Abstract

A field and petrofabric study of the southwestern part of the Pine Mountain thrust sheet shows that most deformation features may be attributed to one of two processes. The first, layer-parallel compression, is responsible for (a) a layer-parallel maximum shortening direction in the mechanically twinned fabric of the limestone and dolomite country rock, (b) transport- parallel extension fractures and joints, (c) transport-normal stylolite surfaces, and (d) some small thrusts. The second, bending at thrust-ramp hinges, is responsible for (a) non-bedding-parallel maximum shortening directions in mechanically twinned calcite from fracture fillings, (b)transport-normal extension fractures and joints, and (c) small normal faults. Possible explanations for the failure of calcite in the country rock to twin in response to bending at the ramp include both (1) an early imposed, pre-thrusting, calcite twin strain hardening and (2) deformation of the country rock by competing mechanisms such as pressure solution. Small normal faults are parallel to the strike of the thrust sheet and seem to have evolved from strike-parallel extension fractures in response to continued extension. Both small thrusts and stylolite surfaces are rare in outcrop. There seems to be no major change in deformation of rocks which have traveled through different fractions of the total ramp length, so long as the rocks have passed onto the ramp.

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