Structural geology of western part of Lemhi Range, east-central Idaho

Abstract

The large Poison Creek anticline, now broken by normal faults, is the major structure in the western part of the Lemhi Range. The fold resulted from late Mesozoic compressional deformation in the hinterland of the Cordilleran thrust belt in central Idaho. Bedrock of the study area is mainly Mesoproterozoic strata that are part of a thrust sheet floored by the Poison Creek fault, a major thrust fault whose trace extends northwest across the northern part of the study area. The thrust fault displaced the Proterozoic rocks over lower Paleozoic formations composed of carbonate and quartzite. In the southern part of the study area, the easterly trending Lem Peak normal fault has vertical displacement of several thousand meters and may cut the Poison Creek thrust fault at depth. Between the trace of the Lem Peak normal fault and the Poison Creek thrust fault lie the Hayden Creek and Bear Valley faults, which in their present configuration show normal displacement. The Hayden Creek fault, however, has a demonstrable thrust displacement prior to normal displacement. Both it and the Bear Valley fault may have formed originally as subsidiary thrusts of the Poison Creek thrust fault, generated during rupture of tight folds within the Poison Creek thrust sheet. Removal of the normal component of slip on the Hayden Creek and Bear Valley faults, restoring rocks to about their pre-normal-slip positions, reveals the original geometry of the eastern part of the large Poison Creek anticline above the Poison Creek thrust fault. A single regionally extensive thrust sheet, the Medicine Lodge thrust system, previously was interpreted as the major structure of the region. Within the southeastern part of the present study area, a window was stated to exist in the vicinity of Hayden Creek and to expose rocks of the subthrust Mesoproterozoic Yellowjacket Formation. My mapping leads me to conclude that neither the Medicine Lodge thrust plate nor the window exists in this part of the range. The northwest-trending structural grain of the Lemhi Range is truncated by the north-striking Salmon River fault at the western margin of the study area. The fault is oblique to the structural grain of the range and is interpreted as a tear fault that formed during Mesozoic thrust faulting and folding of rocks of the Poison Creek thrust sheet. The fault served to compartmentalize the style of deformation that took place on opposite sides of it. Rocks east of the Salmon River fault, the main focus of this report, deformed into the major fold and associated subsidiary thrusts of the Poison Creek sheet. Rocks directly west of the Salmon River fault generally display low dips and locally define a shallow-dipping syncline oriented about parallel to the north-trending fault. A segment of the fault subsequently was reactivated as part of the Lem Peak normal fault during down-dropping of the Lem Peak block. Later, the entire length of the present Salmon River fault was reactivated as a basin-and-range normal fault that dips steeply to the west.