SEISMIC EXPLORATION OF CONTINENTAL STRIKE-SLIP ZONES

Seismic exploration of both Palaeozoic and active strike-slip zones sh ows strike-slip faults that penetrate all or most of the crust. Offset s on the Moho are evident, particularly at young and active zones with a component of compression, such as the Alpine Fault of New Zealand w here a change in crustal thickness of about 20 km is observed. Moho of fsets for the old Palaeozoic strike-slip zones are usually much less p rominent. Careful migration of crustal seismic reflection data from so me of these zones shows that instead of sharp offsets, the Moho struct ure consists of a localized keel-type crustal thickening of a few kilo metres in amplitude and occurs over a zone approximately 10 km wide. T he large Moho offsets of young strike-slip zones may in some cases par tially decay with time. Active strike-slip zones are becoming an impor tant focus of study, but seismic exploration is hampered by complex ne ar-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 inclu de wide-angle seismic reflection, refraction, P-wave delays and the st udy of guided S-waves. Results from California give geophysical images of vertical strike-slip faults that penetrate to the Moho. In contras t, the Alpine Fault of New Zealand appears to be a surface manifestati on of an inclined (similar to 40 degrees) ramp, extending down to the lower crust and along which uplift and exhumation of the continental c rust, and possibly strike-slip motion, is taking place. (C) 1998 Elsev ier Science B.V. All rights reserved.