Volatiles in Magmas

Paul J. Wallace , ... Erik H. Hauri , in The Encyclopedia of Volcanoes (Second Edition), 2015

5.4 Backarc Basin Basalts

Basaltic magmas from backarc basins show wide variations in H 2O concentration, ranging from values as low as those in the most H2O-poor N-MORB (0.1   wt%) to values as high as 2.3   wt% (Figure 7.9). Such variations reflect the range of bulk chemical composition of backarc basin basalts, which can vary from MORB-like to showing many similarities to island arc volcanics, reflecting their formation in spreading centers that overlie subduction zones. Dissolved CO2 in submarine basaltic glass samples from the Mariana Trough are between 100 and 200   ppm. Negative covariation between H2O and CO2 in these glasses and pressure estimates based on experimental solubility data suggest that the samples were vapor saturated at their depth of eruption and quenching, consistent with the presence of vesicles in the glassy pillow rims. Therefore, the bulk magmatic concentrations of H2O and CO2 may be significantly greater than the dissolved concentrations. Chlorine concentrations vary from 80   ppm to nearly 1   wt% and show a positive correlation with H2O in comagmatic suites of glasses. Glasses with the higher values (>2000   ppm) as well as many with values in the range from 200 to 2000   ppm also have high Cl/K2O and therefore appear to have been affected by assimilation of chlorine-rich altered oceanic crust at shallow levels. By contrast, other glasses in the range from 80 to 2000   ppm with a very low Cl/K2O likely reflect variably amounts of subduction-related enrichment from the underlying slab into the mantle wedge.

Some H2O-rich basaltic magmas erupted in backarc basins or the submarine portions of island arcs have a high-enough water concentration that they vesiculate and degas dissolved H2O even during eruption in very deep water. These glasses are distinguished by their relatively high vesicularities (as much as 50% vesicles by volume). As expected, they also have anomalously low concentrations of volatile components, such as S, that exsolve from magma together with H2O.

Measurements of H2O content in backarc basin glasses generally show negative correlations with TiO2, which, as an incompatible element during mantle melting, can be used as a proxy for the extent of melting. The correlation therefore shows that the degree of mantle melting is controlled in part by how much H2O is added to the mantle from the underlying subducted plate. The H2O concentrations estimated to be in the mantle beneath backarc basins, based on these correlations, suggest that mantle H2O contents increase toward the trench, as would be predicted from the presence of a subducted slab that progressively dehydrates with depth. Backarc spreading segments that lie above the mantle with the highest estimated H2O concentrations are at anomalously shallow water depths, probably because the high H2O increases mantle melt productivity and therefore results in a greater thickness of backarc basin crust. These interpretations have been somewhat controversial, however, and an alternative possibility is that the negative correlations between H2O and TiO2 are the result of mixing of magmas from H2O-poor mantle, similar to that beneath midocean ridges, and subduction-modified mantle, like that which is beneath island arcs.

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East Sea

Sung Kwun Chough , in Geology and Sedimentology of the Korean Peninsula, 2013

12.1 Introduction

The East Sea is a semienclosed backarc basin surrounded by the Asian continent and the Japanese islands. It consists of three deep basins separated by submarine topographic highs such as the Korea Plateau, Oki Bank, and the Yamato Ridge ( Figure 12.1). The topographic highs are made of relatively shallow-seated acoustic basements of rifted continental crust and the overlying thin sedimentary succession, in contrast to the adjacent deep basins of extended continental crust (Tamaki, 1988; Chough et al., 2000) (Figure 12.1). The continental backarc basins formed by multiaxial extensional opening since the Oligocene (Chough and Barg, 1987; Tamaki, 1988).

Figure 12.1. Crust types of the East Sea. Contours of water depth in meters.

 Source: Tamaki (1988).

The southwestern part of the sea has recently been under particular attention, from which an array of both multichannel deep-seismic reflection profiles and high-resolution subbottom profiles (Chirp) was obtained (Figures 12.2 and 12.3) ( Lee et al., 2002, 2004 Lee et al., 2002 Lee et al., 2004 ; Bahk et al., 2005; Kwon et al., 2009; Kim et al., 2011). The geology of the eastern continental margin and the Ulleung Basin was summarized at the turn of the century in Marine Geology of Korean Seas (Chough et al., 2000). In this volume, I focus on the new findings on the South Korea Plateau and the Ulleung Interplain Gap.

