Foredeeps versus subduction zones

Abstract

The geologic characteristics of foredeeps and accretionary wedges suggest that these features are distinguishable on the basis of the direction of the associated subduction. East-north-east-dipping subduction-related accretionary wedges show high relief and broad outcrops of metamorphic rocks. They are associated with shallow foredeeps with low subsidence rates. In contrast, west- dipping subduction-related accretionary wedges show low relief and involve mainly sedimentary cover. The related foredeeps are deep and have high subsidence rates. This differentiation is useful both for oceanic and continental subductions, e.g., eastern vs. western Pacific subductions, or east- dipping Alpine vs. west-dipping Apenninec subductions. In a cross section of the Alps the ratio of the area of the orogen to the area of the foredeeps is at least 2:1, whereas this ratio is 0.22:1 for the Apennines. These ratios explain why foredeeps related to east- or northeast-dipping subduction are quickly filled and are bypassed by clastic rocks, whereas foredeeps related to west-dipping subduction maintain a deep-water environment longer. These differences support the presence of an eastward asthenospheric counterflow relative to the westward drift of the lithosphere detected in the hot-spot reference frame, even in the Mediterranean where no hot spots are present. In this interpretation, the Apennines foredeep was caused by the eastward push of the mantle acting on the subducted slab, whereas the foredeeps in the Alps were caused by the load of the thrust sheets and the downward component of movement of the upper Adriatic plate; these forces contrast with the upward component of the eastward mantle flow.