Ocean Transition Research Articles

Chapter 21 Middle thrust sheets in the Caledonide orogen, Sweden: the outer margin of Baltica, the continent–ocean transition zone and late Cambrian–Ordovician subduction–accretion

Abstract Nappes of continental outer and outermost margin affinities (Middle Allochthon) were transported from locations west of the present Norwegian coast and thrust eastwards onto the Baltoscandian foreland basin and platform. They are of higher metamorphic grade than underlying thrust sheets and most are more penetratively deformed. These allochthons are treated here in three groups. The lower thrust sheets comprise Paleoproterozoic crystalline basement (e.g. Tännäs Augen Gneiss Nappe) and greenschist facies, Neoproterozoic, siliciclastic metasedimentary rocks (e.g. Offerdal Nappe). These are overthrust by a Cryogenian−Ediacaran succession intruded by c. 600 Ma dolerites (Baltoscandian Dyke Swarm) with an affinity to mid-ocean ridge basalt containing normal to enriched incompatible element contents (Särv Nappes). The upper sheets are dominated by higher-grade allochthons (Seve Nappe Complex) with similar, mainly siliciclastic sedimentary protoliths, more mafic magmatism and some solitary ultramafic bodies. Within this early Ediacaran continent−ocean transition zone (COT) assemblage, generally metamorphosed in amphibolite facies, some nappes experienced migmatization, and eclogites are present. Evidence of ultrahigh-pressure metamorphism has been obtained from garnet peridotites and eclogites; recently, microdiamonds have been discovered in paragneisses. Subduction of the COT started by the late Cambrian and accretion continued through the Ordovician, prior to the Baltica–Laurentia collision. Thrusting of all these Middle allochthons onto the foreland basin exceeds a distance of 400 km.

Chapter 23 Swedish Caledonides: key components of an early–middle Paleozoic Himalaya-type collisional orogen

Abstract Caledonian collision of continents Laurentia and Baltica, with at least 1000 km of lateral shortening, dominates the bedrock along the northern margins of the North Atlantic Ocean. Scandian (Silurian–Devonian) underthrusting of Laurentia by Baltica resulted in stacking of the main orogenic wedge and its migration onto the platform edge of Baltica. Complementary thrust sheets, exposed in northeastern Greenland, telescoped the Laurentian continental margin. The Swedish part of the Caledonides, comprising the foreland segment along the central half of this mountain belt, includes the key components of: (1) the Baltoscandian inner margin, including Ordovician and Silurian foreland basins; (2) the Neoproterozoic extended outer margin dominated by mafic magma and continent–ocean transition zone; (3) Iapetus oceanic terranes; and (4) evidence that substantial parts of the outermmost Baltoscandian margin experienced deep subduction and high- and ultrahigh-pressure (HP/UHP) metamorphism during late Cambrian–Ordovician accretion. This evidence, integrated with the Norwegian Caledonides, defines an orogenic pro-wedge comparable to that in the Himalaya today. Orthogonal Scandian collision, lasting for about 60 million years ( c. 440–380 Ma), involved late Silurian–Early Devonian HP/UHP metamorphism of the underthrusting Baltoscandian basement. By the Middle Devonian, the hinterland was experiencing orogen-parallel folding and axial extension, accompanying exhumation, while the orogenic pro-wedge continued to migrate eastwards on to the platform.

Crustal velocity structure across the Orphan Basin and Orphan Knoll to the continent–ocean transition, offshore Newfoundland, Canada

