers Formation along a bend of Short Hill Mountain (fig. 8, loc. A). The SHF locally has cut upsection from the Catoctin into the Harpers Formation. It then cuts downsection to the Catoctin Formation on the south side of the Hillsboro gap. So in the quartzite of the Weverton Formation and Sc in the hanging-wall strata generally strike parallel to the fault, even at places where the fault bends. Hornblende gneiss is recognized in the hanging-wall block only east of Hillsboro gap.

Figure 8. Geologic map and cross section of the central region. Figure 9 shows the Hillsboro syncline (HS) south of location A. Figure 10 shows the Weverton core (WC) site north of the Potomac River (PR). PKF, Purcell Knob folds; SHF, Short fault. [Download a high-quality PDF file.]

The geometry of the Hillsboro syncline (HS) was determined where quartzite of the Weverton Formation was preserved on both limbs (Nickelsen, 1956) (fig. 9). The overturned east limb of the syncline contains some Z-shaped parasitic folds and intraformational thrust faults (Southworth, 1995).

Figure 9. Geologic map of the Hillsboro syncline (HS) and equal-area projections (lower hemisphere) of poles to bedding the Weverton Formation (Cw) where rocks on both limbs crop out. SHF, Short Hill fault. [Download a high-quality PDF file.]

The form of the syncline is defined by So in the lower and upper members of the Weverton Formation. The upper member on the normal limb is exposed in only five outcrops north of Hillsboro gap. The lower member on the normal limb is tectonically thinned and outcrops only in two places (Southworth, 1995). A backhoe trench exposed the lower member (fig. 8, loc. A). A tight syncline of the upper member of the Weverton Formation is exposed on the south side of Hillsboro gap above the Catoctin Formation (footwall).

The geometry of the syncline was determined by stereographic projection of poles to bedding because the fold hinge is not exposed. The central part of the syncline plunges 16° N. 68° E. Poles to So on both limbs plot in parallel arrays, so the macroscopic syncline is isoclinal as suggested by exposures at Hillsboro gap. The southern part of the syncline plunges 8° S. 12° E., approximately perpendicular to the regional northeast plunge of the BR-SMA. The limbs remain parallel. The northern part of the syncline is more open (arrays of poles to So on each limb are nearly perpendicular to one another), and plunges 31° S. 85° E.

FOOTWALL

The immediate footwall rocks of the SHF are predominantly hornblende gneiss and garnet monzogranite, but rocks of the Catoctin Formation are exposed in the footwall for 3 km southwest of Hillsboro gap (fig. 8). Metarhyolite of the Catoctin Formation in the immediate footwall south of Hillsboro gap is similar to that in the Catoctin Formation in the immediate hanging wall on the northwest side of the gap. Hornblende gneiss and metabasalt of the Catoctin Formation locally have a gently dipping Scc that cuts the steeper Sc. Both cleavages are parallel to the fault.

THE WEVERTON DRILL SITE AND CORE

The Weverton core was spudded in the Harpers Formation on the north side of the Potomac River (fig. 10) to determine the fault attitude and its kinematics. Hand excavations along U.S. Route 340 in 1989 exposed the SHF, which had placed Harpers Formation on garnet monzogranite. Top-to-the-northwest shear bands exposed that excavation suggested a thrust fault (Southworth, 1991b) the dip which varies from 80° to 40° over a short distance.

Figure 10. A, Geologic map of the Weverton core site. B, Schmidt nets show poles to phyllonitic foliation (Sp) in the Harpers Formation (Ch), bedding (So) of the Weverton Formation (Cw), and great circles of the mean attitude. SHF, Short Hill fault. Topographic contour interval 20 ft. See figure 8 (WC) for location. [Download a high-quality PDF file.]

The Harpers Formation here is a phyllonitic metasiltstone, and So was not recognized. Sp is kinked and cut by shear bands and east-verging backthrusts. To the east, overturned So in the quartzite of the Weverton Formation is parallel to Sc in the Harpers Formation (fig. 10). The garnet monzogranite and intrusive Late Proterozoic metadiabase dikes in footwall outcrops have a strong Sc that is also parallel to the Sp of the hanging wall. The Weverton core (WC) was drilled near an outcrop of Harpers Formation that has Sp that dips 35° SE. The drill hole shows that the SHF also dips 35° SE (fig. 10). Fault rock from the core show phyllonitic Harpers Formation against a phyllonitic Late Proterozoic metadiabase dike (fig. 11). The SHF was identified in the core by the marked increase in chlorite, magnetite, and small clots of calcite that define a difference in lithology. Ssb transects the fault so that the immediate hanging-wall and footwall rocks were deformed by the last active slip surfaces of the fault motion, as described by Woodward and others (1988).

Figure 11. Phyllonitic foliation, shear band cleavage, kinks, and back thrusts in the Weverton core and thin sections of the core (right) across the Short Hill fault (SHF). See figure 12 for photomicrographs of the thin section tracings. [Download a high-quality PDF file.]

DESCRIPTION AND INTERPRETATION OF MICROSTRUCTURES

The southeast-dipping Sp in the hanging-wall and footwall rocks recovered in the core has a greenschist-facies mineral assemblage of quartz, sericite, chlorite, calcite, and magnetite. Asymmetric recrystallized quartz, pressure shadows of quartz around magnetite, and enclaves of sericite of Sp indicate east-over-west motion (fig. 12). Sp is deformed by Ssb that dips at a low angle to the northwest and southeast (fig. 12A). The asymmetry and shear sense of the Ssb and Sp indicate east-over-west motion (fig. 12B). The Ssb contains recrystallized quartz, calcite, and some sericite. Sp and possibly Ssb are folded by northwest-dipping dextral kink bands (fig. 12C). Parallel to the kink bands are back thrusts that cut Sp and possibly Ssb (fig. 12D).

Figure 12. Photomicrographs (plane polarized light) of rocks cored from the Short Hill fault (SHF). A shows a thin section and a sketch of the Harpers Formation (Ch) thrust onto a metadiabase dike (Zmd) summarizing the location and orientation microstructures shown in B–D. B shows Sb cutting Sp (Ch), C shows kinks that fold Sp, and D shows backthrusts that cut Sp and Ssb cutting Sp. Light-colored minerals are quartz, sericite, calcite (decreasing abundance); dark-colored minerals are mostly chlorite, magnetite, and opaque residues. Photomicrographs are of thin sections of the Weverton drill core illustrated in figure 11. [Download a high-quality PDF file.]

The main southeast-dipping SHF plane is roughly parallel to Sp and both were deformed by Ssb. Ssb transects the fault and continues for 2 m into the metadiabase dike in the footwall. Ssb is present in the hanging wall at least 20 m above the SHF. The dextral kink bands continue for 1 m into the footwall, but the backthrusts are restricted to the hanging-wall rocks.

The microstructures have the same geometric orientation as Riedel shears (White and others, 1986). Northwest-directed thrust motion on the SHF formed the Sp and later Ssb. The dextral kink bands and backthrusts are related to further contraction.

SOUTHERN REGION

HANGING WALL

The immediate hanging-wall rocks of the SHF are the Late Proterozoic Swift Run and Late Proterozoic and Lower Cambrian Catoctin Formations of the Hillsboro syncline (HS) that underlie Black Oak Ridge (BOR) (fig. 13). The fault follows the contact of the Swift Run and Catoctin Formations, but in one area it underlies Middle Proterozoic biotite gneiss. Protomylonitic metasandstone and quartz-sericite schist of the Swift Run Formation occur in podiform outcrops along the fault. South of BOR, metasandstone of the Swift Run Formation is in fault contact with porphyroblastic granite gneiss. Farther south, the SHF was mapped as a diffuse zone of Sm as much as 1 km wide along the western margin of the leucocratic metagranite, which is the youngest basement unit in the study area (table 1).

Figure 13. A, geologic map, B, cross section, and C, aeromagnetic image of the southern region; light tones ore magnetic highs, and dark tones are magnetic lows. SHF, Short Hill fault; BOR, Black Oak Ridge; HS, Hillsboro syncline; PC, Purcellville core site. (A color version and description of the aeromagnetic image is in Southworth and others, 2005.) [Download a high-quality PDF file.]

FOOTWALL

The footwall rocks of the SHF are Middle Proterozoic quartzite, garnet graphite gneiss, hornblende gneiss, granite gneiss, and porphyroblastic granite gneiss, the lithologic boundaries of which trend northwest parallel to the steeply northeast dipping Sg (fig. 13). The lithologic boundaries within the gneiss are truncated by the SHF and are indicated on an aeromagnetic image (fig. 13). Middle Proterozoic rocks observed in the footwall are the oldest basement units in the study area. They were not observed in the hanging wall. In the immediate footwall, the Sg and Sc are subparallel to the fault. Quartzite has been strongly tectonized, and quartz veins are common in the gneisses.

