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Timing, style, and significance of Cambrian through Laramide brittle reactivation along the Proterozoic Homestake shear zone, Colorado mineral belt Joseph L. Allen Department of Geology and Physical Sciences, Concord University, Athens, WV 24712, U.S.A. email: [email protected] ABSTRACT Detailed mapping and stratigraphic studies at the intersection of the Proterozoic Homestake shear zone with the early Paleozoic section exposed on the margins of the Laramide Sawatch anticline in central Colorado provide limits on the timing and magnitude of brittle reactivation during Phanerozoic time. An episode of displacement rooted within two distinct ductile branches of the Homestake shear zone in the northeastern Sawatch Range generated an 8-km-wide, Late Cambrian fault block that had 4–20 m of paleotopographic relief prior to depositional onlap by sandstones of the Sawatch Formation. An episode of up-to-south displacement (<30 m) subsequently occurred along the southern part of the shear zone in the northeastern Sawatch Range and in the western Sawatch Range during deposition of the Lower Ordovician Manitou Formation. A third episode of reactivation during Early to Middle Ordovician time (post-Manitou Formation, pre-Harding Sandstone) resulted in renewed, decameter-scale uplift of the Late Cambrian fault block in the northeastern Sawatch Range. A fourth episode of reactivation during Late Cretaceous Laramide deformation produced localized strike-slip displacement along brittle faults on the northeastern flank of the Sawatch Range. The three early Paleozoic episodes of reactivation occurred on a cratonic platform and are genetically linked to extension and thermal uplift during intrustion of a suite of bimodal, rift-related plutonic rocks (Iron Hill and Wet Mountains intrusive rocks). They were emplaced ~135 km south of the Homestake shear zone along the Cimarron–Red Rocks fault and Apishapa fault system. The reactivation history model proposed herein differs from some previous interpretations that cumulatively suggest at least eleven episodes of Phanerozoic reactivation, including relatively large-magnitude late Paleozoic displacement. KEY WORDS: Fault reactivation, intracratonic deformation, tectonic heredity, Southern Rocky Mountains, Homestake shear zone, Cambrian, Ordovician, Colorado, Sawatch Range, Sawatch Formation, Dotsero Formation, Manitou Formation. INTRODUCTION Accurate recognition of the timing and kinematics of subtle fault reactivation is complicated in regions where multiple episodes of deformation have overprinted older structures and extensive unconformities indicate a depleted stratigraphic record. The early Paleozoic craton of west-central North America was covered by a thin stratigraphic succession and overprinted by as many as three younger deformational events. These include late Paleozoic Ancestral Rockies uplift, Late Cretaceous to early Eocene Laramide deformation, and younger extension related to opening of the Rio Grande rift. Basement-fault reactivation was prevalent during these tectonic events (e.g., Weimer, 1980; Kluth, 1986; Kellogg, 1999; Marshak et al., 2000), although the history and spatial extent of earlier reactivation within the craton is largely unknown. A record of pre-Pennsylvanian reactivation is preserved in southwestern Colorado (Baars, 1966; Baars and See, 1968; Thomas and Baars, 1995), but is obscure in other parts of the Southern Rocky Mountains (Tweto, 1977; Ross and Tweto, 1980). In part, this is due to limited exposure, a sparse stratigraphic record, and extensive structural overprinting. In order to evaluate the pre-Pennsylvanian history of cratonic fault reactivation in the Southern Rocky Mountains, basement-fault systems with minimal overprint and a relatively complete stratigraphic record can be examined. One example of a fault system with these characteristics is the Homestake shear zone (HSZ), a system of northeast- Rocky Mountain Geology, v. 39, no. 2, p. 65–84, 9 figs., 2 tables, December, 2004 65 J. L. ALLEN striking, Proterozoic ductile shear zones along the northwestern margin of the Colorado mineral belt (Fig. 1). The HSZ and other Proterozoic shear zones throughout the mineral belt are interpreted to have had a long history of reactivation and to have had regional control on the distribution of intrusions of Late Cretaceous to Tertiary magmas and related hydrothermal mineralization (Tweto and Sims, 1963; Karlstrom et al., 2002; Shaw et al., 2002). The HSZ cuts Proterozoic crystalline basement rocks in the Sawatch Range of central Colorado, and is covered by Paleozoic sedimentary rocks at the margin of the range. The contact between Proterozoic rocks of the HSZ and overlying sedimentary strata is well exposed in the northeastern Sawatch Range, providing an opportunity for accurate determination of the magnitude, location, and timing of reactivation recorded within the sedimentary cover succession (Fig. 1). Figure 1. Location maps. A, Colorado mineral belt (shaded; adapted from Tweto and Sims, 1963). B, Simplified geologic map of northern Sawatch Range (modified from Tweto et al., 1978). Locations of stratigraphic cross sections (Fig. 5 and Figs. 2 and 7) are shown by heavy lines across strike of Homestake shear zone (HSZ). Roman numerals represent general areas discussed in text: (I–III) contact between Paleozoic strata and Proterozoic crystalline basement rocks above the northern (I), central (II), and southern (III) branches of Homestake shear zone; and (IV) location of HSZ near Paleozoic–Precambrian contact in western Sawatch Range. C, Geologic sketch map of the contact between basement-rooted branches of the Homestake shear zone and sedimentary strata along the northeastern flank of Sawatch Range. Strata dip gently (10–15o) to east-northeast. Numbers show location of measured sections in Figures 2 and 7. 