Geological transect across the Southern Alps of New Zealand

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SR_1995-030-pdf
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Cox, S.C. 1995 Geological transect across the Southern Alps of New Zealand . Lower Hutt: Institute of Geological & Nuclear Sciences Institute of Geological & Nuclear Sciences science report 95/30 29 p.

Abstract: The region along the Rangitata River, eastern Southern Alps, comprises large river valleys with Quaternary glacial deposits and alluvial fans, surrounded by mountains and hills of Torlesse Supergroup basement. Mountains are c.2100 m elevation about the Main Divide in the west, and decrease to c. 1400 m in the Canterbury foothills, and terraces further east range from 300 m to sea level across the Canterbury Plains in the east. Torlesse rocks comprise alternating graywackes and argillites of mainly Triassic and Jurassic age in steeply dipping sequences. Calc-alkaline Mount Somers Volcanics intruded the Torlesse during the mid Cretaceous, erupting locally onto a Torlesse erosion surface. Extension and subsidence in Late Cretaceous-Tertiary times resulted in deposition of a transgression-regression sequence, which is now only locally exposed along the western margin of the Canterbury Plains. The region is mantled by glacial du and outwash from at least five late Quaternary glacial advances, with the outwash and recent alluvium coalesced to form the Canterbury Plains in the east. The Southern Alps developed in response to late Cenozoic convergence across the Alpine Fault plate boundary. Major structures in the crustal transect, east of the Alpine Fault, include: the Main Divide Fault - a segmented northwesterly dipping backthrust; a sequence of poorly known east-west trending structures west of the Main Divide - possibly part of the Marlborough fault system; the Lake Heron Fault and a zone of active faults 60 km southeast of the Alpine Fault, and the Range Front Fault along the western margin of the Canterbury Plains. Differential uplift and erosion has exposed a metamorphic and structural transition from prehnite-pumpeuyite facies graywackes in the southeast, through greenschist facies semi-schists, to amphibolite fades schists in the northwest beside the Alpine Fault. Structures and lithologies are summarised in a 1:250 000 strip map and interpretative cross-section. (auth)