HR: 1340h
AN: S23A-1119    [Abstracts]
TI: Crustal thickness variations due to deep faulting and the velocity structure of the Lucky Strike segment (37N Mid-Atlantic ridge)
AU: * Seher, T
EM: seher@ipgp.jussieu.fr
AF: Institut de Physique du Globe de Paris, Boite 89 - 4 place Jussieu, Paris, 75252, France
AU: Crawford, W
EM: crawford@ipgp.jussieu.fr
AF: Institut de Physique du Globe de Paris, Boite 89 - 4 place Jussieu, Paris, 75252, France
AU: Singh, S
EM: singh@ipgp.jussieu.fr
AF: Institut de Physique du Globe de Paris, Boite 89 - 4 place Jussieu, Paris, 75252, France
AB: The 2005 SISMOMAR seismic survey, part of the MOMAR project (Monitoring the Mid-Atlantic Ridge), included 2D and 3D seismic reflection and refraction experiments covering the Lucky Strike segment. Prior results from this cruise demonstrate the presence an axial magma chamber beneath the volcano (1), under which earthquakes are detected. Here we present results from the 3D seismic refraction experiment, which reveals the interplay of tectonics and magmatism throughout the crust. We analyse data from 27 seismic lines extending approximately 40 km along- and across-axis, with inter-shot spacings of 75 and 150 m. The data were recorded by 25 ocean bottom seismometers (OBS) in a 18x18 km grid with 4.5-km inter-OBS spacing, centered on Lucky Strike volcano. The instruments recorded arrivals of crustal turning waves and reflections from the Mohorovi\check{c}i\acute{c} discontinuity (Moho). We performed a joint travel time tomographic inversion on the turning waves and reflections. The turning rays constrain crustal velocities, whereas the reflected rays constrain velocities and the depth to the Moho. The data reveal a low velocity zone beneath the Lucky Strike volcano, spanning the depths between the axial magma chamber reflector and the Moho. The velocity anomaly is at least 0.6 km/s, indicating that melt is present. We also show evidence that upper crustal velocities are slower in the median valley than beyond the valley walls and that the change in velocity occurs rather abruptly at the valley wall faults. This suggests that the crust outside of the valley was emplaced in a different manner than that within the valley or that crustal formation is broadly distributed across the valley and stops at the walls. Finally, the crust is 0.8 km thicker to the west of the volcano than to the east, suggesting that the magma source has recently moved westward relative to the segment or that asymmetric tectonism plays a major role in the formation and modification of the crust. References: (1) S. Singh et al.: Discovery of a magma chamber and faults beneath a Mid-Atlantic Ridge hydrothermal field. Nature 442, 1029-1032 (2006).
DE: 0930 Oceanic structures
DE: 3025 Marine seismics (0935, 7294)
DE: 3035 Midocean ridge processes
DE: 7245 Mid-ocean ridges
DE: 8178 Tectonics and magmatism
SC: Seismology [S]
MN: 2007 Fall Meeting