THE PURCELLVILLE DRILL SITE AND CORES

The Purcellville drill holes (fig 14; loc. 1, 2) were selected to test the existence of the SHF in an area where the map pattern does not indicate a fault (McDowell and Milton, 1992) because both limbs of the Hillsboro syncline are exposed (fig. 14). Quartz-sericite schist and metasandstone of the Swift Run Formation on the fold limbs form ridges, and metabasalt of the overlying Catoctin Formation underlies the valley between them. The metasandstone has a strong southeast-dipping Sc that is parallel to the lower and upper contact. So was recognized neither in outcrop nor hand specimen. Strong lineations on Sc make this an L-S tectonite. To the immediate west, hornblende gneiss southeast-dipping Sc that cuts the dominant Sg that strikes N. 60° W. and dips 68° NE.

Figure 14. A, geologic map and schematic sections of Purcellville cores (1 and 2). B, stereonet showing poles to Sc cleavage and slickenlines in the Swift Run Formation of the west limb of the Hillsboro syncline (HS). C, photomicrograph (plane polarized light) showing west-verging folds of mylonitic biotite gneiss (hanging wall). SHF, Short Hill fault. Topographic contour interval 10 ft. [Download a high-quality PDF file.]

A core hole (1) spudded at the western edge of the Swift Run Formation recovered over 15 m of mylonitized biotite gneiss. A second core bole (2) spudded on metasandstone of the Swift Run Formation recovered 6 m of schist, metabasalt, vein quartz, metasandstone, metasiltstone, and dolomitic marble of the Swift Run Formation above sheared biotite gneiss (fig. 14). The hanging-wall rocks are thus tectonically mixed insofar that marble was only found at the contact of the Swift Run and Catoctin Formations elsewhere in this region. Folds of Sm in the biotite gneiss are overturned to the west (fig. 14). The interpreted fault contact of the hanging-wall biotite gneiss and the footwall hornblende gneiss was not penetrated. The relations here, however, suggest that the SHF locally lies within the gneiss of the allochthonous Hillsboro syncline.

NORTHERN REGION

“The puzzling problem is why the South Mountain sequence is repeated in Elk Ridge to the west and how this area terminates near Rohrersville.” Cloos, 1951

HANGING WALL

The hanging-wall rocks of the SHF are the Lower Cambrian strata on the overturned limb of the Lambs Knoll anticline (LKA) (fig. 15). The SHF was traced across steep topography from Gapland (G) to the valley bottom at Rohrersville (R) as it cuts upsection from the Harpers to the Tomstown Formation. The fault has a sinuous trace indicating a low dip and is outlined by creeks along the east side of Elk Ridge (ER). It wraps around the end of Elk Ridge (footwall) and can be traced south for 8 km. The fault is traced north along the east limb of the Red Hill anticline (RHA). Northward, the SHF was not recognized in a valley underlain by carbonate rocks of the Tomstown Formation.

Figure 15. Geologic map and cross section of the region. Water gap of Elk Ridge is immediately west of locality A. SHF, Short Hill fault; ER, Elk Ridge; ERC, Elk Ridge core; G, Gapland; GF, Gapland fault; LKA, Lambs Knoll anticline; RHA, Red Hill anticline; R, Rohrersville; RC, Rohrersville core. [Download a high-quality PDF file.]

The dominant Sc and Sp in the immediate hanging-wall rocks strike parallel to and dip away from the fault trace, even where the fault bends. Southeast-dipping Scc transects the SHF and is axial planar to northwest-verging inclined folds. A Ssb (similar to that seen in the Weverton core) in one outcrop of the Tomstown Formation dips northeast away from the SHF and parallel is footwall strata (fig. 15, loc. A). Ls on So of the Tomstown Formation have been folded by open, west-verging inclined folds. Pavement outcrops of the Tomstown Formation define broad domes and basins.

FOOTWALL

The SHF cuts rocks of the Swift Run Formation that have been down dropped into gneiss along the Gapland fault (GF) (fig. 15). It then cuts upsection from hornblende gneiss through the Swift Run to the Catoctin Formation. The SHF cuts westward across the Catoctin Formation and turns north and cuts upsection to the Weverton Formation. A reentrant in the SHF is where quartzite of the Weverton lies the end of Elk Ridge (ER). The SHF then cuts upsection (southward) from the Harpers Formation through the Antietam (northward) to the Tomstown Formation of the east limb of the Red Hill anticline (RHA). Sc strikes parallel to the fault in the immediate footwall. Away from the Catoctin Formation was mapped for 2.5 km southwest of the SHF. Lc∧cc fault (south) and microfold axes plunge gently to the northeast or southwest parallel to the folds of Sc

Map patterns and foliation attitudes (fig. 15) suggest that younger rocks (Tomstown Formation) overlie older rocks (Catoctin and Weverton Formations) along a flat-lying fault (Elliott's data in Wojtal, 1989) near Rohrersville (R). Reentrants along the trace of the SHF strongly suggest that the fault was folded over (1) Pleasant Valley at Rohrersville, (2) Elk Ridge, and (3) the valley east of the Red Hill anticline to form a series of antiforms and synforms.

ELK RIDGE DRILL SITE AND CORE

The Elk Ridge core (ERC) was drilled to test the hypothesis that rocks of the Tomstown Formation (hanging wall) overlie rocks of the Weverton Formation (footwall). West-dipping, beds of quartzite and interbedded phyllitic metasiltstone of the Weverton Formation have been cut by a strong, southeast dipping Sc at the north end of Elk Ridge (fig. 16). In the water gap of Elk Ridge to the south (fig. 15, west of loc. A), the lower member of the Weverton Formation has been gently folded and overturned to the west. So containing intrafolial folds in the Tomstown Formation (hanging wall) strikes parallel to and dips away from the SHF, north, east, and west of Elk Ridge.

Figure 16. Geologic maps and cross sections of the A, Elk Ridge core and B, Rohrersville core sites. SHF, Short Hill fault; MBC, Millbrook creek. Topographic contour interval is 10 ft. [Download a high-quality PDF file.]

Highly weathered limestone of the Tomstown Formation above phyllite and quartzite of the Weverton Form was recovered in the core, but the fault rock was not recovered because of the weathered nature of the strata. The fault dips 10° N. at this locality.

ROHRERSVILLE DRILL SITE AND CORE

At Rohrersville, metabasalt, metarhyolite, quartz sericite schist, and phyllite of the Catoctin Formation (footwall) form hilly topography. A karst lowland to the north is underlain by limestone and dolomite of the Tomstown Formation (hanging wall, fig. 16). These rocks are well exposed along Millbrook Creek (MBC) and crop out within 10 m of the SHF (fig, l7).

Figure 17. A, photographs of the Short Hill fault along Millbrook Creek showing west-verging folds of Tomstown Formation Ct (on photograph at left) above west-dipping metabasalt of the Catoctin Formation (CZc). Photograph at left is outcrop shown as a black dot; hammer is 30 cm long. B, enlargement of metabasalt (A, right) showing west-dipping cleavage (Sc) cut by southeast-dipping crenulation cleavage (Scc). Lens cap is 5 cm in diameter. Photomicrographs of rocks from this outcrop are shown in figure 19B. [Download a high-quality PDF file.]

Metabasalt and phyllite of the Catoctin Formation have been gently folded. So is not preserved. Sc dips northwest and Scc dips southeast (fig. l7). Near the fault, the Sp of the Catoctin Formation strikes N. 20° W. and dips 19° NE.

The first outcrop in the hanging wall of the SHF in Millbrook Creek is a large knocker of dolomite of the Tomstown Formation that has abundant veins of calcite and quartz. So could not be recognized. To the west, beds of limestone and dolomite strike east-west and dip 10° N. Strong Scc strikes N. 30° E. and dips 18° SE. These beds contain west-verging intrafolial folds. Mesoscopic, west-verging folds plunge gently to the northeast on the north side of the creek (fig. 17A).

The drill hole was spudded on the flood plain of Millbrook Creek approximately 25 m north of the fault trace projected from outcrops to the south. Sheared rocks of Tomstown Formation above Catoctin Formation were recovered. The fault is marked in the core by a gradation from white dolomite to chloritic phyllonite and metabasalt. Here, the SHF dips 25° N.

