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Inner-Forearc Sequence Architecture in Response to Climatic and Tectonic Forcing Since 150 ka: Hawke's Bay, New Zealand

¹ UMR Geosciences, CNRS Université de Rennes 1, Campus de Beaulieu, 35042 Rennes cedex, France; fabien.paquet{at}univ-rennes1.fr
² UMR Geosciences, CNRS Université de Rennes 1, Campus de Beaulieu, 35042 Rennes cedex, France
³ National Institute of Water & Atmospheric Research (NIWA), Private Bag 14-901, 301 Evans Bay Parade, Greta Point, Wellington, New Zealand
⁴ Department of Geological Sciences, University of Canterbury, Private Bag 4800, Christchurch, New Zealand

The influence of eustasy, tectonic deformation, and sediment flux as controlling parameters on basin stratigraphy and depositional sequence development are largely accepted. Eustasy is usually considered as the dominant mechanism of sequence generation, especially for Pleistocene successions. In active subduction-margin settings, the high rates of tectonic deformation are expected to have a stronger influence on basin-fill architecture, while sediment flux is generally less well constrained, and therefore less frequently considered. The active Hikurangi subduction margin in New Zealand offers the opportunity to quantitatively assess the relative roles of tectonic, climatic, and eustatic drivers.

We present a quantitative source-to-sink-like study of the late Pleistocene succession from the Hawke's Bay sector of the inner forearc domain (c. 150 ka to present). The interpretation of a grid of high-resolution marine seismic data, onland and offshore core and well descriptions, and the integration of geomorphic studies enabled identification of system tracts. In turn these comprise two sea-level-cycle depositional sequences (LPS1 and LPS2), including one complete 100 ka sequence (LPS1). Isopach maps of both sequences reveal changes in sediment distribution and preservation that reflect the relative roles of tectonic deformation and eustasy. Eustasy dominates development of sequence architecture at relatively short time scales (i.e., < 20–30 kyr), whereas tectonic deformation is increasingly important at longer time scales (> 100 kyrs). Four long-lasting depocenters are identified over the inner forearc domain and located in four subsiding basins (Kidnappers, Mahia, Lachlan, and Motu-o-Kura basins). Significant shifts of the depocenter location in the basins are correlated with eustatic sea-level changes. Estimates of sediment volumes and masses from isopach maps indicate higher mass accumulation rates during climato-eustatic extremes, which we correlated to the onland erosional response.

Sediment distribution and landscape evolution are strongly influenced by the interaction of the structural deformation and sediment flux. We present paleogeographic reconstructions for the inner forearc domain coincident with two paleoclimatic extremes (Last Glacial Maximum and Holocene Optimum). These illustrate the importance of eustatic changes, structural deformation, and sediment flux on the pattern of sediment distribution, accumulation, and sequence architecture.