The Siberia Fault Zone, northwest Otago, and kinematics of mid‐Cenozoic plate boundary deformation in southern New Zealand

Abstract

The Siberia Fault Zone (SFZ) in northwest Otago, New Zealand, represents a northern extension of the Moonlight Tectonic Zone. The 40 km long fault strikes northeast subparallel to, and 25 km southeast of, the Alpine Fault and dips steeply northwest. Kinematic data indicate an overall dextral strike‐slip sense of movement, with an estimated 3 ± 2 km of displacement. On the southwest section of the fault, strike‐slip movement is largely transformed into dip‐slip, and actual fault displacement is superimposed on uplift in the core of the Shotover Antiform. This results in a total apparent throw 7000 m. Near Haast Pass, the SFZ steps right c. 3.5 km through a complex zone of northeast‐ and east‐striking faults to connect with the Haast River Fault. Fault rocks from the SFZ are mainly foliated cataclasites. Syn‐kinematic temperature‐depth conditions were in the range 200–250°C and 8–10 km. Timing of fault activity is constrained, by mutually crosscutting relationships with dikes of the Alpine dike swarm, to late Oligocene and early Miocene. The kinematics of the SFZ and other structures in the region are consistent with distributed strain in a regime of dextral transcurrent shear. Previous models for Cenozoic deformation in southwest South Island are refined by combining improved plate tectonic data with reconstructed paleogeometry of Cenozoic structures before regional bending and rotation.