Seafloor Spreading Centers Research Articles

NOAA’s use of U.S. Navy fixed hydrophone arrays for monitoring the Juan de Fuca Ridge

NOAA’s VENTS Program studies the impact of seafloor hydrothermal emissions on the chemical and thermal budgets of the global ocean. The research focus has been on seafloor spreading centers, in particular the Juan de Fuca Ridge in the northeast Pacific Ocean, and early results indicated the episodic nature of activity. Since no method existed for long-range continuous monitoring, NOAA obtained permission to collect acoustic data from U.S. Navy’s sound surveillance system (SOSUS) in the north Pacific Ocean. Two NOAA systems have been installed to process the signals independently and in parallel with the Navy’s operational systems. One system uses outputs from two end hydrophones from five arrays to analyze seismicity through the north Pacific. The second system collects outputs from formed beams directed at the Juan de Fuca Ridge for real-time monitoring. An acoustic beacon (260 Hz), installed on a seamount, allows continuous monitoring of travel times for location correction. Use of the SOSUS system produces an increase in the number of earthquake detections from ∼350 events/year to ∼7500 events/year for the northeast Pacific. In 6 months of operation, the real-time monitoring system has detected two episodes of volcanic activity, later documented by field observations.

Large igneous provinces: Crustal structure, dimensions, and external consequences

Large igneous provinces (LIPs) are a continuum of voluminous iron and magnesium rich rock emplacements which include continental flood basalts and associated intrusive rocks, volcanic passive margins, oceanic plateaus, submarine ridges, seamount groups, and ocean basin flood basalts. Such provinces do not originate at “normal” seafloor spreading centers. We compile all known in situ LIPs younger than 250 Ma and analyze dimensions, crustal structures, ages, and emplacement rates of representatives of the three major LIP categories: Ontong Java and Kerguelen‐Broken Ridge oceanic plateaus, North Atlantic volcanic passive margins, and Deccan and Columbia River continental flood basalts. Crustal thicknesses range from 20 to 40 km, and the lower crust is characterized by high (7.0–7.6 km s−1) compressional wave velocities. Volumes and emplacement rates derived for the two giant oceanic plateaus, Ontong Java and Kerguelen, reveal short‐lived pulses of increased global production; Ontong Java's rate of emplacement may have exceeded the contemporaneous global production rate of the entire mid‐ocean ridge system. The major part of the North Atlantic volcanic province lies offshore and demonstrates that volcanic passive margins belong in the global LIP inventory. Deep crustal intrusive companions to continental flood volcanism represent volumetrically significant contributions to the crust. We envision a complex mantle circulation which must account for a variety of LIP sizes, the largest originating in the lower mantle and smaller ones developing in the upper mantle. This circulation coexists with convection associated with plate tectonics, a complicated thermal structure, and at least four distinct geochemical/isotopic reservoirs. LIPs episodically alter ocean basin, continental margin, and continental geometries and affect the chemistry and physics of the oceans and atmosphere with enormous potential environmental impact. Despite the importance of LIPs in studies of mantle dynamics and global environment, scarce age and deep crustal data necessitate intensified efforts in seismic imaging and scientific drilling in a range of such features.

Large Igneous Provinces

Large igneous provinces (LIPs) are massive crustal emplacements of predominantly iron- and magnesium-rich (mafic) rock that form by processes other than normal seafloor spreading; they are the dominant form of near-surface magmatism on the terrestrial planets and moons of our solar system. On the Earth’s surface, LIP rocks are readily distinguishable from the products of the two other major types of magmatism – mid-ocean ridge magmatism and arc magmatism – on the basis of petrologic, geochemical, geochronological, geophysical, and physical volcanological data. LIPs occur both on the continents and in the oceans, and include continental flood basalts, volcanic divergent margins, oceanic plateaus, submarine ridges, seamount chains, and ocean-basin flood basalts (Figure 1 and Table 1). LIPs and their contemporary small-scale analogues, hotspot volcanoes, are commonly attributed to decompression melting of hot low-density mantle material ascending from the Earth’s interior in mantle plumes, and thus provide a window into mantle processes. This type of magmatism currently accounts for about 10% of the mass and energy flux from the Earth’s deep interior to its crust. The flux may have been higher in the past, but is episodic over geological time, in contrast to the relatively steady-state activity at seafloor spreading centres and subduction zones. Such episodicity reveals dynamic non-steady- state circulation within the Earth’s mantle, perhaps extending far back into Earth history, and suggests a strong potential for LIP emplacements to contribute to, if not instigate, major environmental changes.

