Reconstruction of Mid￿Cenozoic Extension in the Rincon Mountains Area, Southeastern Arizona, USA, and Geodynamic Implications

Item Type Article

Authors Spencer, Jon E.; Richard, Stephen M.; Lingrey, Steven H.; Johnson, Bradford J.; Johnson, Roy A.; Gehrels, George E.

Citation Spencer, J. E., Richard, S. M., Lingrey,S. H., Johnson, B. J., Johnson, R. A., & Gehrels, G. E. (2019). Reconstruction of mid￿ Cenozoic extension in the Rincon Mountains area, south eastern Arizona,USA, and geodynamic implications.Tectonics,38, 2338– 2357. https://doi.org/10.1029/2019TC005565

DOI 10.1029/2019tc005565

Publisher AMER GEOPHYSICAL UNION

Journal TECTONICS

Rights Copyright © 2019. American Geophysical Union.All Rights Reserved.

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Link to Item http://hdl.handle.net/10150/634480 RESEARCH ARTICLE Reconstruction of Mid‐Cenozoic Extension in the 10.1029/2019TC005565 Rincon Mountains Area, Southeastern Arizona, Key Points: • Structural reconstruction of the San USA, and Geodynamic Implications Pedro detachment fault in Jon E. Spencer1 , Stephen M. Richard1 , Steven H. Lingrey1, Bradford J. Johnson2, southeastern Arizona indicates 34– 1 1 38 km of top‐southwest Roy A. Johnson , and George E. Gehrels displacement 1 2 • The San Pedro fault cut through Department of Geosciences, University of Arizona, Tucson, AZ, USA, Arizona Geological Survey, University of Arizona, carbonate tectonites beneath an Tucson, AZ, USA older thrust fault that formed a weak zone in the upper crust • The architecture of core complexes Abstract The Rincon Mountains metamorphic core complex, located east of Tucson, Arizona, consists may have been influenced by upper crustal weak zones inherited from of an arched footwall of foliated crystalline rocks bounded above by the generally outward dipping, older thrust faulting Oligocene‐Miocene San Pedro extensional detachment fault. The southwest trending axes of corrugations in the detachment fault, and in footwall foliation and lithologic layering, parallel mylonitic lineation, and inferred top‐southwest displacement on the fault. An upper plate fault block within a synformal fault Correspondence to: groove on the west side of the Rincon Mountains contains a thrust fault that is interpreted as displaced J. E. Spencer, 34‐38 km westward from an original position adjacent to a similar thrust in the footwall of the San Pedro [email protected] detachment fault. Much of the footwall of the detachment fault in the eastern Rincon Mountains consists of metasedimentary tectonites derived largely from Paleozoic carbonates that were buried Citation: beneath Proterozoic crystalline rocks forming the hanging wall of the Laramide Wildhorse Mountain Spencer, J. E., Richard, S. M., Lingrey, thrust. These tectonites were later exhumed by displacement on the San Pedro detachment fault. S. H., Johnson, B. J., Johnson, R. A., & Gehrels, G. E. (2019). Reconstruction of Structural reconstruction supports the interpretation that the carbonate tectonites localized extensional mid‐Cenozoic extension in the Rincon faulting along the San Pedro detachment fault at crustal depths where carbonates would be weak and Mountains area, southeastern Arizona, deform by crystal plasticity while quartzo‐feldspathic rocks would be strong and brittle. This weak zone is USA, and geodynamic implications. Tectonics, 38, 2338–2357. https://doi. located adjacent to the greatest width of exposed extension‐parallel mylonitic fabrics in southeastern org/10.1029/2019TC005565 Arizona and may have been associated with the earliest initiation of extension in the region. Domains of low‐strength carbonates may be an underappreciated influence on extensional tectonics in cratonic Received 6 MAR 2019 southwestern North America. Accepted 12 JUN 2019 Accepted article online 26 JUN 2019 The Rincon Mountains east of Tucson, Arizona, were uplifted and Published online 13 JUL 2019 Plain Language Summary uncovered from a position in the middle crust, perhaps 10–20 km deep. Uplift and exhumation were the result of horizontal extension of Earth's crust. Recent geologic mapping identified features that are now separated by 34–38 km of horizontal extension but that originally were adjacent to each other. With this new understanding it is possible to identify the approximate initial form of the extensional fault that was active ~15–30 million years ago. This allows identification of features that influenced crustal architecture and the modern form of the Rincon Mountains.

1. Introduction Continental metamorphic core complexes are interpreted as products of tectonic extension in which deep crustal rocks were exhumed by large displacement on gently to moderately dipping normal faults and their downdip continuations as ductile shear zones (e.g., Davis et al., 1986; Platt et al., 2014). Factors that determine core‐complex size, geometry, and distribution, however, are not well understood. A recent analysis by Singleton et al. (2018) determined that carbonates within quartzo‐feldspathic crystalline rocks were likely sites of weakness that influenced core‐complex geometry and distribution in western Arizona. In this study we identify a similar situation in the Rincon Mountains of southeastern Arizona where Paleozoic carbonates that were beneath a Laramide (50–80 Ma) thrust fault appear to have localized a middle Cenozoic extensional detachment fault. A previous interpretation of core‐complex distribution in the Cordilleran orogen had placed core complexes

©2019. American Geophysical Union. above overthickened crustal roots of older areas of crustal shortening and thickening, with the buoyancy of All Rights Reserved. the crustal roots driving core‐complex uplift and exhumation (Coney & Harms, 1984; Spencer & Reynolds,

