USGS B 2123 -- Structural Geology

The Harpers Ferry quadrangle covers the western half of the Blue Ridge-South Mountain anticlinorium, a fault-bend fold that is overturned to the northwest, plunges gently to the northeast, and is allochthonous above one or more blind thrust faults (Harris, 1979; Mitra and Elliott, 1980; Mitra, 1987). Structural elements are complex and include gneissic foliation, mylonitic and phyllonitic foliation, bedding, three types of cleavage, lineations, joints, at least three orders of folds, and multiple generations of faults (fig. 14).

Gneissic foliations in the Middle Proterozoic rocks are planar zones of dark- and light-colored minerals in leucocratic and melanocratic layers that range from 1 cm to 0.5 m thick. The dark minerals include hornblende, hypersthene, almandine, biotite, and opaque minerals. The light-colored minerals are quartz, plagioclase, and alkali feldspar. The compositional foliation formed under upper amphibolite- to granulite-facies metamorphism during the Grenville orogeny (Kunk and others, 1993). Gneissic foliation is best developed and ubiquitous in the hornblende gneiss and is locally in the garnet graphite gneiss, garnet monzogranite (see fig. 4), and biotite gneiss. Steeply dipping gneissic foliation in the hornblende gneiss in Loudoun Valley reverses attitude over short distances and indicates significant folding of this unit. Gneissic foliation in the garnet monzogranite and biotite gneiss is nearly flat lying along the bluffs of the Potomac River east of Short Hill Mountain (fig. 15A).

The deformed basement gneisses commonly have textures ranging from protomylonite to ultramylonite (Higgins, 1971). Mylonitic foliation is marked by planar aggregates of sericite, chlorite, and fine-grained recrystallized quartz in the otherwise massive gneiss. Microscopically, the mylonitic foliation shows evidence of grain-size reduction and dynamic recrystallization. Mylonitic foliation is best developed in the biotite gneiss and is rarely present in massive hornblende gneiss. Most mylonite zones are no wider than several meters and most are less than 20 cm wide. The Bolivar Heights Member of the Tomstown Formation is locally a mylonitic marble (fig. 13) as the result of detachment from the underlying Antietam Formation.

The largest zone of mylonitic foliation is the 1-km-wide Dutchman Creek shear zone. More than seven discrete mylonite zones, ranging from 0.5 to 3 m wide, define the shear zone along the Potomac River and Dutchman Creek. The rocks are predominantly ultramylonite and blastomylonite. Mylonitized gneiss closely resembles rocks of the Swift Run Formation (Keith, 1894; Stose and Stose, 1946; Nickelsen, 1956). Sense-of-shear indicators of asymmetric porphyroclastic augen, fractured books of feldspar, pressure shadows of quartz, sericite "fish," and shear band cleavage consistently show east-over-west thrust motion. The biotite gneiss (Ybg) grades into augen gneiss that, in turn, grades into mylonite as the shear zone is approached from the east. Poles to mylonitic foliation in the shear zone (fig. 15A) shows that it is coplanar with cleavage.

Phyllonitic foliation is recognized as platy, lustrous phyllosilicate-rich rocks that exhibit anastomosing cleavage. Outcrops of phyllonitic foliation were found only along the Short Hill fault in the Harpers Formation along U.S. Route 340. Phyllonitic foliation also occurs in metadiabase dikes in outcrop, as well as drill core, immediately north of the Potomac River. The phyllonitic foliation has microscopic mica "fish" and asymmetric buttons in hand specimen.

Bedding was recognized in most Late Proterozoic and Lower Cambrian rocks. Graded beds and crossbeds occur in the Weverton Formation and are locally present in the Swift Run Formation. Bedding is more difficult to discern in the Harpers Formation and is very difficult to find in phyllite and schist of the Swift Run and Catoctin Formations. Igneous flow structures occur locally in metabasalt of the Catoctin Formation. Bedding can be traced consistently along strike in the Weverton Formation on Short Hill-South Mountain. Elsewhere, bedding attitude changes over short distances and suggest significant folding.

Crossbeds in the Weverton Formation on Short Hill-South Mountain show that bedding is overturned in 93 percent of the observations (fig. 15B). West-dipping beds are upright fold limbs near the "Radio Facilities." The cluster of poles show the homoclinal nature of the strata that underlie the ridge. Approximately 72 percent of the observations of the Weverton Formation were overturned. Here, the strata are complexly folded, and the poles to bedding are shown with fold axes (fig. 15B).