Introduction to Atlantic Hydrothermal Activity

Seafloor hydrothermal research has advanced rapidly from local to global scope through a sequence of discoveries. Hydrothermal research at seafloor spreading centers began in the mid‐1960s with the discovery of hot metalliferous brines and sediments ponded in deeps along the slow spreading (half rate 1 cm yr−1) axis of the Red Sea [Chamock, 1964; Miller, 1964; Swallow and Crease, 1965; Miller et al., 1966; Hunt et al., 1967; Bischoff, 1969]. At the same time a hydrothermal metalliferous component was identified in sediments of the East Pacific Rise [Skomyakova, 1965; Arrhenins and Bonatti, 1965; Boström and Peterson, 1966]. Geophysicists recognized that heat flow measurements at spreading centers could only be explained by convective cooling of the crust with circulating seawater [Elder, 1967; Lister, 1972].

Geophysical observations of the northern Juan de Fuca Ridge system: lessons in sea-floor spreading

By virtue of its proximity to the coastline of North America and to numerous oceanographic institutions, the Juan de Fuca Ridge has been the focus of a large number of marine geological, geochemical, and geophysical investigations. Systematic studies began in the early 1960's with the geophysical survey of A. D. Raff and R. G. Mason, which provided much of the foundation for the development of the extraordinarily successful paradigms of sea-floor spreading and plate tectonics. Subsequent systematic and detailed studies of the plates and plate boundaries of the area by investigators from many academic, industrial, and government agencies, including the Geological Survey of Canada, have provided the basis for much of the fundamental understanding we now have of global plate motions and the processes that are involved in the creation of new oceanic crust at sea-floor spreading centres. Much of the success of these studies can be attributed to the geological diversity found along the Juan de Fuca Ridge. Clear examples are present of "normal" volcanically robust ridge segments, deep extensional rift valleys, stable and evolving transform faults, nontransform ridge offsets, propagating rifts, and off-axis seamount chains. Much has been learned about the nature of hydrothermal circulation through intensive studies of the many active hydrothermal systems and mature hydrothermal deposits that occur in both unsedimented and sedimented environments along the ridge. Better understanding of the way that oceanic crust chemically and physically "ages" is emerging from studies on the ridge and ridge flank. A clear history of the evolution of the ridge and of plate motions is provided by the magnetic anomalies mapped over the ridge and adjacent plates. From this history, lessons have been learned about the causes and consequences of plate motions, fragmentation, and internal deformation. Some of the success of these studies can be attributed to the rapidly evolving geophysical tools which provide ever increasing efficiency of operation and resolution. A new phase of study most recently begun involves the deployment of sea-floor geophysical "observatories" that provide a means by which temporal variations and events can be monitored over extended periods of time. These new studies are expected to yield yet another level of understanding of the processes that have produced two thirds of the Earth's surface as well as many important geologic formations in terrestrial settings.

