Passive margins of the North and Central Atlantic: A comparative study

The main features of the volcanic and nonvolcanic passive margins of the North and Central Atlantic are considered. The margins are compared using rather well-studied reference tectonotypes as examples. The conjugate margins of the Norwegian-Greenland region and the margins of West Iberia and Newfoundland are chosen as tectonotypes of volcanic and nonvolcanic margins, respectively. The structural and magmatic features of the margins and their preceding history are discussed. A complex of interrelated attributes is shown for each tectonotype. The Norwegian-Greenland region close to the Iceland plume is distinguished by narrow zones of stretched continental crust, rapid localization of stretching with breakup of the continent, a high rate of subsequent spreading, and intense magmatism with the formation of a thick new crust at the margin and the adjacent oceanic zone. The Iberia-Newfoundland region, remote from the plumes, is characterized by wide zones of stretched continental crust, long-term and diachronous prebreakup extension propagating northward, extremely restricted mantle melting during rifting and initial spreading, and frequent occurrence of ancient crustal complexes and serpentinized mantle rocks at the margin. Crustal faults and a thin tectonized oceanic crust appear along the margin under conditions of slow spreading. A model of hot and fast spreading with a high degree of melting in the mantle is applicable to the Norwegian-Greenland region, whereas a model of cold and slow amagmatic rifting with a long pre-breakup stretching and thinning of the lithosphere is appropriate to the Iberia-Newfoundland margins. The differences in the development of the margins is determined by the interaction of many factors: deep temperature, rheology of the underlying lithosphere, heterogeneities in the previously formed crust, and the duration and rate of stretching. All of these factors can be related to the effect of deep plumes and propagation of the extension zone toward the segments of the cold Atlantic lithosphere. Both types of margins also reveal similar features, in particular asymmetry. It is suggested that the rotation forces superimposed on the general tectonomagmatic pattern controlled by plumes could have been the cause of structural asymmetry.     

Development of continental margins of the Atlantic Ocean and successive breakup of the Pangaea-3 supercontinent

Comparative tectonic analysis of passive margins of the Atlantic Ocean has been performed. Tectonotypes of both volcanic and nonvolcanic margins are described, and their comparison with other passive Atlantic margins is given. The structural features of margins, peculiarities of magmatism, its sources and reasons for geochemical enrichment of melts are discussed. The important role of melting of the continental lithosphere in the development of magmatism is demonstrated. Enriched EM I and EM II sources are determined for the lower parts of the volcanic section, and a depleted or poorly enriched source is determined for the upper parts of the volcanic section based on isotope data. The conclusions of the paper relate to tectonic settings of the initial occurrence of magmatism and rifting and breakup during the period of opening of the Mesozoic Ocean. It was found out that breakup and magmatism at proximal margins led only to insignificant structural transformations and reduction of the thickness of the ancient continental crust, while very important magmatic events happened later in the distal zone. New growth of magmatic crust at the stage of continental breakup is determined as a typical feature of distal zones of the margins under study. The relationship of development of margins with the impact of deep plumes as the source of magmatic material or a heat source only is discussed. Progradation of the zone of extension and breakup into the areas of cold lithosphere of the Atlantic and the formation of a single tectonomagmatic system of the ocean are under consideration.     

Tectonotype of nonvolcanic passive margins in the Iberia-Newfoundland region

The tectonotype of nonvolcanic passive margins is discussed on the basis of data on the conjugate margins of West Iberia and Newfoundland. Magmatic, structural, and historical aspects are considered. The Late Mesozoic structural elements related to rifting and transition to spreading are considered, as well as the Early Mesozoic sedimentary basins that begin the history of oceanic opening. The problem is set to determine the tectonic conditions of the early opening of the ocean in the framework of the chosen tectonoptype. These conditions are compared with the setting at the volcanic margins. The formation of the conjugate Iberia-Newfoundland margins is reconstructed as an asymmetric rift system developing in an almost amagmatic regime. All three segments of the margins on both sides of the ocean reveal similar features of transverse zoning with zones of the tectonized continental, transitional, and oceanic crust oriented nearly parallel to the margin. Special attention is called to the old age of the continental crust and subcontinental mantle and the absence of newly formed crystalline crust; the stadial tectonic and rheological evolution of the crust and lithospheric mantle; the specific features of the transitional zone; the serpentinization and exhumation of mantle peridotites and their role in the development of detachment at the crust-mantle interface, related listric faults and the Peridotite Ridge, attenuation of the medium, further localization of continental breakup, and the eventual development of asymmetric conjugate margins. Two papers characterizing the tectonotypes of volcanic and nonvolcanic passive margins ([2] and this paper) determine the line of further comparative analysis necessary for insights into the geodynamics of ocean opening.     

