Hydrothermal Vent Complexes Research Articles

Seismic interpretation of the Mero Field igneous rocks and its implications for pre- and post-salt CO2 generation – Santos Basin, offshore Brazil

Understanding igneous rock emplacement is crucial for deciphering any sedimentary basin's evolution. Its occurrence can be a determining factor for elements of the petroleum system, such as hydrocarbon generation, migration, trap formation, and reservoir quality. The high-impedance nature of igneous rocks in relation to their surrounding host rocks renders amplitude anomaly-based attributes sufficient for the identification of igneous bodies within siliciclastic sequences. However, this is not necessarily valid in pre-salt settings because of the high-impedance carbonates that make the impedance contrast subtle. Furthermore, if the emplacement characterization of the various magmatic pulses that occur in a given area over geological time were better evaluated, it would be possible to assess the gases (such as CO2) they produce with higher reliability. In this paper, the principal geometric and petroelastic characteristics of the igneous rocks of the Mero Field are described in detail in order to estimate the thermogenic production of CO2. Both manual and semi-automatic seismic interpretation were carried out to quantitatively and properly reduce the uncertainties inherent to the seismic mapping of these rocks. Thus, a 3D probabilistic igneous facies classification was performed based on a Bayesian inference using the P- and S-impedance well log data available and seismic elastic inversion volumes. These volumes of occurrence probability were useful for semi-automatically extracting maps and geobodies, which, along with conventional manual seismic interpretation, provided hypothetical volumetric scenarios for Mero igneous rocks and also for CO2 production. Five main intrusive features were recognized based on mainly in seismic geometries found in the Mero Field, two of them being Lower Cretaceous rift-related magmatism synchronous with the pre-salt reservoir deposition and the other three being related to the Santonian/Campanian magmatism. The analyzed data suggests that deep intrusions were responsible for the installation of a multiple hydrothermal vent system in the Santos Basin. During pre-salt sedimentation in the Lower Cretaceous, the emplacement of tholeiitic sills, primarily in low-energy facies at structural depressions, may have played a significant role in the thermal alteration of the sedimentary framework and would have released a significant amount of CO2 into the atmosphere. It's possible that the reactivation and reuse of these older structures (i.e., sill-related hydrothermal vent complexes generated by tholeiitic intrusions) during alkaline Santonian/Campanian magmatism might have contributed further to the production of additional carbon dioxide, which would be trapped in pre-salt reservoirs in this instance.

Shallow-water hydrothermal venting linked to the Palaeocene–Eocene Thermal Maximum

The Palaeocene–Eocene Thermal Maximum (PETM) was a global warming event of 5–6 °C around 56 million years ago caused by input of carbon into the ocean and atmosphere. Hydrothermal venting of greenhouse gases produced in contact aureoles surrounding magmatic intrusions in the North Atlantic Igneous Province have been proposed to play a key role in the PETM carbon-cycle perturbation, but the precise timing, magnitude and climatic impact of such venting remains uncertain. Here we present seismic data and the results of a five-borehole transect sampling the crater of a hydrothermal vent complex in the Northeast Atlantic. Stable carbon isotope stratigraphy and dinoflagellate cyst biostratigraphy reveal a negative carbon isotope excursion coincident with the appearance of the index taxon Apectodinium augustum in the vent crater, firmly tying the infill to the PETM. The shape of the crater and stratified sediments suggests large-scale explosive gas release during the initial phase of vent formation followed by rapid, but largely undisturbed, diatomite-rich infill. Moreover, we show that these vents erupted in very shallow water across the North Atlantic Igneous Province, such that volatile emissions would have entered the atmosphere almost directly without oxidation to CO2 and at the onset of the PETM.

Seismogenic Structure and Tectonic Mechanism of the 2021 Mw 5.0 Yancheng Earthquake in the South Yellow Sea Basin, East Asia

