Volcanic Margin Research Articles

The Role of Premagmatic Rifting in Shaping a Volcanic Continental Margin: An Example From the Eastern North American Margin

AbstractBoth magmatic and tectonic processes contribute to the formation of volcanic continental margins. Such margins are thought to undergo extension across a narrow zone of lithospheric thinning (~100 km). New observations based on existing and reprocessed data from the Eastern North American Margin contradict this hypothesis. With ~64,000 km of 2‐D seismic data tied to 40 wells combined with published refraction, deep reflection, receiver function, and onshore drilling efforts, we quantified along‐strike variations in the distribution of rift structures, magmatism, crustal thickness, and early post‐rift sedimentation under the shelf of Baltimore Canyon Trough (BCT), Long Island Platform, and Georges Bank Basin (GBB). Results indicate that BCT is narrow (80–120 km) with a sharp basement hinge and few rift basins. The seaward dipping reflectors (SDR) there extend ~50 km seaward of the hinge line. In contrast, the GBB is wide (~200 km), has many syn‐rift structures, and the SDR there extend ~200 km seaward of the hinge line. Early post‐rift depocenters at the GBB coincide with thinner crust suggesting “uniform” thinning of the entire lithosphere. Models for the formation of volcanic margins do not explain the wide structure of the GBB. We argue that crustal thinning of the BCT was closely associated with late syn‐rift magmatism, whereas the broad thinning of the GBB segment predated magmatism. Correlation of these variations to crustal terranes of different compositions suggests that the inherited rheology determined the premagmatic response of the lithosphere to extension.

The thermal maturity of sedimentary basins as revealed by magnetic mineralogy

AbstractThe thermal evolution of sedimentary basins is usually constrained by maturity data, which is interpreted from Rock‐Eval pyrolysis and vitrinite reflectance analytical results on field or boreholes samples. However, some thermal evolution models may be inaccurate due to the use of elevated maturities measured in samples collected within an undetected metamorphic contact aureole surrounding a magmatic intrusion. In this context, we investigate the maturity and magnetic mineralogy of 16 claystone samples from Disko‐Svartenhuk Basin, part of the SE Baffin Bay volcanic margin. Samples were collected within thermal contact metamorphic aureoles near magma intrusions, as well as equivalent reference samples not affected by intrusions. Rock‐Eval pyrolysis (Tmax), and vitrinite reflectance (Ro) analysis were performed to assess the thermal maturity, which lies in the oil window when 435°C ≤ Tmax ≤ 470°C and 0.6%–0.7% ≤ Ro ≤ 1.3%. In addition, we performed low‐ (<300K) and high‐temperature (>300K) investigations of isothermal remanent magnetization to assess the magnetic mineralogy of the selected samples. The maturity results (0.37% ≤ Ro ≤ 2%, 22°C ≤ Tmax ≤ 604°C) show a predominance of immature to early mature Type III organic matter, but do not reliably identify the contact aureole when compared to the reference samples. The magnetic assemblage of the immature samples consists of iron sulphide (greigite), goethite and oxidized or non‐stoichiometric magnetite. The magnetic assemblage of the early mature to mature samples consists of stoichiometric magnetite and fine‐grained pyrrhotite (<1 μm). These results document the disappearance of the iron sulphide (greigite) and increase in content of magnetite during normal burial. On the other hand, magnetite is interpreted to be the dominant magnetic mineral inside the contact aureole surrounding dyke/sill intrusions where palaeotemperatures indicate mature to over‐mature state. Interestingly, the iron sulphide (greigite) is still detected in the contact aureole where palaeotemperatures exceeded 130°C. Therefore, the magnetic mineralogy is a sensitive method that can characterize normal burial history, as well as identify hidden metamorphic contact aureoles where the iron sulphide greigite is present at temperatures beyond its stability field.

Transition from amagmatic to volcanic margin: Mantle exhumation in the Vøring Basin before the Icelandic plume influence

