Continental Drift, V: Proposed Hypothesis of Earth Tectonics

Abstract

The earth's major tectonic features-orogens, island-arc complexes, and midocean ridges-have formed in response to tangential compression, principally in the lithosphere, but partly in the asthenosphere. This compressive stress field is a response to minor thermal contraction. As the lithosphere accommodates its lower surface to the upper surface of the underlying middle mantle (400-1,000 km), a "surface" called the level of no strain develops between the zone of tension (in the mantle) below and the zone of compression (lithosphere) above. We believe that this surface actually is the "plastic" asthenosphere (low-velocity zone) of the upper mantle. The near absence of this zone beneath the Archean shields implies that the shields are "welded" to the underlying mantle. Cooling and contraction have decreased exponentially through time. Today contraction is limited to two approximately great-circle fracture systems-the Tethyan and the circum-Pacific-and to very mild tectonic activity at and near the midocean ridge crests. Contraction also explains the magnetic anomaly patterns of the ocean basins and continents. The ocean-basin anomalies are roughly concentric around Archean shields. Where they continue into the continents, they "ignore" middle Proterozoic and younger features. Formation of the linear anomaly patterns around shields and in the ocean basins was accomplished by thermoremanent magnetization in a magnetic field of alternating polarity, and in the tangential stress field of the lithosphere and upper asthenosphere. The geometric relations between the magnetic lineations and the Archean nuclei and increasing numbers of Precambrian age dates from the ocean basins provide strong corroboration for the hypothesis presented here.