Modeled interactions of mesoscale eddies with the East Pacific Rise: Implications for larval dispersal

Larval transport from distant populations is essential for maintenance and renewal of populations in patchy and disturbed ecosystems such as deep-sea hydrothermal vents. We use quasi-geostrophic modeling to consider the potential for long-distance dispersal of hydrothermal vent larvae in mesoscale eddies interacting with the northern East Pacific Rise. Modeled eddy dynamics were similar to the observed propagation dynamics of Tehuantepec eddies, including their ability to cross the ridge. Simulated surface anticyclones were associated with coherent cyclones in the deep layer with relatively strong current velocities that could significantly increase the dispersal potential of passive particles. Eddy interactions with ridge topography further enhanced tracer dispersal along the ridge axis through shearing and elongation of the eddy core. Simulations suggest that the passage of an eddy would result in local loss from the vent field and aggregate transport with potential enhancement of dispersal between vent fields separated by up to 270 km. Based on the latitude at which most Tehuantepec eddies cross the ridge, eddy-induced flows would enhance connectivity between the 13°N, 11°N, and 9°N vent fields along the East Pacific Rise asymmetrically with higher transport from northern vent fields to southern vent fields.     

Community structure in mussel beds at Logatchev hydrothermal vents and a comparison of macrofaunal species richness on slow- and fast-spreading mid-ocean ridges

Species lists for vent fields on the Mid‐Atlantic Ridge (MAR) from 14°N to 38°N suggest that there is a northern (>27°N), shallow (<2000 m) fauna and a southern (<27°N), deeper (>3000 m) endemic vent fauna, but little is known about how community structure varies along the ridge axis. In this study, quantitative samples of macrofaunal invertebrates associated with mussels (Bathymodiolus puteoserpentis) were collected at Logatchev (14°45′N), the southern‐most explored vent field on the MAR. Community structure (including species composition, species richness, diversity, and relative species abundances) in mussel beds at Logatchev was compared with that of Snake Pit (23°22′N) and Lucky Strike (37°17′N) mussel beds. The most striking feature of the Logatchev mussel‐bed macrofaunal invertebrate community was the tremendous abundance (up to 2390 individuals per liter of mussel‐volume sampled) and biomass of the ophiuroid, Ophioctenella acies. Logatchev and Snake Pit mussel beds share >50% of their associated macrofaunal species; these two sites share only 20–25% of their macrofaunal species with Lucky Strike. Species–effort curves and univariate measures of diversity (H′, J′) do not support the claim that diversity of vent organisms on the MAR is highest at Logatchev, at least when one assesses this within a habitat type. Multivariate analysis readily differentiates the species‐abundance characteristics of Logatchev, Snake Pit, and Lucky Strike mussel‐bed macrofaunas. The relationship between sea‐floor spreading rate and diversity was explored through comparison of species richness in mussel‐bed habitats on slow‐spreading (MAR), fast‐spreading [northern East Pacific Rise (EPR)], and ultra‐fast‐spreading (southern EPR) mid‐ocean ridges. Species richness was greater in samples from the faster‐spreading ridge axes, where vents are more closely spaced but shorter lived, than on slow‐spreading centers, where vents are further apart but longer lived.     

Heat flux estimate of warm water flow in a low-temperature diffuse flow site, southern East Pacific Rise 17°25′ S

A low-temperature diffuse flow site associated with abundant vent fauna was found by submersible observations on the southern East Pacific Rise at 17°25′ S in 1997. This site was characterized by thin sediment covered pillow and sheet lavas with collapsed pits up to ∼15 m in diameter. There were three warm water vents (temperature: 6.5 to 10.5 °C) within the site above which the vented fluids rise as plumes. To estimate heat flux of the warm water vents, a temperature logger array was deployed and the vertical temperature distribution in the water column up to 38 m above the seafloor was monitored. A stationary deep seafloor observatory system was also deployed to monitor hydrothermal activity in this site. The temperature logger array measured temperature anomalies, while the plumes from the vents passed through the array. Because the temperature anomalies were measured in only specific current directions, we identified one of the vents as the source. Heat flux from the vent was estimated by applying a plume model in crossflow in a density-stratified environment. The average heat flux from September 13 to October 18, 1997 was 39 MW. This heat flux is as same order as those of high-temperature black smokers, indicating that a large volume flux was discharged from the vent (1.9 m³/s). Previous observations found many similar warm water flow vents along the spreading axis between 17°20′ S–30′ S. The total heat flux was estimated to be at least a few hundred mega-watts. This venting style would contribute to form effluent hydrothermal plumes extended above the spreading axis.     

