Fluid Escape Research Articles

Gas hydrate accumulation associated with fluid escape structure in the western margin of South China Sea

Gas hydrate accumulation associated with fluid escape structure in the western margin of South China Sea

Seismic characterization of a fluid escape structure in the North Sea: the Scanner Pockmark complex area

SUMMARYSubsurface fluid escape structures are geological features which are commonly observed in sedimentary basins worldwide. Their identification and description have implications for various subsurface fluid flow applications, such as assuring integrity of overburden rocks to geological CO2 storage sites. In this study, we applied 3-D first-arrival traveltime tomography to a densely sampled wide-azimuth and wide-angle ocean bottom seismometer (OBS) data set collected over the Scanner Pockmark complex, a site of active gas venting in the North Sea. Seismic reflection data show a chimney structure underlying the Scanner Pockmark. The objective of this study was to characterize this chimney as a representative fluid escape structure in the North Sea. An area of 6$\times $6 km2 down to a depth of 2 km below sea level was investigated using a regularized tomography algorithm. In total, 182 069 manually picked traveltimes from 24 OBS were used. Our final velocity model contains compressional wave velocity perturbations ranging from −125 to +110 ms−1 relative to its average 1-D model and compares favourably with a coincident seismic reflection data set. The tomographic velocity model reveals that the chimney as observed in seismic reflection data is part of a larger complex fluid escape structure, and discriminates the genuine chimney from seismic artefacts. We find that part of the seeping gas migrates from a deep source, accumulates beneath the Crenulate Reflector unconformity at ∼250 m below seafloor (mbsf) before reaching the porous sediments of the Ling Bank and Coal Pit formation at <100 mbsf. In addition, the model shows that the venting gas at Scanner Pockmark is also being fed laterally through a narrow NW–SE shallow channel. Quantitative velocity analysis suggests a patchy gas saturation within the gas-charged sediments of the Ling Bank and the Coal Pit formations. Confined to the well-resolved regions, we estimate a base case average gas saturation of ∼9 per cent and in-situ gas volume of ∼1.64 $\times {10^6}\ {{\rm{m}}^3}$ across the Ling Bank and Coal Pit Fm. that can sustain the observed methane flux rate at the Scanner Pockmark for about 10 to 17 yr.

Compactive deformation of incoming calcareous pelagic sediments, northern Hikurangi subduction margin, New Zealand: Implications for subduction processes

Calcareous rocks are commonly found in subduction zones, but few studies have investigated the consolidation and compactive deformation of these rocks prior to subduction, and their potential effects on subduction and accretionary processes are thus poorly understood. Using drilling data obtained during International Ocean Discovery Program (IODP) Expeditions 372 and 375 combined with 2D and 3D seismic reflection data, the structure, growth history, and slip rates of normal faults identified in the incoming pelagic sedimentary sequences of the Hikurangi Margin were investigated. A seismic coherence depth slice and vertical profiles show that these faults exhibit polygonal structure that has rarely been documented at subduction margins. The polygonal faults are closely spaced and layer-bound within sequences dominated by pelagic carbonate and calcareous mudstone of Paleocene-Pliocene age. Kinematic modeling and 2D displacement analysis reveal that fault throws decrease toward the upper and lower tipline. In detail, two groups of throw profiles are defined by locations of displacement maxima, possibly reflecting lateral variations in physical properties. The polygonal fault system (PFS) likely formed by syneresis processes that involve diagenetically induced shear failure and volumetric contraction of the pelagic unit associated with fluid escape. Fault growth sequences reveal multiple, weakly correlated intervals of contemporaneous seafloor deformation and sedimentation and allow estimates of fault slip rates. We find evidence for a significant increase in typical slip rates from 0.5-3 m/Ma during pelagic sedimentation to >20 m/Ma following the onset of terrigenous sedimentation. These observations suggest that rapid loading of the pelagic sediments by the trench-wedge facies was associated with renewed and faster growth of the PFS. The PFS will eventually be transported into the base of the accretionary wedge, enhancing geometric roughness and heterogeneity of materials along the megathrust, and providing inherited zones of weakness. Selective fault reactivation may facilitate deformation and episodic vertical fluid migration in the lower wedge associated with microearthquakes, tremor, and slow slip events.