Morphology of the Blanco Transform Fault Zone-NE Pacific: Implications for its tectonic evolution

The right-lateral Blanco Transform Fault Zone (BTFZ) offsets the Gorda and the Juan de Fuca Ridges along a 350 km long complex zone of ridges and right-stepping depressions. The overall geometry of the BTFZ is similar to several other oceanic transform fault zones located along the East Pacific Rise (e.g., Siquieros) and to divergent wrench faults on continents; i.e., long strike-slip master faults offset by extensional basins. These depressions have formed over the past 5 Ma as the result of continual reorientation of the BTFZ in response to changes in plate motion. The central depression (Cascadia Depression) is flanked by symmetrically distributed, inward-facing back-tilted fault blocks. It is probably a short seafloor spreading center that has been operating since about 5 Ma, when a southward propagating rift failed to ‘kill’ the last remnant of a ridge segment. The Gorda Depression on the eastern end of the BTFZ may have initially formed as the result of a similar occurrence involving a northward propagating rift on the Gorda ridge system. Several of the smaller basins (East Blanco, Surveyor and Gorda) morphologically appear to be oceanic analogues of continental pull-apart basins. This would imply diffuse extension rather than the discrete neovolcanic zone associated with a typical seafloor spreading center. The basins along the western half of the BTFZ have probably formed within the last few hundred thousands years, possibly as the result of a minor change in the Juan de Fuca/Pacific relative motion.

ALVIN investigation of an active propagating rift system, Galapagos 95.5� W

ALVIN investigations have defined the fine-scale structural and volcanic patterns produced by active rift and spreading center propagation and failure near 95.5° W on the Galapagos spreading center. Behind the initial lithospheric rifting, which is propagating nearly due west at about 50 km m.y.−1, a triangular block of preexisting lithosphere is being stretched and fractured, with some recent volcanism along curving fissures. A well-organized seafloor spreading center, an extensively faulted and fissured volcanic ridge, develops ~ 10 km (~ 200,000 years) behind the tectonic rift tip. Regional variations in the chemical compositions of the youngest lavas collected during this program contrast with those encompassing the entire 3 m.y. of propagation history for this region. A maximum in degree of magmatic differentiation occurs about 9 km behind the propagating rift tip, in a region of diffuse rifting. The propagating spreading center shows a gentle gradient in magmatic differentiation culminating at the SW-curving spreading center tip. Except for the doomed rift, which is in a constructional phase, tectonic activity also dominates over volcanic activity along the failing spreading system. In contrast to the propagating rift, failing rift lavas show a highly restricted range of compositions consistent with derivation from a declining upwelling zone accompanying rift failure. The lithosphere transferred from the Cocos to the Nazca plate by this propagator is extensively faulted and characterized by ubiquitous talus in one of the most tectonically disrupted areas of seafloor known. The pseudofault scarps, where the preexisting lithosphere was rifted apart, appear to include both normal and propagator lavas and are thus more lithologically complex than previously thought. Biological communities, probably vestimentiferan tubeworms, occur near the top of the outer pseudofault scarp, although no hydrothermal venting was observed.

NOAA's VENTS program targets oceanic hydrothermal effects

The VENTS Program was established in 1984 to focus interdisciplinary research of the National Oceanographic and Atmospheric Administration on the oceanic effects of hydrothermal activity along seafloor spreading centers. Since then, the accelerating rate of discoveries in this area of research has resulted in increased recognition of the importance of seafloor hydrothermal venting as a fundamental process for transfer of mass and heat from the Earth's interior to its surface—a process that is active intermittently, but not uncommonly, along the entire 60,000 km‐long global spreading‐center system. As the nation's civilian ocean agency, NOAA undertakes research to understand processes that affect the ocean environment. The VENTS program's highest priority is studies that address large‐scale hydrothermal chemical and thermal effects on the ocean, or the potential for such effects, that manifest themselves over relatively short time periods—from years to centuries.

