Concentrated slip zones with subsidiary shears: their development on three scales in the Cerro Brass fault zone, Appalachian valley and ridge

Concentrated slip zones bound subsidiary shears at three scales within the Cerro Brass Fault, which cuts Cambrian dolomites of the Nittany Anticlinorium, Pennsylvania. On the outcrop scale thrust faults along both Cerro Brass fault zone boundaries acted as concentrated slip zones bounding subsidiary shears developed along original bedding planes. The outcrop scale subsidiary shears are zones of concentrated slip bounding handspecimen-scale subsidiary shears. In turn, the hand-specimen-scale subsidiary shears are concentrated slip zones bounding even smaller-scale subsidiary shears. Subsidiary shears in Cerro Brass fault zone are analogous to subsidiary shears (i.e. R1 and R2) commonly found in laboratory gouge-friction experiments. The orientations of subsidiary shears at the outcrop and hand-specimen scales define a ‘Riedel within Riedel’ geometry in which the original bedding played the role of R1 shears at the outcrop scale and, at the same time, operated as boundary faults at a smaller scale. The presence of subsidiary shears on more than one scale suggests that the Coulomb failure theory is not sufficient to explain their origin.