Stress Distribution in Overthrusting Slabs and Mechanics of Jura Deformation

Abstract

Our investigations relate the stability to stress-distribution in overthrusting slabs. Finite-element models have been formulated to evaluate the magnitude and orientation of stress within overthrusting slabs under given boundary conditions of displacement. The Mohr-Coulomb criterion has been adopted to define a zone of failure. In addition to a simple model to describe idealized slabs, the tectonic deformation of the Jura decollement has been modelled.

The computations were carried out with a program which enables the simulation of elastic/ideal plastic behavior. For the first simulation the stability of an overthrusting crustal slab is considered, assuming a left-to-right displacement with the right hand edge fixed expect for the lowest 1 km thick layer. The cases of a completely homogeneous slab, of a slab with one layer of lower strength, and of a slab with two low-strength layers, were considered in different simulation experiments. Deformation of the homogeneous body could not produce a clearly defined detachment-surface. In the one-layered block, the low-strength layer acts as a detachment-surface. The ratio of the length of the thrust plate to its thickness is 4/1 (16 km long for a 4 km thick thrust, or 40 km long for a 10 km thick plate) in this test. However, by varying the composition of the layering, the length/thickness ratio of a multiple thrust complex can be 20/1 or 80 km long for a 4 km thickness plate. This ratio is comparable to that observed in the cover nappes and rigid-basement nappe-complexes of the Alps.

In the case of the Jura-overthrust, two different models have been investigated: by rotation on a concave detachment horizon (“distant-push” theory) and subduction of the crystalline basement under the Aare-massif (“underthrusting” theory). As ductile shear-horizon, we assumed a 100-m thick layer of anhydrite. The material constants have been determined by triaxial deformation experiments. The analyses clearly show tha the Jura mountains could hardly have been deformed in the manner postulated by the “distant-push” theory. On the other hand, the assumption of the underthrusting model can result in satisfactory simulation of field observations.

Contribution No. 145 of the Laboratory of Experimental Geology.