Dextral Strike-Slip Faulting Along the Early Permian Margin of Pangaea (Eastern Australia) and Implications for Oroclinal Bending
Wednesday, 17 December 2014
The breakup of the Pangaean supercontinent was one of the most significant events that affected Phanerozoic global tectonics. Heralding this process, and following the Carboniferous maximum stage of continental assembly, was a period in which the southern part of Pangaea (Gondwana) was subjected to a counterclockwise rotation relative to Laurasia. According to tectonic reconstructions, dextral wrench faulting and oroclinal bending in Varsican Europe and eastern Gondwana accompanied this rotation, but direct evidence for dextral strike-slip faulting in the eastern Gondwanan margin has hitherto not been reported. Here we show evidence from a well-preserved fault zone in eastern Australia (Red Rock fault zone), which occurs along the eastern limb of the Z-shaped Texas/Coffs Harbour orocline. Structural observations show evidence for dextral strike-slip faulting, with a reverse kinematic component, along a sub-vertical fault plane oriented NNE-SSW. Direct geochronological data (Rb-Sr and Ar-Ar) from fault gouge samples associated with this fault zone indicate that brittle faulting occurred in the early-mid Permian (288-264 Ma). In addition, oxygen and hydrogen stable isotope geochemistry indicates that the origin of fluids that circulated in the fault zone was associated with a deep crustal source. These results are consistent with independent constraints on the timing of oroclinal bending, supporting the idea that dextral wrench faulting has directly contributed to the formation of the oroclines. We propose a kinematic model for the formation of the oroclines, attributing the early stage of oroclinal bending to subduction rollback and slab segmentation (at ~300-288 Ma) followed by a period of dextral wrench faulting at 288-264 Ma. In the context of Pangaea, our model suggests that the origin of oroclines along the rim of Gondwana was likely associated with bending in response to migrating plate boundaries, and a subsequent tightening of pre-existing curvatures by dextral wrench faulting.