We provide the first measurement of strike‐slip and shortening rates across the 200‐km‐long right‐lateral strike‐slip Main Köpetdag Fault (MKDF) in Turkmenistan. Strike‐slip and shortening components are accommodated on parallel structures separated by ∼10 km. Using Infra‐red‐stimulated luminescence and reconstruction of offset alluvial fans we find a right‐lateral rate of 9.1 ± 1.3 mm/yr averaged over 100 ± 5 ka, and a shortening rate of only ∼0.3 mm/yr averaged over 35 ± 4 ka across the frontal thrust, though additional shortening is likely to be accommodated locally by folding and faulting, and regionally within the eastern Caspian lowlands to its south. The MKDF is estimated to have ∼35 km of cumulative right‐lateral slip which, if these geological measurements are correct, would accumulate in only 3–5 Ma at the rate we have determined, suggesting that the present tectonic configuration started within that time period. We use the MKDF slip‐rate to form a velocity triangle, from which we estimate the Iran‐South Caspian and Eurasia‐South Caspian shortening rates, and show that the South Caspian Basin moves at 10.4 ± 1.1 mm/yr in direction 333° ± 5 relative to Eurasia and at 4.8 ± 0.8 mm/yr in direction 236° ± 14 relative to Iran. In contrast to both the eastern Köpetdag and the Caspian lowlands the MKDF has little recent or historical seismicity. The rapid slip‐rate estimated here suggests that it is a zone of high earthquake hazard.
The Main Köpetdag fault (MKDF) of Turkmenistan is one of the longest (≈500 km) and the most rapidly straining (9.1 ± 1.3 mm/yr) faults of the Arabia–Eurasia collision zone, and yet, in contrast to adjacent parts of Turkmenistan and Iran, it has little in the way of observed seismicity. Field observations indicate a fresh paleorupture along an ≈100 km long section of the MKDF with abundant streams offset across it. We use WorldView-2 optical satellite imagery to measure geomorphic offsets and compile them into a cumulative offset probability density (COPD) function. The COPD yields two peaks in offset density at 7.5 ± 1.5 and 12.5 ± 1.5 m, when considering the highest and the lowest quality measurements, whereas the medium-quality measurements present a single peak at 9 ± 3 m. We are unable to separate peaks into distinct events when exploring the contributions of offset measurements along strike that have significant variability. The paleorupture displaces archaeological remains, including extensive field boundaries of likely medieval age, and a paleoseismic trench brackets the age of the most recent displacement at 600–800 yr B.P. From our measurements of displacement, we estimate that the most recent paleoearthquake had a moment magnitude (Mw) 7.6 ± 0.4 and yet is missing from earthquake records in nearby Iran, indicating that the paucity of historical earthquakes in Turkmenistan may be misleading. The Baharly, Döwgala, and Gökdepe segments of the MKDF, which extend eastward toward Aşgabat, do not show fresh surface expression. Infrared-stimulated luminescence ages of 3.4 ± 0.5 and 2.5 ± 0.2 ka from a river-cutting exposure along the Baharly segment predate the most recent paleorupture. We conclude that individual segments of the MKDF are capable of rupturing in major (Mw>7.0) earthquakes, with ends of individual ruptures possibly controlled by segment boundaries. The occurrence of major earthquakes and rapid slip rate suggests that the MKDF constitutes a hazard to a populated region of Turkmenistan and northeast Iran.
SUMMARY The Main Köpetdag Fault (MKDF) is a predominantly right-lateral strike-slip fault that dissects the northern edge of the Köpetdag mountains of Turkmenistan and Iran. The fault represents the northernmost expression of deformation from the Arabia-Eurasia collision to the east of the Caspian Sea, and plays an important role facilitating the motion of the South Caspian Basin (SCB). Despite the kinematic significance of the MKDF, previous geodetic measurements of the slip-rate across the fault have been highly variable, with a recent geological slip-rate supporting evidence for rapid motion across the fault. To resolve this ambiguity, we derive Sentinel-1 InSAR time-series in both the ascending and descending LOS (line-of-sight) to measure interseismic motion across the MKDF. This implements a processing strategy for the correction and performance analysis of tropospheric models from GACOS (Generic Atmospheric Correction Online Service), which suggests a 25–40 per cent under-estimation in the amplitude of the tropospheric path delay in this region. Modelling the rate of fault-parallel motion across the MKDF and combining this with a geological slip-rate constrains 9 ±2 mm/yr right-lateral motion, along with a shallow locking depth of ≈6 km. The LOS time-series resolves path delays of greater than 1 mm/yr in both LOS geometries to the north of the MKDF, aligned with mapped frontal thrust scarps to the north of the MKDF. Modelling this as uplift from deformation across these frontal thrusts estimates 4 ±2 mm/yr of shortening across the fault system. This implies that convergence measured with GNSS within the East Caspian Lowlands is partitioned across frontal thrusts to the north of the MKDF. Revising an Iran-Eurasia-SCB velocity triangle suggests that motion along the conjugate MKDF and Shahroud fault zones is representative of the motion of the SCB, which is moving at 10 ±2 mm/yr in a direction of 330○ ± 10○ relative to Eurasia.
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