Strain partitioning in the mid-crust of a transpressional shear zone system: Insights from the Homestake and Slide Lake shear zones, central Colorado

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Journal of Structural Geology 39 (2012) 237e252

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Journal of Structural Geology

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Strain partitioning in the mid-crust of a transpressional shear zone system:Insights from the Homestake and Slide Lake shear zones, central Colorado

P. Elizabeth Lee a,1,*, Micah J. Jessup a, Colin A. Shawb, Gordon L. Hicks III a, Joseph L. Allen c

aDepartment of Earth and Planetary Sciences, University of Tennessee, Knoxville, TN 37996, USAbDepartment of Earth Sciences, Montana State University, Bozeman, MT 59717, USAcDepartment of Physical Sciences, Concord University, Athens, WV 24712, USA

a r t i c l e i n f o

Article history:Received 23 May 2011Received in revised form28 January 2012Accepted 5 February 2012Available online 3 March 2012

Keywords:TranspressionProterozoicLow-angle shear zonesMicrostructuresQuartz fabrics

* Corresponding author.E-mail address: [email protected] (P.E. Lee).

1 Present address: ExxonMobil Production Company77002, USA.

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a b s t r a c t

Kinematic analysis and field mapping of the Homestake shear zone (HSZ) and Slide Lake shear zone(SLSZ) in central Colorado may provide insight into the interaction between subvertical and low-angleshear zones in the middle crust. The northeast-striking, steeply dipping HSZ comprises a w10-km-wide set of anastomosing ductile shear zones and pseudotachylyte-bearing faults. Approximately 4 kmsouth of the HSZ, northenortheast-striking, shallowly dipping mylonites of the SLSZ form three 1e10-m-thick splays. Oblique stretching lineations and shear sense in both shear zones record components of dip-slip (top-up-to-the-northwest and top-down-to-the-southeast) and dextral strike-slip movement duringmylonite development. Quartz and feldspar deformation mechanisms and quartz [c] axis latticepreferred orientation (LPO) patterns suggest deformation temperatures ranging from w280e500 �C inthe HSZ to w280e600 �C in the SLSZ. Quartz [c] axis LPOs suggest plane strain general shear across theshear system. Based on the relative timing of fabric development, compatible kinematics and similardeformation temperatures in the SLSZ and the HSZ, we propose that both shear zones formed duringstrain localization and partitioning within a transpressional shear zone system that involved subverticalshuffling in the mid-crust at 1.4 Ga.

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1. Introduction

During oblique convergence continental tectonics can involveshortening and transpression (e.g. Harland, 1971; Sanderson andMarchini, 1984; Tikoff and Teyssier, 1994), as well as crustalextension (e.g. Wheeler and Butler, 1994). Systems of obliqueconvergence are often associated with wide orogenic zones thatinclude strike-slip shear zones far inboard from active plateboundaries (e.g. White Mountain shear zone and western Idahoshear zone, North American Cordillera), and include transtensionaland transpressional structures (Teyssier et al., 1995; Tikoff andGreene, 1997; Giorgis and Tikoff, 2004; Sullivan and Law, 2007).This contribution focuses on the partitioning of transpressionalstrain into kinematically linked low-angle and steep shear zones atmid-crustal levels during intracontinental deformation of juvenilecontinental lithosphere. The work is relevant to strain partitioning

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in crust that contains inherited anisotropy related to continentalassembly.

The north-to-northeast-striking Slide Lake shear zone (SLSZ) isa 1-km-wide, shallow to moderate dipping mylonite and ultra-mylonite shear zone that is exposed 4-km-south of the Homestakeshear zone (HSZ) near the summit of Homestake Peak (4023m) andabove Slide Lake in central Colorado (Figs. 1 and 2). The 10-km-wide, steeply dipping HSZ has beenmapped as one of the dominantProterozoic shear zones within the Colorado mineral belt (CMB;Tweto and Sims, 1963; Tweto, 1974). These structures are hypoth-esized to have played a role in localizing Cretaceous and Paleogeneplutonism and mineralization within the CMB (Tweto and Sims,1963). Timing of regional metamorphism/thermal events (Shawet al., 2001, 2005), kinematics, and rheology (Shaw and Allen,2007) are constrained for the HSZ. These studies link in discretemylonite, ultramylonite, and pseudotachylyte zones that comprisethe HSZ into several distinct phases of progressive deformationwithin an exhumed Proterozoic brittleeductile seismogenic shearzone (Shaw and Allen, 2007).

These structures cut through highly deformed Paleoproterozoicmid-crustal crystalline rocks that make up the core of the southernRocky Mountains and formed during the Proterozoic growth of

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Fig. 1. (A) Regional tectonic map of Proterozoic assembly in the southwestern U.S. with Proterozoic boundaries (Condie, 1986; Bennett and DePaolo, 1987; Karlstrom and Bowring,1988; Wooden et al., 1988; Wooden and DeWitt, 1991; Jones et al., 2010a). (B) Inset generalized geologic map of the study area (modified from Shaw and Allen, 2007; Tweto, 1979)with the location of Fig. 2.

P.E. Lee et al. / Journal of Structural Geology 39 (2012) 237e252238

Laurentia (Fig. 1A; e.g. Karlstrom and Bowring, 1988; Hill andBickford, 2001). Within this setting, Proterozoic through Phanero-zoic events have left a record of polyphase deformation that marksthe region’s assembly and unroofing e from Paleoproterozoic,ductile movement at middle crustal levels, to Late Cenozoic, upper-crustal brittle fracturing (Fig. 1; Bickford et al., 1989; Bowring andKarlstrom, 1990; Shaw and Karlstrom, 1999a; Shaw et al., 2001,2005; Tyson et al., 2002; Jessup et al., 2005; McCoy et al., 2005;Jessup et al., 2006; Shaw and Allen, 2007; Caine et al., 2010).

We combine a new detailed structural map of the SLSZ withmesoscale observations, microstructural analyses, and quartz [c]axis lattice preferred orientation (LPO) patterns derived fromelectron backscatter diffraction (EBSD) to constrain the kinematicsof the SLSZ and HSZ. As the first major contribution focusing on the

SLSZ, this study determines that the low-angle SLSZ recordsmovement in a system that may be kinematically linked to the HSZ.Our new data suggests that the HSZ and SLSZ are parts of a complexshear system that involved the formation of coeval steep and low-angle shear zones in the mid-crust. Results also provide insightsinto how strainwas partitioned between the steep HSZ and shallowSLSZ during w1.4 Ga tectonism within the continental interior(Shaw et al., 2005).

2. Geologic setting

The evolution of continental crust involves multiple pulses oftectonism, where new crust is assembled onto preexisting crustand structures associated with shortening, extension, and

Fig. 2. (A) Generalized geologic map of the northern Sawatch Range in the vicinity of Homestake and Slide Lake shear zones (see location map in Fig. 1B). Represented structuraldata from this study, Tweto (1974), Tweto et al. (1978), and Shaw and Allen (2007). (B) Geologic cross section of the field area (AeA0). Figure shows the multiple generations offoliation exposed in the field area: S1 early melt-present foliation, S2/F2 upright folds, S3 mylonite, S4 ultramylonite and pseudotachylyte (red-dashed line), and S3(SL) SLSZ myloniteand ultramylonite.

