HR: 13:55h
AN: T33G-02 INVITED     [Abstracts]
TI: The Plutonic Foundation of a MAR Ridge Spreading Segment: The Kane Oceanic Core Complex
AU: * Dick, H J
EM: hdick@whoi.edu
AF: Woods Hole Oceanographic Institution, Dept. of Geology & Geophysics, Woods Hole, MA 02543 United States
AU: Tivey, M A
EM: mtivey@whoi.edu
AF: Woods Hole Oceanographic Institution, Dept. of Geology & Geophysics, Woods Hole, MA 02543 United States
AU: Tucholke, B E
EM: btucholke@whoi.edu
AF: Woods Hole Oceanographic Institution, Dept. of Geology & Geophysics, Woods Hole, MA 02543 United States
AU: Cheadle, M J
EM: cheadle@uwyo.edu
AF: University of Wyoming, Dept. of Geology & Geophysics, Laramie, WY 82071 United States
AB: We mapped an oceanic core complex on the west flank of the Mid-Atlantic Ridge exposing the plutonic foundation of an ancient - 35 km long 2nd order ridge volcanic segment. We identify a restricted zone of melt transport through the mantle to the crust at ~23° 23'N 45° 24'W marked by dunite and spatially associated primitive troctolite. This indicates focused melt flow through the shallow mantle to a narrow ~ 10 km wide zone beneath an ancient volcano now situated on crust of the same age on the opposing tectonic plate. To the north, from, 23° 26'N to 23° 31'N, primitive gabbro is rare, massive depleted peridotite tectonites abundant and dunite nearly absent: showing that little melt crossed the crust-mantle boundary. Abundant highly differentiated ferrogabbro veins cross-cut this mantle section, while diabase, generally altered in the greenschist facies, is scattered over the detachment surface: evidence of lateral melt transport by diking through the crust and shallow mantle to feed the volcanic carapace north of the principle volcanic center. Further north, adjacent to the transform is a secondary magmatic center with less primitive gabbro. The latter is on strike with a larger gabbroic center at the present day inside corner high at the eastern ridge-transform intersection of the Kane F.Z. The secondary magmatic center lacks a significant volcanic carapace on the opposing tectonic plate, but is the locus of off-axis volcanism associated with the formation of intersecting faults formed during uplift of the complex at the ancient inside-corner high. A continuous gabbroic lower crust, then, did not likely form beneath the ancient accretionary segment. Instead, the core complex exposes a crustal section unlike the classic Penrose ophiolite succession of pillow lavas, sheeted dikes and gabbros. Even where this may have existed beneath the ancient volcanic center, with primitive troctolites intruded to the base of the sheeted dikes, it is unlike the internal stratigraphy of many ophiolites where high-level gabbros are typically evolved. The MARK area of the MAR has long been taken as a classic section of "normal slow-spreading ridge. Our results, then, supports the idea that slow-spreading lower ocean crust is atypical of that found in "well-preserved" sections of ancient ocean crust in ophiolite complexes (e.g.: Karson, 1998; Lagabrielle et al., 1998), but shows that the oceanic crust at slow-spreading ridges varies in composition and architecture in both space and time - and cannot be characterized by a single model section.
DE: 3000 MARINE GEOLOGY AND GEOPHYSICS
DE: 3035 Midocean ridge processes
DE: 3600 MINERALOGY AND PETROLOGY
DE: 8000 STRUCTURAL GEOLOGY
SC: Tectonophysics [T]
MN: Fall Meeting 2005