Major off-axis hydrothermal activity on the northern Gorda Ridge

The first hydrothermal field on the northern Gorda Ridge, the Sea Cliff hydrothermal field, was discovered and geologic controls of hydrothermal activity in the rift valley were investigated on a dive series using the DSV Sea Cliff. The Sea Cliff hydrothermal field was discovered where predicted at the intersection of axis-oblique and axis-parallel faults at the south end of a linear ridge at mid-depth (2700 m) on on the east wall. Preliminary mapping and sampling of the field reveal: a setting nested on nearly sediment-free fault blocks 300 m above the rift valley floor 2.6 km from the axis; a spectrum of venting types from seeps to black smokers; high conductive heat flow estimated to be equivalent to the convective flux of multiple black smokers through areas of the sea floor sealed by a caprock of clastic breccia primarily derived from basalt with siliceous cement and barite pore fillings; and a vent biota with Juan de Fuca Ridge affinities. These findings demonstrate the importance of off-axis hydrothermal activity and the role of the intersection of tectonic lineations in controlling hydrothermal sites at sea-floor spreading centers.

Role of hydrothermal precipitates in the geochemical cycling of vanadium

THE input of high-temperature fluids to the ocean along sea-floor spreading centres plays an important part in controlling the composition of sea water1–3. Available data1–3 show that hydrothermal systems are major sources of lithium, manganese, rubidium, iron and silicon and sinks for magnesium and sulphur. Furthermore, reactive precipitates formed in vent systems or emerging plumes4–6 scavenge elements from sea water, thereby also influencing the composition of the sea water. Our work on the Mid-Atlantic Ridge, combined with data from the Pacific Ocean, shows that scavenging by vent-derived iron oxides helps to control the concentration and cycling of vanadium in the ocean. We estimate that 10–60% of the riverine input of vanadium to the ocean is removed from sea water by this mechanism and that such a process is also important for other elements.

New discoveries of massive sulfides on the East Pacific Rise

Newly discovered massive sulfide occurrences were mapped on the East Pacific Rise between 18° and 24°S. Large occurrences of sulfides are restricted to zones in which there is more tectonic strain than usual for sea-floor spreading centers. These sites are also characterized by a relatively shallow bathymetric position and greater coverage with sheet lava than with pillow lava. Several hydrothermal sources with no sulfide precipitate on the sea floor suggest precipitation of sulfide below the sea floor (stockwork mineralization). Animal communities that depend on sulfide oxidation are scarce and small — at least, that is, the fauna that is large enough to be recognized in underwater photographs. The animal communities around hydrothermal sources which profit from the larger food supply provided by the primary fauna are present in dense populations over relatively large areas. The enrichment of the hydrothermal component in the sediments adjacent to a spreading center is not typical of hydrothermal fields along the East Pacific Rise. It can be concluded from the evaluation of the sediment that the hydrothermal activity increases northwards.

A new view of the mid-ocean ridge from the behaviour of ridge-axis discontinuities

High-resolution images covering large areas of the seafloor reveal numerous discontinuities along the mid-ocean ridge. These discontinuities occur at a range of scales (10–1,000 km) and define a fundamental segmentation of seafloor spreading centres. Some are transient; others persist for millions of years, migrating along the mid-ocean ridge and disrupting the structural and geochemical character of approximately 20% of the oceanic lithosphere.

The acoustics of “black smoker” hydrothermal plumes

High-temperature “black smoker” hydrothermal plumes occur when seawater that has penetrated into the oceanic crust and assimilated heat from magma is discharged from vents located at the axis of a mid-ocean ridge. The acidic, metal-rich, discharge mixes with alkaline, oxidizing seawater, and a fine suspension of sulfide particles is precipitated and convected by the flow. Vents fields have now been found at both fast and slow seafloor spreading centers and may be a ubiquitous feature of mid-ocean ridges. A review of the progress made in using underwater acoustics to study black smoker plumes is presented. Both active and passive techniques are being investigated. Active techniques involve a high-frequency monostatic sonar mounted on a submersible. Analysis of the amplitude and phase of the signal backscattered from the plume provides information about the three-dimensional shape of the plume as well as estimates of the flow-velocity field of the discharging fluid. Passive techniques use bottom-mounted hydrophones to listen to the very low-frequency, hydrodynamic noise generated by a plume. These noise signatures have potential use in locating, characterizing, and monitoring plume sites and in determining the contribution plume noise makes to the overall ambient noise field in the ocean.

