EOCENE FLOW OF PARTIALLY MOLTEN CRUST IN THE NORTH AMERICAN CORDILLERA

Orogeny is characterized by crustal thickening, abundant partial melting, and the formation of continental plateaux. Geodynamic models have shown that partially molten crust may flow laterally in a channel or vertically in diapirs and redistribute mass and heat in the orogen. Therefore, channel and diapiric flows are first-order mechanisms controlling the evolution of orogens.

The Shuswap (British Columbia) and Okanogan-Kettle (eastern Washington state) metamorphic core complexes display migmatite terrains that have preserved the memory of channel and diapiric flow in their structures, fabrics, and metamorphic evolution. In these core complexes migmatite terrains are exposed immediately beneath the detachment mylonites. In the Shuswap, the detachment is coated by 60-55 Ma leucogranitic laccoliths in which the study of magmatic to solid-state fabrics, using the anisotropy of magnetic susceptibility, suggests that the metamorphic core complex formed by symmetrical crustal boudinage, with a kinematic axis located at the center of the core complex. This fabric is deformed by the N-S elongated Thor-Odin dome located along the eastern edge of the core complex and right under the Columbia River detachment. The Thor-Odin dome is surrounded by cascading folds and contains subdomes. The Okanogan-Kettle core complex also displays a symmetric kinematic pattern, with the Okanogan detachment in the west, the Kettle detachment in the east, and the development of en-echelon rift basins, such as the Republic graben (~50 Ma fill), at the center of the complex. Two migmatite domes, elongated NW-SE, parallel to the migmatitic as well as mylonitic lineation, are present in the immediate footwall of the Okanogan detachment. Cascading folds on the NE and SW flanks of the elongated domes suggest diapiric rise of the domes. In both the Shuswap and the Okanogan-Kettle domes, high-grade amphibolites display spectacular decompression textures - symplectite of spinel+cordierite on kyanite/sillimanite. Thermobarometric analyses indicate a P history from > 10 kbar to ~ 4 kbar. This decompression is partly related to the diapiric rise of domes that are eventually exhumed by detachment tectonics. Therefore, during the transition from thick crust to collapse stage, the partially molten crust transfers mass and heat vertically and laterally, resulting in the formation of domes.