Flexural Deformation Due to Amazon Fan Loading - a Feedback Mechanism Affecting Sediment Delivery to Margins

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The uplift associated with the peripheral bulge engendered by sediment loads such as the Amazon Fan may provide a potential tectonic feedback mechanism that affects both coastal and fluvial depositional processes. For example, topographic and geologic maps of the Brazilian margin document the existence of a drainage divide affecting coastal river networks that is spatially coincident with the location of the peripheral bulge predicted by our flexural modeling of the fan load. While regions 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 portions 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 location of sediment input into a basin also changes the shape of the cumulative sediment load and thus the resultant deformation. This feedback between sediment loading and the peripheral bulge uplift modifies the spatial and temporal history of sediment delivery to the margin and provides a viable way to change base level without having to invoke glacial eustasy. The efficiency of this feedback is dependent on the flexural wavelength and thus the strength of the lithosphere at the time of loading and the relative position of the sediment load with respect to the continental margin.


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