Abstract

The Beartooth uplift is a large elongate (N.6S° W.) range located along the Montana-Wyoming border. The warped Precambrian peneplaned surface is an asymmetric anticlinal fold. Differential uplift of the basement amounting to more than 15,000 feet is greatest along the northern side of the crest. Along the northern and eastern borders of the uplift the crystalline rocks are thrust over Mesozoic sediments. Magnitude of thrusting is variable and is at least 10,000 feet at the northeastern corner where tears have developed in the thrust. The northeastern slope is dissected by glaciation and divided into large valleys and plateaus. The southwestern slope has a relatively uniformly inclined surface. Relief is more than 7000 feet on the northeastern slope and about 2000 feet on the southwestern slope.

Folded granitic gneisses and migmatites are exposed in the core of the range. The folds trend N.-S. to N. 20° E. and plunge southward. Rocks throughout the uplift are complexly fractured. Fracture studies over 700 square miles of the uplift include more than 25,000 fracture measurements made at 250 stations, located to give coverage of all parts of the uplift. The nature of fracturing is considered for the uplift as a whole, for subdivisions of the uplift, and at individual stations. A regional fracture pattern consists of four prominent trends: N. 15°W., N. 45° W., N. 45° E., and N. 65° W. Other trends are at N. 55°–60° E., N. 15°–20°E., N.-S. to N. 5° E., and N. 85° E. to E.-W. All these fractures are nearly vertical. The pattern is consistent and best developed on the southwestern slope. Low-dipping fractures occur in the northeastern part of the uplift and result from more intense deformation in the overthrust parts of the uplift. The inclined fractures do not form a consistent pattern.

Dikes and large fracture zones are shown on aerial photographs of the southwestern slope of the uplift. Fracture trends on photographs have been studied in detail. Photographs alone are not adequate for studies of fracture patterns, but ground measurements can be correlated with photo lineaments.

Five groups of dikes are found in the uplift; all have trends parallel to fractures: (1) Ortho-amphibolite dikes which may be pregranitization are the oldest dikes. (2) Archean metabasaltic dikes, somewhat younger. (3) Unmetamorphosed late Precambrian quartz dolerites. The cycle of metamorphism and granitization is dated at 2.7 billion years. (4) Olivine dolerite dikes, probably Tertiary. (5) Laramide porphyry dikes. Dikes of different ages occupy the same fracture trends; hence the dikes cannot be used to differentiate the fracture patterns. The dikes show that the fracture patterns are Archean.

The major fracture trends indicate that the pattern was formed by warping with relative shortening along the northwest trend, resulting in two sets of tension fractures parallel and perpendicular to maximum shortening. Two sets of conjugate shears also formed at angles of 25°–30° with the direction of maximum shortening. Other fractures appear to have formed in a later deformation when the areas outlined by early major fracture zones were deformed as blocks. These fractures formed with reference to maximum shortening of the blocklike masses. Movements in the core of the uplift in the Laramian were again governed by the pre-existing fractures.

First Page Preview

First page PDF preview
You do not currently have access to this article.