Fracture Zone Area Research Articles

Deep Argo Observations of Antarctic Bottom Water in the Deep Fracture Zones of the Southwest Indian Ridge

AbstractThe Madagascar Basin is the primary pathway for Antarctic Bottom Water to ventilate the entire western Indian Ocean as part of the Global Overturning Circulation. The only way for this water mass to reach this basin is by crossing the Southwest Indian Ridge through its deep fracture zones. However, due to the scarcity of observations, the Antarctic Bottom Water presence has only been well‐established in the Atlantis II fracture zone. In May 2023, the Deep Madagascar Basin Experiment deployed three Deep SOLO Argo floats in the exit of the fracture zones that were more likely to transport Antarctic Bottom Water: Atlantis II, Novara, and Melville. These floats have been collecting temperature and salinity profiles every 3–5 days with high vertical resolution in the deep ocean. In the present paper, we use the first 7 months of float data to characterize the Antarctic Bottom Water in the deep fracture zone area, revisiting a half‐century puzzle about the Melville contribution. We also collected shipboard‐based profiles to calibrate float salinity and show it is within the Deep Argo program target accuracy. We find Antarctic Bottom Water in both Melville and Novara fracture zones, not only in Atlantis II. This is the first time the Novara contribution has been revealed. The floats also uncover their distinct properties, which may result from the different mixing histories.

Research on active load reducing optimization strategy for 6061Al low cycle fatigue cropping process based on crack propagation mode transformation

Low cycle fatigue cropping technology is a new type of cropping technology with advantages such as no chip waste and low load need. Numerous studies have shown that increasing the fatigue frequency or increasing the displacement load can improve the cropping efficiency, but at the same time, it will reduce the quality of the cross-section and cannot achieve a balance between high efficiency and high quality of cross-section. In response to this issue, this paper found the phenomenon of transition of fracture modes (from fatigue mode to tensile fracture mode) of 6061Al material in LCFC process, and proposed the Active Load Reducing Optimization Strategy for 6061Al in LCFC, which achieves a significant improvement in quality of cross-section with a slight loss of cropping efficiency. Firstly, this paper studied the crack fracture mechanism of 6061Al bar under different displacement loads in the LCFC process. It was found that the transition of crack propagation mode in 6061Al- LCFC process was the main reason for the increase in cross-section roughness. The crack propagation mode changed from fatigue propagation to tensile fracture propagation during the LCFC process, and the roughness of the area significantly increased under the tensile fracture propagation mode. Further research has found that the fundamental reason for the contradiction between the cropping efficiency and section quality of 6061Al is the change in time of crack propagation mode transition and tensile fracture zone area under different displacement loads. Finally, the LCFC process was modeled in a time dimension using acoustic emission(AE) technology, and it was divided into three stages according to the AE signals and the fracture mechanism. As a result, an active load reducing optimization strategy for LCFC process was developed to delay the occurrence of tensile fracture propagation modes. Experiments have shown that compared to the traditional LCFC cropping strategy using constant displacement loads, the active load reducing strategy proposed in this paper can reduce the proportion of tensile fracture area, achieve a significant improvement in quality of cross-section with a slight loss of cropping efficiency, thus achieving a balance between efficiency and section quality to some extent. At the same time, an LCFC process section quality efficiency evaluation index with dual weights of efficiency and section quality has been proposed, which can assist in selecting LCFC active load reducing strategy parameters with consideration of efficiency and section quality weights.

Synchronous Grouting Analysis of Shield Tunneling through High Water Pressure Fault Fracture Zone

Using numerical software to establish a three-dimensional geological model of shield tunneling through fault fracture zones, considering the synchronous grouting effect at the shield tail, the disturbance of various geotechnical structures in the fault fracture zone under deep burial and high water pressure conditions is studied. According to the comparison between the numerical model analysis results and the on-site inspection results of the entire process of Shiziyang B3 section, synchronous grouting at the shield tail is the main reason for the changes in soil and water pressure in the fault fracture zone area. The soil and water pressure changes strongly during shield tunneling, and the maximum excess pore pressure value occurs when the shield tail passes. When the shield tunneling machine is far away, the soil and water stress shows a downward trend. The impact of synchronous grouting decreases with the increase in tunneling distance. Due to the difference in permeability, the variation law of pore pressure in the fault fracture zone is significantly different from the original rock, leading to a faster increase in pore pressure in the fracture zone area and a wider range of influence.