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1990). Applied at the scale of individual core complexes or clusters of core complexes, this interpretation is now problematic because uppermost Cretaceous subduction‐complex rocks have been identified beneath and near‐core complexes in western Arizona (Haxel et al., 2015; Jacobson et al., 2017; Strickland et al., 2018). Postthrusting emplacement of such rocks by subduction at lower crustal levels would have scraped off older thrust‐related crustal roots (Spencer et al., 2018). In a setting of generally thick and hot continental crust, deep crustal rocks may become so mobile that they behave as a fluid at tectonic rates and scales and can flow in response to lateral pressure gradients resulting from topography associated with laterally varying tectonic thinning of the upper crust (Block & Royden, 1990; Wernicke, 1990, 1992). Extension in this setting would form rafts of upper crustal blocks floating on a deep crustal fluid‐like layer that rises to shallow crustal levels where divergence between blocks is large (e.g., Gans, 1987; Kruger & Johnson, 1994; Spencer & Reynolds, 1989). The geometry of upper crustal faults would then be central to the geometry of core complexes rather than the distribu- tion and thickness of buoyant crustal roots. As a result, weak zones in the upper crust that localize normal faults may influence the architecture of core complexes and core‐complex clusters. This study is based largely on the results of geologic mapping in the Rincon Mountains and Johnny Lyon Hills east of Tucson, Arizona. It first describes the geology of the Rincon Mountains, Johnny Lyon Hills, and southwestern Galiuro Mountains, and then outlines the structure of east- ern Tucson basin southwest of the Rincon Mountains. This is followed by a tectonic reconstruction along an extension‐parallel transect across the Rincon Mountains. This reconstruction reveals the significance of an older thrust fault and its footwall carbonates in influencing the geometry of the San Pedro detachment fault and evolution of the Catalina‐Rincon metamorphic core complex.

1.1. Geologic Setting An 800‐km‐long belt of metamorphic core complexes extends southeast- ‐ Figure 1. Map of the southwest North America core complex belt. Sources ward from the eastern Mojave desert in southeastern California to south- of data include the following, from northwest to southeast: Pease & Argent (1999), John & Mukasa (1990), Davis and Lister (1988), Knapp (1993), ern Sonora, Mexico (Figure 1). Unlike core complexes farther north that Strickland et al. (2017), Spencer et al. (2016), Richard et al. (1990), formed within the thick upper Neoproterozoic to Mesozoic Cordilleran Brittingham (1985), Reynolds (1985), Richard et al. (1999), Ferguson et al. miogeocline, the southwest core‐complex belt formed almost entirely (2002), Davis (1980), Force (1997), Bykerk‐Kauffman (2008), Naruk (1986), within cratonic North America (Coney, 1980; Sloss, 1988). As a result, Nourse et al. (1994), Granillo and Calmus (2003), and Wong and Gans detachment faults associated with these core complexes formed largely (2008). within crystalline rocks, including extensive areas of granitic rocks which are typically strong and mechanically isotropic. The cratonic geology of southeastern Arizona and adjacent areas was affected by (1) Middle Jurassic magma- tism; (2) Late Jurassic tectonic extension and associated deposition of the Bisbee Group that continued well into the Cretaceous; (3) Late Cretaceous to Paleogene metaluminous magmatism and thrust faulting during the Laramide orogeny; (4) early Cenozoic peraluminous magmatism at the deep crustal levels revealed in core complexes; (5) Oligocene to Miocene extension, core‐complex uplift, and magmatism; and (6) late Cenozoic high‐angle normal faulting associated locally with modern basin and range topography (Dickinson, 1989, 1991). Laramide and older aspects of geologic history remain obscure because of these complex, superposed events. Isolated exposures of thrust faults, known or inferred to be Laramide in age, indicate dominant shortening in a southwest‐northeast direction, with basement‐cored uplifts similar to such structures in the Colorado Plateau and Rocky Mountains (e.g., Clinkscales & Lawton, 2018; Davis, 1979; Gehrels & Smith, 1991; González‐León & Solari, 2017; Keith & Barrett, 1976; Krantz, 1989).

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The Rincon and Santa Catalina Mountains together form a large core complex on the east and north sides of Tucson basin, respectively (Figure 2; Banks, 1980; Davis, 1980; Force, 1997; Keith et al., 1980; Spencer, 2006). The ranges consist largely of Proterozoic granitoids intruded by voluminous Eocene peg- matitic leucogranites and by two Laramide and two Oligocene plutons (Drewes, 1974; Fornash et al., 2013). Mesoproterozoic to Mesozoic supracrustal rocks dip generally northeastward on the northeastern flanks of the ranges (Bykerk‐Kauffman, 2008; Drewes, 1974; Spencer et al., 2000). Middle Cenozoic mylonitic fabrics related to tectonic exhumation, generally with top‐southwest shear sense, are well developed on the southwest side of the ranges but are rare or absent on the northeast side (Bykerk‐ Kauffman, 2008; Davis, 2013; Perry, 2005; Spencer, 2006). The ranges are generally interpreted as having been exhumed by top‐southwest displacement on the San Pedro detachment fault (also known as the “Catalina” detachment fault) and its downdip continuation as a mylonitic shear zone (Davis, 1987; Dickinson, 1991). Extension began at about 26.4 Ma, as indicated by the oldest precise 40Ar/39Ar dates (Peters et al., 2003) from a tuff near the base of the extensional Cienega basin at the south foot of the Rincon Mountains (Figure 3). Illite and cataclasite from the San Pedro detachment‐fault zone in the western Rincon Mountains yielded conventional K‐Ar dates from four samples that are all ~21 Ma (Damon & Shafiqullah, 2006), suggesting that detachment faulting and cooling during tectonic exhumation were under way at this time. Slip along the detachment fault likely ended in the middle Miocene and was followed by displacement on younger high‐ angle normal faults that cut the detachment fault and contributed significantly to uplift of the modern ranges (Davis et al., 2004).

2. Geology of the Rincon Mountains Transect The following description of the geology of the Rincon Mountains area proceeds from east to west, starting with the breakaway region of the San Pedro detachment fault and proceeding across the Rincon Mountains to Tucson basin.

