Paper No. 2
Presentation Time: 1:45 PM
MANTLE CONVECTION AND THE LATE CENOZOIC EVOLUTION OF SOUTHWESTERN US TOPOGRAPHY
MOUCHA, Robert, Department of Earth Sciences, Syracuse University, 204 Heroy Geology Laboratory, Syracuse, NY 13244, FORTE, Alessandro M., Geotop, Université du Québec à Montréal, Département des Sciences de la Terre et de l'Atmosphère, CP 8888, Succursale Centre-ville, Montreal, QC H3C3P9, Canada, ROWLEY, David B., Department of the Geophysical Sciences, The University of Chicago, 5734 S. Ellis Avenue, Chicago, IL 60637, MITROVICA, Jerry X., Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, MA 02138, SIMMONS, Nathan A., Atmospheric, Earth, and Energy Division, Lawrence Livermore National Laboratory, Livermore, CA 94550-9234 and GRAND, Stephen P., Department of Geological Science, University of Texas, Austin, TX 78712, rmoucha@syr.edu
The Farallon plate (FP) was completely overridden by the North American plate by mid-Cenozoic. Although the FP ceased to exist on the surface, it continues to have a significant tectonic impact on North America. The subducted FP continues to drive a large-scale convective cell below the southern extent of North America, where downward flow in the east is driven by the deep dense Farallon slab and the upward return flow in the west is enhanced by a region of warm mantle beneath the southwestern US [Moucha et al., 2008; 2009]. Herein, we reconstruct the temporal evolution of this flow from a global backward-in-time mantle convection simulation starting with present-day heterogeneity derived from a high-resolution joint seismic-geodynamic global tomography model [Simmons et al., 2009]. A significant result of the joint seismic-geodynamic inversions is that the inferred seismic wave speed to density scalings vary in all three dimensions and thus contain the crucially important effects of intrinsic changes in mantle chemistry and possible partial melting on density. The final density model provides an excellent fit to present-day global geodynamic observables while preserving excellent fit to global seismic data. Additionally, the convection simulation incorporates Newtonian rheology with a viscosity profile that is constrained by global joint inversions of convection-related surface observables and data associated with the response of the Earth to ice-age surface mass loading [Mitrovica and Forte, 2004].
We quantify the effects of this reconstructed flow on the southwestern US tectonics, including the uplift of the Colorado Plateau (CP) and adjacent regions over the last 30 Ma, by presenting a time-dependent numerical model of topography that is supported by convectively maintained vertical stresses generated by the flow in the mantle, termed dynamic topography. We show that: (1) the extension and basaltic volcanism (post 25 Ma) in the central Basin & Range coincides with the arrival and eastward progression of mantle upwelling, and (2) dynamic uplift of the southern CP, totalling about 1 km, transpired in the last 20 Ma. Since 10 Ma, the center of uplift continued northeastward from the southwestern rim of the plateau consistent with a young Grand Canyon model and eastward sweep of magmatism in the CP.
