Temporal Changes in Deformation Mode: From Failure to Flow in the Colorado River Extensional Corridor

Abstract

The Colorado River Extensional Corridor (CREC) of southeastern California, southern Nevada, and western Arizona experienced up to 100% extension between ~23 and 12 Ma. Extension was accommodated by low-angle normal faulting in the upper and middle crust, subvertical brittle failure of the lower crust, emplacement of mantle-derived magma in a crustal-scale fracture, and, later, crustal flow. Evidence for brittle, whole-crustal failure includes geophysical observations, structural analysis, geochronology, thermochronology, geobarometry, and paleomagnetic studies of surface exposures of syntectonic, mantle-derived mafic plutonic sheets emplaced as subvertical, crustal-scale intrusions tracking the zone of maximum extension. Their subsurface extent and geometry are interpreted from the shape and magnitude of the Colorado River gravity high, a 10-20 mGal positive gravity anomaly that follows the belt of core complexes west of the Colorado River for ~150 km along strike. In the southern Sacramento Mountains (latitude 34°40' N), mid-crustal rocks exposed in the footwall of the extensional fault system comprise brittle-deformed, syntectonic intrusive rocks emplaced into variably mylonitized gneiss and granite as three intrusive pulses between 19.1 and ~16 Ma. Mingled, mantle-derived diorite and granite accommodated 5-18 km of magmatic extension (10-20% total extension) in the CREC at this latitude. Analogous regions interpreted to be undergoing brittle failure of the lower crust include the East African, Baikal, and Rio Grande rifts. Each exhibit lower-crustal seismicity on normal faults with dips ≥ 70°. Miocene conditions in the CREC are interpreted to include high strain rates (10-16s-1 to 10-14s-1) and moderate heat flow, both conditions considered necessary for brittle lower-crustal failure in the East African Rift. Lower-crustal flow was initiated after magmatism, buoying mid-crustal rocks to shallow structural levels, as low-angle normal faulting in the upper and middle crust accommodated extension. Ductile flow of the lower crust allowed the maintenance of nearly constant crustal thickness and created domed core complexes in areas of extreme extension. In the Colorado River extensional corridor, the lower and middle crust behaved ductilely only in late stages of extension, after brittle failure of the lower crust. This change from brittle failure to ductile flow indicates a change in style of deformation from narrow rift-zone to core-complex mode.