Contrasting gravity-driven systems along submarine active margins: examples from the Whareama and Castlepoint Basins (southern Hikurangi margin)

Along submarine active margins, predominant tectonic activity strongly influences sediment transport, distribution and preservation, challenging traditional source-to-sink models. These settings instead favor gravity-driven sedimentation styles that are highly variable, intricate and often not readily predictable.

This research aims to (1) provide an overview of the contrasting depositional systems that may develop along submarine active margins, particularly within their sedimentary basins (e.g., trench-slope basins [TSBs]), and (2) offer new insights into the close interplays between deformation and sedimentation.

To achieve this, three fieldwork campaigns were conducted in the emerged southern portion of the Hikurangi subduction wedge (Coastal Ranges, North Island of New Zealand). More than 24 kilometers of coastal outcrops were captured and mapped using integrated fieldwork observations and photogrammetric datasets across two TSBs (Whareama and Castlepoint Basins).

Building on the analysis and integration of the collected data, this study proposes a high-resolution reconstruction of the tectonostratigraphic evolution of both basins. The results document the range of gravity-driven systems that may coexist or evolve through time within short distances along submarine active margins due to the highly complex, structurally controlled submarine topographies and subsequent diversity of sediment sources.

This study highlights that, beyond sea-level fluctuations or changes in sediment supply, tectonic activity focused along the basin-bounding thrust ridges exerted primary control on the stratigraphic architecture of the TSBs. These structures periodically promoted or disrupted the development of the depositional systems and generated distinct styles of mass-transport deposits (MTDs) according to their evolving structural positions.

Although centered on one geographical region, the findings are expected to have broader relevance for other tectonically active settings. They contribute to a better understanding of deep-marine fold-and-thrust belt evolution and support improved geological risk assessments and exploration predictions.