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.

The Franciscan complex of California provides the ideal field laboratory to examine the rock record of subduction. This field trip guide describes a two-day field trip of the 2013 Geological Society of America Cordilleran Section Meeting. The field stops include a stop along the Panoche Road in the southern Diablo Range, and four in the San Francisco Bay region: one on the southern margin of the California State University East Bay campus in the Hayward Hills; one, with additional optional stops, at El Cerrito quarry north of Berkeley; one at Ring Mountain on Tiburon Peninsula; and the final stop at Rodeo Cove of the Marin Headlands. The geology seen at these stops provides insight into subduction initiation processes, subduction accretion and subduction erosion, accretion of ocean plate stratigraphy, mélange evolution, multiple burial-exposure cycles, exhumation of high-pressure metamorphic rocks, localization of subduction megathrust slip, large-scale subduction complex architecture, and spreading ridge deformation. The field trip stops will captivate visitors with their scenic beauty as well as their interesting geology.

Serpentinite matrix mélange represents a significant, if less common, component of many accretionary complexes. There are two principal hypotheses for the origin of serpentinite mélange: (1) formation on the seafloor in a fracture zone–transform fault setting, and (2) formation within a subduction zone with mixing of rocks derived from both the upper and lower plates. The first hypothesis requires that the sheared serpentinite matrix be derived from hydrated abyssal peridotites and that the block assemblage consist exclusively of oceanic rocks (abyssal peridotites, oceanic basalts, and pelagic sediments). The second hypothesis implies that the sheared serpentinite matrix is derived from hydrated refractory peridotites with supra-subduction zone affinities, and that the block assemblage includes rocks derived from both the upper plate (forearc peridotites, arc volcanics, sediments) and the lower plate (abyssal peridotites, oceanic basalts, pelagic sediments). In either case, serpentinite mélange may include true mélange, with exotic blocks derived from other sources, and serpentinite broken formation, where the blocks are massive peridotite. The Tehama-Colusa serpentinite mélange underlies the Coast Range ophiolite in northern California and separates it from high-pressure/temperature (P/T) metamorphic rocks of the Franciscan complex. It has been interpreted both as an accreted fracture zone terrane and as a subduction-derived mélange belt. Our data show that the mélange matrix represents hydrated refractory peridotites with forearc affinities, and that blocks within the mélange consist largely of upper plate lithologies (refractory forearc harzburgite, arc volcanics, arc-derived sediments, and chert with Coast Range ophiolite biostratigraphy). Lower plate blocks within the mélange include oceanic basalts and chert with rare blueschist and amphibolite. Hornblendes from three amphibolite blocks that crop out in serpentinite mélange and sedimentary serpentinite yield 40Ar/39Ar plateau ages of 165.6–167.5 Ma, similar to published ages of high-grade blocks within the Franciscan complex and to crystallization ages in the Coast Range ophiolite. Other blocks have uncertain provenance. It has been shown that peridotite blocks within the mélange have low pyroxene equilibration temperatures that are consistent with formation in a fracture zone setting. However, the current mélange reflects largely upper-plate lithologies in both its matrix and its constituent blocks. We propose that the proto-Franciscan subduction zone nucleated on a large offset transform fault–fracture zone that evolved into a subduction zone mélange complex. Mélange matrix was formed by the hydration and volume expansion of refractory forearc peridotite, followed by subsequent shear deformation. Mélange blocks were formed largely by the breakup of upper plate crust and lithosphere, with minor offscraping and incorporation of lower plate crust. We propose that the methods discussed here can be applied to serpentinite matrix mélange worldwide in order to understand better the tectonic evolution of the orogens in which they occur.

