Stresses in the lithosphere caused by crustal thickness inhomogeneities

Abstract

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

Role of faults, nonlinear rheology, and viscosity structure in generating plates from instantaneous mantle flow models

“…According to the studies by Jeffreys [1970] and Artyushkov [1973], the large topography variation in regions near seamounts and orogenic belts induce large local stresses. With strongly nonlinear stress-weakening rheology (e.g., the pseudo-plasticity or with n=-l), these high stress regions should develop weak fault zones and deform in the same way as plate margins.…”

Did the Variscides collapse or were they torn apart?: A quantitative evaluation of the driving forces for postconvergent extension in central Europe

“…The excess potential energy stored in the thickened crust can result in extension provided it is sufficient to exceed the strength of the lithosphere [e.g., Artyushkov, 1973;Dewey, 1988]. Lithospheric strength is strongly temperature-dependent and will thus decrease if temperatures in the thickened crust rise during postshortening thermal reequilibration owing to the increased contribution of radiogenic heat.…”

Deciphering orogenic evolution

“…The degree of coupling between the crustal and mantle parts of the lithosphere is also a function of the balance among surface forces related to plate tectonics, the gravity force related to lateral variations in lithospheric thickness and the buoyancy forces related to density variations ( [Artyushkov, 1973] , [Ramberg, 1981] , [Molnar and Lyon-Caen, 1988] , [Molnar et al, 1993] , [Chemenda et al, 1995] , [Ellis, 1996] and [0275] ). The impact of the thermal evolution of the crust and lithospheric mantle on the lithosphere's rheology and mechanical behavior is a major issue that is rarely taken into account.…”