The outer Vøring Basin, Norwegian Sea, is an abnormally deep and distal part of the Norwegian passive margin, known as “magma-rich”. Yet, the lastest borehole drilled in its northernmost part led to a drastic change in the interpretation of seismic reflection data. This update motivates a new model for the entire Mesozoic-Eocene rifting period. Here I evaluate the link between tectonics, serpentinization and magmatism in the Norwegian margin from the Ryazanian-Valanginian to the Palaeocene-Eocene magmatic breakup. Maps of the regional Nise sandstones and main regional unconformities specifically highlight the fundamental role of the Nyk-Vema Structure to understand the basin and to question the deformation timing related to deep structures. The top of partially serpentinized mantle is supported by high-amplitude reflectors clearly visible on certain seismic lines, known as “T-Reflector”, covered with gravity-driven pre-exhumation sediments and further deposits. A new structural scheme based on the interpretation of 2D and 3D seismic data shows the complex organization of fault networks and constrains a major deformation period between the Late Campanian and the Late Palaeocene. Ocean-scale complementary arguments are in good agreement with Lower-Cretaceous mantle exhumation. Before becoming a volcanic margin in the Palaeocene, this segment of the Norwegian margin appears as amagmatic until the Late Campanian, at least. I propose an integrated model since the onset of hyperextension in the end of the Jurassic until the cessation of activity of the Aegir Ridge in the Miocene. This model has implications regarding the context-dependent interpretation of lower-crustal bodies, the ubiquity of serpentinization and mantle exhumation worldwide, and on the hydrocarbon system.

Carboniferous ore-controlling volcanic apparatus and metallogenic models for the large-scale iron deposits in the Western Tianshan, Xinjiang

新疆西天山阿吾拉勒成矿带发育一系列大型海相火山岩型铁矿床，是我国十大重要金属矿产资源接替基地之一，备受关注。该成矿带石炭纪火山活动强烈，前人已经识别出该成矿带东段的区域艾肯达坂破火山口，且查岗诺尔、智博、敦德、备战等大型铁矿床均赋存于下石炭统大哈拉军山组火山岩中，其形成受火山机构控制。然而，迄今为止，这些矿床尚未确立具体的火山机构，矿床与矿区发育的火山机构关系有待查明；此外，与查岗诺尔、智博、备战等矿床包含单一的铁元素不同，敦德矿床具有Fe-Zn-Au元素组合，其原因尚不清楚。在前人研究基础上，通过野外地质剖面观察和测量，结合室内研究和遥感解译工作，我们认为阿吾拉勒成矿带东段的4个大型铁矿床均独自发育石炭纪与成矿有关的火山机构，且不同矿床，其火山机构的特点及其演化各不相同。其中，查岗诺尔矿区发育圆形破火山口，面积约为10km²，矿床位于破火山口北缘，矿体赋存于破火山口环状断裂系中；智博矿区发育椭圆形破火山口，面积约为15km²，矿床位于破火山口西南部，矿体赋存于经多次塌陷的破火山口环状断裂系中；敦德矿区发育椭圆形破火山口，面积约为6km²，矿床位于破火山口的中部，矿体赋存于火山通道及其附近的环状和放射状断裂系中；备战矿区发育椭圆形火山口，面积约为4km²，矿床位于火山口北缘，矿体赋存于环状断裂系中。可见，只有敦德矿床的矿体赋存于火山通道断裂系中，其余矿床的矿体均赋存于火山机构环状断裂系中；高渗透性的火山机构断裂系为成矿流体迁移和金属沉淀提供了有利的空间，是铁矿体形成的关键控矿因素和赋矿部位。此外，这些铁矿床的成矿年龄（319~304Ma）滞后于容矿火山岩的年龄（328~319Ma），矿体与围岩界线多为渐变关系；成矿作用以热液作用为主，其中，敦德矿区发育单一的热液成矿作用，具有Fe-Zn-Au元素组合，其它矿床成矿作用复杂，包括热液成矿作用、沉积成矿作用和可能的岩浆熔体成矿作用，仅发育有Fe元素。基于此，可将阿吾拉勒成矿带东段的海相火山岩型铁矿床进一步划分为两个亚类，即火山通道型铁多金属矿床（敦德）和火山边缘型铁矿床（查岗诺尔、智博、备战）；成矿模式包括火山通道相热液富集铁多金属成矿模式和火山边缘相沉积-热液富集铁成矿模式。;Numerous large-scale submarine volcanic iron deposits occurred in the Awulale metallogenic belt in the Western Tianshan, Xinjiang, which is one of the ten most important alternative reserves in China, and it has attracted much attention. The Awulale belt is characterized by intensively Carboniferous submarine magmatic activities. Previous studies have recognized a huge Aikendaban oval caldera which occurs in the eastern Awulale belt, and several large-scale iron deposits, including Chagangnoer, Zhibo, Dunde and Beizhan, are found hosting in the volcanic rocks of the Lower Carboniferous Dahalajunshan Formation, which are controlled by this volcanic apparatus. However, so far, the volcanic apparatus within each iron deposit has not been recognized, the relationship between the deposit and the volcanic apparatus is not identified. In addition, unlike the Chagangnoer, Zhibo and Beizhan deposits which contain single Fe metal, the Dunde deposit is characterized by a Fe-Zn-Au assemblage, and its genetic reason is still unclear. On the basis of previous studies, through field geological profile observation and survey, combined with indoor research and remote sensing interpretation, we believe that some Carboniferous volcanic eruption cycles occurred in each iron deposit, forming itself volcanic apparatus which is closely related to iron mineralization. These volcanic apparatus have different characteristics and evolutionary processes for each other. A circular caldera, with an area of about 10km², occurs at Chagangnuoer, and it experienced the collapse of the cratering. The Chagangnuoer deposit is located in the north of this caldera, and the ore body occurs in the ring fault system of the caldera. An oval caldera, with an area of about 15km², occurs at Zhibo, and it also experienced collapsing; the Zhibo deposit is located in the southwest of the caldera, and the ore body occurs in the ring fault system. An oval caldera, with an area of about 6km², occurred at Dunde; the Dunde deposit is located in the central part of the caldera, and the ore body occurs in the volcanic channel and its surrounding ring and radial fault system. The half crater occurs at Beizhan and the whole crater has an area of about 4km²; the Beizhan deposit is located in the north of the crater, and the ore body occurs in the ring fault system. It is clear that the iron orebodies in all deposits are hosted in the ring fault systems of a caldera, except for the orebodies in the Dunde deposit which are hosted in the volcanic channel and adjacent fault system. The high permeability fault system of the volcanic apparatus acts as channels for the migration of the ore-forming fluids and deposition of iron oxides and is the key ore-controlled factor and important ore-hosted site for the iron orebody. In addition, the published geochronological data show that the ore-forming age of these deposits (319~304Ma) is slightly later than that of their host volcanic rocks (328~314Ma). The orebodies and the host rocks are gradual. The mineralization is characterized by predominant hydrothermal mineralization. The Dunde deposit is characterized by single hydrothermal mineralization with Fe-Zn-Au assemblage; whereas, the Chagangnoer, Zhibo and Beizhan deposits have complicated mineralization, including sedimentary, hydrothermal and possible magmatic melt mineralizations with single Fe metal. Thus, marine volcanic-hosted iron deposits in the Awulale belt can be divided into two sub-types: one is volcanic channel phase-hosted Fe multimetal deposit; and the other is volcanic margin phase-hosted Fe deposit. Based on these, two models are proposed: one is volcanic channel phase-hosted Fe multimetal model with single hydrothermal mineralization; and the other is volcanic margin phase-hosted Fe model with sedimentary-hydrothermal mineralization.