AbstractOn 17 November 2021, a moment magnitude (Mw) 5.0 earthquake occurred off the Yancheng city in the South Yellow Sea. This earthquake has caused ground motions and been felt by people in a few megacities in east China, for example, Shanghai, Nanjing. However, due to limited seismic data, the focal fault and mechanism of this earthquake remain unknown. Here, we combine the seismic profiles, seismicity, focal mechanism, well data with regional tectonics to investigate the seismotectonics of this event and the possible role of fluid migration in triggering the earthquake. Our new results suggest that two sets of preexisting strike-slip faults were developed near the epicenter of the Yancheng earthquake, including the north–northeast-trending fault F1, the North Wunansha fault, and the northwest-trending fault F5. These preexisting faults have been reactivated entirely or partly with dextral and sinistral motions during the Miocene to Quaternary. Fault F1 runs through the epicenter of the Yancheng earthquake and is therefore interpreted to be the seismogenic fault. Various igneous complexes and associated hydrothermal vent complexes have been identified in the vicinity of the reactivated fault F1 and other strike-slip faults. Combined with the published igneous rock ages from nearby region, the igneous and hydrothermal activities were suggested to be continue until the Miocene and Quaternary, which are coincident with the reactivation of the preexisting strike-slip faults. The reactivation of the preexisting fault, and igneous and associated hydrothermal activities were suggested to be caused by the subduction of Pacific and Philippine Sea plates and collision of India and Eurasia plate. The hydrothermal fluids may have migrated through the preexisting fault F1 and other strike-slip faults, which enhanced the reactivation of these faults and in turn promoted the fault slip, eventually triggering the Yancheng earthquake and other historical events in the study area.

Characterizing ancient and modern hydrothermal venting systems

Ancient hydrothermal vent complexes have released large volumes of greenhouse gases in the past causing global warming, and similar modern vent structures are potential geohazards. In the NE Atlantic, thousands of hydrothermal vent complexes were formed during the Paleocene-Eocene Thermal Maximum. In Java, Indonesia, the erupting Lusi sediment-hosted geothermal system caused the displacement of 40,000 people. In order to determine how ancient and modern hydrothermal venting systems are related, we map a well-defined buried hydrothermal vent complex offshore mid-Norway using 3D seismic reflection data and then compare it to the active Lusi eruption (since 2006) and the neighboring inactive Porong Structure. These are characterized using 2D seismic reflection data, borehole data and field observations. The venting structures are subcircular in plan-view and a few kilometers in diameter. They are funnel-shaped in profiles, with inward-dipping beds surrounding the conduits. The hydrothermal vent complex offshore mid-Norway reveals five seismically-distinct vent fill facies units. Importantly, two of the facies units are separated by an angular unconformity, clearly indicating that the depositional events within the vent fill were distinct. Hydrothermal fluids are interpreted to have led to the fluidization of mud-rich sediments which were erupted and deposited in and around the vent complex. Interpretation of a seismically transparent body along the conduit of the Norwegian venting structure, and the abrupt widening of the conduit at the Porong Structure, are interpreted to be caused by changes in fluid-flow dynamics as the fluids rise and get released from the host-rock. The hydrothermal venting systems in Java and offshore mid-Norway are found to be morphologically similar and are interpreted to form as the result of the transport and eruption of fluidized sediments.

Hydrothermal Vent Complexes Control Seepage and Hydrocarbon Release on the Overriding Plate of the Tyrrhenian-Ionian Subduction System (Paola Basin)

Active fluid seeps have been described in a wide range of geological environments and geodynamic contexts, which include continental shelves of non-volcanic passive margins and accretionary wedges. Fluids seeping in hybrid volcanic-sedimentary basins, characterized by the presence of magmatic intrusive complexes, have always received less attention. We detected and imaged dozens of distinct gas flares, as high as 700 m, on the continental slope of the Paola Basin in the southeastern Tyrrhenian Sea, at 550–850 m water depth. The sedimentary basin is surrounded by Pleistocene active and inactive volcanoes and volcanic-intrusive complexes, which formed in the back-arc basin of the Calabrian subduction zone, in response to subduction-induced mantle flow. Gas flares develop above pockmarks, craters and mud flows that form over and along the scarps of mound structures and correspond to seismic zones of free gas accumulation in the sub-seafloor. Here, methane-derived siderite shows enrichment in δ13C and δ18O isotopes likely related to methanogenesis and intermittent venting of deep-sourced CO2. Multichannel seismic reflection data showed that the gas flares develop in correspondence of doming and diapirism apparently originating from the top of the Messinian evaporites and nearby magmatic sills, that are present in the lower part of the Plio-Quaternary succession. These diapiric structures can be related to seafloor hydrothermal vent complexes fed by the igneous intrusions. Our data suggest that the vent complexes acted as fluid migration pathways and gas conduits, which at times are bounded by deep-rooted normal faults, leading to post-explosive near-surface microbial activity and seep carbonate formation. Fluids being mobilised by magmatism in the study area include: hydrocarbons and hydrothermal fluids generated at depth, interstitial water expelled during formation of polygonal faults. The close spatial correlation between seafloor seep manifestations, fluid migration pathways in the sub-surface involving part of the Messinian units and igneous features indicates that magmatic activity has been the main driver of fluid flow and can have a long-term effect in the southern Tyrrhenian Sea.

