Tectonic significance of Cretaceous bivergent extensional shear zones in the Torlesse accretionary wedge, central Otago Schist, New Zealand

Abstract

We describe two shear zones in the Otago Schist of the Torlesse accretionary wedge, South Island, New Zealand: the north-dipping Rise-and-Shine Shear Zone (RSSZ) and the south-dipping Cromwell Gorge Shear Zone (CGSZ). Kinematic indicators (shear bands and asymmetric folds) indicate top-north movement for the RSSZ and top-south transport for the CGSZ. Back rotation of the shear zones into their Late Cretaceous orientation and consideration of the relationship of the shear zones to arching of the Otago Schist show that both shear zones are extensional. Offset of textural zones suggests up to 15 km of dip-slip displacement on the RSSZ and probably a similar amount of slip for the CGSZ. We speculate that the gold-mineralised RSSZ may be the western continuation of the late Mesozoic gold-bearing Hyde-Macraes Shear Zone in eastern Otago, forming a c. 100 km long extensional shear zone on the northern flank of the Otago Schist. Age constraints suggest that shear zone formation took place between 135 and 105 Ma. The shear zones aided the final exhumation of the deeper parts of the Otago Schist. We discuss whether normal shearing is related to syn-orogenic supercritical tapering of the Torlesse wedge, or due to post-orogenic New Zealand-wide Albian rifting. © 2002 Taylor & Francis Group, LLC.