HOW DOES A VISCOUS LAYER AFFECT THE MECHANICS AND KINEMATICS OF ACCRETIONARY WEDGES?

The mechanics of accretionary wedges depend on the physical properties of its basal detachment or the so-called décollement. Several numerical and analogue studies already revealed the key role of the strength of the basal detachment as a major controlling factor. Most of them assume a brittle Mohr-Coulomb rheology for the potential basal detachment (e.g. Mulugeta, 1988, Burbidge et al., 2002).

In some cases décollements are generated in the vicinity of viscous salt layers, e.g. at the Hellenic subduction zone in the eastern Mediterranean. Here the accretionary wedge – the Mediterranean Ridge, is partly underlying by large evaporite horizons which correlate with a shallow décollement responsible for evolution of the outer prism.

Hence, major aim of this study is to verify the influence of an embedded viscous layer on the mechanics of evolving wedges. An extensive parameter sensitivity study varying the viscosity will enable us to investigate does and how does this parameter control wedge geometry, accretion mode, fault geometry, mass transport pattern and the location of the detachment.

We develop 2D numerical ‘sandbox’ model utilizing the Discrete Element Method to simulate the deformation behaviour of accretionary wedges. A mechanically weaker viscous layer based on the Burger’s Model is embedded in the brittle undeformed ‘sediments’. This viscous rheology describes the creep behaviour of natural rocks. We tested different viscosity values from 5*10¹⁶ to 1*10¹⁸ Pas to quantify their influence on the wedge kinematics and the development of the detachment.

Within all experiments at least two active detachments develop at different depth generating a down-steeping décollement. So, a temporary decoupling of the viscous layer from the underlying or overlying brittle strata takes place.

References

Burbidge, D. R., Braun, J., 2002. Numerical models of the evolution of accretionary wedges using the distinct element method. Geophys. J. Int., 148, 542-561.

Mulugeta, G., 1988. Modelling the geometry of Coulomb thrust wedges. J. Struct. Geol. 10, 847-859.