Late Holocene volcanic stratigraphy and eruption chronology of the dacitic Young Doña Juana volcano, Colombia

We present the late Holocene eruption history of the poorly known Doña Juana volcanic complex, in SW Colombia, which last erupted in the twentieth century. This represents a case study for potentially active volcanism in the rural Northern Andes, where tropical climate conditions and a fragmented social memory blur the record of dormant volcanoes. We reconstructed the volcanic stratigraphy of the central-summit vent area by integrating new mapping at 1:5000 scale with radiocarbon ages, sedimentology analysis, and historical chronicles. Our results revealed cyclic transitions from lava-dome growth phases and collapse to explosive Vulcanian and possibly subplinian phases. Pyroclastic density currents were generated by dome collapse producing block-and-ash flows or by pyroclastic fountain/column collapse and were rapidly channelized into the deeply incised fluvial valleys around the volcano summit. The pyroclastic density currents were ∼4−10 × 106 m3 in volume and deposited under granular flow− or fluid escape−dominated depositional regimes at high clast concentrations. In places, more dilute upper portions reached a wider areal distribution that affected the inhabited areas on high depositional terraces. The coefficient of friction (ΔH/L) is higher for block-and-ash flows and dense lava−bearing fountain/low-column-collapse pyroclastic density currents compared to pumice-bearing, column-collapse pyroclastic density currents. Associated mass-wasting processes included syneruptive and intereruptive debris flows, with the last one documented in 1936 CE.

Keratose sponges in ancient carbonates – A problem of interpretation

AbstractReports of diverse vermiform and peloidal structures in Neoproterozoic to Mesozoic open marine to peritidal carbonates include cases interpreted to be keratose sponges. However, living keratose sponges have elaborate, highly elastic skeletons of spongin (a mesoscopic end‐member of a hierarchical assemblage of collagenous structures) lacking spicules, thus have poor preservation potential in contrast to the more easily fossilized spicule‐bearing sponges. Such interpreted fossil keratose sponges comprise diverse layered, network, amalgamated, granular and variegated microfabrics of narrow curved, branching, vesicular–cellular to irregular areas of calcite cement, thought to represent former spongin, embedded in microcrystalline to peloidal carbonate. Interpreted keratose sponges are presented in publications almost entirely in two‐dimensional (thin section) studies, usually displayed normal to bedding, lacking mesoscopic three‐dimensional views in support of a sponge body fossil. For these structures to be keratose sponges critically requires conversion of the spongin skeleton into the calcite cement component, under shallow‐burial conditions and this must have occurred prior to compaction. However, there is no robust petrographic–geochemical evidence that the fine‐grained carbonate component originated from sponge mummification (automicritic body fossils via calcification of structural tissue components) because in the majority of cases the fine‐grained component is homogenous and thus likely to be deposited sediment. Thus, despite numerous studies, verification of fossil keratose sponges is lacking. Although some may be sponges, all can be otherwise explained. Alternatives include: (i) meiofaunal activity; (ii) layered microbial (spongiostromate) accretion; (iii) sedimentary peloidal to clotted micrites; (iv) fluid escape and capture resulting in bird's eye to vuggy porosities; and (v) moulds of siliceous sponge spicules. Uncertainty of keratose sponge identification is fundamental and far‐reaching for understanding: (i) microfacies and diagenesis where they occur; (ii) fossil assemblages; and (iii) wider aspects of origins of animal clades, sponge ecology, evolution and the systemic recovery from mass extinctions. Thus, alternative explanations must be considered.