Tectonotype of volcanic passive margins in the Norwegian-Greenland region

A tectonotype of volcanic passive margins exemplified in the conjugate Norwegian and East Greenland margins is considered, with discussion of the Paleogene igneous complexes and the regional rift structure before continental breakup. Fragments of asymmetrical rift have been retained on both sides of the ocean. Large Cretaceous pre-rift sedimentation basins marking the initial stage of the ocean opening are included into the passive margin as well. The continental breakup was accompanied by intense basaltic magmatism over a short time span. This magmatic episode was distinguished by (1) the formation of widespread plateau-basalt complexes on continents and in near-shore areas of the ocean; (2) the development of thick lava series that are recorded in seaward dipping reflector wedges; (3) thick high-velocity lower crust, resulting from magmatic underplating; (4) asymmetrical accretion of the crust and structure formation. The discussion is based on published seismic data and reference sections selected for each margin with consideration of the composition and thickness of the igneous rocks, their lateral variations, source composition, and eruption and crust formation conditions. The characteristic feature of both sections is the two-member structure of volcanic complexes with substantial geochemical differences between the rocks from the lower and upper parts of the section, which correspond to the pre-breakup and breakup phases. At the initial phase, small magma volumes were melted out from the lithosphere. The geochemical signatures of the upper parts of the sections testify to the melting of the asthenospheric mantle. Their spatiotemporal variations reflect the ascent and melting of the deep plume, which was active during and after continental breakup. In the Greenland area, near the central part of the plume, a N-MORB-type mantle magma source gave way to a depleted Iceland-type mantle, while apart from the central part of the plume, its effect is expressed only in the enormous volume of mantle-derived melt without migration of its source. A variety of evidence is provided for the plume’s activity: the great thickness of the volcanic complexes and the relatively stable composition of the melt; the elevated temperature in the mantle; the specific geochemistry of the breakup-related lavas and their lateral zoning; conclusions on the necessity of dynamic support of volcanic eruptions; and recent results of seismographic tomography. The continental breakup inherited a system of older sedimentary basins in the zone of prolonged extension of the lithosphere in the North Atlantic. The continuous dynamic support of extension was most likely provided by long-term ascent of the Iceland plume. The comparison of the considered tectonotype with other volcanic and non-volcanic margins opens the way to further elucidation of the geodynamic processes responsible for the ocean opening.     

Subduction of spreading ridges as a factor in the evolution of continental margins

The subduction of spreading ridges creates a special geodynamic setting distinguished by the interference of convergent and divergent boundaries between lithospheric plates and their long-term interaction accompanied by the formation of characteristic geological complexes and structures. The available data on subduction of the contemporary Chile Ridge make it possible to reconstruct such settings in the geological past. The subduction of the spreading ridge leads to uplift of the continental margin, cut off the accretionary wedge by means of tectonic erosion, emplacement of a fold-thrust structure and longitudinal strike-slip faults, and creates settings favorable for obduction of the young oceanic lithosphere. A lithospheric window expressed in geological and geophysical features opens beneath the continental margin at the continuation of the ridge axis. The subduction-related volcanic activity ceases above this window, giving way to specific proximal magmatism close to the boundary with the ocean and distal magmatism at a distance from this boundary. The proximal bimodal magmatism was related to the sources of tholeiitic basalts characteristic of the ridge involved in subduction and to the partial melting of its oceanic crust and sediments. The distal basaltic magmatism is a product of melting of the fertile oceanic asthenosphere ascending through the lithospheric window with subsequent transformation of magma in the mantle wedge and the continental crust. The use of the Chilean tectonotype for paleoreconstructions is limited by the diverse settings of ridge subduction. The Paleogene magmatism at the Pacific margin of Alaska, where the kinematics of subduction was close to the Chilean subduction, is similar to the proximal igneous rocks of Chile in composition and zoning, retaining some geological differences. Another aspect of the paleoreconstruction is discussed on the basis of Jurassic and Cretaceous granitoids of the Ekonai Terrane of the Anadyr-Koryak System and terranes of southern Alaska. These localities are known for a special, accretionary type of granitoids in the forearc region related to anatectic magma formation without participation of the plunging ridge. Proceeding from comparison with the Chilean tectonotype, the criteria for the identification of granitoids varying in their origin are considered. The effect of subducting ridges on continental margins changed over geologic time and was subject to the rhythm of supercontinental cycles.     

