A Magmatic Seafloor Source at an Ultraslow-Spreading Ridge

In 2018, two earthquake swarms occurred along segments of the ultra-slow Southwest Indian Ridge (spreading rate: 14-15 mm/a). The first swarm is located at the spreading-ridge intersection with the Atlantis Fracture Zone and comprises 9 Mw > 5.0 events (GCMT catalogue) and about 227 lower magnitude events (ISC catalogue), spanning over 9 days (July 10-18). The second crisis is more of a cluster of events focusing near a discontinuity, 220km away from the Indian Triple Junction and comprises 6 Mw > 5 events (GCMT) and 87 lower magnitude events (ISC catalogue), spanning over 30 days (September 28 to October 27). All focal mechanisms (GCMT) indicate normal faulting for both swarms. These two swarms are examined using hydroacoustic records from the OHASISBIO network with 7 to 9 autonomous hydrophones moored on either side of Southwest Indian Ridge. The first swarm initiates with a Mw=4.9 event (July 10 2018, 03h55) which triggers numerous events with an average of ~250 events per day for the first three days (July 10to 12), propagating in the NE direction. After this, the seismic activity ceases down along with a sparse distribution of events until another burst of activity initiating after July 15, lasting for 3 days and comprising of several high intensity events. Overall, this swarm includes ~1100 hydroacoustic events spanning over 13 days.The second swarm, further east, starts with two events, Mw=5.5 and 5.6 (Sept. 28 2018, 6h21 and 7h06), followed by a few discrete events. After 3 days, a dense cluster of events initiates with a Mw=5.4 event (October 1st, 18h16) and lasts for 7 days (~415 events per day) and decreases till the end of October. Two additional sub-swarms occur on October 1st and on October 6, both propagating towards the NE. Several other high intensity events occur October 10, after which seismic activity propagates towards the SE and fades away until October 27. Overall, this swarm includes ~5000 hydroacoustic events spanning over 33 days.The number of events per day is thus larger for the second swarm than for the first one. Also, event source levels are in average smaller in the second crisis than in the first one. Further analyses of these characteristics, along with the different geographical and time distribution of the ~6000 acoustic events (vs ~300 events in the land-based catalogues), provide insights on the onset and on the tectonic or magmatic origin of these two contrasting swarms.

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Hydroacoustic Observations of Two Contrasted Seismic Swarms along the Southwest Indian Ridge in 2018

Geosciences ◽

10.3390/geosciences11060225 ◽

2021 ◽

Vol 11 (6) ◽

pp. 225

Author(s):

Vaibhav Vijay Ingale ◽

Sara Bazin ◽

Jean-Yves Royer

Keyword(s):

Fracture Zone ◽

Temporal Distribution ◽

Southwest Indian Ridge ◽

Earthquake Swarms ◽

Spreading Ridge ◽

Magmatic Origin ◽

Seismic Swarms ◽

Source Level ◽

Indian Ridge ◽

Spatio Temporal

In 2018, two earthquake swarms occurred along spreading ridge segments of the ultra-slow Southwest Indian Ridge (SWIR). The first swarm was located at the spreading-ridge intersection with the Novara Fracture Zone, comprising 231 events (ISC catalogue) and spanning over 6 days (10 July to 15 July). The second swarm was more of a cluster of events focusing near a discontinuity, 220 km west of the Rodrigues Triple Junction, composed of 92 events and spanning over 31 days (27 September to 27 October). We examined these two swarms using hydroacoustic records from the OHASISBIO network with seven to nine autonomous hydrophones moored on either side of the SWIR. We detected 1109 hydroacoustic events spanning over 13 days (6 July to 18 July) in the first swarm and 4880 events spanning over 33 days in the second swarm (25 September to 27 October). The number of events per day was larger, and the hydroacoustic magnitude (source level) was, on average, smaller during the second swarm than the first. The spatio-temporal distribution of events from both swarms indicates a magmatic origin initiated by dike intrusions and followed by a readjustment of stresses in the surrounding crust.

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Plastic Deformation of Plagioclase in a Gabbro Pluton at a Slow-Spreading Ridge (IODP Hole U1473A, Atlantis Bank, Southwest Indian ridge)

10.5194/egusphere-egu21-4089 ◽

2021 ◽

Author(s):

Maël Allard ◽

Benoît Ildefonse ◽

Émilien Oliot

Keyword(s):

Plastic Deformation ◽

Grain Boundary ◽

Shear Zones ◽

Southwest Indian Ridge ◽

Temperature Deformation ◽

Spreading Ridge ◽

Slip Systems ◽

Thin Sections ◽

Atlantis Bank ◽

Indian Ridge

The crustal architecture of slow-spread ocean crust results from complex interactions between magmatism, hydrothermalism, and tectonics. IODP Hole U1473A (809 m depth) was drilled during IODP Expeditions 360 and 362T at the summit of the Atlantis Bank, a gabbroic massif exhumed at the Southwest Indian Ridge (SWIR). In this study, we identify and quantify plastic deformation processes in oceanic gabbros and active slip-systems in plagioclase from 112 thin sections sampled throughout Hole U1473A.We describe deformed zones using petrographic observations and modern Electron Backscattered Diffraction (EBSD) analyses made all along the core. Ductile deformation is widespread and is sometimes strongly localized. It initiated during accretion under magmatic conditions and continued until late brittle conditions. Porphyroclastic microstructures testify to post-magmatic, solid-state, high-temperature (HT) deformation. Plagioclase represents ~60% of rock&#8217;s volume and is the dominant phase accommodating deformation in the gabbro. It shows strong dynamic recrystallization accommodated by dislocation creep, forming a fine-grained matrix. Strain localizes in mylonitic and ultramylonitic zones, and these shear zones are often overprinted by lower temperature deformation.EBSD analyses reveal weak to moderate crystallographic preferred orientations (CPO) of plagioclase first developed during early magmatic flow, that has produced a primary fabric with a (010) foliation plane and a [100] lineation axis. This CPO is persistent during subsequent plastic deformation and strain localization and is observed in almost all samples. However, a detailed investigation of internal misorientations measured at subgrains reveals the activity of at least 4 to 5 slip systems in plagioclase grains: <img src="https://contentmanager.copernicus.org/fileStorageProxy.php?f=gnp.ccc6371e5dff55120440161/sdaolpUECMynit/12UGE&app=m&a=0&c=6a9c8089187375ae30c9f8697f57bca5&ct=x&pn=gnp.elif&d=1" alt="">, and maybe <img src="https://contentmanager.copernicus.org/fileStorageProxy.php?f=gnp.effe7c6d5dff54400440161/sdaolpUECMynit/12UGE&app=m&a=0&c=33f8e61b26ba87bbb8acbd5b9e5556ab&ct=x&pn=gnp.elif&d=1" alt="">. The strength of CPO is first increasing from slightly foliated gabbros to mylonites before decreasing significantly in ultramylonites, which could be explained by orientation scattering after subgrain rotation recrystallization and grain boundary processes (e.g., nucleation, grain boundary sliding).

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