All Middle Proterozoic through Lower Cambrian rocks in this area have a penetrative cleavage and a consistent southeast dip. This trend, first identified as the South Mountain cleavage by Cloos (1947; 1951), is approximately axial planar to the Blue Ridge-South Mountain anticlinorium. It is a planar fabric characterized by the parallel alignment of the greenschist-facies minerals chlorite, quartz, sericite, magnetite, epidote, and actinolite. The cleavage is continuous into both slaty and schistose textures depending on the scale of layering and the rock competence. Cleavage in the gneiss, dikes, and cover rocks is coplanar and supports other field evidence that the basement and cover rocks were deformed together (fig. 15B).

Cleavage that formed during Paleozoic deformation cuts the gneissic foliation and is marked by biotite, sericite, chlorite, and recrystallized quartz in thin ductile-deformation zones (Mitra, 1978). The cleavage in the basement rocks is coplanar to cleavage in the cover rocks (fig. 15B). Both contain the same greenschist-facies mineral assemblages with the exception of biotite.

Slaty cleavage is best developed in fine-grained phyllite and schist of the Swift Run, Catoctin, and Harpers Formations, where it is the dominant structural element because bedding is largely transposed. Slaty cleavage is less well developed in quartzite of the Weverton Formation. Cleavage surfaces have a shiny luster because of sericite and chlorite crystals. Quartz grains are elongated and flattened in the cleavage plane. The Late Proterozoic and Lower Cambrian rocks in this area have a strong penetrative southeast-dipping cleavage. In the Purcell Knob folds, however, cleavage dips northwest and is cut at a high angle by southeast-dipping crenulation cleavage (fig. 15C). Elsewhere, the cleavage is highly variable in orientation, both along and across strike, as the result of folding after the cleavage formed.

Metadiabase dikes cut the gneissic foliation at a high angle, but they generally parallel the Paleozoic cleavage and mylonitic foliation of their host (fig. 15B). Cleavage in the dike rocks (fig. 15B) is parallel to the contacts and dips southeastward, coplanar to the South Mountain cleavage. Dikes that deviate from a southeast dip are associated with shear zones and the Purcell Knob inverted folds. Some dikes show boudinage and pinch-and-swell structure near contacts with basement rocks. The metadiabase is phyllonitic near the contact, and intrafolial folds occur in the plane of cleavage. During Paleozoic deformation, the dikes acted as shear zones in the basement rocks. Regional variations in the strike and dip of dikes are interpreted to reflect Paleozoic folds of the basement rocks.

Crenulation cleavage is a discontinuous, spaced, zonal, and discrete cleavage (Gray, 1979). The crenulation cleavage is axial planar to F2 folds that fold the cleavage and F1 folds. Insoluble residues concentrated in the laminae suggest that it is a pressure solution cleavage (Gray, 1979). At the Purcell Knob inverted folds, crenulation cleavage uniformly dips to the southeast (fig. 15C) and approximately parallels the regional cleavage. Where crenulation cleavage is well developed, the slaty cleavage shows intrafolial folds and transposed compositional layering in the slaty cleavage plane. Crenulation cleavage is best developed along Blue Ridge-Elk Ridge (fig. 16) and is found locally to the north in the Keedysville quadrangle (Southworth, 1993), to the south in the Round Hill quadrangle (McDowell and Milton, 1992; Southworth and others, in press), and to the east in the Point of Rocks quadrangle (Burton and others, 1995).

Shear band cleavage (White and others, 1986) is recognized only in rocks along the Short Hill fault where it cuts phyllonitic foliation in the Harpers Formation along U.S. Route 340 (fig. 17). Asymmetry of nearly horizontal shear bands with phyllonitic foliation shows consistent east-over-west motion. Dextral kinks fold the shear bands (Southworth, 1993).

Hornblende, garnet, biotite, and chlorite, elongated down the foliation and cleavage planes, are best seen in pavement outcrops of hornblende gneiss (Yhg) in Loudoun Valley. The lineations consistently plunge southeast to northwest. The Paleozoic cleavage is marked with down-dip lineations of elongated quartz, mica, lithic fragments, vesicles, and amygdules (Cloos, 1971).

Both open and quartz-filled annealed joints are common in all rocks of the quadrangle. Systematic joints are most abundant in quartzite of the Weverton Formation, and they are interpreted to be cross (northwest-strike), longitudinal (northeast-strike), and oblique joints formed during folding. The joints are spaced at a minimum of 1 cm apart. Joints are less common in the phyllite and schist. Joints are important conduits for ground water because cleavage planes have little open space. Investigations for ground water should employ a detailed, site-specific analysis of joints because regional compilations (Southworth, 1990, 1991a) give only a random distribution.

Blue Ridge-South Mountain anticlinorium is a first-order fold. Blue Ridge-Elk Ridge and Short Hill-South Mountain are overturned west limbs of second-order folds that are separated by the Short Hill fault. Blue Ridge-Elk Ridge defines a tectonite front (Mitra, 1987) where deformation in the cover rocks is much more extreme than in cover rocks that underlie Short Hill-South Mountain.