Gold in sea-floor polymetallic sulfide deposits

Bulk chemical analyses of 48 samples of hydrothermal precipitates from seven polymstallic sulfide deposits at sea-floor spreading centers in the eastern Pacific reveal distinct patterns of gold enrichment. High gold contents are found in samples from two sulfide deposits: the Axial Seamount (45057 ' N, 130002 ' W), and the southern Explorer Ridge (49045.6 ' N, 130016.2 ' W). A large, 160-kg sample composed of silica, barite, and sphalerite from the top of the Axial Seamount deposit gives analyses up to 6,700 ppb Au, averaging 4,900 ppb Au. Similar material from the southern Explorer Ridge gives analyses up to 1,500 ppb Au, averaging 660 ppb Au. Detailed mineralogical studies together with bulk chemical analyses of a wide range of samples from these two sites reveal strong elemental associations and paragenetic controls on gold deposition. Gold at concentrations of about 200 ppb is associated with high Cu (>1 wt %) and Mo (up to 470 ppm); at concentrations > 800 ppb, with high Zn (>10 wt %), Ba (>3 wt %), and SiO (>20 wt %); and at concentrations > 1,200 ppb, with high Pb (>0.1 wt %), Ag (>100 ppm), As (>300 ppm), and Sb (50-100 ppm). Samples with the highest gold values from both locations contain late, sinterlike, low-temperature sulfosalts of Pb, As, Sb, Ag, and S in a matrix of amorphous silica. These sulfosalts are the probable repositories of the gold. Published analyses of Au-poor ( 300oC), Cu-Fe-rich sulfides and subsequently remobilized by late, sustained, low-temperature (<250oC) fluids and concentrated in SiO-Ba-Zn-rich precipitates near the surface. Suitable fluid chemistry is required to mobilize the gold and a favorable precipitation mechanism is needed to concentrate it. These observations could explain the occurrence of high gold concentrations in selected ore types of some ancient massive sulfide deposits on land.

Median valley tectonics: Air photographs of the Ghoubbet-Asal rift, Afar

The Ghoubbet-Asal rift in Afar is the western continuation of the Gulf of Aden seafloor spreading centre onto land. Air photographs were used to examine its small-scale tectonic structures. A central, relatively flat, zone about 5 km across is the main centre of volcanism. This is bounded by large-throw normal faults spaced at 1–2 km with scarps that face the spreading axis, which are responsible for the gross morphology of the lower walls of the median valley. Short, small-throw, antithetic faults are situated near the base of the main fault scarps in places. These are formed as the main fault blocks ascend the median valley walls, and make only a minor contribution to the morphology of the median valley. Rotation of the main fault blocks away from the spreading centre, as the crust ascends the valley walls, is the main process giving rise to the morphology of the transition from the median valley to the crestal mountains. Antithetic faults close to the bases of major synthetic faults seen on the air photographic data are uncommon observations on Gloria side-scan sonar data for marine slow-spreading centres. If present in greater numbers, however, the scarps of such features would lie in acoustic shadows cast by the major synthetic faults and thus be hidden from Gloria. Other interesting features on the air photographic data include a subsidiary rift that intersects a large-throw normal fault associated with the main rift, and a linear volcanic ridge which constitutes the main centre of volcanism within the spreading centre.

