THERMAL EVOLUTION OF THE SOUTHEASTERN CANADIAN CORDILLERA

The eastern metamorphic culmination of the southern Canadian Cordiller a is a composite core complex, which at low structural levels exposes the Monashee decollement, a major contractional fault with large Late Cretaceous to Paleocene east-directed displacement. The hanging wall o f this fault, the Selkirk allochthon, is a sheared thrust sheet, recor ding metamorphic and deformational events spanning the period from ca. 170 to 60 Ma, with younger kinematic and thermal events recorded at p rogressively deeper levels. The Monashee complex, the footwall terrane of the Monashee decollement, consists of an Early Proterozoic crystal line basement complex overlain by Late Proterozoic and perhaps Phanero zoic metasedimentary rocks. The Monashee complex was significantly met amorphosed 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 yea rs at most, and it provides a strong argument that the heat source for this metamorphism was the overlying hot Selkirk allochthon. A similar to 1.85-1.90 Ga metamorphism also is recorded within the Precambrian basement. The tectonometamorphic chronology of the footwall and hangin g-walt terranes of the Monashee decollement are very different, and on ly share Paleogene thermal-tectonic events when the two were structura lly juxtaposed by deep-seated thrusting. Although this region is the h interland of the foreland belt of the southern Cordillera, the thermal and tectonic history of the metamorphic core zone is analogous to tha t in a thrust belt setting where warmer rocks progressively override c ooler rocks as displacement migrates toward the foreland. In such sett ings, a protracted and more complex thermal history of the hanging wal l is juxtaposed with a simpler thermal history of shorter duration of the footwall. Seismic reflection and chronological information indicat e that the Monashee decollement is the same structure as the basal dec ollement beneath the full width of the southern Rocky Mountains, repre senting its deep-seated continuation in the hinterland. Tectonic denud ation resulting from Eocene extension and crustal-scale tilting, follo wed by late Tertiary erosion, brought these rocks to the surface for s tudy.