SUMMARYOrphan Basin, a massive deepwater rifted basin off the northeastern coast of Newfoundland, was one of the targets of the 2009 SIGNAL (Seismic Investigations off Greenland, Newfoundland and Labrador) experiment to collect refraction/wide-angle reflection (RWAR) data from the Bonavista Platform, through the Orphan Basin, to the Orphan Knoll, and beyond into oceanic crust. Both the data from an earlier RWAR acquisition and the new data were jointly analysed in order to improve on the earlier velocity model and extend its coverage landward and seaward. The resulting velocity model is characterized by an 8–9-km-thick sedimentary package immediately outboard of the Bonavista Platform, which thins toward the Orphan Knoll and beyond. The shallowest modelled sedimentary layer, interpreted as Paleocene and younger post-rift sediments, does not show significant thickness variations and velocities do not exceed 3.3 km s–1. The second modelled sedimentary layer with laterally variable velocities ranging from 2.3 to 5.3 km s–1, interpreted as Late Cretaceous post-rift sediments, is thickest over an interpreted failed rift. The deepest modelled sedimentary layer consists of laterally variable velocities that do not exceed 5.9 km s–1 and is interpreted as possibly Jurassic to Early Cretaceous syn-rift sediments. The crust beneath the Bonavista Platform is subdivided into an upper (5.4–5.9 km s–1), middle (5.9–6.4 km s–1) and lower crust (6.4–6.9 km s–1). The middle crust is modelled as disappearing beneath the seaward limit of the Bonavista Platform at an interpreted failed rift, only to re-appear 100 km further seaward beneath the central Orphan Basin and extend to the seaward limit of the Orphan Knoll, beyond which the crust can be modelled by just an upper (5.0–6.7 km s–1) and a lower (6.7–7.0 km s–1) crustal layer. Towards land, for the first 450 km of the model, velocities generally follow the globally averaged velocity trend for rifted continental crust, albeit with slightly elevated velocities suggestive of magmatic contributions. At the failed rift, within the continental domain, hyperextended crust is modelled, overlying a limited zone of serpentinized mantle. Seaward of Orphan Knoll, the interpretation for the velocity structure is less definitive but an 80-km-wide continent–ocean transition zone consisting of either transitional embryonic oceanic crust or thinned continental crust overlying serpentinized mantle is proposed. Upper mantle velocities as low as 7.7 km s–1 are modelled beneath the interpreted failed continental rift as well as beneath the continent–ocean transition zone, while the rest of the crustal model is underlain by typical mantle velocities of 8 km s–1. Analysis of extension and thinning factors based on the velocity model reveal that the failed rift experienced hyperextension and should have achieved full crustal embrittlement, consistent with localized mantle serpentinization.

The thermal imprint of continental breakup during the formation of the South China Sea

The stretching of continental lithosphere results in asthenospheric upwelling, raising of isotherms, melting during decompression and eventually seafloor spreading. The thermal maturity of overlying sedimentary organic matter from these settings would be expected to be distinctly altered by these processes, however this is still poorly constrained and quantitatively unexplored. International Ocean Discovery Program (IODP) Expeditions 367–368 cored sediments at the Continent Ocean Transition (COT) on the Northern Margin of the South China Sea (SCS). From two settings at the South East China COT we measured and modelled thermal maturity in pre-/syn- to post-rift sediments making use of a range of thermal maturity parameters. Various heat-flow evolutionary scenarios were investigated, with notable jumps in thermal maturity for sediments corresponding to different depositional packages. In order to match observations of thermal maturity, it was found that the deeper and likely pre-rift sediments were heated to temperatures as high as 200°C during initial breakup. Achieving this temperature for the deeper sediments requires that significant additional heat be imparted at shallow depths (e.g. exposure to at least the far-field effects of a magmatic intrusion or subsurface expressions of volcanism). The post-rift sediments have lower thermal maturities which are likely due to limited burial and the absence of late post-rift magmatism. The comparison of the SE China COT with other margin examples highlights some parameters controlling the thermal evolution and its record.

Structure of the Lithosphere in the Zone of the Continent–Ocean Transition in the Southern Part of the Russian Far East: Evidence from the Density Modeling

The quantitative processing of gravimetric data is performed and the 3D density model is calculated for the southern part of the Russian margin of Eurasian continent. Based on the density model, new patterns of disjunctive tectonics at different depth levels are revealed and the position of the region at the global tectonic level is shown. It is established that the transition zone is an integrated lithospheric structure of the northeastern orientation, in which there are mantle blocks with the boundaries corresponding to the borders of depressions and major tectonic objects. The data obtained may be applied for refinement of the deep structure and paleogeodynamics of the studied area.

