Melange and related structures in Torlesse accretionary wedge, Wairarapa, New Zealand

Abstract

Abstract In southeastern North Island, New Zealand, the middle Cretaceous Mangapokia Formation represents the youngest part of the Torlesse accretionary wedge. This accreted submarine fan turbidite sequence is weakly metamorphosed and complexly deformed. Structures including local development of melange fabric, several episodes of shear fracturing, and two fold events, were associated with Cretaceous accretion. An early, extensional fabric is overprinted by F1 folds that were later dissected by tensional fractures and veins. These early structures were observed in melange inclusions and all predate completion of regional stratal disruption and melange formation. Structures associated with regional disruption include attenuated beds, boundins, “pinch and swell” structure, tectonically elongated conglomerate clasts, lozenge (transposition) fabric, and tectonic melange. Varying states of disruption represent arbitrary stages in a progressive structural continuum, with the most deformed sequences acquiring their structural fabric via a combination of repeated movement on discontinuous and often anastomosing bedding‐parallel shears, thrust and normal faults at low angles (<45°) to bedding, and reverse and normal faults at high angles (>45°) to bedding. Tectonic disruption probably occurred through a range of consolidated to semi‐indurated physical states by a combination of intergranular flow and cataclasis. Melange units containing exotic ocean‐floor (basalt, chert, red and green argillite) lithologies occur as sheet‐like bodies 2–25 m thick. They are inferred to represent frontal imbrication thrust faults, separating packets of strata ranging from tens to hundreds of metres thick. Thus, the melanges are tectonic, rather than sheared, chaotic sedimentary deposits. Isoclinal to tight, predominantly asymmetric, mesoscopic, second generation (F2) folds developed, after the initial pervasive disruption of strata, during continuing accretion. Two late‐stage, post‐Torlesse fold events (F3 and F4) are probably responses to Miocene‐Recent subduction at the Hikurangi Margin. Mesoscopic faults at high angles to bedding overprint all fold events. The structural history of the rocks is similar to older Torlesse sequences elsewhere in New Zealand. This reflects the diachronous development of structures as the accretionary wedge evolved.