Origin of light volatile hydrocarbon gases in mud volcano fluids, Gulf of Cadiz — Evidence for multiple sources and transport mechanisms in active sedimentary wedges

Widespread mud volcanism across the thick (≤ 14 km) seismically active sedimentary prism of the Gulf of Cadiz is driven by tectonic activity along extensive strike–slip faults and thrusts associated with the accommodation of the Africa–Eurasia convergence and building of the Arc of Gibraltar, respectively. An investigation of eleven active sites located on the Moroccan Margin and in deeper waters across the wedge showed that light volatile hydrocarbon gases vented at the mud volcanoes (MVs) have distinct, mainly thermogenic, origins. Gases of higher and lower thermal maturities are mixed at Ginsburg and Mercator MVs on the Moroccan Margin, probably because high maturity gases that are trapped beneath evaporite deposits are transported upwards at the MVs and mixed with shallower, less mature, thermogenic gases during migration. At all other sites except for the westernmost Porto MV, δ¹³C–CH₄ and δ²H–CH₄ values of ~ − 50‰ and − 200‰, respectively, suggest a common origin for methane; however, the ratio of CH₄/(C₂H₆ + C₃H₈) varies from ~ 10 to > 7000 between sites. Mixing of shallow biogenic and deep thermogenic gases cannot account for the observed compositions which instead result mainly from extensive migration of thermogenic gases in the deeply-buried sediments, possibly associated with biodegradation of C₂₊ homologues and secondary methane production at Captain Arutyunov and Carlos Ribeiro MVs. At the deep-water Bonjardim, Olenin and Carlos Ribeiro MVs, generation of C₂₊-enriched gases is probably promoted by high heat flux anomalies which have been measured in the western area of the wedge. At Porto MV, gases are highly enriched in CH₄ having δ¹³C–CH₄ ~ − 50‰, as at most sites, but markedly lower δ²H–CH₄ values < − 250‰, suggesting that it is not generated by thermal cracking of n-alkanes but rather that it has a deep Archaeal origin. The presence of petroleum-type hydrocarbons is consistent with a thermogenic origin, and at sites where CH₄ is predominant support the suggestion that gases have experienced extensive transport during which they mobilized oil from sediments ~ 2–4 km deep. These fluids then migrate into shallower, thermally immature muds, driving their mobilization and extrusion at the seafloor. At Porto MV, the limited presence of petroleum in mud breccia sediments further supports the hypothesis of a predominantly deep microbial origin of CH₄.     

Mud volcano eruptions and earthquakes in the Northern Apennines and Sicily, Italy

The relations between earthquakes and the eruption of mud volcanoes have been investigated at the Pede–Apennine margin of the Northern Apennines and in Sicily. Some of these volcanoes experienced eruptions or increased activity in connection with historical seismic events, showing a good correlation with established thresholds of hydrological response (liquefaction) to earthquakes. However, the majority of eruptions have been documented to be independent of seismic activity, being mud volcanoes often not activated even when the earthquakes were of suitable magnitude and the epicentre at the proper distance for the triggering. This behaviour suggests that paroxysmal activity of mud volcanoes depends upon the reaching of a specific critical state dictated by internal fluid pressure, and implies that the strain caused by the passage of seismic waves can activate only mud volcanoes in near-critical conditions (i.e., close to the eruption). Seismogenic faults, such as the Pede–Apennine thrust, often structurally control the fluid reservoirs of mud volcanoes, which are frequently located at the core of thrust-related folds. Such an intimate link enables mud volcanoes to represent features potentially suitable for recording perturbations associated with the past and ongoing tectonic activity of the controlling fault system.     

五大连池火山1720-1721年喷发观测记录

存于黑龙江将军衙门档案中的五大连池火山喷发满文史料 (由吴雪娟发现并译成汉文 ) ,详细记载了五大连池火山在 172 0年 1月 14日至 172 1年 3月 18日喷发形成老黑山、172 1年 4月 2 6日至 172 1年 5月 2 8日喷发形成火烧山的全部过程 ,记述了这 2座火山的喷发时间、地点、喷发状态和火山堰塞湖形成以及参加观测的人员情况等各种史实。这是中国历史上迄今为止对火山喷发仅有的一次有组织的观测活动 ,这些记录为火山观测研究提供了珍贵的第一手资料。同时 ,也表明中国是世界上火山观测开展较早的国家之一。以往认为五大连池老黑山、火烧山火山喷发的时间为公元1719— 172 1年 ,实际应为公元 172 0— 172 1年     

