Recent faulting in the southern Arava, Dead Sea Transform: Evidence from single grain luminescence dating

Abstract

Colluvial sediments in tectonically active areas are associated with faults, and dating them is important for timing earthquakes and assessing seismic hazards. In hyperarid regions organic matter for radiocarbon is scarce, and the dating method of choice is luminescence. However, colluvial sediments are difficult to date due to the short transport distance that is commonly involved and the resulting incomplete bleaching of the sediment. Single grain and small aliquot measurements of fault-related colluvial sediments may isolate well-bleached grains and provide reliable ages of faulting events. Elat, southern Israel, lies on the active Dead Sea Transform and paleoseismic data are crucial for city planners. A small fresh-looking NNE trending graben within the late Pleistocene to Holocene alluvial fan of Nahal Shehoret near Elat was trenched to expose recently active normal faults with a stack of colluvial wedges on the footwalls. Samples for luminescence dating were collected near the eastern fault, from the alluvial fan and from the heels (less than 1 m from the fault plane) and toes (∼2 m away from the fault plane) of a stack of five colluvial wedges. Single grain measurements of quartz were compared with large (thousands of grains) and small (hundreds of grains) single aliquots of quartz and alkali feldspar (KF). KF was measured using the infrared stimulated luminescence (IRSL) signal and the single aliquot added dose (SAAD) or single aliquot regenerative dose (SAR) protocols. Large aliquots and single grains of quartz were measured using the optically stimulated luminescence (OSL) signal and the SAR protocol. Large and small aliquots show large scatter in equivalent dose values, with a distinct population of young ages. Quartz large aliquot ages of samples taken from the toes of the wedges range from 5200 to 2600 years, however the ages of samples taken from the heels, close to the fault, are in reversed order, from 4300 years at the base to 13,000 years at the upper wedge. This results from progressive exposure and erosion of deeper and older beds on the fault scarp with continued faulting, and incorporation of older poorly bleached grains into the upper wedges. Only single grain measurements could isolate a distinct, small population of grains that were fully bleached at the time of deposition, dating the colluvial wedges to between 500 and 1300 years. Large aliquots ages substantially overestimated the time of faulting. The single grain ages record very young faulting events which may be correlated with the known historical earthquakes of the past 1500 years.