HR: 14:05h
AN: U12A-02    [PDF]
TI: First Results From Hole 1256D - a New Ocean-Crust Reference Hole Drilled in Fast-Spread Crust During ODP Leg 206
AU: * Wilson, D S
EM: dwilson@geol.ucsb.edu
AF: Dept. Geological Sci., UCSB, Santa Barbara, CA 93106 United States
AU: Teagle, D A
EM: dat@soc.soton.ac.uk
AF: Southampton Oceanography Ctr, Univ Southampton, Southampton, SO14-3ZH United Kingdom
AU: Acton, G D
EM: acton@geology.ucdavis.edu
AF: Dept Geology, UC Davis, Davis, CA 95616 United States
AB: Sampling a complete section of oceanic crust has been a long term objective since the inception of scientific ocean drilling. Recovery of in situ ocean crust is imperative to understand igneous accretion and the complex interplay between magmatic, hydrothermal, and tectonic processes, as well as a means for calibrating remote geophysical observations, especially seismic and magnetic data. Only by drilling a complete section of upper crust formed in a simple tectonic setting can the processes operating at normal mid-ocean ridges be understood. Leg 206 completed the initial phase of a planned multi-leg project to drill a complete in situ section of the upper ocean crust that will eventually extend through the extrusive layer and sheeted dikes and into gabbros. Drilling was conducted at Site 1256 (6.7N, 91.9W), which resides on 15-Ma oceanic lithosphere of the Cocos plate that was created by superfast seafloor spreading (~220 mm/yr). Given the inverse relationship that exists between spreading rate and depth to axial magma chambers, it should be possible to reach gabbros on a second leg to this site, at depths $<$ 1400 m sub-basement. On Leg 206, we installed a reentry cone with a 16-inch-diameter casing string that extends into basement in Hole 1256D. Beside coring the 250 m of sediment and 88.5 m of basement in pilot holes (Holes 1256A, B, and C), we cored 502 m sub-basement in Hole 1256D. The basement section consists of massive and sheet flows with subordinate pillow lavas, hyaloclastites, and rare dikes that are capped by more evolved, ponded massive flows up to at least 75-m thick. The lavas have N-MORB compositions and exhibit typically slight, low-temperature alteration effects, with little oxidation. The capping sequence found at the top 100 m of basement is chemically and magnetically distinct, remarkably unfractured, and the great flow thickness requires existing abyssal-hill topography, all suggesting deposition at least 5 km from the axis. The low proportion of pillow lavas ($<$10%) indicates typically rapid lava emplacement, consistent with thermal-model predictions of less than 1-km vertical thickness of the dike zone. A complete suite of geophysical wire-line logs confirmed that Hole 1256D is in excellent condition, open to its full depth (752 m below seafloor), and ready for future deepening into the sheeted dikes and gabbros early in the next phase of ocean drilling.
DE: 3035 Midocean ridge processes
SC: U
MN: 2003 Fall Meeting