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| Open-File Report 01–0364 |
Uncompahgran disturbance of Snake River-Wichita tectonic zone
Clastic metasedimentary rocks of Proterozoic age
Reactivation of Proterozoic shear zones
Regional significance of northeast shear zones
Tectonic environment of 1.4 Ga shearing and magmatism
Plate 1. Preliminary Precambrian basement map of Colorado--A geologic interpretation of an aeromagnetic anomaly map
Plate 2. Aeromagnetic map of Colorado
with interpreted Precambrian rocks
Figure 1. Figure showing Proterozoic intrusions and shear zone locations
Figure 2. Map showing principal uplifts exposing Precambrian rocks
Figure 3. Exposed Precambrian basement rocks and major features of Colorado
Figure 4. Simplified Precambrian basement map of Colorado
Figure 6. Rose diagram showing trends of magnetic anomalies, Colorado mineral belt.
Colorado geology has been studied extensively for more than a century, because of its diversity and the state's unsurpassed scenery. A major impetus was the discovery in the mid-1800's of valuable precious and base-metal deposits. Known mineral deposits are principally in the Colorado mineral belt (fig.3), but include the world-class uranium deposits of the Colorado Plateau (Finch, 1967), in the southwestern part (fig.1). Nearly all the metalliferous deposits are of post-Precambrian (Phanerozoic) age, having formed in Laramide and mid-Tertiary times; but it has been shown that ductile shear zones formed in the Mesoproterozoic (1.4 billion years ago, Ga) were important in localizing younger intrusions and genetically related ore deposits, especially those in the Colorado mineral belt (Tweto and Sims, 1963).
Compilation of a new aeromagnetic anomaly map of Colorado (Oshetski and Kucks, 2000), used in plate 2, provides a means to reexamine the geology of the Precambrian rocks. In particular, the map expands knowledge of the distribution and significance of northeast-trending shear zones and associated folds and coeval granite plutons of Mesoproterozoic age, and adds to understanding of the extent of Proterozoic lithologic units in the subsurface. Also, it clarifies the significance and regional extent of the northwest-trending structures that are dominant in southwestern Colorado.
The term Precambrian basement as used in this report applies to all rocks of Precambrian age. In most of the state, the basement consists of crystalline igneous and metamorphic rocks, lying stratigraphically below the Phanerozoic stratified sedimentary rocks. In some places, however, sequences of younger Precambrian sedimentary rocks overlie the crystalline rocks; these sequences are included as basement. The Precambrian rocks of Colorado mainly comprise the Colorado province (fig.1), as defined by Bickford and others (1986). The province is composed largely of Paleoproterozoic metamorphosed volcanic-sedimentary gneisses and associated synectonic calc-alkalic plutons of island arc affinity, which were accreted to the Archean Wyoming craton (province) (Reed and others, 1987) along the Cheyenne belt, in southern Wyoming and northwestern Colorado (Houston and others, 1989) during the interval 1.78-1.75 Ga (Chamberlain, 1998). Deformation during this orogenic episode (Colorado orogeny, Sims and Stein, 2001) culminated during the interval 1735-1705 Ma (Premo and Fanning, 2000). After a stable interval of about 250 million years, voluminous granitic and lesser granodioritic plutons (~1.4 Ga) were intruded into the Paleoproterozoic country rocks. These intrusions were emplaced contemporaneously with, and were localized by, scattered, widely developed Mesoproterozoic northeast-trending shear zones and local slip/shear folds of the same trend. This tectonic episode has been named the Berthoud orogeny (Sims and Stein, 2001). The magmatic activity and shearing were accompanied by pervasive regional remetamorphism of the Paleoproterozoic country rocks, dominantly to amphibolite facies. Following the Berthoud orogeny, but during later Mesoproterozoic, a continental-dimension shear zone (Budnik, 1986)--herein called the Snake River-Wichita tectonic zone--developed diagonally across the Colorado province. It traversed southwestern and central Colorado and markedly disturbed the older structural fabrics in the basement rocks. Reactivation of these shear zones had a strong role in developing the framework of Colorado geology during the Phanerozoic.
The accompanying Precambrian basement map (pl.1) supplements the earlier Precambrian basement map of Colorado (Tweto, 1987), which was prepared simultaneously with the current state geologic map (Tweto, 1979). A digital revision of the map is available on the Web (Green, 1992). In addition to more comprehensive aeromagnetic data, much new geologic and geochronologic data have become available during the past two decades, and the map is an attempt to express these new data in a meaningful way. In the text, precise ages are referred to in millions of years, i.e., 1430+ 20 Ma, whereas generalized ages are given in billions of years, i.e., 1.8 Ga.