At least two different styles of folds are recognized: (1) F1 isoclinal folds having axial plane cleavage and (2) F2 inclined folds having axial plane crenulation cleavage. The F1 and F2 folds plunge gently to the northeast and southwest. The type locality of the fold phases is the Purcell Knob inverted folds. First mapped by Nickelsen (1956), the Purcell Knob inverted folds are complex, refolded folds that include rocks from the Middle Proterozoic hornblende gneiss through the Weverton Formation. An antiformal syncline, a synformal anticline, and several inverted parasitic folds show that the basement is folded with the cover rocks. Younger rocks and cleavage dip westward beneath older rocks and are cut by crenulation cleavage that dips eastward. Westward-inclined F2 antiforms and synforms exhibiting axial plane crenulation cleavage are superimposed on the F1 isoclinal folds and cleavage (fig. 18). The crests of the F2 folds rise and fall along strike.

A culmination of the antiformal syncline exposes metabasalt of the Catoctin Formation (chemical sample 17, table 1) beneath the Swift Run Formation. The rocks are thickened in fold hinges and thinned on inverted limbs, which is consistent with supratenuous folds of passive flow. Deformation of the hornblende gneiss (Yhg) in the synformal anticline is heterogeneous. Mylonitic foliation occurs at the contact with the cover rocks on the inverted limb, but the gneiss is massive in the core of the anticline. This style of folding is unique to the Blue Ridge-South Mountain anticlinorium (Nickelsen, 1956; Elliott, 1970). However, nappe-like recumbent folds of cover rocks can be traced southward into the Round Hill quadrangle (McDowell and Milton, 1992) and northward into the Keedysville quadrangle (Southworth, 1993). The orientation of foliations in the metadiabase dikes and basement gneiss in Loudoun Valley suggests that the Purcell Knob folds continue north to at least the Potomac River.

Short Hill Mountain is underlain by the second-order Hillsboro syncline. This isoclinal syncline is overturned to the northwest and is cored by rocks of the Harpers Formation. The overturned east limb shows monoclines of local third and fourth-order parasitic folds. The Hillsboro syncline can be traced southward for 21 km where it is cored by rocks of the Catoctin Formation (Southworth, 1994; 1995).

The Weverton Formation is intensely folded on Blue Ridge-Elk Ridge (figs. 12, 14, and 15B). Mesoscopic and macroscopic folds are overturned to the northwest, are disharmonic, and have similar fold geometry, (that is, thickened crests and thinned limbs). Second- and third-order, asymmetric, similar folds have wavelengths of 1 to 100 m and plunge gently northeast or southwest. Two large recumbent folds exposed along the Potomac River plunge 10° southwest (figs. 12 and 19). Parasitic folds on these larger structures have extremely thickened strata produced by folding on Blue Ridge. Slickenlines, stretched quartz veins, and boudinage plunge down the dip of the bedding and indicate flexural slip folding. Several anticlines, synclines, and antiformal synclines in the Harpers Formation are recognized by cleavage fans and bedding and cleavage relations along the north bank of the Shenandoah River in the Harpers Ferry National Historical Park (fig. 20).

Two synclines in quartz sericite schist of the Swift Run Formation (Zss) are located east of Short Hill Mountain. The synclines are short and probably shallow. The Dutchman Creek syncline is in the upper plate of a shear zone. The Mt. Olivet syncline has a complex outcrop pattern that suggests refolded folds.

The Short Hill fault, the Dutchman Creek shear zone, the White Rock thrust fault, the Keedysville detachment, and several small thrust faults and normal faults are recognized in the map area (pl. 1). The Short Hill fault was interpreted by Cloos (1951) and Nickelsen (1956) to be a down-to-the-east Triassic normal fault. Field relations along the Short Hill fault reveal younger-on-older rocks typical of extensional faults, but contractional kinematic indicators show east-over-west motion (Southworth, 1993). Drilling of the Short Hill fault (October 1990, south of Weverton, Md., along the Keep Tryst Road; fig. 21) revealed phyllonitic metasiltstone of the Harpers Formation in fault contact with a phyllonitic Late Proterozoic metadiabase dike intruded into Middle Proterozoic garnet monzogranite. The attitude of the southeast-dipping fault was determined to be approximately 35°, but excavations along U.S. Route 340 show that the dip of the fault changes from 8° to 40° over short distances. Microstructures of the core show shear band cleavage that cuts phyllonitic foliation with an east-over-west sense of motion. The fault plane is parallel to the phyllonitic foliation but anastomoses along the shear bands. These foliations are cut by later northwest-dipping dextral kink bands. On the basis of these data, Southworth (1991a and b) interpreted the Short Hill fault as a thrust.

