Fault kinematics and active tectonics at the southeastern boundary of the eastern Alborz (Abr and Khij fault zones): Geodynamic implications for NNE Iran

Abstract

a b s t r a c tThe Alborz is a region of active deformation within the Arabia-Eurasia collision zone. The Abr and the Khij Faults are two NE-trending left-lateral strike-slip faults in the eastern Alborz that correspond to the Shahrud fault system extended through an area of about 95 km × 55 km. Tectonic landforms typically associated with active strike-slip faults, such as deflected stream channels, offset ridges and fault scarps are documented along the mentioned faults. Detailed analyses of satellite images and digital topographic data accompanied by field surveys allowed us to measure horizontal offsets of about 420 ± 50 m and 400 ± 50 m for the Abr and Khij Faults, respectively. A total of 8 quartz-rich samples were sampled and dated from two different fan surfaces using in situ-produced 10 Be cosmogenic dating method. Minimum exposure ages for the abandonment of the alluvial fan surfaces of 115 ± 14 kyr along the Abr Fault and of 230 ± 16 kyr along the Khij Fault imply that both faults are active with slip rates of about 3-4 mm yr −1 and 1-3 mm yr −1 , respectively. The results of our study provide the first direct quantitative geological estimates of slip rate along these two active faults and place a new constraint on slip distribution between the faults in the eastern Alborz. Fault kinematic studies (from fault slip data) indicate a N35 • E-trending maximum stress axis comprising a dominant strike-slip regime in agreement with the geomorphological analyses. The left-lateral strike-slip faulting along the Abr and Khij Faults and their associated fault zones in the eastern Alborz can be due to the westward component of motion of the South Caspian Basin with respect to Eurasia and Central Iran.

Neogene to Present paleostress field in Eastern Iran (Sistan belt) and implications for regional geodynamics

“…Except for east Alborz area where the stress evolution through time shows a clockwise rotation [Javidfakhr et al, 2011a[Javidfakhr et al, , 2011b, most areas thus evidence a counterclockwise rotation of compression with time, from the Miocene to Present (Figure 10). The amount of rotation is higher in central and north-east of Iran than in the west and south regions (Makran and Zagros) where, in some places, the direction of compression remains unchanged during the Neogene [Authemayou et al, 2006].…”

Present-day kinematics and fault slip rates in eastern Iran, derived from 11 years of GPS data

“…Farther north, the E-W left-lateral faults give place to the NW trending, right-lateral, and reverse Kopeh Dagh fault zone ( Figure 1). The left-and right-lateral motions on the E-W faults and on the Kopeh Dagh fault, respectively, transfer into folding and thrusting east and south of the Caspian Sea [e.g., Javidfakhr et al, 2011;Hollingsworth et al, 2006Hollingsworth et al, , 2008Hollingsworth et al, , 2010b.…”

Interseismic deformation of the Shahroud fault system (NE Iran) from space‐borne radar interferometry measurements

“…Although the kinematics of the eastern Alborz are broadly understood from studies of geomorphology, seismicity, and GPS geodesy [Berberian, 1983;Allen et al, 2003;Ritz et al, 2006;Hollingsworth et al, 2010;Djamour et al, 2010;Javidfakhr et al, 2011], there are still several first-order unknowns. In particular, the slip rates of the major faults in the eastern Alborz are not known, which are critical constraints for seismic hazard estimations, as well as being highly relevant for describing the kinematics of Iran and the SC [Jackson et al, 2002;Djamour et al, 2010;Mousavi et al, 2013].…”

“…The Shahroud left-lateral fault system, in the eastern Alborz between longitudes 55-57°E, has been mapped as several discontinuous northeast trending left-lateral fault segments (the Abr, Khij, Jajarm, and Cheshmeh-Nik faults) [Wellman, 1966;Hollingsworth et al, 2008;Javidfakhr et al, 2011]. There have been no large instrumental earthquakes on the Shahroud fault system (SFS)in the last 50 years, but destructive earthquakes are known from historical records, including an event directly north of Shahroud in 1890 [Ambraseys and Melville, 1982].…”

“…The Astaneh fault is likely to have been responsible for the 856 A.D. Qumis earthquake, which killed over 200,000 people [Ambraseys and Melville, 1982;Hollingsworth et al, 2010]. pose a substantial hazard to local populations: the Jajarm fault is 20 km north of Jajarm city and strikes NE for more than 130 km [Hollingsworth et al, 2008[Hollingsworth et al, , 2010Javidfakhr et al, 2011], and the 120 km long Cheshmeh-Nik fault terminates in the east at the city of Bojnord.…”