Figure 12.2. Bathymetric map of the Ulleung Basin and the southern part of the Korea Plateau. Contour interval is 100   m. NUE=North Ulleung Escarpment.

 Source: Courtesy of the National Oceanographic Research Institute.

Figure 12.3. Tracklines of multichannel seismic profiles (A) by the KIGAM, and Chirp sonar survey (B) by the National Oceanographic Research Institute. WSKP=Western South Korea Plateau, ESKP=Eastern South Korea Plateau, UB=Ulleung Basin, UI=Ulleung Island, US=Ulleung Seamout, DI=Dok Island, OB=Oki Bank. Contours in meters.

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Summary

Sung Kwun Chough , in Geology and Sedimentology of the Korean Peninsula, 2013

In the Paleogene, both the volcanic arc and the backarc basin ceased to develop, as volcanic activities shifted eastward, accompanied by rollback of the subduction of the Pacific plate. In the Miocene, pull-apart (right-lateral) basins formed in the eastern continental margin, including the Pohang and Ulleung basins and the associated backarc basins (Youngduk and Hupo basins). The Korea Plateau experienced continental rifting accompanied by extensive volcanism during the extensional opening of the Ulleung Basin. It subsided more than 1000  m below sea level. The Ulleung Interplain Gap represents an extensional opening of the crust between the South Korea Plateau and the Oki Bank.

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Seas of Southeast Asia

J.T. Potemra , T. Qu , in Encyclopedia of Ocean Sciences (Second Edition), 2009

The boundary between the western Pacific Ocean and the Asian continent consists of numerous backarc basins (also called marginal seas), islands, and straits. This region, known collectively as the Southeast Asian Seas, is an extremely dynamic and complex region from the viewpoint of ocean circulation and climate. The region is under the direct influence of seasonally reversing monsoon winds, and ocean circulation and resulting changes in sea surface temperatures in the region may in turn contribute to monsoon variability. The region also provides a low-latitude link between the Pacific and Indian Oceans, thus enabling a warm-water pathway for the so-called global ocean conveyor belt. The various seas and connecting straits are subject to different dynamics, and thus the circulation patterns are different from basin to basin as well as from the ocean surface to the seafloor. In this article, a brief description of the region is given, followed by a general picture of ocean properties and surface currents for the main seas. The connection between the Pacific and Indian Oceans, the Indonesian throughflow, is then described in more detail. Finally, some other considerations and monitoring efforts are given.

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Deep-Sea Sediments

Steven N. Carey , Jean-Luc Schneider , in Developments in Sedimentology, 2011

6.1 Comparison of volcaniclastic aprons

Two of the most significant deep-water volcaniclastic depositional areas are found adjacent to large oceanic islands, such as Hawaii, and in backarc basins bordering active oceanic island arcs. Menard (1956) recognized the volumetric importance of volcaniclastic aprons in relation to large oceanic islands, but it is clear that island arcs create substantial deep-water accumulations as well. Facies associations on sub-marine volcaniclastic aprons are complex and correspond to primary volcanic and volcaniclastic, reworked volcaniclastic, and volcanic epiclastic and non-volcanic products. Inter-stratified non-volcanic, marine sediments (often fossiliferous) are very useful to establish age controls of the apron pile. The architecture of the facies associations is governed by magmatic productivity, and the nature of the transport and sedimentation processes. Non-volcanic processes, such as erosion, that occur sub-aerially on the volcanic source also play a role in sub-marine apron sedimentation (Fisher and Schmincke, 1994; Schneider et al., 1998). The term 'volcaniclastic apron' is used generally for successions in both environments, but there are likely to be important differences between oceanic islands and island arcs. First, oceanic islands are considerably larger than individual arc volcanoes and thus a single oceanic island will likely feed a larger deep-water apron. However, the size of individual eruptions at subduction zones can be much larger than at oceanic islands, and thus individual primary volcaniclastic-sedimentation events recorded in arc aprons may greatly surpass those at oceanic islands. Second, volcaniclastic aprons in backarc basins are generally more elongated in geometry and fed by multiple sources compared to oceanic island aprons that are more circular in shape. Third, volcanic arcs are characterised by more explosive eruptions, and thus the sediments are more highly fragmented and finer grained than those of oceanic islands. And finally, the diversity in magmatic compositions, and thus products, is significantly greater in subduction zones than in oceanic islands.

