Structural architecture during the Mount Isa Rift Event

The most significant Mount Isa Rift Event-aged extensional structures are normal faults that bound E and SE-thickening half graben. There is a notable difference in the geometry of extensional structures between the Mount Isa Rift and the northwestern rift flanks (cf. Figure 1).

In the Mount Isa Rift, N-S and E-W trending normal faults bound half graben (O'Dea et al., 1997a, b). The Mount Isa Rift was approximately 40 km wide during the Mount Isa Rift Event and bounded by a westdipping fault that was located at the approximate position of the Quilalar and the Gorge Creek Faults (Figure 1) (O'Dea et al., 1997a). Normal faults are documented along the eastern Mount Isa Rift (Batson, 1991). These faults dip to the southeast and have marked thickness variations of the Bigie Formation and Surprise Creek Formation across them (~200 and ~950 m respectively (Batson, 1991).

Another important structural element of the Mount Isa Rift is south and north dipping cross rift structures (Lister et al., 1999). These faults initially developed during the Myally Rift Event (O'Dea et al., 1997b) and were reactivated during the Mount Isa Rift Event. They have a significant influence on sedimentation with syn-rift sequences coarsening and thickening in the hanging walls (Nijman et al., 1992a; Derrick, 1 9 8 2 ) . L a rger cross rift faults include the Investigator Fault and the Conglomerate Creek Fault (Figure 1).

Mount Isa Rift Event-aged normal faults are best preserved in the mildly inverted (<25 % shortening) northern Mount Isa terrane (Betts et al. , 1999). NWdipping normal faults and NW-striking transverse faults are preserved. Regional-scale normal faults include the Fiery Creek Fault system and the Jacqueline Fault (Figure 1). Normal faults were active during the deposition of the Bigie Formation and the Gunpowder Creek Formation (Betts et al., 1999). Transverse faults strike perpendicular to normal faults. These faults behaved as tear or scissor faults, and evolved to facilitate differential displacement along normal faults (Betts et al., 1999). The NWstriking Termite Range Fault (Figure 1) is a basin-scale transverse fault.

Major normal faults terminate against this fault system. Differential subsidence across the fault is indicated by the absence of the upper Surprise Creek Formation to the south of the fault system.