Marble of the Tomstown Formation (hanging wall) above phyllite of the Catoctin Formation (footwall) was seen in a 0.5-m-wide pavement outcrop (fig. 18), 0.5 km west of the Rohrersville drill site. The flat-lying SHF is folded with west-verging inclined folds.

Figure 18. A, outcrop of marble and dolomite of the Tomstown Formation (Ct) above phyllite of the Catoctin Formation (M). Hammer is 28 cm long. B, close-up of folded contact of Tomstown Formation on Catoctin Formation along the Short Hill fault. Lens cap is 5 cm in diameter. [Download a high-quality PDF file.]

DESCRIPTION AND INTERPRETATION OF MICROSTRUCTURES

Rocks of the Tomstown Formation (hanging wall) are L-S tectonites that contain west-verging intrafolial folds that have horizontal axial surfaces (fig. 19A). The foliation in the Catoctin Formation (footwall) is Sc and Sp. The metamorphic segregation layering of quartz and phyllosilicates in the Sc of the Catoctin Formation, contains magnetite that has asymmetric quartz pressure shadows that indicate east over west motion (fig. 19D).

Figure 19. Photographs of A, thin section of west-verging intrafolial folds (F1) and B, west-verging inclined folds (F2) in rocks of the Tomstown Formation (Ct; hanging wall). F1, intrafolial folds on the hanging wall are interpreted to synchronous with Sc on the footwall. C and D, photomicrographs of Sc cleavage cut by Scc cleavage in rocks of the Catoctin Formation on the footwall. Scc on the footwall is axial planar to F2 folds. [Download a high-quality PDF file.]

Southeast-dipping Scc cuts Sc (fig. 19C) and transects the fault. Scc is axial planar to northwest-verging F2 (fig. 19B). F2 in the Tomstown Formation fold So, Sc, and Ls. Scc is mostly discrete (Gray, 1979) and separates microlithons with apparent offset (fig. 19C). Insoluble residues of iron oxides that are concentrated in the cleavage laminae (fig. 19D) suggest that it is a pressure solution cleavage. The apparent offset is the result of volume loss (Gray, 1979; Groshong, 1988). Locally, Scc is zonal (Gray, 1979) and has realigned muscovite and chlorite.

At most places, So and Sc (hanging wall) and Sc and Sp (footwall) are parallel to the SHF. Sp (footwall) and the intrafolial folds in the L-S tectonites (hanging wall are related to thrust motion). The southeast-dipping Scc transects the SHF and is axial planar to northwest-verging F2 that fold the SHF.

SUMMARY

Geometry.—The SHF mostly dips 8° to 40 S° at the surface, and the dip changes over short distances. Its surface trace is sinuous. It dips 0 to 25° to any direction in the northern region. A stratigraphic separation diagram (Woodward, 1987) illustrates the geometry of the SHF both along strike (longitudinal) and across strike (fig. 20). Stratigraphic separation diagrams plot the geographic points where faults are at different stratigraphic levels along ramps and flats (Woodward, 1987). Figure 20 shows a profile similar to that of the Cordilleran thrust belt in Idaho and Wyoming (Woodward, 1987), except that the hanging-wall and footwall profiles of the SHF are inverted. In general, the SHF has a ramp geometry where it cuts upsection in the direction of transport (northern region). A longitudinal section (fig. 20) suggests that the geometry results from northeast-plunging fault blocks.

Figure 20. A, stratigraphic separation diagrams (longitudinal A–A' and transport direction, B–B', see index, center). B, the longitudinal cross section on the hanging wall suggests that the Short Hill fault (SHF) was folded with the Blue Ridge-South Mountain anticlinorium. [Download a high-quality PDF file.]

Kinematics.—The SHF is parallel to Sm and Sp that indicate east-over-west motion. Progressive deformation along the Sc produced the Sm (southern region) and Sp (central and northern regions). Intrafolial folds in L-S tectonites are related to thrust transport. Ssb cuts the Sp with east-over-west motion (central region).

Stratigraphic Relations.—Rocks in the hanging wall strike parallel to the SHF. Middle Proterozoic rocks in the footwall are truncated by the SHF and were not found in the hanging wall. The youngest basement rocks (leucocratic metagranite) are juxtaposed on the oldest basement rocks (paragneiss, porphyroblastic granite gneiss, and granitic gneiss) along the SHF in the southern region. The discontinuity of Middle Proterozoic lithologic units and foliations across the SHF suggests that the fault may have been active in the Middle Proterozoic. Late Proterozoic and Lower Cambrian rocks of the Swift Run, Catoctin, and Loudoun Formations are rift facies, but there is no stratigraphic evidence to support Late Proterozoic syndepositional extension along the SHF. Carbonate rocks of the Tomstown Formation, and possibly Waynesboro and Elbrook Formations (Brezinski, 1992), in the hanging wall of the SHF are passive continental margin deposits that postdate the Iapetus rifting event. Early Jurassic dikes that cut the SHF demonstrate that the fault has not been active in the last 200 m.y.

Structural Relations.—The structural elements along the SHF can be related to two phases (D1 and D2) of contractional deformation (table 3).

D1 Sc, axial planar to Fv formed during prograde greenschist-facies metamorphism. The SHF is generally parallel to Sc and both are folded. Sm and Sp are coplanar to and possibly synchronous with Sc. Sp is parallel to the plane of the SHF and Sm is parallel to the trace of the SHF. Sssb is restricted to the SHF where it cuts Sp with east-over-west motion, and they may have formed in a continuum.

D2 Scc is axial planar to F2 west-verging folds. Scc is a pressure solution cleavage formed in a sub-greenschist-facies metamorphic environment. Scc transects the SHF so it postdates it. Lc∧cc are coaxial to Scc and F2.

Time Relations of Folding and Faulting.—F1 of the hanging-wall rocks (Hillsboro syncline and Lambs Knoll anticline) are genetically related to contractional deformation of the SHF. These folds were cut obliquely by the SHF during a contractional reactivation. West-verging F2 (5-cm to 0.5-m wavelength) are interpreted to be related to the folding of the SHF (hanging wall and footwall, northern region). The SHF has been folded over (1) Pleasant Valley at Rohrersville (antiforms and synforms), (2) Elk Ridge (antiform), and (3) the valley east of the Red Hill anticline (synform) (northern region) (figs. 21 and 22). Thus the SHF has been folded (F2) and cut by attendant axial planar Scc (fig. 21). The parallelism between Lc∧cc microfold axes, and statistical fold axes obtained from poles to Sc and Scc along the SHF (fig. 21) and the orientation of the folded fault supports a common F2 folding event. The Locust Grove fault, Lambs Knoll fault, and the Turner Gap fault were subsequent contractional faults that cut the hanging-wall strata in the northern region.

Figure 21. Summary structural map of the Short Hill fault (SHF) showing—cleavage (Sc) traces that are parallel to limbs of minor folds superimposed on the SHF, crenulation cleavage (Scc that transects the SHF (axial planar to F2 folds that fold the SHF), and the antiforms and synforms of the folded SHF. Stereonets illustrate the folded SHF in the northern region. A, structures in the hanging wall (HW; Tomstown Formation, Ct) and B, footwall (FW; Catoctin Formation, Czc) rocks are coplanar (B is trend and plunge of fold as determined by computer stereographic projection); C, poles to Sc cleavage along the SHF define a great circle of the north-northeast-plunging Blue Ridge-South Mountain anticlinorium; D, Scc, and Lc∧cc are related to the folding of the fault. [Download a high-quality PDF file.]

Figure 22. A, side-looking airborne radar image of the northern half of the study area showing location and perspective (open arrow) of photographs B–D, which illustrate the regional F2 folds superimposed on the Short Hill fault (SHF). Note the parallelism in structures of the footwall (FW) and hanging wall (HW) blocks. The westward bend at Gapland (G) is parallel to the bend at Beelers Summitt (BS); the Red Hill anticline (RHA), Elk Ridge antiform (ERA), and Lambs Knoll anticline (LKA) plunge out along the same zone. B, Photograph looking down plunge from Virginia to Lambs Knoll anticline showing the sinuous ridge that is parallel to the SHF; C, Photograph looking up plunge from the upright limb of the Lambs Knoll anticline to the Hillsboro syncline that is out of view due to the sinuous structure. D, Up plunge view of Pleasant Valley showing the broad footwall fold from Beelers Summitt (BS) to the termination of Elk Ridge (ER). [Download a high-quality PDF file.]

