Trailmarkers for arctic

Abstract

The stratigraphic succession of the 3580-2770 m.y. old granite-greenstone terrain of the Pilbara Block is re-assessed by the first- and second-order sequence-stratigraphic techniques that are the standard for Phanerozic terrains, with the view to testing the conformity of stratigraphic cause and response. A holistic tectonostratigraphic framework is presented that pairs the supracrustal and crustal rock records. The resulting sequence scheme is tested against the cause of first-order sequence cycles, viz. global sea-level responses to supercontinent breakup and assembly.Three, first-order, inter-regional unconformities divide the stratigraphic succession of the supracrustal belts into four units of first-order, megasequence status. In order of decreasing age, these are named the Warrawoona, Gorge Creek, Roebourne, and Mount Negri Megasequences. A total of 17 second-order supersequences are also named, although some may represent supersequence sets. The megasequences, and/or their genetic crustal equivalents, represent the preserved rock record of four megacycles of forearc, arc, and/or back-arc geotectonic evolution associated with the convergent margins of a Pilbara Continent. Individual supersequences are the rock record of separate second-order basins or basin phases, and reflect the different geotectonic components of each megasequence.The four megacycles had respective longevities, from oldest to youngest, of 150 m.y., 230 m.y., 160 m.y. and 180 m.y. They are identical in their stratigraphic successions and predicted first-order sea-level cycles to Phanerozoic first-order tectono-eustatic cycles, and are interpreted as the same response to global tectonic change. The Warrawoona and Gorge Creek Megacycles, and the Roebourne and Mount Negri Megacycles are tectonogenetic pairs of the East Pilbara and the West Pilbara Megacycle Sets, respectively. The East Pilbara Megacycle Set evolved over a period of 380 m.y. from 3490 Ma to 3110 Ma, whereas the West Pilbara Megacycle Set evolved over a period of 340 m.y. from 3110 Ma to 2770 Ma. The two megacycle sets are identical in their longevity, geotectonic components, and predicted sea-level cycles to convergent-margin Wilson and Supercontinent Cycles, and are thereby interpreted individually as the oldest established records of the Supercontinent Cycle. The implication of this interpretation is that a steady-state global tectonic regime has been in operation from early in Earth's history, which has important consequences for theoretical modelling of the Earth's early tectonic style and secular changes in its geochemical regime.