Geology of Torlesse and Waipapa terrane basement rocks encountered during the Tongariro Power Development project, North Island, New Zealand

Abstract

Greywacke-type basement rocks from tunnels and other excavations along the western part of the Kaimanawa Range (eastern diversions) and in a buried sequence cut by the Whakapapa-Tawhitikuri tunnel (western diversions) comprise mainly sandstone, siltstone, and argillite, but chert and minor basaltic rock and limestone are also present in the Kaimanawa Range. Rocks of the latter are comparable with Torlesse terrane sequences elsewhere, whereas the Whakapapa-Tawhitikuri tunnel rocks have contrasting petrography and structural style and belong to the younger Waipapa terrane, forming part of the Morrinsville facies. Petrographic, modal, and chemical data show that the Torlesse rocks are more siliceous and contain abundant granitic debris; the Waipapa terrane rocks are considerably less siliceous, and typical sandstones have a high content of intermediate volcanic clasts. Among the Kaimanawa Range rocks, complexly intermixed units, involving sandstone, siltstone, argillite, and chert, approach typical melange. All Kaimanawa Range rocks show pervasive recrystallisation, and in fabric most range from textural zone 1 to 2A. Siltstone and sandstone of textural zone 2B cover only comparatively small areas. They are characterised by steeply dipping lineation and poorly developed schistosity and are classed as semischist. Fissility is marked in associated argillites. The formation of the melange and associated features recorded is probably the result of severe shearing and imbrication during the Rangitata Orogeny.