Although crystalline rocks dominate the footwall of the Buckskin-Rawhide detachment fault in west-central Arizona (USA), we estimate that thin (<1 to 100 m thick) calcite-rich metasedimentary mylonites were present along 25%–35% of the detachment fault, and in parts of the footwall they were continuous for ∼30 km in the slip direction. New field observations, geochronology, and detailed microstructural data provide insight into the origin of these metasedimentary rocks and their role in the structural evolution of the detachment fault system. We propose that calc-mylonites along the Ives Peak footwall corrugation were derived from locally overturned Pennsylvanian–Permian strata that were buried to mid-crustal depths beneath a southeast-vergent Cretaceous thrust fault, which was reactivated in the Miocene by the parallel Buckskin detachment fault shear zone. In some areas these laterally persistent calc-mylonites were smeared out along the detachment fault during incisement into the crystalline footwall, forming a thin carapace of rheologically weak rocks structurally below the original weak zone. Metasedimentary mylonites consistently record top-to-the-northeast simple shear parallel to the detachment fault slip direction. Strain, synmylonitic veins, and paleostress recorded in these mylonites increase toward the detachment fault. Marble mylonites <1 m below the detachment fault preserve strong calcite crystallographic preferred orientations and lack cataclastic deformation that characterizes quartz-rich rocks along the detachment fault. In addition, unlike quartzofeldspathic mylonites, calc-mylonites typically lack extension via postmylonitic normal faults and associated horizontal axis rotation. Paleopiezometry and rheological modeling of the metasedimentary mylonites suggest that when quartzite layers were being sheared at ∼100 MPa and 10−13 to 10−14 s−1 near the brittle-plastic transition, marble layers could have been strained ∼100× faster at ∼20 MPa. Detachment fault strain localized within the metasedimentary rocks, and the calcite marbles exerted significant control on the rheology of the footwall shear zone. This study highlights the important role that inherited weak zones may play in influencing the location, geometry, rheology, and style of deformation associated with detachment fault systems.
Rheologically weak zones may play a critical role in controlling the location, geometry, and style of deformation in any tectonic setting. In particular, weak zones near the brittle-plastic transition, where peak crustal strength typically resides (e.g., Sibson, 1983; Kohlstedt et al., 1995; Behr and Platt, 2014), may reduce strength across a range of structural levels and promote efficient strain localization in mid-crustal shear zones. Strength contrasts between relatively strong feldspar-rich continental crust and weak metasedimentary rocks rich in calcite, quartz, and/or mica are significant at mid-crustal depths, and previous studies have documented numerous examples of strain localization in metasedimentary rocks.
Extensional detachment faults and associated footwall shear zones are commonly associated with metasedimentary rocks. Detachment fault systems in Cordilleran metamorphic core complexes affect all types of rocks, but several are localized in rheologically weak metasedimentary units. Across several regions in the Basin and Range (western U.S.), Proterozoic to Paleozoic marine strata were buried to mid-crustal depths during Mesozoic shortening and subsequently exhumed during Cenozoic detachment faulting. Shear zones associated with these Cenozoic detachment fault systems utilized rheological boundaries and weak zones within these metasedimentary rocks. For example, the northern Snake Range décollemont (Nevada, USA) developed primarily within the Cambrian stratigraphic section (Miller et al., 1983; Lee et al., 1987), and the Raft River shear zone (northwest Utah) localized along an unconformity between Archean rocks and Proterozoic quartzite for at least 24 km in the shear direction (Wells, 2001; Sullivan, 2008). Where present, carbonate-rich strata absorbed significant strain in detachment shear zones. For example, the shear zone in the northern Ruby Mountains–East Humbolt Range (Nevada) developed in the carbonate-rich Neoproterozoic–Paleozoic section (Snoke, 1980), attenuating strata to ∼5% of original thickness (Snoke and Howard, 1984). Much of the shear associated with the Miocene detachment fault system in the Funeral Mountains (Death Valley, California) appears to have been accommodated in a ≤10-m-thick calcite marble mylonite zone derived from Neoproterozoic strata (Hoisch and Simpson, 1993; Beyene, 2011). Detachment shear zones may also localize in quartzite, as documented in the Shuswap metamorphic core complex (British Columbia, Canada; Mulch et al., 2006) and Coyote Mountains metamorphic core complex (south Arizona; Davis et al., 1987).
Detachment fault strain localization in weak metasedimentary rocks has also been documented in several areas outside of the North American Cordillera. Calcite-rich mylonites (calc-mylonites) are widespread along detachment systems in the Greek Aegean (e.g., Dinter and Royden, 1993; Bestmann et al., 2000; Iglseder et al., 2011) and in central Turkey (e.g., Whitney et al., 2007; Lefebvre et al., 2011). Calc-mylonites are common along detachment systems in the Himalayan-Tibetan orogen (e.g., Murphy et al., 2002; Jessup et al., 2006; Cottle et al., 2007; Searle, 2010; Courthouts et al., 2015). Other examples of detachment fault strain localization in calc-mylonites include the Ainslie detachment fault in Nova Scotia, Canada (Lynch and Tremblay, 1994; Lynch and Giles, 1996), the Gubbedalen detachment shear zone in Greenland (Johnston et al., 2010), and the Zuccale detachment fault in Elba (Collettini and Holdsworth, 2004; Collettini et al., 2009).
In the examples cited here rheologically weak metasedimentary mylonites largely parallel their bounding detachment faults and record a significant amount of transport associated with extensional exhumation. The location, geometry, and rheology of structures that accommodate large-magnitude extension was undoubtedly influenced by the presence of metasedimentary rocks in most (if not all) of these areas. Even volumetrically minor amounts of metasedimentary rocks, which are easily overlooked, may play an important role in the structural evolution of detachment systems. This study focuses on calc-mylonites in the Buckskin-Rawhide metamorphic core complex, located in the lower Colorado River extensional corridor in west-central Arizona. Although crystalline rocks dominate mylonitic footwalls of detachment faults in this region, metasedimentary rocks are present in several areas. The relationship between these rocks and the well-known detachment fault systems in the region has not previously been described in detail. The goals of this study are to understand the origin of the footwall metasedimentary rocks and their role in influencing the structural evolution of the Buckskin-Rawhide detachment fault system.
A belt of kinematically related metamorphic core complexes between southern Nevada and Phoenix, Arizona, is one of Earth’s archetypical examples of regional-scale, large-magnitude continental extension (Fig. 1). Mid-crustal mylonites within these core complexes were unroofed primarily during 30–70 km of top-to-the-northeast–directed slip on a detachment fault system in the early to mid-Miocene (ca. 22–12 Ma) (Howard and John, 1987; Spencer and Reynolds, 1991; John and Foster, 1993; Singleton et al., 2014a; Prior et al., 2016). The central portion of this belt is located within and adjacent to the Maria tectonic (fold-thrust) belt, an anomalous zone of Cretaceous shortening characterized primarily by south- to southwest-vergent folds and basement-involved thrusts. This shortening resulted in widespread burial of Paleozoic and Mesozoic strata to mid-crustal depths beneath Proterozoic crystalline thrust sheets. Between the Whipple Mountains in eastern California and the Harcuvar Mountains in west-central Arizona the core complex belt abruptly bends to a west-northwest trend parallel to the adjacent Maria tectonic belt, suggesting that Cretaceous tectonism influenced the location and geometry of Miocene extension (Spencer and Reynolds, 1990; Fig. 1). Spencer and Reynolds (1990) attributed this spatial relationship to the likely presence of a thick crustal welt produced during Cretaceous contraction. Flexural stresses associated with this crustal welt may have influenced the detachment fault geometry and location, and isostatic uplift driven by Miocene denudation of the thick crust may have played an important role in core complex exhumation (Spencer and Reynolds, 1990). It is also possible that metasedimentary rocks buried during Cretaceous shortening influenced Miocene extension by providing rheologically weak zones for detachment shear zone localization. Carbonate-rich Paleozoic strata that are common in the Maria tectonic belt would have been particularly prone to detachment strain localization. However, Spencer and Reynolds (1990) largely discounted this weak zone hypothesis because metasedimentary rocks are volumetrically minor across the west Arizona core complexes.