Deep learning for pockmark detection: Implications for quantitative seafloor characterization

Occurring globally, pockmarks are seafloor depressions associated with seabed fluid escape. Pockmark ubiquity and morphologic heterogeneity result in an irregular seafloor that can be difficult to quantitatively describe. To address this challenge, we test the hypothesis that deep-learning based object detection and segmentation can be used to develop data-driven models for pockmark identification and characterization. This study describes the development, testing, and deployment of eight separate deep learning-based pockmark detection models using publicly available, gridded bathymetric data from the Belfast Bay, Maine, USA, Blue Hill Bay, Maine, USA, and Passamaquoddy Bay, New Brunswick, Canada estuarine pockmark fields. The models tested include three types of convolutional neural network architectures, as well as a generative adversarial network. We find that the data-driven models consistently resolve pockmarks from the background seafloor, allowing for quick and accurate delineation of pockmarks in a variety of seabed habitats. With these delineations we examine and compare the morphology of the muddy estuarine pockmark fields. We then compare these morphometric results to pockmark fields in two distinct settings, the sandy German Bight and the Aquitaine continental slope. We find that in all the pockmark fields a power law relationship, generally, exists between pockmark area and pockmark depth, though this relationship deteriorates with the smallest pockmarks, suggesting that there may be a minimum size needed for geomorphic stability. These results show that the training data and trained models developed here can be applied for quick detection and characterization of pockmarks where other high-resolution bathymetry is available, demonstrating the value of data-driven detection models for characterizing morphologically complex seafloors. Last, the morphologic characteristics of pockmarks identified in this study will aid future studies in relating pockmark size to environmental characteristics like seabed sediment texture and regional gradient.

Seismic evidence of gas hydrate and seafloor fluid escape on the upper Amazon deep-sea fan, Brazilian equatorial margin

The Amazon fan contains a gas hydrate province known from a bottom-simulating reflection (BSR) that lies within an upper slope compressional belt. In this study, the extent and character of the BSR and its relation to thrust-fold structures is examined using a grid of 2D and 3D seismic data. We show the BSR to comprise a series of elongate patches up to 16 km wide that are present along 300 km of the slope in water depths of 750-2250 m and extend over a total area of 6800 km2. The elongate BSR patches show a strong spatial correspondence with the arcuate crests of thrust-fold anticlines. In profile, the BSR patches exhibit convex forms and/or locally irregular relief that rises toward the seafloor. In plan, 3D seismic horizon maps reveal columnar BSR elevations up to 1 km wide, which rise beneath seafloor mounds and depressions, up to 0.5 km wide and 30 m in relief, interpreted as small mud volcanoes and possible pockmarks. The elongate BSR patches are interpreted to record the structurally-controlled rise of warm, gas-rich fluids into the crests of thrust-folds and their leakage into the gas hydrate stability zone, and in places to seafloor, through a near-surface system of faults, hydrofractures and vents.

Opsoclonus Myoclonus Ataxia Syndrome in a HIV Patient as a rare presentation of CSF escape: A Case Report

Cerebrospinal fluid escape is an uncommon phenomenon seen in patients where there is discordance in plasma and cerebrospinal fluid viral load. The presentation of this condition can range from being asymptomatic to the patient showing cognitive dysfunction. In addition, it can also be associated with various opportunistic neurological infections. Opsoclonus Myoclonus Ataxia is very rare entity classically associated with neuroblastoma or viral illness. Opsoclonus Myoclonus Ataxia Syndrome (OMAS) has previously been described as a presentation of Human Immunodeficiency Virus cerebrospinal fluid escape very rarely. We report a case of 55 years old patient diagnosed with HIV/AIDS, who was on first line antiretroviral therapy and presented with Opsoclonus Myoclonus Ataxia Syndrome associated with Cerebral Fluid (CSF) Escape, along with the review of the literature. Moreover, in this article, we're providing a postulation for the plausible cause behind cerebrospinal fluid escape leading to the Opsoclonus Myoclonus Ataxia Syndrome. Keywords: HIV/AIDS, CSF Escape, Opsoclonus Myoclonus Ataxia Syndrome.

