Triangular Zone at the Front of Fold-and-Thrust Belts Due to Either Erosion or Delamination

Yves, Leroy ²; Mary, Baptiste ³; Leroy, Yves ¹; Pubellier, Manuel ¹; Maillot, Bertrand ³
(1) Laboratoire de Geologie, Ecole Normale Supérieure, Paris, France. (2) Laboratory of Computational Geodynamics, Graduate University of Chinese Academy of Sciences, Beijing, China. (3) Departement des Geosciences & Environnement, Université de Cergy-Pontoise, Cergy-Pontoise, France.

The style of deformation at the termination of fold-and-thrust belts is often characterized by triangular zones with forward-dipping ramps (e.g. Southeastern Canadian Cordillera). Two potential mechanisms responsible for the triangular zone are explored based on the maximum strength theorem presented by Maillot and Leroy (JMPS,2006) and applied by Cubas et al. (JGR, 2008) for predicting thrusting sequences. The proposed prototype is a wedge-type, triangular-shaped belt on a weak decollement. Material properties and geometrical attributes are such that the deformation is to the front, based on the critical wedge theory.

The first mechanism results from the coupling between a forward prograding erosion front, the rock failure in conjugate, reverse faults and the activation of the weak decollement. The rock in the bulk and the decollement are assumed to be cohesive and frictional. Numerical results show that the normal forward-thrusting sequence at the front of the belt can be replaced by back-thrusting of the incoming material, thus forming a triangular zone at the tip of the decollement, in conditions of intense erosion and low mechanical weakening on the thrusts. The length of the decollement is determined by the position of the erosion front. The second mechanism to explain the triangular zone relies on the introduction of a weak tensile strength of the decollement typical of evaporites, as for example, in the Northern Appennines and the Southern Pyrenees. It is shown how at shallow depths, the tensile delamination of the decollement and the curved, forward-dipping ramp is characteristic of the triangular zones, in the absence of any erosion process.

It is proposed to apply these two mechanisms to explain tentatively the presence of triangular zones along various segments of the Andes as well in the Longmen Shan. The key parameters are the lithologies, and thus the mechanical properties, the thickness of the frontal region and also, the sedimentation influx and the efficiency of erosion processes. Implications of this theory for the selection of appropriate analogue materials to reproduce triangular zones in the laboratory are also discussed.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.