The Internal Structure of Dilational Stepovers in Regional Transtension Zones

We present a theoretical model and discuss field-based observations from the Carboniferous Northumberland Basin (UK), describing the structures developed at stepovers associated with regionally oblique divergence (i.e., transtension zones). We show that these structures have significantly different geometries and evolution compared to those found in stepovers along strike-slip faults. The development of complex and heterogeneous patterns of structures, accommodating both brittle (fault/fracture mesh) and ductile deformation (folds), is observed at these sites. The dilational mesh structures in the stepover region experience a complex evolution due to finite strain-controlled switches from wrench- to extension-dominated transtension. This disrupts the development of a smoothly evolving structural fabric and may inhibit/perturb the development of a throughgoing fault linking adjacent fault segments. Markedly curvilinear and locally curviplanar folds, compartmentalized by strike-slip faults, are also developed. Significant amounts of hinge-parallel extension are accommodated by calcite-filled tensile veins and conjugate tension gash arrays. The fold-fracture associations described here contrast strongly with the more widely recognized patterns of strike-slip conjugate shear planes and extension fractures associated with folds developed in contractional and wrench tectonic settings. This represents a diagnostic feature that allows transtensional folds in both surface and subsurface environments to be distinguished from structures formed by later episodes of compressional or strike-slip inversion.