The ∼800 km2 Buckskin-Rawhide metamorphic core complex in west-central Arizona, located near the central portion of the Maria tectonic belt, is dominated by mid-crustal mylonites exposed beneath the Buckskin-Rawhide detachment fault (herein referred to as the Buckskin detachment fault for short). These footwall mylonites consistently record a top-to-the-northeast– (top-NE) directed sense of shear parallel to the detachment fault slip direction (Singleton and Mosher, 2012). Based on 1:100,000-scale mapping (Bryant, 1995) and more detailed mapping (e.g., Scott, 2004; Singleton, 2013b; Singleton et al., 2014b), the exposed mylonitic footwall of the Buckskin detachment fault consists of ∼75% layered crystalline gneisses and granitoid gneisses (Proterozoic, Jurassic, and Cretaceous), ∼20% early Miocene granitoids of the Swansea Plutonic Suite, and ∼5% Paleozoic metasedimentary rocks. Most mylonites are exposed along 3 ∼35-km-long antiformal corrugations, referred to as the Planet Peak antiform, Clara Peak antiform, and Ives Peak antiform (Bryant, 1995; Fig. 2). Hornblende and biotite 40Ar/39Ar cooling ages from the Buckskin-Rawhide core complex and adjacent Harcuvar core complex indicate that syndetachment fault mylonitization occurred primarily in the early Miocene (ca. 21–16 Ma) under greenschist facies conditions (Richard et al., 1990; Scott et al., 1998; Wong et al., 2015). The Swansea Plutonic Suite, which dominates the Clara Peak corrugation (antiform), records relatively uniform upper greenschist facies mylonitization conditions across a distance of ∼35 km in the extension direction, suggesting that the footwall shear zone initiated with a subhorizontal dip (Singleton and Mosher, 2012). Pre-Cenozoic crystalline gneisses and Late Cretaceous granites locally record top-NE–directed amphibolite facies mylonitization that likely occurred during the latest Cretaceous to early Paleogene (Wong et al., 2013; Singleton and Wong, 2016). This study focuses on deformation of the metasedimentary mylonites exposed beneath the Buckskin detachment fault. Although these rocks comprise a volumetrically minor component of the footwall, they are widespread along the detachment fault system and played an important role in its evolution.
METASEDIMENTARY MYLONITES IN THE BUCKSKIN-RAWHIDE METAMORPHIC CORE COMPLEX
Distribution and Characteristics
Metasedimentary rocks in the footwall of the Buckskin detachment fault are most abundant near Battleship Peak at the southwestern end of the Ives Peak corrugation, where they crop out over an ∼16 km2 area (Bryant, 1995; Fig. 2). The footwall in this area is dominated by marble, calc-silicate rocks, and quartzite, which are locally intruded by granitic sills of the early Miocene Swansea Plutonic Suite. These sills, which are typically 0.2–3 m thick and comprise 5%–10% of the footwall near Battleship Peak, were emplaced ∼5–6 m.y. before the cessation of mylonitization, so it is unlikely that they have affected microstructures preserved in the metasedimentary mylonites. Based on detailed mapping by Marshak et al. (1987), metasedimentary rocks in this area are structurally up to ∼1 km thick. Variably developed mylonitic foliation parallels compositional layering, dipping gently to moderately southwest across much of this area. Shear sense indicators associated with southwest-plunging mylonitic lineations in these metasedimentary rocks consistently record top-NE–directed sense of shear (Marshak and Vander Meulen, 1989; this study; Fig. 3A). As this southwest-dipping package of metasedimentary mylonite and protomylonite approaches the overlying Buckskin detachment fault exposed near the top of Battleship Peak, foliation and layering rotate to subhorizontal, parallel to the detachment fault (Figs. 4A and 5A). In addition, mylonitic fabrics become better developed toward the detachment fault, and units appear to be significantly attenuated. For example, a distinct white aplite sill that is ∼80 m thick within the southwest-dipping package of metasedimentary rocks is only 3–7 m thick within the subhorizontal metasedimentary section beneath Battleship Peak (Fig. 4A). We cannot rule out the possibility that some of this variation in sill thickness is due to primary igneous emplacement, but the apparent attenuation is consistent with the transition to ultramylonitic fabrics near the detachment fault. The upper part of the southwest-dipping metasedimentary section may be excised by the Buckskin detachment fault, which cuts down to the Battleship Peak area from the inferred breakaway zone in the western Bouse Hills, ∼15–20 km to the southwest (Spencer and Reynolds, 1991; Singleton et al., 2014a). However, it is possible that much of the ∼1-km-thick metasedimentary section is present within the attenuated ∼75-m-thick metasedimentary section beneath Battleship Peak (Fig. 6). In this area the metasedimentary rocks appear to have absorbed almost all of the Miocene ductile strain, as only the top 1–2 m of the underlying crystalline gneiss contains a top-NE mylonitic shear fabric.
Along the northwest flank of the Ives Peak corrugation, metasedimentary mylonites parallel the Buckskin detachment from the Battleship Peak area to the northeast end of the core complex, ∼30 km in the slip direction (Fig. 2). Locally the detachment fault and the metasedimentary section are discordant, but the preservation of an ∼50–100-m-thick section along the detachment between Battleship Peak and Lincoln Ranch basin highlights the dominant parallelism between the detachment fault and layering in this section (Figs. 2, 5, and 6). This parallelism contrasts with the geometric relationship between the detachment fault and mylonitic fabric in quartzofeldspathic crystalline rocks exposed along the Clara Peak and Planet Peak footwall corrugations. Mylonitic foliation in crystalline rocks dips gently to moderately southwest across most of these corrugations, and on average fabrics are ∼15° discordant with the subhorizontal detachment fault (Singleton, 2013b, 2015). Detailed measured sections indicate that ∼72%–93% of the metasedimentary rocks along the Ives Peak corrugation are calcareous (Fig. 6). Siliceous marble and calcareous quartzite are particularly common, and intervals of calcite marble up to ∼8 m thick are present along the detachment fault in the central and northeastern part of the Ives Peak corrugation (Fig. 6). Marble mylonites along the detachment fault have undergone little to no brecciation and preserve coherent mylonitic fabrics within 1 m of the Buckskin detachment fault (Fig. 3). In the Lincoln Ranch basin ≤40 cm of brittle fault rock separate platy calcite marble mylonite from the detachment (Fig. 3G). By contrast, along the same detachment fault surface in the same area, an ∼2-m-thick ultracataclasite and ∼20-cm-thick gouge zone derived from quartzite mylonite is present where marble is absent. These observations suggest that a significant amount of penetrative strain in the footwall marble was accommodated when quartzite mylonites were undergoing cataclastic flow, most likely between the lower temperature limits of quartz dislocation creep (∼280–310 °C; e.g., Stipp et al., 2002; Stöckhert et al., 1999) and calcite dislocation creep (∼180–200 °C; e.g., Burkhard, 1990; De Bresser et al., 2002).
