Analysis of some fault striations in the Proterozoic southern McArthur Basin, Northern Territory, with reference to pre- and post-Roper Group stress fields

Pre- and post-Roper Group regional far field stress patterns in the southern McArthur Basin were related to north-south/northwest-southeast and east-northeast-west-southwest compressions respectively. A number of stress states (up to six in the northern Mallapunyah Dome) reflect local stress conditions that were subject to variation around a fault intersection and 'pop-out' in the northern Mallapunyah Dome. Contrasting extensile stress states, due to stress relaxation or rebound effects, were responsible for normal faulting in areas that suffered compression. These stress states represent periods of crustal extension that may have coincided, in part, with Roper Group sedimentation. The northern Mallapunyah Dome, lying at the intersection of the Mallapunyah and Tawallah Faults, had its origins in a combination of processes that included: (i) the emplacement of a sill or laccolith complex of Settlement Creek Volcanics to higher than usual crustal levels; (ii) uplift caused by wrench faulting and west- or east-directed thrusting, giving the dome its present north-south elongation. Uplift may have been aided by early (pre-Masterton Sandstone) east-side-up movements on the Tawallah Fault contemporaneous with magma emplacement. The fault striation method works best with data sets of 200 or more; however, fewer data (<15) can be used to adequately define the reduced stress tensor in selected areas as long as a wide range of fault orientations is used.