The principal sources of geologic information for the compilation were the excellent state geologic map (Tweto, 1979) and Precambrian basement map (Tweto, 1987). Other more specific sources of data are acknowledged in the text. The reader is urged to refer to the text of U.S. Geological Survey Professional Paper 1321-A that accompanies the earlier published basement map (Tweto, 1987) for detailed descriptions of the major rock units in the state. We benefited from discussions with Jack Reed, Wayne Premo, Kevin Chamberlain, and Karl Evans, and thank Gregory Green and Anna Wilson for providing base map materials. Critical reviews by Karen Kelley and Anna Wilson improved the manuscript. Vicki Rystrom helped develop the web page format. This map and text are dedicated to the memory of Ogden Tweto.
Except for a rather small area in northwestern Colorado, which is underlain by buried Archean rocks (2.5 Ga and older) of the Wyoming province and local, overlying craton-margin rocks (~2.0 Ga) (Houston and others, 1993), the basement rocks of Colorado are Proterozoic in age, mainly Paleoproterozoic (2.5-1.6 Ga), but also Mesoroterozoic (1.6-0.9 Ga). They constitute the Colorado province, a volcanic-plutonic arc terrane that was sutured to the Archean Wyoming craton along the Cheyenne belt (mainly exposed in southern Wyoming). The Paleoproterozoic successions in the Colorado province range in age from about 1,780 Ma to 1,650 Ma. The eastern extension of the Colorado province in the midwest has been called the Central Plains orogen (Sims and Peterman, 1986); the Wyoming segment of the province previously was named the Medicine Bow orogen (Chamberlain, 1998). The southern limit of the Colorado province has not been delineated although geologic mapping and age data suggest that it extends into northern New Mexico and probably beyond (Robertson and others, 1993; Karlstrom and others, 1997). The Colorado province constitutes a substantial segment of the Transcontinental Proterozic provinces (Van Schmus and others, 1993) that extend across the midcontinent and westward from Colorado into New Mexico and Arizona. In New Mexico and Arizona, the approximate time equivalent of rocks in the Colorado province has been termed the Yavapai province (see Shaw and Karlstrom, 1999, and references therein).
Rocks of the Colorado province are exposed mainly in the north-trending mountain ranges that cross central Colorado (fig.2) and extend northward into Wyoming and southward into New Mexico, and underlie younger Phanerozoic rocks at shallow to considerable depths in the remainder (ca. two-thirds) of the state (Tweto, 1987). Proterozoic rocks are also exposed in deep valleys entrenched in rocks of the Uncompahgre Plateau, in southwestern Colorado, and in the Gunnison River gorge and adjacent areas in south-central Colorado (Tweto, 1979).
The supracrustal rocks of the Colorado province consist chiefly of quartz-feldspar gneiss and amphibolite, of mainly volcanic origin, and biotite gneiss and migmatite, chiefly of sedimentary origin (Reed and others, 1987). Most of the supracrustal rocks are metamorphosed to amphibolite grade or, in one area in the Wet Mountains (fig.2), granulite facies. Areas characterized by low-grade metamorphism (fig.1), where primary textures and structures are preserved, are (locality A) the Green Mountain area, in the Sierra Madre, near the Cheyenne belt (Premo and VanSchmus, 1989), (locality B) near the mouth of Big Thompson Canyon in the northern Colorado Front Range (Braddock and others, 1970), and (locality C) in the Gunnison-Salida area in south-central Colorado (Bickford and others, 1989; Reed and others, 1987).
The Paleoproterozoic metavolcanic and metasedimentary rocks are modified by batholithic intrusions of ~1.7, ~1.4, and 1.1 Ga (Tweto, 1987). The oldest and most abundant intrusions are mainly intermediate composition foliated hornblende-biotite granodiorite or monzogranite of calc-alkalic affinity (~1.7 Ga). Generally, these intrusions were synchronous with a regional deformation, herein referred to as the Colorado orogeny, but some were post-tectonic. They have U-Pb zircon ages in the range 1.75-1.65 Ga (Reed and others, 1987; Reed and others, 1993).
The second major intrusive episode, of ~1.4 Ga age, consisted of intrusions of dominantly granitic, but variable compositions, that were emplaced during a major deformational episode, herein designated as the Berthoud orogeny. These plutons commonly are discordant to older structures in the wall rocks and are undeformed except locally. Previously, most authors have classed these intrusions as A-type or "anorogenic", mainly because of their chemical compositions (Anderson and Cullers, 1999, and references therein), but in recent years the appropriateness of classification of these rocks as anorogenic has been questioned (for example, Nyman and others, 1994).
A single, large batholith of partly potassic and partly sodic affinities, the Pikes Peak batholith, is present in the southern part of the Front Range. It has an age of about 1.1 Ga, and has been classed as a "type example" of A-type (anorogenic) granite magmatism (Loiselle and Wones, 1979; Smith and others, 1999).
