When did plate tectonics begin? Evidence from the orogenic record
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Evidence of modern-style plate tectonics is preserved in the continental rock record as orogens and rifts; these orogens represent regions of mountain building resulting from compression between converging plates. Recognition of orogens in the ancient rock record can help identify when plate tectonics began on Earth. Evidence of Paleoproterozoic collisional orogeny is widely accepted. The development, however, of Archean collisional orogens is highly controversial, as is the operation of plate tectonics in general. We review the tectonic evolution of three well-studied Archean terranes—the Pilbara craton of Western Australia, the Barberton granite-greenstone terrane of South Africa, and the Superior Province of Canada—in terms of their geological development and evidence for Archean collisional and accretionary platetectonic processes in the context of secular evolution of the planet. The Pilbara craton preserves geological, geochemical, and geochronological evidence for continental rifting at 3.2 Ga, development of an oceanic-arc subduction complex at 3.12 Ga, and terrane accretion at 3.07 Ga. The Barberton granite-greenstone terrane of the Kaapvaal craton provides thermobarometric evidence for subduction-related high-pressure–low-temperature metamorphism juxtaposed against medium-pressure–high-temperature metamorphism associated with exhumation of high-grade rocks via orogenic collapse, which together are interpreted to represent a paired metamorphic belt. The Superior Province in the Canadian Shield records widespread accretionary and collisional assembly at ca. 2.7 Ga. This evidence argues for "modern-style" plate tectonics on Earth since at least 3.2 Ga.Keywords:
Orogeny
Rodinia
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<p>In a geodynamic, geological and geophysical review of global Archean cratons, we find that the survival of Archean cratons depends on the initial conditions of their formation, as well as the tectonic processes to which they were exposed.&#160; In a sense, we must consider both their nature and how they were nurtured.&#160; In a review of existing literature and models, we use stability regime diagrams to understand the factors that contribute to the intrinsic strength of a craton: buoyancy, viscosity, and relative integrated yield strength. We find that cratons formed in the Archean when thermal conditions enhanced extraction of large melt fractions and early cratonization promoted the formation of stable Archean cratonic lithosphere.&#160; In terms of the cratons' nurturing, processes that may have modified and weaken cratonic lithosphere include subduction and slab rollback, rifting, and mantle plumes, as these processes introduced materials and conditions that warmed and metasomatized the lithosphere.&#160; Examining four Archean cratons that are more stable, and four that are categorized as modified or destroyed, we note that continental lithosphere that was cratonized prior to the end of the Archean has more potential to survive deformation during the last 500 My. Although, the survivability of these cratons is highly dependent on their unique positions relative to larger scale tectonic processes, such as subduction.&#160;&#160; We also observe that once an Archean craton begins to undergo even a small amount of modification, it is more likely to continue to be modified, as it loses the preservation advantage that it had upon birth.<br><br></p>
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The original connections of Archean cratons are becoming traceable due to an increasing amount of paleomagnetic data and refined magmatic barcodes. The Uauá block of the northern São Francisco craton may represent a fragment of a major Archean craton. Here, we report new paleomagnetic data from the 2.62 Ga Uauá tholeiitic mafic dyke swarm of the Uauá block in the northern São Francisco craton, Eastern Brazil. Our paleomagnetic results confirm the earlier results for these units, but our interpretation differs. We suggest that the obtained characteristic remanent magnetization for the 2.62 Ga swarm is of primary origin, supported by a provisionally-positive baked contact test. The corresponding paleomagnetic pole (25.2°N, 330.5°E, A95 = 8.1°, N = 20) takes the present northern part of the São Francisco craton to moderate latitudes. Based on the comparison of the paleolatitudes of cratons with high-quality paleomagnetic data and magmatic barcodes, we suggest that the northern part of the São Francisco craton could have been part of the proposed Supervaalbara supercraton during the Archean. Supervaalbara is proposed as including (but not limited to) the part of the São Francisco craton as well as the Superior, Wyoming, Kola + Karelia, Zimbabwe, Kaapvaal, Tanzania, Yilgarn, and Pilbara cratons.
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The boundaries of rigid cratons can be affected by subsequent orogenic events, leading to ‘metacratonic’ characteristics not often properly recognized and still poorly understood. Major lithospheric thickening is absent and early events such as ophiolites are preserved; however, metacratonic boundaries are affected by major shear zones, abundant magmatism and mineralizations, and local high-pressure metamorphism. West Africa, marked by the large Eburnian ( c. 2 Ga) West African craton, the absence of Mesoproterozoic events, the major Pan-African (0.9–0.55 Ga) mobile belts that generated the Peri-Gondawanan terranes, and the weaker but enlightening Variscan and Alpine orogenies, is an excellent place for tackling this promising concept of metacratonization. The papers in this book consider most of the West African craton boundaries, from the reworking of the Palaeoproterozoic terranes, through the Pan-African encircling terranes, the late Neoproterozoic-early Palaeozoic extension period and the Peri-Gondwanan terranes, the Variscan imprint to the current situation.
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