Hales discontinuity

The Hales discontinuity is a presumed, locally limited boundary layer between two rock layers of the uppermost mantle . It is defined by an increase in seismic velocities at a depth of approx. 80–100 km. It was named after Anton L. Hales , a South African- born geophysicist who discovered such a boundary layer under the central United States in 1969 using seismic methods . Hales explained the increase in seismic speed with an increase in the density of the rock. This, in turn, he traced back to a phase transition from spinel to garnet in the rock. However, theoretical analyzes show that the calculated impedance contrast of this transition is insufficient to explain the observation, so other mechanisms must be considered.

Similar phenomena within the lithosphere have been observed in the Arabian Shield as well as in the Canadian Craton - the old mainland core. Here the speed increases are explained by other effects, such as mineral conversion through metasomatosis or anisotropic propagation speeds in the rock. Due to the few observations and their locally very limited extent, the Hales discontinuity is of little importance with regard to the structures and the predominant processes in the interior of the earth's body.

Individual evidence

1. ↑ Hales, AL, 1969: A seismic discontinuity in the lithosphere. In: Earth and Planetary Science letters , Vol. 7, pp. 44-46
2. ↑ Stixrude, L .; Lithgow-Bertollini, C., 2005: Mineralogy and elasticity of the oceanic upper mantle: Origin of the low-velocity zone. In: Journal of Geophysical Research , Vol. 110, B03204, doi : 10.1029 / 2004JB002965 .
3. ↑ Sandvol, E., Seber, D., Barazangi, M., Vernon, F., Mellors, R., & Al-Amri, A., 1998: Lithospheric seismic velocity discontinuities beneath the Arabian Shield. In: Geophysical Research Letters , Vol. 25, pp. 2873-2876
4. ↑ Bostock, MG, 1998: Mantle stratigraphy and evolution of the Slave province. In: Journal of Geophysical Research , Vol. 103 (B9), pp. 21183-21200