The Kings-Kaweah ophiolite belt of the southwestern Sierra Nevada Foothills was generated in two pulses of mid-oceanic-ridge basalt (MORB) magmatism. The first was in the Early Ordovician, which resulted in the generation of a complete abyssal crust and upper mantle section. The crustal section was rendered from convecting mantle whose Nd, Sr, and Pb isotopic systematics lie at the extreme end of the sub-Pacific mantle regime in terms of time integrated depletions of large ion lithophile (LIL) elements. Semi-intact fragments of this Early Ordovician oceanic lithosphere sequence constitute the Kings River ophiolite. Following ~190 m.y. of residence in the Panthalassa abyssal realm, a second pulse of MORB magmatism invaded the Early Ordovician lithosphere sequence in conjunction with intensive ductile shearing and the development of ocean floor mélange. This Permo-Carboniferous magmatic and deformational regime produced many of the essential features observed along spreading ridge–large-offset transform fracture zones of the modern ocean basins. During this regime, Early Ordovician upper mantle–lower crustal rocks were deformed in the ductile regime along what appears to have been an oceanic metamorphic core complex, as well as along steeply dipping strike-slip ductile shear zones that broke the ophiolite into semi-intact slabs. Progressive deformation led to the development of serpentinite-matrix ophiolitic mélange within the abyssal realm. This (Kaweah) serpentinite mélange constitutes the majority of the ophiolite belt and encases fragments of both disrupted Early Ordovician oceanic lithosphere and crustal igneous-metamorphic assemblages that were deformed and disrupted as they formed by diffuse spreading along the fracture zone. An ~190 m.y. hiatus in abyssal magmatism cannot be readily accommodated in the current configuration of Earth's ocean basins, but it was possible during the mid- to late Paleozoic Panthalassa regime, when the proto-Pacific basin occupied over half of the Earth's surface. The transform history of the ophiolite belt can be directly linked to the late Paleozoic transform truncation of the SW Cordilleran passive margin. Following juxtaposition of the transform ophiolite belt with the truncated margin a change in relative plate motions led to the inception of east-dipping subduction, and the en masse accretion of the ophiolite belt to the hanging wall of the newly established subduction zone. Structural relations and isotopic data on superimposed igneous suites show that the ophiolite belt was not obducted onto the SW Cordilleran continental margin. The accreted ophiolite belt formed the proto-forearc of the newly established active margin. The ophiolite belt never saw high-pressure/temperature (P/T) metamorphic conditions. Rare small blocks of high-pressure metamorphic rocks were entrained from the young subduction zone by serpentinite diapirs and emplaced upward into the ophiolitic mélange within a proto-forearc environment. An Sm/Nd garnet-matrix age on a high-pressure garnet amphibolite block suggests subduction initiation at ca. 255 Ma. This timing corresponds well with the initiation of arc magmatism along the eastern Sierra Nevada region. In Late Triassic to Early Jurassic time proximal submarine mafic eruptions spread across and mingled with hemipelagic and distal volcaniclastic strata that were accumulating above the accreted ophiolite belt. These lavas carry boninitic to arc tholeiitic and primitive calc-alkaline geochemical signatures. By Middle Jurassic time siliciclastic turbidites derived from early Paleozoic passive margin strata and early Mesozoic arc rocks spread across the primitive forearc. In late Middle to Late Jurassic time tabular plutons and dike swarms of calc-alkaline character invaded the ophiolite belt in a transtensional setting. Deformation fabrics that developed in these intrusives, as well as cleavage that developed in the cover strata for the ophiolite belt, imparted components of superimposed finite strain on the ophiolitic mélange structure but did not contribute significantly to mélange mixing. By ca. 125 Ma, copious gabbroic to tonalitic plutonism of the western zone of the Cretaceous Sierra Nevada batholith intruded the ophiolite belt and imparted regional contact metamorphism. Such metamorphism variably disturbed U/Pb systematics in rare felsic intrusives of the ophiolite belt but did not significantly disturb whole rock Sm/Nd systematics. Age constraints gained from the Sm/Nd and U/Pb data in conjunction with Nd, Sr, and Pb isotopic and trace element data clearly define the polygenetic abyssal magmatic history of the ophiolite belt. The variation of Nd and Sr radiogenic isotopes over time from the Paleozoic abyssal assemblages, through early Mesozoic supra-subduction zone volcanism to Early Cretaceous batholithic magmatism, record the geochemical maturation of the underlying mantle wedge without the involvement of SW Cordilleran continental basement.

