Institute o] Physics o] the Earth, Academy o] Sciences of the U$SR, Moscow, U$SRA Physical analysis is given of the state of stresses in the earth's lithosphere floating on the asthenosphere. Crustal thickness strongly varies in a horizontal direction and thus creates large variations in the potential energy stored in the crust. As a result, the crustal matehal tends to spread and to reduce crustal thickness inhomogeneities. A similar situation occurs where the masses of low-density mantle are located under the crust. Large nonhydrostatic stresses arise in the lithosphere from that tendency, even if the rate of spreading is very low. The main stresses are caused by regional and global crustal thickness inhomogeneities. Their magnitudes are of the order of the additional loads associated with the regional and global relief of the crustal surface. The stresses inherent in the variations in the thickness of crustal and low-density mantle layers form a global system. Especially high stresses of the order of thousands of bars exist in the regions of high uplifts on the continents. They may be both tensile and compressive, depending on the position of uplifts with respect to the boundaries between the lithospheric plates. They are responsible for rock deformations and earthquakes. The state of stress in the oceanic lithosphere is mainly defined by the low-velocity mantle distribution beneath the mid-ocean ridges. Compressive stresses of the order of a few hundred bars exist in the lithosphere in the oceanic basins. They may be responsible for sea floor spreading and continental drift. The stresses in the lithosphere produced by other mechanisms such as thermal convection and the lithosphere's sinking into the mantle are also considered. Crustal and low-density mantle thickness inhomogeneities represent the main factor determining the stresses in the lithosphere, and the plate motions are mainly caused by the spreading of low-density mantle material rising below the oceanic ridges. It is shown that the pressure of the oceanic lithosphere on the continents caused by sea floor spreading cannot produce folded mountains on the continents. Numerous geophysical and geological data indicate that large stresses exist in the earth's lithosphere. Investigations of the seismicity of the earth show that these stresses form a global system [Balakina e• al., 1967; Sykes, 1967]. According to the seismic data, tensile stresses prevail in the mid-ocean ridges and their extensions into Africa, Asia, and North America. The boundaries between the lithospheric plates in the circum-Pacific belt are the regions of compression. Geological data indicate that a high level of rock deformation is typical of most orogenic regions, both recent and ancient [Spenser, 1969]. Since the viscosity of rocks is usually high, large stresses should act in the lithosphere for a long time to produce the observed deformations. The stresses in the lithosphere are apparently a cause of various tectonic movements (e.g., ocean floor spreading, continental drift, and

The Baikal rift is the most important of a system of rift valleys located in the USSR. A comparison of geological and geophysical data for the Baikal rift and other rift zones of the earth leads to the conclusion that their origin is determined by similar deep‐seated processes. Some mechanisms of rifting suggested earlier are considered. It is shown that rift‐valley formation cannot result from collapse of the central parts of large crustal arches. The concept of Vening‐Meinesz that considers rifting to result from isostatic subsidence of a wedge‐shaped crustal block also is problematic. A new mechanism for rifting is proposed that is based on modern geological and geophysical data. Rift valleys may be considered to result from crustal extension when neck‐shaped strains in the crust are developed. The lower part of the crust is plastically attenuated, and faults are formed only in the upper layer of the crust, which is highly viscous. It is suggested that large‐scale movements in the mantle created by gravity convection may be the cause of crustal extension. The mechanism of gravity convection has advantages over thermal convection.

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It is now assumed that the earth's crust tends to reach isostatic equilibrium if there are no external forces acting on the crust. This statement is one of the main principles of geophysics, yet it is incorrect. The isostatic state is generally unstable and differs from the stable position of the crust, which actually exists in many regions. These positions coincide only where the crustal and lithospheric thicknesses are constant. They differ considerably when these thicknesses vary strongly in a horizontal direction. Thus many regional isostatic anomalies in orogenic regions are explained. The instability of isostatic equilibrium is associated with the asymmetry of the upper and lower lithospheric boundaries and with large nonhydrostatic stresses inherent in crustal thickness inhomogeneities.

The large hydrocarbon basin of South Caspian is filled with sediments reaching a thickness of 20–25 km. The sediments overlie a 10–18 km thick high-velocity basement which is often interpreted as oceanic crust. This interpretation is, however, inconsistent with rapid major subsidence in Pliocene-Pleistocene time and deposition of 10 km of sediments because the subsidence of crust produced in spreading ridges normally occurs at decreasing rates. Furthermore, filling a basin upon a 10–18 km thick oceanic crust would require twice less sediments. Subsidence as in the South Caspian, of ≥20 km, can be provided by phase change of gabbro to dense eclogite in a 25–30 km thick lower crust. Eclogites which are denser than the mantle and have nearly mantle P velocities but a chemistry of continental crust may occur beneath the Moho in the South Caspian where consolidated crust totals a thickness of 40–50 km. The high subsidence rates in the Pliocene-Pleistocene may be attributed to the effect of active fluids infiltrated from the asthenosphere to catalyze the gabbro-eclogite transition. Subsidence of this kind is typical of large petroleum provinces. According to some interpretations, historic seismicity with 30–70 km focal depths in a 100 km wide zone (beneath the Apsheron-Balkhan sill and north of it) has been associated with the initiation of subduction under the Middle Caspian. The consolidated lithosphere of deep continental sedimentary basins being denser than the asthenosphere, can, in principle, subduct into the latter, while the overlying sediments can be delaminated and folded. Yet, subduction in the South Caspian basin is incompatible with the only 5–10 km shortening of sediments in the Apsheron-Balkhan sill and south of it and with the patterns of earthquake foci that show no alignment like in a Benioff zone and have mostly extension mechanisms.