HR: 12:05h
AN: T22B-08    [Abstracts]
TI: Modelling Strain and Anisotropy Near the Southern Alpine Fault, New Zealand
AU: * Savage, M K
EM: Martha.Savage@vuw.ac.nz
AF: Institute of Geophysics, Victoria University of Wellington, Wellington, 6005 New Zealand
AU: Tommasi, A
T22B-08 AF: Laboratoire Tectonophysique, University of Montpellier II, Montpellier, 34095 France
AU: Ellis, S
T22B-08 AF: Institute of Geological, and Nuclear Sciences, Lower Hutt, 6008 New Zealand
AU: Chery, J
T22B-08 AF: Laboratoire Tectonophysique, University of Montpellier II, Montpellier, 34095 France
AU: Chery, J
T22B-08 AF: Institute of Geological, and Nuclear Sciences, Lower Hutt, 6008 New Zealand
AB: The southern Alpine Fault (AF) marks a sharp boundary in the upper crust between the Australian and Pacific plates, yet strong and widespread anisotropy inferred across the width of New Zealand (NZ) from shear wave splitting suggests that the mantle undergoes diffuse deformation in this region of relatively thin continental crust. To better understand the origin of the anisotropy we calculate the strain from geodynamic models, and compute resultant shear wave splitting to compare to observations. We apply kinematic boundary conditions appropriate for the plate interaction and allow for the evolution of temperature-dependent, linear or nonlinear viscosity and for anisotropy to be determined using polycrystal plasticity theory. Increased thickness of the crust in the continental compared to the adjacent oceanic regions localises the strain under the continental landmass, which may help to explain the large width of the shear zone in NZ. Nonlinear viscosity helps to further localise the strain and seismic anisotropy in the continental landmass. We assume initially random orientation of grains in the mantle, which align with the applied strain. Before the last 10 My motion was nearly purely strike-slip along the AF. During this time period, we assume a 27 km thick continental crust, thinning to 15 and 20 km, respectively, in the oceanic regions of the Tasman Sea and Pacific Ocean. We apply strike slip motion of 35 mm per year at the edges of the plates. For small strains, the pure strike-slip motion causes shear wave splitting fast polarisation directions to align parallel to the extension direction, at 45 degrees to the AF, and delay times are small (less than 1 s for a 100 km thick mantle). Delay times increase to 2.0 s and the fast direction rotates to become closer to fault-parallel with larger strain. The localisation provided by the nonlinear viscosity models causes the delay times to saturate before 20My, at which point the fast directions are 25 degrees from fault-parallel, similar to measurements in southern NZ. Further strain does not increase the delay times, but the fast direction of anisotropy rotates until it is 15 degrees from the fault after 40 My. These values suggest that dynamic recrystalisation, which is not included in our models, might be needed to align the fast directions to be fault-parallel as they are measured along the central AF. To simulate the change in plate motion at 10 My, we add a compressional component of 10 mm/yr to the strike-slip motion, and use the present inferred temperature structure. A cold root inhibits the strain, suggesting that the anisotropy measured now may be "frozen in" from the period of strike-slip deformation that occurred before the development of the present mantle root. We plan to further test this hypothesis by examining the effect that temperature dependent density has on the resulting strain and anisotropy field.
UR: http://www.geo.vuw.ac.nz/research/projects/anisotropy/index.html
DE: 8106 Continental margins: transform
DE: 8108 Continental tectonics: compressional
DE: 8111 Continental tectonics: strike-slip and transform
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8162 Rheology: mantle (8033)
SC: Tectonophysics [T]
MN: Fall Meeting 2005