Key Ecological Function Peaks at the Land–Ocean Transition Zone When Vertebrate Scavengers Concentrate on Ocean Beaches

Ecotones can form hot spots of biodiversity by containing species from multiple ecosystems. Because biodiversity is often linked to ecological function, we posit that rates of key ecological functions are highest at ecotones and decline away from them. Here we test this hypothesis by measuring spatial decays in the function of carrion scavenging on a gradient ranging from ocean beaches upland into abutting coastal dunes. A large field experiment in Eastern Australia, at the ecotone formed by ocean beaches, employed multiple carrion placements and motion-triggered cameras to identify the animals consuming carcasses and the removal rates of necromass. Significantly more carrion was consumed by vertebrate scavengers at the beach interface (50–80% of total necromass removed) and declined significantly with distance (max. 350 m) into the abutting coastal dunes (20–25%). This marked cline was due to the consumption of carrion by both dune-dwelling and beach-dwelling animals at the beach–dune interface and a decline in scavenging activity by both groups farther upland. These spatial effects were consistent between sites, but the lower carrion removal away from the beach became less pronounced as the carcasses putrefied, suggesting that microbial actions can modify carrion suitability for vertebrate scavengers and hence change spatial patterns in ecological function. Our findings provide quantitative support for the widespread notion that ecological transition zones are hot spots of ecological functions and highlight the importance of managing functionally important species at ecotones.

Robotic Solutions for Arctic Transit Corridor Cargo Autonomous Undersea Vehicles

The development of regular navigation through the Arctic Ocean transit transport corridor from Southeast Asia to Western Europe can provide economic advantages comparable in importance with the construction and operation of the main canals between the oceans – the Suez and Panama. Cost-effective year-round navigation on the northern transport corridor is possible with an undersea (under-ice) scheme of cargo delivery. The construction of reinforced concrete Cargo Autonomous Undersea Vehicles (CAUVs) and the lighter transportation scheme allow solving technical and economic problems. The complicated scheme of undersea transportation gives birth to a new class of ships and raises a number of unsolved technical tasks. Among them are the loading of lighters on CAUVs, the formation of CAUVs trains and the locomotion control tasks. All these tasks require control of a large number of parameters and are complex for humans. Robotic CAUVs are regarded as an option of choice to deal with these control tasks.

Laterally Confined Volcanic Successions (LCVS); recording rift-jumps during the formation of magma-rich margins

Seaward Dipping Reflectors (SDRs) are a characteristic feature of magma-rich margins, and represent the generation of large volumes of flood basalts at the point of continental breakup. A number of recent studies provide new insights into the emplacement and tilting of SDRs and conclude that the majority of SDRs are contained within new magmatic crust that has a close affinity to oceanic crust. However, the process by which these initial magmatic systems evolve into a fully established spreading centre remains poorly understood. Several characteristic features of magma-rich margins may be explained by the occurrence of rift-jumps during SDR emplacement, yet the cause and prevalence of such rift-jumps remain unknown.Here we constrain the 3D geometry of the continent–ocean transition in the Orange Basin, offshore South Africa. This allows us to test if, where and why such rift jumps occur. Our results demonstrate an order of along-strike segmentation previously unobserved in these settings. We demonstrate that the SDR belt is disrupted by the occurrence of a volcanic-stratigraphic package, defined as the Laterally Confined Volcanic Succession (LCVS), not previously identified on a rifted margin. We interpret this as a magmatic spreading centre that was abandoned by a subsequent rift-jump. Identification of LCVSs is important for two reasons. First, we argue that the LCVS formed via the same process as SDRs, and hence provides a unique example of SDR geometry prior to their separation onto conjugate plates. Second, as we can map out the 3D geometry of the LCVS and SDRs, we propose that rift-jumps during magma-rich margin formation may be fundamental to the establishment of a laterally continuous incipient spreading centre.