Melt Impregnation of Mantle Peridotite Facilitates High‐Temperature Hydration and Mechanical Weakening: Implications for Oceanic Detachment Faults

AbstractThe footwalls of oceanic detachment faults commonly expose shear zone rocks that appear to have compositions intermediate between those of mantle peridotite and magmatic rocks. These compositions either reflect metasomatic mass transfers or they relate to the impregnation of lithospheric mantle with basaltic or more evolved melts. We studied chlorite‐amphibole‐rich shear zone rocks from a detachment fault zone in the 15°20′N Fracture Zone area, Mid‐Atlantic Ridge, to examine their origin and role in strain localization. Geochemical compositions of these rocks imply that they formed by mixing between peridotite and gabbro. Textural observations indicate a strong contrast between the deformation intensity of these hybrid peridotite‐gabbro rocks and the host serpentinized peridotite. Geothermometry data give formation temperatures of >500 °C for synkinematic amphibole, zircon, rutile, and titanite. Chlorite appears intergrown with these phases and likely grew at similar temperatures. These results are compliant to thermodynamic computations that predict comparable mechanically weak mineralogies when hydrating hybrid rocks at 500 to 600 °C, whereas secondary assemblages after pure peridotite or gabbro are considerably stronger. Consequently, metamorphic weakening takes place to a much greater extent in rocks with a hybrid ultramafic–mafic composition than in purely ultramafic or gabbroic lithologies. Deformation may enhance fluid flow, which will in turn increase the extent of hydration and mechanical weakening. A positive feedback loop between hydration and strain localization may hence develop and facilitate the concentration of extensional tectonics into long‐lived, high‐displacement faults. We suggest that hybrid lithologies may play a key role in detachment faulting at slow spreading ridges worldwide.

Fossilized Life in Subseafloor Ultramafic Rocks

ABSTRACTUltramafic rocks are hypothesized to support a subseafloor hydrogen-driven biosphere because of extensive production of bioavailable energy sources like H2 or CH4 from fluid-rock interactions. Hence, the apparent lack of microbial remains in subseafloor ultramafic rocks, in contrast to their frequent observation in subseafloor basalts, is somewhat of a paradox. Here we report fossilized microbial remains in aragonite veins in ultramafic rocks from the 15°20′N Fracture Zone area on the Mid-Atlantic Ridge (MAR), collected during Ocean Drilling Program (ODP) Leg 209. The microbial remains consist of filamentous structures associated with biofilms. The young age (<1 Myr) and absence of diagenesis result in fossilized microbial communities with a pristine composition characterized by carbonaceous matter (CM) and the enrichment in trace elements such as Ni, Co, Mo and Mn. Our study confirms the presence of the hypothesized deep subseafloor biosphere hosted in ultramafic rocks. We further show that host rock composition may influence the microbial elemental composition, which is recorded during the fossilization.

The ecosystem of the Mid-Atlantic Ridge at the sub-polar front and Charlie–Gibbs Fracture Zone; ECO-MAR project strategy and description of the sampling programme 2007–2010

The ECOMAR project investigated photosynthetically-supported life on the North Mid-Atlantic Ridge (MAR) between the Azores and Iceland focussing on the Charlie–Gibbs Fracture Zone area in the vicinity of the sub-polar front where the North Atlantic Current crosses the MAR. Repeat visits were made to four stations at 2500m depth on the flanks of the MAR in the years 2007–2010; a pair of northern stations at 54°N in cold water north of the sub-polar front and southern stations at 49°N in warmer water influenced by eddies from the North Atlantic Current. At each station an instrumented mooring was deployed with current meters and sediment traps (100 and 1000m above the sea floor) to sample downward flux of particulate matter. The patterns of water flow, fronts, primary production and export flux in the region were studied by a combination of remote sensing and in situ measurements. Sonar, tow nets and profilers sampled pelagic fauna over the MAR. Swath bathymetry surveys across the ridge revealed sediment-covered flat terraces parallel to the axis of the MAR with intervening steep rocky slopes. Otter trawls, megacores, baited traps and a suite of tools carried by the R.O.V. Isis including push cores, grabs and a suction device collected benthic fauna. Video and photo surveys were also conducted using the SHRIMP towed vehicle and the R.O.V. Isis. Additional surveying and sampling by landers and R.O.V. focussed on the summit of a seamount (48°44′N, 28°10′W) on the western crest of the MAR between the two southern stations.

Asteroid fauna of the northern Mid-Atlantic Ridge: additional records

New data on deep-sea starfish from the North Atlantic are presented, based on samples obtained in the Charlie-Gibbs Fracture Zone area on three cruises of the RRS James Cook (years 2007, 2009 and 2010) in the framework of the ECOMAR project. Asteroids were collected at 19 stations at depths between 2272 and 2750 m. The collection includes 30 species, 11 of them recorded for the first time on the northern Mid-Atlantic Ridge. Two new species of Hymenaster are described. For some species geographical and depth ranges were extended. An annotated checklist of asteroids collected on the ECOMAR cruises is provided.