The canadian atlantic margin: A passive continental margin encompassing an active past

The continental margins of Atlantic Canada described in this paper show the effects of plate tectonic motions since Precambrian time and thus represent an ideal natural laboratory for geophysical studies and comparisons of ancient and modern margins. The Grenville Province shows vestiges of Helikian sedimentation on a pre-existing continental block beneath which there may have been southeastward late-Helikian subduction resulting in collision between the Grenville block and the continental block comprised of the older shield provinces to the north. The Grenville block was subsequently split in Hadrynian time along an irregular line so that the southeastern edge of the Grenville exhibited a series of promontories and re-entrants similar to those seen at the present Atlantic continental margin of North America. That margin, which had a passive margin history perhaps comparable with that of the present Atlantic margin, was separated by the lapetus ocean from the Avalon zone whose Precambrian volcanism has been attributed both to that associated with an island arc and with intra-cratonic rifting. However, the lapetus ocean appears to have been subducted in early Paleozoic time with a southeastward dip beneath the Avalon zone, leaving exposures of oceanic rocks in place as in Notre Dame Bay, or transported onto Grenville basement as at Bay of Islands.Plate motions proposed for Devonian and Carboniferous time are numerous, but resulted in the welding of the Meguma block to the Avalon zone of New Brunswick and northern Nova Scotia, extensive faulting within Atlantic Canada which can be correlated with contemporaneous European faulting and extensive terrestrial sedimentation within the fault zones. Graben formation, continental sedimentation and basaltic intrusion in the Triassic represent the tensional prelude to the Jurassic opening of the present Atlantic Ocean.This Jurassic opening produced a rifted margin adjacent to Nova Scotia and a transform margin along the southern Grand Banks. The width of the ocean-continent transition across the transform margin (approx. 50 km) is narrower than for the rifted margin (approx. 100 km). The eastern part of the transform margin is associated with a complex Cretaceous (?) volcanic province of seamounts and basement ridges showing evidence of subsidence. The western portion of the transform margin is non-volcanic, adjacent to which lies the 350 km wide Quiet Magnetic Zone floored by oceanic crust.Development of the margin east of Newfoundland was more complicated with continental fragments separated from the shelf by deep water basins underlain by foundered and atypically thin continental crust. Although thin, the crust appears unmodified, the similarities between the crustal sections of the narrow Flemish Pass and the wide Orphan Basin suggesting that the thinning is not simply due to stretching. The Newfoundland Basin shows evidence for two-stage rifting between the Grand Banks and Iberia with both lateral separation and rotation of Spain, leaving a wide zone of transitional crust in the south. The overall pattern of variations in crustal section for the margin east of Newfoundland is comparable with that of the British margin against which it is located on paleogeographical reconstructions.The major sedimentary unconformities on the shelves (such as the Early Cretaceous unconformity on the Grand Banks) reflect uplift accompanying rifting. Tracing of the sedimentary horizons across the shelf edge is complicated by paleocontinental slopes, which separate miogeocline and eugeocline depositional environments. The subsidence of the rifted margins is primarily due to cooling of the lithosphere and to sediment loading. The subsidence due to cooling has been shown to vary linearly with (time) , similar to the depth—age behaviour of oceanic crust. The consequent thermal history of the sediments is favourable for hydrocarbon generation where other factors do not preclude it.     

