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.

A geologic map represents the melding of field observations with various types of analytical data and earth science concepts. Choosing features to be portrayed is a reflection of the questions posed. Some would claim that in the mapping process, theory meets reality. However, a map is a more subjective product based on the sum of the geologist's prior training, aggregate field experience, and the stage of development of scientific concepts, the complexity of the mapped units, the extent and quality of exposures, the wealth of constraining ancillary data, and the time and thought expended in the mapping. The published map also reflects accommodations to the scientific reviewers' knowledge, and to technical compromises required by the printer-publisher. Because mapping style depends on a geologist's prior experience, it is necessarily a somewhat idiosyncratic process. My own field research has focused chiefly on the petrologic-structural development of Mesozoic and younger contractional orogenic belts, and through them, the tectonic evolution of continental margins. Mapping has been an essential step enhancing my understanding of processes that have shaped convergent portions of Earth's crust. (1) For instance, field relations combined with mineralogic analysis in the Panoche Pass area, southern Diablo Range, central California, indicated relatively high pressure–low temperature recrystallization and postmetamorphic, low-angle faulting of the Franciscan Complex. (2) Mapping of a similar Franciscan terrane in the central Diablo Range identified imbricate, subhorizontal, syn- to postmetamorphic bedding-plane thrust faults and implied accretionary growth in the Pacheco Pass quadrangle. (3) Field study of a structural inversion of high-grade metamorphic rocks tectonically overlying low-grade equivalents in the Sanbagawa belt, central Shikoku, Japan, led to the interpretation of postrecrystallization, ductile nappe emplacement and, as in California and the Western and Eastern Alps, (4) a progressive, relatively high P –low T metamorphism-exhumation subduction-zone model. (5) Mapping the interstratified distal turbidites and mafic lavas, and the discovery of pillow tops in the Sawyers Bar area, documented in situ stages of oceanic-island arc development in the North Fork terrane, central Klamath Mountains, northwestern California. Bulk-rock compositions of interlayered ocean-island basalts and island-arc tholeiites supported this interpretation. (6) Detailed geologic mapping combined with remote sensing in the central White Mountains, easternmost California, demonstrated that the Middle Jurassic Barcroft granodioritic complex is a steeply southeast-dipping slab that intruded previously deformed mid-Mesozoic arc volcanic rocks and Neoproterozoic–Lower Cambrian platform strata along a high-angle reverse fault. Conclusions derived from these studies, as well as more general plate-tectonic syntheses, depended on geologic mapping, and, for me, the field mapping was an enjoyable and scientifically fulfilling experience.