Browsing by Subject "Geophysics."
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Item Seismic anisotropy in Texas and Oklahoma and its relationship to tectonic events that shaped southern Laurentia.(2013-09-24) Comiskey, Cody.; Pulliam, Robert Jay.; Geology.; Baylor University. Dept. of Geology.We present new shear wave splitting measurements of approximately 1500 SKS phases recorded at 176 broadband seismic stations deployed in Texas and Oklahoma. SKS splitting on the North American (NA) craton show NE-SW fast axis polarization directions that are generally parallel to the average motion of the NA plate. Around the Southern Oklahoma Aulacogen the fast axis polarization direction changes to NW-SE and parallels the strike of the Aulacogen. Along the transition zone (COTZ), SKS phases show NE-SW polarization direction and parallel to the NA plate. However, larger delay times are observed on the COTZ. Observed patterns of fast axis directions parallel to plate motions, small delay times, and a shallower anisotropic depth 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 is responsible for the seismic anisotropy observed along the COTZ.Item Seismic tomographic imaging reveals possible lithospheric erosion beneath Trans-Pecos Texas and Southeastern New Mexico.(2011-12-19) Rockett, Carrie V.; Pulliam, Robert Jay.; Geology.; Baylor University. Dept. of Geology.Results from the 1999-2001 La Ristra array revealed a fast seismic velocity anomaly beneath the Rio Grande rift, attributed to a lithospheric “drip” into the mantle, perhaps due to edge-driven convection. To investigate this anomaly, the Seismic Investigation of Edge-Driven Convection Associated with the Rio Grande Rift (SIEDCAR) project deployed a two-dimensional array of seismographs with a typical station spacing of ~35 km. Earthquakes of magnitude 5.0 or greater occurring at epicentral distances of 30- 90° were used to create tomographic images with FMTOMO. We present three-dimensional P and S tomographic models of the crust and upper mantle beneath the edge of the rift that confirm the anomaly’s existence and show that it is more laterally extensive than was indicated previously. Our images reveal that the anomaly is disconnected from and adjacent to the Great Plains craton, suggesting convective lithospheric erosion is a likely cause of the fast seismic structure.