GEOPHYSICAL EVIDENCE FOR THE ROLE OF FLUIDS IN ACCRETIONARY WEDGE TECTONICS

The seismic-reflection technique, by imaging lithological and structural boundaries, can largely define the framework upon which models of the fluid-flow regimes of accretionary wedges are hung. The distribution of fluid loss from the sediments that form the accretionary wedge and that lie beneath it, can be estimated from variation in seismic velocity as a measure of change in porosity, in conjunction with the interpretation of the structural evolution derived from seismic-reflection sections. Seismic techniques have detected regions of pronounced undercompaction believed to be associated with overpressured pore fluid, and detailed modelling has defined zones of localized fluid overpressuring, such as the decollements beneath wedges. The measurement of heat flow, directly, or indirectly from the methane-hydrate seismic reflector, can be used to detect the outflow of fluids, and map its variation in relation to structure and lithology. Geophysical techniques will achieve their full potential in constraining models of the behaviour of accretionary wedges, when calibrated from borehole measurements.