UNRAVELLING THE TECTONIC CONTEXT OF EARLY HIGH-TEMPERATURE SHEAR ZONES

High-temperature shear zones preserve vital information regarding processes of mid- to lower-crustal tectonism, and have been described as critical structures that accommodate deformation in both extensional (e.g. metamorphic core complexes) and compressional (e.g. decollement structures) environments. However, recognition and interpretation of the tectonic context of high-temperature shear zones is often arduous due to reactivation during later tectonism and (retrograde) metamorphism.

Within the Broken Hill Block, New South Wales, Australia, early deformation was accommodated along lithology-parallel high-temperature shear zones that have not previously received deserved attention, hence their role in the evolution of the terrane remains speculative. The shear zones were overprinted during multiple, intense deformation and metamorphic events during the Mesoproterozoic Olarian Orogeny, and the Late Cambrian Delamerian Orogeny. Recognition of these early-formed high-temperature shear zones, and interpretation of their local to regional structural context is difficult, and has resulted in much controversy. Two main schools of thought have evolved. The first proposes an episode of terrane-scale nappe formation, and the other advocates an early ductile extensional event either during basin development, or immediately before the onset of orogenesis. Both models have implications for how we view the high-temperature metamorphic evolution of the Broken Hill Block as well as many other Proterozoic orogenic belts throughout Australia.

Detailed metamorphic analysis in the context of the structural architecture of the Broken Hill Block has revealed three deformation events at high-temperature conditions that have affected the terrane. The youngest of these events is unequivocally related to early orogenesis (Olarian Orogeny), however, the origin of the first two fabrics remains unresolved. The second high-temperature event is related to the development of early lithology-parallel shear zones, and potentially had significant influence on the metamorphic regime and pre-orogenic architecture of the terrane. This study focuses on delineating the role of these shear zones in the early evolution of the Broken Hill Block, and understanding their influence on subsequent deformation.