Spontaneously Exsolved Free Gas During Major Storms as an Ephemeral Gas Source for Pockmark Formation

AbstractAbrupt fluid emissions from shallow marine sediments pose a threat to seafloor installations like wind farms and offshore cables. Quantifying such fluid emissions and linking pockmarks, the seafloor manifestations of fluid escape, to flow in the sub‐seafloor remains notoriously difficult due to an incomplete understanding of the underlying physical processes. Here, using a compositional multi‐phase flow model, we test plausible gas sources for pockmarks in the south‐eastern North Sea, which recent observations suggest have formed in response to major storms. We find that the mobilization of pre‐existing gas pockets is unlikely because free gas, due to its high compressibility, damps the propagation of storm‐induced pressure changes deeper into the subsurface. Rather, our results point to spontaneous appearance of a free gas phase via storm‐induced gas exsolution from pore fluids. This mechanism is primarily driven by the pressure‐sensitivity of gas solubility, and the appearance of free gas is largely confined to sediments in the vicinity of the seafloor. We show that in highly permeable sediments containing gas‐rich pore fluids, wave‐induced pressure changes result in the appearance of a persistent gas phase. This suggests that seafloor fluid escape structures are not always proxies for overpressured shallow gas and that periodic seafloor pressure changes can induce persistent free gas phase to spontaneously appear.

Seismites in Miocene gypsum microbialites: Multiproxy tools for paleoenvironmental reconstruction of saline lakes

This work provides the first evidence of seismites within Miocene gypsum microbialites deposited in inland lakes. Seismites are observed on good quality outcrops in two separated successions of the Madrid and Duero basins (central Spain) that are sited nearby strike-slip faults. The successions consist of sandy gypsum facies interbedded with gypsiferous mudstone and marlstone. The association of microbial induced sedimentary structures (MISS) with mineralogical biosignatures such as intrasediment-grown lenticular gypsum, dolomite with inner biomolds and other biological induced minerals, supports that gypsum beds represent microbialites. Gypsum microbialites are characterized by parallel-lamination, cross-bedding and laminated leveling structures. The rapid mineralization of the microbial mats and biostabilization processes may play a role in the preservation of the depositional structures in sulfates. In addition to MISS, several categories of large-scale soft-sediment deformation structures (SSDSs) reflecting ductile and brittle behavior repeat vertically through the successions. These are classified as load casts, pillows, dish structures, convolute bedding and lamination, plastic intrusions, fluid escape tubes, mud/sand volcanoes, Neptunian dikes and fractures. The deformation structures in crystalline sediments are interpreted to be caused by seismic shocks of magnitude over 5 related with syndepositional fault movements. Micromorphological analysis of gypsum beds reveals distinctive features such as cracked crystals, oriented crystals filling dikes and horizontal microstylolites that also indicate seismic shocks. Comparison between the two basins shows some differences in the type and vertical distribution of structures, and this suggests that earthquakes were less frequent and of higher magnitude in the Madrid Basin. The results confirm that microbial gypsum deposits have both high susceptibility to earthquakes and high potential to preserve a wide range of information of the sedimentary environment, including paleoecology, paleogeochemistry, paleohydrodynamics, and seismogenic behavior of microbial sediments and faults.

First evidence of (paleo)pockmarks in the Bass Strait, offshore SE Australia: A forced regression modulated shallow plumbing system

Pockmarks are morphological expressions of seabed fluid escape along continental margins. Identifying the underlying controls on their formation and spatial distribution is crucial for understanding substrate fluid plumbing systems and has important implications for hydrocarbon exploration, seafloor stability and seabed release of greenhouse gasses. Here, we use 3D seismic reflection dataset and a machine learning approach to present the first evidence for paleo-pockmarks in the Bass Strait, southeast Australia. The paleo-pockmarks are identified in the Bass Basin, within an interval between 250 and 310 m below present-day seafloor, corresponding to the Miocene carbonate-dominated Torquay Group. The paleo-pockmarks have depths ranging from ∼29 to 74 m and areas between ∼0.01 and 0.8 km2, with diameters varying between ∼0.1 and 1.1 km. The absence of an underlying seal-bypass system such as pipes and faults associated with these paleo-pockmarks discounts a deeper thermogenic source or a potential magmatic-driven fluid system. Rather a biogenic fluid system derived from the degradation of organic-rich layers and pore water expelled during early-stage compaction is hypothesised to drive paleo-pockmark formation. The seismic interval comprising these paleo-pockmarks demonstrates a distinctive seaward progradation and stepping-down configuration, indicating a forced regression. We propose this resulted in the destabilization of hydrostatic pressure triggering the creation of the paleo-pockmarks.