Hydrothermal Vent Geology and Biology at Earth’s Fastest Spreading Rates

Earth’s fastest present seafloor spreading occurs along the East Pacific Rise near 31°–32° S. Two of the major hydrothermal plume areas discovered during a 1998 multidisciplinary geophysical/hydrothermal investigation of these mid-ocean ridge axes were explored during a 1999 Alvin expedition. Both occur in recently eruptive areas where shallow collapse structures mark the neovolcanic axis. The 31° S vent area occurs in a broad linear zone of collapses and fractures coalescing into an axial summit trough. The 32° S vent area has been volcanically repaved by a more recent eruption, with non-linear collapses that have not yet coalesced. Both sites occur in highly inflated areas, near local inflation peaks, which is the best segment-scale predictor of hydrothermal activity at these superfast spreading rates (150 mm/yr).     

Intense hydrothermal activity at the axis of the east pacific rise near 13°N: Sumbersible witnesses the growth of sulfide chimney

A submersible study of a fast spreading (12 cm yr^-1) Mid-Ocean ridge segment (East Pacific Rise near 12°50′ N) led to the discovery of intense hydrothermal activity. Twenty four sites with active vents and sixty inactive hydrothermal deposits were found within a narrow graben averaging about 300 m in width along a 20 km long segment of the ridge crest. The graben is floored with fresh basaltic sheet flows including collapsed pits or lava lake structures. From both deep towed camera stations and manned submersible observations, it is estimated that the average spacing between the ridge axis hydrothermal deposits averaging in size 10–50 m in diameter lies between 100 to 200 m. The hydrothermal deposits found in the central graben are believed to have formed rapidly (on the order of a few decades). Detailed investigations of one active site have enabled us to witness the growth of an active chimney which increased its height by 40 cm in 5 days i.e. 8 cm per day. Extensive hydrothermal deposits were discovered on an off-axis seamount located 6 km East of the ridge axis. The hydrothermal deposits found on both the ridge axis and on the seamount are similar in composition and consist essentially of zinc, copper and iron mineral phases. Chemical studies conducted on the venting fluids (320°C) showed that the concentration of the dissolved major metal ions (Fe, Mn, and Zn) is about 0.10–0.13 gl^-1. If we accept an average flow rate of 10ls^-1 based on visual observations the mass of metallic products spewing out from an active chimney must be about 100 kg per day.     

Dispersion of a tracer on the East Pacific Rise (9°N to 10°N), including the influence of hydrothermal plumes

On 12 November 2006, 3 kg of sulfur hexafluoride were released in a 1.2 km long streak in the axial summit trough of the East Pacific Rise at 9°30′N to study how circulation and mixing affect larval dispersion. The first half of a tracer survey performed approximately 40 days after the injection found a small percentage of the tracer on the ridge axis between 9°30′N and 10°10′N, with the main concentration near 9°50′N, a site of many active hydrothermal vents. These observations provide evidence of larval connectivity between vent sites on the ridge. The latter half of the survey detected the primary patch of tracer west of the ridge and just south of the Lamont Seamounts, as a majority of the tracer had been transported off the ridge. However, by the end of the survey, the eastern edge of this patch was transported back to within 10 km of the ridge crest at 9°50′N by a reversal in the subinertial flow, suggesting another pathway for larvae between points along the ridge. Both the horizontal and vertical distributions of the tracer were complex and were likely heavily influenced by topography and vents in the area. Elevated tracer concentrations within the axial summit trough and an adjacent depression on the upper ridge flank suggest that tracers may be detained in such depressions. Correlated tracer/turbidity profiles provide direct evidence of entrainment of the tracer into vent plumes from 9°30′N to 10°N. A comparison of the vertical tracer inventory with neutral density vent-plume observations suggests that on the order of 10% of the tracer injected was entrained into vent plumes near the injection site. The results imply that effluent from diffuse hydrothermal sources and larvae of hydrothermal vent fauna can be entrained in significant quantities into plumes from discrete sources and dispersed in the neutrally buoyant plumes.     

