Tectonic significance of structural successions preserved within low-strain pods: Implications for thin- to thick-skinned tectonics vs. multiple near-orthogonal folding events in the Palaeo-Mesoproterozoic Mount Isa Inlier (NE Australia)

Abstract

Event-wise structural successions and their regional correlation across the orogenic belts are always been problematic, resulting in construction of conflicting tectonic models. However, in recent years, it has been demonstrated that complex deformation and metamorphic overprinting relationships and their correlation across large regions can be resolved through careful quantitative analysis of fabrics preserved within low-strain pods at different scales. In this study, integrated macro, meso and microstructural analyses of the southern Selwyn Range of the Palaeo-Mesoproterozoic Mount Isa Inlier, NE Australia, have revealed that the tectonic evolution involved four dominant fabric-generating events. These differ from those reported previously based on interpretation of the Mount Isa deep seismic section. D 1 is preserved in low-strain zones as outcrop-scale recumbent folds (S 1 subparallel S 0) with E–W trending fold axes. D 2 is characterized by upright to inclined N–S trending folds with a pervasive regional foliation (S 2), subparallel to S 0/S 1. D 3 was very heterogeneously developed in the region and appears to be controlled by rheological contrast. Three structural domains are recognized. The Eastern Domain (ED) and Central Domain (CD) are characterized by shallow dipping S 3 foliation. S 3 foliation is rare in the Western Domain (WD). Deformation during D 4 was intense in the WD and produced complex overprinting patterns. All these observations from macro to microscopic scale suggest that the regional S 2 initially formed as a steep foliation and was deflected to shallow to moderate dips due to the effect of subvertical shortening (D 3). These observations are difficult to reconcile with previously suggested thin- to thick-skinned nappe-style tectonics in the region. Metamorphic field gradient is higher (sillimanite zone) in the ED, which represents deeper crustal levels than the rocks exposed in the CD and WD separated by pre-existing faults. Andalusite porphyroblasts in the WD preserve steeply dipping S 2, whereas staurolite porphyroblasts preserve gently dipping S 3, suggesting staurolite grew after andalusite. This interpretation is consistent with the appearance of mineral phases in the KFMASH system along a prograde P– T path (M 2) for D 2 and D 3. An earlier higher pressure P– T path (M 1) is proposed for D 1 based on the metamorphic mineral assemblages in the MnNCKFMASH system.