The two major episodes of Proterozoic deformation and metamorphism differed substantially in expression, extent, and origin. The older deformation, (Colorado orogeny), of regional extent, took place beginning at about 1.8 Ga, and consisted of several tectonic pulses that differed temporally from place to place (Reed and others, (1987). Deformation associated with the Colorado orogeny culminated at 1735-1705 Ga (Premo and Fanning, 2000). Folding under pressure-temperature conditions generally of the amphibolite facies produced the Precambrian structural framework of the region.
The second major deformation (Berthoud orogeny) took place during the Mesoproterozoic (~1.4 Ga). Although of regional extent, it is expressed by areally local, close-to-wide-spaced ductile shear zones and accompaning folds. Associated metamorphism was pervasive, and mainly of amphibolite grade; emplacement of the coeval plutons was localized by the shear zones. This structural episode produced the northeast-trending ductile shear zones so conspicuous on the geologic basement map (pl.1).
The northwest-trending structures that are dominant in central and southwest Colorado, and compose the Snake River-Wichita tectonic zone, were formed subsequent to the Berthoud orogeny, in late Mesoproterozoic time. They clearly cut and offset rocks as old as 1.4 Ga (Berthoud) plutons. Magmatism did not accompany initiation of the northwest shears. This report emphasizes initiation of the zone in the late Mesoproterozoic--deformation that has been called the "Uncompahgran disturbance" (Barker, 1969).
During the Colorado orogeny a layer-parallel foliation was developed in the Paleoproterozoic metavolcanic and metasedimentary rocks, followed by plastic folding under moderate P-T conditions and intrusion of intermediate calc-alkalic rocks, such as the granodioritic Boulder Creek batholith (fig.1; Gable, 1980). The accompanying amphibolite-facies metamorphism is characterized by sillimanite and, locally, garnet, andalusite, and cordierite. Contemporaneity of emplacement of the granodioritic rocks with folding is indicated by concordant plutonic boundaries and by conformity of the internal structure (of solid-state recrystallization) in the batholith with that in the supracrustal wall rocks. Comparable mineral facies in the country rocks and batholiths (Gable, 1980), indicate that emplacement took place at moderate depths.
Deformation patterns formed during the Colorado orogeny (1.8-1.7 Ga) differ throughout the Colorado province. Adjacent to the Cheyenne belt, and extending across a width of at least 150 km to the south, foliation and upright folds predominantly trend westward. In the northern Front Range sector of this region, geologic mapping, (eg. Braddock and Cole, 1979), demonstrated three generations of northwest-trending folds that pre-date the ~1.4 shear zones (Selverstone and others, 1997). Similar fold patterns are present in the northern Park Range and the Sierra Madre and Medicine Bow Mountains (fig.2: Houston and Karlstrom, 1992; Houston and Graff, 1995). These structural fabrics indicate shortening in a north-south direction and can be explained by collision, subsequent subduction, and continued convergence along the Cheyenne belt (fig. 1). Farther south, more distant from the Cheyenne belt, fold patterns differ materially from those in the northernmost part of the Colorado province. In the north-central Front Range, west of Denver, in an area of >2000 km2 that has been mapped in detail (1:6,000: Moench and others, 1962, and references therein), the older regional folds mainly bear north-northeast; the folds range from broad open, upright folds to tight, upright folds that plunge gently to moderately northeast. In nearby areas to the east and west, however, the folds of the older orogenic event trend northwest (Gable, 2000); field observations indicate that these folds apparently are slightly older than the more prevalent north-trending folds (Gable and Sims, 1969), but both generations are part of the older gneiss-forming episode inasmuch as they are cut by the Boulder Creek batholith (Gable, 2000). Both sets of these folds indicate shortening events resulting from regional stress patterns. Because of the consistency of these fold patterns over relatively large areas, evidence for folding resulting from forceful intrusion of igneous rocks is generally lacking; intrusions of this orogenic event are synkinematic, as exemplified by the much-studied Boulder Creek batholith (fig.1: Gable, 1980).
Subsequent geologic studies further increased knowledge of the shear zones and substantiated early observations of the essential contemporaneity of shearing and emplacement of the Silver Plume Granite. In a study of the Mt. Evans batholith and environs, adjacent to the Idaho Springs-Ralston shear zone on the south (fig.1), the batholith was dated at 1.44 Ga by the U-Pb zircon method (Aleinikoff and others, 1993), and mapping showed that it was transected by mylonitic structures of the shear zone. Mylonite foliations dip 40°-85° NW. Asymmetrical potassium feldspar augen and shear bands indicate synmagmatic sinistral reverse reactivation along the shear zone. A minimum age for the deformation in this area is given by dikes of undeformed 1.422-Ga Silver Plume Granite, which cut the shear zone structures (Graubard, 1991; Graubard and Barker, 1991). Also, a recent study in the northern Park Range (Barinek and others, 1999) showed that the ~1.4 Ga Mt. Ethel Granite pluton was emplaced along the northeast-trending Soda Creek-Fish Creek shear zone and that renewed shearing took place after crystallization of the pluton. The shear zone dips northwest. Kinematic indicators within the shear zone indicate vertical movement opposite to that in the Idaho Springs-Ralston shear zone--a north-side-down sense of shear, and a left-lateral component.
