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

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 (SI) and lineation (Ll)' Lithologic layering inherited from transposed bedding (So) is parallel to SI except in very rare mesoscopic Fl 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. ~ 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 SI during Kaikoura block faulting. Overlying Miocene sedi- Received 13 June 1980, revised 8 October 19805* ments 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. Keywordsdeformation; Haast Schist Zone; folds; metamorphism; faulting; Cromwell; structural analysis; foliation INfRODUCTION Regional settingThe Cromwell map area covers some 1150 km 2 in Central Otago, centred on the town of Cromwell (Fig. 1). The district is dominated by northerly trending block-faulted ranges rising to 2000 m, generally moderately dissected and underlain by well exposed quartzofeldspathic schists and subordinate greenschists of the Haast Schist Zone (Carter et al. 1974). Intermontane fault-angle depressions are infilled with deformed lacustrine sediments of the Miocene Manuherikia Group overlain by extensive Quaternary outwash gravels (Turnbull in press). Park (1906Park ( , 1908 recorded numerous schist foliation attitudes, noted greenschist and metachert (quartzite) bands as well as many faults, and gave petrographic descriptions of schist lithologies. Parts of the Cromwell map ...

Tectonic map of Central Otago based on Landsat imagery

“…The easternmost step is close to the axis of warping of the peneplain surface, which may be related to the Leaning Rock Antiform of Turnbull ( 1981 ). These steps appear not only on the peneplain but also in the gorge of the Clutha River, suggesting that faulting on the steps postdates the cutting of the gorge.…”

“…These faults are not confirmed on the ground; expected evidence for them would be zones of crushed schist and schist gouge on trend with the steps. Turnbull ( 1981) showed that his Leaning Rock Antiform is asymmetric with vergence to the southeast; foliation planes in schist dip steeply toward the Manuherikia Basin to the southeast and gently toward the Cromwell Basin to the northwest. The warping of the peneplain surface is also asymmetric, and the surface is best preserved on the gentle northwestern slope of the Dunstan Range, where the surface is overlain unconformably by Tertiary sediments of the Cromwell Basin (Officers of the New Zealand Geological Survey 1984) (Fig.…”

“…The peneplain surface is preserved a short distance north of the Kawarau River, and Landsat suggests that the surface is warped into an asymmetric anticline with its steep limb on the eastern side. This anticline may be related to the Scrubby Stream Antiform of Turnbull (1981 ), developed on broadly warped foliation surfaces in schist. As with the Dunstan Fault, trenching was done on late Quaternary fault scarps within Cromwell Basin rather than the range front itself.…”

“…and terminates just north of the Caimmuir Fault. This narrow basin is terminated on the west by a fault mapped by Turnbull (1981) and visible on Landsat. Another fault to the east mapped by Turnbull (I 981) is also visible on Landsat.…”

Structural and lithological continuity and discontinuity in the Otago Schist, Central Otago, New Zealand

“…These structures were first described in detail for the Otago region by Wood (1963) and Means (1963Means ( , 1966, and folds of comparable style occur in many parts of the Otago Schist (Brown 1968;Turnbull 1981;Craw 1985;Mortimer 1993a). These structures are commonly viewed as having formed in a continuum of progressive development of macroscopic structures (e.g., Means 1966;Brown 1968;Turnbull 1981;Craw 1985;Cox 1991). Consequently, early formed folds in this continuum are overprinted by later formed structures.…”