Fast dismantling of a mountain belt by mantle flow: Late-orogenic evolution of Pyrenees and Liguro-Provençal rifting

The Pyrenean Belt ends against the Gulf of Lion passive margin. The mechanism responsible for dismantling the mountain belt during Oligocene rifting has not yet found a proper explanation. The Late Eocene and Oligocene period is characterized by a first order change in subduction dynamics in the Mediterranean and the subduction zones started to retreat with back-arc basins forming at the expense of mountain belts build earlier. The slab subducting below Provence and Sardinia-Corsica started a fast south-eastward retreat, forming the Liguro-Provençal Basin. This syn-rift period in the Gulf of Lion margin is coeval with exhumation of the eastern Pyrenean basement, while underthrusting of Iberia continued until Early Miocene. Based on interpretation of seismic lines, we propose a tentative model in which the mantle flow related with Apennine slab retreat has (1) exhumed and thinned the continental mantle below the Gulf of Lion and the eastern Pyrenees and (2) exhumed the lower crust, leading to crustal thinning and subsidence of the Gulf of Lion margin. The wide distribution of syn-rift volcanism in the transition between the Gulf of Lion and Valencia Basin is in line with the geometry observed in the margin suggesting ductile deformation of a weak continental crust, typical of volcanic margins. The direction of SKS-waves seismic anisotropy below the Pyrenees and the observed migration of exhumation toward the west fit this simple model. The concentration of syn-rift magmatism and the ductile behaviour of the crust during rifting can be explained by the high heat flow above the slab tear that separates the future Apennines and Maghreb branches of the West Mediterranean subduction zone. Finally, removal of upper mantle, inducing uplift and an increase of potential energy, may explain why thrusting continued in the Pyrenees while rifting was still active nearby along strike.

