The geology of central southern Fiordland : with emphasis on the cause of polybaric Cretaceous metamorphism in western New Zealand

Author:

Powell, Nicholas Garth.

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Abstract:

Central southern Fiordland, New Zealand, is underlain extensively by metasediments and associated metavolcanics. These are mapped in three lithostratigraphic units, from west to east Edgecumbe Group, Cameron Group and Cumbrae Supergroup. Lower Cameron Group units lithocorrelate with Central Fiordland Belt lithological associations and with those of Fraser Complex, Westland.

Eastern Fiordland Belt metavolcanics and lacustrine metasediments are tectonostratigraphically unrelated to Cameron Group, from which they are separated by the Grebe Fault. They instead have affiliations with the Loch Burn Formation, Largs Volcanics, Drumduan Group and Paterson Group. These units (collectively, "Cumbrae Supergroup") represent remnants of a Triassic-Jurassic calc-alkaline arc.

Six deformational episodes are identified in central southern Fiordland. The earliest, D1, is obliterated by D2 and M2 metamorphism. D3 is restricted to the Southwest Fiordland Block. D4 occupied a brief interval of M3 time. D4 of the Central and Western Fiordland Belts corresponds to earliest deformation in Eastern Fiordland Belt metavolcanics. The Grebe Fault is a left-lateral reverse D4 fault; now vertical, it previously dipped eastward. The Dusky Fault, a reactivated D5 left-lateral transfer structure, accommodated the dip-slip component of displacement at low-angle normal faults during mid-Cretaceous extension. Open folds represent D6. Post-glacial scarps mark the post-D6 Kilcoy and Vincent Faults. Their merged northward continuation is intersected by the tailrace tunnel of the Manapouri Hydroelectric Power Station.

Southwest Fiordland Block pelites were metamorphosed at 665 °C, c. 3 kbar during M2. Early M3 is of contact metamorphic aspect. Late M3 is distinctively polybaric: Central Fiordland Belt kyanite-garnet pelites recrystallised at c. 8.5 kbar after metamorphism in the sillimanite field at c. 3.5 kbar. Western Fiordland Orthogneiss 12 kbar granulite assemblages formed during late M3. South of the Dusky Fault, late M3 is almost asymptomatic. The M3 field gradient is continuous across the Grebe Fault: in the Eastern Fiordland Belt, late M3 staurolite and garnet supersede chloritoid in lacustrine (meta-)sapropel-silts.

The Grebe Fault is an important tectonostratigraphic break; it may separate New Zealand's Western and Eastern Provinces. Its relationship to any "Median Tectonic Zone" is unclear, as no such zone has been found in southeastern Fiordland. Cumbrae Supergroup rocks within the "Median Tectonic Zone" represent the arc that nourished the Eastern Province's Barretts Formation, Murihiku Supergroup and Stephens Subgroup. The Cumbrae arc was 'obducted' westwards during Early Cretaceous continent-arc collision. This event simultaneously halted Eastern Province volcanogenic sedimentation and tectonically buried Fiordland, imposing late M3 pressure increments. Drumduan Group lawsonite is coeval.

Cretaceous collision induced glaciation. Late Cretaceous climatic deterioration and extensional tectonism caused icecap development. The Otago "Peneplain" is a Late Cretaceous subglacial floor. Accumulation of voluminous perennial Cretaceous ice on Earth has hitherto not been inferred.

Facultative psychrophily in New Zealand's ancient endemics and their preference for dark conditions reflect passage through a hitherto-unsuspected evolutionary bottleneck: prolonged winter darkness and harsh climate of near-polar Late Cretaceous New Zealand exerted extraordinary evolutive pressures on ancestral forms after biotic links with Gondwana were severed. New Zealand's ancient endemics are the evolutionary derivatives of a Late Cretaceous near-polar fauna.

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xx, 344 p. : ill., maps ; 30 cm. + 1 map (99 x 84 cm.; folds to 25 x 18 cm.)

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1997Powell

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http://download.otagogeology.org.nz/temp/Abstracts/1997Powell.pdf

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Citation

Powell, Nicholas Garth., “The geology of central southern Fiordland : with emphasis on the cause of polybaric Cretaceous metamorphism in western New Zealand,” Otago Geology Theses, accessed May 22, 2024, https://theses.otagogeology.org.nz/items/show/336.

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