A common feature of thin-skinned fold and thrust belts is the sequential nature of foreland directed thrust systems. As a rule, younger thrusts develop in the footwalls of older thrust, the whole sequence propagating towards the foreland in the transport direction. As each new younger thrust develops, the entire sequence is thickened; particularly in the frontal region. The compressive toe region can be likened to an advancing wave; as the mountainous thrust belt advanced the down-surface slope stresses drive thrusts ahead of it much like a surfboard rider. In attempt to investigate the stresses in the frontal regions of thrustsheets, a numerical method has been devised from the algorithm given by McTigue and Mei <1981>. The algorithm yields a quickly computed approximate solution of the gravity- and tectonic-induced stresses of a two-dimensional homogeneous elastic half-space with an arbitrarily shaped free surface of small slope. A comparison of the numerical method with analytical examples shows excellent agreement.

The numerical method was devised because it greatly facilitates the stress calculations and frees one from using the restrictive, simple topographic profiles necessary to obtain an analytical solution. The numerical version of the McTigue and Mei algorithm shows that there is a region of increased maximum resolved shear stress, &tgr;, directly beneath the toe of the overthrust sheet. Utilizing the Mohr-Coulomb failure criterion, predicted fault lines are computed. It is shown that they flatten and become horizontal in some portions of this zone of increased &tgr;. Thrust sheets are known to advance upon weak decollement zones. If there is a coincidence of increased &tgr;, a weak rock layer, and a potential fault line parallel to this weak layer, we have in place all the elements necessary to initiate a new thrusting event. That is, this combination acts as a nucleating center to initiate a new thrusting event. Therefore, thrusts develop in sequence towards the foreland as a consequence of the stress concentrating abilities of the toe of the thrust sheet. The gravity- and tectonic-induced stresses due to the surface topography (usually ignored in previous analyses) of an advancing thrust sheet play a key role in the nature of shallow foreland thrust propagation. ¿ American Geophysical Union 1990