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The Putnam thrust has long been recognized as an important Mesozoic structure in the northern Portneuf Range, southeastern Idaho. At most localities, the thrust places Ordovician rocks above Permian and Pennsylvanian rocks, although near its southeastern extent, it ramps laterally downsection to the southeast. At its southeasternmost exposures, Cambrian rocks are juxtaposed above Mississippian rocks. New work indicates that the hanging wall of the Putnam thrust contains three imbricate thrust slices or subplates, which are, from structurally lowest to highest (and generally from north to south), the Lone Pine subplate, the Narrows subplate, and the Bear Canyon-Toponce subplate.

The steeply south-dipping, east-trending Narrows thrust overlies the Lone Pine subplate, underlies the Narrows subplate, and is a lateral ramp that merges eastward into the Putnam thrust. Where exposed, the Narrows thrust places Late Proterozoic quartzite of the Brigham Group over Ordovician and Cambrian rocks. The Bear Canyon thrust overlies the Narrows subplate and underlies the Bear Canyon-Toponce subplate, dips eastward along the west side of the Portneuf Range, and places lower Brigham Group quartzite above Cambrian limestone and Cambrian and Late Proterozoic upper Brigham Group quartzite and argillite. At its northern extent, the Bear Canyon thrust curves to the east, where it merges with the Putnam thrust. On the east side of the range, the intensely folded Toponce thrust places upper Brigham Group quartzite above Ordovician rocks; the Toponce is believed to be an eastward extension of the Bear Canyon thrust.

East-dipping rocks within the Lone Pine subplate were not strongly deformed during Cretaceous thrusting, in contrast to rocks within the Narrows subplate, where east-vergent recumbent folds, cleavage directions that fan about northerly strikes, and tectonic thickening and thinning of beds indicate intense, thrust-parallel shear. The deformation and thrust geometry within the Narrows subplate suggest that the Narrows subplate actually consists of several horses within a foreland-dipping duplex.

Late Miocene and younger basin deposits occur in north-trending valleys adjacent t o the northern Portneuf Range and, to the west, the Bannock and Pocatello ranges. At most places, the Neogene deposits dip to the east by as much as 35°, indicating that late Miocene and younger extension and down-to-the-east rotation occurred along mostly west-dipping listric faults that are inferred to merge on at least one regional detachment. Although range-bounding faults account for a large component of extension and rotation, an additional large component was contributed by numerous, relatively small-displacement normal faults within mountain ranges.

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