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The Continental Record and the Generation of Continental Crust

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The Continental Record and the Generation of Continental Crust The continental record and the 1888 2013 generation of continental crust CELEBRATING ADVANCES IN GEOSCIENCE Invited Review P.A. Cawood1,2,†, C.J. Hawkesworth1, and B. Dhuime1,3 1Department of Earth Sciences, University of St. Andrews, North Street, St. Andrews KY16 9AL, UK 2School of Earth and Environment, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia 3Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queens Road, Bristol BS8 1RJ, UK ABSTRACT differs from the younger record that can be more directly related to a plate-tectonic re- 4 Continental crust is the archive of Earth gime. The paucity of this early record has led history. The spatial and temporal distribution to competing and equivocal models invoking of Earth’s record of rock units and events is plate-tectonic– and mantle-plume–domi- 2 Continents – andesite heterogeneous; for example, ages of igneous nated processes. The 60%–70% of the pres- (age range: 4000–0 Ma) crystallization, metamorphism, continental ent volume of the continental crust estimated margins, mineralization, and sea water and to have been present at 3 Ga contrasts mark- 0 atmospheric proxies are distributed about a edly with the <10% of crust of that age ap- series of peaks and troughs. This distribution parently still preserved and requires on going refl ects the different preservation potential of destruction (recycling) of crust and sub- –2 Oceans – basalt rocks generated in different tectonic settings, conti nental mantle lithosphere back into the Elevation (km) (age range: 180–0 Ma) rather than fundamental pulses of activity, mantle through processes such as subduction and the peaks of ages are linked to the tim- and delamination. –4 ing of supercontinent assembly. The physio- chemical resilience of zircons and their INTRODUCTION derivation largely from felsic igneous rocks –6 means that they are important indicators of Earth has a bimodal surface elevation re- the crustal record. Furthermore, detrital zir- fl ecting the contrasting chemical-mechanical cons, which sample a range of source rocks, properties of continental and oceanic crust. The 5101520 provide a more representative record than latter is dense, gravitationally unstable, and % of surface direct analysis of grains in igneous rocks. hence young, whereas continental crust is buoy- Figure 1. Bimodal distribution of surface Analysis of detrital zircons suggests that at ant and represents the archive of Earth history ele vations on Earth, relative to sea-level, em- least ~60%–70% of the present volume of (Fig. 1), not only of the crust itself but through phasizing the contrasting physical-chemical the continental crust had been generated by the rock record of the atmosphere, hydrosphere, properties of continental and ocean crust. 3 Ga. Such estimates seek to take account of and biosphere, and of the mantle through its Figure is adapted from Taylor and McLennan the extent to which the old crustal material is interactions with the crust. Over 4.5 b.y., the (1985, 1996). underrepresented in the sedimentary record , continental crust has evolved to form the envi- and they imply that there were greater vol- ronment in which we live and the resources on umes of continental crust in the Archean which we depend. An understanding of the crust than might be inferred from the composi- and its record is fundamental to resolving ques- how the continental crust was generated, and tions of detrital zircons and sediments. The tions on the origin of life, the evolution and oxy- to discuss constraints on the rates of continen- growth of continental crust was a continu- genation of our atmosphere, past climates, mass tal growth through time and the relationship ous rather than an episodic process, but extinctions, the thermal evolution of Earth, and to tectonic processes, especially the role of the there was a marked decrease in the rate of the interactions between the surfi cial and deep supercontinent cycle in controlling the geologi- crustal growth at ca. 3 Ga, which may have Earth. Yet, when and how the continental crust cal record. We explore the information available been linked to the onset of signifi cant crustal was generated, the volume of continental crust from the zircon record, since igneous zircons recycling, probably through subduction at through Earth history, and whether it provides a preserved as detritus in sedimentary rocks are convergent plate margins. The Hadean and representative record remain fundamental ques- increasingly regarded as a key temporal record Early Archean continental record is poorly tions in the earth sciences. of the activity associated with the generation preserved and characterized by a bimodal Our aim in this paper is fi rst briefl y to outline and evolution of the continental crust. TTG (tonalites, trondhjemites, and grano- the general character of the crust and to clarify Views on the evolution of the continen- diorites) and greenstone association that the terms used, and then to explore the nature of tal crust have changed dramatically as ideas the continental record that is available for study, on geological processes have evolved and as †E-mail: [email protected] to evaluate different approaches for when and methods to interrogate the rock record have GSA Bulletin; January/February 2013; v. 125; no. 1/2; p. 14–32; doi:10.1130/B30722.1; 15 fi gures. 