2.1. Johnny Lyon Hills The Johnny Lyon Hills consist of Paleoproterozoic Pinal Schist and Johnny Lyon granodiorite, both over- lain to the east by generally east to northeast dipping Mesoproterozoic Pioneer Formation of the Apache Group and Paleozoic quartzite, siltstone, and carbonate (Figure 3; Cooper and Silver, 1964). At the north- western end of the Hills, Proterozoic crystalline rocks are thrust over clastic sedimentary rocks of the Mesozoic Bisbee Group along an east dipping brittle thrust fault. A fault sliver of Mesozoic volcanic‐lithic sandstone and conglomerate (Walnut Gap volcanics of Cooper & Silver, 1964) is present in the thrust zone (Figure 4; Drewes, 1974; Spencer, Cook, et al., 2009). At two locations to the northwest, Pinal Schist is thrust over generally overturned Bisbee Group (Figure 4; Dickinson et al., 1987). Directly to the north in the southwestern Galiuro Mountains, the Hot Springs Canyon thrust places Bisbee Group strata over Laramide volcanic and sedimentary rock and is possibly a related thrust (Figure 3; Dickinson et al., 1987). The Johnny Lyon Hills, along with the thrust‐fault exposures, are tilted eastward in the hanging wall block of the mid‐Cenozoic Teran Basin fault and form the footwall block to the mid‐Cenozoic Teran Wash fault (Figure 4). Both moderately dipping normal faults dip beneath the San Pedro River valley and project southwestward beneath the Rincon Mountains. The subhorizontal Lime Peak fault in the southeastern Johnny Lyon Hills has been interpreted as part of the breakaway to the San Pedro detach- ment fault and as unrelated to the normal faults in the northern Johnny Lyon Hills (Figures 2 and 3; Dickinson, 1984).

2.2. Eastern Rincon Mountains The Rincon Mountains are dominated by granitic and metamorphic rocks that form two antiformal corru- gations in the San Pedro detachment system. The northern corrugation includes Tanque Verde Peak and Mica Mountain, while the southern one includes Rincon Peak (Figure 3). These west‐southwest trending ridges are separated by a synformal corrugation represented by Rincon Valley on the west side of the range and Happy Valley on the east. The corrugated form of the San Pedro detachment fault is apparent over the entire range.

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Figure 2. Highly simplified geologic map of the greater Tucson area. Cross section A‐A′′′ is shown in Figure 14a.

The footwall block of the San Pedro detachment fault in the eastern Rincon Mountains consists of Proterozoic Pinal Schist, granitic rocks, and heterogeneous orthogneiss that are overlain by paragneiss derived primarily from Paleozoic sedimentary rocks (Figures 5 and 6). The paragneiss forms a carapace over the underlying granitic and gneissic rocks with a contact between the two that commonly appears as plasti- cally deformed carbonate and calc‐silicate rocks in sharp contact with underlying, brittlely deformed crystal- line rocks (shown as “semibrittle fault” in Figures 5 and 6; Figure 7a). Some contacts within the paragneiss are also mapped as semibrittle faults, especially where they juxtapose siliceous and carbonate rocks (Lingrey, 1982; Spencer, Richard, et al., 2009; Spencer et al., 2011). Foliation in meta‐igneous rocks and the paragneiss carapace, and the form of the carapace and overlying San Pedro detachment fault, define an approximately east‐west trending antiform (Figures 5, 6e, and 6f). Penetrative deformation of the paragneiss was associated with upper greenschist‐grade to locally lower amphibolite‐grade metamorphism, with metamorphic minerals including talc, tremolite, diopside, phlogopite, biotite, muscovite, cordierite, and K‐feldspar (Lingrey, 1982). Metamorphism is older than unmetamorphosed Oligocene andesitic dikes (Lingrey, 1982), and younger than similarly metamorphosed lower to middle Cretaceous Bisbee Group strata just to the north in the eastern Santa Catalina Mountains (Bykerk‐Kauffman, 2008). Abundant irregular intrusions of Eocene pegmatitic leucogranite are present in underlying crystalline rocks and locally intrude the metasedimentary carapace. The intrusions in underlying basement are generally unfoliated but are the inferred source of heat during metamorphism and penetrative deformation in the overlying paragneiss carapace (Lingrey, 1982). Absence of Cambrian quartzite and Mesoproterozoic Apache Group strata above crystalline basement and below carbonate and calc‐silicate tectonites of the carapace indicates that carapace rocks have been dis- placed into their current position above the quartzo‐feldspathic crystalline basement. Structural analysis of fold asymmetry in the tectonites indicates penetrative shearing and transposition generally top

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Figure 3. Simplified geologic map of the Rincon Mountains, Johnny Lyon Hills, and adjacent areas. Shaded relief represents the footwall of the San Pedro detach- ment fault in the Rincon and Santa Catalina Mountains and reveals the abundant linear drainages that parallel extension direction (Spencer, 2000). Semibrittle faults are shear zones in which the silicic rocks on one side of the fault are brittlely deformed and the carbonate rocks on the other side are deformed by crystal‐plastic processes. Data for U‐Pb geochronology samples “a” to “d” are shown in Figure 10; sample “e” is from Eisele and Isachsen (2001). Green line represents cross‐section location as shown in Figure 2. Geologic map sources include Bykerk‐Kauffman (2008), Cooper and Silver (1964), Dickinson et al. (1987), Drewes (1974, 1977), Ferguson et al. (2001), Richard et al. (2001, 2005), Skotnicki and Siddoway (2001), and Spencer et al. (2001), Spencer, Cook, et al. (2009), Spencer, Richard, et al. (2009), Spencer et al. (2011).

southwest before and/or during intrusion of the Eocene leucogranites (Lingrey, 1982). Both fold asymmetry and the generally westward thinning of the carapace are consistent with southwestward transposition and tectonic smearing over underlying crystalline rocks, although lineations that reveal transposition direction are almost entirely absent.

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Figure 4. Geologic map and cross section of the northwestern Johnny Lyon Hills and southwestern Galiuro Mountains (modified from Cooper & Silver, 1964; Dickinson et al., 1987; Spencer, Cook, et al., 2009). Fault symbols as in Figure 3. Green line represents cross‐section location as shown in Figure 2. See Figure 3 for location.