Significance of Pseudomeningocele After Decompressive Surgery for Chiari I Malformation.

BackgroundPseudomeningoceles (PMCs) as abnormal collections of cerebrospinal fluid are quite common findings on follow-up MRI after Chiari decompression surgery (CDS). However, the importance of their identification has not been truly determined, especially when PMCs are described occasionally in the process of radiological follow-up. We retrospectively analyzed surgical outcomes and imaging findings after CDS depending upon the occurrence and thickness of PMCs.MethodsA total of 76 adult patients who underwent CDS were analyzed. The clinical and radiological outcomes of patients with a pseudomeningocele (wPMC) were evaluated and compared to those of patients without a pseudomeningocele (w/oPMC). Radiological morphometric measurements were performed and compared between groups. Comparisons of the maximal PMC thickness were made within the wPMC group.ResultsPMCs were recognized in 27 (35.5%) patients, of whom 3 (11.1%) required reoperation. Differences in satisfactory result rates regarding gestalt assessment and Chicago Chiari Outcome Scale were statistically insignificant between the w/oPMC and wPMC groups (p = 1 and p = 0.56, respectively). The postoperative syringomyelia decrease and cerebellar tonsil elevation were similar between the groups (p = 1 and p = 0.74, respectively) in the long-term follow-up. Additionally, the clinical or radiological outcomes with radiological details were not related to PMC thickness in the long-term follow-up. However, radiological details showed the cooccurrence of PMCs with a postsurgical of cerebello-tentorial distance increase (p < 0.05), basion-pontomedullary sulcus distance decrease (p < 0.05) and tonsillo-graft distance decrease (p < 0.05).ConclusionsWe found no significant relationships between PMC presence or thickness and clinical or radiological outcomes. However, postoperative changes within the posterior fossa associated with PMCs resemble brain sagging, which occurs in intracranial hypotension. Therefore, extradural cerebrospinal fluid escape may also be responsible for symptoms in some patients with PMCs after CDS.

Episodic venting of extreme subsalt overpressure through a thick evaporitic seal

Episodic venting from deep overpressured layers has rarely been documented in sedimentary basins, so the pressure evolution leading to these cyclical phenomena is poorly understood at present. Using three-dimensional (3D) seismic data from the West Nile Deep Sea Fan, we interpret >480 fluid escape pipes within a region of prolific leakage across a > 2 km thick sealing sequence of salt and claystones. Within this region, we map five linear trails of pipes with pockmarks or small mud volcanoes at their outlets and interpret them to have formed by episodic venting of overpressured fluids from beneath the salt, over a ∼2–3 Myr period, coeval with basinward displacement of the pipes by flow of the salt. Importantly, the pipe trails root to the crests of stratigraphic traps at the base of the salt seal. Direct measurements from nearby exploration wells are used to construct pressure-depth and pressure-time plots over the lifetime of the pipe trails and demonstrate that overpressure near to or in excess of the lithostatic pressure must be repeatedly achieved in the stratigraphic traps to breach the salt seal every 50–150 kyrs. This pressure evolution is best described by a sawtooth pattern with overpressure relieved by venting, followed by re-seal of the trap, and pressure recharge. The pressure recharge for this episodic venting can most easily be explained here by biogenic gas generation and aquifer pressure transfer in the traps, with some minor contribution from sustained disequilibrium compaction during the Pliocene to Recent. The conditions necessary for this type of episodic venting should occur more widely, particularly in basins with thick evaporite deposits. The recognition of episodic venting has important implications for prospect de-risking, drilling safety and CO2 and hydrogen storage.

