H. Nazari

Active transtension inside central Alborz: A new insight into northern Iran–southern Caspian geodynamics

The tectonic activity in the Alborz mountain range, northern Iran, is due both to the northward convergence of central Iran toward Eurasia, and to the northwestward motion of the South Caspian Basin with respect to Eurasia inducing a left-lateral wrenching along this range. These two mechanisms give rise to a NNE-SSW transpressional regime, which is believed to have affected the entire range for the last 5 6 2 m.y. In this paper, we show that the internal domain of central Alborz is not affected by a transpressional regime but by an active transtension with a WNW-ESE extensional axis. We show that this transten- sion is young (middle Pleistocene). It postdates an earlier N-S compression and may have been initiated when the South Caspian Basin started moving. Consequently, our results suggest that the South Caspian Basin motion may have taken place more recently than previously proposed.

The 2003 Bam (Iran) earthquake: Rupture of a blind strike-slip fault

A magnitude 6.5 earthquake devastated the town of Bam in southeast Iran on 26 December 2003. Surface displacements and decorrelation effects, mapped using Envisat radar data, reveal that over 2 m of slip occurred at depth on a fault that had not previously been identified. It is common for earthquakes to occur on blind faults which, despite their name, usually produce long-term surface effects by which their existence may be recognised. However, in this case there is a complete absence of morphological features associated with the seismogenic fault that destroyed Bam.

Extrusion tectonics and subduction in the eastern South Caspian region since 10 Ma: REPLY

We examine active deformation of the region surrounding the eastern South Caspian Sea using a combination of seismic, geological, geodetic, and geomorphological data. Global positioning system (GPS) velocities indicate a westward component of motion of the South Caspian basin, relative to Eurasia and central Iran. We identify major zones of shear (the Ashkabad and Shahrud fault zones) that accommodate this westward extrusion on either side of the South Caspian block. Estimates of total strike-slip motion could, at present-day slip rates derived from GPS observations, be achieved in ~10 m.y. Therefore, the northwest extrusion of the South Caspian region, which is accommodated by subduction beneath the North Caspian region (stable Eurasia) along the Apsheron-Balkan sill, may also date from that time. This suggests that the onset of subduction may be older than previous estimates (ranging from 1.8 to 5.5 Ma). Our results are summarized in a new kinematic model that significantly clarifies the active tectonics of this complicated region.

Active tectonics of the east Alborz mountains, NE Iran: Rupture of the left-lateral Astaneh fault system during the great 856 A.D. Qumis earthquake

The 856 A.D. Qumis earthquake (M7.9) is the most destructive earthquake to have occurred in Iran, killing more than 200,000 people and destroying the cities of Damghan and the old Parthian capital of Shahr-i Qumis (Hecatompylos). This study combines evidence of historical seismicity with observations of the geomorphology and paleoseismology to provide the first detailed description of active faulting in the Damghan region of the east Alborz mountains, Iran. Regional left-lateral shear is accommodated on the Astaneh, Damghan, and North Damghan faults. Quaternary alluvial fans have been displaced along the Astaneh fault, with 15–20 m stream offsets recording the cumulative displacement over the last two to five earthquakes. A paleoseismology study from a single trench along a 5–10 km segment of the Astaneh fault reveals a rupture prior to 1300 A.D. and significantly later than 600 B.C. Despite the limitations of a single trench in documenting the spatial and temporal evolution of the fault over the late Quaternary, we are nevertheless able to bracket the last event to a time period consistent with the 856 A.D. earthquake. Two older earthquakes were also identified during the Holocene occurring between 600 B.C. and 4600 B.C. and between 4600 B.C. and 9600 B.C. The location of our trench within a bend on the Astaneh fault, which could act as a barrier to rupture propagation, means the three earthquakes recovered from our trench over the Holocene may represent a minimum. Further trenching will reveal how the Astaneh fault ruptures over repeated earthquakes and, consequently, the magnitude and extent of slip during the 856 A.D. earthquake.

