Microseismicity of the Mid‐Atlantic Ridge at 7°S–8°15′S and at the Logatchev Massif oceanic core complex at 14°40′N–14°50′N

Abstract

[1] Lithospheric formation at slow spreading rates is heterogeneous with multiple modalities, favoring symmetric spreading where magmatism dominates or core complex and inside corner high formation where tectonics dominate. We report microseismicity from three deployments of seismic networks at the Mid-Atlantic Ridge (MAR). Two networks surveyed the MAR near 7 S in the vicinity of the Ascension transform fault. Three inside corner high settings were investigated. However, they remained seismically largely inactive and major seismic activity occurred along the center of the median valley. In contrast, at the Logatchev Massif core complex at 14 45 0 N seismicity was sparse within the center of the median valley but concentrated along the eastern rift mountains just west of the serpentine hosted Logatchev hydrothermal vent field. To the north and south of the massif, however, seismic activity occurred along the ridge axis, emphasizing the asymmetry of seismicity at the Logatchev segment. Focal mechanisms indicated a large number of reverse faulting events occurring in the vicinity of the vent field at 3-5 km depth, which we interpret to reflect volume expansion accompanying serpentinization. At shallower depth of 2-4 km, some earthquakes in the vicinity of the vent field showed normal faulting behavior, suggesting that normal faults facilitates hydrothermal circulation feeding the vent field. Further, a second set of cross-cutting faults occurred, indicating that the surface location of the field is controlled by local fault systems.

Geophysical and geochemical constraints on the evolution of oceanic lithosphere from formation to subduction

“…Whereas previous microearthquake surveys of active oceanic detachment faults and OCCS have identified distinct faulting and deformation structures associated with regional-scale lithospheric extension [Collins et al, 2012;deMartin et al, 2007;ParnellTurner et al, 2017;Grevemeyer et al, 2013], the lack of any such features in our data suggests that if the Rainbow massif was exhumed along a detachment fault, such fault is no longer active. This supports the interpretation of [Andreani et al, 2014], based on the observation of a continuous and undisrupted sediment cover between the western flank of the massif and the nodal basin, that the Rainbow massif is no longer being exhumed.…”

“…With respect to the Logatchev, seismicity at Rainbow is an order of magnitude more abundant 21 with a much higher proportion of small events relative to large events. Seismicity in the vicinity of the Logatchev OCC is asymmetric relative to the spreading axis, while elsewhere along the 15°N segment it is distributed symmetrically within the axial valley [Grevemeyer et al, 2013]. This along-axis difference is interpreted by Grevemeyer et al…”

“…The events are distributed in a region sandwiched in-between magmatic sills imaged by MCS reflections and the seafloor, with no spatial patterns related to a fault surface, indicating that the detachment fault responsible for the exhumation of the massif is likely no longer active, consistent with seafloor geological observations. This a fundamental difference between the seismicity of Rainbow and that observed at other MAR OCCs such as TAG, AtlantisMassif, and Logatchev [Collins et al, 2012;deMartin et al, 2007;Grevemeyer et al, 2013].(2) While we are unable to place robust constraints on the event focal mechanisms, the body of evidence suggests that the observed seismicity was generated by some combination of extension from tectonic stresses and volumetric strain from serpentinization of the massif.(3) The lack of seismicity associated with active detachment faulting argues against models in which active detachment faults are needed to focus and sustain hightemperature hydrothermal activity within the footwall of OCCs [McCaig et al [2007].Instead, our results, combined with seismic imaging results [Canales et al, 2017], indicate that high-temperature hydrothermal discharge can be generated within inactive detachment faulting systems that contain magmatic intrusions in the footwall. …”

“…A deployment of OBSs within the 15°N spreading segment of the MAR, which includes the serpentine-hosted Logatchev hydrothermal field, recorded an average of 24 locatable earthquakes per day on a similar sized array [Grevemeyer et al, 2013]. With respect to the Logatchev, seismicity at Rainbow is an order of magnitude more abundant 21 with a much higher proportion of small events relative to large events.…”

Fault geometry and permeability contrast control vent temperatures at the Logatchev 1 hydrothermal field, Mid-Atlantic Ridge

“…New seismological data were acquired during two deployments in 2009 ( Fig. 1; Grevemeyer et al, 2013) and indicate that deformation is concentrated along the eastern rift mountains in the vicinity of LHF1. Hypocenters of normal-faulting earthquakes roughly line up with the outcrop of a fault zone near LHF1 (Fig.…”