Continental Slopes of the West Africa Region: A Unique Treasure of Hydrocarbons

The West African region embraces a number of coastal sedimentary basins, which continued in deep-water areas of the Atlantic Ocean. It includes the following oil-and-gas-bearing basins: the Gulf of Guinea, the Kwanza–Cameroonian, and the Namibian. The sedimentary cover of the basins of this passive margin is represented by Mesozoic–Cenozoic deposits. The composition of sediments accumulated in them is quite specific and surprisingly units over the vast areas. The tectonic structure of the majority of the continental margins of West Africa makes possible to refer them to the margins of epiplatform orogenic belts. The existence of two systems of linear troughs—internal and external—on the passive margins at the early stages of continent–ocean transition zones relates deep-water hydrocarbon deposits to internal troughs filled by younger sediments: the alluvial fans of submarine rivers and landslide fronts with prograde formations (turbidites, debris flows, etc.). Late Cretaceous and Middle Paleogene clay formations played the role of source beds in the region, so-called “black clays.” An analysis of over 200 hydrocarbon fields, mainly petroleum, discovered in the past 10–15 years in the region revealed a clear tendency of these fields occurring in a productive zone of oil pools extending in a sea depth interval of 400–3000 m on the continental slope and possibly to 4000 m at the continental rise. Moreover, all discovered fields have been estimated in terms of reserves from large to giant. It is also noteworthy that within the shallow of this region, which includes the shelf and the coastal plain, only a number of small, insignificant oil and gas pays have been discovered. The main of oil and gas bearing potential prospects are related to deposits in the middle and lower parts of the continental slope and possibly adjacent areas of the continental rise. In the long term, the drilling objectives will be both postsalt and presalt deep-water oil-and-gas fields.

Estudios de zooplancton marino en Costa Rica: una revisión y perspectivas a futuro

This review summarizes marine zooplankton research performed in the Costa Rican Pacific and Caribbean coasts and at Isla del Coco National Park. Composition, abundance, biomass, distribution and some ecological features of the zooplankton for each system is discussed. Pacific coastal zooplankton composition has been described in Bahía Salinas and Bahía Culebra. The local oceanography of these bays is heavily influenced by the Papagayo wind jet, which produces seasonal coastal upwelling and most likely drives the seasonal changes in zooplankton abundance and biomass. Copepods and ostracods were the dominant taxonomic groups throughout the year with a strong seasonality, furthermore 53 hydromedusae species has been found, where Liriope tetraphylla (Chamisso & Eysenhardt, 1821), Solmundella bitentaculata (Quoy & Gaimard, 1833) and Aglaura hemistoma (Péron & Lesueur, 1810), were the most abundant species. Zooplankton herbivory rates at Gulf of Nicoya estuary was estimated over 50 % removal of primary production. This is a typical value for tropical productive environments. Copepods numerically dominated zooplankton community from the Gulf of Nicoya. Other dominant taxa includes merozooplanktonic decapod larvae and ichthyoplankton. Biomass is high and there are strong crossshelf gradients in the Gulf. Zooplankton collected in the southern Pacific coast (close to Isla del Caño) is also dominated by copepods, while Bahía Coronado has a mixed zooplankton composition of oceanic and coastal transition waters. In Golfo Dulce, zooplankton is highly diverse and dominated by copepods, appendicularians, and ostracods. About 35 % of daily primary production is consumed by microzooplankton, and zooplankton adapt to anomalous conditions of El Niño-Southern Oscillation (ENSO). Coco´s Island, and other oceanic islands in the Eastern Tropical Pacific are “hot spot” of zooplankton diversity, where > 160 zooplankton species have been reported (predominantly copepods). The biomass is dominated by mesozooplankton, and new species records for the Eastern Tropical Pacific have been found in this insular ecosystem. In the other hand, early works of 1980 carried out at Caribbean coast (Cahuita National Park), show a considerably low zooplankton diversity and abundance while research at the end of the 2000 decade recognized a significant increase in zooplankton diversity, specially fish larvae. Recent zooplankton studies have added six new copepod species and several new records of species from different taxonomic groups such as appendicularians, amphipods, chaetognats, euphausiids, gastropods, and polychaetes. Future work should focus on monthly long-term monitoring programs to investigate the effects of ocean acidification and the trophic dynamics associated with fisheries. Alike, longterm studies are needed to see trends of change in planktonic communities, especially comparing areas under human activity in the coastal zone (tourism, port, urban development) with protected areas as targets for study. An increase of sampling efforts must be done in the relatively understudied Northern Caribbean coast.

