Constraining neotectonic orogenesis using an isostatically compensated model of transpression

Current models of transpression allow upward flow of material to compensate for shortening in the horizontal plane. We present an isostatically compensated model of transpression, where material is allowed to flow both upwards to create topographic relief and downwards to form a crustal root. Our model also incorporates the effects of erosion to more accurately examine the developing topography in orogenic systems. The modeling results suggest that topographic relief and crustal root thickness developed in transpressional orogens are most dependent on the magnitude of shortening, the initial thickness of the crust, and the density contrast between the crust and mantle. In contrast, the rate of convergence and final width of the deformed zone are relatively unimportant parameters in this model. Application to the Alpine fault system in New Zealand, a well-constrained active transpressional plate boundary, shows good agreement between topography-based model estimates of shortening and those derived from plate reconstructions. Application of our model to the Central Range fault zone in Trinidad suggests 2 ± 1 km of neotectonic shortening. The strain analysis approach presented here may be useful for isolating the effects of modern deformation in a region that has reactivated an ancient contractional belt.