High rates of hydrothermal activity at the west rift of the Easter Microplate deduced from metalliferous sediment composition

Based on the mineralogy and composition of sediment cores from three locations along the East Pacific Rise, sediments from 12°58’ N are shown to contain about 34% metalliferous sediment (on a carbonate‐ and salt‐free basis) whereas those from 23°50’ S and 27°24’ S each contain about 73% metalliferous sediment. The latter two cores are situated near the West Rift of the Easter Microplate. The metalliferous contents of the sediments at 23°50’ S and 27°24’ S are among the highest recorded on the East Pacific Rise. The distribution of elements with depth in these cores indicates that the hydrothermal activity has lasted more than 100,000 years. These data suggest that the West Rift of the Easter Microplate is a highly prospective site for submarine hydrothermal minerals. This is in accord with the high rate of ocean spreading (15–16 cm/yr) and the complex processes of plate extension and rotation there.     

Spatial structures of hydrothermal vents and vent-associated megafauna in the back-arc basin system of the Okinawa Trough,western Pacific

Spatial patterns and morphology of hydrothermal vents and the occurrence of vent-associated megafauna were investigated in the back-arc basin system of the Okinawa Trough, western Pacific. Amongst hydrothermal vent fields located on the rising slopes to the NE and S of the basin, the Iheya North area has been subjected to a series of intensive diving surveys. Hydrothermal vents demonstrated concentrated patterns of distribution on different spatial scales. In Iheya North, the majority of vents occurred in lines parallel to the NE to W/SW alignment of the spreading axis, within a radius of ca. 200 m around the most active sulphide structure with the highest recorded fluid temperature of over 300°C. The morphology of hydrothermal vents varied greatly from an incipient flat rock with crevices to a 20- to 30-m-tall, multi-flanged structure with concomitant variation in the distribution and abundance of vent-associated megafauna, particularly a galatheid Shinkaia crosnieri and Bathymodiolus mussels. Comparison of active and inactive vents revealed that the spatial extent of Shinkaia ‘aggregation’ (a group of individuals with short nearest-neighbour distances) effectively defined a habitat unit of this species, and active hydrothermal chimneys contained more of these units, leading to greater spatial occupancy by Shinkaia. Given the temporally unstable nature of vent structures as recognised by repeated surveys, vent assemblages are thought to be closely dictated by the spatio-temporal dynamics of vents in the Okinawa Trough back-arc system.     

Benthic fauna associated with mussel beds and shrimp swarms at hydrothermal fields on the Mid-Atlantic Ridge

Macrofaunal assemblages with the prevalence of Bresiliidae shrimp and Mytilidae mussels are abundant in the hydrothermal vents along the Mid-Atlantic Ridge. The mussels inhabit the zone of diffuse seeps of hydrothermal fluids with temperature abnormalities up to several degrees. Shrimps inhabit an extreme biotope in the mixed interface between the seawater and the hydrothermal fluid at a temperature up to 20–30°C. We studied the mussel and shrimp assemblages in three hydrothermal vent fields: the Rainbow, Broken Spur, and Snake Pit. The species richness of the mussel assemblages in at least two regions (Broken Spur and Snake Pit) is higher as compared with the shrimps of the same hydrothermal vent fields. The fauna inhibiting the shrimp swarms lack almost any taxa specific for particular assemblages: almost all the taxa are also present in the mussel beds. The structure of the shrimp assemblage is less homogeneous as compared with that of the mussel assemblage. The population prevalence of one taxon (Copepoda) in the shrimp assemblage is most likely connected with the extreme and unstable conditions of the biotope occupied by the shrimps in the hydrothermal field. The taxonomic similarity between the mussel and shrimp assemblages within one hydrothermal vent field is higher as compared with the similarity between the mussel (or shrimp) assemblages from different fields.     

基于PXRF的西南印度洋脊硫化物表层沉积物地球化学找矿研究

海底热液喷发形成的热液羽状流中富含成矿物质,并沉淀在距离热液喷口不等的范围内。对西南印度洋中脊热液喷口附近、距离喷口中等距离、远离喷口的六个表层沉积物样品开展了不同粒度沉积物的便携式XRF（PXRF）分析。结果表明,PXRF可以有效获得的洋中脊钙质沉积物中成矿元素的富集特征。>40目的沉积物样品具有较高的Cu、Zn、Fe、Mn等元素含量,40目以下沉积物中则相对稳定,可能与粗粒沉积物中玄武岩碎屑/玻璃含量较高有关,与镜下鉴定结果一致。以热液喷口为中心,表层沉积物中表现出Cu、Zn、As、Fe、Mn的元素分带,靠近喷口的沉积物中具有较高的成矿元素的含量,并具有较高的Cu/Fe、Zn/Fe比值或者Cu/Mn与Zn/Mn比值。上述结论表明,洋中脊沉积物中成矿元素的含量主要受与喷口的距离的影响,而与粒度的关系不大,洋中脊沉积物地球化学找矿应采用40目以下粒径沉积物作为分析样品。通过PXRF获得的Cu、Zn、As、Fe、Mn等元素含量及其比值特征可以作为海底多金属硫化物沉积物地球化学找矿的指标,该方法满足快速有效识别洋中脊沉积物中地球化学异常的要求。     

