HR: 13:55h
AN: T23F-02 INVITED     [Abstracts]
TI: Correlation of Seismic Layer 2A and Observed Extrusive/Dike Boundary of Fast-Spreading and Intermediate-Spreading Crust Exposed Near Hess Deep and the Blanco Transform
AU: * Christeson, G L
EM: gail@ig.utexas.edu
AF: Univ of Texas Institute for Geophysics, 4412 Spicewood Springs Rd, Bldg 600, Austin, TX 78759 United States
AU: McIntosh, K D
EM: kirk@ig.utexas.edu
AF: Univ of Texas Institute for Geophysics, 4412 Spicewood Springs Rd, Bldg 600, Austin, TX 78759 United States
AB: The simple picture of oceanic crust, based primarily on the studies of ophiolites, is an upper layer of basaltic lavas, a middle layer of basaltic sheeted dikes, and a lower layer of gabbroic rocks. By correlating seismically defined crustal intervals with specific rock units, many workers have attempted to draw inferences regarding the geology of the oceanic crust including volcanic construction, dike intrusion, and faulting. However, there has been no direct link between seismic and geologic structure for 'normal' oceanic crust except at DSDP Hole 504B. In July 2003 we conducted a seismic survey over fast-spreading crust adjacent to the Hess Deep rift. The program acquired MCS profiles parallel to and perpendicular to the scarp edge adjacent to the region where the lithologic boundary between the dikes and overlying extrusive section had been mapped by submersible dives over a lateral distance of tens of kilometers. A similar experiment was conducted in October 2004 over intermediate-spreading crust adjacent to the Blanco Transform; submersible dives along the Blanco Transform cliff faces have been able to map the uppermost 2 km of the exposed oceanic crust over a lateral distance similar to that at Hess Deep. Surveys at both locations were acquired with similar or identical experimental geometry, source array, and streamer configuration. Average seismic layer 2A thickness is 605 m at Hess Deep and 640 m at Blanco; average layer 2A interval velocity is 3.6 km/s at Hess Deep and 3.5 km/s at Blanco; average velocity at top of layer 2B is 5.05 km/s at Hess Deep and 4.80 km/s at Blanco. At both locations the layer 2A interval velocity and layer 2B velocity is 0.3-0.7 km/s lower for the profiles within 2 km of the scarps. Thus our results indicate nearly identical seismic structure at both locations. However, mapped extrusive thickness is markedly different at Hess Deep and Blanco, with an average extrusive thickness of ~0.4 km at the fast-spreading Hess Deep site and ~1.3 km at the intermediate-spreading Blanco site. Thus the seismic layer 2A/2B boundary is positioned near or deeper than the mapped extrusive-dike boundary at Hess Deep and well above the mapped extrusive-dike boundary at Blanco. This mismatch between seismic and geologic results suggests that it is invalid to use the seismic layer 2A reflector to map the lithologic contact between extrusives and dikes.
DE: 7220 Oceanic crust
DE: 7245 Mid-ocean ridges
SC: Tectonophysics [T]
MN: Fall Meeting 2005