66 Rocky Mountain Geology, v. 39, no. 2, p. 65–84, 9 figs., 2 tables, December, 2004 PHANEROZOIC REACTIVATION OF PROTEROZOIC SHEAR ZONE This paper documents the structural and stratigraphic record of Phanerozoic reactivation along the HSZ. A significant result is the delineation of recurrent intracratonic fault reactivation during the early Paleozoic �anorogenic� interval. Interpretation of the kinematic history of fault reactivation yields insight into the early Paleozoic tectonic history of both the Colorado mineral belt and west-central North American craton. In addition, the reactivation-history model developed in this paper accurately identifies the location of reactivation within the HSZ. The model differs significantly from some prior speculative interpretations that were constructed on the basis of a general coincidence between thickness and facies variations in cover stratigraphy and the approximate, but not specific, location of the shear zone. GEOLOGIC SETTING Early geologic mapping defined the HSZ as anastomosing zones of northeast-striking, subvertical, mylonitic gneisses (initially termed �cataclastic gneisses�) hosted within Paleoproterozoic gneisses and plutonic rocks of granitic to intermediate composition (Tweto and Sims, 1963; Tweto, 1974). Individual mylonitic gneisses were mapped as a series of 10- to 200-m-wide zones (Tweto, 1974) that contain a diverse suite of fault rocks, including mylonite, ultramylonite, pseudotachylyte (e.g., Allen et al., 2002), and subordinate cataclasite and breccia. The mylonitic gneisses are clustered into three 1- to 2km-wide branches that are exposed for more than 15 km across strike in the northeastern Sawatch Range, and are herein referred to as the northern, central, and southern branches of the HSZ (Fig. 1). Recent structural mapping (Shaw et al., 2001, 2002) indicates that the shear-zone branches are primarily subvertical, high-strain zones locally overprinted by subparallel, 1- to 30-m wide mylonite-ultramylonite zones. Within the western Sawatch Range, the HSZ is mapped as a single fault (Tweto et al., 1978), which is contiguous with the central branch of the shear zone in the northeastern Sawatch Range (Fig. 1B). Basement rocks that now comprise distinct branches of the HSZ were initially deformed during an extended orogeny (~1.71–1.63 Ga) that included: high-temperature metamorphism; partial melting; development of a subvertical, northeast-striking foliation; and intrusion of the ~1.7-Ga Cross Creek batholith north of the HSZ (Shaw et al., 2001). The shear zone was complexly reactivated (~1.45–1.38 Ga) and locally overprinted during and after intrusion of the ~1.4-Ga St. Kevin batholith. Narrow mylonite and ultramylonite zones were formed under low-temperature, greenschist-facies conditions (Shaw et al., 2001). Both the high-strain zones that comprise branches of the HSZ and some mylonite-ultramylonite zones that cut high-strain zones are overprinted by northeast-striking brittle faults, indicating that the HSZ was reactivated in the brittle regime after the basement rocks were exhumed to shallower crustal levels. Paleozoic strata crop out along the flanks of the Sawatch Range, a Laramide anticlinal basement uplift in the Rocky Mountain foreland. The Cambrian to Mississippian stratigraphic succession rests nonconformably on the Proterozoic basement and consists of approximately 150–210 m of shallow shelf to peritidal facies that are bounded and punctuated by significant unconformities (Table 1). These strata are disconformably overlain by a variably thick (>500–4000 m) succession of Pennsylvanian and Permian strata that record development of a northwest-trending, intracratonic sedimentary basin (the Central Colorado trough) during uplift of the Ancestral Rockies (Hoy and Ridgway, 2002). Early Paleozoic stratigraphic units are particularly important to this study and consist, in ascending order, of the Cambrian Sawatch and Dotsero Formations, the latest Cambrian to Lower Ordovician Manitou Formation, and the Middle Ordovician Harding Sandstone. The Sawatch Formation consists of three informal members (Johnson, 1944): lower and upper white quartzite members and a middle glauconitic and dolomitic sandstone member. Mixed siliciclastic-carbonate strata referred to as Dotsero Formation in this article were previously mapped as Peerless Formation in the northern Sawatch Range (e.g., Tweto, 1974; Tweto and Lovering, 1977; Wallace and Blaskowski, 1989). However, Myrow et al. (2003) recently rejected the Peerless as a valid lithostratigraphic unit on the basis of new biostratigraphic data. The overlying Manitou Formation is a mixed siliciclastic-carbonate unit that is lithologically similar to the underlying Dotsero Formation northwest of the HSZ. Southeast of the HSZ, the Manitou Formation is a medium- to thick-bedded dolostone characterized by bedded chert. White conglomeratic quartz arenite and sandy green mudstones of the Harding Sandstone disconformably overly the Manitou Formation or older strata where the Manitou is absent due to erosion. Rocky Mountain Geology, v. 39, no. 2, p. 65–84, 9 figs., 2 tables, December, 2004 67 J. L. ALLEN Table 1. General stratigraphy of the northern Sawatch Range (modified from Tweto and Lovering, 1977; Bryant, 1979; Myrow et al., 2003). Age Formation Thickness (m) General Description L. Permian, U. and M. Pennsylvanian Maroon Formation 1280+ Conglomerate, sandstone, and siltstone; grayish red to reddish brown; arkosic. Middle Pennsylvanian Minturn Formation 1900+ Conglomerate, sandstone, mudstone, and local carbonate units; gray to reddish gray; arkosic. Belden Formation 60+ Dark gray shale and subordinate sandstone and limestone. 20–55 Dolostone (limestone north of Colorado mineral belt); medium to dark gray; fine to coarse grained; irregular thickness due to paleokarstic upper surface; primary host rock for replacement ore bodies at Gilman and Red Cliff. Gilman Sandstone 2–15 Sandstone, dolostone, and breccia; thin but laterally persistent; rests on surface; generally irregularly bedded. Dyer Dolomite 15–40 Dolostone; medium gray to black; local medium-grained quartz sand and dark chert; thin to medium bedded. Parting Formation 7–37 Conglomerate and sandstone; white to light gray; medium to coarse grained; local trough cross bedding; very well indurated. 0–21 Sandstone and silty to sandy mudstone, locally conglomeratic; white, gray, and greenish or mottled; medium to very coarse grained; abrupt basal contact. 0–90 Southeastern facies: Upper- Thick-bedded dolostone; Middle- Thick-bedded dolostone and bedded chert;. Lower- Sandy, thick-bedded dolostone. Northwestern facies: Upper- Thin- to thick-bedded dolostone and arenaceous dolostone. Lower- Thin- to medium-bedded dolomitic sandstone. Note: Facies boundary is coincident with the trace of the NE-striking Homestake shear zone. SE facies unconformably overlie older strata everywhere; most NW facies conformably overlie older strata (cf., Myrow et al., 2003, for discussion and detailed description of members). –UNCONFORMITY– Lower Mississippian Leadville Formation –UNCONFORMITY– Upper Devonian –UNCONFORMITY– Middle Ordovician Harding Sandstone –UNCONFORMITY– Lower Ordovician Manitou Formation –UNCONFORMITY (local only; see note above)– Upper Cambrian 68 Dotsero Formation 17–26 Upper: Thin- to medium-bedded sandy dolostone; local flat-pebble conglomerate. Capped by Clinetop bed – A 0.5–1 m thick stromatolitic carbonate bed found only northwest of the Homestake shear zone. Lower: Thin-to medium-bedded dolomitic sandstone; local flat-pebble conglomerate. Sawatch Formation 4–72 Upper member: Sandstone; white; medium