This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).
During the Cenozoic, southwestern North America underwent a shift from Farallon subduction to the present Pacific - North American strike-slip plate boundary, the San Andreas fault system (SAF). As a result, much of the southwestern U.S. experienced extension, in part the result of orogenic collapse prior to and during the transition from convergence to the current transform boundary. Yet deformation and volcanism still occur at great distances from the plate boundary, through the Basin and Range, around the edges of the Colorado Plateau, and in the Rocky Mountains. The structure of the continental lithosphere is linked to structures deeper in the upper mantle beneath each of tectonic provinces, and the details of the lithosphere and deeper structure and the overall response to the change in tectonic regime are not clearly understood. As the USArray Transportable Array rolls across the continent, it is uniformly covering the Western U.S., recording teleseismic earthquakes. These data, plus data integrated from the LA RISTRA, Deep Probe, and CD-ROM experiments in addition to data from the COARSE array in Arizona, will provide a wealth of information on the lithosphere and upper mantle structure. These models will be constrained by previous active source and petrologic data. This project is examining, in 3D, the Earth's discontinuity structure from the crust thru the 660 km discontinuity to systematically look for important tectonic/geodynamic indicators: sources of isostatic support, regions of thermal disequilibrium, partial melt, and rheological heterogeneity, slab fragments and slab interactions with the transition zone. The resulting images will be used to interpret the 4D tectonic and geodynamic evolution of the Colorado Plateau in relation to the evolution of its surrounding tectonic provinces: southern Basin and Range, southern Rocky Mountains, and the Rio Grande Rift.
The research uses a combination of surface wave tomography and P- and S-wave receiver functions to clearly image the base of the crust, the lithosphere-asthenosphere boundary (LAB), and the upper mantle structure through the transition zone beneath the Four Corners area of the southwestern U.S. USArray Transportable Array teleseismic data combined with previous broadband array studies have been used to make common conversion point (CCP) stacked PdS and SdP receiver function and surface wave tomography image volumes. The receiver functions have been made with two types of scattered waves: P converted to S (PdS), and S converted to P (SdP). The use of both PdS and SdP allows for independent models of the same area, and provides different frequency bands of investigation and different raypaths to image lithospheric and upper mantle structure. Since receiver functions and surface wave dispersion have different sensitivities to velocity structure jointly inverting the receiver functions and the shear velocity values provides independent estimates of structure. A receiver function images velocity-density discontinuities, not the absolute velocity structure. It senses the high frequency velocity fluctuations, and in some cases density fluctuations in the vertical direction. Phase velocities of surface waves, on the other hand, are most sensitive to the absolute shear velocity structure. In principle the two can be inverted jointly to overcome the non-uniqueness of the receiver-function inversion and the lower vertical resolution of surface-wave tomography.
Mapping 3D variations in the Moho, the LAB, and the transition zone discontinuities with multiple methods will provide a consistent framework for interpreting 4D Cenozoic extension, compression, and local convection features (upwellings, decompression melts, and mantle drips) beneath the southwestern U.S. Results of this study will be useful to non-seismologists interested in continental evolution, extensional dynamics, geodynamics, volcanology, and structural geology, both in the western U.S. and elsewhere. The methodology will be applicable to other regional array data, including future USArray data. Two graduate students will be trained in the use of seismic array data and integrated geophysical interpretation.
In addition to the research objectives of this project, the award will support a new investigator at the University of Southern California and the education and training of two graduate students.
