Microseismicity constraints on brittle lithosphere thickness at a nearly amagmatic spreading corridor of the ultraslow Southwest Indian Ridge

Abstract:

The eastern Southwest Indian Ridge (SWIR) is characterized by low melt supply and this melt is focused on axial volcanos, so that the intervening regions of the ridge spread in a nearly amagmatic fashion, producing extensive ultramafic seafloor and flip-flop detachment faults (Cannat et al., 2006; Sauter et al., 2013). Schlindwein and Schmid (2016) studied microseismicity near an axial volcano at 65°30'E and found that maximum earthquake depth increased from ~7 km near the volcano, to 20 km in a magma-starved region at 65°10'E. They also reported a seismic gap at depths of 0-10 km in this magma-starved region. Here, we report on a microseismicity experiment with 6-OBSs deployed for 20 days in a magma-poor to magma-starved region of the ridge around 64°30'E (ROVSMOOTH cruise; Dec 2016). Serpentinized peridotites make up most of the seafloor in this region, with patches of hummocky basalts in the western part of the area (Sauter et al., 2013). The presently active detachment fault has been imaged seismically with a dip of ~50° down to ~5 km, and the seismic crust is ~5 km (Momoh et al., 2017). Our purpose is to relocate the recorded earthquakes in order to constrain the tectonic activity in the axial detachment context and the thickness of the brittle lithosphere.

More than 200 microearthquakes of local magnitudes between -0.3 and 2.5 were recorded and relocated using HypoDD and a 1-D P wave velocity model from Momoh et al. (2017). These events occur in the footwall (41%), in the hanging wall (21%), and in detachment fault region (38%). Most events occur in the western, more volcanically active part of the survey area. A seismic swarm occurred in the footwall of this volcanic area during Dec 22-23, with progressively shallower events that could tentatively be attributed to a diking event. Footwall events probably correspond to bending of the lithosphere. Hanging wall events could be related to minor normal faults observed in seafloor maps and seismic reflection images (Momoh et al., 2017 and 2020). The deepest relocated earthquakes occur near the fault zone about 13 km below seafloor, suggesting that the 64°30'E magma-poor region has a thinner brittle lithosphere than proposed by Schlindwein and Schmid (2016) for the adjacent region at 65°10'E.