In the northwestern corner of Happy Valley, a well‐lineated mylonitic fabric with top‐west shear‐sense indi- cators overprints granitic rocks within a few tens of meters structurally below the San Pedro detachment fault (Spencer et al., 2011). This fabric is weaker to the east and dies out at about the point where carbonate tectonite appears below the fault (Figures 5 and 6a). Carbonate tectonites with similarly oriented lineations are present locally below the detachment fault over a distance of 3–4 km to the east of this point. This lineated fabric is increasingly localized to the fault zone so that in eastern exposures they form an ~1‐m‐thick zone directly beneath the detachment fault (just north of the eastern end of cross‐section line A‐A′ in Figure 5). Farther east, the detachment fault is truncated by a north trending high‐angle normal fault (Figure 5). A lineated fabric associated with the detachment fault was not identified east of this fault. The upper plate of the San Pedro detachment fault in the eastern Rincon Mountains consists of Paleoproterozoic Pinal Schist and Johnny Lyon granodiorite and unmetamorphosed Mesoproterozoic, Paleozoic, Mesozoic, and middle Cenozoic strata. In several fault blocks Proterozoic crystalline rocks are thrust over clastic sedimentary rocks of the Bisbee Group (Figures 5 and 6a–6d). The thrust fault is well exposed in the northeastern Rincon Mountains where it dips gently eastward and is truncated and deposi- tionally overlain by Oligocene to lower Miocene strata that now dip moderately to steeply to the northeast (Figures 5, 6b, and 6c). In most areas displaced strata form large tilted slabs, but in some areas a younger gen- eration of normal faults has broken the tilted slabs (Figures 5 and 6a). These lithologic and structural asso- ciations are similar to those of the northern Johnny Lyon Hills. Foliated metasedimentary rocks in the footwall of the San Pedro detachment fault are overthrust by Johnny Lyon granodiorite on the north flank of Wildhorse Mountain (Figure 8a). Mylonitic fabrics in the granodior- ite, present within ~5–10 m of the underlying thrust contact, include subhorizontal lineations with an aver- age trend of 067° and top‐northeast shear‐sense indicators (Figures 8b and 8c). Two small exposures of this thrust are present to the west (Figure 5). The trace of the thrust at Wildhorse Mountain projects northeast- ward toward the thrust exposed in the northwestern Johnny Lyon Hills (Figure 3) and the two are presumed to be correlative.

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Figure 5. Geologic map of the eastern Rincon Mountains (modified from Dickinson, 1987; Drewes, 1974; Lingrey, 1982; Spencer, Cook, et al., 2009; Spencer, Richard, et al., 2009; Spencer et al., 2011). Fault symbols as in Figure 3. See Figure 3 for location.

The San Pedro detachment fault is broken by two generations of north striking, moderately to steeply dipping normal faults. The older faults are associated with moderately tilted Miocene strata (Paige gravels of Lingrey, 1982) present primarily in a north trending belt in the eastern Rincon Mountains (Figures 5 and 6d; Dickinson, 1991). The Teran Wash fault in the southwestern Galiuro Mountains (Figures 3 and 4) may be a member of this fault generation. Tilting is minor for the youngest faults such as the Martinez Ranch fault (Figure 3) that are also associated with development of significant topographic relief (Davis et al., 2004).

2.3. Rincon Valley The western side of the central Rincon Mountains is characterized by a deep embayment in the trace of the San Pedro detachment fault that coincides with Rincon Valley (Figure 3). Upper plate rocks consist mostly of

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Figure 6. Geologic cross sections of the eastern Rincon Mountains. See Figure 5 for locations. Complex folding of the semibrittle fault at the base of the tectonites is shown schematically.

coarse‐grained granite and/or granodiorite interpreted as part of a single intrusive suite that was thrust over overturned strata of the Bisbee Group (Figure 7b), with thrust slivers of Mesozoic volcanic‐lithic sandstone and fragmental volcanic rocks present along the fault (Figure 9; Richard et al., 2005). The thrust is characterized by brittle deformation and the juxtaposed rock units form a low‐relief landscape. At Sentinel Butte east of the overturned Bisbee Group and at Pistol Hill to the south (Figure 3; Richard et al., 2001), Cambrian Bolsa Quartzite rests depositionally on the Mesoproterozoic Pioneer Formation, which is the lowest of the three units that make up the Apache Group (Wrucke, 1989). This stratal juxtaposition is the same as in the Johnny Lyon Hills (Cooper & Silver, 1964) but is unlike the Santa Catalina Mountains to the north where Cambrian Bolsa Quartzite rests on stratigraphically higher units of the Apache Group (Bykerk‐Kauffman, 2008; Force, 1997). Granite in Rincon Valley yielded a U‐Pb date, by laser‐ablation mass spectrometry of single zircons, of 1,644 ± 17 Ma, which overlaps within analytical uncertainty with three U‐Pb dates of the Johnny Lyon granodiorite in the eastern Rincon Mountains (Figure 10). All four these dates overlap within analytical uncertainty with a U‐Pb date of 1,643 ± 5 Ma from the Johnny Lyon granodiorite in the Johnny Lyon Hills (Eisele & Isachsen, 2001). It thus appears that the rock units above the San Pedro detachment fault in Rincon Valley are essentially identical to those of the Johnny Lyon

