The relationship between microstructure and fluid flow traced by hydrogen isotope ratios (δD) is examined within the Wildhorse detachment system of the Pioneer metamorphic core complex in south-central Idaho. Within the detachment footwall, 100-m-thick mylonitic quartzite containing minor white mica and K-feldspar displays a NW-trending stretching lineation and consistent top-to-the-NW sense of shear criteria. Microstructures within the detachment footwall comprise two groups: quartz ribbons and relict quartz grains flattened within the foliation, with porphyroclastic white mica fish; and intensely deformed and recrystallized quartz with high-aspect-ratio white mica arranged within C′ shear bands. White mica δD values are highly negative and cluster around −145‰ in high-aspect-ratio white mica and around −120‰ in porphyroclastic white mica fish. The most negative values are interpreted to reflect interaction with meteoric fluids from a high-elevation catchment (3000–4000 m), and the less negative values are interpreted to represent incomplete hydrogen isotope exchange between the meteoric fluid and the pre-extensional metamorphic fluid δD values in the white mica porphyroclasts. A suite of tightly clustered 40Ar/39Ar ages from synkinematic white mica in the detachment footwall dates deformation, recrystallization, fluid-rock interaction, and therefore the presence of high topography at 38–37 Ma; these ages are consistent with the cooling/exhumation history of the high-grade core of the Pioneer metamorphic core complex in the late Eocene. The 38–37 Ma 40Ar/39Ar ages are substantially younger than previously published ages of high topography in British Columbia to the north (49–47 Ma), in line with the hypothesis that high topography propagated from north to south in the northern segment of the North American Cordillera through Eocene time.
During crustal-scale extension and metamorphic core complex formation, low-angle detachment faults facilitate exhumation of mid- to lower-crust rocks (e.g., Coney, 1980; Armstrong, 1982; Coney and Harms, 1984; Davis and Lister, 1988; Rey et al., 2009b, Whitney et al., 2013). Within the mylonitic footwall of these detachments, deformation is localized into shear zones that form a thermal boundary between the cool, upper crust and the hot mid- to lower crust (e.g., Rey et al., 2009b). Frequently, such mylonite zones are kinematically linked to a system of extensional brittle faults that collectively maintain high porosity and permeability within the upper crust. Cool surface temperatures and heat input at depth generated by the exhumation of high-grade metamorphic rocks produce high heat flow in the shallow crust that can drive convective fluid flow, with potential fluid circulation between the mylonitic footwall and Earth’s surface (e.g., Mulch et al., 2004, 2006; Person et al., 2007; Campani et al., 2012; Gébelin et al., 2012, 2015; Gottardi et al., 2013).
Extensional detachments are favored fluid pathways within extending crust, owing to deformation-induced porosity and permeability. Stable isotope studies of hydrous minerals, such as white mica, have shown that fluids within detachment systems can be derived from numerous sources, including meteoric, magmatic, or metamorphic origins (e.g., Wickham and Taylor, 1987; Mulch et al., 2004; Holk and Taylor, 2007). Studies focused on mylonitic quartzite in such extensional detachment footwalls have documented recrystallized and neocrystallized white mica that equilibrated with meteoric fluids (e.g., Famin et al., 2004; Mulch et al., 2004, 2007; Gébelin et al., 2011, 2012, 2013; Gottardi et al., 2011; Gottardi and Teyssier, 2013; Campani et al., 2012; Quilichini et al., 2015). Dating using 40Ar/39Ar geochronology in conjunction with stable isotope studies of hydrous phases allows for determination of the timing of convective fluid flow and movement along the detachment system (e.g., Mulch et al., 2004, 2005, 2006, 2007).
Hydrogen isotopes, measured by their deuterium content (δD), are a good tracer of fluid sources in the crust (e.g., Wickham and Taylor, 1987; Fricke et al., 1992; Mulch et al., 2004, 2006; Person et al., 2007). The δD value of meteoric water varies with latitude and elevation and is also controlled by the presence of orographic barriers. Generally, higher latitudes and higher elevations correspond to more negative δD values (Dansgaard, 1964; Poage and Chamberlain, 2001). Present-day precipitation δD values from high elevations in the western North American Cordillera range from −90‰ to −150‰ (Kharaka and Thordsen, 1992), whereas magmatic and metamorphic fluid δD values are generally less negative, ranging from −20‰ to −80‰ (Hoefs, 1987).