P.E. Lee et al. / Journal of Structural Geology 39 (2012) 237e252 239

transcurrent movement are created. Such structures can evolveinto persistent intracontinental tectonic zones through repeatedreactivation during continental deformation (Harland, 1971;Molnar and Tapponier, 1975; Molnar, 1988; Bowring andKarlstrom, 1990; Teyssier et al., 1995). Major northeast-strikingshear zones throughout the southwestern United States recorddeformation associated with the assembly of the North Americancontinent and reactivation of structures by subsequent intra-continental tectonism (Tweto and Sims, 1963; Bowring andKarlstrom, 1990; Nyman et al., 1994; Karlstrom and Humphreys,1998, Shaw et al., 2001). Proterozoic rocks are exposed from theCheyenne belt of southern Wyoming (e.g. Karlstrom and Houston,1984; Duebendorfer et al., 1987; Karlstrom and Bowring, 1988;Bowring and Karlstrom, 1990; Bickford and Hill, 2007) southwardto New Mexico. The Proterozoic rocks exposed in central Coloradoare part of a w1200-km-wide swath of juvenile lithosphere andblocks of older material that were assembled onto the southernmargin of Laurentia during two phases of deformation from about1.8 to 1.6 Ga (Fig. 1A; Tweto and Sims, 1963; Tweto, 1974; DePaolo,1981; Karlstrom and Bowring, 1988; Bowring and Karlstrom, 1990;

Shaw and Karlstrom, 1999a; Hill and Bickford, 2001; Shaw et al.,2001; Tyson et al., 2002; Jessup et al., 2005, 2006; McCoy et al.,2005; Shaw and Allen, 2007). The first phase (1.78e1.70 Ga)assembled the Yavapai province, consisting of arc-derived rocksand fragments of older continental crust, across a complex systemof northeast- and northwest-striking subduction zones(Duebendorfer et al., 1987; Shaw and Karlstrom, 1999a; Hill andBickford, 2001; Jessup et al., 2005, 2006). The second phase(1.68e1.65 Ga) involved continued shortening and the accretion ofthe Mazatzal province, a mosaic of tectonostratigraphic terranes,along a northeast-striking convergent margin (Fig. 1A; e.g.Karlstrom and Bowring, 1988; Shaw and Karlstrom, 1999a).

Following the 1.68e1.65 Ga Mazatzal orogeny, a w200-m.y.period of continental stability ensued, ending at w1.4 Ga withmagmatism and the reactivation of earlier structures (Karlstromand Bowring, 1988; Williams, 1991; Aleinkoff et al., 1993; Reedet al., 1993; Nyman et al., 1994; Duebendorfer and Christensen,1995; Kirby et al., 1995; Karlstrom and Humphreys, 1998;Williams et al., 1999; Jessup et al., 2005, 2006; Shah, 2010; Joneset al., 2010a, b). Anorogenic interpretations based on geochemical


data suggest magma emplacement occurred in an extensionalsetting (Anderson, 1983; Frost et al., 2001); however, field- andlaboratory-based structural investigations of these granites suggestthat emplacement was accompanied by northwest-directedshortening and strike-slip deformation (Graubard and Mattinson,1990; Shaw et al., 2001; Nyman et al., 1994; Jessup et al., 2006;Jones et al., 2010b). This deformation has been attributed to far-field stresses invoked by distal subduction or transpression onthe southeastern margin of Laurentia (Nyman et al., 1994;Duebendorfer and Christensen, 1995; Ferguson et al., 2004; Joneset al., 2010a).

Fig. 3. Field observations from the HSZ. (A) View from Hornsilver Campground towards tnortheast-striking, steeply dipping Homestake shear zone and shaded band at skyline repreHigh-temperature (bt þ qtz þ sil þ fsp) migmatite (S1) characteristic of the region exposedalong a vertical outcrop surface at Hornsilver Campground. (D) Holy Cross City subvertical ultsouthwest. (E) Image viewed towards the southwest along strike on horizontal outcrop sursubvertical S2 fabric; a pseudotachylyte injection-vein network is localized southeast of the

3. Geology of the Homestake and Slide Lake shear zones

The HSZ, a regional northeast-striking shear zone system,consists of Mesoproterozoic mylonites, ultramylonites, and pseu-dotachylyte zones that cut w1.8e1.6 Ga high-temperature schist,gneiss, and migmatite. We will use the deformation history of theHSZ (Tweto, 1974; Shaw et al., 2001; Allen, 2005; McCoy et al.,2005; Shaw and Allen, 2007) to calibrate our investigation intothe evolution of the SLSZ. The chronology of deformation presentedhere uses the terminology established by Shaw et al. (2001)(D1eD4), which accounts for progressive development of

he southwest along Homestake Creek; shaded bands in the foreground represent thesents the low-angle Slide Lake shear zone exposed near Homestake Peak (4023 m). (B)on a horizontal outcrop surface. (C) S1 foliation folded and transposed into steep F2/S2ramylonite on a horizontal outcrop surface (between dashed lines), viewed towards theface; northeast-striking, subvertical pseudotachylyte fault vein (top arrow) overprintsfault vein (fsp ¼ feldspar leucosome).

N(S3/S4) = 91N(S4) = 53

N(S3(SL)) = 173

N(S3(SL)) = 16

N(L3/L4) = 101C.I. = 2.0 sigma

Holy Cross CityHornsilver Camp.Homestake Valley

Slide Lake cirqueHomestake ridgeline

Bennett ridgeline

N(L3(SL)) = 128

N(L3(SL)) = 10

C.I. = 2.0 sigma

C.I. = 2.0 sigma

HSZA

C SLSZ

SLSZB

(Shaw andAllen, 2007)

L3

L4

S3(SL)

S3/S4

S3(SL)

S4

Fig. 4. Lower hemisphere equal area stereograms showing foliation and lineationrelationships in the field area. Black planes represent average foliation plane andshaded contours represent poles to foliation for all measured planes. Stretching line-ations from this study represented by dashed contour lines. Stretching lineations fromShaw and Allen (2007) denoted with “x” and “o”. (A) Homestake shear zone S3 (056,79�SE), L3 (64� / 213) and L4 (78� / 120). Mean orientation for S4 pseudotachylyterepresented by the dashed plane (057, 75�SE). (B) SLSZ Bennett ridgeline moderatelydipping splay, S3(SL) (048, 60�SE) shaded contours and L3(SL) (59� / 121) dashedcontours. (C) SLSZ low-angle splays, S3(SL) (007, 24�SE) are shaded contours and L3(SL)(9� / 165) are dashed contours.


distinctive foliations and lineations during each phase of defor-mation. Shear sense indicators presented in this study are observedin the XZ plane (parallel to lineation and perpendicular to foliation).

3.1. Deformation history and mesoscale observations of theHomestake shear zone

The HSZ (Figs. 2 and 3) is exposed on glacially polished outcropsalong Homestake Creek as well as above tree line at the old mininglocale of Holy Cross City. The country rock in the vicinity of the HSZconsists of locally migmatized biotite gneiss and schist(bt þ grt þ sil þ qtz þ fsp þ ms), calc-silicate gneiss(hbl þ cal þ qtz þ fsp þ ms), and dispersed maficeultramafic podsand alkaline mafic dikes (Tweto, 1974), all of which are cut bypegmatite dikes and Precambrian granites (Fig. 2A and B). Theoverall northeast-striking shear zone is exposed along HomestakeCreek as a series of anastomosing splays (0.1e3-m-thick). Startingat the southwest end of the valley and trending toward thenortheast, the shear zone thins and splits into smaller splaystoward the northeastern part of Homestake Creek (Figs. 2A and 3A).

