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

Ingo Grevemeyer,Florian Schmid,Cord Papenberg,Harm J.A Van Avendonk,Nicholas W Hayman,Anke Dannowski,Laura Gómez De La Peña,Dietrich Lange,Christine Peirce

doi:10.1130/g46577.1

Abstract

AbstractThe 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.

Highlights

Seismicity is generally restricted to the mechanically strong lithosphere (Chen and Molnar, 1983), which, along mid-ocean ridges (MORs), may extend for several kilometers beneath the seafloor
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, occur deeper than along most other slow-spreading ridges
The new MCSC data and Southwest Indian Ridge (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

Summary

INTRODUCTION

Seismicity is generally restricted to the mechanically strong lithosphere (Chen and Molnar, 1983), which, along mid-ocean ridges (MORs), may extend for several kilometers beneath the seafloor. Larger earthquakes are generally rare along MORs, so instead, local microseismicity data offer a means to study the maximum depth of seismic faulting (e.g., Kong et al, 1992; Wolfe et al, 1995; Tilmann et al, 2004; Korger and Schlindwein, 2014). The velocity-depth structure as well as geological investigations suggest that volcanic crust occupies most of the axial valley seafloor (Haughton et al, 2019), exhumed mantle occurs locally (Hayman et al, 2011). The AVR runs roughly parallel to the trend of the MCSC Both the Von Damm hydrothermal vent field at Mount Dent and the Beebe vent field to its northeast within the median valley (Connelly et al, 2012) show little seismicity (Fig. 1). Schlindwein and Schmid (2016) suggested that, in contrast to the MCSC, the SWIR seismicity pattern is significantly different from that expected from the global temperature-depth

16 No of earthquakes earthquakes extend to greater depth as spreading

B SWIR original versus new