Secular trends in the geologic record and the supercontinent cycle☆
Research highlights
► A compilation of over 100 geologic secular trends is presented. ► About 20 of these show an uneven pulse that can be linked on geologic grounds to the supercontinent cycle. ► Based on trends during the Pangea cycle, supercontinent tenure is a time of few passive margins and low zircon abundance. ► Zircon lows at 2400–2100, 1600–1275, and 900–725 Ma mark tenures of the supercontinents Sclavia–Superia, Nuna, and Rodinia.
Introduction
Global paleogeography and plate interactions are reasonably well understood back to the assembly of Pangea in the late Paleozoic (Fig. 1). Multiple lines of evidence go into these reconstructions—marine magnetic anomalies, passive-margin matchups, geologic interpretation of orogenic belts, paleomagnetism, paleobiogeography of fossils, and distribution of climatically sensitive strata—and the results are synthesized into a scenario that is consistent with the rules of plate kinematics. Like genealogy, however, the plate-reconstruction approach to Earth history gets harder and harder back though time, as each trail of evidence gets fainter. It would be pointless, for example, to attempt a global plate reconstruction at 4030 Ma, with only the Acasta Gneiss to go on. Precambrian reconstructions are easiest when continents were clustered together and there were fewer objects; but even these times of supercontinent tenure are a challenge. For example, Neoproterozoic reconstructions of Rodinia (Moores, 1991, Karlstrom et al., 2001, Li et al., 2008, Evans, 2009) show West Africa in very different positions. If it is that hard to decide where it goes, the assumption that West Africa was part of a Rodinia supercontinent is itself open to question.
A complementary approach to Earth history is the analysis of secular trends (e.g., Condie, 2005, Dewey, 2007, Reddy and Evans, 2009, Condie et al., 2011, Goldfarb et al., 2010). Countless geologic variables can be tracked through time, even times when plate reconstructions are out of the question. This makes it possible to sidestep the unknown specifics of plate paleogeography and instead focus on evolution of the Earth system as a whole. Two precepts guide this approach: (1) every secular trend can have only one correct explanation (complicated though it might be); and (2) any viable explanation for one secular trend must honor all the rest.
The supercontinent cycle (Fig. 2) is manifested in a number of secular trends (e.g., Worsley et al., 1984, Worsley et al., 1986, Nance et al., 1986, Barley and Groves, 1992, Condie, 2005, Condie et al., 2009b, Hawkesworth et al., 2010) and is well suited to study by this means because the opening and closing of ocean basins impact many parts of the Earth system. In this paper, I examine the timing of past supercontinent cycles using a new compilation of secular trends for most of the geologic datasets for which this sort of information is now available: age distributions of rocks and minerals, geochemical trends, censuses of tectonic settings, numerical model results, and more. Many new global secular trends are published every year. The most informative secular trends for present purposes are those that show fluctuations related to the assembly, tenure, and disassembly of Pangea. It will be shown that the tenure of Pangea was a time of few passive margins and low zircon abundance. These two variables can be tracked deeper into the past to deduce, on uniformitarian grounds, the tenures of putative Precambrian supercontinents. A number of secular trends show this same irregular pulse, with maxima and minima at several hundred million years' spacing (Figs. 11-23). Other secular trends show little or no sign of it, however, but instead paint a changing backdrop against which the supercontinents have come and gone (Figs. 24-45).
The terms “secular trend”, “time trend”, and “time series” are used here as synonyms for a set of ordered pairs (x, y) where y is a geologic variable and x is its age. The new compilation expands on previous compendia of secular trends by Garrels and Mackenzie, 1971, Meyer, 1981, Meyer, 1988, Nance et al., 1986, Hallam, 1992, Barley and Groves, 1992, Groves et al., 2005, Condie, 1997, Condie, 2005, Veizer and Mackenzie, 2003. A few plots are new, either constructed from data that were given only in tabular form in the original publications, or from my own compilations. The plots are divided between the main body of the paper (Figs. 3-10, Figs. 11-45; Table 1) and a Supplementary data section (Figs. A1-A86; Table A1), available online. Tables 1 and A1 serve as figure captions. The Supplementary data section includes discussions of the data or model assumptions behind some plots.
