Bradley,D.C.(2011): Secular trends in the geologic record and the supercontinent cycle. Earth-Science Reviews, 108, 16-33.


 Geologic secular trends are used to refine the timetable of supercontinent assembly, tenure, and breakup. The analysis rests on what is meant by the term supercontinent, which here is defined broadly as a grouping of formerly dispersed continents. To avoid the artificial pitfall of an all-or-nothing definition, quantitative measures of “supercontinentality” are presented: the number of continents, and the area of the largest continent, which both can be gleaned from global paleogeographic maps for the Phanerozoic. For the secular trends approach to be viable in the deep past when the very existence of supercontinents is debatable and reconstructions are fraught with problems, it must first be calibrated in the Phanerozoic against the well-constrained Pangea supercontinent cycle. The most informative geologic variables covering both the Phanerozoic and Precambrian are the abundances of passive margins and of detrital zircons. Both fluctuated with size of the largest continent during the Pangea supercontinent cycle and can be quantified back to the Neoarchean. The tenure of Pangea was a time represented in the rock record by few zircons and few passive margins. Thus, previously documented minima in the abundance of detrital zircons (and orogenic granites) during the Precambrian (Codie et al., 2009a, Gondwana Research 15, 228-242) now can be more confidently interpreted as marking the tenures of supercontinents. The occurrences of carbonatites, granulites, eclogites, and greenstone-belt deformation events also appear to bear the imprint of Precambrian supercontinent cyclicity. Together, these secular records are consistent with the following scenario. The Neoarchean continental assemblies of Superia and Sclavia broke up at ca. 2300 and ca. 2090 Ma, respectively. Some of their fragments collided to form Nuna by about 1750 Ma; Nuna then grew by lateral accretion of juvenile arcs during the Mesoproterozoic, and was involved in a series of collisions at ca. 1000 Ma to form Rodinia. Rodinia broke up in stages from ca. 1000 to ca. 520 Ma. Before Rodinia had completely come apart, some of its pieces had already been reassembled in a new configuration, Gondwana, which was completed by 530 Ma. Gondwana later collided with Laurentia, Baltica, and Siberia to form Pangea by about 300 Ma. Breakup of Pangea began at about 180 Ma (Early Jurassic) and continues today. In the suggested scenario, no supercontinent cycle in Earth history corresponded to the ideal, in which all the continents were gathered together, then broke apart, then reassembled in a new configuration. Nuna and Gondwana ended their tenures not by breakup but by collision and name change; Rodinia's assembly overlapped in time with its disassembly; and Pangea spalled Tethyan microcontinents throughout much of its tenure. Many other secular trends show a weak or uneven imprint of the supercontinent cycle, no imprint at all. Instead, these secular trends together reveal aspects of the shifting background against which the supercontinents came and went, making each cycle unique. Global heat production declined; plate tectonics sped up through the Proterozoic and slowed down through the Phanerozoic; the atmosphere and oceans became oxidized; life emerged as a major geochemical agent; some rock types went extinct or nearly so (BIF, massif-type anorthosite, komatiite); and other rock types came into existence or became common (blueschists, bioclastic limestone, coal).

Keywords: Supercontinent; Secular trends; Time series; Precambrian geology; Passive margin; Detrital zircon』

1. Introduction
2. What constitutes a supercontinent?
3. Pangea
4. Secular trends that reflect the supercontinent cycle
 4.1. Area of the largest continent and number of continents
 4.2. Abundance, start dates, and end dates of passive margins
 4.3. Granites and detrital zircons
 4.4. Isotopic composition of seawater strontium
 4.5. Deformation ages of greenstone belts
 4.6. Eclogite- and granulite-facies metamorphism
 4.7. Carbonatites
 4.8. Large igneous provinces
5. Proposed supercontinent timetable
 5.1. Vaalbara, Superia, and Sclavia (Bleeker, 2003) or Kenorland (Williams et al., 1991)
 5.2. Nuna
 5.3. Rodinia
 5.4. Gondwana
6. Supercontinent cycles and other secular variation
 6.1. Global heat production and rates of plate tectonics
 6.2. Oceanic crust and passive-margin proxies
 6.3. Obducted ophiolites
 6.4. High-pressure, low-temperature metamorphic rocks
 6.5. Mantle-derived igneous rocks
 6.6. Massif-type anorthosites
 6.7. Orogenic gold deposits
 6.8. Sedimentary rocks
 6.9. Sedimentary recycling
 6.10. Oxygenation of the atmosphere and oceans
 6.11. Glaciation
 6.12. Sea level
 6.13. Mississippi Valley-type lead-zinc deposits
 6.14. Other secular trends
7. Summary
Appendix A. Supplementary data

Figs. 3-10. Secular trends for 0 to 550 Ma. (Table 1 takes the place of these captions).

Figs. 11-45. Secular trends for 0 to 4560 Ma. (Table 1 takes the place of these captions).

Fig. 46. (A) Published assessments of the tenures of various supercontinents according to the identified authors. (B) Age distributions (in black) of variables that have bear on the tenures of supercontinents, from sources cited in Figs. 11, 14, 15, 16, and 20. For each plot, the blue, green, lavendar, orange, and pink swaths indicate tenures of supercontinents as inferred from minima in those data alone. Dimmer and brighter colors represent more and less inclusive interpretations, respectively. The colored swaths agree in general but diffe in many details. (C) Proposed tenures of supercontinents based on the present study, combining information from Phanerozoic plate reconstructions, passive-margin age distributions, and zircon age distributions.

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