Velocity Structure Revealing a Likely Mud Volcano off the Dongsha Island, the Northern South China Sea

The Dongsha Island (DS) is located in the mid-northern South China Sea continental margin. The waters around it are underlain by the Chaoshan Depression, a relict Mesozoic sedimentary basin, blanketed by thin Cenozoic sediments but populated with numerous submarine hills with yet less-known nature. A large hill, H110, 300 m high, 10 km wide, appearing in the southeast to the Dongsha Island, is crossed by an ocean bottom seismic and multiple channel seismic surveying lines. The first arrival tomography, using ocean bottom seismic data, showed two obvious phenomena below it: (1) a low-velocity (3.3 to 4 km/s) zone, with size of 20 × 3 km2, centering at ~4.5 km depth and (2) an underlying high-velocity (5.5 to 6.3 km/s) zone of comparable size at ~7 km depth. MCS profiles show much-fragmented Cenozoic sequences, covering a wide chaotic reflection zone within the Mesozoic strata below hill H110. The low-velocity zone corresponds to the chaotic reflection zone and can be interpreted as of highly-fractured and fluid-rich Mesozoic layers. Samples dredged from H110 comprised of illite-bearing authigenic carbonate nodules and rich, deep-water organisms are indicative of hydrocarbon seepage from deep source. Therefore, H110 can be inferred as a mud volcano. The high-velocity zone is interpreted as of magma intrusion, considering that young magmatism was found enhanced over the southern CSD. Furthermore, the origin of H110 can be speculated as thermodynamically driven, i.e., magma from the depths intrudes into the thick Mesozoic strata and promotes petroleum generation, thus, driving mud volcanism. Mud volcanism at H110 and the occurrence of a low-velocity zone below it likely indicates the existence of Mesozoic hydrocarbon reservoir, which is in favor of the petroleum exploration.

Characterization of Archaeal and Bacterial Communities Thriving in Methane-seeping on-land Mud Volcanoes, Niigata, Japan.

Submarine mud volcanoes (MVs) have attracted significant interest in the scientific community for obtaining clues on the subsurface biosphere. On-land MVs, which are much less focused in this context, are equally important and they may provide insights also for astrobiology of extraterrestrial mud volcanism. Hereby, we characterized microbial communities of the two active methane-seeping on-land MVs, Murono and Kamou, in central Japan. Metataxonomic 16S rRNA sequencing analysis of those sites recovered the dominant archaeal taxa affiliated with methanogens. Anaerobic methanotrophs (ANME), with the subgroups ANME-1b and ANME-3, were also recovered from the Murono site albeit a greatly reduced abundance compared to typical submarine MVs. ANME-3 was in fact identified for the first time for land-based MVs. The bacterial sequences affiliated to Atribacteria, sulfate-reducing bacteria (SRB), and Fe(III)-reducing bacteria were recovered. SRB and ANME form a syntrophic consortium, which is often found at the sulfate-methane transition zone of submarine MVs where diffused sulfate (SO42-) is constantly enriched from the ocean. Previous investigations speculate that the erupted materials from Murono are originated from the Miocene marine strata, and we hypothesize that the old sea-related juvenile water is the source of additional sulfur-related components for the SRB-ANME consortium at Murono.

Shallow depth, substantial change: fluid-metasomatism causes major compositional modifications of subducted volcanics (Mariana forearc)

Mass transfer at shallow subduction levels and its ramifications for deeper processes remain incompletely constrained. New insights are provided by ocean island basalt (OIB) clasts from the Mariana forearc that experienced subduction to up to ~25–30 km depth and up to blueschist-facies metamorphism; thereafter, the clasts were recycled to the forearc seafloor via serpentinite mud volcanism. We demonstrate that the rocks were, in addition, strongly metasomatized: they exhibit K2O contents (median = 4.6 wt.%) and loss on ignition (median = 5.3 wt%, as a proxy for H2O) much higher than OIB situated on the Pacific Plate, implying that these were added during subduction. This interpretation is consistent with abundant phengite in the samples. Mass balance calculations further reveal variable gains in SiO2 for all samples, and MgO and Na2O increases at one but the loss of MgO and Fe2O3* at the other study site. Elevated Cs and Rb concentrations suggest an uptake whereas low Ba and Sr contents indicate the removal of trace elements throughout all clasts.The metasomatism was likely induced by the OIBs’ interaction with K-rich fluids in the subduction channel. Our thermodynamic models imply that such fluids are released from subducted sediments and altered igneous crust at 5 kbar and even below 200°C. Equilibrium assemblage diagrams show that the stability field of phengite significantly increases with the metasomatism and that, relative to not-metasomatized OIB, up to four times as much phengite may form in the metasomatized rocks. Phengite in turn is considered as an important carrier for K2O, H2O, and fluid-mobile elements to sub-arc depths.These findings demonstrate that mass transfer from subducting lithosphere starts at low P/T conditions. The liberation of solute-rich fluids can evoke far-reaching compositional and mineralogical changes in rocks that interact with these fluids. Processes at shallow depths (<30 km) thereby contribute to controlling which components as well as in which state (i.e., bound in which minerals) these components ultimately reach greater depths where they may or may not contribute to arc magmatism. For a holistic understanding of deep geochemical cycling, metasomatism and rock transformation need to be acknowledged from shallow depths on.

