Folded basement-cored tectonic wedges along the northern edge of the Amadeus Basin, Central Australia: evaluation of orogenic shortening

Abstract

Intracontinental deformation at the northern margin of the Amadeus Basin, Central Australia, is characterised by the presence of basement-cored, downward-facing antiformal synclines. These structures were formed during the Devonian–Carboniferous Alice Springs Orogeny and in the past were explained as erosional relics of fold nappes indicating large horizontal crustal shortening within the Australian continent. This notion is in stark contrast with both the uncomplicated homoclinal structure that contains the `nappes' and with seismic data suggestive of a simple crustal structure formed during only modest intraplate shortening. Balanced and sequentially restored sections across the Razorback `nappe' and its vicinity show that such downward-facing synformal anticlines can be produced by emplacement of a basement cored tectonic wedge along shortcut thrusts in the footwall of a crustal-scale (Redbank) shear zone which initially formed during the Mesoproterozoic. The forelandward-propagating wedge proper is formed by basement and the lower two successions of the Amadeus Basin. Higher sedimentary successions accommodate shortening by hinterlandward backthrusting above a detachment horizon which is hosted in carbonates and evaporites of the Bitter Springs Formation. During the formation of the homocline which characterises much of the northern margin of the Amadeus Basin, the tectonic wedge was rotated through 90° and now forms a downward-facing antiformal syncline. The orogenic shortening indicated by this succession of structural events is about 19 km which is compatible with seismic data. The proposed model explains the existence of downward-facing synformal anticlines as folded basement-cored tectonic wedges; although they resemble parts of fold nappes, these folded tectonic wedges do not necessarily imply large amounts of crustal shortening.