Deciphering continental breakup in eastern Australia using low‐temperature thermochronometers

Abstract

[1] First-order topographical features at high-elevation passive margins have the potential to store information about the early stages of continental rifting, which are usually lost in the sediment record. In this study we present a new combination of apatite fission track and (U-Th)/He analyses from two coast-perpendicular traverses to decipher the time of formation and evolution of the escarpment at the southeastern Australian margin. A combination of inverse and forward modeling of the apatite fission track (AFT) and He data indicates that the coast experienced a rapid denudational event starting at 110 ± 10 Ma, at least 15 Myr before seafloor spreading (85 Ma), in agreement with the hypothesis that rifting at magma-poor margins evolves slowly. Thus the enhanced denudation is not related to the synbreakup base level drop, but it may have been related to a thermally driven transient surface uplift. Considerations on isostatic flexure and rigidity of the lithosphere indicate that unless significant synrift brittle movements are assumed to have occurred along margin-parallel faults that are no more visible, the coast must have experienced a higher geothermal gradient than normal. The evolution of the rest of the coastal plain may have taken up to $90 Myr, in agreement with numerical models of passive margins evolution. The (U-Th)/He ages across the coastal plain are consistent with an in-place-excavation scenario of a high rift shoulder, and they rule out the possibility of a constant retreat of the escarpment. AFT + He data from the plateau indicate that the highlands remained stable throughout continental breakup, experiencing rates of erosion of 5-10 m/Myr since, at least, 200 Ma.

Sedimentation record in the Konkan–Kerala Basin: implications for the evolution of the Western Ghats and the Western Indian passive margin

“…However, the emerging view is that the development of these escarpments may be rapid and shortlived, with the escarpment reaching essentially its modern configuration in a relatively short time after rifting (Cockburn et al, 2000;Matmon et al, 2002;Persano et al, 2002Persano et al, , 2005.…”

Long‐term landscape evolution: linking tectonics and surface processes

“…This pattern of kilometre-scale post-breakup denudation on the coastal plain, between the escarpment and the coast, and a marked episode of accelerated denudation broadly coincident with continental breakup, has now been broadly confirmed for several margins (e.g. southern Africa, Brown et al, 2000aBrown et al, , 2000bEritrea, Balestrieri et al, 2005), as well as being very clearly evident in more recent data for the SE Australian margin (Persano et al, 2002(Persano et al, , 2005. All these various data unequivocally confirm the critical role of flexural isostasy in passive margin evolution, and make it extremely unlikely that the downwarp model is sustainable on the southern African and southeastern Australian PCMs.…”

“…the Drakensberg escarpment in southeastern Africa - Brown et al, 2002) or a secondary, but still major, drainage divide between coastal rivers and rivers that initially flow inland before turning back out to the sea (e.g. the escarpment in far southeastern Australia - Persano et al, 2002Persano et al, , 2005. On type-3 escarpments, various data, including TCN measurements, point to high rates of gorge extension where the coastal rivers cross the escarpment (Nott et al, 1996;Weissel and Seidl, 1998).…”

“…The most recent thermal modelling of AFTT and apatite-He data from several high-elevation PCMs convincingly points to either very rapid erosion of at least the bulk of -and in some cases virtually all of -the full width of the coastal plain with its backing escarpment within 10 Myr or so of rifting and breakup, and an essentially stable escarpment thereafter (e.g., Brown et al, 2000b;Cockburn et al, 2000;Persano et al, 2002Persano et al, , 2005. These low rates of escarpment retreat after the escarpment was formed are confirmed by Late Cenozoic denudation rates of the escarpment and the coastal plain based on terrestrial cosmogenic nuclide (TCN) analysis (southern Africa, Fleming et al, 1999;Brown et al, 2000b;Bierman and Caffee, 2001;Cockburn et al, 2000;Van der Wateren and Dunai, 2001; southeastern Australia, Heimsath et al, 2000).…”

“…Monte Carlo simulation of AFTT age and track length distribution provides a family of possible time-temperature histories of the mineral's trajectory to the surface as a result of denudation (Gallagher, 1995;Ketcham and Donelick, 2003;Gallagher et al, 2005). These AFTT-based time-temperature histories are then interrogated to select those that reproduce apatite-He ages (Balestrieri et al, 2005;Persano et al, 2005).…”