Tectonomorphic scenarios in the Southern Alps of New Zealand

Temperature-time histories of rocks are often used to constrain the kinematics of tectonic events, the evolution of landforms, erosion rates, and/or amount of faulting in tectonically active areas. However, interpretations based on thermal histories are not always straightforward as they rely on several interdependent mechanisms such as heat conduction, heat advection, or the presence of transient topography that must be taken into account. Using a three-dimensional finite element code recently developed by one of the authors, we have calculated temperature-time histories to interpret an existing thermochronological data set (K-Ar and FT) complemented by new low-T thermochronometer data ((U-Th)/He and FT) from the Southern Alps of New Zealand. Combined with inversion methods (Genetic Algorithm and Neighbourhood Algorithm), the model is used to derive from the data information on the tectonomorphic development of the orogen during the Pliocene and Pleistocene epochs. Assuming a quasi-geomorphic steady state, we can constrain the rate of tectonic horizontal advection and vertical uplift as well as the geometry of the main structural boundary, i.e., the Alpine Fault. We can also explain the along-strike geometry of the metamorphic isograds and thermochrons in relationship to surface topography. Furthermore, we show that if one assumes that the landscape has not been horizontally transported by tectonic movement, the relief on the west coast of the Southern Alps has increased. This relief increase could have been initiated at any time between 1.5 Ma and 100 ka. Any relief reduction during this period is ruled out by our modeling. Alternatively, if the landscape is advected horizontally in the direction normal to the trace of the Alpine Fault, a relief increase is not required to explain the thermochronological data.