Tectonic Geomorphology of the Old Man Range and Garvie Mountains, Central Otago.

Author:

Stirling, MW

Year:

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Abstract:

The late Cenozoic tectonics and geomorphology of a 700 square kilometre area of remnant peneplain topography comprising the Old Man Range, Obelisk Range and Garvie Mountains, Central Otago, have been investigated. This study combined with existing data from Otago show that the peneplain was originally devoid of significant relief (maximum paleorelief = 500m/10km, in west) and is underlain by a broadly low angle schistosity (c. 0-20°). Tertiary sediments in the area indicate that the peneplain surface has been vertically offset by up to c. 1500 metres. Quartzose Potters Gravels (new name) have a palynologically determined Waipipian maximum age, and were reworked from Manuherikia Group sediments during early uplift of the schist ranges.
Mapping has revealed a consistently low angle relationship between schistosity and peneplain attitude, and dominance of distributed deformation (flexure) over fault displacement deformation. Undulations in topography are thus mirrored by equivalent undulations in underlying schistosity, and the study area has been divided into nine fault bounded tectonotopographic blocks (new term). Three fault provinces are also recognised. Tabulation of joint orientations shows D (ESE) sets to dominate in the west and C and D (ENE-ESE) sets to dominate in the east. They are geometrically and genetically related to pre-Cenozoic faults (Nevis-Cardrona and Old Man fault systems, the latter of which does not displace the peneplain significantly), and reactivated according to flexure of blocks. Fold axes of blocks trend north-northeast in the west and north-northwest in the east. Heterogeneous strain distribution is evidenced by variable shortening percentages/axes of blocks and contrasts with bordering tectonic basins. Shortening by folding is calculated through measurement of angular spread of poles to topography and schistosity attitudes on stereonets. Shortening values of 3.1% (schistosity-derived) and 3.8% (peneplain-derived) on an axis 081 are calculated for the study area. The axis is similar to geodetic PHS directions and plate convergence vectors for Central Otago (Blick 1986, Walcott 1978). A transpressional en echelon folding model has been developed with the aid of Surface II graphics and Fourier analysis. Regional peneplain and schistosity-derived values of 1% shortening are calculated in the east (Taieri Ridge area), and up to 26% in the west (Cardrona-Moonlight block). Shortening values are similar to those required by the crustal bending model (Norris 1979). Late Cenozoic bending and rotation is evidenced by a c. 10° northward convergence in schist lineation trend (equivalent to Old Man and Nevis fault convergence). Considerable shortening is indicated by steep schistosity attitudes in west Otago/Northern Southland, where the peneplain is not preserved. Schistosity attitude could be used in late Cenozoic strain analysis outside Otago (e.g. Marlborough schists).
Landform studies show the upland peneplain to have suffered minimal degradation in the late Cenozoic. Tors have been formed by erosion of a weathered zone of irregular depth (0 to 10 metre depth at the Potters depression), and are thus genetically inseparable from lower altitude tors. They occur where jointing is well developed (i.e. dependant on degree of flexure) and in areas relatively sheltered from the southwest. XRD analysis of clays from the study area and from Quaternary-late Tertiary sediments in the upper Clutha Valley has constrained kaolinite age as pre dating deposition of the Maori Bottom Formation. Additionally, erosion of the peneplain has been limited to removal of the weathered zone. A degradation rate (3-10 mm/1000 y) is calculated, based on known thicknesses of the weathered zone. This degradation rate is similar to rates determined in tectonically inactive cold temperate and arctic environments. Clay and landform studies could be used for identification of the peneplain surface outside Central Otago.

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xiv. 179 p., ill. Maps, ; 30 cm

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OU geology Identifier:

1988Stirling

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http://download.otagogeology.org.nz/temp/Abstracts/1988Stirling.pdf

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Citation

Stirling, MW, “Tectonic Geomorphology of the Old Man Range and Garvie Mountains, Central Otago.,” Otago Geology Theses, accessed May 22, 2024, https://theses.otagogeology.org.nz/items/show/225.

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