Distribution and geological controls of the seabed fluid flow system, the central‐western Bohai Sea: A general overview

AbstractThorough understanding of seabed fluid flow system is of great significance to geohazard identification and hydrocarbon exploration as it can reshape the seabed and act as an indicator of subsurface hydrocarbon resources. For the first time, an integrated study of side‐scan sonar, single‐ and multi‐channel seismic data and magnetic data reveals a complex fluid flow system composed of various seafloor expressions (i.e. pockmarks and mounds) and shallow fluid migration pathways in the central‐west Bohai Sea off northeast China. Gas chimneys, mud diapirs and a dense network of Quaternary faults are the main fluid migration pathways in the shallow subsurface. The gas chimneys can be classified into three categories (Type A formed by relatively rapid gas escape, Type B formed by episodic fluid expulsion and Type C formed by fluid escape from mud diapirs), based on their distribution and seismic character, implying variability in the formation processes. Sediment remobilization and basement‐involved faults contribute to deep fluid migration into shallow depths. As a seal for up‐moving fluids, the nature and thickness of Holocene marine sediments generally decide the permeability and overburden pressure that may control the distribution of pockmarks and mounds since they are almost distributed above relatively thin Holocene deposits (thickness &lt;20 m) and localized coarse surface sediments. The results of the interpretation gain an improved understanding of the geological processes controlling the genesis and spatial distribution of gas chimney formation and show the significance of gas chimney classification. The distribution pattern of different types of gas chimneys may signify the difference of geological background and fluid flow process, like fluid migration through faults or flow of mobilized sediments, that is crucial for the evaluation of global petroleum systems and Carbon Capture and Storage studies.

TECTONOSTRATIGRAPHIC EVOLUTION AND HYDROCARBON PROSPECTIVITY SOUTH OF GWADAR BAY, MAKRAN ACCRETIONARY WEDGE, OFFSHORE SW PAKISTAN

The Makran accretionary wedge developed as a result of subduction of the Arabian Plate beneath the southern margin of Eurasia since the Eocene. Interpretation of 2D seismic profiles calibrated to offshore well data in a study area to the south of Gwadar Bay (SW Pakistan) indicates a major period of accretion from the mid‐Miocene, as evidenced by the occurrence of thick growth strata associated with large‐scale imbricate thrusts. The thrust faults originate from a deep detachment within the mud‐rich Oligocene interval, and well‐developed piggy‐back basin successions occur in thrust hanging walls. In the study area, the thrust structures are sealed by a thick, progradational Pliocene to Recent interval in which the presence of submarine canyons, up to 2.5 km across, indicate that sedimentary transport was from the north.Fluid escape pipes and associated amplitude anomalies are observed in the seismic profiles studied and may be related to upward migration of thermogenic hydrocarbons from depth, as heavy hydrocarbon fractions &lt;C3 have been reported from nearby wells. The hydrocarbons are believed to have been sourced from the Oligocene Hoshab Shale and to have then migrated up through a sedimentary succession in which permeability barriers are largely absent. Hanging wall anticlines mapped in the study area could provide structural traps, and turbidites in the Lower Miocene Panjgur Formation may represent a potential reservoir. Amplitude anomalies are also observed adjacent to shallow fluid escape pipes within the topsets of clinoforms in the Pleistocene Chatti and Omara Formations, and probably indicate the presence of biogenic hydrocarbons sourced from distal mudstones in bottomset strata.

Management of a human immunodeficiency virus case with discordant antiviral drug resistance profiles in cerebrospinal fluid compared with plasma: a case report

BackgroundHuman immunodeficiency virus-1-associated neurocognitive disorder is a known complication in individuals treated with antiretroviral therapy. Cerebrospinal fluid escape, which is defined as discordant higher cerebrospinal fluid viremia than plasma, may occur in antiretroviral therapy-experienced individuals. Different cerebrospinal fluid versus plasma mutation patterns have been observed in individuals with cerebrospinal fluid escape.Case presentationA 46-year-old adult African male with human immunodeficiency virus-1 infection and acquired immunodeficiency syndrome based on cerebral toxoplasmosis and a chronic hepatitis B virus infection developed cerebrospinal fluid escape. A different human immunodeficiency virus-1 genotypic drug resistance profile was observed in plasma compared with cerebrospinal fluid. Brain biopsy and cerebral magnetic resonance imaging indicated the development of human immunodeficiency virus encephalopathy. A discordant protease inhibitor mutation/wild-type T74PT in plasma but not in cerebrospinal fluid indicated poor central nervous system penetration due to the selective pressure of drug therapy. An intensified antiretroviral therapy regimen including dolutegravir with good central nervous system penetration improved conditions.ConclusionsThis case shows the importance of measuring human immunodeficiency virus drug resistance in cerebrospinal fluid, which might differ from resistance detected in plasma samples and target effective antiretroviral therapy treatment accordingly.

