Mesozoic to early Tertiary tectonic evolution of the Shuswap metamorphic complex in the Vernon area, southeastern Canadian Cordillera.

摘要

Low-angle shear zones exposed near the western margin of the Shuswap metamorphic complex, part of the Okanagan Valley fault system, have been previously interpreted as crustal-scale shear zones. These shear zones are well-exposed in the Vernon area, where a semi-intact carapace of greenschist-facies superstructure extends across the width of the Shuswap metamorphic complex.; Detachment faults are absent in the Vernon area and the superstructure has not been thinned by an array of extensional faults. Superstructure and infrastructure are juxtaposed by 1-2 km thick Paleocene to Early Eocene low-angle shear zones characterized by a well-developed stretching lineation and top to the west shear-sense indicators. To the east, in the Vidler Ridge-Pinnacles area, the transition from infrastructure to superstructure is characterized by a gradational, but attenuated metamorphic sequence; no shear zones are apparent.; Mineral assemblages and P-T estimates indicate peak metamorphic conditions of 8-10 kbar and 800-850°C for the infrastructure. Greenschist-facies mineral assemblages and conodont colour alteration index values indicate maximum temperatures of 440°C for the superstructure. Geochronological data (U-Pb zircon, titanite; chemical monazite dates) from infrastructure rocks exposed southeast of Vernon indicate a complex thermotectonic evolution. Metamorphic zircon growth occurred at 155-150, 100-90, and 70-50 Ma. Magmatism included emplacement of diorite at ∼232 Ma, tonalite at 155-150 Ma, granodiorite at 102 Ma, and monzonite at 50 Ma. In contrast, greenschist-facies superstructure exposed west of Vernon records evidence of ductile deformation and emplacement of the Okanagan composite batholith between 170 and 155 Ma.; The minimal gaps in the superstructure, the lack of normal faults, and the lack of a regional detachment fault indicate that the infrastructure was not exhumed by removal of the superstructure during motion on a low-angle detachment. On the basis of new and previously published mapping, geochronological, metamorphic, and geophysical data, and comparisons with thermal-mechanical modeling of large, hot orogens, exhumation of the infrastructure and the formation of gneiss domes is hypothesized to have resulted from Late Cretaceous to Early Eocene channel flow of partially molten rocks. Exhumation is proposed to have occurred as the zone of channel flow encountered a crustal-scale ramp in the underlying Paleoproterozoic basement.