Lithospheric Structure and Tectonic Processes Constrained by Microearthquake Activity at the Central Ultraslow‐Spreading Southwest Indian Ridge (49.2° to 50.8°E). Issue 8 (25th August 2018)

• Record Type: Journal Article
• Authors: Yu, Zhiteng; Li, Jiabiao; Niu, Xiongwei; Rawlinson, Nicholas; Ruan, Aiguo; Wang, Wei; Hu, Hao; Wei, Xiaodong; Zhang, Jie; Liang, Yuyang
• Is Part Of: Journal of geophysical research. Volume 123:Issue 8(2018)
• Page Start: 6247
• Page End: 6262
• Publication Date: 2018-08-25
• Subjects: microearthquake -- Southwest Indian Ridge -- detachment fault -- axial magma chamber -- lithospheric structure; Geomagnetism -- Periodicals; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 551.1
• DOI: 10.1029/2017JB015367
• Languages: English
• ISSNs: 2169-9313

Abstract

Abstract: 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. Key Points: A thick brittle lithosphere, ~20‐km thick, is revealed by the microseismicity at the ultraslow‐spreading SWIR The presence of an active detachment fault geometry is confirmed in the Dragon Flag hydrothermal vent field A seismic gap at the center of magmatic Segment 27 is new evidence for a large magma supply