A low-angle detachment fault revealed: Three-dimensional images of the S-reflector fault zone along the Galicia passive margin

A new 3-D seismic reflection volume over the Galicia margin continent–ocean transition zone provides an unprecedented view of the prominent S-reflector detachment fault that underlies the outer part of the margin. This volume images the fault's structure from breakaway to termination. The filtered time-structure map of the S-reflector shows coherent corrugations parallel to the expected paleo-extension directions with an average azimuth of 107°. These corrugations maintain their orientations, wavelengths and amplitudes where overlying faults sole into the S-reflector, suggesting that the parts of the detachment fault containing multiple crustal blocks may have slipped as discrete units during its late stages. Another interface above the S-reflector, here named S′, is identified and interpreted as the upper boundary of the fault zone associated with the detachment fault. This layer, named the S-interval, thickens by tens of meters from SE to NW in the direction of transport. Localized thick accumulations also occur near overlying fault intersections, suggesting either non-uniform fault rock production, or redistribution of fault rock during slip. These observations have important implications for understanding how detachment faults form and evolve over time. 3-D seismic reflection imaging has enabled unique insights into fault slip history, fault rock production and redistribution.

Oxygen minimum zones in the early Cambrian ocean

The relationship between the evolution of early animal communities and oceanic oxygen levels remains unclear. In particular, uncertainty persists in reconstructions of redox conditions during the pivotal early Cambrian (541-510 million years ago, Ma), where conflicting datasets from deeper marine settings suggest either ocean anoxia or fully oxygenated conditions. By coupling geochemical palaeoredox proxies with a record of organic-walled fossils from exceptionally well-defined successions of the early Cambrian Baltic Basin, we provide evidence for the early establishment of modern-type oxygen minimum zones (OMZs). Both inner- and outer-shelf environments were pervasively oxygenated, whereas mid-depth settings were characterised by spatially oscillating anoxia. As such, conflicting redox signatures recovered from individual sites most likely derive from sampling bias, whereby anoxic conditions represent mid-shelf environments with higher productivity. This picture of a spatially restricted anoxic wedge contrasts with prevailing models of globally stratified oceans, offering a more nuanced and realistic account of the Proterozoic-Phanerozoic ocean transition.

New Data on the Composition of Cretaceous Volcanic Rocks of the Alazeya Plateau, Northeastern Yakutia

This work presents new data on the composition of volcanics, developed within the Alazeya Plateau of the Kolyma–Indigirka fold area (Northeast Russia), which indicate essential differences in their composition and, accordingly, different geodynamic settings of the formation of rocks. The studied igneous rocks are subdivided into two groups. Volcanics of the first group of the Late Cretaceous age, which are represented by differentiated volcanic rock series (from andesitobasalts to dacites and rhyolites), were formed under island arc conditions in the continent–ocean transition zone. Volcanics of the second group are ascribed to the tholeiitic series and were formed under the other geodynamic setting, which is associated with the regime of extension and riftogenesis, manifested in the studied area probably at the later stage.

Tie points for Gondwana reconstructions from a structural interpretation of the Mozambique Basin, East Africa and the Riiser-Larsen Sea, Antarctica

Abstract. Movements within early East Gondwana dispersal are poorly constrained, and there is debate about conjugate geologic structures and the timing and directions of the rifting and earliest seafloor spreading phases. We present a combined structural interpretation of multichannel reflection seismic profiles from offshore of northern Mozambique (East Africa) and the conjugate Riiser-Larsen Sea (Antarctica). We find similar structural styles at the margins of both basins. At certain positions at the foot of the continental slope close to the continent–ocean transition, the basement is intensely deformed and fractured, a structural style very untypical for rifted continental margins. Sediments overlying the fractured basement are deformed and reveal toplap and onlap geometries, indicating a post-breakup deformation phase. We propose this unique deformation zone as a tie point for Gondwana reconstructions. Accordingly, we interpret the western flank of Gunnerus Ridge, Antarctica as a transform margin similar to the Davie Ridge offshore of Madagascar, implying that they are conjugate features. As the continental slope deformation is post-rift, we propose a two-phase opening scenario. A first phase of rifting and early seafloor spreading, likely in NW–SE direction, was subsequently replaced by a N–S-directed transform deformation phase overprinting the continent–ocean transition. From previously identified magnetic chrons and the sediment stratigraphy, this change in the spreading directions from NW–SE to N–S is suggested to have occurred by the late Middle Jurassic. We suggest that the second phase of deformation corresponds to the strike-slip movement of Madagascar and Antarctica and discuss implications for Gondwana breakup.

