Comparison of the ‘strike‐slip’ versus the ‘episodic rift‐sag’ models for the origin of the Isa Superbasin

Abstract

In this paper we assess two competing tectonic models for the development of the Isa Superbasin (ca 1725–1590 Ma) in the Western Fold Belt of the Mt Isa terrane. In the ‘episodic rift‐sag’ tectonic model the basin architecture is envisaged as similar to that of a Basin and Range province characterised by widespread half‐graben development. According to this model, the Isa Superbasin evolved during three stages of the Mt Isa Rift Event. Stage I involved intracontinental extension, half‐graben development, the emergence of fault scarps and tilt‐blocks, and bimodal volcanism. Stage II involved episodic rifting and sag during intervening periods of tectonic quiescence. Stage III was dominated by thermal relaxation of the lithosphere with transient episodes of extension. Sedimentation was controlled by the development of arrays of half‐grabens bounded by intrabasinal transverse or transfer faults. The competing ‘strike‐slip’ model was developed for the Gun Supersequence stratigraphic interval of the Isa Superbasin (during stage II and the beginning of stage III). According to this model, sinistral movements along north‐northeast‐orientated strike‐slip faults took place, with oblique movements along northwest‐orientated faults. This resulted in the deposition of southeast‐thickening ramp sequences with local sub‐basin depocentres forming to the west and north of north‐northeast‐ and northwest‐trending faults, respectively. It is proposed that dilation zones focused magmatism (e.g. Sybella Granite) and transfer of strike‐slip movement resulted in transient uplift along the western margin of the Mt Gordon Arch. Our analysis supports the ‘episodic rift‐sag’ model. We find that the inferred architecture for the strike‐slip model correlates poorly with the observed structural elements. Interpretation is made difficult because there has been significant modification and reorientation of fault geometry during the Isan Orogeny and these effects need to be removed before any assertion as to the basin structure is made. Strike‐slip faulting does not explain the regional‐scale pattern of basin subsidence. The ‘episodic rift‐sag’ model explains the macroscopic geometry of the Isa Superbasin and is consistent with the detailed sedimentological analysis of basin facies architecture, and the structural history and geometry.