Age of metamorphism and uplift in the Haast Schist Group at Haast Pass, Lake Wanaka and Lake Hawea, South Island, New Zealand

Abstract

Potassium-argon ages are reported for mineral and total-rock sampies from the progressively metamorphosed Haast Schist sequence in the Haast valley, Haast Pass, Lake Wanaka, and Lake Hawea areas of Otago, South Island. These rocks extend from low-grade (chlorite zone greenschist facies) near Lake Hawea, through medium-grade (biotite and gamet zone greenschist facies) in the Haast valley, to high-grade (amphibolite facies) near the Alpine Fault. The ages of total-rock and micas show a general eastwards decrease from the Alpine Fault in three zones. (I) More than 25 km east of the fault, the ages increase steadily from 120 to I 73 Ma near Lake Hawea. These ages are interpreted as a cooling age pattern resulting from continued slow uplift, which began after the main (Rangitata) metamorphism at least 173 Ma aga (mid Jurassic) and which continued until at least 120 Ma aga (mid Cretaceous). (2) In a transitional zone, between 15 and 25 km from the fault, the mineral and totalrock ages increase rapidly eastwards from about 20 Ma to 120 Ma. It is possible that these ages may also be a cooling age pattern, reftecting the continuation of uplift at a much slower rate throughout the late Cretaceous and Cenozoic, but it is more probable that the ages represent a section in the metamorphic sequence where only partial accumulation of argon has taken place while buried at depth. (3) In a narrow 10 km zone adjacent to the fault, the mica and total-rock ages are very young (12-30 Ma) but the coexisting hornblende and plagioclase ages are very widely discordant (40-1600 Ma). This indicates that excess argon is definitely present in the plagioclase and hornblende and possibly throughout the rocks as a wh oie. Amounts of excess argon in the high-grade schists are 1-4 nljg, and they decrease westwards to less than 0.5 nljg in the biotite zone greenschist facies and to zero

The approach to steady-state thermochronological distribution following orogenic development in the Southern Alps of New Zealand

“…Prior to this, as far back as the latest Oligocene to early Miocene (Cooper and others, 1987), the Alpine Fault existed as a predominantly strike-slip or mildly transtensional structure linking the Hikurangi and Fiordland subduction zones (Kamp, 1986;Sutherland, 1996;Walcott, 1998), with minimal associated uplift and topography (Nathan and others, 1986;Tippett and Kamp, 1993). Cooling ages vary in systematic fashion across the Southern Alps and can be divided into two main groups or domains of consistent behavior (Adams and Gabites, 1985;Tippett and Kamp, 1993;Batt and others, 2000). In the interior of the orogen, remote from the Alpine Fault, ages are at early Cenozoic to Mesozoic levels .…”

“…Farther south than the Paringa River ( fig. 2), K-Ar mica ages are commonly affected by excess argon contamination (Adams and Gabites, 1985;Batt and others, 2000). Assessing and correcting for the presence of such contamination, however, yields a consistent trend of K-Ar ages increasing progressively southward, from approx 5 Ma at Fox Glacier to as high as 15 Ma for muscovite at Haast Pass (figs.…”

“…A 5 to 20 km wide zone adjacent to the Alpine Fault ( fig. 2) along much of the length of the orogen displays patterns of cooling age variation indicative of the modern dynamics of convergence between the Australian and Pacific plates (Adams and Gabites, 1985;Kamp, Green, and White 1989;Tippett and Kamp, 1993;others, 1999, 2000). Recent assessment of the thermochronological record within this zone others, 1999, 2000;Batt and Braun, 1999) indicates that the orogen is still adjusting dynamically to the development of the present tectonic regime at about 5 Ma.…”

Transpressional development of the Australia‐Pacific boundary through southern South Island, New Zealand: Constraints from Miocene‐Pliocene sediments, Waiho‐1 borehole, South Westland

“…Mineral cooling ages from the Otago Schist indicate that greenschist facies metamorphism occurred during the 253 Cretaceous (Adams & Gabites 1985;Tippett & Kamp 1993). Alpine Schist reaches amphibolite facies near to the Alpine Fault, and samples commonly yield mineral cooling ages of <5 Ma (Adams 1981;Adams & Gabites 1985;Tippet & Kamp 1993).…”

“…Alpine Schist reaches amphibolite facies near to the Alpine Fault, and samples commonly yield mineral cooling ages of <5 Ma (Adams 1981;Adams & Gabites 1985;Tippet & Kamp 1993). The pattern of high-grade metamorphic rocks and young mineral cooling ages near to the Alpine Fault is now accepted by most authors as being related to late Cenozoic uplift and erosion owing to oblique thrusting on the Alpine Fault (e.g., Norris et al 1990;Tippett & Kamp 1995).…”