2012 IRIS Workshop

Structure and Support of the Southern Rocky Mountains from CREST and TA Seismic Data

Steven M. Hansen: University of Wyoming, Ken Dueker: University of Wyoming

A) CREST array base map with simplified geology. The location of all EW cross sections is denoted by the black line B) Ps receiver function section C) Sp receiver function section D) Surface wave tomography section E) Crustal shear wave velocity overlain by density (g/cc) contours F) Mantle temperature model at 75 km depth overlain by velocity (km/s) contours G) Comparing topography and flexure modeling H) Residual topography and scaled free-air gravity I) Observed and predicted Bouguer gravity

Full-resolution graphics file in original format: 0120.ai

With an average elevation >3 km the Southern Rocky Mountains (SRM) in central Colorado crest a broad N-S trending topographic swell demarcating stable North American from the active Western US. Rising from sea-level since the Cretaceous, evidence suggests continuing epeirogenic uplift. High topography is associated with slow seismic velocities, reduced Bouguer gravity and elevated surface heat flow suggesting lithospheric scale processes are at work. The structure and support of the SRM is investigated by surface wave tomography and receiver function imaging from data collected by the Transportable Array and the temporally coincident CREST array. Moho depths derived from Ps imaging show ~48 km thick crust; however, Moho depths are consistently shallower beneath the SRM relative to the adjacent Colorado Plateau (CP) and Great Plains (GP), precluding Airy support. Thickest crust is observed beneath the Cheyenne Belt and a NE trend near the inferred Yavapai-Mazatzal boundary and thus Proterozoic accretion likely played a prominent role in crustal structure. Slowest mean crustal shear wave velocities are observed in the SRM with a N-S trend proximal to Oligocene calderas suggesting crustal felsification during the ignimbrite flare-up. The SRM upper-mantle is slow relative to the adjacent regions, modeled as a temperature increase of ~400°C estimated from the anelastic olivine model from Jackson and Faul (2010). Modified upper-mantle is supported by Sp results which image negative arrivals at 110 and 160 km depth beneath the CP and GP while the SRM exhibits broad arrivals at 80 km. Crust and mantle velocities are mapped to density and the resulting model accurately predicts long wavelength Bouguer gravity and flexural topography. Residual elevation is well correlated with free-air gravity suggesting elastic support. These results indicate that regional topography can be accounted for by lithospheric density variations and thus exogenous forces are not required.

Acknoweldgements: NSF Continental Dynamics program

Keywords: receiver_function, lithosphere, topography, gravity

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