Drilling of superfast spreading rate ocean crust during Leg 206 will characterize the nature of magmatic accretion and the primary and secondary chemical composition, as well as the tectonic and seismic structure of the uppermost oceanic crust (the target depth for Leg 206 is 600800 m subbasement). These cores will provide an essential link to relate geology to remote geophysical observations (seismics and magnetics) and ground-truth the relationship between seismic stratigraphy and lithostratigraphy. Paleomagnetic studies will establish the relative contributions of the major lithologic units to marine magnetic anomalies and the position of our site (~50 k.y. from a magnetic reversal) will provide information on crustal cooling rates and the contribution of deep plutonic rocks to surface magnetic anomalies. The holes drilled during Leg 206 will provide the first test of the lateral variability of the ocean crust and provide an essential comparison for the models of crustal accretion, hydrothermal alteration, and the secondary mineral/metamorphic stratigraphy principally developed from ODP Hole 504B. This will refine models for the vertical and temporal evolution of ocean crust, including the recognition and description of zones of hydrothermal and magmatic chemical exchange. Physical properties measurements of cores recovered from fast-spreading ocean crust will yield information on the porosity, permeability, and stress regime as well as the gradients of these properties with depth. A full suite of wireline logs will supplement geological, chemical, structural, and magnetic observations and physical properties studies on the core. The careful integration of borehole observations with measurements of the recovered core is imperative for the quantitative estimation of chemical exchange fluxes between the ocean crust and oceans.
A major objective of Leg 206 is to establish a cased reentry hole that is open for future drilling to the total depth penetrated during the leg. Though not impossible, the total depth will unlikely be deep enough to reach the dikegabbro transition zone during Leg 206. Our efforts, however, will provide the groundwork for a second leg to return to the site and investigate the geological nature of the geophysically imaged "axial melt lens" believed to be present close to the gabbrodike transition. Drilling of this boundary in situ will allow the relationships between vigorous hydrothermal circulation, mineralization, dike injection, and the accretion and freezing of the plutonic crust to be investigated.
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