Stratigraphic, structural, and metamorphic relationships in the dansey pass area, otago, new zealand

Abstract

In the Dansey Pass area, greywacke, semischist, and schist on the north-eastern margin of the Otago schist arc show a progressive increase in metamorphic grade from prehnite-pumpellyite to greenschist facies, or in terms of textural development, from non-foliated greywacke of textural zone 1 to laminated schist of textural zone 3B. Locally a stratigraphic sequence some 15000 m thick can be recognised, in which the metamorphic grade increases towards the top. The rocks have been affected by four periods of folding. The earliest phase, probably initially penecontemporaneous with the later stages of sedimentation, resulted in steeply dipping strata, overturned throughout extensive areas. Phase II folding may have commenced as the early phase progressed, as early Phase II structures show evidence of soft sediment deformation, although other evidence indicates that Phase II continued until well into the metamorphic event. Isograds and isotects are not folded by Phase II structures, however, indicating that Phase II deformation was completed before the early Cretaceous metamorphic climax. Following metamorphism, post-metamorphic folds in both the foliation and in isogradic and isotectic surfaces formed on the outer edge of the schist belt, and were accompanied by important pre-peneplanation fault movements leading to deposition of the fanglomeratic middle-to-late-Cretaceous Kyeburn Formation in a major fault-angle depression. Deposition of the Kyeburn Formation was followed by regional peneplanation in the late Cretaceous and early Tertiary, before a thin veneer of middle Tertiary sediments was laid down. Plio-Pleistocene deformation during the Kaikoura Orogeny was dominated by faulting in the basement rocks, with locally strong folding of the covering strata. Large scale gentle warping of the foliation may have occurred at this time. The fault pattern seems to indicate vertical uplift rather than lateral shortening or Extension, and the rejuvenation of older faults, commonly with a reversal in the sense of movement, suggests a potentially reversible process is a necessary requirement of the fundamental mechanism. © 1974 Taylor & Francis Group, LLC.