COUPLED EVOLUTION OF OROGRAPHIC PRECIPITATION AND OROGENIC WEDGES

We study the fully coupled co-evolution of precipitation, topography, and rock deformation in orogenic wedges at critical taper. In response to a change in climate or tectonic influx, an orogen will gain or lose mass as it evolves toward a steady-state configuration. As the wedge grows or shrinks in size, the magnitude and pattern of orographic precipitation will change, influencing patterns of erosional efficiency and hence patterns and rates of rock deformation and the rate of mass loss or gain. Depending on prevailing conditions, this climatic response may act as either a positive or negative feedback. In order to explore the geodynamic and geologic implications, here we combine recently developed analytical models for (1) orographic precipitation as a function of mean elevation over asymmetric triangular mountain ranges and (2) the transient evolution of frictional orogenic wedges at critical taper. Recent work has addressed the transient evolution of frictional orogenic wedges in response to step-function changes in either erosional efficiency (climate) or accretionary flux, but where precipitation and thus erosional efficiency was held constant and not allowed to vary as a function of orography. The precipitation model that we couple this geodynamic model to is an extension of the classic upslope model that incorporates an explicit representation in the vertical dimension and represents the finite growth time of hydrometeors, their downwind advection by the prevailing wind, and evaporation. Inclusion of these factors allows more realistic prediction of precipitation patterns on both the windward and leeward slopes. The net influence of orographic precipitation depends on both the height and the width of mountain ranges. The atmospheric moisture scale height emerges as a critical control on orogen evolution; ranges that reach elevations that exceed this level will always experience a net drying with further growth, and will suffer an extreme rain shadow on the leeward slopes, potentially leading to a run-away condition that precludes attainment of mass balance. We discuss implications of initial conditions, prevailing wind direction, tectonic accretionary flux, and the efficiency of recycling of sediment temporarily stored in the foreland basin.