Sedimentation in the Conway Trough, a deep near-shore marine basin at the junction of the Alpine transform and Hikurangi subduction plate boundary, New Zealand

ABSTRACT The Conway Trough is a 40 km × 10 km, 1000 m deep, rectangular, tectonically controlled sedimentary basin situated on an active plate boundary. The basin lies at the junction of the Alpine transform and Hikurangi subduction sectors of the Indo‐Australian/Pacific plate boundary. It is incised into Cainozoic sedimentary rocks and contains a thick fill of late Neogene sediment. Study of the continental shelf near the Conway Trough indicates: (1) that transport mechanisms operate in a net north‐easterly direction, and (2) that the neritic sediment drift system is confined to within a few kilometres of the shoreface, with the result that (3) large areas of the shelf south of the trough comprise bare bedrock, or relict late Pleistocene sediment. Despite reaching to within 3 km of shore, the Conway Trough receives little modern bedload sediment, as indicated by a paucity of sand fill. However, the trough has intercepted some Holocene bedload, since the main channel contains redeposited graded gravel and sand with a C‐14 age of 4670 yr. Satellite imagery and piston cores reveal that abundant mud is provided to the trough, much of it from river sources up to 200 km to the south. Some of this sediment is inferred to move by turbid layer transport through Conway Trough and out via Kaikoura Canyon into nearby Hikurangi Trough. Nonetheless, sedimentation rates in the Conway Trough may be as high as 1‐7 m (1000 yr) 1. The Conway Trough forms a modern example of a transform basin, varying Neogene examples of which are found widely in north‐eastern South Island. Sedimentation models are constructed to compare transform and transduction basins, particularly with respect to the effects of Pleistocene sea‐level changes.