Archaeal lipid diversity, alteration, and preservation at the Cathedral Hill deep sea hydrothermal vent, Guaymas Basin, Gulf of California, and its implications regarding the deep time preservation paradox

Archaea are among the earliest evolved organisms. Today, they exist in nearly every habitable environment and their highly recalcitrant membrane lipids are both abundant and easily detected in modern sediments. Yet, unlike bacteria and eukaryotes, the lipid biomarker signatures of archaea are almost entirely absent in the ancient rock record (>145 Ma). We present a comprehensive study of archaeal lipids from the Cathedral Hill hydrothermal vent complex in Guaymas Basin, Gulf of California. Here, porewaters reach 155 °C by 21 cm below the sea floor (cmbsf), which enables the near-complete tracking of sedimentary organic matter (SOM) accumulation, diagenetic alteration and thermochemical transformation into petroleum-like hydrocarbons within and beyond the habitable zone of life. Identified intact polar lipids (IPLs) of living archaea included mono- and di-glycosidic archaeols (1G-AR, 2G-AR) and glycerol dialkyl glycerol tetraethers (1G- and 2G-GDGTs). The 1G- and 2G-ARs, potentially derive from methane-oxidizing archaeal groups ANME-1 and ANME-2 and anaerobic thermophilic methanogens, reach sediment depths of ∼ 135 cm and 50 °C temperatures. The 1G-and 2G-GDGT lipids reach a depth of ~180 cm where vent fluid temperatures are ~145 ± 15 °C. Core lipids (CLs) include archaeols (AR) and isoprenoidal and branched GDGTs (iGDGTs, brGDGTs). The core GDGTs (cGDGTs) closest to the vent center have high rates of lipid loss. Up to 95% of all identified archaeal biomarkers, including lipids supplied from the upper water column, are degraded or recycled in the surface sediments and do not reach conditions of late diagenesis and catagenesis. Only ∼ 0.11%, proportional to the lipid bound component in the protokerogen, marks thermochemically cracked biphytane – a prominent archaeal hydrocarbon biomarker. This extreme reduction indicates archaeal lipids do not necessarily become incorporated into kerogen thereby helping to explain the deep time archaeal lipid preservation paradox.

Widespread hydrothermal vents and associated volcanism record prolonged Cenozoic magmatism in the South China Sea

Abstract The continental margin of the northern South China Sea is considered to be a magma-poor rifted margin. This work uses new seismic, bathymetric, gravity, and magnetic data to reveal how extensively magmatic processes have reshaped the latter continental margin. Widespread hydrothermal vent complexes and magmatic edifices such as volcanoes, igneous sills, lava flows, and associated domes are confirmed in the broader area of the northern South China Sea. Newly identified hydrothermal vents have crater- and mound-shaped surface expressions, and occur chiefly above igneous sills and volcanic edifices. Detailed stratigraphic analyses of volcanoes and hydrothermal vents suggest that magmatic activity took place in discrete phases between the early Miocene and the Quaternary. Importantly, the occurrence of hydrothermal vents close to the present seafloor, when accompanied by shallow igneous sills, suggest that fluid seepage is still active, well after main phases of volcanism previously documented in the literature. After combining geophysical and geochemical data, this study postulates that the extensive post-rift magmatism in the northern South China Sea is linked to the effect of a mantle plume over a long time interval. We propose that prolonged magmatism resulted in contact metamorphism in carbon-rich sediments, producing large amounts of hydrothermal fluid along the northern South China Sea. Similar processes are expected in parts of magma-poor margins in association with CO2/CH4 and heat flow release into sea water and underlying strata.

Shale gas leakage in lower Ecca shales during contact metamorphism by dolerite sill intrusions in the Karoo Basin, South Africa

