HR: 0800h
AN: T41A-08    [Abstracts]
TI: Structural and morphological evolution of thrust wedges above a ductile layer with different viscous behavior
AU: * Cerca, M
EM: mcerca@geociencias.unam.mx
AF: Centro de Geociencias, UNAM, campus Juriquilla, Queretaro, QRO 76230, Mexico
AU: Barrientos, B
EM: bb@cio.mx
AF: Centro de Investigaciones en Optica, Loma del Bosque #115 Col. Lomas del Campestre, Leon, GTO 37150, Mexico
AU: Garcia-Marquez, J
EM: jgarciam@cio.mx
AF: Centro de Investigaciones en Optica, Loma del Bosque #115 Col. Lomas del Campestre, Leon, GTO 37150, Mexico
AU: Portillo-Pineda, R
EM: rokdrigo14-83@hotmail.com
AF: Centro de Geociencias, UNAM, campus Juriquilla, Queretaro, QRO 76230, Mexico
AU: Hernandez-Bernal, C
EM: caridad@geol-sun.igeolcu.unam.mx
AF: Departamento de Geoquimica, Instituto de Geologia, UNAM, Ciudad Universitaria, Mexico, DF 04510, Mexico
AB: A series of scaled physical experiments illustrate the importance of differences in density and viscous behavior of décollement in the structural evolution of thrust wedges during shortening. In particular, we have analyzed the effect of changes in viscosity in the morphological evolution and strain of the brittle overburden surface. Ten models properly scaled in geometry and mechanical behavior of natural geological materials were deformed at the Modeling Laboratory (LAMMG) of UNAM. Mechanical stratification of the models included basal and upper brittle layers of 1 and 2 cm, respectively; separated by an intermediate viscous layer of 0.5 cm. Brittle layers were constructed with grains of quartz sand following a Mohr-Coulomb criterion of faulting and bulk density of ca. 1300 kg m-3. The viscous layer was composed of silicon-sand mixtures having differences in dynamic viscosity (Pa s) and density (kg m-3) as the following cases: (A) 2.0 e 4 and 978, (B) 3.3 e 4 and 1195, (C) 4.7 e 4 and 1270. The experiments were carried out in a Plexiglas box of 40x15x10 cm and deformed by moving a vertical wall at a constant velocity of 1.5 cm hr-1. Cross sections of the experiments were obtained for values of bulk shortening of ca. 20 and 40 percent. The modeling results suggest a close relation of structural style of the thrust wedge with the initial conditions of décollement viscosity. Low viscosity models have a structural development characterized by low angle napes and detachment folds with limb rotation indicating a predominant vergence towards foreland. High viscosity models have a greater mechanical coupling between décollement and overburden and develop preferentially detachment folds with higher elevation and undefined vergence. The evolution of the surface in two models with different initial dynamic viscosity, cases A and B, was analyzed at the optical interferometry laboratory of CIO with two full-field optical techniques: fringe projection and laser speckle photography. The use of these combined techniques permitted to obtain a full-field and high resolution map of the changes in elevation during deformation and the instantaneous displacement field of the particles in the surface. The detailed analysis of the instantaneous displacement field also suggests an intimate link of the surface strain with viscous flow of the ductile layer. The presence of displacement instabilities, such as small vortex-like structures and changes in displacement direction, suggests non-steady flow of granular material in the surface of models during the initial steps of deformation in both cases. However, quasi-steady displacement field is reached significantly earlier in the case B (ca. 6 mm of bulk shortening) than in the case A model (ca. 31 mm of bulk shortening). Finally, taking into account the limitations of modeling (e.g., no erosion and deposition, no thermal evolution) the comparative analysis of the models with natural examples can give insights into the structural evolution of thrust wedges developed above décollement layers. Some characteristics of the Laramide shortening and its kinematics reconstruction in southern Mexico can thus be constrained with these models.
DE: 8020 Mechanics, theory, and modeling
SC: Tectonophysics [T]
MN: 2007 Joint Assembly