Mid-ocean-ridge seismicity reveals extreme types of ocean lithosphere

Abstract

Along ultraslow-spreading ridges, where oceanic tectonic plates drift very slowly apart, conductive cooling is thought to limit mantle melting and melt production has been inferred to be highly discontinuous. Along such spreading centres, long ridge sections without any igneous crust alternate with magmatic sections that host massive volcanoes capable of strong earthquakes. Hence melt supply, lithospheric composition and tectonic structure seem to vary considerably along the axis of the slowest-spreading ridges. However, owing to the lack of seismic data, the lithospheric structure of ultraslow ridges is poorly constrained. Here we describe the structure and accretion modes of two end-member types of oceanic lithosphere using a detailed seismicity survey along 390 kilometres of ultraslow-spreading ridge axis. We observe that amagmatic sections lack shallow seismicity in the upper 15 kilometres of the lithosphere, but unusually contain earthquakes down to depths of 35 kilometres. This observation implies a cold, thick lithosphere, with an upper aseismic zone that probably reflects substantial serpentinization. We find that regions of magmatic lithosphere thin dramatically under volcanic centres, and infer that the resulting topography of the lithosphere-asthenosphere boundary could allow along-axis melt flow, explaining the uneven crustal production at ultraslow-spreading ridges. The seismicity data indicate that alteration in ocean lithosphere may reach far deeper than previously thought, with important implications towards seafloor deformation and fluid circulation.

Quasi‐3‐D Seismic Reflection Imaging and Wide‐Angle Velocity Structure of Nearly Amagmatic Oceanic Lithosphere at the Ultraslow‐Spreading Southwest Indian Ridge

“…A broad zone of distributed instantaneous deformation could form if the rheology of the footwall rocks is such that strain localization is not efficient. This could be favored both by the scarcity of the weak mineral talc in the fault rocks dredged in the area (Cannat et al, ) and by the thick brittle axial lithosphere inferred from the depth of microearthquakes (>20 km; Schlindwein & Schmid, ).…”

“…A passive seismic experiment carried out at a volcanic center (Segment 8 volcano) located on‐axis the SWIR at longitude 65°E to 66°E, east of our study area, reveals local earthquakes down to depths of 25 km below the seafloor, with a trend to deeper earthquakes in less magmatically robust regions of the axis (Schlindwein & Schmid, ). Such deep earthquakes suggest that the brittle part of the lithosphere there is at least 25 km thick, and therefore substantially thicker than at the faster‐spreading Mid‐Atlantic Ridge (Toomey et al, ; Wolfe et al, ).…”

“…Such deep earthquakes suggest that the brittle part of the lithosphere there is at least 25 km thick, and therefore substantially thicker than at the faster‐spreading Mid‐Atlantic Ridge (Toomey et al, ; Wolfe et al, ). Schlindwein and Schmid () also show that most earthquakes in the less volcanic regions of the axis occur in a 10 to 15 km thick band and that the upper lithosphere there is aseismic, which they propose due to substantial serpentinization.…”

Modeling Porosity Evolution Throughout Reaction‐Induced Fracturing in Rocks With Implications for Serpentinization

“…Moreover, the reaction‐induced fracturing process induces large stress concentrations within the medium, promoting the transient generation of potentially large shear stresses (Figure ). These large shear stresses could explain the existence of microseismic activity observed near slow spreading oceanic ridges which was interpreted as resulting from active serpentinization at depth (Horning et al, ; Schlindwein & Schmid, ).…”

The geochemistry of methane isotopologues

“…At magma-poor slow-spreading ridges, extension is accommodated primarily by detachment faulting, as opposed to magmatic emplacement of new crust that characterizes fast-spreading ridges (Buck et al, 2005;Dunn, 2007). Low-angle, large-offset, and long-lived (>1 Myr) normal faults near vent fields at slow-spreading ridges allow for fluid penetration deep into plutonic rocks of layer 3, enabling access to fresh gabbroic material and/or inclusions to be leached (Kelley, 1996;Schroeder et al, 2002;Schlindwein and Schmid, 2016). In contrast, in fast spreading environments such as the East Pacific Rise, shallow melt lenses at 1 to 2 km below seafloor may limit the depth of circulation (e.g., Hasenclever et al, 2014;and references in Alt, 1995).…”