我国海底热液活动研究的阶段性进展与展望

海底热液活动调查研究是深海进入、深海探测和深海开发的切入点之一。近十年来,中国在西太平洋弧后盆地、东太平洋海隆、大西洋洋中脊和印度洋脊,发现了一批新的海底热液活动区,围绕着热液活动区的硫化物、流体、热液柱、生物等热液产物开展了调查研究,构建了海底热液地质学,提出了热液活动、冷泉及天然气水合物的同源异汇假说,出版了《海底热液地质学》、《现代海底热液硫化物成矿地质学》、《现代海底热液活动》、《东太平洋海隆热液地质》专著,获得了一批调查研究成果。未来,聚焦海底热液活动的深部过程及其资源环境效应关键问题,发展海底热液活动探测技术,拓展极地海底热液活动调查研究新领域,围绕烟囱体、热液柱、含金属沉积物、流体以及热液区生物等热液产物,开展深入、系统的调查研究工作,无疑将推动海底热液地质学取得新的进展。     

The morphology and tectonics of the Mark area from Sea Beam and Sea MARC I observations (Mid-Atlantic Ridge 23° N)

High-resolution Sea Beam bathymetry and Sea MARC I side scan sonar data have been obtained in the MARK area, a 100-km-long portion of the Mid-Atlantic Ridge rift valley south of the Kane Fracture Zone. These data reveal a surprisingly complex rift valley structure that is composed of two distinct spreading cells which overlap to create a small, zero-offset transform or discordant zone. The northern spreading cell consists of a magmatically robust, active ridge segment 40–50 km in length that extends from the eastern Kane ridge-transform intersection south to about 23°12′ N. The rift valley in this area is dominated by a large constructional volcanic ridge that creates 200–500 m of relief and is associated with high-temperature hydrothermal activity. The southern spreading cell is characterized by a NNE-trending band of small (50–200 m high), conical volcanos that are built upon relatively old, fissured and sediment-covered lavas, and which in some cases are themselves fissured and faulted. This cell appears to be in a predominantly extensional phase with only small, isolated eruptions. These two spreading cells overlap in an anomalous zone between 23°05′ N and 23°17′ N that lacks a well-developed rift valley or neovolcanic zone, and may represent a slow-spreading ridge analogue to the overlapping spreading centers found at the East Pacific Rise. Despite the complexity of the MARK area, volcanic and tectonic activity appears to be confined to the 10–17 km wide rift valley floor. Block faulting along near-vertical, small-offset normal faults, accompanied by minor amounts of back-tilting (generally less than 5°), begins within a few km of the ridge axis and is largely completed by the time the crust is transported up into the rift valley walls. Features that appear to be constructional volcanic ridges formed in the median valley are preserved largely intact in the rift mountains. Mass-wasting and gullying of scarp faces, and sedimentation which buries low-relief seafloor features, are the major geological processes occurring outside of the rift valley. The morphological and structural heterogeneity within the MARK rift valley and in the flanking rift mountains documented in this study are largely the product of two spreading cells that evolve independently to the interplay between extensional tectonism and episodic variations in magma production rates.     

Spatial distribution, diversity and composition of bacterial communities in sub-seafloor fluids at a deep-sea hydrothermal field of the Suiyo Seamount

Spatial distribution, diversity, and composition of bacterial communities within the shallow sub-seafloor at the deep-sea hydrothermal field of the Suiyo Seamount, Izu-Bonin Arc, Western Pacific Ocean, were investigated. Fluids were sampled from four boreholes in this area. Each borehole was located near or away from active vents, the distance ranging 2–40 m from active vents. In addition, fluids discharging from a natural vent and ambient seawater were sampled in this area. We extracted DNA from each sample, amplified bacterial 16S rRNA genes by PCR, cloned the PCR products and sequenced. The total number of clones analyzed was 348. Most of the detected phylotypes were affiliated with the phylum Proteobacteria, of which the detection frequency in each clone library ranged from 84.6% to 100%. The bacterial community diversity and composition were different between hydrothermal fluids and seawater, between fluids from the boreholes and the vent, and even among fluids from each borehole. The relative abundances of the phylotypes related to Thiomicrospira, Methylobacterium and Sphingomonas were significantly different among fluids from each borehole. The phylotypes related to Thiomicrospira and Alcanivorax were detected in all of the boreholes and vent samples. Our findings provide insights into bacterial communities in the shallow sub-seafloor environments at active deep-sea hydrothermal vent fields.     

