COOLING AND EXHUMATION ALONG THE CURVED ALBANY-FRASER OROGEN, WESTERN AUSTRALIA

The Albany-Fraser Orogen is a Grenville-aged orogenic belt, putatively formed by the collision of the Yilgarn and Mawson Cratons during Rodinia assembly. The orogen is marked by a distinct curvature between its eastern and western domains, with ca 45° difference in strike along its 12000 km extent. Recently reported results from the western Albany-Fraser Orogen indicate extremely fast cooling rates of ca 22 – 39°C/Ma (Scibiorski et al. 2015, Precam. Res. 265, 232-248). We use ⁴⁰Ar/³⁹Ar thermochronology to test whether different cooling rates characterise these large-scale differently-striking domains in a curved orogenic belt.

In the eastern Albany-Fraser Orogen, hornblende yields ca 1192 Ma cooling ages, 20 My older than the ca 1170 Ma hornblende cooling ages from the western part of the orogen. Muscovite and biotite grains from the eastern Albany-Fraser Orogen yield ca 1180 – 1150 Ma cooling ages, similar to ca 1172 – 1144 Ma cooling ages in the west. We also conducted a Monte Carlo simulation to constrain mineral closure temperatures and post-metamorphic cooling rates. Simulation of cooling from hornblende to biotite closure temperatures in the eastern Albany-Fraser Orogen yields median cooling rates of ca 7 – 9°C/Ma.

Differences in the timing and rate of cooling may be related to the difference in deformation style between the eastern and western domains, which results from the curvature of the orogen. Slow cooling in the eastern Albany-Fraser Orogen is similar to that observed in other Mesoproterozoic orogens, which typically record 1 – 5°C/Ma cooling driven by isostatic mechanisms such as post-orogenic extension and erosion. In contrast, fast cooling in the western Albany-Fraser Orogen suggests rapid exhumation in an active tectonic setting. The interpreted driver of this fast exhumation is the transpressional tectonic activity associated with deformation in the west, which is an underrepresented tectonic regime in the Mesoproterozoic cooling record.

The similarity of muscovite and biotite cooling ages across the orogen requires a regionally homogenous cooling history, independent of tectonic setting or deformation style. This suggests that tectonically-driven exhumation had ceased by ca 1180 – 1150 Ma , and that exhumation was subsequently driven by post-orogenic exhumation mechanisms, such as erosion.