Fluid impregnation and development of fault breccias in the Tavetsch basement rocks (Sedrun, Central Swiss Alps)

Microstructural, mineralogical and microthermometric studies provide an insight into the brittle deformation mechanisms and the P-T-t-x evolution of fault breccias from the Tavetsch massif. The fault breccias are overprinted by several pre- to early Alpine deformation events (summarized as deformation stage I), and at least five Alpine to late Alpine deformation stages (II to VI). Stages III to VI were responsible for the generation of cohesionless fault breccias. The predominant loss of cohesion occurred at a depth of 9-6 km, 280-190 degrees C and fluid pressures of 2.3-0.6 kbar. These conditions prevailed between deformation stages IV and V. The loss of cohesion is interpreted to be the result of pore space interconnection induced by brittle deformation. This process was accompanied by a decrease in fluid pressure towards hydrostatic conditions. Finally stage VI generated fault gouge and further fault breccias due to small displacements (millimetre to centimetre range) along discrete faults and fractures. The fluid evolution shows a retrograde trend to lower salinities, indicating a meteoric influence during hydrostatic conditions. (C) 1998 Elsevier Science B.V. All rights reserved.