Analysis of transpression within contractional fault steps using finite- element method Seyed Tohid Nabavi a, * , Seyed Ahmad Alavi a , Soheil Mohammadi b , Mohammad Reza Ghassemi c , Marcel Frehner d a Faculty of Earth Sciences, Department of Geology, Shahid Beheshti University, Tehran, Iran b High Performance Computing Laboratory, School of Civil Engineering, University of Tehran, Tehran, Iran c Research Institute for Earth Sciences, Geological Survey of Iran, Tehran, Iran d Geological Institute, NO E3, ETH Zürich, Sonneggstrasse 5, Switzerland article info Article history: Received 15 November 2016 Received in revised form 4 January 2017 Accepted 18 January 2017 Available online 21 January 2017 Keywords: Transpression Strike-slip fault Fault segment Contractional step Finite-element method abstract Two-dimensional finite-element modelling of elastic Newtonian rheology 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. 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 (20e90  ). The orientations of the local s 1 (the maximum compressive stress) and s 3 (the minimum compressive stress) directions strongly depend on the structural position within the transpression zone. For neutral and overlapping fault steps, there is a contractional linking damage zone between the fault segments. For overlapping faults, the s 1 trajectories within the transpression zone deflects significantly forming 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 the stress field and reflects the heterogeneous nature of deformation. A lozenge- (for under- lapping steps), rhomboidal- (for neutral steps), and sigmoidal-shaped (for overlapping steps) trans- pression zone developed 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, 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. © 2017 Elsevier Ltd. All rights reserved. 1. Introduction On geological maps, strike-slip fault systems are often appar- ently linear and relatively continuous (Dasgupta and Mukherjee, 2016). However, in nature they are typically discontinuous and segmented on various scales. Such sub-parallel discontinuous fault segments often exhibit en- echelon, non-coplanar geometries and include steps and bends in the master fault that demarcate the boundaries of the strike-slip zones. The individual fault segments are separated from each other and interact through their stress/ strain fields. The movement on the fault segment parallel each other and initiates shear fracture/fault (Segall and Pollard, 1983; Pollard and Segall, 1987; Aydin et al., 2006). The represent step- overs between two fault segments represent the locations of extensional or contractional heterogeneous deformations depending on the sense of fault step with respect to the sense of slip along the main strike-slip fault system (Fig. 1). The sense of step is described as left- and right-stepping. Releasing or extensional stepovers result where the sense of step is the same as the sense of the overall slip (e.g., a left-step along a left-lateral fault) (Biddle and Christie-Blick, 1985; Christie-Blick and Biddle, 1985; Woodcock and Fischer, 1986; Sylvester, 1988; Woodcock and Schubert, 1994; Westaway, 1995; Cunningham and Mann, 2007; Mann, 2007; Crider, 2015; Cao and Neubauer, 2016; Kattenhorn et al., 2016; * Corresponding author. E-mail addresses: Tohidnabavi@gmail.com, T_nabavi@sbu.ac.ir (S.T. Nabavi). Contents lists available at ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com/locate/jsg http://dx.doi.org/10.1016/j.jsg.2017.01.004 0191-8141/© 2017 Elsevier Ltd. All rights reserved. Journal of Structural Geology 96 (2017) 1e20