The SYN-RIFT Supersequence and Crustal Boundary

Abstract

Isotopic dates of anorthosites from the Moon and meteorites indicate that the age of the Solar System is about 4.6 b.y. Modal lead ages of volcanic rocks and ores suggest that the age of the Earth is 4.55 to 4.6 b.y.; thus the major melting and crustal-forming events closely followed accretion of the Earth and the other planetary bodies. Until 1.0 b.y. ago plate growth may have been accommodated mostly by vertical or horizontal displacements and buckling and shearing of plates (Kröner, 1981). According to Condie (1982), continental rifting began about 2.0 b.y. ago and became widespread about 1.0 b.y ago, when continental fragmentation occurred and oceanic basins were produced and destroyed by sea-floor spreading. This cycle was important for the Pan-African system in northern Africa, Brazil, and other areas around the North Atlantic Ocean. However, most of the Pan-African mobile belts are ensialic (only sial), suggesting that this was a time of transition between dominantly ensialic tectonics and modern plate tectonics. Continental fragmentation followed by sea-floor spreading was the dominant tectonic style with the opening of the North Atlantic (Iapetus oceanic basin) 700 to 500 m.y. ago (Fig. 101). This Paleozoic sea-floor spreading episode was followed by closing of the Iapetus basin and then by renewed sea-floor spreading that initiated the present ocean-floor cycle during the Mesozoic Era (Fig. 101, 103) and produced the present structural configuration of the Atlantic Ocean. The position and morphology of the transition from continental to oceanic crust was controlled by the construction of the mega-continent Pangaea, and the subsequent breakup of this landmass.