Narrow, discontinuous bands of high-grade subophiolitic metamorphic rocks, comprising predominantly amphibolite facies metabasites with rare metasediments, are observed at the contact between the complexes and subjacent mélanges of the Mesohellenic Ophiolite exposed in northwestern Greece. Both conventional and pseudosection thermobarometry have been used to yield estimated peak pressure-temperature (P-T) conditions of these tectonic sheets. Toward the leading edge of the ophiolite, subophiolitic rocks of the Vourinos Complex record peak metamorphic temperatures of 770 ± 100 °C. Pressures of 4 ± 1 kbar beneath the Vourinos are estimated on the basis of hornblende composition and are similar to the expected pressures from ophiolitic overburden. Beneath the exposed Dramala Complex, at the trailing edge of the ophiolitic body southwest of the Vourinos, estimated temperatures reached 800 ± 40 °C and 12.00 ± 1.27 kbar at the top of an apparent inverted metamorphic gradient imposed by discrete phases of accretion. High pressure assemblages beneath ophiolitic bodies imply exhumation relative to the overlying ophiolite. Estimated homologous temperatures in the upper plate are similar to those inferred for channeled exhumation during continental collision. Mineral assemblages lower in the Dramala sole indicate reduced temperatures and peak pressures. Similar pressures obtained within lower temperature sole rocks beneath Vourinos and Pindos suggest that a shallowly dipping thrust may have been responsible for obduction. Peak temperatures and pressures are in agreement with those estimated for secondary thrust propagation beneath a proto-arc after subduction in an intra-oceanic setting.

We investigated the field relations, metamorphic and deformation conditions, age, and chemistry of basaltic, plutonic, and metamorphic blocks in the Mineoka ophiolite mélange belt, Boso Peninsula, central Japan, to clarify their emplacement mechanisms. We considered internal and external deformation of the blocks in the context of the complicated processes by which the ophiolite mélange belt was formed in a forearc setting. A two-stage history leading to the present-day forearc sliver fault zone was revealed: an early stage of deep ductile deformation followed by an episode of brittle deformation at shallower levels. Both stages were the result of transpressional stress conditions. The first stage produced subduction-related schistosity with microfolding and mylonitization and then brecciation during exhumation in the intraoceanic subduction zone, from a maximum depth of garnet-amphibolite facies or eclogitic facies. The second stage was characterized by strong, brittle shear deformation as the rocks were incorporated into the present-day fault zone. The first incorporation of the oceanic plate to the side of the Honshu arc might have occurred during the Miocene, and was followed by right-lateral oblique subduction that has continued ever since the Boso triple junction arrived at its present-day position, thus forming the paleo-Sagami trough plate boundary.

