Tectonic relationships between the Proterozoic gawler and willyama orogenic domains, Australia

Stratotectonic and morphotectonic data from the two principal exposed domains (pre-Adelaidean rocks) of the Gawler sub-province are used to characterize the Proterozoic Olarian orogeny and to distinguish its effects from those of the later Phanerozoic Delamerian orogeny. The principal metasedimentary sequences in the Gawler domain and in the Willama domain are inferred to have been deposited in a single broad zone of early Proterozoic shallow-water sedimentation on older (presumed Archaean) continental crust. The sequence becomes more pelitic upwards and may be interpreted as a transgressive sequence with more distal facies to the east. Three main phases of deformation are recognized, and each phase has similar characteristics and age in both domains. D₁and D₂can be dated between 1850 and 1650 Ma, while D₃appears to be about 1650-1540 Ma. In high grade rocks, D1 gave rise to a layer-parallel schistosity, while D₂is characterized by tight folds with a high-grade axial-plane schistosity. The whole sub-province was characterized by high geothermal gradients so that medium- to high-grade metamorphism affected the lower parts of the succession before and during the D₁and D₂deformation episodes. No distinct tectonic zones can be recognized but large-scale stratigraphic inversions (i.e. nappe tectonics) during D₁have been recognized only in the east of the Willyama domain. The higher parts of the stratigraphic succession are generally less deformed and exhibit only low-grade metamorphism. D₃produced relatively open, upright macroscopic folds and was characteristically associated with retrogression, but was demonstrably of pre-Adelaidean age. The Gawler domain exhibits D₃structures although it lies in the platform west of the Adelaide Geosyncline and was not affected by deformation during Adelaidean sedimentation or by the subsequent Delamerian orogeny. A network of retrograde shear zones is the principal expression of post-Olarian deformation in the Willyama domain which forms part of the basement to the Adelaide Geosyncline. The trends of D₂and D₃folding in the two domains are similar and it is shown therefore that no large-scale rotations of one domain relative to the other has been produced by the Delamerian orogeny. Large-scale translations on discrete faults or on broad zones of simple shear in the basement are not easily ruled out, but if they exist, are probably largely of pre-Adelaidean age. However, a significant relationship between Olarian structures and variable Adelaidean fold trends has been deduced. The Olarian orogeny may have occurred in close proximity to a continental margin to the east and may thus be related to subduction processes. It differs from linear gneissic belts in Phanerozoic orogenies since it occurs in a more stable stratotectonic environment and over a wider area.