Geology of the Haast area

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Rattenbury, M.S.; Jongens, R.; Cox, S.C. (comps) 2010 Geology of the Haast area. Lower Hutt: GNS Science. Institute of Geological & Nuclear Sciences 1:250,000 geological map 14 58 p. + 1 folded map

Abstract: The Haast 1:250 000 geological map covers 6348 km2 of onshore South Westland and northwest Otago in the South Island of New Zealand, and straddles the boundary between the Pacific and Australian tectonic plates. The map area is crossed by the Alpine and Livingstone faults, amongst others, which juxtapose a variety of Paleozoic to Mesozoic rocks belonging to six tectonostratigraphic terranes. Northwest of the Alpine Fault, early Paleozoic metasedimentary and plutonic basement rocks are a fragment of the Gondwana supercontinent. They include the early Paleozoic Buller terrane (part of the Western Province), represented by the metasedimentary Greenland Group, and the intruding mid-Paleozoic granitic rocks of the Karamea and Paringa suites. Southeast of the Alpine Fault are a series of Mesozoic rocks (part of the Eastern Province) that were accreted to the Gondwana margin. They include deformed and attenuated parts of the Permian-Jurassic Brook Street, Murihiku, Dun Mountain-Maitai, Rakaia and Caples terranes, with metamorphosed parts of the latter two collectively termed the Haast Schist. Convergent margin tectonism prevailed until the late Early Cretaceous, when rocks of the Eastern and Western provinces were uplifted and formed a comparatively stable basement to younger Cretaceous and Cenozoic sedimentation. The break-up of Gondwana began in the Early Cretaceous and led to the opening of the Tasman Sea. Localised fault activity and clastic sedimentary basin development occurred in the late Early to Late Cretaceous, and again in Late Cenozoic time, with intervening deposition of widespread limestone and mudstone in the passive margin environments of the Paleocene, Eocene and Oligocene. Sedimentation over this period was punctuated by intraplate basaltic volcanism. The modern Australian-Pacific plate boundary began developing as a major structural entity through the region during the Early Miocene. The influx of clastic sedimentation during the Late Cenozoic reflects development of the present oblique-compressional plate boundary and uplift of the Southern Alps and other ranges. Glacial-interglacial climatic fluctuations resulted in the widespread deposition of unconsolidated Quaternary tills and glacial outwash gravels. During warmer interglacial periods, high sea levels cut marine terraces and left isolated remnants of beach deposits that have subsequently been uplifted. Growth of the mountains by folding and faulting has continued to the present day throughout much of the map area. Mined mineral resources within the map area include alluvial gold and rock aggregate. Pounamu (nephrite/greenstone) and kyanite are locally treasured non-metallic minerals. Other recorded metallic and non-metallic minerals include chromium, copper, nickel, ilmenite and scheelite. There are no recorded seeps or shows of oil and gas in the area, and there has been no commercial hydrocarbon extraction nor any petroleum exploration wells. Gently dipping Paleogen e source and reservoir sedimentary rocks potentially occur 3 to 5 km below sea level offshore.The Haast map area is subject to severe natural hazards, including high potential for strong Alpine Fault earthquake shaking causing landsliding, liquefaction and ground rupture. Three magnitude 6 or greater earthquakes with epicentres within, or in close proximity to, the map area have occurred in the last 70 years. Slope instability, including landslides, rock avalanches, rockfalls and debris flows,is a major hazard in hill and mountain terrain. Erosion, floodingand sedimentation hazards exist near watercourses. Low-lying areas along the coast and lake shores are potentially at risk from tsunami. (auth)

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