GEOMETRIC EVOLUTION OF DETACHMENT FAULTS AND FOOTWALL SHEAR ZONES IN THE LOWER COLORADO RIVER EXTENSIONAL CORRIDOR, USA

Forty years after publication of the seminal GSA Memoir “Cordilleran Metamorphic Core Complexes” the geometric evolution of detachment faults and their relationships to footwall mylonite zones remain important research topics in extensional tectonics. Integratation of observations and published data from detachment fault footwalls in the Plomosa, Harquahala, Harcuvar, Buckskin-Rawhide, and Whipple Mountains metamorphic core complexes in the lower Colorado River extensional corridor (SE California and W Arizona) reveal key components of the structural evolution of core complexes. In the uppermost exposed parts of footwalls, early Miocene strata deposited ≤2 Myr before the inception of rapid footwall cooling typically dip ~40–70° SW, indicating that detachment faults initiated with moderate to steep northeast dips near the surface. Thermochronology across the nonmylonitic parts of footwalls suggest ~20–50° of footwall tilting, whereas dikes near the mylonitic front suggest ≤30° tilting. The structurally deepest exposed parts of footwalls record early Miocene mylonitization at ≤~500°C, suggesting footwall shear zones initiated with a low dip (≤10°). Altogether these observations support a geometric model in which detachment faults initiate as broadly listric structures that flatten into mid-crustal shear zones. Magmatism and mylonitization likely initiated ~1–4 Myr before the inception of detachment slip, and the majority of penetrative strain was localized in early Miocene plutons, weak metasedimentary rocks, and/or pre-existing fabrics. Inherited weak zones and thermal softening associated with magmatism thus largely controlled the development of footwall shear zones, which in turn exerted a major influence on the initiation of detachment faults. Rotation of the stress field adjacent to the subhorizontal mid-crustal shear zones most likely gave rise to listric detachment fault geometries. Locally, calcareous mylonites continued to influence detachment fault geometry until the late stages of extension, but in most places detachment faults incised across footwall mylonite zones during exhumation.