Multiple foliation development during thrusting and synchronous formation of vertical shear zones

Abstract

Three orthogonal foliations developed during large-scale N-S thrusting in the central portion of the Mount Isa Block, Australia. Two of these foliations, which are well preserved in their original orientations in porphyroblasts of pelitic schists but considerably affected and commonly destroyed in the schist matrix due to the effects of subsequent deformation and metamorphism, formed synchronously and have a common stretching lineation, which is near-horizontal and trends N-S. One foliation formed near-horizontal and the other near-vertical, striking N-S. Both developed during N-S-directed thrusting, with the N-S vertical foliation accommodating sinistral differential displacement between the thrust belt to the west and that to the east. The third foliation which dips steeply to the north and strikes E-W, formed during N-S shortening of the thrust belt and tends to overprint the other foliations formed in the same orogeny. The N-S vertical foliation is well preserved within a narrow belt of gneissic rocks (called the Wonga-Duchess Belt) that also locally contains the subsequently folded remains of the foliation that formed near-horizontal. This belt separates multiply thrust-repeated beds to the west from large-scale, apparently less repetitive thrusts to the east. The direction of thrusting appears to change across this belt from N to S on the west to S to N on the east. The differential deformation to either side of the belt was taken up by sinistral shear along this belt. Numerous related, but smaller-scale vertical shear zones occur up to 3 km away to either side of this narrow zone. The primary vertical foliation within the Wonga-Duchess Belt appears to have formed during transform-like faulting by progressive inhomogeneous simple shear with no component of orthogonal shortening.