Accurate dating of the time of metamorphism during the Mesoproterozoic tectonothermal event has been elusive until recently. In a study within the northern Front Range, Shaw and others (1999) identified a low-pressure metamorphism represented by the presence of paragenetically late staurolite, andalusite, cordierite, and garnet porphyroblasts that overprint older, presumed Colorado orogeny assemblages. Using 40Ar/39Ar geochronology, they dated muscovite and biotite from the late metamorphic assemblages; these minerals yielded ages from 1.4 to1.34 Ga. They interpreted these ages as representing the cooling phase of the ~1.4 Ga metamorphic event. At about the same time, Sims and Stein (1999) dated Precambrian-hosted molybdenites from three localities in the Front Range; they yielded a Re-Os isochron age of 1,430±20 Ma (Stein and Sims, 2001). This age is interpreted as the time of peak thermal activity. It compares closely with U-Pb zircon ages of several Mesoproterozic plutons exposed in the Colorado province (Reed and others, 1993).
The northeast-trending shear zones and folds of the Berthoud orogeny are interpreted as resulting from northwest-southeast contractional shortening. The consistency in orientation of the shears and folds suggests that deformation was transpressive and probably involved a horizontal couple of regional extent.
The Snake River-Wichita tectonic zone has been recognized as a major continental structural feature for more half a century (Ham, 1950), but its early (pre-Phanerozoic) history has been poorly understood. Accordingly, knowledge of its origin, duration, and tectonic evolution has been incomplete. Widespread exposures of Proterozoic basement rocks within the tectonic zone in Colorado (pl.1) and updated potential field data provide a means to reappraise the early development of the tectonic zone and refine its role in subsequent geologic events. Previous studies (Budnik, 1986, and references therein) have focused mainly on the southeastern segment in southern Oklahoma, commonly called the Wichita lineament (Sales, 1968) or the Wichita megashear (Walper, 1970), and its significance in development of the Ancestral Rocky Mountains (Budnik, 1986). These studies mainly involved stratigraphic and tectonic features developed during the late Paleozoic within the tectonic zone. Tweto (1980) was the first to recognize and delineate ancestral Precambrian structures within the zone in Colorado.
Initial structures within the tectonic zone are ductile to brittle shear zones having a systematic pattern that cut Precambrian crystalline basement rocks of 1.4 Ga and older ages. This tectonism is referred to here as the "Uncompahgran disturbance" of the Snake River-Wichita tectonism; the name was first proposed by Barker (1969) for a post-1.7-Ga deformation of the Mesoproterozoic Uncompahgre Formation in the Needle Mountains (fig.2).
The regional fault pattern developed during the Uncompahgran disturbance is shown in figure 5. The fractures can be grouped into three fault sets: (1) west-northwest, sub-parallel to the trend of the Snake River-Wichita tectonic zone; (2) northwest to north-northwest; and (3) north-northeast. The west-northwest faults, represented by the Cimarron-Red Rocks shear zone (fig.5), are long, continuous steep fractures with left-lateral displacements; they are sub-parallel to the length of the Snake River-Wichita shear and apparently were the first to form. The Cimarron-Red Rocks shear zone has a demonstrable strike-slip displacement of about 10 km (Tweto, 1980b, p.44), as indicated by offset of Proterozoic rock units. The second set, northwest to north-northwest faults, are the dominant structures in central and northern Colorado. They have demonstrable left-lateral displacements, as shown clearly on the geologic map of the Central Front Range (Gable, 2000). This fault set includes the "breccia reefs" and "breccia dikes" of Lovering and Goddard (1950). Faults of this set appear to cut and displace shears of the west-northwest set (fig.5). A mafic dike (Iron dike) in a fault northwest of Denver, in the central Front Range, gives a Rb-Sr isochron age of 1316±50 Ma (Braddock and Peterman, 1989), thus establishing a minimum age for the northwest-oriented faults. A fault of this generation in the Central City mining district in the central Front Range (Sims and others, 1963, fig.6) displaces Proterozoic rock units about 200 m and a Tertiary igneous dike about 30 m, establishing definite left-lateral strike-slip movement during the Precambrian. In the same area, north-northeast-trending (antithetic) faults with right-lateral displacement are common (op.cit., fig.6).