In this paper, we use a combination of microstructural, hydrogen isotopic (δD), and 40Ar/39Ar geochronological data to evaluate the relationship between quartz and white mica microstructure and fluid flow and to determine the timing of microstructure development and fluid-rock interaction in a mylonitic quartzite within the footwall of the Wildhorse detachment in the Pioneer metamorphic core complex (Fig. 1). The Pioneer metamorphic core complex is an important area in which to resolve these relationships because it represents a transition in terms of timing of metamorphic core complex–style extension and geographic position in the Northern Cordillera. Our new 40Ar/39Ar and δD results from synkinematic hydrous minerals of the Wildhorse detachment footwall complement a significant body of paleoaltimetry studies that record proxy mineral (muscovite, biotite, amphibole) δD values ranging from −80‰ to −160‰, suggesting high topographic elevation during the initiation of metamorphic core complexes in the North American Cordillera for the middle to late Eocene (Fricke and O’Neil, 1999; Mulch et al., 2004, 2006, 2007; Gébelin et al., 2011, 2012, 2015; Gottardi et al., 2011; Gottardi et al., 2013; see also Chamberlain et al., 2012). The Pioneer metamorphic core complex δD data are consistent with other studies suggesting that high topography in the Cordillera was long-standing during the early to middle Cenozoic (e.g., Wolfe et al., 1998; Mulch et al., 2007; Best et al., 2009, 2013; Mix et al., 2011; Chamberlain et al., 2012; Cassel et al., 2014).
The North American Cordillera experienced orogenic thickening owing to collision and terrane accretion associated with subduction of the Farallon plate during Mesozoic to early Cenozoic time (e.g., Coney and Harms, 1984; DeCelles, 2004). Immediately after thickening, extension and partial melting of the mid- to lower crust are recorded in a discontinuous chain of Eocene metamorphic core complexes situated in the hinterland of the western North American Cordillera (Armstrong, 1982; Vanderhaeghe et al., 1999; Foster et al., 2001, 2007; Teyssier et al., 2005; Gordon et al., 2008; Kruckenberg et al., 2008; Whitney et al., 2013). The localization of extension in a few metamorphic core complex–style, rolling-hinge detachment systems that accommodated tens of kilometers of motion is attributed to the presence of low-viscosity lower crust that was likely partially molten (Teyssier et al., 2005). Decoupling between lower-crustal flow and upper-crustal brittle extension favored the generation of metamorphic core complex–style detachment systems and their continued localization as the deep crust was progressively exhumed (e.g., Armstrong, 1982; Constenius, 1996; Teyssier et al., 2005; Whitney et al., 2013).
Subsequent to thickening, rollback of the Farallon slab led to extension (Bendick and Baldwin, 2009) and voluminous magmatism, represented by the Challis-Absaroka-Kamloops volcanics that erupted at ca. 55–25 Ma (Best et al., 2009, 2013; Humphreys, 2009). In Idaho, the Challis volcanics erupted from ca. 50 to 45 Ma, and NW-SE Eocene extension is recorded in the upper crust and the exhumed mid- to lower crust (Bennett, 1986; Janecke, 1992; Janecke et al., 1997; Vogl et al., 2012).
From north to south, the Cordilleran metamorphic core complexes record a general decrease in age of extension, magnitude of exhumation, and influence exerted by partially molten crust (as represented by migmatite domes), leading to the definition of northern, central, and southern belts (Fig. 1; e.g., Crittenden et al., 1980; Armstrong, 1982; Vanderhaeghe and Teyssier, 2001; Rey et al., 2009a, 2009b; Vogl et al., 2012; Whitney et al., 2013). The northernmost metamorphic core complexes include the Shuswap, Valhalla, Okanogan, Kettle, and Priest River metamorphic core complexes, characterized by voluminous migmatite domes that represent former partially molten crust. The northern metamorphic core complexes preserve Paleocene–Eocene crystallization of partially molten crust (U-Pb zircon and monazite ages 55–50 Ma) that immediately followed exhumation-related cooling as documented by 40Ar/39Ar mica (49–47 Ma) and apatite fission-track ages (Ewing, 1980; Vanderhaeghe et al., 1999; Foster et al., 2001, 2007; Teyssier et al., 2005; Hinchey et al., 2006; Mulch et al., 2006, 2007; Gordon et al., 2008; Kruckenberg et al., 2008; Toraman et al., 2014,;Quilichini et al., 2015), recording fast cooling and exhumation to within 1–2 km of the Eocene land surface only a few million years after high-temperature crystallization (Toraman et al., 2014).
