Core complex geometries and regional scale flow in the lower crust

Abstract

A simple flexural model that treats the lithosphere as a thin elastic plate overlying a fluid asthenosphere produces an excellent fit to the domal detachment fault geometries observed in typical core complexes of the Basin and Range Province. However this mode of isostatic compensation requires an unacceptably large reduction in local topographic elevation (3–5 km) above core complex structures. Because topographic lows of this magnitude are not observed above core complexes today, it requires mountain ranges 3–5 km higher than the surrounding areas to have existed at the site of each future core complex and requires these mountain ranges to have had large crustal roots that were subsequently uplifted to form a flat Moho as the core complexes developed. An alternate mode of isostatic compensation, whereby compensation occurs primarily by regional scale flow of material within the lower crust beneath an elastic midcrust, provides an equally good fit to the observed detachment fault geometries, while requiring little reduction in local topographic elevation above developing core complex structures and providing an obvious means of maintaining a flat Moho beneath these zones of upper crustal doming. If this interpretation is correct, then the effective elastic plate thickness below the Basin and Range Province probably cannot exceed a maximum of about 4 km, with a best guess estimate of about 0.5 to 1 km. Computation of the temperature structure associated with crustal flow during doming shows that an increase in temperature at the Moho of about 100°C follows doming by several million years, perhaps explaining the observed coincidence of core complexes with crustally derived igneous rocks. Subsequent cooling of the mid to lower crust beneath the core complex could result in “freezing” of the initially ductile lower crust within the core of the dome, effectively locking the domal shape into the crust even after the detachment surface is cut by younger faults.