Emergence of pediments, tors, and piedmont junctions from a bedrock weathering-regolith thickness feedback

摘要

Sediment erosion laws form the basis for most landscape evolution models and guide geomorphologists in the pursuit of understanding how landscapes evolve. This focus on the alluvial surface, however, ignores the role of intrinsic feedbacks between sediment transport and bedrock weathering in shaping Earth's landforms. Here, we present a new, parsimonious explanation for the origin and maintenance of pediments, piedmont junctions, and tors, which emerge spontaneously in a numerical model coupling bedrock weathering and sediment transport. The spatial uniformity of the thin regolith mantle that often characterizes pediments is a manifestation of a negative feedback between bedrock weathering and regolith thickness: if regolith thins (thickens) by sediment transport, the regolith production rate will increase (decrease), maintaining an equilibrium regolith thickness on the piedmont. We propose that high infiltration capacities and the instability of ephemeral channel banks in arid and semiarid environments suppress fluvial incision and promote the smoothness of pediments. A positive feedback between bedrock weathering and regolith thickness causes tor growth: if regolith thins locally below a critical value, regolith production slows while surrounding areas continue to weather and erode more rapidly. We suggest that many pedimented and tor-studded landscapes may therefore be a consequence of intrinsic sediment transport–weathering feedbacks mediated by climatic and tectonic conditions, not by lithologic templates.