LAYER PARALLEL SHORTENING: A MECHANISM FOR DETERMINING THE SEQUENCE OF DEFORMATION OF THE LITTLE WATER SYNCLINE, TENDOY MOUNTAINS, SOUTHWESTERN MONTANA

The Tendoy Range and Little Water syncline are located in Beaverhead County, southwestern Montana. This region was subjected to two significant orogenic events: The Sevier Orogeny (140-50 Ma) and the Laramide Orogeny (75-35 Ma). The Sevier Orogeny is generally characterized by the roughly N-S-trending, W-dipping thin-skinned fold-thrust belt, while Laramide structures involve thick-skinned, basement-involved deformation and localized magmatism. The large amplitude, NE-striking fold of the Little Water syncline is associated with NW-SE-shortening generally attributed to the Laramide-style (thick-skin) uplift, while the NNW-striking Four Eyes Canyon and Tendoy thrusts are structural features associated with NE-SW-shortening related to the Sevier-style (thin-skinned) fold and thrust belt. Photographs and samples were collected from the NNW-trending overturned western limb, the NE-trending overturned northern limb, and the NNE-trending upright southern and eastern limb of the Little Water syncline, using the Thaynes Formation, a well-lithified limestone that contains significant numbers of the Pentacrinus sp. and peloid fossils along some bedding surfaces. Photographs taken perpendicular to bedding were analyzed using the normalized Fry method. If the objects are on bedding surfaces from an overturned limb, then the shape of the central vacancy field, preserved in the overturned limb, gives a clear visualization of the initial direction of S_Hmax when the beds were originally horizontal, i.e., LPS-strain. The Fry plots and hand-fit ellipses of individual Pentacrinus sp. fossils were retro-deformed and rotated to match the trend of the Little Water Syncline, and thusly revealed an average shortening direction of 220°. This trend suggests that initial LPS was associated with the Sevier Orogeny, and subsequently reveals that the Sevier-associated thrusting occurred prior to the development of the Laramide-associated, NE-trending Little Water Syncline.