SEISMIC VELOCITY STUDIES IN THE SOUTHERN ALPS, NEW ZEALAND

Abstract

Seismic velocities of outcropping basement rocks were measured at 35 sites east and west of the Alpine Fault. Velocities range from 3–20 km/s to 5–15 km/s and although there are no statistical differences between the velocity distributions east and west of the fault, local velocity variations have geological implications. Estimates of the contribution to velocity of metamorphic grade and alignment of schistosity are made on the basis of field and laboratory results. In some areas velocity is controlled by the degree of crushing of the rock although such low velocities have not been traced deeper than 100 m. The garnet oligoclase schist close to the Alpine Fault consistently has a low velocity (3-2-3·6 km/s). Reflections 11–21 km east of the fault are tentatively explained as being derived either from the top of an old magma chamber, or from the Alpine Fault plane should this dip south-east. The Haast Schist Group was found to have velocities similar to rocks of Mesozoic age.