Isostatic constraint for 2D nonlinear gravity inversion on rifted margins

We have developed a nonlinear gravity inversion for simultaneously estimating the basement and Moho geometries, as well as the depth of the reference Moho along a profile crossing a passive rifted margin. To obtain stable solutions, we impose smoothness on basement and Moho, force them to be close to previously estimated depths along the profile and also impose local isostatic equilibrium. Different from previous methods, we evaluate the information of local isostatic equilibrium by imposing smoothness on the lithostatic stress exerted at depth. Our method delimits regions that deviate and those that can be considered in local isostatic equilibrium by varying the weight of the isostatic constraint along the profile. It also allows controlling the degree of equilibrium along the profile, so that the interpreter can obtain a set of candidate models that fit the observed data and exhibit different degrees of isostatic equilibrium. Our method also differs from earlier studies because it attempts to use isostasy for exploring (but not necessarily reducing) the inherent ambiguity of gravity methods. Tests with synthetic data illustrate the effect of our isostatic constraint on the estimated basement and Moho reliefs, especially at regions with pronounced crustal thinning, which are typical of passive volcanic margins. Results obtained by inverting satellite data over the Pelotas Basin, a passive volcanic margin in southern Brazil, agree with previous interpretations obtained independently by combining gravity, magnetic, and seismic data available to the petroleum industry. These results indicate that combined with a priori information, simple isostatic assumptions can be very useful for interpreting gravity data on passive rifted margins.

Nature and origin of the volcanic ash beds near the Permian–Triassic boundary in South China: new data and their geological implications

AbstractPermian–Triassic boundary (PTB) volcanic ash beds are widely distributed in South China and were proposed to have a connection with the PTB mass extinction and the assemblage of Pangea. However, their source and tectonic affinity have been highly debated. We present zircon U–Pb ages, trace-element and Hf isotopic data on three new-found PTB volcanic ash beds in the western Hubei area, South China. Laser ablation inductively coupled plasma mass spectrometry U–Pb dating of zircons yields ages of 252.2 ± 3.6 Ma, 251.6 ± 4.9 Ma and 250.4 ± 2.4 Ma for these three volcanic ash beds. Zircons of age c. 240–270 Ma zircons have negative εHf(t) values (–18.17 to –3.91) and Mesoproterozoic–Palaeoproterozoic two-stage Hf model ages (THf2) (1.33–2.23 Ga). Integrated with other PTB ash beds in South China, zircon trace-element signatures and Hf isotopes indicate that they were likely sourced from intermediate to felsic volcanic centres along the Simao–Indochina convergent continental margin. The Qinling convergent continental margin might be another possible source but needs further investigation. Our data support the model that strong convergent margin volcanism took place around South China during late Permian – Early Triassic time, especially in the Simao–Indochina active continental margin and possibly the Qinling active continental margin. These volcanisms overlap temporally with the PTB biocrisis triggered by the Siberian Large Igneous Province. In addition, our data argue that the South China Craton and the Simao–Indochina block had not been amalgamated with the main body of Pangea by late Permian – Early Triassic time.

Hotspot origin for asymmetrical conjugate volcanic margins of the austral South Atlantic Ocean as imaged on deeply penetrating seismic reflection lines

We have interpreted 27,550 km of deep-penetrating, 2D-seismic reflection profiles across the South Atlantic conjugate margins of Uruguay/Southern Brazil and Namibia. These reflection profiles reveal in unprecedented detail the lateral and cross-sectional, asymmetrical distribution of voluminous, postrift volcanic material erupted during the Barremian-Aptian (129–125 Ma) period of early seafloor spreading in the southernmost South Atlantic. Using this seismic grid, we mapped the 10–200 km wide, continental margin-parallel limits of seaward-dipping reflector (SDR) complexes — that are coincident with interpretations from previous workers using seismic refraction data from the South American and West African conjugate margins. Subaerially emplaced and tabular SDRs have rotated downward 20° in the direction of the mid-Atlantic spreading ridge and are up to 22 km thick near the limit of continental crust. The SDR package is wedge shaped and thins abruptly basinward toward the limit of oceanic crust where it transitions to normal, 6–8 km thick oceanic crust. We have developed a model for the conjugate rifted margins that combine diverging tectonic plates and northwesterly plate motion relative to a fixed mantle position of the mantle plume. Our model explains an approximately 30% higher volume of SDRs/igneous crust on the trailing Namibian margin than on the leading Brazilian margin during the syn- and postrift phases. Our model for volcanic margin asymmetry in the South Atlantic does not require a simple shear mechanism to produce the asymmetrical volcanic material distribution observed from our data and from previously published seismic refraction studies. Determining the basinward extent of the extended continental basement is crucial for understanding basin evolution and for hydrocarbon exploration. Although these conjugate margins have evolved asymmetrically, their proximity during the early postrift stage suggests a near-equivalent, early basin evolution and similar hydrocarbon potential. Understanding the tectonic and magnetic processes that produce these observed asymmetries is critical for understanding volcanic passive margin evolution.