Abstract Contact metamorphism along widespread dolerite sills and dykes, emplaced at 182 to 183 Ma through the sedimentary host rocks of the Karoo Basin, triggered devolatilization of carbon-rich shales of the Lower Ecca Group. Hornfel samples collected from drill cores that intersect dolerite sills were analyzed for mineral phase equilibria, chemistry and porosity to characterize thermal aureoles at various distances from sill intrusions. Andalusite-chiastolite and cordierite porphyroblasts with biotite and muscovite occur within 10 to 20 m of many intrusive contacts. These metamorphic minerals crystallized when host shales attained maximum temperatures ranging between 450 and 600°C. Scanning electron microscopy imaging confirms that the hornfels are compact and that their metamorphic minerals limit porosity along grain boundaries. In few cases intra-mineral porosity occurs within individual crystals such as calcite, andalusite and cordierite. Disequilibrium metamorphic textures such as irregular grain boundaries, and inclusions in andalusite and cordierite reveal that the elevated temperatures were too short-lived to accomplish complete (re)crystallization. Thermal modeling results are consistent with the observed metamorphic mineral assemblages. Gas leakage calculations along a 7 m and a 47 m thick dolerite sill that intrude toward the top of the Whitehill Formation suggest that methane volumes ranging between 8 to 15 Tcf were generated during the sill emplacement. Methane was likely released into the atmosphere through hydrothermal vent complexes that are well preserved in the western Karoo Basin. If such loss was widespread across the entire basin, the implications for paleo-climate change and preserved shale gas reserves in the Karoo Basin of South Africa would be significant.

Hydrocarbon transformations in sediments from the Cathedral Hill hydrothermal vent complex at Guaymas Basin, Gulf of California – A chemometric study of shallow seep architecture

Guaymas Basin isa submarine depression in the Gulf of California marking the northern end of the East Pacific Rise mid-oceanic spreading ridge.The basin receives high input of sedimentary organic matter (SOM) from elevated productivity in the overlying surface waters and runoff from the surrounding continent. This, coupled with high sedimentation rates, produces near-uniform compositions of SOM.Various hydrothermal vent complexes occur along this margin. One of these is Cathedral Hill, a hydrothermal mound with sulfide chimneys surrounded by mats dominated by sulfide-oxidizing Beggiatoa, covering sediments stained by locally produced oil. We collected four push cores along a transect at this site (4 m × 0.21 m) extending from near the vent center to the ambient sediment just outside the microbial mat cover. Porewater temperatures near the mound center were projected to reach 155 °C by 21 cm below the sea floor (cmbsf). Within these conditions, a kinetic model based on vitrinite reflectance equivalence (%Re) predicts petroleum formation as shallow as 15–18 cmbsf, with metagenesis commencing at <60 cmbsf. Bulk extract data (total lipid extracts as well as polar and apolar fractions), solvent-extracted sediment TOC (herein referred to as protokerogen TOC), and molecular thermal maturation parameters support these generation estimates. In recent years, the application of chemometric techniques tocomprehensive two-dimensional gas chromatographic (GC × GC) analyses has allowed comparison of thousands of unique hydrocarbons within oils. Here we reconstruct the shallow subsurface petroleum system by applying multiway principal component analysis (MPCA) and hierarchical cluster analysis (HCA)directly to GC × GC chromatograms. We then compare the resulting multivariate models to a systematic survey of subtracted GC × GC chromatograms, a transect heat map of sample hydrocarbon compound diversities, and profiles of various thermal maturation parameters to elucidate how these hydrocarbon matrices are attenuated by production, migration, and/or thermochemical oxidation. Sample matrices have up to 5800 unique compounds spanning a range ofnormal and branched alkanes, saturated and unsaturated biomarkers, substituted and unsubstituted polycyclic aromatic hydrocarbons (PAHs), perhydro-PAHs, and benzothiopheneswith up to six ring-cycles (i.e., benzoperylenes, dibenzopyrenes, dibenzochrysenes, and indenopyrenes). These matrices display systematic temperature-dependent trends. Generation likely begins in 6–10 and 15–18 cmbsf where sediments are exposed to ∼115 °C vent porewater temperatures, which is shallower than predictions based on our kinetic model. Independent of these sites of generation is ubiquitous staining of the sediments from advected oil that is heavily dominated by PAHs as well as two stratigraphic bandsof migrated oil that extend horizontally across the transect at 0–2 and ∼6–10 cmbsf, respectively.The MPCA models along with non-statistical validation techniques show evidence of decreasingdiversities and concentrations of alkylated aromatic hydrocarbons concomitant with elevated abundances of dealkylated PAHs and/or the migration of unsubstituted PAHs from deeper basin depths as sediments become exposed to more severe hydrothermal conditions. These results indicate that even at relatively small spatial scales, the petroliferous sediments at hydrothermal vent sites can be highly complex.

