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

A Forearc Stratigraphic Response to Cretaceous Plateau Collision and Slab Detachment, South Island, New Zealand

“…"Thickening and coarsening up cycles" up to 200 m thick within Pahau Terrane units in the Clarence Valley have been attributed to deep marine gravity-flow processes (Reay, 1993), although descriptions of these units include sandy conglomerates, soft-sediment deformation and abundant plant material (including rare coals, see also Figure 6)-features that collectively indicate deltaic processes (Bhattacharya, 2006;Orton & Reading, 1993;Reading, 1996). Furthermore, the descriptions closely resemble cyclical Urutawan-Motuan Pahau Terrane and cover units in the study area interpreted as shelfal/deltaic (Challis, 1960;Gardiner & Hall, 2021;Prebble, 1976;Reay, 1993).…”

“…Furthermore, present day deposition along the Cascadia margin is impacted by slab flexure/flat-slab subduction (Goes et al, 2017;Mc-Crory et al, 2012), analogous to second-order subduction processes of interest to this study. Similar depositional conditions (low forearc gradients, high sediment volumes) are broadly interpreted along the East Gondwana margin during mid-Cretaceous (Adams et al, 2013;Gardiner & Hall, 2021;Kamp, 1999). Importantly, the Cascadia analog produces conservative (i.e., shallow) estimates for depositional water depth owing to its profile, and hence resultant topographic oscillation, relative to steeper margins (e.g., Central Chile).…”

“…Thirty-one stratigraphic sections compiled from locations across Marlborough were translated into decompacted PWD and PT subsidence curves (Figure 5, note differing vertical scales). Curves are grouped by fault block and designated as either inboard/southwest (orange) or outboard/northeast (blue) relative to the interpreted paleo-shoreline (Gardiner & Hall, 2021). PWD curves converge to 0 m RSL between 100 and 95 Ma (Figures 5a-5d), described stratigraphically as evolution from highstand to lowstand conditions, marked inboard by a shift from marine to terrestrial deposition (>0 m RSL) and outboard from marine to fluvio-deltaic (marginal marine) deposition.…”

“…Areally extensive Motuan and Ngaterian deltaic units were deposited largely subaerially (i.e., >0 m RSL), but with deposition of significant siltstone units in shallow marine interdistributary settings that are nominally categorized as estuarine (−3.7 m RSL). In contrast, lithologically indistinguishable siltstone units interbedded with gravity-flow deposits (turbidites, debrites) are clearly marine (Gardiner & Hall, 2021). Respective depositional depths for the siltstones were therefore interpreted from stratigraphic context with coeval depositional facies.…”

“…The stratigraphic successions of interest were likely deposited during the Urutawan, a 5 Ma period that is largely indistinguishable from the ensuing Motuan (Crampton et al, 2004). The limited number of detrital zircons extracted for this period provide little age constraint and may indicate declining arc magmatism (Figure 3c), but may alternatively reflect a decline in exhumation of arc zircons, or zircon sequestration within the terrestrial hinterland (Adams et al, 2013(Adams et al, , 2016Gardiner & Hall, 2021). Furthermore, magnitudes of RSL change are based predominantly on marine strata and effectively omit information above 0 m RSL.…”