grained; very well sorted and rounded; medium to thick bedded. Middle glauconitic member: Interbedded sandstone and dolomitic sandstone/mudstone (subordinate); medium- to thick-bedded with thin to very thin dolomitic partings; local brachiopod fragments at base. Lower member: Sandstone and conglomeratic sandstone; white to brownish white; medium to very coarse grained; thick bedded. Rocky Mountain Geology, v. 39, no. 2, p. 65–84, 9 figs., 2 tables, December, 2004 PHANEROZOIC REACTIVATION OF PROTEROZOIC SHEAR ZONE EVALUATION OF REACTIVATION HISTORY Introduction Several previous studies have attributed variations in stratigraphic thickness or facies in the general vicinity of the northern Sawatch Range to at least eleven episodes of Phanerozoic reactivation of the HSZ (Table 2). The previously cited evidence is mostly speculative, and in some cases, contradictory. For example, Tweto and Sims (1963) report the Late Devonian Parting Formation to be thicker across strike of the HSZ, and DeVoto (1990) reports the Parting Formation to thin in the vicinity of the shear zone. Neither study presents any specific supporting data, such as measured stratigraphic sections across strike of the HSZ. In addition, the kinematics Table 2. Previously inferred reactivation episodes for the Homestake shear zone (HSZ). Time Criteria for Interpretation and Kinematics (if specified) Late Cretaceous-Tertiary (Laramide) Coincidence of location of HSZ and brittle faults in lower Paleozoic sedimentary rocks. Reference: Lovering et al. (1978). Late Pennsylvanian to Permian Thick basin (1000–4500 m) of nonmarine Maroon and State Bridge Formations located several tens of km northwest of HSZ; subsidence inferred to be related to reactivation of HSZ with large-magnitude, down-toNW displacement implied. Reference: DeVoto et al. (1986); DeVoto (1990). Late Mississippian Paleokarst distribution around the northern Sawatch Range area. Up to 150 m of down-to-NW displacement along an unspecified component of the HSZ suggested as causal mechanism. Reference: Tschauder et al. (1990). post-Early Mississippian Erosional thickness variations (12 m) on post-Leadville Formation paleokarst surface. Kinematics unspecified. Reference: Tweto and Sims (1963). Late Devonian Parting and Dyer Formations thin and change facies to inferred shoreline deposits in the general vicinity of the northern Sawatch Range. Kinematics, magnitude, and number of reactivation events unspecified. Reference: DeVoto (1990). Late(?) Devonian Parting Formation reported to be thicker and more conglomeratic across HSZ. Kinematics and magnitude unspecified. Reference: Tweto and Sims (1963). post-Late Ordovician, preLate Devonian Pinch-out of Harding Formation SE of HSZ on NE flank of Sawatch Range; presence of Harding and Fremont Formations only SE of shear zone on west flank of range. Kinematics and magnitude unspecified. Reference: Tweto and Sims (1963). post-Middle Ordovician, pre-Late Devonian Map relationships in the northeastern Sawatch Range show Middle Ordovician Harding Sandstone absent on the southeastern block of a basement-rooted fault. Relationships could be interpreted to suggest up-tosouth displacement along the central branch of the HSZ after deposition of Middle Ordovician Harding Sandstone and prior to deposition of Late Devonian Parting Formation. Reference: Tweto (1974). post-Early Ordovician (1) Wedge-out of Manitou Formation near SE margin of HSZ in the NE Sawatch Range; regional thinning of Manitou northwestward across HSZ in the western Sawatch Range. Kinematics and magnitude unspecified. Reference: Tweto and Sims (1963). (2) Biostratigraphically constrained absence of upper Manitou strata northwest of HSZ due to erosional removal beneath sub-Harding Formation disconformity. Stratigraphic relationships imply up-to-north displacement; specific location within the HSZ unspecified. Reference: Myrow et al. (2003). Early Ordovician (synManitou Formation) (1) Regional facies variations inferred to be caused by synsedimentary uplift of HSZ to form a shallowwater “threshold” separating two distinct depositional sub-basins. Kinematics and magnitude unspecified. Reference: Gerhard (1972). (2) Biostratigraphically constrained southeastward onlap of Manitou Formation onto broad, NW-tilted uplift southeast of HSZ. Magnitude and specific location with HSZ undetermined. Reference: Myrow et al. (2003). Late Cambrian (pre-Sawatch Formation) Abrupt thickness variations in Upper Cambrian Sawatch Formation across general vicinity of HSZ. Kinematics unspecified, although ~23 m down-to-NW displacement along an unspecified fault or faults could possibly be inferred. Reference: Tweto and Sims (1963). Rocky Mountain Geology, v. 39, no. 2, p. 65–84, 9 figs., 2 tables, December, 2004 69 J. L. ALLEN Figure 2. Cross section of Upper Cambrian–Lower Ordovician rocks across trend of Homestake shear zone (HSZ) in northeastern Sawatch Range. The Sawatch Formation is interpreted to onlap a paleotopographic high bounded by the central and southern branches of the shear zone as shown by southward thinning and pinchout of the informal middle member of the Sawatch Formation at the margin of the central branch of the shear zone. Measured sections indicated by numbered vertical lines; see Figure 1 for locations. and magnitude of reactivation are unspecified in all but one previous interpretation, and the exact location of any reactivation episode across the 15-km width of the HSZ has never been previously determined. Some studies have inferred reactivation on the basis of data from stratigraphic sections located tens of kilometers from the HSZ (e.g., Gerhard, 1972; DeVoto et al., 1986). In order to test for reactivation and evaluate the kinematic history, segments of the HSZ were remapped in detail in the northeastern Sawatch Range where Pro- 70 terozoic rocks comprising the northern, central, and southern branches of the shear zone are in direct contact with Paleozoic cover strata (Fig. 1C). Structural and stratigraphic cross sections across the trend of the HSZ are compiled from geologic mapping and a series of new, closely spaced measured stratigraphic sections. Additional measured sections are presented across the HSZ in the western Sawatch Range for regional control (Fig. 1B). Since evidence for subtle episodes of fault displacement are likely to have been preferentially Rocky Mountain Geology, v. 39, no. 2, p. 65–84, 9 figs., 2 tables, December, 2004 PHANEROZOIC REACTIVATION OF PROTEROZOIC SHEAR ZONE Late Cambrian Reactivation (pre-Sawatch Formation) in Northeastern Sawatch Range Figure 3. Isopach map of the Sawatch Formation (thickness in meters). Lightly shaded area indicates where original thickness has been reduced by erosion below the Early