Abstract
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 36Cl 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 ∼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 ∼8 and ∼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.
Highlights
Tectonic deformations in Iran result from the Arabia–Eurasia convergence
The Kopeh Dagh Mountains form a NW–SE active belt separating central Iran from Eurasia (Turan platform) (Fig. 1). This mountain range accommodates a significant part of the Arabia–Eurasia convergence not absorbed by the Makran subduction (Vernant et al 2004), involving thrust faulting, left-lateral strike-slip in the Western, and right-lateral strike-slip in the CentralEastern Kopeh Dagh (Afshar Harb 1979; Jackson & McKenzie 1984) mainly accommodated along a large intracontinental fault system
Morpho-tectonic investigations in the Kopeh Dagh Mountains 1057 review of the geological setting and structural framework of the Kopeh Dagh Mountains, we present the active faulting evidence along the main fault system (Bakharden–Quchan Fault) within this mountain belt
Summary
Tectonic deformations in Iran result from the Arabia–Eurasia convergence. This convergence took place by crustal shortening and strike-slip faulting in different intracontinental deformation zones, such as the Zagros, Alborz and Kopeh Dagh mountain ranges, and the active subduction zone of the Makran. According to the available geodetic data, this northward motion should be accommodated in northeastern Iran (mainly in the Kopeh Dagh Mountains) at a rate ranging from 4 to 10 mm yr−1 (Vernant et al 2004; Reilinger et al 2006; Masson et al 2007; Tavakoli 2007), given angular relationships between block motions and major structures (Fig. 1). The Kopeh Dagh Mountains form a NW–SE active belt separating central Iran from Eurasia (Turan platform) (Fig. 1) This mountain range accommodates a significant part of the Arabia–Eurasia convergence not absorbed by the Makran subduction (Vernant et al 2004), involving thrust faulting, left-lateral strike-slip (on minor faults) in the Western, and right-lateral strike-slip in the CentralEastern Kopeh Dagh (Afshar Harb 1979; Jackson & McKenzie 1984) mainly accommodated along a large intracontinental fault system. Analysing post-folding brittle deformation combined with the obtained fault slip rates leads to our estimate that the strike-slip faulting within the Kopeh Dagh Mountains began roughly 4 Ma
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