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 Zealand 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 temperatures allowing partial radiogenic product retention during exhumation (and thus effectively instantaneous thermochronological closure), this record presents important general insight into the transient physical and thermal effects of changes in tectonic conditions. The tectonothermal response of the Southern 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 temperature prior to the change in dynamics at 5 Ma results in an initial decrease 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 ages reflecting the new steady-state dynamics of the region. This two-step response is a direct result of the relationship between cooling and exhumation and illustrates the care needed in reconstructing physical histories from isotopic ages and cooling rates in tectonically active regions.