Abstract

The Walker Mountain-northern Piney Creek area, located along the northeast flank of the Bighorn Mountains, Wyoming displays oppositely verging Precambrian basement blocks, which are on trend with one another. Southwest-verging structures within the study area are interpreted to be the combined product of subsidence along the axis of the Powder River Basin, uplift along the axis of the Bighorn Mountains, and regional horizontal compression. The subsidence-uplift couple necessitated a shortening of the outer surface of the crust, which resulted in the establishment of a local, compressive stress regime superimposed on the regional stress field. Deformation of the upper crust was localized along the steep-dipping, west side of the basin. The northeast-verging Piney Creek thrust-block is interpreted to be the exclusive product of a regional, horizontal compressive stress, which uplifted and ultimately thrust the steep-dipping, east-central flank of the Bighorn Mountains over the adjacent Powder River Basin.

The present dip of basement-involved faults in the Walker Mountain-northern Piney Creek area is the result of the initial dip of the fault at the time of rupture and any subsequent rotation associated with the general uplift of the Bighorn Mountains. The southwest-verging faults nearest to the mountain flank demonstrate considerable rotation, indicating their development prior to significant uplift of the range. The northeast-verging Piney Creek Thrust demonstrates no significant rotation, indicating it developed after the primary uplift of the range.

Subsurface data indicate the Piney Creek Thrust dips 25-30° southwest and displays approximately 15,200 feet (4,630 m) of horizontal separation and 7,000 feet (2,130 m) of vertical separation. The three miles (4.8 km) of proven left oblique-slip on the northeast-striking Piney Creek "tear fault" allowed the emplacement of the Piney Creek Thrust-block over the Powder River Basin. Structure on opposite sides of this "tear fault" did not develop simultaneously. The younger, northeast-verging Piney Creek Thrust was part of a large-scale tectonic event which was superimposed upon the older, southwest-verging structures.

There is no evidence in support of high-angle movement along northwest-striking faults as predicted by the vertical block-uplift theory of foreland deformation. All northwest-striking faults identified within the study area were either observed or interpreted to be genetic thrust faults with primary dip angles of 25-45°.