Xiongwei Niu

Seismic observation of an extremely magmatic accretion at the ultraslow spreading Southwest Indian Ridge

The oceanic crust is formed by a combination of magmatic and tectonic processes at mid-ocean spreading centers. Under ultraslow spreading environment, however, observations of thin crust and mantle-derived peridotites on the seafloor suggest that a large portion of crust is formed mainly by tectonic processes, with little or absence of magmatism. Using three-dimensional seismic tomography at an ultraslow spreading Southwest Indian Ridge segment containing a central volcano at 50°28′E, here we report the presence of an extremely magmatic accretion of the oceanic crust. Our results reveal a low-velocity anomaly (−0.6 km/s) in the lower crust beneath the central volcano, suggesting the presence of partial melt, which is accompanied by an unusually thick crust (~9.5 km). We also observe a strong along-axis variation in crustal thickness from 9.5 to 4 km within 30–50 km distance, requiring a highly focused melt delivery from the mantle. We conclude that the extremely magmatic accretion is due to localized melt flow toward the central volcano, which was enhanced by the significant along-axis variation in lithosphere thickness at the ultraslow spreading Southwest Indian Ridge.

Along-axis variation in crustal thickness at the ultraslow spreading Southwest Indian Ridge (50°E) from a wide-angle seismic experiment

The Southwest Indian Ridge (SWIR) is characterized by an ultraslow spreading rate, thin crust, and extensive outcrops of serpentinized peridotite. Previous studies have used geochemical and geophysical data to suggest the presence of a thicker crust at the central and shallowest portions of the SWIR, from the Prince Edward (35°30′E) to the Gallieni (52°20′E) fracture zones. Here we present a new analysis of wide-angle seismic data along the ridge 49°17′E–50°49′E. Our main conclusions are as follows: (1) we find an oceanic layer 2 of roughly constant thickness and steep velocity gradient, underlain by a layer 3 with variable thickness and low velocity gradient; (2) the crustal thickness varies from ∼5 km beneath nontransform discontinuities (NTDs) up to ∼10 km beneath a segment center; (3) the melt supply is focused in segment centers despite a small NTD between adjacent segments; (4) the presence of a normal upper mantle velocity indicates that no serpentinization occurs beneath this thick crust. Our observation of thick crust at an ultraslow spreading ridge adds further complexity to relationships between crustal thickness and spreading rate, and supports previous suggestions that the extent of mantle melting is not a simple function of spreading rate, and that mantle temperature or chemistry (or both) must vary significantly along axis.

Lithospheric Structure and Tectonic Processes Constrained by Microearthquake Activity at the Central Ultraslow‐Spreading Southwest Indian Ridge (49.2° to 50.8°E)

21 Beneath ultra-slow spreading ridges, the oceanic lithosphere remains poorly understood. Using 22 recordings from a temporary array of ocean bottom seismometers, we here report a ~17-days23 long microearthquake study on two segments (27 and 28) of the ultra-slow spreading Southwest 24 Indian Ridge (49.2° to 50.8° E). A total of 214 locatable microearthquakes are recorded; seismic 25 activity appears to be concentrated within the west median valley at segment 28 and adjacent 26 nontransform discontinuities (NTDs). Earthquakes reach a maximum depth of ~20 km beneath 27 the seafloor, and they mainly occur in the mantle, implying a cold and thick brittle lithosphere. 28 The relatively uniform brittle/ductile boundary beneath segment 28 suggests that there is no 29 focused melting in this region. The majority of earthquakes are located below the Moho 30 interface, and a 5-km-thick aseismic zone is present beneath segment 28 and adjacent NTDs. At 31 the Dragon Flag hydrothermal vent field along segment 28, the presence of a detachment fault 32 has been inferred from geomorphic features and seismic tomography. Our seismicity data show 33 that this detachment fault deeply penetrates into the mantle with a steeply dipping (~65°) 34 interface, and it appears to rotate to a lower angle in the upper crust, with ~55° of rollover. There 35 is a virtual seismic gap beneath magmatic segment 27, which may be connected to the presence 36 of an axial magma chamber beneath the spreading centre as well as focused melting; in this 37 scenario, the increased magma supply produces a broad, elevated temperature environment 38 which suppresses earthquake generation. 39