Despite these differences, there are features that both share in common. In the proximal parts of the aprons, the steep slopes display a seismically chaotic facies. These slopes are dominated by coarse deposits, and are generally incised by numerous gullies and canyons. Volcaniclastic turbidites related to the discharge of volcaniclastic gravity flows or triggered from delta slopes are inter-calated in the sedimentary succession (Carey and Sigurdsson, 1984; Ollier et al., 1998; Schmincke et al., 1995). Proximal debris-avalanche deposits are related to sub-aerial collapse events, but can also originate on the apron slopes. Numerous primary volcanic and volcaniclastic deposits are frequent in proximal volcaniclastic aprons. More distally, usually over 1500   m in depth, the facies become more organized and stratified. Volcaniclastic turbidites fed from sub-marine canyons and fallout deposits dominate. They are inter-stratified with the normal marine sediments that form the background sedimentation. Distal debris-avalanche deposits are also present in these distal deep-water parts of the aprons. The mean grain size of the deposits in distal aprons is finer than in proximal areas.

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Volume 3

Yong-Fei Zheng , in Encyclopedia of Geology (Second Edition), 2021

Oceanic-Oceanic Convergent Boundaries

Where two oceanic plates converge, the colder and denser one sinks beneath the other, resulting in the formation of an oceanic subduction zone. Such a convergent boundary is composed of a seafloor trench at shallow depths and a subduction zone with earthquakes at greater depths, as well as a forearc basin, an oceanic arc and a backarc basin above the subduction zone ( Fig. 1). An example of an oceanic-oceanic convergent boundary can be found between the western Pacific and Philippine plates, where active subduction is taking place due to faster movement of the lower plate than the upper plate (Doglioni et al., 2009). This boundary includes the Mariana island arc and the Mariana subduction zone encompassing the Mariana trench, containing the Challenger Deep, the deepest part of world's ocean. The majority of oceanic arcs occur along the western margin of the Pacific Ocean, where they form arcuate chains of volcanic islands, such as the Mariana and Kermadec. The few exceptions are the East Indian and the West Indian arcs and the Scotia arc in the South Atlantic.

Fig. 1

Fig. 1. Schematic diagram showing an oceanic-oceanic convergent plate boundary as exemplified by the Mariana subduction zone in the western side of Pacific Ocean. Oceanic arc volcanics develop above the western Pacific subduction zone that has a divergent subduction hinge relative to the upper oceanic plate, a steep subduction dip and a fast subduction rate, showing a low topographic envelope and a high foreland monocline (Doglioni et al., 2009).

 Modified from Zheng YF, Chen YX, Dai LQ and Zhao ZF (2015) Developing plate tectonics theory from oceanic subduction zones to collisional orogens. Science China: Earth Sciences 58: 1045–1069.

An oceanic arc is a line of volcanoes created by partial melting of the mantle wedge overlying the subducting slab (Wyllie, 1973; Ringwood, 1974). The forearc basin is located between the trench and the arc while the backarc basin is on the opposite side of the arc. Oceanic lithosphere has an average thickness of 100   km, with crustal thickness only in 7–8   km. The tectonic coupling at lithospheric depths prevails between the two converging plates in the early stage of subduction, leading to compressional heating for regional metamorphism at the interplate interface. The mantle wedge is progressively cooled by the subducting cold slab, resulting in the lowest temperatures at the slab-wedge interface (Zheng, 2019). With further subduction to asthenospheric depths, the downgoing slab may become rolled back as soon as its gravity overcomes its buoyancy during plate convergence (e.g., Kusky et al., 2014; Zheng et al., 2016; Stern and Gerya, 2018). This may cause the tectonic decoupling between the two plates, resulting in extensional heating for partial melting of the mantle wedge and thus basaltic magmatism in the late stage of subduction (Zheng, 2019). The common occurrence of backarc basins behind oceanic arcs indicates that such oceanic subduction zones have developed into the highly mature stage.

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The Phanerozoic Tectonic and Sedimentary Evolution of North America

Andrew D. Miall , Ronald C. Blakey , in The Sedimentary Basins of the United States and Canada (Second Edition), 2019

Abstract

The Phanerozoic history of North America, when viewed from the perspective of Pangea, can be divided into three broad phases with gradational time boundaries, as described in the following paragraphs.

