Beneath ultraslow‐spreading ridges, the oceanic lithosphere remains poorly understood. Using recordings from a temporary array of ocean bottom seismometers, we here report an ~17‐days‐long microearthquake study on two segments (27 and 28) of the ultraslow‐spreading Southwest Indian Ridge (49.2° to 50.8°E). A total of 214 locatable microearthquakes are recorded; seismic activity appears to be concentrated within the west median valley at Segment 28 and adjacent nontransform discontinuities. Earthquakes reach a maximum depth of ~20 km beneath the seafloor, and they mainly occur in the mantle, implying a cold and thick brittle lithosphere. The relatively uniform brittle/ductile boundary beneath Segment 28 suggests that there is no focused melting in this region. The majority of earthquakes is located below the Moho interface, and a 5‐km‐thick aseismic zone is present beneath Segment 28 and adjacent nontransform discontinuities. At the Dragon Flag hydrothermal vent field along Segment 28, the presence of a detachment fault has been inferred from geomorphic features and seismic tomography. Our seismicity data show that this detachment fault deeply penetrates into the mantle with a steeply dipping (~65°) interface, and it appears to rotate to a lower angle in the upper crust, with ~55° of rollover. There is a virtual seismic gap beneath magmatic Segment 27, which may be connected to the presence of an axial magma chamber beneath the spreading center and focused melting; in this scenario, the increased magma supply produces a broad, elevated temperature environment, which suppresses earthquake generation.

Oceanic transform faults, a key element of plate tectonics, represent the first-order discontinuities along mid-ocean ridges, host large earthquakes, and induce extreme thermal gradients in lithosphere. However, the thermal structure along transform faults and its effects on earthquake generation are poorly understood. Here we report the presence of a 10- to 15-kilometer-thick in-depth band of microseismicity in 10 to 34 kilometer depth range associated with a high-temperature (700° to 900°C) mantle below the brittle lithosphere along the Romanche mega transform fault in the equatorial Atlantic Ocean. The occurrence of the shallow 2016 moment magnitude 7.1 supershear rupture earthquake and these deep microearthquakes indicate that although large earthquakes occur in the upper brittle lithosphere, a substantial amount of deformation is accommodated in the semibrittle mylonitic mantle that resides at depths below the 600°C isotherm. We also observe a rapid westward deepening of this band of seismicity indicating a strong lateral heterogeneity.

We present results from an ocean bottom seismometer experiment surveying the fossil spreading centre in the Southwest Sub‐basin of the South China Sea. The detailed velocity model shows that oceanic layer 2 exhibits across‐axis variations in thickness and velocity, whereas oceanic layer 3 displays a variation in crustal thickness. A low‐angle (24°) SE‐dipping oceanic detachment fault is proposed to explain the anomalous structure on the NW side of the spreading centre, which exhibits uplifting of the upper mantle beneath a thinned oceanic crust. The inferred oceanic detachment fault was at its initial stage, localized within the basaltic crust and did not exhume lower crust. We suggest that the low‐velocity (7.6–7.9 km/s) body located within the upper mantle beneath the footwall of the detachment fault is caused by both mantle serpentinization and partial melting. The difference in crustal thickness in the Southwest Sub‐basin indicates that the magma supply varied in time and space during or even after the seafloor spreading. Compared with other fossil spreading ridges, the fossil spreading centre of the northeast Southwest Sub‐basin studied here represents a third type of fossil spreading ridge, characterized by a reduced melt supply at the waning stage of spreading and a strong post‐spreading magmatism. Copyright © 2016 John Wiley & Sons, Ltd.