Numerical modeling of spatially-variable precipitation and passive margin escarpment evolution

Passive margin escarpments are long-lasting landforms which can remain steep and high over tens of millions of years. Precipitation variability has seldom been considered as a contributor to passive margin escarpment evolution. However, large and distinct variability in precipitation rates with elevation is observed in mountainous landscapes. We assess the potential impacts of spatially variable precipitation on passive margin escarpment evolution with a numerical model that couples a simple elevation-dependent precipitation rule with the CASCADE surface processes model and a simple flexural uplift model. Modeled topography is more sensitive to the elevation of the precipitation maximum than to the effective elastic thickness of the lithosphere. If precipitation is maximized at the escarpment crest, the escarpment decreases in elevation, slope and proximity to the coast more rapidly than if precipitation is focused at low elevations. Low elevation precipitation maxima contribute to increasing escarpment slope over time and preservation of a high-elevation escarpment. Using Tropical Rainfall Measuring Mission (TRMM) precipitation radar data, we establish precipitation/elevation relationships at high-elevation passive margins across the tropics and document the occurrence of patterns similar to those included in our model. Despite uncertainty as to the robustness of precipitation patterns and the simplified tectonic and geomorphic framework of our model we conclude that spatial variability in precipitation is a potentially significant contributor to passive margin escarpment evolution.