GEOMETRY OF A MIDDLE PROTEROZOIC EXTENSIONAL DECOLLEMENT IN NORTHEASTERN AUSTRALIA

An 80 km-long belt of gneissic and mylonitic rocks is exposed in the core of a major anticlinorium in the Middle Proterozoic Mt. Isa Inlier of northeastern Australia. These rocks, in the Mary Kathleen Fold Belt, represent an early, originally horizontal, mid-crustal shear zone which is interpreted as an extensional decollement between a lower ductile sheet and an upper, more brittle plate. The decollement was subhorizontal over a distance of at least 80 km and its depth was perhaps as shallow as 7 km. Shearing was accompanied by the syn-tectonic intrusion of a bimodal complex of granitic and basic rocks over a period of 70-90 m.y. Moderate, to extreme, ductile shearing was pervasive throughout the 1-1.5 km thickness of the lower plate that is now exposed. Massive dilation accompanied shearing and approximately 70% of the volume of-the shear zone is occupied by igneous intrusive rocks (which have been variably deformed to gneissic granite, mylonite, and amphibolite). Strain softening driven by this high thermal input is responsible for the pervasive ductile response at such shallow depths. Peak metamorphic conditions at the decollement were approximately 625-degrees-C to 675-degrees-C at 2 kbar. Intense ductile shearing in the upper plate is restricted to the lower few hundred metres. Above this zone, brittle processes dominate, although evidence is presented that the sheet as a whole has undergone a component of ductile shearing, probably heterogeneously distributed. Large, sheet-like granite and dolerite bodies were introduced into dilation sites associated with synthetic faults in this plate but, unlike their synchronous counterparts in the lower plate (about 1 km below), these are internally undeformed. The Middle Proterozoic history of the area involves a cycle of extension and shortening with the shortening axis perpendicular to the earlier extension axis. We speculate that the driving mechanism for both extension and shortening is mantle convection, with switches in stress related to mantle upwelling.