Orogenesis in Paleo‐Mesoproterozoic Eastern Australia: A response to Arc‐Continent and Continent‐Continent Collision During Assembly of the Nuna Supercontinent

Abstract

Northern and southern Australia preserve a common record of 1790-1655 Ma intracontinental rifting and backarc extension culminating in formation of a marginal sea that subsequently collapsed following a 1650-1640 Ma reversal in plate motion and onset of arc-continent collision. Arc-continent collision was accompanied by intermediate-pressure Barrovian-type metamorphism (6-8 kb) and widespread crustal thickening as reflected in a clockwise pressure-temperature-time path. Later metamorphism during the 1620-1580 Ma Isa and Olary orogenies occurred under intrusion-enhanced lower pressure conditions (4-6 kb) and followed a counterclockwise pressure-temperature-time path incompatible with either significant amounts of crustal thickening or tectonic models which have Australia and Laurentia commencing collision at this time. Continent-continent collision more likely followed on closely behind arc-continent collision (Riversleigh Tectonic Event), precipitating a switch in crustal shortening from northeast-southwest to west-east as the older rift template and underlying basement structures were reactivated in a transpressive or multicollisional tectonic regime not unlike that which produced the Himalayan-Tibet orogenic system. As in the latter, ongoing collision was accommodated by thrusting, extensional collapse and lateral extrusion of still thermally weak crust on orogen-parallel strike-slip faults, resulting in formation of the west-east-striking Isa Superbasin, isothermal decompression and granite intrusion from 1620 Ma. A comparable record of deformation, metamorphism, and magmatic intrusion from 1650 until 1600 Ma has been documented for northern and southern North America (Forward/Racklan and Mazatzal orogenies) indicating that Laurentia and Australia were likely proximal to each other throughout this period.

Basin inversion and structural architecture as constraints on fluid flow and Pb-Zn mineralisation in the Paleo-Mesoproterozoic sedimentary sequences of northern Australia

“…subsequent to the start of deposition in the River and Term supersequences (Gibson et al, 2020). Along with rocks of Calvert age in the core of the Punjaub Structure, these two sequences were deformed during the 1650-1640 Ma Riversleigh Tectonic Event for which a NE-SW shortening direction has been proposed (Gibson et al, 2020;Gibson et al, 2017). As such, shortening during the earlier stages of basin inversion in the Punjaub Structure would have been approximately orthogonal to strike in the Calvert Superbasin and its NW-SE basin-bounding normal faults.…”

“…It is further evident from the abrupt truncation of older Calvert-age stratigraphy at the base of the River Supersequence in seismic section 91Bn33-91Bn28 ( Fig. 6a) that rocks of the Lawn Hill platform were subjected to an earlier phase of basin inversion, uplift, and erosion before sedimentation in the Isa Superbasin had even started (Bradshaw et al, 2000;Bradshaw et al, 2018;Gibson et al, 2020;Gibson et al, 2017;Scott et al, 1998;Southgate et al, 2000b). As much as 1700m of sedimentary section is estimated to have been lost from the Calvert Superbasin during this inversion event (Bradshaw et al, 2000), a significant amount of which may have been redeposited farther south in the Leichhardt River Fault Trough (Fig.…”

“…3), giving rise to the 1640-1580 Ma Isa Superbasin (Fig. 2) and deposition of a further 6-8 km of sandstone, carbonaceous shales, and dolomitic siltstones (Riversleigh and Term Supersequences) in fault-bounded basins predominantly oriented ENE-WSW (Bradshaw et al, 2000;Bradshaw et al, 2018;Gibson et al, 2020;Gorton and Troup, 2018). Despite the resumption in crustal extension, basaltic rocks are absent and, save for a few tuff beds, there was no corresponding resurgence in felsic magmatism until after the Isa Orogeny had concluded at ca.…”

“…The absence of any significant magmatism is in stark contrast to the two older superbasins, leading some researchers to conclude that the Isa Superbasin represents a sag basin, albeit one periodically punctuated by crustal extension (Betts et al, 2003;Betts et al, 2006), whereas others have argued for deposition in a foreland setting (McConachie and Dunster, 1996), pull-apart basin (Scott et al, 1998;Scott et al, 2000;Southgate et al, 2000a) or syn-orogenic basin in which extension was facilitated by orogen-parallel strikeslip faulting and lateral extrusion of continental crust (Gibson et al, 2020;Gibson et al, 2017). This episode of orogenesis concluded at ca.…”

“…This episode of orogenesis concluded at ca. 1590 Ma (Gibson et al, 2020) or possibly as late as 1580 Ma (Pourteau et al, 2018) before being followed by further crustal extension and successive episodes of pluton-enhanced low pressure-high temperature metamorphism at 1560-1540 Ma and 1520-1490 Ma (Duncan et al, 2011;Foster and Rubenach, 2006;. Granitic rocks associated with this late metamorphism have both A-and S-type compositions and are mainly to be found in the east where they are demonstrably of post-tectonic origin, truncating and cutting across folds and axial plane fabrics produced during the Isan Orogeny (Foster and Austin, 2008;Giles et al, 2006;Page and Sun, 1998;Pollard and McNaughton, 1997;Pollard et al, 1998;Withnall and Hutton, 2013).…”