Extent and tectonic significance of the Central Otago shear-strain anomaly

Abstract

Recent geodetic crustal strain determinations confirm that a region in Central Otago, New Zealand, is characterised by a principle axis of relative compression with an azimuth lying between 40° and 70°. This region now appears to extend at least as far west as Lake Wakatipu. The northern boundary of this zone lies in the upper Clutha valley, 10 km south of Lake Wanaka. Within the Central Otago region, strain rates increase to the west, with maximum shear-strain rates of 0.6 μrad/yr in the Kawarau area.

Strain orientations of 40–70° are difficult to reconcile with the regional tectonic pattern dominated by orthogonal northeast and northwest trending thrust faults, because the azimuth of the principle axis of relative compression is oblique to both sets of faults, which would be consistent with strike-slip rather than dip-slip movement on the faults. A model by which the geodetic strain measurements and the regional tectonics of Central Otago can be reconciled involves simultaneous oblique compression on two orthogonal fault traces. If all of the strains are eventually to be relieved by known structures within Central Otago, this model requires that a substantial amount of strike-slip movement occurs on both the northeast and northwest trending structures in the area. Slip rates as high as 8 mm/yr for the Nevis-Cardrona and Moonlight Faults are indicated by this model. There is currently little evidence for strike-slip movement of this magnitude in Central Otago suggesting that some of the strain may be released on structures outside the region.

Keywords:

• shear strains
• geodetic triangulation
• late Quaternary
• tectonics
• Central Otago