ABOVE AND BELOW ARE EXAMPLES OF SLICKENSIDES, FOUND IN RELATION TO ROTATING AND SHEARING CORIOLIS CELLS IN WASHINGTON COUNTY, UT: SLICKENSIDES EXIST ONLY ON THE FACE OF ROCK, WHERE SLIDING, SHEARING, AND ETCHING TRANSPIRE. COMPARE THE METAMORPHIC ROCK ON THE LEFT WITH SLICKS ON THE SURFACE OF A SEDIMENTARY SPECIMEN:
Slickensides as INDICATORS of LATERAL SHEAR AND ROTATION
Striated and hard-faced coating on outcrops (which are softer than the scale, coating them) yield valuable information about movements of the CRUST OF THE EARTH. Should the striations have parallel lines or scratches, they indicate that the movement was along the direction of the striations. Additionally, should the striations have an orientation other than vertical, there is an additional bit of information; the diagonal or non-vertical scratches or grooves indicate that the movement was more than a simple vertical displacement. An example is shown below, where the striation is inclined to the horizontal by 60 degrees. This case has the horizontal component ½ that of the vertical. That is, the lateral movement was half of the vertical movement (tan apprx.= sin 30 degrees= ½). The vertical drop in a NORMAL FAULT is usually the one sought by field workers, since this indicates the most-easily understood and plotted displacement. This masks the most valuable movement- that of lateral or sideways displacement- and is the most difficult to determine, since there is NO SEDIMENTARY ROCK DEPOSITIONAL REFERENCE. The case shown is located on the edge of the Colorado Plateau, CP, along the Hurricane fault, Hf, and it yields the rare glimpse that CP is moving CCW- south along Hf:
Interpreting this is assisted by finding that the graben drops vertically while sliding southward on both sides (track a pebble, making striations by etching the lines, while the etched face moves southward). I HAVE TAKEN A PHOTO AFTER A SNOW, SO THAT THE FLAT COATING IS PRONOUNCED.
Slickensides are valuable for documenting lateral movement, and fortunately they are preserved by the extremely hard facing- composed of chert-like silica, because of drop of pressure at the plane of sliding. The silica is dissolved from limestone by the extreme stress of rotation and heat (laterally or diagonally), and it remains in solution until the fracturing or lateral faulting creates strain- the movement associated with stress relief. Limestone allows easy recognition, because of the contrast of hardness and composition. Other rocks such as striated basalt, are more difficult to spot, since the rock is already a silicate, and the color may be similar to the host. The striations or grooves are the most valuable finding, and they might not be preserved any more than the host- which degrades with weathering.Below are striations which were found in Sullivan knoll basalt, which occur on a loose boulder and therefore cannot yield the orientation of the basalt which was rotating after cooling:
Notice that the striations are almost vertical; the usual case I have found is that there is always a lateral component,SUCH AS IN THE B&R LATERAL FAULT BELOW.