Perturbed mantle: a unifying characteristic of plate boundaries

Abstract

Ocean-spreading ridge-, basin-and-range-, graben-, young-alpine-, island-arc-, and intra-continental- geosynclinal types of terrestrial crust are indicative of 'unstable' lithosphere. Whether undergoing tensional or compressional tectogenesis, these areas have certain geological and geophysical characteristics in common. Observations and measurements include anomalously high heat flow (>1.8 HFU), shallow intermediate P-wave velocities (7.2-7.8 km/s), low Q (i.e. a high attenuation factor), attenuated S-wave velocities (<4.5 km/s), and high electrical conductivity. Most 'unstable' areas are also marked by shallow seismicity and outpourings of basaltic volcanics which testify to shallow mantle sources. The data suggest welts of raised mantle as a common denominator under all types of 'unstable' crust. Therefore, seismicity, which presently defines the plate boundaries in plate tectonic theory is only a conspicuous effect of a deeper and more fundamental global phenomenon. I propose that the concept of plate boundaries be redefined on the basis of the wider range of geological and geophysical characteristics indicative of sublithospheric perturbations.