HR: 12:05h
AN: V31F-07    [PDF]
TI: Post-Laramide Volcanism and Upper Mantle Dynamics in the Western US: Role of Small-Scale Convection
AU: * Hernlund, J W
EM: hernlund@ess.ucla.edu
AF: Department of Earth and Space Sciences, Univ. Calif., Los Angeles, 3806 Geology Bldg., Los Angeles, CA 90095-1567 United States
AU: Tackley, P J
EM: ptackley@ess.ucla.edu
AF: Department of Earth and Space Sciences, Univ. Calif., Los Angeles, 3806 Geology Bldg., Los Angeles, CA 90095-1567 United States
AU: Tackley, P J
EM: ptackley@ess.ucla.edu
AF: Institute of Geophysics and Planetary Physics, Univ. Calif., Los Angeles, 3806 Geology Bldg., Los Angeles, CA 90095-1567 United States
AB: Seismic tomography in the Western U.S. reveals ubiquitous small-scale (~200-600 km wavelength) seismic velocity variations at 100 km depth. Many slow anomalies lie beneath recent volcanic fields and exhibit a magnitude that is difficult to explain on the basis of temperature variations alone, implying the presence of partial melt in the asthenosphere. Two- and three-dimensional numerical convection simulations, including the effects of melt and residual composition changes, show that these effects add buoyancy to the system and increase both the vigor and temperature contrast in the resulting small-scale convection relative to cases in which no melting occurs. In our models, the viscosity contrast between lithosphere and asthenosphere is regulated by the activation energy for temperature-dependent viscosity. A high viscosity contrast results in small horizontal temperature anomalies and involves only small portions of the lithosphere (the "compact mode"). A lower viscosity contrast gives rise to larger temperature anomalies by involving a greater portion of lithosphere in the convection, resulting in substantial thinning of the lid by erosion in the form of detached drips (the "drippy mode"). Because the drippy mode occurs for small viscosity contrasts, it is consistent with a weak lithosphere that might arise due to the presence of volatiles. A temporal transition from a "wet" drippy mode (resulting in substantial lithospheric thinning and melt production) to a "dry" compact mode (following volatile removal in melts) may provide an explanation for the observed diachronous character of tectonic and volcanic events in the Western U.S. geologic record. This process would have initiated with the mid-Tertiary ignimbrite flare-up following removal of the flat (and presumably volatile-rich) Farallon slab, and continued through the Miocene when much of the volcanism became basaltic and effusive in character. Such a model is consistent with numerous observations, and perhaps importantly involves only basic small-scale convection processes without requiring the presence of deep-seated mantle plumes. In addition, by experimenting with various imposed strain rates in a moving lithosphere, we have also been able to produce time-progressive volcanism generated by small-scale convection in the form of Richter rolls that presents a viable alternative to the hot spot idea.
UR: http://geodyn.ess.ucla.edu/~hernlund
DE: 8109 Continental tectonics--extensional (0905)
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8145 Physics of magma and magma bodies
DE: 8499 General or miscellaneous
DE: 9604 Cenozoic
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting