Observations of Seafloor Fluid Venting Along the Transform Margin Between the North American and Pacific Plates in the Gulf of California

Lundsten, Eve; Anderson, Krystle; Paull, Charles K.; Caress, Dave W.; Thomas, Hans; Gwiazda, Roberto; Herguera, Juan Carlos; McGann, Mary; Edwards, Brian; Hinojosa, Alejandro; Mercado, Beatriz Eugenia Mejia; and Sanchez, Ana Cristina
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Detailed surveys of the seafloor morphology at nine representative sections of the North American – Pacific Plate boundary on the floor of the Gulf of California were conducted by the Monterey Bay Aquarium Research Institute (MBARI) during a two ship expedition in March and April 2012. One of the objectives of this program was to better understand the distribution and impact of seafloor fluid venting along an active transform fault. At issue is whether the fault itself forms an active fluid flow conduit. An autonomous underwater vehicle (AUV) provided detailed bathymetry of the seafloor, and a remotely operated vehicle (ROV) allowed ground-truth observations and sampling of the surveyed area. The AUV surveys provide high-resolution multibeam bathymetry with a vertical precision of 0.15 m, horizontal resolution of 1.0 and 2-10 kHz CHIRP seismic reflection profiles. The nine surveys captured the morphology of a highly active fault zone in unprecedented detail. Among the features highlighted by these surveys are areas of distinctive seafloor textures. This distinctive morphology commonly occurs on the surface of elevated features that are up to 10 m higher than the surrounding seafloor and is characterized by considerable fine scale surface topography observed in both the AUV and ROV surveys. The surface topography is made up of broken and irregular blocks of methane-derived carbonates, separated by seafloor craters and open cracks that are 1 to 2 m deep and up to 10 m across. Individual patches of this distinctive morphology which are commonly ~100 m across occur within two of the nine survey areas. The presence of active chemosynthetic biological communities in open karst-like cracks show many of these sites are areas of active fluid seepage. In some places methane bubble venting was observed. Seepage was only observed where truncated strata exposed permeable layers adjacent to the fault rather than directly along the fault trace.

AAPG Search and Discovery Article #90162©2013 Pacific Section AAPG, SPE and SEPM Joint Technical Conference, Monterey, California, April 19-25, 2013