Ordovician, midRossodus unconformity of Myrow et al. (2003); the Sawatch Formation is progressively truncated below the unconformity southward towards the zero isopach east-southeast of Buena Vista. Patterned regions indicate position of late Paleozoic Ancestral Rockies uplifts where most Paleozoic strata have been removed. Sources of data from Allen (unpublished measured sections), Anderson (1970), Bass and Northrop (1953, 1963), Bryant (1979), Bush (1973), Campbell (1972), Dings and Robinson (1957), Johnson (1944), Mackay (1953), Soule (1992), Stevens (1961), Taranik (1974), and Tweto and Lovering (1977). preserved within the thin cratonic succession overlying the HSZ, measured sections focus on the Cambrian to Mississippian interval. The overlying late Paleozoic section (Belden and Minturn Formations; Table 1) was not measured in detail because of great thicknesses of strata and potential correlation problems. However, the late Paleozoic section was examined on the northeastern flank of the range during the course of geologic mapping in order to identify possible synsedimentary structures or abrupt facies changes overlying the HSZ. Measured sections within the Cambrian–Ordovician stratigraphic succession in the northeastern Sawatch Range provide the framework for a stratigraphic cross section across strike of the HSZ (Fig. 2). The Sawatch Formation thins by 35 m within a distance of less than 10 km across strike of the HSZ, a general observation also noted by Tweto and Sims (1963). In detail, however, the change in thickness is abrupt at two locations that precisely coincide with the location of the central and southern branches of the HSZ. Similarly, an isopach map of the Sawatch Formation reveals anomalous abrupt thinning on a regional scale as indicated by the sharp embayment in the 40- and 60-m contours across the central and southern branches of the HSZ in the northeastern Sawatch Range (Fig. 3). Facies variations in the Sawatch Formation are associated with its abrupt changes in thickness across both the central and southern branches of the HSZ (Fig. 2). In the more northerly sections, the basal 10–15 m contains decimeter-scale lenses of granule conglomerate, and dispersed, well-rounded pebbles and granules of white and gray quartz as large as 1 cm. Conglomeratic strata are uncommon between the central and southern branches of the HSZ, although Tweto (1949) reported local conglomeratic strata within outcrop-scale basement depressions. Near the southern branch of the HSZ, maximum observed clast size is much larger (2–5 cm) than to the north; however, the clasts are less abundant and much more widely dispersed. North of the central branch, the middle member of the Sawatch is represented by a tongue of thin beds and partings comprised of shaly and glauconitic dolostone interbedded with medium- to thick-bedded quartzarenite. Field correlations completed by walking out distinctive beds demonstrate that this lithofacies interfingers and grades into quartzarenite to the south ~3.5 km north of the central branch, and is not present where the Sawatch is thinner than 40 m (Fig. 2). The Dotsero Formation conformably overlies the Sawatch Formation, suggesting that abrupt thinning is not an artifact of post-Sawatch erosion. In addition, no indication of soft-sediment deformation or growth faulting was observed in the well-exposed Sawatch Formation above mapped branches of the shear zone, suggesting that abrupt thinning is not a manifestation of syndepositional faulting. Facies and thickness variations observed in the Sawatch Formation are therefore Rocky Mountain Geology, v. 39, no. 2, p. 65–84, 9 figs., 2 tables, December, 2004 71 J. L. ALLEN interpreted to be a result of depositional onlap onto a paleotopographic high that was bounded by the central and southern branches of the HSZ. The paleotopography apparently provided a local source for an apron of quartz-pebble conglomerate in the basal Sawatch north and south of the paleohigh, and may have provided a local basin margin for lower-energy marine deposition of glauconitic and dolomitic mudstone beds that characterize the middle member. The origin of the paleotopographic high may be interpreted as either: (1) the eroded stump of a fault block uplifted prior to deposition of the Late Cambrian Sawatch Formation, or (2) an artifact of differential erosion of crystalline basement rocks of contrasting lithology. Field evidence favors an origin by fault reactivation because the location of thinning of the Sawatch is aligned with the position of the central and southern branches of the HSZ rather than Precambrian lithologic contacts. Furthermore, the Sawatch Formation abruptly changes thickness beneath the Dotsero Formation by as much as 4 m across a distance of less than 20 m above the southern branch of the HSZ. The abrupt step does not overlie a change in basement lithology, which suggests the presence of a relict fault scarp. Coincidences of wellconstrained abrupt thinning above the exposed location of both the southern and central branches of the HSZ provide evidence for fault reactivation as a cause for the development of pre-Sawatch paleotopography. The twodimensional geometry of the paleohigh is that of an ~8km-wide dissected horst, and abrupt thickness variations in the Sawatch Formation suggest uplift may have exceeded 20 m along the central branch of the HSZ and 4 m along the southern branch. The Cambrian–Precambrian contact is commonly abrupt where exposed across the paleohigh, whereas outcrops to the north exhibit a regolith as much as 2 m thick on the pre-Sawatch depositional surface. Therefore, reactivation presumably occurred sometime in the Late Cambrian prior to Sawatch deposition, as a significantly earlier date may not have preserved the paleohigh from significant weathering and erosion. Evidence for Ordovician and Younger Reactivation Introduction Structural and stratigraphic data presented in this section support three additional, post-Cambrian episodes of brittle reactivation: (1) Early Ordovician (during deposition of the Manitou Formation), (2) Early to 72 Middle Ordovician (post-Manitou Formation, pre-Harding Sandstone), and (3) Laramide or younger. Data are discussed from four locations in the northeastern and western Sawatch Range (Roman numerals in Fig. 1B). Central Branch of Homestake Shear Zone in Northeastern Sawatch Range Along the northeastern flank of the Sawatch Range, faults subparallel to the central branch of the HSZ (area II, Fig. 1B) are well exposed both along strike within the basement and within strata above the Cambrian–Precambrian contact. New detailed mapping in the sedimentary succession above the central branch of the shear zone focused on a pair of brittle faults that are herein referred to