EROSION AS A DRIVING MECHANISM OF INTRACONTINENTAL MOUNTAIN GROWTH

In nature, mountains can grow and remain as localized tectonic feature s over long periods of time (> 10 m.y.). By contrast, according to cur rent knowledge of lithospheric rheology and neglecting surface process es, any intracontinental range with a width that exceeds that which ca n be supported by the strength of the lithosphere should collapse with in a few tens of millions of years. For example, assuming a quartz-dom inated crustal rheology, the relief of a range initially 3 km high and 300-400 km wide is reduced by half in about 15 m.y. as a result of la teral spreading of its crustal root. We suggest that surface processes might actually prevent such a ''subsurface collapse.'' Removal of mat erial from topographic heights and deposition in the foreland oppose s preading of the crustal root and could eventually drive a net influx o f material toward the orogeny. We performed a set of numerical experim ents in order to validate this hypothesis. A section of a lithosphere, with a brittle-elasto-ductile rheology, initially loaded by a mountai n range is submitted to horizontal shortening and to surface processes . If erosion is intense, material is removed more rapidly than it can be supplied by crustal thickening below the range, and the topography is rapidly smoothed, For example, a feature 3 km high and 300-400 km w ide is halved in height in about 15 m.y. for an erosion coefficient k = 10(3) m(2)/yr (the erosion rate is of the order of a few 0.1 mm/yr). This regime might be called ''erosional collapse.'' If erosion is not active enough, the crustal root spreads out laterally and ''subsurfac e collapse'' occurs. In the third intermediate regime, removal of the material by erosion is dynamically compensated by isostatic rebound an d inward flow in the lower crust so that the range can grow. In this ' 'mountain growth'' regime the range evolves toward a characteristic gr aded shape that primarily depends on the erosion law. The erosion rate may be high (e.g., 0.5-0.9 mm/yr), close to the rate of tectonic upli ft (e.g., 0.7-1.1 mm/yr), and few times higher than the rate of topogr aphic uplift (0.15-0.2 mm/yr). These experiments show that surface pro cesses can favor localized crustal shortening and participate in the d evelopment of an intracontinental mountain. Surface processes must the refore be taken into account in the interpretation and modeling of lon g-term deformation of continental lithosphere. Conversely, the mechani cal response of the lithosphere must be accounted for when large-scale topographic features are interpreted and modeled in terms of geomorph ologic processes.