In the Lincoln Ranch basin area, the Buckskin detachment fault appears to cut out the ∼50–100-m-thick metasedimentary section toward the crest of the Ives Peak corrugation. Cross sections based on detailed mapping (Singleton et al., 2014b) suggest that ∼2 km southeast of the margin of the Lincoln Ranch basin the detachment system incised 190–200 m below the metasedimentary section (Fig. 4B). However, where the detachment fault is exposed beneath a klippe of hanging-wall basalt, an ∼0.25-m-thick zone of marble mylonite is present at the top of the footwall (Fig. 4B). The marble layer has undergone relatively little brecciation and preserves a clear S-C fabric indicating top-NE sense of shear (Fig. 3F). This exposure and adjacent thin (<5 m) exposures of calc-mylonites along the detachment fault suggest that weak metasedimentary rocks were smeared along the detachment fault during footwall incisement.
Previous mapping identifies crystalline mylonite along the detachment fault in almost all areas outside of the northwest flank of the Ives Peak corrugation (Shackelford, 1989; Spencer and Reynolds, 1989; Bryant, 1995). However, along well-exposed portions of the detachment fault it is clear that metasedimentary mylonites are present below most, if not all, of the detachment fault northeast of the Lincoln Ranch basin and along the northwest flank of the Planet Peak corrugation. Near Alamo Lake at the northeastern end of the core complex, a 7–8-m-thick interval of marble and calc-silicate mylonite at the top of the footwall parallels the detachment fault. Near Pride Mine, at the southwestern end of the Planet Peak corrugation, an ∼2-m-thick calcite marble mylonite zone marks the top of the footwall. This marble interval records a top-NE S-C fabric and lacks cataclastic deformation that characterizes crystalline footwall rocks along the detachment (Fig. 3I). In the Mineral Wash–Planet Wash area along the northwest flank of the Planet Peak corrugation, an ∼5-m-thick zone of calc-mylonite is present along the detachment, separating Mesozoic sedimentary and volcanic rocks in the hanging wall from crystalline mylonites in the footwall (Fig. 3H). In the Rawhide Mountains to the northeast, quartzite and calc-silicates are present along many of the detachment fault exposures. These exposures suggest that metasedimentary rocks are continuous along the northwest flanks of the Planet Peak and Ives Peak corrugations (Fig. 7). Although metasedimentary rocks compose only ∼5% of the eroded footwall today, we estimate that calcareous-bearing metasedimentary units were present beneath 25%–35% of the Buckskin detachment fault (Fig. 7). These units thus played an important role in controlling the rheology of the detachment fault system.
Calc-mylonites do not appear to be present in the adjacent Little Buckskin Mountains and Harcuvar Mountains, which are dominated by Late Cretaceous leucogranitoids. However, an interval of chloritically altered meta-arkose and quartzite mylonite up to ∼200 m thick marks the top of the footwall along the flanks of Little Buckskin Mountains (Singleton, 2011) and along an ∼4-km-long portion of the Harcuvar Mountains near Burnt Well (Figs. 2 and 7). Although this unit largely lacks calc-silicates, the higher quartz and mica content makes these metasedimentary rocks rheologically weaker than the granitoids and crystalline gneisses that comprise most of the footwall. This rheology contrast may explain why penetrative mid- to low-greenschist facies strain associated with widespread chloritization appears to have been preferentially absorbed in the quartzite and meta-arkose unit, which are significantly more altered than underlying granitoid mylonites (Singleton and Wong, 2016).
Age of Metasedimentary Protoliths
Based on the carbonate-rich nature of metasedimentary rocks in the footwall of the Buckskin detachment fault, previous workers have interpreted the protolith as Paleozoic, likely correlative with the well-known Grand Canyon strata (Marshak and Vander Meulen, 1989; Bryant, 1995), which are locally exposed in the hanging wall (e.g., Shackelford, 1989; Spencer and Reynolds, 1989, 1990). However, previous studies have not suggested specific correlations at the formation level. Using laser ablation–inductively coupled plasma–mass spectrometry we determined detrital zircon U-Pb ages for three quartzite mylonite samples: two from the metasedimentary section near Battleship Peak and one from the chloritically altered meta-arkose–rich section along the northwest flank of the Harcuvar Mountains near Burnt Well (Fig. 2; Data Repository Tables DR1, DR21). Following data analysis procedures of Gehrels et al. (2011) for detrital zircons from Grand Canyon strata, we determined detrital zircon age spectra using analyses that record <20% discordance and <5% reverse discordance (92–96 zircons per sample).
Our detrital zircon ages from the Battleship Peak area come from a quartzite ∼3 m above the base of the metasedimentary section (BP-190) and a micaceous quartzite (∼25% muscovite) ∼57 m above the base of the section (16-3-BP1; Fig. 6). BP-190 contains zircon age maxima of ca. 1.8 Ga (Yavapai province), 1.68–1.62 Ga (Mazatzal province), 1.08–1.0 Ga (Grenville orogen age), ca. 435–420 Ma and ca. 350–335 Ma (Fig. 8). The Paleozoic zircons were likely derived from the Appalachian orogen (Dickinson and Gehrels, 2003; Gehrels et al., 2011). A weighted average of the youngest three zircon grains that define a single population (range 334 ± 6 Ma to 339 ± 6 Ma) yields a maximum depositional age of 336 ± 3 Ma. This zircon age distribution resembles those from Permian strata in the Grand Canyon (Gehrels et al., 2011). Kolmogorov-Smirnov (K-S) statistical comparison of the BP-190 detrital zircon data with the Gehrels et al. (2011) detrital zircon data from the Grand Canyon suggests that BP-190 correlates with the middle part of the Permian Hermit Formation (BP-190; middle Hermit Formation K-S P value = 0.991; Table 1; Table DR3). This correlation with the mudstone-dominated Hermit Formation is consistent with the lithology in this part of the section, as the ∼23 m of section above BP-190 are dominated by slope-forming, greenish-gray, platy calcareous phyllite (Fig. 6). Similar phyllite is also present near the base of the metasedimentary section in the southern Lincoln Ranch basin (Fig. 6). Metamorphosed equivalents of the Hermit Formation are described as green calcareous schist in eastern California (Stone et al., 1983) and greenish-gray phyllite in the hanging wall in the westernmost Buckskin Mountains (Reynolds and Spencer, 1989). In the westernmost Buckskin Mountains the Hermit Formation is typically 2–5 m thick (Reynolds and Spencer, 1989), so the ∼24-m-thick phyllite-rich mylonite in the Battleship Peak area is most likely either structurally thickened and/or does not all correlate with the Hermit Formation.
