Comparison of the 'strike-slip' versus the 'episodic rift-sag' models for the origin of the Isa Superbasin

In this paper we assess two competing tectonic models for the development o f the Isa Superbasin (ca 1725-1590 Ma) in the Western Fold Belt of the Mt I sa terrane. In the 'episodic rift-sag' tectonic model the basin architectur e is envisaged as similar to that of a Basin and Range province characteris ed by widespread half-graben development. According to this model, the Isa Superbasin evolved during three stages of the Mt Isa Rift Event. Stage I in volved intracontinental extension, half-graben development, the emergence o f fault scarps and tilt-blocks, and bimodal volcanism. Stage II involved ep isodic rifting and sag during intervening periods of tectonic quiescence. S tage III was dominated by thermal relaxation of the lithosphere with transi ent episodes of extension. Sedimentation was controlled by the development of arrays of half-grabens bounded by intrabasinal transverse or transfer fa ults. The competing 'strike-slip' model was developed for the Gun Supersequ ence 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 mov ements along northwest-orientated faults. This resulted in the deposition o f southeast-thickening ramp sequences with local sub-basin depocentres form ing 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 suppor ts the 'episodic rift-sag' model. We find that the inferred architecture fo r the strike-slip model correlates poorly with the observed structural elem ents. Interpretation is made difficult because there has been significant m odification and reorientation of fault geometry during the Isan Orogeny and these effects need to be removed before any assertion as to the basin stru cture is made. Strike-slip faulting does not explain the regional-scale pat tern of basin subsidence. The 'episodic rift-sag' model explains the macros copic geometry of the Isa Superbasin and is consistent with the detailed se dimentological analysis of basin facies architecture, and the structural hi story and geometry.