The nature of the Mohorovicic discontinuity, A compromise

The available experimental data and steady-state calculations make it difficult to explain the M discontinuity beneath both oceans and continents on the basis of the same phase change. The oceanic M discontinuity may be a chemical discontinuity between basalt and peridotite, and a similar chemical discontinuity may thus be expected beneath the continents. Since available experimental data place the basalt-eclogite phase change at about the same depth as the continental M discontinuity, intersections may exist between a zone of chemical discontinuity and a phase transition zone, the transition being either basalt-eclogite or feldspathic peridotite-garnet peridotite. Detection of the latter transition by seismic techniques may be difficult. The M discontinuity could therefore represent the basalt-eclogite phase change in some localities (e.g. mountain belts) and the chemical discontinuity in others (e.g. oceans and continental shields). Variations in the depth to the chemical discontinuity and in the positions of geoisotherms produce great flexibility in orogenetic models. Intersections between the two zones at depth could be reflected at the surface by major fault zones separating large structural blocks of different elevations