Sample 16–3-BP1 is from the middle portion of an ∼35-m-thick interval that is dominated by resistant calcareous quartzite and siliceous marble with 0.3–3-m-thick intervals of calcite marble (Fig. 6). Detrital zircons yield a range of ca. 1.8–1.0 Ga ages without a well-defined Proterozoic maxima (Fig. 8). The Paleozoic maximum is ca. 460–420 Ma, and the youngest zircon is 366 ± 6 Ma (Fig. 8). Due to the lack of well-defined Proterozoic maxima, this sample is more difficult to correlate with the Grand Canyon section. K-S statistics suggest potential correlation with the base of the upper Mississippian Surprise Canyon Formation (K-S P value = 0.471) or correlation with the upper part of the Permian Kaibab Formation (K-S P value = 0.461; Table 1). Of these options, the Paleozoic zircon ages in 16–3-BP1 do not correlate well with the upper Kaibab Formation, which includes abundant ca. 301–273 Ma zircons (Gehrels et al., 2011). The ca. 460–420 Ma maximum in 16–3-BP1 is similar to the Paleozoic maxima in the base of the Surprise Canyon Formation (Fig. 8).
Based on the Paleozoic zircon population and the lithologies at Battleship Peak, we favor the interpretation that the protolith of 16–3-BP1 is older than the protolith of BP-190, which requires that the section is at least locally overturned. The calcareous quartzite and siliceous marble that dominate the top part of the section near Battleship Peak most resemble the metamorphosed Pennsylvanian Supai Group. If the section was not overturned, the equivalent of the Permian Coconino Sandstone should be above the Hermit Formation near the base of the section. The Coconino Sandstone is a distinct light gray to pinkish-tan vitreous quartzite up to 80 m thick in the westernmost Buckskin Mountains hanging wall (Reynolds and Spencer, 1989), which does not resemble quartzite in the Battleship Peak section (Fig. 6).
The meta-arkose–rich metasedimentary mylonite section along the flanks of the Little Buckskin Mountains and Harcuvar Mountains has not previously been correlated with other formations. We determined detrital zircon ages from a quartzite mylonite sample (16–3-BW1) collected <10 m below the Buckskin-Bullard detachment fault near Burnt Well in the Harcuvar Mountains. All zircon ages in this sample are ≥1.56 Ga, and the dominant population is 1.75–1.69 Ga (Table DR2). This detrital zircon age distribution is similar to that documented in Mesoproterozoic quartzite east of Phoenix (e.g., in the Yankee Joe Formation and units in the Four Peaks area; Doe et al., 2012; Mako et al., 2015) and to Mesoproterozoic conglomerate and arkose in the Kingston Range in eastern California (the base of the Crystal Spring Formation; Mahon et al., 2014). The age distribution does not match any Paleozoic or Neoproterozoic units from the Grand Canyon (Gehrels at al., 2011). We have not seen evidence that this meta-arkose unit was ever continuous with the package of Paleozoic metasedimentary units along the detachment fault in the Buckskin-Rawhide Mountains.
Burial of Metasedimentary Rocks to Mid-Crustal Depths
The preservation of Paleozoic sedimentary rocks in the Maria tectonic belt has been attributed to burial by Mesozoic thrust faults (Spencer and Reynolds, 1990). Near the Buckskin-Rawhide and Harcuvar metamorphic core complexes, major Mesozoic thrust faults are well documented in the Granite Wash Mountains (e.g., Laubach et al., 1989; Reynolds et al., 1991), western Harcuvar Mountains (Reynolds and Spencer, 1993), and westernmost Buckskin Mountains (Reynolds and Spencer, 1989). The oldest deformation event involving Paleozoic–Mesozoic units within this part of the Maria tectonic belt (D1) is associated with southeast- to south-southeast–vergent thrust faulting (Laubach et al., 1989; Reynolds and Spencer, 1993). In the Billy Mack and Eagle Nest areas in the westernmost Buckskin Mountains, a southeast-vergent thrust system juxtaposes Proterozoic crystalline rocks over Devonian through Jurassic rocks (Reynolds and Spencer, 1989). The footwall rocks of the thrust faults, which are predominantly southeast facing and overturned, include Pennsylvanian and Permian strata of the Grand Canyon section (Reynolds and Spencer, 1989). Based on the distribution and interpreted age of Paleozoic metasedimentary rocks in the footwall of the Buckskin detachment fault, we propose that a related Mesozoic thrust system was responsible for burial of the metasedimentary rocks to mid-crustal depths. The prevalence of these units along the northwest flanks of the Planet Peak and Ives Peak corrugation is consistent with their burial beneath gently northwest-dipping thrust faults (Fig. 7). The relatively uniform thickness of the metasedimentary section across a distance of ∼20 km along the Ives Peak corrugation (Fig. 6) suggests that this northwest-dipping geometry predates Miocene detachment faulting. If the orientation and distribution of these metasedimentary rocks were significantly modified by Miocene shear, this section would likely record significant attenuation in the shearing direction, as seen where the detachment fault cuts into the metasedimentary section at the southwestern end of the Ives Peak corrugation (Fig. 4A). Proterozoic crystalline rocks that may have originally been thrust over the metasedimentary rocks along the northwest corrugation flanks are locally present in the dismembered Buckskin detachment fault hanging wall (e.g., Bryant, 1995; Singleton et al., 2014b). Our interpretation of an overturned section near Battleship Peak that includes the Permian Hermit Formation near the base and the Pennsylvanian Supai Group in the upper half is also consistent with burial in the footwall of a thrust sheet, similar to overturning of Paleozoic strata in the footwall of thrusts in the Granite Wash Mountains and westernmost Buckskin Mountains (Reynolds et al., 1991; Reynolds and Spencer, 1989). Calc-silicates in the metasedimentary mylonites across the Buckskin Mountains locally contain diopside (Bryant and Wooden, 2008; this study), indicating that the section was buried to amphibolite facies conditions. These calcite-rich rocks would have very low strength under such conditions, forming a weak zone that locally extended across the entire detachment fault footwall in the slip direction.
MYLONITIZATION AND RHEOLOGICAL BEHAVIOR OF METASEDIMENTARY ROCKS
Field relationships described herein indicate that a significant amount of detachment fault-related strain was localized in the calcite-rich Paleozoic metasedimentary section. To better understand the rheology of this strain and the relationship between the Buckskin detachment fault and the metasedimentary mylonites we investigated deformation conditions through detailed microstructural analysis of >60 oriented thin sections (Table DR7). These analyses include determination of deformation mechanisms, dynamically recrystallized grain sizes, grain shape fabrics, and crystallographic preferred orientations. In addition, we present some constraints on kinematic vorticity, and model the rheology of the brittle-plastic transition using quartzite and calcite marble flow laws.
Recrystallized grain sizes were quantified by tracing >150 well-defined grains in areas of relatively pure (≥95%) calcite on photomicrographs. For samples with ≤∼10 µm grain sizes, we traced grains on photomicrographs taken in both cross-polarized light and reflected light that yielded similar results. We present mean grain sizes as the arithmetic mean ± standard deviation. For all of the calcite marble mylonite samples analyzed in this study, the arithmetic mean, geometric mean, and root mean square grain size are within 2 µm (Table DR4). In samples from the Battleship Peak area we evaluated the orientation of calcite grain shape fabrics in marble mylonites by measuring the angle between the trace of foliation (X:Y plane) and the long axes of ≥200 grains on photomicrographs of X:Z thin sections. We also quantified the area of veinlets by measuring veinlet apertures across thin sections on a grid with an ∼3.25 mm spacing between vertical and horizontal lines. Crystallographic orientation patterns were determined with electron backscatter diffraction (EBSD) at Colgate University using a step size of 1–5 µm. Crystallographic axes in pole figures were reduced to show one point per grain using a misorientation of 10° as a grain boundary threshold. EBSD data collection focused on areas of pure calcite with complete recrystallization, and twinned grains were included in the pole figures.
