Abstract

Even though “rift” and “grain” (R/G) are New England quarrymens' terms for ease-of-fracture, somewhat similar structures appear in basement exposures throughout the Rocky Mountains of Montana and Wyoming. However, the nature, origin, and structural utility of these subtle features are generally unrecognized in the western uplifts. R/G represent directions of easy mesoscopic splitting produced by closely spaced microscopic fracture planes in quartz, now healed into thin zones of very tiny fluid inclusions. At hand-specimen to outcrop-scale these represent strength anisotropies that can exert major controls in the orientation of a host of younger types of joints and other fractures, one of many types of tectonic heredity.

In the Rocky Mountain uplifts, strike of R/G can remain constant over 100's of km2, commonly with strikes parallel to adjacent mountain fronts. A late to post-Laramide age is indicated by dips remaining vertical even in strongly tilted mountain fronts. It is argued that these microfractures were produced by stresses generated by differential thermal contraction along the several crystallographic axes of quartz grains. Thermally induced grain-scale stresses directionally augmented by even weak regional stress fields produced the observed systematic regional patterns. Temperatures well above 150°C have been documented for some microcrack origins, but these Rocky Mountain examples are limited by stratigraphic cover and association with eroding mountain fronts to formation at temperatures below 80°C, possibly as low as 50–60°C, and depths as shallow as 1–2 km.

Some of the R/G orientational domains represent older stress fields, but the mountain-front parallel ones are essentially snapshots of the last gasps of dying Laramide stress fields. The relationships suggests formation by volumetric expansion and interchange of σ1, σ2, and σ3 stress fields as decreasing confinement by adjacent basin fill allowed the uplifted mountain mass to begin gravitational spreading.

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