Abstract

The eastern metamorphic culmination of the southern Canadian Cordillera is a composite core complex, which at low structural levels exposes the Monashee décollement, a major contractional fault with large Late Cretaceous to Paleocene east-directed displacement. The hanging wall of this fault, the Selkirk allochthon, is a sheared thrust sheet, recording metamorphic and deformational events spanning the period from ca. 170 to 60 Ma, with younger kinematic and thermal events recorded at progressively deeper levels. The Monashee complex, the footwall terrane of the Monashee décollement, consists of an Early Proterozoic crystalline basement complex overlain by Late Proterozoic and perhaps Phanerozoic metasedimentary rocks. The Monashee complex was significantly metamorphosed and deformed in Paleogene time (60–55 Ma), on the basis of U–Pb data presented in this paper. Analysis of U–Pb titanite data show that the duration of this metamorphic event was but a few million years at most, and it provides a strong argument that the heat source for this metamorphism was the overlying hot Selkirk allochthon. A ~1.85–1.90 Ga metamorphism also is recorded within the Precambrian basement. The tectonometamorphic chronology of the footwall and hanging-wall terranes of the Monashee décollement are very different, and only share Paleogene thermal–tectonic events when the two were structurally juxtaposed by deep-seated thrusting. Although this region is the hinterland of the foreland belt of the southern Cordillera, the thermal and tectonic history of the metamorphic core zone is analogous to that in a thrust belt setting where warmer rocks progressively override cooler rocks as displacement migrates toward the foreland. In such settings, a protracted and more complex thermal history of the hanging wall is juxtaposed with a simpler thermal history of shorter duration of the footwall. Seismic reflection and chronological information indicate that the Monashee décollement is the same structure as the basal décollement beneath the full width of the southern Rocky Mountains, representing its deep-seated continuation in the hinterland. Tectonic denudation resulting from Eocene extension and crustal-scale tilting, followed by late Tertiary erosion, brought these rocks to the surface for study.

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