New 40Ar/39Ar data reveal ages and thermal discontinuities that identify mapped and unmapped fault boundaries in the Potomac terrane in northern Virginia, thus confirming previous interpretations that it is a composite terrane. The rocks of the Potomac terrane were examined along the Potomac River, where it has been previously subdivided into three units: the Mather Gorge, Sykesville, and Laurel Formations. In the Mather Gorge Formation, at least two metamorphic thermal domains were identified, the Blockhouse Point and Bear Island domains, separated by a fault active in the late Devonian. Early Ordovician (ca. 475 Ma) cooling ages of amphibole in the Bear Island domain reflect cooling from Taconic metamorphism, whereas the Blockhouse Point domain was first metamorphosed in the Devonian. The 40Ar/39Ar data from muscovites in a third (eastern) domain within the Mather Gorge Formation, the Stubblefield Falls domain, record thrusting of the Sykesville Formation over the Mather Gorge Formation on the Plummers Island fault in the Devonian. The existence of two distinctly different thermal domains separated by a tectonic boundary within the Mather Gorge argues against its status as a formation. Hornblende cooling ages in the Sykesville Formation are Early Devonian (ca. 400 Ma), reflecting cooling from Taconic and Acadian metamorphism. The ages of retrograde and overprinting muscovite in phyllonites from domain-bounding faults are late Devonian (Acadian) and late Pennsylvanian (Alleghanian), marking the time of assembly of these domains and subsequent movement on the Plummers Island fault. Our data indicate that net vertical motion between the Bear Island domain of the Mather Gorge complex and the Sykesville Formation across the Plummers Island fault is east-side-up. Zircon fission-track cooling ages demonstrate thermal equilibrium across the Potomac terrane in the early Permian, and apatite fission-track cooling ages record tilting of the Potomac terrane in the Cretaceous or later with the west side up at least 1 km.

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