ODP-IODP Site 1256 (East Pacific Rise): an in-situ section of upper oceanic crust formed at a superfast spreading rate

The igneous rocks cored in Holes 1256C and 1256D during Ocean Drilling Program (ODP) Leg 206 and Integrated Ocean Drilling Project (IODP) Expedition 309 are dominated by thin (10's of cm to 3 m) basaltic sheet flows separated by chilled margins, with several massive flows (> 3 m thick), minor pillow basalt and hyaloclastites, and rare small dikes. In Hole 1256D, the percentage of massive flows decreases downhole. One notable feature of both holes cored is the presence of a very thick massive lava (̃35 m thick in Hole 1256C and ̃75 m thick in Hole 1256D) near the top of each hole. The structural aspect of this thick massive unit are here described. It has been interpreted as a lava pond emplaced in a topographic depression. Although the ocean crust drilled at Hole 1256C and Hole 1256D partly fits the "Penrose" model for showing the superposition of volcanics on sheeted dikes and intrusives, the relative thickness of the lava-dike sequence could reflect a combination of two or more of the following processes: local (spatial) heterogeneity along ridge-axis, (temporal) variability, essentially linked to a more or less intense activity of the magma chamber, and off-axis eruptions.