On sheared passive continental margins

Studies in intra-continental and intra-oceanic shear zones reveal structures that may be developed during the formation of a sheared passive continental margin.During the intra-continental shear stage of margin development, rapid vertical movement of the crust may occur resulting in small, tectonically-active basins containing thick sedimentary sequences. At deeper levels in the continental crust, more plastic deformation may lead to a zone of strongly sheared rocks that widens downwards. The tectonic fabric in this zone may exert some control over the subsequent development of the continent-ocean transition under the influence of regional stresses.The thermal event related to asthenosphere upwelling at sheared margins is a transient one and thus of less effect than the event on rifted margins. Nevertheless, following the event the cooling and contraction of oceanic crust against the continent may throw the oceanic crust into tension and lead to normal, block faulting in the oceanic regions analogous to the faulting seen in oceanic fracture zones. The subsidence of oceanic crust as it ages at the margin will either drag down the adjacent continental crust or, more likely, cause the oceanic crust to slip down by normal faulting along the continent-ocean boundary. The kinds of compressional features observed in oceanic fracture zones may also occur at sheared margins.     

被动陆缘洋陆转换带和岩石圈伸展破裂过程分析及其对南海陆缘深水盆地研究的启示

作为伸展陆壳和正常洋壳之间重要的过渡和衔接,洋陆转换带(ocean-continent transition,简写为OCT)蕴含有丰富的地壳岩石圈伸展破裂过程的信息。文中通过系统的资料调研,在总结OCT研究历史、现状和发展趋势的基础上,阐明了OCT的现代概念、类型及其识别标志;详细介绍了以OCT为基础而建立的被动陆缘地壳岩石圈结构构造单元划分方案、表层沉积盆地构造地层格架及重要的构造变革界面特征;分析了大型拆离断层在地壳岩石圈薄化、地幔剥露过程中的控制作用;揭示了陆缘变形集中、迁移和叠合的规律,建立了被动陆缘岩石圈伸展、薄化、剥露和裂解模式。最后,论文对比了国际非岩浆型被动大陆边缘与我国南海OCT的研究,介绍了南海OCT和陆缘深水超深水盆地研究的新发现,提出深入研究南海OCT将为南海陆缘构造演化、洋盆扩张过程和深水超深水盆地的成因机制研究提供新的启示。     

Rifted arch basins and post-breakup rim basins on passive continental margins

In its evolution by plate divergence to a passive continental margin, a continental arch marked by narrow rift valleys (intra-arch basins) and flanked by broad basins (inter- and extra-arch basins) is most likely to break up along a rift valley boundary fault. The resulting dismembered arch at the continental margin is a rim that constitutes the oceanward flank of a rim basin, and the rim basin succeeds one or other of the basins related to the previous arch. In offshore Western Australia, the juxtaposition of Mesozoic reservoir rock at a rift shoulder and source rock of the succeeding rim basin provide a mechanism for concentrating a large gas deposit.     

Rheologic properties of the lithosphere and applications to passive continental margins

Thermal and petrologic models of the crust and upper mantle are used for calculating effective viscosities on the basis of constant creep rates. Viscosity—depth models together with pressure—depth models are calculated for continental and oceanic blocks facing each other at continental margins. It is found from these “static models” that the overburden pressure in the lower crust and uppermost mantle causes a stress which is directed from the ocean to the continent. The generally low viscosity of 10²⁰–10²³ poise in this region should permit a creep process which could finally lead to a “silent” subduction. In the upper crust static stresses act in the opposite direction, i.e. from the continent to the ocean, favouring tension which could produce normal faulting in the continent. Differences between observations and the results obtained from the static models are attributed to dynamical forces.     

Lithospheric extension from rifting to continental breakup at magma-poor margins: rheology,serpentinisation and symmetry

The symmetry or asymmetry of the process of continental breakup has been much debated over the last 20 years, with various authors proposing asymmetric simple shear models, others advocating more symmetric, pure shear models and some combinations of the two. The unroofing of vast expanses of sub-continental mantle at non-volcanic margins has led some authors to argue in favour of simple shear models, but supporting evidence is lacking. Subsidence evidence from conjugate margin pairs is equivocal, and the detailed crustal and lithospheric structure of such pairs not generally well enough known to draw firm conclusions. In the Porcupine Basin, where the final stages of break-up are preserved, the development of structural asymmetry is demonstrable, and apparently related to late stage coupling of the crust to the mantle following the complete embrittlement of the crust. This agrees with theoretical modelling results, which predict that asymmetric models can develop only on a lithospheric scale when the crust and mantle are tightly coupled. However, whether such asymmetry is maintained during continued exhumation of the mantle is unclear.     