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Hills in three respects: the rock types in thrust juxtaposition, the details of their Cambrian–Mesoproterozoic stratigraphy, and the U‐Pb age of Paleoproterozoic granitoids. 2.4. Rincon Mountains Lower Plate Mylonitic lineations directly below the San Pedro detachment fault are interpreted to indicate displacement direction under conditions of crystal‐plastic shearing downdip from the active fault and before cooling to brittle conditions during extensional exhumation. Studies of mylonitic fabrics near the toe of Tanque Verde Ridge identified consistent evidence for top‐southwest shearing (Davis, 2013; Perry, 2005). From the tip of Tanque Verde Ridge on the west to the northwest margin of Happy Valley to the east, a distance of ~25 km, mylonitic lineations gradually change orientation from ~240° to ~255° (dotted line in Figure 11). The crest of Tanque Verde Ridge parallels mylonitic lineations (Figure 11), consistent with the interpretation that the ridge is an antiformal corruga- tion in the San Pedro detachment fault. The axis of Tanque Verde Ridge projects eastward to align with the axis of the antiformal corrugation in the San Pedro detachment fault and the antiform defined by foliation and lithologic layering in underlying paragneiss and orthogneiss in the eastern Rincon Mountains (Figures 3 and 5). The crest of the antiform is represented as three contiguous linear segments on Figure 3 that approxi- mately align with the trend of mylonitic lineations (Figure 11). 2.5. Tucson Basin The San Pedro detachment fault dips generally southwestward beneath the deepest part of Tucson basin, which is an elongate, north‐south trough that is over 3 km deep in its central area (Figures 2 and 12). A 3,827‐m‐ deep drill hole (Exxon State (32)‐1) at the center of the basin penetrated, from top to bottom, the following: (1) 1,880 m of weakly consolidated clas- tic strata interpreted as postdating detachment faulting, (2) 1,176 m of Figure 7. (a) Tectonite derived from calcareous Glance Conglomerate of the consolidated clastic strata and volcanic rocks that are lithologically simi- Bisbee Group smeared over brittlely deformed chloritic granitoid. Red line lar to the middle Cenozoic Pantano Formation and associated volcanic marks the contact between the two units. (b) Overturned Glance rocks deposited and tilted during detachment faulting, (3) 602 m of Conglomerate channel. Red arrows point to sandy beds that indicate planar Mesozoic Bisbee Group, and (4) 169 m of granitoids (Houser et al., fi bedding before channel incision and channel lling with conglomerate. 2005). Interpretation of penetrated rock units was based largely on exam- ination of drill cuttings, which is potentially problematic because of ero- sion of sediments from the uncased part of the well bore by drilling fluids and mixing with cuttings from the bottom of the well. U‐Pb dating of individual zircons from drill cuttings in the bottom ~120 m of the well, interpreted as granitic rock from visual examination of well cuttings, yielded a range of dates so diverse that the age of what appeared to be granite is not apparent (Spencer et al., 2015). This is possibly due to contam- ination from higher levels in the well or it is an indication that the lowest unit penetrated is a middle Cenozoic clastic sedimentary rock unit and overlying rocks interpreted as Bisbee Group are mid‐Cenozoic rock‐avalanche breccias derived from Bisbee Group. A29‐km‐long, east‐west seismic reflection profile that passes just north of the Exxon well (Figure 12) reveals some of the shallow structure associated with the central Tucson basin and the area penetrated by the well (Wagner & Johnson, 2006, 2010). Reflections reveal a bedrock shoulder ~2–3 km east of the Exxon drill hole that coincides with a steep lateral Bouguer gravity gradient (Rystrom, 2003). This shoulder was interpreted as the footwall block of a normal fault that dips westward under the deeper units penetrated by the Exxon drill hole (Figure 12; Wagner & Johnson, 2006). An eastward dipping fault was interpreted from seismic data on the west side of the basin, placing the Exxon well near the center of a buried graben. Identification of the east dipping normal fault is uncertain, however, and we propose an alternative interpretation in which east dipping normal faults are minor features and the basin is primarily a half graben above the west dipping nor- mal fault. In another alternative interpretation, both normal faults are younger than, and cut, the

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detachment fault. However, a tuff unit penetrated by the Exxon drill hole at ~2,600‐m depth yielded an 40Ar/39Ar incremental‐release date of 26.91±0.19 Ma (Houser et al., 2005), which indicates that lower graben‐ filling clastic and volcanic rocks are associated with earliest detachment faulting. In our interpretation, these rock units were deposited during detachment faulting in an upper plate half graben. Downdip projection of the San Pedro fault, at the 8.4° plunge of the crest of lower Tanque Verde Ridge, would result in intersection of the fault with the Exxon drill hole at ~3‐km depth. Because the fault was not encoun- tered in the drill hole, and pegmatitic leucogranite and mylonitic rocks were not identified in cuttings, we infer that the fault is at greater depth than the ~4‐km depth of the well. The detachment fault is shown in the cross section of Figure 12 with a slightly convex‐upward form over the ~17‐km distance from the range front to the drill hole. On the east‐west seismic reflection profile, reflections that were previously interpreted as derived from density contrasts associated with the detachment fault flat- ten away from the range front and then steepens to 8–9‐km depth near the Exxon drill hole (Wagner & Johnson, 2006, 2010). This apparent flat- tening and other variations in dip of the interpreted fault plane reflections are at least partly the result of lateral velocity variations in overlying rock units that are not fully accounted for in the time‐ or depth‐converted seis- mic profiles. For example, higher seismic velocities in the overlying bed- rock shoulder, which is recognized in seismic profiles and supported by gravity data, would cause apparent upward arching of the fault plane reflections. The reflectors attributed to the detachment fault could repre- sent lithologic contrasts in gneissic rocks that make up most of the lower plate of the San Pedro detachment fault. The cross section of Figure 12 shows the fault as projected from the foot of the Rincon Mountains with- out modification based on the deep, arched seismic reflections shown by Wagner and Johnson (2006, 2010).

3. Total Extension and Hypothetical Reconstruction Restoration of displacement on the San Pedro detachment fault requires knowledge of both displacement magnitude and direction. The well‐ developed, corrugated form of the detachment fault footwall, especially on the west side of the range, provides an indication of displacement Figure 8. (a) Geologic map of the Wildhorse Mountain thrust (from direction, as do nearly parallel mylonitic lineations (Figure 11). We base Spencer, Cook, et al., 2009). PzYms = metasedimentary tectonites derived our reconstruction on displacement directions indicated by the two linear from Paleozoic and Mesoproterozoic strata, YXg = 1.4–1.7‐Ga granitoids; segments of Tanque Verde Ridge (240° and 253° trends) and the contigu- orange color represents mylonitic granitoids derived from map unit YXg. See ous antiform in the eastern Rincon Mountains (263° trend), as shown in Figure 5 for location. (b) Lower hemisphere stereonet plot of mylonitic Figures 3 and 13. In an alternatively interpretation, the lower plate rotated foliations and lineations measured along the Wildhorse Mountain thrust. Stereonet and eigenvalues were produced with Stereonet 10.0 software clockwise while extension direction remained constant. In the analysis (Allmendinger et al., 2012; Cardozo & Allmendinger, 2013). (c) S‐C mylo- that follows, we do not consider this alternative but rather simply evaluate nites at the extreme southwest end of the Wildhorse Mountain thrust indi- displacement relative to footwall rocks. cating top‐northeast shearing. Field of view is ~6 cm wide. The geomorphology of the western side of the Rincon Mountains is marked by numerous linear drainages that parallel the crest of Tanque Verde Ridge, with a change in drainage trend that parallels the bend in the Ridge (Figure 13). These drai- nages have been interpreted as revealing displacement direction due to incision of the freshly denuded foot- wall along small fault grooves (Spencer, 2000) or to preferential incision along joints aligned with extension direction (Pelletier et al., 2009; see Davis & Hardy, 1981, Figure 13, for a cross section that shows fault cor- rugations in down‐plunge view). Regardless of origin, it appears that drainages reflect extension direction. Note that the linear drainages southwest of Rincon Peak parallel the crest of Tanque Verde Ridge west of

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Figure 9. Simplified geologic map and cross section of the Loma Alta area of the western Rincon Mountains (from Richard et al., 2005). See Figure 3 for location and Figure 5 for fault symbols. Bedding symbols with dots at end of symbol indicate that stratigraphic‐top direction is known from sedimentary structures.

