Structure of the Lithosphere Beneath the Colorado Rocky Mountains Using Multiple Geophysical Datasets

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Structure of the lithosphere beneath the Colorado Rocky Mountains using multiple geophysical datasets Jonathan MacCarthy Geophysics Program, EES Department, NM Tech The Aspen anomaly is a low velocity (-4.5% Vs and -2.5% Vp) upper mantle feature that approximately underlies the highest elevations of the Colorado Rocky Mountains. It is geographically associated with Proterozoic structure in the lithosphere underlying the Colorado Mineral Belt. The coincidence of high topography, evidence of Cenozoic uplift, slow mantle velocities, magmatism, and possible inherited Proterozoic lithospheric structure and associated rheological weakness, together suggest that the Aspen anomaly may be a significant mantle geodynamic influence on the evolution of the central Colorado Rocky Mountains through to the present day. Competing end-member models for the origin of the anomaly are: 1) upwelling asthenosphere associated with Cenozoic mantle modification, and 2) thermal, rheological, and/or compositional variations in the lithosphere resulting from reactivation of much older lithospheric structures. Hybrid models involving interaction between recent mantle reorganization and older lithospheric flaws are also possible. In August of 2008, we deployed 59 IRIS PASSCAL broadband seismographs above the Aspen anomaly with a mean station spacing of 26 km, which will remain in place until October 2009. This IRIS PASSCAL deployment was embedded within the 70-km spacing USArray Transportable array and 4 NEIC sites. In total, this composite array is 94 seismic sites which will provide improved resolution to transition zone depths beneath the Colorado Rockies. We report on resolution tests for seismic and joint seismic/gravity inversion and results from early data from this experiment in the context of Aspen anomaly hypotheses and predictions for three-dimensional upper-mantle velocity heterogeneity. .

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