The architecture and evolution of intracontinental orogens: a structural, metamorphic and geochemical characterisation.

Abstract

Intracontinental orogens are irreconcilable with conventional plate tectonics theory, which confines mountain building to plate-margin settings alone. However, the comparative rarity of such orogenic systems in the geological record means that their architecture and evolution are poorly understood. In order to address this deficiency, this thesis presents an integrated framework that characterises the structural, metamorphic and geochemical features of intracontinental reworking in central and southeastern Australia. The axial zone of the Ediacaran–Cambrian (600–530 Ma) Petermann Orogen, western Musgrave Province, is characterised by pervasive mylonitic deformation and low geothermal gradient metamorphism that formed at deep crustal levels (P = 10–14 kbar and T = 700–800 °C). Peak metamorphic conditions were attained at c. 570 Ma, followed by slow cooling to 600–660 °C by c. 540 Ma, at an average rate of 2–6 °C Myr⁻¹. The macroscopic structural, kinematic and metamorphic architecture of this terrane is broadly comparable in style to the Himalayan‒ Tibetan Orogen, suggesting that both systems are dominantly shaped by the gravitationally-driven flow of deep crustal material. This similarity also extends to their spatial and temporal scales, overall cooling histories, average geothermal gradients, levels of exhumation and extents of crustal thickening, implying that the basic anatomy of intracontinental orogens is analogous to that of typical collisional belts. The Ordovician–Carboniferous (450–300 Ma) Alice Springs Orogen is characterised by intensely metasomatised ductile shear zones that dissect the eastern Arunta Region. Similar alteration features are also observed in the Cambrian–Ordovician (514–490 Ma) Delamerian Orogen, southern Curnamona Province. In both cases, isotopic datasets confirm that substantial volumes of surface-derived fluids were involved in the rehydration of the deep crust. Calculated δ¹⁸O and δD fluid values are as low as ~2‰ and –60‰ for the former, whereas garnet porphyroblasts from the latter exhibit equilibrium δ¹⁸O fluid values of ~4‰. It is argued these surficial fluid signatures are imposed in the vicinity of the brittle‒ductile transition by the burial and dehydration of hydrothermally-altered fault panels, rather than the deep drawdown of a mobile fluid phase via hydraulic forcing. The resultant accumulation of increasing fluid volumes in penetrative fault networks promotes extensive metasomatism and reaction softening at the locus of stress transmission from plate-boundary sources. It is therefore concluded that the interaction of externally-derived fluids with refractory crustal material is a key contributing factor to critical reductions in lithospheric strength, ultimately providing strong impetus for the initiation and advancement of intracontinental orogenesis.