The principal faults in the N.25-35°W. set form a topographic corridor--as well as geologic and geophysical lineaments--that separates the Front Range from uplifts to the west (fig.5). In the Late Paleozoic, during development of the Ancestral Rocky Mountains, this zone of structural weakness separated the Front Range and Apishapa Highlands from the Uncompahgre-San Luis Highland to the west (Tweto, 1980a, fig.1). In the Neogene, uplift of the Sangre de Cristo and Gore Ranges took place along this ancestral fault set (Tweto, 1980).
Folding related to the west-northwest wrench faulting has been recognized in the Needle Mountains (fig.2). This deformation folded rocks of the Mesoproterozoic Uncompahgre Formation (see following section) and is distinctly younger (Barker, 1969) than the older structures developed in crystalline basement during the Colorado orogeny.
The fault system developed during the Uncompahgran disturbance can be accounted for by east-northeast west-southwest compression resulting from a left-lateral shear couple. Budnik (1986) suggested a similarly oriented stress field for the late Paleozoic deformation of Paleozoic rocks in the Wichita segment of the tectonic zone. Also, he documented a left-lateral displacement of Paleozoic rocks across the zone of 120-150 km.
The shear zones and faults of this system can be traced along strike by aeromagnetic anomalies for a distance of about 200 km. The tectonic zone extends from the western margins of the continent to the southern margin of the continent and is an intraplate tectonic feature. The southern margin of the continent in the Mesoproterozoic was at least several tens of kilometers to the south of the Snake River-Wichita tectonic zone.
Tectonism during the Uncompahgran disturbance was amagmatic. The lack of magmatism and prograde metamorphism indicates that the deformation took place at moderate P-T conditions. The crust had cooled substantially after the slightly (?) earlier Berthoud orogeny.
Accumulations of clastic sedimentary rocks that are younger and less metamorphosed than underlying gneisses are present in three areas within Colorado. These deposits comprise the Uncompahgre Formation and the Vallecito Conglomerate in the Needle Mountains, the Uinta Mountain Group in the Uinta Mountains, and the Las Animas Formation in southeast Colorado. The clastic units in the Needle Mountains (Barker, 1969) and Uinta Mountains (Hansen, 1965) are well exposed. The Las Animas Formation (Tweto, 1983) is known only in the subsurface (pl.1). Reinterpretation during this study indicates that the three sedimentary deposits probably formed more or less contemporaneously in the late Mesoproterozoic during the Uncompahgran disturbance of the Snake River-Wichita tectonic zone. The deposits formed subsequent to the Berthoud orogeny and before the Grenville orogeny, probably in the interval 1.4 to 1.2 Ga.
Factors that favor a unifying concept for deposition of the separate sedimentary deposits include: (1) Each of the sedimentary deposits lie within the bounds of the Snake River-Wichita tectonic zone, as delineated herein; (2) the exposed stratigraphic units in the Needle and Uinta Mountains consist of immature clastic sedimentary rocks indicative of a terrestrial source; (3) deposition in each basinal area was structurally controlled, mainly by similarly oriented west-northwest-trending shear zones and faults, and (4) deposition was followed by folding along westerly trends that demonstrably post-dates development of older tectonic fabrics related to the Colorado (Paleoproterozoic) and Berthoud (Mesoproterozoic) orogenies. Also, no other interval of tectonic instability favorable for development of such deposits is now known to have existed in the Proterozoic
The Uncompahgre Formation and its probable lateral equivalent, the Vallecito Conglomerate, have been studied for more than a century, most recently by Barker (1969). The Uncompahgre succession is a thick body (~3,000 m) of quartzite, slate, and phyllite that disconformably overlies highly deformed (~1.7 Ga) gneisses. Previously, the clastic sedimentary rocks were considered as older that the 1.46-Ga Eolus Granite (Barker, 1969), but evidence that this granite cuts, and is later, than the Uncompahgre Formation is equivocal.
The Uinta Mountain Group consists dominantly of massive to cross-bedded quartzite, with subordinate beds of shale and conglomerate (Hansen, 1965). Hansen estimated that the succession is about 8,000 m thick. It unconformably overlies much more highly deformed, amphibolite-facies quartzite (Red Creek Quartzite).
The Las Animas Formation, defined by Tweto (1983), is a thick sequence of moderately metamorphosed sedimentary and volcanic rocks that lies beneath the plains of southeastern Colorado. It consists largely of slate, phyllite, graywacke, and chert, as determined from boreholes (Tweto, 1987). Total thickness is as much as 1,700 m. The succession overlies 1.4-Ga granite and gneiss (pl.1). It resembles and probably is equivalent to the Tillman Metasedimentary Group of Ham and others (1964).
