HR: 0800h
AN: T41D-1340    [Abstracts]
TI: Off-Axis Deformation near the Equatorial Mid-Atlantic Ridge in the Region of the North American-South American-African Triple Junction
AU: * Smith, D K
EM: dsmith@whoi.edu
AF: Woods Hole Oceanographic Institution, MS 22, Woods Hole, MA 02543 United States
AU: Escartin, J
EM: escartin@ipgp.jussieu.fr
AF: CNRS-IPGP, 4 Pl. Jussieu (Case 89), Paris, 75252 France
AB: The equatorial Mid-Atlantic Ridge (MAR) is especially interesting because of its complex tectonic history associated with movement of the North American-South American-African (NA-SA-AF) triple junction and the opening of the Atlantic Ocean. It is also notable because of an unusual zone of hydrophone-recorded seismicity that occurs 70 km west of the axis (5.5 My old crust) between 14.3N and 12.8N, and trends parallel to the ridge for 100 km. At the adjacent MAR, focal mechanisms from teleseismic earthquakes show extension directions that are consistent with NA-AF or SA-AF plate spreading (E-W). In contrast, focal mechanisms from earthquakes located in the off-axis strip show extension directions consistent with NA-SA spreading (NNW-SSE). In June 2005, we conducted an 8-day survey of this region of the MAR extending south from 14.25N to past the Marathon FZ, and extending 100 km west of the axis and 30 km east of it. Multibeam bathymetry, gravity and magnetic data were collected. This survey was a first step towards understanding how slow-spreading lithosphere is deforming in the NA-SA-AF triple junction region. Multibeam bathymetry data (including data from previous studies) show that the MAR is well defined between the Fifteen Twenty and Marathon FZs. In our study region, striated massifs are observed close to the axis, but not necessarily associated with axis offsets. The off-axis seismicity zone is characterized by irregular topography and flat sediment-filled grabens (widths of up to 10 km). They are not unique though: similar grabens are observed on the east flank of the MAR. Sidescan images, derived from the multibeam data, show no unusual reflectivity in the off-axis region that would be indicative of recent eruptions, faulting, or mass wasting. Preliminary results from the gravity data show no unusual crustal thickness variations. As of now, analysis of the magnetic data is ongoing. Since the hydrophone-recorded seismicity indicates that anomalous off-axis seismicity begins south of the Fifteen Twenty FZ and extends to at least 10N we speculate that our survey region is part of a broad zone that is responding to the NA-SA plate motions. At this point though, we do not understand why the strain is accommodated on lithosphere that is 70 km west of the axis.
UR: http://www.whoi.edu/oceanus/viewArticle.do?id=5521
DE: 3035 Midocean ridge processes
DE: 3040 Plate tectonics (8150, 8155, 8157, 8158)
DE: 3045 Seafloor morphology, geology, and geophysics
DE: 3075 Submarine tectonics and volcanism
DE: 7245 Mid-ocean ridges
SC: Tectonophysics [T]
MN: Fall Meeting 2005