Post-rift evolution of the southeastern Australia Great Escarpment from apatite 4He/3He thermochronology

High-relief great escarpments are prominent geomorphic features characterizing many passive continental margins, extending for hundreds to thousands of kilometers subparallel to the continent-ocean boundary and connecting coastal plains with upland plateaus. Initial formation of these escarpments is most often attributed to oceanic rifting preceding passive margin development. However, in many cases, including our study area in SE Australia, these escarpments have persisted for tens to hundreds of millions of years after rifting, raising questions about their geomorphic origin and evolution. One of the challenges to understanding the evolution of great escarpments is that erosion-driven exhumation produced during their retreat from the coast is expected to be too small in magnitude to be recorded by conventional thermochronometers, such as apatite fission track (AFT). Here, we present apatite ⁴He/³He thermochronology results from a bedrock transect across the SE Australian escarpment. Existing AFT and conventional (U-Th)/He data in SE Australia appear to lack sufficient resolution to fully document the timing of cooling associated with escarpment retreat. Apatite ⁴He/³He thermochronology, on the other hand, is sensitive to temperatures as low as 35 ºC, making it suitable for detecting cooling signals from the estimated 1-1.5 km total exhumation associated with escarpment retreat in this region. Preliminary thermal history models based on our initial apatite ⁴He/³He data document an increase in cooling rates across the coastal plain ca. 120-80 Ma. This late Cretaceous signal overlaps with the initiation of rifting of the Tasman Sea and is consistent with a plateau degradation style of escarpment evolution, where the escarpment formed and retreated to near its present-day position rapidly after rifting. Ongoing acquisition of additional apatite ⁴He/³He data will allow us to further assess the extent of late Cretaceous cooling along the coastal plain and better constrain landscape evolution models of escarpment development.