Relationships of Surface Geological and Geophysical Characteristics with the Deep Structure of the Mid-Atlantic Ridge According to Seismic Tomography Data

Abstract

—A retrospective analysis of seismic tomography models with different levels of detail along the Mid-Atlantic Ridge (MAR) has shown that mantle inhomogeneities depicted by variations of seismic waves velocities in low-detail models look larger than their true sizes determined by more detailed modern models. They also confirm that there are two different types of upwelling in the Atlantic mantle: active plume, which has an impulsive character, and passive axial, which occurs as a response to spatial appearance during the drift of lithospheric plates. The structure of mantle velocity and density inhomogeneities, determined by low-frequency gravity anomalies and their reductions, has a consistent interpretation based on the thermal state of the mantle. Detailed models of the velocities under the MAR can serve as a basis for comparison with the geochemical characteristics of basalts. The decomposition of a single axial tomographic anomaly in detailed models into a chain of vertical lenses under slow spreading conditions corresponds to geochemical data on discrete manifestations of different basalt associations along the MAR. The attribute δ(Vp/Vs) section along the MAR in the mantle above ~700 km contains cold lenses with a thickness of 200–300 km, which are spatially related to the following structural and tectonic phenomena: transform faults with maximum lateral rift displacement; geochemical segmentation of the MAR; modulation of the total lengths of transform faults with increased values above the “cold” lenses; asymmetry features of the half-spreading rates, which decreases to zero above the cold lenses and reaches maximum values between the cold lenses. The cold mantle lenses at an average depth of ~500 km and the corresponding lithospheric geophysical characteristics reflect the background conditions, and areas above hot segments are disturbances that occur when plume branches interfere with the MAR. Inhomogeneous spreading rates can lead to movements of blocks inside large plates and tectonic deformations of the intraplate space. Along the 4 Ma isochron in the Northern Hemisphere, the half-spreading rates on the eastern flank of the MAR exceed the half-spreading rates on the western flank. In the Southern Hemisphere, the pattern is reversed, which indicates the possible influence of the Earth’s rotation effects on geodynamic processes along the MAR. Against the general trend, there are local inversion zones from the western predominance of increased rates to the eastern and, conversely, in the Northern and Southern hemispheres. The main demarcation faults of the Atlantic differ in seismic events by their maximum energy release and are located near the “cold” mantle lenses and the contrasting lateral transition to the hot regions. The distribution of the total seismic moment in the depth intervals of 0‒13 and 13‒35 km also has less intense extremes near the branches of plumes with a predominantly crustal position of the hypocenters. The seismicity along the main MAR space associated with standard basalt magmatism has a background character and does not significantly contribute to the total released seismic moment compared to shear zones.