Tanque Verde Peak (240° trend) rather than the ridge crest east of Tanque Verde Peak (253° trend). As a result of this relationship, we infer that the displacement vector passing through Rincon Peak would bend 13° at the peak (Figure 13). Minimum displacement of upper plate rocks in the Rincon Valley area is the amount necessary to restore Paleoproterozoic granitoids to an eastern position above similar age granitoids rather than above older Pinal Schist (Dickinson, 1991). Following Dickinson (1991), we assume that granitoid plutons had broadly convex intrusive tops and did not spread out, sill‐like, at upper crustal levels. With this assumption, the mini- mum displacement necessary to restore all Paleoproterozoic granitoids in Rincon Valley so that they are not placed structurally above Pinal Schist is 28 km (vector number “4” in Figure 13). Maximum displacement is indicated by three features. The Loma Alta thrust fault in Rincon Valley inter- sects and is cut by the San Pedro detachment fault. Restoration of this point 40 km to the east (vector number “1” in Figure 13) would place it directly above the thrust fault in the northwestern Johnny Lyon Hills, with

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Figure 10. U‐Pb geochronology of four samples of granitoids in the Rincon Mountains that are correlated with the Johnny Lyon granodiorite of Cooper and Sliver (1964). Figures “a,”“b,”“c,” and “d” correspond to U‐Pb sample locations shown in Figure 3. See Spencer et al. (2015, samples 51, 56, 57, and 58) for analytical data. All four dates overlap, within given uncertainties, with a 1,643 ± 5‐Ma zircon U‐Pb date from Johnny Lyon granodiorite in the Johnny Lyon Hills determined by Eisele and Isachsen (2001) by isotope‐dilution thermal‐ionization mass spectrometry.

similar hanging wall and footwall rocks, including small slivers of volcanic‐lithic sedimentary rocks of probable Jurassic age in the footwalls of thrust faults in both areas (Figures 4 and 9). Restoration of more than 40‐km displacement would place the thrust above Johnny Lyon granodiorite and unconnected to the east dipping thrust in the footwall. Similarly, restoration of Mesoproterozoic and Cambrian strata at Sentinel Butte eastward 41 km would align these rocks with similar, east dipping correlative strata in the footwall (vector “2”; Figure 13), whereas greater displacement would place these strata updip from younger Paleozoic strata in the footwall, representing a mismatch. Finally, displacement of granitic rocks in Rincon Valley to greater than 41.5 km (vector “3”) would place the granitoids above Mesoproterozoic and Paleozoic strata, whereas less displacement would juxtapose similar granitoids. We conclude that displacement of upper plate rocks in Rincon Valley due to displacement on the San Pedro detachment fault was 28 to 40 km (34 ± 6 km). Features represented by vectors “1” and “2” indicate that restoration of maximum 40‐km displacement would align similar detachment footwall and hanging wall rocks and structures. This alignment suggests that detachment‐fault displacement was closer to maximum 40‐km displacement rather than minimum 28 km. Postdisplacement erosion of footwall rocks would have moved the trace of the relevant, east dipping footwall contacts eastward, so that, for example, 1 to 2 km of erosion would have moved these contacts ~1–3 km eastward. Minor extension on younger normal faults also produced perhaps 1–3 km of extension. As a result of these considerations we infer that actual displacement of the Rincon Valley upper plate fault block was ~34–38 km.

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Figure 11. Blue dots indicate individual measurements of lower plate mylonitic‐lineation trend versus UTM Easting. (Lineation trend is transposed 180° where the measured value is <180°.) Dotted line is a least squares fit of these data points, which are from Richard et al. (2005) and Spencer et al. (2011). Dark red line labeled 237° represents average of >260 mylonitic‐lineation measurements from Perry (2005). Magenta lines labeled 240°and 253° represent orientation of the crest of Tanque Verde Ridge; line labeled 263° represents antiform axis on east side of Rincon Mountains (see Figures 3 and 5).

Figure 12. Eastern Tucson basin, showing depth to bedrock (from Richard et al., 2007), adjacent bedrock geology, and geophysical features. In cross section B‐B′, note that designation of pre‐Cenozoic, upper plate bedrock as Mesozoic sedimentary rocks is based only on the fact that such rocks are exposed along the cross‐ section line to the northeast. Most likely the composition of the buried bedrock is diverse and its structure complex.

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Figure 13. Reconstruction vectors for maximum (1, 2, 3) and minimum (4) displacement of geologic features in the upper plate of the San Pedro detachment fault. Vectors parallel crest of Tanque Verde Ridge and eastern Rincon antiform (magenta line).

A90‐km‐long cross section through the Tucson basin, Rincon Mountains, and Johnny Lyon Hills (Figure 14a) is shown with upper crustal rocks reconstructed in Figure 14b (enlarged to 130% for clarity). Features shown in the footwall of the San Pedro detachment fault east of the crest of the Rincon Mountains are restored to positions that are progressively deeper to the west (numbers 1–3). Metasedimentary tectonite is shown transposed westward by ductile shearing and displacement across foot- wall crystalline rocks and across the thrust fault (number 4 in Figures 14a and 14b). Transposition occurred primarily before middle Cenozoic detachment faulting, probably during heating associated with intrusion of voluminous Eocene leucogranites. Transposition displaced the tectonites across the thrust (point “P” in Figures 14a and 14b) as if the carbonates were mobilized and intruded laterally in sheet‐like form from below the inclined thrust to above it. The reconstruction of Figure 14b thus reflects restoration of the para- gneiss carapace to its position before detachment faulting but after its penetrative deformation and south- westward transposition (Lingrey, 1982). The Loma Alta area, with its thrust juxtaposition of Paleoproterozoic granitoids over Bisbee Group, is restored to a position directly adjacent to the thrust fault that juxtaposes similar units in the northwestern Johnny Lyon Hills (number 5 in Figure 14), which is the basis of the 40‐km estimate of maximum displacement for the Loma Alta area. Note that in Figures 5 and 8a the Wildhorse Mountain thrust is truncated westward by a north striking nor- mal fault. There are no other indications of a continuation of the Wildhorse Mountain thrust in the detachment‐fault lower plate in the Rincon Mountains except for two small exposures 1 and 4 km west of the above‐mentioned truncation point (Figure 5; Spencer et al., 2011). The westernmost exposure could be part of a quartzo‐feldspathic tectonic block displaced southwestward with the carapace tectonites. Rather than continuing west, the thrust fault must turn northward and continue beneath the detachment‐fault upper plate and beneath paragneiss smeared southwestward over the truncated trace of the thrust fault.

