| Abstract: | Laboratory samples from the upper oceanic crust (tholeiitic basalt flows) that have not been significantly weathered, hydrothermally altered or fractured have a typical Poisson's ratio of 0.30 (
) and a compressional velocity of 6.0 km s−1; from the middle crust (dolerite sheeted dykes) a ratio of 0.28 (
) and a velocity of 6.7 km s−1; from the lower crust (gabbro) a ratio of 0.31 (
) and a velocity of 7.1 km s−1; and from the uppermost mantle a ratio of 0.24 (
) and a velocity of 8.4 km s−1. These sample values are representative of the large scale insitu values for the middle and lower crust and for the upper mantle. The upper crust is modified by several processes that decrease the velocity and generally increase Poisson's ratio: (1) the formation of an irregular layer of low temperature weathering generally less than 50 m thick; (2) large scale porosity in the form of drained pillows and lava tubes, of talus and rubble and of large open fractures; (3) where there was a high sedimentation rate over the ridge that formed the crust, hydrothermal alteration and intercalation of basalt and sediments. The Poisson's ratios of both high velocity sediments and of crystalline continental crustal rocks generally are significantly lower than the ratios of oceanic crustal rocks of similar compressional wave velocity. Thus, the use of shear wave velocities should permit the separation of these different formations which frequently cannot be distinguished on the basis of compressional wave seismic refraction data alone. |
