My PhD was done at Cambridge, under the supervision of James Jackson, entitled 'Active faulting and tectonics of eastern Iran'.
Iran provides a relatively compact example of a young continent-continent collision zone, in which the active faults and earthquake distributions are for the most part restricted within the political borders of the country. In addition, the arid desert climate, with an almost complete lack of vegetation and human habitation across much of the country, aids the preservation in the landscape of very subtle indications of surface deformation due to active faulting. Interpreting the development of landscapes in actively deforming parts of Iran can provide detailed information on the ways in which faults grow, evolve and interact. However, relatively few studies have attempted to unravel the local and regional scale tectonics of Iran.
I looked at the surface expression of active faulting in eastern Iran, utilising recent advances in satellite imagery and interpretation software to examine the geomorphology of the active regions, seismological investigation of recent earthquakes to constrain the distribution and nature of active faults, and field investigation in a number of key areas. Much of the discussion concerns the active fault systems that surround the Dasht-e-Lut desert (Figure 1). However, I also assess the active deformation in the central parts of Iran (the central Iranian plateau and Dasht-e-Kavir), which appears to be part of the same tectonic story, even though there is no record of earthquakes in these central parts of Iran.
Although I provide several results that are important for an appreciation of Iranian tectonics, there are likely to be similarities in the ways that continental fault zones develop irrespective of geographical location. Therefore, determining how faults are manifest at the Earth's surface in Iran, and using the surface expression of faulting to infer details about the growth and evolution of the faults, will provide results that can be applied to other regions of active continental deformation, where the geomorphology might not be preserved to the same level.
The active deformation of Iran is controlled by Arabian-Eurasian convergence. Shortening is mainly accommodated by distributed faulting in high mountains of the Zagros (Z) in the south, and the Alborz (A) and Kopeh Dagh (K) in the north. The surrounding regions to the north and east are aseismic and appear not to be deforming (Figure 1).
(a) Seismicity of Iran 1964-1998 (from the catalogue of Engdahl etal, 1998). (b) A velocity field for Iran estimated from variation in strain rates indicated by earthquakes (Jackson etal, 1995).
Arabia-Eurasia convergence is about 26 to 30 mm/yr at the longitude of eastern Iran. It seems likely that about half of this is taken up in the Zagros, leaving about 15 mm/yr to be accommodated in the Alborz and central Iran. This must cause the same amount of right-lateral shear in eastern Iran. There are several indications that the present-day tectonic configuration dates from around 5 million years ago. We therefore have some idea of how much deformation must be accounted for on the active faults.
Eastern Iran is a region of widespread active faulting (Figure 2). Right-lateral shear is taken up on several north-south right-lateral fault systems that surround the Lut desert (another aseismic block that is probably not deforming). In the north, the right-lateral shear is seen as clockwise rotation about vertical axes of east-west left-lateral faults (the Doruneh and Dasht-e-Bayaz faults). Shortening components associated with the strike-slip faults result in widespread thrust faulting. These thrust faults often fail to reach the surface (termed 'blind' faults).
Fault map of eastern Iran.
Many of these fault systems have been responsible for destructive earthquakes, and pose a serious seismic hazard to local populations. However, little is known of their evolution, development and rate of slip. I hope to contribute to an understanding of the deformation in eastern Iran by investigating the long-term development and total Late Tertiary offset on the faults. By investigating regions that have had destructive earthquakes, I can combine geomorphological, geological and seismological data to gain a better understanding of the ways in which strike-slip and thrust faults evolve. This also has importance for other regions of active continental shortening.
Abstracts and images from published parts of my thesis are shown below. A more complete list of publications and conference abstracts can be reached through the links at the bottom of the page. If you would like any further information please don't hesitate to email me at the address given below.
We use drainage reconstructions to estimate long-term offsets on the Gowk fault, an oblique right-lateral strike-slip fault in eastern Iran, on which there have been a number of recent large earthquakes. A 3~km horizontal offset is inferred from well-preserved geomorphology. We further identify a total cumulative offset of ~12~km, which produces a simple reconstruction of geomorphology across the fault, filling in pull-apart basins, restoring rivers to linear courses across the fault trace and aligning structural and bed-rock features. The probable age of the Gowk fault, and K-Ar dating of offset basalts north of the study region, suggest an overall slip rate of ~1.5-2.4 mm/yr. This is small compared to the overall 10-20 mm/yr of shear expected between central Iran and Afghanistan and the deficit is likely to have been accommodated on other faults east of the Gowk fault. Drainage displaced by dextral movement leads to the development of a series of basins along the fault and a longitudinal topographic profile that resembles an asymmetric saw-tooth. The geomorphic evolution of the fault zone at the surface includes both normal and reverse faulting components, reflecting a probable ramp-and-flat structure in cross-section. This interpretation is consistent with evidence from the analysis of seismological, radar and surface rupture data in recent earthquakes.
Landsat image of the Gowk region. The parts of the fault that have ruptured the surface in recent earthquakes are marked with a white line. The thickened line represents the section that ruptured in 1981, and then again in 1998. Focal mechanisms of the five major earthquakes are included.
Left-hand panels: East-west cross-sectional views of the Gowk fault. The earthquakes and geomorphology indicate a rather complicated fault zone, with a 'ramp-and-flat' structure on an oblique thrust fault causing uplift of mountains to the east of the Gowk fault valley. The 1981 earthquakes ruptured deeper parts of the structure (1), and the 1998 Fandoqa earthquake ruptured the upper parts (2) with a slight normal component of motion. This structure is unstable and must evolve with time. Right-hand panels: The influence of structure on landscape development within the Gowk fault valley. Vertical fault movements restrict rivers to a few deeply incised canyons. Lateral movements lengthen river channels and result in a distinctive 'saw-tooth' topography along the valley. Continued lateral movement may result in capture of the river by closer outlets.