Reutilisation of hydrothermal vent complexes for focused fluid flow on continental margins (Modgunn Arch, Norwegian Sea)

AbstractConventional three‐dimensional (3D) seismic data reveal abundant igneous activity on the Modgunn Arch, mid‐Norwegian margin. Magmatic sills and associated hydrothermal vent complexes located at various depths prove the repeated utilisation of Paleocene‐Eocene magmatic conduits. In total, 125 sills and 85 hydrothermal vent complexes were identified and mapped, with vent complexes ranging in diameter from 300 to 3,100 m and sills from 0.5 to 50 km. Three examples of stacked vent complexes are presented, revealing large eruptions of hydrothermal fluids vertically through the same conduit, from sills to the palaeo‐sea floor. The vent complexes are found throughout Paleocene strata (66–56 Ma), whilst at least ten (10) vents were active during the Eocene. This study emphasises the importance of characterising ancient magmatic structures, as hydrothermal conduits and vent structures were, and may still be, reutilised as preferential fluid flow pathways to shallower strata. A minimum of four phases of hydrothermal vent complex formation are inferred. Cretaceous faults are both bypassed and used for magma and fluid flow. The reutilisation of magmatic structures here described may bring to light previously overlooked plays and renew interest in exploring magma‐rich continental margins.

Potential for Sullivan-type Pb-Zn-Ag deposits in modern sedimentary basins

Numerous modern sedimentary basins contain geological features similar to those associated with the giant (150 Mt) Sullivan Pb-Zn-Ag deposit (British Columbia) in the Mesoproterozoic Belt-Purcell Basin. Important features at Sullivan include a large synsedimentary mud volcano that hosts the stratabound and partly stratiform sulfide ores, an underlying vent complex of altered and mineralized rocks, and mafic sills in the shallow to deep footwall. High-resolution 3D seismic profiles of the North Sea region and the South China Sea, as well as other late Mesozoic to Cenozoic basins such as the Irish Rockall Basin, suggest the presence of mafic sill complexes with overlying fluid-escape pipes, mud volcanoes, and hydrothermal vent complexes of diverse size and shape. Although fluid-escape pipes and related discordant features in modern sedimentary basins are widely accepted as being linked to hydrocarbon generation and resources, potential for metal concentrations in these basins should also be tested. Evaluating this possibility can be done using high-resolution seismic surveys followed by deep drilling.

Mercury anomalies across the Palaeocene–Eocene Thermal Maximum

Abstract. Large-scale magmatic events like the emplacement of the North Atlantic Igneous Province (NAIP) are often coincident with periods of extreme climate change such as the Palaeocene–Eocene Thermal Maximum (PETM). One proxy for volcanism in the geological record that is receiving increased attention is the use of mercury (Hg) anomalies. Volcanic eruptions are among the dominant natural sources of Hg to the environment; thus, elevated Hg∕TOC values in the sedimentary rock record may reflect an increase in volcanic activity at the time of deposition. Here we focus on five continental shelf sections located around the NAIP in the Palaeogene. We measured Hg concentrations, total organic carbon (TOC) contents, and δ13C values to assess how Hg deposition fluctuated across the PETM carbon isotope excursion (CIE). We find a huge variation in Hg anomalies between sites. The Grane field in the North Sea, the most proximal locality to the NAIP analysed, shows Hg concentrations up to 90 100 ppb (Hg∕TOC = 95 700 ppb wt %−1) in the early Eocene. Significant Hg∕TOC anomalies are also present in Danish (up to 324 ppb wt %−1) and Svalbard (up to 257 ppb wt %−1) sections prior to the onset of the PETM and during the recovery period, while the Svalbard section also shows a continuous Hg∕TOC anomaly during the body of the CIE. The combination with other tracers of volcanism, such as tephra layers and unradiogenic Os isotopes, at these localities suggests that the Hg∕TOC anomalies reflect pulses of magmatic activity. In contrast, we do not observe clear Hg anomalies on the New Jersey shelf (Bass River) or the Arctic Ocean (Lomonosov Ridge). This large spatial variance could be due to more regional Hg deposition. One possibility is that phreatomagmatic eruptions and hydrothermal vent complexes formed during the emplacement of sills led to submarine Hg release, which is observed to result in limited distribution in the modern era. The Hg∕TOC anomalies in strata deposited prior to the CIE may suggest that magmatism linked to the emplacement of the NAIP contributed to the initiation of the PETM. However, evidence for considerable volcanism in the form of numerous tephra layers and Hg∕TOC anomalies post-PETM indicates a complicated relationship between LIP volcanism and climate. Factors such as climate system feedbacks, changes to the NAIP emplacement style, and/or varying magma production rates may be key to both the onset and cessation of hyperthermal conditions during the PETM. However, processes such as diagenesis and organic matter sourcing can have a marked impact on Hg∕TOC ratios and need to be better constrained before the relationship between Hg anomalies and volcanic activity can be considered irrefutable.

