The approach to steady-state thermochronological distribution following orogenic development in the Southern Alps of New Zealand

A diachronous sequence of isotopic ages along the Southern Alps of New Zeal and illustrates details of the development of the modem tectonic regime of this orogen at about 5 Ma. Coupled with the rapid cooling rates experienced in the Southern Alps, which ensure negligible residence time at temperatur es allowing partial radiogenic product retention during exhumation (and thu s effectively instantaneous thermochronological closure), this record prese nts important general insight into the transient physical and thermal effec ts of changes in tectonic conditions. The tectonothermal response of the So uthern Alps to the change in dynamical conditions at 5 Ma is resolved into two evolutionary stages that are observed with progressive exhumation. The removal by erosion of material that had cooled below the relevant closure t emperature prior to the change in dynamics at 5 Ma results in an initial de crease in age. This is followed by a sharp drop to younger ages as material subjected to thermal re-equilibration associated with the modern orogenic regime is exposed, culminating in the eventual exposure of time-invariant a ges reflecting the new steady-state dynamics of the region. This two-step r esponse is a direct result of the relationship between cooling and exhumati on and illustrates the care needed in reconstructing physical histories fro m isotopic ages and cooling rates in tectonically active regions.