Extension at the coast of the Makran subduction zone (Iran)

Raphaël Normand,Guy Simpson,Abbas Bahroudi

doi:10.1111/ter.12419

Abstract

AbstractIn the Makran subduction zone, earthquake focal mechanisms and geodetic data indicate that the deforming prism currently experiences N–S compression. However, palaeostress inversions performed on normal faults observed along the coast reveal local stress components consistent with N‐S extension. Previously proposed mechanisms such as gravitational collapse are not favoured by N–S compression and surface uplift. We propose that the observed kinematics result from transient stress reversals following large earthquakes. During the interseismic period (now), the region experiences N–S compression. However, following a large reverse rupture on the subduction interface, stresses in the inner wedge relax, enabling a brief period of extensional faulting before a compressive stress state is re‐established. This mechanism, also observed in other subduction zones, requires low overall stresses in the upper plate and that the margin ruptures in large megathrust earthquakes that result in nearly complete stress drops.

Highlights

Subduction plate boundaries are areas of intense folding and thrusting, emphasizing the general state of compressive stress in the overriding plate
Extensional faulting could result from sediment underplating, causing steepening of the taper followed by gravitational collapse (Platt et al, 1985), facilitated by an increase in fluid overpressure 135 with depth (Ruh, 2017) and/or by sediment loading (Ellouz-Zimmermann et al, 2007)
Numerical models and sand-box experiments have shown that these phenomena could be cause of the large listric normal faults observed near the Makran shelf break with seismic reflection (Ellouz-Zimmermann et al, 2007; Grando and McClay, 2007; Ruh, 2017)

Summary

Introduction

Subduction plate boundaries are areas of intense folding and thrusting, emphasizing the general state of compressive stress in the overriding plate. Some convergence is being accommodated within the emerged part of the prism, as evidenced by the presence of deformed late Quaternary fluvial and marine terraces (Haghipour et al, 2012; Normand et al, 2019b) In addition to this compressional deformation, a narrow region near the coast is characterized by the presence of normal faults (e.g., Hosseini-Barzi and Talbot, 2003; Burg et al, 55 2013; Dolati and Burg, 2013) and seismic sections reveal the presence of south-dipping listric normal faults about 50 km offshore of the study area (Ellouz-Zimmermann et al, 2007; Grando and McClay, 2007). The downthrown fault block forming the Chabahar headland could be an orthorhombic fault system consistent with N-S 115 extension (e.g., Reches, 1978; Hosseini-Barzi and Talbot, 2003)

Discussion

Conclusions

185 Acknowledgements