Elsevier

Geomorphology

Volume 237, 15 May 2015, Pages 98-118
Geomorphology

Co-seismic, geomorphic, and geologic fold growth associated with the 1978 Tabas-e-Golshan earthquake fault in eastern Iran

https://doi.org/10.1016/j.geomorph.2013.02.016Get rights and content

Highlights

  • Combine seismology, geomorphology, geology to investigate the 1978 Tabas earthquake

  • Warping of alluvial fan surface suggests multiple earthquakes in the Holocene.

  • Strike–slip is accommodated by distributed deformation within hanging-wall anticline.

  • Environment is likely the most important factor in landscape evolution of the region

Abstract

We describe the seismicity and late Cenozoic deformation associated with a blind thrust fault at Tabas-e-Golshan (hereafter referred to as Tabas), eastern Iran, which generated a devastating Mw 7.3 earthquake on the 16th September 1978. Measurements from a structural transect through the Sardar anticline segment indicate fault-propagation folding above a gently ~ 20° eastward-dipping blind thrust fault. The thrust flattens into a horizontal detachment at a depth of only ~ 2 km. Tightening of the fold forelimb is accommodated by flexural slip along numerous bedding planes, with many of the slip surfaces showing fresh striations with a large component of right-lateral strike–slip. A steeply dipping fault zone showing almost pure strike–slip is also developed within the forelimb of the fold. Our field observations are consistent with the source parameters of the 1978 Tabas earthquake, and additional events in 1979 and 1980, which all involved slip on a shallowly-dipping thrust with a significant component of right-lateral slip. The surface of an alluvial fan, which is likely to have been abandoned at 8–10 ka, has been folded as it crosses the Sardar anticline. The age constraints, combined with topographic profiles along the deformed fan surface and constraints on the dip of the fault at depth, provide an approximate rate of horizontal shortening of ~ 1.5 mm/yr. Shortening at Tabas appears to result from transpressional bending at the north end of the Nayband strike–slip fault. A northward continuation of the Nayband Fault, which may be slipping at rates of > 2 mm/yr, is identified along the base of the Shotori Mountains ~ 10–20 km east of the Tabas thrust. The range-front fault did not move in 1978 and constitutes an additional threat to local populations.

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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