ESR Plateau dating of fault rocks

Abstract

<p>Past movement on faults can be dated by measurement of the intensity of ESR signals in quartz. These signals are reset by lattice deformation and heating on grain contacts during faulting. The ESR signals then grow back as a result of bombardment by ionizing radiation from surrounding rock. The age is obtained from the ratio of the equivalent dose, Dᴇ, needed to produce the observed signal, to the dose rate. Fine grains are more completely reset during faulting, and a plot of age vs. grain size shows a plateau for grains with radius r < 75 μm; these grains are presumed to have been completely zeroed by the last strain event, the age of which they record with a precision of 5-15 %.</p> <p>Two major fault zones (San Gabriel and Santa Susana-Sierra Madre) and folds are developed in the Little Tujunga region, in southern California. The trace 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.</p> <p>Type I and II fault zones in a single outcrop can give us the age of each stage of the evolution of fault rock zones. For Type III fault zone, we can determine only the age of last movement. The sequence of ESR plateau ages is consistent with the sequence of fault movement recognized by geological intersection relationship.</p> <p>ESR ages from both the main strands and subsidiary faults range from 1170 to 40 ka. The ages show temporal clustering into active and inactive periods, analogous to that seen in historic and Holocene earthquake fault activity. Within a given active period, activity is spread out at 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 km away from the main strand.</p> <p>The San Gabriel fault zone was formerly considered to be an exhumed ancient fault of the San Andreas fault system. The results suggest that although much less active than the San Andreas fault zone (average recurrence interval in Pallett Creek: 132 a; Sieh et aI., 1989), the long term (80-120 ka) cyclic fault activity of the San Gabriel fault zone continued during the Pleistocene.</p> <p>I have studied the temporal distribution of paleoseismic records of earthquakes in several regions of California over time scales ranging from decades to several hundreds of thousands years, using dates obtained by ESR plateau dating of fault gouge, ¹⁴C dated sediments from fault zones and records of historic earthquakes. The historical record in the San Andreas fault zone of central California, and the paleoseismic record of the San Gabriel fault zone both exhibit self similarity with a fractal dimension of 0.43-0.46. The fractal dimension of the San Andreas fault zone in Southern California both in historic and paleoseismic time scales is 0.67 indicating more evenly distributed fault movements than that of the central California. On time scales < 1 y, the distribution is largely random.</p> <p>Seven out of ten samples collected at the URL site of AECL, Pinawa, Manitoba were saturated. The three ESR plateau ages appear to lie in interglacials, which is consistent with a model in which movement on the thrust fault is triggered by melting of an ice sheet at the end of a glacial stage.</p>