Abstract

Remapping of upper Proterozoic and Cambrian rocks in the northern Bannock Range south and east of Pocatello, Idaho indicates a polyphase deformation history characterized by both shortening and extension. Mapscale folds and faults, related to shortening associated with Mesozoic thrusting have been offset and rotated along superimposed normal faults.

The oldest structure in the area is the north-trending, east-vergent Rapid Creek fold. This fold involves upper Proterozoic and Cambrian strata in the hanging wall of the Putnam thrust fault. The fold is truncated by east-striking subvertical faults that are inferred to be coeval tear faults. Fold and fault geometries, along with evidence that the area underwent major uplift prior to deposition of Neogene rocks, suggests the Rapid Creek fold formed over a footwall ramp whose location may have been controlled by basement normal faults related to Proterozoic rifting.

Proterozoic and Cambrian rocks are cut by numerous north-striking, west-dipping normal faults that were previously interpreted as thrusts. Restoration of balanced cross sections to restore slip on normal faults and to return tilted Neogene rocks to horizontal indicates the region was extended east-west approximately 40%.

Several east- to northeast-striking, subvertical faults are present, that offset north-striking normal faults. These east-striking faults are approximately parallel to the south edge of the Snake River Plain and may be related to formation of this structural basin. Some of these may be reactivated Mesozoic tear faults.