Assembling Laurentia—Integrated Theme Sessions on Tectonic Turning Points

Michael L. Williams, Dept. of Geosciences, University of Massachusetts, Amherst, Massachusetts 01003, USA, [email protected]; Dawn A. Kellett, Geological Survey of Canada–Atlantic Division, Natural Resources Canada/Government of Canada, 1 Challenger Drive, Dartmouth, Nova Scotia B2Y 4A2, Canada, [email protected]; Basil Tikoff*, Dept. of Geoscience, University of Wisconsin– Madison, 1215 W. Dayton Street, Madison, Wisconsin 53706, USA, [email protected]; Steven J. Whitmeyer, Dept. of Geology and Environmental Science, James Madison University, 801 Carrier Drive, MSC 6903, Harrisonburg, Virginia 22807, USA, [email protected]

The North American continent records the headlined by a Pardee Symposium, which and the broader implications of, these changes, evolution of tectonic processes and tectonic will provide an overview of the tectonic evo- and to widen the scope of investigation environments from the earliest Archean to lution of Laurentia and an introduction to the beyond a particular boundary or regional geo- modern times. The continent hosts a rich concept of key “Turning Points.” Seven logical event to the scale of Laurentia itself. Archean (and possibly Hadean) record, at related topical sessions, under the general The time slices for the topical sessions are as least three great Proterozoic orogenic belts, heading “Assembling Laurentia,” will span follows (with brief explanations from each and a wide range of Phanerozoic tectonic, the GSA meeting. Each session will scruti- session proposal): sedimentary, and paleobiologic environ- nize key periods in the long history of the con- ments, including active plate boundaries. In tinent when the character, rate, or style of tec- 2.8–2.5 Ga: Neoarchean Crust many ways, Laurentia (ancestral North tonic processes may have changed or when Formation and Cratonization America) might be thought of as the proto- the plate tectonic process itself may have The Neoarchean time interval was the last, typical continent, with a little—if not a lot— changed in some fundamental way. The ulti- great episode of Precambrian crust forma- of everything. With its long record of chang- mate goal is to identify potential drivers for, tion prior to the Paleoproterozoic assembly ing tectonic settings, supercontinent cycles, and geologic outcomes, one question per- sists: Does Laurentia preserve a record of fundamental, relatively short duration changes in tectonic processes on Earth, or rather, a gradually changing configuration of continents in a slowly evolving plate tectonic system? The resolution of this question requires a continent-wide perspective on tectonics through time, and it requires the inte- gration of many fields of geoscience. One way to approach the question is to ask (1) what is the holistic geological character of Laurentia at particular times in Earth history?; and (2) what are the apparent controlling factors at those times, from mantle dynamics to plate interactions, to surface weathering, to biological evolution? The GSA 2020 Annual Meeting provides an opportunity to address the tectonic evolution of Laurentia and the evolution of tectonic processes in general as an international geo- Figure 1. The tapestry of North science community. A meeting-long series of American geology, showing regions topical sessions have been organized that will of exposed Archean and Protero- zoic rocks in red-orange colors, focus on important times—which we call Paleozoic rocks in purple-blue col- “Turning Points”—in the assembly, disas- ors, Mesozoic rocks in green colors, and Cenozoic rocks and sedi- sembly, modification, and growth of North ments in yellow colors. After Barton America (Fig. 1). These sessions will be et al. (2003).

