Paleomagnetic and Rock Magnetic Signature of Upper Oceanic Crust
Generated by Superfast Seafloor Spreading: Results from ODP Leg 206

Gary Acton¹ and Doug Wilson^2
1Department of Geology, University of California, Davis, CA 95616
² Department of Geological Sciences and Marine Sciences Institute, UC Santa Barbara, CA 93106

During Ocean Drilling Program (ODP) Leg 206, Site 1256 (6.736°N, 91.934°W) was cored deep into a 15-Ma section of oceanic crust that is part of the Cocos Plate formed by superfast spreading (>200 mm/yr) at the East Pacific Rise. Three holes penetrated through 250 m of sediment and into igneous basement, with the two deepest holes, Holes 1256C and D, reaching 88.5 and 502 m sub-basement, respectively. The igneous section consists of an uppermost massive ponded flow that is >70 m thick, underlain by thin flows (<3 m thick), massive flows (>3 m thick; some of which may be dikes), pillows, and hyaloclastites. The uppermost units, particularly the massive ponded flow, record an anomalously steep paleomagnetic direction for this near equatorial site and commonly retain only a few percent of their natural remanent magnetization (NRM) after being demagnetized in peak alternating fields (AF) of only 20 mT. The steep direction appears to be the characteristic remanent magnetization (ChRM) as it can be separated from an even steeper drilling overprint. Units below this have shallow inclinations and retain 5% to 15% of their NRM after 20 mT AF, both characteristics being more typical of what would be expected for near equatorial oceanic basalts. Rock magnetic results based on hysteresis, FORC, and thermomagnetic measurements indicate that there are insignificant differences in the paleomagnetic carriers for the units with steep and shallow inclinations. In both cases, pseudo-single domain titanomagnetite and titanomaghemite are the main carriers. This supports the interpretation that the uppermost basalts were extruded in a weak transitional or excursional field. Given that the site was cored 5 km east of the transition zone between marine magnetic anomalies 5Bn.2n and 5Br, the massive ponded lava most likely recorded this transitional field after traveling ~5 km from the spreading axis.