A stratigraphic record from syn to post subduction sedimentation in Marlborough, New Zealand, and implications for Gondwana breakup

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Sediments that accumulate within marine basins at convergent plate margins can provide important insights into the evolution and tectonic stability of the subduction wedge. Long-term sediment dispersal and stratigraphic patterns can also be influenced by geodynamic reconfigurations of the plate margin, such as changes in plate dip or velocity, or the subduction of anomalous oceanic crust. An understanding of tectonic context and complexity is therefore critical for any interpretation of convergent margin stratigraphy. Unfortunately, ongoing subduction and wedge deformation tends to obscure or destroy the stratigraphic record of such events as sediments become progressively incorporated into the wedge.The Awatere Valley in New Zealand's South Island contains well-preserved forearc stratigraphic record that accumulated within coastal-plain and marine basins during the final stages of East Gondwana subduction. We assign seven facies associations (FA) to characterise post-subduction units, showing that a transition from syn to post-subduction deposition is contained within largely marine strata from the New Zealand Urutawan to Ngaterian stages (c. 108 to 95 Ma). In contrast to previous accounts of deposition within nascent rift-basins, restored basin geometries are most consistent with those of terraced subduction wedges. We find no evidence that extension affected deposition prior to 95 Ma. Our results instead document a transition from highly deformed sedimentary wedge (pre 108 Ma) to largely undeformed sedimentary cover units (∼100 Ma) that is predominantly gradational.Up to 2 km of basin fill documents a relative sea-level (RSL) rise and fall between 108 and 95 Ma. A period of relative sediment starvation during the Urutawan (108–103 Ma) was abruptly terminated by synchronous mass-wasting, including the remobilisation of metre-scale blocks and rafts, that signals a dramatic increase in depositional energy relative to preceding patterns. Within the context of a large RSL fluctuation, diminishing compressional tectonism and forearc volcanism (from 98 Ma), this increase is anomalous and appears to signal a significant change in underlying subduction dynamics.