Michael G. Prior

Timing, rate, and magnitude of slip on the Buckskin-Rawhide detachment fault, west central Arizona

We present thermochronologic and geochronologic data that constrain the slip history of the Buckskin-Rawhide detachment fault in west central Arizona, one of the largest extensional fault systems in the North American Cordillera. (U-Th)/He zircon and apatite thermochronology, integrated with 40Ar/39Ar geochronology of postdetachment volcanic rocks, indicate that large-magnitude extension associated with the detachment fault initiated at ~21–20 Ma and continued until ~12–11 Ma in the southwestern portion of the Buckskin-Rawhide metamorphic core complex. (U-Th)/He footwall cooling ages from the breakaway zone in the western Bouse Hills to upper greenschist-facies mylonites in the southern Buckskin Mountains indicate that the slip rate on the detachment fault was 3 + 1.5/−1 km/Myr during the early Miocene. Space-time patterns of hanging wall tilting suggest that at 17–16 Ma, a secondary detachment fault breakaway developed ~12 km northeast of the primary detachment fault breakaway. Proximal conglomerates deposited in a supradetachment basin adjacent to the secondary breakaway scarp were displaced 6–11 km northeast in the middle Miocene by the Buckskin-Rawhide detachment fault at a slip rate of 1.2–2.7 km/Myr. The total displacement across the detachment fault in the southwestern portion of the core complex is 24 ± 10 km, well short of the previous estimate of 66 ± 8 km across the entire core complex. Based on these data and new observations, we propose that total displacement on the Buckskin-Rawhide detachment fault system increases in the slip direction to ~40–50 km at the northeastern end of the exposed footwall, corresponding to time-averaged slip rates that ranged from ~2 km/Myr to ≤6 km/Myr across the entire core complex.

Miocene slip history of the Eagle Eye detachment fault, Harquahala Mountains metamorphic core complex, west-central Arizona

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 complexes in the early 1980s. We present new geo-thermochronometry data from the Harquahala Mountains in west-central Arizona to determine the timing of extension, displacement magnitude, and slip rates along the Eagle Eye detachment fault (EED) during large-magnitude Miocene extension. Zircon and apatite (U-Th)/He data (ZHe and AHe, respectively) from 31 samples along a ~55 km extension-parallel transect indicate active slip along the EED occurred between ~21 ± 1 Ma until ~14 Ma. The spatial extent of ZHe ages and exhumation of the zircon partial retention zone indicated ~44 ± 2 km of total displacement, whereas lithologic similarity and identical U-Pb ages between correlated footwall rocks in the Little Harquahala Mountains and breccia clasts at Bullard Peak in the NE Harcuvar Mountains indicated ~43-45 km of displacement across the EED. AHe and ZHe data indicated slip rates of ~6.7 + 7.8/-2.3 km/Myr, and ~6.6 + 7.1/-2.0 km/Myr, respectively, both consistent with the duration and displacement estimates. The EED initiated as a listric fault with an ~34 ± 9° dip that decreased to ~13 ± 5° below ~7 km depth. Secondary breakaway development and footwall exposure occurred by ~17 Ma, during active EED slip. Lithologic and geo-thermochronometric offset constraints show excellent agreement and provided a rare opportunity to fully resolve the timing, rates, and total displacement magnitudes along a major continental detachment fault.