ABSTRACT

The northern part of the Ruby Range of the Rocky Mountain foreland of Montana consists of a Precambrian metamorphic basement overlain by Paleozoic sedimentary strata. During the Laramide orogeny, the Ruby Range basement broke into blocks that underwent differential uplift concurrent with rotation to the northeast and tilting to the northwest. Northeast-trending faults with as much as 2,000 m of throw separate the blocks and are nearly vertical to the depth to which they can be observed.

Deformation of the sedimentary strata was in direct response to differential movement of the basement blocks, forming an anticline over the edge of each uplifted block and a syncline over the adjacent downthrown block Each syncline is asymmetric, with a steeply upturned limb parallel to the fault boundary of the adjacent upthrown block and a moderately inclined limb dipping about 30 to 40 degrees, in accordance with the inclination of the underlying basement block Each syncline is most intensely deformed toward the southeast, becoming progessively narrower, overturned, and eventually faulted, corresponding to a narrowing of the underlying basement block.

Strata adjacent to an upthrown block are under compression owing to folding into a syncline. In addition, a subhorizontal compressive stress is exerted on the strata by rotation of the underlying basement block through the 30 to 40 degrees of arc that corresponds to the northeast inclination of each block This stress may be transmitted laterally through the strata across the entire width of the block if not absorbed by faulting and folding. Faults formed in the nose area of a syncline develop parallel to the axial plane of the fold They are nearly flat, low in the sedimentary section, and steepen rapidly upward. Basement is not cut by these faults.