MICROSTRUCTURAL EVOLUTION OF A DEFORMATION ZONE IN THE UPPER OCEAN CRUST - EVIDENCE FROM DSDP HOLE 504B

The deepest reference section through in-situ ocean crust lies at DSDP site 504B. Structural relations combined with existing detailed lithostratigraphy and alteration studies have been used to constrain the nature of deformation histories within a zone between 840 and 958.5 m. This zone lies close to the lithological transition zone between pillows and dykes. It is one of the most deformed intervals in the 504B core. Several discrete fault planes have been identified within this interval. Microstructures from these faults provide evidence for polyphase deformation persisting through one or more stages of alteration. A wide range of structures were generated during slip including cataclastic zones, isoclinal microfolds, crystallographic preferred orientation in quartz and preferred orientation of phyllosilicates. Repeated sequences of fracturing overprinted by non-coaxial (brittle and plastic) deformation are observed. The deformation zone is interpreted as a long-lived fault zone which may have nucleated as a result of the mechanical and transient thermal boundary at the top of the transition zone. Stockwork-like alteration may have developed in a dilational site generated during slip on a fault. Alteration histories imply that the fault was active during motion out of the main rift zone and local rotations of the minimum principal stress directions may have been related to flexure during this process. Reactivation of faults with a change in shear sense as material moved out of the rift may also account for the structures. Alternatively, the local stress field could have been modified during the evolution of a process zone associated with fault propagation.