The earliest stage of Paleoproterozoic deformation (D1) is char-acterized by high-temperature, melt-present flow in the countryrocks of the shear zones. The main foliation (S1) is subhorizontaland resulted from viscous flow near the granite solidus at1708 � 6 Ma (Shaw et al., 2001). The presence of prismatic silli-manite, biotite, and garnet within HSZ samples implies conditionswithin the sillimanite stability field. Alternating bands of leuco-somes and biotite-rich melanosomes in migmatitic gneiss(bt þ grt þsil þ qtz þ fsp þms ) define S1 in HSZ (Fig. 3B). This earlyfoliation (S1) is present in the HSZ as well as throughout much ofthe CMB (McCoy et al., 2005).

The second stage of deformation (D2) also occurred during thePaleoproterozoic at amphibolite facies conditions and involvednorthwest-directed shortening, forming northeastesouthwest-trending upright tight to isoclinal folds. Within the HSZ, this mid-crustal shortening event steepened and transposed S1, creating S2,a steep northeast-striking axial-surface foliation at 1658 � 5 Ma(Fig. 3C; Shaw et al., 2001). The steeply dipping foliation (S2: 059,79�SE) contains zones of high-strain rocks that record general shearin the region at w1.65 Ga (Shaw et al., 2001).

Following w200-m.y. of stability, Mesoproterozoic deformationrepresents a major shift in deformation style across the region fromthe distributed high-temperature, melt-present deformationduring the Paleoproterozoic to moderate temperature deformationand localized solid-state shear zone development. The initial stageof Mesoproterozoic deformation (D3) is recorded by mylonitedevelopment within the HSZ along anastomosing systems (S3) thatlocally reactivated and overprinted the steep S2 foliation. Near theHornsilver Campground and Holy Cross City areas, narrow (1e3-m-thick) bands of quartzofeldspathic rocks that contain interspersedribbon quartz and phyllosilicate-rich layers with rigid feldsparporphyroclasts make up a pervasive mylonitic foliation (S3: 056,79�SE) that contains a stretching lineation (L3: 73� / 213; Figs. 3Dand 4A). Narrow (1e10 cm-thick) mylonitic quartz veins occuralong some of the mylonite splays. Feldspar porphyroclasts andshear bands record top-down-to-the-southeast sense of shearduring mylonite development (D3) that occurred between 1.45 and1.38 Ga (Fig. 2B; Shaw et al., 2001).

The final stage of reactivation (D4) involved the development ofultramylonite fabrics (S4: 056, 79�SE) confined to straight, narrowshear zones that record top-up-to-the-northwest shear sense aswell as pseudotachylyte that cuts S2 and S3 (Figs. 3E and 4A). D4ultramylonite contains a steeply plunging stretching lineation (L4:78� / 120; Fig. 4A) and records dextral and top-up-to-the-northwest (reverse) sense of shear (Shaw and Allen, 2007). In situ

Fig. 5. Field observations of the Slide Lake shear zone (SLSZ). (A) View of Slide Lake cirque as from the summit of Homestake Peak. Three major splays of the SLSZ shaded red withlocation of field photos denoted on image. Bennett Gulch is located north of Bennett ridgeline. (B) Bennett ridgeline with mylonite splay outlined in white. (C) Quartzoefeldspathicmylonite from the Bennett ridgeline with shear sense indicators in S3(SL) foliation with top-down-to-the-southeast shear sense. (D) Fabric truncation between the steep fabric (S2)within the hanging wall of the upper splay and the shallowly plunging foliation (S3(SL)) of the SLSZ mylonite. (E) Porphyroclast within calc-ultramylonite (cal þ qtz þ bt þ chl) of theHomestake ridgeline splay, top-down-to-the-southeast sense of shear. One Indian Rupee coin (25 mm diameter) for scale.



monazite geochronology yields ages for the formation of ultra-mylonites (S4) at 1375 � 14 Ma in the HSZ (Shaw et al., 2001).

Pseudotachylyte occurs as black, linear fault veins and intricatenetworks of associated injection veins in migmatite and biotitegneiss host rock (Fig. 3E). In the HSZ, pseudotachylyte has beendivided into more than twelve northeast-striking, steeply dippingzones, each of which is generally 5e15-m-wide and 1.5e7.3-km-long (Allen, 2005). Each zone consists of numerous pseudotachy-lyte fault veins (057, 75� SE) that are concordant with the steep,

Fig. 6. Photomicrographs of HSZ; crossed polars unless otherwise noted. The orientation mar(qtz þ fsp þ bt þ ms) containing SeC fabric with top-up-to-the-southeast shear. (B) Mylonrecording top-up-to-the-northwest sense of shear; plane light. (C) Quartz vein with quaboundaries displaying high-temperature quartz texture. (E) Quartz mylonite with well-deveoblique grain-shape fabric. (F) Quartz mylonite (qtz þ fsp þ bt þ ms) with elongated ribbo

northeast-striking foliation (S2: 059, 79�SE; Fig. 4A). Field relations(Allen, 2005) and rare slickenlines on subvertical fault veins raking15e20� to the southwest and northeast indicate dextral oblique,strike-slip kinematics. Both plastically deformed and undeformedpseudotachylyte veins are locally present within the D4 ultra-mylonite (Shaw and Allen, 2007). The existence of coeval ductileultramylonite with pseudotachylyte points to unique conditions,suggesting that local changes in temperature, grain size, fluidpressure, and strain rate controlled the prevalence of mid-crustal

kers define the trend and the plunge of the lineation for each section. (A) Ultramyloniteite (qtz þ fsp þ bt þ ms) with polycrystalline feldspar porphyroclasts in quartz matrixrtz subgrains and mica fish that records top-up-to-the-northwest shear. (D) Grainloped quartz subgrains and top-down-to-the-southeast sense of shear recorded by ann quartz subgrains displaying SeC fabric, top-up-to-the-northwest sense of shear.

Table 1Summary of HSZ shear sense and temperature data.

Sample Rock type Shear sense Deformationtemperature (�C)

Temperatureindicatorb

Homestake shear zone e Hornsilver CampgroundHS08-01 qtz my t-NW 300e400 q.d., m.a.Homestake shear zone e Holy Cross City transectHS08-07 qtzefsp my t-NW 450e500 q.d., m.a.HS08-08 qtzefsp my t-NW 400e500 q.d., m.a.HS08-09 qtz my t-NW 350e450 q.d., m.a.HS08-10 qtz my t-NW 450e500 q.d., m.a.HS08-11 qtz my t-NW 350e450 q.d., m.a.HS08-12a qv t-NW 450e500 q.d., m.a.HS08-13a qv t-NW 450e500 q.d., m.a.HS08-14 qtz my t-NW 450e500 q.d., m.a.HS09-03 qtz my t-NW 400e500 q.d, m.a.HS09-04 qtz my t-NW 300e450 q.f.d., m.a.

Abbreviations: qtz my, quartz mylonite; c.s. my, calc-silicate mylonite; fsp my,feldspar mylonite; qv, quartz vein; t-SE, top-down-to-the-southeast; t-NW, top-up-to-the-northwest.

a Samples analyzed with EBSD.b Temperature indicators; samples used q.d, quartz deformation textures; q.f.d.quartz and feldspar deformation textures; m.a., mineral assemblage.


ductile vs. brittle deformation within an exhumed seismogeniczone (Allen, 2005; Shaw et al., 2005; Shaw and Allen, 2007).