Sediment deformation atop the Lomonosov Ridge, central Arctic Ocean: Evidence for gas-charged sediment mobilization?

We have used a hovercraft platform drifting with the sea ice to acquire the first digitally recorded seismic reflection data transects across the Canada/Greenland (89°N-85°N) section of the Lomonosov Ridge, central Arctic Ocean. The flat-lying, laterally uniform Cenozoic sediment package on top of the ridge at 87°N, 60° W shows at least four sites with local seismic amplitude anomalies. The common feature is a column (<600 m wide) of partly discontinuous or chaotic bright reflection events at the center of a <1.5 km wide dome (amplitude <25 m) terminating at the seabed in a 8–12 m deep depression. The amplitude anomalies are interpreted as gas-charged fluid escape pipes marked by a pockmark at the seabed. Gas and fluids introduced from below have mobilized the overlying high porosity, low density Eocene bio-siliceous ooze causing the doming. The gas and fluids appear to originate from the top of rotated fault blocks and sub-basalt sediments of Mesozoic or older age deposited when the Lomonosov Ridge was part of the pre-Late Cretaceous continental margin north of Franz Josef Land.

Sedimentological analysis of ash-rich pyroclastic density currents, with special emphasis on sin-depositional erosion and clast incorporation: The Brown Tuff eruptions (Vulcano, Italy)

The sedimentological, lithological and textural characteristics of the Brown Tuffs (BT) pyroclastic deposits, combined with their grain-size, componentry and geochemical glass compositions, are here investigated to obtain information on the transport and depositional mechanisms of the corresponding pyroclastic density currents (PDCs). The BT are widespread reddish-brown to grey, ash-rich pyroclastic deposits generated by pulsating hydromagmatic explosive activity from the La Fossa Caldera on Vulcano island during the c. 80–6 ka time-stratigraphic interval, and then distributed on most of the Aeolian Islands and Capo Milazzo peninsula (Sicily) and in the Tyrrhenian and Adriatic Sea regions. Near the source area on Vulcano, the BT are characterised by alternating massive and planar to cross stratified lithofacies that result from the stepwise, repeating aggradation of discrete PDC pulses. This alternance is regulated by either fluid escape or granular flow depositional regimes at high clast concentration or grain by grain traction deposition in the waning diluted stages of the PDCs. Most of the BT on Vulcano show intermittently stratified and massive ash deposits resulting from a pervasive post-depositional disruption of the primary structures. This is induced by upward fluid expulsion associated with dissipation of pore pressure between layers at different grain size (fine to coarse ash) and porosity, as outlined by distinctive upwards bends and pillar-type escape structures through the fluid-filled cracks and rupture points. Massive BT deposits with a faint colour and grain-size banding are widely recognised on Lipari, the nearby island of Vulcano. Based on the presence, at the base of BT depositional units, of cm-thick amalgamation bands containing pumice lapilli, scoria and lithic clasts ripped-up and embedded from the loose underlying pyroclastic units, they are interpreted as deposited by ash-rich PDCs laterally spreading from La Fossa Caldera and moving to Lipari. During their motion to Lipari these currents (likely) crossed a narrow and shallow sea-water inlet which did not stop their advancement but influenced the grain size distribution of those spreading on the Lipari mainland. In this paper, the mechanism of clast erosion and incorporation is outlined across the whole island of Lipari by means of field study, grain-size, and geochemical glass analyses on the different components of the mixed basal bands of the BT. This suggests that the BT PDCs maintained enough flow power as to erode the substratum, hence likely impacting the territory, over a distance up to at least 16–17 km from the volcanic source. Evidence that the BT PDCs exerted a high shear-stress over the loose substratum is also provided by undulated, recumbent flame and rip-up structures at the base of some depositional units in southern and central Lipari. In order to form such bed granular instabilities between the BT and the underlying deposits we calculate that the currents had at least a shear velocity of ca. 2 m s−1 and a shear stress in the range of 1‐4.5 kPa. These results add new insights on the large-scale hazard at the Aeolian Islands and shed new lights on the widespread transport and depositional dynamics of ash flows spreading over the sea and reaching nearby islands, and their interactions with the substratum and the pre-depositional topography.

