Thrust fault geometry of the Snake River Range, Idaho and Wyoming

Abstract

The Snake River Range is located at the north end of the Absaroka fault system in the Idaho-Wyoming-Utah thrust belt, and it adjoins the Teton Mountains. Nine imbricate sheets of the Absaroka system form a shingled array of overlapping thrusts. Most fault nomenclature in the range dates from early, discontinuous, reconnaissance studies wherein names were applied from the top thrust down whether or not the thrust sheets separated by this method are, or ever were, connected as mechanical blocks. Fault names are revised here to consider the lateral continuity of sheets as the major criteria for defining the block to which a name should be applied. The Absaroka thrust and St. John thrust are recognized to be equivalent different parts of a major transfer zone, using the continuity of sheets as the main criteria. The map pattern of the range is dominated by major re-entrants to the west in the traces of the St. John and Absaroka thrusts. The re-entrants in both major faults are caused by folding in their foot walls related to lateral ramping of each thrust upward to the southeast. The folding of the St. John thrust above the Indian Creek Culmination also folds the thrust sheets above it, helping to mark the emplacement of the imbricate sheets as being from top to bottom and west to east. The Absaroka system thrusts formed from 20 to 40 km west of their present position. They are folded by underlying faults of the Jackson or, farther south, the Darby system, indicating that the Absaroka system was emplaced into its present position by motion on these underlying faults. Discussions concerning mechanical interaction between the thrust belt and a foreland buttress (ancestral Teton or Targhee uplift) concentrate on intensities of deformation and changes in geometry adjacent to the proposed buttress. The individual imbricate geometries and regional changes in trend are more related to a changing stratigraphic package and the lateral thrust ramps than to geographic proximity to any proposed buttress.