Mapping Seismic Discontinuinuities in the Crust and Upper Mantle beneath the Southwestern United States from S and P Receiver Functions

The southwestern United States consists of distinctive tectonic provinces including the Great Plain (GP), Rio Grande Rift (RGR), Colorado Plateau (CP) and Basin and Range province (BRP), hence constitutes a prime location for studying mantle dynamics related to continental accretion and rifting. This dissertation images seismic discontinuities in the crust and upper mantle across the southwestern US using P and S receiver functions from teleseismic data recorded at 74 seismic stations from La RISTRA 1.0 and La RISTRA 1.5 arrays. We first estimated the crustal and lithosphere thicknesses across three provinces, GP, RGR and CP from S receiver functions recorded by La RISTRA 1.0 array. Individual receiver functions were stacked according to ray piercing points with moveout corrections to improve the signal-to-noise ratio of the converted Sp phases. A mantle discontinuity, which is interpreted as the lithosphere-asthenosphere boundary (LAB), is observed along the profile with the depth ranging from 80 km beneath the RGR to 100 km beneath GP and 120–180 km beneath the CP. The shallow LAB beneath the RGR is indicative of lithosphere extension and asthenosphere upwarp. We then analyzed both P and S receiver functions at the 17 stations of La RISTRA 1.5 array across the boundary of the CP and BRP. The estimated Moho depth from both P and S receiver functions are in agreement with each other, with the Moho depth of ~30 km in the eastern BRP, increasing gradually to ~45 km beneath the CP. In addition, the image of Sp phases indicates the depth of LAB at ~70 km beneath the BRP, deepening to ~100 km in the transition between the BRP and CP, and further to ~160-180 km beneath the interior of the CP. The images from both seismic arrays were tied together to form a long profile across the four tectonic provinces. The thinning of the crust and the shallow LAB beneath the BRP and RGR are consistent with the rifting processes in the areas. The thick Moho and LAB under the CP indicates that the lithosphere of CP has not been thinned as its neighboring provinces. Two local discontinuities beneath the southeastern and southwestern CP are imaged at ~250 km and ~300 km, which could be the top and base of the eroded lithosphere, respectively. The large variation of the LAB across the BRP-CP and CP-RGR boundaries provides evidence for edge-driven, small-scale mantle convection beneath the area, which is further supported by the image of delaminated lithosphere under the CP. These observations suggest that edge-driven, small-scale convection is probably the mantle source for recent extension and uplift in the southwestern US.