HR: 0800h
AN: T41E-1357    [Abstracts]
TI: Where the Gabbros are. A Different View of Evidence from Seismic Profiles and Lab Studies
AU: * Carlson, R L
EM: carlson@geo.tamu.edu
AF: Department of Geology & Geophysics, Texas A&M University, College Station, TX 77843-3115 United States
AB: Though there is good evidence for a significant volume of ultramafic rocks in crust formed at slow spreading ridges, lower crustal Vp/Vs ratios from both the Atlantic and Pacific are most consistent with a lower crust composed of gabbroic rocks. Nevertheless, the top of the gabbro has not been recognized in seismic profiles. The layer 2/3 transition is nowadays defined by velocities characteristic of the lower crust (~6.7 km/s) and by a marked decrease of the vertical velocity gradient from values typical of layer 2 (~1/s). In Hole 504B such a transition has been shown to occur at or very near the top of the 1-km section of diabase dikes that has been penetrated by drilling, and where it is associated with a lithologically-controlled reduction of the porosity of the formation. Though this correlation is well supported by the available data, it leaves unanswered a question of primary importance: Where in the seismic structure is the top of the gabbros that are thought to comprise the bulk of the lower oceanic crust? It has also been noted that the velocity at the putative layer 2/3 transition in Hole 504B is unusually low (<6.5 km/s) and that the gradient in the underlying dikes (~0.5/s) is much higher than gradients that are typical of the lower crust (0.2/s). Another transition has been shown to occur where P-wave velocities reach 6.6 km/s or more and the gradient decreases to <0.2 /s. Several lines of evidence suggest that this transition marks the top of the massive gabbro section: 1) these transitions are common in both Atlantic and Pacific P- and S-wave profiles; 2) these transitions occur just where the seismic profiles meet gabbro velocity vs depth profiles estimated from laboratory data; and 3) seismic velocities below the transition are in excellent agreement with the seismic properties of altered oceanic gabbros. Parsimony suggests that the transition to vertical velocity gradients less than 0.2/s at P-wave velocities near 6.7 km/s marks the top of the gabbros.
DE: 1021 Composition of the oceanic crust
DE: 1032 Mid-oceanic ridge processes (3614, 8416)
SC: Tectonophysics [T]
MN: Fall Meeting 2005