On steady homogeneous sand–water flows in a vertical conduit

In this paper, an idealized model of the steady‐state phase of the flow in a vertical conduit leading to a sand volcano eruption is developed from first principles. The model assumes that a sand–water mixture flows upwards, driven by an overpressure at the base of a vertical cylindrical conduit (or a two‐dimensional fracture) and opposed by gravity, viscous resistance and turbulent drag. The conditions for flow are analysed in detail, and the mechanisms controlling the eruption rates are studied quantitatively. The flow predictions are in accordance with our observations of analogous vigorous sand eruptions at deepwater oil fields. For sufficiently high flow velocities (u > 10u_T) and small sand fractions (s < 0·2), the flow may be well mixed and homogeneous. If these conditions are not met, the flow may either become two phase or does not develop. Combining geological considerations with the steady homogeneous model, it is possible to predict the behaviour of the vigorous quasi‐steady stage of a sand volcano eruption. It is shown that, based on the average density of the overlying sediments, there are a range of overpressures for which sand volcanoes may develop.     

Focal mechanism solutions and its tectonic significance in the trench of the eastern South China Sea

Introduction South China Sea (SCS) is located in the convergence zone between Euro-Asian plate, Pacific plate (Philippine plate) and Indian plate. Interactions of three plates made the crust of this region suffer tectonic stress in many directions and made the South China Sea be in the complex environ-ment of the tectonic stress. There are four different marginal types in the surrounding of the South China Sea: The tectonic zone of the rifting margin in the north of SCS, the NS direct…     

Recent activity of Nisyros volcano (Greece) inferred from structural, geochemical and seismological data

This study summarizes the results of structural, geochemical and seismological surveys carried out at Nisyros volcano (Aegean Sea, Greece) during 1999–2001. Field mapping and mesostructural measurements at the summit caldera (Lakki plain) indicate that faults follow two main strikes: NE-SW and N-S. The N-S striking fault depicts extensional features accommodating the left-lateral component of motion of the NE-SW- striking main faults. The NE-SW preferred strike of the Lakki faults and of the mineral-filled veins as well as the distribution and NE-SW elongation of the hydrothermal craters indicate that tectonics plays a major role in controlling the fluid pathway in the Nisyros caldera. The same NE-SW trend is depicted by CO₂ anomalies revealed through detailed soil CO₂ flux surveys, thus indicating a structural control on the pattern of the hydrothermal degassing. Degassing processes account for a thermal energy release of about 43 MW, most of which occurs at Lofos dome, an area that was affected by hydrothermal eruptions in historical times. The seismic study was conducted in June 2001, using a deployment specifically aimed at detecting signals of magmatic-hydrothermal origin. Our instruments recorded local and regional earthquakes, a few local long-period events (LP), and bursts of monochromatic tremor. Local earthquake activity is concentrated beneath the caldera, at depths generally shallower than 6 km. Plane-wave decomposition of tremor signal indicates a shallow (<200 m) source located in the eastern part of the caldera. Conversely, LP events depict a source located beneath the central part of the caldera, in the area of Lofos dome, at depths in the 1–2-km range. In agreement with geochemical and structural measurements, these data suggest that both the deeper and shallower part of the hydrothermal system are subjected to instability in the fluid flow regimes, probably consequent to transient pressurization of the reservoir. These instabilities may be related to input of hot fluids from the deeper magmatic system, as suggested by the variations in geochemical parameters observed after the 1997–1999 unrest episode. The significance of seismological and geochemical indicators as precursors of hydrothermal explosive activity at Nisyros is discussed.Editorial responsibility: H. Shinohara     

论新托尔巴奇克火山对五大连池近代火山研究的借鉴意义

新托尔巴奇克火山是世界上著名的大裂隙喷发火山，前苏联火山学家对这次喷发有准确预报，而且从喷发的第一天起就做了详细观测记录，取得了丰富的观测研究成果，本文从火山锥体的形成，喷发区特征和监测预报三个方面论述这些成果对研究我国五大连池近代火山的借鉴意义。     

Development of lava tubes in the light of observations at Mauna Ulu,Kilauea Volcano,Hawaii

