Serpentinization of the forearc mantle wedge in subduction zones: revisiting Roy D. Hyndman’s seminal contributions 25 years later

Abstract

In this paper we focus on the serpentinization of the forearc mantle wedge, just one of Roy Hyndman’s many contributions to our understanding of subduction zones. Over the past 25 years, numerous advances in geophysics, petrology, and geology clearly document that H2O-rich fluids, derived from the subducting plate, hydrate portions of the overlying mantle to form serpentinite. The extent of mantle-wedge serpentinization depends, to first approximation, on the thermal evolution of the subducting plate. Dehydration reactions in warm subducting slabs occur at shallow (< 100 km) depth making relatively large amounts of H2O available for forearc mantle-wedge hydration; in cool subduction zones, dehydration reactions occur at greater depth and less H2O is available to directly hydrate the shallow mantle wedge. High-resolution seismological studies, complemented by numerical modelling, reveal that serpentinization in a subduction zone varies spatially, with strong evidence for a serpentinite layer at the base of the mantle wedge. Serpentinite mineralogy plays an important role in controlling the rheologic behaviour of the subduction plate interface, but appears not to control the downdip extent of large thrust earthquakes as originally proposed. Weak serpentinite along the base of the mantle wedge acts to mechanically isolate the forearc mantle wedge from induced corner flow, and serpentinized regions of the forearc mantle wedge may localize deformation. Rare mantle-wedge earthquakes may reflect the subvertical flow of fluids along fractures. Future work in well-instrumented subduction zones is expected to clarify the spatial distribution and extent of serpentinization in the forearc mantle wedge.