Growth and deformation history of alpine schist garnets, Franz Josef Glacier, Westland, New Zealand

Abstract

Late Cenozoic dextral reverse slip along the Alpine Fault and its one km thick mylonite zone in the central South Island has exhumed a mid crustal section of metamorphic rocks. The Alpine Schist is a 25 km wide zone of metamorphosed Torlesse supergroup, increasing from prehnite-pumpellyite fades in the east to amphibolite facies at the Alpine Fault to the west. Although it has been the subject of considerable theoretical modelling studies, field data constraining the mechanism of uplift of the Alpine Schist belt on the Pacific Plate to the east of this fault remains scant. Previous structural studies of the Alpine Schist in the Waiho Valley have focussed on brittle and brittle-ductile structures at the mesoscopic scale (e.g., Holm et al., 1989), while metamorphic petrologists have concentrated on mapping metamorphic isograds and constructing pressure-temperature-time paths (e.g., Grapes & Watanabe, 1992). Until now there has been little detailed structural geological work, especially at the microscopic scale of observation. Microtextural studies of garnet and other minerals provide key insights into the deformational history of the Alpine Schist as a whole. Thermobarometric, compositional zoning and microtextural study of garnets from the Alpine Schist in the Waiho Valley suggest that all garnet growth predates late Cenozoic mylonitisation, but that several diachronous phases of growth took place, possibly including ones in both the Late Cretaceous and Cenozoic. Based on new thermobarometric data, garnets are inferred to have nucleated at depths of 26 - 34 km, with initial growth overgrowing a pre-existing foliation. As garnet growth continued, the remainder of garnet growth was syntectonic with crenulation microfolding of this pre-existing fabric. Peak metamorphism resulted in the formation of a garnet isograd, which is here determined to be strike ~ 060 and dip ~ 85° southeast. The garnet isograd in the Roberts Point region may have been folded by the Roberts Point anticline, supporting the interpretation that garnet growth in part predates D3. New thermobarometric data suggest crustal thinning of the metamorphic pile perpendicular to the foliation since formation of the garnet isograd on the order of 75%. Ubiquitous internal fracturing affects garnets throughout non-mylonitic and mylonitic parts of the Alpine Schist. The radial nature of internal fracturing in garnets and the circular pattern of strain shadows adjacent to garnets, indicates a strain pattern that is oblate, with compression perpendicular to the foliation and extension parallel to it. Estimates of stretch from microstructural analysis of garnet strain shadows suggests extension parallel to the foliation on the order of 100 % (stretch = 2). This equates in isovolumetric perfectly oblate strain to shortening on the order of 75 % (stretch = 0.25). An analysis of deformed veins in the rocks also suggests an oblate pattern of strain with the shortening direction approximately orthogonal to the dominant Alpine Foliation (S3), which strikes ~ 045° and dips ~ 70° southeast. Minimum estimates of stretch for boudinaged veins suggest extension sub-parallel to the foliation greater than 50 % (stretch >1.5). Hornblende prisms in the Alpine Schist are also fractured, and the fractures have been infilled by a retrogressive mineral assemblage consisting of calcite, quartz and chlorite, the same assemblage that fills fractures in garnets. In rare cases grunerite (a sub-calcic amphibole) is formed as an alteration product along hornblende fracture margins. This alteration is interpreted to be the result of fluid infiltration, occurring at depths of ~ 22 km (~ 480°C and ~ 6 kbars). 39Ar/40Ar dating of hornblende by Chamberlain et al. (1995) suggests hornblende cooled through 500°C at 3 - 5 Ma. This suggests fracturing in hornblende and, by inference garnet, is very young. This deformation is interpreted to have been imprinted on the garnets in the late Cenozoic, as the Alpine Schist moved westward across the Southern Alps transpression zone prior to being translated up the Alpine Fault. A statistically significant mineral alignment lineation observed in the planar zone of the Alpine Schist in the Waiho Valley is oriented in the down dip direction, and is interpreted as the direction of maximum finite stretch. This is consistent with flow in a ductile transpressive regime involving a component of vertical thickening. At the base of the Alpine Fault ramp, the rocks are inferred to be subject to dextral normal shearing as they 'turn the corner' up the Alpine Fault. Dextral reverse translation and uplift along the Alpine Fault and associated mylonitisation are the youngest deformation events observed in the rocks.