Mechanism of thin-skinned detachment in the Amazon Fan: assessing the importance of fluid overpressure and hydrocarbon generation

Abstract

Since about 10 Ma, a wedge of clastic sediment up to 10 km in thickness has accumulated near the mouth of the Amazon River, forming the Amazon Fan. Thin-skinned extensional and contractional structures root into a common basal detachment, hundreds of kilometers long. Extensional faults have formed landward of the prograding delta slope and contractional folds and faults have formed seaward of it. By plan-view restoration, we show that extension and contraction balance. We therefore infer that slope instability has provided the driving force for thin-skinned deformation. On seismic data, the detachment appears as a sharp surface within Late Cretaceous strata. However, there is no evidence at that level for soft layers, such as evaporites or muds. Abnormal fluid pressure provides a more reasonable mechanism for the detachment. At depths of several thousand meters, deep wells have encountered fluid overpressures. Amongst the possible causes of abnormal fluid pressures are compaction and hydrocarbon generation. Although compaction may have contributed to fluid overpressures encountered in wells, we argue that hydrocarbon generation provides the best explanation for detachment in Cretaceous rocks, which were old and lithified when it occurred. In particular, the broadly triangular area of detachment coincides with an inferred pod of Cenomanian–Turonian source rock, which is interpreted to have been in the gas window during detached deformation. We have developed new techniques of sandbox modelling to investigate the geometries of fault patterns produced by fluid flow through porous media. For the Amazon Fan, we have been able to reproduce the structural pattern, on the assumption that fluid overpressures existed over a broadly triangular area beneath the detachment.