Unpairing metamorphic belts: P–T paths and a tectonic model for the Ryoke Belt, southwest Japan

Abstract

Southwest Japan is divided into Outer and Inner Zones by the Median Tectonic Line (MTL), a major transcurrent fault. The Outer Zone is composed of the Sambagawa (high-pressure intermediate or high P/T type metamorphism), Chichibu and Shimanto Belts. In the Inner Zone, the Ryoke Belt (andalusitesillimanite or low P/T type metamorphism) was developed mainly within a Jurassic accretionary complex. This spatial relationship between high P/T type and low P/T type metamorphic belts led Miyashiro to the idea that metamorphic belts were developed as 'paired' systems. Textural relationships and petrogenetically significant mineral assemblages in pelites from the Ryoke Belt imply peak P-T conditions of #5 kbar and up to 850°C in migmatitic garnet-cordierite rocks from the highest-grade metamorphic zone. It is likely that the thermal anomaly responsible for metamorphism of the Ryoke Belt was related to a segment of the Farallon-Izanagi Ridge as it subducted under the eastern margin of the Asian continent during the Cretaceous. The sequence of mineral assemblages developed in pelites implies a metamorphic field gradient with shallow dP/dT slope, inferred to have been generated by a nested set of hairpin-like 'clockwise' P-T paths. These P-T paths are characterized by limited prograde thickening, minor decompression at peak-T , and near-isobaric cooling, features that may be typical of P-T paths in low P/T type metamorphic belts caused by ridge subduction. A ridge subduction model for the Ryoke Belt implies that juxtaposition of the high-P/T metamorphic rocks of the Sambagawa Belt against it was a result of terrane amalgamation. Belt-parallel ductile stretching, recorded as syn-metamorphic, predominantly constrictional strain in both Ryoke and Sambagawa Belt rocks, and substantial sinistral displacement on the MTL are consistent with left-lateral oblique convergence. Diachroneity in fast cooling of the Ryoke Belt is implied by extant thermochronological data, and is inferred to relate to progressive SW to NE docking of the Sambagawa Belt. Thus, an alternative interpretation of 'paired' metamorphic belts in Japan is that they represent laterally contemporaneous terranes, rather than outboard and inboard components of a trench/arc 'paired' system. Amalgamation of laterally contemporaneous terranes during large translations of forearcs along continental margins may explain other examples of 'paired' metamorphic belts in the geological record.

Viscous flow and deformation of regional metamorphic belts at convergent plate boundaries

“…Considering the lateral displacement associated with both viscous flow and the MTL, it is suggested that the Ryoke-Sambagawa belts have been formed in a single domain. Also considering the overlapping metamorphic ages described, the Ryoke and Sambagawa metamorphic rocks must have been formed by a series of the common events within a single trench-arc system, supporting the concept of ''paired metamorphic belts'' proposed by Miyashiro [1961], rather than by accidental amalgamation of two metamorphic belts formed under separate events and systems [e.g., Brown, 1998]. …”

Coupled micro-faulting and pressure solution creep overprinted on quartz schist deformed by intracrystalline plasticity during exhumation of the Sambagawa metamorphic rocks, southwest Japan

“…Miyashiro (1961) originally suggested that the low-and high-P/T type paired Ryoke and Sambagawa metamorphic belts were formed under a volcanic arc and at a subduction zone, respectively with a 100-200 km separation distance. However, recent studies have suggested that they originated from an accretionary prism at places far from each other along a common subduction zone to the Pacific Ocean, which were later juxtaposed as a result of Cretaceous left-lateral transcurrent faulting along the MTL oblique to the arc (Taira et al, 1989;Brown, 1998;Sakashima et al, 2003).…”

Initial 87Sr/86Sr ratio heterogeneity in Kamihara Tonalite, Ryoke belt, southwest Japan: Evidence from strontium isotopic analysis of apatite

“…The protoliths of the metamorphic rocks are mainly chert, shale, and sandstone of the Mino terrane (Paleozoic − Mesozoic; e.g., Mizutani et al, 1981;Wakita 1987;Adachi and Suzuki, 1992). The peak pressure and peak temperature of the Ryoke metamorphism are estimated to be approximately 5 kbar and 850 °C, respectively (Brown, 1998). The Kamihara Tonalite is the oldest series of granitoid intrusions in the Cretaceous Ryoke belt.…”