ON THE EDGE OF THE ICOSAHEDRON: ANATOMY OF THE MONTANA-TENNESSEE STRUCTURAL CORRIDOR

Several researchers have noted a trans-Laurentian structural corridor extending from Montana southeast to Tennessee.The corridor is 2600 km long and 600 km wide, and is marked by discontinuities in basement aeromagnetic anomalies, the Clearwater re-entrant of the west Laurentian margin, a wide zone of en echelon fault zones, dike swarms, rift basins, extensive anorogenic magmatic provinces, and the Alabama promontory. The corridor includes the Belt-Purcell, Uinta, and Anadarko basins. It was established in early Mesoproterozoic time, with widespread magmatism at 1.47 and 1.37 Ga, and was active intermittently through later geologic time. It influenced isopach and facies patterns throughout the Phanerozoic, especially during tectonically active periods such as Pennsylvanian and Cretaceous. Many structures within the corridor were inverted during the growth of the Rockies. The corridor marks a lithospheric discontinuity between the southwestern and central parts of Laurentia. It comprises one edge of a very precise, continental-scale truncated icosahedral fracture tessellation that limits the Precambrian basement of Laurentia. Each edge of a truncated icosahedron is a great circle segment that subtends 23 degrees of arc (2600 km at the earth’s surface), and edges intersect at precise angles to outline hexagonal and pentagonal tiles. The tessellation defines the fractal limit for hexagonal tiling of a spherical shell, and the least-work configuration for nucleation and propagation of fractures under uniform, layer-parallel extension. It probably resulted from uniform lithospheric extension of Laurentia’s parent supercontinent above an insulated and expanding asthenosphere. Pressure-relief melting of the asthenosphere beneath the fractures generated anorogenic magma that episodically invaded the tessellation. The Cordilleran, Arctic, Caledonian, and Appalachian edges of the tessellation and the southeast part of the Montana-Tennessee corridor opened into ocean basins in Early Cambrian time, and miogeoclines draped the rifted edges. Because of incomplete rifting, the Montana-Tennessee corridor preserves and reveals the internal structure of one of the tessellation edges. Episodic rotation of the conjugate lithospheric tiles resulted in uplift, subsidence, or inversion of structures within the corridor.