PUBLICATIONS Tectonics RESEARCH ARTICLE Miocene slip history of the Eagle Eye detachment 10.1002/2016TC004241 fault, Harquahala Mountains metamorphic core Key Points: complex, west-central Arizona • Total displacement on the Eagle Eye detachment fault is ~44 ± 2 km Michael G. Prior1, Daniel F. Stockli1, and John S. Singleton2 • Apatite and zircon (U-Th)/He slip rates along the Eagle Eye detachment are 1Department of Geological Sciences, University of Texas at Austin, Austin, Texas, USA, 2Department of Geosciences, ~6.6 + 7.8/À2.3 km/Myr and ~6.6 + 7.1/À2.0 km/Myr, respectively Colorado State University, Fort Collins, Colorado, USA • Active extension along the Eagle Eye detachment fault from ~21 ± 1 Ma to ~14 Ma Abstract The structural and thermal evolution of major low-angle normal faults in the Colorado River extensional corridor has been a controversial topic since the pioneering studies of metamorphic core Supporting Information: complexes in the early 1980s. We present new geo-thermochronometry data from the Harquahala Mountains • Supporting Information S1 in west-central Arizona to determine the timing of extension, displacement magnitude, and slip rates along the • Table S1 Eagle Eye detachment fault (EED) during large-magnitude Miocene extension. Zircon and apatite (U-Th)/He • Table S2 • Table S3 data (ZHe and AHe, respectively) from 31 samples along a ~55 km extension-parallel transect indicate active • Table S4 slip along the EED occurred between ~21 ± 1 Ma and ~14 Ma. The spatial extent of ZHe ages and exhumation • Table S5 of the zircon partial retention zone indicated ~44 ± 2 km of total displacement, whereas lithologic similarity Correspondence to: and identical U-Pb ages between correlated footwall rocks in the Little Harquahala Mountains and breccia M. G. Prior, clasts at Bullard Peak in the NE Harcuvar Mountains indicated ~43–45 km of displacement across the EED. AHe [email protected] and ZHe data indicated slip rates of ~6.7 + 7.8/À2.3 km/Myr, and ~6.6 + 7.1/À2.0 km/Myr, respectively, both consistent with the duration and displacement estimates. The EED initiated as a listric fault with an ~34 ± 9° dip Citation: that decreased to ~13 ± 5° below ~7 km depth. Secondary breakaway development and footwall exposure Prior,M.G.,D.F.Stockli, andJ.S.Singleton occurred by ~17 Ma, during active EED slip. Lithologic and geo-thermochronometric offset constraints show (2016), Miocene slip history of the Eagle Eye detachment fault, Harquahala excellent agreement and provided a rare opportunity to fully resolve the timing, rates, and total displacement Mountains metamorphic core complex, magnitudes along a major continental detachment fault. west-central Arizona, Tectonics, 35, doi:10.1002/2016TC004241. 1. Introduction Received 18 MAY 2016 Accepted 18 JUL 2016 Low-angle normal faults (detachment faults) are major extensional structures that accommodated large- Accepted article online 22 JUL 2016 magnitude displacements (greater than or equal to tens of kilometers) and exhumed midcrustal rocks within Cordilleran metamorphic core complexes from British Columbia to Sonora, Mexico [Coney, 1980]. The signifi- cant crustal strain and large-magnitude displacements accommodated by detachment fault systems within a variety of extensional tectonic regimes underscores the importance of fully understanding their geometric, thermal, and temporal evolution. Many fundamental concepts regarding the structural evolution of detach- ment faults and metamorphic core complexes were developed in the Colorado River extensional corridor (CREC) [Lister and Davis, 1989; Spencer and Reynolds, 1991; John and Foster, 1993] and subsequently applied to understanding detachment faulting within numerous extensional settings such as the Greek Cycladic islands [Lister et al., 1984; Brichau et al., 2006, 2010], the Appenines [Collettini et al., 2006], and the Rio Grande rift [Ricketts et al., 2015]. Low-temperature thermochronometry has been an important tool for quan- tifying the timing and magnitudes of exhumation and cooling along detachment faults within these varied extensional tectonic regimes [e.g., Stockli, 2005, and references therein]. Numerous studies within the Colorado River extensional corridor of west-central Arizona and eastern California (Figure 1) have focused on determining slip rates, timing of extension, and displacement magnitudes within Cordilleran meta- morphic core complexes [Reynolds and Spencer, 1985; Howard and John, 1987; Davis and Lister, 1988; Spencer and Reynolds, 1991; Foster and John, 1999; Brady, 2002; Singleton et al., 2014]. Despite these previous investigations, the estimates for total displacement magnitudes and slip rates within individual core complexes of the CREC still vary significantly, largely due to the