Masaru Terabayashi

Blueschists and eclogites of the world and their exhumation

High-P/T metamorphic belts were classified into types A and B according to their protoliths. The A-type (collision-type) blueschists possess passive-margin protoliths characterized by platform-type carbonates, bimodal volcanics, and peraluminous sediments. B-type (Cordilleran-type) blueschists consist of active continental-margin protoliths in an accretionary complex characterized by bedded chert, MORB and ocean-island basalts, reef limestones, and graywackes. The spatiotemporal distribution of blueschists and eclogites of the world was compiled; among 250 recognized high-P/T belts, about 20% belong to the A type and the rest to the B type. Most A-type zones lie in Europe and the Tethyan domain, include ultrahigh-pressure metamorphic terranes, and have metamorphic pressure up to 45 kbar. B-type zones occur mainly in the circum-Pacific orogenic belts and intracontinental orogens in Asia, and were recrystallized at P <12 kbar. Associated peridotites include garnet peridotite in the A type and strongly serpe...

Well-preserved underplating structure of the jadeitized Franciscan complex, Pacheco Pass, California

Duplex structures several hundred metres thick bounded by marker beds exhibiting basalt-chert association have been found in the Franciscan complex near Pacheco Pass, central Diablo Range, California. The terrane was regionally metamorphosed and deformed under blueschist facies conditions at 150 °C and 7–8 kbar. Detailed mapping of the basalt-chert sequence has clarified the change of environment from mid-oceanic ridge to subduction zone. Eastward convergence of duplexing indicates eastward subduction of the oceanic plate during Early Cretaceous time. The presence of duplexing and a reconstruction of oceanic plate stratigraphy recorded within the underthrust unit demonstrate the accretionary origin of protoliths of the Franciscan coherent unit, which has long been believed to have an in situ origin, interpreted as either near-trench volcanism or accreted terranes in which “geosynclinal-type” volcanism occurred.

Accretionary Complex Origin of the Mafic-Ultramafic Bodies of the Sanbagawa Belt, Central Shikoku, Japan

In the high-grade Cretaceous Sanbagawa high-pressure (HP) metamorphic belt, our new 1:5000 scale mapping of eclogitic mafic-ultramafic bodies and their surrounding epidote-amphibolite—facies schists has revealed a duplex structure formed by the subduction of the Izanagi-Pacific oceanic plate. Lithologies of the two largest mafic-ultramafic bodies in the Sanbagawa belt, the Iratsu eclogite and the Higashi-Akaishi peridotite, strike WNW-ESE and dip N; the upper boundary with the surrounding schist is a normal fault, whereas the lower boundary is a thrust. The Iratsu body is subdivided into at least two tectonic units; the unit boundary is subparallel to a lithological boundary. Protoliths of the upper unit are gabbro, basalt, minor quartz rock, and pelite, and those of the lower unit are pyroxenite, gabbro, basalt, chert, and marble, in ascending order. The lower unit is characterized by layers of alternating eclogitic metagabbro and pyroxenite. The layers are extensive at the bottom of the Iratsu eclogite, and transient toward the Higashi-Akaishi body. Eclogitefacies metapsammite is intercalated between the Iratsu and Higashi-Akaishi bodies. Our mapping has revealed the following: (1) a duplex structure of the mafic-ultramafic bodies indicating their accretionary complex origin; (2) reconstructed oceanic plate stratigraphy in ascending order of peridotite, gabbro, basalt, limestone, minor chert, and pelite, suggesting that different parts of the protolith were derived from a mid-oceanic topographic high, an oceanic island or plateau, and an overlying trench turbidite; and (3) a change in the convergent motion of the oceanic plate from NW to NE during the accretion of the large oceanic island or plateau.

Large pressure gap between the Coastal and Central Franciscan belts, northern and central California

Abstract The nature of the metamorphism of the Central belt of the Franciscan Complex has long been debated. Did the whole Central belt, or its melange matrix undergo high-P/T metamorphism? To solve this problem, we have investigated the regional distribution of aragonite in these rocks in northern and central California. We have discovered more than 300 new localities of aragonite, using the staining method by Friedman (1959) . Aragonite occurs as network veins or pods in both metagraywacke and matrix-forming metashale, and is invariably partially replaced by calcite on its rim. Thus, we conclude that aragonite is stable in all parts of the Franciscan Central belt from the lower boundary between the Coastal and Central belts to the topmost part of the Central belt, and that both the matrix-forming melange shale and exotic blocks of the Central belt suffered high-P/T metamorphism. We traced the boundary between the Central and Coastal belts in northern California using the staining method. Along the gently eastward-dipping thrust plane between the underlying Coastal and overlying Central belts, more than 2.5 kbar pressure break is obtained through the lawsonite-laumontite-aragonite-quartz relations. Such an observed large pressure gap was formed during the exhumation process of the Central belt of the Franciscan Complex.

Exhumation Tectonics of the Sanbagawa High-Pressure Metamorphic Belt, Southwest Japan—Constraints from the Upper and Lower Boundary Faults

We have determined the exhumation process of the Sanbagawa Belt based on kinematic analyses of the upper and the lower boundary faults. The Sanbagawa Belt is tectonically intercalated as a thin subhorizontal sheet between overlying, weakly metamorphosed Jurassic and underlying Cretaceous accretionary complexes (e.g., Maruyama et al., 1996). On the lower boundary in Kii Peninsula, pumpellyite—actinolite facies metabasites have undergone semibrittle deformation, indicating a top-to-the south sense of thrusting. The upper boundary in central Shikoku is north-vergent, and indicates a top-to-the north sense of shear; this suggests that the original normal fault on the boundary was warped by later doming. These results support a model of selective exhumation of the Sanbagawa Belt from a depth of 30 km, and its juxtaposition against the over-and underlying accretionary complexes by orogen-orthogonal movements.