FAULT REACTIVATION DURING INTRACONTINENTAL DEFORMATION: THE TOKO SYNCLINE AND TOOMBA FAULT, GEORGINA BASIN, CENTRAL AUSTRALIA

The Toko Syncline is a 200 km long, asymmetric, northwest-trending structural trough located on the southwestern edge of the Georgina Basin in central Australia. Approximately 5000 m of Neoproterozoic to Paleozoic strata are preserved in the structure, with Middle Cambrian units considered to be prospective for petroleum. The Toomba Fault forms the southwestern edge of the syncline and juxtaposes Proterozoic crystalline basement of the Arunta Block with Paleozoic sedimentary units preserved in the syncline. Due to complex structure and poor surface exposure, the structural history of the Toomba Fault is difficult to determine. Regional geophysical studies from the 1970's indicate that the Toomba Fault is a high-angle reverse fault dipping west-southwest, with an estimated offset of 6.5 km (Harrison, 1980), but a more in-depth investigation into the subsurface structure of the syncline and structural history of the fault has been lacking.

We analyzed reprocessed seismic reflection profiles from the Toko Syncline and constructed detailed cross-sections that, when combined with constraints from regional magnetic and gravity data, provide a more detailed understanding of the syncline geometry and exact nature of the Toomba Fault. Reverse offset on the Toomba Fault is the result of transpressional deformation during the mid-Paleozoic intraplate Alice Springs Orogeny. Our field work indicates that the Toomba Fault, along with other high-angle reverse faults in the region, reactivates a previous rift-bounding normal fault, originally formed during the Neoproterozoic breakup of Rodinia. Such Neoproterozoic rifting possibly connects this region in central Australia to the Neoproterozoic western margin of North America. Fault reactivation during the Alice Springs Orogeny in central Australia is similar in structural style to the Cretaceous Laramide Orogeny of western North America. Reactivation of former extensional normal faults appears to be a primary mechanism of intraplate deformation. These high-angle normal faults form fundamental weaknesses in the craton, which may then be prone to reactivation as a result of far-field stresses.