SPHERICAL SHELL TECTONICS : ON THE BUCKLING OF THE LITHOSPHERE AT SUBDUCTION ZONES 球殻テクトニクス : リソスフェアの沈み込みにおける座屈現象について

The buckling phenomena of the subducting lithosphere dueto the sphericity or the earth are studied by a nonlinearfinite element method, and the results are compared with thebuckling experiments by Yamaoka et a1. (1986b). Thesubduction or the lithosphere is modeled by a hemisphericalshell squeezed at its circumferential edge. For simplicity,two parameters are employed in scaling the subductinglithosphere. One is the thickness or the lithosphere andthe other is the length or its deformable portion (i.e. , thelength of the descending slab). Uniform inward load isapplied to the free circumferential edge or the deformableportion. At the opposite edge (i.e., at the trench),built-in boundary condition is imposed; no displacements andno rotations are allowed. Investigations are made mainly onthe following three subjects: (I) How do the two parameterscontrol the wavelength of buckling? The results show thatshorter waves are associated with thinner and shortershells. The irregularities such as tears or seamounts donot alter the buckling wavelength very much. (2) How doshells deform in their postbuckling state. Sharp cuspspointing outward appear in the postbuckling undulationwithout any irregularities in shell property and in appliedload･The shape or the cuspidal undulation is quite similart.o that of the actual subducting lithosphere. (3) Canbuckling take place in the subducting lithospbere underlikely conditions in the earth? The examination is madewith respect to the buckling load, the shell deflection, andthe buckling wavelength. The results strongly suggest thatthe arcuate shapes of subduction zones are in fact generatedby buckling or the subducting lithosphere.