Seafloor collapse and methane venting associated with gas hydrate on the Blake Ridge: Causes and implications to seafloor stability and methanerelease
Abstract
A large irregular topographic depression exists at the crest of the Blake Ridge about 280 km off the South Carolina coast, southeastern United States. The feature, which is believed to be a collapse structure, is characterized by normal faulting of strata within the gas hydrate stability zone (GHSZ) on its SW side and by gravity gliding and folding of these subsided sediments in a nappe-like structure on its NE side. Sediments beneath the pre-collapse GHSZ appear to have been chaotically disturbed. The event apparently occurred more than 75 Ka. Subsidence of the seafloor exceeded 100 m over an area of >25 km2 and the depression represents a volume loss of >13 km3. This depression is difficult to account for by extension, deep structural movements, landslides, or erosion. We suggest this model: 1. free gas was trapped beneath the GHSZ and overpressures developed; 2. sediments below the GHSZ were mobilized and flowed, causing inflation of the ridge crest; 3. mobilized sediment erupted with significant volume loss; 4. strata within the GHSZ subsided by normal faulting and gravity gliding and folding. If such a sequence of events had occurred on a continental slope/rise, it would probably have triggered a major slide, but on the gentle slopes of the Blake Ridge, the strata above the GHSZ simply collapsed into the evacuated zone. Thus, if this model is correct, this example provides an unusual opportunity to study the process of mobilization of surface layers by gas hydrate-related processes. The expulsion of sediment may have released large volumes of methane to the ocean/atmosphere system, perhaps 1-4% of the present atmospheric content from the Blake Ridge event, so the process, when also accompanied by landslides, probably has been significant to the global greenhouse.
- Publication:
-
Geophysical Monograph Series
- Pub Date:
- 2001
- DOI:
- 10.1029/GM124p0211
- Bibcode:
- 2001GMS...124..211D