Basin structure and prospectivity of the NE Atlantic volcanic rifted margin: cross-border examples from the Faroe–Shetland, Møre and Southern Vøring Basins

Interpreted horizons within the Atlantic Margins (AM) area and borehole synthetic seismogram for the Tulipan 6302/6-1 borehole compared to the AMS fast track processed seismic data.

The nature of Earth’s first crust

Recycling of crust into the mantle has left only small remnants at Earth’s surface of crust produced within a billion years of Earth formation. Few, if any, of these ancient crustal rocks represent the first crust that existed on Earth. Understanding the nature of the source materials of these ancient rocks and the mechanism of their formation has been the target of decades of geological and geochemical study. This traditional approach has been expanded recently through the ability to simultaneously obtain U-Pb age and initial Hf isotope data for zircons from many of these ancient, generally polymetamorphic, rocks. The addition of information from the short-lived radiometric systems 146Sm-142Nd and 182Hf-182W allows resolution of some of the ambiguities that have clouded the conclusions derived from the long-lived systems. The most apparent of these is clear documentation that Earth experienced major chemical differentiation events within the first tens to hundreds of millions of years of its formation, and that Earth’s most ancient crustal rocks were derived from these differentiated sources, not from primitive undifferentiated mantle. Eoarchean rocks from the North Atlantic Craton and the Anshan Complex of the North China Craton have sources in an incompatible-element-depleted mantle that dates to 4.4–4.5 Ga. Hadean/Eoarchean rocks from two localities in Canada show the importance of remelting of Hadean mafic crust to produce Eoarchean felsic crust. The mafic supracrustal rocks of the Nuvvuagittuq Greenstone Belt are a possible example of the Hadean mafic basement that is often called upon to serve as the source for the high-silica rocks that define continental crust. Many, but not all, ancient terranes show a shift in the nature of the sources for crustal rocks, and possibly the physical mechanism of crust production, between 3.0–3.6 Ga. This transition may reflect the initiation of modern plate tectonics. Eoarchean/Hadean rocks from some terranes, however, also display compositional characteristics expected for convergent margin volcanism suggesting that at least some convergent margin related magmatism began in the Hadean. The persistence of isotopic variability in 142Nd/144Nd into the mid-Archean, and the eventual reduction in that variability by the end of the Archean, provides new information on the efficiency by which mantle convection recombined the products of Hadean silicate-Earth differentiation. The rate of crust production and recycling in the Hadean/Archean, however, is not resolved by these data beyond the observation that extreme isotopic compositions, such as expected for Hadean evolved, continent-like, crust are not observed in the preserved Eoarchean rock record. The lack of correlation between 142Nd/144Nd and 182W/184W variation in Archean rocks suggests that these two systems track different processes; the Sm-Nd system mantle-crust differentiation while Hf-W is dominated by core formation. The major silicate differentiation controlling Sm/Nd fractionation occurred at ∼4.4 Ga, possibly as a result of the Moon-forming impact, after the extinction of 182Hf.

The role of mantle melts in the transition from rifting to seafloor spreading offshore eastern North America

The Eastern North American Margin (ENAM) formed in the early Jurassic (∼195 Ma) with the breakup of supercontinent Pangea. This rifting event was accompanied by volcanism along the continental shelf of the eastern United States, giving rise to the East Coast Magnetic Anomaly (ECMA). Numerical models show that magmatic diking can assist rifting by reducing the effective stress required to rupture the lithosphere. Therefore, magma-rich rifts can develop a rapid transition from continental breakup to seafloor spreading. Alternatively, the onset of seafloor spreading may be accompanied by a long period of persistent thermal weakening and erosion of the lithosphere. In this paper we present analyses of active-source wide-angle seismic data collected during the 2014 ENAM Community Seismic Experiment in an area that extends farther offshore than previous seismic surveys along the eastern U.S. Seaward of the ECMA lies a zone of thin oceanic crust with high compressional seismic velocity (up to 7.5 km/s in the lower crust), whereas thicker normal oceanic crust lies ∼200 km farther offshore along another prominent magnetic anomaly, the Blake Spur Magnetic Anomaly (BSMA). We reconcile our seismic images with a petrologic model to evaluate the mantle melting conditions during the late stages of continental breakup and early seafloor spreading in the Central Atlantic. We propose that the zone of thin crust between the ECMA and BSMA with high seismic velocity is best explained by the presence of a ∼15–20 km thick lithospheric lid at the time of early seafloor spreading, which prevented melting in the shallow mantle. Our analysis suggests that the volcanism that produced the nearshore ECMA did not lead to complete lithospheric breakup of the conjugate North American and African margins. Normal seafloor spreading began at the BSMA under a moderately high mantle potential temperature of ∼1395–1420°C, ∼25 million years after the initial formation of the volcanic margin. These results imply that stretching, thermal weakening, and rupture of the lithosphere may take millions of years, even in the presence of mantle melts.

