Structural and mineralogical study of the Alpine fault zone, and an investigation into the post-glacial environment : Mahitahi, South Westland

Higham, Caroline Hamilton Rose.

The Mahitahi region lies astride the boundary between the Pacific and Australian Plates, 65 km north of Haast in South Westland. The Alpine Fault transects the area, striking 060° along the margin of the range front. Aerial photos reveal a spectacularly linear recent trace. An older trace of the fault in the form of an abandoned thrust nappe extends we~tward from the presently active fault. Although both the leading edge of the thrust nappe and the active trace can be seen cutting across the rainforest and creeks on aerial photos, field investigation reveals a distinct lack of a major cataclasite-bearing crush zone. This suggests that the active trace of the Alpine Fault at Mahitahi is relatively young and still developing, with insufficient uplift and erosion to expose a significant thickness of cataclasite along the fault surface. The Alpine Schist east of the Alpine Fault has been metamorphosed to oligoclase zone. Albite and oligoclase coexist in the mylonites adjacent to the fault, with zoning profiles indicating a prograde relationship. Zoned garnet porphyroclasts within mylonite also developed during prograde metamorphism. The presence of peristerite plagioclase compositions and garnet zoning indicate the mylonite adjacent to the Alpine Fault was metamorphosed to a maximum of garnet zone, and is, therefore, not a result of more recent retrogression. The mechanics of uplifting garnet zone mylonite west of oligoclase zone protomylonite and schist may involve inversion of metamorphic isograds at depth due to shearing of the ductile Alpine Fault zone (15-20 km depth). Post-glacial sediments are exposed on the northern bank of the Mahitahi River. Here, a sedimentary sequence provides evidence of a post-glacial rise in sea level at Mahitahi approximately 13,000 yrs ago, with the invasion of the sea as a fiord. The sea then regressed, depositing a thick sequence of sands. At the top of the sequence lies 6 m of rhythmically deposited lacustrine mud, which is inferred to have two possible origins; (1) formation of a lake a by the damming of the Mahitahi River, due to infilling of a glacial hollow or an earthquake-related landslide, or (2) a·Late Glacial event involving the creation of a lake behind a loop of moraine, extending westward onto the valley floor. The age of the rhythmite unit is unknown. If the second option is true, it may be a Younger Dryas-related event (10- 11,000 yrs B. P). A Younger Dryas age of the rhythmites exposed on ' the banks of the Mahitahi River would have great significance on a regional scale.

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

1996Higham

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