Flexural rigidity and lithospheric stress in collisional orogens from constrained 3D density models

3D modeling of gravity and terrain reduced geoidal undulations have been carried out in the interdisciplinary research groups of the European TRANSALP consortium and the German-South American "Collaborative Research Center 267" (Deformation Processes in the Andes) to provide insight into the structure of the lithosphere in the areas of the Alpine-Adriatic (African) and Pacific collision zone, respectively. The density models are well constrained by modern seismic imaging and other geophysics. In particular the model resolution of superficial crustal structures is sufficient to calculate gravity effects and the subsurface loads. Both are used to calculate lithospheric flexural rigidity and/or effective elastic thickness from the gravity field and the geoid by the aid of two methods: (1) coherency function for the Andes and (2) convolution for Alps and Andes. The aim of this poster is to compare the results of the two methods and demonstrate the advantage of the convolution over the coherency method. Although the results of flexural rigidity are rather similar we figures out that convolution overcomes a series of numerical problems we have with the coherency method. The regional tectonical environments of Andes and Alps clearly differ from each other which is reflected by different pattern of flexural rigidity. For maps rendering of flexural rigidity (effective elastic thickness of the lithosphere) from topographic and subsurface loads we obtained for the Western South American Continental margin and the high Andean plateau low values (10 E22 to 10 E23 Nm) for Central Andes and 10 E23 to 5x10 E 23Nm in the continental back arc region which corresponds with an effective elastic thickness of 35 to 45 km. In the case of the Alps rigidity/effective elastic thickness was calculated only by the convolution method. Here the flexural rigidity shows small values (10 E21 Nm) and correlates clearly with the crust-mantle boundary and the shape of the orogen. In a last step we used the curvature of lithospheric layers of our 3D model to calculate the stress distribution which is produced by flexured layers. First results can show that the main stress is connected with the crust-mantle boundary and the transition zones of the different plates, however, additional patterns can be seen which were correlated with the results of Vp- tomography and the distribution of electrical conductivity (volcanic arc and forearc areas in the Andes) and in the Eastern Alps abnormal stress is caused by crustal domains in the area of the Vicenza gravity high.