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SUMMARYEdge-based data structures are used to improve computational e ciency of inexact Newton methods for solving ÿnite element non-linear solid mechanics problems on unstructured meshes. Edge-based data structures are employed to store the sti ness matrix coe cients and to compute sparse matrix-vector products needed in the inner iterative driver of the inexact Newton method. Numerical experiments on three-dimensional plasticity problems have shown that memory and computer time are reduced, respectively, by factors of 4 and 6, compared with solutions using element-by-element storage and matrix-vector products.

Bookshelf type mechanisms are commonly used to explain antithetic accommodation of deformations caused by direct shear. Stress analysis of such mechanisms have been proposed, mainly through the use of classic Mohr circle graphical constructions. The present work introduces an alternative framework for analysis, the mechanics of generalized Cosserat continua, although maintaining the same tools (Mohr circle construction). In this type of mechanics, kinematics of the material points incorporates independent rotational degrees of freedom besides the classic translational ones. In the equilibrium equations, quantities involving moments are generated and the stress tensor is in the general case non-symmetric. Such characteristics of generalized Cosserat continua make it suitable for the analysis of bookshelf mechanisms, as these are, in essence, rotational antithetic mechanisms induced by shear stresses having different magnitudes in perpendicular planes. The paper applies the mechanics of generalized continua to the analysis of a specific bookshelf mechanism, related to crustal extension. In the Appendices, a review of the generalized Cosserat continua and details of the graphical Mohr circle representation of stresses for such media are presented.

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The Southeastern Brazilian Margin presents a NE-SW-striking structural framework, which is characteristic of the Neoproterozoic Ribeira Belt. Other important structural trends along the area are associated with E-W and NW-SE-striking structures, being most related to the Mesozoic-Cenozoic tectonic evolution. A succession of Cenozoic tectonic events has been described by many authors in the Southeastern Brazilian. Among those Cenozoic tectonic events, a Neogene to Quaternary E-W dextral strike-slip regime (EW-DT) shows a wide distribution, presenting some variations in the stress field orientation. This research investigates the influence of the preexisting structural framework on the variation of the paleostress field orientation associated with the EW-DT tectonic event in a selected area in the onshore continental margin of the Southeastern Brazil. The study was carried out in three main stages: lineament analysis, paleostress field analysis, and numerical mechanical modeling. The paleostress field related to the EW-DT tectonic event in the studied area presents: NNW-SSE-striking maximum horizontal stress, and counterclockwise rotation influenced by the structural framework. Regarding this, the regional variation of the paleostress field is influenced by a preferential reactivation of NE-SW-striking structures while sets of approximately E-W-striking structures are the main local control of counterclockwise rotation of the stress field.

We provide a structural analysis and propose a geomechanical model for an extensional fault system observed in seismic data from the deep water Santos Basin, offshore Brazil. The system includes low-angle normal faults (LANFs) associated with underlying listric faults and salt rollers. The LANFs cut through shallow sedimentary successions up to the seafloor, reaching displacements of 1 km during the last 5 myr, at least one order of magnitude larger than the displacement of the coeval Andersonian high-angle normal faults. We suggest that the LANFs were produced by clockwise rotation of the stress field caused by the presence of salt intruded along the footwall of the main listric normal faults of the extensional system. Numerical models indicate that the proposed geomechanical model can be attained with strain weakening and widespread plastic yielding along the main normal faults. We suggest that the studied LANFs are related to salt flow in the same way that the LANFs described in regional extensional provinces are related to flow of either the lower crust or the mantle. The rheological analogy is remarkable and therefore we suggest that this work will contribute to the solution of the LANFs paradox.

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