Penetrative Deformation in the Battleship Peak Area and Vertical Strain Patterns
The relationship between the Buckskin detachment fault and penetrative strain in the footwall metasedimentary rocks is best seen in the Battleship Peak area, where the metasedimentary section is rotated into parallelism with the subhorizontal detachment fault (Fig. 4A). Southwest of Battleship Peak, where the southwest-dipping metasedimentary section is 0.5–1.0 km below the projected detachment fault (Fig. 4A), mylonites record recrystallization under low stress conditions. For example, a quartzite mylonite sample from this area is characterized by ∼100-µm-diameter recrystallized grains with irregular boundaries that are commonly pinned by micas (e.g., see Singleton and Mosher, 2012, fig. 14F therein), suggesting grain boundary migration recrystallization at ≤30 MPa differential stress (based on the extrapolation of quartz grain size piezometers; e.g., Twiss, 1977; Stipp and Tullis, 2003). A marble mylonite sample from the southwestern end of the core complex records an average calcite grain size of ∼125 µm. As in all other parts of the footwall, kinematic indicators in these relatively coarse grained metasedimentary mylonites record a top-NE sense of shear (Fig. 3A).
The rotation and attenuation of the metasedimentary section <100 m below the detachment fault corresponds with a pronounced change in microstructures. In the metasedimentary mylonites that parallel the subhorizontal detachment fault, marble mylonites have average recrystallized calcite grain sizes of <30 µm, and quartzite mylonites have average recrystallized quartz grain sizes of <20 µm (Fig. 6). In the marble mylonites the presence of calcite subgrains and the polygonal, relatively uniform calcite grain size indicate that subgrain rotation recrystallization was dominant (Fig. 9A), whereas the small and locally blurry grains along quartz ribbons indicate that bulging was an important recrystallization mechanism in quartzite (Fig. 9B). Ti-in-quartz analyses by Seymour et al. (2016) provide some constraints on dynamic recrystallization temperature in this area. Quartzite sample BP-194, which is located ∼48 m below the detachment fault near Battleship Peak, has Ti-in-quartz concentrations ranging from 2.3 to 3.8 ppm. At greenschist facies conditions Ti diffusion is sluggish in quartz, and complete equilibration during bulging recrystallization is difficult (Grujic et al., 2011). We estimate a temperature of 342 ± 22 °C in this sample by taking the average Ti concentration of the lowest 2 spots (2.45 ppm) and assuming a pressure range of 1.5–2.6 kbar (∼6–10 km) and Ti-in-quartz activity range of 0.6–1.0 (e.g., Ghent and Stout, 1984).
Grain size analysis and EBSD data from marble mylonite samples collected along a vertical transect <80 m below the detachment near Battleship Peak reveal a spatial correlation between mylonitic strain and the detachment fault. Mean calcite recrystallized grain size in the marble mylonites in this area progressively decreases from ∼25–28 µm near the base of the section to ∼10 µm right below the detachment fault, which is marked by a 1.5–2-m-thick resistant ultracataclasite layer derived from calcareous quartzite (Figs. 3C, 6, and 10). The marble mylonites near Battleship Peak have a well-defined crystallographic preferred orientation (CPO) characterized by calcite c-axes oriented at a high angle to the foliation and a-axes distributed along a great circle at a low angle to foliation (right column in Fig. 11). These marble mylonites all record a clear top-NE sense of shear, but the asymmetry of calcite c-axis maxima varies from synthetically rotated with the sense of shear (BP-193, BP-199, BP-204), to normal to foliation (BP-201), to antithetically rotated with respect to the sense of shear (BP-189; Fig. 11). Antithetically rotated calcite c-axis maxima are commonly attributed to e-twinning, whereas symmetric or synthetically rotated c-axis maxima are characteristic of samples in which intracrystalline slip dominates over deformation twinning (e.g., Schmid et al., 1987; Lafrance et al., 1994; Trullenque et al., 2006; Spanos et al., 2015). Twinned recrystallized calcite grains are common in all of the Battleship Peak marble mylonite samples, and because we did not distinguish twinned grains from non-twinned grains in the pole figures, we suggest that the c-axis asymmetry reflects the relative significance of e-twinning versus intracrystalline slip. Overall the strength of the calcite CPO increases toward the detachment fault, as seen in the decrease in the minimum eigenvalue (e3) for c-axes (e.g., e3 varies from 0.27 at 77 m below the detachment fault to 0.15 at 16 m below the detachment; Fig. 11). The one exception to this pattern is that the marble mylonite 2.5 m below the detachment fault (BP-204a) records a lower strength CPO (e3 = 0.19) than the marble mylonite below. This decrease in CPO strength could reflect the influence of minor brittle deformation in this sample and/or potential contributions from grain boundary sliding, which may be an important deformation mechanism in fine-grained samples like this marble (e.g., Schmid et al., 1987; Bestmann and Prior, 2003; Rogowitz et al., 2014). Marble mylonite ∼2–3 m below the detachment fault lacks cataclastic deformation, but siliceous marble ∼3–4 m below the detachment fault records pervasive foliation-parallel cataclasis.
To evaluate the spatial relationship between kinematic vorticity and the Buckskin detachment fault, we analyzed calcite grain shape-preferred orientations in 6 marble mylonite samples from the Battleship Peak area (Fig. 12). In all of these samples the dominant orientation of recrystallized grain long axes is ∼10°–20° counterclockwise of the northeast side of the lineation, which is consistent with a top-NE sense of shear (Fig. 12). The long axes of recrystallized grains in mylonites typically initiate parallel to the instantaneous stretching axis (ISA1) and rotate during progressive shear, so grains that form the largest angle with respect to foliation should approximate the orientation of the ISA1 (e.g., Law et al., 1984; Dell’Angelo and Tullis, 1989; Wallis, 1995; Spanos et al., 2015). In the 6 marble mylonite samples the maximum angle between grain shape orientation and foliation (not including outliers that are not within a continuous range of the mode orientation) ranges from 30° to 43°, with an average of 37° (excluding BP-201, which does not have a strong calcite grain shape-preferred orientation, the average is 38°; Table DR5). There is no clear spatial relationship between these angles and the Buckskin detachment fault (Fig. 12), and the relative consistency of this angle across the ∼80-m-thick mylonite zone suggests the oblique grain shape fabrics can be used to estimate the ISA1. Adding the angle between the foliation and the shear plane (flow apophysis) to the maximum angle between the grain axes and foliation would provide the angle Θ, where the kinematic vorticity number (Wk) is equal to sin2Θ. The shear plane orientation is unclear from calcite CPO patterns, but given the large amount of strain taken up by the metasedimentary rocks near Battleship Peak and the parallelism between the mylonitic fabric and the detachment fault, the angle between foliation and the shear plane is likely small. For example, 2 km of top-NE shear distributed across the ∼80 m of footwall mylonitic fabrics in this area would result in a ≤2° angle between foliation and the shear plane. Adding just 1° to the maximum angle between grain axes and foliation to estimate Θ would yield kinematic vorticity values ranging from 70% to 98% simple shear (Wk = 0.89–1.0). Regardless of the kinematic vorticity number, it is clear from the oblique grain shape fabrics that the marble mylonites record dominantly top-NE–directed simple shear.
