HR: 09:15h
AN: T41F-06    [Abstracts]
TI: Predicting Folding Sequences Based on the Maximum Rock Strength and Mechanical Equilibrium
AU: * Cubas, N
EM: cubas@geologie.ens.fr
AF: Laboratoire de Géologie, CNRS, Ecole Normale Supérieure, 5 rue Lhomond, Paris, 75005, France
AU: Souloumiac, P
EM: souloumiac@geologie.ens.fr
AF: Laboratoire de Géologie, CNRS, Ecole Normale Supérieure, 5 rue Lhomond, Paris, 75005, France
AU: Souloumiac, P
EM: souloumiac@geologie.ens.fr
AF: LMSSMAT, CNRS, Ecole Centrale Paris, Grande Voie des Vignes, Chatenay Malabry, 92290, France
AU: Maillot, B
EM: bertrand.maillot@u-cergy.fr
AF: Laboratoire de Tectonique, CNRS, Université de Cergy-Pontoise, 5 mail Gay Lussac, Neuville-sur-Oise, 95000, France
AU: Leroy, Y M
EM: leroyy@geologie.ens.fr
AF: Laboratoire de Géologie, CNRS, Ecole Normale Supérieure, 5 rue Lhomond, Paris, 75005, France
AB: The objective is to propose and validate simple procedures, compared to the finite-element method, to select and optimize the dominant mode of folding in fold-and-thrust belts and accretionary wedges, and to determine its stress distribution. Mechanical equilibrium as well as the constraints due to the limited rock strength of the bulk material and of major discontinuities, such as décollements, are accounted for. The first part of the proposed procedure, which is at the core of the external approach of classical limit analysis, consists in estimating the least upper bound on the tectonic force by minimisation of the internal dissipation and part of the external work. The new twist to the method is that the optimization is also done with respect to the geometry of the evolving fold. If several folding events are possible, the dominant mode is the one leading to the least upper bound. The second part of the procedure is based on the Equilibrium Element Method, which is an application of the internal approach of limit analysis. The optimum stress field, obtained by spatial discretisation of the fold, provides the best lower bound on the tectonic force. The difference between the two bounds defines an error estimate of the exact unknown tectonic force.
To show the merits of the proposed procedure, its first part is applied to predict the life span of a thrust within an accretionary prism, from its onset, its development with a relief build up and its arrest because of the onset of a more favorable new thrust (Cubas et al., 2007). This life span is sensitive to the friction angles over the ramp and the décollement. It is shown how the normal sequence of thrusting in a supercritical wedge is ended with the first out-of sequence event. The second part of the procedure provides the stress state over each thrust showing that the active back thrust is a narrow fan which dip is sensitive to the friction angle over the ramp and the amount of relief build up (Souloumiac et al., 2007). The stress state is dominated by a concentration at the root of the active ramp and the presence of the back thrust.
Analogue experiments with sand demonstrate the ability of the first part of the proposed procedure to predict the position and the lifetime of thrusts, the topographic evolution, as well as the value of the compressive force. The simplicity and lightness of the procedure allows to determine probability distributions of the friction angles of the décollement, the ramps, and the pristine material using an inverse problem formalism. Applied to a section at the front of Nanka'i's wedge, Southeast Japon, the two parts of the method confirm the relative weakness of the basal décollement. From the first part, we conclude that the active thrust is necessarily weaker than the incipient thrust and that the frontal section is likely inhomogeneous. The second part shows that, close to criticality, for minutes changes in the décollement friction angle, the stress concentrations, marking the onset of thrusting, can be positioned at very different locations, the details of which depend on relief irregularities.
DE: 8005 Folds and folding
DE: 8010 Fractures and faults
DE: 8020 Mechanics, theory, and modeling
DE: 8045 Role of fluids
DE: 8169 Sedimentary basin processes
SC: Tectonophysics [T]
MN: 2007 Fall Meeting