Abstract

Microjoints, defined as four or more macroscopic subparallel fractures spaced closer than 3 mm, appear as pervasive fracture systems in about half the Precambrian basement outcrops of Montana and Wyoming. Six thousand five hundred orientation measurements of the microjoints indicate that the common local pattern is two nearly vertical, perpendicular sets. Persistent vertical orientation at most, but not all, mountain fronts which have undergone demonstrable Laramide rotation of basement indicates microjoints developed subsequent to the bulk of Laramide deformation. The microjoints are interpreted as expansion features in Laramide block mountains, created as the blocks were lifted free of the confines of the adjacent basin floors.

Microjoint systems simulate the orientations of ordinary joints, of Precambrian dikelets, and of most planes of microscopic fluid inclusions but are independent in degree of development. In thin section the microjoints are represented by one class of very tiny fluid inclusions paralleling and locally reopening older planes of inclusions. The microscopic fluid inclusion planes are interpreted as a system of Precambrian weakness directions partially controlling the orientations of all subsequent fracture systems including Laramide microjoints.

On a regional scale the microjoints have a number of recurring directions similar to Spencer's (1959) common joint directions in the Beartooth Mountains, Montana. Thus, the available measurements suggest a regional system of eight recurring fracture directions.

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