Rocky Mountain Section - 64th Annual Meeting (9–11 May 2012)

Paper No. 5
Presentation Time: 9:15 AM


HANSEN, Steven M., Cires, University of Colorado, 216 UCB, Boulder, CO 80309 and DUEKER, Ken G., Geology and Geophysics, Univ of Wyoming, Laramie, WY 82071,

Lithospheric structure is of the Southern Rocky Mountain (SRM) region is investigated via surface wave tomography, Ps and Sp receiver function imaging and topographic modeling. Ps imaging finds Moho depths of 40 to 56 km and a negative correlation between topography and Moho depth which rules out Airy isostasy. Conversion of crustal shear velocity to density is performed using empirical fits (Christensen and Mooney, 1995). Mantle temperatures are estimated from anelastic olivine measurements (Jackson and Faul, 2010) and mapped to density via thermal expansion coefficient (Afonso et al. 2005). Calculated buoyancy forces are applied to a 30 km thick elastic plate which matches >200 km scale topography with the exception of the uncompensated Front Range block. A positive free air anomaly and high upper-mantle velocities indicate a strong lithosphere in this region. Our flexure model predicts 1200m of differential topography between the Southern Rock Mountains (SRM) and surrounding Colorado Plateau (CP) and Great Plains (GP) with near equal contributions from crust and mantle density variations. Proterozoic accretion of the Colorado Province appears to have had a prominent role in crustal thickness. Thick crust is found near the Cheyenne Belt and in an E-NE lineament along the inferred location of the Yavapai-Mazatzal boundary (Shaw and Karlstrom, 1999). A marked contrast in mean crustal shear wave velocity is found between the slow SRM and the faster GP and CP regions. Slowest velocities (<3.5 km/s) are found in a N-S trend from the Mt Princeton batholith to the San Juan volcanic field suggesting crustal scale felsification during the Oligocene ignimbrite flare-up, consistent with Bouguer gravity lows and high surface heat flow. The SRM upper-mantle is ~500° warmer although sub-adiabatic and the spatial distribution of temperatures is complex, consistent with variable shear wave splitting and P wave tomography results. We speculate that the mantle lithosphere is currently being eroded although the exact geodynamic mechanisms remain unclear, e.g. post-Oligocene delamination or small scale convection. A modified lithosphere is consistent with Sp imaging results which find negative amplitude arrivals at 110 and 160 km depth beneath the CP and GP in contrast to the SRM which exhibits a broad negative arrival at 80 km.