HR: 13:40h
AN: T43D-01 INVITED     [Abstracts]
TI: Using Satellite-derived Gravity Data to Infer the Existence of Localised Convective Motions in the Mantle Associated with Mid-Atlantic Ridge Transforms
AU: * WILSON, M
EM: M.Wilson@earth.leeds.ac.uk
AF: Institute of Geophysics and Tectonics, School of Earth and Environment University of Leeds, Leeds, LS2 9JT United Kingdom
AU: Fairhead, D
EM: jdf@getech.leeds.ac.uk
AF: Institute of Geophysics and Tectonics, School of Earth and Environment University of Leeds, Leeds, LS2 9JT United Kingdom
AU: Fairhead, D
EM: jdf@getech.leeds.ac.uk
AF: GETECH, University of Leeds, Leeds, LS2 9JT United Kingdom
AB: Bonatti (1996) proposed that long-lived, large (greater than 200 km), transform offsets of the Mid-Atlantic Ridge in the Equatorial region are located above upper mantle thermal minima, and that they may represent semi-permanent structural and/or geochemical boundaries between adjacent ridge segments. The sites of these active transforms are regions of decreased magma supply, resulting in deep bathymetric troughs, imaged using the satellite-derived free air gravity data of Sandwell and Smith (1997) as negative anomalies, reflecting the positive correlation of free air gravity with bathymetry and the presence of low density sediments within the topographic lows. Within the South Atlantic, where the SW end of the St. Helena Seamount Chain intersects the mid-ocean ridge, we have observed distinct, southward directed, "V"-shaped, seamount trails (positive free air gravity anomalies) on the oceanic crust centred on the ridge axis. These are interpreted as an indication of excess magma production associated with a ridge-centred melting anomaly which is moving relatively, and progressively, southwards with time with respect to the underlying asthenosphere. Similar features also occur close to the ridge axis at the SW end of the Walvis Ridge and the SE end of the Rio Grande Rise. The orientation of these seamount trails is broadly concordant with the GPS-derived plate motion of Africa and South America. North of the Equatorial Fracture Zone similar, but inverted (i.e. northward directed), "V"-shaped volcanic lineaments are observed, bounded by E-W trending flowlines; these may also be associated with time-progressive migration of shallow melting anomalies, in this case moving relatively northward with respect to the asthenosphere within a particular ridge segment. The changing orientations of these near-ridge seamount trails north and south of the Equatorial Fracture Zone suggests that the source of the excess magmatism is closely associated with shallow-level seafloor spreading processes, rather than with conventional mantle plumes or hotspots. Our interpretation of the geometry of the seamount trails is consistent with localised flow directions within the asthenosphere converging in the Equatorial Zone (i.e. southward asthenosphere flow to the north of the Fracture Zone and northward flow to the south), supporting the model of Bonatti (1996). Bonatti, E. (1996) Long-lived oceanic transform boundaries formed above mantle thermal minima. Geology 24, 803-806. Sandwell,D.T. and Smith W.H.F. (1997) Marine gravity anomaly from Geosat and ERS 1 satellite altimetry. Journal of Geophysical Research 102:10,039-10,054.
DE: 9325 Atlantic Ocean
DE: 8155 Plate motions--general
DE: 5480 Volcanism (8450)
DE: 1744 Tectonophysics
DE: 1219 Local gravity anomalies and crustal structure
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting