Abstract

The Laurentian margin of the Appalachians is divided into external and internal zones on the basis of metamorphic and structural contrasts. In the southern Quebec internal zone, Silurian to Early Devonian southeast-verging structures are superimposed on northwest-verging structures, whereas most of the external zone lacks such overprints. Regional backthrust faults define a major upper plate–lower plate boundary; the external-zone rocks are in the hanging wall, and internal-zone rocks are in the footwall. Metamorphic rocks with Silurian–Early Devonian 40Ar/39Ar ages (430–410 Ma) characterize the lower plate. To the east, the Saint-Joseph fault and the Baie Verte–Brompton line are southeast- dipping normal faults that crosscut the upper plate–lower plate boundary. Metamorphic rocks with Middle Ordovician 40Ar/39Ar ages (469–461 Ma) and rocks of the external zone both occur in the downthrown side of the Saint-Joseph fault and the Baie Verte–Brompton line. U-Pb and 40Ar/39Ar ages suggest that the northwest-verging structures are related to ophiolite obduction and crustal thickening during the Taconian orogeny (ca. 480–445 Ma), whereas the southeast-verging structures formed during Silurian–Early Devonian backthrusting and normal faulting. The revised structural interpretation has implications for the Salinian orogeny and involves (1) southeast-directed transport of the Taconian crustal wedge of the upper plate, followed by normal faulting and juxtaposition with the lower plate along the Saint-Joseph fault and the Baie Verte–Brompton line, and (2) the formation of fault-bounded sedimentary basins, such as the Connecticut Valley–Gaspé trough.

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