Geologic Evolution of the Beartooth Mountains, Montana and Wyoming. Part 4. Relationship Between Precambrian and Laramide Structures in the Line Creek Area

摘要

The Line Creek area is at the eastern margin of the Beartooth Mountain block, Montana and Wyoming. The range front here changes in trend from due north to N. 45° E. The Precambrian rocks of the range were uplifted and thrust toward the Bighorn basin during Laramide time, over-turning Paleozoic and later strata. The sedimentary rocks at Line Creek were folded about steep, nonparallel axes. The Precambrian rocks include migmatites, granitic gneisses, metasedimentary rock, and ultra-mafic and mafic intrusions. The Archean history is one of sedimentation, folding, igneous intrusion, metamorphism, and granitization which ended 2750 million years ago. All rocks were later intruded by felsic Laramide porphyry. In the core of the range the crystalline rocks preserve open folds trending north to N. 20° E. Small-scale structures show that during granitization at Line Creek a large synform developed, the axis of which trends N. 45° E. The division of the map area into three subareas within which fabric directions are consistent results from the analysis of the penetrative fabric. The three subareas are: a north-central subarea of north-northeast-trending folds; a central subarea of northeast-trending folds which includes the Line Creek synform; and a southeast area of mainlv east-trending folds. Most folds plunge west to southwest at shallow angles. A causal relationship between the northeast-trending folds and the boundary fault is implied by the proximity and parallelism of the two. The northeast-trending Laramide boundary fault follows trends established during the granitization episode. In Precambrian and overturned Paleozoic rocks steep fractures parallel the trends of the range core and also Precambrian dikes. Fractures at the range front indicate that after their formation gravity spreading was not an important process. Planar shear zones dip westward at moderate angles and southeastward at steep angles. They lie in a position predicted by a theory of vertical uplift. Steep folds in the overturned sedimentary rocks bordering the range have resulted from Laramide thrusting of the two oblique-trending boundary fault segments. An area of northeast-southwest compression occurs where the segments join. Horizontal slip took place along the N. 45° E.-trending segment of the frontal thrust during the last stages of Laramide uplift when a large east-west tear fault developed at the northeast corner of the Beartooth Range. This slippage caused stretching along the mountain front and formed two gaps in the sedimentary palisades that mantle the range.