Seismic exploration of continental strike-slip zones

Abstract

Seismic exploration of both Palaeozoic and active strike-slip zones shows strike-slip faults that penetrate all or most of the crust. Offsets on the Moho are evident, particularly at young and active zones with a component of compression, such as the Alpine Fault of New Zealand where a change in crustal thickness of about 20 km is observed. Moho offsets for the old Palaeozoic strike-slip zones are usually much less prominent. Careful migration of crustal seismic reflection data from some of these zones shows that instead of sharp offsets, the Moho structure consists of a localized keel-type crustal thickening of a few kilometres in amplitude and occurs over a zone approximately 10 km wide. The large Moho offsets of young strike-slip zones may in some cases partially decay with time. Active strike-slip zones are becoming an important focus of study, but seismic exploration is hampered by complex near-surface geology, 3D structure and the difficulty of imaging steeply dipping structure in the subsurface. In order to meet this challenge, a wide range of seismic techniques is now being deployed. These include wide-angle seismic reflection, refraction, P-wave delays and the study of guided S-waves. Results from California give geophysical images of vertical strike-slip faults that penetrate to the Moho. In contrast, the Alpine Fault of New Zealand appears to be a surface manifestation of an inclined (∼40°) ramp, extending down to the lower crust and along which uplift and exhumation of the continental crust, and possibly strike-slip motion, is taking place.