The classic mélanges of the Franciscan Complex of California comprise a variety of structural-tectonic settings and give insight into mélange forming processes and material movement patterns within subducting plate margins. Structural settings of mélanges include (1) shale matrix mélanges that occur within, cutting, and bounding coherent nappes, and (2) serpentinite matrix mélanges that occur at the structurally highest levels and commonly cut blueschist and higher grade coherent nappes. Although nappe-bounding zones may have accommodated tens of kilometers or more of movement and represent paleosubduction megathrust zones, exposures at El Cerrito Quarry in the eastern San Francisco Bay area suggest that most displacement is accommodated in a narrow zone (5 m wide in this example) of brittle faults between the nappe-bounding mélange and the coherent nappes. Intranappe mélanges, or their boundaries, accommodated smaller displacements owing to similar or identical units bounding them. Blueschist-facies sedimentary breccias present in the northwest Diablo Range range from nearly undeformed to strongly foliated rocks that appear to be similar or identical to classic foliated shale mélange matrix. Most of the breccia clasts exhibit blueschist-facies metamorphic mineral growth that predated sedimentation, indicating exhumation of source material from blueschist depths, followed by deposition and resubduction to blueschist depth. Published apatite fission-track data suggest that the source of the clasts may have been completely eroded since exhumation. Blueschist-facies sedimentary breccia on one side of a high-grade blueschist block within serpentinite-matrix mélange in southern Sonoma County suggests that the mélange originated as sedimentary serpentinite, following exhumation of blueschist and higher grade rocks early in Franciscan subduction history, prior to deposition of metaclastic rocks (absent as blocks in the mélange). The strongly foliated serpentinite matrix apparently recrystallized during resubduction to blueschist-facies depth, erasing sedimentary textures, in contrast to the unstrained and unmetamorphosed sedimentary serpentinite in the basal Great Valley Group that may be a temporal equivalent. These relationships suggest an early forearc history in which exhumed serpentinite and associated high-grade blocks were shed into the trench and into what later became the forearc basin, prior to forearc high development and significant clastic sedimentation. The occurrence of high-grade metamorphic blocks in structurally low mélanges, as well as their preferential localization along the boundaries of mélanges, also suggests sedimentary mixing. Thus, many Franciscan mélanges, including the nappe-bounding mélanges that might be expected to have accommodated large displacement, show evidence of early sedimentary mixing. Most of the block-in-matrix fabric and introduction of exotic blocks may have resulted from sedimentary processes rather than tectonic strain. In contrast, some mélanges that cut across the bedding or foliation of coherent units apparently had a diapiric rather than a sedimentary origin.

Structural field studies and geochemical and age analyses of the Eldivan ophiolite, which is dismembered within the Ankara Mélange, indicates that it developed as a supra-subduction zone basin within the İzmir-Ankara-Erzincan Ocean, which later subducted to form the İzmir-Ankara-Erzincan suture zone through continental block collision. Whole-rock and mineral geochemical evidence show a supra-subduction zone tectonomagmatic affinity for the ophiolitic crust and mantle, revealing that this basin formed in the upper plate of an intra-oceanic subduction zone. Structural restoration of the sheeted dike complex reveals that the supra-subduction zone spreading ridge of the Eldivan ophiolite was nearly parallel to the Sakarya-Pontide continental margin. U/Pb age analyses of detrital zircon in sandstone within the mélange and in the unconformably overlying Karadağ Formation indicate maximum depositional ages for the units of 143.2 ±2 Ma, and 105.2 ±5 Ma, respectively. Thus, thrust imbrication of the ophiolite and the development of serpentinite mélange were mostly complete by 105 Ma, as indicated by an angular unconformity between the ophiolitic units and the overlying Karadağ Formation. These results reveal how and when the Eldivan ophiolite was constructed, destructed, and incorporated into the serpentinite Ankara Mélange and İzmir-Ankara-Erzincan suture zone. The tectonic evolution of the İzmir-Ankara-Erzincan Ocean is similar to that of the Philippine Sea and Banda Sea ocean basins.

