3D Seismic Traveltime Tomography of the Central South Island, New Zealand

Abstract

The three-dimensional (3D) evolution of the Australian-Pacifi c late boundary in the central South Island of New Zealand is investigated by analysing seismic data from the South Island GeopHysical Transect (SIGHT) project and by using a novel 3D tomography inversion method, FMTOMO. A 380 km-long, 350 km-wide and 56 km-deep 3D tomography image of the P-wave velocity structure and interface geometry of the crust and upper-mantle is constructed by inverting for 164,048 traveltime picks. The picks are both coincident (in-line) and oblique (cross-line) to the survey geometry. The traveltime picks and station elevations were static corrected and reduced to basement level, respectively, to eliminate the highly variable sedimentary component of the inversion process. Synthetic testing of the model space was carried out to help the interpretation of the solution model features. Some model features are consistent with previous results. Usual crustal velocities (5.5 km/s close to the surface and 6.3 km/s at the bottom of the crust) are found at distal ends of the collision zone. Lower velocities (5.7 km/s) intrude the mid-crust of the Australian plate to depths of about 20 km, which is consistent with the downward flexure of the Australian plate. A low velocity zone (5.9 - 6.1 km/s) is situated to the southeast of the Alpine fault, which is consistent with the Alpine fault low velocity zone. Furthermore, a high-velocity body (6.3 km/s) is observed in the top 10 km of the upper-crust immediately above the thickened crust between the west coast of the South Island and the Main Divide of the Southern Alps. This body is interpreted as a drier, more rigid body of schist. A zone of low velocity (5.8 km/s reaching 8 km depth) is observed immediately to the southeast of the aforementioned high velocity body. The feature is interpreted as a back-shearing faulting structure through which fluid escape towards the surface. A flexural analysis of an apparent flexure profile of the Australian Plate along SIGHT line 01 yielded a flexural parameter, a, of 89 km, an elastic thickness, Te, of 14 km and a flexural rigidity, D, of 1.5 : 10^(23) N.m. These results are consistent with results of a flexural analysis of SIGHT line 02W [Harrison 1999]. The following features are derived from the solution model. An apparent gradient in uppermantle anisotropy is observed with seismic velocities increasing towards the south of the model. Also, the geometry of the Mohorovicic discontinuity is apparently smooth between the two main SIGHT transects. The tomography method used in this project proves to be complementary to other coarser-scale and finer-scale seismic studies of the region in that it brings out features that were not seen by them. Notwithstanding that the interface inversion process remains to be perfected in the software, the velocity inversion produced a satisfactory solution model.