3.2. Mesoscale structural observations of the Slide Lake shear zone

The shallow to moderately dipping (w20e60�) SLSZ is exposedw1200-m-above Homestake Valley (Fig. 3A) at and above tree line,and spans two prominent ridgelines and glacially carvedcirques (Fig. 2A and B). The SLSZ comprises mylonite andultramylonite that cross-cuts amphibolite facies biotite gneiss(bt þ grt þ sil þ qtz þ fsp þms ), quartzofeldspathic gneiss(qtz þ fsp þ ms þ bt), calc-silicate gneiss (hbl þ cal þ qtz þ chl), andmigmatite (bt þ grt þ sil þ qtz þ fsp þ ms), all of which are cut bypegmatite and granite. We subdivide the northenortheast-strikingSLSZ into three mylonite and ultramylonite splays: (1) Bennett ridge-line, (2)Homestake ridgeline, and (3) the Slide Lake andBennettGulchpavement (Figs. 2 and 5A). We use the Proterozoic deformationalevents of the HSZ to calibrate our investigation of the SLSZ, and werecognize that these may represent a wide range of timing sequencesthat include distinct and/or protracted deformational events.

High-temperature rocks from the first (D1) and second (D2)stages of Paleoproterozoic deformation are found in thehanging wall and footwall of the SLSZ. Migmatitic gneiss(bt þ grt þ sil þ qtz þ fsp þ ms) is similar to that observed in theHSZ area, with leucosomes and biotite melanosomes characterizingthe high-temperature melt-present subhorizontal flow (S1). Themid-crustal shortening event (D2) steepened and transposed S1,creating an axial-surface foliation (S2: 059, 79�SE) similar to thatseen in the HSZ.

The SLSZ is exposed along Bennett ridgeline (Fig. 5B and C) andconsists of at least two, w1-m-thick moderately dipping (048,60�SE) mylonite (qtz þ fsp þ bt) bands bound by high-strain zonesthat consist of grain-size reduced biotite and quartz. The mainmylonitic fabric in the SLSZ is designated S3(SL) because it cross-cutsS2 in the country rock, but cannot be definitively correlated to S3 inthe HSZ. Exposure of this splay is confined to a narrow band ofhigh-strain rock and mylonite interspersed with foliated quartz-ofeldspathic gneiss on the ridge that divides Bennett Gulch fromSlide Lake cirque (Figs. 2 and 5A). Moderately dipping mylonite(qtz þ fsp þ bt) contains rigid pink feldspars set in a matrix ofphyllosilicates (bt þ ms) and quartz ribbons. Mylonitic foliation(S3(SL)) in the hanging wall and footwall of this splay is parallel withthe moderately dipping earlier high-temperature foliation (Fig. 5B).A well-developed, southeast-plunging stretching lineation (L3(SL):59� / 121) defined by quartz and feldspar aggregates wasobserved on the moderately dipping foliation surface (S3(SL): 048,60�SE; Fig. 4B). Mesoscale shear sense indicators (e.g. asymmetrictails on porphyroclasts, shear bands) reveal dominant top-down-to-the-southeast sense of shear.

Homestake ridgeline (Fig. 5A) is the most laterally extensiveexposure of the SLSZ that wasmapped along the Continental Dividefrom the saddle southwest of Homestake Peak to the unnamedpeak that divides Bennett Gulch and Slide Lake cirque (Fig. 2A). Onthe saddlew10-m-below and southwest of the Homestake summit,the low-angle calc-silicate (calþ qtzþ btþmsþ chl) ultramyloniteis traceable along-dip for 100þ m from the saddle down thesoutheastern side of the Continental Divide. On the northeasternside of Homestake summit, low-angle (S3(SL): 003, 17�SE) greens-chist facies ultramylonite (qtz þ bt þ fsp þ ms � cal þ chl) isexposed. Ultramylonite is composed of small feldspar porphyr-oclasts and ribbon quartz with alternating layers of phyllosilicates(bt þ ms þ chl). Quartz and feldspar grains form a shallowlyplunging lineation (L3(SL): 05� / 166; Fig. 4C). Mylonite containsmesoscopic shear sense indicators (e.g. porphyroclasts, SeC fabric,shear bands) that display both top-up-to-the-northwest and top-

down-to-the-southeast (Fig. 5E) sense of shear, possibly suggest-ing overprinting of an earlier fabric. Foliation relationships (Fig. 2B)at the northeast end of this splay (Fig. 5D) reveal a sharp contactbetween the low-angle SLSZ splay (S3(SL): 003, 17�SE) and theoverlying steep, high-temperature fabric (S2: 054, 78�SE). The shearzone truncates the steeply dipping fabric (S2; Fig. 5D).

The structurally lowest splay of the SLSZ was mapped as low-angle (bt þ qtz þ fsp þ ms) mylonite and ultramylonite thatoccurs on the glacially carved pavement in both Slide Lake cirqueand Bennett Gulch (Figs. 2 and 5). This splay consists of several thin(1e3-m), shallowly dipping (S3(SL): 015, 29�SE) ultramylonitestrands (bt þ fsp þ qtz þ ms þ sil) that contain a shallowlyplunging, southeast-trending stretching lineation (L3(SL):16� / 164). Ultramylonite contains rigid pink feldspar, ribbonquartz, and phyllosilicates (bt þ ms) in a matrix of bt þ ms þ chl.Thin strands of ultramylonite occur as discontinuous, anastomosingsplays bounded by sections of migmatite and biotite gneiss andsome high-strain domains. High-strain domains were extensive inSlide Lake cirque, consisting of grain-size reduced biotite andquartz that anastomose (i.e. follow and crosscut the foliation)throughout the outcrop. Similar to the Homestake ridgelineoutcrop, mesoscopic shear sense indicators (e.g. rigid feldsparporphyroclasts with tails, shear bands) record dominant top-down-to-the-southeast and minor top-up-to-the-northwest sense ofshear.

4. Shear sense indicators

4.1. Homestake shear zone

Mylonite from the Hornsilver Campground splay (Fig. 5A) of theHSZ is characterized by aligned biotite and muscovite interlayeredwith quartz-rich domains that define a penetrative foliation (S3:056, 79�SE). The well-developed stretching lineation is defined byaggregates of quartz, feldspar, and muscovite (L3: 64� / 213).Quartz subgrains occur as elongate ribbons (S fabric) that are drawninto shear bands (C fabric) of aligned biotite and muscovite(Fig. 6A). This fabric (S3) contains s-type feldspar porphyroclastswith tails of quartz subgrains and biotite mica fish that record top-up-to-the-northwest with a minor component of top-down-to-the-southeast sense of shear (Fig. 6B; Table 1). One interpretationfor the varying shear sense indicators within the same sample maysuggest different deformational episodes, with the more recent


events partially overprinting a previous event (e.g. top-up-to-the-northwest overprints top-down-to-the-southeast), where analternative interpretation would suggest a component of pureshear with themaximumprincipal shortening at a high angle to theshear zone (e.g. contemporaneous top-up-to-the-northwest andtop-down-to-the-southeast).

Mylonite and ultramylonite in the Holy Cross City splay of theHSZ contain aligned biotite and muscovite interlayered withquartz-rich domains. Rigid porphyroclasts are interspersed within

Fig. 7. Photomicrographs of SLSZ; crossed polars unless otherwise noted. The orientation m(qtz þ ms þ bt þ fsp) with SeC fabric that records top-down-to-the-southeast sense of sheaquartz and biotite tails with top-down-to-the-southeast sense of shear; plane light. (C) Mphyllosilicate laths. (D) Mylonite with quartz subgrains showing oblique grain-shape fabric,quartz grains (GBM) and feldspar porphyroclasts rims that display core and mantle structReduction (GBAR).

mylonitic quartz veins composed of quartz subgrains with isolatedmuscovite and biotite grains. Two generations of well-developedstretching lineation are defined by quartz, feldspar, and musco-vite (L3: 64� / 213; L4: 78� / 120). Most porphyroclasts in thesemylonites appear as mono- and polycrystalline rounded feldsparporphyroclasts with and without tails (Fig. 6B). Thin sections ofmylonite and ultramylonite from the Holy Cross City splay containthe greatest quantity and variety of shear sense indicatorsincluding; d- and s-type porphyroclasts (Fig. 6B), rhomboidal and

arkers define the trend and the plunge of the lineation for each section. (A) Myloniter. (B) Mylonite (qtz þ fsp þ bt) with rigid feldspar porphyroclasts showing asymmetricylonite (qtz þ bt þ fsp þ ms) with large lobate quartz domains (GBM) pinned bytop-up-to-the-northwest sense of shear. (E) Mylonite (qtz þ fsp þ bt þ ms) with largeures (BLG). (F) Semi-annealed quartz grains in quartz mylonite Grain Boundary Area


lenticular (Fig. 6C) mica fish, oblique grain-shape fabric in quartz(Fig. 6C), and C’-type shear bands. Oblique grain-shape fabriccreated by quartz subgrain alignment exists at steep angles(32e53�) to foliation and mica fish orientation. Shear sense indi-cators record top-down-to-the-southeast and top-up-to-the-northwest shear sense, either evidence for general shear duringmultiple stages of deformation or a component of pure shearduring one major deformational event.

