QUATERNARY LOW-ANGLE NORMAL FAULTING

Seismological analysis of normal-fault earthquakes has been interpreted to indicate that such faults are not active at dips less than ~25°, and led to the inference that inactive, gently dipping normal faults were tilted from steeper dips after faulting ended. In contrast, several Quaternary normal faults exhume footwalls that reveal gentle fault dip, including the following: (1) The tectonically exhumed footwall at the north foot of Dayman dome in Papua New Guinea dips 15° to 24°. (2) The Gurla Mandhata detachment fault in the Himalaya dips southward 10° to 19°. (3) The deep-sea core complex at 13° 19′ N latitude on the mid-Atlantic ridge is bounded by a normal fault with a dip of 17°. (4) The normal fault at the northern foot of the Tokorondo Mountains in central Sulawesi, Indonesia, dips 11° to 18°. (5) The normal fault at the northern foot of the Pompangeo Mountains, also in central Sulawesi, dips 13° to 4°. Fitting a critical-taper stability field to data points from these complexes, and several others, suggests that the theoretical minimum normal-fault dip for stable sliding is 3 degrees, far less than inferred from seismological studies. This discrepancy may have several explanations: (1) In all cases, exhumed footwalls are arched, forming core complexes. Downward projection of curved faults leads to greater fault dip at depth. Most seismic moment release may be on the deeper, steeper part of such faults, with displacement of the thinly tapered end of the hanging-wall block representing only a small fraction of moment release identified by moment tensor solutions. (2) Active low-angle normal faults are rare and possibly have not yielded a major earthquake during recent decades of modern seismology. In conclusion, active low-angle normal faults are real, and it is not unreasonable to incorporate them into interpretations of the evolution of orogenic belts.