Thematic Articles

The Ryoke belt represents the root of a volcanic arc exposed across SW Japan. It records successive deformation phases, high-temperature metamorphism, and several magmatic pulses that occurred during the Late Cretaceous. Successive magma intrusions at different crustal levels raised the overall geothermal gradient of the arc crust, and their thermal influence was contrastingly recorded in metamorphic zircon and monazite. Despite a broadly similar duration of magmatic activity (20–30 My) along the belt, the timing and periodicity of magma pulses varied. An along-arc variation in lower crustal magma generation together with a fluctuating crustal stress regime likely controlled the formation and evolution of this magmatic arc section.

Recent advances in geochronological studies have helped establish the Sanbagawa belt as an important site for studying metamorphism related to subduction. Application of several dating methods yield the following important results. 1) Metamorphism and deformation related to subduction started ~120 Ma and were complete by ~50 Ma. 2) Subduction to eclogite facies, followed by return to the surface, was rapid and occurred within a few million −1 years or less (at ~89 Ma), indicating exhumation rates of at least 1–2 cm•y−1. 3) The age of the slab during the peak eclogite facies metamorphism was ~60 My. These results help redefine the geological history of SW Japan and provide important constraints for mechanical and thermal models of subduction zones in general.

Mantle wedge domains beneath the forearc Moho are unique regions of Earth’s interior where mantle encounters subducting oceanic plates. Crystal-plastic deformation and fluid-induced reactions in the supra-subduction mantle control global material circulation, arc volcanism, and seismicity within subduction zones. The Sanbagawa metamorphic belt contains numerous ultramafic blocks in its higher-grade zones, some of which likely originated as lower crustal arc cumulates that were subsequently incorporated into the mantle wedge and transported to the slab–mantle inter- face by mantle flow. Properties of these ultramafic rocks provide a valuable opportunity to understand the dynamic processes of the mantle wedge up to 80 km depth, including mantle flow, hydration/dehydration, and fluid–rock interactions near the slab–mantle interface of a warm subduction zone.

The Sanbagawa belt is a “coherent” oceanic subduction-type metamorphic region representing a rock package predominantly derived from oceanic crust and accreted at depths of 20–80 km (300–700 °C). The thermal structure and lithological layers are complexly deformed but semi- continuous, in contrast to more commonly reported subduction-related domains dominated by mélange. The coeval Shimanto accretionary complex records accretion at depths <15 km and the rocks are primarily terrigenous sediments. The Sanbagawa belt has a greater proportion of mafic rocks than the Shimanto complex, implying progressive peeling-off of oceanic plate stratigraphy with more basaltic oceanic crust slices accreted at deeper levels. Tectonic exhumation can be explained by three separate phases dominated by buoyancy-driven upflow, ductile thinning, and normal faulting.

The Sanbagawa-Ryoke pair of geological units in southwest Japan is the classic example of paired metamorphism originally identified by Akiho Miyashiro. Together these belts represent an important study area for developing and testing ideas about how convergent margins behave over geological time based on studies of the rock record including petrology, geochemistry, deformation, and geochronology. The two sides of the pair represent ancient examples of a subduction zone in the Sanbagawa belt and an associated volcanic arc in the Ryoke belt. This issue of Elements brings together the results of a wide range of different approaches summarizing the current state of knowledge about the Sanbagawa-Ryoke pair and how this informs our understanding of convergent margins in general.