During the first phase, which lasted from the late Precambrian to the Pennsylvanian, Pangea was under construction and the North American craton was marked by various, persistent, sedimentary basins—the Michigan, Hudsons Bay, Appalachian, Black Warrior, Arkoma, Tobosa, Illinois, and Williston. The western continental margin was either a divergent (passive) margin, facing the paleo-Pacific Ocean (Panthalassa ) or a backarc basin bordering that ocean, while the eastern margin, beginning in the Middle Ordovician, underwent convergent and collisional tectonism, during the various phases of the Appalachian orogen.

Phase two, which included the final assembly of Pangea, extended through the Pennsylvanian and Permian, and continued into the Triassic. Most of the cratonic basins became inactive as new stresses were imposed on the continent related to Pangean assembly. The greater Appalachian foreland basin developed adjacent to the Appalachian orogen. The Sverdrup basin formed as a successor basin to the Franklin orogen in the Canadian Arctic. The Slide Mountain and Havallah back-arc basins extended from the Yukon to western Nevada, and the Maritime basins of eastern Canada formed in complex rift-transform settings. The southwestern margin of the continent was affected by oblique-slip displacement between North America and Gondwana (including portions of what are now Mexico and the southwestern United States), with the development of an orogenic highland called the Ancestral Rockies. A host of new basins were formed throughout the region of Southwestern North America affected by the Ancestral Rocky Mountains orogen, an event with unclear relations to the Ouachita-Marathon orogen.

Phase three, commencing in the Late Triassic or Early Jurassic, corresponds to the Pangea breakup phase, during which North America drifted northwestwards (relative to a hot-spot reference frame). The eastern continental margin became the modern extensional Atlantic margin, while the western margin underwent accretionary tectonism leading to the assembly of the Cordilleran orogen. Various rift-related basins of eastern North America extended from the Bisbee trough of Arizona to Southeastern Canada. Complex trench, forearc, and backarc basins developed across the Cordillera and the largest North American basin, formed in response to the Sevier orogen, became the site of the Western Interior Seaway. Late Cenozoic basins in the western Cordillera formed in response to widespread transform processes.

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Continental Margins

Sanjeev Rajput , Naresh Kumar Thakur , in Geological Controls for Gas Hydrate Formations and Unconventionals, 2016

1.5 Convergent Active (Pacific) Margins

Plate tectonic forces drive continental plates to diverge or converge leading to creation or destruction of land masses at the plate edges or boundaries. Divergent margins develop as consequences of the extensional stresses whereas compression dominates the processes in the formation of convergent margins. When two plates converge, the surface expression on the ocean floor is deep-sea trenches. Unlike divergent margins, the convergent margins and adjacent seafloor belong to different tectonic plates. When two plates with different physical or geological characteristics interact the result is their deformation and consumption by convergent processes. The various types of convergent plate boundaries are sometimes defined as active or seismic margins because they are often marked by shallow to deep earthquake as well as volcanism, crustal deformation and metamorphism. They form much of the margin around the Pacific Ocean as well as parts of the southern Atlantic Ocean, the Northeast Indian Ocean and the Southwest Pacific region. Convergent margins are sites of subduction (the consumption of one segment of lithosphere beneath another adjacent segment) of the ocean and continental crust characterized by deep-sea trenches at their seaward (forearc) margin. Most of them have a volcanic arc and many have rifted backarc basins behind the volcanic arc. A variety of crustal interactions may occur resulting in the complex, structural configuration of the convergent margin ( Fig. 1.9).

Figure 1.9. Figure shows that oceanic lithosphere is being subducted beneath the edge of a continent. The sediments from the ocean floor and pieces of the oceanic crust are scraped from the descending oceanic plate and plastered against the edge of the overriding continent. (From United States Geological Survey.)