as the Homestake Creek fault system (HCFS; Fig. 4A). In the pre-Pennsylvanian sedimentary cover, the HCFS consists of distinct southeastern and northwestern faults that strike N45o�E; several secondary and unmapped minor faults branch from these faults at acute (8–25o) angles. The HCFS decreases in displacement stratigraphically upward, and neither fault cuts the Pennsylvanian section (Fig. 4). The southeastern and northwestern faults are marked in outcrop by 100-m-wide zones expressed as linear, steep-sided topographic gullies lined with both brecciated and unbrecciated blocks of Sawatch Formation talus. An elongate linear ridge of brecciated and silicified Sawatch Formation that is folded into a tight asymmetric anticline marks the junction between the southeastern fault and a prominent secondary fault. In the immediate vicinity of the HCFS, slickenlines on bedding planes and northeast-striking minor faults plunge gently to the northeast, subparallel to the regional dip of bedding. A cross-strike cross section of the central branch of the HSZ illustrates two kinematically contrasting episodes of reactivation along the HCFS (Fig. 4B). The southeastern fault has four important characteristics: (1) The upper part of the Dotsero Formation and the entire overlying Manitou Formation are absent due to erosion from the upthrown southern block; (2) both the upthrown and downthrown blocks are overlain by a sheet of Harding Sandstone that is equally thick on both sides; (3) a small uplifted anticlinal wedge of Sawatch Formation and Proterozoic basement is bounded by the fault; and (4) the fault roots within, and is parallel to, a narrow basement mylonitic zone indicating that it is a reactivated segment of the central branch of the HSZ. Abrupt erosional thinning of the Dotsero and Manitou Formations across the southeastern fault indicates that Rocky Mountain Geology, v. 39, no. 2, p. 65–84, 9 figs., 2 tables, December, 2004 PHANEROZOIC REACTIVATION OF PROTEROZOIC SHEAR ZONE Figure 4. A, Geologic map of intersection of central branch of HSZ with Paleozoic cover along northeast flank of the Sawatch Range (area II, Fig. 1B). Geology of Proterozoic basement adapted from Tweto (1974). B, Cross section illustrates absence of uppermost Dotsero Formation and all of overlying Manitou Formation south of the southeastern boundary of Homestake Creek fault system (HCFS) (VE=4.7X). Note that Quaternary sediments at the surface are not shown along the cross section line. These relationships suggest 17 m up-to-south fault reactivation (post-Manitou, pre-Harding). The distribution of the Harding Sandstone and Manitou Formation above the central branch of the HSZ is important to this study and significantly differs from previous mapping (Tweto, 1953, 1974), which shows Harding Sandstone absent southeast of the HCFS and Manitou Formation absent everywhere near the central branch. These previously mapped relationships imply an episode of post-Harding reactivation along the HCFS, which is not supported by new mapping presented in this figure. Strata originally mapped as Peerless Formation north of the HCFS (Tweto, 1974) are shown here to consist of Dotsero Formation and the overlying Manitou Formation. This reassignment is justified by the presence of the stromatolitic Clinetop bed in this area (outcrops can be found along the mapped contact between Cambrian rocks and the Manitou Formation 50–110 m southeast of the northwestern HCFS fault near the location of measured section 21). Rocky Mountain Geology, v. 39, no. 2, p. 65–84, 9 figs., 2 tables, December, 2004 73 J. L. ALLEN Figure 5. Cross section of pre-Pennsylvanian strata in western Sawatch Range; line of section shown on Figure 1B. The cross section indicates that facies and thickness variations in post-Manitou strata are non-systematic with respect to location of shear zone. Thin vertical lines illustrate locations of sections measured by author, except for sections at Aspen, East Lake Creek, and Woods Lake, which were compiled from Bush (1973), Campbell (1970), Johnson (1944), Mackay (1953), and Nadeau (1972). one component of the central branch of the HSZ was reactivated as a steep fault after deposition of the Lower Ordovician Manitou Formation and prior to deposition of the Middle Ordovician Harding Sandstone. A displacement of approximately 17 m down-to-northwest is indicated by the amount of pre-Harding erosion immediately southeast of the fault. The uplifted wedge of Upper Cambrian Sawatch Formation (Fig. 4B) is juxtaposed against Lower Mississippian rocks along the same fault that was reactivated prior to Middle Ordovician Harding Sandstone deposition, and clearly indicates an episode of post-Early Mississippian reactivation. In cross section, the anticlinal wedge of Sawatch Formation has the geometry of a 74 flower structure. This, combined with the presence of northeast-striking, bed-parallel slickenlines, suggests an origin in response to strike-slip displacement. Apparent truncation of a sill of the ~70-Ma Pando Porphyry (K-Ar date of Pearson et al., 1962) along the southeastern fault suggests Late Cretaceous or younger reactivation, probably related to Laramide uplift of the Sawatch anticline. This structural style is consistent with the observation of minor strike-slip faults along bedding planes observed in now-flooded mine workings above the northern branch of the HSZ. These minor faults had previously been attributed to Laramide flexure during uplift of the Sawatch anticline (Lovering et al., 1978). Rocky Mountain Geology, v. 39, no. 2, p. 65–84, 9 figs., 2 tables, December, 2004 PHANEROZOIC REACTIVATION OF PROTEROZOIC SHEAR ZONE Western Sawatch Range The HSZ in the western Sawatch Range is contiguous with the central branch of the shear zone in the northeastern Sawatch Range (area IV, Fig. 1B). A series of measured sections of pre-Pennsylvanian rocks provides the basis for a stratigraphic cross section across strike of the shear zone in the western Sawatch Range (Fig. 5). Proterozoic basement rocks comprising the HSZ are not exposed in contact with Paleozoic cover rocks in this area; however, stratigraphic data across the position of the HSZ suggest two episodes of Ordovician reactivation in the western Sawatch Range. The Manitou Formation consists of two distinct lithofacies: (1) a southeastern dolostone facies characterized by the presence of bedded chert within the middle part of the formation, and (2) a northwestern facies that does not contain abundant bedded chert and is characterized by flat-pebble conglomerate, sandy dolostone, and shaly dolostone (Fig. 5). Both the northwestern and southeastern facies contain distinctive lithofacies associations used to define members (Bass and Northrop, 1953; Gerhard, 1967, 1972; Myrow et