TECTONIC INHERITANCE DURING EXTENSION IN RIFTS AND PASSIVE MARGINS: A REVIEW

Data compiled from 27 geologically well constrained rift basins worldwide suggest that inheritance is a significant influence during extension. In basins where fault reactivation has taken place during rifting, the mean duration of the rift initiation to climax transition is 2.9 Myr compared to 8.8 Myr in basins where there is no reactivation of pre-existing structures.

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.