SEQUENTIAL AND INCREMENTAL FORMATION OF CONJUGATE SETS OF FAULTS

The evolution of a system of strike-slip faults in porous Entrada Sandstone of Arches National Park, Utah, is investigated here. The area includes domains of parallel faults, oriented either N30E or N60E, and domains of conjugate sets. The age relations indicate that conjugate faults do not form simultaneously, but that a few members of one set form and then are offset by a few members of a second set. Then, new members of the first set form and offset some members of the second set. In this way, a conjugate pattern is built up. Cross-cutting relations also indicate that individual faults do not form simultaneously along their lengths. The traces of individual faults are segmented, with individual segments ranging from 10 cm to 60 m. The cross-cutting age relations indicate that the individual faults form sequentially by longitudinal growth and coalescence of segments. In an area of two sets of conjugate faults, segments in the sets form alternately, so a segment in one set is older than some segments and younger than other segments in the other set. The field observations of temporal and geometrical relations among conjugate faults in porous sandstone are consistent with experimental observations of growing faults in clay. Domains of a single set of faults, the sequential growth of fault segments, and the alternating growth of two sets all have been observed in carefully controlled experiments by Reches. Members of one set of conjugate faults interfering with members of another set, recognized by Oertel, is reflected in multiple fault splays in the vicinity of intersecting faults. The similarity between the field and experimental observations suggests that the processes controlling these relations are not restricted to faulting in porous rocks.