SYSTEMATIC SPATIAL VARIATION OF SEISMIC ANISOTROPY BENEATH THE COLORADO ROCKIES AND ADJACENT REGIONS REVEALED BY CREST

Seismic anisotropy within the Colorado Rockies and adjacent regions measured via teleseismic shear wave splitting has long been noted to feature significant numbers of indeterminate (“null”) measurements, probably attributed to low degrees of azimuthal anisotropy and/or variable vertical anisotropic layering resulting in illegible net anisotropy. The CREST project, in association with interleaved EarthScope USArray stations, has produced a uniform set of splitting parameters for this region using SKS, SKKS, and PKS data with unprecedented spatial resolution. The splitting parameters, observed at about 100 stations spanning the northern Rio Grande Rift (RGR), the Colorado Rockies, and the San Juan Mountains (SJM), display significant and systematic spatial variations. A wide variability in fast directions, ranging from near absolute plate motion (~60^o E of N) for the RGR and SJM regions, to ENE-WSW to E-W directions in northern Colorado are observed. Shear wave splitting times as large as ~2.0 s are estimated at the majority of stations, implying significant mantle anisotropy. The greatest amplitudes are found in the northern RGR and SJM, suggestive of thinner lithosphere and/or increased absolute plate motion-subparallel asthenospheric shear and/or recrystallized olivine matrix in these regions. This broad southern pattern is consistent with that observed for the central RGR region and SE Colorado Plateau in RISTRA and other prior deployments. To the NE of the SJM, however, beneath the Aspen region, fast azimuths rotate coherently by approximately 90^o (to roughly NW-SE), and subsequently rotate back to NE-SW beneath the Front Range. If teleseismic shear wave splitting in south through central Colorado is predominantly due to sub-lithosphere flow, this indicates significantly different flow patterns beneath the two predominant upper mantle low velocity zones imaged beneath the Colorado Rockies by CREST, the San Juan and the Aspen Anomalies. However, such a simple interpretation is complicated by pervasive NE-striking Proterozoic orogenic fabrics across the region that could also significantly influence these near-vertically-integrated anisotropic observables.