HR: 08:30h
AN: T11B-03    [PDF]
TI: Evidence for Highly Focused Magmatic Accretion Along the Ultra-Slow Southwest Indian Ridge
AU: * Lin, J
EM: jlin@whoi.edu
AF: Woods Hole Oceanographic Institution, Department of Geology and Geophysics, Woods Hole, MA 02543 United States
AU: Dick, H J
EM: hdick@whoi.edu
AF: Woods Hole Oceanographic Institution, Department of Geology and Geophysics, Woods Hole, MA 02543 United States
AU: Schouten, H
EM: hschouten@whoi.edu
AF: Woods Hole Oceanographic Institution, Department of Geology and Geophysics, Woods Hole, MA 02543 United States
AB: The Southwest Indian Ridge (SWIR) spreads at effective slow to ultra-slow rates of only 8-15 mm/yr at the $7\deg$-$16\deg$E region, depending on its local obliquity. During Dec. 2000-Jan. 2001 and Jan.-Feb. 2003, we carried out an extensive SeaBeam bathymetry, gravity, and magnetic survey, as well as rock dredging, of a 400-km-long section of the SWIR, including the Shaka transform. The results reveal dramatic variations in seafloor morphology, residual gravity anomaly, crustal magnetization, and rock types within the surveyed region. A 50-km-long segment centered on $14\deg$30' E, which we call the "Narrowgate" segment, has a well-defined narrow axial rift valley, a bull-eye-shaped mantle Bouguer gravity low with along-axis amplitude of 25 mGal, and clear off-axis record of magnetic stripes to crustal age of ~7 Ma. These ridge-axis features resemble a classic slow-spreading ridge segment in the northern Mid-Atlantic Ridge. However, this ridge segment appears to be less robust prior to ~7 Ma. To the west, a 200-km-long stretch of the ridge axis at $11\deg$45'-$14\deg$15'E lies obliquely to the spreading direction, resulting in effective ultra-slow full spreading rates of only 8 mm/yr. The relatively deep rift valley, systematically positive mantle Bouguer gravity, and extremely low amplitude of crustal magnetization suggest that while lithosphere is formed there, it has very little magmatic crust. Such interpretation is collaborated by the abundance of ultramafic rocks dredged on this stretch of the ridge. Further westward, an apparently robust ridge segment, called the Joseph Mayes Seamount segment, is found near $11\deg$20'E. This 40-km-long ridge segment is associated with major mantle Bouguer anomaly with along-axis amplitude of 40 mGal as well as strong crustal magnetization. However, this ridge segment appears to have become magmatically robust only 3-4 Ma. Together the above evidence suggests that magma accretion along this stretch of the SWIR axis is highly focused in space and time. We propose a model in which the overall low magma supply rate and dominant lithospheric cooling at a slow/ultra-slow ridge will result in temporal centers of highly focused magma accretion. The extremely heterogeneous thermal and rheological boundaries in the lithosphere should help lateral migration of melts to be pooled towards such temporal centers of magmatic accretion.
DE: 3005 Geomagnetism (1550)
DE: 3010 Gravity
DE: 3035 Midocean ridge processes
DE: 3040 Plate tectonics (8150, 8155, 8157, 8158)
DE: 3045 Seafloor morphology and bottom photography
SC: Tectonophysics [T]
MN: 2003 Fall Meeting