Relationship between polarity of extensional fault arrays and presence of detachments

Extensional fault arrays are often dominated by a single fault vergence, forming ‘domino’ fault blocks. The polarity of faults can be defined as synthetic or antithetic in relation to the overall shear sense across the fault system. Observations of the geometry of a number of field examples of extensional fault arrays vs. the lithologies (as a proxy for strength profile) show that synthetic extensional arrays from the North Sea and Bristol Channel Basin detach on salt layers, whereas examples of antithetic arrays from the North Sea are found to pin out downwards and no basal detachment is present. In the Sacramento Mountains core complex of the Basin and Range, the major low-angle detachment fault post-dates antithetic shears that evolved at mid-crustal level and are preserved in the granitic footwall. These antithetic shears were cut by the main low-angle detachment, whose hanging wall disintegrated into an array of synthetic faults. The kinematics of synthetic vs. antithetic arrays dictate that faults within synthetic arrays must branch onto a basal detachment, whereas faults within antithetic arrays may die out downwards. Therefore, the results emphasise that fault polarity in domino arrays may be related to the boundary conditions of the fault blocks and therefore may be diagnostic of the strength profile of the faulted stratigraphy.