USING ORIENTATION STATISTICS TO TEST SHEAR ZONE MODELS: AN EXAMPLE FROM THE MARGIN OF THE PALEOARCHEAN MT EDGAR GRANITIC DOME, WESTERN AUSTRALIA

The statistical analysis of field foliations-lineation measurements was used to quantitatively test two competing geologic hypotheses for the formation mechanism of the 3.47-2.8 Ga Mt Edgar dome, East Pilbara terrane, Western Australia. The Mt Edgar dome is a large (50 km diameter), ovoid granitic dome that represents some of the oldest felsic continental crust in the rock record. A nearly continuous shear zone that incorporates both dome rocks and wall rocks extends around ~75% of the dome’s circumference. The orientation of stretching lineations within this marginal shear zone was used to test between two competing hypotheses for dome formation: 1) The Mt Edgar dome represents a vertically rising diapir; or 2) The Mt Edgar dome is an extensional core complex that exhumed along a detachment. Each of these hypotheses make specific predictions about the orientation of lineation in the marginal shear zone. In the diapir model, lineations plunge radially away from the dome center, perpendicular to the strike of the dome margin. In the core complex model, lineations are unidirectional, trending in the direction of extension. We test these predictions by analyzing foliation-lineation pairs collected throughout the marginal shear zone. We formulate each prediction as a two-sample hypothesis test using orientation statistics. Our statistical workflow involves dividing the marginal shear zone into four geographic segments and computing the mean lineation and a confidence interval about that mean for each segment. The mean and confidence interval of each segment is compared with: 1) Synthetic lineations that trend perpendicular to foliation strike; and 2) Mean lineations from other segments. The results from these two statistical analyses indicate that the orientations of lineations within the marginal shear zone are not compatible with either of the predicted systematic patterns. We suggest that the marginal shear zone is not kinematically coherent across the structure, possibly resulting from local flow variations during dome emplacement. The statistical approach taken here is broadly applicable to the study of shear zones, and has particular utility where field structures provide important context for other datasets.