Leg 206

Fast Spreading Crust

Leg 206 is the first leg in a proposed two-leg program designed to penetrate a complete upper crustal section to the gabbro in 15 Ma oceanic crust at one site on a superfast spreading ridge in the eastern Pacific Ocean. When successfully completed, the section at this site will represent one end member of mid ocean ridge evolution, where a steady state melt lens generates the idealized "layercake stratigraphy" of plutonic rocks, underlying a sheeted dike sequence, in turn underlying an extrusive basalt sequence. Leg 206 is dedicated to coring the upper section and initiating a single cased reentry hole in 3650 m water depth. One primary site will be cored and will focus on determining the depth to and nature of the dike/gabbro (layer 2/3) contact or transition zone. The fast spreading rates (~200 mm/yr) at the location should result in this contact being relatively shallow (1300-1800 mbsf). In addition, the age of the lithosphere should result in lower heat flow with depth than was encountered at Site 504B, resulting in reduced thermal stresses during drilling. The rapid sedimentation at the site may have increased cementation and reduced fracturing, also producing more favorable drilling conditions. Other topics which may be investigated on Leg 206 include: fluid flow in and alteration of oceanic crust; petrology and geochemistry of typical oceanic crust; paleomagnetic signature of oceanic crust; the relationship between seismic boundaries and observed lithologic contacts; and further deep biosphere studies.

At the primary drilling target, 240 m of nannofossil ooze will be cored in a pilot hole that will also penetrate as deeply as 20 m into the extrusive basalt section. A second hole will provide cores across the sediment basement interface and to a depth of approximately 100 m into basement. In preparation for a deep penetration effort, a third hole will be cased to 100 m below the sediment basement interface and the remainder of the leg will be directed at achieving maximum depth of penetration while coring. Our tentative goal is to exceed 1km of basement penetration during Leg 206.