Stable isotope constraints on the fluid source of hydrothermal breccia pipes in the Tankwa Karoo depocentre, South Africa: Breakdown of authigenic minerals during sill intrusion

AbstractThe Karoo Basin covers much of South Africa and is an area of prospective shale gas exploration, with the Whitehill Formation the target shale unit. However, the sedimentary succession, including the Whitehill, has been intruded by a series of sills and dykes associated with the Karoo Large Igneous Province (~183 Ma), which are expected to have modified the thermal history of the basin dramatically. Here, we investigate a secondary effect of these intrusions: a series of hydrothermal vent complexes, or breccia pipes, focusing on using O, H, and C isotopes to constrain the origin and evolution of fluids produced during the intrusion of basaltic sills. A cluster of breccia pipes have been eroded down to the level of the Ecca Group at Luiperdskop on the western edge of the Karoo basin; a small isolated pipe of similar appearance crops out 13 km to the east. The Luiperdskop pipes are underlain by a Karoo dolerite sill that is assumed to provide the heat driving fluidization. The pipes consist of fine‐grained matrix and about 8% clasts, on average, of mostly sedimentary material; occasional large rafts of quartzite and dolerite are also present. The presence of clasts apparently from the Dwyka Group is consistent with the depth of formation of the pipes being at, or near, the base of the Karoo Supergroup, between 400 and 850 m below present surface. The presence of chlorite as the dominant hydrous mineral is consistent with an emplacement temperature between 300 and 350°C. The major and trace element, and O‐ and H‐isotope composition of the Tankwa breccias is homogenous, consistent with them being derived from the same source. The δ18O values (vsVSMOW) of the breccias are relatively uniform (7.1‰–8.7‰), and are similar to that of the country rock shale, and both are lower than expected for shale. The water content of the breccia is between 2.7 and 3.1 wt.% and the δD values range from −109‰ to −144‰. Calcite in vesicles has δ13C and δ18O (VSMOW) values of −4.2‰ and 24.0‰, respectively. The low δD value of the breccia rocks does not appear to be due to the presence of methane in the fluid. Instead, it is proposed that low δD and δ18O values are the result of the fluid being derived from the breakdown of clay minerals that formed and were deposited at a time of cold climate at ~290 Ma.

Episodic fluid flow as a trigger for Miocene‐Pliocene slope instability on the Utgard High, Norwegian Sea

AbstractMass wasting is triggered on many continental slopes by a number of mechanisms, including seismic shaking, high sedimentation rates, the presence of weak geological units and gas hydrate dissociation. In this study, the morphology of a Late Miocene–Early Pliocene mass‐transport complex (MTC) on the Utgard High is unravelled and discussed in relation to possible trigger mechanisms. The approach used here includes 3D seismic interpretation and the analysis of variance attribute maps. The interpreted MTC is located on the crest and flanks of the Utgard High and is composed of three mass‐transport deposits with seismic characters varying from transparent and chaotic seismic facies at the base to slightly deformed layers composed of mounds and rafted blocks in the middle and chaotic to transparent reflections at the top. Lithologically, the MTC consists predominantly of claystone with high gamma ray and low density and resistivity values, demonstrating that the associated mounds represent remobilized ooze sediments. A vertical stack of six magmatic sills emplaced from 55.6 to 56.3 Ma into the Upper Cretaceous shales is interpreted at depths of 3,000–5,500 ms two‐way travel time (TWTT). In association with these magmatic sills are several hydrothermal vent complexes that interacted with the top MTC horizon, signifying that episodic and secondary fluid‐venting events might be the principal mechanism facilitating mass wasting in the study area. In addition, the remobilization of ooze sediments into mounds is hypothesized to be dependent on fluids and clayey layers. As a corollary of this work, the importance of relict and recurrent episodes of fluid flow in the Vøring Basin and their influence on the geotechnical integrity of the overburden and later mass wasting is established.

Three-dimensional (3-D) seismic imaging of conduits and radial faults associated with hydrothermal vent complexes (Vøring Basin, Offshore Norway)

