Coring an In Situ Section of Upper Oceanic Crust formed by Superfast Seafloor Spreading: Shipboard Results from Ocean Drilling Program Leg 206
Abstract
Drilling a complete section of oceanic crust has been a long term objective since the inception of scientific ocean drilling. Recovery of in situ oceanic crust is imperative to understand igneous accretion and the complex interplay between magmatic, hydrothermal, and tectonic processes, as well as a means for calibrating remote geophysical observations, particularly seismic and magnetic data. Only by drilling a complete section of upper crust formed far from fracture zones can the processes operating at normal mid-ocean ridges be understood. Leg 206 completed the initial phase of a planned two-leg project to drill a complete in situ section of the upper oceanic crust that will eventually extend through the extrusive lavas and sheeted dikes and into gabbros. Drilling was conducted at ODP Site 1256 (6.736N, 91.934W), which resides on 15-Ma oceanic lithosphere of the Cocos plate that was formed by superfast seafloor spreading (>200 mm/yr) at the East Pacific Rise. Given the observation of an inverse relationship between spreading rate and depth to axial low-velocity zones, it should be possible to reach gabbros on a second leg to this site. Preparatory work at Site 1256 recovered a complete sequence of the 250 m sedimentary overburden and 88.5 m of basement in pilot holes (Holes 1256A, B, and C), before we installed a reentry cone with a 16-inch-diameter casing string extending 20 m into basement. Hole 1256D was then drilled to a total depth of 502 m sub-basement with high recovery (50%). The basement section consists of sheet flows and massive flows with subordinate pillow lavas, hyaloclastites, and rare dikes, capped by a sequence of evolved, massive flows (up to 75 m thick), which apparently flowed several kilometers off axis. The lavas have N-MORB compositions and are only slightly affected by low temperature hydrothermal alteration. Our shipboard results include characterization of the petrology, alteration, structure, physical properties, paleomagnetism, chemistry, heat flow, microbiology, and the digital scanning of basalt whole-rounds for core orientation after integration with the wireline logs. A complete suite of geophysical downhole tools, including the first deployment in a basement hole of the Ultrasonic Borehole Imager, confirmed that the hole is in excellent condition. Hole 1256D is open to its full depth (752 m below seafloor) and primed for future deepening into the sheeted dikes and gabbros early in the next phase of ocean drilling.
- Publication:
-
EGS - AGU - EUG Joint Assembly
- Pub Date:
- April 2003
- Bibcode:
- 2003EAEJA.....7721W