TRANSFER ZONES OF DEFORMATION IN THRUST WEDGES - AN EXPERIMENTAL-STUDY

Although numerous transfer zones of deformation have been observed in thrust wedges, their mechanisms and conditions of development are Poorly understood. In this paper three basic questions are addressed: (1) what are the parameters controlling the genesis of transfer zones? (2) what are the kinematics of thrust propagation during transfer zone development? and (3) which kinds of geological structure link the two compartments on either side of the transfer zone? We attempted to solve these problems using a 3-D experimental approach. Sand models were used to simulate the foreland, a rigid, but mobile, backstop was used to produced a thrust Coulomb wedge which was studied during the various stages of its formation. Shortening led to the development of transfer zones in the wedge, separating two domains of homogenous behaviour and connecting thrust planes in different domains. The experimental results show that the development of transfer zones is controlled by mechanical and/or geometrical parameters such as: (1) variations in the sedimentary thickness in the foreland; (2) the steepness of the backstop; (3) domains with different basal friction in the foreland; (4) kinked backstop geometry. During deformation of the models, two types of thrust sequence were identified: alternative thrust propagations on both sides of the transfer zone and synchronous thrust propagations which are anastomosed within the transfer zone. 3-D reconstructions revealed that the thrust deformations observed in the transfer zone are oblique ramps. The frontal thrust bending of the Barbados accretionary complex, as well as the imbrication of thrust in the ''Castellane'' arc and the oblique ramp in the Zanskar Range, are discussed in relation to the mechanisms and structures revealed by the models. The results of analogue modelling suggest that the development of transfer zones is directly linked to variations in the boundary conditions (mechanical and/or geometrical) between two adjacent parts of a thrust wedge.