The Bitterroot, Anaconda, and Pioneer metamorphic core complexes form a transitional region between the northern and central Cordilleran belts based on geographic position, and they document a more protracted cooling and exhumation history than the northern belt. The western portions of the Bitterroot and Anaconda metamorphic core complexes cooled below 300–350 °C by 47 Ma, whereas the eastern portions did not cool below 300–350 °C until ca. 40 and 38 Ma, respectively (Foster et al., 2001, 2007, 2010). The gap between high-temperature metamorphism and cooling is more pronounced in the Pioneer metamorphic core complex. Exhumation and associated cooling of the high-grade core below 300–350 °C and subsequent exhumation did not occur until 38–35 Ma, a full 10 m.y. after cessation of crystallization of partially molten crust (Vogl et al., 2012). This gap in time may be due to decreasing exhumation rates moving southward toward the Pioneer metamorphic core complex (Vogl et al., 2012; Whitney et al., 2013). This places the Pioneer metamorphic core complex in an important geographic and temporal position at the junction of the northern and central belts of metamorphic core complexes.
The central belt of metamorphic core complexes includes the Raft River–Albion–Grouse Creek metamorphic core complex (Utah), located just south of the Snake River Plain, and the Ruby Mountain–East Humboldt and Snake Range metamorphic core complexes farther south in Nevada; these metamorphic core complexes display Eocene to Miocene extension (e.g., McGrew and Snee, 1994; MacCready et al., 1997; Miller et al., 1999; Sullivan and Snoke, 2007; Wells et al., 2000; Colgan et al., 2010; Gans et al., 2011; Konstantinou et al., 2012, 2013; Methner et al., 2015). The southern belt of metamorphic core complexes, from Arizona to Mexico, records significant Miocene extension, lower magnitudes of exhumation, and small amounts of partially molten crust (e.g., Foster et al., 1993; Scott et al., 1998).
PIONEER METAMORPHIC CORE COMPLEX
Development of the Pioneer metamorphic core complex initiated during the Eocene owing to midcrustal extension and emplacement of magma and partially molten rocks (Vogl et al., 2012). Footwall rocks of the Pioneer metamorphic core complex were exhumed along the extensional Wildhorse detachment in late Eocene time (Silverberg, 1990). The Wildhorse detachment system, which includes a N-dipping segment and a W-dipping segment, separates Paleozoic low-grade metasediments unconformably overlain by Eocene Challis volcanics from underlying metamorphic and plutonic footwall rocks (Fig. 2; Wust, 1986; O’Neill and Pavlis, 1988; Silverberg, 1990). The deepest footwall rocks, the Wildhorse gneiss complex, comprise a core of Archean to Proterozoic gneisses that experienced amphibolite-facies metamorphism and partial melting in the Eocene; these rocks were intruded and transected by granitic plutons, including the Pioneer intrusive suite, at ca. 50–47 Ma (Vogl et al., 2012). The Wildhorse gneiss complex forms two NW-trending domes, the larger Wildhorse Dome and the smaller Kane Creek Dome (Fig. 2). The Wildhorse Dome is E-verging with a subvertical to overturned eastern limb. In the NW portion of the Wildhorse gneiss complex, stretching lineations trend predominantly NW, which is consistent with stretching lineations in the mylonitic footwall of the Wildhorse detachment. Ordovician metasedimentary units that reached amphibolite-facies metamorphism in the Eocene, and possibly in the Cretaceous, overlie the Wildhorse gneiss complex in the southwestern portion of the Pioneer metamorphic core complex footwall (Wust, 1986; O’Neill and Pavlis, 1988; Silverberg, 1990; Vogl et al., 2012). The metasediments consist of quartzite, calc-silicate, and marble that have been substantially folded and sheared. Folded and crosscutting granitic dikes indicate that many of the preserved deformation structures in the metasediments are Eocene in age (Vogl et al., 2012).
The footwall of the Wildhorse detachment consists of a 200–500-m-thick mylonitic section of intensely sheared quartzite, calc-silicate, marble, granodiorite, and migmatite (Fig. 3; O’Neill and Pavlis, 1988; Vogl et al., 2012). The mylonitic shear zone is transected by steeply dipping (∼40°–60°) normal faults and overprinted by minor chloritic brittle shears. Along the northern segment of the Wildhorse detachment footwall, the mylonitic footwall rocks display moderately dipping SW-NE–striking foliations and NW-trending, shallowly plunging lineations. In the NW corner of the Wildhorse detachment footwall, foliations within the Kinnikinik Quartzite strike WSW-ENE with dips ranging from 25° to 45°, and stretching lineations, defined by stretched quartz and white mica, trend NW and plunge 10°–20° (Fig. 2).
