COUPLING BETWEEN CRUSTAL FLOW AND DETACHMENT TECTONICS DURING EXHUMATION OF THE NORTHERN CORDILLERAN METAMORPHIC CORE COMPLEXES

Two types of detachments form during flow of orogenic crust: a channel detachment and a rolling-hinge detachment, coupled by flow of the partially molten crust. The channel detachment, on the hinterland side of the orogen, represents the long-lived interface that separates the partially molten crust flowing in a channel from the rigid upper crustal lid. A rolling-hinge detachment forms and dips toward the foreland side of the core complex. The geometry of this crustal-scale detachment is governed by asymmetric boudinage that localizes strain at the critical interface between cold foreland and hot hinterland. A kinematic hinge (switch in sense of shear) occurs between the two detachments. Activation of the rolling-hinge detachment drives rapid decompression and substantial melting of orogenic crust, leading to the diapiric rise of migmatite domes in the footwall of detachments.

In the northern Cordillera, from eastern British Columbia to eastern Washington and Idaho, the metamorphic core complexes record coeval crustal melting and development of detachments during orogenic collapse. The relationships of flow of orogenic crust, detachment evolution, and partial melting are particularly well developed at the latitude of the Thor-Odin dome in the Shuswap metamorphic core complex. Structural, metamorphic, and geo/thermochronologic investigations, combined with an anisotropy of magnetic susceptibility (AMS) study of leucogranites concentrated in the detachments, suggest that this part of the orogen collapsed rapidly through the development of a crustal boudinage instability in early Eocene time (~55-50 Ma). A kinematic hinge originated in the immediate footwall of the rolling-hinge detachment (Columbia River fault) and is currently located 40 km west of the fault. Accordingly, the rate of exhumation (> 5 km/Myr) explains the near-isothermal decompression path recorded in the migmatite dome and the positive feedback between rapid decompression and crustal melting. Emplacement of the migmatite dome at shallow crustal levels transfers heat and mass efficiently and marks the end of crustal flow and orogeny.