- Email: [email protected]
Active tectonics and strain partitioning along dextral fault system in Central Iran: Analysis of geomorphological observations and geophysical data in the Kashan region
Journal of Asian Earth Sciences 40 (2011) 1015–1025
Contents lists available at ScienceDirect
Journal of Asian Earth Sciences journal homepage: www.elsevier.com/locate/jseaes
Active tectonics and strain partitioning along dextral fault system in Central Iran: Analysis of geomorphological observations and geophysical data in the Kashan region Farshad Jamali a, Khaled Hessami a,*, Manoochehr Ghorashi b,c a b c
International Institute of Earthquake Engineering and Seismology, Tehran, Iran, P.O. Box 19395-3913, Tehran, Iran Geological Survey of Iran, PO Box 13185-1494, Tehran, Iran Islamic Azad University, North Tehran Branch, Tehran, Iran
a r t i c l e
i n f o
Article history: Available online 1 June 2010 Keywords: Strain partitioning Kashan Central Iran Active fold Active fault
a b s t r a c t This paper uses high-resolution images and field investigations, in conjunction with seismic reflection data, to constrain active structural deformation in the Kashan region of Central Iran. Offset stream beds and Qanats indicate right-lateral strike slip motion at a rate of about 2 mm/yr along the NW–SE trending Qom-Zefreh fault zone which has long been recognized as one of the major faults in Central Iran. However, the pattern of drainage systems across the active growing folds including deep incision of stream beds and deflected streams indicate uplift at depth on thrust faults dipping SW beneath the anticlines. Therefore, our studies in the Kashan region indicate that deformation occurs within Central Iran which is often considered to behave as a non-deforming block within the Arabia–Eurasia collision zone. The fact that the active Qom-Zefreh strike-slip fault runs parallel to the active folds, which overlie blind thrust faults, suggests that oblique motion of Arabia with respect to Eurasia is partitioned in this part of Central Iran. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction and tectonic setting The Kashan region is located in the southwestern part of Central Iran which is often considered to behave as a non-deforming block within the Arabia–Eurasia collision zone (Jackson and McKenzie, 1988; Vernant et al., 2004a) (Fig. 1). The motion of the Arabian plate with respect to Eurasia causes deformation along pre-existing structures striking oblique to the convergence direction in the Kashan region. Tectonic movement, oblique to plate boundaries is observed worldwide and can produce two different modes of tectonic regimes. The first and general case is wrench tectonics with oblique slip and relatively strong coupling along faults (Wilcox et al., 1973; Lee et al., 1998). The second mode was introduced first by Fitch (1972) as decoupling and then adopted by others as partitioning (e.g. Lettis and Hanson, 1991; McCaffrey, 1992). In this case, the oblique strain in the lower crust and/or lithospheric mantle partitions into nearly normal and tangential components in the brittle crust (Lettis and Hanson, 1991). In seismically active regions, the sort of tectonic regime could be recognized based on seismic activity (e.g. Seeber and Pecher, 1998), however, when seismic data is not available, or in aseismic regions,
* Corresponding author. Tel.: +98 21 22831116; fax: +98 21 22299479. E-mail addresses: [email protected], [email protected] (K. Hessami). 1367-9120/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jseaes.2010.05.013
