Linking orogeny and orography in the Southern Alps of New Zealand: New observations from detrital fission-track thermochronology of the Waiho-1 borehole

Abstract

Numerical modeling coupling erosion with crustal deformation predicts that development of an orographic rain shadow may explain the asymmetric exhumation of convergent plate boundary orogens. This prediction is consistent with observations from the Southern Alps of New Zealand, where bedrock thermochronology indicates crustal exhumation has been concentrated along the wet, windward side of the mountains. While the spatial correspondence of exhumation and precipitation patterns is compelling, a robust evaluation of the link between orographic and orogenic processes further requires a temporal comparison between exhumation history, plate reconstructions and paleoclimatic records. We present a detailed exhumation history of the Southern Alps from detrital apatite and zircon fission-track thermochronology of the Waiho-1 borehole, a 3.6 km-thick sequence of proximal foreland basin sediments. Inverse thermal modeling of a 2062-grain dataset predicts two periods of rapid exhumation in the Early and Late Miocene. Rapid exhumation in the Early Miocene was synchronous with the development of transpression along the Alpine Fault and a ca. 20–22 Ma pulse of exhumation may reflect cooling during inversion of preexisting extensional basins. Rapid exhumation in the Late Miocene was not synchronous with a discrete change in plate convergence but increased exhumation rates after 7.4 Ma may instead reflect localization of plate boundary deformation along the Alpine Fault as orographic precipitation concentrated exhumation in the Alpine Fault hanging wall. We propose that, similar to prior interpretations from bedrock thermochronology, detrital thermochronology of the Southern Alps foreland basin is consistent with numerical model predictions linking asymmetric exhumation of the orogen to the growth of an orographic rain shadow.