DYNAMIC LITHOSPHERE BENEATH THE SOUTHWESTERN US

The deployment of EarthScope Transportable Array (TA) seismometers provides the ability to examine the crust and upper mantle beneath the US in unprecedented detail. We use data from the TA and other openly available seismic stations to image sharp structural layers and measure bulk shear wave velocities for the southwestern US. The goal is both to better understand the Cenozoic evolution of the region and to explore the relationship between crustal and upper mantle processes in an area of active tectonism.

Initial results from this work support the hypothesis that the upper mantle in the southwestern US is actively deforming and that this deformation is driving many of the processes occurring in the upper crust. Receiver functions show significant variations in crustal thicknesses and crustal shear velocities, calculated using Rayleigh wave tomography, are largely consistent within individual physiographic provinces. Within the upper mantle, we identify a high shear-velocity layer, which we interpret as lithosphere. A lower-velocity layer, interpreted as asthenosphere, typically underlies this. Based on these velocity measurements we estimate lithospheric thicknesses for the region which show significant variation in the depth of the lithosphere asthenosphere boundary (LAB). Sharp gradients in LAB depth often correlate with physiographic boundaries, regions of active volcanism, topography, and regions of concentrated strain. Based on LAB depths we identify several regions of thinned mantle lithosphere. This thinning is likely caused by shear stresses related to extension or to the mechanical removal of lithospheric material. This process of lithospheric removal has previously been hypothesized for several areas in the southwestern US based on body-wave tomography studies that show high-velocity features in the upper asthenosphere.

Our results suggest that the upper-mantle within the southwestern SW US has been significantly reworked since the end of the Laramide orogeny. This has produced significant variations in lithospheric thickness that correlate well with topography, volcanism, seismicity and crustal thickness, suggesting that the mantle lithosphere has a large impact on strain within the crust.