The Doruneh left-lateral strike-slip fault is one of the longest, and most prominent active faults in Iran, stretching from the centre of the country to the Afghan border. The fault is oriented east-west, and accommodates up to 15 mm/yr of north-south right-lateral shear by vertical axis clockwise rotation. The fault shows numerous indications of cumulative left-lateral slip over a range of Quaternary time-scales. Measurements of ~26 m of left-lateral displacement in young (~10 ka) alluvial fan and river terrace deposits provide a slip-rate estimate of ~2.5 mm/yr. No major recent or historical earthquakes are recorded on the Doruneh fault. Relatively fresh scarps and partially-infilled fractures appear to be the preserved surface ruptures from a single earthquake event of unknown age. A series of small streams showing left-lateral displacements of 4 to 5.5 m record the likely magnitude of slip during this earthquake, which from scaling relationships would have had a magnitude of ~7.5, and ruptured the fault over a length of >100 km. At the estimated slip-rate of ~2.5 mm/yr, the average recurrence time between large-magnitude earthquakes on the Doruneh fault is ~2,000 years. Components of shortening are spatially separated (partitioned) onto thrusts which run parallel to the main strike-slip strand. Parts of these thrust faults have generated destructive historical earthquakes. Observations from the field and satellite imagery provide information on the structure of the folds, and examples of late Cenozoic and Holocene fault movement.
Simple geomorphological observations of active faulting are applied to a part of Kerman province in S.E. Iran which is often considered to behave as a rigid block within the Arabia-Eurasia collision zone. GPS studies show that present-day rates of deformation are not resolvable with the existing coverage, and must be less than 2~mm/yr. In addition very little seismicity is recorded. The remote sensing observations reveal a major oblique fold-and-thrust belt, active in the late Quaternary, which may be capable of producing destructive earthquakes in the future.The active faults appear to link the Sabzevaran right-lateral strike-slip fault in southeast Iran to other strike-slip faults within the interior of the country and may provide the means of distributing right-lateral shear between the Zagros and Makran mountains over a wider region of central Iran. This study shows how widely available remote sensing data can be used to provide information on the distribution of active faulting across large areas of deformation, with implications both for interpreting the tectonics of deforming regions, and also as a first step in identifying potential sources of earthquake risk.
20 mm/yr of north-south shortening between Arabia and Eurasia is accommodated across Iran. A large proportion of this is taken up on thrust faults that are capable of generating large magnitude earthquakes, as shown in the instrumental and historical records of seismicity. The styles of active thrusting, and the ways in which the faults are manifest at the Earth’s surface, show considerable variation across the country. We use the patterns of seismicity, the source parameters of large earthquakes, interpretation of the landscape in both remote sensing imagery and fieldwork, and structural geology to study the thrust faults and the ways in which they accommodate Arabia-Eurasia convergence on a transect from the Zagros mountains in the south, through the desert depressions of central Iran, through to the Alborz and Kopet Dagh in the northeast of the country. Between 5 and 10 mm/yr of Arabia-Eurasia shortening is taken up across the Zagros mountains of southern Iran. Detachment folding within the ~10 km thick sedimentary sequence contains one of the largest reserves of hydrocarbons in the world. Cross-sections drawn from exposed surface geology can be restored both as purely thin-skinned deformation within the sediments, or as detachment folding decoupled from steeply-dipping basement reverse faulting along weak evaporitic horizons. The presence of widespread, and relatively deep, earthquakes suggests that the basement reverse faults are active beneath the Zagros, with implications for the evolution of the deformation. Within the desert interior of Iran, Arabia-Eurasia convergence and right-lateral shear between Iran and surrounding regions, is accommodated by movement on large (>100 km long) strike-slip and associated thrust faults. The thrusts are commonly expressed at the Earth’s surface as folding within alluvium and lake-bed marls. Surface ruptures observed following a number of recent earthquakes (e.g. Torud in 1953; Tabas in 1978; Sefidabeh in 1994) are indicatative of co-seismic folding and suggest that, in at least some cases, all or most of the slip on the seismogenic faults fail to reach the surface. Results from seismology and InSAR where we have them are also compatible with interpretations of blind thrust faulting (for example at Sefidabeh and at Shahdad). In several examples within Iran, a basinward migration of fault activity away from regions of elevated topography is seen. Often, the thrusts are situated close to, and parallel with, large strike-slip faults, and are thought to accommodate oblique convergence by ‘partitioning’. North-south shortening along the northeastern margin of Iran is taken up on thrust faults bounding parallel linear mountain ranges. There are very few instrumentally recorded large magnitude reverse-faulting earthquakes within this part of the country. However, the geomorphology of the region shows that thrust faulting in the Late Quaternary is widespread. Historical records of seismicity indicate that the thrust faults do move in large earthquakes and have the potential to cause large amounts of damage to local population centres.