大陆解体与被动陆缘的演化

火山型被动陆缘是大陆解体过程中形成的一类陆缘类型,其演化过程与活动陆缘一样复杂多变。随着近年来对大陆解体过程与被动陆缘演化的深入研究,对其沉积过程、岩浆活动以及变质作用研究都有了很大的进展。陆壳减薄解体的过程有许多不同的模式,不对称的简单剪切模式可能是火山型被动陆缘的成因,其机制是软流圈隆起的最大位置从剖面上看与地壳减薄最大位置不在一条垂线上,造成软流圈上升的岩浆在解体的大陆一侧形成火山型被动陆缘。被动陆缘的沉积建造由两套沉积物组成,一套是大陆解体的裂谷阶段所形成的陆相沉积物和双模式火山岩组合,另一套是稳定陆缘的复理石组合;岩浆作用中基性岩类反应了物质直接源于上地幔的主要特点,并有部分受到地壳混染的特征;变质作用中高温低压环境主要发生在裂谷作用阶段,其特点反映了大陆解体过程中随着时间的增温和减压过程,而拆离伸展阶段则被脆性变形所代替。     

Wilson cycle passive margins: Control of orogenic inheritance on continental breakup

Rifts and passive margins often develop along old suture zones where colliding continents merged during earlier phases of the Wilson cycle. For example, the North Atlantic formed after continental break-up along sutures formed during the Caledonian and Variscan orogenies. Even though such tectonic inheritance is generally appreciated, causative physical mechanisms that affect the localization and evolution of rifts and passive margins are not well understood.We use thermo-mechanical modeling to assess the role of orogenic structures during rifting and continental breakup. Such inherited structures include: 1) Thickened crust, 2) eclogitized oceanic crust emplaced in the mantle lithosphere, and 3) mantle wedge of hydrated peridotite (serpentinite).Our models indicate that the presence of inherited structures not only defines the location of rifting upon extension, but also imposes a control on their structural and magmatic evolution. For example, rifts developing in thin initial crust can preserve large amounts of orogenic serpentinite. This facilitates rapid continental breakup, exhumation of hydrated mantle prior to the onset of magmatism. On the contrary, rifts in thicker crust develop more focused thinning in the mantle lithosphere rather than in the crust, and continental breakup is therefore preceded by magmatism. This implies that whether passive margins become magma-poor or magma-rich, respectively, is a function of pre-rift orogenic properties.The models show that structures of orogenic eclogite and hydrated mantle are partially preserved during rifting and are emplaced either at the base of the thinned crust or within the lithospheric mantle as dipping structures. The former provides an alternative interpretation of numerous observations of ‘lower crustal bodies’ which are often regarded as igneous bodies. The latter is consistent with dipping sub-Moho reflectors often observed in passive margins.     

Dimensions of the Late Cretaceous-Paleocene Northeast Atlantic rift derived from Cenozoic subsidence

The distribution of Cenozoic subsidence across Northeast Atlantic volcanic margins have been evaluated to define the width of the rift zone and magnitude of extensional deformation. The subsidence profiles are corrected for the effects of lower-crustal magmatic bodies emplaced during continental break-up. The dimensions of the bodies have been derived from the crustal velocity structure. The width of the Late Cretaceous-Paleocene Northeast Atlantic rift zone was more than 300 km, and the lithospheric extension factor increases gradually towards the line of continental separation. A large number of high-quality seismic reflection data tied to scientific and commercial wells reveals that the initiation of extensional deformation preceded continental separation by ˜ 18 m.y. on the Vøring margin, off Norway. These results show that the Northeast Atlantic volcanic margins, commonly considered as typical volcanic margins indeed, have similar dimensions as non-volcanic margins, and as continental rifts. Thus, these margins contrast significantly with previously suggested evolutionary models based on narrow rift zones and formation during rapid lithospheric failure. The wide rift is compatible with volume of igneous rocks observed along these margins, and with a thermal anomaly similar to that associated with production of Northeast Atlantic oceanic lithosphere.     

