Abstract:
Leg 206 will be the first leg of a multi-leg program to recover a complete section of the upper oceanic crust including volcanic extrusive rocks, sheeted dikes, and gabbros as well as the geologically important transition zones between these rock types. Leg 206 is dedicated to coring the upper section of 15-Ma crust on the Cocos plate generated during superfast seafloor spreading ( approximately 200 mm/yr) in the eastern Pacific (6 degrees 44.19'N, 91 degrees 56.06'W; water depth = 3655 m) and will initiate a single cased deep hole for future reentry. This site is optimal for this objective for a number of reasons. First, the depth to axial low-velocity zones, interpreted to be magma chambers at mid-ocean ridges, decreases with increasing seafloor spreading rate. The top of the low-velocity zone, which by inference is the lid of the magma chamber, is thought to correspond to the location of the dike-gabbro transition in normal oceanic crust. Given the superfast spreading rate at the proposed site, the dike-gabbro transition should be relatively shallow, possibly as shallow as 900-1300 m subbasement. Second, the 15-Ma age of the lithosphere should result in lower heat flow with depth than was encountered for the 6-Ma lithosphere in Hole 504B, resulting in reduced thermal stresses during drilling. Third, rapid sedimentation at the site should have increased cementation in the basement rocks, producing favorable drilling conditions. In addition, the resulting sediment thickness of 240 m is more than sufficient for installing a reentry cone with 20-in casing and establishing a reentry hole for deep penetration into basaltic basement. Fourth, the location of the proposed site, <3 days transit from Panama, provides maximum time on site for a 2-month drilling cruise and makes the site more accessible for return visits during future cruises. In one or two legs we will sample the fast spreading rate end-member of mid-ocean-ridge upper crust geometry, where a steady-state melt lens generates the idealized "Penrose stratigraphy" of plutonic rocks underlying a sheeted dike sequence in turn underlying an extrusive basalt sequence. One primary site will be cored and will focus on determining the depth to and nature of the dike-gabbro transition and the seismic Layer 2-Layer 3 transition. Other topics to be investigated during 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 <10 m into the extrusive basalt section. A second hole will provide cores across the sediment basement interface and to a depth of approximately 120 m into basement. In preparation for a deep penetration effort, a third hole will be cased through the sediment and into the upper basement, and the remainder of the leg will be directed at achieving maximum depth of penetration while coring. Our tentative goal is to exceed 600 m of basement penetration, possibly coring as deep as 1 km subbasement, during Leg 206.