Toward a mechanistic understanding of larval dispersal: insights from genomic fingerprinting of the deep-sea hydrothermal vent tubeworm Riftia pachyptila

Larval dispersal is critical for the maintenance of species populations in patchy and ephemeral hydrothermal vent habitats. On fast‐spreading ridges, such as the East Pacific Rise, rates of habitat turnover are comparable to estimated lifespans of many of the inhabiting species. Traditionally, dispersal questions have been addressed with two very different approaches, larval studies and population genetics. Population genetic studies of vent‐endemic species have been informative for determining whether patterns of dispersal are suggestive of stepping stone or island models and estimating rates of gene flow (effective migrants per generation) over broad geographic ranges. However, these studies leave fundamental questions unanswered about the specific mechanisms by which larvae disperse and species maintain their populations and biogeographic ranges. With the goal of examining genetic structure and elucidating alternative larval dispersal mechanisms, we employed a genomic DNA fingerprinting technique, amplified fragment length polymorphisms (AFLPs). To assess the potential utility of AFLPs, and genetic structure of the hydrothermal vent tubeworm Riftia pachyptila, genomic ‘fingerprints’ were recovered from 29 individuals from five vent fields spanning a distance of up to c. 5000 km along the East Pacific Rise. In contrast to previous population genetic studies that found little to no genetic structure using allozymes and mitochondrial DNA, genetic analyses of 630 polymorphic AFLP loci identified distinct subclades within R. pachyptila populations. Significant levels of differentiation were observed among populations from all vent regions as well as within each region. Discrete assemblages of tubeworms separated by as little as c. 400 m within a given vent region were genetically distinguishable and cohorts (based on size‐frequency distribution) within an aggregation were found to be most closely related. These results suggest that mechanisms of larval dispersal act to retain cohort fidelity in R. pachyptila.     

Morphology and tectonics of the Galapagos Triple Junction

We describe the results of GLORIA and SEABEAM surveys, supplemented by other marine geophysical data, of the Galapagos Triple Junction where the Pacific, Cocos and Nazca plates meet. The data allowed detailed topographic and tectonic maps of the area to be produced. We located each spreading axis with a precision of about 1 km. All three plate boundaries change character as the triple junction is approached to take on morphologies typical of slower spreading axes: the fast-spreading East Pacific Rise develops the morphology of a medium-spreading rise, and the medium-spreading Cocos-Nazca Rise takes on the appearance of a slow-spreading ridge. The axis of the East Pacific Rise was found to be completely continuous throughout the survey area, where it runs along the 102°05 W meridian. The Cocos-Nazca axis, however, fails to meet it, leaving a 20-km-wide band of apparently normal East Pacific Rise crust between its tip and the East Pacific Rise axis. As a consequence there must be considerable intra-plate deformation within the Cocos and Nazca plates. A further 40 km of the Cocos-Nazca axis is characterised by oblique faulting that we interpret to be a sign of rifting of pre-existing East Pacific Rise crust. We infer that true sea-floor spreading on the Cocos-Nazca axis does not begin until 60 km east of the East Pacific Rise axis. Other areas of similar oblique faulting occur on the Pacific plate west of the triple junction and along the rough-smooth boundaries of the Galapagos Gore. We present a model involving intermittent rifting, rift propagation, and sea-floor spreading, to explain these observations.     

东太平洋海隆13°N表层含金属沉积物物质组成与元素赋存状态

为了解不同类型含金属沉积物在物质组成和元素赋存状态的差异,对东太平洋海隆13°N洋中脊两侧表层含金属沉积物(站号:E271和E53)进行了矿物学、地球化学分析,顺序提取实验,并与前人对轴部表层沉积物(站号:17A-EPR-TVG1)的研究结果作了对比分析。研究结果表明, E271和E53是远端含金属沉积物,由非浮力热液羽状流中颗粒物沉降所形成的;17A-EPR-TVG1沉积物是近喷口含金属沉积物,由黑烟囱或者热液硫化物丘状体崩塌、堆积,或者由热液羽状流中Fe-Mn氧化物和硫化物快速沉淀而形成的,近喷口沉积物比远端沉积物更富集Fe、Cu和Zn等元素。元素在两种含金属沉积物中的赋存状态基本相同,除了Fe,Cu,Zn,Mo和稀土元素(REE)等元素在远端沉积物中主要存在于Fe-Mn氧化物相,在近喷口沉积物中主要在残留相中。远端沉积物中REE页岩标准化配分模式与海水相似,表明REE主要来自海水,而近喷口沉积物中REE配分模式与热液流体相似,说明REE以高温热液流体来源为主。相关研究结果加深了对热液沉积作用研究的认识。     

