Makawhio geology : Alpine Fault Zone structure and tectonometamorphic evolution, Makawhio River, South Westland

McClintock, M. K. (Murray Kevin)

The Alpine Fault at Makawhio River comprises a series of north or north-east trending thrust-dominated sections linked by east-west trending, predominantly strike-slip segments. The Alpine Fault in the Makawhio area appears to be evolving from a geometry dominated by serial partitioning toward a simpler structure dominated by oblique slip, through abandonment of westward-thrusting nappes and 'out of sequence' imbrication in thrust sheets. High erosion rates drive both fault death (by inducing loss of criticality in the thrust wedges), and the development of oblique thrusts (via concentration of mechanical energy on a single structure). Metabasites in the mylonites adjacent to the Alpine Fault have protoliths that derive from two distinct petrotectonic settings; one suite of metabasaltic rocks shows affinities with MORB-type basalts, and the other shows a geochemical signature characteristic of GIBtype basalts. The association of these contrasting pre-metamorphic oceanic lithologies, in association with ultramafic, metapelagic, quartzofeldspathic and calc-silicate mylonites suggests extensive tectonic disruption during, and prior to, incorporation into the high strain plate boundary zone. The distinct character ofthe mylonites at Makawhio River may have significance in interpretation of the position of the Torlesse-Caples terrane boundary within the Alpine Fault zone, and/or of strain magnitudes within the mylonites. Kyanite amphibolites are reported in situ for the first time. The kyanite-bearing rocks are associated with monomineralic chlorite, indicative of an ultramafic association, and contain anthophyllite and gedrite, the first reported from the Haast Schist terrane. The postglacial sediments at Makawhio River are correlated with the Paringa sequence to the south. Both sequences show similarities in tectonic influence, suggesting some measure of tectonic control on sedimentation may operate at a regional scale in W estland, and that tectonics be more important in generating widespread change in the sedimentary record than is often assumed.

89 p. : ill. (some col.), maps (some folded) ; 30 cm.

1999McClintock

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