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 Isotope dilution thermal ionization mass spectrometry U–Pb dating and coupled Lu–Hf solution inductively coupled plasma mass spectrometry analyses of zircon were acquired from magmatic rocks along two transects across the Scandinavian Caledonides in the Troms–Ofoten region of Norway to explore possible correlations and gain insight into the evolution of far-travelled nappes within the Upper and Uppermost Allochthons. One pulse of magmatic activity was recorded at c. 489 Ma in the Tromsø Nappe. In the underlying Nakkedal Nappe, a magmatic pulse was recorded at c. 450 Ma, being contemporaneous with eclogite facies metamorphism in the area. Tonalites in the structurally underlying Lyngen and Gratangseidet ophiolitic complexes, both forming the substratum to carbonate–schist–quartzite sequences (Balsfjord and Evenes groups, respectively), yielded ages of 481 and 474 Ma. Obtained ɛ Hf(t) values are, however, distinctly different and indicate a juvenile origin for the Gratangseidet tonalite (ɛ Hf(474) =+9.57) and the presence of Palaeoproterozoic source material for the Lyngen tonalite (ɛ Hf(481) =−16.8 to −2.3). The 474 Ma age of the Snaufjellet granite intruding the Bogen Group structurally above the Evenes Group requires a thrust between the two units. An age of 435 Ma on the Heia gabbro of the Nordmannvik Nappe is comparable to that of the Råna norite (437 Ma) in the Narvik area, supporting a correlation with the Narvik Nappe Complex. The youngest intrusion dated to 425 Ma is a leucogranite that intruded the Balsfjord Group. The Lu–Hf data indicate a common source for several of the magmatic rocks in the Uppermost Allochthon, as well as Laurentian arc granitoids from East Greenland and possibly from Newfoundland and related areas.

The Pine Mountain window contains the southernmost Grenville basement massif to be found in the Appalachians. Granulite- and upper-amphibolite-facies granitic gneisses that form the basement complex are isotopically dated at 1.1–1.0 Ga. Locally, the gneisses contain rare mafic injections and supracrustal and plutonic xenoliths. The Pine Mountain Group cover sequence nonconformably overlies Grenville basement and is interpreted to correlate with Blue Ridge units as follows: Halawaka/Sparks Schist = Ocoee Supergroup (Late Proterozoic, rift), Hollis Quartzite = Chilhowee Group (Late Proterozoic-Cambrian, rift-to-drift), and Chewacla Marble = Shady Dolomite (Cambro-Ordovician, drift). Facies variations within the sedimentary cover units were cited as evidence for a southward decrease in the extent of the Ocoee rift basins, but new mapping documents the continuity of thick packages of Halawaka (i.e., Ocoee) rocks extending southward beneath the Gulf Coastal Plain. In contrast to upper amphibolite- and granulite-facies metamorphism of the basement during the Grenville event, cover rocks contain staurolite and staurolite-kyanite zone assemblages reflecting Paleozoic Appalachian metamorphism. Sensitive high-resolution ion microprobe (SHRIMP) and conventional single-grain U-Pb datings of detrital zircons from the basal Hollis Quartzite document a distinct population of clear, subrounded zircons of ca. 1.09 Ga, which were most likely derived from underlying Grenville-age gneiss. An older, white/gray population found in the lowermost Hollis is ca. 2.4–2.3 Ga, an age restricted to Gondwanan continents and very limited occurrences in northern Laurentia. Tectonic reconstructions of Unrug (1997) and others depict southeast Laurentia proximal to the Amazonia and Rio de la Plata cratons during the Neoproterozoic, offering the possibility that they may be the source for 2.4–2.3-Ga zircons in Hollis sediments. Alternatively, the AUSWUS (Australia/Western United States) reconstruction (Karlstrom et al., 2001) places east Antarctica and the Australian Gawler craton, both of which contain abundant 2.4 Ga granites, proximal to the southwestern United States during this time. Depending on the stream systems present during the Neoproterozoic, zircons from the Gawler may have been transported to the vicinity of the Pine Mountain window. In addition, three clear zircons yield ages of 1.4 Ga, and may have been derived from either the Laurentian Mid-continent granite-rhyolite province or the Rondonian Province of South America. A Chilhowee Group sandstone sample contains a similar mixture of Grenville and Mid-continent/Rondonian-age zircons, but none with ages of 2.4–2.3 Ga.

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