Microearthquake activity, lithospheric structure, and deformation modes at an amagmatic ultraslow spreading Southwest Indian Ridge segment

Abstract

While nascent oceanic lithosphere at slow to fast spreading mid‐ocean ridges (MOR) is relatively well studied, much less is known about the lithospheric structure and properties at ultraslow MORs. Here we present microearthquake data from a 1 year ocean bottom seismometer deployment at the amagmatic, oblique supersegment of the ultraslow spreading Southwest Indian Ridge. A refraction seismic experiment was performed to constrain upper lithosphere P‐velocities and results were used to construct a 1D velocity model for earthquake location. Earthquake foci were located individually and subsequently relocated relative to each other to sharpen the image of seismically active structures. Frequent earthquake activity extends to 31 km beneath the seafloor, indicating an exceptionally thick brittle lithosphere and an undulating brittle‐ductile transition that implies significant variations in the along‐axis thermal structure of the lithosphere. We observe a strong relation between petrology, microseismicity distribution, and topography along the ridge axis: Peridotite‐dominated areas associate with deepest hypocenters, vast volumes of lithosphere that deforms aseismically as a consequence of alteration, and the deepest axial rift valley. Areas of basalt exposure correspond to shallower hypocenters, shallower and more rugged axial seafloor. Focal mechanisms deviate from pure extension and are spatially variable. Earthquakes form an undulating band of background seismicity and do not delineate discrete detachment faults as common on slow spreading ridges. Instead, the seismicity band sharply terminates to the south, immediately beneath the rift boundary. Considering the deep alteration, large steep boundary faults might be present but are entirely aseismic.

Lithospheric Structure and Tectonic Processes Constrained by Microearthquake Activity at the Central Ultraslow‐Spreading Southwest Indian Ridge (49.2° to 50.8°E)

“…Our tomography results reveal high‐velocity anomalies beneath the detachment fault at Segment 28 (Figures , S11, and S12), which Zhao et al () suggested is composed primarily of gabbro. Moreover, compared to the oblique Supersegment of the SWIR (Schmid & Schlindwein, ), which is underlain by a thick aseismic lithosphere (~15 km), our aseismic zone is thinner (~5 km). At the TAG and 13°20′ detachments on the MAR, the aseismic zones are much thinner (~3 km; deMartin et al, ; Parnell‐Turner et al, ), which may indeed be a result of a mostly gabbroic crust.…”

“…In general, the deepest microearthquakes (with reference to the seafloor) are known to occur in the crust/shallow mantle at fast‐/slow‐spreading ridges: for example, 1.8 km at 9°50′N on the fast‐spreading East Pacific Rise (EPR; Bohnenstiehl et al, ; Tolstoy et al, ), 4 km at 35°N on the slow‐spreading Mid‐Atlantic Ridge (MAR; Barclay et al, ), 6 km at 14°45′N on the MAR (Grevemeyer et al, ), 7–8 km at 26°N and 5°S on the MAR (deMartin et al, ; Kong et al, ; Tilmann et al, ; Toomey et al, ), and 10 km at 13°20′N on the MAR (Parnell‐Turner et al, ). By contrast, microseismicity at ultraslow‐spreading ridges tends to feature deep hypocenters: for example, 14 km at the Lena Trough (Läderach et al, ), 15 km at the Segment 8 volcano of the SWIR (Schmid et al, ), 20 km beneath the Knipovich Ridge (Schlindwein et al, ), and 31 km at the orthogonal supersegment of the SWIR (Schmid & Schlindwein, ). From our results in Figures and , the maximum focal depth is ~20 km below the seafloor (bsf) or equivalently 16 km below the Moho discontinuity.…”

“…A majority of events are located in the depth range 8–18‐km bsl. As with Schlindwein and Schmid (), we consider the maximum hypocenter depth as equivalent to the brittle‐ductile transition (cross‐section A in Figure ), which likely coincides with the 600 °C isotherm (McKenzie et al, ; Schmid & Schlindwein, ). This isotherm should be shallower in magmatically robust sections of the ridge (Schlindwein & Schmid, ).…”

“…The hypocenter distribution along the axial valley reveals an obvious shallow aseismic zone above the Moho discontinuity, with a thickness of ~5 km, although a small number of scattered crustal events can be observed within the Dragon Flag hydrothermal vent field and the axial valley (Figure ). A 15‐km‐thick aseismic upper lithosphere has also been found at the oblique supersegment of the SWIR (Schmid & Schlindwein, ), which probably reflects substantial serpentinization (Schlindwein & Schmid, ). Serpentinization can reduce the strength of the lithosphere and promote strain localization (Escartín et al, ), and a very low degree of serpentinization (10%–15%) can cause a dramatic change in rheological behavior, which favors the formation of large‐offset and low‐angle faults (Escartín et al, ).…”

“…Microearthquakes usually result from brittle processes and occur in the region above the brittle to ductile transition. Microseismic monitoring of mid‐ocean ridges by ocean bottom seismometers (OBSs) has been widely used on MORs with a variety of spreading rates; this has proven to be a direct and efficient approach to advance our understanding of hydrothermal activity (Bohnenstiehl et al, ; Tolstoy et al, ), faulting modes (deMartin et al, ; Parnell‐Turner et al, ), tectonic extension (Läderach et al, ; Smith et al, ; Tilmann et al, ; Toomey et al, ), lithosphere deformation (Schmid & Schlindwein, ), and magmatic processes (Dusunur et al, ; Schlindwein et al, ; Wilcock et al, ). To date, only two long‐term OBS experiments have been carried out along the SWIR: one at the western end (Schmid & Schlindwein, ) and the other at the eastern end (Schlindwein & Schmid, ; Schmid et al, ).…”