Extrusion tectonics and subduction in the eastern South Caspian region since 10 Ma
- 11COMET (Center for the Observation and Modelling of Earthquakes and Tectonics), Bullard Laboratories, Department of Earth Sciences, University of Cambridge, Madingley Road, Cambridge CB3 0EZ, UK
- 22COMET, Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, UK
- 33Geological Survey of Iran, Azadi Square, Meraj Avenue, P.O. Box 13185–1494, Tehran, Iran
Abstract
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.
INTRODUCTION
This paper is concerned with the active tectonics of the eastern South Caspian region (Fig. 1), and the role this area currently plays in the collision between Arabia and Eurasia. The Alborz and Kopeh Dagh mountain belts, which have experienced many destructive earthquakes over the past few thousand years (e.g., Ambraseys and Melville, 1982; Jackson et al., 2002), surround the relatively aseismic South Caspian basin (Fig. 1).
Summary fault map of eastern South Caspian region. Small arrows show slip vectors (south relative to north) from large thrust (blue) and strike-slip (red) earthquakes; numbers indicate earthquake depth (km). Large arrows show GPS velocities from central Iran (black) and the South Caspian block (orange) (in mm/yr) (Masson et al., 2007). White arrow shows South Caspian velocity, averaged from stations MAHM and KORD. Estimates of central Iran northward velocity (N-vel) at the longitude of Damghan and Jajarm are shown along the bottom of the map. Abbreviations: ABS—Apsheron-Balkan Sill; T—Turkmenbashi; N—Nebit Dagh; U—Ushak; KA—Kizyl Arvat; BH—Bakharden; A—Ashkabad; K—Kara Kala; G—Gorgan; D—Damghan; S—Shahrud; J—Jajarm; B—Bojnurd; Ne—Neyshabur. Inset shows detailed fault map of region. Key to faults: right-lateral (red); left-lateral (green); thrust (blue).
Although global positioning system (GPS) velocities indicate a westward component of motion of the South Caspian block relative to central Iran and Eurasia (Masson et al., 2007; Fig. 1), little is known about how this westward extrusion is accommodated by active faults. The aim of this paper is to identify active faults in the eastern South Caspian region using a combination of recent and historical earthquake data, offsets of the bedrock geology, and Quaternary geomorphology. Interpreting our new fault map in light of published GPS velocities (Fig. 1), we provide a new and coherent model for the present-day kinematics of this region that provides a much clearer insight into the geodynamics of the Arabia-Eurasia collision zone. Furthermore, over two million people live in the area surrounding the eastern South Caspian basin. Fault slip rates estimated in this study will therefore provide important constraints on the seismic hazard posed by faults across this region.
Tchalenko (1975) first identified a series of active N-NW–S-SE right-lateral strike-slip faults that cut obliquely across the central Kopeh Dagh (Fig. 1). Hollingsworth et al. (2006) suggested that these faults rotated counterclockwise, thereby accommodating N-S shortening, with the further requirement that elongation occurs along strike of the range. Hollingsworth et al. (2006) tentatively concluded that this elongation may be accommodated by the westward extrusion of the eastern South Caspian lowlands (Fig. 1).
A map of the major active faults identified in the eastern South Caspian region is shown in Figure 1 inset (for fault locations, see Tables DR1–DR3 in the GSA Data Repository1). Along the northern margin of the eastern South Caspian lowlands between 53°E and 57.5°E, N-NW shortening is partitioned onto separate NW-SE thrust and right-lateral strike-slip fault segments comprising the Ashkabad fault zone.
Deformation across the east Alborz mountains (between 53°E and 57.5°E) is partitioned onto the Khazaar thrust fault, which bounds the range to the north, and the left-lateral Shahrud fault system, which bounds the range to the south. These fault systems are discussed separately in the following sections.
ASHKABAD FAULT ZONE: WEST OF 55°E
The Ashkabad fault zone runs along the northern margin of the eastern South Caspian lowlands and northwest Kopeh Dagh between Bakharden and Turkmenbashi (BH and T in Fig. 1). West of 55°E, the fault zone bends ~40 km to the south, where it passes south of the Balkan anticline (see Fig. 1). Many large earthquakes are known from the Balkan region; Figure 1 shows slip vectors of earthquakes for which the source parameters have been constrained by body-waveform modeling and the likely fault planes identified (Priestley et al., 1994; Jackson et al., 2002; this study). Slip vectors are partitioned onto separate thrust and right-lateral faults that strike NW-SE. The thrust earthquakes are notable for their relatively deep hypocenters (31–42 km, increasing northward), compared with typical earthquake depths in central Iran of ~15 km (Engdahl et al., 2006). Receiver function estimates of Moho depth at Nebit Dagh and Turkmenbashi (N and T in Fig. 1) are between 40 and 50 km, and sediment thickness is 10–15 km (K. Priestley, 2008, personal commun.), which indicates that these earthquakes occur in the base of the crust as the South Caspian block is thrust beneath the Balkan region. Thrust faulting in this region is south-vergent and appears to form a landward continuation of the Apsheron-Balkan sill, which extends across the central South Caspian sea and across which the South Caspian block is thought to be subducting beneath the North Caspian (Allen et al., 2002; Jackson et al., 2002).
