Did the East Pacific rise subduct beneath the North America plate (western Mexico)?

New data collected between the northernmost tip of the East Pacific rise (18°05'N, 105°35'W) and the Middle America trench provide evidence that the seafloor, which lacks significant sedimentary cover, has a typical spreading-derived abyssal hill topography. The tectonic fabric of this seafloor is concave to the west, as it is today at the tip of the East Pacific rise. Farther to the east, at the outer wall of the trench, the seafloor topography exhibits a north-south trending fabric. We suggest that this fabric originated along the East Pacific rise, as it reached the trench and possibly subducted beneath the North America plate prior to the development of the complex connection of the East Pacific rise with the Rivera transform.     

Regional shape and tectonics of the equatorial East Pacific Rise

The geography of the East Pacific Rise (EPR) between 10°N and 6°S, redetermined by new surface ship surveys, is characterized by long spreading axes orthogonal to infrequent transform faults. Near 2°10N the EPR is intersected by the Cocos-Nazca spreading center at the Galapagos triple junction. The present pattern was established 27-5.5 m.y.b.p. by a complex sequence of rise-crest jumps and reorientations from a section of the Pacific-Farallon plate boundary. Transverse profiles of the rise flanks can be matched by thermal contraction curves for aging lithosphere, except between the triple junction and 4°S, where the east flank is anomalously shallow and almost horizontal. Most sections of spreading axis have the 10–30 km wide, 100–400 m high, axial ridge that is characteristic of fast spreading centers. However, within 60 km of the triple junction the rise crest structure is atypical, with an axial rift valley and elevated rift mountains, despite a spreading rate of 140 mm/yr. With the exception of this atypical section, the bathymetric profile along the spreading axis is remarkably even, with continuous, gentle slopes for hundreds of kilometers between major transform faults, where step-like offsets in axial depths occur. Most of the observations can be accommodated by a model in which the long spreading axes are underlain by continuous crustal magma chambers that allow easy longitudinal flow of magma, and whose size controls the style and dimensions of EPR crestal topography.Contribution of the Scripps Institution of Oceanography, new series.     

Magnetic properties of some young basalts from the East Pacific Rise

We report the results of a study of the magnetic properties of basalts recovered from the axis and from 0.7 m.y. old crust at 21° N and 19°30 S on the East Pacific Rise as well as from the 9°03 N overlapping spreading centers. The natural remanent magnetization of the samples from 21° N and 19°30 S decreases from the axis to 0.7 m.y. old crust as a result of low-temperature oxidation. In addition, the magnetic properties of the samples from the 21° N sites indicate that: (1) the magnetic susceptibility and the Koenigsberger ratio decrease with low-temperature alteration, (2) the Curie temperature, the median demagnetizing field and the remanent coercivity increase with maghemitization, (3) the saturation magnetization measured at room temperature does not change significantly with age. The magnetic properties of the basalt samples from the 9°03 N overlapping spreading centers indicate the presence of a high magnetization zone at the tip of the eastern spreading center. This high magnetization zone is the result of the high percentage of unaltered, fine-grained titanomagnetites present in the samples. These measurements are consistent with the results of the three-dimensional inversion of the magnetic field over the 9°03 N overlapping system [Sempere et al., 1984] as well as with detailed tectonic and geochemical investigations of overlapping spreading centers (Sempere and Macdonald, 1986a; Langmuir et al., 1986; Natland et al., 1986). The high magnetization zone appears to be the result of the eruption of highly fractionated basalts enriched in iron associated with the propagation of one of the limbs of the overlapping system into older lithosphere and not just to rapid decay, due to low-temperature oxidation, of the initially high magnetization of pillows extruded in the neovolcanic zone.     

