EVOLUTION OF THE PRECAMBRIAN ROCKS OF YELLOWSTONE NATIONAL PARK (YNP): STRUCTURAL GEOLOGY

The Precambrian rocks of northern YNP exhibit a complex history of polyphase deformation. Pelitic schists experienced at least three folding events that include early isoclinal folds (F₁) that are only locally preserved and at least two stages of crenulation folds (F₂ and F₃). Generally, S₁ schistosity is parallel with bedding, S₀, and changes orientation from 010/045 in the western part of the study area to 350/015 in the east. F₂ crenulation fold hinges and L₂ mineral lineations plunge shallowly to the N-NE or S-SW. L₃ mineral lineations, though rare, are SE-trending and nearly orthogonal to L₂ and F₂. These structural relations are particularly well-defined near the confluence of Bear Creek and the Yellowstone River. A well-developed slaty cleavage cuts the bedding and primary schistosity at a high angle, and on the microscopic scale, mica grains are observed to have been rotated into this crenulation cleavage. A meter- to regional -scale open fold system has been identified in this area based on poles to foliation with calculated fold axis with trend and plunge of 173/22. This open fold system may be related to the finer-scale F₂ crenulation. Field observations and stereonet analysis confirms the structural interpretations of Jablinski et al. (1992). This area has also experienced extensive mylonitization, particularly in the igneous plutons and dikes and sills that intrude the metasedimentary sequences. The mylonites occur in distributed shear zones. Shear zones in the Blacktail Creek and Garnet Hill areas contain kinematic indicators that preserve a sinistral sense of shear, and have an average orientation of 359/38 whereas to the east in the Lamar River Valley to Slough Creek the average orientation is 018/58. Mylonites measured in the Hellroaring Creek area have an average orientation 333/41. Feldspars underwent both brittle and plastic deformation, as evidenced by fractures and minor nucleation of subgrains. These deformation mechanisms imply temperatures of roughly 500° C on samples with more plastic feldspars and 400-450°C for samples with more brittle feldspars. Some kinematic indicators such as asymmetric mica fish, tailed porphyroclasts, and shear bands indicate simple-shear plastic deformation. Folding of the metasediments occurred prior to emplacement of ca. 2.8 Ga plutons, but mylonitization post-dates granite emplacement