Multiple geological processes affect the distribution of hydrothermal venting along a mid‐ocean ridge. Deciphering the role of a specific process is often frustrated by simultaneous changes in other ...influences. Here we take advantage of the almost constant spreading rate (65–71 mm/yr) along 2500 km of the Southeast Indian Ridge (SEIR) between 77°E and 99°E to examine the spatial density of hydrothermal venting relative to regional and segment‐scale changes in the apparent magmatic budget. We use 227 vertical profiles of light backscatter and (on 41 profiles) oxidation‐reduction potential along 27 first and second‐order ridge segments on and adjacent to the Amsterdam‐St. Paul (ASP) Plateau to map ph, the fraction of casts detecting a plume. At the regional scale, venting on the five segments crossing the magma‐thickened hot spot plateau is almost entirely suppressed (ph = 0.02). Conversely, the combined ph (0.34) from all other segments follows the global trend of ph versus spreading rate. Off the ASP Plateau, multisegment trends in ph track trends in the regional axial depth, high where regional depth increases and low where it decreases. At the individual segment scale, a robust correlation between ph and cross‐axis inflation for first‐order segments shows that different magmatic budgets among first‐order segments are expressed as different levels of hydrothermal spatial density. This correlation is absent among second‐order segments. Eighty‐five percent of the plumes occur in eight clusters totaling ∼350 km. We hypothesize that these clusters are a minimum estimate of the length of axial melt lenses underlying this section of the SEIR.
Key Points
Most extensive hydrothermal plume survey anywhere on the mid‐ocean ridge
Hydrothermal frequency tracks magmatic budgets at regional and segment scales
Plume clusters and MBA lows imply a melt lens beneath 14–33% of the ridge axis
Abstract
Using a combined approach of seafloor mapping, MAPR and CTD survey, we report evidence for active hydrothermal venting along the 130°‐140°E section of the poorly‐known South‐East Indian ...Ridge (SEIR) from the Australia‐Antarctic Discordance (AAD) to the George V Fracture Zone (FZ). Along the latter, we report Eh and CH
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anomalies in the water column above a serpentinite massif, which unambiguously testify for ultramafic‐related fluid flow. This is the first time that such circulation is observed on an intermediate‐spreading ridge. The ridge axis itself is characterized by numerous off‐axis volcanoes, suggesting a high magma supply. The water column survey indicates the presence of at least ten distinct hydrothermal plumes along the axis. The CH
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:Mn ratios of the plumes vary from 0.37 to 0.65 denoting different underlying processes, from typical basalt‐hosted to ultramafic‐hosted high‐temperature hydrothermal circulation. Our data suggest that the change of mantle temperature along the SEIR not only regulates the magma supply, but also the hydrothermal activity. The distribution of hydrothermal plumes from a ridge segment to another implies secondary controls such as the presence of fractures and faults along the axis or in the axial discontinuities. We conclude from these results that hydrothermal activity along the SEIR is controlled by magmatic processes at the regional scale and by the tectonics at the segment scale, which influences the type of hydrothermal circulation and leads to various chemical compositions. Such variety may impact global biogeochemical cycles, especially in the Southern Ocean where hydrothermal venting might be the only source of nutrients.
Key Points
Intense and contrasted hydrothermal activity has been evidenced along the South‐East Indian Ridge in the Furious Fifties
Ultramafic circulation is evidenced in the George V FZ, which is the first observation of this type along an intermediate‐spreading ridge
Chemical compositions of the plumes reveal various regional and local controls on the hydrothermal circulation
Non-hot spot, intraplate volcanism is a common feature near the East Pacific Rise or Pacific-Antarctic ridge. Volcanic ridges and seamount chains, tens to hundreds of kilometers long, are ...asymmetrically distributed about the ridge axis, with most volcanic features occurring on the Pacific plate. Their origins remain controversial. We have analyzed off-axis volcanic ridges near the Pacific-Antarctic ridge from bathymetry, backscatter, gravity, and geochemistry data of the Pacantarctic 2 cruise. K/Ar dating of samples dredged on these structures reveals a contrast of up to 3 Ma between the volcanoes and the underlying crust. The volcanic activity, as suggested by the strong backscatter in sonar images, appears to be limited to areas of seafloor younger than about 3 Ma. All surveyed ridges north of the Menard transform fault (TF) show recent activity close to the ridge axis and are not affected by faults. The off-axis volcanic ridges south of the Menard TF show recent volcanic flows in their center and are affected by N-S extension. Two different types of volcanoes can be characterized: conical, flat-topped ones and rough, elongated ones associated with narrow, E-W trending volcanic ridges, some of them showing strong backscattering on the EM12 imagery. From the morphology of the seamount chains and the ages of the lava samples, we infer that the magma source for the volcanic ridges is related to the feeding of the ridge axis through three-dimensional mantle convective circulation. We suggest that a change in the relative plate motion since about 5 Ma might have induced an offset of the mantle upwelling circulation under the ridge axis so that anomalously hot mantle rises under the Pacific ridge flank. The kinematic change is also likely responsible for the tectonic deformation in the young lithosphere south of Menard TF.