14 For permission to copy, contact [email protected] © 2013 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/125/1-2/14/3713781/14.pdf by guest on 23 September 2021 The continental record and the generation of continental crust advanced through developments in stratigraphic analysis, petrography, paleontology, geochemis- try, geochronology, geophysics, and modeling. Crucially, our understanding of the processes in- volved in the generation and the evolution of the continental crust has grown enormously through the latter part of the twentieth and beginning of the twenty-fi rst centuries following on from the development and acceptance of plate-tectonic theory. This has focused our research on plate margins, the sites of continental crust forma- tion and stabilization, and it has resulted in a fundamental change in the way we approach our interrogation of Earth and its record from a descriptive documentation of units and events into investigation into the processes controlling these features. A factor critical to determining these processes is an understanding of rates of change, and this has been facilitated by devel- opments in data collection and analysis. This expansion of knowledge has been particularly important in further understanding not just the exposed surfi cial rock record, but in gaining in- sight into the composition and development of the whole crust. In particular, this has led to new ideas into what shaped the record, and how rep- resentative, or unrepresentative, it may be. Figure 2. Contour map of the thickness of Earth’s crust (developed from model CRUST 5.1). SOME FACTS AND TERMINOLOGY The contour interval is 10 km (45 km contour interval is also shown to provide greater detail on the continents). To a fi rst approximation, the continents and their margins are outlined The total area of continental crust is 210.4 × by the 30 km contour. Source of fi gure: http://earthquake.usgs.gov/research/structure/crust 6 2 10 km , or some 41% of the surface area of /index.php. Earth, and the volume is 7.2 × 109 km3, which constitutes some 70% of Earth’s crustal vol- ume (Cogley, 1984). The crust extends ver- is conductive, and the base is a rheological a hetero geneous middle crust assemblage of tically from the surface to the Mohorovičić boundary with the isothermal convecting mantle ortho gneiss and paragneiss at amphibolite discontinuity (Moho) and laterally to the break (Sleep, 2005). facies to lower granulite facies (Fig. 3C), and a in slope in the continental shelf (Rudnick and Continents include cratons, areas of stable lower crust consisting of granulite-facies coun- Gao, 2003). The Moho is defi ned as the jump in crust, orogenic belts, and regions of continental try rocks and basic intrusive rocks and/or cumu- seismic primary waves (P-waves) to greater than extension, which form either intracratonic rifts lates (Rudnick and Fountain, 1995; Wedepohl, ~7.6 km s–1. This change in seismic-wave veloc- or develop into zones of continental breakup 1995; Rudnick and Gao, 2003). Thicknesses of ity is taken as a fi rst-order approximation of the and thermal subsidence (passive margins). the three crustal layers vary, but the upper and boundary between mafi c lower-crustal rock and Orogens evolve through one or more cycles of middle crustal sections generally form around ultramafi c mantle peridotite: the crust-mantle sedimentation, subsidence, and igneous activ- 30% each of a typical crustal profi le, with the boundary. Petrologic studies from exposed sec- ity punctuated by tectonothermal events (orog- lower crust forming the remaining 40% (Fig. 5; tions of ocean fl oor (ophiolites) and from xeno- enies), involving deformation, metamorphism, Rudnick and Gao, 2003; Hawkesworth and liths in continental settings suggest that in some and igneous activity, which result in thickening Kemp, 2006a). situations, the crust-mantle boundary may dif- and stabilization of the lithosphere (Figs. 3A The bulk composition of the crust is equiva- fer from the Moho (Malpas, 1978; Griffi n and and 3B). Cratons are ancient orogens that have lent to andesite (Fig. 3D) and requires two O’Reilly, 1987). generally been undeformed and tectonically stages of formation involving extraction of The mean elevation of the continental crust stable for long periods of time, often since the mafi c magmas from the mantle and their dif- is around 125 m (Fig. 1), and some 31% of Archean, and are divisible into shields, which ferentiation through either fractional crystalliza- the crustal area is below sea level.
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