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

Abstract

Beneath ultra‐slow spreading ridges, the oceanic lithosphere remains poorly understood. Using recordings from a temporary array of ocean bottom seismometers, we here report a ~17‐days‐long microearthquake study on two segments (27 and 28) of the ultra‐slow 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 (NTDs). 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 are located below the Moho interface, and a 5‐km‐thick aseismic zone is present beneath segment 28 and adjacent NTDs. 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 centre as well as focused melting; in this scenario, the increased magma supply produces a broad, elevated temperature environment which suppresses earthquake generation.