al., 2003). The boundary between the distinctive facies of the Manitou Formation was interpreted by Gerhard (1972) to be near the location of the HSZ and related to uplift of an unspecific feature termed a �shallow threshold� which separated the facies into separate sub-basins. The facies boundary was mapped between two very widely spaced measured sections, and no evidence was presented that more closely tied the boundary to the location of the shear zone. A field traverse between stratigraphic sections measured for this study demonstrated that the facies boundary is remarkably abrupt, although it lies in a region of poor exposure across a distance of less than 3 km (between Hunter Creek and Woody Creek, Figs. 1B, 5). Published geologic mapping illustrates a 1-km-wide zone of sheared Proterozoic basement between the two locations (Bryant, 1971; Fridrich et al., 1998), and the zone represents the southwestern-most exposure of the HSZ (Tweto et al., 1978). New trilobite and conodont biostratigraphic data from Cambrian and Ordovician rocks across Colorado demonstrate that the Manitou Formation is older in the northwestern Sawatch Range and younger to the south with only minor overlap in age between strata of the two areas (Myrow et al., 1999, 2003). Myrow et al. (2003) attribute this to progressive regional uplift and erosion southeast of the HSZ during development of a Lower Ordovician unconformity within the Rossodus mani- touensis conodont zone of the upper part of the Manitou Formation. This unconformity, first documented by Berg and Ross (1959), shows progressive truncation of underlying earliest Ordovician and Late Cambrian units from north to south and later onlap of young deposits of the Manitou Formation southward. Approximately 60–80 km southeast of the HSZ, the upper Manitou Formation above the unconformity rests on Precambrian basement. The location of the Manitou–basement contact corresponds to the zero isopach of the Sawatch Formation in the southeastern corner of Figure 3. The relationship between older and younger Manitou facies across the shear zone is critical to interpretation of Ordovician reactivation. Importantly, the younger Manitou strata are not preserved northwest of the HSZ (Myrow et al., 2003). Since the facies boundary is herein documented to be coincident with a mapped component of the HSZ, it seems reasonable to suggest post-Manitou, up-to-northwest reactivation as a brittle fault in the western Sawatch Range and erosion of young Manitou rocks to the northwest. Young, chert-bearing facies that characterize the southeastern facies of the Manitou Formation are juxtaposed against older northwestern strata across the HSZ between Hunter Creek and Woody Creek in the western Sawatch Range (Fig. 5). This strongly suggests two episodes of reactivation: (1) Early Ordovician (syn-Manitou), up-to-southeast displacement, followed by (2) post-Manitou, up-tonorthwest displacement. The magnitude of displacement cannot be accurately determined, but an up-to-southeast displacement of less than 30 m would be sufficient to account for a lack of older Manitou facies immediately southeast of the shear zone. A second, up-to-northwest displacement of approximately 30–50 m would account for post-Manitou erosional removal of younger facies northwest of the shear zone. Southern Branch of Homestake Shear Zone in Northeastern Sawatch Range New field mapping (area III, Fig. 1B), combined with mapping of structure contours of the Cambrian– Precambrian contact, indicates the presence of a brittle fault that projects upward from the southern branch of the HSZ in the northeastern Sawatch Range (Camp Hale fault; Fig. 6). The Camp Hale fault is a zone more than 10 m wide where exposed within the Sawatch Formation and incorporates a silicified tectonic breccia similar to breccia along the HCFS. A mostly covered brittle fault, herein termed the Rule Gulch fault, also cuts basement Rocky Mountain Geology, v. 39, no. 2, p. 65–84, 9 figs., 2 tables, December, 2004 75 J. L. ALLEN 76 Rocky Mountain Geology, v. 39, no. 2, p. 65–84, 9 figs., 2 tables, December, 2004 PHANEROZOIC REACTIVATION OF PROTEROZOIC SHEAR ZONE Figure 6. (facing page) Geologic map (A), and cross section (B) of intersection of southern branch of HSZ with Paleozoic cover strata along northeastern flank of the Sawatch Range (area III, Fig. 1B). Structure contours on Cambrian–Precambrian contact illustrate lateral extent of Rule Gulch fault rooted in the southern branch of the HSZ. Measured sections at Camp Hale and South Fork, located north and south of the Rule Gulch fault, illustrate absence of Manitou Formation north of fault, and its presence south of fault (noted by bold arrows). Geology of Proterozoic basement adapted from Tweto (1956, 1974). Figure 7. Cross section of pre-Pennsylvanian strata along the northeastern flank of the Sawatch Range; line of section shown on Figure 1B. Section illustrates post-Manitou, pre-Harding fault reactivation along central and southern branches of the HSZ. Facies and thickness variations in post-Manitou strata are non-systematic with respect to mapped components of the shear zone. and cover parallel to strike of the southern branch of the HSZ. The southern branch was reactivated in Late Cambrian time (Fig. 2), and a stratigraphic cross section of pre-Pennsylvanian strata compiled from new measured sections in the northeastern Sawatch Range (Fig. 7) suggests the possibility of two additional episodes of Ordovician reactivation along the southern branch of the shear zone. Chert-bearing strata of the southeastern facies of the upper Manitou Formation are preserved south of the southern branch, and older, sandy dolostones of the northwestern facies are preserved just 8 km to the north above the central branch; the Manitou Formation is absent between these two locations. Manitou strata in both locations are capped by the Middle Ordovician Harding Sandstone. Biostratigraphic and age relationships in the Manitou are similar to those in the western Sawatch Range (discussed previously), and suggest: (1) Early Ordovician (syn-Manitou), up-to-south reactivation to account for an apparent lack of older northwestern facies strata south of the southern branch; and (2) Early to Middle Ordovician (post-Manitou, pre-Harding), up-to-north reactivation to account for a lack of young southeastern facies strata north of the central branch. The Early Ordovician (syn-Manitou) reactivation episode may have occurred on either the central or the southern branch of the HSZ, or both. The magnitude of displacement is inferred to be similar to estimated dis- Rocky Mountain Geology, v. 39, no. 2, p. 65–84, 9 figs., 2 tables, December, 2004 77 J. L. ALLEN placement in the western Sawatch Range (~30 m, up-tosouth). Early