The geology of the Wairaka Block, west Wellington coast, North Island, New Zealand

Abstract

Rocks of the Torlesse Terrane within the Wairaka Block are dominated by clastic sediments of Late Triassic (Oretian-Otamitan) age. Thick-bedded sandstones, alternating interbedded sandstones and argillites, and very thick-bedded argillites belong to two facies associations that were deposited as turbidites, probably in a migrational mid-submarine fan environment. Unlike many other large exposures of Torlesse rocks, basalt and chert are absent. The sediments have a detrital composition that is quartzo-feldspathic (average bulk composition = Q40 F45 L15) and are thought to be derived from a dissected magmatic arc that was shedding detritus mainly of plutonic origin with a minor component of volcanic origin. Geochemical analyses support a magmatic arc provenance. Deposition in a trench associated with an active convergent plate margin was followed by accretion to the inner-trench wall during subduction to form an accretionary prism. During accretion the rocks were metamorphosed to prehnite-pumpellyite grade. Structure is complex with soft sediment deformation (F0) followed by three stages of tectonic deformation (F1, F2 and F3), reflecting a changing tectonic regime. F1 produced inclined, tight to isoclinal folds which plunge gently NE; F2 is represented by upright open to close folds which plunge gently both to the north and south and has an associated axial surface cleavage. It has been disrupted by pervasive bedding-parallel shear and faulting, and refolded by variably orientated, open F3 folds. Later shearing, developed locally, is thought to relate to relatively recent movement along the Pukerua Fault and has resulted in associated ductile or drag folding (F4). Structural features and facing evidence are consistent with the accretionary prism model. The current physiographic pattern of the Wairaka Block relates to activation of the Wellington Fault System and accompanying terrigenous sedimentation. Three phases of sand have been deposited subaerially during glacial phases of sea-level retreat. They are thickest in the Pukerua Corridor, a tectonically controlled basin flanking the upfaulted Wairaka Ridge.