Tectonic evolution of the Pine Creek Inlier, Northern Territory

Abstract

The Pine Creek Inlier (c. 66 000 km 2) contains Early Proterozoic sediments and volcanics deposited ∼ 1900 Ma ago in a basin formed by crustal extension of c. 2500 Ma granitic basement. About 10 km of supracrustals accumulated in < 20 Ma in shallow marine to continental environments during sagging related to post-extensional subsidence. Deposition in this ‘Pine Creek Geosyncline’ was succeeded by an orogenic period lasting for 180 Ma (1870-1690 Ma, the ‘Top End Orogeny’). Felsic and mafic intrusives were emplaced before and after the main deformation, which ranged in style from open to tight upright folds in low-grade areas in the centre of the region to reclined multiple isoclinal folds in areas of medium- to high-grade metamorphism in the northeast and west. During late orogenesis, rift-related felsic volcanics and continental sediments were deposited on the metamorphosed sequence to form the ‘Katherine Volcanic Sequence’, representing a second period of crustal extension. The lower part of this sequence was itself mildly deformed by the last significant perturbation of the Top End Orogeny. The hiatus between these Early Proterozoic events and Middle Proterozoic deposition is marked by a saprolitic weathered profile, indicating a stable subaerial period perhaps as long as 150 Ma. The cover rocks are the basal part of the McArthur Basin sequence, which extends eastward into Queensland. In the Pine Creek region they are plateau-forming sandstone generally ∼ 600 m thick, thickening to 2000 m in basins near Katherine formed by further movement of the late Early Proterozoic rift systems. The region has remained stable since Proterozoic time with only minor reactivation of rift structures to allow accumulation of locally thicker Mesozoic and Eocene sequences and intrusion of minor mafic to intermediate dykes in the Late Proterozoic and Palaeozoic. Later eustatic movement led to incursion of shallow Palaeozoic and Mesozoic seas.