Ductile and brittle deformations in the northern snake range, nevada

Abstract

The northern Snake Range, east-central Nevada, is one of the metamorphic core complexes of the Sevier hinterland. Within the range a major décollement separates an ‘upper plate’ composed of brittlely deformed Paleozoic sedimentary rocks (mostly carbonates), from a ‘lower plate’ composed of metamorphic Upper Precambrian-Lower Cambrian rocks, intruded by gneissic granites. A study of the geometry and kinematics of structures and fabrics at outcrop scale and in thin sections indicates that: the northern Snake Range décollement has been a zone of intense non-coaxial E-vergent shear and transport in a ∼- N115°E direction. Outstanding asymmetric boudinage within the marble sheet capping the lower plate testifies for late ductile shear strains (γ) of at least 20. The interface between brittlely and ductilely deformed rocks seems too sharp to represent a regional rheological transition, but might result from two distinct phases of deformation. Ductile deformation in and below the décollement could have occurred before brittle deformation in the upper plate. Brittle faulting in the upper plate related to Basin and Range extension reactivated the upper surface of the ductile shear zone. High topographic relief on the normal faults bounding the range triggered the slide of olistolites from the upper plate into the adjacent Oligo-Miocene basins.