RHEOLOGICAL HETEROGENEITY, MECHANICAL ANISOTROPY AND DEFORMATION OF THE CONTINENTAL LITHOSPHERE

This paper aims to present an overview on the influence of rheological heterogeneity and mechanical anisotropy on the deformation of contine nts. After briefly recapping the concept of rheological stratification of the lithosphere, we discuss two specific issues: (1) as supported by a growing body of geophysical and geological observations, crust/ma ntle mechanical coupling is usually efficient, especially beneath majo r transcurrent faults which probably crosscut the lithosphere and root within the sublithospheric mantle; and (2) in most geodynamic environ ments, mechanical properties of the mantle govern the tectonic behavio ur of the lithosphere. Lateral rheological heterogeneity of the contin ental lithosphere may result from various sources, with variations in geothermal gradient being the principal one. The oldest domains of con tinents, the cratonic nuclei, are characterized by a relatively cold, thick, and consequently stiff lithosphere. On the other hand, rifting may also modify the thermal structure of the lithosphere. Depending on the relative stretching of the crust and upper mantle, a stiff or a w eak heterogeneity may develop. Observations from rift domains suggest that rifting usually results in a larger thinning of the lithospheric mantle than of the crust, and therefore tends to generate a weak heter ogeneity. Numerical models show that during continental collision, the presence of both stiff and weak rheological heterogeneities significa ntly influences the large-scale deformation of the continental lithosp here. They especially favour the development of lithospheric-scale str ike-slip faults, which allow strain to be transferred between the hete rogeneities. An heterogeneous strain partition occurs: cratons largely escape deformation, and strain tends to localize within or at the bou ndary of the rift basins provided compressional deformation starts bef ore the thermal heterogeneity induced by rifting are compensated. Seis mic and electrical conductivity anisotropies consistently point toward s the existence of a coherent fabric in the lithospheric mantle beneat h continental domains. Analysis of naturally deformed peridotites, exp erimental deformations and numerical simulations suggest that this fab ric is developed during orogenic events and subsequently frozen in the lithospheric mantle. Because the mechanical properties of single-crys tal olivine are anisotropic, i.e. dependent on the orientation of the applied forces relative to the dominant slip systems, a pervasive fabr ic frozen in the mantle may induce a significant mechanical anisotropy of the whole lithospheric mantle. It is suggested that this mechanica l anisotropy is the source of the so-called tectonic inheritance, i.e. the systematic reactivation of ancient tectonic directions; it may es pecially explain preferential rift propagation and continental break-u p along pre-existing orogenic belts. Thus, the deformation of continen ts during orogenic events results from a trade-off between tectonic fo rces applied at plate boundaries, plate geometry, and the intrinsic pr operties (rheological heterogeneity and mechanical anisotropy) of the continental plates. (C) 1998 Elsevier Science B.V. All rights reserved .