Abstract
Space probes in our solar system have examined all bodies larger than about 400 km in diameter and shown that Earth is the only silicate planet with extant plate tectonics sensu stricto. Venus and Earth are about the same size at 12 000 km diameter, and close in density at 5 200 and 5 500 kg.m-3 respectively. Venus and Mars are stagnant lid planets; Mars may have had plate tectonics and Venus may have had alternating ca. 0.5 Ga periods of stagnant lid punctuated by short periods of plate turnover.
In this paper, we contend that Earth has seen five, distinct, tectonic periods characterized by mainly different rock associations and patterns with rapid transitions between them; the Hadean to ca. 4.0 Ga, the Eo- and Palaeoarchaean to ca. 3.1 Ga, the Neoarchaean to ca. 2.5 Ga, the Proterozoic to ca. 0.8 Ga, and the Neoproterozoic and Phanerozoic. Plate tectonics sensu stricto, as we know it for present-day Earth, was operating during the Neoproterozoic and Phanerozoic, as witnessed by features such as obducted supra-subduction zone ophiolites, blueschists, jadeite, ruby, continental thin sediment sheets, continental shelf, edge, and rise assemblages, collisional sutures, and long strike-slip faults with large displacements. From rock associations and structures, nothing resembling plate tectonics operated prior to ca. 2.5 Ga. Archaean geology is almost wholly dissimilar from Proterozoic-Phanerozoic geology. Most of the Proterozoic operated in a plate tectonic milieu but, during the Archaean, Earth behaved in a non-plate tectonic way and was probably characterised by a stagnant lid with heat-loss by pluming and volcanism, together with diapiric inversion of tonalite-trondjemite-granodiorite (TTG) basement diapirs through sinking keels of greenstone supracrustals, and very minor mobilism. The Palaeoarchaean differed from the Neoarchaean in having a more blobby appearance whereas a crude linearity is typical of the Neoarchaean. The Hadean was probably a dry stagnant lid Earth with the bulk of its water delivered during the late heavy bombardment, when that thin mafic lithosphere was fragmented to sink into the asthenosphere and generate the copious TTG Ancient Grey Gneisses (AGG). During the Archaean, a stagnant unsegmented, lithospheric lid characterised Earth, although a case can be made for some form of mobilism with “block jostling”, rifting, compression and strike-slip faulting on a small scale. We conclude, following Burke and Dewey (1973), that there is no evidence for subduction on a global scale before about 2.5 Ga, although there is geochemical evidence for some form of local recycling of crustal material into the mantle during that period. After 2.5 Ga, linear/curvilinear deformation belts were developed, which “weld” cratons together and palaeomagnetism indicates that large, lateral, relative motions among continents had begun by at least 1.88 Ga. The “boring billion”, from about 1.8 to 0.8 Ga, was a period of two super-continents (Nuna, also known as Columbia, and Rodinia) characterised by substantial magmatism of intraplate type leading to the hypothesis that Earth had reverted to a single plate planet over this period; however, orogens with marginal accretionary tectonics and related magmatism and ore genesis indicate that plate tectonics was still taking place at and beyond the bounds of these supercontinents. The break-up of Rodinia heralded modern plate tectonics from about 0.8 Ga. Our conclusions are based, almost wholly, upon geological data sets, including petrology, ore geology and geochemistry, with minor input from modelling and theory.
Air Pollution and Sea Pollution Seen from Space
