Structural complexity in the Curnamona Province (South Australia): Polyphase strain partitioning and reactivation
Résumé
In this paper, we examine the apparent timing relationships between the structural and metamorphic events of the amphibolite facies sequence in the Walter-Outalpa Shear Zone (WOSZ) area in the southwestern part of the Palaeo- to Mesoproterozoic Curnamona Province. The local structural geometry and style are largely due to heterogeneous, non-coaxial flow in which foliation development is related to the superposition of two main independent structural events (D2* and D3*). Strain was progressively partitioned into high-strain zones in the southern part of the study area in contrast to the low-strain areas in the north of the study area. D2* deformation developed under amphibolite facies conditions and is partitioned between zones of thrusting and flattening (±folding) related to nappe formation. D3* deformation developed under greenschist facies conditions and is partitioned between strike-slip faulting and folding. D3* high-strain zones form discontinuous, NW-trending anastomosing strands that contain relics of the D2* thrust-related foliation. These relicts preserve the higher grade mineral assemblages that are partially overprinted by D3* retrograde assemblages. Strike-slip movement along the WOSZ is, therefore, interpreted to have re-used the pre-existing D2* thrust such that D2*-strain partitioning influenced renewed D3* strain partitioning. The high-grade metamorphism and phase of pervasive deformation (D2*/M2) are ascribed to the D2 Mesoproterozoic Olarian Orogeny that occurred in the Curnamona Province between ca. 1600 and 1590 Ma. The tectonic context of the D3*/M3 shearing event is highly uncertain. Recent geochronological Sm–Nd isotope ages from garnet from within the WOSZ shear zone yield ca. 520–490 Ma Delamerian ages [Dutch, R.A., Hand, M., Clark, C., 2005. Cambrian reworking of the southern Australian Proterozoic Curnamona Province; constraints from regional shear zone systems. Geol. Soc. London 162; 5, 763–775]. However, the D3*M3 shear foliation are truncated by the Neoproterozoic unconformity flooring the Adelaide Rift Complex stratigraphy, suggesting pre-Delamerian Orogeny activity along the shear zone. It is, thus, likely that the D3* movement along the shear zone records a combination of late Olarian Orogenic movement with a component of reactivation during the Delamerian Orogeny.