Porphyry Cu fertility of the Loch Lilly-Kars Belt, Western New South Wales, Australia

This study investigates the prospectivity for porphyry Cu mineralisation of the early Paleozoic Loch Lilly-Kars Belt, which follows the southern margin of the Paleoproterozoic Broken Hill (Willyama) Inlier in western New South Wales, Australia. The belt is thought to have resulted from the 515–510 Ma collision of a volcanic passive margin and an intra-oceanic island arc above a west-dipping subduction zone, with subsequent post-collisional magmatism. To identify fertility signals of selected magmatic intrusive rocks, we combined whole-rock analyses with in-situ zircon trace-element and isotopic measurements. Data reveal that both Cambrian (ca 496 Ma) and Devonian (ca 393 Ma) intrusive suites are present in the Loch Lilly-Kars Belt. Cambrian dioritic rocks are largely non-fertile for porphyry Cu mineralisation, whereas Devonian monzodiorites and monzonitic porphyries carry strong fertility signals that include high whole-rock Sr/Y (>35) and V/Sc (>10) values, associated with the occurrence of equant, stubby zircons showing relatively constant Eu/Eu* values without any significant Eu anomalies. Although only few samples were analysed and more intensive exploration is required, the results of this study indicate significant potential for porphyry Cu mineralisation in this poorly known belt.

Seismic velocity structure of seaward-dipping reflectors on the South American continental margin

Seaward dipping reflectors (SDRs) are a key feature within the continent to ocean transition zone of volcanic passive margins. Here we conduct an automated pre-stack depth-migration imaging analysis of commercial seismic data from the volcanic margins of South America. The method used an isotropic, ray-based approach of iterative velocity model building based on the travel time inversion of residual pre-stack depth migration move-out. We find two distinct seismic velocity patterns within the SDRs. While both types show a general increase in velocity with depth consistent with expected compaction and alteration/metamorphic trends, those SDRs that lie within faulted half grabens also have high velocity zones at their down-dip ends. The velocity anomalies are generally concordant with the reflectivity and so we attribute them to the presence of dolerite sills that were injected into the lava pile. The sills therefore result from late-stage melt delivery along the large landward-dipping faults that bound them. In contrast the more outboard SDRs show no velocity anomalies, are more uniform spatially and have unfaulted basal contacts. Our observations imply that the SDRs document a major change in rift architecture, with magmatism linked with early extension and faulting of the upper brittle crust transitioning into more organised, dike-fed eruptions similar to seafloor spreading.

A ~2.5 Ga magmatic arc in NE China: New geochronological and geochemical evidence from the Xinghuadukou Complex

This study reports new LA‐ICP‐MS U–Pb zircon ages from the Xinghuadukou Complex from the Erguna Block, NE China. A suite of the Palaeoproterozoic orthogneiss and a suite of the Vendian to Cambrian paragneiss are recognized with the co‐magmatic age of 2,464 ± 26 Ma and the maximum depositional age of 561 ± 10 Ma for their igneous and sedimentary protolith, respectively. The trace element geochemical analysis suggests a strong arc affinity for our orthogneiss, characterized by the enriched LREE and LILE, the elevated incompatible and fluid‐mobile elements and the depleted yet negligible fractionated HREE and HFSE. By comparing our samples with a suite of Cenozoic mantle‐derived granitoids from Hokkaido and a suite of Mesozoic active continental margin volcanics from NE China, it is suggested that the igneous protolith of our near Archaean orthogneiss formed in an arc environment and probably derived from the prior arc crust. The trace element features of the paragneiss highly resemble that of the orthogneiss, showing arc affinity, which suggests an arc basin environment for the sedimentary protolith of our Vendian to Cambrian paragneiss. Based on the synthesized evidence from the Xinghuadukou Complex, a new tectonic scenario is proposed that the basement of the Erguna Block might be forged by the accretion and collision of multi‐episodic igneous arcs and their correlated arc basins, which was probably lasting through the Proterozoic and followed by the Pan‐African period high‐grade metamorphism. The consolidated Erguna Block was later involved into the CAOB tectonic domain, modified and disrupted with the assembly of NE China.