Large-scale ocean connectivity and planktonic body size

Global patterns of planktonic diversity are mainly determined by the dispersal of propagules with ocean currents. However, the role that abundance and body size play in determining spatial patterns of diversity remains unclear. Here we analyse spatial community structure - β-diversity - for several planktonic and nektonic organisms from prokaryotes to small mesopelagic fishes collected during the Malaspina 2010 Expedition. β-diversity was compared to surface ocean transit times derived from a global circulation model, revealing a significant negative relationship that is stronger than environmental differences. Estimated dispersal scales for different groups show a negative correlation with body size, where less abundant large-bodied communities have significantly shorter dispersal scales and larger species spatial turnover rates than more abundant small-bodied plankton. Our results confirm that the dispersal scale of planktonic and micro-nektonic organisms is determined by local abundance, which scales with body size, ultimately setting global spatial patterns of diversity.

МОРФОСТРУКТУРНЫЙ АНАЛИЗ ПРИ РЕШЕНИИ ГЕОЛОГИЧЕСКИХ ПРОБЛЕМ ВОСТОКА АЗИИ, "Геоморфология"

Окраинные моря Западно-Тихоокеанской зоны перехода континент-океан рассматриваются как вихревые структуры. В Япономорском регионе (ЯР) применение линеаментного анализа (комплекса геоморфологических, геологических и геофизических методов) позволило выявить в континентальной части Япономорского региона рудоконтролирующие зоны (эндогенного оруденения, месторождений алмазов и др.), скрытых разломов фундамента и магматических очаговых структур, а на дне Японского моря разломных структур. В регионе: 1) выявлены три литосферные вихревые морфоструктуры - Япономорская (ЯМ), Нижнеамурская (НА) и Сунляо (СЛ), 2) выделены два структурных этажа: мантийный и литосферный, включающие, соответственно, тектонический каркас и вихревые структуры, тесно связанные с подстилающей астеносферой. ЯМ морфоструктура отражает процесс раскрытия Японского моря - как результат взаимодействия Евроазиатской и Тихоокеанской плит; НА - соответствует межгорная впадина, с которой на западе сочленяется нефтегазоносная депрессия Сунляо вихревого типа. В ЯР развиты нефтегазоносные мезокайнозойские осадочные бассейны, представляющие литосферные вихри. Линеаментный анализ позволит по-новому прогнозировать характер миграции углеводородов в земной коре и выявлять предполагаемые места их скопления. Этот метод может быть использован в процессе подготовительного этапа при исследовании нефтегазоносности Западно-Тихоокеанских окраинных морей, а также Арктических бассейнов.

Recurrent patterns of microdiversity in a temperate coastal marine environment

Temperate coastal marine environments are replete with complex biotic and abiotic interactions that are amplified during spring and summer phytoplankton blooms. During these events, heterotrophic bacterioplankton respond to successional releases of dissolved organic matter as algal cells are lysed. Annual seasonal shifts in the community composition of free-living bacterioplankton follow broadly predictable patterns, but whether similar communities respond each year to bloom disturbance events remains unknown owing to a lack of data sets, employing high-frequency sampling over multiple years. We capture the fine-scale microdiversity of these events with weekly sampling using a high-resolution method to discriminate 16S ribosomal RNA gene amplicons that are >99% identical. Furthermore, we used 2 complete years of data to facilitate identification of recurrent sub-networks of co-varying microbes. We demonstrate that despite inter-annual variation in phytoplankton blooms and despite the dynamism of a coastal–oceanic transition zone, patterns of microdiversity are recurrent during both bloom and non-bloom conditions. Sub-networks of co-occurring microbes identified reveal that correlation structures between community members appear quite stable in a seasonally driven response to oligotrophic and eutrophic conditions.