Active faults in Iran have caused many destructive earthquakes in recent years, for example at Tabas in 1978; at Rudbar in 1990; and at Bam in 2003, and further disasters will continue to occur in the future. However, relatively little is known at present about the distribution of active faulting in Iran, the rates of deformation and earthquake recurrence in individual regions, and the ways in which the faults in Iran accommodate Arabia-Eurasia convergence. We present an overview of collaborative research between the Geological Survey of Iran and the Universities of Cambridge and Oxford aimed at enlarging our understanding of the active faulting and tectonics of this earthquake prone region. North-south convergence between Arabia and Eurasia results in ~25-30 mm/yr of shortening across Iran, which occurs as movement on strike-slip and thrust faults. The Arabia-Eurasia collision in Iran is relatively young, and 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, making Iran an ideal region for the study of active faulting using remote sensing, geomorphology and seismology. A significant number of recent earthquakes in Iran have occurred on faults that were not previously known to exist, as the faults responsible are often 'blind' (or partially blind) and expressed at the Earth's surface as folding in relatively soft and easily eroded Quaternary deposits. One aim of our study is to use simple observations of the landscape to identify zones of 'blind' faulting. By studying recent earthquakes (such as the events at Tabas in 1978; at Sefidabeh in 1994; at Avaj in 2002; and at Bam in 2003), we have found diagnostic geomorphic indications of faulting at depth, which we use to recognize other active faults in areas that have no record of earthquakes. The ability to identify zones of active faulting in Iran is essential both for hazard assessment and for understanding the regional tectonics. The low rates of erosion and pristine geomorphology of Iran allow us not only to identify active faults, but also to infer how they have grown, evolved and interacted over relatively long periods of time - results that also have implications for fault evolution in other regions of active faulting. Particularly useful insights are gained from geomorphic investigation of large strike-slip fault systems (such as the seismically active Dasht-e Bayaz fault in north-east Iran), where substantial vertical axis rotation of fault bounded blocks is likely to occur - resulting in the formation, rotation, and eventual abandonment of successive generations of active fault systems. We combine our observations of the geomorphology, the distribution of earthquake epicenters (both recent and historical), and the source parameters of recent earthquakes to map the major zones of deformation in Iran. We also use the patterns of active faulting, and information on the ways the faults have evolved over relatively long time-scales preserved in the landscape to infer how Arabia-Eurasia convergence is accommodated across Iran, and how the present-day strain is distributed across the many active faults. Some of our interpretations are speculative at present, but we aim to provide quantitative estimates of deformation styles and rates in the near future. Thus our results provide a framework to which later work will be added.
A right-lateral strike-slip fault (herein named the Jid fault) runs north-south for 60 km in the eastern Kharkhiraa Range of the Mongolian Altay. The Altay Mountains contain seismically active faults that have generated numerous recorded earthquakes of magnitude > 7. Identifying active structures such as the Jid fault has implications for estimates of seismic hazard and for understanding the regional tectonics. The fault shows indications of recent movements at scales ranging from drainage re-organisation at the kilometre scale, to ground deformations likely to result from movements during a single earthquake. From the scale of the ruptures and the observed length of roughly 60 km the most recent earthquake may have had a magnitude of about 7.5. In addition, numerous horizontal offsets of about 10-15 m, and vertical scarps of < 5 m in the youngest alluvial deposits show the likely cumulative Holocene movement. The fault has many excellent examples of features associated with oblique-slip active faults, including stream displacements, pull-apart basins, shutter ridges, and river piracy. Along most of the Jid fault trace there is a slight vertical component of motion. The vertical component is introduced by bending along the fault and is normal in some places, and reverse in others. Parts of the fault show structural complexity at the surface with numerous parallel strands having both normal and reverse components of motion. The complexities probably form a 'flower structure' in cross-section. Assuming that the last period of alluvial fan deposition was at the last glacial maximum (approximately 10-15 ka ago), the horizontal offsets of 10-15 m would imply roughly 1 mm/yr of strike-slip motion. If the fault fails in magnitude 7.5 earthquakes with 3 m of slip in each, the average time between events will be about 3,000 years.
