HR: 12:05h
AN: T32B-07    [Abstracts]
TI: Mapping of Seismic Layer 2A at Intermediate-Spreading Crust Exposed Near Blanco Transform
AU: * Christeson, G L
EM: gail@ig.utexas.edu
AF: Institute For Geophysics, University of Texas, Austin, TX 78759, United States
AU: McIntosh, K D
EM: kirk@ig.utexas.edu
AF: Institute For Geophysics, University of Texas, Austin, TX 78759, United States
AU: Karson, J A
EM: jakarson@syr.edu
AF: Department of Earth Sciences, Syracuse University, Syracuse, NY 13244, United States
AB: We present results from a seismic survey over intermediate-spreading crust adjacent to the Blanco Transform using a source array optimized to image the layer 2A boundary. The program acquired ten MCS profiles parallel to and four MCS profiles perpendicular to the transform scarp edge covering a region approximately 10 km (ridge- parallel) x 40 km (ridge-perpendicular). The average two-way travel time to the layer 2A event in our dataset is 0.36 s, which is similar to values measured elsewhere along the Juan de Fuca Ridge and East Pacific Rise. An independent method to estimate two-way travel time to the layer 2A/2B boundary is to model layer 2B refractions recorded on the 6-km streamer; this method gives similar two-way travel time values. The average layer 2A interval velocity calculated from analyses on selected CDP supergathers is 2700 m/s; converting from two-way travel time using this interval velocity results in an average layer 2A thickness of 490 m. Our seismic study was conducted adjacent to a region where the lithologic boundary between the dikes and overlying extrusive section has been mapped by submersible dives over a lateral distance of tens of kilometers. A comparison of the lithologic and seismic boundaries indicates that the mapped depth to the top of the sheeted dikes (average depth 1150 m) is well below the imaged layer 2A/2B boundary. We argue (Christeson et al., Nature, 2007) that the primary control on the depth of the layer 2A/2B boundary is crack closure enhanced by hydrothermal alteration and sealing, and not the structural boundary between lavas and dikes. Mapping reveals regions in the ridge-parallel direction where the layer 2A thickness is consistently thin or thick; these regions do not correlate strongly with seafloor bathymetry. These patterns suggest a temporal nature to the processes controlling the layer 2A/2B boundary.
DE: 3025 Marine seismics (0935, 7294)
DE: 3035 Midocean ridge processes
SC: Tectonophysics [T]
MN: 2007 Fall Meeting