Seaward dipping reflectors and the continent-ocean boundary at passive continental margins

The definition of the continent-ocean boundary at passive continental margins has proved to be an elusive task. Even the relatively direct method of seismic refraction experiments has yielded results that cannot always be interpreted unequivocally. Multichannel seismic reflection profiles on many passive margins have revealed the presence of remarkable suites of arcuate reflectors, dipping seaward to form a wedge-shaped structure. Their general characteristics and velocity structure suggest that they may be largely volcanic in nature, but there is no agreed upon model for their origin. Nevertheless it is generally thought that they lie at or close to the boundary between continent and ocean, and as such they are extremely important structural markers that may yield important evidence concerning the structure and evolution of passive margins.     

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.     

Evolution of the passive continental margins of India—a geophysical appraisal

The passive continental margins of India have evolved as India broke and drifted away from East Antarctica, Madagascar and Seychelles at various geological times. In this study, we have attempted to collate and re-examine gravity and topographic/bathymetry data over India and the adjoining oceans to understand the structure and tectonic evolution of these margins, including processes such as crustal/lithosphere extension, subsidence due to sedimentation, magmatic underplating and so on. The Eastern Continental Margin of India (ECMI) seems to have evolved in a complex rift and shear tectonic settings in its northern and southern segments, respectively, and bears similarities with its conjugate in East Antarctica. Crustal extension rates are uniform along the stretch of the ECMI in spite of the presence or absence of crustal underplated material, variability in lithospheric strength and tectonic style of evolution ranging from rifting to shearing. The Krishna-Godavari basin is underlain by a strong ( 30 km) elastic lithosphere, while the Cauvery basin is underlain by a thin elastic lithosphere ( 3 km). The coupling between the ocean and continent lithosphere along the rifted segment of the ECMI is across a stretched continental crust, while it is direct beneath the Cauvery basin. The Western Continental Margin of India (WCMI) seems to have developed in an oblique rift setting with a strike-slip component. Unlike the ECMI, the WCMI is in striking contrast with its conjugate in the eastern margin of Madagascar in respect of sedimentation processes and alignment of magnetic lineations and fracture zones. The break up between eastern India and East Antarctica seems to have been accommodated along a Proterozoic mobile belt, while that between western India and Madagascar is along a combination of both mobile belt and cratonic blocks.     

Elevated passive continental margins: Numerical modeling vs observations. A comment on Braun (2018)

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Growth and breakup of supercontinents and evolution of oceans and continental margins during the global tectonic megacycles

