Limited arc magmatism and seismicity due to extensive mantle wedge serpentinization in the Makran subduction zone V. Teknik a,* , I.M. Artemieva b,c,d,* , H. Thybo b,d,e,* a Istanbul Technical University, Istanbul, Turkey b SinoProbe Laboratory, Chinese Academy of Geological Sciences, Beijing, China c Section of Marine Dynamics, GEOMAR Helmholtz Center for Ocean Research, Kiel, Germany d State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Sciences, China, University of Geosciences, Wuhan, China e Eurasia Institute of Earth Sciences, Istanbul Technical University, Istanbul, Turkey A R T I C L E INFO Editor: J.P. Avouac Keywords: Mantle wedge Accretionary prism Low-angle subduction Seismicity Arc magmatism Serpentinization ABSTRACT Subduction processes usually involve extensive seismicity and create voluminous magmatic arcs by mantle wedge melting caused by dehydration of the subducting slab, but the Makran subduction zone has anomalously low seismicity and magmatism. Here we explain these anomalous features by 60–65% serpentinization in the peridotitic shallow mantle wedge based on our new integrated seismic, magnetic, gravity and isostatic model across the Makran subduction zone. The low-angle, slow Makran subduction provides ample time for the slab to release sufficient amounts of fluids for creating a large volume of rheologically weak serpentinite. This reduces seismicity by lowering the friction between the slab and surrounding rocks. Further, very little fluid is left in the slab when it reaches the melting depth, which explains the limited arc magmatism. Around 100 km depth, the subduction switches from low-angle to almost vertical. Our model demonstrates the combined effects of sub- duction rate and dip on mantle serpentinization with implication for assessment of seismic and volcanic hazards in subduction systems. 1. Introduction Most disastrous seismic, volcanic and tsunami activity is associated with active subduction zones. Traditionally, the subduction dynamics is thought to be controlled by parameters related to the tectonic type of the subducting lithospheric plates (oceanic or continental), their conver- gence rate and the age of the downgoing oceanic plate, which might control the dip angle (Stern, 2002; Artemieva et al., 2016; Lallemand and Heuret, 2017; Hu and Gurnis, 2020). The dip angle, among other parameters, controls the location of the volcanic arc (England et al., 2004), subduction geodynamics and, to some extent, the seismicity in the Wadati-Benioff zone (Nagaya et al., 2016). Due to the interplay of many parameters, where the amount of trapped water in the slab is critical, classic models of subduction zone dynamics include a large di- versity of subduction types and associated hazards (e.g. Stern, 2002; Hyndman and Peacock, 2003; Lallemand et al., 2005). Serpentinization of shallow oceanic mantle has long been proposed as an important wide-spread geodynamic process in subduction settings (Martin and Fyfe, 1970; Moody, 1976; Stern, 2002; Ulmer and Trommsdorff, 1995). The chemical reaction includes alteration of anhydrous ferromagnesian ultramafic rocks (e.g. peridotite and pyrox- enite) into hydrous serpentine group minerals (with composition similar to mantle rocks, but with 12–16 wt.% H 2 O), and it also forms other minerals such as brucite in case of a magnesium-rich (e.g. dunite) parent rock and magnetite in case of an iron-rich (e.g. pyroxenite) parent rock (Moody, 1976; Deschamps et al., 2013). These reactions require water-rich conditions at low temperatures, usually <400 ◦ C, and are most effective at ~270 ◦ C (Martin and Fyfe, 1970). Thus the rate and degree of serpentinization depend on (i) water availability, with water flux into the shallow mantle being facilitated by normal faulting of the subducting lithosphere (Ranero et al., 2003) and by thin impermeable sedimentary layers permitting seawater percolation, (ii) P-T conditions appropriate for serpentinization, which primarily exist in >10–15 Ma oceanic lithosphere (Grevemeyer et al., 2018) to ensure a cold (<400 ◦ C) shallow mantle wedge. Fluid fluxes expected in subduction zones may be sufficient to serpentinize the entire forearc mantle wedge in tens of My (Hyndman and Peaccock, 2003). These requirements limit typical oceanic geodynamic settings for * Corresponding authors. E-mail addresses: vahid.teknik@gmail.com (V. Teknik), iartemieva@gmail.com (I.M. Artemieva), h.thybo@gmail.com (H. Thybo). Contents lists available at ScienceDirect Earth and Planetary Science Letters journal homepage: www.elsevier.com/locate/epsl https://doi.org/10.1016/j.epsl.2024.118950 Received 24 December 2023; Received in revised form 13 July 2024; Accepted 12 August 2024 Earth Planet. Sci. Lett. 645 (2024) 118950 Available online 28 August 2024 0012-821X/© 2024 Elsevier B.V. All rights are reserved, including those for text and data mining, AI training, and similar technologies.