KINEMATICS OF THE SHORT HILL FAULT

A composite cross section of the three regions was made to define the prethrust geometry and to determine the kinematics of the SHF (fig. 23A). The restored geometry of the SHF is schematic because the following assumptions had to be made: (1) thrust displacement on the SHF was small and (2) the restored SHF dips 30° SE.

Figure 23. A, composite cross section of the Short Hill fault (SHF) and B, with hanging-wall faults restored. Cross section A–A' in A corresponds to A–A' in figure 15. C, restored cross section of B showing the normal fault geometry. D, schematic illustration of the contractional reactivation of the interpreted normal fault. [Download a high-quality PDF file.]

Contractional faults in the hanging-wall block (LGF, LKF, and TGF) were restored first (fig. 23B). The cutoff points (where the SHF crosses a stratigraphic boundary) (Woodward, 1987) along the SHF are pinned. The folded fault and strata were straightened to the relative positions prior to contractional deformation. The Hillsboro syncline (HS) and the Lambs Knoll anticline (LKA) restore to a gently dipping sequence of strata along a southeast-dipping fault (fig. 23C). Therefore, the SHF is a normal fault that predated contractional deformation. as suggested in the northern region by Wojtal (1989) and Brezinski (1992). The kinematics of the SHF may be explained by inversion tectonics, in which early extensional faults control the geometry of contractional structures during later deformation (Cooper and Williams, 1989). The mechanics of this process were reconstructed in figure 23D. Contractional reactivation of the early normal fault formed a hanging-wall ramp fold (HS and LKA).

Figure 24 illustrates a map view of the sequence of deformation and the chronology of foliations. In T1, the early normal fault was approximately parallel to its present regional northeast trend. In T2, contractional reactivation of the SHF formed the hanging-wall folds (HS and LKA) during prograde greenschist-facies metamorphism. Progressive deformation of Sc formed Sm and Sp. and Ls formed during thrust transport. In T3, the normal limb of the HS was cut by the reactivated SHF (fig. 24, loc. A), a splay (LGF) cut hanging-wall strata, and the LKF duplicated the lower member of the Weverton Formation of the LKA. Sb recorded the last slip on the thrust fault. In T4, the SHF was folded to form the Elk Ridge (ER) antiform, the Red Hill anticline (RHA), and the intervening synform. Scc axial planar to F2, Lc∧cc and dextral kinks and backthrusts are related to folding of the SHF. In T4, the out-of-sequence Turner Gap fault (TGF) cuts the LKF and LGF.

Figure 24. Map view of the interpreted sequence of deformation (T1–T4). T1 is post-Early Cambrian normal faulting, T2 and T3 are late Paleozoic contractional reactivation of the Short Hill fault (SHF), and T4 is the latest Paleozoic folding of the SHF. Foliations that are related to the deformation sequences are shown at the bottom. Locality A in T3 is where the reactivated SHF cuts the axial surface of the Hillsboro syncline. ER, Elk Ridge; GF, Gapland fault; HS, Hillsboro syncline; LGF, Locust Grove fault; LKA, Lambs Knoll anticline; LKF, Lambs Knoll fault; RHA, Red Hilt anticline; TGF, Turner Gap fault. [Download a high-quality PDF file.]

Brezinski's (1992) Rohrersville fault is coincident with the Short Hill fault (South Mountain fault of Brezinski, 1992) because they are part of the same younger-over-older fault strand (fig. 25). The South Mountain fault, Rohrersville fault, and the Eakles Mills fault of Brezinski (1992) are younger-over-older faults that are here interpreted to be the SHF. The South Mountain fault and the Sans Mar fault of Brezinski (1992), as well as the Locust Grove fault, are late thrust faults that splayed from the South Mountain fault system and imbricated the hanging-wall strata during contractional reactivation of the Short Hill fault.

Figure 25. Structural map showing the relationship of the Short Hill fault (SHF) to faults of Brezinski (1992) in Maryland. [Download a high-quality PDF file.]

REGIONAL ANALOGIES

Faults that share many of the same characteristics of the SHF are found in the Blue Ridge province from central Virginia to Pennsylvania (fig. 26). The southern region of the SHF is similar to the Rock Fish Valley fault (RFVF) (Bartholomew and others, 1991) that placed Middle Proterozoic amphibolite-facies rocks of the Lovingston massif on granulite-facies rocks of the Pedlar massif. The RFVF is either a (1) Grenvillian terrane boundary (Bartholomew and others, 1991), (2) the result of Late Proterozoic extension, Paleozoic contraction, and retrogressive hydration (Evans, 1991), or (3) a Paleozoic thrust fault (Bailey and Simpson, 1993). Kinematic indicators along the southern RFVF, however, indicate that a Late Proterozoic(?) extensional fabric has been overprinted by a Paleozoic contractional fabric (Simpson and Kalaghan, 1989). The RFVF marks the westernmost boundary between rigid and highly extended crust (Wehr and Glover, 1985). The main extension occurred to the east (Bailey and Simpson, 1993).

Figure 26. Index map of the Blue Ridge-South Mountain anticlinorium (BR-SMA) showing faults analogous to the Short Hill fault (SHF). RFVF, Rock Fish Valley fault; MRS, Mechum River synclinorium. A, Snowden fault (SF) and related faults along James River (modified from Spencer and others, 1989); B, faults near Tye River (modified from Werner, 1966); C, South Mountain anticlinorium (SMA) (modified from Berg and others, 1980); ACF, Antietam Cove fault; 1 and 2, faults that place Tomstown Formation on Catoctin Formation. [Download a high-quality PDF file.]

The Mechum River synclinorium (MRS) extends for 96 km along the core of the BR-SMA. The MRS was filled by rift facies sediments of the Mechum River Formation (Schwab, 1986). It is overturned to the west and locally faulted (Gooch, 1958; Nelson, 1962; Allen, 1963; Lukert and Nuckols, 1976; Bailey and Simpson, 1993). It has a zone of mylonitic gneiss along its the western margin (Conley, 1989). The MRS lies between the RFVF and the Robertson River Igneous Suite and suggests that the axial region of the BR-SMA had been a rift zone that was reactivated by contractional deformation. The SHF and Hillsboro syncline may be an en echelon continuation of this axial rift zone.

South of Charlottesville, Va., at least five younger-over-older faults have contractional structures like the SHF. The southeast-dipping Snowden fault (SF) (Brown and Spencer, 1981) (fig. 26A) has placed the Lower Cambrian Unicoi and Harpers Formations on the Middle Proterozoic Pedlar Formation. It was interpreted as a folded Early Cambrian synsedimentary normal fault (Spencer and others, 1989; Simpson and Eriksson, 1989). To the north, at least two faults have brought the Unicoi Formation onto Pedlar Formation (Werner, 1966) (fig. 26B). Bloomer and Warner (1955) and Werner (1966), however, interpreted these as thrusts that have truncated closely spaced folds.

The north-plunging BR-SMA is structurally discordant to the south-plunging South Mountain anticlinorium (SMA) in Pennsylvania (MacLachlan, 1991) along the younger-over-older Antietam Cove fault (ACF) (Root, 1970) (fig. 26C).

The doubly plunging SMA can be divided into at least three bounded blocks on the basis of structure and the distribution and chemistry of the Catoctin Formation (Smith and others, 1991). The faults that bound these blocks were considered to be normal (Late Proterozoic cross-strike faults of Root and Hoskins, 1977, and Mesozoic strike-parallel faults of Root, 1989). TWo of the northeast-striking younger-over-older faults (fig. 26C, 1 and 2) place Tomstown Formation on Catoctin Formation (Berg and others, 1980) as does the SHF.

Serial cross sections across the BR-SMA suggest that the SHF may be part of a complex fault system that records the history of reactivation and reversal of motion (fig. 27). Cross sections of the study area (C–C' and E–E') are related to regional cross sections (north and south) that involve the analogous younger-over-older faults.

Figure 27. Geologic map and cross sections of the Blue Ridge-South Mountain anticlinorium (BR-SMA) showing the Short Hill fault (SHF), Mechum River syncline (MRS), Rock Fish Valley fault (RFVF), Horner Run fault (HRF), Turner Gap fault (TGF), Antietam Cove fault (ACF), Locust Grove fault (LGF), and the South Mountain fault (SMF). Cross sections C–C', D–D', and E–E' are of the study area; A–A' and H–H' are modified from Berg and others (1980) and Boyer and Mitra (1988), respectively. [Download a high-quality PDF file.]

