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High-resolution magnetic imaging of extrusive and intrusive crust at Hess Deep and a comparison with Juan de Fuca and Hole 504B results
M A Tivey1 and G L Christeson21Woods Hole Oceanographic Institution, Woods Hole, MA 02543 USA
2Institute for Geophysics, University of Texas, Austin TX 78759
Deep submersible ALVIN dives in 1999 carried out magnetometer profiles of the exposed crustal section on the northern scarp of Hess Deep in the eastern Pacific Ocean. The scarp has exposures of ocean crust for both the transition from extrusive lava to intrusive sheeted-dike and the transition from sheeted-dike to gabbro. Approximately seven dives yielded interpretable magnetic results. The age of the crustal section surveyed is between 1 M.y. and 1.7 M.y. which implies that the crust should be comprised of predominantly reversely polarized remanent magnetization. Dive 3372 represents the most easterly and oldest crustal traverse and crossed both extrusive crust and sheeted dikes including the intervening 75 m thick transition zone. The extrusives appear to be uniformly reversely magnetized at 12 A/m and overlie a relatively weak to non-magnetic transition zone. The underlying sheeted dike section is clearly reversely magnetized with a slightly greater intensity of about 15 A/m. Dive 3380 is the most westerly and youngest crustal traverse (~1 M.y.) and shows multiple polarity units within the extrusive lava section i.e. a normal polarity zone sandwiched between reversely polarized lavas. This normal zone could be the Jaramillo chron. The underlying transition zone is also only weakly magnetized.
All the Hess deep dives tend to show that the sheeted-dike section is intensely magnetized, equivalent or more magnetic than the extrusive section. This observation contrasts with the results from the Blanco fracture zone on the Juan de Fuca ridge which found that the dikes underlying the extrusives were only very weakly magnetized (< 1A/m). However, this discrepancy may be explained by the lack of exposure of a true sheeted-dike section at the Blanco compared with the clearly defined exposures of sheeted dikes at Hess deep. The Hess deep dives show that the transition between the extrusive and dike sections are very weakly magnetized which is compatible with results from Hole 504B. The weakly magnetized dikes underlying the extrusives at Blanco may therefore be more representative of the transition zone rather than the sheeted dike section. This pattern may thus be diagnostic of the transition zone within ocean crust.
Magnetic profiles across gabbro exposures also show that they are magnetized although possibly to a lesser degree than the dike section.
Last Modified: November 24, 1999
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