The tectonic evolution of the Southern Alps, New Zealand: insights from fully thermally coupled dynamical modelling

Abstract

We present here the results of numerical models of the Southern Alps in which the thermal and dynamical developments of the orogen are fully coupled to one another. This interlinkage allows the use of a wide range of thermal and physical features of the orogen as constraints on the applicability of our models. In particular, the thermal aspect of this method enables the prediction of the distribution of thermochronological ages at the surface of the model. Perturbation of the geothermal structure due to rapid uplift and exhumation causes an intrinsic weakening and concentration of strain along the equivalent of the Alpine Fault zone in these models. This thermal weakening of the crust also produces a zone of high strain antithetic to the Alpine Fault in the upper crust that is comparable to the Main Divide Fault Zone within the Southern Alps. The introduction of orographic rainfall and erosional processes into the model leads to the development of high, asymmetric topography comparable to that of the Southern Alps. This topographic profile results from the capture of available precipitation by the windward side of the orogen and the resultant rain shadow on the leeward side. Due to the time required to accumulate sufficient topography for this rain-capture effect to become significant, the establishment of this high, asymmetric topography lags the initiation of surface uplift by several million years. Comparison of the observed thermal history of the Southern Alps with that seen in models based on differing hypotheses of the tectonic evolution of the South Island shows that the present tectonic regime of the orogen most probably developed in a single rapid reorganization of plate motions at approximately 5 Ma with relative stability of the tectonic regime since that time. The variation in isotopic ages along the Southern Alps is consistent with that expected from variation in accumulated uplift and exhumation along the orogen arising from the obliquity of convergence of the Australian and Pacific plates.