The SHF placed hanging-wall rocks of Short Hill-South Mountain on footwall rocks of Blue Ridge-Elk Ridge in the BR-SMA (fig. 27, C–C'). The BR-SMA was stacked en echelon with the South Mountain anticlinorium. (Pennsylvania) by the younger-over-older ACF (A–A'). Late faults of the South Mountain fault (SMF) system imbricated the strata of the west limb (BR-SMA) and cut the ACF (B–B') (Brezinski and others 1991; Campbell and others, 1992); the LGF is part of this system (C—C'). To the south, the SHF may link with a shear zone in the basement gneiss (J.W. Clarke, U.S. Geological Survey, oral commun., 1991), where felsic metavolcanic and metavolcaniclastic rocks of unknown affinity are overlain by rocks of the Fauquier Formation in the hanging wall (P.T. Lyttle. U.S. Geological Survey, oral commun., 1992) (fig. 27, G–G'). These rocks on the east limb of the BR-SMA have been cut by the Late Proterozoic Horner Run fault (HRF) (Espenshade, 1986), and the western contact may be down-faulted along the axial rift zone. Outliers of Mechum River (Lukert and Nuckols, 1976; Conley, 1989) and Fauquier Formations (Clarke, 1984) to the south may link the shear zone with the axial rift zone (fig. 27, H–H').

These regional relationships suggest that the SHF and analogous faults are probably Late Proterozoic to Early Cambrian normal faults that were reactivated contractionally and folded in the late Paleozoic. Late thrust faults imbricated the hanging-wall strata of the SHF (northern region) and are here interpreted to be splays from the reactivated SHF.

TECTONIC FRAMEWORK

The regional cross sections (fig. 27) and the interpretation of a seismic reflection profile were used to portray the tectonic framework of the BR-SMA in this region (fig. 28). My interpretation of a seismic reflection profile (Southworth, 1993a) suggests that faint reflectors beneath the surface trace of the SHF do not displace the underlying reflector. The reflector is probably Cambrian and Ordovician carbonate platform rocks in the footwall of the Blue Ridge thrust sheet. Because the reflector is continuous, the North Mountain footwall ramp is beneath the Culpeper basin, rather than beneath the BR-SMA (see fig. 1, B–B') (Mitra, 1987; Evans, 1988) as suggested by Lampshire and others (1994) (fig. 28). Reactivation of the footwall tectonic ramp during Mesozoic extension formed the western border fault of the basin. The fault-bend fold geometry of northeast-plunging BR-SMA and the folding of the SHF are the result of late doming by an antiformal duplex in the footwall of the Blue Ridge thrust sheet as described elsewhere by Hatcher (1991). The SHF was simply exhumed and folded by a subthrust duplex during a late stage of the Alleghanian orogeny.

Figure 28. A, block diagram; B, schematic cross sections showing the Early Paleozoic setting; and C, cross section A–A' interpreted tectonic framework of the region (modified from Evans, 1988). BRFR, Blue Ridge footwall ramp; NMFR, North Mountain footwall ramp; NMF, North Mountain fault; MBF, Mesozoic border fault; SHF, Short Hill fault; BR-SMA, Blue Ridge-South Mountain anticlinorium. [Download a high-quality PDF file.]

RELATION OF THE SHORT HILL FAULT TO OROGENIC EVENTS

The SHF resulted from the tectonic inversion of a half graben. The half graben must have postdated the Lower Cambrian Tomstown Formation and possibly the Middle and Upper Cambrian Elbrook Formation (Brezinski, 1992). It must have predated the regional greenschist-facies metamorphism and contractional deformation that was probably synchronous with the Alleghanian orogeny (Burton and others, 1992b; Kunk and others, 1993).

Analogous faults in the BR-SMA, which are characterized by younger-over-older and low-grade on high-grade rocks, support a general interpretation that the BR-SMA exposes exhumed and reactivated normal faults of the rifted continent during Paleozoic orogenesis. Compression during the Alleghanian orogeny has obscured the evidence; uplift and subsequent erosion has exposed the evidence. Contractional reactivation of early extensional faults in orogenic belts was common in the western Cordillera, Alps, Pyrenees, and Caledonides (LePichon and others, 1982; Soper and Anderton, 1984; Cooper and Williams, 1989; Mitra, 1993).

The prevailing tectonic model, based largely on rocks exposed west of the BR-SMA, illustrates a passive continental margin from the deposition of the Lower Cambrian Tomstown Formation until the late Middle and early Late Ordovician Taconic orogeny (Read, 1989a). Extensional faults in Middle Proterozoic through Ordovician rocks in the Appalachian orogen were involved in (1) the development of the presuccessor basins in the southern Appalachian Blue Ridge (Tull and Groszos, 1990), (2) the breakup of the Taconic shelf in the Taconides of the northern Appalachians (Zen, 1967; Schamel and others, 1984), (3) Late Proterozoic to Early Cambrian rifting of the Iapetus Ocean (Gresko, 1985; Spencer and others, 1989), and (4) the latest Early to Middle Cambrian Rome trough (Wagner, 1976; Webb, 1980; Read, 1989a; Ryder, 1992; Ryder and others, 1992). The Early Cambrian Snowden fault (Spencer and others, 1989) involves rocks only as young as the Harpers Formation, but this could be a function of the erosion level. Faults in the core of the SMA in Pennsylvania that place the Tomstown Formation above the Catoctin Formation, like the SHF, may provide more evidence of Early Cambrian extension.

The SHF and similar faults in the Appalachians suggest that extensional faulting occurred in the early Paleozoic (fig. 29). The normal faults could be the result of (1) continental rifting associated with the Rome trough (Read, 1989b), (2) early growth faults in the Appalachian basin, similar to faults described by Stewart and others (1993), or (3) breakup of the continental shelf at the onset of the Taconic orogeny, as discussed by Bradley (1989). The extensional tectonic regime of the Rome trough is poorly understood, but it provides strong evidence that continental rifting was active in Cambrian time.

Figure 29. Tectonic map and schematic restored cross section (modified from Read, 1989a) of the central Appalachians showing the interpreted extensional faults of Cambrian age (modified from Ryder, 1992; Ryder and others, 1992). Cambrian carbonate platform and slope rocks are Tomstown (T), Rome and Waynesboro (R), and Elbrook (E) Formations. [Download a high-quality PDF file.]

The Short Hill fault is an early Paleozoic normal fault that was contractionally reactivated during late Paleozoic orogenesis. Thrust motion and folding resulted from the emplacement of an antiformal duplex at depth during the Alleghanian orogeny.

Similar faults in Virginia and Pennsylvania warrant investigation to determine if contractional reactivation of extensional faults (tectonic inversion) was a common process in Appalachian orogenesis. The BR-SMA may be a composite of fault blocks that constituted the Late Proterozoic and Early Cambrian rifted continental margin. Faults that do not fit the prevailing tectonic models for the region need to be reappraised. This reappraisal may ultimately be the basis for new concepts on the evolution of the Appalachian orogenic belt.

REFERENCES CITED

Aleinikoff, J.N., Walter, Marianne, Lyttle, P.T., Burton, W.C., Leo, G.W., Nelson, A.E., Schindler, J.S., and Southworth, C.S., 1993, U-Pb zircon and monazite ages of Middle Proterozoic rocks, Northern Blue Ridge, Virginia [abs.]: Geological Society of America Abstracts with Programs, v. 25, no. 2, p. 2.

Aleinikoff, J.N., Zartman, R.E., Walter, Marianne, Rankin, D.W., Lyttle, P.T., and Burton, W.C.,1995, U-Pb ages of metarhyolites of the Catoctin and Mount Rogers Formations, central and southern Appalachians—Evidence for two pulses of Iapetan rifting: American Journal of Science, v. 295, no. 4, p. 428–454.

Allen, R.M., Jr., 1963, Geology and mineral resources of Greene and Madison Counties: Virginia Division of Mineral Resources Bulletin 78, 102 p.

Badger, R.L., and Sinha, A.K., 1988, Age and Sr isotopic signature of the Catoctin volcanic province—Implication for subcrustal mantle evolution: Geology, v. 16, no. 8, p. 692–695.

Bailey, C.M., and Simpson, Carol, 1993, Extensional and contractional deformation in the Blue Ridge province, Virginia: Geological Society of America Bulletin, v. 105, no. 4, p. 411–422.

Bartholomew,, M.J., Lewis, S.E., Hughes, S.S., Badger, R.L., and Sinha, A.K., 1991, Tectonic history of the Blue Ridge basement and its cover, central Virginia, in Schultz, A.M., and Compton-Gooding, Ellen, eds., Geologic evolution of the eastern United States; Field trip guidebook of the Northeast-Southeast section of the Geological Society of America: Virginia Museum of Natural History Guidebook, p. 57–90.

