Contortions in the schists of the Cromwell district, Central Otago, New Zealand

Abstract

Abstract Regional mapping of schists in the Cromwell (S133) sheet district, Central Otago, has demonstrated the presence of mesoscopic and macroscopic folds developed during 4 major deformational phases. Folds are described from textural zone IV, chlorite and biotite zone schists. Quartz-rich and mica-rich quartzofeldspathic schists, as well as marker units of greenschist and metachert, have been mapped. Macroscopic folds have been delineated by marker horizons, and from changes in mesoscopic fold vergence, the attitudes of faltenspiegel and axial plane surfaces and the degree of development of axial plane foliation. Following folding, the whole district was broken by major faults attributed to both Rangitata and Kaikoura orogenies. Phase 1 deformation coincided with the metamorphic climax and produced a pervasiye mineral segregation foliation (S1) and lineation (L1). Lithologic layering inherited from transposed bedding (S0) is parallel to S1 except in very rare mesoscopic F1 fold hinges. No macroscopic Phase 1 folds have been proven. Phase 2 deformation produced numerous tight to isoclinal macroscopic folds and abundant mesoscopic folds. S2 axial planes are frequently marked by widespread penetrative foliation. S2 surfaces have generally low dips; F2 axes trend NW, SE, N-S or NE-SW. Folds close to the NW, SE, NE, or SW. Some metamorphic recrystallation occurred during Phase 2. Phase 3 folds are generally isolated mesoscopic features, but 2 macroscopic folds are present. These refold Phase 2 macroscopic folds and have penetrative S3 axial plane foliations. Minor metamorphic recrystallisation accompanied Phase 3 deformation in these folds. Subsequent deformation was limited to mesoscopic kink folding or warping adjacent to major faults and to much larger scale warping of S1 during Kaikoura block faulting. Overlying Miocene sediments were also deformed during block faulting. Some kink folding probably accompanied Rangitata faulting. The present wide range of macroscopic fold orientations is attributed to major Rangitata fault movements on both low-angle and subvertical faults.