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

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 S₁ 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 S₁ 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.