Abstract Phanerozoic reactivations of basement fault zones are documented in 5000 m of basement core recovered from beneath the updip Atlantic Coastal Plain underlying the US Department of Energy Savannah River Site (SRS) in South Carolina. These basement fault zones are adjacent to the excised Rheic Ocean suture. Meta-intrusive rocks from c. 620 and 625 Ma contain a mylonitic fabric and intrude foliated mafic metavolcanic rocks. At c. 305 Ma, granulite facies orthogneisses were thrust over amphibolite facies meta-igneous rocks in the transpressive Tinker Creek Nappe. The overturned limb of the nappe localizes the Triassic Dunbarton Basin Border Fault. The border fault acted as a conduit for fluids in the Mesozoic and Cenozoic. At c. 220 ± 5 Ma, a potassium and silica metasomatic event affected the SRS basement. A propylitic event flushed reducing fluids through rocks as young as the Santonian. The remains of a Triassic sub-basin were identified in the northwesten part of the site. A Cretaceous and younger vein paragenesis overprints the previous events. More than 30 pseudotachylytes are found in the SRS basement and are preferentially localized on metasomatized Alleghanian chloritic fractures. Pseudotachylyte post-dates mineralized fractures. The Pen Branch Fault offsets the basement–Cretaceous unconformity and is present in c. 242 m of core between PBF-7-419 m and PBF-7-660.8 m. The Pen Branch Fault cross-cuts mineralized fractures and must post-date strike-normal zeolites.

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.