ANOMALOUS POST-RIFT FEATURES PRESERVED IN THE LITHOSPHERE OF THE PASSIVE MARGIN OF THE TEXAS GULF COAST

The passage of the EarthScope USArray and a targeted broadband deployment across the Texas Gulf Coast provides data for an integrated seismic investigation of the lithosphere of the Texas Gulf Coast. Seismic results include single station analysis resulting in estimates of SKS anisotropy, estimates of depths to the Moho and Vp/Vs ratios (from RF analysis); 2-D RF (Sp and Ps) profiles from Matagorda Island (a barrier bad) and Johnson City, Tx; Moho P-velocity map from Pn tomography; and 3-D tomographic images of the regional Vp, and Vs. The RF images identify a large region of anomalous lower crust/upper mantle that appears to be depleted mantle as a result active rifting during the 140 Mya rifting event that lead to the opening of the Gulf of Mexico (GoM). This feature appears to be layered, reflecting a less dense, lower velocity upper layer of depleted material underlain by denser more iron rich layer resulting from fractionation in the magma chamber during the rifting event. A portion of this lower layer appears to have delaminated from in the region near the Balcones Fault Zone which may coincide with an episode of post rift volcanism. A negative anomaly in Sp RFs at a depth just over 100 km is interpreted to be the traditional LAB but a positive anomaly appears at a depth of ~180 to ~200 km. Additional semi-contiguous phases are observed in the Sp RF image between the 100 and 200 km depth which we attribute to a ~ 100km thick transitional LAB characterized as a shear zone. The Ps RFsappear to image the LAB but at a depth consistent with a semi contiguous horizon in the middle of the transitional LAB from the Sp image. Tomography finds normal P-velocities throughout this layer but low S-velocities. The lacks of a P-velocity anomaly in the presence of slow S-velocities together with strong SKS anisotropy supports the interpretation of the anomalous transitional LAB as a region of extreme shearing.