Deformation path partitioning within the transpressional White Mountain shear zone, California and Nevada

Abstract

Most numerical simulations of transpression zones predict a change in finite stretching direction from subhorizontal to subvertical for simple shear-dominated zones. We provide a detailed description of a dextral transpression zone, the White Mountain shear zone (WMSZ), with a range of lineation orientations and compare these natural data to numerical models. The WMSZ is characterized by steeply dipping foliations, with dominant shallowly plunging lineations and coeval subordinate domains of steeply plunging lineations. Within shallowly lineated domains, foliation geometry, shear sense indicators and quartz c-axis fabrics indicate a large component of simple shear, while microstructural and quartz c-axis fabric data from steeply lineated domains indicate a large component of pure shear. Geometric relationships between foliations and lineations and quartz c-axis fabrics demonstrate that lineation orientation has remained constant during much of the deformation history. Comparison of numerical models with the data collected from WMSZ shows that the shear zone geometry and the observed strain path partitioning do not match any of these models. We propose a conceptual kinematic model for the WMSZ involving stable segregated coeval kinematic domains of simple shear-dominated fabrics and pure shear-dominated fabrics that accommodate the transcurrent and contractional components of deformation respectively.