Postdepositional injections of uranium-rich solutions into East Pacific Rise sediments

An 8.5 m long apparently undisturbed core from a hilltop on the crest of the East Pacific Rise has uranium and thorium isotope distributions that are very unusual. The core is very poor in ²³²Th, and very rich in U, particularly at the 500-cm level, where a value of about 150 ppm is reached. At the same depth the ²³⁰Th_xs reaches very large negative values. These facts could be accounted for if one assumes that solutions rich in U and poor in Th had been postdepositionally injected into the sediments about 90,000–110,000 years ago. The top of the sediment received much of its U from seawater, judging from the ${}^{234}\text{U}{}^{238}\text{U}$ ratio. Possibly carbonate rich solutions were the carriers of the injected uranium.     

Gold and rare-earth elements in sediments from the East Pacific Rise

The distribution of rare-earth elements and gold have been measured in surface sediments of cores collected along a traverse across the East Pacific Rise at approximately 39° S latitude. The shale-normalized rare earth pattern of crestal sediments supports a seawater source. In samples from the flanks of the rise, concentrations of individual rare earths increase with an increase in Al₂O₃, suggesting that the contribution by lithogenic material becomes significant, in addition to the seawater contribution.The concentration of gold in flank sediments also suggests a lithogenic origin. In crestal sediments, however, unlike the rare earths, its concentration increases over its concentration in flank sediments. It is concluded that similar to iron, which also exhibits a maximum in crestal sediments, a fraction of the gold in these sediments has a volcanogenic source.     

东太平洋海隆深海热液区沉积物古菌多样性分析

采用PCR-RFLP方法对东太平洋海隆深海热液区3个站位沉积物中的古菌多样性进行了初步研究.结果显示,从古菌16S rRNA基因文库中随机挑取的296个阳性克隆分属奇古菌门（Thaumarchaeota,47.64%）、广古菌门（Euryarchaeota,44.93%）、泉古菌门（Crenarchaeota,6.77%）和未分类古菌（0.68%）,其中优势菌群为奇古菌门的亚硝化侏儒菌属（Nitrosopumilus,35.47%）和广古菌门的热原体纲（Thermoplasmata,27.03%）,DHVE3、DHVE5、DHVE6、MBGB和MBGE类群在沉积物样品中也均有发现.另外,3个站位沉积物中古菌类群组成存在差异,S5-TVG1站位样品文库的97个古菌克隆分属奇古菌门（49.48%）、广古菌门（49.48%）和泉古菌门（1.03%）,S14-TVG10站位样品文库的103个古菌克隆由奇古菌门（84.47%）和广古菌门（15.53%）组成,S16-TVG12站位样品文库的96个古菌克隆包括广古菌门（71.88%）、泉古菌门（19.79%）、奇古菌门（6.25%）和未分类古菌（2.08%）.研究结果表明,东太平洋海隆深海热液区沉积物中古菌多样性丰富,存在着许多新的古菌菌群;不同站位古菌菌群结构以及多样性存在差异,这与其所处环境的热液活动密切相关.     

Tectonics at the intersection of the East Pacific Rise with Tamayo Transform fault

During the Fall of 1979, a manned submersible program, utilizing DSRV ALVIN, was carried out at the intersection of the East Pacific Rise (EPR) with the Tamayo Transform boundary. A total of seven dives were completed in the vicinity of the EPR/Tamayo intersection depression and documented the geologic relationships that characterize the juxtaposition of these types of plate boundaries. The young volcanic terrain of the EPR axis can be traced into and across the Tamayo Transform valley but becomes buried by sedimentary talus that is being shed from sediment scarps along the unstable sediment slope that defines the north side of the intersection depression. Within 4 km of the transform boundary, the dominant trend (000°) of the fissures and faults that disrupt the rise-generated volcanics is markedly oblique to the regional direction of sea floor spreading (120°). Since no evidence was found to suggest that these structures accommodate significant amounts of strike-slip displacement, they are taken to reflect a distortion of the EPR extensional tectonic regime by a transform generated shear couple. The floor of the Tamayo Transform valley in this area is inundated by mass-wasted sediment, and the principal transform displacement zone is characterized at the surface by a narrow (<1.5 km) interval of fault scarps in sediment that trends parallel with the transform valley. Extrapolated to the west, this zone links with zones of transform deformation investigated during earlier submersible studies (CYAMEX and Pastouret, 1981). Evidence of low-level hydrothermal discharge was seen at one locality on the EPR axis and at another 8 km west of the axis at the edge of the zone of transform deformation.     