Depending on the density contrast and morphological configuration, one of the incoming plates is tectonically inserted or subducted under the other plate, which mostly has a terrestrial crust. The morphological as well as structural features of the subducting plate prior to the subduction is inherited from the constructive phase of the extensional regime, volcanic activity and sediment deposition on the abyssal plain (Aubouin, 1989) and the basement relief of the plate is controlled by the rate of sediment accumulation on the abyssal plain (Jarrad, 1989). The overriding terrestrial plate is 20–40 km thick. In most places the plate of oceanic lithosphere descends into the mantle along the Benioff–Wadati zone. Such subduction can occur under the oceanic (Mariana type) or continental (Andean type); in both cases the outer trench slope (seaward slope) is formed in response to the downward flexure of the subducting plate. This provides a mechanism for the downward displacement of deep-sea sediments several kilometers beneath the convergent margin. It is also widely recognized that tectonic processes at the convergent margins constantly fashion new regions of continental crust from fragments of the older or preexisting crustal material. A juvenile mass of terrestrial crust is added to the overriding crust through arc volcanism or magmatism or accretion of sedimentary strata (accrectionary prism). Arc magmatism is probably the most important process sustaining earth stock of terrestrial matter (Reymer and Schubert, 1984). At the convergent ocean margin the lower plate is the principal contributor to the input material. The lower plate is constructed of igneous rocks of the ocean crust with an overlying cover of ocean basin sediments. Ocean basin sediments primarily constitute pelagic ooze, hemipelagic mud and clayey far-traveled terrigenous or continental detritus contributed by continental erosion; it may even contain turbidites. The incoming oceanic sediments are scraped off through skimming of the upper part of the sedimentary layer during subduction and accreted to the overriding plate by a mechanical transfer process. Sediments may also underthrust the seaward position of resistive consolidated sediments and rock framework (buttress) of overriding plate (von Huene and Scholl, 1991). Under certain conditions the sedimentary deposits and oceanic crust may bypass the buttress and subduct, resulting in the consumption of incoming masses. Tectonic removal into the subduction zone of the down-going slab including the sediments and oceanic crust is called consumption (Kennett, 1982). The convergence process also may lead to loss of landmass by erosion of the lower part of the upper terrestrial crust and the eroded material may be taken by the subducting plate into deeper parts of the earth (Underwood and Moore, 1995). The main control on the morphology and geological structure of the convergent margin depends on the amount of sediment entering the trench or backarc region. Sediment input is quite variable, depending on the distance of the trench or backarc basin from major river systems. Moreover, the variation depends on whether the continental or oceanic crust is subducted in the forearc or rifted in the backarc region. The observed seismic activity over Chile and its absence in the Mariana may be ascribed to prevailing compressional and tensional stresses produce the pattern of backarc basin opening in these regions (Uyeda and Kanamori, 1979). Interaction of converging plates at the convergent margin produces complex structural patterns and their surface expressions on the seafloor.

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Tectonic and Basin maps of the world

A.W. Bally , ... A. Sinkewich , in Regional Geology and Tectonics: Phanerozoic Passive Margins, Cratonic Basins and Global Tectonic Maps, 2012

Active margin fold and thrust belts (AMFTBs)

AMFTBs are best subdivided, into accretionary wedge systems (i.e., 1.1 on Table 25.3), as well as FTBs that deform adjacent forearc and backarc basement and their overlying sediments (i.e., 1.2 on Table 25.3). Common strike-slip related tectonics and associated transpressional and transtensional, as well as inversion, tectonics are included in this group.

AMFTBs and their associated Mesozoic/Tertiary basins are limited to the Circum-Pacific and South East Asia active margins and range from intra-oceanic arcs to continental volcanic arcs systems. Forearc basins are located on the oceanward side of the often still active arcs or else the deactivated batholithic roots of former arcs. Backarc basins are located behind the arcs. Both basin types are typically underlain by peneplaned, sometimes rifted, basements that merge with outcropping igneous rocks of the arc or else underlie them. Both forearc and backarc basins are members of the episutural basin class which is discussed in Chapter 4, Vol. 1A (Roberts and Bally) and Tables 25.2 and 25.3 therein.

Most AMFTBs differ greatly from FFTBs because they involve relatively "young basements" formed during a continuing Mesozoic/Cenozoic orogenic process. Associated episutural basins and their tectono-stratigraphic megasequences also have sediment provenances differing drastically from their continent-verging FFTBs counterparts. AMTFBs often involve the continental forearc or else backarc basements, that is, peneplaned earlier continental arcs systems, for example, the Sierran or Franciscan basement of some Californian basins. Part of this older arc-related basement may also outcrop in adjacent mountains. Igneous continental arcs may intrude earlier, that is, much older basement. Following our respective definitions of orogens and basements, older reworked basement would be dated by the latest regional igneous and metamorphic reworking episode (e.g., Plate 20 Continental basement map). AMFTBs by definition coincide with Cenozoic/Mesozoic active margins. For some years Von Huene and Scholl (1991) and Scholl and von Huene (2007) have differentiated accreting margins from non-accreting (eroding) margins whereby accretionary wedges are representative of FTBs overriding the subducting slab adjacent to deep sea trenches. These appear as wedges of folds and/or faulted, imbricated oceanic sediments that in a broad sense may be viewed as "conjugate" to continental FFTB wedges (Plates 25.17 and 25.18).

