Flexure of Lithosphere Beneath Apennine and Carpathian Foredeep Basins: Evidence for an Insufficient Topographic Load

Abstract

The Pliocene foredeep basin external to the Apennine thrust belt and the late Miocene foredeep basin external to the outer East Carpathians are considered typical molassic foredeeps that formed contemporaneously with thrusting in the adjacent mountain belts. A simple mechanical model in which the response of the lithosphere to loading was approximately by a semi-infinite (broken) elastic plate overlying a weak fluid was used to analyze the deflection of the foreland lithosphere beneath the Apennine and outer East Carpathian foredeep basins along three transects perpendicular to the thrust belts. Comparison between the calculated and observed depth to basement suggests that the emplacement of thrust sheets onto the foreland lithosphere during mountain building failed to pr duce a sufficient load to create the observed foredeep basins and observed topography regardless of the position of the slab end or the flexural rigidity assumed for the foreland lithosphere. We infer that an additional subsurface load must be present and that its magnitude must be roughly equal to the load created by that part of the thrust belt that is above sea level. For the two Apennine transects, an acceptable fit to the shape and size of the observed foredeep basin was generated by assuming effective elastic plate thicknesses of 8 and 15 km (4.9 and 9.3 mi) and applied vertical forces of 2 × 1015 dynes/cm and 1.3 × 1015 dynes/cm at the slab end. For the Carpathian transect, an effective plate thickness of 30 km (18.6 mi) and an applied force of 1.5 #215; 1015 dynes/cm generated an acceptable fit. This study suggests a method whereby an estimate of the depth to autochthonous basement rocks beneath overthrust belts can be made in some areas where this information is not available from seismic or drilling data. Such an approach may enable exploration geologists to determine if crystalline rocks encountered in drilling overthrust terranes are a part of the autochthonous basement, or if they are part of an overthrust sheet. In the latter situation, oil-bearing sequences of sedimentary rocks might lie below crystalline rocks contained in the allochthonous sheet.