Origin of a vertical lineation in conjugate transcurrent shear-zones at Broken Hill, Australia

Abstract

Shear-zones at Little Broken Hill are interpreted as conjugate transcurrent structures. They developed before dolerite dyke emplacement in response to N–S horizontal shortening. They were reactivated as conjugate transcurrent structures after dyke emplacement in response to horizontal E–W shortening. Shear-zone deformation was accompanied by retrograde metamorphic conditions for at least the later part of the history. Displacement on the shear-zones was accommodated by coeval deformation within the shear-zone-bound blocks. This deformation involved shear on the pre shear-zone S 1 foliation and resulted in incomplete retrogression of earlier peak metamorphic mineral assemblages. It also resulted in a vertical extension and a component of dip slip on the shear-zones. A near-vertical stretching-lineation occurs parallel to the intersection of the shear-zones and is interpreted as a localised response to an overall vertical extension. Localisation of the lineation is explained in terms of grain-scale partitioning of deformation mechanisms. In the country rock, vertical extension was achieved by shear on S 1 with very little intragranular deformation. Consequently this deformation failed to produce a lineation. In the shear-zones, where all fabric elements were tending to vertical, the only deformation mechanism available for vertical extension was intragranular deformation, which resulted in a shape fabric, viz the stretching-lineation. Shear within the zones had both horizontal and vertical components parallel to the shear-zone foliation. Because the shear was achievable by slip on the foliation and did not involve intragranular mechanisms, it did not modify the lineation. Late shear-zone movement resulted in local crenulation of the shear-zone foliation about the early-formed stretching-lineation. The result was vertical plunging folds and a more pronounced “stretching” lineation. It is concluded that stretching-lineations in shear-zones should not be considered indicators of movement direction or of instantaneous or finite strain axes.