Last Interglacial shoreline successions in southeastern Australia: A framework for identifying a waning mantle upwelling, neotectonic movements and sea-level change

Nicolas Flament, Colin V Murray-Wallace

Relict shoreline successions are critically important for investigations of recent tectonism, as they are commonly amenable to dating and may provide information about surface displacement and changes in sea level since their deposition. In this study, Last Interglacial (MIS 5e; 128–116 ka) shoreline successions from 47 locations across southeastern Australia are reviewed. The surface displacement of shoreline successions since their deposition is inferred from their present-day elevation and paleosea levels derived from sedimentary lithofacies and molluscan faunal assemblages. The paleosea levels suggest that marine isotope stage (MIS) 5e sea level peaked at 4 ± 1 m above present sea level in southeastern Australia, about two metres lower than the commonly assumed global sea level maximum. Although this remains to be investigated with models, we speculate that this difference could be explained by glacial isostatic adjustment and topography associated with mantle convection in the region. There is no evidence for two MIS 5e sea-level peaks in paleoshoreline successions from southeastern Australia.

The inferred uplift since MIS 5e is largest in Tasmania (up to 31 m at Stumpys Bay), where it is likely caused by the waning Cosgrove mantle upwelling. Seismic tomographic models and MIS 5e shoreline data suggest that the Cosgrove mantle upwelling is centred presently under northeastern Tasmania, farther east than previously proposed. Volcanism above a steep gradient in lithospheric thickness caused about 10 m of eastward, upward tilt of the Woakwine Range on the Coorong coastal plain since MIS 5e time. On the Fleurieu Peninsula, 6.5 m of upward displacement of the block to the southeast of the Willunga Fault is inferred. In contrast, the Gawler Craton stands out as tectonically stable, with possible limited local subsidence. The careful characterisation of shoreline successions is a powerful way to define subtle geodetic changes.