Deformation and fluid-flow in magma-poor margins : A study of the Tasna Ocean-Continent transition, SE Switzerland

Abstract: In this study, interaction between fluids and deformation during the final stages of magma-poor rifting was investigated. The Tasna Ocean-Continent transition, located in the Swiss Alps, was studied and a large data set was obtained from profiles oriented perpendicular to two detachment faults. One of these juxtaposed serpentinized mantle against continental crust and the other exhumed both mantle and continental crust to the seafloor. Deformation associated with detachment faulting showed many common features but also some phenomena which were unique to each fault, confirming their sequential activity and differing roles in the exhumation process. Oxygen isotopes indicated the presence of both pervasive and channeled fluid phases, either accompanying or post-dating serpentinization. Deformation in the fault zone occurred in previously serpentinized mantle indicating that serpentinization pre-dated final exhumation. Most strain localization and displacement occurred in fault cores which are narrow zones beneath the detachments. These are underlain by wide zones characterized by more distributed strain. Transitional fabrics as well as reactivated and/or overprinted deformation structures indicated that the final phase of rifting is complex. Fault cores acted as fluid conduits or barriers. Thus, the most deformed zones may become the least permeable. Hence the coupling between deformation and fluid flow is complex in a study area subjected to several phases of deformation and fluid flow. Finally the importance of serpentinization in the evolution of magma-poor rifting was investigated. It was found that serpentinization is the consequence rather than the reason for strain localization at magma-poor margins. However, serpentinization may be an important process which can accelerate exhumation rates in the very latest stages of magma-poor rifting. The pre-existing deformation history of the crust may also be of importance for the development and location of margins.