ASHKABAD FAULT ZONE: EAST OF 55°E
In contrast with the Balkan region, shortening across the Ashkabad fault zone east of 55°E is partitioned onto north-vergent thrust faults (Fig. 2A) and right-lateral strike-slip fault segments (Fig. 2B). The structural change from south-vergent to north-vergent thrusting near 55°E is obscured by sediments of the old Oxus River delta.
A: Quickbird satellite image (GoogleEarth) showing Quaternary alluvial deposits east of Kizyl Arvat that have been uplifted by south-dipping Kizyl Arvat thrust fault (between black arrows). Inset shows topographic profile across fault (extracted from Shuttle Radar Topography Mission [SRTM] data). B: Quickbird satellite image (GoogleEarth) near Ushak village, showing Kazandzhik fault segment (between black arrows). Yellow pointers highlight 10 m right-lateral displacements in a river bank and irrigation tunnel (qanat). C: Geological reconstruction (35 km) of Cretaceous and Neogene deposits either side of Ashkabad fault system near Bakharden. Ct—Cretaceous; Ng—Neogene; Qt—Quaternary; gvl—gravel; mrl—marl; sst—sandstone; shl—shale; lmst—limestone.
The eastern Ashkabad fault zone is made up of separate and parallel thrust (Fig. 2A) and right-lateral (Fig. 2B) fault segments (Tchalenko, 1975; Trifonov, 1978). Figure 2A shows an area east of Kizyl Arvat city (KA in Fig. 1), where Quaternary alluvial deposits have been uplifted by the Kizyl Arvat thrust fault segment and through which rivers are actively incising. On the separate right-lateral Kazandzhik fault segment, which is 10 km south of the Kizyl Arvat thrust, near the town of Ushak (U in Fig. 1), recent right-lateral motion has caused the displacement of an underground irrigation tunnel (qanat) and river bank by ~10 m. Trifonov (1978) claimed that this qanat was constructed no later than 500 B.C., indicating a maximum right-lateral slip rate of 4 mm/yr. An estimate of the total offset can be made near the town of Bakharden (BH in Fig. 1), where a new reconstruction of a displaced anticline indicates a total right-lateral displacement of 35 km (Fig. 2C) across the fault. Although the broad-scale geomorphology (Figs. 2A and 2B) and seismicity (Fig. 1) indicate that shortening across the Ashka-bad fault zone is partitioned onto separate right-lateral and thrust segments, it should be noted that a thrust component of motion cannot be ruled out for this fault segment. Oblique motion across this fault would result in an apparent right-lateral offset created by thrusting of the hanging wall over the footwall, resulting in an overestimate of the total right-lateral displacement. Therefore, 35 km should be treated as an upper limit.
KHAZAAR THRUST FAULT
Little is known about the Khazaar fault (Fig. 1), which runs along the northern margin of the Alborz mountains, due to the geomorphology and geology being obscured by dense vegetation on the north side of the range. The absence of a clear fault scarp may result from shortening in the upper crust being accommodated by ductile deformation of water-rich sediments deposited in the South Caspian basin. This interpretation is consistent with the 2004 Baladeh earthquake, which occurred in the central Alborz mountains at a depth of 22 km; aftershocks between 10 and 30 km indicate a south-dipping fault zone that projects to the surface along the Khazaar fault (Tatar et al., 2007).
Three relatively deep earthquakes (30–32 km) located in the rigid eastern South Caspian lowlands north of Gorgan (G in Fig. 1) occur in the base of the crust where receiver function estimates of the Moho depth are ~35 km, and sedimentary cover is 7 km (K. Priestley, 2008, personal commun.). These earthquakes may be related to a deep and relatively young thrust splay north of the range front, or possibly bending in the base of the South Caspian block as it is thrust beneath central Iran along the Alborz range.
SHAHRUD FAULT SYSTEM: ASTANEH FAULT
The Shahrud fault zone (Fig. 1) is a system of discontinuous left-lateral fault segments that strike NE-SW along the southern margin of the east Alborz range between 53°E and 58°E. Virtually nothing has been published on this 300-km-long fault system since the early study of Wellman (1966), who first identified it using air photos. This active system is probably responsible for the devastating Qumis earthquake of A.D. 856, which killed more than 200,000 people in the Damghan region (D in Fig. 1; see also Ambraseys and Melville, 1982). However, there are few recent earthquakes with fault plane solutions to confirm its proposed left-lateral slip style.
