No mountains to snow on: major post-Eocene uplift of the East Victoria Highlands; evidence from Cenozoic deposits

Abstract

Since the idea of the Pliocene Kosciusko Uplift in the Southeastern Highlands of Australia was first introduced, there has been considerable debate about the validity of this Cenozoic uplift event. Until the mid 1990s, most researchers argued that most highland relief was present by the Cretaceous. Since the late 1990s, there has been a paradigm shift that extensive young Cenozoic uplift created much of the high relief. In this paper, we synthesise Cenozoic stratigraphic and structural data from the East Victoria Highlands to assess the timing and origin of uplift. New high-resolution radar topography data indicate extensive east-northeast- and northeast-trending vertical and horizontal fault block displacement of the Cenozoic volcanic and sedimentary paleovalley infill. We suggest that regional uplift and exhumation of the East Victoria Highlands took place along these faults, initiated during the Late Eocene to Early Oligocene, and movements continue to the present day. By a combination of block faulting and epeirogenic uplift the divide migrated 40 km north reaching the present position by Pliocene time. Paleocurrent, lateral stream and magnetic basaltic valley flow directions indicate northward paleoflow directions for many of the Eocene – Oligocene valleys even those south of the present divide. Paleovalleys close to the Gippsland Basin show southward flow directions. The uplift that began in the Eocene causing valley cut and infill, eroded an Early Cenozoic paleoplain surface. Remnants of Late Eocene to Oligocene ligneous sediments are preserved as sub-basaltic, lowland valley, fluvio-lacustrinal sediment on this surface. Three large low-gradient paleodrainage systems that begin south of the present divide flowed north over 100 km to the Murray Basin where they are overlain by younger sediments. In contrast, paleodrainage systems flowing south from the present coastal escarpment to Gippsland, were shorter and steeper. The similarities of palynofacies of this infill to the adjacent basins suggest the valleys were low-relief/low-altitude paleodrainage systems that extended over the East Highlands. Based on our palynology results from Mt Hotham (present-day height 1800 m), previous macrofossil estimates of 800 m maximum Eocene relief could be overestimates of the paleo-height. Due to Cenozoic uplift, the strata (where present) are preserved as hilltop deposits and flows tilted away from the present divide. The Late Eocene to Pliocene uplift is probably primarily responsible for the topographic relief of the present East Victoria Highlands, is of the order of several hundreds of metres to a kilometre, and commenced in the Late Eocene at a divide closer to the Gippsland Basin than at present. This uplift continues to the present day as shown by the active seismicity in the area.