Palaeoproterozoic mid-basin inversion in the northern Mt Isa terrane, Queensland

The Mellish Park Syncline is located in the northern part of the Mt Isa ter rane. It has an axial trace that transects the remnants of the unconformity -bounded Palaeoproterozoic Leichhardt and Isa Superbasins. The syncline is separated into a lower and upper component based upon variation in fold geo metry across the basin-bounding unconformity. The lower syncline, in the Le ichhardt Superbasin, is tight and has an inclined west-dipping axial plane. The upper syncline, in the isa Superbasin, is open and upright. The geomet ry of the lower syncline is a consequence of a period of shortening and bas in inversion which post-dated the Leichhardt Rift Event (ca 1780-1740 Ma) a nd pre-dated the Mt Isa Rift Event (ca 1710-1655 Ma), forming on open and u pright north-oriented syncline. Subsequent southeast tilting and half-grabe n development during the Mi Isa Rift Event resulted in the lower syncline b eing tilted into its inclined geometry. Sequences of the Isa Superbasin wer e then deposited onto the eroded syncline. The geometry of the upper syncli ne reflects regional east-west shortening during the Isan Orogeny (ca 1590- 1500 Ma). The position of the upper syncline was largely controlled by the pre-existing lower syncline. At this time the lower syncline was reactivate d and tightened by flexural slip folding.