Role of Fluids in Penetrative Deformation in the Battleship Peak Area
Microstructures preserved in mylonites in the Battleship Peak area highlight the role of fluids in penetrative deformation. Calcite ± barite veinlets are present in every marble and calc-silicate mylonite sample, and calcite ± quartz ± barite ± chlorite veinlets are present in every quartzite mylonite sample. Most of these veinlets (herein referred to as veins) are <1 mm thick (and commonly <250 µm thick) and oriented either parallel or perpendicular to foliation, or they dip moderately to steeply northeast with respect to foliation. Veins also typically display various degrees of dynamic recrystallization, indicating that they formed synkinematically with respect to mylonitization. Some calcite veins are completely recrystallized and are difficult to distinguish from the host marble. Recrystallized quartz and barite within veins locally exhibit bulging recrystallization, consistent with deformation at high stress–low temperature conditions. A late-kinematic quartz vein at a high angle to foliation within an aplite sill ∼40 m below the detachment fault has a recrystallized grain size of ∼6–7 µm (6.5 ± 2 µm; Fig. 13A). This is the smallest quartz grain size observed in this study or previously reported from the Buckskin-Rawhide-Harcuvar core complexes (e.g., Singleton and Mosher, 2012; Singleton and Wong, 2016). Recrystallized quartz in this veinlet has a moderately developed CPO that likely formed near peak stress conditions for dislocation creep-dominated deformation in the area (Fig. 13B).
The abundance of veins in the metasedimentary mylonite section near Battleship Peak increases with proximity to the detachment fault. Veins compose <1% of a marble mylonite sample ∼55 m below the detachment, whereas marble mylonite samples 9–35 m below the detachment have ∼9%–10% calcite ± barite veins, and a marble mylonite sample ∼2.5 m below the detachment has ∼19% calcite + barite veins (Fig. 6). The concentration of fluid flow along brittle detachment fault systems in the region is obvious from well-documented detachment-hosted mineralization (e.g., Wilkins and Heidrick, 1982; Spencer and Welty, 1986; Kerrich, 1988), but the abundance of recrystallized veins in the metasedimentary rocks near the detachment fault indicates that fluid flow was also widespread during at least the late stages of footwall mylonitization. Hydrolytic weakening associated with this fluid flow could have contributed to relatively low temperature plasticity in the marble and calc-silicate mylonites (e.g., Liu et al., 2002). Fluids also played an important role in deformation of Swansea Plutonic Suite granitoid sills within the metasedimentary mylonite section. Where present, the early Miocene Swansea Plutonic suite is mylonitic to protomylonitic across most of the footwall, typically recording 16%–40% feldspar recrystallization (Singleton and Mosher, 2012). Swansea sills are present within the crystalline rocks near Battleship Peak, where they are typically only weakly strained and record little to no feldspar recrystallization. Within the metasedimentary section, however, the sills are ultramylonitic and not obviously distinguished from mylonitic quartzite. We determined a U-Pb zircon crystallization age of 22.2 ± 0.2 Ma for an ultramylonitic sill ∼37 m below the detachment fault near Battleship Peak (Fig. 6; Table DR6). Most feldspar porphyroclasts in this sample have broken down to a very fine grained mixture of recrystallized feldspar, quartz, and white mica, and relic porphyroclasts are intensely sericitized (Fig. 9C). These observations suggest that reaction softening due to fluid-driven chemical breakdown of feldspar played an important role in the penetrative deformation of relatively strong granitoid sills within rheologically weaker metasedimentary rocks.
Penetrative Deformation in Marble Mylonite along the Buckskin Detachment Fault
In addition to our detailed studies of mylonites near Battleship Peak, we investigated microstructures in metasedimentary mylonites along the Buckskin detachment fault from several other locations across the core complex (Figs. 2 and 3). These metasedimentary rocks are dominated by calcite marble and calc-silicate marble that record a top-NE sense of shear parallel to the detachment fault slip direction (Fig. 3). The marble mylonite layers locally have clasts of unstrained detrital quartz surrounded by recrystallized calcite matrix (Fig. 9D), and they consistently record much less brittle deformation than interlayered quartzite, highlighting the significant rheological difference between these units. As at the Battleship Peak area, mean dynamically recrystallized calcite grain size in marble mylonite <70 m below the detachment fault is <28 µm, and grain sizes generally decrease toward the detachment (Fig. 10). Recrystallized calcite grain sizes <10 m below the detachment are ≤13 µm, and are locally as small as 4–5 µm within 0.5 m of the detachment (Fig. 10). Calcite CPOs in these marble mylonite samples are also similar to those at Battleship Peak, with c-axes oriented at a high angle to the foliation and a-axes distributed along a great circle at a low angle to foliation (Fig. 11). The samples with the strongest calcite CPO are from 0.5 to 1 m below the detachment fault (14–5-274, 14–5-289, AL-26), with minimum c-axis eigenvalues between 0.11 and 0.14 (Fig. 11). The lack of cataclastic deformation and the presence of a strong CPO in footwall rocks so close to this major brittle fault confirm that nearly all strain in the marble footwall was accommodated by dislocation creep. Two marble samples collected from immediately below the detachment principal slip plane also have a calcite CPO. Sample 3–137 is a sliver of marble mylonite interlayered within siliceous ultracataclasite ∼0.2 m below the principal slip plane in the western Buckskin Mountains. Sample EM-188 is from the top of an ∼0.25-m-thick zone of marble mylonite along the detachment fault in the southern Lincoln Ranch basin (discussed in Distribution and Characteristics; Fig. 3F). Both of these samples are fractured and contain numerous generations of veinlets, but they preserve a relic S-C fabric and a moderately strong calcite CPO (e3 = 0.21–0.23; Fig. 9E). These data suggest that the majority of detachment fault slip occurred at conditions in which footwall temperatures were above the calcite brittle-plastic transition.
Pervasive dynamic recrystallization and the presence of calcite CPOs in all of the marble samples indicate that dislocation creep was the dominant deformation mechanism in these rocks. However, three marble samples from the western and central Buckskin Mountains also have stylolitic seams, indicating that pressure solution was locally important. In all of these samples stylolitic seams and synkinematic calcite veins show mutually crosscutting relationships. For example, in one sample from the Pride Mine area (14–5-274) the following deformation sequence is preserved at a centimeter scale: (1) formation of calcite veinlets, which are dynamically recrystallized; (2) development of foliation-parallel pressure solution seams; (3) formation of calcite veinlets, which are twinned; and (4) development of foliation-parallel pressure solution seams associated with a top-NE sense of shear (Fig. 9F). Multiple generations of veins with recrystallized or twinned calcite are also present in most marble mylonite samples lacking evidence for pressure solution.
