LONG-TERM LINK BETWEEN OROGRAPHIC PRECIPITATION AND ROCK EXHUMATION IN THE SOUTHERN ALPS OF NEW ZEALAND CONFIRMED BY DETRITAL ZIRCON (U-TH)/HE AND FISSION-TRACK DOUBLE-DATING AND RAMAN SPECTROSCOPY OF CARBONACEOUS MATERIAL IN THE WAIHO-1 BOREHOLE

The Southern Alps of New Zealand are an ideal setting to evaluate links between orographic precipitation and crustal deformation. Transpression between the Pacific and Australian plates has uplifted > 15 km of bedrock in the hanging wall of the Alpine Fault. Intense erosion asymmetrically focused rock exhumation to a ~25 km wide region along the western side of the orogen. In the Late Miocene a migration of the Southern Hemisphere Westerlies, directed moisture-laden winds into a near orthogonal intersection with Southern Alps topography. Foreland basin deposits in the West Coast Basin preserve the history of Southern Alps exhumation, and are used to reconstruct the exhumation history of the mountain belt.

The Waiho-1 borehole in the West Coast Basin, recovered a 3.7 km thick sequence of proximal foreland basin sediments representing a nearly continuous record over the last 14 Myr. Detrital apatite and zircon fission-track thermochronology of borehole sediments indicate that exhumation rates in the Southern Alps source area abruptly increased after 7.4 Ma. We build on this work by adding (U-Th)/He dating of zircons previously dated with fission-track thermochronology. Single-grain zircon “double dating” provides a direct constraint on mineral cooling rates. We also use Raman spectroscopy of carbonaceous minerals (RSCM) to independently constrain the peak metamorphic temperatures exposed in the Southern Alps source area at the time of deposition.

We use simple analytical thermal modeling to interpret zircon double dates within the context of rock exhumation. This dataset constrains the response timescale over which the Southern Alps orogen adjusted to a change in precipitation rate. Our results indicate catchment averaged exhumation rates increased linearly over a period of 3.2 Myr from 0.21 km/Myr prior to 7.4 Ma to 0.82 km/Myr by 3.4 Ma, and remain at elevated rates to the present day. Preliminary RSCM results indicate lower peak temperature distributions and a high proportion of charcoal material preserved in early foreland basin deposits with higher temperatures recovered in late foreland basin deposits, consistent with unroofing models in which deeper crustal levels were progressively exhumed. These results support interpretations of a long-term link between orographic precipitation and rock exhumation.