We investigated temporal and spatial patterns of Quaternary fault activity of the San Gabriel fault zone in the Little Tujunga region, southern California, using the electron spin resonance (ESR) plateau dating method. Two major fault zones (San Gabriel and Santa Susana–Sierra Madre) and folds are developed in the Little Tujunga region. The traces of thrust faults and folds of the Santa Susana–Sierra Madre fault zone are roughly parallel to the San Gabriel fault zone, indicating that the maximum horizontal stress was nearly perpendicular to the San Gabriel fault zone. Bends in the main strands of the San Gabriel fault zone yielded local transpressive regimes and changed the direction of maximum horizontal stress to a lower angle to the main strands, resulting in the development of subsidiary faults and folds oblique to the main trend of the fault. These structural features can be explained by the process of low drag-decoupled shear combined with transpression. The temporal pattern of partition between compressional and strike-slip deformation in the Little Tujunga region is not clear yet.
ESR dates from both the main strands and subsidiary faults range from November 1970 to 40 ka. The dates show temporal clustering into active and inactive periods, analogous to those seen in historic and Holocene earthquake fault activity. Within a given active period, fault activity is widely distributed along a restraining bend in the San Gabriel fault zone. Historic earthquake faults similarly show that fault strain was prohibited in restraining bends and that fault activities spread out up to several kilometers away from the main strand. Some faults were reactivated, producing well-defined bands of reactivated fault gouge within old zones of host fault rock, which exhibit younger ESR dates than those of the host fault rocks.
The San Gabriel fault zone was formerly considered to be an exhumed ancient fault of the San Andreas fault system. Results from this study suggest that, although much less active than the San Andreas fault zone (average recurrence interval in Pallett Creek, 132 yr), the long-term (80–120 k.y.) cyclic fault activity of the San Gabriel fault zone continued into the Pleistocene. These results show that ESR dating can be used to evaluate the recurrence intervals of paleoearthquake fault movements and crustal stability with a temporal resolution of 5%–10%.