RIFT AND GRAIN (MICROJOINTING) IN BASEMENT AS THERMALLY-TRIGGERED RECORDS OF TRANSIENT LARAMIDE STRESS FIELDS DURING UNROOFING OF THE MIDDLE ROCKY MOUNTAINS

Patterns of microjoints in basement exposures throughout the Middle Rocky Mountains were described by Wise in 1964. Near-vertical dips of these fractures even in strongly rotated Laramide mountain fronts prove a late or even post-Laramide age whereas domains of sub-parallel orientation extending over areas of 100’s of km2 indicate origin in some broader stress fields. Microfractures or microjoints in granitic basement, the rift and grain (R&G) of quarrymen, have received some limited attention in structural geology literature in recent years. Such features are now generally recognized as partially healed mode I microcracks best developed in quartz grains but their PT conditions and tectonic environment of origin are poorly constrained. Temperatures in the 150-250ºC range are typically cited values. Some current discussions suggest origin at high differential stress levels but modifying factors may be involved. Strong differences in coefficients of thermal contraction along crystallographic axes of irregular quartz grains intertwined with other minerals can generate significant local strains. At higher temperatures, crystal-plastic processes preclude or heal microfractures but below some closing temperature any newly formed fractures survive. In an isotropic stress field, such fractures should have no preferred orientation but presence of even a weak stress field could determine which orientations of incipient fractures actually develop. The Laramide examples suggest maximum conditions of 3-4 km depth from stratigraphic constraints and temperatures around 100º C as recorded by fission track, He retention, and regional geothermal gradients. Comparatively uniform development of the R&G across entire uplifts, orientations parallel to or at high angles to uplift axes or mountain fronts, and a late to post-frontal rotation age suggest possible origin by gravitative spreading of the uplifted mountain mass. Once temperatures in the unroofed basement passed through 125 - 75ºC isotherms, the snapshots of transient stress fields had been taken, R&G-forming events ended, and more normal jointing processes took over, albeit influenced by recently acquired microfracture anisotropy.