Geological Background

The tectonic history of the Mount Isa terrane can be divided into two major phases. The early phase is dominated by intracontinental extension and basin development (ca 1800-1590 Ma). Three major episodes of extension are recognised in the Western Fold Belt. The Leichhardt Superbasin (ca 1800 Ma- 1740 Ma) (Figure 1 and 2) (Southgate et al., 1998; Page and Sweet, 1998) formed during superimposed E-W and N-S extensional events termed the Leichhardt Rift Event and Myally Rift Event respectively (O'Dea et al.,1997b). Coarse clastic, quartzite and tholeiitic basalt sequences were deposited into a N-S rift zone (Leichhardt Rift) in which the deformed remnants are preserved in the Leichhardt River Fault Trough (Figure 1). The Myally Subgroup (Figure 3) was deposited into north thickening half graben that were bounded by EW normal faults (O'Dea et al., 1997b). Laterally extensive quartzite-carbonate sequences (Quilalar Formation: Figure 3) were then deposited during thermal subsidence (Derrick et al., 1980; Jackson et al., 1990).

Figure 2. Tectono-stratigraphic column

Tectono-stratigraphic column

Tectono-stratigraphic column of the Western Fold Belt. Summary of the major lithostratigraphic formations, regional unconformities, tectonic events, and age of igneous rocks. Data is compiled from Wyborn et al., 1988; Page, 1983; Page and Sweet, 1998; Page et al., 1994; Southgate et al., 1997. Modified from O'Dea et al., 1997a.

A period of depositional hiatus then ensued (ca 1740 and ~1710 Ma). The Leichhardt Superbasin was uplifted and eroded, and a regional unconformity developed. The cause of uplift is conjectural. O'Dea et al. (1997a, b) interpreted the uplift as the manifestation of mid-crustal extension in the Eastern Fold Belt (Wonga Event: Holcombe et al., 1991; Oliver et al., 1991). A mid-basin inversion event has also been identified in the Western Fold Belt at this time (Betts, 1999).

Sequences belonging to the Isa Superbasin (Figure 1) (ca 1710-1590 Ma) were deposited on to this unconformity (Figure 2). This basin initially developed during the NW-SE directed extension (Mount Isa Rift Event: ca 1710-1655 Ma). A new rift axis (Mount Isa Rift) was superimposed onto the Leichhardt Rift (Betts et al., 1998). Sedimentation during the Mount Isa Rift Event is dominated by W to NW tapering sequences (Betts et al., 1999; Batson, 1991; O'Dea et al., 1997a, b; Derrick, 1982; Lister et al., 1999).

During the post-rift evolution of the basin (ca 1655- 1595 Ma), shallow marine to lacustrine carbonates of the Mount Isa Group and Lower McNamara Group (Figure 2) were deposited (McConachie and Dunster, 1996; Dunster and McConachie, 1998). The basin depocentre shifted to the northern Mount Isa terrane and the upper McNamara Group was deposited in an outer shelf to deep water environment (Andrews, 1996). There are no temporal or lateral equivalents of the Upper McNamara Group preserved in the Leichhardt River Fault Trough.

Development of the Isa Superbasin was interrupted by the Isan Orogeny (ca 1590-1500 Ma; Blake, 1987). Early N-S shortening and basin inversion (Bell, 1983; Lister et al., 1999; O'Dea et al., 1997a) was followed by a period of regional E-Wshortening, resulting in the development of crustal-scale upright folds. Late in the Isan Orogeny strike-slip faults were active in a brittle wrench environment (Lister et al., 1999).