The Namaras rock avalanche: Evidence of mid-to-late Holocene paraglacial activity in the Central Taurus Mountains, SW Turkey

The Namaras rock avalanche (NRA) deposit originated from the northern flank of the glaciokarstic Geyikdağ Mountain in the Central Taurus Range, SW Turkey. The deposit has an area of ~0.430 km2 and an estimated average thickness of 10 m, corresponding to 4.3 million m3 volume. The fan-shaped deposit area consists of house-sized Jurassic-Cretaceous neritic limestone boulders that overlie lateral and hummocky moraines down into the valley. We used geomorphological mapping and 36Cl surface exposure dating to obtain six boulder ages which unravelled the NRA age, the number of events and their geometric features. Our results indicate that the NRA consists of two successive mid-to-late Holocene events; the first high magnitude main event with a weighted average age of 4.59 ± 0.25 ka followed by a second low magnitude event with a weighted average age of 3.77 ± 0.20 ka. 36Cl exposure dating of the lateral moraine covered by the rock avalanche deposit yielded approximately an age of ~12.30 ± 1.20 ka. The older event with a maximum runout distance of 1550 m and 600 m elevation loss yielded in a travel angle of 21°. Similarly, the younger event with 1720 m maximum runout length and 640 m vertical elevation loss resulted in a travel angle of 20°. The bedded limestone with cross-joints in the glacial cirque preconditioned the NRA slope failure. The significant lag time between deglaciation and the rock avalanche indicates that the glacial erosion and debuttressing acted as preparatory factors. Based on the low seismic activity in the Central Taurus Range and the synchronicity of the rock avalanche with mid-to-late Holocene climatic transitions, we propose that climatic factors may have triggered the NRA failure. The age of the older main event coincides with the warm drought period of the mid-Holocene associated with torrential rainfalls in the Taurus Mountains, while the age of the younger event correlates with the long wet period in the late Holocene. Thus, the main NRA event may have been triggered by the influence of warm temperatures and intense rainfall, while the second event may have been triggered by prolonged period of high precipitation. The two events of the NRA are coeval with the enhanced rock slope failures dating back to 5–3 ka in the Alps.