Crustal structure of the continental margin of Korea in the East Sea (Japan Sea) from deep seismic sounding data: evidence for rifting affected by the hotter than normal mantle

Despite the various opening models of the southwestern part of the East Sea (Japan Sea) between the Korean Peninsula and the Japan Arc, the continental margin of the Korean Peninsula remains unknown in crustal structure. As a result, continental rifting and subsequent seafloor spreading processes to explain the opening of the East Sea have not been adequately addressed. We investigated crustal and sedimentary velocity structures across the Korean margin into the adjacent Ulleung Basin from multichannel seismic (MCS) reflection and ocean bottom seismometer (OBS) data. The Ulleung Basin shows crustal velocity structure typical of oceanic although its crustal thickness of about 10 km is greater than normal. The continental margin documents rapid transition from continental to oceanic crust, exhibiting a remarkable decrease in crustal thickness accompanied by shallowing of Moho over a distance of about 50 km. The crustal model of the margin is characterized by a high-velocity (up to 7.4 km/s) lower crustal (HVLC) layer that is thicker than 10 km under the slope base and pinches out seawards. The HVLC layer is interpreted as magmatic underplating emplaced during continental rifting in response to high upper mantle temperature. The acoustic basement of the slope base shows an igneous stratigraphy developed by massive volcanic eruption. These features suggest that the evolution of the Korean margin can be explained by the processes occurring at volcanic rifted margins. Global earthquake tomography supports our interpretation by defining the abnormally hot upper mantle across the Korean margin and in the Ulleung Basin.     

Seismic models of inner parts of the Euro-Asian continent and its margins

Analogies are drawn between continental and continental margin structures on the basis of seismic data on the crustal structure of Eurasia and its Atlantic margins. Crustal thinning from the inner parts of the continent to its margins is observed to be a general feature common to the formation of deep midland depressions and sedimentary basins of shelf zones. The latter are characterized by crustal thinning and its assimilation. These phenomena cannot be explained solely be sea-floor spreading effects in the process of active rifting and formation of oceanic crust. It appears that the main role in the formation of the margins in played by processes of mantle erosion in connection with heating at continental margins and with the migration of mantle material to the lower part of the crust.     

扬子板块北部古被动大陆边缘的地球化学特征

本文初次提出扬子板块北部古被动大陆边缘的地球化学特征是:a、裂陷早期阶段,发育碱性双峰式火山岩,但在裂陷的晚期和移离的早期阶段,发育碱性而不具双峰特征的火山岩、岩脉群;b、发育具两类不同地球化学特征的砂页岩,裂陷阶段形成的砂页岩与活动大陆边缘形成的砂页岩具相似的地球化学特征,移离阶段所形成的砂页岩才真正具被动大陆边缘砂页岩的地球化学特征。     

Evolution of Cambrian and Early Ordovician arcs in the Kyrgyz North Tianshan: Insights from U-Pb zircon ages and geochemical data

Geochronological, geochemical, and structural studies of magmatic and metamorphic complexes within the Kyrgyz North Tianshan (NTS) revealed an extensive area of early Palaeozoic magmatism with an age range of 540–475 Ma. During the first episode at 540–510 Ma, magmatism likely occurred in an intraplate setting within the NTS microcontinent and in an oceanic arc setting within the Kyrgyz-Terskey zone in the south. During the second episode at 500–475 Ma, the entire NTS represented an arc system. These two phases of magmatism were separated by an episode of accretionary tectonics of uncertain nature, which led to obduction of ophiolites from the Kyrgyz-Terskey zone onto the microcontinent. The occurrence of zircon xenocrysts and predominantly negative whole-rock ɛ_Nd(t) values and ɛ_Hf(t) values of magmatic zircons suggest a continental setting and melting of Precambrian continental sources with minor contributions of Palaeozoic juvenile melts in the generation of the magmatic rocks. The late Cambrian to Early Ordovician 500–475 Ma arc evolved mainly on Mesoproterozoic continental crust in the north and partly on oceanic crust in the south. Arc magmatism was accompanied by spreading in a back-arc basin in the south, where supra-subduction ophiolitic gabbros yielded ages of 496 to 479 Ma. The relative position of the arc and active back-arc basin implies that the subduction zone was located north of the arc, dipping to the south. Variably intense metamorphism and deformation in the NTS reflect an Early Ordovician orogenic event at 480–475 Ma, resulting from closure of the Djalair-Naiman ophiolite trough and collision of the Djel'tau microcontinent with the northern margin of NTS. Comparison of geological patterns and episodes of arc magmatism in the NTS and Chinese Central Tianshan indicate that these crustal units constituted a single early Palaeozoic arc and were separated from the Tarim Craton by an oceanic basin since the Neoproterozoic.     

