Preliminary Results on the Structure of Ocean Crust from new Holes Drilled in Fast-Spread Crust During ODP Leg 206

ODP Leg 206 successfully accomplished the initial phase of a multi-leg drilling program that aims to sample a complete section of upper oceanic crust through the extrusive lavas, the sheeted dike complex, and into the 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). Two holes, 1256C and 1256D were drilled into the basement to a depth of 340.3 mbsf (89.6 m sub-basement) and 752 mbsf (502 m sub-basement) respectively. The main stratigraphy of upper oceanic crust at Site 1256 consists of a sequence of massive flows and thin sheet flows with minor amounts of pillow basalts and breccias. The sequence is slightly altered and has N-MORB composition. Structural analysis carried out on board on the recovered cores from both Holes 1256C and 1256D revealed the occurrence of primary igneous as well as post-magmatic structures. Primary igneous features include magmatic fabrics, laminations and flattened vesicles, folds and shear-related structures, late magmatic veins, and fracturing. Postmagmatic structures include veins, shear veins, microfaults, joints, and breccia. Veins are the most prominent structural features and show a variety of morphologies ranging from planar and curviliear to anastomosing. In many cases veins are oriented in en echelon, and Riedel-shear arrays, giving in such case useful shear indications. Shear veins are mostly present in massive coarser-grained lithologic units and are filled with fibrous clay minerals. Shear veins and microfaults indicate both strike-slip and oblique apparent senses of shear. In Hole 1256D shear veins show a change in the sense of shear, from reverse to normal, from ~645 mbsf to the bottom of the hole. More than 600 veins and joints from the basement units of Hole C and more than 1700 features from Hole D were measured in the archive half relative to the core barrel reference frame. True dip data show that structures inHole 1256D are mostly gently dipping, having most common frequency dip angles of ~15° . Other dip angles are represented nearly by the same frequency throughout the hole. In Hole 1256C, true dip angles show a maximum in frequency between 10° and 20° ; however, dip values around 50° -55° and 90° are common as well. Late magmatic veins are mostly gently dipping in the two holes, showing the highest frequency at 15° and 5° . By contrast, shear veins are moderately to steeply dipping in the two holes (maximum frequency ranges from 45° to 75° ). In Hole 1256C, the distribution of true dips per lithologic unit (i.e., massive flows and sheet flows vs. pillowed, brecciated or glassy units) shows that, in the upper units (sheet flows), the dip values are bimodally distributed in sets making an angle of 50° -60° . This is linked to the presence of conjugate systems of veins in the upper part of the hole, whereas in the middle and lower parts, true dips are mostly clustered in one group. In the lower three igneous units (sheet flows), structures mainly have gentle dips. In Hole 1256D, the distribution of true dip angles with depth does not show any systematic variation. The variation in dips of the veins and in their density seem to be related mainly to the physical properties and morphology of the lithologic units rather than to the depth of their occurrence. Further investigations and elaboration of our results are in progress, with the aim of understanding the link between structural features and local vs. regional stress field.