HR: 0800h
AN: OS41D-0523    [Abstracts]
TI: Craters in the Seabed of the Gulf of California Caused by Volcanic Explosion? Volcanic Subsidence? Meteoroid Impact? Hydrocarbon Venting? or Dissolution?
AU: * Lonsdale, P
EM: plonsdale@ucsd.edu
AF: Scripps Institution of Oceanography, 9500 Gilman Drive, La Jolla, CA 92093 United States
AU: Eakins, B
EM: beakins@ucsd.edu
AF: Scripps Institution of Oceanography, 9500 Gilman Drive, La Jolla, CA 92093 United States
AU: Castillo, P
EM: pcastillo@ucsd.edu
AF: Scripps Institution of Oceanography, 9500 Gilman Drive, La Jolla, CA 92093 United States
AB: Several circular enclosed depressions were mapped at depths of 500 to 2500m during a multibeam sonar survey of the Gulf of California. Those on the summits and flanks of volcanic seamounts were caused by volcanic explosions or by subsidence following magma withdrawal, but the origins of the others, in both oceanic and continental crust, are more problematic. Multiple working hypotheses are being tested with geophysical data and rock-sample analysis. A fresh circular crater similar in size to Meteor Crater, Arizona but deeper (2km diameter, with a flat floor at 2185m below sea-level, 500m deeper than parts of its rim) indents young, faulted oceanic crust in rift mountains just 6km from the North Pescadero spreading axis. This axis, believed to spread a half-rate of 24km/Myr, accretes mainly a sediment-sill complex, with very localized eruptions of tholeiitic lava. Dredging of the steep (30 to $45\deg$) crater walls and rim recovered mudstone and angular boulders of basalt, and destroyed one dredge that grabbed a boulder too large to lift. The faulted rift walls 2km from the crater yielded mudstone and basalt fragments, plus rounded boulders (crater ejecta?) composed mainly of fresh tholeiitic glass. The most plausible, perhaps equally improbable, genetic hypotheses are (i) volcanic explosion when magma intruded off-axis into the sediment-sill complex and interacted with pore water, and (ii) meteoroid impact. Craters of a different type dissect the thick deformed sediment on the 1700 to 400m-deep crest of a transform ridge along the Guaymas Basin margin. Shallow pockmarks around discharges of hydrothermally generated hydrocarbons have been explored with submersibles on the deeper part of the ridge. The larger craters (0.5 to 2km diameter, 100 to 400m deep) that we found where the crest is 600 to 500m deep probably have the same origin (explosive release through a near-surface clathrate horizon of hydrocarbons that migrated up-dip from Guaymas Basin); a possible alternative is dissolution of evaporite strata, for which we have no direct evidence.
DE: 8499 General or miscellaneous
DE: 9350 North America
DE: 8105 Continental margins and sedimentary basins
DE: 5420 Impact phenomena (includes cratering)
DE: 3045 Seafloor morphology and bottom photography
SC: Ocean Sciences [OS]
MN: 2004 AGU Fall Meeting