TECTONIC EVOLUTION OF THE BROKEN HILL REGION, SOUTH-CENTRAL AUSTRALIA AS A KEY TO IMPROVED RECONSTRUCTIONS OF RODINIA

Lying just inboard of the late Neoproterozoic rift margin of Australia (represented by the Tasman Line), the Broken Hill region occupies a critical position in reconstructions of Rodinia, combining an older basement geology (Willyama Supergroup) with a well preserved record of 827 Ma basement faulting, dyke intrusion and syn-rift sedimentation. These events not only constrain the timing and initial direction of late Neoproterozoic continental extension but provide a means of testing competing reconstructions of Rodinia in which south-central Australia is juxtaposed against western Laurentia. Some of these reconstructions postulate a continuation of 1100-1300 Ma Grenville-age rocks into the Broken Hill region whereas others require juxtaposition of the Broken Hill and Mojave-Oaxaca terranes (southern USA) along the Sonora-Mojave megashear, a major palaeo-transform fault which in some restorations of Rodinia (AUSWUS) becomes aligned with northwest-trending late Neoproterozoic normal faults in the Broken Hill region. Besides the obvious mismatch of structures with different geometry and kinematic history, there is no convincingly documented equivalent of the Grenville Orogen in the Broken Hill region.

Greater credence can be placed in reconstructions that combine the late Neoproterozoic tectonic histories with older basement events. In the case of Broken Hill, basement events include episodic crustal extension and clastic sedimentation between 1710 and 1640 Ma, bimodal magmatism at 1670-1690 Ma and accompanying early low pressure-high temperature amphibolite-granulite facies metamorphism. These events were followed by crustal thickening, higher pressure granulite facies metamorphism (anticlockwise P-T-time path) and northeast-directed nappe formation, heralding the onset of the Olarian Orogeny which culminated around 1600 Ma in an orogenic belt 50-60 km thick. Further crustal shortening occurred between 1595 and 1590 Ma when the Willyama was subjected to northwest-directed thrust faulting. Validation of existing reconstructions for Rodinia requires a greater range of equivalent events be present in western Laurentia than is presently recognised.