The geology of the oceanic crust: Compressional wave velocities of oceanic rocks

A diverse suite of rocks has been recovered from escarpments in the North Atlantic: basalts, dolerites, meta-basalts (zeolite and greenschist-facies), meta-dolerites (greenschist facies), gabbros, meta-gabbros (greenschist- and amphibolite-facies), serpentinized peridotite, and actinolitic rocks. Since these rocks have been sampled from tectonic escarpments, it is difficult, based on petrology alone, to separate those rock types resulting from localized tectonization from those rocks reflecting regional geologic trends. In an attempt to resolve this ambiguity, the Compressional wave velocities (confining pressure range 1 bar to 7 kb) of representative samples were measured as a function of pressure. Compressional wave velocity measurements at confining pressures equivalent to the in situ conditions were compared with published seismic refraction results. These comparisons are compatible with an oceanic basement (3.7–6.0 km/sec) composed of basalt, dolerite, and meta-basalt. The oceanic layer (6.7–6.9 km/sec) is composed of gabbro. The measured velocities of meta-gabbro (greenschist- and amphibolite-facies), serpentinized peridotite, and actinolite-rich rocks are not compatible with the generalized velocity structure of the ocean basins, thus suggesting that these rock types do not occur in abundance within the oceanic crust but instead reflect tectonic processes associated with the escarpments on which they are sampled.