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

Abstract

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 40 Ar/ 39 Ar 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/À1 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.

Shortening in the upper plate of the Buckskin-Rawhide extensional detachment fault, southwestern U.S., and implications for stress conditions during extension

“…(U‐Th)‐He dates are generally 1–3 Myr younger and show similar west‐to‐east cooling [ Brady , ; Singleton et al , ]. The eastern, mylonitic tip of the Harquahala Mountains yielded six (U‐Th)‐He zircon dates at 15–16 Ma [ Prior et al , ], similar to three (U‐Th)‐He zircon dates of 15–17 Ma from mylonitic rocks in the southwestern Buckskin Mountains [ Singleton et al , ]. Both these areas are near the southwestern limit of mylonitic footwall fabrics and so were at temperatures of mylonitization (> ~300°C) when extension began and cooled below ~180°C [ Wolfe and Stockli , ] at 15–17 Ma.…”

“…The basal depositional contact was interpreted as a footwall cutoff of the Buckskin‐Rawhide detachment fault, with the equivalent hanging wall cutoff displaced ~68 km east‐northeast to beneath Date Creek basin (Figure ) [ Spencer and Reynolds , ]. A (U‐Th)‐He thermochronologic profile from the western Bouse Hills northeastward into the southern Buckskin Mountains reveals early Cenozoic cooling in southwesternmost exposures probably due to gradual erosional exhumation, including a date of 64.5 ± 4.3 Ma from zircon [ Singleton et al , ]. The western, nonmylonitic parts of the Harcuvar and Harquahala Mountains and the adjacent Granite Wash and Little Harquahala Mountains yielded (U‐Th)‐He apatite dates as old as 35 Ma and zircon dates as old as 55 Ma [ Carter et al , ; Prior et al , ].…”

The transition from large-magnitude extension to distributed dextral faulting in the Buckskin-Rawhide metamorphic core complex, west-central Arizona

“…Spencer and Reynolds [] correlate lower plate crystalline basement and overlying Oligo‐Miocene strata in the western Bouse Hills with upper plate rocks in the Poachie and Artillery Mountains (Figure ), suggesting ~50–70 km of displacement accumulated across the Buckskin‐Rawhide detachment fault (herein referred to as the Buckskin detachment fault for short). Based on (U‐Th)/He thermochronology and the correlation of a proximal upper plate conglomerate with its lower plate source, Singleton et al [] interpret the total Buckskin detachment fault displacement to increase from 25 ± 11 km across the southwestern portion of the core complex to ~40–50 km at the northeastern end of the exposed lower plate. Slickenlines along the Buckskin detachment fault and lower plate mylonitic fabrics indicate that most extension during the early to middle stages of core complex development was directed toward 040–047° [ Singleton and Mosher , ; Singleton , ].…”

“…Rapid lower plate cooling associated with slip on the Buckskin detachment fault did not initiate until ~21–20 Ma [ Scott et al , ; Singleton et al , ], but extensional basin development locally initiated ~27 Ma [ Lucchitta and Suneson , ]. In the western and central parts of the core complex, quartzo‐feldspathic lower plate rocks most likely cooled to brittle conditions (<350°C) by ~17–16 Ma [ Richard et al , ; Foster et al , ; Scott et al , ; Brady , ; Singleton et al , ]. Low‐temperature thermochronologic data indicate that extensional unroofing in the northeastern end of the core complex continued until ~12–11 Ma [ Bryant et al , ; Brady , ].…”

Synextensional dike emplacement across the footwall of a continental core complex, Chemehuevi Mountains, southeastern California

“…22-21 Ma Swansea Plutonic Suite predates the inception of extension ca. 21-20 Ma (Singleton et al, 2014). We show in the Chemehuevi Mountains, the 21.5-19.2 Ma dikes predate rapid slip ca.…”