Structural architecture of the southern Mount Isa terrane in Queensland inferred from magnetic and gravity data

Abstract

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 ∼250km 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. 1790–1730Ma Leichhardt Superbasin. Regional low magnetic and low gravity responses may reflect basinal sequences deposited during the formation of the ca. 1725–1690Ma Calvert Superbasin and the ca. 1675–1595Ma 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. 1595–1580Ma 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. 1570–1550Ma Middle Isan Orogeny produced inversion of major NNW-trending rift faults and resulted in regional scale culminations. The ca. 1550–1540Ma 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. 1540–1520Ma 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.