Co-seismic, geomorphic, and geologic fold growth associated with the 1978 Tabas-e-Golshan earthquake fault in eastern Iran
Introduction
On September 16, 1978, the oasis town of Tabas in eastern Iran was destroyed, and ~ 20,000 people were killed, by an earthquake of Mw 7.3 (Berberian, 1979; Fig. 1). This earthquake, which occurred on a series of previously unrecognised blind thrust faults, was one of the largest and most destructive to have occurred in Iran in recent decades. Despite having a long history of occupation, there are no records of destructive earthquakes in the area prior to 1978 (e.g. Berberian, 1979, Ambraseys and Melville, 1982). Evidence of long-term active faulting is, however, preserved in the landscape in the form of anticlinal folding in Neogene basin deposits, deformation of late Quaternary alluvial fan deposits, and widespread river incision (Walker et al., 2003). No estimates exist of the rate of slip, and hence the average interval between large earthquakes, on the Tabas Fault system.
The Tabas folds, and presumably the thrust faults that underlie them, are segmented (Fig. 2). For the purposes of this study we focus primarily on the segment located close to the Sardar River in the northern part of the system (Fig. 3). We choose this segment for three reasons. (1) The Sardar fold is the only visible fold segment present at this latitude; farther south, several parallel folds appear to be active simultaneously, thus making shortening estimates much more difficult to determine. (2) Uninterrupted exposures of folded and faulted strata outcrop along the walls of the deeply incised Sardar River allowing a detailed structural cross-section to be produced. (3) The Sardar River is incised into the surface of a large alluvial fan, which is continuous across the fold, and, when combined with estimates of its age, can be used to determine a rate of shortening across the fold.
In the following sections, we first describe the tectonic setting of the Tabas earthquake. To aid our description of the earthquake we present improved epicentres of seismicity in the Tabas region obtained from a multiple-event relocation technique. We then use the deformation of an alluvial fan crossing the Sardar fold segment, combined with age constraints from luminescence and cosmogenic 36Cl exposure dating on the abandonment of the fan surface, to estimate a rate of uplift across the underlying fault. With constraints on the dip of the Tabas Fault at depth (obtained from seismology and a structural cross-section) we convert this rate of uplift into rates of horizontal shortening (important for regional tectonic studies) and a rate of slip along the fault (useful for estimating the average interval between earthquakes). Finally, we assess the implications of our study for the source processes of the 1978 earthquake, the tectonics of eastern Iran, and for the evolution of the landscape observed at the present-day near Tabas.
Section snippets
Tectonic setting
The active tectonics of Iran are controlled by the northward motion of Arabia, at a velocity of ~ 25 mm/yr at longitude 60°E, with respect to the interior of Eurasia (Fig. 1A; Vernant et al., 2004). Deformation resulting from this northward motion is broadly confined to within the political borders of Iran, and surrounding parts of Pakistan, Afghanistan and Turkmenistan appear to behave as non-deforming parts of stable Eurasia. Northward motion of central and northern Iran with respect to
Geology and geomorphology of the Tabas thrust fault system
The Tabas folds are situated between the Tabas playa depression (~ 600 m above sea level) and the Shotori Mountains, with peak elevations of ~ 2900 m (Fig. 2). The folds are expressed in the topography as a series of low rounded hills with no more than ~ 100 m of relief above the surrounding gravel apron. The Shotori Mountains are composed of heavily deformed Palaeozoic and Mesozoic rocks (Stöcklin and Nabavi, 1969). Changes in sediment thickness from the Shotori Mountains towards Tabas indicate
Seismicity of the Tabas region
The 16 September 1978 (Mw 7.3) Tabas earthquake is the largest instrumentally recorded earthquake in Iran. It devastated the entire region, killing 85% of the population of Tabas (e.g. Berberian, 1979). The earthquake occurred prior to the development of geodetic techniques for imaging earthquake ground deformation; though seismological, remote-sensing, and field investigations have all helped to develop a broad understanding of the Tabas earthquake as the result of slip on a series of
A structural transect through the Sardar fold
Examination of the cumulative deformation and folding recorded in the geology of the Tabas region has the potential to provide additional constraints on the source of the 1978 Tabas earthquake. Fig. 2 shows the major segments of the Tabas blind thrust system. Along most of the length of the system there are several parallel folds, each of which is likely to be underlain by faults, and each of which will contribute to the overall rate of shortening. North of Tabas town, however, only one main
Late Quaternary deformation of the Sardar alluvial fan surface
Deposits of the F1 Sardar alluvial fan extend over a distance of ~ 20 km from the Shotori Mountain range-front to the edge of the Tabas playa (Fig. 3). The fan surface is traced continuously across the Sardar anticline, and is preserved without interruption in three narrow channels. There is no visible evidence for displacement of the F1 fan surface as it crosses the Shotori range-front (Walker et al., 2003).
To provide an additional constraint on the structure of the Sardar blind thrust we
Dating the Sardar alluvial fan abandonment
To provide an estimate of the slip-rate on the Tabas thrust faults we must first determine the age of the F1 fan surface. We excavated a single 2-m-deep pit into the fan surface at 33°39′12.9″N 57°05′54.4″E (Fig. 8). The site was selected for its planar surface and the absence of obvious disturbance by small distributary channels on the fan surface. The exposed alluvial deposits are primarily composed of limestone with rare chert clasts. The material is very coarse, with clasts typically of
Discussion
The active faulting at Tabas is important for a number of reasons. It was the site of a devastating earthquake in 1978 and is potentially an important feature in the active tectonics of Iran. It is also an important region for understanding the tectonic and environmental controls on landscape development. Our study, which has combined seismological, geological and geomorphological investigations, yields results relevant both to tectonics and to landscape evolution, and we deal with these two
Conclusions
The seismological, geomorphological and geological constraints on the structure of the Tabas blind thrusts help to constrain the subsurface structure of the faults and hence the source of the devastating 1978 earthquake. The Tabas Faults are likely to flatten into a decollement at a shallow depth and, in addition to faulting on the Tabas blind thrusts, the 1978 earthquake is likely to have involved rupture on deeper parts of the fault system beneath the Shotori Mountains. A component of
Acknowledgements
We thank the University of Birjand, the Geological Survey of Iran, and the local government of Tabas for their support of this project. We are grateful to Mr. Arabi and Mr. Califi for their careful driving during the two field visits to Tabas. We also express our thanks to A. Dolati and R. Tajik for their help in making the GPS measurements. Accelerator Mass Spectrometer measurements were performed by Colin Maden at SUERC, and the AMS data reduction by Stewart Freeman and Colin Maden. RTW is
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2018, Journal of GeodynamicsCitation Excerpt :The deflection of the stream channels as well as offset of U-shape valleys and moraine ridge presented above suggested continuous activity and accumulative offset along the SF. Fault scarp is one of the major landform features accompanying strike-slip faulting and has produced fateful effects for identifying fault traces either on DEM data and satellite imagery or in the field (e.g., Lin et al., 2002; Oguchi et al., 2003; Zielke et al., 2010; Burbank and Anderson, 2011; Walker et al., 2015). The scarps developed on alluvial fans or ridges alternate along the fault trace from upstream facing to downstream facing, which has been termed “scissoring structure” as a common characteristic of strike-slip faults (Yeats et al., 1997; Lin et al., 2003).