A simple geometric model for the syn-kinematic erosional denudation of thrust fronts

The syn-kinematic denudation history of contractional orogenic systems is explored using a simple geometric construction. The purpose is to gain a better appreciation of the relationship between thrust-related shortening and erosion at the front of an orogen, specifically in cases where rocks with marked differences in metamorphic grade are juxtaposed across faults. Although this analysis examines an obvious feature of orogenic fronts, that erosion and thrusting occur synchronously, the magnitude of the necessary erosional flux has not been appreciated. The front of an orogen is modeled as a wedge comprising one component represented by the elevation above the level of the thrust toe and a second, subsurface component, between the dipping slab and the level of the toe of the thrust. Because pressure is a simple function of overburden it is possible to calculate the horizontal distance from the toe to the point along the slab at any given pressure. Thrusting by itself does not effect pressures in the hanging wall, and hence the footwall would be expected to assume the pressure gradient of the hanging wall. If a thrust juxtaposes rocks that record markedly different equilibrium pressures, material must have been removed from the hanging wall syn-kinematically. If the thrust bounds the metamorphic front of an orogen and deposition occurs synchronously with thrusting in the foreland, removal of material is required to occur syn-kinematically, otherwise the thrust will bury the foreland. The amount of material that must be removed syn-kinematically is dependent on the pressure differential across the thrust and the dip of the thrust. For any given dip an increase in the differential pressure requires an increase in cross-sectional area that must be removed, both above and in the foreland direction of this point. From these simple relationships it is possible to calculate that at least 1 × 107 km3 of material was eroded from the Main Central Thrust of the Himalayas between about 20 and about 15 Ma, and that a comparable amount must have been removed from the southern front of the Dabie Shan-Sulu region of eastern China between the Middle Triassic and Middle Jurassic. In both, approximately equivalent sediment volumes are preserved in the sedimentary basins adjacent to the orogens. In addition, the rapid increase in the 87Sr/86Sr of the seawater during the interval from about 20 to 15 Ma is compatible with erosion of this amount of material.