The results from a 53‐km‐long, wide‐angle seismic profile across the rift valley of the Mid‐Atlantic Ridge south of the Kane transform (near 23°20′N, MARK area) provide new constraints on models of ...tectonic extension and magmatic accretion along slow spreading mid‐ ocean ridges. Anomalously low middle and lower‐crustal P wave velocities beneath the neovolcanic Snake Pit ridge are consistent with elevated axial temperatures and with the presence of 4±1% partial melt evenly distributed within the lower crust in preferentially oriented, elongated thin films. If the melt inclusions have larger aspect ratios, melt fractions can be up to 17±3%. This and other geological observations suggest that the study area is presently in a magmatically active period. The igneous crust is anomalously thin beneath both flanks of the median valley (≤2.3–2.5 km). Thus the mantle rocks observed along the western rift valley wall at Pink Hill were probably emplaced at shallow levels within the valley floor during a period of very low magma supply and were later exposed on the valley walls by normal faulting. The crust within the eastern rift valley and flanking rift mountains is seismically heterogeneous, with igneous crustal thickness variations of ≥2.2 km over horizontal distances of ∼5 km. This heterogeneity indicates that the magma supply in the area has fluctuated during the last ∼2 m.y. Thus magmatic and amagmatic periods at slow spreading ridges may alternate over much shorter temporal scales that previously inferred from sea surface gravity data.
We present the interpretation of a new set of closely spaced marine magnetic profiles that complements previous data in the northeastern and southwestern parts of the South China Sea (Nan Hai). This ...interpretation shows that seafloor spreading was asymmetric and confirms that it included at least one ridge jump. Discontinuities in the seafloor fabric, characterized by large differences in basement depth and roughness, appear to be related to variations in spreading rate. Between anomalies 11 and 7 (32 to 27 Ma), spreading at an intermediate, average full rate of ≈50 mm/yr created relatively smooth basement, now thickly blanketed by sediments. The ridge then jumped to the south and created rough basement, now much shallower and covered with thinner sediments than in the north. This episode lasted from anomaly 6b to anomaly 5c (27 to ≈16 Ma) and the average spreading rate was slower, ≈35 mm/yr. After 27 Ma, spreading appears to have developed first in the eastern part of the basin and to have propagated towards the southwest in two major steps, at the time of anomalies 6b‐7, and at the time of anomaly 6. Each step correlates with a variation of the ridge orientation, from nearly E‐W to NE‐SW, and with a variation in the spreading rate. Spreading appears to have stopped synchronously along the ridge, at about 15.5 Ma. From computed fits of magnetic isochrons, we calculate 10 poles of finite rotation between the times of magnetic anomalies 11 and 5c. The poles permit reconstruction of the Oligo‐Miocene movements of Southeast Asian blocks north and south of the South China Sea. Using such reconstructions, we test quantitatively a simple scenario for the opening of the sea in which seafloor spreading results from the extrusion of Indochina relative to South China, in response to the penetration of India into Asia. This alone yields between 500 and 600 km of left‐lateral motion on the Red River‐Ailao Shan shear zone, with crustal shortening in the San Jiang region and crustal extension in Tonkin. The offset derived from the fit of magnetic isochrons on the South China Sea floor is compatible with the offset of geological markers north and south of the Red River Zone. The first phases of extension of the continental margins of the basin are probably related to motion on the Wang Chao and Three Pagodas Faults, in addition to the Red River Fault. That Indochina rotated at least 12° relative to South China implies that large‐scale “domino” models are inadequate to describe the Cenozoic tectonics of Southeast Asia. The cessation of spreading after 16 Ma appears to be roughly synchronous with the final increments of left‐lateral shear and normal uplift in the Ailao Shan (18 Ma), as well as with incipient collisions between the Australian and the Eurasian plates. Hence no other causes than the activation of new fault zones within the India‐Asia collision zone, north and east of the Red River Fault, and perhaps increased resistance to extrusion along the SE edge of Sundaland, appear to be required to terminate seafloor spreading in the largest marginal basin of the western Pacific and to change the sense of motion on the largest strike‐slip fault of SE Asia.
We analyze the structure and evolution of two propagators along the Pacific–Antarctic Ridge (PAR) that we surveyed during the Pacantarctic cruise of the N/O L’Atalante. A large propagator at 63°30′S, ...167°W shows a N50°E-trending segment of the PAR propagating southwestward, while the adjacent, N45°E-trending segment retreats. The propagating and doomed ridges are offset by about 43 km. They both curve into the offset to define an overlap zone about 25 km long. The inner pseudofault is juxtaposed to a series of E–W-trending ridges inferred to represent the failed axes. Their direction and arrangement suggest an evolution as an overlapping propagator with cyclic rift failure. The pseudofaults are 35±5° oblique to the propagating ridge, which implies a rate of propagation of 44±8 mm/yr, using a 62 mm/yr full spreading rate, comparable to that of other propagators with similar morphology. The age of the initiation of the propagation from the Heirtzler fracture zone is estimated to be 5–6 Ma, which coincides with the age of a clockwise change in spreading direction. A second, smaller, southwestward propagator is observed northeast of the major one, at 63°15′S, 165°10′W, with a morphology very similar to that of the larger one. It is inferred to have started near 1 Ma, again at the time of a clockwise change in spreading direction along the PAR. These two propagators are likely to have evolved from extensional relay zones which developed within the Heirtzler transform fault (TF) valley following clockwise changes in spreading direction. The present-day axial discontinuity is less than 40 km in offset and may not be a TF anymore. The development of propagators in this area of the PAR appears to be triggered by kinematic changes rather than by thermal gradients along the ridge. Other propagators that have left similar signatures on the flanks of the PAR, appear to have developed at similar spreading rates near 50–60 mm/yr full rate, as a result of kinematic changes.
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