U-Pb sensitive high-resolution ion microprobe (SHRIMP) and laser-ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) analyses of zircon from migmatites of the Wildhorse gneiss complex and plutonic rocks of the Pioneer intrusive suite yield crystallization ages ranging from ca. 52 to 46 Ma (Vogl et al., 2012). The migmatites and plutonic rocks preserve NW-trending mineral stretching lineations that record high-temperature extensional strain. In the metasedimentary sequence, an isoclinally folded felsic dike and a crosscutting undeformed felsic dike yield U-Pb zircon ages of ca. 48–47 and 46 Ma, respectively (Vogl et al., 2012). The U-Pb crystallization age of the undeformed, crosscutting dike and the crystallization age range for migmatite and plutonic rocks bracket high-temperature extensional strain between ca. 52 and 46 Ma. Hornblende 40Ar/39Ar ages record subsequent cooling below 500 °C between ca. 47 and 42 Ma. Within the mylonitic footwall of the Wildhorse detachment and the core of the Pioneer metamorphic core complex, white mica and biotite 40Ar/39Ar ages range from 38 to 35 Ma, and 40Ar/39Ar low-temperature 39Ar release steps in K-feldspar have ages of ca. 33 Ma (Silverberg, 1990). Collectively, these thermochronological data indicate that cooling from migmatite crystallization between ca. 52 and 46 Ma (U-Pb zircon) through the hornblende closure temperature was relatively slow. The core complex rocks remained above the closure temperature for argon in mica (300–400 °C) until rapid cooling of the core complex, likely related to extensional unroofing around 38–35 Ma; cooling below 175 °C prior to 33 Ma occurred rather rapidly. Apatite (U-Th)/He ages from the Pioneer Mountains record cooling below 70–80 °C between ca. 11 and 8 Ma. This period of cooling has been attributed to uplift and exhumation of the Pioneer Mountains during hotspot-related magmatism and flexure (Vogl et al., 2014).
Ten mylonitic Kinnikinik Quartzite samples were analyzed from a 100 m vertical transect within the footwall of the Wildhorse detachment (Fig. 3). The Kinnikinik Quartzite within the footwall of the Wildhorse detachment displays intense foliation and lineation defined by stretched quartz. Meter-scale C′ shear bands, isoclinal folds with hinge lines subparallel to the stretching lineation, and centimeter- to millimeter-scale asymmetric structures are present (Fig. 3B). The quartzite is composed of >90% quartz with minor amounts of white mica fish and K-feldspar porphyroclasts. Thin sections reveal white mica fish, sigma-type K-feldspar porphyroclasts, secondary oblique foliation, and S-C-C′ structures that consistently record top-to-the-NW shear sense (Fig. 4). Large, flattened, high-aspect-ratio quartz grains define the foliation, and small, recrystallized quartz grains define a secondary foliation oblique to the mylonitic foliation (Figs. 4A and 4B). Discontinuous C′ shear bands are oblique to the mylonitic foliation (Fig. 4B) and are defined by recrystallized, high-aspect-ratio white mica, recrystallized tails of white mica fish, and small, recrystallized quartz grains (Figs. 4B and 4C). Thin rutile needles are abundant within the large quartz grains and are commonly boudinaged. Recrystallized quartz grains commonly mantle the asymmetric K-feldspar porphyroclasts.
Within the mylonitic quartzite, we observe two types of quartz microstructure: Type 1 displays large, high-aspect-ratio, relict grains that are flattened within the foliation (Fig. 4A). Quartz grains range from 0.5 to 2.0 mm in length and have an approximate mean aspect ratio of 4:1; some quartz ribbons with considerably higher aspect ratios are locally preserved. Flattened grains show undulose extinction and visible subgrains. Newly recrystallized grains mantle the larger, deformed grains and are typically 50–150 µm in diameter (Fig. 4B); subgrains and recrystallized grains have a similar size, indicating that the quartzite dynamically recrystallized by dominant subgrain rotation with minor grain boundary bulging (Hirth and Tullis, 1992; Stipp et al., 2002).
Type 2 microstructures include intensely deformed and recrystallized quartz grains. The recrystallized grains range from 40 to 120 µm, have irregular grain boundaries, define a shape-preferred orientation (SPO) oblique to the mylonitic foliation, and display a strong crystallographic-preferred orientation (CPO; Malekpour, 2012). Recrystallized grains display sutured and weakly lobate grain boundaries (Figs. 4A and 4B). These microstructures are indicative of dynamic recrystallization by subgrain rotation with a contribution of grain boundary bulging (Hirth and Tullis, 1992; Stipp et al., 2002). Oblique foliation and S-C-C′ structures are prevalent within the quartzite and systematically indicate top-to-the-NW shear sense. Small, recrystallized quartz grains occur within the C′ shear bands. Recrystallized quartz foliations are deflected adjacent to the large feldspar porphyroclasts and C′ shear bands (Fig. 4B).
