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 approximately 11-20 m.y. ago, Site 1256 (6.736 degrees N, 91.934 degrees W) on approximately 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 ( approximately 300 m), the upper gabbros, thought to be the frozen axial melt lens, are predicted at approximately 1275-1550 meters below seafloor (mbsf). To fully characterize the sedimentary overburden and establish depths for the casing strings, three pilot holes were cored, recovering a nearly complete section of the 250.7 m of sediment overlying basement and penetrating 88.5 m into basement with very good recovery (61.3%). The sediments can be subdivided into two main lithologies: Unit I (0-40.6 mbsf) is clay rich with a few carbonate-rich intervals, whereas Unit II (40.6-250.7 mbsf) is predominantly biogenic carbonate. Following installation of a reentry cone with a 16-in-diameter casing string that extended 20 m into basement in Hole 1256D, >500 m of young Pacific extrusive lavas was cored with moderate to high rates of recovery. These upper lavas comprise sheet flows with subordinate pillow lavas, hyaloclastites, and rare dikes, capped by an evolved, massive flow >74 m thick and other sheet flows that probably ponded in small faulted depressions several kilometers off axis. The lavas have normal mid-ocean-ridge basalt chemistries, with evolved compositions more common upsection, and heterogeneous incompatible element ratios. Hole 1256D was exited cleanly, leaving the hole clear of debris, open to its full depth, and primed for the future deepening into the sheeted dikes and gabbros that took place early in the next phase of scientific ocean drilling during Integrated Ocean Drilling Program Expeditions 309 and 312. This Scientific Results volume comprises a collection of papers, mostly data reports, of the initial postcruise research undertaken by the ODP Leg 206 science party. This synthesis also briefly describes research published to date in the external scientific literature. With follow- up cruises to deepen Hole 1256D occurring soon after Leg 206, we anticipate that numerous future manuscripts and data that investigate the complete upper crustal section at Site 1256 will soon be available in the wider scientific literature.

West: -91.5600 East: -91.5600 North: 6.4400 South: 6.4400

Expedition: 206

Site: 206-1256

Provider: SEDIS Publication Catalogue

Data set link: http://sedis.iodp.org/pub-catalogue/index.php?id=10.2973/odp.proc.sr.206.001.2007 (c.f. for more detailed metadata)

Data download: application/pdf

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