TRANSIENT RESPONSE OF LANDSCAPES TO TECTONIC AND CLIMATIC PERTURBATIONS

Negative feedbacks between the processes of mountain growth and surface erosion tend to drive an active orogenic system towards a steady state (SS) in which the rate of tectonic mass accretion into the orogen is equal to the erosional flux out of the system. Although there has been considerable analysis of the steady-state system, there are still important questions regarding the time required to reach a SS and the timescale of response of a landscape to perturbations in the climatic or tectonic forcings that drive the system. We present a coupled model of orogenic growth and erosion, find analytical and numerical solutions to topographic evolution, and evaluate the timescale of response to climatic and tectonic perturbations. The tectonic model is based on wedge mechanics such that an orogen grows self-similarly with an orogen-perpendicular "critical" mean slope. The erosion model is based on a channel network which incises bedrock with a power-law dependence on channel slope and discharge. These processes are represented in an analytical model that solves for orogen width as a function of accretionary flux, precipitation rate, erosion parameters and critical slope. The solution provides scaling relationships for the orogen width (and height) to the flux and precipitation rate. In all commonly considered cases, the model predicts that the width is more sensitive to the accretionary flux than to the precipitation rate. To address the time-dependent problem as well as aspects of the two-dimensional river network, we construct a numerical model that couples a planform surface process model with a vertical-section, plane-strain tectonic model for orogenic wedge formation. This is compared to the steady-state analytical model with positive results and is subsequently used to evaluate transient problems. Changes to the precipitation rate (climate) or to the accretionary flux (tectonics) induce a change in the SS width and height of a mountain belt with a characteristic time for this change to be completed. Landscape response is recorded by characteristics of the topography and sediment flux from the orogen. Climate changes are evidenced by rapid change in sediment flux, but with no net change. Tectonic change produces a slower response in sediment flux, but results in a net change in sediment flux.