Off-axis compression triggered by a seafloor spreading event on the Northern Mid-Atlantic Ridge, 54ºN
Authors: Olive Jean-Arthur , Ekström Göran , Buck W. Roger , Liu Zhonglan , Escartín Javier , Bickert Manon 
Published: February 2023

Mid-ocean ridges are quintessential sites of extensional deformation, where large-magnitude compressional seismicity is rare and typically confined to transpressional ridge-transform intersections. Here we report on a recent, unusual seismic sequence that included 12 thrust faulting events with magnitudes up to 6, ~25 km off-axis on both sides of the Mid-Atlantic Ridge (MAR) at 54&#186;N. These compressional events were preceded by a rapidly-migrating swarm of extensional on-axis earthquakes with M&#8805;4.2. We relocated a total of 124 earthquakes and calculated their focal mechanisms using a surface wave-based method. We then modeled the stress state of the ridge flanks to construct a mechanically-consistent interpretation of the sequence, and discuss its significance in terms of seafloor spreading processes.The sequence started on September 26th, 2022 at 6:07 UTC with a M4.8 normal faulting earthquake at 54&#186;01&#8217;N on the Northern MAR, ~125 km north of the Charlie-Gibbs fracture zone. Over 80 normal faulting earthquakes (4.5&#8804;M&#8804;5.8) occurred over the next 3.5 days, with locations steadily migrating southward at ~0.6 km/hr. Earthquake locations form a narrow band that closely follows the axial valley of the symmetric, abyssal hill-bearing 53&#186;30N segment, which is bound by non-transform offsets both to the north and south. Extensional seismicity continued in this band for ~27 more days without a clear propagation pattern. 80 hours into the earthquake swarm, a magnitude-5.7 thrust earthquake occurred ~25 km east of the extensional band. Between September 29, 2022 and January 4, 2023, 11 more thrust events occurred on N-S striking planes east and west of the axis, outlining two narrow bands ~25 km away from the neovolcanic zone. Some of these events seem well aligned with off-axis normal fault scarps, suggesting a possible reactivation of these faults on both flanks.To better understand this remarkably symmetric pattern of off-axis compression, we model the absolute stress state of the ridge flanks, and the relative stress changes imparted by the on-axis extensional event. 2-D visco-elasto-plastic simulations of slow mid-ocean ridges show that unbending of the lithosphere as it moves out of the axial valley imparts horizontal compression in the cross-axis direction within ~10 to ~40 km away from the ridge axis, and down to ~3 km below seafloor. While this deviatoric compression can reach the brittle yield stress, the associated strain rates are so low that a seismic manifestation of this phenomenon should be extremely rare. On the other hand, the on-axis intrusion of a vertical dike up to a depth of ~5 km below seafloor can put the shallow axis in tension while imparting excess compression on the shallow lithosphere ~25 km off-axis on both sides. Our preferred interpretation is therefore that the extensional swarm represents the southward migration of a blind dike within the neovolcanic axis, which drove both ridge shoulders to compressional failure. Off-axis shortening may thus be an integral component of seafloor spreading that usually operates aseismically, but can be highlighted by certain types of on-axis intrusion events.