Crustal structure of Central Sicily

We processed crustal seismic profile SIRIPRO, acquired across Central Sicily. To improve the seismic image we utilized the wave equation datuming technique, a process of upward or downward continuation of the wave-field between two arbitrarily shaped surfaces. Wave equation datuming was applied to move shots and receivers to a given datum plane, removing time shifts related to topography and to near-surface velocity variations. The datuming procedure largely contributed to attenuate ground roll, enhance higher frequencies, increase resolution and improve the signal/noise ratio. Processed data allow recognizing geometries of crust structures differentiating seismic facies and offering a direct image of ongoing tectonic setting within variable lithologies characterizing the crust of Central Sicily. Migrated sections underline distinctive features of Hyblean Plateau foreland and above all a crustal thinning towards the Caltanissetta trough, to the contact with a likely deep Permo-Triassic rifted basin or rather a zone of a continent to oceanic transition. Inhomogeneity and fragmentation of Sicily crust, with a distinct separation of Central Sicily basin from western and eastern blocks, appear to have guided the tectonic transport inside the Caltanissetta crustal scale syncline and the accumulation of allochthonous terrains with south and north-verging thrusts. Major tectonic stack operated on the construction of a wide anticline of the Maghrebian chain in northern Sicily. Sequential south-verging imbrications of deep elements forming the anticline core denote a crust wedge indenting foreland structures. Deformation processes involved multiple detachment planes down to decoupling levels located near crust/mantle transition, supporting a presence of high-density lenses beneath the chain, interrelated to a southwards push of Tyrrhenian mantle and asthenosphere.

An aeromagnetic survey system based on an unmanned autonomous helicopter: Development, experiment, and analysis

ABSTRACTChina has very long coastline, but special surface conditions make it very difficult to conduct ground magnetic and marine geomagnetic surveys over beach–shallow sea transition areas such as mariculture areas, harbours, and wharfs; as a result, marine survey data cannot be linked to continental survey data, giving rise to missing data. To address this issue, an aeromagnetic survey technology based on an unmanned autonomous helicopter is proposed. Not only is an unmanned autonomous helicopter capable of aeromagnetic surveying the beach–shallow sea transition area inaccessible to vehicles and vessels but its high efficiency and low cost also enable it to be deployed on a vessel, thus making it suitable for aeromagnetic surveying oceanic islands, too. The setup of the aeromagnetic survey system based on an unmanned aerial vehicle (UAV) is introduced, and its trial flight information and processed and analysed magnetic survey data obtained in trial flights are discussed. The measurement accuracy of UAV aeromagnetic data is shown to be comparable to that of marine geomagnetic survey data, and the system effectively compensates for missing magnetic data from inshore and continent–ocean transition zones.

Isostasy as a tool to validate interpretations of regional geophysical datasets – application to the mid-Norwegian continental margin

Abstract Isostasy is a well understood concept, yet rarely applied to its full capacity in regional interpretations of crustal structures. In this study, we utilize a recent density model for the entire NE Atlantic, based on refraction seismic data and gravity inversion, to calculate isostatically balanced bathymetry along the mid-Norwegian margin. Since gravity and isostatically balanced elevation are independent observables but both depend on the underlying density model, consistencies and discrepancies point towards model deficits, erroneously interpreted or poorly understood areas. Four areas of large isostatic residuals are identified. Along the outer Vøring Margin, a mass deficit points to more extensive high-density bodies or a shallower Moho than currently mapped. Farther seaward, along the Vøring Marginal High, a mass excess indicates inaccurate mapping of the continent–ocean boundary and surrounding structures. A number of eclogitic bodies along the proximal mid-Norwegian margin have been described in recent publications and their presence is now also confirmed by isostatic calculations. Major elevation and gravity residuals along the transition between the Vøring and Møre margins signify that the structure of this region is poorly understood and modifications to the mapped continent–ocean transition may be required.