The Mw 6.4 Changureh (Avaj) earthquake occurred on the 22 June, 2002 in Qazvin province, NW~Iran. We use observations from seismology, field investigation and analysis of satellite imagery and digital topography to suggest that slip on a previously unrecognised thrust fault (herein named the Abdarreh fault) was responsible for the earthquake. Inversion of long-period P and SH body-wave seismograms shows rupture on a thrust fault dipping 49 degrees to the southwest and with a centroid depth of about 10 km. Multiple-event relocation of the main-shock and aftershock epicentres, and discontinuous surface ruptures observed after the earthquake are compatible with a NW propagating rupture on a SW-dipping thrust, but maximum recorded displacements are much less than expected from seismology, suggesting that much of the slip failed to reach the surface and was accommodated as folding at the surface instead. Long-term folding is difficult to see in the topography of the epicentral region as the Abdarreh fold is growing through a relict Neogene topography. Anticlinal uplift can however be inferred from drainage disruption and stream incision. The 22 June, 2002 Changureh earthquake shows the importance of being able to interpret diagnostic features of active faulting in the landscape.
Folding identified near the town of Mahan in S.E. Iran has no record of historical activity, and yet there are clear geomorphological indications of recent fold growth, presumably driven by movements on underlying thrust faults. The structures at Mahan may still be capable of producing destructive earthquakes, posing a considerable hazard to local population centres. We describe a drainage evolution that shows the effect of lateral propagation of surface folding and the effect of tilting above an underlying thrust fault. River systems cross and incise through the fold segments. Each of these rivers show a distinct deflection parallel to the fold axis. This deflection starts several kilometres into the hanging-wall of the underlying thrust fault. Remnants of several abandoned drainage channels and abandoned alluvial fans are preserved within the folds. The westward lateral propagation of folding is also suggested by an increase in relief and exposure of deeper stratigraphic layers across fold segments in the east of the system, implying a greater cumulative displacement in the east than in the west. The preservation of numerous dry valleys across the fold suggests a continual forcing of drainage around the nose of the growing fold, rather than an along strike variation in slip-rate.
Eastern Iran is a region of rapid active deformation, with abundant strike-slip and thrust faulting that poses a serious seismic hazard to local populations. About 15--20~mm/yr of north-south right-lateral shear occurs between central Iran and western Afghanistan. We suspect that the present-day tectonic configuration dates from about 5~Ma ago, a time of major re-organisation in both the Zagros and Alborz mountains. South of 34\dgr N, right-lateral shear is accommodated on north-south right-lateral faults. North of 34\dgr N, the right-lateral shear is taken up on east-west left-lateral faults that rotate clockwise about vertical axes. However, little is known of the late Tertiary and younger offsets and slip-rates on these active fault systems, results that are important for estimating potential seismic hazard in the region, and also for understanding the regional tectonics. We use the distribution of historical and instrumental seismicity, and indications of Holocene fault movements observed using high-resolution satellite imagery, to determine the present-day distribution of active faulting in eastern Iran. We then use displaced geological and geomorphic markers, as well as the overall morphology and orientation of the major fault systems, to estimate the total cumulative late Tertiary displacements across the fault zones. By assuming that the pattern of active faulting dates from \aprx 5~Ma, we can obtain rough estimates of the slip-rates on the major strike-slip faults, that are in agreement with dated offsets where we have them. The measured cumulative offsets across the faults compare well with the total amount of right-lateral shear (75 to 100~km) that is expected across the region in the last 5~Ma.
Eastern Iran is a region of rapid active tectonics, with abundant strike-slip and thrust faulting that poses a serious seismic hazard to local populations. However, little is known about Late Tertiary and present-day slip-rates of the many active faults. The first aim, therefore, is to determine cumulative offsets across the major strike-slip faults, by restoration of drainage networks, geomorphic features, and bed-rock lithologies cut by the faults. This provides an estimate of the Late Tertiary to Recent strain distribution, which is important, both for an understanding of Iranian tectonics and for potential seismic hazard. A second aim is to use the interactions between fault movement and surface geomorphology, exceptionally preserved in the arid desert climate, to investigate the development of strike-slip and thrust fault systems through the Quaternary. This gives insight into the ways in which the faults grow and evolve. Understanding how faults develop in Iran has relevance to other regions of active continental shortening and strike-slip deformation, where surface indications of faulting might be less well preserved. Satellite imagery, aerial photography, and digital elevation models (DEMs) are used to identify indications of active faulting. Where we have studied the surface geomorphology of a fault, earthquake seismology provides insight into the nature of the faulting at depth. By showing how subsurface faulting can drive surface deformation, we can look for evidence of active faulting in regions not known to be at risk from earthquakes. This again has importance for potential seismic hazard.
