Geology of Torlesse rock around the Wellington coast between Paekakariki and Pencarrow Head

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Suneson, N.H. 1992 Geology of Torlesse rock around the Wellington coast between Paekakariki and Pencarrow Head. Lower Hutt: Institute of Geological & Nuclear Sciences. Institute of Geological & Nuclear Sciences science report 92/08. 52 p.

Abstract: The Wellington coast between Paekakariki and Pencarrow Head is underlain mostly by Late Triassic Torlesse terrane (Rakaia subterrane). The principal rock types are fine-to medium-grained greywacke and argillite that have been metamorphosed to prehnite-pumpellyite facies. These strata were deposited by turbidity currents and mass-flow processes. Diamictites containing greywacke, chert, basalt, and limestone clasts in an argillite matrix are rare and originated as subaqueous debris flows. Lithofacies relations and sedimentary structures suggest that the sediments were deposited on a series of overlapping fans in a base-of-slope slope-apron environment. Melange zones, consisting of isolated blocks or lenses of greywacke in a sheared argillite matrix, are subparallel to bedding within the turbidites. Individual structures vary from compressional to extensional and brittle to ductile; stratal disruption resulted from bedding-parallel extension. The melange zones represent decollements and, at least locally, are at the tops of coherent sedimentary sequences. Most of the strata strike north-northeast, dip steeply, and face west. Shear planes subparallel to bedding are pervasive. Megascopic and some larger folds plunge gently to moderately parallel to the regional strike of bedding; some map-scale folds plunge steeply oblique to bedding. West of Wellington, the strata are folded into steeply plunging, map-scale upright and inverted anticlines and synclines. The structural history of the Wellington-area Torlesse is based on cleavage-fold relations and similarity of fold styles and is consistent with deformation in an accretionary prism (AP) over a west-dipping subduction zone: (1) Formation of melange beneath the toe of an AP. (2) Underplating to the base of the AP and tectonic thickening. (3) Subhorizontal asymmetric folding and cleavage development. (4) Imbrication, stacking, and coaxial rotation of strata to subvertical. (5) Folding due to layer-parallel oblique-slip. Subsequent events less clearly related to deformation within the AP area. (6) Early Cretaceous shearing and mineralization and (7) Cenozoic right-slip folding and faulting.