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

Abstract

Brittle fault data from the Buckskin-Rawhide metamorphic core complex, west-central Arizona, document the Miocene transition from large-magnitude, NE directed extension to distributed E-W extension and dextral faulting. The Buckskin-Rawhide detachment fault locally records a clockwise rotation of the slip direction from dominant top-NE directed slip to ENE and E directed slip during the last stages of activity. Postmylonitic brittle deformation is dominated by normal and dextral slip along moderately to steeply NE dipping faults. Relative timing relationships indicate that most postmylonitic normal faulting was coeval with middle Miocene detachment fault slip, whereas dextral and oblique dextral faulting postdates detachment fault slip (post~11 Ma). Based on the persistent SW dip of lower plate fabrics and NE dip of abundant normal faults, the amount of NE-SW brittle extension across the mylonitic lower plate is estimated to be 20-30% in several areas. The end of detachment faulting in the Buckskin-Rawhide metamorphic core complex was marked by a transition to dominantly E-W extension and associated dextral-normal faulting. At least 10 NE dipping postdetachment faults record~0.1-1 km dextral or oblique dextral displacement. The cumulative amount of dextral shear across the core complex is probably 7-9 km, which is the amount needed to restore the topographic trend of lower plate corrugations into alignment with the dominant extension direction. Postdetachment dextral/transtensional faulting across the core complex reflects the increasing influence of the diffuse Pacific-North America transform plate boundary toward the end of the middle Miocene.

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

“…Both displacement paths are based on the inference that the western Bouse Hills is the footwall cutoff for the base of Oligocene to lower Miocene strata with an equivalent hanging wall cutoff that is displaced to the axis of the Date Creek basin half graben. Breaks in the lines represent postdetachment strike‐slip displacement, inferred to be substantial by Singleton []. Note that the northern Plomosa Mountains detachment fault in the southwestern corner of the map area [ Spencer et al , ] projects below the Bouse Hills and the Harcuvar core complex.…”

“…The arcuate path of the proposed extension vector is not unique, as a similar arcuate extension vector characterized exhumation of the Godzilla deep‐sea core complex in the Philippine Sea [ Spencer and Ohara , ]. An alternative restoration that places the western Bouse Hills beneath the Artillery Mountains (Figure , green line) is more consistent with mylonitic lineation trends in the Buckskin and Rawhide Mountains [ Singleton , ] but is discordant to the Harcuvar groove and its mylonitic lineations. With a conservative estimate of 10 km preextension average depth to mylonitic footwall rocks, either displacement path represents removal of ~20,000 km 3 of rock from above the complex with almost none remaining.…”

Distributed Neogene faulting across the western to central Arizona metamorphic core complex belt: Synextensional constriction and superposition of the Pacific–North America plate boundary on the southern Basin and Range

“…In western Arizona (USA), NW-striking dextral faults cut an extensional detachment fault system that accommodated tens of kilometers of top-tothe-NE slip (Singleton, 2015). This dextral faulting may have initiated near the end of detachment fault slip in response to eastward migration of Pacific-North America shear in the middle Miocene, resulting in a transition from large-magnitude extension to distributed transtension (Singleton, 2015). Alternatively, dextral faults and conjugate north-striking sinistral faults may have initiated during detachment faulting to accommodate subhorizontal y-axis shortening in a constrictional strain regime.…”

“…While there is some uncertainty in when and why strike-slip faulting initiated across the western Arizona core complex belt, dextral and oblique dextral slip on NW-striking faults clearly represents the dominant style of faulting following cessation of detachment slip at ca. 14-12 Ma (Richard et al, 1990;Singleton, 2015), accommodating a minor component of distributed Pacific-North America shear. In the Buckskin-Rawhide core complex, postdetachment dextral faults reoriented the range-scale topographic trend of mylonitic footwall corrugations to an orientation clockwise of the detachment fault slip direction (Singleton, 2015).…”

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

“…Assuming no preexisting anisotropies, dikes are predicted to intrude roughly orthogonal to the extension direction (Anderson, 1951), as they do in the Newberry Mountains (Spencer, 1985), Whipple Mountains (Gans and Gentry, 2016), Harquahala Mountains (Richard, et al, 1990), and Buckskin Mountains (Singleton, 2015). Large populations of Miocene dikes in the Chemehuevi Mountains strike east-west or northeast-southwest, parallel or oblique to the extension direction.…”