Abstract
The topography of the southern part of the North Island of New Zealand has been constructed by relative tectonic uplift and subsidence above an active west-dipping subduction zone. In contrast, the nearby northern South Island has been constructed during active transcurrent faulting. There is a ~100 km tectonic transition zone between these contrasting structural regimes, within which the Cook Strait has opened between the islands. There is little preserved geological record of the timing and geometry of past landscape changes within the tectonic transition zone over the past million years. A biological approach to Pleistocene geomorphology can fill this knowledge gap using quantification of genetic divergences of species of freshwater biota that now inhabit that topography. Riverine fish populations, especially Galaxias divergens, show that there must have been extensive freshwater drainage interconnections across the transition zone prior to ~1 Ma. Genetic divergence of populations of G. divergens show that the islands became separated at ~0.7–1 Ma, and this is supported by isolation of populations of freshwater crayfish Paranephrops planifrons. South Island river drainage interconnections and severance events have been dominated by glacial and interglacial erosion and sedimentation over the past 500 ka. North Island drainage evolution has been primarily driven by fault-dominated uplift of ranges that resulted in several river capture events that have been dated by genetic divergences of G. divergens between 700 and 100 ka. Colonisation by South Island subalpine stonefly species constrains timing of development of North Island upland environments to >400 ka. An apparently antecedent gorge on the Manawatu River through rising North Island bedrock ranges has developed after a tectonically-driven river capture event at ~500 ka. Tectonically and geometrically similar post-capture antecedent style gorges have developed elsewhere in the world, including in the Himalaya where geomorphic and biological evidence supporting river capture still persists despite rapid uplift and deep incision.