Geophysical processes and biosphere: article

Аnnotation: The analysis of the tectonic situation (according to the literature data) and seismicity in the vicinity of the Mid-Atlantic Ridge (MAR) is carried out. It is confirmed that the spreading of the axial part of the MAR plays a decisive role in the seismotectonics of the region under consideration. The result of spreading is a pronounced rift valley, no more than several tens of kilometers wide, torn into separate segments by transform faults transverse to its strike. Displacements along these faults range from several tens to several hundred kilometers. The southern half of the Atlantic Ocean is characterized by significantly greater tectonic fragmentation (greater frequency of closely parallel faults following each other). Perhaps this is why its significantly lower seismic activity is associated with it: not a single earthquake with M = 8 has been recorded here for more than 100 years, while three such events took place in the Central Atlantic. The vast majority of earthquake epicenters closely mark the axial part of the MAR occupied by the rift valley, clearly following its stepped appearance caused by displacements along transform faults. On the basis of which it is concluded that the main potentially seismically active elements of the MAR are spreading areas characterized by a discharge type of seismotectonic deformation, according to the construction solutions of the composite focal mechanisms. A reconstruction of the composite focal mechanisms was carried out based on fairly large aggregates of known individual solutions in various spatial samples in the immediate vicinity of the MAR. The predominance of normal fault seismotectonic deformation has been established in almost all spatial samples within the MAR, which corresponds well with spreading processes in the axial part of the MAR. The exception is the largest shear zone separating the southern and central parts of the Atlantic Ocean. Pronounced shear deformations are observed within this zone, mainly due to right-sided shear movements along the system of transverse to the strike of the axial part of the MAR faults. Their number is more than 80 % of the total number of individual focal mechanisms (274 events) involved in this reconstruction. Despite the obvious predominance of normal fault seismotectonic deformation within the MAR, there is a noticeable proportion of shear (about 30 %) and, to a lesser extent, thrust (about 10 %) dislocations in different ratios (depending on the sample). This result can be considered as evidence of a more complex nature of the deformation of the moving lithospheric plates than their simple spreading relative to the axis of the MAR. One of the interesting elements of the internal deformation of the MAR lithosphere is the ability, established during the study, to stretch individual blocks of the lithosphere as a single element along the transform faults that bound them from the south and north. These extensions can lead to tectonic crowding of crustal and mantle masses due to the processes of thrust formation, which is observed in geological studies. Thus, it can be assumed that these subhorizontal compression stresses find their continuation in the lithosphere of neighboring continents. It is shown that seismic events recorded both by the global teleseismic network and on dense networks by bottom equipment (microearthquakes) represent a single class of tectonic earthquakes. Composite solutions of focal mechanisms of micro-earthquakes occurring at prevailing depths of 3–9 km under the bottom of the middle rift valley indicate normal (discharge) faulting along fault planes parallel to the axis of the MAR, which fall in the direction of the axis of the MAR at angles of 30° or more. These results confirm that the ridge undergoes brittle fracture during stretching (spreading). The study of the focal mechanisms of micro-earthquakes has shown that against the background of the predominance of discharge movements in the spreading regime of the seismotectonics of the MAR, a noticeable amount of discharging and thrusting movements occurring there. These movements are located at depths of 3–7 km inside the sole of the recumbent wing of the detachment fault in the area where the detachment abruptly changes its immersion from 60–70° with the horizon to a much more gentle occurrence of 20–30°, forming a compression stress area under the lower side of the detachment.