Paleoearthquakes of the past 30,000 years along the North Tehran Fault (Iran)

The North Tehran Fault (NTF) is located at the southernmost piedmont of Central Alborz and crosses the northern suburbs of the Tehran metropolis and adjacent cities, where ∼15 million people live. Extending over a length of about 110 km, the NTF stands out as a major active fault and represents an important seismic hazard for the Iranian capital after historical seismicity. In order to characterize the activity of the NTF in terms of kinematics, magnitude and recurrence intervals of earthquakes, we carried out a first paleoseismological study of the fault within its central part between Tehran and Karaj cities. We opened a trench across a 3 m-high fault scarp affecting Quaternary deposits. Our study shows that the scarp is the result of repeated events along a main N115°E trending shallow dipping thrust fault, associated with secondary ruptures. From the trench analysis and Infrared Stimulated Luminescence (IRSL) dating of fault-related sediments, we interpreted between 6 and 7 surface-rupturing events that occurred during the past 30 kyrs. Their magnitudes (estimated from the displacements along the faults) are comprised between 6.1 and 7.2. The two last events - the largest - occurred during the past 7.9 ± 1.2 ka, which yields a Holocene slip rate of ∼0.3 mm/yr. The 7 earthquakes scenario suggests a regular periodicity with a mean recurrence interval of ∼3.8 kyrs. However, the two most recent events could correspond to the two largest historical earthquakes recorded in the area (in 312-280 B.C. and 1177 A.D.), and therefore suggest that the NTF activity is not regular.

Late Pleistocene‐Holocene right slip rate and paleoseismology of the Nayband fault, western margin of the Lut block, Iran

The 290-km-long, Nayband strike-slip fault bounds the western margin of the Lut block and cuts across a region thought to have been quiescent during the last few millennia. Cl-36 cosmic ray exposure (CRE) and optically stimulated luminescence (OSL) dating of cumulative geomorphic offsets are used to derive the long-term slip rate. The measured offsets at two sites along the fault range between 9 ± 1 m and 195 ± 15 m with ages from 6.8 ± 0.6 ka to ∼100 ka, yielding minimum and maximum bounds of late Pleistocene and Holocene slip rates of 1.08 and 2.45 mm yr-1, respectively. This moderate slip rate of 1.8 ± 0.7 mm yr-1, averaged over several earthquake cycles, is compared to the paleoseismic record retrieved from the first trench excavated across the fault. Combining the paleoseismic evidence with 18 OSL ages obtained from this trench site demonstrates the occurrence of at least four large (M ∼7) earthquakes during the last 17.4 ± 1.3 ka and of two older wearthquakes, one before ∼23 ka and another before 70 ± 5 ka. The exposed sediment succession also indicates a significant gap at the end of MIS-2 and the beginning of MIS-1. The age of the most recent regional incision is accurately bracketed between 6.1 ka and 7.4 ka. Sediments from the last ∼7 ka contain evidence of the three younger earthquakes. Interestingly, the penultimate and antepenultimate events occurred between 6.5 ± 0.4 ka and 6.7 ± 0.4 ka within a time interval lasting at most 1 ka whereas the most recent earthquake occurred within the last millennium. Such an irregular earthquake occurrence suggests the seismic behavior of the Nayband fault is not strictly time dependent but possibly related to clustering. From this and taking into account the occurrence of the most recent earthquake within the last 800 years, the imminence of an earthquake along the Nayband fault cannot be discarded. Although the most recent surface-rupturing event seems to have occurred after AD 1200, this event went unnoticed in the historical records. This provides a marked illustration of the incompleteness of the historical seismic catalogs in Central Iran, challenging any assessment of regional seismic hazard without appropriate geologic and geochronological information. Large and infrequent earthquakes are characteristic of the seismic behavior of the slow-slipping strike-slip faults slicing Central and Eastern Iran. Also, the slip rates summed across Central and Eastern Iran from the Iran Plateau up to the Afghan lowlands appear in agreement with the most recent GPS data.