The Middle Mountain shear zone is exposed in the Idaho portion of the Albion- Raft River-Grouse Creek metamorphic core complex. The structure strikes north- northeast-south-southwest and dips shallowly to the west. It is defined on the basis of well-developed mylonitic fabrics and had an initial thickness of at least 4 km. At least some movement on the shear zone occurred at depths of 22-26 km during Eocene amphibolite-facies metamorphism, but shear-zone fabrics may be found in some granite bodies of late Oligocene age. The zone may thus have been active in the ductile regime over a period of as much as 10-15 m.y.

Mylonitic rocks within the Middle Mountain shear zone display both type I and type II S-C fabrics. Augen structures, pressure shadows, and quartz c-axis orientations are strongly asymmetric. These fabrics and textures indicate that deformation within the zone was characterized by noncoaxial laminar flow, during which the hanging wall of the structure moved down and to the west-northwest relative to the footwall. We interpret the Middle Mountain shear zone as a major detachment that played an important role in middle Tertiary extension of this part of the eastern Great Basin. The top-to-the-west displacement sense contrasts with late Cenozoic, predominantly top-to-the-east extension in this area, implying a major change in extension geometry in early to middle Miocene time.

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