The Newport line off Oregon - Studies in the North East Pacific

The Newport Hydrographic (NH) Line along 44.65°N off central Oregon was sampled seasonally during two epochs: 1961-1971 through the TENOC program and 1997-2003 through the GLOBEC Northeast Pacific Long Term Observations Program (LTOP); some observations are available for 2004 and 2005. During TENOC, the line extended 305 km offshore to 128°W, with stations 18 km apart over the continental shelf and 36 km offshore. During LTOP, the line was shorter (to 126°W) with closer station spacing over the continental shelf (9 km apart) and slope (18 km apart). LTOP cruises included biochemical sampling and underway current measurements. During both TENOC and LTOP, the seasonal cycle is very strong (accounting for >50% of the variance in surface layer properties), with rapid transitions in spring and fall. The summer regime is subject to coastal upwelling driven by southward winds, equatorward surface currents, and advection of low-salinity waters from the Columbia River. The winter regime off Newport is subject to coastal downwelling and poleward surface currents driven by northeastward winds. Comparison between TENOC and LTOP summer regimes shows the near-surface layer (0-100 m) at most locations is significantly warmer and fresher during LTOP than TENOC, and steric heights over the continental margin are significantly higher. Comparison of LTOP and TENOC winters shows that average differences at most locations were not statistically significant, but that the variance of steric height and shelf-break temperatures was significantly higher during LTOP than TENOC. Interannual variability of climate indices is also stronger during LTOP, which included a rare Subarctic invasion in 2002 as well as the strong 1997-1998 El Niño. During both TENOC and LTOP, interannual variability of steric height is closely related to the El Niño/La Niña cycle. Nutrient concentrations and nitrate-to-phosphate ratios of upwelling-source waters vary inversely with halocline temperature. Both reflect alongshore advection by coastal currents: southward currents bring cool, nitrate-rich waters in summer (especially during the Subarctic invasion), and northward currents bring relatively warm, nitrate-poor waters to the NH line in winter (especially during El Niño). Seasonal and interannual variations in the nutrient level of upwelling-source water are reflected in time series of vertically-integrated chlorophyll over the LTOP survey region (about 150 km by 300 km). Seasonal variations in chlorophyll and currents are congruent with seasonal variations in copepod biomass and diversity. We were not successful in establishing a clear connection between chlorophyll levels and interannual variations in copepod biomass or diversity, nor in explaining the large decrease in the survival rate of coho salmon between TENOC (6%) and LTOP (3%).     

Segmentation and disruption of the East Pacific Rise in the mouth of the Gulf of California

Analysis of new multibeam bathymetry and all available magnetic data shows that the 340 km-long crest of the East Pacific Rise between Rivera and Tamayo transforms contains segments of both the Pacific-Rivera and the Pacific-North America plate boundaries. Another Pacific-North America spreading segment (Alarcon Rise) extends 60 km further north to the Mexican continental margin. The Pacific-North America-Rivera triple junction is now of the RRR type, located on the risecrest 60 km south of Tamayo transform. Slow North America-Rivera rifting has ruptured the young lithosphere accreted to the east flank of the rise, and extends across the adjacent turbidite plain to the vicinity of the North America-Rivera Euler pole, which is located on the plate boundary. The present absolute motion of the Rivera microplate is an anticlockwise spin at 4° m.y.^–1 around a pole located near its southeast corner; its motion has recently changed as the driving forces applied to its margins have changed, especially with the evolution of the southern margin from a broad shear zone between Rivera and Mathematician microplates to a long Pacific-Rivera transform. Pleistocene rotations in spreading direction, by as much as 15° on the Pacific-Rivera boundary, have segmented the East Pacific Rise into a staircase of en echelon spreading axes, which overlap at lengthening and migrating nontransform offsets. The spreading segments vary greatly in risecrest geomorphology, including the full range of structural types found on other rises with intermediate spreading rates: axial rift valleys, split shield volcanoes, and axial ridges. Most offsets between the segments have migrated southward, but within the past 1 m.y. the largest of them (with 14–27 km of lateral displacement) have shown dueling behavior, with short-lived reversals in migration direction. Migration involves propagation of a spreading axis into abyssal hill terrain, which is deformed and uplifted while it occupies the broad shear zones between overlapping spreading axes. Tectonic rotation of the deformed crust occurs by bookshelf faulting, which generates teleseismically recorded strike-slip earthquakes. When reversals of migration direction occur, plateaus of rotated crust are shed onto the rise flanks.     