Due to the westward-directed off-centre rotation of the spinning Earth around the gravitational centre of the Earth-Moon (-Sun) system the lower mantle should be displaced eastwards in relation to the upper mantle-crust system (principle of hypocycloid gearing). In consequence, the shape of the Pacific is displaced eastwards above gravity anomalies of the lower mantle in relation to the Earth's crust (once around the globe in 200 to 250 my; 20 to 16 cm/y East drift), thus causing the Global Tectonic Megacycles (Oceanic/Wilson Cycle, Orogenic Cycle, Cycle of the Collisional Mountain Belt, etc.)The continents migrate westwards around the shape of the Pacific in the N and S. They collide sequentially W of the Pacific continuously adding segments to a collisional mountain belt, that becomes older towards the W (zip fastener principle) and since the Permian has lapped some 1 1/3 times around the cratonic nucleus of Laurasia in the form of a spiral (explanation for lateral continental growth and the cyclical repetition of orogenic events for a certain continental margin). Following half an E drift lapping of the Earth's crust by the shape of the Pacific, the Pacific appears again in the W. In the Mediterranean/Caribbean setting (tongue of the Pacific) the continents of the N and S hemispheres that had previously collided sequentially W of the Pacific separate again (rift propagation towards the E), whereby parts of the N margins of the S continents remain attached to the N continents in the form of tectonostratigraphic terranes, which will subsequently migrate westwards around the shape of the Pacific in the N.The Earth's crust is subdivided into a Pacific area and a continental or Pangaea area with Intra-Pangaea Oceans (Atlantic, Red Sea-Indian Ocean, etc.). The Pangaea area in turn is subdivided into a North Pangaea area and a South Pangaea area with the North and South Pangaea continents broadly distributed over the N and S hemispheres. The Earth's history appears to be subdivided into alternating North Pangaea growth/South Pangaea breakup eras (Permian to present Alpine Cycle; Late Proterozoic Panafrican-Brasiliano Cycle) and South Pangaea growth/North Pangaea breakup eras (Late Proterozoic and Early to Middle Paleozoic Baikalian-Caledonian Cycle; Middle Proterozoic Kibaran-Grenvillian Cycle).In the hemisphere of the Pangaea growing (since the Permian the N hemisphere) the continents are subjected to pendular movements (alternating clockwise and counterclockwise rotations combined with movements between high and low latitudes). They always face either the equator or the Pacific with the same margin. Otherwise, a collisional mountain belt would not form. The remaining two margins alternate between an Arctic- and a North Atlantic-type setting. The Cordilleran-type margin of the NE-Pacific is therefore the forerunner of the NW-Pacific island arc-type and both types are one-sided, embryonic states of the two-sided collisional mountain belt forming at the equator W of the Pacific. Since the Jurassic/Cretaceous, the Pacific margins of the N hemisphere are remobilized segments from the older lap of the North Pangaea collisional mountain belt spiral.In the hemisphere of the Pangaea breaking up (since the Permian the S hemisphere) a continent passing through the Antarctica setting rotates through approximately 120° (clockwise during a South Pangaea breakup — Permian to present; counterclockwise during a North Pangaea breakup — Late Proterozoic and Early to Middle Paleozoic) and breaks up into several India-, Australia- and Antarctica-size fragments. The one-sided Andes-type margin of the SE-Pacific (previously evolved from a West Africa-type margin) develops therefore into a one-sided New Guinea-type and into the equatorwards facing thrust zone of the two-sided collisional mountain belt forming at the equator W of the Pacific. On the other hand the SW-Pacific island arc-type margin has evolved from a North-type that might still carry fragments from the older lap of the collisional mountain belt (Atlas, parts of the N Andes and West Antarctica, New Zealand), the main parts of which migrate around the shape of the Pacific in the N in the form of tectonostratigraphic terranes.Due to the pendular movements of a continent from the Pangaea growing and the 120° rotation of a continent from the Pangaea breaking up passing through the Antarctica setting, between its birth in a rift and its death in the collision zone at the equator W of the Pacific, a continental margin will normally need much more time (Cycle of Continental Margins) than the 200 to 250 my necessary for one E drift lapping of the Earth's crust by the shape of the Pacific and the ocean states of the Wilson Cycle or Oceanic Cycle. The eugeosynclinal evolution of an ocean in most cases will therefore be comparatively shorter than the miogeosynclinal evolution of the continental margins bordering that ocean.     

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。     

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。     

The state of stress at continental margins

Three sources of stress at active (Andean) continental margins are considered: body forces on the plates which drive their motion, thermal stresses generated within the cooling lithosphereand bending stresses due to the flexure of the lithosphere at an ocean trench. It is argued that the bending stresses dominate. The evolution of passive (Atlantictype) continental margins is also considered. Models for the free and locked flexure of the continental and oceanic lithosphere are given. Based on observed gravity anomalies, it is argued that the continental margin fault system must remain active throughout much of the evolution of the margin. These displacements accommodate both the subsidence of the oceanic lithosphere due to its cooling and thickeningand the sedimentary loading. This loading may be responsible for the seismicity on the eastern continental margin of the United States e.g., the Charleston, South Carolina earthquake of 1884.     

Accretion of oceanic plateaus at continental margins: Numerical modeling

The accretion of oceanic plateaus has played a significant role in continental growth during Earth's history, which is evidenced by the presence of oceanic island basalts (OIB) and plume-type ophiolites in many modern orogens. However, oceanic plateaus can also be subducted into the deeper mantle, as revealed by seismic tomography. The controlling factors of accretion versus subduction of oceanic plateaus remain unclear. Here, we investigate the dynamics of oceanic plateau accretion at active continental margins using a thermo-mechanical numerical model. Three major factors for the accretion of oceanic plateaus are studied: (1) a thinned continental margin of the overriding plate, (2) “weak” layers in the oceanic lithosphere, and (3) a young oceanic plateau. For a large oceanic plateau, the modes of oceanic plateau accretion can be classified into one-sided and two-sided subduction–collisional regimes, which mainly depend on the geometry of the continental margin (normal or thinned). For smaller-sized seamounts, accretion occurs only if all three factors are satisfied, of which a thinned continental margin is the most critical. Possible geological analogues for the two-sided subduction–collisional mode include the Taiwan orogenic belt and subduction of the Ontong Java Plateau. The accretion model for small oceanic plateaus applies to the Nadanhada Terrane in Northeast China.     