Seismic atlases that include many of examples from active margins around the world were published by Bally (1983) and Von Huene (1987). In recent years, many more detailed studies have been published. With the important exception of ocean-verging accretionary wedges, AMFTBs typically correspond to deformed forearc-, backarc-, and also strike-slip-related basin margins that in the literature are commonly viewed as sedimentary basins and not as folded belts. Thus, the many compressional/transpressional fold and thrust systems associated with the basins of California are commonly presented in the overall context of the evolution of their sedimentary basins. For a more detailed list of AMFTB examples, see Table 25.3 and also page 1090 etc: Vol. 1C : Basins located within orogenic belts (episutural basins), Vol. 1C.

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Volume 4

C.R. van Staal , in Encyclopedia of Geology (Second Edition), 2021

Peri-Gondwanan terranes of the Dunnage domain

The peri-Gondwanan terranes incorporated into the Dunnage domain occur east of the Red Indian Line (Figs. 1 and 3 ) and formed part of Ganderia before its late Early Cambrian break-up into two terranes: the Penobscot arc on its leading edge and the passive Gander margin on its trailing edge, due to back-arc spreading and formation of the intervening oceanic Penobscot backarc basin (510–490  Ma). The Penobscot arc and backarc rocks are collectively referred to as the Penobscot complex (515–482   Ma). Penobscot backarc ophiolitic rocks have been preserved in central Newfoundland and southern New Brunswick. The Gander margin remained attached to West Gondwana until the Middle Cambrian (~   503   Ma), when it became separated by opening of the Rheic Ocean, which led to a transgression characterized by deposition of a blanket of black shales. The ensialic basement to the Penobscot arc mainly comprises, arc plutonic rocks, which are at least as old as 565   Ma (Fig. 5). The Penobscot arc was reunited with the Gander margin after closure of the Penobscot backarc basin, which took place during the short-lived Penobscot orogeny (483–478   Ma) and led to southeast-directed obduction of backarc ophiolites (510–490   Ma) onto the Gander margin. The cause of the Penobscot orogeny is not well understood, but convergence between Laurentia and Penobscot arc was highly oblique, such that the latter accreted to composite Laurentia at the latitude of central New England during this orogenic event. The Penobscot arc and its successor, the Popelogan-Victoria arc, remained separated from composite Laurentia by Iapetan oceanic lithosphere further to the northeast, until the Late Ordovician (ca. 455   Ma).

The composite crust of the Penobscot complex and underthrusted Gander margin sedimentary rocks are disconformably overlain by younger rocks related to the Floian to Darriwilian Popelogan–Victoria arc/Tetagouche–Exploits back-arc system, which was active between 478 and 454   Ma in Maine, Maritime Canada and Newfoundland. The Popelogan–Victoria arc/Tetagouche–Exploits back-arc system has been traced from Newfoundland through central New Brunswick into Maine, mainly on basis of a remarkably consistent and distinct lithological association of coeval plutonic, volcanic and sedimentary rocks. The Popelogan–Victoria arc is continuous with the Bronson Hill arc in New England, although the arc polarities were in part probably opposite due to a diachronous, northeast propagating arc-polarity reversal following the onset of Penobscottian arc-Laurentia collision in central New England (Fig. 1). The Popelogan–Victoria arc was fully accreted to composite Laurentia by ca. 455   Ma according to field relationships, faunal and sparse paleomagnetic evidence, while the passive (Gander) margin side of the Tetagouche–Exploits back arc was positioned further south due to concurrent back-arc spreading. Rifting and seafloor spreading in the Tetagouche–Exploits back-arc is necessary to account for the formation of Dapingian–Darriwilian back-arc oceanic crust (470–459   Ma), locally preserved as incomplete ophiolite complexes and large structural slices of highly tectonized back-arc oceanic basalt and gabbro in mélange belts in New Brunswick and Newfoundland. Fossil and paleomagnetic evidence suggest that the Tetagouche–Exploits back-arc nowhere achieved a width of more than ~   1000   km.

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