TECTONIC INHERITANCE DURING EXTENSION IN RIFTS AND PASSIVE MARGINS: A REVIEW
Inheritance results from stress concentration and shear localisation manifested at all scales in the continental lithosphere. Lithosphere-scale controls include crustal thickness, thermal age and plate tectonic boundary conditions. Grain-scale controls include local environmental controls (depth, stress, etc), rock composition, grainsize, fabric intensity and the presence of fluids. Multi-scale geometric controls are largely related to the size, orientation and interconnectivity of pre-existing anisotropies. If reactivation occurs, it likely requires a combination of processes across all three scale ranges to be favourable. This can make the unequivocal recognition of inheritance and reactivation difficult.
Most pre-existing crustal structures are significantly oblique (<70°) to regional extension directions. Transtensional bulk strains are therefore widespread during reactivation leading to strain partitioning and/or multimodal fracturing where the deformation cannot be described or reconstructed in single 2D cross-sectional or map view.
Crustal-scale pre-existing structures are especially important due to their ability to efficiently concentrate stress and localise strain across a broad scale range. This aids the rapid propagation of high displacement rift-bounding faults. Large structures are also prone to reactivation due to the development of strongly anisotropic, weak phyllosilicate-rich fault rocks (friction coefficients <0.2).
In summary, pre-existing structures can influence deformation patterns across a range of scales. The deformation magnitudes associated with reactivation events may be modest compared to the regional-scale deformation of the crust. However, reactivation will almost always influence the development of smaller-scale (<1km) geological architectures and this in turn will impact on crustal processes such as fluid flow and the accumulation of natural resources.