Intraplate deformation: the role of rheological heterogeneity and anisotropy of the lithospheric mantle (Invited)

Although most tectonic activity occurs at or close to plate boundaries, a large body of evidence suggests that intraplate deformation is a common process in continental plates. The occurrence of intraplate deformation requires that forces applied at plate boundaries are transferred over long distances, triggering strain localization within the plate. How can we account for this process? Continental plates are built over long periods of time through successive extensional and compressional cycles. Variations in tectonic age and lithosphere thickness result in lateral variations in the geotherm within a plate, which result in rheological heterogeneity of the continental lithosphere. The oldest domains of continents, the cratons, are characterized by a relatively cold, thick, and consequently stiff lithosphere. Rifting also modifies the rheological structure of the lithosphere. Depending on the relative stretching of the crust and upper mantle, a rifted domain may represent a stiff or a weak heterogeneity within a continental plate. The geotherm may also be locally steepened by metasomatism-induced heat production in the lithospheric mantle. Numerical models show that the presence of stiff and weak rheological heterogeneities modifies the large-scale deformation of the continental lithosphere. Cratons largely escape deformation and strain tends to localize within or at the boundary of rift basins or metasomatized domains provided deformation starts before the thermal heterogeneity is relaxed. Lithospheric-scale strike-slip faults allow strain to be transferred between the rheological heterogeneities. In addition, seismic and electrical conductivity anisotropy measurements highlight the existence of preferred orientations of olivine crystals that are coherent over tens to hundreds of km scales in the subcontinental lithospheric mantle. Because the mechanical properties of the olivine crystal are anisotropic, i.e., dependent on the orientation of the applied forces relative to the dominant slip systems, pervasive olivine crystal preferred orientation frozen in the lithospheric mantle results in large-scale mechanical anisotropy. Lateral variation in the mantle fabric may also lead to strain localization within a plate. Numerical models that explicitly consider an evolving anisotropic viscosity controlled by the orientation of olivine crystals in the mantle by coupling a viscoplastic self-consistent simulation of the deformation of a polycrystal to a 3D finite element model of the plate deformation show indeed strain localization in domains where shear stresses on the inherited mantle fabric are high. We propose that this mechanical anisotropy is the source of the so-called tectonic inheritance, that is, the systematic reactivation of ancient tectonic directions; it may in particular explain preferential rift formation and continental break-up along pre-existing orogenic belts.