The Alpine Fault Zone of the Waitaha Valley Area.

Author:

Green, DC

Year:

Project type:

Abstract:

The geology of an area about the Waitaha River, Westland is studied. The A1 pine-Fraser Fault zone separates Greenland Group and Mt Bonar Granite to the west from Haast Schist to the east.
Greenland Group west of the Fraser Fault is interlayered hornfels and schist reflecting original compositional bedding. It is intruded by granite and granodiorite of the Mt Bonar Granite.
Haast Schist is dominated by quartzofeldspathic schist, but amphibolite, quartz schist and marble do occur. Three localities of Pounamu Ultramafics with poorly developed metasomatic zonation are described. Metamorphic grade increases monotonically from albite (biotite) zone to the east, to a K-feldspar bearing zone to the west. Zonation in quartzofeldspathic schist is consistently retarded with respect to interlayered amphibolite; the areal offset of equivalent mineral isograds is about lkm. Three Rangitatan episodes of folding and of recrystallisation of the schist are recognised, the ultimate metamorphic grade occurring during the latest episode.
A progressive increase in the metamorphic grade, of incipiently developed retrogressive mineral assemblages toward the west is interpreted to result from thermal metamorphism associated with activity on the Alpine Fault. Structural deformation associated with fault activity has resulted in a lkm wide belt of fault rocks to the western margin of the Haast Schist zone. Strike slip motion on the Alpine Fault has produced conjugate kinks, joints and faults; uplift of the schist produces joints, faults, schist overthrusting westward, hydrothermal veining, and tilting of Quaternary gravels.
Sandwiched between Haast Schist and lower Paleozoic rocks, the Fraser Group is a collage of fault bounded blocks: chiefly of fault rocks derived from various original litho1ogies to the east and west of the group, inc1uding pre-Tuhuan paragneiss and orthogneiss. The latest fault rock textures were impressed in the Plio-Pleistocene to the east, and earlier to the west, and are thought to have formed during relative movement of the blocks.
Study of the microstructures of minerals and textures in the fault rocks indicates that (i) strain rate has consistently been greater to the east of fault planes than to the west, (ii) typical distributed shear stress during faulting was 60 MPa, (iii) shear deformation of individual minerals is complex, with several deformation mechanisms active in each phase simultaneously.

Named Localities:

Thesis description:

vii, 164 p, photos, map (folded in pocket), 30 cm.

Department:

OU geology Identifier:

1982Green

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Location (WKT, WGS84):

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Files

http://download.otagogeology.org.nz/temp/Abstracts/1982Green.pdf

Collection

Citation

Green, DC, “The Alpine Fault Zone of the Waitaha Valley Area.,” Otago Geology Theses, accessed October 25, 2020, http://theses.otagogeology.org.nz/items/show/154.

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