Tectono-Magmatic Evolution of the South Atlantic Continental Margins with Respect to Opening of the Ocean

The history of the opening of the South Atlantic in Early Cretaceous time is considered. It is shown that the determining role for continental breakup preparation has been played by tectono-magmatic events within the limits of the distal margins that developed above the plume head. The formation of the Rio Grande Rise–Walvis Ridge volcanic system along the trace of the hot spot is considered. The magmatism in the South Atlantic margins, its sources, and changes in composition during the evolution are described. On the basis of petrogeochemical data, the peculiarities of rocks with a continental signature are shown. Based on Pb–Sr–Nd isotopic studies, it is found that the manifestations of magmatism in the proximal margins had features of enriched components related to the EM I and EM II sources, sometimes with certain participation of the HIMU source. Within the limits of the Walvis Ridge, as magmatism expanded to the newly formed oceanic crust, the participation of depleted asthenospheric mantle became larger in the composition of magmas. The role played by the Tristan plume in magma generation is discussed: it is the most considered as the heat source that determined the melting of the ancient enriched lithosphere. The specifics of the tectono-magmatic evolution of the South Atlantic is pointed out: the origination during spreading of a number of hot spots above the periphery of the African superplume. The diachronous character of the opening of the ocean is considered in the context of northward progradation of the breakup line and its connection with the northern branch of the Atlantic Ocean in the Mid-Cretaceous.     

Character of formation of the Erdenet-Ovoo porphyry Cu-Mo magmatic center (northern Mongolia) in the zone of influence of a Permo-Triassic plume

The Erdenet-Ovoo magmatic center (EOMC) lies within the North Mongolian magmatic area formed through the interaction of a Permo-Triassic plume with the lithosphere in an environment of active continental margin. Two stages are recognized in the EOMC history: subduction stage with participation of basalt-andesite-dacite-rhyolite series and rifting stage with trachybasalt series. The granitoid magmatism (258–220 Ma) is expressed as the Selenge, Shivota, and ore-bearing porphyry complexes. The formation of the Selenge and Shivota granitoids was preceded by the intrusion of gabbroids. Trachybasalts formed during the granitoid magmatism after the Selenge complex, nearly synchronously with the Shivota and ore-bearing porphyry complexes. At the subduction stage of the EMC evolution, the plume influence is documented from the appearance of gabbros both depleted and enriched in lithophile trace elements similar to volcanic rocks of trachybasalt series and basaltoids of bimodal series in northern Mongolia. The Rb-Sr and Sm-Nd isotope characteristics of the enriched gabbros suggest the participation of a lower mantle source in their formation. The plume, as a heat carrier, led to a large-scale manifestation of volcanism and, obviously, a wide development of basic rocks of this stage at depth. The basic rocks were the source of granitoid magma that produced the Selenge granitoids. The protolith melted in the >50 km thick crust preventing the wide manifestation of basaltoid volcanism in that period. The increased plume influence, rifting, uplift of the region, and extension of the crust favored the basaltoid and granitoid (Shivota and ore-bearing porphyry) magmatism activity.     