Brezinski and others (1991) and Campbell and others (1992) interpreted the Short Hill fault at the Potomac River to be the "South Mountain fault," which is part of a late-stage, linear, en echelon fault system that extends to Pennsylvania. Brezinski (1992) considers this strand of the South Mountain fault to be coincident with the Rohrersville fault (fig. 22), which he interprets to be an early Paleozoic normal fault. The South Mountain fault, the Rohrersville fault, and the Eakles Mill fault of Brezinski (1992) are younger-on-older faults that are interpreted by Southworth (1993) to be the continuations of the Short Hill fault. Brezinski's (1992) South Mountain fault (north of Rohrersville) and Sans Mar fault are interpreted by Southworth (1993) to be late thrust faults that splay from the Short Hill fault. These thrust faults imbricated the hanging-wall strata of the Short Hill fault during Late Paleozoic contractional reactivation.

The Short Hill fault has been traced for more than 60 km northward from a shear zone in Middle Proterozoic gneiss in Fauquier County, Va. (Howard, 1991; Southworth, 1993; 1994), to the west side of Elk Ridge in Maryland where Lower and Middle(?) Cambrian rocks overlie Lower Cambrian rocks (Brezinski, 1992; Southworth, 1993). Near Rohrersville, Md., the Short Hill fault is folded and transected by pressure solution crenulation cleavage similar to that in the Purcell Knob inverted folds (Southworth, 1993). The Short Hill fault is interpreted to be an early Paleozoic normal fault that was reactivated by contraction in the Alleghanian orogeny (Wojtal, 1989; Brezinski, 1992; Southworth, 1993).

The Dutchman Creek shear zone is a 1-km-wide fault zone that places mylonitic biotite gneiss (Ybg) on garnet monzogranite (Ygt). Kinematic indicators in the mylonitic foliation show east-over-west motion. However, the structural level and preservation of quartz sericite schist (Zss) of the Swift Run Formation in the hanging wall of the Dutchman Creek syncline may be the result of an early normal fault. Similar to the Short Hill fault, the Dutchman Creek shear zone may be a reactivated early extensional fault.

The White Rock thrust fault is interpreted to be a detachment structure that involves quartzite of the Maryland Heights Member of the Weverton Formation on the crest of Short Hill Mountain. The thrust fault is marked by a zone of cataclastic vein quartz as much as 10 m thick. In places, foliated quartz cataclasite constitutes 80 percent of the fault zone in which 0.25- to 0.5-m-thick slabs of quartzite breccia float in a white quartz matrix (fig. 23) (pl. 1, ref. loc. 18). South of White Rock, the east slope of Short Hill Mountain is underlain by colluvial blocks of similar deformed vein quartz. This area is interpreted to be the sole of a bedding-parallel detachment in the Buzzards Knob Member(-Cwb) of the Weverton Formation.

The Keedysville detachment is marked by aerially extensive mylonitic marble near the base of the Bolivar Heights Member of the Tomstown Formation (Brezinski, 1992). The mylonitic marble, which Brezinski termed the Keedysville marble bed, is very light gray, to white, with streaks of tan dolomite, ranges in thickness from 3 to 15 m, and has a pervasive east-west oriented stretching lineation (fig. 13). Overlying the marble, 25 to 35 m of mylonitized limestone exhibits asymmetric shear indicators that suggest a top-to-the-northwest sense of movement. Marble has been folded along with adjacent strata indicating that it originated prior to major folding episodes. The Keedysville marble bed has, at present, been traced from Berryville, Va., northward into Franklin County, Pa., a distance of more than 100 km. The Keedysville marble bed is interpreted to be a bedding-parallel detachment that decoupled the overlying carbonate rocks from underlying siliciclastic rocks of the Chilhowee Group. Since the mylonitic rock is folded with the adjacent Blue Ridge cover sequence, detachment predated the major folding episodes that produced the Blue Ridge-South Mountain anticlinorium.

Intraformational thrust faults in the Weverton Formation are exposed at Blue Ridge-Elk Ridge and Weverton Cliffs north of the Potomac River (figs. 11 and 12). One of the thrust faults at Weverton Cliffs has indeterminate displacement but truncates isoclinal folds in the footwall (fig. 24).

Highly cleaved phyllite of the Harpers Formation is interpreted to be in thrust contact with right-side-up metasiltstone of the Harpers Formation within the Harpers Ferry National Historical Park. Where recognized, bedding in the Harpers Formation to the east is nearly horizontal to inverted (Lessing and others, 1991). The thrust fault is intraformational with indeterminate displacement and is interpreted to be a late fault that is partly responsible for the wide outcrop pattern of the Harpers Formation.

West of Potomac Wayside, a northwest-striking vertical fault in the Weverton Formation with slickenside striations raking 35° to 40° southeast is parallel to the normal faults and shear zones along the B&O tracks and may indicate minor oblique faulting along the Potomac River gorge.