4.2. Slide Lake shear zone

Mica fish, asymmetric tails on rigid feldspar porphyroclasts, C-and C0- type shear bands, and oblique grain-shape fabric in quartzrecord both top-down-to-the-southeast and top-up-to-the-northwest shear sense for three major splays of the SLSZ (Figs. 2and 7A and B; Table 2). The Bennett ridgeline splay of moderatelydipping quartzofeldspathic mylonite contains well-defined asym-metric feldspar porphyroclasts, mica fish, C-type shear bands(Fig. 7C), and oblique grain-shape fabric in quartz (Fig. 7D). Thepervasive foliation is defined by bands of quartz and feldspar thatalternate with zones of interlayered white mica and biotite (S3(SL):048, 60�SE) and contain a well-developed lineation (L3(SL):59� / 121). Lenticular mica fish are set in a matrix of quartz with

Table 2Summary of SLSZ shear sense and temperature data.

Sample Rock type Shear sense Deformationtemperature (�C)

Temperatureindicatorb

Slide Lake shear zone e Bennett ridgeline splayHS09-17 c.s. my e 300e350 q.d., m.a.HS09-42a qtzefsp my t-SE 450e600 q.d., m.a.HS09-43 qtzefsp my t-SE 450e600 q.d., m.a.HS09-44 qtzefsp my t-SE 450e600 q.d., m.a.HS09-45 qtzefsp my t-SE 450e600 q.d., m.a.HS09-46 qtzefsp my t-SE 450-600 q.d., m.a.HS09-47 qtzefsp my t-SE 400e600 q.d., m.a.Slide Lake shear zone e Homestake ridgeline splaySL08-08 c.s. my t-S 450e550 q.d., m.a.SL08-07 c.s. my t-NW 400e500 m.a.SL08-06 c.s. my t-SE 400e450 q.d., m.a.SL08-05 mbl e 400e450 q.d., m.a.SL08-04a qv t-SE 350e450 q.d., m.a.SL08-03 c.s. my t-SE 300e400 q.d., m.a.SL08-02 c.s. my t-SE 300e400 q.d., m.a.SL08-01 c.s. my t-NW 300e400 q.d., m.a.HS09-54 qtz my t-SE 350e400 q.d., m.a.HS09-31 calc my t-SE 500e650 m.a.HS09-32 qtz my t-NW 500e650 m.a.HS09-33 qtz my t-SE 650þ q.d., m.a.HS09-34 qtz my t-SE 650þ m.a.HS09-35 qtz my t-SE 450e650 q.d., m.a.HS09-36 qtz my t-NW 650þ q.d., m.a.HS09-37 gns t-NW 300e400 q.d., m.a.Slide Lake shear zone e Slide Lake cirque splayHS09-21 qtz my t-SE 350e450 q.d., m.a.HS09-22 qtz my t-SE 300e400 q.d., m.a.HS09-23 qtz my t-SE 450e550 q.d., m.a.HS09-24 qtz my t-SE 450e550 q.d., m.a.HS09-25 c.s.my t-SE 450e550 q.d., m.a.HS09-27 qtz my t-SE 350e400 q.d., m.a.HS09-29 qtz my t-NW 450e550 q.d., m.a.HS09-30 qtz my t-SE 500e600 q.d., m.a.HS90-39 qtz my t-SE 500e650 q.d., m.a.HS09-40 qtz my t-S 450e500 q.d., m.a.HS09-41 qtz my t-SE 450e500 q.d., m.a.

Abbreviations: qtz my, quartz mylonite; c.s. my, calc-silicate mylonite; fsp my,feldspar mylonite; gns, gneiss; mbl, marble; qv, quartz vein; t-SE, top-down-to-the-SE; t-NW, top-up to-the-NW; t-S, top-down-to-the-S.

a Samples analyzed with EBSD.b Temperature indicators; samples used q.d., quartz deformation textures; m.a.,mineral assemblage.

polygonal grain boundaries that record grain-boundary areareduction at high temperatures (quartz domains; Fig. 7E). Mostfeldspar porphyroclasts record top-down-to-the-southeast(normal) shear sense (Fig. 7B and C), while others record top-up-to-the-northwest sense of shear.

The upper w100-m-thick Homestake ridgeline splay (Fig. 5A)records dominantly top-down-to-the-southeast sense of shearwith several samples that record a contribution of top-up-to-the-northwest motion along the shallowly dipping shear zone(Table 2). The pervasive foliation is defined by white mica fish andbiotite laths interlayered with quartz and feldspar grains (S3(SL):003, 17�SE). Aligned quartz and muscovite define a shallowlyplunging and weakly developed stretching lineation (L3(SL):05� / 166). Between mica-rich domains, quartz and feldspargrains exist in a matrix of calcite, quartz, and biotite. S-C (Fig. 7A),C0-type shear bands, and polycrystalline porphyroclasts record top-down-to-the-southeast and top-up-to-the-northwest in the uppersplay. An oblique grain-shape fabric exists in quartz-rich regions ata steep angle to foliation (w57�) and records top-up-to-the-northwest shear sense (Fig. 7D).

Mylonite samples (n ¼ 11) from the Slide Lake and BennettGulch pavements record dominant top-down-to-the-southeastmotion. White mica and biotite domains interlayered with quartzand feldspar grains define the pervasive foliation in the mylonite(S3(SL): 015, 29�SE). A combination of quartz, feldspar, and musco-vite make up aweak- to well-developed stretching lineation (L3(SL):16� / 164). Mylonite samples contain mica fish, SeC0 fabric, silli-manite boudins, and d-type porphyroclasts. C0-type shear bandsrecord top-down-to-the-southeast sense of shear. Other samplescontain lenticular and rhomboidal mica fish that are set in a matrixof dynamically recrystallized quartz and record top-down-to-the-southeast sense of shear. Mica fish along with quartz subgrainscreate C0-type shear bands that also record top-down-to-the-southeast shear.

5. Deformation temperatures

Deformation temperatures in the HSZ and SLSZ were assessedusing a combination of quartz deformation textures (Hirth andTullis, 1992; Stipp et al., 2002a, b), feldspar deformation textures(Pryer, 1993), mineral assemblages, and quartz LPOs (Mainpriceet al., 1986; Tullis and Yund, 1992). Quartz microstructures reflecta transition in deformation mechanisms with higher temperaturesduring dynamic recrystallization. Assuming constant strain rateand fluid composition these microstructures can be used to esti-mate temperature conditions during deformation. The phases ofgrain-boundary mobility are defined by bulging (BLG,w280e400 �C), subgrain rotation (SGR, w400e500 �C), and grain-boundary migration (GBM, >500 �C; Stipp et al., 2002a, b). Thesestages represent the dynamic recrystallization of quartz fromdislocation glide and creep (BLG) to climb-accommodated dislo-cation creep (SGR) and into high-temperature grain-boundarymigration (GBM), where recrystallization-accommodated creepreduces internal strain energy and decreases dislocation density.Grain-boundary straightening results in polygonal grain bound-aries that allow for the lattice to progress towards a dislocation-freelattice and Grain Boundary Area Reduction (GBAR; Bons and Urai,1992; Kruhl, 2001).