Figure 3A shows a topographic map of the southern part of the east Alborz range front near Damghan city. The Astaneh fault system (part of the wider Shahrud fault system) cuts across the topography north of the city. Where it bends within the mountains, it has produced a large (30–40 km long) left-lateral pull-apart basin. Figures 3B and 3C show the Astaneh fault, where the youngest alluvial fan deposits have been displaced 45 m in a left-lateral sense, and with no significant vertical displacement. If these fan deposits date from the last major regional incision event, found to be 12 ± 2 ka at various sites in eastern Iran (e.g., Regard et al., 2005; Fattahi et al., 2006), this would indicate a slip rate of 3–5 mm/yr. Furthermore, the total left-lateral offset across the Astaneh fault may be estimated by restoring Cambrian Lalun Formation sandstones and limestones, and Triassic Elikah Formation dolomites, by 30–40 km (Fig. 3A). This restoration also fully closes the Astaneh pull-apart basin.
A: Topographic fault map of southern east Alborz margin near Damghan. Inset shows simplified geology across Astaneh fault, which has been offset 30–40 km in left-lateral sense; orange shows Cambrian Lalun Formation, blue shows Triassic Elikah Formation. B, C: Air photos showing youngest alluvial fan material (T0), which has been displaced 45 m against older fan material (T1). Blue line shows a deflected river.
SHAHRUD FAULT SYSTEM: JAJARM FAULT
The left-lateral Jajarm fault (part of the wider Shahrud fault zone) is 20 km north of Jajarm city (J in Fig. 1), and strikes E-NE–W-SW, parallel to the east Alborz range, for more than 130 km. The fault cuts across Quaternary alluvial fans, producing a fault scarp clearly visible in satellite imagery (Fig. 4). Left-lateral motion across a bend in the fault near 56°10′E has produced a small restraining bend at the point of inflection, uplifting Devonian shales and gypsum (Khoshyeilagh Formation), which appear dark purple on Landsat imagery (Fig. 4). A deflected south-draining river (shown by yellow line) and displaced alluvial fans rich in Khoshyeilagh material south of the Jajarm fault (outlined in red) indicate 4–5 km of left-lateral motion during the Pliocene–Quaternary.
Landsat image of Jajarm fault (between black arrows). Alluvial fans rich in Khoshyeilagh Formation shales (outlined in red) have been displaced from Khoshyeilagh shales exposed in the restraining bend. Yellow line shows a river deflected 4–5 km. Green triangles show offset river channels, which are now dry.
DISCUSSION
The coarse distribution of GPS velocities across northeastern Iran (Fig. 1) allows only crude estimates to be made on the relative importance of thrust and strike-slip motion in the northwest Kopeh Dagh and east Alborz mountains (using data from Masson et al., 2007). The northward velocity of central Iran, south of the Alborz, decreases linearly to the east from 11.6 mm/yr near Tehran (TEHN), to 9.0 mm/yr at Semnan (SEMN), and 5.8 mm/yr at Kashmar (KASH) (Fig. 1). Based on this decrease, values for northward velocity may be estimated at Damghan (9.2 mm/yr) and Jajarm (7.2 mm/yr). The northwest velocity of the South Caspian basin, relative to Eurasia, may be estimated from the average velocity of stations MAHM and KORD (7.4 mm/yr; white arrow in Fig. 1).
The deformation currently being accommodated across the east Alborz range near Damghan is estimated as 3.3 mm/yr (NE-SW) from the difference in GPS velocities on either side of the range (all shortening and fault slip rates in this section are estimated using velocity triangle constructions; Fig. DR1 [see footnote 1]). Resolving this velocity onto the Khazaar and Astaneh faults indicates slip rates of 0.8 mm/yr thrust, and 3.3 mm/yr left-lateral strike slip, respectively. At the longitude of Jajarm, the South Caspian block is moving 2.5 mm/yr to the west, relative to central Iran. As this motion is subparallel to the strike of the range, it is mostly accommodated by left-lateral strike slip (2.4 mm/yr) on the Jajarm fault.
