Geological controls on palaeo-environmental change in a tectonic rain shadow, southern New Zealand

This study is a multidisciplinary synthesis that quantifies the geological processes that control surface palaeo-environmental change, and the associated biota, in an evolving Pliocene–Quaternary orographic rain shadow, South Island of New Zealand. The processes are relevant to evolution of older and more complex rain shadows of the Andes, western North America, and the Himalaya. Local rain shadow aridification can be distinguished from global climate change using asymmetry of stable isotope signatures across a steep orographic mountain rainfall gradient, with δ18O and δD values as low as − 13‰ and − 95‰, respectively. Salination in the rain shadow results from evaporative concentration of marine aerosols on impermeable rocks, with only minor water–rock interaction, and a distinct flora has developed on these sites. Natural coal fires, burning in excess of 1000 °C, contributed to maintenance of savannah environments. Uplift of fold mountains, especially in the Quaternary, has maintained salination sites and their flora as well as lignites for coal fires, with these environments progressively migrating laterally at 1–10 m/thousand years. Numerous river capture events accompanied Quaternary mountain uplift in the driest part of the rain shadow because of low erosion rates, and these have facilitated radiation of freshwater galaxiid fish into nine sister species, whereas an adjacent region with less complex tectonics and topography has only one species from that lineage. The combination of tectonic uplift and rain shadow aridification has two main effects on the surface environment: (a) topographic evolution and biological evolution are occurring at similar rates, and growth of mountains has facilitated fish speciation; (b) mountain uplift causes local environmental rejuvenation, so that salination and local coal fires, and the biological effects of these phenomena, are maintained over long time scales by lateral migration.