MIOCENE-PLIOCENE LOW-ANGLE NORMAL FAULTS WERE ACTIVE AT LOW ANGLES: BANNOCK DETACHMENT SYSTEM, SOUTHEAST IDAHO

Geologic mapping of the Clifton 7.5’ quadrangle indicates that the bulk of Cenozoic extension in southeast Idaho was accommodated by low-angle normal faults of the Bannock detachment system. The Bannock detachment system is a regionally extensive system of low-angle normal faults with a possible N-S extent of >130 km. The detachment system was active from ~12 Ma to 4 Ma (Janecke et al., in press) and accommodated >15 km of WSW-directed extension. The footwall is composed of Neoproterozoic Pocatello Formation and Tertiary intrusions that were metamorphosed to greenschist facies. A mafic sill invaded a low-angle fault of the Bannock system during extensional exhumation and contains both ductile and brittle fabrics. Cross-cutting relationships show that the master detachment fault, the Clifton fault, is the youngest low-angle normal fault of the system, was active at a low-angle, and has not been rotated to a low-angle geometry through time. Map patterns and stratigraphic relationships indicate that the hanging wall to the detachment system began as a cohesive block that later broke up along listric and planar normal faults, some spaced as little as 200 m apart, that either sole into or are cut by the Clifton fault. After break up of the hanging wall began, the Clifton fault excised into the hanging wall. The structural geometry and evolution of the Bannock detachment system strongly resembles metamorphic core complexes and we suggest that the Bannock detachment system is a proto-metamorphic core complex. Two large extensional anticlines were produced by slip across the Bannock detachment system. The first, the Oxford Peak anticline, is an ESE-plunging anticline that formed in a cohesive hanging wall of Clifton fault during initial translation on the detachment. 3-D inclined shear may have produced the anticline. The second anticline trends NNW, parallel to the strike of the detachment system, and likely formed due to isostatic rebound of the footwall of the Clifton fault. We interpret the Oxford Ridge anticline to be the protocore of the Bannock detachment system. High-angle, Basin and Range normal faults cut and expose faults of the Bannock detachment system in the southern Bannock Range. Basin and Range extension began no earlier than 4-5 Ma in this area and continues today. Altogether there was about 60% WSW-ENE extension.