The effects of a lateral variation in lithospheric strength on foredeep evolution: Implications for the East Carpathian foredeep

Abstract

Lateral variations in lithospheric strength have been adopted often in flexural modeling (both 2D and 3D) to better fit the observed basement deflections, typically supported by gravity data. This approach provides essentially a “snap-shot” of the role of lithosphere strength in determining the present day geometry. In contrast, we investigate and quantify the effects of a lateral change in lithospheric strength on the evolution of the foredeep in front of an advancing orogen. Transitions in lithospheric strength are common in the foreland of orogens and show large variations in the width of the transition zone and the strength difference. Former passive margins, for instance, will display strength changes distributed over several tens to hundreds of kilometers. Other transitions may originate from juxtaposition or accretion of pieces of lithosphere with different properties and may be characterized by a much smaller width than former passive margins. In our modeling, a constant load, representing an advancing orogenic belt, is displaced towards and across a transition from a weak to a strong plate in a 2D elastic thin plate model. The effect of different transition widths and strength contrasts on foredeep geometry and bending stress is investigated. Interference of flexural wavelengths across the transition affects foredeep geometry by causing rapid basin widening, oscillation of the bulge and volume increase. The bending stresses are found to concentrate and amplify around the strength transition. Large transition gradients, i.e. large strength contrast or small transition width, cause the highest rates of change. Basin widening caused by the orogenic load advancing towards the transition between the East European Craton and the Moesian Platform, appears to control the Sarmatian transgression over the East Carpathian foreland in Romania.