Continental-scale rheological heterogeneities and complex intraplate tectono-metamorphic patterns: insights from a case-study and numerical models

Abstract

Continental plates are built over long periods of time through successive extensional and compressional cycles. They are therefore rheologically heterogeneous. This heterogeneity should significantly influence the mechanical response of the continental lithosphere during collision processes. The study of the Neoproterozoic Borborema shear zone system of northeast Brazil highlights a systematic link between marked changes in its tectono-metamorphic pattern and the pre-existing structure of the plate, that is characterized by juxtaposition of continental domains either comprising an old basement (Palaeo- to Eoproterozoic) or accreted during an extensional event between 1.0 and 0.7 Ga. In Neoproterozoic time, when the shear zone system was developed, these domains displayed different geotherms and lithospheric thicknesses, and therefore contrasted rheological behaviours. We use numerical models simulating the mechanical evolution of a continental plate comprising multiple thermally-induced rheological heterogeneities submitted to compression to investigate how these heterogeneities may affect strain localization and the distribution of deformation regimes and vertical strain within the plate. From the very beginning of the deformation, weak and stiff heterogeneities induce strain localization, due to a lower initial effective viscosity or to stress concentrations at their tips, respectively. Shear zones propagate from the heterogeneities and finally coalesce, forming a network of high-strain zones bounding almost underformed blocks. Within this network, shear zones transfer strain between the different heterogeneities and model boundaries. The evolution of the system depends essentially on the geometrical distribution of heterogeneities and on their strength contrast relative to the surrounding lithosphere. The resulting finite-strain field is heterogeneous and displays rapid lateral variations in vertical and/or rotational deformation. Such a heterogeneous strain distribution may induce contrasted magmatic, metamorphic and uplift evolutions within an orogenic belt, as observed in the Borborema shear zone system and other collisional belts.