The King Ridge Road mélange is a unit of the Franciscan Complex, cropping out in an area of at least 50 km2 around the town of Cazadero, coastal California. This unit is an olistostrome with a massive, unfoliated sandstone matrix, containing >232 large meta-igneous and chert blocks of greatly varying size, lithology, and metamorphic history within the study area. This sandstone matrix is litharenite or arkosic arenite and exhibits prograde prehnite-pumpellyite facies and retrograde zeolite facies metamorphism. It is devoid of megascopic textures except for rare simple bedding. No fossils have been found, and no Bouma units or other graded beds are present. Detrital zircon geochronology has established the maximum age of deposition of the sandstone matrix at 83 Ma, whereas apatite fission-track data indicate cooling of the olistostrome below 100 °C at ca. 35–38 Ma. The 232 exotic blocks sampled in the study area are dominantly low- to medium-grade greenstones and cherts, together with fewer high-grade blocks partly composed of blue amphibole and/or omphacitic pyroxene, and some amphibolites. Thus, many of the blocks have higher grade metamorphic assemblages than the matrix. All block types are well mixed together, so none greatly predominate anywhere. Blocks of oceanic-island-arc plutonic rocks, including granitoids and recemented breccias, are particularly distinctive for this mélange. One granitoid block has a zircon U-Pb age of 165 ± 1 Ma. The massive sandy matrix of the olistostrome formed by accumulation of hyperconcentrated sedimentary density flows (grain flows) sourced primarily from the Klamath-Sierra continental magmatic arc. Many of the blocks record a pre-mélange history of metamorphism and exhumation, followed by partial subduction and reburial with the matrix after 83 Ma. Cooling below 100 °C took place at 35–38 Ma, probably associated with partial exhumation of the unit, with subsequent removal of ~10 km of cover.

Mélanges represent a significant part of the Miocene Nabae accretionary complex. Such mélanges show sheath folds with D1 axial plane pressure-solution cleavage, whereas the coherent unit shows asymmetric folding with D1 slaty cleavage. In addition, the mélanges are characterized by D1 asymmetric shearing, which includes both thrust and right-lateral-sense components, in contrast to D1 pure shear that characterizes the coherent unit. Thus, this tectonic style acted on the climax of prism development can be referred to as a tectonic mélange. However, because the D1 shear displacement is almost negligible, D0 normal faults and basaltic dikes, operated when matrix sediments were not consolidated, are not disrupted. Oceanic materials such as basalt and chert cannot be incorporated into terrigenous matrix, given the small displacement associated with the D1 shearing. Exotic blocks of chert and sandstone show D minus 1 (D–1) cleavage, which is apparent in the older, probably Eocene, accretionary prism. When this prism was exhumed, it supplied debris to the Miocene trench, and then underwent additional D1 deformation, which included the above asymmetric shearing. This sedimentary and two-way-street tectonic process was recycled within the prism as the latter developed. Thus, as the block-in-matrix fabric was originally sedimentary and labeled D0, the tectonic mélange process that forms block-in-matrix fabric is only conjectural for the Nabae complex. Also it is suggested that these deformations are not progressive nor distinctive for each other.

Two types of thrust duplex structures were identified in excellent exposures of the deep level of the Jurassic to Cretaceous accretionary complex in the Kanto Mountains, central Japan, and the thickening ratio and shortening ratio were calculated. Simple (S type) and composite (C type) duplexes are mapped in an excavation site 100 × 40 m in extent. The beds of the S type and C type duplexes were thickened by factors of 5.8 and 6.0, respectively; however, the C type duplex includes four orders of smaller duplexes within it that underwent their own shortening. Thus the total thickening factor may attain at least 6–13, indicating a comparable degree of thickening at the level of greenschist facies conditions (approximately10 km or more in depth) in the accretionary prism.

