FLUID FLOW AND TECTONICS IN AN ACTIVE TRANSFORM CONTINENTAL MARGIN: THE MONTEREY BAY REGION, CENTRAL CALIFORNIA

Past and modern evidence of fluid and gas flow have been reported in the Monterey Bay region. In coastal outcrops of Miocene biosiliceous rocks, fluid-flow-related structures include large vertical intrusions of hydrocarbon saturated sandstone striking ~NE, and authigenic carbonate vent slabs and chimneys whose elongations and orientations of the conduits parallel fractures and faults in the host rocks striking NE and NW. Both examples indicate that fluid flow was focused along Miocene fractures and faults. Seep carbonate pavements and chimenys are also documented on the seafloor of the Monterey Bay: along faulted exposures in the Monterey Canyon, and occur in linear trends (NE) on the continental slope.

With our investigation we intend to assess whether these seep structures might be part of a larger fluid system, to determine what is the main driving mechanism for fluid flow in an active transform margin, and why the prominent orientation of the seep structures (NE) does not parallel the dominant orientation of the San Andreas Fault System (SAFS), a swath of anastomosing right-lateral NW-trending faults that outline the boundary between the North American Plate, the Salinian Block (a dominantly granitic fragment) and the Pacific Plate.

Combined, geophysical and geologic data from the region (e.g. MC seismic profiles, location of earthquakes, and gravimentry), indicate that most seep sites are located on NW-trending basement highs, next to basin depocenters of Tertiary organic-rich sediments which are the most likely source for the hydrocarbon-rich fluids, suggesting that lithostatic pressure is the main mechanism controlling the upward migration of flow of fluid and gases. We discovered for the first time NE-trending left-lateral strike-slip and thrust faults that offset the axis of the basement highs and basins. This system of cross faults is well outlined by linear trends of earthquake epicenters, and by large vertical-axis clockwise rotations indicated by paleomagnetic directions.

In conclusion, cross faults in rotated blocks have been focusing most of the shear stress between conjugate fault systems of the SAFS and may provide important conduits for fluid flow in strike-slip settings.