RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 4, NO. 1, PAGES 1–17, doi:10.2205/2002ES000083, 2002

Global stresses in the Western Europe lithosphere and the collision forces in the Africa-Eurasia convergence zone

Schmidt United Institute of Physics of the Earth, Russian Academy of Sciences, Moscow, Russia

Abstract

[1]   The experimentally determined directions of the maximum horizontal compressive stress S_{H, max} in the western European stress province (WESP) of the West European platform are mostly oriented northwest with an average azimuth of 325^o 26^o. The traditional approach to the mathematical modeling of the stress field in the Western Europe lithosphere (as well as in other stable lithospheric blocks) is based on an elastic model of the medium and boundary conditions that specify stresses and/or displacements at the entire perimeter of the region studied. In the particular case of Western Europe, poorly constrained boundary conditions should be set at the southern (in the collision zone of the African and Eurasian plates) and eastern boundaries of the region. For this purpose, experimental directions of S _{H, max} are used as constraints on the sought-for solution. However, even if the experimental directions of S _{H, max} agree well with their theoretical estimates, model stress magnitudes are still sensitive to the choice of model boundary conditions.A basically different approach proposed and implemented in this work for determining the field of tectonic stresses uses the experimental directions of S_{H, max} as input information rather than constraints on the sought-for solution. The spatially persisting strike of the S_{H, max} axis makes it possible to construct the field of straight trajectories of principal stresses on the WESP territory. The problem of stress determination is then reduced to the hyperbolic-type problem of integrating the equilibrium equations that does not require postulating constitutive relations, and the boundary conditions are only specified on some part of the boundary of the model region. The lithosphere material can be mechanically anisotropic and inhomogeneous. Stresses t^R produced by the ridge push were specified in this work on a segment of the Mid-Atlantic Ridge, and stresses t^C due to the Africa-Europe collision were specified in the convergence zone. No boundary conditions are required at the eastern boundary of the region studied. Supposedly, straight trajectories of stresses can be extended into oceanic lithosphere areas adjacent to the continent. The formulation of the problem presented in the paper provides substantial constraints on the collision stresses t^C. These constraints directly result from the equilibrium conditions of the Western Europe lithosphere rather than from the plate convergence kinematics in the collision zone. A simple analytical expression obtained for the tensor of global tectonic stresses in the study region indicates that the S_{H, max} magnitude decreases in the NW direction. The minimum horizontal stress in the WESP region is shown to be sensitive to the direction of the collision stresses t^C. This stress whose modulus increases in the SE direction is compressive, zero or tensile depending on whether the vector of collision stresses t^C deviates westward from the S_{H, max} direction, coincide with it or deviates eastward from it. The modification of the inferred solution incorporating stresses applied at the base of the lithospheric plate is discussed.

Received 8 February 2002; published 8 March 2002.

Keywords: stress orientation, stress trajectory, stress field, platform, Western Europe, Africa, collision force.

Copyright 2002 by the Russian Journal of Earth Sciences