Tectonic geomorphology of the Rock and Pillar Range and Taieri Ridge, Otago, New Zealand

Abstract

In the South Island of New Zealand, east Otago is a region of low seismicity with many potentially active faults, and profound data gaps. I use geomorphic observations, and Optically Stimulated Luminescence (OSL) dating methods applied to Quaternary sediments, to constrain the geometry and quantify the slip rates of the Hyde and Taieri Ridge Faults. The two faults are significant in east Otago, and are responsible for the uplift of the Rock and Pillar Range and Taieri Ridge respectively. Across Otago, the linearity of the range fronts, ~15km spacing of the ranges, their asymmetric cross-sectional profile, and a ~15km crustal depth suggest the faults dip ~45˚NW, and are located at, or near the base of the southeast range fronts.

An earlier than expected mid-Miocene age for the initiation of substantial uplift of both ranges is postulated based on sedimentary grounds. This leads to long-term slip rates of 0.02mm/yr for Taieri Ridge, and 0.11mm/yr. for the Rock and Pillar Range, derived from the offset of the ~planar Cretaceous-Tertiary Waipounamu Erosion Surface perpendicular to the north-east-trending ranges, and assuming the faults dip at ~45˚.

A late-Quaternary slip-rate was not established for the Taieri Ridge Fault, but evidence suggesting Holocene activity at the North of the range is presented. A late Quaternary sliprate of ~0.11mm/yr is postulated for the Hyde Fault, based on offsets of late Quaternary surfaces. This consistent with the long-term slip rate of 0.11mm/yr required to produce the present-day topography, if significant uplift was initiated in the mid-Miocene (~50Ma).