The Paparoa Metamorphic Core Complex, New Zealand: Cretaceous extension associated with fragmeotation of the Pacific margin of Gondwana

Abstract

In Westland‐Nelson provinces of New Zealand, high‐grade metamorphic and granitic basement rocks showing mylonitic ductile deformation are juxtaposed beneath low‐grade metasedimentary rocks and undeformed granites by uplift on low‐angle detachment faults. Several metamorphic core complexes analogous to those described from western North America are recognized. In the Paparoa Range, basement rocks include late Precambrian(?) paragneiss and granitic rocks of both Paleozoic and Cretaceous ages. Cover rocks include Ordovician turbidites, Paleozoic and Cretaceous granites, and mid‐Cretaceous breccia‐conglomerates. Brittle deformation and hydrothermal alteration (silica, chlorite, hematite, carbonate ± fluorite, uranium) characteristic of the detachment zone are also superimposed on uppermost lower‐plate mylonites. Kinematic indicators in the mylonitic rocks including composite S‐C febrics indicate that the detachment faults on the northeast and southwest sides of the Paparoa Core Complex had opposite senses of shear, with cover rocks on both sides moving away from the metamorphic core. Ductile deformation postdates several 114±18 Ma granitic plutons but by 108 Ma had ceased to affect at least some of the rocks currently exposed. Mylonitic rocks were uplifted to the surface and eroded into evolving half‐grabens by 105–100 Ma. Uplifted basement yields K‐Ar dates as young as 88 Ma, and tilting of the graben sediments indicates detachment continued well into the Late Cretaceous, when both cover and basement were intruded by alkali lamprophyre dykes. The Nelson‐Westland core complexes occur within an Early Cretaceous granitic province characterized by relatively radiogenic strontium. The boundary of this province, the NW trend to mid‐Cretaceous half‐grabens, the NNE trend of stretching lineations in mylonitic rocks, and the ESE trend of late Cretaceous lamprophyre dykes indicate that regional extension was maintained in a NNE direction for much of the Cretaceous. This regional extension may be part of an “extension corridor” which traversed the entire Gondwana continental margin from NE Queensland, Australia, to Marie Byrd Land, Antarctica. Extension preceded opening of the Tasman Sea between New Zealand and Australia at approximately 84 Ma and closely followed long‐lived compression on the Pacific convergent margin of Gondwana. The presence of core complexes in western New Zealand contrasts with the Australian margin to the Tasman Sea and lends support to simple shear models of continental rifting.