Rheology of the Metasedimentary Mylonite Footwall Shear Zone
The concentration of penetrative strain within metasedimentary rocks below the Buckskin detachment fault across ∼25%–35% of the core complex indicate that these rocks exerted significant control on the rheology of the footwall shear zone. To understand the rheology of this shear zone, we modeled quartzite and calcite marble flow laws based on constraints from microstructural data. Frictional slip strength profiles were created at µ = 0.85 (Byerlee, 1978), µ = 0.60, and µ = 0.40, assuming a vertical σ1 from an average overburden of 2650 kg/m3 and hydrostatic fluid pressure (λ = 0.38). We modeled quartzite dislocation creep using the Hirth et al. (2001) flow law, assuming a surface temperature of 15 °C and a synextensional geothermal gradient of ∼40 °C (e.g., Foster et al., 1991; John and Foster, 1993). We used a water fugacity of 13.8 MPa near the brittle-plastic transition assuming a temperature of 335 °C (approximate recrystallization temperature of quartzite sample BP-194 from Ti-in-quartz analysis) and hydrostatic fluid pressure at 8 km (calculated using T. Withers’ fugacity calculator, www.esci.umn.edu/people/researchers/withe012/fugacity.htm). Modeling calcite dislocation creep is more difficult, as no single experimental flow law or power law creep equation accurately predicts the relatively large range of temperature and stress conditions recorded in natural shear zones (De Bresser et al., 2002; Renner et al., 2002). We used the dislocation creep flow law of Renner et al. (2002), which includes a Peierls stress relationship describing resistance to glide and a Hall-Petch relationship describing the strength decrease with increasing grain size. The Renner et al. (2002) flow law appears to be well suited for low-temperature shear zones dominated by calcite dislocation creep (Rogowitz et al., 2014). Figure 14 shows the composite strength profile from these models.
To evaluate the metasedimentary mylonite samples within the modeled strength profiles, we estimated differential stresses in the metasedimentary mylonites using empirically determined piezometric relationships. For quartz, we use the popular paleopiezometer of Stipp and Tullis (2003) with the Griggs apparatus molten salt assembly correction of Holyoke and Kronenberg (2010). Paleopiezometers for recrystallized calcite vary considerably and must be applied cautiously. For example, for a calcite grain size of 10 µm the Schmid et al. (1980) piezometer gives a differential stress of 47 MPa, whereas the Barnhoorn et al. (2004) piezometer gives a differential stress of 82 MPa, and the Rutter (1995) piezometer for rotation recrystallization gives a differential stress of 107 MPa. All three of these piezometers are based on deformation experiments on the Carrara marble, but the Schmid et al. (1980) and Rutter (1995) experiments did not achieve high enough strains to result in steady-state deformation behavior. Herwegh et al. (2005) noted that the Rutter (1995) piezometer may overestimate stress. Applied to the metasedimentary mylonites in the Buckskin-Rawhide footwall shear zone, the Rutter (1995) piezometer gives calcite marble stress values equal to or higher than those for the interlayered quartzite in 4 out of 5 cases, which is inconsistent with clear evidence that calcite marbles were weaker than the quartzite (Table DR4). We applied the Barnhoorn et al. (2004) calcite piezometer to marble mylonite samples because: (1) it yields stresses that are most consistent with stresses inferred for interlayered quartzite, and (2) the Barnhoorn et al. (2004) piezometer is based on experiments in which very large shear strains were accommodated by calcite dislocation creep, similar to the marble mylonites in the Buckskin-Rawhide footwall.
In the Battleship Peak area quartzite sample BP-194 underwent recrystallization close to the brittle-plastic transition at ∼342 ± 22 °C and ∼94 +24/–15 MPa (grain size 8 ± 2 µm). These constraints suggest strain rates between 10−13 s−1 and 10−14 s−1, consistent with inferred strain rates near the brittle-plastic transition in the adjacent Whipple Mountains metamorphic core complex (Behr and Platt, 2011; Fig. 14). Based on an ∼7 µm recrystallized quartz grain size associated with the brittle-plastic transition (see discussion Role of Fluids in Penetrative Deformation in the Battleship Peak Area), we infer that the peak strength of the quartzite was ∼105 MPa. The quartzite mylonite data from the Battleship Peak area are consistent with an average upper crust friction µ ≈ 0.85 (Byerlee’s Law; Fig. 14). The average recrystallized calcite grain size from all samples <10 m from the Buckskin detachment fault is ∼9 µm, suggesting that peak stress in the marble mylonite was ∼90 MPa. We do not have constraints on deformation temperature, but compilations of recrystallized calcite grain size versus temperature in natural shear zones indicate that a grain size of 9 µm typically correlates with recrystallization temperatures ∼300 °C (De Bresser et al., 2002; Ebert et al., 2008). A marble brittle-plastic transition corresponding to ∼90 MPa and 300 °C would correspond to a strain rate of 10−11 s−1 (using the Renner et al., 2002, flow law and µ ≈ 0.85; Fig. 14). These results are consistent with our observations that marble layers were sheared at temperatures below the brittle-plastic transition in rheologically stronger quartzite layers. This stress profile modeling predicts that when quartzite layers were being sheared at a peak stress of ∼105 MPa at the brittle-plastic transition, marble layers at the same depth and temperature could have been strained ∼100× faster at ∼20 MPa (Fig. 14). At relatively high strain rates of 10−12 s−1 the marble mylonites were capable of being sheared at ∼83 MPa at 275 °C, whereas at 10−12 s−1 the quartzite intervals would have behaved brittlely below ∼410 °C and µ ≈ 0.85. These results underscore the relative weakness of calcite marble mylonite in this shear zone and highlight the important role that marble played in controlling the rheological behavior of the detachment fault system.
Despite representing a volumetrically minor component of the Buckskin detachment fault footwall, calcite-rich metasedimentary mylonites are widespread along the detachment system and appear to have absorbed a disproportionally large amount of penetrative strain compared to the crystalline rocks that dominate the footwall. Along the northwest flank of the Ives Peak corrugation, calc-mylonites are continuously present for ∼30 km in the Miocene shear direction, forming a prominent weak zone. In addition, a thinner package of calc-mylonites is most likely present along the entire northwest flank of the Planet Peak corrugation (Fig. 7). We interpret these metasedimentary rocks to have been derived from Paleozoic strata (including locally overturned Pennsylvanian–Permian units) that were buried beneath crystalline rocks to mid-crustal depths along southeast-vergent Cretaceous thrust faults, similar to relationships exposed in structurally higher thrusts in the region (e.g., Reynolds and Spencer, 1989). The parallelism between metasedimentary layering and the Buckskin detachment fault along the flanks of these corrugations is consistent with the interpretation that the detachment fault shear zone directly reactivated this thrust fault system. If the detachment shear zone originally transected these packages of metasedimentary rocks and rotated layering into parallelism, the sections should be progressively attenuated and eventually cut out toward the northeast. However, the metasedimentary section along the Ives Peak corrugation has a relatively uniform ∼50–100 m thickness across a distance of ∼20 km in the shear direction (Fig. 6). Regardless of whether our interpretation of a Cretaceous southeast-vergent thrust system is correct, the geometric relationships documented in this study support the idea that the detachment fault shear zone initiated parallel to the metasedimentary rocks, and it is clear that detachment fault strain was localized within the zone of calc-mylonites along the northwest flank of the Ives Peak corrugation. The attenuation and rotation of the southwest-dipping metasedimentary section near Battleship Peak into parallelism with the detachment fault indicates that locally the detachment fault transected the metasedimentary rocks as it initially penetrated the ductile middle crust (Fig. 4A). We suggest that the southwest-dipping package of metasedimentary rocks southwest of Battleship Peak represents a different Cretaceous structural domain than the metasedimentary rocks along the northwest flanks of the corrugations.
