HR: 13:40h
AN: V22F-01 INVITED     [PDF]
TI: How Variable Slow-Spread Ocean Crust?
AU: * Dick, H J
EM: hdick@whoi.edu
AF: Dept. of Geology & Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543 United States
AU: Kelemen, P
EM: pkelemen@whoi.edu
AF: Dept. of Geology & Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543 United States
AU: Kikawa, E
AF: Japan Marine Science and Technology Center, Natsushima-cho, Yokosuka, 2-15 Japan
AU: Cheadle, M J
AF: Department of Geology, University of Wyoming, Laramie, WY 82070 United States
AU: Michael, P J
EM: pjm@utulsa.edu
AF: University of Tulsa, 600 S College Ave., Tulsa, OK 74104 United States
AU: Snow, J
AF: Max-Planck Institut f�r Chemie, Abt. Geochemie, Postfach 3060, Mainz, D-55020 Germany
AU: Schouten, H
EM: hschouten@whoi.edu
AF: Dept. of Geology & Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543 United States
AU: Lin, J
EM: jlin@whoi.edu
AF: Dept. of Geology & Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543 United States
AU: Hirth, G
EM: jhirth@whoi.edu
AF: Dept. of Geology & Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543 United States
AU: Leg 209 Scientific Party, .
AF: Ocean Drilling Program, 1000 Discovery Drive, College Station, TX 77845 United States
AB: Field studies at slow and ultra-slow spreading ocean ridges reveal a diversity of crustal architecture. At the extreme, ultra-slow spreading ridges consist of linked magmatic and amagmatic accretionary segments. The latter replace transform faults and en-echelon magmatic segments to accommodate ridge obliquity. They are sparsely volcanic and expose large areas of peridotite on the sea floor. Gabbros are largely absent in dredges and dikes uncommon. At magmatic accretionary segments, sheeted dikes and pillow lavas, and gabbros are exposed in abundance, including primitive layered gabbros, and the crustal section appears to fit the Penrose ophiolite model. At slow spreading ridges, by contrast, en-echelon magmatic segments linked to non-transform discontinuities and transforms accommodate ridge obliquity. Diabase, gabbro and peridotite are dredged largely from transform walls and locally at non-transform offsets. Lithologic relationships inferred from dives and dredges suggest a non-Penrose stratigraphy. However, mapping at the Atlantis Bank oceanic core complex suggests that this may be due to sampling fault faces that localize late serpentine diapirism and intrusions of mostly highly differentiated gabbro. There, detachment faulting rooted at the dike-gabbro transition has dismembered Penrose type oceanic crust exposing a 400 km2 gabbro massif less than ~5 km thick, that tapers out at variable distances towards the fracture zone. Large mantle outcrops, however, are exposed along rift valley walls away from transforms in the $15\deg$20'N region of the MAR. ODP Leg 209 drilling at 8 widely spaced sites found more abundant gabbro than expected from the dredging. Here, a non-Penrose stratigraphy consists of numerous gabbro plugs intruded at varying depth in a mantle section beneath the ridge axis. These, in turn, are locally cross-intruded by dikes and covered by a veneer of pillow basalts. Most gabbros are highly evolved, suggesting more primitive gabbros are abundant at depth. The abundance of dredged peridotite likely reflects strain localization in the roof zone where peridotites are intruded by gabbros, commonly exposing deformed intrusion breccias and net vein complexes in massive metamorphosed peridotites, rather than the main gabbro body. With this exception, oceanic crust at slow spreading ridge magmatic accretionary segments likely largely conforms to a Penrose model, while that at amagmatic accretionary segments, spreading discontinuities, and transforms have an attenuated stratigraphy, often quite different than the Penrose model of massive pillow basalt, sheeted dikes, isotropic and layered gabbro overlying a massive peridotite tectonite.
DE: 3035 Midocean ridge processes
DE: 3045 Seafloor morphology and bottom photography
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting