Tectonic settings of Archaean (3325-2775 Ma) crustal-supracrustal belts in the West Pilbara Block

The Archaean Pilbara Block comprises six fault-bounded tectonostratigraphic domains. The West Pilbara Block contains, from east to west. Domains 4, 5 and 6. Review of stratigraphic. magmatic, structural and geochronologic data. and new field mapping, indicates that the crustal and supracrustal rock records correlate to three megacycles. Megacycle IVU (3325-3135 Ma) is recorded by an intra-are basin and are granitoids in Domain 6, and a back-are basin in Domains 4 and 5. The domains were an are-continent orogen from similar to 3200-3155 Ma, during which the outboard are setting was accreted to the back-arc continental margin. Although Domain 6 is allochthonous with respect to Domains 5 and 4, it was not exotic with respect to the tectonic settings of the Pilbara continent. Megacycle VL (3135-2955 Ma) is recorded by, in decreasing age: (i) a back-are basin and are granitoids in Domain 5, (ii) a deep-marine, retroarc or remnant-ocean basin in Domain 4 and a plutonic belt in Domain 5: (iii) a plutonic belt in Domains 5 and 6, and possibly also in Domain 4; and (iv) craton-wide deformation belts. coeval with the Whim Creek basin and granitoid plutons in Domain 5. The Whim Creek basin was a half-graben that developed at a releasing bend (or overstep) along the boundary fault to Domains 4 and 5, and, to the northeast, Domains 4 and 6. The basin was closed by sinistral transpression as the boundary fault between Domains: 4 and 6 propagated to the southwest from the releasing bend, and thereby generating a restraining bend geometry. Fundamental strike-slip faults. which are interpreted to have been tectonic-escape responses to a cause external to the preserved Pilbara Block, juxtaposed domains craton-wide. The initial stage of Megacycle VU (2955-2775 Ma) is recorded by the eroded remnants of a rift province. Preserved elements include the Negri basin (Domain 5 only), mafic-ultramafic sub-volcanic layered intrusions, and granitic plutons and related high-level sills, Two periods of granitoid intrusion are preserved. Base-metal sulphide deposits, hosted by the Whim Creek basin, were probably sourced from extensional geofluids in the rift province. A plume origin is likely For the komatiitic parent magmas to sub-volcanic intrusions. and to basalts of the Negri basin. Plume magmatism is interpreted to have been coincident with, but not the cause of, lithospheric extension because plume-related volcanism post-dates the first period of plutonic intrusion by similar to 25 million years. Extension was followed by orogenic deformation that converted Domains 4, 5 and 6 to a hinterland fold-thrust brit. Deformation is interpreted to have been caused by collision orogeny during terrane accretion along the western Pilbara margin, Collision orogeny was the last event of a long history of Pacific-type tectonics along the margins of the Archaean Pilbara continent. (C) 1998 Elsevier Science B.V.