Here, we document a suite of radial faults associated with hydrothermal vent complexes in the Vøring Basin, offshore Norway. These complexes have pyramid-shaped, cylindrical- and conical-shaped conduits, with a dome-, or eye-shaped morphology at their summit, intruding on Paleogene sedimentary rocks. Hydrothermal vents are intimate with the tips of magmatic sills that were emplaced at depths ranging between 1800 and 5800 ms Two Way Travel Time (TWTT). At shallower depths of 1800 to 3000 ms TWTT and intermediate depths of 3000 to 5000 ms TWWT, magmatic sills regularly intersect the lower parts of the vent conduits, which are characterized here as pipes. An important parameter that is used to characterize the morphology of a hydrothermal vent conduit is the width of the conduit, which is defined as the longest axis marking the extent of the vents' conduit within the surrounding host-rock strata. Our findings reveal that radial faults are commonly associated with the summits of hydrothermal vents, implying the existence of local stress fields around the vents, where the maximum compressive stress is radial and minimum stress is circumferential, which overrides the regional stress field and indicate variable stress regimes as opposed to tectonic faults. Importantly, circumferential stretching due to catastrophic plumbing of hydrothermal fluids, differential compaction and intensive fracturing enabled the polygonal faults to realign in a radial pattern resulting in the formation of radial faults at the vent summit. As a corollary of this work, we hypothesize that pyramid-shaped hydrothermal conduits are possibly markers of protracted sill emplacement in sedimentary basins.

Integrated 3D Geological Modeling to Gain Insight in the Effects of Hydrothermal Alteration on Post-Ore Deformation Style and Strain Localization in the Flin Flon Volcanogenic Massive Sulfide Ore System

3D geological modeling of lithogeochemical and geological data provides insight into the role of the sulfide ore horizon and associated footwall hydrothermal alteration in localizing shear strain in the Flin Flon volcanogenic massive sulfide deposits, Canada, as deformation evolved from brittle-ductile to ductile regimes during collisional stages of the 1.9–1.8 Ga Trans-Hudson orogeny. 3D spatial characterization of hydrothermal alteration based on the Ishikawa index (AI) and normative corundum percentages outline sericite + chlorite-rich high strain zones, consisting of Al-enriched and Na-depleted felsic and mafic volcanic rocks in the footwall of the sulfide ore horizon. The hydrothermal vent complex, from which these sheared alteration zones originated, was stacked together with the ore horizon by W-vergent thrust faults during an early collisional deformation regime, imbricating molasse-type clastic sediments with the ore-hosting volcanic and volcaniclastic rocks of the Flin Flon arc assemblage. Chlorite-rich planar zones marked by high values of the Carbonate–chlorite–pyrite index (CCPI) are laterally more extensive and outline a later system of ductile shear zones, in which phyllosilicates, quartz and chalcopyrite in stringer zones localized shear strain and enhanced transposition of the hydrothermal vent stockwork. The contrasting deformation styles of these two thrusting events and their localization within the ore horizon and hydrothermal vent stockwork have important implications for vectoring towards undiscovered ore in this mature mining camp that are possibly also relevant to other strongly deformed VMS ore systems.

Mafic intrusions, hydrothermal venting, and the basalt-sediment transition: Linking onshore and offshore examples from the North Atlantic igneous province

The emplacement of large intrusive complexes into sedimentary basins can have profound effects on host sedimentary rocks including deformation, thermal aureole metamorphic reactions, alteration of fluid-flow pathways, and the formation of associated hydrothermal vent complexes (HVCs). These processes can in turn have major implications for petroleum systems on the local and regional scale, and can contribute to global climate change due to the production and outgassing of greenhouse gases, such as [Formula: see text] and [Formula: see text]. Imaging these features and assessing their implications from seismic data beneath extrusive volcanic cover is challenging due to heterogeneities in the volcanic pile and at the basalt-sediment transition. We have evaluated combined field and laboratory petrophysical data from the Isle of Skye, Scotland, where we identify a seismic-scale example of extensive intrusions interacting with the base-basalt transition. We have also evaluated a unique onshore example of a linked sill and associated HVC cutting up through the lava sequence. We compare these field results with HVCs from reflection seismic data across the Vøring Marginal High, offshore Norway, where subbasalt saucer-shaped intrusions are also seen associated with HVCs cutting the lava sequence. Seismic imaging problems associated with the velocity heterogeneity of volcanic sequences, along with a historical lack of high-quality 3D data in volcanic regions worldwide, is suggested as having largely precluded the identification of these features in the past. The under-representation of these hydrothermal vents in the literature has key implications for the future appraisal of intrusion-related outgassing effects on the global climate such as those related to the Palaeocene Eocene Thermal Maximum, along with subbasalt petroleum prospectivity where they may alter maturation and migration pathways.

