Late Holocene Slip Rate for the North Anatolian Fault, Turkey, from Cosmogenic 10Be Geochronology: Implications for the Constancy of Fault Loading and Strain Release Rates

Measurement of a stream offset and cosmogenic dating (¹⁰Be) of the alluvial surface into which the stream incised yield a preferred late Holocene slip rate of 19 ± 3.5 mm yr^-1 for the central part of the North Anatolian fault at Tahtaköprü, Turkey; use of variable cosmogenic production rate (VPR) models yields a slightly slower rate of ~16.5 ± 4.5 mm yr^-1. The offset drainage (Karanlık Dere), which flows southward almost perpendicular to the east-west trace of the NAF at the site, is displaced right-laterally by 57.5 ± 7.5 m, with no vertical displacement. A ¹⁰Be age of ~3 ka (~3.5 ka VPR) from the top of a boulder on the best-preserved part of the incised alluvial surface provides the most reliable maximum age for the onset of incision; eleven 10Be ages from cobbles collected from the cultivated surface to the south yield younger ages, consistent with exhumation, erosion and mechanical mixing during plowing. Our 19 ± 3.5 mm yr^-1 rate is similar to other geologic slip rates measured along the NAF, all of which cluster between 15-20 mm yr^-1 over a wide range of (10³-10⁵) time scales. All of these geological rates, however, are slower than the 25 ± 1 mm yr^-1 short-term rate of elastic strain accumulation measured geodetically. This disparity suggests the possibility that the NAF is experiencing a strain transient in which the lower crust beneath the fault is deforming faster than its long-term rate, possibly in response to long-lived effects of the 20^th century sequence of large magnitude NAF earthquakes.