Mantle Deformation and Anisotropy beneath a Continental Transform Fault, Offshore
New Zealand
Anne Sheehan and Peter Molnar, University of Colorado, Bradford H. Hager,
Massachusetts Institute of Technology; John A. Collins and J. Gregory Hirth, Woods
Hole Oceanographic Institution
We propose an onshore/offshore passive source seismic experiment at New Zealand to
explore mantle deformation associated with large scale continental transform faulting
and/or shear in the underlying asthenosphere. The deformation will be studied through
measurements of seismic anisotropy and associated geodynamic modeling for strike-slip
shear and different rheological structures of the asthenosphere.
We plan a multidisciplinary approach, including seismology, geodynamics, and mineral
physics. We envision the use of 20 broadband ocean bottom seismometers, with most
deployed on the submerged continental lithosphere northwest of the Alpine Fault. The
OBS's will be supplemented by existing and planned land-based stations operated by
New Zealand colleagues at IGNS and Victoria University. We plan to constrain amounts
and orientations of anisotropy using S-wave splitting, surface wave dispersion, azimuthal
variations in Pn and Sn travel times, and receiver functions. We seek to determine
whether
the thin viscous sheet model of deformation as a continuum in the mantle lithosphere can
account for the anisotropy and is appropriate for continental transform faulting, or if
instead deformation is localized on faults in the mantle lithosphere and anisotropy
develops because of shear of the underlying asthenosphere.
Both the depth and width of the anisotropic zone will be used to determine how localized
deformation is in the lithosphere and the relative contributions of lithosphere and
ansthenosphere to the measured anisotropy. These results can be compared to those from
a proposed study of an oceanic transform boundary at the Macquirie ridge. The difference
in rheology between oceanic and continental lithosphere should produce significantly
different flow fields. The contribution of fluids and thermal effects will be explored
through the joint use of velocity and attenuation tomography. Secondary studies will
include the exploration of the Lehmann discontinuity on adjacent continental and oceanic
lithosphere and its interpretation as either a thermal or an anisotropic boundary layer.