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The basement rocks of the southern Mount Isa terrane are concealed under younger sedimentary units and its crustal architecture is understood using constrained regional potential field analysis. Prominent N-S- to NNW-trending geophysical anomalies extend for ∼250 km south of the exposed Mount Isa Inlier and are abruptly terminated by the NE-trending Cork Fault. Palaeoproterozoic basinal successions and major Palaeo- to Mesoproterozoic structures recorded in the Mount Isa Inlier are interpreted to continue southward under the Palaeozoic cover. The intensely positive geophysical signature of the region is mostly attributed to shallowing of the Barramundi-aged basement and distribution of metamorphosed sedimentary and volcanic rocks deposited during the formation of the ca. 1790–1730 Ma Leichhardt Superbasin. Regional low magnetic and low gravity responses may reflect basinal sequences deposited during the formation of the ca. 1725–1690 Ma Calvert Superbasin and the ca. 1675–1595 Ma Isa Superbasin. Short wavelength magnetic anomalies and co-located low intensity Bouguer gravity anomalies are interpreted to represent shallow and variously magnetized granitic intrusions. The geophysical signature of the southern Mount Isa terrane reflects the bipartite fault architecture of the region. In the Western Fold Belt, Palaeoproterozoic multiple basin formation was controlled by west-dipping faults in half graben setting, within the NNW-trending Leichhardt River Fault Trough. To the east of the Pilgrim Fault, thick Palaeoproterozoic sedimentary and volcanic successions were controlled by major NNW-trending, east-dipping normal faults. The Mesoproterozoic Isan Orogeny reactivated the existing extensional fault network. Deformation during the ca. 1595–1580 Ma Early Isan Orogeny is not apparent in the southern Mount Isa terrane and may have been confined to the northern part of the terrane. The ca. 1570–1550 Ma Middle Isan Orogeny produced inversion of major NNW-trending rift faults and resulted in regional scale culminations. The ca. 1550–1540 Ma wrench tectonics is represented by the activation of NW- and NE-oriented strike-slip faults overprinting the prominent N-S- to NNW-oriented structures. The emplacement of elongated batholitic bodies may be coeval to the emplacement of the ca. 1540–1520 Ma Williams Supersuite recorded in the Mount Isa Inlier and thus may represent its southern extension. The southern Mount Isa terrane also appears to have been affected by post-Isan tectonics that resulted in deposition and magmatic intrusions focused along the northern side of the NE-trending Cork Fault. [ABSTRACT FROM AUTHOR]

The basement rocks of the poorly understood Thomson Orogen are concealed by mid-Paleozoic to Upper Cretaceous intra-continental basins and direct information about the orogen is gleaned from sparse geological data. Constrained potential field forward modelling has been undertaken to highlight key features and resolve deeply sourced anomalies within the Thomson Orogen. The Thomson Orogen is characterised by long-wavelength and low-amplitude geophysical anomalies when compared with the northern and western Precambrian terranes of the Australian continent. Prominent NE- and NW-trending gravity anomalies reflect the fault architecture of the region. High-intensity Bouguer gravity anomalies correlate with shallow basement rocks. Bouguer gravity anomalies below –300 µm/s2define the distribution of the Devonian Adavale Basin and associated troughs. The magnetic grid shows smooth textures, punctuated by short-wavelength, high-intensity anomalies that indicate magnetic contribution at different crustal levels. It is interpreted that meta-sedimentary basement rocks of the Thomson Orogen, intersected in several drill holes, are representative of a seismically non-reflective and non-magnetic upper basement. Short-wavelength, high-intensity magnetic source bodies and colocated negative Bouguer gravity responses are interpreted to represent shallow granitic intrusions. Long-wavelength magnetic anomalies are inferred to reflect the topography of a seismically reflective and magnetic lower basement. Potential field forward modelling indicates that the Thomson Orogen might be a single terrane. We interpret that the lower basement consists of attenuated Precambrian and mafic enriched continental crust, which differs from the oceanic crust of the Lachlan Orogen further south. [ABSTRACT FROM AUTHOR]

The crustal architecture as well as the kinematic evolution of the Thomson Orogen in Queensland is poorly resolved because the region is concealed under thick Phanerozoic sedimentary basins and the basement geology is known from limited drill holes. Combined potential field and seismic interpretation indicates that the Thomson Orogen is characterized by prominent regional NE- and NW-trending structural grain defined by long wavelength and low amplitude geophysical anomalies. The ‘smooth’ magnetic signature is interpreted to reflect deeply buried source bodies in the mid- to lower crust. Short wavelength positive magnetic features that correlate with negative gravity anomalies are interpreted to represent shallower granitic intrusions. They appear to be focused along major fault zones that might have controlled the locus for magmatism. The eastern Thomson Orogen is characterized by a prominent NE structural grain and orthogonal faults and fold interference patterns resulting in a series of troughs and highs. The western Thomson Orogen consists of a series of NW-trending structures interpreted to reflect reverse faults. Sedimentation and basin development are interpreted to have initiated in the Neoproterozoic to Early Cambrian during E–W- to ENE–WSW extension, possibly related to the Rodinia break-up. This extensional event was followed by Late Cambrian shortening recorded in the Maneroo Platform and the Diamantina River Domain which possibly correlates with the Delamerian Orogeny. Renewed deposition and volcanism occurred during the Ordovician and may have continued until Late Silurian, resulting in thinned Proterozoic basement crust and extensive basin systems that formed in a distal continental back-arc environment. Our interpretation places the Thomson Orogen to the west of the Neoproterozoic passive margin preserved in the Anakie Inlier. The region is likely to represent the internal extensional architecture during the Rodinia break-up that has been subsequently extensively modified by multiple extensional basin forming events and transient episodes of crustal shortening and basin inversion. [ABSTRACT FROM AUTHOR]