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.
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 used satellite-derived gravity anomaly maps and bathymetry data to analyze the distribution of off-axis seamounts on young crust on the flanks of the East Pacific Rise (EPR) and northern ...Pacific–Antarctic Ridge (PAR), from 17°N to 56°S latitude. We observed large-scale variations in the distribution of the volcanoes which we attribute to variations of the regional magmatic budget of the underlying mantle. Two distinct settings for off-axis volcanism were observed at the scale of second-order segments. The first one favors volcano growth near first- or second-order discontinuities. The second type occurs preferentially near the centers of second-order segments. We infer that the distribution of seamounts is controlled both by the availability of melt under the flanks of the ridge and by the vulnerability of the young lithosphere. Near the northern EPR and in some places near the PAR, the off-axis volcanism results mostly from fracturing of the crust near axial discontinuities, probably due to the thermomechanical stresses that develop in the cooling lithosphere. In the southern EPR, especially in the 14–19°S area, the young lithosphere is probably much more vulnerable to off-axis volcanism. The seamount distribution seems to reflect variations in the off-axis melt production at the scale of second-order segments (∼150 km). We infer that the difference in volcanic setting (close to segment ends or close to segment centers) between the northern and southern EPR results from the difference in spreading rate and, consequently, the variation in rheology of the young lithosphere.
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