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by DANIEL Romuald - 2 October 2011

Date de Mission: Mai 2005 - Juillet 2005

Chef de Mission: Singh Satish

co-chef de Mission: Crawford Wayne

Sismique active, avec tirs sur lignes 2D et 3D. Aussi sismique réflexion avec flute 4.5 km, mesures gravi, bathy et magnéto.

The 2005 SISMOMAR experiment was a seismic reflection/refraction study of the Lucky Strike segment and its central volcano, at 37°N on the Mid-Atlantic Ridge. The main objective of the cruise was to determine the crustal structure beneath the hydrothermal vent fields on the Lucky Strike volcano and to place the volcano in the magmatic/tectonic context of the segment.

We conducted 4 surveys:
- a 3D multi-channel streamer seismic reflection study of the central volcano, consisting of 39 18 km-long lines spaced 100 m apart with 37.5 m shot spacing to a 4.5 km, 360-element streamer towed behind the ship (yellow box, figure 1 and all of figure 2).
- a 3D refraction experiment within an 18x18 km box, shot to 25 seafloor OBSs with 150 m shot spacing (black lines, figure 1).
- 2 long 2D refraction lines, each shot to 15 OBSs, with 425 m shot spacing. One line went along the rise axis, the other went across (red lines, figure 1)
- 5 seafloor compliance measurements using broadband seafloor seismometers that were also used to record airgun shots. We also recorded microearthquakes using the OBSs, and obtained detailed bathymetry, gravity, 3.5 kHz chirp, and magnetics data from the ship thanks to the tight line spacing. The streamer was left deployed for all of the 3D refraction experiment to obtain reflection images and some of these lines were reshot using a tighter shot spacing to obtain a better reflection fold.

Preliminary analysis of the data reveal a melt reflector beneath the Lucky Strike volcano, with axial bounding faults diving down to near its edges. We are currently processing the data to determine the crustal thickness along- and across-axis, to constrain the 3D velocity structure within the volcano’s upper crust, to study variations in the thickness of layer 2A along- and across-axis, and to constrain the brittle/ductile transition within the segment using earthquake locations.

Rapport de mission

SISMOMAR Cruise Report - 17.6 Mb

Rapport de mission SISMOMAR