--> Abstract: Active Faulting, Gas Hydrate Formation, and the Growth of Seafloor Blisters in Santa Monica Basin, California; #90063 (2007)

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Active Faulting, Gas Hydrate Formation, and the Growth of Seafloor Blisters in Santa Monica Basin, California

 

Paull, Charles K.1, William R. Normark2, William Ussler1, David W. Caress1, Rendy Keaten1 (1) Monterey Bay Aquarium Research Institute, Moss Landing, CA (2) United States Geological Survey, Menlo Park, CA

 

Relationships among anticline growth associated with active transpressional faulting, formation of Holocene mud-volcano-like mounds, methane migration and gas hydrate formation were investigated through detailed seafloor surveys in Santa Monica Basin, offshore of Los Angeles, CA. Two distinct deep-water (~800 m water depth) topographic mounds were surveyed using MBARI's Mapping AUV (carrying multibeam and sidescan sonars and a chirp sub-bottom profiler) and one was explored with the ROV Tiburon. The mounds are domal bathymetric features (~15 m high, 90 m wide). Local vertical displacement needed to form them suggests expansion within near surface sediment. The mounds are associated with gas venting. Continuous streams of methane gas bubbles emanate from the eastern mound crest, extensive methane-derived authigenic carbonate pavements and chemosynthetic communities cover the mound surface, and a piston core taken on the mound flank recovered gas hydrate at approximately 2 m below the seafloor. No allochthonous materials were found associated with the mounds. These mounds protrude from crests of broader anticlines (~20 m high and ~1 to 3 km long) formed within latest Quaternary-aged seafloor sediments associated with active faults. Locally, offset is observed on limbs of the anticlines ~100 m from the crests. The steps in a conceptual model for mound formation involve focused gas migration along a fault to the crest of an actively forming anticline through sand layers cut by the fault, formation of gas hydrate in the shallow subsurface, and upward expansion associated with continued gas hydrate growth supported by the flux of methane blisters the seafloor, generating the mounds.

 

AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California