SEISMIC ANISOTROPY IN TEXAS AND OKLAHOMA: INVESTIGATION INTO TECTONIC EVENTS THAT SHAPED SOUTHERN LAURENTIA

Understanding the causes, strength, and orientation of seismic anisotropy in can provide clues to the tectonic processes that shape the Earth. We present new shear wave splitting measurements using approximately 1500 SKS phases recorded at 189 transportable array and permanent broadband seismic stations deployed in Texas and Oklahoma. Measurements from SKS phases on the craton show a NE-SW fast axis polarization direction that is parallel to subparallel to the absolute motion of the North American plate. In the vicinity of the Southern Oklahoma Aulacogen the fast axis polarization direction changes to NW-SE and parallels the strike of the Aulacogen. This orientation is consistent with other rifts of comparable age and suggests rift-parallel dike intrusions as the cause for the observed anisotropy.

An analysis of analysis of the distribution of SKS splitting measurements and orientations reveals that the depth to the anisotropic layer beneath continental-oceanic transition zone (COTZ) is significantly greater than beneath the craton. Along the COTZ, SKS phases show a NE-SW fast axis polarization and are again consistent with the motion of the North American plate. However, large delay times, approaching 2.3s, are observed on the COTZ, compared to delay times of less than 1.5s observed on the craton. We performed an analysis that uses the geographic distribution of measurements to calculate the depth to the anisotropic layer beneath each particular study area. Our findings indicate the depth to the anisotropic layer for the craton at approx. 180km and a depth approaching 300km for the COTZ region. This observed pattern of fast axis directions parallel to plate motions, small delay times, and a shallower depth to the anisotropic zone are consistent with a lithospheric origin for the observed anisotropy on the craton. Larger delay times and a deeper anisotropic source suggest that asthenospheric flow around the keel of the North American craton is responsible for the seismic anisotropy observed along the COTZ.