Shear zones developed during the Mesoproterozoic were reactivated repeatedly in the Phanerozoic, as shown by multiple brecciation of mylonitic shear fabrics, late faulting and brittle fracturing, alignment of younger intrusions, and structural and topographic features. Also, pronounced differences in stratigraphic thicknesses and facies of Paleozoic and Mesozoic strata are common on opposite sides of major shears. Reactivation of the Precambrian structures played a prominent role in the subsequent geologic development of Colorado, as has been emphasized for other multiply-deformed terranes by Richards (2000).
Reactivation of the northeast-trending shear zones formed during the Berthoud orogeny affected the entire state, but was most pronounced within the Colorado mineral belt (Tweto and Sims, 1963). Tweto (1980a) has summarized the complex relations of differential uplift and sedimentation during the Paleozoic and Mesozoic, and Tweto and Sims (1963) have documented the importance of Mesoproterozoic shear zones in localizing the Colorado mineral belt.
The new geologic mapping (pl.1) provides a means to refine the relationship of the Colorado mineral belt to Mesoproterozoic northeast structures (fig.3). In the Sawatch Range, the Homestake shear zone marks the northwest margin of the mineral belt. In the Front Range, the Idaho Springs-Ralston shear zone closely bounds the southeast margin, where the mineral belt is relatively narrow. In southern Colorado, the southwestward projection of the Arkansas River shear zone (named herein) marks the approximate southeast limit of the mineral belt, in the vicinity of the Creede caldera (fig.3). That this shear zone is a fundamental crustal structure is suggested also by its apparently truncating the south end of the Front Range and marking the approximate northern limit of granulite-facies gneisses in the Wet Mountains; it also marks the southern limit of the 1.1-Ga Pikes Peak batholith (fig.1). On the geologic map of Colorado (Tweto, 1979) the Arkansas River shear zone and its apparent southwestward projection makes a pronounced lineament, extending from the eastern side of the Front Range to the Needle Mountains (pl.1). This structure is herein named the Needle Mountains-Wet Mountains lineament. Its full significance remains to be determined. Features deserving future studies are the remarkable northeast trends of contacts of Cretaceous and Tertiary stratigraphic units cropping out on the High Plains, east of the Front Range, and a northeast trend of many modern streams in the same region (Tweto, 1979).
Reactivation of ancestral structures within the Snake River-Wichita tectonic zone had a profound role in the geologic development of the region during the Phanerozoic, especially in central and southwestern Colorado. Extension along the zone in the Cambrian and Cambro-Ordovician led to emplacement of small alkalic intrusions in the vicinity of the Gunnison River (Iron Hill) and in the Wet Mountains and a bimodal dike swarm (Larson et al., 1985), particularly along and near the west-northwest-trending Cimarron-Red Rocks shear zone in the Gunnison area. During the late Paleozoic, the Ancestral Rocky Mountains were developed along the tectonic zone in Colorado and northeast New Mexico (Tweto, 1980a; Budnik, 1986). Contemporaneously with the uplift, northwest-trending clastic sedimentary rocks were formed. For example, differential movement along the shear zone during the Pennsylvanian produced nearly 2 km of structural relief between the Uncompahgre uplift and the adjacent Paradox basin to the southwest (Ohlen and McIntyre, 1965). In the Mesozoic, the tectonic zone was not notably active, but many of the Laramide faults that produced the Rocky Mountains are reactivated northwesterly-oriented Proterozoic structures (Erslev, 1993), and Laramide intrusions and mineral deposits were partly controlled by northwest-to-north-trending faults. It is noteworthy that the Colorado mineral belt widens appreciably where it encounters abundant northwest shears between the Sawatch Range and the Front Range (figs. 3 and 5).
The aeromagnetic anomaly map of Colorado reproduced here as plate 2, (Oshetski and Kucks, 2000), is characterized by grossly east-west-trending positive and negative anomalies of moderate amplitude and width, and particularly in the northern half of the state, by conspicuous northeast-trending positive and negative anomalies. In the central and southwestern part of the state, northwest-trending anomalies are dominant. Basement rocks in the north-trending mountain ranges cause sharp, high-frequency positive and negative anomalies, whereas rocks in the covered areas to the east and west are expressed by more subdued anomalies. Case (1966) has demonstrated that the magnetic anomalies in western Colorado, and presumably elsewhere throughout Colorado, are caused mainly by contrasts in the magnetism of the Precambrian crystalline basement and that the overlying Phanerozoic sedimentary rocks are essentially non-magnetic. Laramide and Tertiary intrusions are generally small and do not appreciably disturb the magnetic patterns. Magnetic anomalies do not correlate with position of Phanerozoic basins. However, in southwestern Colorado, over the San Juan volcanic field, anomalies caused by a veneer of volcanic and subvolcanic rocks mask the magnetic expression of the underlying Precambrian basement; accordingly we have not attempted to interpret basement lithologic units in much of the area south of the Cimarron-Red Rocks shear zone (pl.1).
