ROV Observations of Fluid Expulsion in Monterey Bay, California

Daniel L. Orange, J. Barry, N. Maher, D. Stakes, J. Paduan, H. G. Greene, J. B. Martin, B. McAdoo, and J. Yun

ROV dives in Monterey Bay have been used to examine the relationship of fluid flow to tectonic and stratigraphic conduits along an active transpressional continental margin. We used side-scan sonar to identify dive targets for the ROV, since anomalous reflectivity can be caused by the presence of biological "cold seep" communities or authigenic carbonate. On a compressional ridge west of the San Gregorio Fault, cold seep clams are found along with extensive fields of authigenic carbonate in an elliptical region of anomalous reflectivity ~400m in diameter. The reflectivity and fluid expulsion suggest that this feature is an active mud volcano. Analyses of push cores from the ridge site indicate high concentrations of both methane and sulfide and the presence of higher-orde hydrocarbons. Many carbon isotopic ratios of the carbonate crusts indicate a methane carbon source; some values represent a mixture of methane carbon and normal marine carbon. Fluids charging the seeps west of the San Gregorio Fault may originate in tectonically-compacted sediments affected by residual Pacific-North America plate convergence, and may have an additional component of hydrocarbon charging from the underlying Monterey Formation. At the intersection of the Monterey Fault Zone and the Monterey Canyon a number of cold seeps occur in headless side canyons characterized by intense fracturing. This supports the hypothesis that submarine canyons act as hydrologic sinks for any overpressured fluid flowing toward the surface. On the San Gregorio Fault itself we have found en echelon ridges of carbonate. The fluids seeping out along fault zones may originate deep in the section and utilize the deformation-induced fracture permeability of the fault zone. Alternatively, aquifer-forcing from the uplifted Santa Cruz Mountains may provide a source of fluids venting along these fault zones (aquicludes?) and at seeps east of the fault zones.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California