Width of mantle deformation across a continental transform: Evidence from upper mantle (Pn) seismic anisotropy measurements

Seismic anisotropy (delta V(P)) of at least 6.5% +/- 2.5% in the upper mantle (Pn phase) is reported 200 km east of the Alpine fault in South Island, New Zealand. When these new data are combined with a previous Pn study and published SKS splitting results, two distinct domains of mantle deformation are inferred within South Island. In southern South Island, the deforming zone in the mantle lithosphere is as wide as 335 km, whereas in mid-South Island the deforming zone narrows to similar to 200 km. Both regions display wider zones of seismic anisotropy than found at other continental transforms of the world. The width and orientation of mantle anisotropy in southern South Island are consistent with a model of simple shear in a zone that has undergone similar to 800 +/- 200 km of right-lateral shear displacement-a value close to that predicted by geological reconstructions. Hence displacement can be accommodated by distributed shear without faulting being required in the mantle.