The Old Man Range and Garvie Mountains: Tectonic geomorphology of the Central Otago peneplain, New Zealand

Abstract

A study of the late Cenozoic tectonic geomorphology of a 700 km² area of remnant peneplain comprising the Old Man Range and Garvie Mountains, Central Otago, has shown the peneplain to have had a maximum regional Miocene paleorelief of 500 m/10 km, and to be underlain by a low-angle schistosity (dipping 0–20°). As much as 1500 m vertical displacement of the peneplain is evident from preserved Tertiary sediments at varying altitude. Sedimentology of mid-Pliocene (Waipipian) quartzose sediments found at 1200 m suggests that reworking of older Miocene sediments (Manuherikia Group) has occurred during early stages of uplift.

Mapping reveals a consistently low angle relationship between schistosity and peneplain attitude, and dominance of distributed deformation (flexure) over faulting. Topography and schistosity are sympathetically warped into northerly trending fault-bounded folds (tectonic blocks). Joint orientations are related to orientations of major faults (Nevis-Cardrona Fault System and Old Man Fault), and reactivated (opened) during flexure of blocks. Shortenings of 1.7% (schistosity derived) and 2.4% (peneplain derived) on an axis 081° are calculated, and a transpressional folding model developed. Late Cenozoic bending is suggested by a c. 10° northward convergence in schist lineation trend over the study area. Regional peneplain and schistosity-derived shortening of 1% is calculated for fault blocks in eastern Central Otago, and up to 26% in western Central Otago. Considerable shortening is indicated by steeply dipping schistosity in west Otago/northern Southland, where the peneplain is not preserved.

Keywords:

• Old Man Range
• Garvie Mountains
• Central Otago
• peneplain
• schistosity
• tectonic geomorphology
• neotectonics