干旱区生态环境知识资源中心

P>The Kopeh Dagh is accommodating a large portion of the northward motion of central Iran with respect to Eurasia, involving a major right-lateral strike-slip fault system (Bakharden-Quchan). This fault system corresponds to the northeastern boundary of the Arabia-Eurasia collision, and can be considered as a lithospheric-scale tectonic feature. We present a well-constrained estimation of late Quaternary slip rates along two major strike-slip faults (the Baghan and Quchan faults) in this fault system, using in situ-produced Cl-36 nuclide to date two offset alluvial fan surfaces. Combining detailed satellite image and digital topographic data analyses complemented with geomorphic fieldwork allows quantifying the cumulative offset values of 940 +/- 100 and 360 +/- 50 m of the fan surfaces along the Baghan and Quchan faults, respectively. A total of 12 carbonate boulders from the fan surfaces were collected and dated. This yields minimum age of two episodes of fan abandonment at 280 +/- 16 (Baghan fault) and 83 +/- 4 ka (Quchan fault). Age estimates and measured offsets of the fans are consistent with respective maximum long-term fault slip rates of 2.8 +/- 1.0 and 4.3 +/- 0.6 mm yr(-1) for the Baghan and Quchan faults over the Middle-Late Pleistocene. Applying the slip rates to cumulative post-folding offsets along the Baghan and Quchan faults indicates that strike-slip motion within the Kopeh Dagh may have started similar to 4 Ma. This constrains the timing of a major tectonic reorganization in the Kopeh Dagh, previously recorded through Arabia-Eurasia collision between 3 and 7 Ma. At the regional scale, the sum of total cumulative strike-slip offsets is about 35-40 km, which implies a total maximum slip rate of about 9 +/- 2 mm yr(-1) in the Central-Eastern Kopeh Dagh. This is resolved to average northward and westward slip rates of similar to 8 and similar to 4 mm yr(-1), respectively, for the Western Kopeh Dagh with respect to Eurasia. Our results also suggests that the localized strike-slip faulting in the Central Kopeh Dagh can be considered as an intercontinental movement between northeast Iran and Eurasia, accommodating about 80 per cent of northward motion of central Iran with respect to Eurasia.