The northern margin of the Capricorn Orogen, Western Australia—an example of an Early Proterozoic collision zone

Abstract

The Early Proterozoic (2200-1600 Ma) Capricorn Orogen is a major zone of deformation, metamorphism and granite emplacement between the Archaean Yilgarn and Pilbara Cratons of northwest Australia. Capricorn Orogen structures are recognized along the southern margin of the Pilbara Craton deforming the Archaean granite-greenstone terrain of the Sylvania Inlier together with rocks of the Hamersley, Ashburton and Blair Basins. In the Sylvania Inlier deformation is characterized by large-scale shear zones interpreted as NNE-directed thrusts. In the Hamersley Basin, orogenic deformation forms the Ophthalmia Fold Belt and can be divided into two groups of structures: in the southwest large-scale dome-and-basin folds (pre-Ashburton Basin) are present; in the southeast folds and faults are part of a younger (syn-Ashburton Basin) N-directed foreland fold and thrust belt directly linked to deformation in the Sylvania Inlier. The deformed Ashburton Basin and adjacent parts of the Hamersley Basin form the Ashburton Fold Belt. Two periods of deformation are recognized: early recumbent folding ( D 1a, post-Ashburton Basin-pre-Blair Basin), and later dextral wrench faulting and associated folding ( D 2a, syn- or post-Blair Basin). Capricorn Orogen structures are interpreted as the result of an oblique continent-continent collision between the Pilbara and Yilgarn Cratons. Along the southern Pilbara margin, collision first occurred in the southeast and migrated northwest, with the Ashburton Basin evolving as a foreland basin. Late-stage dextral wrench faulting was the result of westward extrusion of material caught between the opposing craton margins.