Criteria for complete zeroing of ESR signals during faulting of the San Gabriel fault zone, southern California

In using electron spin resonance (ESR) to date the time of the last movement on a fault, it is essential to demonstrate that the ESR signal was fully zeroed at the time of faulting; if the signal was not completely reset, then the ESR age obtained will be too great. We have refined the criteria for complete zeroing of ESR signals at the time of fault movement. Samples of fault rocks were collected from surface outcrops of the San Gabriel fault zone in the Little Tujunga region near Los Angeles, southern California. Using quartz grains of < 250 μm, the estimated ages for either E' or OHC signals in all fault rock samples analyzed decrease with grain size down to a plateau value for particles smaller than some critical grain size. In some cases, the Al signal in the smallest grains yields the same age as the plateau age of either the E' or OHC signal. Plateau ages of reactivated younger gouges were indeed smaller than those of the older fault host rocks. Within the plateau region, different grain size fractions show different radiation sensitivity for a single ESR signal, but all converge to the same DE (equivalent dose) value. This indicates that the complete resetting probably occurred during fault movement. The depth of fault rocks during fault movement can be estimated from uplift rates and the ESR ages. The estimated uplift rates was about 0.3 cm/yr for the Holocene and 0.03 cm/yr for the late Pleistocene (Menges et al., 1979). The estimated plateau ESR ages ranged from 1200 to 135 ka. The depths at the time of the last fault movements in the Little Tujunga region were in the range of 68–360 m. The measured maximum horizontal stresses at this depth in southern California range from 5 to 10 MPa. For a depth of 70 m, the calculated vertical stress is 1.16 MPa and measured maximum horizontal stress is about 5 MPa. The angle between the strike of the San Gabriel fault and the direction of maximum horizontal stress is about 85°. Using above values, calculated shear stress on the fault is about 0.67 MPa and the normal stress to the fault is about 5 MPa. If we assume mean displacements of 4 cm (M 4) to 1 m (M 7), these conditions were sufficient to reset ESR signals in quartz below critical grain size at the time of fault activity.