The Messinian mélange of the Tertiary Piedmont Basin is the product of different but interrelated processes (tectonic, gravitational, and diapiric) that operated sequentially over a short time span (intra-Messinian time) and in a geodynamic environment (episutural basin) for which mélanges have so far been poorly described. It is composed of different mappable bodies of (non-metamorphic) mixed rocks characterized by a strong facies convergence. Their geometric and stratigraphic position, the internal organization, and the nature of the bounding surfaces allow the defining of some criteria to distinguish different units of mixed rocks (tectonically disrupted unit, gravity-driven sedimentary unit, and diapiric disrupted unit), in each of which the role of a different prevailing mélange-forming process can be inferred. None of these processes operated in isolation. They were linked by complex and intimate mutual interactions and triggered by intra-Messinian tectonics. The latter produced self- generating processes of mélange formation in which gravitational and diapiric processes triggered and affected each other. Different pulses of overpressured fluids (often rich in methane) strongly governed sediment deformation and also played a crucial role in influencing the time relationships and causative links between the different mélange-forming processes. Faulting may have triggered gas hydrate dissociation, promoting the upward rise of overpressured fluids. These fluids reduced the shear strength of the overlying sediments, promoting large-scale gravity-driven phenomena. Loading provided by rapid emplacement of the gravity-driven sedimentary bodies could have, in turn, developed new overpressured conditions necessary to promote the upward rise of poorly consolidated sediments and shale diapirism.

The Pliocene-Pleistocene Chikura Group, southern tip of the Boso Peninsula, central Japan, occurs northeast of the present Sagami trough of the Philippine Sea plate subduction boundary. This group has many bedding-parallel shortening structures, including thrust-anticlines, duplexes, and small-scale conjugate sets of thrusts in addition to various kinds of chaotic deposits. The group forms one large synclinorium with smaller scale folds, but its relationship to accretionary prism evolution has not been explained. On the basis of geological structures examined on uplifted coastal benches, we propose that the lower half of the group was deposited on the subduction plate boundary as trench fill. When the trench was filled, the frontal thrust jumped seaward, causing landward tilting of the earlier trench fill deposits, after which the upper part of the group was deposited in a slope basin setting. The key observation to unravel the sedimentation and deformation is the recognition of the chaotic deposits, specifically whether they have a methane-bearing, fluid-supported chemosynthetic biocommunity (Calyptogena) and calcite-cemented sediments (chimneys or pipes). The chaotic deposits that bear such methane-related materials suggest that the deposition has occurred on the thrust at the landward slope foot, and that the emplacement or depositional mechanism is either as a debris flow or an injection (diapir). As a result, it is concluded that at least the lower half of the Chikura Group is a kind of accretionary prism of the trench-fill type, similar to the Sagami Basin at the present time. We conclude that the lower half of the Chikura Group records accretionary prism development in a trench-fill environment, similar to the present day Sagami Basin.

Thin, planar, dark, lamination-like bands are found in host siltstones in the Miocene-Pliocene metamorphosed Miura-Boso accretionary prism, southern Boso Peninsula, Japan. We classified the bands into four types on the basis of distribution, crosscutting relations, and internal textures. Type 1-1 dark bands are developed parallel to the bedding plane and do not include crushed or deformed grains within the band. Type 1-2 bands are also developed parallel to the bedding plane, but grain alignment within the band cuts obliquely across that in the host rock. Type 2 bands include ductilely deformed grains similar to an S-C′ structure, whereas type 3 bands have cataclastic grains. All the dark bands except type 1-1 (being an open fracture with little displacement) are shear bands or slip planes formed from sedimentation to accretion, although the formation mechanisms between the four types are different. These deformation bands are affected by the state of consolidation and magnitude of stress during formation, reflecting the deformation processes. Type 1-1 bands show evidence of independent particulate flow from excess pore-fluid-pressure generation, which occurs just after sedimentation. Type 1-2 bands are flexural-slip faults formed during formation of folds; type 2 bands are sliding planes formed from submarine landslides, whereas type 3 bands are thrust faults formed during accretion.