White Mica Microstructure
White mica content in the mylonitic quartzite ranges from 1% to 10%. White mica is commonly concentrated in 50–100-μm-thick layers that define the mylonitic foliation in the shear zone. Thin white mica folia also define a secondary foliation (C′). Numerous relict white mica grains form fish-shaped clasts that indicate top-to-the-NW shear sense. Most white mica fish display lenticular shapes with recrystallized tails. Rare fish are isoclinally folded. The white mica grain boundaries are commonly fuzzy, and quartz inclusions occur in numerous white micas indicative of dissolution-precipitation processes (e.g., ten Grotenhuis et al., 2003; Vernon, 2004; Mulch et al., 2006).
White mica microstructures can be separated into two groups based on their aspect ratio, location, recrystallization, and deformation. Group 1 white mica grains are mostly smaller grains with high aspect ratios ranging from 5:1 to 20:1 that define the C and C′ fabric in the quartzite (Figs. 4C and 4E). The white mica grains are commonly in contact with K-feldspar porphyroclasts. Rare white mica fish in these samples display rhomboidal shapes with (001) cleavage planes parallel to the long side of the fish and the C′ foliation (cf. ten Grotenhuis et al., 2003). In the samples from this group, white mica comprises <5% of the thin section. Group 2 white mica fish are larger relict grains up to 1 mm in length with an average aspect ratio of 4:1. White mica comprises 5%–10% of the thin section in these samples. The fish are predominantly lenticular with tips inclined in the direction of foliation (Figs. 4D and 4F). The (001) planes are parallel to the secondary recrystallized quartz foliation. The relict white mica fish display tails with small, recrystallized grains. Rare white mica grains are isoclinally folded.
WHITE MICA CHEMISTRY
White Mica Mineral Chemistry
Electron microprobe analyses (EPMA) were used to assess the range of compositions for shear band and porphyroclastic white mica from four samples. WLM-1 and WLM-2 consist predominantly of shear band white mica, whereas WLM-7 and WLM-8 consist of predominantly porphyroclastic white mica fish. Mineral chemistry for each sample displays compositional variation with respect to Altot, Ti, and IVSi (Fig. 5). Most porphyroclastic white mica grains plot close to the muscovite end member, with a few exceptions in sample WLM-8. The shear band white mica grains also predominantly plot within the muscovite field, but grains have a broader range of Al/Si ratios, with numerous grains plotting in the phengite field. The Ti content for all analyzed white mica also displays substantial variability. Shear band white mica–dominated WLM-1 has the lowest Ti content, with values ranging up to 0.03 pfu, whereas sample WLM-2 has Ti content, ranging up to 0.13 pfu (Fig. 5). In addition, the porphyroclastic white mica–dominated samples, WLM-7 and WLM-8, have Ti contents ranging from 0.01 to 0.23. The white mica compositions fall within a similar range to white mica from quartzites in other Cordilleran detachment footwalls (e.g., Mulch et al., 2006; Gébelin et al., 2015; Quilichini et al., 2015).
White mica δD values were analyzed in 10 of the mylonitic quartzite samples over the 100 m vertical transect of the footwall of the Wildhorse detachment (Figs. 3C and 6; Table DR11; see Appendix for analytical methods). Synkinematic white mica from mylonitic quartzite forms two groups of δD values, both characterized by highly negative δD values. δD values in group 1 (samples WLM-1, 2, 4, 5, 9, and 10) range from −141‰ to −148‰, whereas δD values in group 2 (samples WLM-6, 7, 8, and 12) range from −119‰ to −127‰ (±3‰; all values with respect to Vienna standard mean ocean water [VSMOW]).
Four white mica separates from the transect across the mylonitic quartzite of the Wildhorse detachment footwall were analyzed by multigrain furnace step-heating 40Ar/39Ar geochronology (see Tables DR2 and DR3 [see footnote 1]). White mica samples yield relatively flat argon release spectra with minor complexity and exhibit minimal disturbance in the low-temperature heating steps (Fig. 7). Calculated plateau ages for all samples include >85% of total 39Ar released. Over ∼100 m of the mylonitic quartzite section, 40Ar/39Ar ages are very consistent. The 40Ar/39Ar plateau ages of white mica are 37.51 ± 0.12 Ma (WLM-12; 10 m below hanging wall), 37.38 ± 0.14 Ma (WLM-9; 30 m below hanging wall), 37.71 ± 0.16 Ma (WLM-8; 40 m below hanging wall), and 37.00 ± 0.09 Ma (WLM-2, 3; 90 m below hanging wall).