absence of unambiguous offset structural, lithological, and/or thermochronometric markers. The robust quantification of displacement magnitudes, slip rates, and the timing of extension along low-angle normal faults is crucial to understanding how detachment fl ©2016. American Geophysical Union. faults in uence the thermal structure of continental crust during extension. This study presents new geo- All Rights Reserved. thermochronometric data from the footwall and hanging wall of the Eagle Eye detachment fault system in PRIOR ET AL. HARQUAHALA MOUNTAINS MIOCENE SLIP 1 Tectonics 10.1002/2016TC004241 Figure 1. Simplified geologic map of the lower Colorado River extensional corridor of west-central Arizona and southeastern California (modified from Spencer and Reynolds [1989], Richard et al. [2000], and Jennings [1977]). the Harquahala Mountains metamorphic core complex. We integrated these new data with additional lithologic constraints to provide new insights into the slip history and thermal conditions along large- displacement detachment faults. Quantifying the displacement history and geometric evolution of the Eagle Eye detachment contributes new insights on the classic CREC detachment fault system by constraining initial and evolving fault geometries, synextensional deposition and displacement of hanging wall rocks, footwall surface exposure and fault slip rates, which are all key to further understanding how low-angle normal faults evolve within varied extensional tectonic settings. 2. Geologic Background 2.1. Colorado River Extensional Corridor The Colorado River extension corridor (CREC) in the central Basin and Range province contains extensive exposures of highly extended continental crust characterized by numerous type examples of Cordilleran metamorphic core complexes [Coney, 1980]. Metamorphic core complexes within the CREC of west-central Arizona and eastern California contain low-angle normal faults (detachment faults) that exhumed midcrustal mylonites during the Miocene (Figure 1) [Rehrig and Reynolds, 1980; Spencer, 1984; Davis et al., 1986; Howard and John, 1987; Spencer and Reynolds, 1991; Singleton et al., 2014]. The Harquahala Mountains metamorphic PRIOR ET AL. HARQUAHALA MOUNTAINS MIOCENE SLIP 2 Tectonics 10.1002/2016TC004241 core complex is the southernmost exposure of Tertiary mylonites exhumed along the regional corrugated detachment fault system connecting the lower plate of the Chemehuevi, Whipple, Buckskin-Rawhide, Harcuvar, and Harquahala Mountains within the Whipple tilt domain (Figure 1) [Spencer and Reynolds, 1991]. Metamorphic core complexes within the CREC and the Whipple tilt domain have an elongate topo- graphic expression of ~ N60°E that is aligned subparallel to the regional extensional direction of ~ N50° ± 10°E based on mylonite lineations and synextensional dikes (Figure 1) [Richard et al., 1990; Singleton, 2013]. The extension-parallel topographic trend of metamorphic core complexes is distinct from the trend of fault blocks exhumed by high-angle “Basin and Range style” faulting. The prominent topographic expres- sion reflects the corrugated geometry of the regionally correlative detachment fault system that links indivi- dual core complexes within the Whipple tilt domain [Rehrig and Reynolds, 1980; Spencer and Reynolds, 1991]. Published displacement estimates for the Buckskin-Rawhide detachment fault have varied from as much as ~70 km [Spencer and Reynolds, 1991] to more recent estimates of ~40–50 km [Singleton et al., 2014]. Displacement estimates within the adjacent Harcuvar Mountains are ~40–50 km [Reynolds and Spencer, 1985] based on a hypothesized lithologic correlation. We further test the original hypothesis of Reynolds and Spencer [1985] to determine displacement magnitude along the Eagle Eye detachment fault, which is correla- tive with the Bullard detachment fault that bounds the Harcuvar Mountains. South of the Harquahala Mountains distributed high-angle normal faults in the Big Horn Mountains predominate and mark the southern boundary of the Whipple tilt domain [Spencer and Reynolds, 1991]. The location of the Harquahala Mountains metamorphic core complex makes this an excellent location to evaluate changes in detachment geometry, timing of extension, and displacement magnitudes at the southern boundary of this major tilt domain. 2.2. Harquahala Mountains Metamorphic Core Complex The Harquahala Mountains metamorphic core complex is bound to the northeast by the Eagle Eye detach- ment fault, which exhumed primarily Proterozoic and Mesozoic crystalline basement rocks during Miocene extension [Richard et al., 1990]. The Eagle Eye detachment fault is exposed at Eagle Eye Peak (Figure 2) at the northeast end of the Harquahala Mountains and along the southern flank of