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 ∼2.8 mm/yr at the Alpine fault to a minimum of ∼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 ∼3 km near Mount Cook, to 10 km in the south at Lake Wanaka.

Oblique continental convergence across the Alpine fault since the mid-Miocene has loaded the Australia plate with the leading edge of the Pacific plate and formed a foreland basin. The basin occurs mostly offshore beneath the continental shelf, but remnants of the marine basin fill overlie Ordovician basement in a narrow coastal strip 10-20 km wide in Westland, between the Tasman Sea and the Alpine fault. The results of fission track analyses of apatite and zircon separates from this basement, integrated with the regional structure and stratigraphy, indicate that the foreland basin formerly extended across the whole of Westland and that its succession increased in thickness eastward toward the Alpine fault, reaching a maximum of about 4 km. Continuing covergence across the main thrust (Alpine fault) has caused its footwall to thicken by reverse fauting in a transitional zone 10-20 km wide immediately west of the mountain front (Southern Alps). Consequently, the inner margin of the foreland basin has been inverted and the succession almost completely eroded. In the southern part of Westland, upper crustal shortening in the Australia plate amounted to about 2 km; associated uplift started about 5 m.y. ago and involved only one major fault-bounded block. In northern Westland, upper crustal shortening amounted to about 12 km; two major faultbounded blocks were involved, and uplift was underway by 10 Ma. Assuming a preuplift geothermal gradient of 25øC/km, the long-term rate of uplift ranged between 0.8 and 2.0 mm/yr in southern Westland and between 0.4 and 1.2 mm/yr in different parts of northern Westland.

Here we analyze topographic and fission track data to quantify the response of the surface of the Pacific plate in South Island, New Zealand, to late Cenozoic oblique continental convergence across the Alpine fault. Over the central 350 km length of the Southern Alps mountain chain we derive and map the rates of mean surface uplift, the rates of working associated with mean surface uplift during the late Cenozoic mountain building, the amounts and rates of denudation and consequent isostatic rebound, and the tectonic component of rock uplift. The rate of mean surface uplift ranges from <0.1 mm/yr adjacent to the Alpine fault to >0.3 mm/yr over most of the area east of the Main Divide. The highest rates of mean surface uplift occur to the southeast of the regions of highest mean elevation and relief. The rate of working against gravity during uplift of the mean surface ranges from ~2.5 mW m '2 in the southwest to ~10 mW m '2 in the central eastern parts of the Alps. Areas of lower mean elevation uplifted most recently have received rates of energy input similar to that of areas of higher mean elevation where uplift started earlier. The amount of denudation is large compared with the mean surface uplift and ranges from ~18 km adjacent to the Alpine fault to ~2 km along the southeast margin of the Southern Alps. The rate of denudation ranges from ~2.5 to ~0.5 mm/yr with increasing distance from the Alpine fault across the Alps to the southeast. The amount of isostatic uplift ranges from a maximum of 14 km adjacent to the Alpine fault to ~2 km along the southeast margin of the Alps. The tectonic component of uplift varies from ~4 km along the Alpine fault to ~1 km along the eastern margin of the Alps. 16,118 KAMP AND TIPPETT: CRUSTAL DYNAMICS, SOUTH ISLAND, NEW ZEAlaND Allis, R.G., Mode of crustal shortening adjacent to the Alpine fault, New Zealand, Tectonics, $, 15-32, 1986. Brown, R.W., Rackstacking spatitc fission-track "stratigraphy": A method for resolving the erosional and isostatic rebound components of tectonic uplift histories, Geology, 19, 74-77, 1991. Cooper, A.F., B.A. Rarreiro, D.L. Kimbrough, and I.M. Martinson, Lamprophyre dike intrusion and age of the Alpine fault, New Zealand, Geology, 15, 941-944, 1987. England, P., and P. Molnar, Surface uplift, uplift of rocks, and exhumation of rocks, Geology, 18, 1173-1177, 1990. Field, B.D., G.H. Browne and others, Cretaceous and Cenozoic sedimentary basins and geological evolution of the Canterbury region,

The Southern Alps are the topographic expression of late Cenozoic ( 4 Ma ago) uplift of the crust of the leading edge of the Pacific plate in South Island, New Zealand. New fission track data on the basement exposed in the Southern Alps quantify the age, amount, and rate of rock uplift, and in combination with geomorphic parameters permit the construction of a new model of the geomorphic evolution of the Southern Alps. The model emphasizes the development over time and space of rock uplift, mean surface elevation, exhumation of crustal section, and relief. The earliest indications of mean surface uplift are between 4 and 5 Ma ago at the Alpine Fault. Mean surface uplift, which lagged the start of rock uplift, propagated southeastward from the Alpine Fault at a rate of 30 km/Ma. By about 4 Ma ago, exhumation had exposed greywacke basement adjacent to and east of the entire 300 km long central section of the Alpine Fault. At 3 Ma ago, greenschist was exposed in the southern parts of the Southern Alps near Lake Wanaka, and since then has become exhumed along a narrow strip east of the Alpine Fault. The model infers that amphibolite grade schist has been exhumed adjacent to the Alpine Fault only in the last 0.3 Ma. The age of the start of rock uplift and the amount and rate of rock uplift, all of which vary spatially, are considered to be the dominant influences on the development of the landscape in the Southern Alps.The Southern Alps have been studied in terms of domains of different rock uplift rate. At present the rate of rock uplift varies from up to 8-10 mmja adjacent to the Alpine Fault to 0.8-1.0 mmja along the southeastern margin of the Southern Alps. This spectrum can be divided into two domains, one northwest of the Main Divide where the present rock uplift rates are very high (up to 8-10 mmja) and exceed the long-term value of 0.8-1.0 mm/a, and another to the southeast of the Main Divide where the long-term rate is 0.8-1.0mm/a. A domain of no uplift lies immediately to the east of the Southern Alps, and is separated from them by a 1.0-1.5 km step in the basement topography. We argue that this spatial sequence of uplift rate domains represents a temporal one.The existing models of the geomorphic development of the Southern Alps-the dynamic cuesta model of J. Adams and the numerical model of P. Koons-are compared with the new data and evolutionary model. Particular constraints unrealized by these two earlier models include the following: the earlier timing of the start of rock uplift of the Southern Alps (8 Ma ago); the spatial variation in the timing of the start of rock uplift (8 Ma ago to 3 Ma ago); the lower long-term rock uplift rate (0.8-1.0mm/a) of the Southern Alps for most of the late Cenozoic; the lag between the start of rock uplift and the start of mean surface uplift; and the patterns of the amounts of late Cenozoic rock uplift and erosion across the Southern Alps.

Fission track data for a suite of basement rock samples from the Southern Alps is used to assess the role of faulting in the late Cenozoic rock uplift of the Pacific plate. The amount of rock uplift derived from sites on both sides of the Moonlight, Ostler, Harper, Torlesse, Porters Pass, and Hope Faults shows that for all faults the vertical offset lies within the uncertainty of the data, typically ±2 km, and is <30% of the surrounding uplift. Major faults are conspicuously absent in the zone of greatest uplift. Over a scale of kilometres, the pattern of rock uplift across the Southern Alps is continuous and regular. The amount of rock uplift increases nearly exponentially with increasing proximity to the Alpine Fault, and the pattern is maintained with little variation over the central 350 km long segment of the Southern Alps. This pattern is primarily the result of southeastward tilting of the middle and upper crust of the Pacific plate, which has ramped up the Alpine Fault in response to oblique convergence and crustal shortening. The geometry of the rock uplift implies that at least part of the Alpine Fault has a listric profile at depth.