Fluid Flow and Microstructure
Hydrogen isotopic ratios in white mica within mylonitic quartzite are a robust tracer of fluid migration because white mica commonly undergoes syntectonic recrystallization at temperatures near the brittle-ductile transition and remains (meta-)stable during further exhumation. Highly negative δD values within white mica require hydrogen isotopic exchange on the lattice scale at times when meteoric fluids were able to exchange with the mylonitic quartzite in the ductilely deforming detachment footwall. White mica from the mylonitic quartzite of the Wildhorse detachment footwall typically have highly negative δD values ranging from −119‰ to −148‰ (Fig. 6). Such negative values are indicative of equilibration with meteoric water and suggest that at the time of deformation, the porosity and permeability structure of the extending crust permitted hydraulic connectivity between the mylonitic footwall of the Wildhorse detachment and Earth’s surface. Studies in other northern Cordillera metamorphic core complexes record highly negative δD values ranging from −80‰ to −160‰ (Fricke et al., 1992; Holk and Taylor, 1997; Mulch et al., 2004, 2006, 2007; Gottardi et al., 2011; Gébelin et al., 2011, 2015; Quilichini et al., 2015).
The two mica groups display distinct microstructures that likely correlate with the disparate isotopic values. The highly negative δD values are found in quartzite with small, high-aspect-ratio, shear band–dominated white mica (Figs. 4C and 4E); this quartzite has <5% white mica. The less negative values correspond to quartzite with mostly large, relict white mica that make up to 10% of the rock and form asymmetric mica fish (Figs. 4D and 4F). The two groups do not show significant differences in white mica chemistry with respect to Al or Si (Fig. 5). However, shear band white mica grains have generally lower Ti contents, whereas the porphyroclastic white mica grains display a wide range of Ti contents. In rocks with a Ti-saturating phase such as rutile, Ti concentration can be correlated with metamorphic grade, indicating the lower Ti concentrations may represent lower deformation temperatures (e.g., Guidotti, 1986; Mulch and Cosca, 2004). In the Shuswap metamorphic core complex, lower Ti concentrations and lower Si/Al ratios on muscovite tips were correlated with decreasing temperature during deformation (Mulch et al., 2006). However, the range of chemistry for porphyroclastic and shear band white mica from the Pioneer metamorphic core complex makes reaching a similar conclusion difficult.
We interpret different degrees of isotopic exchange between meteoric and metamorphic fluids within the white mica to have played a role in the different populations of δD values. The C′ structures may have acted as fluid pathways that enhanced hydrogen isotopic exchange and promoted the development of highly negative δD values within the recrystallizing muscovite during progressive deformation, whereas the relict porphyroclasts may have only experienced partial hydrogen isotope exchange with meteoric water. It is likely that the porphyroclastic white micas were not fully recrystallized and that the δD values are a mix of inherited, predeformation metamorphic δD and syntectonic meteoric δD. This would place the δD values on a mixing line between meteoric and metamorphic fluids.
The development of quartz microstructures may be an important component for fluid migration within the mylonitic footwall rocks (e.g., Gottardi et al. 2015; Quilichini et al., 2015). The highly negative δD samples display intensely deformed and small, recrystallized quartz grains (Fig. 4B). The less negative δD samples consist of quartz ribbons, large relict grains that are flattened in the foliation, and small recrystallized grains, except for WLM-7, which displays the same quartz microstructure as the highly negative δD samples (Fig. 4A). These microstructures represent dynamic recrystallization with some component of grain boundary bulging. The general correspondence between highly negative δD values and intensely deformed and recrystallized quartz grains indicates a possible relationship between quartz microstructure and fluid flow. Quartz microstructure may influence fluid flow by enhancing or hindering pathways for fluid migration. For example, grain-size reduction during dynamic recrystallization increases the total surface area of grain boundaries. Quartz grain boundaries can be efficient pathways for fluids to migrate through mylonitic rock, feeding meteoric water to mica grains and controlling their δD values. These results suggest that quartz and white mica microstructures, as well as isotopic compositions, are sensitive recorders of detachment-related fluid flow and that the shear band white mica may have interacted more strongly with meteoric water.
When considering different amounts of isotopic exchange with meteoric fluids, strain localization into meter-scale shear bands within the mylonitic quartzite may also provide insight into the different white mica isotopic groups. The highly negative mica δD values encountered in intensely deformed and recrystallized quartzite and shear band white mica occur as alternating bands with depth (Figs. 3 and 6). These bands may represent shear bands that would be capable of enhancing fluid flow (e.g., Selverstone et al., 1991; Fricke et al., 1992; Bauer et al., 2000), which could have led to complete isotopic exchange with meteoric water for the highly negative δD values in white mica.