Isotopic constraints on crustal architecture and Permo‐Triassic tectonics in New Guinea: Possible links with eastern Australia

New U–Pb zircon ages and Sr–Nd isotopic data for Triassic igneous and metamorphic rocks from northern New Guinea help constrain models of the evolution of Australia's northern and eastern margin. These data provide further evidence for an Early to Late Triassic volcanic arc in northern New Guinea, interpreted to have been part of a continuous magmatic belt along the Gondwana margin, through South America, Antarctica, New Zealand, the New England Fold Belt, New Guinea and into southeast Asia. The Early to Late Triassic volcanic arc in northern New Guinea intrudes high‐grade metamorphic rocks probably resulting from Late Permian to Early Triassic (ca 260–240 Ma) orogenesis, as recorded in the New England Fold Belt. Late Triassic magmatism in New Guinea (ca 220 Ma) is related to coeval extension and rifting as a precursor to Jurassic breakup of the Gondwana margin. In general, mantle‐like Sr–Nd isotopic compositions of mafic Palaeozoic to Tertiary granitoids appear to rule out the presence of a North Australian‐type Proterozoic basement under the New Guinea Mobile Belt. Parts of northern New Guinea may have a continental or transitional basement whereas adjacent areas are underlain by oceanic crust. It is proposed that the post‐breakup margin comprised promontories of extended Proterozoic‐Palaeozoic continental crust separated by embayments of oceanic crust, analogous to Australia's North West Shelf. Inferred movement to the south of an accretionary prism through the Triassic is consistent with subduction to the south‐southwest beneath northeast Australia generating arc‐related magmatism in New Guinea and the New England Fold Belt.     

被动大陆边缘张-破裂过程与岩浆活动:南海的归属

岩浆在被动大陆边缘的张-破裂过程中起到决定性作用.南海东北部陆缘发育厚度达10 km的下地壳高速体,其成因机制长期存在争议,影响了对南海东北部陆缘构造归属的界定.为了分析南海共轭陆缘的张破裂机制,本文调研了国内外最新研究进展,系统分析了南海南北陆缘的地壳结构和岩浆活动特点,提出:南海陆缘和海盆中发育有大量岩浆活动,但东西陆缘存在较大差异,底侵高速体东厚西薄,推测为同张裂成因.根据地壳结构与底侵岩浆的量,将被动陆缘划分为5个子类,南海陆缘东侧为多岩浆型,向西变为少岩浆型.东西差异除与伸展速率有关,可能还与东侧陆缘发生了板缘破裂,而西侧陆缘发生了板内破裂有关.      

被动大陆边缘：从大陆张裂到海底扩张

被动边缘是研究大陆张裂、破裂到海底扩张的关键。ODP103、149、173航次对伊比利亚-纽芬兰非火山型共轭边缘的研究,证实了洋陆过渡带和低角度拆离断层的存在,其中洋陆过渡带中广泛出现蛇纹岩化地幔橄榄岩,钻探结果支持不对称单剪模式。ODP104、152、163航次对挪威-格陵兰东南火山型共轭边缘的调查,揭示了典型的向海倾斜反射层（SDRS）的特征,反映了岩浆活动在边缘形成中的主导作用。为了进一步了解大陆从张裂到破裂到洋底扩张过程的一系列学术问题,需要在IODP阶段继续对共轭被动边缘以及包括冲绳海槽和南海在内的典型地区,通过钻探、采样和观测进行更深入的研究。     

