Precambrian Tectonic Evolution of Earth: an Outline
Total Page：16
File Type：pdf, Size：1020Kb
J.F. DEWEY, E.S. KISEEVA, J.A. PEARCE AND L.J. ROBB Precambrian tectonic evolution of Earth: an outline J.F. Dewey University College, Oxford, United Kingdom e-mail: [email protected] E.S. Kiseeva School of Biological, Earth and Environmental Sciences, University College Cork, Ireland e-mail: [email protected] J.A. Pearce School of Earth and Ocean Sciences, Cardiff University, United Kingdom e-mail: [email protected] L.J. Robb Department of Earth Sciences, University of Oxford, United Kingdom DSI-NRF CIMERA, University of the Witwatersrand, South Africa e-mail: [email protected] © 2021 Geological Society of South Africa. All rights reserved. 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. SOUTH AFRICAN JOURNAL OF GEOLOGY 2021 • VOLUME 124.1 PAGE 141-162 • doi:10.25131/sajg.124.0019 141 PRECAMBRIAN TECTONIC EVOLUTION OF EARTH: AN OUTLINE 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. Introduction Plate tectonics sensu stricto (Isacks et al., 1968; Le Pichon, 1968; present is the key to the past” perhaps with the notion that plate McKenzie and Parker, 1967; Morgan, 1968; Wilson, 1965) is the tectonics sensu stricto is too good a mechanism to waste. Since relative motion among torsionally rigid plates with narrow the advent of plate tectonics (Wilson, 1965), there has been a boundary deformation zones in the oceans but, commonly, naturally enthusiastic, but perhaps over-zealous, tendency to wider in the continents. Significant intra-plate deformation interpret the whole recorded history of Earth in these terms. in the oceans is rare, except very close to ridge axes, but Uniformitarianism has meant a variety of things to geologists. deformation and wide plate boundary zones are common in the The episodicity, periodicity, and catastrophic geologically- continents, except for the stiff Archaean cratons with a thick instantaneous nature of many, if not most, processes indicates lithosphere, which are almost earthquake-free and commonly that uniformitarianism is not strictly true, though this statement wrapped around by younger orogenic terrains. This definition depends upon the time period over which processes are of plate tectonics, involving torsional rigidity with substantial differentiated and integrated. It is clear that tectonic processes deformation and metamorphism largely confined to relatively were not uniform through time and that there have been narrow boundary zones, is its very essence and that which we various forms of secular evolution, as shown in the compilation follow strictly in this paper. We acknowledge that other forms by Bradley (2011) of “everything through time”. As Bradley of lesser non-rigid mobilism as envisaged, for example, by demonstrates, heat production, the temperature of the Bédard (2006), probably took place on Archaean Earth and may asthenosphere, and heat flow have evolved with time whatever have involved block fragmentation and jostling. Subduction form of tectonics was operating, for example from three times zones involve the insertion of torsionally-rigid lithospheric slabs the heat production in the early Archaean compared to today, into the asthenosphere, but it is likely that other ways of with a corresponding progressive thickening and stiffening of transporting surface and shallow materials into the mantle were the lithosphere. prevalent during the Archaean; these have been called There is a plethora of opinions on the existence and origin sagduction, drip tectonics, or foundering. of plate tectonics, most of which do not adhere to the sensu Plate tectonics is a highly-efficient mechanism for global stricto definition of torsional rigidity and narrow boundaries. heat-loss by magmatism and hydrothermal convection at They tend to ignore that most Archaean geological patterns the oceanic ridges and cooling by subduction, with minor are blobby and rarely form narrow belts between older conductive heat-loss, supplemented, episodically, by intra-plate cratons. There are exceptions, however, such as the superb magmatism in large igneous provinces and hotspots above and challenging reviews of the Archaean by Bleeker (2002) and mantle plumes. If there was a time on Earth before plate Hamilton (1998). tectonics, neither conduction nor radiation can account for the The following is a partial list of opinions on the start time necessary heat-loss so that the only credible mechanism would of plate tectonics. At the ‘young end’, Hamilton (2003) and Stern have been pervasive plume upwelling and related massive mafic (2005; 2020) argue, from plate subductability, blueschists, ruby, magmatism with, possibly, some form of “oceanic” crustal- jadeite, and ophiolites, that modern-style plate tectonics began lithospheric fragmentation and foundering, to transport surface in the Neo-Proterozoic (0.8 Ga), preceded by a stagnant lid materials into the mantle. Mantle temperature and heat-loss have during the so-called ‘boring billion’ (1.8 to 0.8 Ga), itself been diminishing since the origin of Earth (Bickle, 1978; Bickle, preceded by a period of plate tectonics from about 2.1 to 1.8 Ga, 1986; Korenaga, 2013); therefore, one might expect changes in then a stagnant lid before that. Hamilton (2007) argues for a tectonic style, involving a stagnant non-segmented lithospheric 2.0 Ga start based upon the first large, narrow deformation belts lid changing to plate tectonics as Earth cooled. such as the Limpopo Belt. Burke and Dewey (1973), Dewey Planetary-scale plate tectonics in our solar system appears (2018, 2019), Brown et al. (2020) all argue for a start between to be restricted to Earth at the present time but may have taken 2.2 and 2.1 Ga based on a range of structural and metamorphic place in other silicate planets (especially Venus) in the past and criteria. Grieve (1980) argues for a Neoarchaean start. A large may be occurring now in moons of the giant gas planets. There group favour the Palaeoarchaean to Neoarchaean transition, has been a natural tendency to lean on the doctrine of “the from 3.2 to 3.0 Ga (Dhuime et al., 2011, 2012, 2015; Condie and 142 SOUTH AFRICAN JOURNAL OF GEOLOGY J.F. DEWEY, E.S. KISEEVA, J.A. PEARCE AND L.J. ROBB Kröner, 2008; Cawood et al., 2006; Dewey, 2007; Pease et al., geochemistry and petrology, that define tectonic regimes 2008; Smithies et al., 2005a, 2007; Van Kranendonk et al., 2007). and involve mostly field observations and geological maps. In There are advocates for the Early Archaean mostly on the simple terms, the question is asked “do geological patterns, at all basis of uniformitarian magmatic and structural comparitors, scales, resemble those that we know were generated by plate including: de Wit (1991) for a 3.4 Ga start; Nutman et al.