Structure and Formation of the Continental Tectosphere

Abstract

This paper reviews the evidence for deep continental structure and the arguments against simple cooling-plate models as explanations of this structure. High-resolution seismological studies of the upper mantle confirm the existence of a thick (> 300 km) thermal boundary layer (TBL) beneath the ancient cratonic nuclei. Petrological and gravimetric data suggest that the continental TBL is stabilized against convective disruption by a buoyant, viscous, chemical boundary layer (CBL) depleted in basaltic constituents and enriched in large-ion lithophile (LIL) elements relative to the source mantle of mid-ocean ridge vulcanism. Geothermal constraints indicate high heat production within the CBL and low heat flow through its base. It is inferred that very little of the internal heat being convectively transported out of the earth is escaping through the continental cratons. The most plausible mechanism for the formation of this continental tectosphere is by the advective thickening of a basalt-depleted, LIL-charged CBL during major episodes of compressive orogenesis, particularly those accompanying the assembly of supercontinents. The stability of the continental tectosphere and the discrepancy in the heat fluxing from the deep mantle beneath continents and oceans are evidence that the surficial CBL is strongly coupled to large-scale convective flow in the mantle. A model of the earth is proposed consisting of four major dynamical systems: the two convecting shells of the mantle and core, and the two CBLs at the free surface and core-mantle interface. Strong interactions among the low-wave number states of these four systems offer new possibilities for explaining the earth's large-scale, long-term behavior.