Although mélanges are exciting, puzzling, and controversial to geologists, it is geopractitioners and contractors who must work with them to engineer the constructed works of Society. Geopractitioners include geotechnical engineers, geological engineers, engineering geologists, and rock engineers. Mélanges are the most intractable bimrocks (block-in-matrix rocks), complex geological mixtures composed of hard blocks of rocks surrounded by weaker matrix, and are famously exemplified by those within the Franciscan Complex of Northern California. Bimrocks also include olistostromes, weathered rocks, fault rocks, and lahars. The conventional characterization, design, and construction procedures used by geopractitioners for well-behaved stratified rocks and soils are not well suited to mélanges. The considerable engineering and construction difficulties related to mélanges burden Society to the extent that they can be considered “antisocial.” Case histories exemplify a recommended systematic procedure for characterization, design, and construction with mélanges. Geopractitioner approaches to characterizing California's chaotic Franciscan mélanges are applicable to geologists and geopractitioners working in fault zones, weathered rocks, lahars, and other bimrocks, and suggestions are offered for collaborative research between geologists and geopractitioners.

Ophiolites have long been recognized as on-land fragments of fossil oceanic lithosphere, which becomes an ophiolite when incorporated into continental margins through a complex process known as ‘emplacement’. A fundamental problem of ophiolite emplacement is how dense oceanic crust becomes emplaced over less dense material(s) of continental margins or subduction-accretion systems. Subduction of less dense material beneath a future ophiolite is necessary to overcome the adverse density contrast. The relationship of subduction to ophiolite emplacement is a critical link between ophiolites and their role in the development of orogenic belts. Although ophiolite emplacement mechanisms are clearly varied, most existing models and definitions of emplacement concern a specific type of ophiolite (i.e. Oman or Troodos) and do not apply to many of the world’s ophiolites. We have defined four prototype ophiolites based on different emplacement mechanisms: (1) ‘Tethyan’ ophiolites, emplaced over passive continental margins or microcontinents as a result of collisional events; (2) ‘Cordilleran’ ophiolites progressively emplaced over subduction complexes through accretionary processes; (3) ‘ridge-trench intersection’ (RTI) ophiolites emplaced through complex processes resulting from the interaction between a spreading ridge and a subduction zone; (4) the unique Macquarie Island ophiolite, which has been subaerially exposed as a result of a change in plate boundary configuration along a mid-ocean ridge system. Protracted evolutionary history of some ocean basins, and variation along the strike of subduction zones may result in more complicated scenarios in ophiolite emplacement mechanisms. No single definition of emplacement is free of drawbacks; however, we can consider the inception of subduction, thrusting over a continental margin or subduction complex, and subaerial exposure as critical individual stages in ophiolite emplacement. AbstractOphiolites have long been recognized as on-land fragments of fossil oceanic lithosphere, which becomes an ophiolite when incorporated into continental margins through a complex process known as ‘emplacement’. A fundamental problem of ophiolite emplacement is how dense oceanic crust becomes emplaced over less dense material(s) of continental margins or subduction-accretion systems. Subduction of less dense material beneath a future ophiolite is necessary to overcome the adverse density contrast. The relationship of subduction to ophiolite emplacement is a critical link between ophiolites and their role in the development of orogenic belts. Although ophiolite emplacement mechanisms are clearly varied, most existing models and definitions of emplacement concern a specific type of ophiolite (i.e. Oman or Troodos) and do not apply to many of the world’s ophiolites. We have defined four prototype ophiolites based on different emplacement mechanisms: (1) ‘Tethyan’ ophiolites, emplaced over passive continental margins or microcontinents as a result of collisional events; (2) ‘Cordilleran’ ophiolites progressively emplaced over subduction complexes through accretionary processes; (3) ‘ridge-trench intersection’ (RTI) ophiolites emplaced through complex processes resulting from the interaction between a spreading ridge and a subduction zone; (4) the unique Macquarie Island ophiolite, which has been subaerially exposed as a result of a change in plate boundary configuration along a mid-ocean ridge system. Protracted evolutionary history of some ocean basins, and variation along the strike of subduction zones may result in more complicated scenarios in ophiolite emplacement mechanisms. No single definition of emplacement is free of drawbacks; however, we can consider the inception of subduction, thrusting over a continental margin or subduction complex, and subaerial exposure as critical individual stages in ophiolite emplacement.