HR: 1330h
AN: T12D-0493    [PDF]
TI: A Wide-Angle Survey of the Mid-Atlantic Ridge at $5\deg$ South
AU: * Planert, L
EM: lplanert@geomar.de
AF: GEOMAR, Research Center for Marine Geosciences, Wischhofstr. 1-3, Kiel, 24148 Germany
AU: Tilmann, F
EM: tilmann@esc.cam.ac.uk
AF: Bullard Laboratories, Department of Earth Sciences, University of Cambridge, Madingley Road, Cambridge, CB3 0EZ United Kingdom
AU: Weinrebe, W
EM: wweinrebe@geomar.de
AF: GEOMAR, Research Center for Marine Geosciences, Wischhofstr. 1-3, Kiel, 24148 Germany
AU: Flueh, E
EM: eflueh@geomar.de
AF: GEOMAR, Research Center for Marine Geosciences, Wischhofstr. 1-3, Kiel, 24148 Germany
AU: Reston, T
EM: treston@geomar.de
AF: GEOMAR, Research Center for Marine Geosciences, Wischhofstr. 1-3, Kiel, 24148 Germany
AB: Slow spreading mid-ocean ridges are characterized by along-axis segmentation where crustal composition and structure varies significantly within a segment and across transform faults and other ridge axis discontinuities. In 2000, the GERSHWIN experiment investigated two spreading segments adjacent to the $5\deg$S transform fault. A set of intersecting wide-angle profiles were acquired running both parallel and perpendicular to the median valley and extending from the center of one segment across the transform well into the next segment. The lines focused on a number of topographic features (median valley, inside corner high, outside corner, transform fault) in order to resolve related velocity variations inside the crust and the uppermost mantle. This ridge transform intersection is unusual in that the inside corner high south of the $5\deg$S fracture zone has been split by a change in location of active seafloor spreading resulting in an outside corner massif and the absence of a volcanic ridge in the northernmost part of the median valley. For assessing velocity models we chose a combination of forward modeling and first-arrival tomographic inversion. For profiles with sufficient Moho reflections a joint refraction and reflection travel-time tomography was used. Energy propagation varies strongly but in most cases reaches for more than 40km, sometimes up to 90km. Modeling results show a velocity structure which differs significantly from normal oceanic crustal structure. In the median valley of the southern segment models show an unusual thin crust of about 4km thickness (shallowing towards the transform fault in the north) underlain by a low velocity upper mantle (Vp$\sim$7.5km/s). North of the fracture zone, median valley seafloor depths show a bathymetric high near the middle of the segment. Here, velocities reach up to 7.5km/s within depths of 6.5-7.5km below seafloor (starting with beneath seafloor velocities of about 3km/s). In contrast, velocity depth profiles in the region of the inside corner high and the outside corner massif in the southern segment show either very high near surface velocities ($>$6.0km/s) or very high velocity gradients causing crustal velocities to reach up to 6.5km/s within the first 1000m below seafloor. Below, velocities increase steadily up to between 7.5-7.8km/s at 4.0 to 6.0km depth below seafloor. By assuming that velocities of 7.5km/s are indicative for the upper mantle, models suggest crustal thicknesses of 4.0-5.0km at the eastern flank of the inside corner high and 4.5-5.5km at the outside corner.
UR: http://geomar.de/projekte/gershwin/
DE: 3035 Midocean ridge processes
SC: Tectonophysics [T]
MN: 2003 Fall Meeting