Igneous intrusions in the Faroe Shetland basin and their implications for hydrocarbon exploration; new insights from well and seismic data

Igneous sills and dykes that intrude pervasively into prospective sedimentary basins are a common occurrence in volcanic margins, impacting the petroleum system and causing geological and technical drilling challenges during hydrocarbon exploration. The Faroe-Shetland Basin (FSB), NE Atlantic Margin, has been the focus of exploration for over 45 years, with many wells penetrating igneous intrusions. Utilising 29 FSB wells (with 251 intrusions) and 3D seismic data, this study presents new insights into the impacts that igneous intrusions have on hydrocarbon exploration. Examination of cores reveals additional igneous material in individual wells, compared to estimates using seismic or petrophysical data alone, leading to potential underestimation of the volume of the igneous component in a basin. Furthermore, analysis of petrophysical data shows that within the FSB there are silicic intrusions such as diorite and rhyolite, in addition to the commonly encountered mafic intrusions. These silicic intrusions are difficult to recognise in seismic and petrophysical data due to their low density and compressional velocity and have historically been misidentified on seismic reflection data as exploration targets. Drilling data acquired through intrusions provide valuable insight into the problems exploration wells can encounter, often unexpectedly, many of which can be detrimental to safe drilling practice and result in prolonged non-productive time.

The thermal structure of volcanic passive margins

Over the past ten years we have numerically modelled the properties of the magmatism generated at four of the key areas where the ‘mantle plume–volcanic margin hypothesis’ is expected to be valid: the North Atlantic, South Atlantic, India–Seychelles and Afar. Our model incorporates many of the original assumptions in the classic White and McKenzie model, with pure shear of the lithospheric mantle, passive upwelling and decompressional melting. Our model is however two- rather than one-dimensional, can capture the rift history (extension rate changes and axis jumps) and tracks mantle depletion during melting. In all four of our study areas we require the sub-lithospheric mantle to be 100 – 200°C hotter than ‘normal’, non-volcanic margins to explain the characteristics of the magmatism. In the three passive margin cases we find this excess temperature is limited to a 50 – 100 km thick layer. We require this layer temperature to drop along-strike away from the proposed sites of plume impact at the base of the lithosphere. However, we also find that lithospheric thickness and rift history are as important as temperature for controlling the magmatism. Our work therefore lends support to the hypothesis that the excess magmatism at volcanic margins is due to a thermal anomaly in the asthenosphere, albeit with consideration of extra parameters. Supplementary material: Numerical model description is available at https://doi.org/10.6084/m9.figshare.c.3924505

Foraminiferal εNd in the deep north-western subtropical Pacific Ocean: Tracing changes in weathering input over the last 30,000 years

This study focuses on the neodymium isotopic composition (εNd) of mixed planktonic foraminifera species in sediment cores collected on the north-western margin of the South China Sea (SCS) and in the western equatorial Pacific. The aim was to identify temporal changes in the εNd of the Upper Circumpolar Deep Water (UCDW) during its northward penetration in the western subtropical Pacific and to re-evaluate the significance of deep-water εNd changes at the northern margin of the SCS over the past 25kyr. The past seawater εNd record derived from sediments of the subtropical western Pacific displays a narrow range from −2.9±0.3 to −1.6±0.4. This suggests that the εNd of the UCDW is greatly modified during its northward penetration through the process of boundary exchange along the western volcanic margin of the South Pacific and that the unradiogenic εNd signature of the UCDW observed in the South Pacific (about −6) has not been propagated beyond the equatorial Pacific, at least not over the last 25kyr. Seawater εNd values obtained from the northern SCS display large variations with values that are lower (−6.8±0.3 to −5.6±0.2) for the last glacial period than for the late Holocene (−5.7±0.3 to −4.1±0.2). Such glacial εNd cannot be attributed to a higher proportion of southern-sourced water (SSW) as εNd values of the UCDW in the equatorial western Pacific are more radiogenic than those observed in the SCS. The evolution of the seawater εNd values of the northern SCS during the last glacial period and the last climatic transition is coeval with variations in the contribution of sediments from the Pearl River. The low sea level stand of the last glacial period resulted in a more proximal position of the Pearl River mouth relative to core MD05-2904, which a higher input of freshwater and, in particular, detrital sedimentary particles originating from the Pearl River that partially dissolved during sinking led to a decrease in the seawater εNd along the north-western margin of the SCS. Enhanced rainfalls associated with intensified monsoon and more frequent typhoon activities during the early Holocene Climatic Optimum (from 9 to 7calkyrBP) are linked to a decrease in sea surface salinity (lower δ18O values of planktonic foraminifera) and a decrease in εNd values (−6.2±0.3) induced by lithogenic Nd input from pronounced freshwater and sediment discharge from distal rivers.

