| 41,460 Sp receiver functions calculated from teleseismic events in 30-120 delta range recorded at Transportable Array and PASSCAL stations are migrated into a common-conversion-point 3D image volume spanning from the west coast of the United States to the Mid-West. This volume covers three separate tectonic regions - 1) Western region consisting of the Pacific, Basin-and-Range, and Columbia Plateau provinces, 2) Proterozoic Colorado Plateau and Rocky Mountains, and 3) Cratonic Great Plains and Central Lowlands. The boundaries of these regions correlate with a change in velocity gradient structure as found in this volume of receiver functions. Moreover, each region has distinct characteristics that bear a unique interpretation. In the western region, there is a pervasive negative velocity gradient (NVG) of ~8% contrast ranging from 70-90 km depth which is interpreted as the Lithosphere-Asthenosphere Boundary. East of the Cordillera hinge-line, the NVG signal beneath the Colorado Plateau distinctly steps down to 120-150 km depth, with some images indicating a doublet signal at both 90 km and 150 km depth. The cratonic portion of this volume contains a NVG signal that is lower in amplitude than the west, with 3% contrast at 80-120 km depth as well as a deeper NVG at 180-220 km depth. Here we interpret the shallower of these two signals within the craton as a Mid-Lithospheric Discontinuity (MLD) and the deeper NVG as the Lithosphere-Asthenosphere Boundary, which has not been previously detected in this region with receiver functions and is a significant finding for those interested in continental dynamics and formation processes and upper mantle structure. |
