Geometry and mechanics of secondary fracturing around small three-dimensional faults in granitic rock

The orientations, locations, sizes, and relative abundances of secondary fractures observed along small natural faults can be accounted for by a three-dimensional elastic model. Secondary fractures along small subvertical left-lateral strike-slip faults in massive granitic rock of the Sierra Nevada of California (1) consistently strike 25 degrees +/- 10 degrees counterclockwise from their host faults and dip at angles greater than 80 degrees; (2) generally are absent along the central portions of the fault traces; (3) are numerous near the ends of some fault traces but absent along others; and (4) in rare cases form echelon arrays either centered along a fault trace or just past the fault trace ends. These observations are consistent with secondary fractures that nucleated near the perimeter of an elliptical fault along a "cohesive rim" of high slip resistance and propagated in three dimensions normal to the local most tensile stress. The fracture orientations relative to the faults reflect small stress drops during slip on the faults. The observations and model together have direct implications for how faults grow and conduct fluids. Secondary fractures are likely to be larger at the ends of small strike-slip faults rather than at their tops and bottoms. As a result, if strike-slip faults grow in an unrestricted manner, they are more likely to be linked end-to-end rather than top-to-bottom, especially where slip is small. Hydraulic conductivity is likely to be enhanced at the linkages between faults, so highly conductive regions along linked strike-slip faults are more likely to be vertical rather than horizontal.