LOWER PLATE INFLUENCES ON FOLD AND THRUST BELT ARCHITECTURE: LESSONS LEARNED FROM 3-D SEISMIC REFLECTION STUDIES OF THE BARBADOS RIDGE ACCRETIONARY WEDGE

Accretionary wedges and continental fold and thrust belts share common elements of mechanics and architecture. An extensive study of 3-D seismic reflection data, across the toe of the Barbados accretionary wedge, has yielded valuable insights into the important role lower-plate tectonism plays in governing deformation in the overlying prism. Seismic amplitude mapping, within the décollement, reveals laterally heterogeneous structure and fluid compartmentalization. Heterogeneity within the décollement zone can be related to structures mapped in the lower plate and readily apparent in 3-D visualizations of the subducted oceanic basement. These structures include a major NE-trending strike-slip fault zone, rooted in the basement and channeling deeply-sourced fluids within the décollement. Differences in the spacing and style of thrusting, within the accretionary wedge, can be related to the fluid content in the décollement. Detailed mapping of near-surface structure in the wedge indicates a change in structural style arcward of the accretionary front. NNE-trending out-of-sequence faults obliquely cut early NS accretionary structures and show a complicated history of coeval compression and extension. The NNE trends mimic relic basement topography and active normal faults mapped at the surface of the incoming oceanic plate. Mapping of the gross structure of the décollement indicates NNE down steps along lower plate normal faults that propagate through the overlying accretionary wedge. The “down-steps” in the décollement, in turn, localize compression forming thrust ramps that cut the earlier normal faults. Continued normal faulting cuts across the newly formed thrust ramps and perpetuates the cycle. 3-D seismic reflection studies give structural geologists an unparalleled view into the architecture of fold and thrust systems. Across the toe of the Barbados Ridge accretionary wedge this view shows great complexity in style. This complexity underscores the influence of lower plate tectonism on the evolution of the overlying prism. The importance of subdécollement tectonism on fold and thrust systems may be greatly under appreciated. Surface geologic studies must be integrated with adequate subsurface control to fully understand geospatial and temporal relationships within fold and thrust systems.