Apatite (U – Th)/He age constraints on the Mesozoic and Cenozoic evolution of the Bathurst region, New South Wales: evidence for antiquity of the continental drainage divide along a passive margin

Abstract

The apatite (U – Th)/He and fission track thermochronometers are combined to constrain the Mesozoic and Cenozoic denudational history of the Bathurst region in New South Wales. New apatite (U – Th)/He ages across the continental drainage divide range from 83 ± 14 to 114 ± 18 Ma (±2σ), supporting the evidence from previously published apatite fission track (AFT) data that the region underwent a period of rapid denudation during the mid-Cretaceous. The apatite He-derived thermal histories constrain the amount of erosion in the area and suggest that spatial variations in denudation of the Bathurst Batholith correspond to three broad regions with different histories. The area of the continental drainage divide separates the eastern flank where denudation was rapid at 120 – 90 Ma, and >3 km of crust was removed in about 30 million years, from the western flank, where denudation was less rapid and less prominent. The coincidence of some of the AFT and He ages with the timing of continental extension and Tasman Sea opening suggests that the rapid denudation east of the continental drainage divide was associated with the rifting processes. Such an interpretation means that the escarpment that bounds the Blue Mountains on the east was not formed at the time of rifting and may be a later topographic feature, connected with exhumation of an earlier structural feature and/or related to post-Mesozoic tectonic activity. The evolution of the area west of the continental drainage divide is characterised by rates of denudation higher than on the rest of the plateau but lower than on the eastern flank in the Bathurst area. The reason for this pattern is still unclear. It is possible that faults of the Lachlan Fold Belt were reactivated during the Cretaceous extension and later, but no faults are mapped across the batholith. Overall, the data imply that the continental margin in the Bathurst region evolved by plateau downwearing pinned to the continental drainage divide. This means in turn that the continental drainage divide is a longstanding topographic feature, dating from before Tasman Sea breakup.