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.     

快速扩张的东太平洋海隆侧翼年轻深海丘陵的热液释放现象

大洋板块从洋中脊扩张后,经过热传导和热平流,大洋岩石圈冷却下来。据估算,50%的水热损失(5.5×1012W)和流体通量(2.3×1012m3/a)来自于5Ma或者更年轻的海底。来自0.1～5Ma的地壳流体循环的大量矿物沉降,可以解释地震速度的快速增加,上地壳孔隙度和渗透率随年龄增加而降低,以及504B孔岩心中约6.9Ma时的玄武岩蚀变和矿脉层。虽然发现了洋脊侧翼通量增大的证据,但是由于缺少勘探和足够的知识来了解洋脊侧翼的热液如何释放至海底,对于如此大范围的海底热液喷口的直接观察进行地很少。沿着洋中脊顶部已经发现了上百座活动喷口,在这里已经进行…     

Symmetric conjugation of continent-ocean boundary structures along the Norwegian and East Greenland Margins

A characteristic structural feature of the Norwegian continental margin that has been described from multichannel seismic data, as well as DSDP and ODP drilling, is a major buildup of layered extrusive basaltic rocks forming wedges that dip seaward along the entire margin. Models for their origin describe them as either products of a late-stage intra-continental rift phenomenon, with the volcanic rocks overlying attenuated continental crust, or as an integral component of the oceanic crust formed during the first few million years of seafloor spreading in the Norwegian-Greenland Sea. From studies of the Norwegian margin alone it was not possible to unequivocally distinguish between the two proposed models so an investigation of the conjugate portions of the East Greenland margin was undertaken which used two-ship multichannel seismic acquisition as its focus. We found that the wedges were not only present on the East Greenland margin, but that details of their along-strike variability are mirrored on both margins, together with their spatial relationship to the oldest magnetic lineation pattern. The wedges are relatively small in the northern part of the margin where they occur almost entirely landward of lineation #24. Moving south we see that the wedges broaden, encroach into the pattern of lineation #24, then develop into two separate wedges, the seaward of which is associated with lineation #23. Lineations #24 and #23 on both margins are distorted when associated with wedges. We believe that these data lend strength to a tectonic association of the wedges with nascent plate boundary processes. Their observation in many margins throughout the world suggest that a prolific volcanic phase may be commonly associated with the inception of a divergent plate boundary, and we believe that the most likely mechanism to generate the excess volcanism is the ‘convective partial melting’ scheme recently proposed by Mutter and Buck (1986) and Mutter et al. (in press).     

