Anticlustering of small normal faults around larger faults

The Solite quarry in the Mesozoic Danville rift basin contains normal faults that conform to two spatial and size distributions. Larger master normal faults (20 cm < length [L] < 200 cm) are not numerous and have spanned the mechanical layer. The other faults are numerous, small (~0.1 cm < L < 20 cm), and exhibit anticlustering with respect to the larger structures, defining crack shields around the master faults. The shields are ellipsoidal in shape and geometrically similar to the elastic deformation fields of the master faults, and correspond to a critical stress-reduction shadow that prevented the nucleation of smaller faults in the vicinity of the master faults. The master faults likely formed earlier than the smaller faults, which nucleated en masse when some critical strain thrreshold was exceeded outside the master fault deformation field. The anticlustered spatial distribution of faults described here is the first field documentation for the existence of stress reduction shadows around normal faults.