Seismic Velocity Studies in the Southern Alps, New Zealand

Abstract

Seismic velocities of outcropping basement rocks were measured at 3 5 sites east and west of the Alpine Fault. Velocities range from 3·20km/s to 5·15km/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 deep!'r 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.

Magnetic anomalies over southern New Zealand

“…Within this belt there are highs near Mt Stephenson (north of Haast River), Mt Awful, Apollo Peak, and Mt Aspiring. This belt of anomalies is of particular interest because it crosses the Haast Pass road between Oarke Bluff* and Halfway Bluff where unusual reflections from horizons within the Haast Schist Group have been reported (Garrick & Hatherton 1973). The travel time of the first reflection lies between 0·8 sand 1·0 s indicating that the reflecting body is probably between 2 km and 3 km below the ground surface.…”

“…Therefore, it was suggested (Garrick & Hatherton 1973) that the reflecting horizon could be the surface of an intrusive body from which the dike swarm was derived. Little is known about the physical properties of the dikes but Cooper notes the presence of magnetite as accessory mineral in the hornblende-mica peridotites and camptonites.…”

Upper crustal structure beneath the eastern Southern Alps and the Mackenzie Basin, New Zealand, derived from seismic reflection data

“…The expected vertical profile of different rock types is outlined in Table 1, with depths recalculated into seismic two-way travel times using a 5.0-6.5 km/s velocity range that has been experimentally measured for these rock types (Garrick & Hatherton 1973;Okaya et al 1995;Godfrey et al 2000). Fission tracks in apatite and zircon help to constrain the section in Table 1.…”

Crustal anisotropy in the vicinity of the Alpine Fault Zone, South Island, New Zealand

“…The results of this study quantify the amounts of velocity anisotropy in this orogen. Garrick & Hatherton (1973) provided velocity measurements obtained from seismic refraction studies and simple laboratory measurements; this study presents velocities measured from rock samples using a pressure range that represents depths of 0-30 km.…”