Structural evolution of the middle Archean coastal Pilbara terrane, Western Australia

The middle Archean coastal Pilbara terrane is composed of two lithotectonic units: the Karratha and Cleaverville-Roebourne supercomplexes. The Karratha supercomplex lies tectonically beneath the Cleaverville-Roebourne supercomplex and consists of 3270 ∼ 3250 Ma granitic batholiths and metamorphosed terrigenous sediments. It is identified as a juvenile continental crust with tonalite-trondhjemite-granodiorite type granites. The Cleaverville-Roebourne supercomplex is interpreted as a dismembered immature island arc which has a restored crustal thickness of ∼20 km. This supercomplex consists of 3195 Ma bimodal volcanic-chemical sedimentary sequences, metabasite, low-K granitic gneiss and periodotite. Regionally distributed granite-porphyries (circa 3020 ∼ 3000 Ma) have intruded all of the coastal Pilbara terrane. Detailed structural analyses and zircon U/Pb dating suggest that the coastal Pilbara terrane records two phases of deformation. The first phase is an immature island arc and micro-continental crust collision and is represented by top to the northwest thrusting (D₁, circa 3100-3020 Ma). This phase of deformation (an arc-continent collision) appears to represent an important transition in the formation of continental crust. The 3020 Ma granite-porphyry event may be related to syn-post collision igneous activity and may reflect a second crustal thickening associated with magmatic underplating. The second phase of deformation is characterized by regional-scale left-lateral strike-slip faulting (D₂, circa 3020-2925 Ma) and by more localized right-lateral strike-slip faulting (D₃, circa 2925-2770 Ma). This phase of craton-scale strike-slip faulting may represent stabilization of the newly formed continental crust.