to Middle Ordovician reactivation (postManitou, pre-Harding) is interpreted to have occurred along both the central and southern branches. The central branch exhibits 17 m up-to-south, post-Manitou displacement (Fig. 4B); the southern branch is inferred to be similar to estimated displacement in the western Sawatch Range (~30–50 m, up-to-north). Northern Branch of Homestake Shear Zone in Northeastern Sawatch Range The northern branch intersects Paleozoic cover strata between Gilman and Red Cliff (area I, Fig. 1B). Published mapping of the area illustrates several northeast-striking, basement-rooted brittle faults at the Cambrian–Precambrian contact that lie along strike of the northern branch of the HSZ (Tweto and Lovering, 1977; Lovering et al., 1978). The faults record only minor, meter-scale offsets of the Cambrian–Precambrian contact (<3 m common; 12 m maximum). Nearly half of the brittle faults cut only the Precambrian basement, and the remainder die out stratigraphically upward within the Upper Cambrian Sawatch Formation, Lower Mississippian Leadville Limestone, and shale-rich strata within the basal part of the Pennsylvanian section. Faults penetrating the base of the Pennsylvanian section have the largest observed offsets and cut the Late Cretaceous Pando Porphyry (Tweto and Lovering, 1977; Lovering et al., 1978), suggesting a Laramide age for displacement on those faults. Facies and thickness variations are non-systematic with respect to brittle faults along the northern branch (Fig. 7), suggesting no significant displacement along these faults during Paleozoic time. REACTIVATION-HISTORY MODEL Timing and Kinematics The evidence summarized above illustrates that faults rooted within specific branches of the HSZ have been reactivated several times as brittle structures throughout the Phanerozoic and that the kinematic behavior of this shear zone has changed through time. In the northeastern Sawatch Range, brittle faults that comprise the HCFS cut the central branch of the Proterozoic HSZ, and the Rule Gulch and Camp Hale faults cut the ductile southern branch. Prior to deposition of the Upper Cambrian Sawatch Formation, a paleotopographic high was formed as a result of uplift of an 8-km-wide, 78 Figure 8. Interpretive summary of timing and kinematics of Phanerozoic fault reactivation along the HSZ. Rocky Mountain Geology, v. 39, no. 2, p. 65–84, 9 figs., 2 tables, December, 2004 PHANEROZOIC REACTIVATION OF PROTEROZOIC SHEAR ZONE block-faulted horst between the central and southern branches of the HSZ (Fig. 8A). The block was bounded by the HCFS on the north, and the Rule Gulch and Camp Hale faults on the south. Abrupt thickness and facies variations in the Sawatch Formation, which onlapped the paleohigh during Late Cambrian transgression, suggest uplift of at least 20 m up-to-the-south along the central branch and 4 m up-to-the-north along the southern branch. The HSZ was reactivated in both the western and northeastern Sawatch Range during deposition of the Early Ordovician Manitou Formation. Up-to-south displacement of ~30 m is estimated (Fig. 8B). The HSZ was subsequently reactivated in Early to Middle Ordovician time after deposition of the Lower Ordovician Manitou Formation and prior to deposition of the Middle Ordovician Harding Sandstone (Fig. 8C). In the northeastern Sawatch Range, the reactivation generated an asymmetric, north-tilted fault block that exploited the same faults that were active preceding deposition of the Sawatch Formation. In the western Sawatch Range, the Early to Middle Ordovician reactivation resulted in upto-north displacement of approximately 30–50 m across the HSZ. During Laramide uplift of the Sawatch Range, both the central and northern branches of the HSZ were reactivated in the northeastern Sawatch Range. The central branch was reactivated as a strike-slip fault along the HCFS (Fig. 8D). Lovering et al. (1978) report meterscale, strike-slip and dip-slip Laramide displacement above the northern branch of the HSZ in now-flooded mine exposures. Discussion Mapped northeast-striking faults in the northeastern Sawatch Range only displace the Cambrian–Precambrian contact by a few meters and only a few faults exhibit larger displacements of a few tens of meters at most (Tweto, 1974; Tweto and Lovering, 1977). The present structural position of the Cambrian–Precambrian contact therefore supports only limited episodes of small displacement, which is consistent with the reactivation history model presented here. In addition, uniformity of stratigraphic thicknesses and facies across fault zones in post-Manitou to pre-Middle Pennsylvanian strata suggests that the HSZ was not reactivated as a brittle fault between Middle Ordovician and latest Paleozoic, and likely not before latest Cretaceous. These conclusions contrast with some previously proposed hypotheses that imply significant middle to late Paleozoic reactivation along the HSZ (Table 2). For example, some workers have suggested that late Paleozoic, down-to-northwest reactivation of the HSZ accommodated 1000– 4500 m of subsidence in the Eagle basin northwest of the Sawatch Range (DeVoto et al., 1986; DeVoto, 1990). In addition, Tschauder et al. (1990) imply 150 m of Mississippian (post-Leadville Formation) displacement along the HSZ. Primary evidence supporting these interpretations includes thickness and facies variations in cover strata in the general vicinity of the northern Sawatch Range, although facies variations were not demonstrated to coincide with any specific components of the shear zone. The decameter-scale offset of the Cambrian–Precambrian contact precludes large middle to late Paleozoic episodes of reactivation. Apparently, the regional stratigraphic changes within the vicinity of the northern Sawatch Range were controlled by factors other than reactivation along the HSZ. Similar geometric relationships in cover rocks are also exhibited by Devonian strata (Parting and Dyer Formations), which thin from north to south near, but not directly above, the shear zone (Fig. 5). The axis of thinning strikes northeasterly and is parallel to the HSZ as indicated by regional Devonian isopach maps (Campbell, 1970), a relationship loosely suggestive of reactivation. However, Devonian strata are well exposed across the shear zone in the northeastern Sawatch Range and there are no abrupt thickness variations or synsedimentary structures that can be specifically attributed to fault reactivation along any mapped components of the shear zone. Although no definitive evidence supports Devonian fault reactivation along the HSZ, it is possible that the shear zone vicinity served as a subtle hinge line accommodating flexural subsidence to the northwest. TECTONIC IMPLICATIONS Although the HSZ is