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ABSTRACT

The M1 blueschist to epidote amphibolite metamorphism that defines the named metamorphic zones of the Sambagawa belt of Japan and coeval ductile D1 deformation overprinted and replaced formerly more extensive eclogite-facies rocks and obscured the original subduction-accretion architecture. Based on new field, structural, and petrographic observations, integrated with published geochronologic, structural, and metamorphic petrologic data, we propose that the eclogites were emplaced both as intact slabs as well as blocks-in-mélange. Some of the latter may record earlier eclogite burial, exhumation to the surface, sedimentation, and resubduction to eclogite-facies conditions. Syneclogitic D0 fabrics include widely distributed granoblastic fabrics, as well as fabrics defined by planar and linear preferred orientations. These eclogitic fabrics collectively indicate strain localization along the subduction interface at the depth of eclogite metamorphism (~50–80 km). Elongate bodies of metamorphosed pelagic sediments associated with mafic rocks and trench-fill turbidites show that coherent imbricates and duplexes with subordinate mélange characterized the original subduction complex architecture of the Sambagawa belt. Eclogite-facies metamorphism spans a range of ages that may define discrete pulses at ca. 120–110 Ma and ca. 90 Ma or more temporally intermediate subduction-accretion events associated with an extended period of subduction. D1 exhumation fabrics exhibit a west-vergent sense of shear antithetic to the rarely preserved east-vergent early (shallow) subduction fabrics (D-1). These early fabrics may have been rotated since their development. D1 fabrics are overprinted by south-vergent D2 brittle and brittle-ductile structures associated with an internal extrusional wedge that was subsequently cut by a major out-of-sequence fault, duplexed, and folded. Exhumation of the eclogite to the depth of the M1 overprint (0.5–1.5 GPa pressure difference between M0 and M1) may have taken place as extruded slabs accommodated by D1 penetrative shear in multiple events, whereas some blocks may have been exhumed early in Sambagawa history in serpentinite diapirs through the forearc mantle wedge or in serpentinite mélange along the subduction interface. The earliest eclogite metamorphism may have taken place shortly after initiation of a new subduction zone in nascent arc crust.

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