Thermochronological analysis of the dynamics of the Southern Alps, New Zealand

The isotopic ages of material exposed close to the Alpine fault in the Sout hern Alps, New Zealand, vary in a consistent fashion along the orogen. For the K-Ar system, muscovite and biotite ages display two distinct spatial tr ends that meet in the area of Fox Glacier, where a strong relationship is o bserved between K-Ar age and altitude. Excess argon is present in samples b etween Mount Kinnaird and Haast Pass in the southwest of the Southern Alps, giving unrealistically old, inconsistent K-Ar ages across this region. Inc orporation of excess argon appears to be highly variable, however, and mini mization of its influence on dynamical interpretations of thermochronology is possible through considering only the youngest age from a given area, Su ch treatment indicates that excess argon contamination does not play an imp ortant role in the systematic K-Ar age variation identified along the South ern Alps, The observed age distribution is consistent with estimates of the modern ra pid uplift and exhumation of the Southern Alps beginning ca, 5 Ma. K-Ar mic a ages older than this south of the Copland Valley indicate that substantia l late Cenozoic exhumation occurred along the Southern Alps prior to the de velopment of the modern tectonic regime, but at rates Lower than at present . Total exhumation during the current phase of tectonism in the Southern Al ps decreases from the Whataroa River southward, North of Fox Glacier, the S outhern Alps have attained a dynamical steady state, such that the observed distribution of exhumation will not change significantly with further defo rmation and denudation under current conditions.