shown to have had only small-scale Phanerozoic reactivation, the effects and implications of recurrent reactivation are nonetheless significant. Most of the reactivation history of the HSZ occurred during the early Paleozoic �anorogenic� interval in a cratonic setting. The timing of the first three episodes of Phanerozoic reactivation between the Late Cambrian and Early to Middle Ordovician is coincident with the intrusion of early Paleozoic igneous rocks ~135 km southeast and southwest of the HSZ (Fig. 9). In southcentral Colorado, the basement is cut by two broadly en echelon Proterozoic fault systems; a northwest-striking swarm of faults in the eastern part of Colorado (Ap- Rocky Mountain Geology, v. 39, no. 2, p. 65–84, 9 figs., 2 tables, December, 2004 79 J. L. ALLEN Figure 9. Relationship of the Colorado mineral belt (shaded area) and HSZ to early Paleozoic intrusive rocks and northwest-striking, basement-fault systems in south-central Colorado. Basement structure and data for intrusive rocks compiled from Tweto (1987). Abbreviations: Gfz – Gore fault zone; Hsz – Homestake shear zone; Mfz – Mosquito fault zone; A – Aspen; GS – Glenwood Springs; L – Leadville. ishapa fault system), and a northwest-striking fault zone (Cimarron–Red Rocks fault) in the west. Field relations indicate that the Cimarron–Red Rocks fault was both initiated and reactivated as a strike-slip system during the Proterozoic prior to additional Phanerozoic dip-slip reactivation (Tweto, 1987). The Proterozoic fault systems were intruded by both bimodal alkalic complexes that have crystallization ages of ~570 Ma (Iron Hill intrusive rocks) and ~535–511 Ma (Wet Mountains intrusive rocks) (Olson et al,. 1977; Bickford et al., 1989), and northwest-striking diabase and syenite dikes that have crystallization ages of ~495 and 497 ± 16 Ma (Olson et al., 1977; Larson et al., 1985). These rocks have been interpreted to be lithologically and geochemically characteristic of an intraplate, extensional rift environment (Larson et al., 1985). Recent geochronologic data place the beginning of the Late Cambrian at 500 Ma, and the beginning of the Ordovician at 490 Ma (Davidek et al., 1998; Encarnacion et al., 1999). The early Paleozoic episodes of reactivation across the HSZ (Fig. 8) temporally overlap emplacement of the early Paleozoic intrusive rocks. The Iron Hill intrusive rocks were emplaced in latest Pro- 80 terozoic time well before deposition of the Upper Cambrian Sawatch Formation, whereas the Wet Mountains intrusives were emplaced just prior to (~536–511 Ma) deposition of the Sawatch. The west-northwest-striking tholeiitic dikes (497 ± 16 and ~495 Ma) were intruded during latest Cambrian or earliest Ordovician and are coincident with the timing of Early Ordovician reactivation. Although these intrusive rocks are associated with fault systems that are distal to the HSZ, the timing of their emplacement suggests a possible causal mechanism for early Paleozoic reactivation. The intrusive rocks define a west- to northwest-striking zone of magmatism spanning more than 75 m.y. (Larson et al., 1985). Biostratigraphic relationships suggest that Early Ordovician uplift occurred south of the HSZ during deposition of the Manitou Formation (Myrow et al., 2003), and stratigraphic data indicate that the Manitou Formation onlaps the Proterozoic basement 60–80 km south of the HSZ (Fig. 3), just north of the belt of Cambrian–Ordovician intrusive rocks. If the intrusives were associated with a long-lived thermal anomaly, then thermally driven uplift may have generated a basement high in south-central Colorado during Manitou deposition and provided the mechanism for intracratonic reactivation along the HSZ. Thus, thermal uplift and associated extension along the northwest-striking belt of intrusives is inferred to have provided a transtensional setting for reactivation along the northeast-striking HSZ from Late Cambrian through Early Ordovician time. Since extension related to magmatism and thermal uplift seems to explain the early Paleozoic kinematic history of the HSZ, this suggests the possibility that other Proterozoic fault zones in central Colorado may have been reactivated in early Paleozoic time. For example, some other features in the region that might be related to Cambrian extension are north- to northwest-striking sandstone dikes that intruded crystalline basement rocks along fault zones in the central and southern Front Range. These dikes have been described and mapped by others (e.g., Cross, 1894; Crosby, 1897; Vitanage, 1954; Scott, 1963; Harms, 1965; Scott and Wobus, 1973) and are composed of quartz sandstone that resembles the Sawatch Formation in thin section, rather than any younger formations. It is possible that these features represent extensional opening of pre-existing fault zones before lithification of the Sawatch Formation. If so, early Paleozoic extensional or transtensional fault reactivation may have been widespread in Colorado. Rocky Mountain Geology, v. 39, no. 2, p. 65–84, 9 figs., 2 tables, December, 2004 PHANEROZOIC REACTIVATION OF PROTEROZOIC SHEAR ZONE CONCLUSIONS The northeastern Sawatch Range offers rare exposure of an early Paleozoic, reactivated basement-fault system in the Southern Rocky Mountains. In the Colorado mineral belt, discrete high-strain zones in the Homestake shear zone were brittlely reactivated on a decameter scale. Inferred dip-slip faults formed at least three times during the early Paleozoic, prior to minor strike-slip and dip-slip reactivation during Laramide uplift of the Sawatch Range. The shear zone appears to have been dormant during middle and late Paleozoic time, and large Phanerozoic displacement is precluded by an observed small offset (meters to decameters) of the Cambrian–Proterozoic contact in the northeastern Sawatch Range. The first episode of Phanerozoic reactivation generated a Late Cambrian fault block with 4–20 m of paleotopographic relief prior to deposition of clastic sediments of the Sawatch Formation. During Early Ordovician time, the shear zone was reactivated across the width of the Sawatch Range (~30 km up-to-south) during deposition of the Manitou Formation. A third episode of reactivation prior to Middle Ordovician time resulted in up-to-north faulting across the width of the Sawatch Range, and renewed uplift (~17–50 m) of the Late Cambrian fault block in the northeastern Sawatch Range after deposition of the Manitou Formation. The early Paleozoic reactivation history of the Homestake shear zone occurred in an intracratonic setting and is temporally coincident with late stages of intrusion of a northwest-striking belt of alkalic and mafic rift-related igneous rocks south of the shear zone. 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