Abstract

Geologic mapping and structural analysis in the Bannock Range, SE Idaho, indicate that the bulk of Cenozoic extension in southeast Idaho was accommodated by slip on the low-angle Bannock detachment system. The Bannock detachment system consists of low-angle normal faults with a likely N–S extent of >130 km. It was active from ca. 10.3 Ma to ca. 3–4 Ma and accommodated >15 km of top-to-the-WSW extension. The master low-angle detachment fault cuts steeper, older normal faults and extensional folds in its hanging wall.

Crosscutting relationships and prior work show that the hanging wall to the detachment system began as a cohesive block that later broke up along normal faults that either sole into or are cut by the master detachment fault. After breakup began, the master detachment fault was folded isostatically by a NNW-trending extensional anticline, the Oxford Ridge anticline. The folding produced a new listric low-angle normal fault east of the Oxford Ridge anticline to replace the back-tilted portion of the detachment fault. This new low-angle normal fault excised part of the hanging wall, cut hanging-wall structures, and served as a secondary breakaway for the detachment. West of the Oxford Ridge anticline, the master detachment fault initiated and slipped at a low angle, is the youngest low-angle normal fault of the system, and has not been rotated to a low-dip angle through time.

The exhumed Bannock detachment system, although somewhat smaller, shorter lived, and more disrupted by younger Basin and Range normal faults, is similar to the buried Sevier Desert detachment system beneath central Utah. Both top-to-the-west systems formed near the transition between the Sevier thrust belt and its hinterland, collapsed broad thrust-related culminations eroded to Cambrian rocks, are paired with partly coeval top-to-east metamorphic core complexes ∼100–160 km to the west, lie east of major thrust ramps, formed and slipped at low angles in the most extended part of the system, and exhibit a translation and breakup phase of deformation. Both are late Cenozoic in age and have voluminous lacustrine synrift deposits that record saline and alkaline conditions during early extension. Similar processes likely produced both detachment systems.

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