Variations in deep-sea hydrothermal vent communities on the Mid-Atlantic Ridge near the Azores plateau

Near the Azores Triple Junction as the Azores Plateau is approached, the ridge axis becomes shallower; its depth decreases from ca. 2400 m in the Rainbow vent field (36°13′N) to ca. 850 m in the Menez Gwen vent field (37°35′N). In this area, extensive mussel beds of the mytilid Bathymodiolus azoricus dominate the hydrothermal vent fauna, along with populations of three shrimps (Rimicaris exoculata, Mirocaris fortunata and Chorocaris chacei). The main physical and chemical characteristics of the vent habitat were studied by discrete sampling, in situ analysis and sediment trap moorings. The vent fauna is distributed along a variable band where the vent fluids and seawater mix, with R. exoculata living in the most concentrated areas and Bathymodiolus azoricus in the most diluted zones. Various non-endemic species live at the border of the vent field. The variations observed in structure and composition of the communities along the depth gradient are most likely due to changes in vent fluid toxicity (metallic and sulphide content) and suspended mineral particles, which render the fluids harsher for species living there. The main faunal differences observed between Lucky Strike and Menez Gwen hydrothermal fields are due to an impoverishment in the hydrothermal endemic species and to the penetration of bathyal species. The comparison of the three studied vent fields suggests the existence of a succession of several biogeographic islands rather than a single province.     

The morphology and structure of the West O’Gorman Fracture Zone

The West O’Gorman Fracture Zone is an unusual feature that lies between the Mathematician Ridge and the East Pacific Rise on crust generated on the East Pacific Rise between 4 and 9 million years ago. We made a reconnaissance gravity, magnetic and Sea Beam study of the zone with particular emphasis on its eastern (youngest) portion. That region is characterized by an elongate main trough, a prominent median ridge and other, smaller ridges and troughs. The structure has the appearance of large-offset fracture zone, possibly in a slow spreading environment. However, magnetic anomalies indicate that the offset, if any, is quite small, and the spreading rate during formation was fast. In addition, the magnetic profiles do not support earlier models for a difference in spreading rate north and south of the fracture. The morphology of the fracture zone suggests that flexure may be responsible for some of the topography; but gravity studies indicate some of the most prominent features of the fracture zone are at least partially compensated. The main trough is underlain by a thin crust (or high density body), similar to large-offset fracture zones in the Atlantic, while the median ridge is underlain by a thickened crust. Sea Beam data does not unambiguously resolve between volcanism or serpentinization of the upper mantle as a mechanism for isostatic compensation. Why the West O’Gorman exists remains enigmatic, but we speculate that the topographic expression of a fracture zone does not require a transform offset during formation. Perhaps the spreading ridge was magma starved for some reason, resulting in a thin crust that allowed water to penetrate and serpentinize portions of the upper mantle.     

Investigation of faulting and abyssal hill formation on the flanks of the East Pacific Rise (21°N) using ALVIN

A wealth of geologic information has been collected during studies of the Matuyama/Brunhes magnetic reversal boundary on the East Pacific Rise at 21°N. Five ALVIN and two CYANA dives, and a series of deep-tow traverses show that abyssal hills in this region of the Pacific are created near the spreading axis by inward dipping normal faulting and by back-tilting of these fault blocks. Outward dipping faults occur but are of less importance in the creation of relief. Tectonic disruption of the crust, particularly through tilting, is less pronounced than in the Atlantic. Small volcanoes approximately 50 m high and 400 m wide are common on the abyssal hills. A significant number of the volcanoes may have split apart at the spreading axis attesting to the narrowness of the crustal accretion zone on the East Pacific Rise. Active faulting is restricted to less than 10 to 12 km off-axis, although minor recent faulting may have been detected 23 km off-axis. Crustal sections exposed by faulting reveal that massive lava flows and sheet flows are common in the upper portion of oceanic layer two, but are less abundant than pillow lavas.