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Figure 14. (a) Geologic cross section from Interstate highway 19 to Allen Flat. See text for discussion of features and Figure 2 for location. Minor faults are not shown. (b) Cross‐section reconstruction enlarged to 130% from “a.” Oligocene to lower Miocene sedimentary and volcanic rocks are shown to help visualize the reconstruction, but these rocks had not been deposited at the time of initial faulting.

The approximate location where the inferred thrust‐fault trace is truncated and concealed by paragneiss is shown with the letter “P” in Figure 14. Displacement of mid‐Cenozoic strata and older rock units penetrated in the Exxon State (32)‐1 well must have been greater than that for the Loma Alta area. Bedrock between Loma Alta and the Exxon well is bur- ied by upper Cenozoic strata and its composition and structure are largely unknown. It could consist of mul- tiple fault blocks as well as sedimentary and volcanic rocks deposited during extension. A single fault block representing these rocks is shown between the two areas with a form consistent with seismic and gravity data (number “6” in Figure 14a). The gap shown between upper and lower plates of the detachment fault in the reconstruction in Figure 14b is the inferred initial position of bedrock represented by fault block 6 in Figure 14a. Fault block 6 is the material derived from this gap or deposited during displacement. According to the reconstruction of Figure 14b, total displacement on the San Pedro detachment fault below the Exxon drill hole is ~60 km. Restoration of displacement of this magnitude seems necessary to place mylonitic rocks in western Happy Valley at sufficient depth for mylonitization, as shown in Figure 14b. Maximum displace- ment is estimated at 64 km because restoration of a greater amount would place pre‐Cenozoic rocks pene- trated deep in the Exxon drill hole above middle Cenozoic strata in the hanging wall block of the Teran Basin fault (Figure 14a). In an alternative reconstruction, displacement of deep rocks in the Exxon well relative to upper plate rocks in the Loma Alta area was not parallel to mylonitic lineation and the inferred southwesterly displacement on the San Pedro detachment fault but rather was to the west or northwest, perpendicular to the northeast strik- ing Santa Rita fault (e.g., Figure 2; Johnson & Loy, 1992; Pearthree & Calvo, 1987). The Santa Rita fault is within the upper plate of the San Pedro detachment fault and was interpreted as truncated northward by

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the San Pedro detachment fault or merging with it (Figure 12; Wagner & Johnson, 2006). This alternative is disfavored here because there is no evidence from mylonitic lineations that more western and structurally deeper footwall rocks were subjected to top‐west or top‐northwest shearing.

4. Geodynamic Implications Mylonitic fabrics are widely distributed in the western Rincon Mountains but in Happy Valley they are loca- lized to within a few tens of meters beneath the detachment fault and are only present in the northwestern corner of the Valley (Figure 5). The eastern geographic limit of this fabric is interpreted as the exhumed, east‐ tilted brittle‐ductile transition in quartzo‐feldspathic rocks during earliest detachment faulting (Figure 14b). This boundary coincides with the westernmost exposures of carapace carbonate tectonites. A lineated fabric in these carbonates, with similarly oriented lineations and located directly below the detachment fault, is interpreted as representing penetrative shearing during ductile deformation in carbonates (dislocation creep and diffusion creep; e.g., De Bresser et al., 2002). These carbonates would have been a zone of weakness near the base of an otherwise brittle, quartzo‐feldspathic upper crust (Brodie & Rutter, 2000). Furthermore, such a zone of weakness would be most significant at the strongest part of the brittle upper crust, at temperatures of ~250–400 °C (e.g., compare dislocation‐creep flow‐law parameters of Hirth et al. (2001) for quartz and Renner et al. (2002) for calcite, as plotted by Singleton et al. (2018, Figure 14)). Such weakening would be especially effective in the cratonic setting of Arizona's core complexes because of the abundance of quartzo‐feldspathic crystalline rocks. The geometry of the Catalina‐Rincon core complex appears to reflect the influence of weak carbonates on the initial form of the San Pedro detachment fault. Most obviously, carbonate tectonites in the large antiform in the eastern Rincon Mountains may be responsible for the initial antiformal detachment‐fault corrugation. Genesis of western Tanque Verde Ridge was perhaps by shaping of the ductile footwall during tectonic exhu- mation from the middle crust (Spencer, 1999). As noted above, mylonitic fabrics in lower plate rocks in the Tanque Verde Ridge area are exposed over ~25 km parallel to lineation, a greater lineation‐parallel distance than in any other core complex in southwestern North America except the Harcuvar complex in western Arizona (Singleton & Mosher, 2012; Spencer et al., 2016). As with the Rincon Mountains, the geometry of the detachment‐fault system in the Buckskin and Rawhide Mountains of the Harcuvar complex was appar- ently influenced by the distribution of carbonate tectonites that form slivers and sheets within or above quartzo‐feldspathic rocks in the detachment‐fault footwall (Singleton et al., 2018). This association suggests that these weak zones led to greater extension‐parallel exposures of mylonitic quartzo‐feldspathic footwall rocks than in other southwestern core complexes. Furthermore, in both the Rincon and Harcuvar com- plexes, the widest areas of extension‐parallel mylonite exposures are near the lateral termination of each local belt of core complexes (Figure 1). This relationship suggests that carbonates in the footwalls of older thrust faults influenced the form of core‐complex belts. Finally, we note that both the Rincon and Harcuvar complexes are associated with the earliest known gen- esis of mid‐Cenozoic extensional basins in the 800‐km‐long core‐complex belt, as indicated by sanidine 40Ar/ 39Ar dates from tuffs in basal clastic rocks in Cienega basin south of the Rincon Mountains (26.45 ± 0.01 Ma; average of two dates from Peters et al. (2003)) and in the Artillery Mountains adjacent to the Harcuvar core complex (26.28 ± 0.01 Ma; Lucchitta & Suneson, 1993). This chronology suggests that weak zones at the brittle‐ductile transition were the sites of initial extensional faulting before widespread extensional basin genesis. Furthermore, the essentially synchronous character of initial extension in two huge core complexes 350–400 km apart suggests that regional tectonic processes involving continental‐margin tectonics triggered extension in both areas simultaneously.