Acoustic imaging of high-temperature hydrothermal plumes at seafloor spreading centers

We explore the possibility of using active sonar techniques to acoustically image high-temperature ‘‘black smoker’’ hydrothermal plumes. We examine recent sonar images of a hydrothermal vent field at 11°N on the East Pacific Rise obtained from DSRV Alvin which may show the presence of plumes. For vent fields on the East Pacific Rise estimates are obtained of the minimum detectable concentration of precipitates as a function of the range between the sonar and the plume boundary. These estimates are compared with measured concentrations. Some of the anticipated advantages of acoustic imaging are discussed. We conclude that acoustic imaging techniques have the potential of providing a coherent framework for point sampling of physical and chemical properties of hydrothermal plumes and for determining the dynamics of their injection into the surrounding water mass.

Black smokers, massive sulphides and vent biota at the Mid-Atlantic Ridge

The discovery of black smokers, massive sulphides and vent biota in the rift valley of the Mid-Atlantic Ridge demonstrates that this assemblage of hydrothermal phenomena is not limited to intermediate-to-fast-spreading oceanic ridges. Hydrothermal exchange processes may thus be important at the ridges which extend through the Atlantic Ocean and western Indian Ocean, comprising more than half of the 55,000-km global length of seafloor spreading centres.

Venting sites studied by NOAA

Evidence of hydrothermal venting at the north end of the Gorda Ridge was identified by a team of researchers from the National Oceanic and Atmospheric Administration (NOAA) this summer on the research vessel Surveyor. If high‐temperature venting actually occurs at the seafloor spreading center at the site, deposits of sulfide minerals may also be present there. Located off the coast of Oregon, the Gorda Ridge is within the U.S. Exclusive Economic Zone (EEZ).

Hydrothermal models for the generation of massive sulfide ore deposits

The discovery of massive sulfide ore deposits at certain sites at active seafloor spreading centers provides a basis for considering the energetics of processes that have concentrated similar large ore deposits that have been preserved in volcanic rocks on land. In this paper we construct transport models based on hydrothermal convection systems in the oceanic crust. We investigate models in which the circulation is driven by heat extracted from the permeable crustal rocks themselves as well as by heat conducted through the roof of a replenished, vigorously convecting crustal magma chamber. If the hydrothermal fluids carry 100 ppm dissolved iron as found in black smokers on the East Pacific Rise and exit at 350°C or greater, calculations show that the heat content of the oceanic crust is insufficient to account for ore deposits of 3 Mt or greater. Heat extraction through the roof of a magma chamber, in conjunction with the formation of a thin (∼1 km) layer of plated gabbro, may account for ore bodies of the order of 3 Mt, provided the permeability of the oceanic crust in the magma‐hydrothermal fluid contact zone is 10−15 m2 or less. If the conducting boundary layer were only 1 m thick, locally, for 103years, the permeability could be as high as 10−13 m2 The permeability in the discharge zone could, conceivably, be two orders of magnitude greater than the permeability at depth. More work is needed on the details of the effects of magma chamber replenishment and off‐axis dike injection and on the role of magmatic fluids. Very large ore deposits (i.e., ∼100 Mt) are probably formed in multiple hydrothermal cycles.

Hydrothermal activity on the Gorda Ridge

Near‐bottom plumes of materials indicative of discharge of metal‐rich hot springs were discovered at sites on the Gorda Ridge by a research team of government and university scientists on a cruise of the National Oceanic and Atmospheric Administration (NOAA) ship Surveyor during May 1985 as part of the NOAA Vents Program. The Gorda Ridge, off northern California and Oregon, is the only seafloor spreading center within the proclaimed 200‐mile U.S. Exclusive Economic Zone (370 km wide) of the conterminous United States and is one of the last oceanic ridges to be explored for metal‐rich hot springs. One reason for this neglect is that the Gorda Ridge is slow spreading, with half‐rates ranging from 1.1 cm/yr in the southern portion to 2.2 cm/yr in the northern portion. Slow spreading centers have not been fully evaluated with regard to hydrothermal activity by many members of the research community, who have concentrated their attention on the faster spreading East Pacific Rise to the south and the Juan de Fuca Ridge to the north of the Gorda Ridge.