Abstract The Variscan Orogen was formed during the closure of the Rheic Ocean and the final collision between the North American and West African cratons in the Late Paleozoic. This collision led to the multistage building of the Mauritanide Belt to the east of the Variscan suture and to the building of the well-known Appalachian Belt to the west. Both led to opposite vergences in this part of the Variscan belt. The earliest records of the main collision episode begin at ∼360 Ma and end about 250 myr ago, while a late extensional phase lasted until ∼190 Ma. Three distinct stages are recognized in West Africa. The first stage (c. 350–300 Ma) records the indentation of the Reguibat Shield into the central Appalachian margin of Laurentia. This indentation led to thrusting of the Souttoufide and Akjoujt ‘nappes’ onto the Reguibat Shield, to southward motion of the Senegalese block (SB), and to strike-slip motion in the Appalachians. The motion of the SB to the south is coeval with: (1) folding of the northern part of the Bové Basin, (2) north–south sinistral strike-slip motions in the central Mauritanides, and (3) the end of sedimentation in the Bové and Taoudeni Basins by the Late Devonian. The second stage (c. 300–250 Ma) involves the eastward motion of the Western Thrust Block (WTB) against the SB and, likely, some of the westward thrusts in the Appalachians. This second ‘Variscan’ event includes: (1) closure of parts of the lower Diourbel Carboniferous basin, which is now concealed beneath the Senegalo-Mauritanian Basin, (2) thrusting to the east of the Simenti Group over the Koulountou Group in the Bassaride Belt, (3) thrusting to the east of the Wa-Wa Group, (4) thrusting of the Mauritanide Belt onto the Taoudeni Basin in the central Mauritanide Belt, and finally (5) thrusting of the Agualilet Group over the Akjoujt nappes and eastward motion of the western units over the Dhloat Ensour (Late Ordovician to early Devonian) autochthonous unit in the Souttoufides. West of the supposed ‘Variscan’ suture, Appalachian thrusting affected parts of Appalachian Belt. The third stage (c. 250 to 190 Ma) began with the opening of Triassic rift basins in the Senegalo-Mauritanian basin and also in the north of Florida. As numerous previous correlations across the Variscan system do not include the West African part, our sythesis is intended to enhance these correlations.

ABSTRACT The Archean Wyoming Province formed and subsequently grew through a combination of magmatic and tectonic processes from ca. 4.0 to 2.5 Ga. Turning points in crustal evolution are recorded in four distinct phases of magmatism: (1) Early mafic magmatism formed a primordial crust between 4.0 and 3.6 Ga and began the formation of a lithospheric keel below the Wyoming Province in response to active plume-like mantle upwelling in a “stagnant lid”–type tectonic environment; (2) earliest sialic crust formed in the Paleoarchean by melting of hydrated mafic crust to produce rocks of the tonalite-trondhjemite-granodiorite (TTG) suite from ca. 3.6 to 2.9 Ga, with a major crust-forming event at 3.3–3.2 Ga that was probably associated with a transition to plate tectonics by ca. 3.5 Ga; (3) extensive calc-alkalic magmatism occurred during the Mesoarchean and Neoarchean (ca. 2.85–2.6 Ga), forming plutons that are compositionally equivalent to modern-day continental arc plutons; and (4) a late stage of crustal differentiation occurred through intracrustal melting processes ca. 2.6–2.4 Ga. Periods of tectonic quiescence are recognized in the development of stable platform supracrustal sequences (e.g., orthoquartzites, pelitic schists, banded iron formation, metabasites, and marbles) between ca. 3.0 and 2.80 Ga. Evidence for late Archean tectonic thickening of the Wyoming Province through horizontal tectonics and lateral accretion was likely associated with processes similar to modern-style convergent-margin plate tectonics. Although the province is surrounded by Paleoproterozoic orogenic zones, no post-Archean penetrative deformation or calc-alkalic magmatism affected the Wyoming Province prior to the Laramide orogeny. Its Archean crustal evolution produced a strong cratonic continental nucleus prior to incorporation within Laurentia. Distinct lithologic suites, isotopic compositions, and ages provide essential reference markers for models of assembly and breakup of the long-lived Laurentian supercontinent.

ABSTRACT The record of Permian–Triassic evolution in eastern North America indicates an important change in the tectonic regime from compression to extension as eastern Laurentia transitioned from the Alleghanian orogeny to continental rifting associated with the breakup of Pangea. The temporal pace (e.g., gradual vs. episodic, diachronous vs. synchronous), the accommodating structures, and the influential processes that characterized this transition provide critical insights into the late Paleozoic evolution of Laurentia and rifted continental margins in general. Connections between the formation of the South Georgia basin and regional cooling of the southernmost Appalachian crystalline rocks, along with the distribution of normal faults and discontinuities in metamorphic grade, indicate extensional collapse of the Alleghanian orogen along an extensive detachment system that was active from ca. 295 to 240 Ma. The 40 Ar/ 39 Ar cooling ages of biotites from low-angle normal shear zones cutting migmatitic gneisses of the southernmost Appalachians are interpreted to document extensional faulting ca. 280 Ma and to provide a snapshot of the prolonged orogenic collapse. The timing, orientation of structures, extent of reactivation, and character of late Alleghanian extension in the central and northern Appalachians provide an orogen-scale framework for this tectonic transition. This contribution focuses on correlations between the beginning of orogenic collapse and the initiation of continental rifting along with the tectonic processes that transformed eastern North America from a convergent to divergent plate boundary following the Alleghanian orogeny.

ABSTRACT This contribution attempts to recount our collective progress in understanding the Archean–Hadean Earth system over the past 50 yr. Many realms of the geological sciences (geochemistry, petrology, geophysics, structural geology, geobiology, planetary science, and more) have made substantive contributions to this effort. These contributions have changed our understanding of the Archean–Hadean Earth in five major areas: (1) the expanse of Archean–Hadean time; (2) tectonics and lithospheric evolution, particularly possible analogs for the sites of modern, primary crust production and mantle differentiation (e.g., magmatic arcs, ocean ridges, and large igneous provinces); (3) evolution of the atmosphere-hydrosphere system, and its impact on the evolution of Earth’s endogenic and exogenic systems; (4) the history of liquid water, particularly at the ocean scale; and (5) the origin and development of the biosphere and its impact on the geologic record. We also emphasize that much of the progress made in understanding the evolution of early Earth systems over the past 50 yr has been fueled by important technological advances in analytical geochemistry, such as the advent of ion probes for U-Pb zircon geochronology, inductively coupled plasma–mass spectrometry for trace-element and Hf isotopic analyses, Raman spectroscopy in organic geochemistry, and molecular reconstructions in biology. Within this context, we specifically review progress in our understanding of the Eoarchean history of southern West Greenland as an example of the value of continuous integration of careful geologic observation and mapping with evolving technology, which have combined to further open this window into Earth’s earliest systems.