Structural development of the Soldiers Cap Group in the Eastern Fold Belt of the Mt Isa Inlier: a succession of horizontal and vertical deformation events and large-scale shearing

Six discrete episodes of sequential, alternating horizontal (D-1, D-3, D-5) and vertical (D-2,D- D-4,D- D-6) compressive deformation have been recorded in the Proterozoic Soldiers Cap Group of the eastern Mt Isa Inlier. A seventh compressive horizontal deformation (D-7) is recognised locally. The deformations range in age from ca 1600Ma to post-1480Ma. Quartz inclusion trails in garnet porphyroblasts preserve the orientations of early foliations that have been correlated with foliations recorded at the outcrop. Both inclusion trails and foliations in outcrop, are consistent with north-south shortening during Di, east-west shortening during D-3 and Ds, and north-south shortening during D7 Field and microstructural relationships are consistent with an orogenic model involving three episodes of horizontal shortening each of which was followed by gravity collapse of the overthickened orogenic rock mass resulting in widespread extension at the centre of the orogen and thrusting along its sides. Deep within the orogen, shallowly dipping cleavages (S-2,S- S-4,S- Sd) imparted by the vertical compression of the gravitationally collapsing rock masses overprinted the steep cleavages formed by the earlier horizontal compression and were in turn overprinted themselves by the steep cleavages of the next horizontal compression. Local unfolding of Dg folds by reactivation of Dg cleavage allowed the formation of several small mineralised enclaves within the Soldiers Cap Group.