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Abstract

We propose a theoretical model, supported by a field study, to describe the patterns of fault/fracture meshes formed within dilational stepovers developed along faults accommodating regional scale wrench-dominated transtension. The geometry and kinematics of the faulting in the dilational stepovers is related to the angle of divergence (α), and differs from the patterns traditionally predicted in dilation zones associated with boundary faults accommodating strike-slip displacements (where α = 0°). For low values of oblique divergence (α<30°) and low strain, the fault–fracture mesh comprises interlinked tensile fractures and shear-extensional planes, consistent with wrench-dominated transtension. At higher values of strain, a switch occurs from wrench- to extension-dominated transtension, leading to the reactivation and/or disruption of the early-formed structures. These structural processes lead to the development of a geometrically complex and kinematically heterogeneous fault pattern, which may affect and/or perturb the development of a through-going fault linking and facilitating the slip transfer between the two overlapping fault segments. As a result, dilational stepover zones will tend to form long-lived sites of localized extension and subsidence in regional transtensional tectonic settings. Cyclic increases/decreases of structural permeability will be related to slip on the major boundary faults that control the distribution of fluid-flow paths and, consequently, the long- and short-term structural evolution of these sites. Our model also predicts complex and more realistic subsurface fluid migration pathways relevant to our current understanding of hydrothermal ore deposits and hydrocarbon migration and storage.

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