UNLOCKING NEW GEOMETRIC CONSTRAINTS OF SHEAR/FAULT ZONE KINEMATICS PLOTTED ON STEREONETS

Lower hemisphere, equal-area, stereographic projections, or Schmidt stereonets, have long been used to help understand the geometry of complex geologic structures. This study, in an effort to solve multiple problems, is an attempt at unlocking key aspects of stereonets, which can be used in shear/fault zone kinematic analyses to geometrically and stereographically elucidate the direction of movement. Although the stereographic technique to confine a shear/fault zone's motion to dip-slip, strike-slip, or oblique-slip exists, we present a new technique that enables one to solve the specific sense of movement in both the dip-slip and strike-slip directions. Focus was put upon ductily/plastically deformed rocks from southeastern Appalachian shear/fault zones, i.e. mylonites, phyllonites, tectonites, gneisses, button-schists, etc. The key to success of this new technique is first to understand the field relationship(s) of the penetrative mylonitic fabric(s) in the rock, then to plot and analyze the measured data on a stereonet. The direction of the fault/shear zone movement, either locally or regionally, may be determined using the relationships between the schistosity (S) and cisaillement (C) planes, the plotted poles to these planes, and their relation to the each other along with the primitive and center of the stereonet. This new technique allows for the solving of movement direction in the dip-slip (reverse/normal), strike-slip (dextral/sinistral), and oblique-slip motion(s). Paired with detailed field and petrographic analysis, it is a critical piece of the puzzle in understanding shear/fault zone kinematics geometrically and stereographically to ultimately resolve the overarching structure(s).