Berg, T.M., Edmunds, W.E., Geyer, A.R., Glover, A.D., Hoskins, D.M., MacLachlan, D.B., Root, S.E., Sevon, W.D., and Socolow, A.A., 1980, Geologic map of Pennsylvania: Harrisburg, Pennsylvania Department of Environmental Resources, Geological Survey, 4th Series, scale 1:250,000.

Bloomer, R.O., and Bloomer, R.R., 1947, The Catoctin Formation in central Virginia: Journal of Geology, v. 55, no. 1, p. 94–106.

Bloomer, R.O., and Werner, H.J., 1955, Geology of the Blue Ridge region in central Virginia: Geological Society of America Bulletin, v. 66, no. 5, p. 579–606.

Boyer, S.E., and Mitra, Gautam, 1988, Relations between deformation of crystalline basement and sedimentary cover of the basement/cover transition zone of the Appalachian Blue Ridge province: Geological Society of America Special Paper 222, p. 119–136.

Bradley, D.C., 1989, Taconic plate kinematics as revealed by fore-deep stratigraphy, Appalachian orogen: Tectonics, v. 8, no. 5, p. 1014–1049.

Brezinski, D.K., 1992, Lithostratigraphy of the western Blue Ridge cover rocks in Maryland: Maryland Geological Survey Report of Investigations, no. 55, 69 p.; map scale 1:62,500.

Brezinski, D.K., Campbell, P.A., and Anderson, T.H., 1991, Progressive deformation of cover rocks at the western margin of the Blue Ridge in Maryland [abs.]: Geological Society of America Abstracts with Programs, v. 23, no. 1, p. 10.

Brown, C.E., and Spencer, E.W., 1981, Geologic map of the James River Face Wilderness, Bedford and Rockbridge Counties, Virginia: U.S. Geological Survey Miscellaneous Field Studies Map MF-1337-A, scale 1:24,000.

Burton, W.C., Froelich, A.J., Schindler, J.S., and Southworth, Scott, 1992a, Geologic map of Loudoun County, Virginia: U.S. Geological Survey Open-File Report 92-716, 16 p, map scale 1:100,000, 1 sheet.

Burton, W.C., Kunk, M.J., and Lyttle, P.T., 1992b, Age constraints on the timing of regional cleavage formation in the Blue Ridge anticlinorium, northernmost Virginia [abs.]: Geological Society of America Abstracts with Programs, v. 24, no. 2, 5.

Burton, W.C., and Southworth, C.S., 1993, Garnet-graphite paragneiss and other country rocks in granitic Grenvillian basement, Blue Ridge anticlinorium, northern Virginia and Maryland [abs.]: Geological Society of America Abstracts with Programs, v. 25, no. 4. p. 6.

Campbell, P.A., Brezinski, D.K., and Anderson, T.H., 1992, Ductile deformation zones along the west Bank of the northern Blue Ridge [abs.]: Geological Society of America Abstracts with Programs, v. 24, no. 1, p. 11.

Clarke, J.W., 1984, The core of the Blue Ridge anticlinorium in northern Virginia: Geological Society of America Special Paper 194, p. 155–160.

Cloos, Ernst, 1951, Structural geology of Washington County, in The physical features of Washington County: Maryland Department of Geology, Mines, and Water Resources, p. 124–163.

———1971, Microtectonics along the western edge of the Blue Ridge, Maryland and Virginia: Baltimore, Md., The Johns Hopkins University Studies in Geology, no. 20, 234 p.

Conley, J.F., 1989, Stratigraphy and structure across the Blue Ridge and Inner Piedmont in central Virginia: American Geophysical Union Field Trip Guidebook T207, 23 p.

Connelly, J.B., and Woodward, N.B., 1992, Taconian foreland-style thrust system in the Great Smoky Mountains, Tennessee: Geology, v. 20, no. 2, p. 177–180.

Cooper, M.A., and Williams, G.D., eds., 1989, Inversion tectonics: Geological Society of London Special Publications, no. 44, 375 p.

Dean, S.L., Kulander, B.L., and Skinner, J.M., 1988, Structural chronology of the Alleghanian orogen in southeastern West Virginia: Geological Society of America Bulletin, v. 100, no. 2, p .299–310.

Demicco, R.V., 1985, Platform and off-platform carbonates of the Upper Cambrian of western Maryland, U.S.A.: Sedimentology, v. 32 no. 1, p. 1–22.

Dennis, A.J., and Secor, D.T., Jr., 1990, On resolving shear direction in foliated rocks deformed by single shear: Geological Society of America Bulletin, v. 102, no. 9, p. 1257–1267.

Drake, A.A., Jr., Sinha, A.K., Laird, Jo, and Guy, R.E., 1989, The Taconic orogen, in Hatcher, R.D., Jr., Thomas, W.V., and Viele, G.W., eds., The Appalachian-Ouachita Orogen in the United States; The Geology of North America: Boulder, Colo., Geological Society of America, v. F–2 p. 362–366.

Elliott, David, Fisher, G.W., and Snelson, S., 1982, A restorable cross section through the central Appalachians [abs.]: Geological Society of America Abstracts with Programs, v. 14, no. 7, p. 482.

Espenshade, G.H., 1986, Geology of the Marshall quadrangle, Fauquier County, Virginia: U.S. Geological Survey Bulletin 1560, 60 p.

Evans, M.A., 1988, The structural geometry and evolution of foreland thrust systems, northern Virginia: Geological Society of America Bulletin, v. 101, no. 3, p. 339–354.

Evans, N.H., 1991, Latest Precambrian to Ordovician metamorphism in the Virginia Blue Ridge—Origin of the contrasting Lovingston and Pedlar basement terranes: American Journal of Science, v. 291, no. 5, p. 425–452.

Fauth, J.L, 1977, Geologic map of the Catoctin Furnace and Blue Ridge Summit quadrangles, Maryland: Maryland Geological Survey, scale 1:24,000.

Favret, P.D., and Williams, R.T., 1988, Basement beneath the Blue Ridge and Inner Piedmont in northeastern Georgia and the Carolinas—A preserved, Late Proterozoic, rift continental margin: Geological Society of America Bulletin, v. 100, no.12, p. 1999–2007.

Geiser, P.A., and Engelder, T., 1983, The distribution of layer parallel shortening fabrics in the Appalachian foreland of New York and Pennsylvania: Evidence for two non-coaxial phases of the Alleghanian orogeny: Geological Society of America Memoir 158, p. 161–175.

Gooch, E.O., 1958, Infolded metasediments near the axial zone of the Catoctin Mountain-Blue Ridge anticlinorium in Virginia: Geological Society of America Bulletin, v. 69, no. 5, p. 569–574.

Gray, D.R., 1979, Microstructure of crenulation cleavages-An indicator of cleavage, origin: American Journal of Science, in 279, no. 1, p. 97-128.

Gray, Mary Beth, and Mitra, Gautam, 1993, Migration of deformation fronts during progressive deformation—Evidence from detailed structural studies in the Pennsylvania Anthracite region, U.S.A.: Journal of Structural Geology, v. 15, no. 3–5, p. 435–449.

Gresko, M.J., 1985, Analysis and interpretation of compressional (P-wave) and shear (SH-wave) reflection seismic and geologic data over the Bane Dome, Giles County, Virginia: Blacksburg, Virginia Polytechnic Institute and State University, Ph.D. dissertation, 110 p.

Groshong, R.H., Jr., 1988, Low-temperature deformation mechanisms and their interpretations: The Geological Society of America Bulletin, v. 100, no. 9, p. 1329–1360.

Hadley, J.B., and Goldsmith, Richard, 1963, Geology of the eastern Great Smoky Mountains, North Carolina and Tennessee: U.S. Geological Survey Professional Paper 349–B, 118 p.

Harris, L.D., 1979, Similarities between the thick-skinned Blue Ridge anticlinorium and the thin-skinned Powell Valley anticline: Geological Society of America Bulletin, v. 90, pt. 1, p. 525–539.

Hatcher, R.D., Jr., 1989, Tectonic synthesis of the U.S. Appalachians, in Hatcher, R.D., Jr., Thomas, W.A., and Viele, G.W., eds, The Applachian-Ouachita orogen in the United States; The Geology of North America: Boulder, Color., Geological Society of America, v. F–2, p. 511–535

———1991, Interactive property of large thrust sheets with footwall rocks—The Subthrust Interactive Duplex Hypothesis,, a mechanism of dome formation in thrust sheets: Tectonophysics, v. 191, p. 237–242.