New data on glass sponges (Porifera, Hexactinellida) of the northern Mid-Atlantic Ridge. Part 1. Farreidae

Four specimens of Farrea herdendorfi Duplessis & Reiswig, 2004 are described from the depth 2428–2630 m in the Charlie-Gibbs Fracture Zone area, northern Mid-Atlantic Ridge. Specimens were collected on cruises of the RRS James Cook (in 2007 and 2010) as part of the ECOMAR project. The species was originally reported from off South Carolina (USA), and subsequently from Campos Basin (SE Brazil). New specimens slightly deviate from specimens in the type series, apparently owing to complete lack of onychohexasters, which are considered here to be of minor importance.

Significant improvements in marine gravity from ongoing satellite missions

Incorporating new altimeter data from CryoSat-2 (30 months), Envisat (18 months), and Jason-1 (7 months) satellites into an updated marine gravity field yields significant reduction in noise and improved resolution. Compared to an older gravity field that did not include the new altimeter data, incoherent power is reduced globally by approximately 2.9 dB at 15 km, 1.6 dB at 20 km, and 1.0 dB at 25 km wavelengths. Coherence analyses between the updated gravity and recent multibeam surveys distributed throughout the world’s oceans shows an average increase of ~0.023 in mean coherence in the 20–160 km waveband, with the biggest increase (>0.08) over fast spreading ridges and smallest ( 3.5 km) over fast spreading ridges and smallest (<1.5 km) over slow spreading ridges and continental shelves. In the Clipperton fracture zone area these improvements result in seamounts that are more accurately located, the detection of smaller seamounts, and the expression of north–south trending abyssal hill fabric. As more altimeter data from the ongoing satellite missions are incorporated into future gravity field updates, finer-scale details of the seafloor will continue to emerge.

Holothuroidea of the Charlie Gibbs Fracture Zone area, northern Mid-Atlantic Ridge

Holothurians are among the most species-rich taxa in the megabenthos on the northern Mid-Atlantic Ridge at depths of 2200–3700 m. Extensive new collections of 32 holothurian species were made in 2007–2010 in the Charlie Gibbs Fracture Zone area as part of the ECOMAR project. New material includes samples taken using a trawl and the ROV Isis. Samples and in situ observations from the ROV were of particular value because the morphological details of a number of holothurian species could be clarified. Many of these species are gelatinous and fragile and were damaged in trawls. Three species of elasipodid holothurians are described as new to science. An annotated check-list of all species of deep-sea holothurians collected in the Charlie Gibbs Fracture Zone area is provided. The checklist includes synonyms, distribution data and morphological descriptions as well as photographs taken in situ and in vivo. Ecological remarks are given for some species.

Fe-Ni-Co-O-S Phase Relations in Peridotite-Seawater Interactions

Serpentinization of abyssal peridotites is known to produce extremely reducing conditions as a result of dihydrogen (H2,aq) release upon oxidation of ferrous iron in primary phases to ferric iron in secondary minerals by H2O. We have compiled and evaluated thermodynamic data for Fe^Ni^Co^O^S phases and computed phase relations in fO2,g^fS2,g and aH2,aq^aH2S,aq diagrams for temperatures between 150 and 4008C at 50 MPa. We use the relations and compositions of Fe^Ni^Co^O^S phases to trace changes in oxygen and sulfur fugacities during progressive serpentinization and steatitization of peridotites from the Mid-Atlantic Ridge in the 15820 0 N Fracture Zone area (Ocean Drilling Program Leg 209). Petrographic observations suggest a systematic change from awaruite^magnetite^pentlandite and heazlewoodite^magnetite^pentlandite assemblages forming in the early stages of serpentinization to millerite^pyrite^polydymite-dominated assemblages in steatized rocks. Awaruite is observed in all brucite-bearing partly serpentinized rocks. Apparently, buffering of silica activities to low values by the presence of brucite facilitates the formation of large amounts of hydrogen, which leads to the formation of awaruite. Associated with the prominent desulfurization of pentlandite, sulfide is removed from the rock during the initial stage of serpentinization. In contrast, steatitization indicates increased silica activities and that highsulfur-fugacity sulfides, such as polydymite and pyrite^vaesite solid solution, form as the reducing capacity of the peridotite is exhausted and H2 activities drop. Under these conditions, sulfides will not desulfurize but precipitate and the sulfur content of the rock increases. The co-evolution of fO2,g^fS2,g in the system follows an isopotential of H2S,aq, indicating that H2S in vent fluids is buffered. In contrast, H2 in vent fluids is not buffered by Fe^Ni^Co^O^S phases, which merely monitor the evolution of H2 activities in the fluids in the course of progressive rock alteration.The co-occurrence of pentlandite^awaruite^magnetite indicates H2,aq activities in the interacting fluids near the stability limit of water. The presence of a hydrogen gas phase would add to the catalyzing capacity of awaruite and would facilitate the abiotic formation of organic compounds.

