WHAT DRIVES CORE-COMPLEX FORMATION IN WESTERN TURKEY -- A NUMERICAL MODELLING APPROACH

The mode of continental extension is determined by the mechanical strength of the lithosphere, which is largely dependent on its thermal structure. In the case of a high geotherm the lower crust will be hot and of very low effective viscosity, resulting in a weak layer between the brittle-elastic upper crust and the highly viscous upper mantle. Stretching of this system is accommodated by strong partitioning of deformation between ductile flow of the lower crust and brittle faulting of the upper crust. In the case of metamorphic core complexes deformation is localized onto few normal fault systems in the brittle upper crust, each accommodating large displacements, and resulting in local exhumation of the lower crust.

We explore the sensitivity of brittle faulting and lower crustal flow to material property distributions using the two-dimensional Lagrangian Integration Point finite element code ELLIPSIS (Moresi et al., 1999). This approach is unique in its capability to allow simulation of very large strains, which is an essential requirement when extreme localization, such as detachment faults, need to be accurately modeled on the crustal scale. Our results indicate that the spacing of brittle faults is controlled by lateral strength heterogeneities and vertical contrasts in rheology.

We further apply ELLIPSIS to simulate rolling-hinge tectonics as proposed for the Central Menderes Metamorphic core complex (CMCC) in western Turkey. The CMCC has been formed by two opposite-facing detachment faults since the late Miocene (Gessner et al. 2001). Since information about subsurface structure in the area is poor, it is not known as to whether deformation is limited to the crust or affects the upper mantle as well. Numerical simulations with ELLIPSIS indicate that the strength contrast between a weak lower crust and strong upper crustal layers may be sufficient to produce the observed geometry without any deflection or offset of the mantle. The simulations also show that a low viscosity lower crust is likely to mechanically decouple the lithosphere by drastic partitioning of deformation processes.

Gessner K, Ring U, Johnson C, Passchier CW, Güngör T (2001) Geology 29:611-614; Moresi L, Mühlhaus HB, Dufour F (1999). In: Mühlhaus HB et al. (Eds): Bifurcation and Localization in Soils and Rocks, Balkema, Rotterdam.