25–28 Sept. GSA 2016. Share your science, network, and explore Colorado. SEPTEMBER | VOL. 26, 2016 9 NO. A PUBLICATION OF THE GEOLOGICAL SOCIETY OF AMERICA® Tectonics and crustal evolution SEPTEMBER 2016 | VOLUME 26, NUMBER 9 Featured Article GSA TODAY (ISSN 1052-5173 USPS 0456-530) prints news and information for more than 26,000 GSA member readers and subscribing libraries, with 11 monthly issues (March/ April is a combined issue). GSA TODAY is published by The SCIENCE Geological Society of America® Inc. (GSA) with offices at 3300 Penrose Place, Boulder, Colorado, USA, and a mail- 4 Tectonics and crustal evolution ing address of P.O. Box 9140, Boulder, CO 80301-9140, USA. Chris J. Hawkesworth, Peter A. Cawood, and GSA provides this and other forums for the presentation of diverse opinions and positions by scientists worldwide, Bruno Dhuime regardless of race, citizenship, gender, sexual orientation, religion, or political viewpoint. 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Section Meeting 42 Why GSA Membership is Important to Me 44 Preliminary Announcement and Call for Papers: 2017 GSA Northeastern and North-Central Joint Section Meeting 48 Geoscience Jobs & Opportunities 53 GSA Foundation Update Erratum: The 2016 new GSA Fellows article (July 2016, v. 26, no. 7, p. 17–21) did not include Brandon Schmandt, who advanced to GSA Fellowship as the 2015 GSA Young Scientist (Donath Medal) awardee. GSA regrets this error. Cover photo inset: Denver Museum of Nature & Science, photo by Rich Grant. Used with permission by Visit Denver. Tectonics and crustal evolution Chris J. Hawkesworth, Department of Earth Sciences, University peaks and troughs of ages. Much of it has focused discussion on of Bristol, Wills Memorial Building, Queens Road, Bristol BS8 1RJ, the extent to which the generation and evolution of Earth’s crust is UK; and Department of Earth Sciences, University of St. Andrews, driven by deep-seated processes, such as mantle plumes, or is North Street, St. Andrews KY16 9AL, UK, c.j.hawkesworth@bristol primarily in response to plate tectonic processes that dominate at .ac.uk; Peter A. Cawood, Department of Earth Sciences, University relatively shallow levels. of St. Andrews, North Street, St. Andrews KY16 9AL, UK; and Bruno The cyclical nature of the geological record has been recog- Dhuime, Department of Earth Sciences, University of Bristol, Wills nized since James Hutton noted in the eighteenth century that Memorial Building, Queens Road, Bristol BS8 1RJ, UK even the oldest rocks are made up of “materials furnished from the ruins of former continents” (Hutton, 1785). The history of ABSTRACT the continental crust, at least since the end of the Archean, is marked by geological cycles that on different scales include those The continental crust is the archive of Earth’s history. Its rock shaped by individual mountain building events, and by the units record events that are heterogeneous in time with distinctive cyclic development and dispersal of supercontinents in response peaks and troughs of ages for igneous crystallization, metamor- to plate tectonics (Nance et al., 2014, and references therein). phism, continental margins, and mineralization. This temporal Successive cycles may have different features, reflecting in part distribution is argued largely to reflect the different preservation the cooling of the earth and the changing nature of the litho- potential of rocks generated in different tectonic settings, rather sphere. In this contribution, we explore the extent to which than fundamental pulses of activity, and the peaks of ages are changes in tectonic processes have shaped the geological record linked to the timing of supercontinent assembly. Isotopic and and the surface environments through Earth’s history. Where elemental data from zircons and whole rock crustal compositions possible these are linked to changing thermal conditions as the suggest that the overall growth of continental crust (crustal addi- earth cooled. tion from the mantle minus recycling of material to the mantle) The cooling earth influenced the depths and hence the has been continuous throughout Earth’s history. A decrease in the geochemical signatures at which melt generation takes place rate of crustal growth ca. 3.0 Ga is related to increased recycling (McKenzie, 1984; Nisbet et al., 1993) and the rheology of the crust associated with the onset of plate tectonics. and lithosphere (Gerya, 2014; Sizova et al., 2010). That in turn We recognize five stages of Earth’s evolution: (1) initial accre- influenced tectonic processes, including the initial onset of tion and differentiation of the core/mantle system within the first subduction and the subsequent onset of “cold” subduction that few tens of millions of years; (2) generation of crust in a pre-plate was prevalent throughout the Phanerozoic (Brown, 2006, 2014; tectonic regime in the period prior to 3.0 Ga; (3) early plate Stern, 2005), which shaped the surface environments on Earth. tectonics involving hot subduction with shallow slab breakoff over Subduction and plate tectonics resulted in the development of the period from 3.0 to 1.7 Ga; (4) Earth’s middle age from 1.7 to supercontinents and enhanced cooling that led to thickening of 0.75 Ga, characterized by environmental, evolutionary, and litho- the lithosphere and increased crustal reworking. This, in turn, spheric stability; (5) modern cold subduction, which has existed for resulted in higher erosion fluxes, and changes in the Sr isotope the past 0.75 b.y. Cycles of supercontinent formation and breakup ratios of seawater and the chemistry of the oceans (Cawood et al., have operated during the last three stages. This evolving tectonic 2013; Flament et al., 2013; Shields, 2007; Spencer et al., 2014). The character has likely been controlled by secular changes in mantle development of the continental crust is illustrated schematically temperature and how that impacts on lithospheric behavior. in Figure 2. Magma oceans may have persisted for 5–10 m.y. after Crustal volumes, reflecting the interplay of crust generation and initial accretion of the earth, and a crust, which is likely to have recycling, increased until Earth’s middle age, and they may have been mafic in composition, will have developed at a late stage in decreased in the past ~1 b.y. the differentiation and solidification of the magma ocean (e.g., Elkins-Tanton, 2008). The mafic crust is thought to have been BACKGROUND thickened by continuing mafic and ultramafic magmatism until The geological record is incomplete—some rock types are more remelting and the generation of felsic magmas could occur, likely to be preserved than others, and breaks in the rock archive resulting in the bimodal silica distribution that is a feature of are marked by breaks in the depositional record in the upper crust Archean crust (Fig.