Transpressional development of the Australia‐Pacific boundary through southern South Island, New Zealand: Constraints from Miocene‐Pliocene sediments, Waiho‐1 borehole, South Westland

Abstract

Abstract Clast compositions of Waiho‐1 conglomerate horizons record dextral movement of the borehole site past different basement terranes on the opposite side of the Alpine Fault. Metavolcanic and volcanogenic metagreywacke clasts in sediments aged c. 4.5 Ma indicate deposition adjacent to exposures of Caples, Dun Mountain—Maitai, or Brook Street basement terranes. Sediments aged c. 4 Ma contain Torlesse Terrane and Haast Schist clasts. Relocation of the Waiho‐1 borehole site using published finite rotations, combined with the observed change in provenance at c. 4.5 Ma, requires that the source rocks had relationships similar to those observed at the present day, that is, that most curvature of basement terranes in southern South Island, New Zealand, was already in place at 4.5 Ma. Major increases in sedimentation rate recorded by Waiho‐1 borehole at c. 12 and 5 Ma are linked to changes in the Australia‐Pacific plate vector. Subsidence before 12 Ma may have been related to thermal subsidence of the Emerald Basin, or/and middle Miocene foreland basin development. Clast compositions of middle—late Miocene (c. 12–5 Ma) sediments record reverse movement on the South Westland Fault Zone and uplift southeast of the Alpine Fault in areas where meagreywacke basement rocks were exposed. Fiordland was low lying and contributing little sediment to the basin in middle—late Miocene time. Large volumes of Pliocene—Quaternary (c. 5–0 Ma) sediment derived from southeast of the Alpine Fault reflect widespread uplift of the Southern Alps. Schist clasts in an early Pliocene sample indicate that amphibolite facies Alpine Schist was exposed by c. 4 Ma.