Linking the Timescales of Orogenic growth and Climatic Feedback in Southern New Zealand: A Pilot Study

Orogenic systems are the rejuvenating engines of the Global Earth system, converting the thermal and kinetic energy of tectonic processes into uplift of the landscape that, in turn, drives erosion and renews the fertility of the biosphere. Understanding the relationships between tectonic uplift, erosion, and local climate, however, lags behind quantitative understanding of other areas of Earth Systems Science. The challenge in developing this field lies in the requirement of independently documenting the long-term evolution of these three systems for a given region. Although sensitive chronometric constraint has long been successfully derived for climatic and structural systems, an inability to constrain the temporal evolution of topography has represented a fundamental deficiency in our understanding. Without such chronometric constraint, correlation and testing of models of fundamental links between tectonic forcing and climate variation remain essentially arbitrary and artificial. We present here the results of a pilot investigation into the feasibility of dating authigenic illite extracted from Miocene-Recent paleosols and weathered surfaces formed during the development of the Southern Alps of New Zealand. Chamberlain et al. (1999) have previously attributed a stable isotope signature resolved in Kaolinite from these weathered horizons as recording development of strongly asymmetric climatic zonation as a consequence of Mio-Pliocene growth of the Southern Alps. By re-visiting this approach to assessing the topographic record of the orogen through isotopic proxy measurements, and combining this with direct chronological constraint of the weathered materials making up this record, we hope to examine the potential of developing a robust framework in which to investigate and model the linked processes of mountain growth and local scale climatic effects. Initial results have been mixed. Clay fractions have been separated from 7 variably weathered paleosols and terrestrial deposits whose stratigraphic position places their age within the phase of progressive growth of the Southern Alpine topographic barrier. Although bearing substantial populations of illite, initial K-Ar ages of the coarse clay fraction range from Cretaceous to Triassic, substantially pre-dating the Southern Alpine orogeny and indicating heavy bias towards inherited material. Through work continuing to purify and characterise finer grain size fractions, we hope to determine whether a stratigraphically consistent age can be isolated for the authigenic component of the clay fraction. Whether this end proves feasible or not, the results of this investigation may have significant bearing on the assumption of closed system behaviour implicit in deriving geochemical time-series information from such weathered systems.