Herz, Norman, and Force, E.R., 1984, Rock suites in Grenvillian terrane of the Roseland district, Virginia—Part 1. Lithologic relations, in Bartholomew,, M.J., ed., The Grenville event in the Appalachians and related topics: Geological Society of America Special Paper 194, p.187–199.

Howard, J.L, 1991, Lithofacies of the Precambrian basement complex in the northernmost Blue Ridge province of Virginia: Southeastern Geology, v. 31, no. 4, p. 191–202.

Hutson, F.E., and Tollo, R.P, 1992, Mechum River formation—Evidence for the evolution of a late Proterozoic rift basin, Blue Ridge anticlinorium, Virginia [abs.]: Geological Society of America Abstracts with Programs, v. 24, no. 2, p. 22.

Johnson, P.R., Zietz, Isidore, and Thomas, W.A., 1994, Possible Neoproterozoic-early Paleozoic grabens in Mississippi, Alabama, and Tennessee: Geology, v. 22, no. 1, p. 11–14.

Jonas, A.I., and Stose, G.W., 1939, Age relation of the Precambrian rocks in the Catoctin Mountain–Blue Ridge and Mount Rogers anticlinoria in Virginia: American Journal of Science, v. 237, no. 8, p. 575–593.

Keith, Arthur, 1894, Harpers Ferry, Virginia, West Virginia, and Maryland, of Geologic Atlas of the United States: U.S. Geological Survey, Folio 10, 5 p.; scale 1: 125,000, 4 sheets.

King, P.B., 1950, Geology of the Elkton area, Virginia: U.S. Geological Survey Professional Paper 230, 82 p.

———1964, Geology of the central Great Smoky Mountains, Tennessee: U.S. Geological Survey Professional Paper 349–C, 148 p.

Kline, S.W., 1991, Phyllonitic zones in the Grenville terrane of the northern Virginia Blue Ridge [abs.]: Geological Society of America Abstracts with Programs, v. 23, no. 1, p. 54.

Kline, S.W., Lyttle, P.T., and Schindler, J.S., 1991, Late Proterozoic sedimentation and tectonics in northern Virginia, in Schultz, A.J., and Compton-Gooding, Ellen, eds., Geologic evolution of the eastern United States; Field trip guidebook of the Northwest-Southeast Section of the Geological Society of America: Virginia Museum of Natural History Guidebook 2, p. 263–294.

Kunk, M.J., Froelich, A.J., and Gottfried, D., 1992, Timing of emplacement of diabase dikes and. sheets in the Culpeper basin and vicinity, Virginia and Maryland—40Ar-39Ar age spectrum results from hornblende and K-feldspar in granophyres [abs.]: Geological Society of America Abstracts with Programs, v. 24, no. 1, p. 25.

Kunk, M.J., Lyttle, P.T., Schindler, J.S., and Burton, W.C., 1993, Constraints on the thermal history of the Blue Ridge in northernmost Virginia—40Ar/39Ar age dating results [abs.]: Geological Society of America Abstracts with Programs, v. 25, no. 2, p. 31.

Lampshire, L.D., Coruh, Cahit, and Costain, J.K., 1994, Crustal structures and the eastern extent of Lower Paleozoic shelf strata within the central Appalachians—A seismic interpretation: Geological Society of America Bulletin, v. 106, no. 1, p. 1–18.

LePichon, X., Angelier, J., and Sibuet, J.-C., 1982, Plate boundaries and extensional tectonics: Tectonophysics, v. 81, p. 239–256.

Lukert, M.T., and Nuckols, E.B., III, 1976, Geology of the Linden and Flint Hill quadrangles, Virginia: Virginia Division of Mineral Resources Report of Investigations 44, 83 p.

MacLachlan, D.B., 1991, A tale of two mountains—Both south, in 56th Annual Field Conference of Pennsylvania Geologists Guidebook: Pennsylvania Geological Survey, Harrisburg, p. 47–53.

McDowell, R.C., and Milton, D.J., 1992, Geologic map of the Round Hill quadrangle, Clarke and Loudoun Counties, Virginia and Jefferson County, West Virginia: U.S. Geological Survey Geologic Quadrangle Map GQ–1702, 11 p.; scale 1:24,000, 1 sheet.

Mitra, Gautam, 1978, Ductile deformation zones and mylonites—The mechanical processes involved in the deformation of crystalline basement rocks: American Journal of Science, v. 278, no. 8, p. 1057–1084.

Mitra, Gautum, and Elliott, David, 1980, Deformation of basement in the Blue Ridge and the development of the South Mountain cleavage, in Wones, D.R., ed., The Caledonides in the U.S.A.; Proceeding of the International Geological Correlation Program Project 27—Caledonide orogen, 1979: Blacksburg, Va., Virginia Polytechnic Institute and State University Department of Geological Sciences Memoir 2, p. 307–311.

Mitra, Shankar, 1981 Regional variations in deformation mechanisms and structural styles in the central Appalachian orogenic belt: Geological Society of America Bulletin, v. 98, no. 5, p. 569–590.

———1993, Geometry and kinematics evolution and inversion structures: The American Association of Petroleum Geologists Bulletin, v. 77, no. 7, p. 1159–1191.

Nelson, W.A., 1962, Geology and mineral resources of Albemarle County: Virginia Division of Mineral Resources Bulletin, no. 77, p. 92.

Nickelsen, R.P, 1956, Geology of the Blue Ridge near Harpers Ferry West Virginia: Geological Society of America Bulletin, v. 67, no. 3, p. 239–269.

Onasch, C.M., 1986, Structural and metamorphic evolution of a portion of the Blue Ridge in Maryland: Southeastern Geology, v. 26, no. 4, p. 229–238.

Parker, P.E., 1968, Geologic investigations of the Lincoln and Bluemont quadrangles, Virginia: Virginia Division of Mineral Resources Report of Investigations 14, 23 p.

Platt, J.A., and Vissers, R.L.M., 1980, Extensional structures anisotropic rocks: Journal of Structural Geology, v. 2, no. 4, p. 347–410.

Rankin, D.W., Drake, A.A., Jr., Glover, Lynn, III, Goldsmith, Richard, Hull, L.M., Murray, D.P., Ratcliffe, N.M., Read, J.F., Secor, D.T., Jr., and Stanley, R.S., 1989, Pre-orogenic terranes, in Hatcher, R.D., Jr., Thomas, W.A., and Viele, G.W., eds.,The Appalachian-Ouachita orogen in the United States; The Geology of North America: Boulder, Colo., Geological Society of America, v. F–2, p. 7–100.

Ratcliffe, N.M., 1987, High Ti02 metadiabase dikes of the Hudson Highlands, New York and New Jersey—Possible Late Proterozoic rift rocks in the New York recess: American Journal of Science, v. 287, p. 817–850.

Read, J.F., 1989a, Evolution of Cambro-Ordovician passive margin, U.S. Appalachians, in Hatcher, R.D., Jr., Thomas, W.A., and Viele, G.W., Boulder, Colorado: Geological Society of America, The Geology of North America, v. F–2, p. 42–57.

———1989b, Controls on evolution of Cambrian-Ordovician passive margin, U.S. Appalachians, in Crello, P.D., Song, L.F., and Read, J.F., eds., Controls on carbonate platform and basin development: Society of Economic Paleontologists and Mineralogists Special Publication 44, p. 147–165.

Reed, J.C., Jr., 1955, Catoctin Formation near Luray, Virginia: Geological Society of America Bulletin, v. 66, no. 7, p. 871–896.

Robinson, G.R., Jr., Lesure, R.G., Marlowe, J.I., II, Foley, N.K., and Clark, S.H., 1992, Bedrock geology and mineral resources of the Knoxville 1°×2° quadrangle, Tennessee, North Carolina, and South Carolina: U.S. Geological Survey Bulletin 1979, 73 p., 2 pls.

Root, S.I., 1970, Structure of the northern terminus of the Blue Ridge in Pennsylvania: Geological Society of America Bulletin, v. 81, no. 3, p. 815–830.

———1989, Basement control a structure in the Gettysburg rift basin, Pennsylvania and Maryland: Tectonophysics, v. 166, p. 281–292.

Root, S.I., and. Hoskins, D.M., 1977, Latitude 40°N fault-zone, Pennsylvania—A new interpretation: Geology, v. 5, no. 12, p. 719–723.

Ryder, R.T., 1992, Stratigraphic framework of Cambrian and Ordovician rocks in the central Appalachian basin from Morrow County, Ohio, to Pendleton County, West Virginia, in Evolution of sedimentary basins—Appalachian basin: U.S. Geological Survey Bulletin 1839–G, p. G1–G25.