东太平洋海隆(EPR)13°N热液区附近沉积物粒度特征

2003年中国海底热液硫化物调查航段在EPR西侧取得了E271与E272站位的小箱体柱状样品,通过对这些样品采用酸化和未酸化两种前处理方法的粒度分析,结果表明样品受热液活动影响不强烈;粒度分布曲线表现为多峰分布,为多种物源共同叠加沉积的结果,其中以生物源与火山源为主。因此该地区的沉积物源虽然有热液活动的贡献,但是较其它来源的沉积物所占的比例较小。     

Tectonic and structural features of the Pacific/Indo-Australian plate boundary in the North Fiji-Lau Basin regions,southwest Pacific

The SOPAC GLORIA survey covered principally an east—west region of the southwest Pacific approximately along latitude 16°S between longitudes 167°E and 171°W. Although a main objective was to determine the potential for seabed resources, the survey covered parts of the boundary between the Pacific and Indo-Australian plates and therefore addresses fundamental tectonic processes in these areas. The boundary is complicated and in some regions is not completely known. It is made more complicated by being unstable in places, making its history unclear. This paper brings together the general results of the survey as discussed by other contributions in this special issue to provide an overall interpretation, particularly as it relates to the major plate boundary through the region.     

Hydrothermal vent meiobenthos associated with mytilid mussel aggregations from the Mid-Atlantic Ridge and the East Pacific Rise

Deep-sea hydrothermal vents occur along the mid-ocean ridges and back-arc basins around the globe. There are very few community analyses of vent meiobenthos. The central objectives of this study were to identify and quantify for the first time the entire metazoan meiobenthic community associated with mussel aggregations of Bathymodiolus thermophilus Kenk and Wilson, 1985 from the EPR, 11°N and of Bathymodiolus puteoserpentis Cosel et al., 1994 from the Mid-Atlantic Ridge (MAR), 23°N. Using a quantitative sampling method, abundance, biomass, sex ratio, species richness, diversity, evenness, and trophic structure were studied based on three samples from each site. Meiobenthic abundance in each sample was unexpectedly low, but similar between sites. The community was composed of nematodes, copepods, ostracods, and mites, with a total of 24 species at EPR vents, and 15 species at MAR vents. While most copepod species were vent endemics within the family Dirivultidae, nematodes and harpacticoid copepods belonged to generalist genera, which occur at a variety of habitats and are not restricted to hydrothermal vents or the deep sea. The meiobenthos of hydrothermal-vent mussel beds constitutes a unique community unlike those of other sulfidic habitats, including the thiobios of shallow-water sediments and the meiobenthos of deep-sea, cold-seep sediments. The trophic structure was dominated by primary consumers, mainly deposit feeders, followed by parasites. Predatory meiofaunal species were absent.     

Smectite formation in metalliferous sediments near the East Pacific Rise at 13°N

A 43 cm long E271 sediment core collected near the East Pacific Rise(EPR) at 13°N were studied to investigate the origin of smectite for understanding better the geochemical behavior of hydrothermal material after deposition.E271 sediments are typical metalliferous sediments. After removal of organic matter, carbonate, biogenic opal,and Fe-Mn oxide by a series of chemical procedures, clay minerals(2 μm) were investigated by X-ray diffraction,chemical analysis and Si isotope analysis. Due to the influence of seafloor hydrothermal activity and close to continent, the sources of clay minerals are complex. Illite, chlorite and kaolinite are suggested to be transported from either North or Central America by rivers or winds, but smectite is authigenic. It is enriched in iron, and its contents are highest in clay minerals. Data show that smectite is most likely formed by the reaction of hydrothermal Fe-oxyhydroxide with silica and seawater in metalliferous sediments. The Si that participates in this reaction may be derived from siliceous microfossils(diatoms or radiolarians), hydrothermal fluids, or detrital mineral phases. And their δ30 Si values are higher than those of authigenic smectites, which implies that a Si isotope fractionation occurs during the formation because of the selective absorption of light Si isotopes onto Feoxyhydroxides. Sm/Fe mass ratios(a proxy for overall REE/Fe ratio) in E271 clay minerals are lower than those in metalliferous sediments, as well as distal hydrothermal plume particles and terrigenous clay minerals. This result suggests that some REE are lost during the smectite formation, perhaps because their large ionic radii of REE scavenged by Fe-oxyhydroxides preclude substitution in either tetrahedral or octahedral lattice sites of this mineral structure, which decreases the value of metalliferous sediments as a potential resource for REE.     

