Stresses in the lithosphere caused by crustal thickness inhomogeneities

Abstract

A 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 material 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.