MIOCENE DEXTRAL TRANSTENSION AND SYNEXTENSIONAL CONSTRICTION DRIVEN BY OBLIQUE DIVERGENCE ACROSS THE WESTERN TO CENTRAL ARIZONA METAMORPHIC CORE COMPLEX BELT

Across the western to central Arizona metamorphic core complex belt, detachment faults and their mylonitic footwalls record a minor component of horizontal shortening perpendicular to the Miocene extension direction. This shortening was coeval with extension, producing constrictional strain and folding detachment systems into extension-parallel corrugations. In the Harquahala, White Tank, and South Mountains, brittle constriction was accommodated by WNW- to NW-striking dextral, normal, and oblique dextral-normal faults that record extension roughly parallel to the direction of earlier ductile stretching. We interpret constriction to have been largely driven by the oblique geometry of extension with respect to the core complex belt orientation. The ~250 km-long belt of kinematically related core complexes from the Whipple Mountains to the South Mountains trends 115–120°, which is ~60–75° clockwise of the 045–060° direction of large-magnitude extension. This obliquity resulted in a component of pure shear-dominated dextral transtension. Modeled as a transtensional system with a 60–75° angle of divergence and vertical deformation zone boundaries, 100% NE-SW extension associated with core complex development would correspond to a slightly prolate strain geometry with ~1–5% NW-SE shortening, which matches the amount of shortening during folding of detachment fault corrugations. Oblique divergence thus may account for most (if not all) of the constrictional strain, although far-field stresses associated with the Pacific-North America plate boundary may have also contributed. Parallelism between ductile and brittle strain axes indicates negligible clockwise rotation due to dextral transtension in the ductile regime, and corrugation folding and constriction are primarily recorded by brittle structures. These observations suggest that dextral transtension may have not initiated until the middle to late stages of core complex development, after kinematic linkage between detachment systems established the boundaries of the obliquely-trending core complex belt. This oblique trend may have been initially guided by the location of a Mesozoic crustal welt, by mid-crustal weak zones inherited from Mesozoic tectonism, and/or by basement structures that originated during Proterozoic rifting.