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 obliquely with the Australia plate in South Island, New Zealand. A result of this convergence has been the growth of a major mountain range (the Southern Alps) at the leading edge of the Pacific plate. The results of fission track analysis of 140 samples from 13 transects across the Alps reported here establish the late Cenozoic vertical kinematics (amount, age, and rate of rock uplift) of the Pacific crust underlying the Alps. The late Cenozoic rock uplift of the Pacific crust is asymmetrical with respect to the Alpine fault, being a maximum (19 km) immediately east of the central part of the fault, with lesser values at the eastern (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 being 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 variation reflects the longer time over which the southern parts of the Alps have been in collision. The rate of propagation of rock uplift southeastward into the Pacific plate has been 30 mm/yr, nearly 4 times the late Cenozoic average rate of convergence normal to the plate boundary. Late Cenozoic mean rock uplift rates range from a maximum of approximately 2.8 mm/yr at the Alpine fault to a minimum of approximately 1.0 mm/yr in the east and have been sustained for periods of 3-8 m.y. Accompanying denudation has exhumed amphibolite grade rocks immediately east of the Alpine fault. The rock uplift has been controlled by oblique-slip displacement on the Alpine fault. A continental crustal section at 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 uplift has accelerated over time, but only significantly since 1.3 +/- 0.3 Ma. The amount of Mesozoic uplift ranged from minimal amounts north of Arthur's Pass, to approximately 3 km near Mount Cook, to 10 km in the south at Lake Wanaka.