TOPOGRAPHIC RELIEF AS A STRAIN MARKER IN NEOTECTONIC SETTINGS: THE EFFECTS OF ISOSTATIC COMPENSATION AND EROSION ON TRANSPRESSIONAL MODELS OF OBLIQUELY CONVERGENT PLATE BOUNDARIES

Transpressional models have greatly increased our understanding of the processes active at obliquely convergent plate boundaries. The original model involves strike-slip motion parallel to the boundary accompanied by contraction across the boundary and elongation in the vertical dimension. Vertical elongation results entirely in the flow of material upwards towards surface – i.e. the development of topographic relief when applied at the plate boundary scale. When this traditional form of transpression is used at large scales it ignores the effects of erosion and isostatic compensation. We present a two-dimensional numerical model of the convergent component of transpression that incorporates both of these factors. Airy isostasy is used to describe the effects of isostatic compensation. Previous workers developed an empirical relief vs. erosion rate relation which is used to describe the effects of erosion. The model is used to investigate the effects of erosion rate and rate of convergence on development of topography, crustal roots, exhumation rates, and uplift rates. Model results suggest the topographic relief in convergent settings is more dependant on the total amount of contraction than the rate of plate motion or the rate of erosion. However, rate of exhumation and rate of uplift are strongly dependent on both the rate of plate motion and the total amount of contraction. Application to a well constrained tectonic setting, the Alpine fault zone in New Zealand, shows the model does a reasonable job of describing the known topography, crustal root thickness, and uplift/exhumation rates. This suggests that application of this model to less well constrained settings may provide additional information on the tectonic history of such areas. In particular, it implies that topographic relief may be used as a strain marker for estimating the total amount of deformation in neotectonic settings where other markers are unavailable.