Influence of erosion and deposition on deformation in fold belts

Abstract

This paper investigates whether surface processes such as erosion and deposition have any influence on folding of the upper crust at small to intermediate spatial scales (i.e., <200 km). This problem is studied using a simple mathematical model comprising a thin elastic-plastic beam overlying a viscous substrate. A critical nondimensional parameter that controls whether surface processes influence folding is the ratio of deformation to fluvial erosion characteristic time scales, denoted as R. When R << 1 (i.e., the rates of surface processes are slow relative to the rate of deformation), numerical results show that surface processes have a negligible influence on folding. Deformation in this regime is characterized by the formation of a train of low-amplitude, short-wavelength folds, which develop serially outward with time. When R >> 1, a new folding mode is entered whereby erosion and deposition dramatically amplify the folding instability leading to localization of rock uplift and exhumation on a single, long-wavelength mega-antiform with relatively high topography. Thus, relatively rapid erosion and deposition at the scale of individual folds can significantly modify the nature, amplitude, and wavelength of folding and can lead to higher topography than in the absence of surface processes. Surface processes influence folding mainly through reducing the influence of gravity but also by modifying the strength distribution. Results indicate that the response of folding to surface processes is dynamic in origin and threshold-like in behavior.