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 Poor ly 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 de velopment? and (3) which kinds of geological structure link the two co mpartments on either side of the transfer zone? We attempted to solve these problems using a 3-D experimental approach. Sand models were use d to simulate the foreland, a rigid, but mobile, backstop was used to produced a thrust Coulomb wedge which was studied during the various s tages 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 resul ts show that the development of transfer zones is controlled by mechan ical and/or geometrical parameters such as: (1) variations in the sedi mentary 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 thru st sequence were identified: alternative thrust propagations on both s ides of the transfer zone and synchronous thrust propagations which ar e anastomosed within the transfer zone. 3-D reconstructions revealed t hat 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 t he oblique ramp in the Zanskar Range, are discussed in relation to the mechanisms and structures revealed by the models. The results of anal ogue modelling suggest that the development of transfer zones is direc tly linked to variations in the boundary conditions (mechanical and/or geometrical) between two adjacent parts of a thrust wedge.