Quaternary evolution of the Alpine Fault Zone, and a mineralogical/microstructural study of the schist-mylonite transition, Paringa, south Westland

Simpson, Guy D. H. (Guy Donald Hugh), 1970-

The 50 km2 area mapped, situated at Paringa, 50 km northwest of Haast, lies astride the Alpine Fault. Prior to c. ·15000 years B.P. the Alpine Fault bent across the Paringa River region. Increased thrusting on the bend led to the formation of an extensive nappe. More recently, the nappe has been abandoned to form a segmented structure made up of northerly striking thrust faults linked with more easterly striking, strike slip faults. The result is the formation of a complex compressional region (jog), characterised by anomalous uplift rates approaching 11 mm I year, in which the Quaternary sediments have been intensely deformed.
Despite post-glacial sea level rise, the Paringa Formation records a local rapid marine regression in response to the development of the segmented structure. The presence of the first angular unconformity (attributed to differential uplift within the compressional jog) at about 12000 years B.P. suggests that the segmented fault system had developed at, or before this time. Following regression of the sea, a braided fluvial system soon became established. However, enhanced uplift in the compressional region about 9000 years B.P. led to damming of the Paringa River, and the subsequent flooding of forests by a lake. The flooding of forests occurred at least twice more before 3500 years B.P., when the present day fluvial regime became established.
Haast Schist east of the Alpine Fault has been metamorphosed to the oligoclase zone of the amphibolite facies. The coexistence of albite and oligoclase within the mylonites is not thought to indicate retrograde metamorphism. Instead the peristerite feldspars are attributed to be part of a pod (of unknown extent) of upper garnet zone rock incorporated into the fault zone during the transition from dominantly transcurrent to oblique reverse motion on the Alpine Fault. There is, however, good evidence to suggest retrograde metamorphism has played an important role in dictating mineralogy within mylonites and cataclastic rocks close to the fault. The exponential increase in the phengite component of muscovite towards the Alpine Fault is thought to represent retrogression under lower temperature conditions. The presence of abundant epidote and actinolite (surrounding hornblende) within these rocks is also attributed to retrograde reequilibrium to greenschist facies conditions.

168 leaves : ill. (some col.), maps ; 30 x 44 cm.

1992Simpson

Abstract Only

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