Parameterizing Surface Processes and their Response to Tectonic and Climatic Forcings
The Earth’s topography is the product of surface vertical motions caused by tectonic processes and modulated by erosional processes that cause redistribution of mass at the Earth’s surface over a wide range of scales. The efficiency of these gravity-driven processes scales with slope (and thus topography). This also implies that the time scale needed for an orogenic system to reach steady-state between tectonic uplift and erosion must scale with the height of the topography, the tectonic uplift rate and the degree of isostatic compensation. Using simple observations from a range of presently active orogenic systems, we estimate that this time scale is comprised between 1 and 15 Myrs. From these estimates, we can also derive a “generic” erosion law based on the Stream Power Law (SPL) to be used in geodynamic models. We also show that the degree of non-linearity of the erosional laws (i.e., the value of the slope exponent) controls the rate at which topography decays once tectonic uplift ceases. This simple behavior of any slope-dependent erosion law may explain the post-orogenic longevity of Earth’s topography.
The same processes that shape the Earth’s topography are, for the most, functions of the availability of moisture from the atmosphere. This has led to the conclusion that there must be a strong link between the efficiency of surface processes and climate. This link is however difficult to establish from observational evidence. For example, the effect of the Cenozoic cooling of the Earth’s climate on the efficiency of erosion in orogenic systems remains highly debated. We propose that this question is difficult to address because the wide range of erosional processes active at the Earth’s surface (such as fluvial erosion, hillslope processes, glacial abrasion, peri-glacial processes, chemical weathering, etc.) are characterized by different response times to climate perturbations. These response times may also depend on the dimensions of the topographic feature being eroded, the mean slope, the mean precipitation (or accumulation) rate and the nature of the rocks being eroded. It is therefore not surprising that a global correlation between climate change and erosional efficiency is difficult to evidence.
Recent work has also shown that erosional efficiency is strongly dependent on the variability of climate and, in particular, of precipitation. We will show how this climate variability has been introduced in fluvial erosional models using a simple stochastic approach. This requires, however, that mean precipitation and precipitation variability (or storminess) be translated into mean discharge and discharge variability. This can be achieved through the use of an eco-hydrological model that requires a limited number of parameters only.
To conclude we will use a surface processes model to demonstrate how tectonics, surface processes and climate interact with each other over geological time scales to create landforms that will ultimately exert a strong control on biodiversity, species richness and endemism. We will illustrate this point using the island of Madagascar as a case example.