Ryder, R.T., Harris, A.G., and Repetski, J.E., 1994 Stratigraphic framework of Cambrian and Ordovician rocks in the central Appalachian basin from Medina County, Ohio, through southwestern and south-central Pennsylvania to Hampshire County, West Virginia, in Evolution of sedimentary basins—Appalachian basin: U.S. Geological Survey Bulletin 1839–K, p. K1–K32.

Schamel, S., Hatcher, R.D., Jr., and Post, P., 1984, Style of foreland thrusting in western New England influenced by pre-Taconic structure [abs.]: Geological Society of America Abstracts with Programs, v. 16, no. 6, p. 645.

Schwab, F.L., 1986, Latest Precambrian-earliest Paleozoic sedimentation, Appalachian Blue Ridge and adjacent areas—Review and speculation, in McDowell, R.C., and Glover, L., eds., The Lowry volume—Studies in Appalachian geology: Virginia Tech Department of Geological Sciences Memoir 3, p. 115–137.

Simpson, Carol, and Kalaghan, T., 1989, Late Precambrian crustal extension preserved in Fries fault zone mylonites, southern Appalachians: Geology, v. 17, no. 2, p. 148–151. 1.

Simpson, E.L., and Eriksson, K.A., 1989, Sedimentology of the Unicoi Formation in southern and central Virginia—Evidence for Late Proterozoic to Early Cambrian rift to passive margin transition: Geological Society of America Bulletin, v. 101, no. 1, p. 42–54.

Simpson, E.L., and Sundberg, F.A., 1987, Early Cambrian age for synrift deposits of the Chilhowee Group of southwestern Virginia: Geology, v. 15, no. 2, p. 123–126.

Sinha, A.K., and Bartholomew, M.J., 1984, Evolution of the Grenville terrace in the central Virginia Appalachians, in Bartholomew, M.J., Force, E.R., Sinha, A.K., and Herz, D.L., eds., The Grenville event in the Appalachians and related topics: Geological Society of America Special Paper 194, p. 175–186.

Smith, R..E, II, Berkheiser, S.W., Jr., and Barnes, J.A., 1991, Pennsylvania version of the Catoctin metabasalt story, in 56th Annual Field Conference of Pennsylvania Geologists Guidebook: Pennsylvania Geological Survey, Harrisburg, p. 5–20.

Soper, N.J., and Anderton, R., 1984, Did the Dalradian slides originate as extensional faults?: Nature, v. 307, p. 357–360.

Southworth, C.S., 1991a, Geologic map of the Loudoun County, Virginia, part of the Harpers Ferry quadrangle: U.S. Geological Survey Miscellaneous Field Studies Map MF–2173, scale 1:24,000.

———1991b, Geology of the Loudoun County, Virginia, part of the Harpers Ferry quadrangle—The Blue Ridge–South Mountain anticlinorium revisited after R.P. Nickelsen [abs.]: Geological Society of America Abstracts with Programs, v. 23, no. 1, p. 132.

———1993a, Geologic map of the Bluemont quadrangle, Loudoun and Clarke Counties, Virginia: U.S. Geological Survey Geologic Quadrangle Map GQ–1739, scale 1:24,000.

———1993b, Kinematics of the Short Hill Fault—Contractional reactivation of an extensional fault Blue Ridge–South Mountain anticlinorium, northern Virginia and Maryland: College Park, University of Maryland, Master's thesis, 150 p.

———1995, Geologic map of the Purcellville quadrangle, Loudoun County, Virginia: U.S. Geological Survey Geologic Quadrangle Map GQ-1755 , scale 1:24,000.

Southworth, Scott, and Brezinski, D.K., 1996, Geology of the Harpers Ferry quadrangle, Virginia, Maryland, and West Virginia: U.S. Geological Survey Bulletin 2123, p., map in pocket, scale 1:24,000.

Southworth, Scott, Burton, W.C., Schindler, J.S., Froelich, A.J., and Drake, A.A., Jr., 2005, Geologic map of Loudoun County, Virginia, with contributions on the geology of the Piedmont province by Drake, A.A., Jr., and with an aeromagnetic Survey by Daniels, D.L., Hanna, B.F., and Bracken, R.E.: U.S. Geological Survey Miscellaneous Investigations Series Map 2533, scale 1:50,000.

Spencer, E.W., Bell, J.D., and Kozak, SJ., 1989, Valley and Ridge and Blue Ridge traverse, central Virginia; 28th International Geological Congress: American Geophysical Union Field Trip Guidebook T157, 69 p.

Stewart, W.D., Dixon, O.A., and Rust, B.R., 1993, Middle Cambrian carbonate-platform collapse, southeastern Canadian Rocky Mountains: Geology, v. 21, no. 8, p. 687–690.

Stose, A.J., and Stose, G.W., 1946, Geology of Carroll and Frederick Counties, in The physical features of Carroll County and Frederick County: State of Maryland Department of Geology, Mines, and Water Resources, p. 11–131.

Stose, G.W., 1906, The sedimentary rocks of South Mountain, Pennsylvania: Journal of Geology, v. 14, p. 201–220.

Stromberg, M.J., 1978, K-Ar ages of muscovite and biotite from the Blue Ridge province, northern Virginia: Ohio State University, B.A. thesis, 19 p.

Tollo, R.R, and Aleinikoff, J.N., 1992, The Robertson River Igneous Suite, Virginia, Blue Ridge—A case study in multiple-stage magmatism associated with the early stages of Iapetan rifting [abs]: Geological Society of America Abstracts with Programs, v. 24, no. 2, p. 70.

Tollo, R.P., Lowe, T.K., Arav, S., and Gray, K.J., 1991, Geology of the Robertson River Igneous Suite, Blue Ridge Province, Virginia, in Schultz, A.J., and Compton-Gooding, Ellen, eds., Geologic evolution of the eastern United States; Field trip guidebook of the Northeast-Southeast section of the Geological Society of America: Virginia Museum of Natural History Guidebook 2, p. 229–262.

Tull, J.F., and Groszos, M.S., 1990, Nested Paleozoic “successor” basins in the southern Appalachian Blue Ridge: Geology, v. 18, no. 11, p. 1046–1049.

Wagner, W.R, 1976, Growth faults in Cambrian and Lower Ordovician rocks of western Pennsylvania: American Association of Petroleum Geologists Bulletin, v. 60, no. 3, p. 414–427.

Webb, E.J., 1980, Cambrian sedimentation and structural evolution of the Rome trough in Kentucky: University of Cincinnati, Ph.D. thesis, 98 p.

Wehr, Frederick, 1985, Stratigraphy of the Lynchburg Group and Swift Run Formation, Late, Proterozoic (730–570 MA), central Virginia: Southeastern Geology, v. 25, p. 225–239.

Wehr, Frederick, and Glover, L., III, 1985, Stratigraphy and tectonics of the Virginia-North Carolina Blue Ridge—Evolution of a Late Proterozoic-early Paleozoic hinge zone: Geological Society of America Bulletin, v. 96, p. 285–295.

Werner, H.J., 1966, Geology of the Vesuvius quadrangle, Virginia: Virginia Division of Mineral Resources Report of Investigations 7, 53 p.

Whitaker, J.C., 1955, Direction of current flow in some lower Cambrian clastics in Maryland. Geological Society of America Bulletin, v. 66, no. 6, p. 763–766.

White, S.H., Bretan, P.G., and Rutter, E.H., 1986, Fault-zone reactivation—Kinematics and mechanisms: Philosophical Transactions of the Royal Society of London, v. 317A, p. 81–97.

Wilson, J.T., 1966, Did the Atlantic close and then re-open?: Nature, v. 211, p. 676.

Wojtal, Steven, 1989, Day one—Blue Ridge anticlinorium and Massanutten synclinorium in western Maryland, in Woodward, N.B., ed., Geometry and deformation fabrics in the central and southern Appalachians Valley and Ridge and Blue Ridge; 28th International Geological Congress: American Geophysical Union Field Trip Guidebook T357, p. 4–14

Woodward, N.B, 1987, Stratigraphic separation diagrams and thrust belt structural analysis, 38th Field Conference, 1987: Wyoming Geological Association Guidebook, p. 69–77.

Woodward, N.B., Wojtal, Steven, Paul, L.B., and Zadins, Z.Z., 1988, Partitioning of deformation within several external thrust zones of the Appalachian orogen: Journal of Geology, v. 96, p. 351–361.

Zen, E-an, 1967, Time and space relationships of the Taconic allocthon and autochthon: Geological Society of America Special Paper 97, 107 p.

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