During the 1969–1974 Mauna Ulu eruption on Kilauea's upper east rift zone, lava tubes were observed to develop by four principal processes: (1) flat, rooted crusts grew across streams within confined channels; (2) overflows and spatter accreted to levees to build arched roofs across streams; (3) plates of solidified crust floating downstream coalesced to form a roof; and (4) pahoehoe lobes progressively extended, fed by networks of distributaries beneath a solidified crust. Still another tube-forming process operated when pahoehoe entered the ocean; large waves would abruptly chill a crust across the entire surface of a molten stream crossing through the surf zone. These littoral lava tubes formed abruptly, in contrast to subaerial tubes, which formed gradually. All tube-forming processes were favored by low to moderate volume-rates of flow for sustained periods of time. Tubes thereby became ubiquitous within the pahoehoe flows and distributed a very large proportionof the lava that was produced during this prolonged eruption. Tubes transport lava efficiently. Once formed, the roofs of tubes insulate the active streams within, allowing the lava to retain its fluidity for a longer time than if exposed directly to ambient air temperature. Thus the flows can travel greater distances and spread over wider areas. Even though supply rates during most of 1970–1974 were moderate, ranging from 1 to 5 m³/s, large tube systems conducted lava as far as the coast, 12–13 km distant, where they fed extensive pahoehoe fields on the coastal flats. Some flows entered the sea to build lava deltas and add new land to the island. The largest and most efficient tubes developed during periods of sustained extrusion, when new lava was being supplied at nearly constant rates. Tubes can play a major role in building volcanic edifices with gentle slopes because they can deliver a substantial fraction of lava erupted at low to moderate rates to sites far down the flank of a volcano. We conclude, therefore, that the tendency of active pahoehoe flows to form lava tubes is a significant factor in producing the common shield morphology of basaltic volcanoes.     

Role of olivine cumulates in destabilizing the flanks of Hawaiian volcanoes

The south flank of Kilauea Volcano is unstable and has the structure of a huge landslide; it is one of at least 17 enormous catastrophic landslides shed from the Hawaiian Islands. Mechanisms previously proposed for movement of the south flank invoke slip of the volcanic pile over seafloor sediments. Slip on a low friction décollement alone cannot explain why the thickest and widest sector of the flank moves more rapidly than the rest, or why this section contains a 300 km³ aseismic volume above the seismically defined décollement. It is proposed that this aseismic volume, adjacent to the caldera in the direction of flank slip, consists of olivine cumulates that creep outward, pushing the south flank seawards. Average primary Kilauea tholeiitic magma contains about 16.5 wt.% MgO compared with an average 10 wt.% MgO for erupted subaerial and submarine basalts. This difference requires fractionation of 17 wt.% (14 vol.%) olivine phenocrysts that accumulate near the base of the magma reservoir where they form cumulates. Submarine-erupted Kilauea lavas contain abundant deformed olivine xenocrysts derived from these cumulates. Deformed dunite formed during the tholeiitic shield stage is also erupted as xenoliths in subsequent alkalic lavas. The deformation structures in olivine xenocrysts suggest that the cumulus olivine was densely packed, probably with as little as 5–10 vol.% intercumulus liquid, before entrainment of the xenocrysts. The olivine cumulates were at magmatic temperatures (>1100°C) when the xenocrysts were entrained. Olivine at 1100°C has a rheology similar to ice, and the olivine cumulates should flow down and away from the summit of the volcano. Flow of the olivine cumulates places constant pressure on the unbuttressed seaward flank, leading to an extensional region that localizes deep intrusions behind the flank; these intrusions add to the seaward push. This mechanism ties the source of gravitational instability to the caldera complex and deep rift systems and, therefore, limits catastrophic sector failure of Hawaiian volcanoes to their active growth phase, when the core of olivine cumulates is still hot enough to flow.     

Monitoring Quiescent Volcanoes by Diffuse CO2 Degassing: Case Study of Mt. Fuji, Japan

Since the 8th century, more than seventeen eruptions have been recorded for the Mt. Fuji volcano, with the most recent eruption occurring in 1707 (Hoei eruption). For the past 300 years the volcano has been in a quiescent stage and, since the early 1960s, has exhibited neither fumarolic nor thermal activity. However, the number of low-frequency earthquakes with a hypocentral depth of 10–20 km increased significantly beneath the northeastern flank of Mt. Fuji in 2000–2001, suggesting a possible resumption of magmatic activity. In this study, diffuse CO₂ efflux and thermal surveys were carried out in four areas of the volcano in 2001–2002 in order to detect possible signs of the upward movement of deep magma. At all survey points, the CO₂ efflux was below the detection limit with the exception of a few points with biological CO₂ emission, and ground temperatures at a depth of 20–30 cm were below ambient, indicating no surface manifestations of gas or heat emission. Should magma rise into the subsurface, the diffuse CO₂ efflux would be expected to increase, particularly along the tectonically weakened lineation on the Mt. Fuji volcano, allowing for the early detection of pre-eruptive degassing.