The anisotropic properties of granites – effects of tectonic emplacement mode on potential crystalline host rocks for nuclear waste deposits in Germany

Abstract. For permanent nuclear waste disposal sites, crystalline rocks, especially granitic/granodioritic batholiths, are considered an appropriate host rock. Principally, three types of granitic plutons occur in the extra-alpine crystalline basement of Germany that were consolidated during the late Paleozoic Variscan orogeny of Central Europe: (i) Pre-Variscan voluminous granodiorites that are hardly affected by the subsequent continent–continent collision; (ii) voluminous granites in various tectonic settings intruded during the late orogenic stage of the Variscides; (iii) post-orogenic granites related to vast Permian intracontinental extension. Thus, in terms of the syn-intrusive tectonic setting and post-intrusive processes there are significant differences. Although it can be expected that different tectonic environments caused significant differences in the material properties, for Germany, however, there is no systematic study regarding the fabric of such plutonites. In order to find the most suitable “granite” we investigate the primary anisotropy of granites evolved during the emplacement and crystallization of the melt. For this we sample rocks of all three principal types and various syn-intrusive tectonic settings, i.e., compression, extension, strike-slip, transtension, and transpression. By means of combined measurements of the “Anisotropy of the Magnetic Susceptibility” and the “Shape Preferred Orientation” we characterize the syn-intrusive flow pattern, i.e., the magmatic foliation and lineation. The Crystallographic Preferred Orientation is analyzed by a combination of neutron time-of-flight experiments and electron backscatter diffraction measurements at the Frank Laboratory of Neutron Physics at JINR, Dubna, Russia, and the TU Bergakademie Freiberg respectively. Furthermore, special attention is given to the systematic mapping of annealed microcracks evolved during late magmatic fluid escape and/or post-crystallization hydrothermal activity. In a second step we compare the primary anisotropy with the post-magmatic fracture pattern of the particular granites. Those fractures constitute probable fluid pathways and, thus, the first-order risk for a potential permanent nuclear waste disposal. All datasets are organized in a Geological Information System allowing for a complete traceability of the different investigation steps. The results of this study will serve as a basis for a future detailed exploration.

Submarine landsliding in carbonate ooze along low-angle slopes (Inner Sea, Maldives)

Multichannel seismic and hydroacoustic data from the Maldives carbonate platform in the northern Indian Ocean document 4–8 km wide and are 70–170 m thick submarine landslides at water depths greater than 500 m, potentially associated with extensive fluid escape and the occurrence of gas hydrates. The mobilized sediments are part of giant drift bodies that consist of fine-grained carbonate ooze. Their downslope extent is located adjacent to large gateways that connect the semi-enclosed inner basin of the carbonate platform with the open Indian Ocean. Exposed at the sea-floor are headwall and translational domain of the landslides, showing different kinematic indicators typical for mass transport complexes. The submarine landslides evolved at slightly inclined slopes (<1°) along a listric basal shear surface (BSS) by downslope progradation and retrogressive failure. Styles of motion include sediment creeping, sliding and slumping. Where the BSS is flat, sediments translate as a large body. Where the BSS is irregular, folded, thrusted and rotated blocks/ridges occur, associated with secondary listric shear faults. Characteristically for the headwall domain are arcuate depressions which are related to rollover structures in the displaced slide block. The distal toe domain of the submarine landslides is located at the steep (>20°) part of the platform slope which faces the Indian Ocean. Data show that landslide activity is ongoing.The likely trigger of slope failure is a combination of gravitational forces and the reduction of sediment shear strength moderated by fluid ascent and the destabilization of gas hydrates. Gas hydrates are suggested to play an important rule, because the top of the gas hydrate stability zone coincides with a significant steepening of the platform slope from 1-3° to 10–20°. It is therefore proposed that the dissociation of the gas hydrates, for example during sealevel lowstands might have been an additional trigger of slope failure. During sealevel highstands, gas-hydrate coating of sediment grains may facilitate low-velocity plastic deformation of the strata.