Accretion on the long-lived continental margin of northeastern Australia

Abstract

The northern extremity of the late Neoproterozoic-Paleozoic Tasman Orogenic zone exposed in north Queensland forms a narrow belt of tectonised rock assemblages abutting Paleoproterozoic - Mesoproterozoic rocks of the North Australian craton. The craton-orogen contact (Tasman Line) is extensively exposed, a unique circumstance for Australia. Sedimentary protoliths of the cratonic rocks were mainly deposited between 1700-1600 Ma and multiply deformed between 1600-1500 Ma. The Lynd Mylonite Zone, one expression of the Tasman Line, separates rocks of the late Neoproterozoic - Ordovician Thomson Orogen from those of the craton. The succeeding Silurian - Devonian Mossman Orogen is generally faulted against the Thomson Orogen, but in its northern extent it may directly abut the craton along the Palmerville Fault, also an expression of the Tasman Line. These two orogenic systems are dominantly of active margin association and E-stepping but deep seismic imaging indicates that they are extensively underlain by crust of Archean or Paleoproterozoic age. The Tasman Orogenic Zone in its southern part represents a broad tract of crust c. 1,000 km across, added to the cratonic core of Australia in a phase of rapid accretion. In contrast, for its north Queensland development a much smaller volume of new crust was generated, expressing slow accretion. For this region the orogenic system laps extensively onto cratonic crust, a geometry which at least in part reflects overthusting during episodes of Paleozoic contractional orogenesis. As a consequence of little orogenic accretionary outgrowth of the north Queensland continental margin, three large-scale, successive igneous assemblages of active margin association generated throughout the Paleozoic form largely co-located and overprinting belts with plutonic suites stitching the Tasman Line and extending into the craton.