Paper No. 8
Presentation Time: 3:30 PM
COMPARING GEOLOGIC AND GEODETIC INFERENCES OF VISCOSITY: EFFECTS OF NON-LINEAR VISCOSITY AND DEPTH SENSITIVITY
HETLAND, Eric A., Department of Earth and Environmental Sciences, University of Michigan, 2534 C. C. Little Building, 1100 North University Avenue, Ann Arbor, MI 48109-1005 and LUI, Ka Yan Semechah, Department of Geological Sciences, University of Michigan, 2534 C. C. Little Building, 1100 North University Avenue, Ann Arbor, MI 48109-1005, ehetland@umich.edu
Estimates of the viscosity of the lower crust from geological studies are often lower than those estimated from geodetic studies. The former studies are based on deformation over time scales up to millions of years, while geodetic studies are either over decadal time scales, for transient postseismic deformation, or time scales of 1-100 kyrs, for steady strain-accumulation. Due to the differing time scales of deformation in these two types of studies, and the possible differing strain-rates of flow in the lower crust, geologic or geodetic studies may be sensitive to different effective viscosities in a non-linear rheology in the lower crust. It also is important to note that all of the estimates of viscosity are model-based, and although the viscosity, or effective viscosity, of the lower-crust is expected to be depth dependent, models usually approximate the lower crust as a single homogeneous viscous layer. Due to this simplification, the inferred lower crust viscosities in either geologic or geodetic studies are bulk values, and the different inferences may merely reflect differing sensitivities to a depth-dependent viscosity.
We use estimates of the effective viscosities from multiple geologic and geodetic studies in order to constrain likely material parameters of the lower crust. For instance, we find that lower-crust viscosity estimates made in Tibet are consistent with a flow law with an exponent of 2-3, and depending on the lower crustal temperatures, is preliminarily consistent with either a quartzite (dry) or diorite (wet) flow law. We also illustrate how inferences of a bulk lower-crustal viscosity made from models of geologic time scale flow are biased to the lowest depth-dependent viscosities in the lower crust, while inferences made from geodetic studies are biased to the highest viscosities. It is also possible that geologic model-based inferences of the bulk lower crustal viscosity may be lower than the lowest actual viscosity in the lower crust. In particular, the low viscosities inferred from geologic studies in Tibet may be quite compatible with the higher viscosities inferred in geodetic studies, without even taking into account the possible non-linearity of the lower crust rheology.
