Crustal structure in the central South Island, New Zealand, from the Lake Pukaki seismic experiment

The crustal structure of the central South Island, New Zealand, has been investigated using explosion seismology measurements. A series of 114 shots were fired in Lake Pukaki and recorded in three modes: on a 120 channel, 6 km long, seismic reflection array rolled along a 27 km profile on the eastern margin of Lake Pukaki; on a Reftek seismograph array consisting of 40 units spread over a 52 km long line, partially coincident with the reflection profile; and wide angle reflections from the lower crust recorded on permanent stations of the New Zealand Seismograph Network that were located between 80 and 120 km from the shots. The data show that the greywackeschist forming the mid-upper crust of the Pacific plate is c. 25 km thick near the east coast and thickens as a crustal root to >35 km beneath the Southern Alps. A strong reflection was recorded from a depth of 25 km and is interpreted to be from the base of the greywacke-schist crust overlying an old oceanic crust. Below 6 km depth the compressional seismic velocity is relatively uniform and increases only slightly from 6.2 km/ s to 6.25 km/s at the lower crustal interface. A relatively large velocity gradient of 0.22 (km/s)/km is derived for the upper 6 km of the greywacke basement. Whether this velocity gradient, derived from a relatively short profile, is regional in extent needs to be verified. A Poisson's ratio of 0.21 +/- 0.03 was determined from apparent velocities of Pg and Sg for the greywacke/schist and is interpreted to indicate fracturing in this layer. Reflections, inferred to be from segments of the downdip extension of a broad Alpine Fault Zone, occur at depths of c. 22 and 28 km and indicate a dip of 33 +/- 5 degrees. A width of 7.5 km is estimated for the fault zone at this depth. The dipping reflector segments would intercept the surface west of the Alpine Fault, and suggest that the fault zone is steeper at shallower depths. The total amount of material, which has been subjected to uplift, has been estimated from the amount of crustal shortening and the shape of the crustal root. These estimates indicate that 50% of this material has not been accounted for in previous studies, and suggests either a non-exponential distribution, or larger rates of uplift southeast of the Alpine Fault than thought previously.