Slip rates for the thrust and strike-slip fault segments that make up the Ashkabad fault system may be estimated from GPS velocities in the same way as for the east Alborz range (see the Data Repository). Resolving the average velocity of South Caspian motion across the western Ashkabad fault zone (Balkan region) indicates ~5 mm/yr N-NE shortening on thrust faults, and 5–6 mm/yr right-lateral slip on W-NW–E-SE strike-slip faults. These values decrease to ~2 mm/yr thrust and 3–4 mm/yr strike slip across the eastern Ashkabad fault zone, near Bakharden (BH in Fig. 1). The slip rates estimated above are not well constrained; likely errors are 1–2 mm/yr based on the errors in the GPS measurements and assumptions used to derive these slip rates. Nevertheless, they provide the first slip-rate estimates for this region, which are important in understanding both the tectonic evolution of the region and the seismic hazard posed by these faults. A tectonic map summarizing the present-day kinematics, along with our slip-rate estimates for the eastern South Caspian region, is shown in Figure 5.
Summary tectonic map of the eastern South Caspian region. Dark gray arrows show approximate velocities across the region (mm/yr), relative to Eurasia, determined by GPS measurements. Fault slip rates estimated in this study are shown in black; those in gray, at Sabzevar (Sabz.) and Neyshabur, are determined from optically stimulated luminescence dating of displaced geomorphology (Fattahi et al., 2006; M. Fattahi, 2008, personal commun.). Faults: BF—Baghan; QF—Quchan; BjF—Bajgiran.
Assuming that the GPS-derived slip rates estimated above have been constant over the late Cenozoic, and the total lateral offsets estimated for the Ashkabad (Fig. 2C) and Shahrud fault zones (Fig. 3A) were produced by pure strike-slip motion, it would take ~10 m.y. to accommodate these displacements. This value is consistent with a regional tectonic reorganization at this time implied by increased exhumation of the west Alborz range at 12 Ma (Guest et al., 2006). However, if these offsets have been exaggerated by significant thrust motion, the time required to accommodate them could be as little as ~5.5 m.y., consistent with the onset of subduction of the South Caspian block, as indicated by subsidence data (Allen et al., 2002); 3.4 m.y., as indicated by the onset of folding of strata in the South Caspian basin (Devlin et al., 1999); or even 1.8 m.y., as indicated by paleo-seismology investigations in the central Alborz range (Ritz et al., 2006). Nevertheless, if the South Caspian block currently moves ~7 mm/yr in a N-NW direction, relative to Eurasia, it would take ~10–15 m.y. to subduct South Caspian material to a depth of 70–80 km beneath the North Caspian region, as indicated by two deep thrust earthquakes, 19 December 1987 and 31 August 1993, which are thought to represent down-dip elongation in the subducting slab (see Jackson et al. [2002] and their Fig. 6). Also, the present-day extension across the central Kopeh Dagh, estimated from GPS measurements, is ~2.3 mm/yr (the difference between stations SHIR and MSHN, Fig. 1). Hollingsworth et al. (2006) estimated 30 ± 6 km total W-NW–E-SE extension across this zone, which would also require ~10–15 m.y. to accumulate. Therefore, the extrusion and probable subduction of the South Caspian basin beneath the Apsheron-Balkan sill may have been occurring for longer than previously suggested: ~5.5 m.y. (Allen et al., 2002) and 1.8 m.y. (Ritz et al., 2006).
CONCLUSION
Our results indicate that the westward extrusion of the South Caspian basin, relative to central Iran and Eurasia, occurs along right- and left-lateral fault systems in the northwest Kopeh Dagh and east Alborz mountains, respectively. Regional shortening across these systems is not accommodated purely by strike slip, but is partitioned onto separate thrust and strike-slip thrust components. At present-day slip rates, the total right- and left-lateral offsets of ~35 km would be accommodated in ~10 m.y., suggesting that the current kinematic pattern in this region, and the possible onset of subduction of the South Caspian block, may also date from this time.
Acknowledgments
We thank M.T. Korehie, M. Ghorashi, and M. Talebian of the Geological Survey of Iran for their continued collaboration and generous support of our field work in Iran. M. Fattahi (Institute of Geophysics) and A. Bahroudi (University of Tehran) also provided valuable field assistance. This work benefited from helpful discussions with J. Ritz, N. White, B. Lovell, A. Copley, and K. Priestley, and careful reviews by M. Allen and P. Vernant. This work was supported by a Natural Environment Research Council–funded COMET (Center for the Observation and Modelling of Earthquakes and Tectonics) research studentship to Hollingsworth. This is Cambridge Earth Science contribution ES.9221.
Footnotes
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↵GSA Data Repository item 2008199, Tables DR1–DR3, with data points (long/lat) for thrust (DR1), strike-slip (DR2), and normal faults (DR3) in the eastern South Caspian region; and Figure DR1 (velocity triangle constructions for faults in the eastern South Caspian region), is available online at www.geosociety.org/pubs/ft2008.htm, or on request from editing{at}geosociety.org or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.
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- Received 11 April 2008.
- Revision received 5 June 2008.
- Accepted 11 June 2008.
- © 2008 Geological Society of America
