The influence of mechanical properties on the link between tectonic and topographic evolution

In actively deforming orogens, the material strength at the Earth's surface is controlled in part by strain localization determined by the local stress fields which in turn are driven by contributions from local topography and far field plate velocities. Material weakening associated with strain localization imparts partially predictable, and entirely inescapable heterogeneity to the material fabric of an orogen. The characteristic damage structure of individual fault zones that undergo strain weakening, as imaged in dam site excavations, deep drill holes and geological observations, results in geomorphically relevant strength variations normal to the fault of many orders of magnitude. The sensitivity of hillslope and fluvial erosion to the strength parameters coupled with the large and oriented strain-related strength variations, cause the topographic evolution to be dominated by tectonically driven rheological behavior at multiple wavelengths. Using three-dimensional, lithospheric scale modeling of two oblique orogens, Southern Alps, New Zealand and the Eastern Himalayan Syntaxis, we examine the generation of model surface strength fields that occur as a consequence of a simple strain softening upper crustal rheological model. Mapping of topographic anisotropy of the Southern Alps and the Eastern Himalaya Syntaxis indicates azimuthal control on correlation distances that are spatially related to the different strain regimes of the two orogens. By defining landscape evolution in terms of mechanical failure in the conventional motion—stress mechanical framework, the behavior of Earth's surface can be brought into the same theoretical framework as the behavior of the sub-surface and many of the observational—theoretical inconsistencies arising from application of dominantly potential field theory can be obviated. Heterogeneity and anisotropy of material strength are fundamental aspects of active orogens and description of the strength field in terms of mechanical evolution can significantly extend present Earth surface models. These basic aspects of mechanics are expressed in landscape geomorphometrics of anisotropy and spatial patterns of complexity.