Constraining the maximum depth of brittle deformation at slow- and ultraslow-spreading ridges using microseismicity

Abstract

The depth of earthquakes along mid-ocean ridges is restricted by the relatively thin brittle lithosphere that overlies a hot, upwelling mantle. With decreasing spreading rate, earthquakes may occur deeper in the lithosphere, accommodating strain within a thicker brittle layer. New data from the ultraslow-spreading Mid-Cayman Spreading Center (MCSC) in the Caribbean Sea illustrate that earthquakes occur to 10 km depth below seafloor and, hence, occur deeper than along most other slow-spreading ridges. The MCSC spreads at 15 mm/yr full rate, while a similarly well-studied obliquely opening portion of the Southwest Indian Ridge (SWIR) spreads at an even slower rate of ∼8 mm/yr if the obliquity of spreading is considered. The SWIR has previously been proposed to have earthquakes occurring as deep as 32 km, but no shallower than 5 km. These characteristics have been attributed to the combined effect of stable deformation of serpentinized mantle and an extremely deep thermal boundary layer. In the context of our MCSC results, we reanalyze the SWIR data and find a maximum depth of seismicity of 17 km, consistent with compilations of spreading-rate dependence derived from slow- and ultraslow-spreading ridges. Together, the new MCSC data and SWIR reanalysis presented here support the hypothesis that depth-seismicity relationships at mid-ocean ridges are a function of their thermal-mechanical structure as reflected in their spreading rate.

Deep high-temperature hydrothermal circulation in a detachment faulting system on the ultra-slow spreading ridge

“…1c). Although some microearthquakes at ultra-slow ridges have been detected deeper than at the Dragon Horn area, e.g., 16 km bsf for 85°E/85°N Gakkel Ridge 20 and 17 km bsf for 12.5°1 4.5°E SWIR 21,22 , no high-temperature hydrothermal activities were found along these ridges. The maximum depth of microearthquakes in these regions reflect the variation in crustal thickness along-axis because of poor and discontinuous melt supply at these ultra-slow spreading centers 22 .…”

Controls on the magmatic fraction of extension at mid-ocean ridges

“…This dataset is however very scarce at slow MORs. There, the seismogenic depth as measured from OBS-based micro-earthquake surveys (e.g., Grevemeyer et al, 2019) constitutes a reasonable estimate of H (squares in Fig. 2a).…”

Magmatism versus serpentinization—crustal structure along the 13°N segment at the Mid-Atlantic Ridge

“…The general lack of shallower seismicity beneath the exhumed terrane may result from serpentinization and hydration of the fractured mantle, enabled by the passage of water along faults (Reston and Pérez Gussinyé, 2007), with Schlindwein and Schmid (2016) suggesting that the considerable thickness of the brittle layer results from cooling by Vp/Vs structure along the 13°N segment, MAR 6 extensive circulation of water. However, Grevemeyer et al (2019) demonstrate that Schlindwein and Schmid's (2016) apparent lack of shallow seismicity and the observation of apparent deep-seated seismicity is, instead, a result of station corrections being applied that do not take account of the thin layer of low velocity sediment present. When updated, the pattern of seismicity conforms to that expected for the global temperature-depth relationship.…”