Evidence for multiphase deformation in the Archean basal Warrawoona Group in the Marble Bar area, east Pilbara, Western Australia

The results of a structural analysis of the Marble Bar Greenstone Belt in the McPhee Reward area of the eastern Pilbara Block in Western Australia indicate that the area has undergone a multiphase deformation on shears which approximately parallel the trace of stratigraphy (cf. primary igneous layering) around the Talga Talga Anticline. Early extensional movements were associated with the formation of the Warrawoona Group (chiefly the Duffer Formation) and indicate a WNW-ESE extensional direction similar to the syn-Warrawoona extension found in the area of the North Shaw Batholith. Extension was followed by thrusting with transport to the east. The Talga Talga anticline is a culmination which formed during this event by thrust stacking. A major shear in the area, the Duffada Shear, was reactivated during granodiorite emplacement with transport to the NE. In their present geometry, the shears have thrust movements on the south side of the Talga Talga anticline but normal movements on the north side. After the reactivation came a NW-SE shortening which was accompanied by conjugate shear activity on north-trending sinistral and west-trending dextral shears, and constrained to similar to 2950 Ma in the North Shaw area. The final movements were associated with a N-S shortening which resulted in an E-W trending thrust, which cuts the northern flank of the Talga Talga anticline, and NW and NE trending conjugate faults. There is no evidence of radially oriented stretching lineations or slickensides which would be expected if the granitoid bodies which make up the Mount Edgar Batholith were emplaced by a solid-state diapiric process. The net result of deformation was tectonic disruption of the stratigraphic stacking within the type area of the Talga Talga Subgroup in the Warrawoona Group. This may offer an additional explanation for anomolously young dates of the Talga Taiga Subgroup with respect to the Duffer Formation, and also indicates that the type section for the Talga Talga Subgroup is a lithotectonic sequence. (C) 1998 Elsevier Science B.V.