HR: 0830h
AN: T41C-0235    [PDF]
TI: On the Relationship Between Rheology and Sublithospheric Small Scale Convection
AU: van Hunen, J
EM: hunen@colorado.edu
AF: Department of Physics, University of Colorado, PO box 390, Boulder, CO 80309 United States
AU: * Zhong, S
EM: szhong@spice.colorado.edu
AF: Department of Physics, University of Colorado, PO box 390, Boulder, CO 80309 United States
AB: Sublithospheric small scale convection (SSC) has been proposed to explain the deviation in oceanic topography and heatflow from the cooling half-space model for oceanic plates spreading away from the mid-ocean ridge. The dynamics of this SSC process have been previously studied in several numerical and laboratory experiments. The process of SSC is the rheology of lithosphere and mantle. Studies with an Newtonian Arrhenius rheology show that the lithospheric age at the onset of SSC $\tau_c$ is related to the Rayleigh number Ra, and rheological activation energy $E^*$ as $\tau_c \sim {\mathrm Ra}^{-0.68} E^{*0.74}$. In the presence of shearing due to plate motion, longitudinal rolls, and not transverse rolls, are the preferred mode of instability, and a moderate dependence of Richter roll onset time on the amount of shearing exists. We further elaborated the rheological influence by exploring the effects of non-Newtonian rheology, in which the deformation rate is related to the ambient stress to the power n. In that case, shearing by plate motion couples to the effective viscosity through the stress-dependence of the rheology. We use a numerical 2-D longitudinal-roll model with imposed background shearing to determine the relationship between $\tau_c$ and the rheological parameters for the general case of non-Newtonian rheology, and results are tested against 3-D models with plate motion. For models with different rheological stress-dependence n, but with a comparable mantle viscosity, a higher n gives a significant reduction in the onset of SSC. Thermal structure of lithosphere above SSC can be compared to the cooling halfspace model, which leads to a 'thermal age' that due to SSC may deviate from the real plate age . We found that numerical models of SSC reproduce the 'thermal age' revealed from a seismic surface wave tomography model. This strongly supports the existence of SSC, and provides constraints on the rheological parameters of the upper mantle.
DE: 7218 Lithosphere and upper mantle
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8155 Plate motions--general
DE: 8162 Rheology--mantle
DE: 8168 Stresses--general
SC: Tectonophysics [T]
MN: 2003 Fall Meeting