We re-examine recent earthquakes in eastern Iran that ruptured blind thrusts near Shahdad (14th March, 1998), Tabas (16th September, 1978) and Ferdows (1st September, 1968). The desert climate and sparse human habitation is well suited to the study of the surface expression of movement on these faults. Movement of ~8 cm was detected by SAR interferometry on the Shahdad blind thrust which was probably triggered by a Mw 6.6 strike-slip earthquake (14th March, 1998) on the nearby Gowk fault. Fieldwork and Landsat image interpretation show clear folding of Quaternary deposits and typify the type of structures that we would expect to develop above blind thrust faults. We also re-examine two regions of eastern Iran that have suffered destructive earthquakes on previously unidentified blind structures. We can demonstrate that the surface geomorphology is consistent with folding above blind faults and that the variety of minor ruptures developed during the earthquakes are consistent with co-seismic fold growth. The Ms 7.4 Tabas earthquake of 16th September, 1978 devastated the surrounding area and produced an abundance of minor faulting and bedding-plane slip along a series of low elongate folds immediately to the east of Tabas. We assess the geomorphology through fieldwork and Landsat image interpretation and find abundant indications of holocene uplift and folding of Quaternary river terraces and alluvial gravel surfaces. The folds are typically broken by secondary faults and slip along bedding planes. Back-thrust scarps on several fold segments define box folds of wavelength ~4 km. However, incision of drainage systems flowing perpendicular to the folds record subtle surface deformations over a greater wavelength than the folding and probably reflect uplift on deeper, master faults. The Ms 6.3 Ferdows earthquake (1st September, 1968) was overshadowed by the much larger Dasht-e-Bayas strike-slip earthquake of the day before and so attracted little study. However, a clear elongate region of pronounced river incision containing abundant uplifted river terraces and alluvial surfaces can clearly be seen on Landsat imagery and aerial photographs. This folded region is offset from the bed-rock mountain front by some 10 km, and suggests a possible migration of faulting away from the mountain front.
This paper summarises active research projects on Iranian neotectonics, involving several major hydrocarbon provinces. Neotectonic deformation across Iran is a consequence of the Arabia-Eurasia collision, but strain is accommodated by very different mechanisms across the country, resulting in a wide range of deformation styles. At 35N 55E (roughly Tehran) the Arabia/Eurasia motion is 33 mm/yr (direction 005). Strain partitioning in the northwestern Zagros (Lurestan) occurs by NW-SE folds and thrusts and dextral slip on the Main Zagros Recent Fault. Fold geometries indicate that more than one detachment level operates. It is unclear how exposed folds are coupled with seismogenic faults in the basement. N-S dextral faults within the Simple Fold Zone link Lurestan to the E-W trending structures of Fars, in the eastern Zagros. Central Iran is less active than the Zagros seismically, and has subdued relief and elevations. Major active strike-slip faults deform this region. In easter Iran, the Gowk fault is one of several major N-S dextral faults accommodating right-lateral shear between deforming Iran and Afghanistan. The Alborz range of northern Iran borders the South Caspian Basin. Whereas thrusts in the Alborz accommodate part of the regional Arabian convergence, range-parallel sinistral faults indicate westward motion of the South Caspian Basin relative to Iran. Several detachment levels operate in the Alborz, but seismicity indicates basement faulting too. As with the Zagros, it is a key question how and to what extent basement and cover are coupled.
The Gowk fault in Kerman province, eastern Iran is one of several major north-south strike-slip faults accommodating right-lateral shear along the eastern margin of Iran. Offset drainage networks are used to restore movement on this ~200 km long fault. The drainage reconstruction was aided by the unusual topography around the fault. Mountains flank the fault on both sides such that the course of drainage flowing eastwards across the fault zone is restricted to several deep gorges and cannot re-adjust to incremental fault movements. Total cumulative right-lateral offsets on the fault are shown to be ~12 km. Study of the geomorphology of the fault zone and identification of uplifted and abandoned drainage systems show active uplift to the east of the fault. The rocks on the uplifting eastern side of the fault control the preservation of geomorphic features. In the north, drainage incises through soft Quaternary alluvium and the preservation of subtle geomorphic features allows detailed reconstruction of the last ~3 km of fault movement, with processes of stream capture forcing repeated phases of drainage re-organisation. Further south, harder Mesozoic rocks have been uplifted and many drainage channels have been abandoned, leaving only one active gorge. Because of this, the processes of stream capture and draiange re-organisation seen in the north cannot operate and so the draiange still retains information on the total cumulative fault offset. The ~12 km total offset is small compared to the >100 km of right-lateral shear expected to have occurred in eastern Iran in the last ~5 Ma, and suggests a possible slip-rate of only ~2 mm/yr. The abundance of recent earthquakes on this fault is therefore probably not representative of the longer-term activity.