Quartz LPOs were also used as a quasi-independent method toestimate temperature. At lower temperature conditions(280e400 �C), crystal lattice slip occurs as basal <a>slip, pro-gressing into moderate temperatures (400e500�) where disloca-tion creep involves prism <a> slip, and lastly into hightemperatures (>500 �C), where prism <c> slip dominates defor-mation (Fig. 8A; Wilson, 1975; Lister and Dornsiepen, 1982;

Fig. 8. (A) Quartz LPO patterns for the [c] axes and <a> axes with increasing temperature for non-coaxial, plane strain deformation (after Stipp et al., 2002b; Passchier and Trouw,2005; Langille et al., 2010b). (B) EBSD generated LPOs for HSZ (qtzefsp) mylonite (HS08-12, HS08-13) display patterns characteristic of plane strain and prism <a> slip. (B) SLSZmylonite (qtzefsp) SL08-04 displays prism <a> slip and plane strain patterns, and HS09-42 displays a complex pattern with possible prism <a> and rhomb <a> slip (see text fordetails).


Mainprice et al., 1986; Law, 1990; Tullis and Yund, 1992; Kruhl,1998; Langille et al., 2010b). Electron backscatter diffraction(EBSD) was used to obtain LPO diagrams. Diffraction patterns werecollected using a Zeiss Supra 55 VP scanning electron microscopecoupled with a HKL Nordlys S EBSD camera at Montana StateUniversity. HKL Channel 5 software was used to index diffractionpatterns and to create plots and maps. Pole figures (Fig. 8) weremade using raw data corrected for pseudosymmetry misindexingwith no extrapolation to unindexed pixels.

Feldspar deformation mechanisms were also used to broadlybracket deformation temperatures in the shear zones. Feldspargrain boundaries develop core and mantle structures characteristicof bulging and dislocation climb (BLG, 450e600 �C; Borges andWhite, 1980; Gapais, 1989; Gates and Glover, 1989; Tullis andYund, 1991; Shigematsu, 1999). Above 600 �C feldspar grainsdeform via SGR and BLG recrystallization that may involve thegrowth of myrmekite (Vidal et al., 1980; Olsen and Kohlstedt, 1985;

Tullis and Yund, 1987; Simpson and Wintsch, 1989; Pryer, 1993;Kruse and Stünitz, 1999; Altenberger and Wilhelm, 2000).

5.1. Homestake shear zone

Quartz deformation textures within HSZ mylonite and ultra-mylonite are dominated by subgrains that occur as small, individualgrains (Fig. 6E) and elongated ribbon subgrains (Fig. 6F), bothevidence for subgrain rotation (SGR, 400e500 �C). The mineralassemblage contains minor sillimanite, cordierite, and garnet thatare legacy to the earlier Paleoproterozoic (D1) high-temperature,GBAR-dominated flow (Fig. 6D) found throughout the HSZ.Quartz subgrain development varies from the Hornsilver Camp-ground splay into the Holy Cross City splay of the HSZ. Quartz grainboundaries in the Hornsilver Campground mylonite contain elon-gated quartz ribbons with bulging grain boundaries (BLG,280e400 �C) and undulose extinction in the interior of the grain.


Holy Cross City mylonite contains ribbon quartz grains and smaller,well-defined subgrains (SGR, 400e500 �C) that align to form anoblique grain-shape fabric that was used as a shear sense indicator(Fig. 6C). Feldspar in the Hornsilver Campground mylonite lacksevidence for dynamic recrystallization, however in the Holy CrossCity mylonite, some feldspar porphyroclasts display core andmantle structures that are evidence for bulging (BLG, 450e600 �C)dynamic recrystallization (Pryer, 1993) that may be part of earlier,high-temperature deformation.

In the Holy Cross City segment, quartz subgrains in mylonite canbe used as evidence for shear band development associatedwith D3in the HSZ. Quartz [c] axes plot in the center of the LPOs, with<a> axes plotting along the primitive circle for two samples(Fig. 8B). One HCC mylonitic quartz vein, HS08-13 (Figs. 6C and 8B),contains a well-developed quartz subgrain texture with obliquegrain-shape fabric and mica fish that record top-up-to-the-northwest shear sense. LPO plots derived from the XZ planesuggest that the [c] axes of quartz subgrains were aligned duringplane strain deformation. LPO patterns can also be used to estimatedeformation temperatures (Stipp et al., 2002b; Langille et al., 2010b)during quartz recrystallization. Both LPO plots suggest prism<a> slip (>500 �C) as the dominant mechanism for deformation,suggesting the possibility of even higher temperatures than the(Fig. 8A) quartz textures observed (SGR, 400e500 �C; Fig 6).

5.2. Slide Lake shear zone

Bennett ridgeline quartzofeldspathic mylonite contains inter-lobate quartz-rich domains that indicate high-temperature GBM(>500 �C) textures and are pinned on the foliation plane by alignedbiotite and muscovite domains (Fig. 7C). Some rigid feldspar por-phyroclasts are set within the quartz matrix that is composed ofquartz grains with polygonal grain boundaries that record a semi-annealed fabric (Fig. 7C and F). A few feldspar grain margins werefound to display core and mantle structures (Fig. 7F) indicative ofmoderate temperature feldspar grain-boundary mobility (BLG,450e600 �C; Tullis and Yund,1991; Pryer, 1993). Feldspar subgrains(BLG) only occur as haloes around larger, rigid porphyroclasts andare not widespread in the samples, implying either a transitionfrommedium- to higher-grade feldspar textures or legacy to earlierD1 and D2 high-temperature deformation. Where the majority offeldspar grains display undulatory extinction, a minority displaythe core and mantle structures indicative of medium temperature(BLG, 450e600 �C) feldspar textures.

Quartz [c] axis LPO plot HS09-42 (Fig. 8C), representative of theBennett ridgeline mylonite, displays LPO patterns that occur as twodistinct groupings of [c] axes data near the middle of the plot, with<a> axes scattered around the outer rim. This pattern may suggestupper prism <a> slip (>500 �C) in an undefined strain regime,possibly due to multiple phases of activation within the shear zonesplay, with one of the [c] axes partially overprinting [c] axes from anearlier event. Prism <a> slip (>500 �C) supports estimates fordeformation temperatures based on quartz and feldspar micro-structural criteria.

The upper Homestake ridgeline splay is composed of calc-mylonite (qtz þ fsp þ cal þ bt þ ms þ chl). Ultramylonitesamples from this part of the shear zone contain quartz grainssegregated into narrow bands that alternate between feldspar- andcalcite-rich domains. Quartz grain boundaries contain small strain-free grains with undulose extinction in the interior of grainboundaries that are interpreted as core and mantle structures (BLG,280e400 �C). Earlier Paleoproterozoic (D1 and D2) high-temperature deformation is recorded in polygonal quartz grainsthat display GBAR. Feldspars lack evidence for internal deformation(<450 �C; Pryer, 1993). Sillimanite retrogressed to muscovite was

also found in the upper splay of SLSZ; similar observations weremade in the HSZ.

