I collected this in ~1999 while working with Alex Bump along the Comb Ridge monocline on the east flank of the Monument Uplift. It was another beautiful day working on the Colorado Plateau, and we were positioned inside the steeply dipping limb of the giant Comb Ridge monocline. Surrounded by such BIG structure, it was quite something to see the delicacy of small-scale tectonic adjustments within individual outcrops, and to be reminded again of how each lithology may have its own way (its own mechanisms) for accommodating and expressing deformation. In this case we are seeing semi-brittle faulting.
This is a perfect example of a crystal-fiber vein with crystal-fiber lineation. The source of the calcite comprising the vein is undoubtedly the limestone itself. When clayey limestone is subjected to tectonic stress, it creates an environment where calcite becomes more soluble, even in the absence of elevated temperature. Pressure solution causes the calcite to go into solution. It then re-precipitates nearby in tensional openings. This particular tensional opening, strictly speaking, was not a joint, but rather a fault marked both by shear and dilation. The fault progressively opened and was simultaneously filled by the calcite. Mineral lineation orientations disclose the slip direction of shear. The actual sense of shear can be determined as well, for the crystal fibers meet the wall of the fracture at a small acute angle. The tiny acute angle between fiber and a given wall “points” in the direction of relative movement for given wall rock to which the vein is attached.
Age & Formation
Lithographic limestone from the Jurassic Carmel Formation, ~165 Ma. Time of vein-filling of fracture/fault not known, but probably in the latest Cretaceous/earliest Tertiary (~65 Ma) or mid-Tertiary (~25 Ma).