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.