In situ evidence for the nature of the seismic layer 2/3 boundary in oceanic crust

Details In situ evidence for the nature of the seismic layer 2/3 boundary in oceanic crust In situ evidence for the nature of the seismic layer 2/3 boundary in oceanic crust

Jul 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994natur.370..288d&link_type=abstract

Nature, Volume 370, Issue 6487, pp. 288-290 (1994).

Mathematics

Logic

36

Scientific paper

THE igneous oceanic crust is typically thought of as comprising two layers: an upper crust ('seismic layer 2') characterized by a rapid increase in seismic velocity with depth, and a thicker lower crust ('seismic layer 3') which is distinguished from layer 2 by both a higher P-wave velocity (6.69 +/- 0.26 km s -l) and a much smaller vertical velocity gradient (<1 km s-1 km-1)1-3. A direct correlation has never been established between this seismic layering and the in situ lithological and physical properties of oceanic crust. The transition between seismic layers 2 and 3 has been variously interpreted as a change in igneous rock texture from doleritic sheeted dykes to gabbro4,5, an increase in metamorphic grade from greenschist- to amphibolite-facies rocks2,6-9, or a change in bulk crustal porosity with depth2,10. We have re-examined available seismic refraction data from around Hole504B, the deepest (>1.8 km) continuous hole drilled into the oceanic crust11-13, and find that at this location the seismic layer 2/3 boundary lies within the sheeted-dyke complex, where it is associated with gradual downhole changes in crustal porosity and alteration, not a litho-logical transition from sheeted dykes to gabbro.

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