HR: 17:45h
AN: T34A-08 INVITED     [Abstracts]
TI: On the Role of Asthenosphere in Thermomechanical Evolution of Oceanic Lithosphere
AU: * Zhong, S
EM: szhong@anquetil.colorado.edu
AF: University of Colorado at Boulder, Dept. of Physics, University of Colorado at Boulder, Boulder, CO 80309 United States
AB: Depending on asthenospheric viscosity, sublithospheric small-scale convection (SSC) may occur to destabilize and erode the bottom portion of lithosphere. This process of SSC has important effects on thermal structure and evolution of lithosphere and surface heat flux and topography. Although the SSC is equally important for continental lithosphere, in this presentation we focus on the SSC in oceanic asthenosphere. We discuss the physical causes and rheological conditions for the onset of SSC, examine the consequences of the SSC to surface heat flux, topography, and lithospheric thermal structure, and compare with observations. The deviations of heat flux and topography from the conductive cooling model at old seafloor have been attributed to the SSC and explained in terms of a plate model. Recent seismic surface wave studies [Ritzwoller et al., 2004] reveal that the Pacific lithosphere and shallow upper mantle for crustal ages older than 70 Ma are significantly warmer than predicted from the conductive cooling model. We found that the seismically determined Pacific structure and their dependence on crustal age are best explained in terms of SSC that starts at 70 Ma [van Hunen et al., 2005]. As the amount of thermomechanical erosion to lithosphere by the SSC depends critically on mantle rheology, we found that to reproduce the seismic results, the upper mantle rheology needs to be non-Newtonian with exponent of 3 and activation energy of ~400 KJ/mol, which is consistent with laboratory results. In the model calculations that satisfactorily reproduce seismic results, the asthenospheric viscosity is ~2x10¹⁹ Pa s. While the SSC explains the elevated surface heat flux at old seafloor relative to the conductive cooling model, the effects of the SSC on surface topography are more complicated. The SSC increases the temperature in the lithosphere by replacing the bottom portion of lithosphere with relatively hot asthenospheric flow, but this process also cools the asthenosphere below. Since the topography is sensitive to thermal structure of the entire mantle, the SSC may not necessarily lead to reduced topography or the plate model. We found that the SSC in plate-scale mantle convection with significant internal heating (60%) may produce increased lithospheric temperature while maintaining uniform mantle temperature, leading to elevated heat flux and reduced topography after onset of the SSC [Huang and Zhong, 2005]. However, the model calculations show that after onset of SSC, there remains significant subsidence in topography, which differs from the topographic flattening as predicted from the plate model but is consistent with seafloor topography with corrections for sediments, seamounts and large igneous provinces and also with seismic results.
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8121 Dynamics: convection currents, and mantle plumes
SC: Tectonophysics [T]
MN: Fall Meeting 2005