ANALYSIS OF TRANSPRESSION WITHIN CONTRACTIONAL PLANAR FAULT STEPS USING THE FINITE-ELEMENT METHOD

Two-dimensional finite-element modelling, using a commercial FEM software, ABAQUS, is used to compute stress distribution and strain localization patterns in a transpression zone between two pre-existing right-stepping, left-lateral strike-slip fault segments as a function of the orientation of the applied far-field compressive stress. Linear elastic rheology and plane strain conditions are assumed. Regional compression is applied at different convergence angles relative to the fault strike. Three representative fault segment interactions are modelled: underlapping, neutral and overlapping. The numerical results indicate that at the onset of deformation, displacement vectors are oblique to the regional compression direction. The orientations of the local σ₁ and σ₃ strongly depends on structural position within the transpression zone. In low-σ₃ zones, which occur close to fault tips, wing cracks and horsetail arrays would propagate outwards along a curvilinear path. For neutral and overlapping fault steps, there is a contractional linking damage zone, mostly crosscutting P-shears, between the fault segments. For overlapping faults, the σ₁ direction within the transpression zone is highly deflected and forms a sigmoidal pattern, which is created by two rotational flow patterns close to the fault tips. These flow patterns are related to friction effects and different shear deformation, from pure shear outside of the fault steps toward simple shear along the fault segments. Interaction between the two fault segments perturbs stress field and reflects the heterogeneous nature of deformation. A lozenge- (for underlapping steps), rhomboidal- (for neutral steps) and sigmoidal-shaped (for overlapping steps) transpression zone is bounded by curved, oblique-slip reverse faults that cause block rotation between the two segments. The modelled mean stress pattern shows a similar pattern to that of the contractional steps, and decrease and increase in underlapping and overlapping fault steps, respectively. Comparison of the Kuh-e-Hori transpression zone, a doubly plunging asymmetric anticlinal structure, between the Esmail-abad and West Neh left-stepping right-lateral strike-slip fault segments in SE Iran with the modelling results shows strong similarities with the neutral step configuration.