In preparing the basement map, qualitative analysis was made of magnetic anomalies. Anomaly patterns observed over known rock units and known structural features were used to extrapolate geologic features in covered areas. In general, the Paleoproterozoic metavolcanic and metasedimentary rocks are essentially non-magnetic, whereas the abundant coeval granitic intrusions are mainly moderately magnetic. The Mesoproterozoic intrusive rocks are in part magnetic and in part non-magnetic, depending on their iron-oxide mineralogy. Ilmenite-bearing plutons, such as the Log Cabin and Sherman batholiths (fig. 1)--north of the Skin Gulch shear zone (Selverstone and others, 1997)--are non-magnetic; whereas, magnetite-bearing plutons, such as the Vernal Mesa and Eolus batholiths in southwestern Colorado (fig. 1), are moderately magnetic. The third type of 1.4 Ga plutons--peraluminous, two-mica granitic intrusions (Anderson and Cullers, 1999)--which dominate plutons of this age in central Colorado and include the Silver Plume Granite, are mainly moderately magnetic, although some bodies are not uniformly magnetized. The late Mesoproterozoic sedimentary sequences are not magnetic. The only known 1.1-Ga pluton, the Pikes Peak batholith in the southern Front Range, causes a pronounced negative anomaly, which extends eastward for a short distance under a cover of Phanerozoic strata.
The prominent northeast-trending negative anomalies mainly reflect the 1.4 Ga ductile shear zones, which were superposed on rocks previously deformed during the Paleoproterozoic. The Idaho Springs-Ralston shear zone in the Front Range, and the Soda Creek-Fish Creek shear zone in the northern Park Range, in particular, produce prominent magnetic lows (cf.pls.1 and 2). In covered areas, many northeast-trending lows are likewise interpreted as representing 1.4-Ga shear zones.
The northwest magnetic fabric that dominates anomalies in the western part of the state reflects structures formed during the beginning (late Mesoproterozoic) stage of the Snake River-Wichita tectonic zone. These ancestral structures are superposed on the older east to northeast magnetic fabrics and in some areas nearly obliterate them. The most prominent zone of these north-northwest-trending anomalies traverses Colorado from the eastern side of the Front Range westward to the Needle Mountains; they reflect major north-northwest-oriented ductile shears, in particular the zone encompassed by the combined Ilse and Gore faults (fig.5). The magnetic fabric is clearly shown in figure 6, a rose diagram showing trends of magnetic anomalies in the Colorado mineral belt. The diagram shows that northwest-trending zones of weakness had a more significant role than thought previously (Tweto and Sims, 1963) in localizing intrusions and mineral deposits in the Colorado mineral belt.
Judging from the abundance and regional distribution of the ~1.4 Ga northeast shear zones and their close association with coeval granitic intrusions in the Colorado province, such shears could have been a major factor in localizing the abundant granitic intrusions within the larger Transcontinental Proterozoic provinces (Van Schmus and others, 1993). The aeromagnetic map of the United States (Anonymous, 1993) shows numerous linear magnetic lows of northeast trend in Kansas; such trends were first highlighted by Zietz and others (1971). The anomalies should be investigated. In north-central New Mexico, northeast-trending folds and shear zones are also mainly of ~1.4 Ga age (Karlstrom and others, 1997; Marcoline and others, 1999), and similar structural trends characterize the Mesoproterozoic rocks in Arizona (Shaw and Karlstrom, 1999). Inasmuch as the shearing in Colorado is contemporaneous with the ~1.4 Ga plutonism, it could provide a key for developing a unifying model to explain the origin and nature of the Transcontinental Proterozoic provinces, at least for southwestern United States.
The contemporaneity of shearing, magmatism, and metamorphism at ~1.4 Ga (Berthoud orogeny) in Colorado (Sims and Stein, 2001) and adjacent New Mexico (Karlstrom and others, 1997) indicates that the intrusions of this age are orogenic in origin. Having accepted an orogenic origin of the ~1.4 Ga magmatism, Nyman and others (1994) and Karlstrom and others (1997), among others, have proposed compressional or transpressional tectonism from 1.5 to 1.3 Ga along southern Laurentia and subduction to account for the ~1.4 Ga magmatism. They interpreted the shear zones associated with the magmatism as having been initiated in late Paleoproterozoic time (Nyman and others, 1994, p.904), and having been strongly reactivated during the Mesoproterozoic.
As a working hypothesis to account for the contemporaneity of ~1.4 Ga shearing, magmatism, and metamorphism in the Colorado province, as determined herein, we propose that the granite melts were generated along high-strain sectors of the numerous, widespread shear zones during contractional deformation and that the melts moved upward and consolidated along and adjacent to the shears; heat transferred by this mechanism can account for the widespread pervasive prograde metamorphism. This general model of feedback relations has been described recently by several authors, notably Solar and others (1998) and Brown and Solar (1998), for contemporaneous shearing, metamorphism, and magmatism in the northern Appalachian Mountains, and elsewhere in Phanerozoic terranes.
