SHRIMP U-Pb zircon dating of Archean core complex formatio and pancratonic strike-slip deformation in the East Pilbara Granite-Greenstone Terrain

Sensitive high-resolution ion microprobe (SHRIMP) U-Pb dating of zircons from granitic rocks in the East Pilbara Granite-Greenstone Terrain has provided time constraints for main tectonic events in the Shaw Granitoid Complex and has shown that deformation was intricately related to granitoid intrusion. The earliest recognized deformation activity occured within the Split Rock Shear Zone (SRSZ), probably between 3470 and 3420 Ma. The timing, geometry, and kinematics of this shear zone suggest that it was a midcrustal detachment in a core complex type extensional setting. It was active during the intrusion of synkinematic sheeted granodiorite of the North Shaw Suite and brittle extensional faulting within the upper crust. Samples of gray migmatic gneiss and a diorite that occur structurally below the SRSZ were dated at 3451±1 Ma and 3463±2 Ma, respectively, indicating that melting and metamorphism took place in the lower units during extension in the middle to upper crust. The 3470-3420 Ma extensional event is most consistent with extensional collapse, possibly as a result of delamination or convectional thinning of part of the lithosphere. Activity on the pancratonic strike-slip Mulgandinnah Shear Zone (MSZ) was constrained by the crystallization age of a synkinematic granitic dyke to have occured at 2934±2 Ma. The MSZ and activity on the Sholl Shear Zone of the western Pilbara at 2960 Ma [Smith et al., 1998] are consistent with a regional NW-SE directed compressional stress field at this time. Steepening of greenstone belt stratigraphy has been attributed to diapirism, but probably occured after a 3300-3200 Ma compressional event and following deposition of the youngest clastic sediments in the eastern Pilbara stratigraphic sequence. Diapiric processes did not play a major role during the early evolution of the eastern Pilbara Craton from 3470 to 3300 Ma