Alpine fault segmentation and range front structure between Gaunt Creek and Little Man River, near Whataroa, central Westland, New Zealand

Read, Stephen Edward.

The Alpine Fault south and east of Whataroa consists of two northeasterly striking thrust segments ~8 kilometres long linked by an easterly striking strike-slip segment -6 kilometres long. This structural assemblage is one of the largest geometrical_ irregularities south of the Maruia bend. The geometry of the segmentation requires that the dip of the Alpine Fault at depths of a few kilometres should be greatest near the eastern end of the strike-slip segments where they intersect the southwest ends of the thrust segments. Orientations of the mylonitic foliation, which appear to mirror the Alpine _Fault, are compatible with this requirement, dipping -40° SE at the western end of the strike-slip segment and -56° SE near the southwestern end of the eastern thrust segment. The outcrop distribution of the mylonites is compatible with the suggested segmentation geometry, with higher grade rocks towards the western end of the strike-slip segment. Large scale block rotation is evident within a zone -4 kilometres long and up to several hundred metres across. This zone is bounded to the north by the strike-slip segment striking 080° and to the south by a large fault with a linear trace striking 070°. Foliation within the zone appears to have been rotated 40° in a clockwise sense with respect to foliation to the south of the 070° striking fault. The change in strike occurs within a distance of 5-l 0 metres of the latter fault. Foliation dip does not change abruptly across the fault, suggesting that the rotation occurred about a subvertical axis. The clockwise sense of rotation of the foliation is compatible with a dextral sense of shear between the river flats to the north and the range-front to the south. Foliation strike within 1-2 kilometres of the extrapolated intersection of the western thrust segment and the strike-slip segment swings -37° northward as the intersection is approached from the south and southwest. This change is coupled with a lessening in the dip of -24°. These changes are compatible with gravity driven flexure of the hanging wall as it nears the surface. An approximately 3.5 kilometre long, steeply west-dipping normal fault scarp on the true right of the Whataroa River records collapse of the range front during the late Pleistocene. iii I I I I I I I I I I I I I I I r Most of the feldspars in. the mylonites have both oligoclase cores and rims indicating that: (1) the protolith for the mylonites was oligoclase zone schist, and (2) most of the mylonitisation occurred under oligoclase zone, amphibolite facies conditions. Between -50 and 200 metres away from the Alpine Fault a pod of ultramylonite has been retrogressed. In one of these rocks calcic-albite rims (-An7) around sodic-oligoclase cores ( -An12) indicate that the recrystallisation occurred at the top of the peristerite solvus near the second oligoclase isograd. This indicates that at least some of the mylonitisation occurred unde'r garnet zone, transitional amphibolite facies conditions. c Retrogression of biotite to chlorite occurred under greenschist facies conditions below the biotite isograd in these ultramylonites. The feldspar grains did not re-equilibrate to greenschist facies conditions. Xenoliths of probable Greenland Group in shears around the margins of two granite intrusions suggest that the granitoid intrusions west of the Alpine Fault were intruded into Greenland Group basement. IV I ' !

vii, 112, xiv leaves : ill. (some col.), maps (some col.) ; 30 cm.

1994Read

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