Leg 206 synthesis: initiation of drilling an intact section of upper oceanic crust formed at a superfast spreading rate at Site 1256 in the eastern equatorial Pacific

Abstract

Drilling a complete section of oceanic crust has been an unfulfilled ambition since the inception of scientific ocean drilling. Recovery of in situ oceanic crust is imperative to understanding igneous accretion and the complex interplay between magmatic, hydrothermal, and tectonic processes, as well as providing a means for calibrating remote geophysical observations, particularly seismic and magnetic data. Ocean Drilling Program (ODP) Leg 206 was the initial phase of the Superfast Spreading Rate Crust campaign to ODP Site 1256, a multicruise program to exploit the observed relationship between the depth to axial low-velocity zones imaged at active mid-ocean ridges and spreading rate. Because of the known difficulties of drilling young ocean basement, targeting a region where gabbros should occur at their shallowest optimizes the chances of reaching gabbro in intact ocean crust by reducing the distance to be drilled and the time required on site. Following the recent recognition of an episode of superfast spreading (200–220 mm/yr) on the East Pacific Rise ~11–20 m.y. ago, Site 1256 (6.736°N, 91.934°W) on ~15-Ma oceanic lithosphere of the Cocos plate was identified as the optimal site for a new deep drill hole into ocean crust. Even allowing for significant burial by lavas that flowed off axis (~300 m), the upper gabbros, 1Teagle, D.A.H., and Wilson, D.S., 2007. Leg 206 synthesis: initiation of drilling an intact section of upper oceanic crust formed at a superfast spreading rate at Site 1256 in the eastern equatorial Pacific. In Teagle, D.A.H., Wilson, D.S., Acton, G.D., and Vanko, D.A. (Eds.), Proc. ODP, Sci. Results, 206: College Station, TX (Ocean Drilling Program), 1–15. doi:10.2973/odp.proc.sr.206.001.2007 2National Oceanography Centre, School of Ocean and Earth Science, University of Southampton, European Way, Southampton S014-3ZH, UK. dat@noc.soton.ac.uk 3Department of Earth Science and Marine Science Institute, University of California, Santa Barbara, Santa Barbara CA 93106-9630, USA. Initial receipt: 15 December 2006 Acceptance: 29 April 2007 Web publication: 4 June 2007