Metamorphism and structure of the Cambrian-Ordovician sediments in southwest Nelson district

摘要

The geology of the Victoria Range and the Maruia Upper Grey depression in Springs Junction district has been examined with particular emphasis on metamorphisms and structures. The 'Z?' sediments in the Victoria Range are correlated with the Aorere Group of NW Nelson on the basis of their consistency with Cooper's (1975) Lower Paleozoic sedimentary model, and Shelley's (1975) paired metamorphic belt model, These sediments have been deformed and metamorphosed twice, the first regional metamorphism is considered to be related to an Upper Ordovician Orogeny, and the second metamorphism thought to be associated with the Tuhua Orogeny. The latter metamorphism was at temperatures ranging from 400° to 550° C, and pressures not exceeding 2 to 3 kb. The volcanics and volcanogenic sediments of the Koura Formation are correlated with the Haupiri Group of NW Nelson also on the basis of their consistency with Cooperis (1975) sedimentary and shelley’ s (1975) paired metamorphic belt models. These rocks have been metamorphosed under P-T conditions of 350° to 450° C, and moderate to high pressure. This metamorphism is considered to be related to the Upper Ordovician Orogeny. The Lower Paleozoic (Cambrian? - Upper Ordovician) deposits in the Maruia-Upper Grey depression are correlated with the Mt. Arthur Group of NW Nelson. These rocks were regionally metamorphosed during the Tuhua Orogeny under conditions of 350° to 640° C, and not less than 4 to 6 kb. The deformation accompanying this metamorphism has generated three orders of folds. a) a NE-trending, gently plunging, and inclined major anticline, b) microscopic folds whose attitudes are similar to that of the major anticline, and c) microscopic crenulations whose axial planes are oriented parallel to the axial plane of the major anticline. The Koura Volcanics, the Victoria Range rocks, and the Lower Paleozoic sediments of the Maruia-Upper Grey depression were juxtaposed by movement(s) of the Koura Fault- a proposed Tuhuan major structure which coincided with the Tuhuan median boundary. The Lower Paleozoic rocks of the Maruia-Upper Grey Valley were again metamorphosed during the Rangitata Orogeny. The thermal metamorphism at 500° to 580° C, and 1.5 to. 2.5 kb was accompanied by an episode of deformation which resulted in the microscopic folding of one limb of the microscopic crenulations produced during the Tuhua Orogeny. The Rangitatan event was closely followed by dextral movement(s) along the Kaura Fracture System which resulted in the folding about a vertical axis of the Victoria Range meta-sediments and the Lower Paleozoic sequence bordering the major fracture. The deformed Victoria Range rocks were subsequently refolded about a gently plunging or horizontal axis. The formation of the Rahu Fault and sinistral movement(s) along this major structure during or subsequent to the Rangitata Orogeny has displaced the Mesozoic rocks in the Brown Grey Valley. The Lower Paleozoic sediments in the Maruia-Upper Grey depression were refolded about a gently plunging, N-S trending axis during the Kaikoura Orogeny. In addition, renewed activities of the Rahu Fault have displaced the Lower (?) Tertiary rocks west of Springs Junction.