Modelling fluid flow in clastic eruptions: Application to the Lusi mud eruption

Clastic eruptions involve the rapid ascension of sedimentary clasts together with fluids, gas and/or liquid phases that may further deform and brecciate the host rocks. These fluids transport the resulting mixture, called mud breccia, to the surface. Such eruptions are often associated with geological structures such as mud volcanoes, hydrothermal vent complexes and, more generally, piercement structures. They involve various processes, acting over a wide range of scales, which makes them a complex and challenging multi-phase system to model. Although piercement structures have been widely studied and discussed, only a few attempts have been made to model the dynamics of such clastic eruptions. The ongoing Lusi mud eruption, in the East Java back-arc basin, which began in May 2006, is a spectacular large scale clastic eruption. The Lusi eruptive behaviour has been extensively studied over the past decade and thus represents a unique opportunity to better understand ongoing clastic eruptions and thus fossil clastic systems. We use both analytical formulations and numerical models to investigate simple relationships between the mud breccia properties (density, viscosity, gas and clast content) and the volumetric flow rate. Our results show that the conduit radius of such piercement systems cannot exceed a few metres at depth, and that clasts, if not densely packed, will not affect the flow rate when they are smaller than a fifth of the conduit size. Using published data for the annual gas fluxes at Lusi, we infer a maximal depth at which exsolution starts. This occurs between 1800 m and 3200 m depth for methane and between 750 m and 1000 m for carbon dioxide. Based on annual gas fluxes, we estimate that the conduit radius should be no larger than 1.5 m to match the maximal mud discharge, recorded at Lusi.

Evidence of a modern deep water magmatic hydrothermal system in the Canary Basin (eastern central Atlantic Ocean)

AbstractNew seismic profiles, bathymetric data, and sediment‐rock sampling document for the first time the discovery of hydrothermal vent complexes and volcanic cones at 4800–5200 m depth related to recent volcanic and intrusive activity in an unexplored area of the Canary Basin (Eastern Atlantic Ocean, 500 km west of the Canary Islands). A complex of sill intrusions is imaged on seismic profiles showing saucer‐shaped, parallel, or inclined geometries. Three main types of structures are related to these intrusions. Type I consists of cone‐shaped depressions developed above inclined sills interpreted as hydrothermal vents. Type II is the most abundant and is represented by isolated or clustered hydrothermal domes bounded by faults rooted at the tips of saucer‐shaped sills. Domes are interpreted as seabed expressions of reservoirs of CH4 and CO2‐rich fluids formed by degassing and contact metamorphism of organic‐rich sediments around sill intrusions. Type III are hydrothermal‐volcanic complexes originated above stratified or branched inclined sills connected by a chimney to the seabed volcanic edifice. Parallel sills sourced from the magmatic chimney formed also domes surrounding the volcanic cones. Core and dredges revealed that these volcanoes, which must be among the deepest in the world, are constituted by OIB‐type, basanites with an outer ring of blue‐green hydrothermal Al‐rich smectite muds. Magmatic activity is dated, based on lava samples, at 0.78 ± 0.05 and 1.61 ± 0.09 Ma (K/Ar methods) and on tephra layers within cores at 25–237 ky. The Subvent hydrothermal‐volcanic complex constitutes the first modern system reported in deep water oceanic basins related to intraplate hotspot activity.

3D structure and formation of hydrothermal vent complexes at the Paleocene-Eocene transition, the Møre Basin, mid-Norwegian margin

The mid-Norwegian margin is regarded as an example of a volcanic-rifted margin formed prior to and during the Paleogene breakup of the northeast Atlantic. The area is characterized by the presence of voluminous basaltic complexes such as extrusive lava and lava delta sequences, intrusive sills and dikes, and hydrothermal vent complexes. We have developed a detailed 3D seismic analysis of fluid- and gas-induced hydrothermal vent complexes in a [Formula: see text] area in the Møre Basin, offshore Norway. We find that formation of hydrothermal vent complexes is accommodated by deformation of the host rock when sills are emplaced. Fluids are generated by metamorphic reactions and pore-fluid expansion around sills and are focused around sill tips due to buoyancy. Hydrothermal vent complexes are associated with doming of the overlying strata, leading to the formation of draping mounds above the vent contemporary surface. The morphological characteristics of the upper part and the underlying feeder structure (conduit zone) are imaged and studied in 3D seismic data. Well data indicate that the complexes formed during the early Eocene, linking their formation to the time of the Paleocene-Eocene thermal maximum at c. 56 Ma. The well data further suggest that the hydrothermal vent complexes were active for a considerable time period, corresponding to a c. 100 m thick transition zone unit with primary Apectodinium augustum and redeposited very mature Cretaceous and Jurassic palynomorphs. The newly derived understanding of age, structure, and formation of hydrothermal vent complexes in the Møre Basin contributes to the general understanding of the igneous plumbing system in volcanic basins and their implications for the paleoclimate and petroleum systems.