HR: 1330h
AN: T12D-0503    [PDF]
TI: The Terceira Rift as Hyper-Slow, Hotspot-Dominated Oblique Spreading Axis: A Comparison With Other Slow-Spreading Plate Boundaries
AU: * Vogt, P R
EM: vogt@qur.nrl.navy.mil
AU: Jung, W
EM: jung@qur.nrl.navy.mil
AB: We suggest the 550km long Terceira Rift (Azores Plateau) is the world­_s slowest-spreading (hyper-slow, 4mm/a plate separation; 2.3-3.8mm/a perpendicular to oblique axial segments) organized accreting plate boundary. In its slightly sinuous (ca. 300km radius of curvature) axial trace, its oblique spreading angles (ca. 40$\deg$-65$\deg$), and in frequency and first motions of earthquakes, the TR resembles better-known ultra- or super- slow spreading ridges (e.g., Gakkel and Southwest Indian ridges). Interpreted simply as volcanically unfilled rift valley segments, the inter-island basins (e.g., the 3200m deep Hirondelle Basin) are slightly wider (30-60 km), but not significantly deeper (1000-2200 m) than the Mid-Atlantic Ridge median valley (20-28mm/a; 10$\deg$N-53$\deg$N). However, along-axis segmentation wavelengths (ca. 100km) are double those along the central MAR, but make TR comparable to the ultra-slow (15-16 mm/a) Southwest Indian and Gakkel (7-13 mm/a) ridges. If this segmentation wavelength reflects Rayleigh-Taylor instabilities, the viscosity contrast between the overlying axial lithosphere and the partial melt zones is about an order of magnitude greater at ca. 4-16 mm/a than at 20-30 mm/a. The Terceira Rift differs dramatically from ultra-slow ridges only in the large amplitude of along-strike topography (2000-4000m; 4200m totalvariation) owing perhaps to a copious melt flux from the Azores hotspot, combined with a spreading-rate-determined greater axial flexural strength and plate thickness, and slower export of volcanics from the rift axis. The probable TR youth (ca. 1 Ma, requiring less than 4km new oceanic crust) indicates lack of steady-state spreading conditions, which may explain the published gravity evidence against TR spreading. Absolute plate motions support the creation of the Azores Plateau by successive NE jumps of the rift axis to maintain its position over a fixed hotspot.
DE: 3035 Midocean ridge processes
DE: 3040 Plate tectonics (8150, 8155, 8157, 8158)
DE: 3045 Seafloor morphology and bottom photography
DE: 8150 Plate boundary--general (3040)
DE: 8158 Plate motions--present and recent (3040)
SC: Tectonophysics [T]
MN: 2003 Fall Meeting