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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
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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.
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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
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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
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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-
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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
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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
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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
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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-
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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-
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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. I propose that
these intrusives were associated with thermal uplift that
generated an extensional or transtensional regime in central Colorado that localized reactivation of Proterozoic
shear zones along the northwestern margin of the Colorado mineral belt.
ACKNOWLEDGMENTS
This research was supported by grants from the Colorado Scientific Society (Ogden Tweto Memorial Fund),
Southeastern Section of the Geological Society of America, Mobil Corporation, and the Petroleum Research
Fund of the American Chemical Society. The author
thanks William A. Thomas for research guidance, and
Wm. Jay Sims III for assistance in the field. The ideas
and presentation were enhanced by comments and/or
discussion with E. Erslev, K. Kellogg, S. Marshak, P. Myrow, A. Wallace, and RMG reviewers Bruce Bryant and
Charles F. Kluth.
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MANUSCRIPT SUBMITTED OCTOBER 7, 2003
REVISED MANUSCRIPT SUBMITTED JULY 16, 2004
MANUSCRIPT ACCEPTED SEPTEMBER 13, 2004
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