Metamorphic Duality in SW Japan: The Sanbagawa-Ryoke Classic Example of Paired Metamorphism
Simon R. Wallis, Kazuhiro Miyazaki, and Ulrich Knittel – Guest Editors
Table of Contents
Overview
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2024 Topics
Overview
Subduction, where one plate dives beneath another, controls long-term whole-Earth cycling of rocks, fluids, and energy. Plates subduct faster than they heat up, making them the coldest parts of the Earth’s interior. Fluids released from these cold plates rise into hotter overlying rocks, forming magma that feeds surface volcanism. Cold deep conditions associated with subduction complemented by hot shallow conditions under volcanic arcs are reflected in the presence of pairs of metamorphic belts, representing sites of ancient subduction. This issue of Elements guides readers through a premier example of paired metamorphism: the Cretaceous Sanbagawa-Ryoke metamorphic pair of Japan. Estimates of pressure, temperature, the age and duration of metamorphism, and the tectonic framework in which meta morphism took place help us to develop quantitative models—both for the evolution of SW Japan and subduction systems in general.
- Paired Metamorphism of SW Japan and Implications for Tectonics of Convergent Margins
- Sanbagawa Subduction: What Went in, How Deep, and How Hot did it Get?
- Ultramafic Rocks from the Sanbagawa Belt: Records of Mantle Wedge Processes
- Geochronology of the Sanbagawa Belt: Younger and Faster than Before
- Inside the Ryoke Magmatic Arc: Crustal Deformation, High-T Metamorphism, and Magmatic Pulses
- Linking Pacific Plate Motions to Metamorphism and Magmatism in Japan During Cretaceous to Paleogene Times
- Thermal Modeling of the Sanbagawa and Ryoke Belts
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CRATONS TO CONTINENTS
Guest Editors: Paul Mueller (University of Florida, USA) and Carol Frost (University of Wyoming, USA)
Archean continental crust is present on every continent, but does not constitute a dominant part of any continent’s surficial exposures. Nevertheless, Archean cratons are the longestlived coherent physical structures on earth. Viewed holistically they comprise a welded com bination of continental crust and as subcontinental lithospheric mantle keel. They are survivors of what may, or may not, have been a more numerous and varied population of protocontinents. Many of these crustal blocks have origins in the Hadean and have survived for billions of years through many super continent cycles. Consequently, these cratonkeel structures have influenced the physical and chemical evolution of the silicate earth. This issue of Elements provides an overview of Archean cratons and the information they retain about the early development of Earth’s continental crust.
- Archean Cratons: Time Capsules of the Early Earth Carol D. Frost (University of Wyoming, USA) and Paul A. Mueller (University of Florida, USA)
- Earth’s Earliest Crust Jonathan O’Neil (University of Ottawa, Canada), Hanika Rizo (Carleton University, Canada), Jesse Reimink (Pennsylvania State University, USA), Marion Garçon (Université Clermont Auvergne, France), and Richard W. Carlson (Carnegie Institution for Science, USA)
- At the Dawn of Continents: The Archean Tonalite-Trondhjemite- Granodiorite Suites Oscar Laurent (CNRS, Géosciences Environnement Toulouse, France), Martin Guitreau (Université Clermont Auvergne, France), Emilie Bruand (Université Clermont Auvergne and CNRS, Geo-Ocean, Brest, France), and JeanFrançois Moyen (CNRS, GeoOcean, Brest, France)
- Archean Geodynamics Underneath Weak, Flat, and Flooded Continents Patrice F. Rey (University of Sydney, Australia), Nicolas Coltice (Université Côte d’Azur, France), and Nicolas Flament (University of Wollongong, Australia)
- Embracing the Complexity at Depth Catherine M. Cooper (Washington State University, USA) and Meghan S. Miller (Australian National University, Australia)
2024 Topics
- Extraterrestrial Organic Matter (February 2024)
- Paired Metamorphic Belts of SW Japan: Metamorphic Records of a Subduction System (April 2024)
- Cratons to Continents (June 2024)
- The Invisible Ocean: Hydrogen in the Deep Earth (August 2024)
- Behind and Beyond Luminescence Imaging (October 2024)
Thematic Articles
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.
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.
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.
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.
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.
Plate reconstructions of oceanic domains are generally based on paleo- magnetic and seafloor spreading records. However, uncertainties associated with such reconstructions grow rapidly with increasing geological age because the original oceanic plates have been subducted. Here we synthesize advances in seismic tomographic mapping of subducted plates now lying within the mantle that assist plate reconstructions. Our proposed Japan–NW Pacific subduction histories incorporate tomography results and show three distinct stages comparable to those revealed by geochronology, petrology, and geochemistry. We propose major revisions to previously accepted ideas about the age, kinematics, and identity of the plates outboard of Japan during the Cretaceous–Paleogene Sanbagawa-Ryoke paired metamorphism. These revisions require updates to relevant plate convergence boundary conditions and thermo-dynamic models.
The Sanbagawa and Ryoke belts were formed in a convergent plate boundary along the eastern margin of Eurasia. Thermal modeling using the geological records of these belts as constraints allows quantitative estimates of both shear heating along the Wadati-Benioff zone and magma fluxes beneath the volcanic arc. In contrast to real-time observations of crustal movement and heat flow, rocks record changes in pressures and temperatures that occur over periods of several million years and can be used to examine conditions from the surface to the mantle. Thermal modeling combined with such geological records helps to bridge the gap in our knowledge between real-time observations of ongoing geological processes and the development of orogenies in convergent plate margins over geological time.
