HR: 1340h
AN: T33B-1364    [Abstracts]
TI: Layer 2A thickness variations and upper crustal emplacement on the Lucky Strike segment (37N), Mid-Atlantic Ridge
AU: Crawford, W
EM: crawford@ipgp.jussieu.fr
AF: Institut de Physique du Globe, Boite 89 - 4 place Jussieu, Paris, 75252, France
AU: * Seher, T
EM: seher@ipgp.jussieu.fr
AF: Institut de Physique du Globe, Boite 89 - 4 place Jussieu, Paris, 75252, France
AU: Singh, S
EM: singh@ipgp.jussieu.fr
AF: Institut de Physique du Globe, Boite 89 - 4 place Jussieu, Paris, 75252, France
AU: Combier, V
EM: combier@ipgp.jussieu.fr
AF: Institut de Physique du Globe, Boite 89 - 4 place Jussieu, Paris, 75252, France
AB: We present seismic reflection images from a grid of 29 "2D" shot lines spaced approximately 2.5 km apart collected along and across the Lucky Strike segment of the Mid-Atlantic Ridge during the 2005 SISMOMAR cruise, part of the MOMAR (MOnitoring the Mid Atlanic Ridge) program. These shot lines provide reflection data out to the segment end and up to 30 km off-axis. The air guns were tuned to generate a single bubble and had an array volume of 8410 cubic inches for 25 of the lines (150 m shot spacing) and 5658 cubic inches for the remaining four lines (75 m shot spacing). Data were recorded on a 360 channel, 4.5 km long digital streamer. We use a 2D data processing approach for each line, applying frequency filtering, normal move out correction, stacking and 2D poststack depth migration. Each line was stacked using the same 1D subseafloor velocity profile to assure line- to-line comparability. The velocity profiles for the depth migration were derived from a coincident 3D seismic refraction experiment using 25 ocean bottom seismometers. The central along-axis reflection line reveals that the layer 2A reflector depth is nearly constant over the middle 44 km of the approximately 65 km long segment. In this region, the 2A reflector and the 4.5 km/s isovelocity contour lie about 0.6 km beneath the seafloor, except within the central 4 km of the segment, where the reflector shallows to about 0.45 km. At both segment ends the 2A reflector shallows whereas the isovelocity contour deepens. If the 2A reflector marks the transition between extrusive and intrusive volcanics, the extrusive crustal thickness is nearly constant along two thirds of the segment length, far beyond the extents of the central magma chamber. The thinning of layer 2A and thickening of the isovelocity contour beyond this zone suggests a transition from a magmatic/diking regime to a tectonic regime, in which the extrusive layer marked by layer 2A reflection is formed by surface lava flows from the magmatic/diking zone or by occasional horizontal or vertical magma injections, whereas rock fracturation has lowered the upper crustal velocities. We present a 30x40 km image of layer 2A thickness over the central part of the segment. The 4.5 km/s isovelocity surface varies little along axis, except for some thickness anomalies at the central volcano and its northwest ridge, which we interpret as sites of recent magmatic accretion. The isovelocity surface varies more across axis, thinning significantly off-axes at the valley bounding faults. We will correlate these variations with variations in the reflector depth and interpret the results in terms of emplacement processes and hydrothermal and mechanical alteration.
DE: 3025 Marine seismics (0935, 7294)
DE: 3035 Midocean ridge processes
DE: 7220 Oceanic crust
DE: 7245 Mid-ocean ridges
DE: 8178 Tectonics and magmatism
SC: Tectonophysics [T]
MN: 2007 Fall Meeting