In several parts of the core complex, thin (<1 to 8 m thick) zones of calc-mylonite are present along the detachment fault, separating variably strained crystalline rocks below from unstrained hanging-wall rocks above. In the southern Lincoln Ranch basin area, calc-mylonites are present along the detachment fault even where the metasedimentary section appears to have been cut out by incisement (discussed in Distribution and Characteristics). These relationships suggest that packages of calcite-rich rocks were smeared along the detachment system, creating a thin footwall carapace of rheologically weak rocks. Figure 15 illustrates how such smearing could have been accommodated during detachment fault incisement. During incisement a detachment fault system bites into the footwall to create a more favorable fault geometry (e.g., Lister and Davis, 1989). Where a large rheological contrast exists within the footwall, such as calcite-rich metasedimentary rocks overlying quartzofeldspathic crystalline rocks, the incisement strand could form a ramp-flat geometry, with the flat portions (parallel to layering) composing ductile shear zones in the metasedimentary rock and the ramps cutting through brittle crystalline rocks (Fig. 15A). As slip on this system progresses at temperatures above the calcite brittle-plastic transition, the metasedimentary rocks become smeared out along the ramps, resulting in the juxtaposition of thin calc-mylonite zones between crystalline footwall rocks and unstrained upper plate rocks (Fig. 15B). In this process, volumetrically minor amounts of metasedimentary rocks can become laterally extensive along the detachment fault, and thus may play a significant role in forming a weak detachment zone. In the case of the Buckskin-Rawhide core complex, this process enabled the detachment system to remain weak during the later stages of development, even though incisement favored removal of the rheological weak zone along the active detachment fault.
Regardless of how calc-mylonites originated at the top of the footwall, once present they dominated the rheology of the detachment fault and influenced the style of deformation. The lack of brecciation in the calcite marble mylonites and the preservation of a strong CPO within 1 m of the detachment fault suggest that in these areas more extensional exhumation was accommodated by crystal plastic flow of calcite than by frictional slip along a brittle detachment surface. At temperatures below the brittle-plastic transition for quartzite and granitoids (∼300–400 °C), dislocation creep in calcite would have accommodated top-NE–directed simple shear at significantly lower stresses than frictional slip (Fig. 14). Based on calcite flow laws, strain rates would have been significantly higher in these calcite marble–lined portions of the detachment fault compared to strain rates in quartz-rich lithologies. The structural architecture of the footwall also appears to be influenced by the presence of metasedimentary rocks along the detachment fault. In areas where metasedimentary mylonites are absent, the crystalline footwall underwent a significant amount of brittle deformation. For example, across most of the Clara Peak footwall corrugation, the Swansea Plutonic Suite is cut by northeast-dipping normal faults that have back-tilted mylonitic fabrics to southwest dips (Singleton, 2013b, 2015). The average discordance between these mylonitic fabrics and the subhorizontal detachment fault is ∼15°, and the total amount of northeast-directed postmylonitic extension within the Swansea Plutonic Suite is likely ∼20%–30% on average (Singleton, 2015). Many of the postmylonitic normal faults contain epidote and chlorite mineralization, suggesting that they were active at temperatures in which the calc-mylonites were capable of being ductilely sheared. By contrast, in areas where the detachment fault is lined by calcite-rich metasedimentary rocks, most footwall mylonitic fabrics parallel the detachment fault, and postmylonitic normal faults are far less common (Fig. 4; Singleton et al., 2014b). These observations are consistent with the interpretation that the calcite-rich metasedimentary rocks absorbed footwall strain via crystal plastic flow while the crystalline footwall in areas absent of a calc-mylonite carapace absorbed strain via brittle normal faults and associated horizontal axis rotation.
The data and observations presented in this study highlight the significant role that the rheologically weak metasedimentary rocks played in the structural evolution of the Buckskin detachment fault. It is also important to highlight that quartzofeldspathic crystalline rocks are present along 65%–75% of the detachment fault, so the metasedimentary rocks alone cannot be responsible for the general location and overall configuration of the Buckskin-Rawhide core complex. However, taking into account previous studies of footwall deformation, it appears that the vast majority of penetrative footwall deformation was structurally or rheologically controlled. Within the quartzofeldspathic footwall, mylonitic deformation occurred primarily within the early Miocene Swansea Plutonic Suite (e.g., Bryant, 1995; Singleton and Mosher, 2012) or within layered gneisses that underwent latest Cretaceous to early Paleogene top-NE–directed mylonitization (Wong et al., 2013; Singleton and Wong, 2016). Thus, the footwall shear zone across most of the core complex was either localized in synkinematic plutons and their heated wall rocks (∼25% of the footwall), preexisting shear zones with anisotropies that were well oriented for Miocene shear, or rheologically weak metasedimentary rocks (25%–35% of the footwall). Given the pre–detachment fault tectonic complexity that characterizes many metamorphic core complexes, future studies of footwall deformation will likely provide additional evidence for structural and/or rheological controls on mid-crustal strain.
The Buckskin-Rawhide metamorphic core complex is dominated by mylonitic crystalline rocks, yet metasedimentary mylonites were present along 25%–35% of the Buckskin detachment fault when it was active. Measured sections indicate that 72%–93% of these mylonites are calcareous. Along the northwest flank of the Ives Peak footwall corrugation (and most likely along the northwest flank of the Planet Peak footwall corrugation) these mylonites were laterally continuous for 30 km in the slip direction. Based on this distribution and evidence for an overturned section near Battleship Peak, we propose that the metasedimentary rocks were buried to mid-crustal depths by a southeast-vergent Cretaceous thrust system that was reactivated by the Miocene Buckskin detachment fault shear zone. Top-NE–directed penetrative shear associated with this detachment fault system was localized in the rheologically weak metasedimentary rocks. Locally calcite marble was smeared along the detachment fault during incisement, forming thin intervals of marble mylonite between crystalline footwall rocks and brittlely deformed hanging-wall rocks. Marble <1 m below the detachment fault preserves coherent mylonitic fabrics and strong calcite crystallographic preferred orientations, whereas interlayered quartz-rich rocks have undergone pervasive cataclasis, indicating that a significant amount of detachment fault strain was accommodated at temperatures between the lower limits of dislocation creep in calcite and quartz (∼200–300 °C). In areas where the detachment fault is lined with calc-mylonites, brittle extension via postmylonitic normal faulting is rare, and mylonitic fabrics generally parallel the detachment fault. Marble mylonites have dynamically recrystallized calcite grain sizes ranging from ∼4 to 28 µm, and grain size generally decreases toward the detachment fault. Synmylonitic fluid flow was significant in the metasedimentary section, contributing to the weak rheology of these rocks. Paleopiezometery and rheological modeling of the metasedimentary mylonites suggest that near the brittle-plastic transition quartzite was sheared at ∼100–110 MPa and 10−13 to 10−14 s−1, whereas calcite marble was sheared at ∼80–90 MPa and 10−11 to 10−12 s−1. These data and observations demonstrate that volumetrically minor calc-mylonites played an important role in controlling the location, geometry, rheology, and style of deformation associated with the Buckskin detachment fault system.
This research project was funded by National Science Foundation Tectonics Program award 1557265 to J. Singleton and M. Wong and U.S. Geological Survey EdMap Program award G13AC00232 to Singleton. We thank Michael Wells and Paris Xypolias for helpful review comments that improved this paper. We also thank Colgate student Austin Sun for his help collecting EBSD data. This research has benefited from discussions with Steve Reynolds and Jon Spencer.