Previously unrecognised thrust faults in eastern Iran were responsible for a destructive earthquake at Tabas (16 September 1978), which produced over 80 km of distributed and discontinuous surface ruptures above a series of low anticlinal hills to the west of a major range-front. Analysis of long-period body-wave seismograms shows a simple rupture on a gently dipping (~16 degrees) thrust, with a slight right-lateral component. This is compatible with the locally recorded aftershock distribution. Body wave analysis of two later, smaller events show similar source orientations. Several indicators of long-term active folding at Tabas can be recognised in the geomorphology, and surface ruptures from 1978 are consistent with co-seismic fold growth. Drainage incision also indicates uplift at depth on thrust faults dipping eastwards beneath the folds. Body-wave seismograms for two earthquakes near Ferdows, 150 km east of Tabas, in September 1968 also show thrust faulting at depths of ~10 km. Again, the surface geomorphology indicates a region of folding above a thrust fault, which is ~10 km west of a fault bounded range-front. In both Tabas and Ferdows, the active faulting appears to show Quaternary migration away from the range-front, possibly in response to stresses produced by the elevated topography. The identification of zones of active thrust faulting is important for earthquake hazard assessment and also for an understanding of the local tectonics. We conclude that the structures which gave rise to the 1968 and 1978 earthquakes could have been recognised beforehand from the clear signals in the geomorphology, had we known what to look for at that time.
Landsat image of the Tabas region with focal mechanisms of major earthquakes including the M 7.4 Tabas event (the most southern earthquake). The western margin of anticlinal folds in Quaternary alluvium are marked with black arrows. These folds align with surface ruptures mapped after the 1978 event (Berberian, 1979)
Schematic cross-section through the Tabas thrust fault. The main thrust fault at depth is partially blind, and deformation at the Earth's surface is mainly the result of anticlinal folding. We can however infer faulting at depth from the patterns of stream incision, as uplift of the hanging-wall of the thrust fault results in widespread incision in the areas to the east of the surface folding
Field photographs of river incision in the Sardar canyon near Tabas. The incision results from hanging-wall uplift on a thrust fault at depth, and stops at the western margin of the folds
The Dasht-e-Bayaz left-lateral strike-slip fault system in NE Iran ruptured over a length of ~120 km during two earthquakes in 1968 and 1979. We constrain the source parameters of the 1968 August 31 (Dasht-e-Bayaz) and 1979 November 27 (Khuli-Buniabad) earthquakes by analysing long-period body wave seismograms. Both earthquakes involved complex rupture processes, with at least two subevents in each case. Coseismic surface ruptures and cumulative scarps in alluvium indicate fault segmentation on numerous short (~20 km long) strands with small pull-aparts between them. The earthquake subevents seen in the seismograms probably relate to this segmentation. The total, cumulative, offset on the fault system is estimated at ~4-5 km. This is small compared to the total amount of Late Tertiary deformation expected in this part of Iran, and indicates that the Dasht-e-Bayaz fault may be relatively young. Distributed strike-slip faulting is widespread in the region and there are indications that the Dasht-e-Bayaz fault is evolving from several short faults that are coalescing. These results are important not only for understanding the regional tectonics but for the development and evolution of strike-slip faults in general.
Landsat imagery of the Dasht-e-Bayaz region, showing mapped faults and mechanisms of major earthquakes.
The Dasht-e-Bayaz fault mapped from ruptures observed after the 1968 and 1979 earthquakes (Ambraseys and Tchalenko, 1969: Tchalenko and Berberian, 1975). Although the fault appears to be moving rapidly, and shows evidence for several earthquakes during human history, the total offset of bedrock and geomorphological features is relatively small and the fault appears to be very young. We may be seeing the initiation of a new through-going fault from short segments that are coalescing.
Right-lateral shear between central Iran and Afghanistan is accommodated on N-S right-lateral strike-slip faults surrounding the aseismic Dasht-e-Lut. North of ~34 degrees N, the right-lateral shear is accommodated on left-lateral faults that rotate clockwise about vertical axes. Little is known of the late Tertiary and younger offsets and slip-rates on the active fault systems, results that are important for understanding the regional tectonics. We use observations from satellite imagery to identify displaced geological and geomorphological markers, that we use in conjunction with the overall morphology and orientation of the active fault systems, to estimate the total cumulative right-lateral shear. Estimates of cumulative fault movements from offset features and inferred vertical axis rotation of fault-bounded blocks, suggest that the late Cenozoic strain is concentrated towards the eastern margin of Iran, along the Sistan Shear Zone, where bed-rock offsets of at least 70 km are observed across the active faults. The geomorphology of the Deh Shir, Anar and Great Kavir strike-slip faults in central Iran suggest that although little shortening is accommodated across this region, they might still be active, and hence capable of producing earthquakes. Present-day activity on these faults in central Iran would not be expected from distributions of instrumental and historical earthquakes. Although speculative, the late Tertiary strain distribution described in this paper is consistent with what we know of the present-day rates of shear in eastern Iran and provides a framework to which later, more detailed, work can be added.
Summary of the model of long-term deformation in eastern Iran. The largest amounts of late Cenozoic shear are found on faults in Sistan province, in the far east of the country. This may mean that the present-day rates of slip are highest in Sistan. An uneven distribution of right-lateral shear across eastern Iran may explain the initiation of the E-W Dasht-e-Bayaz fault and the bending along the Doruneh fault to the north - as more clockwise rotation of these E-W faults would be expected in the far east.