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

Abstract

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 peridotite. 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.