Tectonostratigraphic architecture and uplift history of the Eastern ...

n=6826N~100kmFigure 5: Regional foliation data for the study area with a close up of the Leonora areashowing a consistently NNW trending foliation direction.P-T dihedraAll fault/shear zone data from which a movement vector along a plane could be determined havebeen displayed as P-T dihedra. This method was initially developed by Angelier and Mechler(1977) (for English see Angelier (1984)) as a means of inverting brittle fault slip data to constrainthe quadrants (or dihedra) in which σ 1 (dilation or P dihedral) and σ 3 (compression or T dihedral)lie, analogous to earthquake focal mechanisms (Fig. 7). A P-T dihedra plot is established by plottingthe plane of the fault on an equal area lower hemisphere stereonet and the movement vector on thefault plane. The axillary to the fault plane is then constructed by plotting a plane which laysperpendicular to the movement vector and passes through the pole to the shear plane (Fig. 8). Theright dihedra method of stress inversion is based on the Wallace-Bott assumption (Wallace, 1951;Bott, 1959) that any slip on a fault plane is parallel to the maximum resolved shear stress on theplane. There has been some controversy in the literature as to whether the P-T dihedra methodresolves stress or strain (Twiss and Unruh, 1998). Recent studies by Blenkinsop (in press) indicatethat the P-T dihedral method does resolve stress and hence the Wallace-Bott assumption holds truefor large data sets even though experiments show violations of the Wallace-Bott assumption causedby fault interactions (Pollard et al., 1993).Historically this method has only been applied to brittle fault slip data (i.e. faults with slickenlines)presumably due to the method’s origin in seismological studies. However if the Wallace-Bottassumption holds true this method can be applied to any fault/shear data for which a movementvector along a shear plane can be resolved and hence in this study it has been applied to ductilestructures. The reservation of applying the P-T dihedra method to ductile structures stems from aconcern that at high strains there is a lot of rotation of structures within a shear zone. However therotation of structures such as boudins and fold axes occurs within a consistently oriented shear zoneand typically towards a well established stretching lineation, hence the orientation of the shear planeand lineation can be confidently used to construct P-T dihedra.9