关于中国大陆动力学与造山带研究的几点思考

根据人类社会和我国发展的新需求和当代地学的新发展，就我国大陆动力学造山带研究的学术思路、起点、科学目标和关键科学问题与主要研究内容及方法进行了分析讨论，提出了几点思考建议。面对地学 发展的挑战与机遇，制定地学前沿领域研究战略，面向全球，从我国大陆地质实际出发，充分发挥地域优势与特色，突出中国大陆动力学关键科学问题，建立持续研究基地，重点解剖，重点突破。以大陆动力学研究为突破口、源头创新，参与国际地学发展与竞争，进入世界地学先进行列，为我国从地学大国走向地学强国而努力，作为中国应有的贡献。     

Magmatism at continental passive margins inferred from Ambient‐Noise Phase‐velocity in the Gulf of Aden

Non‐volcanic continental passive margins have traditionally been considered to be tectonically and magmatically inactive once continental breakup has occurred and seafloor spreading has commenced. We use ambient‐noise tomography to constrain Rayleigh‐wave phase‐velocity maps beneath the eastern Gulf of Aden (eastern Yemen and southern Oman). In the crust, we image low velocities beneath the Jiza‐Qamar (Yemen) and Ashawq‐Salalah (Oman) basins, likely caused by the presence of partial melt associated with magmatic plumbing systems beneath the rifted margin. Our results provide strong evidence that magma intrusion persists after breakup, modifying the composition and thermal structure of the continental margin. The coincidence between zones of crustal intrusion and steep gradients in lithospheric thinning, as well as with transform faults, suggests that magmatism post‐breakup may be driven by small‐scale convection and enhanced by edge‐driven flow at the juxtaposition of lithosphere of varying thickness and thermal age.     

Oil source rocks at the mesozoic and cenozoic continental margins: Communication 1. Oil source rocks at continental margins in the Triassic-Jurassic and Neocomian-Aptian

The presence of rocks capable of generating hydrocarbons (HC) in the section of sedimentaryrock basins is an essential criterion for their qualification as structures with oil and gas pools. Although organic matter (OM) is always present as dissemination in genetically different sediments, it is believed that rocks enriched with OM of the sapropel series (2 to 3% C_org) can generate a significant amount of liquid HC. However, rock sequences with the C_org ranging from 3–5 to 15–20% are considered oil source formations. The rock section of large petroliferous basins usually includes one or two source sequences, which generated liquid and gaseous HCs after submergence to high temperature and pressures zones. In the basin confined to the Arctic slope of Alaska, one of the main producers of liquid HC is represented by the Upper Triassic clays and limestones of the Shublik Formation. In the Barents Sea and North Sea basins, such rocks are represented by the Spekk Formation and the Kimmeridge Clay, respectively; in the West Siberian basin, by the Bazhenovo Formation; in the Persian Gulf, by the Fahlian, Sargelu, and Garau formations; in basins of the Caribbean region, by marls and clayey limestones of the La Luna Formation. In perioceanic basins of the South Atlantic, the major source sequences are represented by the Neocomian and Barremian clays and marls. The source rocks are identified as the Lagoa Feia Formation in the Campos and Santos basins. They are cognized as the Black Marlstone or Bukomazi Formation in the Lower Congo, Kwanzaa-Cameroon, and Angola basins.     

秦岭大别造山带地壳化学结构研究

大陆地壳不同结构层之间化学组成(含元素和同位素组成)的系统性差异可以称为地壳化学结构，地球化学参考模型(GERM)能够作为参照标准进行地壳化学结构(元素组成)的对比分析，据此提出了反映地壳演化特征的蛛网图标准化方法。对秦岭—大别造山带的尝试发现，造山带总体上具有物质组成分异较弱的特点；造山带及邻区太古亩结晶基底太华群、崆岭群的化学组成都不能代表全部的下地壳，可能仅代表了下地壳上部，而大别群可能经历了复杂的构造熟化过程。通过对造山带地壳化学结构的分析，指出铕负异常可能并非为地壳拆沉作用所造成。     

Mafic Archean continental crust prohibited exhumation of orogenic UHP eclogite

The absence of ultrahigh pressure (UHP) orogenic eclogite in the geological record older than c. 0.6 Ga is problematic for evidence of subduction having begun o...