Some remarks on the origin of foredeeps

Abstract

Abstract Geological observations show that there are two main types of foredeep basins on the base of their subsidence rate and their relationship with the associated orogen or accretionary wedge. High rates of subsidence up to 1600 m/Myr and a ratio lower than 1 between the area of the elevated belt and the area of the basin characterize the foredeeps associated with W-dipping subduction. Low rates of subsidence up to 300 m/Myr and a ratio higher than 1 between the area of the orogen and the area of the basin characterize the foredeeps associated with E- or NE-dipping subductions. This observation enables us to interpret the slow filling of foredeeps of the first type and the much faster filling of foredeeps of the second type. Moreover, “W-dipping” subductions have a steeper and faster monocline bounding the base of the deep foredeep in contrast to “E- or NE-dipping” subductions which have a shallower monocline. This differentiation supports the notion of the “eastward” mantle flow with respect to the lithosphere detected in the hot-spot reference frame. In this view, the foredeep depth in W-dipping subductions is mainly controlled by the roll-back of the subduction hinge pushed by the relative “eastward” mantle flow while foredeep depth in E- or NE-dipping subductions is instead mainly generated by the load of the thrust sheets and by the roll-back of the subduction hinge due to the advancing upper plate, contrasting the upward push of the mantle. The shape of the foredeeps is regularly arcuate in case of W-dipping subduction, while is linear or following the shape of the inherited continental margin in case of E- or NE-dipping subduction. Fold development in the two opposite foredeeps is significantly different: in W-dipping subduction the folds are transported down in subduction while they are forming, and consequently they are poorly eroded. Syntectonic sediments drape a preserved fold. In the E-NE-dipping subduction, folds and thrust sheets are instead uplifted and deeply eroded. This is also predicted by the envelope of the crests in the two distinct accretionary wedges: in the W-dipping case the envelope may dip toward the subduction, while it is rising toward the hinterland in the other cases. This approach may explain the differences in terms of geometries, amount and rates of subsidence between the foredeeps around the Adriatic plate. For instance the foredeep of the Southern Alps (back-thrust-belt foredeep) is deformed in the western part by the later Apenninic foredeep development, associated with the sinistral oblique ramp of the W-dipping Adriatic plate subduction. The relationship in the frontal parts of a thrust belt between accretion and eustacy has to be analyzed in terms of when, where and how fast thrusts propagation took place with respect to the randomly distributed sea-level third-order fluctuations, and by the type of subduction which is connected to the foredeep and thrust belt or accretionary wedge. Therefore the stratigraphic patterns in foredeep basins are peculiar and casual successions that record the combination of regional tectonic evolution with independent wider-scale sea-level changes.