Structure and topographic evolution of the Main Divide in the Landsborough‐Hopkins area of the Southern Alps, New Zealand

Abstract

The Landsborough‐Hopkins area of the Southern Alps lies between the tectonically active central Southern Alps and the south Southern Alps which were most active in the Miocene. The Landsborough‐Hopkins area is underlain by interleaved tectonic slices (kilometre scale) of textural zones 2A, 2B, and 3 schist surrounding a thin sliver of unfoliated greywacke. Foliation and bedding are steeply dipping and northeast striking. This structural pattern is a northern extension of Miocene deformation style which dominates to the south. Late Cenozoic faults, particularly the Main Divide Fault Zone of the Mt Cook area to the north, are not well developed in the Landsborough‐Hopkins area. The northeast structural grain controlled pre‐Quaternary drainage orientation of the Landsborough and Clarke Rivers, and these rivers flowed into major river systems to the east of the topographic divide at that time. These rivers have since been captured by the west‐flowing Haast River, and the topographic divide has stepped progressively to the east in this area. This eastward stepping of the topographic divide was driven by tectonic shortening in the northern part of the area. The Landsborough River capture was facilitated by erosion at a sinistral cross‐fault, the Creswicke Fault. At the capture zone, both Landsborough River and Main Divide cut sharply across the regional structural grain imposed by Miocene deformation. Uplift at the capture zone has raised a c. 2 km high divide since capture. In contrast, after the Clarke River was captured by the Haast River, the Wills River (a former Clarke tributary) maintained the Haast Pass as a topographically low capture point. Hydrothermal alteration and vein formation occurred in the immediate area of the Landsborough River capture zone. Fluid flow was structurally controlled, and deposited ankerite, quartz, and/or sulfide minerals in faults and associated fractures.