Reply to the comment of Talwani et al. (2017) on the Sibuet et al. (2016) paper entitled “Thinned continental crust intruded by volcanics beneath the northern Bay of Bengal”

The northern Bay of Bengal velocity-depth profiles do not follow the velocity-depth curve for the North Atlantic volcanic margins, and only partially the Kerguelen plume velocity-depth curves. Compared with the South China Sea northern margin proxy, we still suggest that the Bay of Bengal crust is thinned continental crust intruded by post-rifting volcanics, as also shown by the interpretation of the numerous high-quality deep multichannel seismic profiles we collected there. What was supposed to be underplating might be sills intruded through the lower thinned continental crust.

A new tectono-magmatic model for the Lofoten/Vesterålen Margin at the outer limit of the Iceland Plume influence

The Early Eocene continental breakup was magma-rich and formed part of the North Atlantic Igneous Province. Extrusive and intrusive magmatism was abundant on the continental side, and a thick oceanic crust was produced up to a few m.y. after breakup. However, the extensive magmatism at the Vøring Plateau off mid-Norway died down rapidly northeastwards towards the Lofoten/Vesterålen Margin. In 2003 an Ocean Bottom Seismometer profile was collected from mainland Norway, across Lofoten, and into the deep ocean. Forward/inverse velocity modeling by raytracing reveals a continental margin transitional between magma-rich and magma-poor rifting. For the first time a distinct lower-crustal body typical for volcanic margins has been identified at this outer margin segment, up to 3.5km thick and ∼50km wide. On the other hand, expected extrusive magmatism could not be clearly identified here. Strong reflections earlier interpreted as the top of extensive lavas may at least partly represent high-velocity sediments derived from the shelf, and/or fault surfaces. Early post-breakup oceanic crust is moderately thickened (∼8km), but is reduced to 6km after 1m.y. The adjacent continental crystalline crust is extended down to a minimum of 4.5km thickness. Early plate spreading rates derived from the Norway Basin and the northern Vøring Plateau were used to calculate synthetic magnetic seafloor anomalies, and compared to our ship magnetic profile. It appears that continental breakup took place at ∼53.1Ma, ∼1m.y. later than on the Vøring Plateau, consistent with late strong crustal extension. The low interaction between extension and magmatism indicates that mantle plume material was not present at the Lofoten Margin during initial rifting, and that the observed excess magmatism was created by late lateral transport from a nearby pool of plume material into the lithospheric rift zone at breakup time.

Growth and demise of a Paleogene isolated carbonate platform of the Offshore Indus Basin, Pakistan: effects of regional and local controlling factors

Based on high-resolution seismic and well datasets, this paper examines the evolution and drowning history of a Paleocene–Eocene carbonate platform in the Offshore Indus Basin of Pakistan. This study uses the internal seismic architecture, well log data as well as the microfauna to reconstruct factors that governed the carbonate platform growth and demise. Carbonates dominated by larger benthic foraminifera assemblages permit constraining the ages of the major evolutionary steps and show that the depositional environment was tropical within oligotrophic conditions. With the aid of seismic stratigraphy, the carbonate platform edifice is resolved into seven seismic units which in turn are grouped into three packages that reflect its evolution from platform initiation, aggradation with escarpment formation and platform drowning. The carbonate factory initiated as mounds and patches on a Cretaceous–Paleocene volcanic complex. Further, the growth history of the platform includes distinct phases of intraplatform progradation, aggradation, backstepping and partial drownings. The youngest succession as late-stage buildup records a shift from benthic to pelagic deposition and marks the final drowning in the Early Eocene. The depositional trend of the platform, controlled by the continuing thermal subsidence associated with the cooling of volcanic margin lithosphere, was the major contributor of the accommodation space which supported the vertical accumulation of shallow water carbonate succession. Other factors such as eustatic changes and changes in the carbonate producers as a response to the Paleogene climatic perturbations played secondary roles in the development and drowning of these buildups.