PRODUCTION OF MULTIPLE FOLIATIONS IN GARNET ZONE ROCKS: A RESPONSE TO SEVIER CRUSTAL THICKENING AND EXTENSION IN THE PEQUOP MOUNTAINS, NEVADA

The Pequop Mountains of northeast Nevada present an ideal location for studying metamorphism and ductile deformation of sedimentary layers caused by thrust burial. Two phases of contraction in the region occurred by motion on the Windermere and Independence thrusts during the Cretaceous Sevier orogeny. Tertiary extension caused uplift and rotation of the rocks, exposing garnet through chlorite metamorphic zones.  Most of the hanging walls of the thrust faults have been eroded away, leaving the metamorphosed footwall layers exposed at the surface. However, a sliver of the hanging wall of the smaller and younger Independence thrust still remains. We spent six days field mapping predominantly within the footwall of the Independence thrust in the Pequop Mountains. Outcrop and thin section analysis of 38 samples revealed that the dominant foliation in the region is S₂, which dips moderately southeast, subparallel to compositional layering.  Biotite porphyroblasts, which we interpret to be peak-metamorphic, predate S₂.  Locally, especially in the highest grade rocks, S₂ is a high-strain, penetrative foliation that formed concurrently with a well developed east-trending mineral stretching lineation.  In other cases S₂ is a variably developed crenulation cleavage that overprints an S₁ foliation still measurable in thin section.  Inclusion trails in biotite porphyroblasts in rocks containing penetrative S₂ dip more shallowly southeast than S₂; we interpret these to be S₁.  If compositional layering was mostly subhorizontal throughout the formation of S₁ and S₂, then S₁ would have formed with a shallow NW dip.  We agree with Camilleri (1998, JSG) that the dominant foliation (the one we call S₂) was produced by nearly coaxial vertical shortening of the crust following thrusting.  This may have been an adjustment to the taper of the Sevier orogenic wedge as deformation propagated eastward.  Our results indicate, however, that there was an earlier foliation (S₁) that we interpret to have formed during southeast-directed Windemere thrusting.  GIS analysis of our field data, combined with the thin section study, revealed no evidence of additional fabric development, metamorphism or even disruption of metamorphic isograds across the later Independence thrust.