Interpretation of 40Ar/39Ar Data
The new 40Ar/39Ar white mica ages we obtained from the Wildhorse detachment footwall cluster around ca. 38–37 Ma and may be interpreted as cooling ages or deformation-recrystallization ages like in other detachment shear zone systems in the North American metamorphic core complexes (e.g., Mulch et al., 2004, 2006). In addition, mica Ar ages throughout the Pioneer metamorphic core complex are also concentrated around 38–35 Ma, suggesting that Ar ages in both the mylonitic footwall of the Wildhorse detachment and the core of the complex record cooling. However, if detachment mylonites had formed much earlier than ca. 38 Ma and had remained at temperatures above the closure temperature for argon diffusion in white mica (ca. 400 °C; Harrison et al., 2009) for a significant period of time, we would expect the mylonitic quartz microstructures to have been thoroughly annealed. Instead, the quartz grains in the mylonite are characterized by undulatory extinction and deformation bands that indicate high dislocation density, limited recovery, and absence of annealing (Hirth and Tullis, 1992; Stipp et al., 2002).
Therefore, while the Ar ages obtained from the Wildhorse detachment mylonitic footwall may represent cooling ages, they must be very close to the age of deformation during activity on the detachment system. Extensional detachments are self-exhuming systems that cool rapidly, resulting in a close relation between deformation and cooling, possibly within the resolution of the method. In addition, activity on the detachment system is responsible for exhumation of the core complex, which explains why cooling ages obtained from the core of the system are similar to cooling-deformation ages within the mylonitic footwall of the detachment.
Eocene Paleotopography of the Pioneer Metamorphic Core Complex
Hydrothermal fluid circulation along extensional detachment systems is typically driven by basal heat sources beneath the detachment and the hydraulic head generated in high-relief regions (Person et al., 2007). Tracing such fluid flow through hydrogen isotope analyses of syntectonic and/or recrystallized white mica in Cordilleran metamorphic core complex detachment footwalls has documented that low δD fluids characterize a series of Cordilleran metamorphic core complexes, including: the Shuswap (Holk and Taylor, 1997; Mulch et al., 2004, 2006), Kettle Dome (Mulch et al., 2007; Quilichini et al., 2015), Bitterroot (Kerrich and Hyndman, 1986; Quilichini, 2012), Raft River (Gottardi et al., 2011; Methner et al., 2015), Ruby Mountains (Fricke et al., 1992), Snake Range (Gébelin et al., 2011, 2015), and Whipple Mountains (Morrison and Anderson, 1998; Gébelin et al., 2012) metamorphic core complexes. Stable isotope paleoaltimetry (e.g., Chamberlain et al., 1999; Garzione et al., 2000; Rowley et al., 2001; Mulch and Chamberlain, 2007; Mulch et al., 2004, 2008; Rowley and Currie, 2006; Rowley and Garzione, 2007; Garzione et al., 2008; Lechler et al., 2013; Saylor and Horton, 2014) provides one of the few methods that can quantitatively reconstruct the evolution of topography in the world’s largest mountain and plateau regions (Gébelin et al., 2013). Paleoaltimetry reconstructions are derived from systematic changes in the hydrogen or oxygen isotope ratios of meteoric water, which ultimately can be related to changes in paleotopography. Measured differences in δD (or δ18O) of precipitation between sea-level and high-elevation sites [Δ(δD)] estimate paleoelevation as changes in δD during orographic ascent of cloud systems along the windward fronts of orogens, which are primarily related to changes in landscape elevation (e.g., Dansgaard, 1964; Garzione et al., 2000; Poage and Chamberlain, 2001; Rowley et al., 2001). One of the advantages of determining δD values of meteoric water through the analysis of extensional detachment systems is that such stable isotope paleoaltimetry data can be directly linked to the tectonic evolution of extensional detachment systems. These, in turn, play an important role in controlling surface uplift and block tilting during regional extension and development of metamorphic core complexes (e.g., Mulch and Chamberlain, 2007).
The tectonic processes responsible for along-strike extension and topographic evolution of the Northern Cordillera in the Cenozoic remain an outstanding question that can in part be addressed through stable isotope paleoaltimetry studies. One possible scenario is that a long-standing high-elevation, low-relief Nevadaplano plateau began collapsing in the early Paleogene (DeCelles, 2004; Best et al., 2009). Another scenario is the development of high topography in the early Cenozoic due to an influx of asthenosphere related to slab rollback (Humphreys, 2009; Gébelin et al., 2012; Mix et al., 2011; Chamberlain et al., 2012; Smith et al., 2014). There is increasing evidence that the north-to-south development of metamorphic core complexes in the Northern Cordillera paralleled changes in regional topography (e.g., Mulch and Chamberlain, 2007; Mix et al., 2011; Chamberlain et al., 2012). It has been postulated that the southward encroachment of an Eocene plateau (SWEEP model of Chamberlain et al., 2012) occurred in concert with mantle-derived heat input, volcanism, and core complex–related extension at least for the northern and central belts of Cordilleran metamorphic core complexes. Independent of the relative roles of changes in lithospheric configuration, heat distribution, and detachment formation for paleotopography, compilations of stable isotope data from various proxy materials in western North America document that no later than ca. 40 Ma, strong isotope-in-precipitation gradients existed on the western and eastern sides of the Rocky Mountains and Great Basin regions (e.g., Mix et al., 2011; Chamberlain et al., 2012). The δD data of the Pioneer metamorphic core complex fit nicely into this context.
