FISSION-TRACK ANALYSIS OF THE LATE CENOZOIC VERTICAL KINEMATICS OF CONTINENTAL PACIFIC CRUST, SOUTH ISLAND, NEW-ZEALAND

During the late Cenozoic the Pacific plate has been converging oblique ly with the Australia plate in South Island, New Zealand. A result of this convergence has been the growth of a major mountain range (the So uthern Alps) at the leading edge of the Pacific plate. The results of fission track analysis of 140 samples from 13 transects across the Alp s reported here establish the late Cenozoic vertical kinematics (amoun t, age, and rate of rock uplift) of the Pacific crust underlying the A lps. The late Cenozoic rock uplift of the Pacific crust is asymmetrica l with respect to the Alpine fault, being a maximum (19 km) immediatel y east of the central part of the fault, with lesser values at the eas tern (3 km), northern (10 km), and southern (8 km) extremities of the Alps. The age of the start of rock uplift varies spatially across the Southern Alps, the earliest indications from fission track analysis be ing at 8 Ma at the southern end of the Alps, decreasing to 5 Ma at the northern end and 3 Ma along the southeastern margin. This age variati on reflects the longer time over which the southern parts of the Alps have been in collision. The rate of propagation of rock uplift southea stward into the Pacific plate has been 30 mm/yr, nearly 4 times the la te Cenozoic average rate of convergence normal to the plate boundary. Late Cenozoic mean rock uplift rates range from a maximum of approxima tely 2.8 mm/yr at the Alpine fault to a minimum of approximately 1.0 m m/yr in the east and have been sustained for periods of 3-8 m.y. Accom panying denudation has exhumed amphibolite grade rocks immediately eas t of the Alpine fault. The rock uplift has been controlled by oblique- slip displacement on the Alpine fault. A continental crustal section a t least 19 km thick has been uplifted on the Alpine fault. Comparison of the late Cenozoic mean rock uplift rates with uplift rates derived from reset zircon data (2-10 mm/yr) near the Alpine fault shows that u plift has accelerated over time, but only significantly since 1.3 +/- 0.3 Ma. The amount of Mesozoic uplift ranged from minimal amounts nort h of Arthur's Pass, to approximately 3 km near Mount Cook, to 10 km in the south at Lake Wanaka.