Active tectonics of the Kopeh Dagh mountains, NE Iran

Category Tectonic & Seismotectonic
Group GSI.IR
Location The26 Symposium of Geoscience
Holding Date 05 February 2008
Active tectonics of the Kopeh Dagh mountains, NE Iran

 

James Hollingsworth 1, Amir Bolourchi 2, James Jackson 1, Nasser Niemi 2,  Richard Walker 3

1 Bullard Laboratories, Madingley Road, Cambridge CB3 0EZ, UK

2 Geological Survey of Iran, Azadi Sq., Meraj street, P.O. Box 13185-1494, Tehran, Iran

3 Department of Earth Sciences, University of Oxford, Oxford OX1 3DP, UK

 

 

Active deformation in NE Iran is confined to a series of sub-parallel mountain ranges, which separate relatively aseismic basins. Using field and remote observations of the geology and geomorphology we investigate the active tectonics of the Kopeh Dagh mountain range, which forms the northern limit of Alpine-Himalayan deformation in NE Iran.

 

The Central Kopeh Dagh has experienced a number of large historical and recent earthquakes. A system of NNW-SSE right-lateral strike-slip faults (Bakharden-Quchan fault zone) cut obliquely across this part of the range, terminating in thrust faults along the Atrak valley to the south, which separates the Kopeh Dagh from the Alborz. These thrust faults form small regions of uplift, across which rivers have incised, and are typical of thrust terminations seen in systems of rotating strike-slip faults. Although the pattern of faulting in the Central Kopeh Dagh is relatively complicated, the tectonics are dominated by three main faults: the Baghan, Quchan and Bajgiran faults, which are probably responsible for the known historical earthquakes. Their significance to the regional tectonics appears to be in achieving an across-strike shortening and along-strike extension by rotating about vertical axes. This is confirmed by the anticlockwise rotation of geology across the fault zone. Simple calculations using the cumulative offset between faults, and their associated rotation, can account for ~60 km of NS shortening, since the onset of strike-slip faulting. This value is similar to estimates of the Late Quaternary NS right-lateral shear between central Iran and Afghanistan, which must be accommodated in NE Iran. The strike-slip faults also require ~30 km of along-strike extension of the Kopeh Dagh, which is taken up by the westward component of motion between the South Caspian Basin and both Eurasia and Central Iran. This is partly accommodated by subduction of the South Caspian beneath the Apsheron-Balkan sill and Talesh mountains.

 

West of the Bakharden-Quchan fault zone, shortening in the NW Kopeh Dagh is partitioned onto strike-slip and thrust faults along its north and southern edges, with some along-strike extension also accommodated by diffuse left-lateral fault systems within the range. The westward elongation of the Central Kopeh Dagh is accommodated by right lateral shear along the northern margin of the range (Ashkabad and Kum Dagh fault zones), and left-lateral shear along the southern margin (Shahrud fault zone). Geological offsets along the Ashkabad and Shahrud fault zones indicate ~30–40 km westward motion of the NW Kopeh Dagh-South Caspian region, which is consistent with estimates from the Central Kopeh Dagh. A reverse in the polarity of thrusting in the Balkan region has caused a 40 km bend in the Kum Dagh-Ashkabad fault system. This may indicate the westward motion and subduction of the South Caspian may have been active for longer than previously thought.

 

In the SE Kopeh Dagh (i.e. east of the elongating central zone), regional shortening is mostly accommodated by thrust faults either side of the range, and minor partitioning onto right-lateral faults south of the range. Therefore, the tectonics of the Kopeh Dagh vary dramatically along strike, from simple crustal thickening in the east, along-strike extension in the centre, and NW-extrusion (relative to Central Iran and Eurasia) between strike-slip faults in the west. Roughly 50% of the present day shortening across NE Iran (i.e. 6 mm/yr) is accommodated in the Kopeh Dagh. The westward motion of the South Caspian by 30–40 km would be accommodated in ~10 Ma, at present day rates from GPS studies. Figure 1 shows a summary tectonic map for NE Iran, highlighting the different styles of deformation in the NW, Central and SE Kopeh Dagh.

 

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