Laramide deformation was mainly responsible for the northerly-trending mountains in Colorado (fig. 2) and elsewhere throughout the Rocky Mountains. Uplift of the mountains resulted principally from generally northeast-southwest shortening and thrust faults that penetrated the basement, bringing the basement and overriding younger strata to relatively high levels in the crust. The Laramide thrusts mainly followed fractures developed in the late Mesoproterozoic. Subsequent erosion has yielded the spectacular scenery visible today. Major west-verging thrusts bound the west margins of the Front Range, Wet Mountains, and the combined Park and Gore Ranges (pl.1). Thrusts that verge eastward bound the east margins of the southern Front Range and the Sangre de Cristo Mountains (pl.1).
Sales (1968) proposed a west-northwest left-lateral couple to account for the thrusting, a mechanism that can explain the gross Laramide structural features. Erslev (1993) has presented an elegant model for the Laramide deformation that expands on Sales' earlier model and explains it as interplay between the subducting Farallon plate and the North American overriding plate during low-angle oblique subduction.
The geologic framework of the Colorado province was established as a result of collision, subduction, and continued convergence of the Paleoproterozoic arc terrane with the Archean Wyoming craton, to the north, along the Cheyenne belt, an east-west-trending, north-vergent ductile shear zone (Duebendorfer and Houston, 1987). Collision occurred at ~1.78Ga and continued at least to ~1.74 Ga (Chamberlain, 1998), deforming and metamorphosing Paleoproterozoic continental-margin sedimentary rocks and Archean marginal gneisses and granitoids north of the Cheyenne belt (Sims and other, 2001), as well as the arc rocks of the Colorado province. Adjacent and near to the Cheyenne belt paleosuture, the Paleoproterozoic arc rocks were folded along steep axial planes subparallel to the suture, whereas those more distant in the Colorado Province from the suture were deformed in diverse patterns. Colorado orogeny deformation was episodic, lasting at least to ~1.70 Ga (Reed and others, 1987), and judged from age dating of syntectonic plutons (Premo and Fanning, 2000), culminated during the interval 1,735--1,705 Ma. Deformation was accompanied by metamorphism, dominantly to middle-upper amphibolite grade. A type locality for this thermotectonic episode is an area in the central Front Range (Moench and others, 1962), where structures formed during the Colorado orogeny are clearly distinguishable from those formed during the younger thermotectonic episode.
The Mesoproterozoic (~1.4 Ga) thermotectonic episode, the Berthoud orogeny, was a major orogenic event, extending southward into northern New Mexico; it is characterized by abundant northeast-trending ductile shear zones and associated folds of the same trend. This intracratonic deformation was accompanied by intrusion of voluminous, coeval alkali-calcic and peraluminous granitic plutons that are a major component of the Proterozoic Transcontinental Provinces in southwestern United States (Van Schmus and others, 1993). Pervasive, dominantly amphibolite-grade metamorphism was associated with this thermotectonism. This orogenic event within the Colorado province is the northern segment of thermotectonic activity in northern New Mexico (Karlstrom and others, 1997), which was dominated by folding on northeast trends and includes extensive ductile shearing.
The Precambrian ancestry of the Snake River-Wichita megashear is established herein for the first time, although Tweto (1980a) recognized that prominent northwest- and north-trending ancient shears existed within the zone as now known in central and southwestern Colorado. This shear zone originated in the late Mesoproterozoic as a left-lateral wrench fault of continental dimensions and was recurrently a major zone of weakness during the Phanerozoic.
In Laramide time, the basement and overlying Paleozoic and Mesozoic sedimentary strata were profoundly disturbed by thrust faults involving northeast-southwest shortening. The major factor controlling the position of the present-day mountains was northwest-trending faults developed initially in the late Mesoproterozoic.
The aeromagnetic anomaly map (pl. 2; Oshetski and Kucks, 2000), together with drill holes into basement rocks (Tweto, 1987), provide an important tool for extending known rock units and major Proterozoic structures in outcrop areas into the vast area in the subsurface (pl. 1). In particular, the aeromagnetic data greatly expand our knowledge of the extent and significance of the Mesoproterozoic (~1.4 Ga) northeast-trending shear zones and closely associated granitic plutons and the slightly younger Snake River-Wichita tectonic zone. The shear zones were reactivated repeatedly in Phanerozoic time (Tweto and Sims, 1963) and played a significant role in determining Phanerozoic stratigraphic, structural, and igneous patterns, as well as controlling localization of significant mineral deposits.
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