Assuming hydrogen isotope exchange and deformation temperatures commonly encountered in deformed quartzite exhumed through the brittle-ductile transition of detachment footwall shear zones (350–450 °C), the δD values of associated (meteoric) fluid can be calculated. Using the calibration of Suzuoki and Epstein (1976), the lowest δD white mica values observed in the Wildhorse detachment footwall are therefore consistent with meteoric fluids having δDfluid = –110‰ ± 10‰ (or δ18O = –15‰ ± 1.5‰ if meteoric water followed the global meteoric water line). Such δD (and δ18O) values are characteristic for large parts of the Eocene western North American Cordillera, yet data are sparse at the latitude of the Pioneer metamorphic core complex (Mix et al., 2011; Chamberlain et al., 2012). When combined with the 38–37 Ma 40Ar/39Ar Wildhorse detachment footwall syntectonic recrystallization ages, δDfluid values of −110‰ ± 10‰ lend further support to the existence of high-elevation topography that accompanied the development of metamorphic core complexes within the northern and central belts in the western North American Cordillera until the late Eocene. Without referencing to sea-level δDprecipitation, it is difficult to establish quantitative constraints on paleoaltimetry in continental interiors based on spatially isolated δDfluid values. However, when integrated in the context of large compilations of hydrogen and oxygen isotopes in precipitation over western North America (Mix et al., 2011; Chamberlain et al., 2012), the Pioneer metamorphic core complex δD data are consistent with the presence of extensive highlands on the order of 3000–4000 m along the Eocene atmospheric moisture trajectories that delivered precipitation to the Pioneer metamorphic core complex.
The relationships among microstructure, lithology, and hydrogen isotope compositions from the mylonitic footwall of the Wildhorse detachment highlight the kinematic link between the flow of surface fluids and the formation of extensional detachment systems. Our combined δD, 40Ar/39Ar, and microstructure data are consistent with isotopic studies in similar Cordilleran metamorphic core complex detachment footwalls, which suggest that the catchments feeding the near-surface fracture-controlled downward-percolation of meteoric fluids were characterized by low δD values of meteoric water and thus stood at high elevations during the late Eocene (38–37 Ma). We observe two groups of δD values from the quartzites of the Wildhorse detachment footwall that can be directly tied to the associated quartz and white mica microstructures in the detachment footwall. The more negative δD values for white mica in shear bands, with δD values around –145‰, represent equilibration with meteoric waters during detachment initiation and protracted extensional strain, whereas the less negative δD values around −120‰ for porphyroclastic white mica denote isotope exchange between meteoric fluids and metamorphic fluids. The correspondence of δD values and mylonitic microstructures suggests that zones with intense white mica recrystallization, deformed and recrystallized small quartz grains, and C′ structures may have been instrumental in supporting enhanced fluid flow within the actively deforming detachment footwall.
McFadden acknowledges a minigrant from Salem State University. Mulch acknowledges support through the LOEWE program of Hesse’s Ministry of Higher Education, Research, and the Arts. Teyssier acknowledges support from National Science Foundation grant EAR-0838541. The paper has greatly benefited from reviews by E. Cassel and J. Vogl and editorial comments by E. Kirby.
Hydrogen Isotope Analyses
The δD values of white mica were determined by continuous flow mass spectrometry using a high-temperature elemental analyzer (Thermo Finnigan TC/EA) coupled to an isotope ratio gas mass spectrometer (Delta V Advantage) at Leibniz University, Hannover, Germany. Three internationally referenced standard materials and additional in-house working standards were run with the samples. After correction of mass bias, daily drift of the thermal combustion reactor, and offset from the certified reference values, NBS30 (biotite), NBS22 (oil), CH7 (polyethylene foil) had δD = −66‰, −121‰, and −105‰, respectively. Repeated measurements of various standards and unknowns gave a precision of ±2‰ for δD. All isotopic ratios are reported relative to Vienna standard mean ocean water (VSMOW).