142 GONDWANALAND AND GONDWANA Tectonic–Climatic Model of the Pangaea-to-Dispersed-Continents Cycle The heightened radioactivity of continental rocks led to the idea that a system of ascending convection (‘monsoonal’) currents beneath an insulating supercontinent would spread out at the top and eventually break the supercontinent into individual continents Changes in the pattern and vigour of mantle convection would increase atmospheric carbon dioxide, creating a greenhouse effect Heat was released in five stages (Figure 12) Stage (platform lacuna) The Earth comprises the single continent (Pangaea) and ocean (Panthalassa) The amounts of spreading and subduction are minimal Minimal turnover of mantle material leads to minimal venting of carbon dioxide and an icehouse climatic state Maximal continental freeboard is due to, first, the short and narrow mid-ocean ridge displacing less water, second, the continental crust having a maximal mean thickness because its ocean frontage of thin (rifted) crust is minimal, and, third, the self-induced Pangaean heat store and accelerated mantle plumes generating a geoid high The Earth is dominated by dry land Stage The heat impounded beneath Pangaea soon leads to localized thinning of the Pangaean crust and lithosphere, initially by sagging of cratonic basement and rifting of orogenic basement (extension I) Stage Continued crustal thinning leads to rifting between the incipient continents (extension II) Stage Pangaea breaks up by spreading of intraPangaean rift oceans to form dispersed continents and oceans The mid-ocean-ridge spreading and subduction are maximal, leading to maximal carbon dioxide venting and greenhouse conditions Low continental freeboard arises from, first, the midocean ridges displacing more water, second, the crust of the continents having a minimal average thickness because its ocean frontage of thin crust is maximal, and, third, the rapidly depleting Pangaean heat store and decelerated mantle plumes supporting a lower geoid The Earth is dominated by ocean Stage The depleted heat store leads to slower spreading and subduction and preferential closing of the rift oceans, so that eventually the continents reform Pangaea and the oceans reform Panthalassa in a return to Stage Earliest Permian (302–280 Ma) Following a lacuna (Pangaean stage 1), the heat beneath Pangaea drove differential subsidence of the Gondwanaland platform (stage 2, extension I) to trap sediment released from the ice (Figure 13) Terranes left the northern margin, and rift zones penetrated East Africa and between India and Australia Granite moved into transtensional rifts, driven by the right-lateral shear between Laurussia and Gondwanaland Coal with the Glossopteris flora succeeds glacigenic sediment in all parts of Gondwanaland except tropical South America and Africa A magmatic and orogenic zone along the Panthalassan margin (Alexander Du Toit’s Samfrau Geosyncline) developed in South America The Sakmarian (288 Ma) postglacial shoreline ran along the margin in North Africa and made broad indentations across Arabia and India (the India– Australia Rift Zone) and narrow indentations across north-west Australia It lapped the magmatic zone of eastern Australia and enclosed a gulf between Australia and Antarctica and another between Antarctica and South America The Gamburtsev upland shed glacial sediment into southern Africa, India, and south-west Australia, and the Beardmore-Ross upland continued to shed sediment into south-east Australia and New Zealand Ice extended across the Congo Basin into North Africa and Arabia, with outwash material along the margin The inception of Gondwanan glaciation is linked to the lowered input of carbon dioxide in Pangaean stage 1, the removal of atmospheric carbon dioxide during accelerated erosion and weathering of the uplands, and the blocked oceanic circulation at the equator Glossopteris of the Gondwana palaeobotanical province made voluminous coals during the entire Permian (Figure 14) The Eurameria province extended into northern South America and Africa, and all three provinces – Eurameria, Gondwana, and Cathaysia – were juxtaposed in Arabia The boundary between the Eurameria and Gondwana provinces approximates the ice limit (Figure 13) The Variscides also contain coal but without Glossopteris Late Permian (255–250 Ma) In the Late Permian the sea retreated to the margins except in the north-east (Figure 15) The magmatic orogen (Gondwanides I) had propagated past northern Queensland It was backed by the Karoo foreland basin, which was occupied by a vast lake with endemic nonmarine bivalves (preserved in the Waterford Formation, which extends into the Estrada Nova Formation of South America) Smaller lakes crossed the rest of southern and central Africa In eastern Australia, volcanogenic sediment flowed across a foreland basin and forebulge into