Quartz [c] axes LPOs plot in the center with <a> axes plottingaround the primitive circle for SL08-04, representative of theHomestake ridgeline splay (Fig. 8C). This [c] axis pattern is indicatesplane strain deformation conditions. This LPO pattern also suggestsrhomb to upper prism <a> slip (>500 �C) as the dominantmechanism for deformation, roughly corresponding with the upperend of temperature estimates for quartz subgrain development(SGR, 400e500 �C) and feldspar grain-boundary immobility(<450 �C; Pryer, 1993; Stipp et al., 2002b; Langille et al., 2010a, b).The discrepancy between this moderate temperature (SGR,400e500 �C) quartz fabric and the lower temperature core andmantle structures (BLG, 280e400 �C) within other mylonitesamples within the SLSZmay be due to a later stage overprint of thehigher temperature fabric.

Quartz grains within mylonites exposed on the Slide Lake andBennett Gulch pavements display well-developed boundaries thatrecord high-temperature dynamic recrystallization (GBM,>500 �C)and GBAR (Fig. 7E and F). Phyllosilicates pin quartz grains in thesesamples, contributing to elongated quartz grain boundaries.Remnant fibrous sillimanite and quartz textures display largepolygonal quartz grains that suggest GBAR may be associated withthe earlier amphibolite grade (D1) deformation. Mylonite andultramylonite in the upper and middle splays of the SLSZ containretrograde chlorite and white mica that record a greenschist faciesconditions and overprint localized splays within high-temperaturebiotite and calc-silicate host rock.

Temperature estimates from deformation mechanisms outlinedabove agree with broad constraints for ca. 1.4 Ga temperatures inthe study area based on 40Ar/39Ar thermochronology (Shaw et al.,2005). Two muscovite and six biotite samples exhibit plateauages or disturbed plateau ages (with evidence of argon loss in lowertemperature steps) ranging between 1411 � 3Ma and 1385 � 2 Ma.This distribution of ages is consistent with slowemoderate coolingthrough closure temperatures for argon diffusion in micas(w300e350 �C; McDougall and Harrison, 1999). In contrast, horn-blende (closure temperature w550 �C; McDougall and Harrison,1999) ages from the study area yield highly disturbed saddle-shaped spectra with total gas ages ranging from 1622 � 2 Ma to1321 � 2 Ma. Although detailed interpretation of these complexspectra is probably not possible, the aggregate data is mostconsistent with either a long residence at elevated temperaturesfrom w1650 Ma to w1400 Ma or partial resetting by a tempera-tures exceeding w500e550 �C around 1420e1380 Ma coincidentwith the resetting of mica ages (Shaw et al., 2005). In light ofevidence for regional metamorphism (e.g. Shaw et al., 1999b, 2005;Daniel and Pyle, 2006) and deformationwithin the HSZ (Shaw et al.,2001) at ca. 1400 Ma the later explanation is preferred.

6. Discussion

6.1. Comparison of SLSZ and HSZ deformation history andkinematics

Meso- and microstructural observations of kinematic indicatorsand fabric relationships in HSZ and SLSZ mylonite and ultra-mylonite demonstrate that mid-crustal Paleo- and Mesoproter-ozoic deformation involved similar structural (e.g. shear sense,lineations, and strike) and deformational (e.g. pure shear, defor-mation temperatures, and shear sense) components (Tables 1 and2). Within the HSZ, 1.4 Ga deformation is subdivided into twoevents that are characterized by a system of steeply dipping (056,79�SE) mylonite, ultramylonite, and pseudotachylyte zones: (1) D3is associated with S3 mylonite development and a steeply plunging


lineation (L3: 64� / 213) that records oblique dextral, top-down-to-the-southeast sense of shear, and (2) D4 ultramylonite witha steeply plunging lineation (L4: 78� / 120) that records obliquedextral, top-up-to-the-northwest sense of shear (Fig. 4A), andpseudotachylyte development that records dextral, oblique strike-slip motion (Fig. 4A). Stretching lineations for L3 vary greatly alongS3, a factor that may be attributed to late stage strain in thedevelopment of S4.

Comparatively, the SLSZ is a shear system composed of at leasttwo low-angle (S3(SL): 007, 24�SE) splays with a shallow south-southeast-plunging lineation (L3(SL): 09� / 165; Fig. 4B), and onemoderately dipping mylonite splay (S3(SL): 048, 60�SE) witha steeper southeast-plunging lineation (L3(SL): 59� / 121; Fig. 4C).All three SLSZ splays record dextral top-down-to-the-southeastsense of shear with minor top-up-to-the-northwest shear sensecomponent. The upper contact of the Homestake ridgeline splay(Fig. 2A) occurs where the shallowly dipping, northenortheast-striking SLSZ foliation (S3(SL)) truncates the steep northeast-striking high-temperature foliation (S2) in the hanging wall(Fig. 5D). In the Bennett ridgeline splay, the shear zone fabric (S3(SL))is parallel with the steep, high-temperature fabric (S2) in thehanging wall and footwall (Fig. 5B). Consequently, the shear zonesplays are interpreted (Figs. 2A and 9) to represent two differentcomponents of the HSZ and SLSZ system. The oblique steeplyplunging HSZ lineations record right-lateral strike-slip and oblique,shallowly plunging SLSZ records a moreminor component of right-lateral motion, both are associated with the top-down-to-the-southeast and the top-up-to-the-northwest events (Fig. 9).

Mesoscale observations are supported by estimates of defor-mation temperatures using quartz and feldspar microstructures,quartz [c] axis LPOs, and shear sense. Deformation temperaturesderived from quartz and feldspar grain boundaries, metamorphicmineral assemblages, and quartz LPO-derived slip systems rangefrom 280e500 �C in the HSZ to 280e600 �C in the SLSZ (Tables 1and 2). Quartz subgrains and LPO data from the HSZ samplessupport a slightly higher temperature range (400e>500 �C) thanoriginally proposed by Shaw et al. (2001). Assuming an averagegeothermal gradient of w25 �C/km, constant strain rate and fluidswould imply that deformation occurred at similar mid-crustalpositions (w12e24 km) within both the HSZ and SLSZ.

Estimates of deformation temperatures and microstructures insplays of the SLSZ and the HSZ indicate thatw1.4 Ga deformation atthese mid-crustal positions included general shear deformation.Quartz LPO plots (Fig. 8B; Lister et al., 1978; Law,1990) indicate that

Fig. 9. Block diagram of the Slide Lake and Homestake shear zones viewed to the south. Linrepresent top-down-to-the-southeast; gray arrows represent top-up-to-the-northwest sendomains) highlighted in dark gray.

the HSZ Holy Cross City ultramylonites and one sample from theHomestake ridgeline were dominated by plane strain deformation.

6.2. Relative age of the SLSZ

Although it is impossible to directly establish a relative chro-nology of HSZ/SLSZ deformation with currently available data, thephysical proximity, kinematic compatibility, and similarity indeformation mechanisms indicate that the two systems formed atsimilar crustal levels. 40Ar/39Ar data for the area (Shaw et al., 2005)suggests that regional temperatures were 400e550 �C at w1.4 Ga.Monazite ages from HSZ mylonite (Shaw et al., 2001) and field-derived fabric relationships provide a proxy for the age of theonset of mylonite development within the SLSZ to be w1.4 Ga.Monazite ages from both top-down-to-the-southeast mylonite andtop-up-to-the-northwest ultramylonite within the HSZ are indis-tinguishable, although morphology and microstructures suggestthat they were formed either during two separate events (D3/D4) orduring different phases of a single tectonic event involvinga reversal of dip-slip shear sense e with the same strike-slip shear(Shaw et al., 2001). Because mylonite (top-down-to-the-southeast)and ultramylonite (top-up-to-the-northwest) development withinSLSZ cannot be uniquely related to the D3/D4 (1.42e1.38 Ga) chro-nology and kinematics of the HSZ, we group these into thew1.4 Gaevent based on the similarity of inferred temperatures andkinematics.