A large submarine debris flow in the path of the Pacific deep western boundary current off New Zealand

The Pacific deep western boundary current (DWBC) encounters an unstable continental margin where it flows across the New Zealand convergent plate boundary. Seismic profiles show the DWBC was intercepted by several submarine landslides, the latest (~38-100 ka) being the newly discovered Matakaoa debris flow. Occupying ~650 km³, the flow extends 200 km from Matakaoa re-entrant to Kermadec Ridge to form a 37-68 m high lobe in the current's path. This deposit appears to have (1) reduced the size of gaps in the western boundary, thereby reducing leakage of the DWBC, and (2) temporarily reduced the terrigenous supply into the flow by impeding the passage of turbidity currents from New Zealand.     

The distribution of geothermal fields along the East Pacific Rise from 13°10′ N to 8°20′ N: Implications for deep seated origins

In 1983 a combined SeaMARC I, Sea Beam swath mapping expedition traversed the East Pacific Rise from 13°20 N to 9°50 N, including most of the Clipperton Transform Fault at 10°15 N, and a chain of seamounts at 9°50 N which runs obliquely to both the ridge axis and transform fault trends. We collected temperature, salinity and magnetic data along the same track. These data, combined with Deep-Tow data and French hydrocasts, are used to construct a thermal section of the rise axis from 13°10 N to 8°20 N.Thermal data collected out to 25 km from the rise axis and along the Clipperton Transform Fault indicate that temperatures above the rise axis are uniformly warmer by 0.065°C than bottom water temperatures at equal depths off the axis. The rise axis thermal structure is punctuated by four distinct thermal fields with an average spacing of 155 km. All four of these fields are located on morphologic highs. Three fields are characterized by lenses of warmed water 20 km in length and 300 m thick. Additional clues to hydrothermal activity are provided in two cases by high concentrations of CH₄, dissolved Mn and ³He in the water column and in another case by concentrations of benthic animals commonly associated with hydrothermal regions.We use three methods to estimate large-scale heat loss. Heat flow estimates range from 1250 MW to 5600 MW for one thermal field 25 km in length. Total convective heat loss for the four major fields is estimated to lie between 2100 MW and 9450 MW. If we add the amount of heat it takes to warm the rest of the rise axis (489 km in length) by 0.065.°C, then the calculated axial heat loss is from 12,275 to 38,525 MW (19–61% of the total heat theoretically emitted from crust between 0 and 1 m.y. in age).     

Experimental evidence of habitat provision by aggregations of Riftia pachyptila at hydrothermal vents on the East Pacific Rise

Habitat created or modified by the physical architecture of large or spatially dominant species plays an important role in structuring communities in a variety of terrestrial, aquatic, and marine habitats. At hydrothermal vents, the giant tubeworm Riftia pachyptila forms large and dense aggregations in a spatially and temporally variable environment. The density and diversity of smaller invertebrates is higher in association with aggregations of R. pachyptila than on the surrounding basalt rock seafloor. Artificial substrata designed to mimic R. pachyptila aggregations were deployed along a gradient of productivity to test the hypothesis that high local species diversity is maintained by the provision of complex physical structure in areas of diffuse hydrothermal flow. After 1 year, species assemblages were compared among artificial aggregations in low‐, intermediate‐, and high‐productivity zones and compared to natural aggregations of R. pachyptila from the same site. Hydrothermal vent fauna colonized every artificial aggregation, and both epifaunal density and species richness were highest in areas of high chemosynthetic primary production. The species richness was also similar between natural aggregations of R. pachyptila and artificial aggregations in intermediate‐ and high‐productivity zones, suggesting that complex physical structure alone can support local species diversity in areas of chemosynthetic primary production. Differences in the community composition between natural and artificial aggregations reflect the variability in microhabitat conditions and biological interactions associated with hydrothermal fluid flux at low‐temperature hydrothermal vents. Moreover, these local ecological factors may further contribute to the maintenance of regional species diversity in hydrothermal vent communities on the East Pacific Rise.