POTENTIAL SOURCES OF CRUSTAL ANISOTROPY IN THE WYOMING PROVINCE: INSIGHTS FROM BASEMENT STRUCTURES OF THE BIGHORN MOUNTAINS, WYOMING

The Archean crystalline rocks that form the core of the Bighorn Mountains arch have enjoyed comparative stability during the Proterozoic and Phanerozoic Eons, with the most recent magmatism—emplacement of the Bighorn Batholith –completed by 2.84 Ga (Frost & Chamberlain, CJES 2006) and a position in the upper crust attained in and maintained since Precambrian time (Reiners & Farley 2001). Over wide areas of exposure in the northern Bighorns, biotite-quartz-feldspar rocks of the Bighorn Batholith lack systematic foliation and evidence of shape preferred orientation; the rocks are isotropic at the macro- and megascopic scale. The monotony is interrupted by zones of mylonite and of brecciation up to 50 m wide corresponding to topographic lineaments transecting the Bighorn range, studied as a component of the NSF Earthscope Bighorn Project (bighorns.org) and Keck Geology Consortium 2010 program. Cataclasites in the steep zones are distinctly anisotropic, with penetrative foliation, increased mica and/or chlorite content compared to the source rock, presence of veins and planes of mineralization parallel to tectonic foliation, and increased brittle fracture density. Field studies determine the prevalent orientation of the subvertical ductile to brittle strike-slip zones to be approximately E-W (with variation ±10°), NE-SW, and NNE-SSW. The predominant orientations of mesoscopic brittle shears from field studies and larger-scale fracture arrays identified on satellite imagery fall within those three groups, also. The mineralogy, spacing, scale and extent of the structures is such that there is potential for influence on seismic wave propagation. Possible evidence of this is the correspondence of the ~E-W and NE-SW structures to the fast directions of shear wave propagation within mantle lithosphere that are dominantly E-trending in the Bighorn Mountains and Basin, and NE-trending in the Powder River basin (Anderson et al. 2011 Earthscope).