The Northern Sacramento Mountains, southwest United States. Part I: Structural profile through a crustal extensional detachment system

Abstract

Abstract Miocene exhumation of the Sacramento Mountains metamorphic core complex in the Colorado River extensional corridor, southwest United States, produced a range of deformational fabrics: pervasive, synthetic ductile deformation (with associated syntectonic intrusion) occurred at depth, discrete antithetic ductile through to brittle extensional shear zones developed at intermediate crustal levels and brittle cataclasis occurred at upper crustal levels. The region divides into three structural domains on the basis of these fabrics: (1) the eastern domain is dominated by a penetrative, ductile fabric with a consistent mineral alignment and stretching lineation, and sense of shear synthethic with detachment fault motion (top-to-northeast); (2) the central domain is characterized by a continuum of overprinting ductile through to brittle extensional shear zones with motion antithetic (topto-southwest) to detachment faulting; and (3) the western domain, where deformation occurred entirely by brittle cataclasis. Structural and petrofabric data from the Sacramento Mountains suggest that increasing structural depth/temperature is exposed in the direction of tectonic transport (N60E), consistent with detachment faulting. This region is interpreted as a profile through a crustal-scale shear zone in which the central domain of discrete antithetic shear zones formed in response to a combination of non-coaxial shear and vertical shortening, and represents an exhumed brittle-ductile transition. Rapid uplift and cooling of the northern Sacramento Mountains in the Miocene facilitated the preservation of this relatively transient regime.