HR: 17:30h
AN: S22E-07    [PDF]
TI: Strength, strain, and structure of the Australian continental lithosphere
AU: van der Hilst, R D
EM: hilst@mit.edu
AF: Massachusetts Institute of Technology, M.I.T. Room 54-514, Cambridge, MA 02139 United States
AU: * Simons, F J
EM: fjsimons@alum.mit.edu
AF: Princeton University, Guyot Hall 321B, Princeton, NJ 08540 United States
AB: The diverse geologic makeup of Australia makes the continent an ideal candidate to investigate the relation of lithospheric strength to the age of the overlying crust. Australia is made up from Archean cratons, Proterozoic orogens and Phanerozoic formations in a west-to-east age decrease. Surrounded by active earthquake belts, Australia is ideally suited for regional seismic tomographic studies. High-quality seismic data sets from portable SKIPPY instruments have been inverted to yield detailed models of the three-dimensional wave speed structure, including its anisotropy. Australia's topography is subdued, which makes strength measurements using traditional admittance/coherence techniques between gravity and topography rather difficult. However, the development of advanced spectral techniques has made measurements of the relative strength of the lithosphere possible. In particular, the application of multitaper techniques for cross-spectral analysis has enabled us to study elastic thickness variations with location (and thus age) as well as azimuth (measuring strength anisotropy). Using our seismic wave speed model we have estimated the thickness of the high-velocity lid underlying the Australian continent and compared it to coherence estimates of its elastic thickness. The variations in seismic thickness within broad age divisions of the Australian continent are larger than the differences between the means over the age groups. This is especially surprising for the Australian Archean, whose high-velocity lid is far less pronounced than traditional evolution models would have suggested. Following a similar pattern, the mechanical strength of the lithosphere increases with age to first order only, and substantial strength differences exist within domains of equal crustal age. Seismically thicker continental keels are not necessarily mechanically stronger, and shallow mechanical strength does not appear to control the preservation of such keels. The seismic data set and the two-dimensional coherence measurements can respectively be analyzed for anisotropy in the wave speed deviations or mechanical strength variations. Surface-wave tomography and gravity-topography analysis can thus provide independent measures of elastic anisotropy (one instantaneous, the second long-term) and, by implication, strain in the lithospheric upper mantle. The depth variation of their relation resolves a change both in the character of seismic anisotropy and in its relation to strain near $\sim$200 km depth in the Australian subcontinental lithospheric mantle. In our interpretation, the top 200 km of the Australian lithosphere primarily records the coherent signature of past deformation episodes, whereas below 200 km, active processes related to current plate motion provide the dominant explanation for the observed seismic anisotropy. The alignment of the fast axes in the flow direction is consistent with the deformation of a dry olivine mantle by simple shear. The correspondence between seismic fast axes and plate motion of Australia is best when the latter is expressed in a hot-spot reference frame. Thus, seismic anisotropy can add information on plate motion with respect to the underlying mantle that is independent from geodetic and plate-circuit constraints. Finally, the comparison of our results with mantle convection simulations suggests how seismic and mechanical models of the lithosphere are approaching resolutions at which they can be treated as ``data'' to refine forward models, thereby strengthening the crucial links between seismology, tectonics, and geodynamics.
DE: 1236 Rheology of the lithosphere and mantle (8160)
DE: 5104 Fracture and flow
DE: 5120 Plasticity, diffusion, and creep
DE: 7218 Lithosphere and upper mantle
DE: 8164 Stresses--crust and lithosphere
SC: Seismology [S]
MN: 2003 Fall Meeting