DYNAMIC TOPOGRAPHY IN TRANSPRESSIONAL REGIMES: AN EXAMPLE FROM THE NEW ZEALAND PLATE BOUNDARY ZONE

Zones of oblique plate convergence typically are characterized by high, narrow mountain topography and deformation that is partitioned variably into orogen-parallel strike-slip faults, thrust and normal faults, and penetrative ductile fabrics. The Southern Alps of New Zealand resulted from the oblique collision between two fragments of continental crust along the boundary between the Australian and Pacific plates beginning about ~25 million years ago. Excellent exposure of Late Tertiary fault zones up to 3 km wide in the southermost segment of the mountain range allowed us to investigate how different styles of tectonically active strike-slip and contractional deformation within this orogen relate to variations in the topography and geomorphology of the mountain range within a well defined plate setting. We present fault-slip data, kinematic models, geochronologic data, and topographic data that show how oblique-slip and reverse displacements on curved fault zones created localized (~1000-5000 km2), fault-bounded topographic uplifts east of the modern Australian-Pacific transform plate boundary. Approximately 3.5-5.5 km of cumulative vertical displacement and 6.5-7.0 km of oblique-dextral displacement occurred on faults that display curved, palm-tree-style geometries and subhorizontal detachments in profile. The fault-bounded uplifts are asymmetric in profile, deeply incised by steep-walled river networks, and are approximately 1 km higher than the adjacent topography. The principal axes of instantaneous strain determined from faults correlate well with principal stresses derived from earthquakes and allowed us to infer the stress fields and displacement patterns most likely responsible for the localized uplifts. Our data indicate that variations in topography within the Southern Alps are directly related to the curved geometry and kinematics of slip on transpressional faults and an increase in the angle and rate of convergence between the Australian and Pacific plates near the end of the Miocene (6.4 Ma).