Search for hydrogeochemical indicators of the genetic relation between mud volcanism and oil and gas fields

The paper reports the results of a comparative analysis of the chemical and isotope composition (δ180 and δD) of mud volcanic waters and formation waters from oil and gas fields. Studies show that the waters discharged by mud volcanoes in most cases are very similar to formation waters. The most characteristic geochemical traits of both waters are elevated concentrations of hydrocarbonate ions, iodine, boron, bromine, and a low content of sulfate ions.

Mud volcanism as a dangerous phenomenon for oil and gas facilities

The research dwells on the danger of mud volcanism for human economic activity, namely, oil and gas production. We performed quantitative assessment of mud volcanoes activities, using Azerbaijan and Kerch-Taman region as examples. Average annual number of mud volcanoes eruptions is 3–4 for Azerbaijan and 1–2 for Kerch-Taman region. We estimate the catalogues of mud volcanic eruptions for those areas to be 52 % and 39 % complete, respectively. Mud volcanoes eruptions are quite frequent. In both regions, over 50 % of all recorded eruptions occur within ten years of the latest eruption. Analysis of mud volcanic eruptions catalogues shows that the volume of breccia ejected during an eruption is practically not related to how long the mud volcano was quiescent. Analysis of potential impact of seismicity on mud volcanic activity shows that the probability of mud volcanoes responding to an earthquake is 6 % and 10 % for Azerbaijan and Kerch-Taman region, respectively.

Geoelectric heterogeneities of the Kerch iron ore basin

The three-dimensional geoelectric model of the Earth’s crust and upper mantle of the Kerch Peninsula has been built for the first time based on the results of experimental observations of the Earth’s low-frequency electromagnetic field, carried out in 2007—2013 by the Institutes of the National Academy of Sciences of Ukraine. Its physical and geological interpretation and detailing of the near-surface part were carried out according to the data of the audiomagnetotelluric sounding method to study the deep structure of the Kerch iron ore basin. To the east of the Korsak-Feodosiya fault along the southern part of the Indolo-Kuban trough (in the north of the South Kerch and almost under the entire North Kerch zones), a low-resistance anomaly (ρ=1 Ohm∙m) was found at depths from 2.5 km to 12 km about 20 km wide. Its eastern part is located in the consolidated Earth’s crust and is galvanically connected with surface sedimentary strata, while the western part is completely in sedimentary deposits. The anomaly covers the territory of the Kerch iron ore basin and occurrences of mud volcanism. The characteristics of the upper part of the layered section of the Kerch Peninsula in the interval of the first hundreds of meters were obtained from the results of one-dimensional inversion of the audiomagnetotelluric sounding data (frequency range 8—4000 Hz). It is shown that the first 15 m of the section, corresponding to Quaternary deposits, have resistivity values up to 1 Ohm∙m. Below, in the Neogene sediments, the electrical resistance increases to values of 5 Ohm∙m and more. Both horizontally and vertically, the distribution of resistivity values has a variable character, manifesting as a thin-layered structure with low resistivity values. Possibly, such areas have a direct connection with the channel for transporting hummock material and gases. A connection is assumed between the low-resistivity thin-layered near-surface areas, a deep anomaly of electrical conductivity in the upper part of the Earth’s crust, and the likely high electrical conductivity of rocks at the depths of the upper mantle with iron ore deposits, as well as the manifestation of mud volcanism. The heterogeneity of the crustal and mantle highly conductive layers may indicate a high permeability of the contact zones for deep fluids.

The mud volcanoes at Santa Barbara and Aragona (Sicily, Italy): a contribution to risk assessment

The Santa Barbara and Aragona areas are affected by mud volcanism (MV) phenomena, consisting of continuous or intermittent emission of mud, water, and gases. This activity could be interrupted by paroxysmal events, with an eruptive column composed mainly of clay material, water, and gases. They are the most hazardous phenomena, and today it is impossible to define the potential parameters for modelling the phenomenon. In 2017, two digital surface models (DSMs) were performed by drone in both areas, thus allowing the mapping of the emission zones and the covered areas by the previous events. Detailed information about past paroxysms was obtained from historical sources, and, with the analysis of the 2017 DSMs, a preliminary hazard assessment was carried out for the first time at two sites. Two potentially hazardous paroxysm surfaces of 0.12 and 0.20 km2 for Santa Barbara and Aragona respectively were defined. In May 2020, at Aragona, a new paroxysm covered a surface of 8721 m2. After this, a new detailed DSM was collected with the aim to make a comparison with the 2017 one. Since 2017, a seismic station was installed in Santa Barbara. From preliminary results, both seismic events and ambient noise showed a frequency of 5–10 Hz.

TECTONIC STRUCTURE OF KERCH-TAMAN FOLD ZONE OF AZOV-KUBAN TROUGH

The nrotectonic structure of the Kerch-Taman folded zone, the southern deformed part of the Azov-Kuban piedmont trough, has a number of structural features and a history of deformation development. The authors divide the zone into five transverse segments, differing in the age of the main phase of folding. In the central Taman segment, folding is still going on, while in the framing segments, the main phases of folding took place in the Pliocene. In the peripheral segments, the main phase of folding is older - Early-Middle Miocene. The boundaries of the Kerch-Taman zone, as well as its youngest, the Taman segment, coincide with the transverse fault (fault-flexure) zones. It is shown that the southern frame of the Kerch-Taman folded zone is the continuation of the structures of the Crimean Mountains and the Northwestern Caucasus. The correlation of mud volcanism manifestations with tectonic elements of the Taman segment is noted to be ambiguous.