A Crustal Cross-Section for a Terrain of Superimposed Shortening and Extension: Ruby Mountains-East Humboldt Range Metamorphic Core Complex, Nevada

Abstract

Geologic mapping coupled with geochronological, thermobarometric, and seismic reflection studies provide the data for constructing a crustal cross-section through a Tertiary extensional orogen in the eastern Great Basin, western U.S. Cordillera. Oligocene-Miocene sedimentary and volcanic rocks were deposited during brittle, upper-level crustal extension dominated by high-angle normal faults, rotation of the strata and the faults themselves, and the progressive evolution of a low-angle detachment fault system. Together with nonmetamorphosed to very low-grade rocks of the Cordilleran miogeocline, the synextensional strata comprise an upper crustal suprastructure that was attenuated during Tertiary crustal extension. Structurally below the suprastructure and commonly separated from it by a regional detachment fault is a transitional metasedimentary/granitoid zone which preserves a principally Mesozoic magmatic, metamorphic, and deformational history. In turn, this zone grades downward abruptly into a 1.5- to 2.0-km-thick, upper amphibolite facies Tertiary shear zone that forms the top of a mylonitic to nonmylonitic migmatitic infrastructure of variable age. This infrastructural zone clearly records a complex Mesozoic history, but is in part characterized by a Tertiary magmatic-metamorphic-deformational history that appears to increase in intensity with depth.