Duplex Underplating, Sediment Dehydration and Quartz Vein Mineralization in the Deep Tremor Source Region

Deep tectonic tremor downdip of the seismogenic zone in warm subduction zones is thought to occur in the region of high fluid pressures. However, the deformation and fluid processes responsible for tremor are poorly understood. We examined the Tomuru metamorphic rocks on Ishigaki Island, southern Ryukyu Arc, deformed at ∼40 km depth and ∼450°C under epidote‐blueschist metamorphism comparable to the tremor source region in northern Cascadia subduction zone. Here, quartz vein‐rich metapelite and metabasite are repeated many times as a result of duplex underplating. The spatiotemporal relationship between clustered quartz veins and the juxtaposition of metapelite and metabasite suggests that quartz vein formation and duplex underplating are contemporaneous. Viscous shear in metapelite is accommodated by dissolution‐precipitation creep. Metabasite records heterogeneous dehydration, resulting in rheological heterogeneity characterized by greenschist lenses in the foliated blueschist matrix. Viscous shear in the blueschist matrix was mainly accommodated by dissolution‐precipitation creep of glaucophane. Geochemical and strontium‐neodymium isotope analyses indicated that quartz veins were derived from sediment dehydration, whereas dehydration from oceanic crust contributed neither to quartz vein formation nor to fluid overpressures. We suggest that high fluid pressure in the deep tremor source region is primarily controlled by dehydration of subducting sediments, and the clustered quartz veins in underplated rocks correlate with the overpressured tremor source in low shear‐wave velocity zones. Plain Language Summary Tectonic tremor is a low‐amplitude, noise‐like seismic signal. It is typically observed downdip of the locked seismogenic zone in warm‐slab environments such as the Cascadia and Nankai subduction zones. Geophysical observations have shown that tremor occurs in the region of high fluid pressure. However, the tremogenic deformation and fluid processes remain unknown. We studied the exhumed metamorphic rocks on Ishigaki Island, southern Ryukyu Arc, which were deformed in a deep subduction zone comparable to the tremor source region in northern Cascadia. Our geological observations show that the tremor source region is represented by multiple stacks of sediments and oceanic crust by duplex underplating and ubiquitous quartz veining as a result of high fluid pressure and silica precipitation. Geochemical analyses indicate that dehydration of subducting sed