*Corresponding author. of Laurentia. These continental nuclei form 190–70 Ma Growth of the Western appropriate time to think about the tectonic Laurentia’s core. Continental Margin by Subduction, record in the broadest possible terms. With or Without Terrane Accretion Geophysical data sets are now widely avail- 1.9–1.7 Ga: Turning Points in This 190–70 Ma time slice saw the forma- able, and workers are processing and inte- Paleoproterozoic Tectonic Evolution tion of the wide North America Cordillera grating the data in new ways to gain insight The Paleoproterozoic time interval saw and the opening of the Gulf of Mexico, into 3D implications of surface observations. the formation of the first recognizable pas- involving a number of processes, including Actualistic models of plumes, drips, slabs, sive margins, formation and accretion of extension, shortening, lateral translation, slab tears, mantle wind, Moho variation, and oceanic crust, arcs, back-arc basins, foreland subduction, magmatism, metamorphism, discontinuities at all levels of the crust, litho- basins, and other environments arguably erosion, and sedimentation. sphere, and deeper mantle can be integrated characteristic of modern plate tectonics. The choice of time slices is based, at least with the surface record in order to build a in part, on the current literature and recent truly 4D model of Earth tectonics. 1.5–1.4 Ga: GEON 14 Enigmas meeting symposia that have highlighted new The second reason is more specifically and Advances in Understanding insights and interpretations, as well as unre- related to the current time and environment the Crustal Evolution and solved questions about Laurentia’s geologic in which we are living—itself a distinct Paleogeography of the Early history. The selection of particular time “Turning Point.” As we write this article, we Mesoproterozoic North America slices and the duration of the time windows are all sequestered in our homes in the hope Mesoproterozoic Laurentia was dominated will certainly be debated, but it is hoped that that we can help mitigate the current pan- by an Andean-scale active, but still very enig- the discussion will articulate critical ques- demic threat. We are also unsure whether the matic margin, spanning ~5000 km, from east- tions or outstanding problems and foster 2020 GSA Montréal meeting will happen, ern Canada to the southwestern United States future collaboration and innovation across and if so, in what form. This gathering of and Mexico. the geological and geophysical research many geoscience disciplines under the communities. One aim is to gather momen- framework of Laurentia’s evolution will be a 1.2–1.0 Ga: Mesoproterozoic to tum for developing new tools for open- Early Neoproterozoic Tectonic source, widely available, collaborative inte- celebration of the creative, collaborative, and Evolution of Laurentia and Its Role gration of time-slice data sets across geologic international efforts that have made the sci- within the Supercontinent Rodinia disciplines and across the continent. As indi- entific advances possible. Plate tectonics is The late Mesoproterozoic to early Neo- cated previously, we hope to distinguish our unifying theory, and as such, inherently proterozoic spans the rise and fall of the times when Laurentia’s plate tectonic setting involves all the fields of the geological sci- Grenville orogen at the center of the super- led to major changes in tectonic style from ences. Moreover, understanding the evolu- continent Rodinia, the formation of the times when the controlling tectonic pro- tion of Laurentia has always required an Midcontinent Rift, and the initial breakup of cesses themselves may have shifted. international effort, although most critically Rodinia. Two or three session proposers/organiz- engaging people located in North America. ers—including Canadian, U.S., and Mexican We are not just “in it together,” but with 700–540 Ma Neoproterozoic to researchers—have volunteered to chair and respect to Laurentia, we are “on it together.” Cambrian Rifting and Continental coordinate the individual topical sessions. Our individual research programs have Margin Evolution during Breakup of The goal is to solicit the broadest range of always benefitted from the research of col- Rodinia and Pannotia researchers, research areas, and research dis- leagues, working in other places and on dif- The Neoproterozoic to Cambrian time ciplines (including sedimentology, structural ferent problems in their respective subfields, interval marked the breakup of Rodinia and geology, petrology, geophysics, geobiology, leading us to insights and discoveries that Pannotia, and formation of the margins of etc.) in order to capture all aspects of the would have otherwise been out of reach. It Laurentia. The Neoproterozoic was also a evolving continent (including now external has become more important than ever to time of profound climatic and biologic Laurentian fragments and traces) and its assemble (whether in person or virtually), change, which may have been related to consequent implications during a particular make new connections across our fields, and these changing tectonic conditions. time slice. The Pardee Symposium and the together establish our collective understand- seven associated topical sessions have been ing of the place we all share. 420–340 Ma Paleozoic Mobile endorsed by several GSA Divisions as well Margins as the Mineralogical Society of America. REFERENCE CITED The Silurian–Carboniferous closure of the There are two reasons that this is a particu- Barton, K.E., Howell, D.G., and Vigil, J.F., 2003, Iapetus and Rheic oceans and the progressive larly opportune time to take such an adventur- The North America Tapestry of Time and Ter- growth of the Appalachian-Caledonian oro- ous approach at a national meeting. First, the rain: U.S. Geological Survey, Geologic Investi- gations Series I-2781. gen along eastern Laurentia marked the onset conclusion of the EarthScope project in the of a series of tectonic events that propagated U.S., the LITHOPROBE project in Canada, Manuscript received 3 Apr. 2020 around Laurentia between the Devonian and and many other more regional geology- Revised manuscript received 15 Apr. 2020 Permian. geophysics collaborations make this an Manuscript accepted 24 Apr. 2020