6.3. Mid-crustal heterogeneity

1.4 Ga shear zone development and magmatism in intra-continental Laurentia is inferred to represent an inboard response tofar-field shortening between southern Laurentia and a continentallandmass farther south (Nyman et al., 1994; Duebendorfer andChristensen, 1995; Karlstrom and Humphreys, 1998; Jones et al.,2010a). Thermal structure beneath an orogenic plateau (e.g.modern Tibet, Andean Antiplano) at 1.4 Ga may explain magmaemplacement near the brittleeductile transition as well as reshuf-fling of thermally weakened blocks via oblique and dip-slip motionalong shear zones (Andronicos et al., 2003; Shaw et al., 2005). In thenorthern Sawatch Range, 1.4 Ga mid-crustal deformation can becharacterized as a complex shufflingof crustal blockswithin theHSZand SLSZ involving inversion of the component of shear (e.g. reverseto normal) within an overall dextral transcurrent flow. Emplace-ment of the St. Kevin granite (Doe and Pearson, 1969; Shaw and

eation-foliation relationships and shear sense are displayed in the figure. Black arrowsse of shear. Shear zones (mylonite, ultramylonite, pseudotachylyte, and high-strain


Allen, 2007) into this milieu may have influenced deformation bychanging the temperature and competency of the country rocks.Shear zone development is attributed to a varied ductileebrittletransition (12e24-km-depth) that may have acted as a barrier formagma ascent contributing to accumulation of magma in the mid-crust that is represented today by the large volume of w1.4 Gagranitic in the central RockyMountains (Shaw et al., 2001, 2005). Anoscillating balance between tectonically driven crustal thickeningand gravitational extension may be the ultimate cause of thecomplex interplay between transtensional (top-down-to-the-southeast) and transpressional (top-up-to-the-northwest) defor-mation recorded in the HSZeSLSZ shear zone systems.

Low-angle structures similar to the SLSZ have been documentedin modern collisional settings where strain is partitioned in anunstable middle crust (Selverstone, 1985; Burchfiel et al., 1992).Studies from the Tibetan Plateau show that some low-angle shearzones cut across anisotropic structures that developed duringshortening, suggesting that low-angle structures can developwithout preexisting features (Kapp et al., 2008) and may be similarto the Homestake ridgeline splay of the SLSZ.

6.4. 1.4 Ga transpression

Stretching lineations can be used to determine flow movement,but should be usedwith caution in transpressional (i.e. 3D and non-plane strain) systems (Tikoff and Greene, 1997; Teyssier and Tikoff,1999). Subvertical stretching lineations can form in high-straintranspressional shear zones that are dominated by pure shear andhave inmany studies been found tooccurwith a subvertical foliation(e.g. Robin and Cruden, 1994; Tikoff and Greene, 1997; Teyssier and

z

xy

SE

A

SLSZ HSZ

SE

dextral, top-down-to-the-southeast

dextral, top-up-to-the-northwest

SLSZHSZ

C

B

L3(SL)

L3(SL)

L4

Fig. 10. Schematic block diagram illustrating one possible kinematic model for HSZ and SLdetermine kinematics in this study. Shear sense indicators and vorticity vector denoted onKinematics of dextral, top-up-to-the-northwest deformation. Not to scale.

Tikoff,1999; Hudleston,1999).Within theHSZ and SLSZ, the obliquestretching lineations include both steeply and shallowly plunginglineations (Figs. 4, 9 and 10). Variation in the orientation ofstretching lineations across a broad shear system (Fig. 10) has beendocumented in other transpressional models (Tikoff and Greene,1997) where fabric symmetry or lack thereof has been used toexplain the differences in the plunge of lineations across a shearsystem (Lister and Williams, 1983; Robin and Cruden, 1994).

We are unable to determine definitively whether or not the HSZand SLSZ were active during the same deformational event becauseof the lack of relative timing constraints. However, the kinematicdata presented above show a strong similarity between the meso-and microstructural indicators of kinematics, deformation mecha-nisms, and temperatures in the SLSZ and HSZ (Figs. 9 and 10) thatstrongly suggest that the two systems developed at similar crustallevels and in a similar tectonic regime. Quartz [c] axis LPOs, andshear sense analyses (Fig. 10A) from the Bennett ridgeline splay ofthe SLSZ and Holy Cross City splay of the HSZ suggest one possiblemodel. This model combines general shear in plane strain regimesas associated with two types of shear zone movement: (1) top-down-to-the-southeast, dextral general shear (Fig. 10B) and (2)top-up-to-the-northwest, dextral general shear (Fig. 10C) at similarmid-crustal positions (12e24 km). An alternative interpretationcould suggest that these two types of movement, if contempora-neous, indicate pure shear deformation (e.g. Rongbuk formation;Law et al., 2004). Our model supports suggestions by other workersthat instability in the middle crust influenced the development ofdiscrete shear zones at around 1.4 Ga that may be associated withregional transpression (e.g. Nyman et al., 1994; Duebendorfer andChristensen, 1995; Shaw et al., 2001, 2005; McCoy et al., 2005).

z

xy

EXPLANATION

dextral strike-slip

SE-NW shortening

SE-NW extension

S-C fabric

vorticity vector

shear zone movement

foliation trace

stretching lineation trace

rigid porphyroclast withtails (shear sense)

NW

NW

L3

SZ deformation. (A) Oriented blocks from each shear zone show the XZ plane used tothe XZ plane. (B) Kinematics of dextral, top-down-to-the-southeast deformation. (C)


7. Conclusions

The SLSZ is a low to moderate-angle structure that accommo-dated normal (top-down-to-the-southeast), reverse (top-up-to-the-northwest), and dextral movement. This study documents thenorthenortheast-striking SLSZ as sharing similar deformationalstyles with the subvertical, northeast-striking HSZ. Mylonite andultramylonite from both shear zones record top-down-to-the-southeast, top-up-to-the-northwest, and dextral movement atsimilar mid-crustal ductile deformation temperatures (HSZ:w280e500 �C; SLSZ w280e600 �C) and general shear in planestrain conditions. This suggests that mid-crustal heterogeneity,possibly influenced by anisotropic D1/D2 foliation, may have par-titioned transpression into the w1.4 Ga shear zones of centralColorado. This data generally supports models based on previouswork (McCoy et al., 2005; Shaw et al., 2005) on shear zones withinthe Colorado mineral belt positing that Mesoproterozoic defor-mation was associated with the subvertical reshuffling of blocksduring transpression to accommodate far-field deformation alongthe evolving margin of Laurentia.

Acknowledgments

We would like to thank Tim Bell and Mark Swanson for reviewsthat strengthened the previous version of this manuscript. Wethank R. D. Hatcher Jr., W. M. Dunne, and J. Langille for early reviewsof this manuscript. K. Karlstrom also contributed insights into theSLSZ while M. Jessup was at the University of NewMexico. Fundingwas provided by grants from the College of Arts and Science at theUniversity of Tennessee, Southeast Geological Society of America,the George D. Swingle award from the Department of Earth andPlanetary Sciences at the University of Tennessee, and NSF grantsEAR-0635920 and 0635894 to C. Shaw and J. Allen.

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Strain partitioning in the mid-crust of a transpressional shear zone system: Insights from the Homestake and Slide Lake shear zones, central Colorado