Reinterpretation of the erosion profile across the southern portion of the Southern Alps, Mt Aspiring area, Otago, New Zealand

Abstract

The geology of the Mt Aspiring area of Otago, New Zealand, before late Cenozoic uplift and erosion, has been reconstructed using regional structure of schistosity, continuity of metavolcanic horizons, projection of the lower boundaries of the Caples and Torlesse Terranes, mapping and projection of an early‐middle Miocene erosion surface, and estimation of paleosurface above late Oligocene to early Miocene diatremes. Less than 500 m of late Cenozoic erosion has occurred on the Pisa Range, and amount of erosion increases westwards to the Moonlight Fault where c. 3 km of erosion has occurred at valley level and c. 1 km at ridge level. Between the Moonlight Fault and the Alpine Fault, schistosity and metavolcanic horizons are nearly horizontal and display remarkable continuity, and meta‐morphic grade is uniform, indicating little late Cenozoic disruption. The amount of erosion estimated west of the Moonlight Fault depends on the original dip of the schistosity. Originally horizontal schistosity implies <4 km erosion at valley level right across the range. An original schistosity dip of 10°NW implies progressively greater erosion towards the west, to c. 10 km near the Alpine Fault. These geological estimates of late Cenozoic erosion are incompatible with most erosion estimates derived from fission track studies, which imply at least 8 km of erosion at valley level in the mountains and >4 km erosion from the top of the Pisa Range. Some reconciliation of different estimates may be possible if thermochronological assumptions are adjusted and a component of mid‐Tertiary erosion is assumed for the region.

Keywords:

• Southern Alps
• erosion
• uplift
• structure
• fission tracks
• Otago Schist