All secular trends have been plotted or replotted with time as the x-axis and the present on the left1. Plots are presented at two time scales, either covering the Phanerozoic plus the very end of the Neoproterozoic (550 Ma to present), or spanning Earth history in its entirety (4560 Ma to present). Numerical age assignments use the time scale of Gradstein and Ogg (2004), except for a few plots that have an older time scale inextricably embedded in the construction.
Section snippets
What constitutes a supercontinent?
Multiple lines of evidence suggest that there have been times when some formerly independent continents came together, and other times when larger continents fragmented into smaller ones. When is a grouping of continents big enough to earn the name supercontinent? Does this semantic distinction even matter? Many tectonicists use the term supercontinent in the sense of Hoffman (1999): “a clustering of nearly all the continents” or Rogers and Santosh (2003): “an assembly of all or nearly all the
Pangea
Pangea is the most recent and best understood supercontinent. Its basic configuration was appreciated a century ago by the early proponents of continental drift, Taylor and Wegener. Pangea's late Paleozoic to early Mesozoic tenure (ca. 310 to 180 Ma) is well constrained. Working back from the present, Pangea is reconstructed simply by fitting the continents that border Atlantic-type oceans: Europe–Greenland, Africa–North America, India–Antarctica, Australia–Antarctica, and so on.
The Pangea fit
Area of the largest continent and number of continents
Although differing in detail, all published Phanerozoic plate reconstructions (e.g., Scotese, 1997, Stampfli and Borel, 2002) plainly show that the continents were dispersed, then gathered together, and then dispersed once again. Two secular trends, derived for the present study from the maps of Scotese (1997), serve as quantitative indices of “supercontinentality”.
In Fig. 3-10, the area of the largest single continent was tracked through time by summing the areas of the main pieces, using
Proposed supercontinent timetable
The tenures of supercontinents are shown from published literature in Fig. 46A. Fig. 46B shows the principal individual secular records that bear on the timing of supercontinents. A proposed supercontinent timetable is shown in Fig. 46C, modified from Bradley (2008) subject to the age constraints provided by the time trends shown. The present study does not bear on the details of particular supercontinent reconstructions.
Supercontinent cycles and other secular variation
Using the revised timetable (Fig. 46C) as a baseline, I next discuss secular trends that bear on the evolving context of the succession of supercontinents and on the properties of particular cycles. The differences between cycles can be attributed to factors such as the vagaries of plate geometry, long-term decline of Earth's radiogenic heat production, long-term increase in viscosity of the residual mantle, emergence of life as a major geochemical agent, and oxygenation of the oceans and
Summary
Two complementary approaches to Earth history—plate reconstructions and analysis of secular trends—have been brought together in this assessment of the supercontinent cycle. A global plate reconstruction for a particular time provides a unified rationale for the origins of, interrelations between, and geographic distribution of the rocks of that age. In general, this approach gets harder and less certain back through time. A few intervals in the Precambrian, however, are more tractable: the
Acknowledgments
I am grateful to reviewers Alison Till, Francis Macdonald, Kent Condie, Rich Goldfarb, and Alfred Kröner for their many constructive suggestions. This paper grew out of the 2004 Penrose Conference on Secular Variation in Tectonics and Allied Fields (Bradley and Dewey, 2005), which received support from the USGS Venture Capital Fund. The global perspective needed for this kind of synthesis came from Kevin Burke and John Dewey. Theresa Taylor, Heather Bleick, and Sam Friedman helped with the
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Bradley, D.C., 2011. Secular trends in the geologic record and the supercontinent cycle. Earth-Science Reviews ##, ##-##.