Discordant forearc deposition and volcanism preceding late-Cretaceous subduction shutdown in Marlborough, north-eastern South Island, New Zealand

Abstract

Compression during normal continental-oceanic subduction leads to the development of a rugose subduction wedge landward of the trench within the marine forearc. Fault-bounded highs across the wedge partition clastic deposition between sub-basins, producing wide variations in sediment thickness over 10’s of kilometres. Partitioning, exacerbated by localised mass-wasting, produces forearc successions that are characterised by a broad spectrum of depositional processes and grainsizes. Second order subduction events (e.g. ridge or plateau collision) can disrupt these processes over several millions of years and elicit depositional responses that are discordant within the forearc stratigraphy. These responses thus constitute stratigraphic signals of such events, but can be difficult to decipher owing to the stratigraphic complexity and inherently destructive nature of marine forearcs.Stratigraphic interpretations for well-preserved clastic and volcanic successions in Marlborough, in New Zealand’s South Island, have long held that between 105 and 95 Ma deposition took place into a series of nascent rift basins, consistent with extension elsewhere in New Zealand. We propose a new stratigraphic framework for Marlborough for the New Zealand Urutawan-Motuan (108.4 to 99.5 Ma) and Ngaterian (99.5 to 95.2 Ma) that instead describes deposition within a coastal plain and attached marine basins during ongoing subduction of the Phoenix Plate. We posit that between 108.4 and 95.2 Ma, hiatus conditions, coarse mass-wasting (including olistostromes), prolific highstand deposition and forearc exhumation punctuated by forearc volcanism represent a significant disruption of preceding depositional patterns. The synchronicity, long-term cyclicity (1 to 5 Ma) and regional extent of these stratigraphic changes indicate that regional tectonism underpinned deposition, which we attribute to second order subduction events along the mid-Cretaceous convergent margin. The stratigraphic framework presented here is consistent with regional geochronologic, stratigraphic and plate reconstruction data that independently argue for continued Phoenix Plate subduction after 92 and as late as 86 Ma.This study underlines the complexity of plate margin dynamics and the complications produced by second order subduction events. The framework presented here indicates that subduction and continental extension were contemporaneous, which following subduction shutdown led to continental breakup from 85 Ma. A stratigraphic record of these events is preserved within the exhumed forearc in Marlborough, which continued to accumulate sediments despite the profound plate-scale events taking place below.