5. Conclusion Oligocene to Miocene displacement on the San Pedro detachment fault and associated uplift of the Rincon Mountains metamorphic core complex uncovered the large, extension‐parallel Tanque Verde Ridge and its eastern continuation as an antiform. This feature, along with parallel mylonitic lineations and linear drai- nages, curves slightly to the southwest in more westerly locations. We interpret this geometry to indicate that displacement direction became gradually more southwesterly during tectonic extension or, alternatively, that the lower plate rotated clockwise during extension. Displacement of the Loma Alta‐Rincon Valley

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fault block along a slightly curved trajectory parallel to Tanque Verde Ridge and its eastern continuation as an antiform is constrained at 34 ± 6 km, with a best estimate of 34–38 km. Greater displacement, perhaps as much as 60 km, is suggested for the deepest rock units penetrated by the Exxon State (32)‐1 well near the center of Tucson basin. The core complex includes an extensive zone of carbonate tectonites in the footwall of the San Pedro detach- ment fault in the eastern Rincon Mountains. These carbonates were derived primarily from Paleozoic sedi- mentary rocks buried by overthrusting of crystalline rocks along the Laramide Wildhorse Mountain thrust and its correlative in the northwestern Johnny Lyon Hills. Top‐northeast shear‐sense indicators in mylonitic granitic rocks adjacent to the thrust indicate a typical northeastward Laramide shortening direction. Fragments of the thrust system are also present in tilted fault blocks in the upper plate of the San Pedro detachment fault. Lineated carbonate tectonites identified directly below the San Pedro detachment fault in northern Happy Valley are directly adjacent to the eastward lateral termination of quartzo‐feldspathic core‐complex mylo- nitic fabrics. These lineated carbonates are interpreted as representing crystal‐plastic deformation at shal- lower crustal levels than the brittle‐ductile transition in quartzo‐feldspathic rocks. We propose that the San Pedro detachment fault in the eastern Rincon Mountains was localized within the previously buried carbonates because of their weakness in this crustal setting. This shear‐zone localization has several pro- posed implications, including the interpretation that extension began earlier and led to more effective mylonitic footwall exhumation because of strength reduction at the depth of the brittle‐ductile transition in cratonic crust.

Acknowledgments References We thank Kurt Constenius for the Allmendinger, R. W., Cardozo, N., & Fisher, D. M. (2012). Structural geology algorithms: Vectors and tensors. Cambridge, England: comments on an early draft. J. Spencer Cambridge University Press. https://doi.org/10.1017/CBO9780511920202 acknowledges Stan Keith for insisting Banks, N. G. (1980). Geology of a zone of metamorphic core complexes in southeastern Arizona. In M. S. Crittenden, Jr., P. J. Coney, & G. over the past 35 years that Laramide H. Davis (Eds.), Cordilleran metamorphic core complexes, Memoir, (Vol. 153, pp. 177–215). Boulder, CO: Geological Society of America. thrust tectonics were an https://doi.org/10.1130/MEM153‐p177 underappreciated aspect of Rincon Block, L., & Royden, L. H. (1990). Core complex geometries and regional scale flow in the lower lithosphere. Tectonics, 9(4), 557–567. Mountains geology. U‐Pb https://doi.org/10.1029/TC009i004p00557 geochronologic analyses were produced Brittingham, P. L. (1985). Structural geology of a portion of the White Tank Mountains, central Arizona (Master's thesis). Tempe, AZ: by the Arizona LaserChron Center at Arizona State University. the University of Arizona with support Brodie, K. H., & Rutter, E. H. (2000). Deformation mechanisms and rheology: Why marble is weaker than quartzite. Journal of the from NSF grant EAR 1649254. Field Geological Society, 157(6), 1093–1096. https://doi.org/10.1144/jgs.157.6.1093 mapping by S. Richard and J. Spencer in Bykerk‐Kauffman, A. (2008). Geologic map of the northeastern Santa Catalina Mountains, Pima County, Arizona. (CM‐08‐A). Tucson, AZ: the Rincon Valley area in the 2003– Arizona Geological Survey. 2004 field season was supported by the Cardozo, N., & Allmendinger, R. W. (2013). Spherical projections with OSXStereonet. Computers & Geosciences, 51, 193–205. https://doi. U.S. Geological Survey National org/10.1016/j.cageo.2012.07.021 Cooperative Geologic Mapping Clinkscales, C. A., & Lawton, T. F. (2018). Mesozoic–Paleogene structural evolution of the southern U.S. Cordillera as revealed in the Little Program under STATEMAP grant and Big Hatchet Mountains, southwest New Mexico, USA. Geosphere, 14(1), 162–186. https://doi.org/10.1130/GES01539.1 03HQAG0114. Field mapping by J. Coney, P. J. (1980). Cordilleran metamorphic core complexes: An overview. In M. S. Crittenden, Jr., P. J. Coney, & G. H. Davis (Eds.), Spencer and S. Richard in the eastern Cordilleran metamorphic core complexes, Memoir, (Vol. 153, pp. 7–31). Boulder, CO: Geological Society of America. https://doi.org/ Rincon Mountains in the 2006–2007 10.1130/MEM153‐p7 field season was supported by Coney, P. J., & Harms, T. A. (1984). Cordilleran metamorphic core complexes: Cenozoic extensional relicts of Mesozoic compression. STATEMAP grant 06HQAG0051. Field Geology, 12(9), 550–554. https://doi.org/10.1130/0091‐7613(1984)12<550:CMCCCE>2.0.CO;2 mapping by J. Spencer and B. Johnson Cooper, J. R., & Silver, L. T. (1964). 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