FLEXURAL DEFORMATION DUE TO AMAZON FAN LOADING - A FEEDBACK MECHANISMAFFECTING SEDIMENT DELIVERY TO MARGINS

The uplift associated with the peripheral bulge engendered by sediment loads such as the Amazon Fan may provide a potential tectonic feedbac k mechanism that affects both coastal and fluvial depositional process es. For example, topographic and geologic maps of the Brazilian margin document the existence of a drainage divide affecting coastal river n etworks that is spatially coincident with the location of the peripher al bulge predicted by our flexural modeling of the fan load. While reg ions in close proximity to the fan load are depressed, regions farther from the load are lifted (i.e., peripheral bulge) by approximately25- 50 m, which is sufficient to cause subaerial exposure of large portion s of the shelf and modification of existing fluvial drainage networks. Furthermore, our modeling results suggest that the development of the peripheral bulge may have deflected the sediment-laden waters of the Amazon River progressively southward through time. This prediction is consistent with the existence of canyons off the Amapa coast north of the present-day Amazon canyon and with the distribution of Amazon Fan sediments, both of which suggest that the proto-Amazon River delivered sediment farther north along the coast. However, changing the locatio n of sediment input into a basin also changes the shape of the cumulat ive sediment load and thus the resultant deformation. This feedback be tween sediment loading and the peripheral bulge uplift modifies the sp atial and temporal history of sediment delivery to the margin and prov ides a viable way to change base level without having to invoke glacia l eustasy. The efficiency of this feedback is dependent on the flexura l wavelength and thus the strength of the lithosphere at the time of l oading and the relative position of the sediment load with respect to the continental margin.