Seawater‐peridotite interactions: First insights from ODP Leg 209, MAR 15°N

We present first results of a petrographic study of hydrothermally altered peridotites drilled during Ocean Drilling Program (ODP) Leg 209 in the 15°20′N fracture Zone area on the Mid‐Atlantic Ridge (MAR). We find that serpentinization is extensive at all drill sites. Where serpentinization is incomplete, phase relations indicate two major reaction pathways. One is reaction of pyroxene to talc and tremolite, and the other is reaction of olivine to serpentine, magnetite, and brucite. We interpret these reactions in the light of recent peridotite‐seawater reaction experiments and compositions of fluids venting from peridotite massifs at a range of temperatures. We suggest that the replacement of pyroxene by talc and tremolite takes place at temperatures &gt;350°–400°C, where olivine is stable. The breakdown of olivine to serpentine, magnetite, and brucite is favored at temperatures below 250°C, where olivine reacts faster then pyroxene. High‐temperature hydrothermal fluids venting at the Logatchev and Rainbow sites are consistent with rapid reaction of pyroxene and little or no reaction of olivine. Moderate‐temperature fluids venting at the Lost City site are consistent with ongoing reaction of olivine to serpentine and brucite. Many completely serpentinized peridotites lack brucite and talc because once the more rapidly reacting phase is exhausted, interaction with the residual phase will change fluid pH and silica activity such that brucite or talc react to serpentine. At two sites we see strong evidence for continued fluid flow and fluid‐rock interaction after serpentinization was complete. At Site 1268, serpentinites underwent massive replacement by talc under static conditions. This reaction requires either removal of Mg from or addition of Si to the system. We propose that the talc‐altered rocks are Si‐metasomatized and that the source of Si is likely gabbro‐seawater reaction or breakdown of pyroxene deeper in the basement. The basement at Site 1268 is heavily veined, with talc and talc‐oxide‐sulfide veins being the most common vein types. It appears that the systems evolved from reducing (oxygen fugacity buffered by magnetite‐pyrrhotite‐pyrite and lower) to oxidizing (dominantly hematite). We propose that this transition is indicative of high fluid flux under retrograde conditions and that the abundance of hematite may relate to the Ca‐depleted nature of the basement that prevents near‐quantitative removal of seawater sulfate by anhydrite precipitation. At site 1272 we find abundant iowaite partly replacing brucite. While this is the first report of iowaite from a mid‐ocean ridge setting, its presence indicates, again, fairly oxidizing conditions. Our preliminary results indicate that peridotite‐seawater and serpentinite‐seawater interactions can take place under a wider range of temperature and redox conditions than previously appreciated.

Upper mantle heterogeneity below the Mid‐Atlantic Ridge, 0°–15°N

Small‐scale variations in composition of mantle‐derived peridotites have been investigated in the 0°–15°N portion of the Mid‐Atlantic Ridge (MAR), thanks to a relatively close‐spaced peridotite sample coverage achieved by combining samples collected by Russian and U.S. expeditions. Areal variations in the composition of mantle‐equilibrated minerals olivine, orthopyroxene, clinopyroxene, and spinel have been interpreted as due primarily to regional variations in the initial composition, degree of partial melting, and thermal structure of the upper mantle. Mantle rocks from the eastern part of the Romanche transform frequently contain a trapped fraction of basaltic melt, while undepleted mantle prevails in the western part of the Romanche, suggesting a “cold” upper mantle thermal regime in this region, which prevented significant melting. Immediately to the north, the St. Paul Fracture Zone (FZ) upper mantle shows intermediate degrees of melting, except for St. Peter‐Paul Island which exposes metasomatized mantle rocks chemically and isotopically different from other oceanic peridotites. Between St. Paul FZ and 4°N (Strakhov FZ) we have an area of strongly depleted upper mantle. Farther north the Doldrums FZ area (∼8°N) appears to be underlain by moderately depleted upper mantle with some melt entrapment. The Vema FZ (11°N) is underlain by relatively homogenous upper mantle which has undergone a rather low degree of melting. The Mercurius and Marathon transforms (between 12° and 13°N) expose moderately depleted peridotites. Finally, the 15°20′ FZ area shows relatively undepleted upper mantle on the northern side of the transform and at sites distant from the MAR axis and strongly depleted mantle south of the transform. The strongly depleted mantle from the 2°–3°N and 14°–15°N regions is associated spatially with light rare earth element enriched mid‐ocean ridge basalt showing a “hot spot”‐type geochemical signature. The areal association of refractory peridotites with enriched basalt and with zero‐age topographic highs in the 2°–3°N and 14°–15°N regions can be explained either by the influence of mantle thermal plumes or by the presence in the mantle of metasomatized, H2O‐rich domains which would cause enhanced melting and provide a source for basalt enrichment. These mantle domains might be relicts of an originally subcontinental mantle.