(1) U.S. Geological Survey, Woods Hole, MA
(2) Monterey Bay Aquarium Research Institute, Moss Landing, CA
(3) Woods Hole Oceanographic Institution, Woods Hole, MA
ABSTRACT: Blake Ridge Collapse Structure: Causal Relationship With The Underlying Gas Hydrate And Influence On Geochemistry And Methane Flux?
A collapse structure covering ~400 km2 is sited at the crestal region of the Blake Ridge, a large, NW-trending sediment drift accumulation, located off the southeastern United States. Within the structure, sediment mass movements created maximum subsidence at the ridge crest. Strata having approximately the thickness of the gas hydrate stability zone (GHSZ) subsided by means of NW-directed block normal faults on the SW flank of the ridge and by SW-directed gravity gliding nappes on the NE flank. Overpressures beneath the GHSZ probably facilitated mobilization of these strata. The volume loss of the depression, >13 km3, may have resulted from release of mobilized, fluid-rich sediment from beneath the GHSZ, or by a slump, which bulged out the NE flank of the Blake Ridge and was subsequently eroded away. A composite chronology based on magnetic susceptibility, stable isotopes, planktic forams and calcareous nannofossils is being constructed from cores to determine the age of sediments deposited within the structure. Preliminary evidence indicates that the structure may have formed at least 75 ka, much older than previously estimated. The base of the GHSZ, which was certainly disrupted by the collapse event, has largely reestablished itself at a nearly constant sub-bottom depth, as indicated by bottom-simulating reflections (BSRs) in seismic profiles. Sulfate gradients indicate that the methane flux appears to be greater at the crests of fault blocks/folds, where BSRs are shallowest and strongest, suggesting that the new shape of the base of the GHSZ may help to focus methane flux.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado