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 geoscience community. A meeting-long
series of topical sessions have been organized that will focus on important times—which we call “Turning
Points”—in the assembly, disassembly, modification, and growth of North America (Fig. 1). These sessions
will be headlined by a Pardee Symposium, which will provide an overview of the tectonic evolution of
Laurentia and an introduction to the concept of key “Turning Points.” Seven related topical sessions,
under the general heading “Assembling Laurentia,” will span the GSA meeting. Each session will
scrutinize key periods in the long history of the continent when the character, rate, or style of
tectonic processes may have changed or when the plate tectonic process itself may have changed in some
fundamental way. The ultimate goal is to identify potential drivers for, and the broader implications
of, these changes, and to widen the scope of investigation beyond a particular boundary or regional
geological event to the scale of Laurentia itself. The time slices for the topical sessions are as
follows (with brief explanations from each session proposal):
The tapestry of North American geology, showing regions of exposed Archean and Proterozoic rocks in
red-orange colors, Paleozoic rocks in purple-blue colors, Mesozoic rocks in green colors, and Cenozoic
rocks and sediments in yellow colors. After Barton et al. (2003).
2.8–2.5 Ga: Neoarchean Crust Formation and Cratonization
The Neoarchean time interval was the last, great episode of Precambrian crust formation prior to the
Paleoproterozoic assembly of Laurentia. These continental nuclei form Laurentia’s core.
1.9–1.7 Ga: Turning Points in Paleoproterozoic Tectonic Evolution
The Paleoproterozoic time interval saw the formation of the first recognizable passive margins, formation
and accretion of oceanic crust, arcs, back-arc basins, foreland basins, and other environments arguably
characteristic of modern plate tectonics.
1.5–1.4 Ga: GEON 14 Enigmas and Advances in Understanding the Crustal Evolution and Paleogeography of
the Early Mesoproterozoic North America
Mesoproterozoic Laurentia was dominated by an Andean-scale active, but still very enigmatic margin,
spanning ~5000 km, from eastern Canada to the southwestern United States and Mexico.
1.2–1.0 Ga: Mesoproterozoic to Early Neoproterozoic Tectonic Evolution of Laurentia and Its Role within
the Supercontinent Rodinia
The late Mesoproterozoic to early Neoproterozoic spans the rise and fall of the Grenville orogen at the
center of the supercontinent Rodinia, the formation of the Midcontinent Rift, and the initial breakup of
700–540 Ma Neoproterozoic to Cambrian Rifting and Continental Margin Evolution during Breakup of Rodinia
The Neoproterozoic to Cambrian time interval marked the breakup of Rodinia and Pannotia, and formation of
the margins of Laurentia. The Neoproterozoic was also a time of profound climatic and biologic change,
which may have been related to these changing tectonic conditions.
420–340 Ma Paleozoic Mobile Margins
The Silurian–Carboniferous closure of the Iapetus and Rheic oceans and the progressive growth of the
Appalachian-Caledonian orogen along eastern Laurentia marked the onset of a series of tectonic events
that propagated around Laurentia between the Devonian and Permian.
190–70 Ma Growth of the Western Continental Margin by Subduction, With or Without Terrane Accretion
This 190–70 Ma time slice saw the formation of the wide North America Cordillera and the opening of the
Gulf of Mexico, involving a number of processes, including extension, shortening, lateral translation,
subduction, magmatism, metamorphism, erosion, and sedimentation.
The choice of time slices is based, at least in part, on the current literature and recent meeting
symposia that have highlighted new insights and interpretations, as well as unresolved questions about
Laurentia’s geologic history. The selection of particular time slices and the duration of the time
windows will certainly be debated, but it is hoped that the discussion will articulate critical
questions or outstanding problems and foster future collaboration and innovation across the geological
and geophysical research communities. One aim is to gather momentum for developing new tools for
open-source, widely available, collaborative integration of time-slice data sets across geologic
disciplines and across the continent. As indicated previously, we hope to distinguish times when
Laurentia’s plate tectonic setting led to major changes in tectonic style from times when the
controlling tectonic processes themselves may have shifted.
Two or three session proposers/organizers—including Canadian, U.S., and Mexican researchers—have
volunteered to chair and coordinate the individual topical sessions. The goal is to solicit the broadest
range of researchers, research areas, and research disciplines (including sedimentology, structural
geology, petrology, geophysics, geobiology, etc.) in order to capture all aspects of the evolving
continent (including now external Laurentian fragments and traces) and its consequent implications
during a particular time slice. The Pardee Symposium and the seven associated topical sessions have been
endorsed by several GSA Divisions as well as the Mineralogical Society of America.
There are two reasons that this is a particularly opportune time to take such an adventurous approach at
a national meeting. First, the conclusion of the EarthScope project in the U.S., the Lithoprobe project
in Canada, and many other more regional geology-geophysics collaborations make this an appropriate time
to think about the tectonic record in the broadest possible terms. Geophysical data sets are now widely
available, and workers are processing and integrating the data in new ways to gain insight into 3D
implications of surface observations. Actualistic models of plumes, drips, slabs, slab tears, mantle
wind, Moho variation, and discontinuities at all levels of the crust, lithosphere, and deeper mantle can
be integrated with the surface record in order to build a truly 4D model of Earth tectonics.
The second reason is more specifically related to the current time and environment in which we are
living—itself a distinct “Turning Point.” As we write this article, we are all sequestered in our homes
in the hope that we can help mitigate the current pandemic threat. We are also unsure whether the 2020
GSA Montréal meeting will happen, and if so, in what form. This gathering of many geoscience disciplines
under the framework of Laurentia’s evolution will be a celebration of the creative, collaborative, and
international efforts that have made the scientific advances possible. Plate tectonics is our unifying
theory, and as such, inherently involves all the fields of the geological sciences. Moreover,
understanding the evolution of Laurentia has always required an international effort, although most
critically engaging people located in North America. We are not just “in it together,” but with respect
to Laurentia, we are “on it together.” Our individual research programs have always benefitted from the
research of colleagues, working in other places and on different problems in their respective subfields,
leading us to insights and discoveries that would have otherwise been out of reach. It has become more
important than ever to assemble (whether in person or virtually), make new connections across our
fields, and together establish our collective understanding of the place we all share.
- Barton, K.E., Howell, D.G., and Vigil, J.F., 2003, The North America Tapestry of Time and Terrain:
U.S. Geological Survey, Geologic Investigations Series I-2781.