Cenozoic extensional processes and tectonics in the northern Rocky Mountains

Extension has shaped the crust of the Northern Rocky Mountains for more than a billion years. Rifting produced the Belt basin shortly after 1.5 Ga and initiated the Paleozoic passive margin. Mesozoic to early Cenozoic convergence produced the Cordilleran fold-and-thrust belt, the uplifts of the Rocky Mountain foreland (Laramide) and an unusually broad Challis arc in the northern Rocky Mountains. The structural grain of the contractional belts, basement features, and the rheology of the Idaho batholith influenced the locus, kinematics and geometry of the >55 m.y. of extension that followed. Sparse data suggest that extension may have begun in the Cretaceous during the orogeny, partially collapsing large culminations of the thrust belt near Salmon Idaho. Widespread extension started during and after middle Eocene time. The large amount of core complex-related extension, starting within the massive Eocene Challis-Sanpoil-Absaroka volcanic flare up, is highlighted by the unusual presence of the paired Bitterroot and Anaconda metamorphic core complexes. Paleogene extension is the first of many kinematically distinct episodes of extension. Some areas near the Eastern Snake River Plain preserve as many as 6 different episodes of normal faulting with extension directions ranging from NE to NW to N. The normal faults with the largest displacement typically parallel the curving contractional structures of the Late Mesozoic to Early Tertiary Sevier belt or formed parallel to basement thrusts in the Rocky Mountain foreland. This parallelism shows that crustal inheritance has controlled the locus of faults for more than 45 m.y. after the end of shortening. Paleogene supradetachment basins are localized in a fairly narrow N-S trending belt between the Eastern Snake River Plain in the south and the Lewis-and-Clark fault zone in the north. Provenance and paleocurrents show persistent southward flow along the axis of the extended zone from much more highly extended areas in the core complexes to less extended regions of the Grant and MuddyNicholia protobasins. Feldspathic and tuffaceous sediment accumulated within the supradetachment basins during their translation phase of development, and facies patterns resemble those of typical rift basins with steep basin-bounding normal faults. The youngest, currently active system of normal faults has a regular spacing and consistent N to NW trends. More faults dip west than east within the former fold-and-thrust belt. Faults become more widely spaced westward in the strong and isotropic Idaho batholith. Normal faults east of the thrust belt have a range of strikes and reactivate Laramide structures with NE, NW and N and E trends. The Rocky Mountain Basin-and-Range province narrows northward and terminates near the Canadian border. The spatial pattern and geometry of the faults suggest clockwise rotation about an axis near the north edge of the province, in agreement with inversion of GPS data. Active normal faults display additional complexities near the Yellowstone hotspot and it is clear that it influences the deformation in the Basin-and-Range province. Normal faults within the neotectonic, seismic, and topographic parabola centered on Yellowstone are unusually active, whereas structures beneath the Eastern Snake River Plain are unusually quiescent because normal faults no longer extend across the Eastern Snake River Plain at the surface (Anders et al., 1989). The 70° NW Northwest Geology, v. 36, 2007 p. 111-132 The Journal of the Tobacco Root Geological Society 112 plunge of the Yellowstone plume to 500-600 km (Yuan and Dueker, 2005) may be responsible for the asymmetry in extensional belts NW and SE of the Eastern Snake River Plain (Pierce and Morgan, 1992). Earthquakes in the greater Yellowstone region show domains of anomalous N-S extensional strains north of the Eastern Snake River Plain from the anomalous north-dipping Centennial fault eastward. Flexure toward the Eastern Snake River Plain plus SW subsidence away from Yellowstone could produce a northeastward migrating domain of E-W striking normal faults NW of the Eastern Snake River Plain and NNE-striking normal faults south of the Eastern Snake River Plain. Lateral flow of lower crustal rocks from beneath the Eastern Snake River Plain might also contribute to this pattern.

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