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

We present here the results of numerical models of the Southern Alps in whi ch the thermal and dynamical developments of the orogen are fully coupled t o 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 pr ediction 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 equiva lent of the Alpine Fault zone in these models. This thermal weakening of th e 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 pr ocesses into the model leads to the development of high, asymmetric topogra phy comparable to that of the Southern Alps. This topographic profile resul ts 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 r equired 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 s een in models based on differing hypotheses of the tectonic evolution of th e South Island shows that the present tectonic regime of the orogen most pr obably developed in a single rapid reorganization of plate motions at appro ximately 5 Ma with relative stability of the tectonic regime since that tim e. The variation in isotopic ages along the Southern Alps is consistent wit h that expected from variation in accumulated uplift and exhumation along t he orogen arising from the obliquity of convergence of the Australian and P acific plates.