Microseismicity and lithosphere thickness at a nearly amagmatic mid-ocean ridge

Abstract

Successive flip-flop detachment faults in a nearly-amagmatic region of the ultraslow-spreading Southwest Indian Ridge (SWIR) at 64°30'E accommodate ~100% of plate divergence, with mostly ultramafic seafloor. As magma is the main heat carrier to the oceanic lithosphere, the nearly-amagmatic SWIR 64°30'E is expected to have a very thick lithosphere. Here, our microseismicity data shows a 15-km thick seismogenic lithosphere, actually thinner than the more magmatic SWIR Dragon Flag detachment with the same spreading rate. This challenges current models of how spreading rate and melt supply control the thermal regime of mid-ocean ridges. Microearthquakes with normal focal mechanisms are colocated with seismically imaged damage zones of the detachment and reveal hanging-wall normal faulting, which is not seen at more magmatic detachments at the SWIR or the Mid-Atlantic Ridge. We also document a two-day seismic swarm, interpret as caused by an upward-migrating melt intrusion in the detachment footwall (6-11 km), triggering a sequence of shallower (~1.5 km) tectonic earthquakes in the detachment fault plane. This points to a possible link between sparse magmatism and tectonic failure at melt-poor ultraslow ridges.

Mid-Ocean Ridges: Geodynamics Written in the Seafloor

“…2F). These faults root to a depth of at least ~16 km, as evidenced by the distribution of microseismicity (Chen et al, 2020). A few tens of km further along the ridge, the seafloor sharply transitions to a more symmetric morphology, and mostly consists of basaltic rocks.…”

“…If magmas preferentially pool near the base of the lithosphere, the conductive boundary layer separating the magmatic heat source from the hydrothermal system would be kilometers wide, resulting in very low heat fluxes ( < 1 W/ m 2 ). Such systems would not be particularly efficient at cooling the oceanic lithosphere, and are unlikely to account for the thick seismogenic zone (~16 km: Chen et al, 2020) observed at quasi-amagmatic sections of the Southwest Indian Ridge, unless the heat advected from the q CBL rising asthenosphere is particularly low and the supply of magma is particularly scarce (Fan et al, 2021). Alternatively, a model in which fluids can convect vigorously down to ~15 km (e.g., Tao et al, 2020), perhaps aided by tectonically driven fracturing, would straightforwardly account for a thick axial lithosphere underlain by a thin boundary layer that allows efficient heat extraction.…”

Thermo‐Mechanical State of Ultraslow‐Spreading Ridges With a Transient Magma Supply

“…Its "smooth" seafloor consists of serpentinized mantle exhumed by a succession of large-offset detachment faults of alternating polarity, overlain in places by a scattered basaltic carapace (Cannat et al, 2006;Cannat, Sauter, et al, 2019;Sauter et al, 2013). OBS surveys conducted west of this section (Schlindwein & Schmid, 2016) as well as preliminary results from a recent OBS deployment (Chen et al, 2020) suggest that microearthquakes occur down to at least 15 km below seafloor at the axis. The recent ROVSMOOTH cruise also identified a low-temperature hydrothermal site named "Old City" midway up the scarp of the active axis-bounding detachment fault (Cannat, Agrinier, et al, 2019).…”

“…The low-temperature Old City field (64ºE, Figure 1b), for example, sits on the footwall of an active fault primarily exposing serpentinized mantle (Cannat, Agrinier, et al, 2019), while the high-temperature Longqi field (49ºE, Figure 1c and 1d) sits on a detachment which clearly dissects a magmatically robust axis, as evidenced by the largely basaltic, hummocky seafloor that surrounds it (Tao et al, 2020;Zhao et al, 2013). Interestingly, microseismicity data suggests that both Old City and Longqi lie above ∼15 km thick brittle lithosphere (Chen et al, 2020;Yu et al, 2018). This illustrates the complex relationship between the thermal state of the ridge axis and the vigor of hydrothermal convection.…”

“…Dent(Harding et al, 2017). (b) Amagmatic corridor at 64°40'E on the Southwest Indian Ridge (SWIR)(Chen et al, 2020;Sauter et al, 2011). (c) Segment 28 of the SWIR near 49°E.…”