Evolution of calc-alkaline magmas of the Okhotsk-Chukotka volcanic belt

Petrological, geochemical, and isotope geochronological aspects of the evolution of calc-alkaline magmatism were investigated in the Western Okhotsk flank zone, the Okhotsk segment, and the Eastern Chukchi flank zone of the Okhotsk-Chukotka volcanic belt (OCVB). The OCVB is a tectonotype of continental margin volcanic belts comprising much greater volumes of felsic ignimbritic volcanics compared with mature island arcs (MIA, Kuril-Kamchatka and Aleutian) and the Andean continental margin. The volcanic rocks of continental margin volcanic belts (OCVB and Andean belt) are enriched in K, Ti, and P compared with the rocks of MIA and show a trend toward the field of high-potassium calc-alkaline series. Primitive andesite varieties (Mg# > 0.6) were not yet found in the OCVB, but there are relatively calcic varieties unknown in Andean-type structures and a significant fraction of moderately alkaline rocks, which are not typical of MIA. Variations in trace and major element characteristics in the basalts and andesites of the OCVB were interpreted as reflecting the competing processes of assimilation/mixing and fractional crystallization during the evolution of the parental basaltic magma. Significant lateral variations were established in the composition of the mantle sources of calc-alkaline magmas along the OCVB over more than 2500 km. The initial isotopic ratios of Sr, Nd, and Pb in the volcanics of the Okhotsk segment are relatively depleted and fall near the mixing line between PREMA and BSE. The magma source of the Western Okhotsk flank zone is most enriched and approaches EMI, whereas that of the central and eastern Chukchi zones contains an admixture of the EMII component. The geochronological characteristics of all the main stages of OCVB magmatism were comprehensively studied by U-Pb SHRIMP and ID-TIMS zircon dating (86 samples) and ⁴⁰Ar/³⁹Ar analysis (73 samples). In general, a discontinuous character was established for the OCVB magmatism from the middle Albian to the early Campanian (106–77 Ma). The volcanism is laterally asynchronous. There are several peaks of volcanism with modes at approximately 105, 100, 96, 92.5, 87, 82, and 77 Ma. The Coniacian-Santonian peaks correspond to the most extensive stages of the middle and late cycles of felsic volcanism. A decreases and a hiatus in magmatic activity were reconstructed for the end of the Cenomanian and the beginning of the Turonian. The volcanism was terminated by plateau basalts with ages of 76–78 Ma, which mark a change in the geodynamic setting from frontal subduction to the regime of a transform margin with local extension in zones normal to the slip direction. A catastrophic character of eruptions with rather narrow ranges of volcanism (<2 Myr) were established taking into account new reliable age estimates for some individual large calderas. The accumulation rate of volcanic materials in such structures was up to 0.15–0.36 km³/yr and even higher.     

Geochronological and geochemical constraints on the Erguna massif basement,NE China – subduction history of the Mongol–Okhotsk oceanic crust

We present new geochronological and geochemical data for granites and volcanic rocks of the Erguna massif, NE China. These data are integrated with previous findings to better constrain the nature of the massif basement and to provide new insights into the subduction history of Mongol–Okhotsk oceanic crust and its closure. U–Pb dating of zircons from 12 granites previously mapped as Palaeoproterozoic and from three granites reported as Neoproterozoic yield exclusively Phanerozoic ages. These new ages, together with recently reported isotopic dates for the metamorphic and igneous basement rocks, as well as Nd–Hf crustal-residence ages, suggest that it is unlikely that pre-Mesoproterozoic basement exists in the Erguna massif. The geochronological and geochemical results are consistent with a three-stage subduction history of Mongol–Okhotsk oceanic crust beneath the Erguna massif, as follows. (1) The Erguna massif records a transition from Late Devonian A-type magmatism to Carboniferous adakitic magmatism. This indicates that southward subduction of the Mongol–Okhotsk oceanic crust along the northern margin of the Erguna massif began in the Carboniferous. (2) Late Permian–Middle Triassic granitoids in the Erguna massif are distributed along the Mongol–Okhotsk suture zone and coeval magmatic rocks in the Xing’an terrane are scarce, suggesting that they are unlikely to have formed in association with the collision between the North China Craton and the Jiamusi–Mongolia block along the Solonker–Xra Moron–Changchun–Yanji suture zone. Instead, the apparent subduction-related signature of the granites and their proximity to the Mongol–Okhotsk suture zone suggest that they are related to southward subduction of Mongol–Okhotsk oceanic crust. (3) A conspicuous lack of magmatic activity during the Middle Jurassic marks an abrupt shift in magmatic style from Late Triassic–Early Jurassic normal and adakite-like calc-alkaline magmatism (pre-quiescent episode) to Late Jurassic–Early Cretaceous A-type felsic magmatism (post-quiescent episode). Evidently a significant change in geodynamic processes took place during the Middle Jurassic. Late Triassic–Early Jurassic subduction-related signatures and adakitic affinities confirm the existence of subduction during this time. Late Jurassic–Early Cretaceous post-collision magmatism constrains the timing of the final closure of the Mongol–Okhotsk Ocean involving collision between the Jiamusi–Mongolia block and the Siberian Craton to the Middle Jurassic.