The stress state of Venusian crust and variations of its thickness: Implication for tectonics and geodynamics |
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| Authors: | Marchenkov Konstantin I Zharkov Vladimir N Nikishin Anatoly M |
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| Institution: | (1) Dept. of Theoretical Physics, Institute of Physics of the Earth, Moscow, U.S.S.R.;(2) Dept. of Geological Sciences, Moscow State University, Moscow, U.S.S.R. |
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| Abstract: | The data obtained for the heights of the relief and the external gravitational field of Venus for spherical harmonics with degree and order up to 18 allow one to start theoretical analysis of the crust-mantle boundary (Venusian Moho) and stress state of the planetary interior. We suppose that Venusian convection is confined by floating massive crust. Apparently the convection in the upper mantle of Venus is separated from that one in the lower mantle and its lateral scale must be essentially smaller than on Earth. So, the convection is reflected to a larger degree of the gravitational field of the planet than for Earth. The spherical harmonic expansion of the topography for Venus correlates with corresponding expansion of the non-equilibrium part of the gravitational potential for n = 3–18. At the same time the relief of Venus is significantly compensated. It is reasonable to suppose that the gravity field for these harmonics is due to crustal thickness variations and, probably, to variations of crustal density. Thus, in the proposed scheme the Moho's relief causes the partial isostatic compensation of the topography.All calculations are carried out for the series of realistic models of Venus taking into consideration an asthenosphere. The asthenosphere is modeled either by a weakened (shear modulus is reduced), or by a liquid inviscid layer. We also suppose that the asthenosphere extends from the base of crust to a depth of 418 km, and the density contrast across the Moho boundary is –0.4 g * cm–3. If the actual density contrast across the Moho is less than the supposed one by some factor, then one must increase the amplitudes of the roots and inverse roots by the same factor. The results for the Moho's relief and stresses in the crust are presented for the case of the mean thickness of the crust of 50 km, which satisfies the probable upper (connected with phase transitions in waterless basalts) and lower (appearing in the framework of our interpretation) limits.On the whole, the crust-mantle boundary on Venus is evidently smooth, and the stress level in the crust is appreciably smaller than the crustal stresses on the Earth. The strong sensitivity of the stresses character to the parameters of the model of external layers of Venus together with geological data allow us to begin a preliminary investigation of the tectonical structure and geodynamics of the planet.'Geology and Tectonics of Venus', special issue edited by Alexander T. Basilevsky (USSR Acad. of Sci. Moscow), James W. Head (Brown University, Providence), Gordon H. Pettengill (MIT, Cambridge, Massachusetts) and R. S. Saunders (J.P.L., Pasadena). |
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