tectonic geomorphology may help to find out the active mode of deformation. Central Iran is delineated by major boundary faults, which appear to be inherited from old geological times and limited by the Lut block to the east, the Alborz and Kopeh Dagh Mountains to the north, the Sanandaj-Sirjan zone and Zagros Mountains to the southwest, and Makran accretionary wedge to the southeast (Berberian and King, 1981) (Fig. 1). Recent GPS studies indicate that the actual motion of the Arabian plate relative to Eurasia is about 22 ± 2 mm/yr along the 51° east longitude (Nilforoushan et al., 2003; Vernant et al., 2004a; Masson et al., 2007). From this amount, about 7 ± 2 mm/yr of shortening occurs across the Zagros Mountains (Vernant et al., 2004a; Hessami et al., 2006), 5 ± 2 mm/ yr in Alborz Mountains, and 3 ± 2 in Central Iran, south of the Alborz and north of the Sanandaj-Sirjan zone (Vernant et al., 2004a,b). The rest of this motion of about 7 mm/yr is consumed in the south Caspian basin (Vernant et al., 2004b) (Fig. 1). Our goal is to determine where and how the 3 mm/yr convergence that occurs across Central Iran, between the Alborz Mountains and the Sanandaj-Sirjan zone, is accommodated. Relatively few historical earthquakes have occurred in the Kashan region (e.g. Kashan, 1755 and 1778, and Qohrud, 1844, see Ambraseys and Melville, 1982) with only minor seismicity in the instrumental period of observation. We combine observations from high-resolution images and field investigations, with seismic reflection data, to
1016
F. Jamali et al. / Journal of Asian Earth Sciences 40 (2011) 1015–1025
Fig. 1. Location map of Iran. Solid lines are active faults and thicker lines are rightlateral strike-slip faults that accommodate shear between Sanandaj-Sirjan zone and Central Iran; NTF: North Tabriz Fault, QZF: Qom-Zefreh Fault, DSF: Dehshir Fault. The box outlines the area in Fig. 2. Urmieh-Dokhtar magmatic arc running along the NE margin of the Sanandaj-Sirjan zone is marked in grey. The arrow at the bottom is the convergence direction between Arabia and Eurasia at this longitude (Vernant et al., 2004a).
constrain the active tectonics of this region. We show that active tectonics in the study region is manifested by both active faulting and folding. This information leads us to a partitioned model along this part of Central Iran rather than a wrench tectonic model.
2. Active faulting The southwestern margin of Central Iran, in the Kashan region, is marked by the Qom-Zefreh fault zone (Nabavi, 1977) which cuts
across the overall structure and morphology of the region (Fig. 2). Although, Nadji (1972) and Amidi (1975) studied two different locations of this long fault, Nabavi (1977) first identified this fault zone as a major right-lateral strike-slip fault. Occurrences of historical and recent earthquakes along the Qom-Zefreh fault zone and its NW continuation (Ambraseys and Melville, 1982; Walker et al., 2005) highlight the active deformation accommodated along the NE boundary of the Urmieh-Dokhtar magmatic arc (i.e. southwestern margin of Central Iran). The Qom-Zefreh fault zone forms the topographic boundary between Eocene volcanic rocks on the southwest and elongated depressions to the NE (Fig. 2). This fault zone is made of several segments trending NW–SE, along which systematic right-lateral offsets of stream beds and alluvial fans are evident (Figs. 3–6). The SW block is up-thrown relative to the northeastern one as a result of a reverse component of motion on the fault segments which dip southwest. Here we present the surface exposures and horizontal offsets of geomorphic features in two localities to document recent activity and the sense of movement along the Kashan segment of the Qom-Zefreh fault zone. 1. The Gabrabad locality is named after the Gabrabad village (Fig. 3) situated on an alluvial fan which is clearly offset in a right-lateral sense along the Kashan fault trace (Fig. 5a). The alluvial fan, incised by drainages, show a 570 ± 30 m horizontal offset and a 25 ± 5 m vertical displacement. Offsets of several stream beds elsewhere along this section of the fault also contain evidence for cumulative displacements by several individual offset events, however without quantitative age constraints, these offsets are difficult to interpret (a, b and c in Fig. 5). The most spectacular feature along this section of the Kashan fault is a line of Qanat (underground water tunnels, marked by lines of access shafts) displaced in a right-lateral sense by 11 ± 1 m (d in Fig. 5). Since Qanats in the Iranian plateau have been constructed since 5000 yr ago (Neely and Wright, 1994), it implies a minimum horizontal slip rate of about 2.2 mm/yr along this section of the Kashan fault segment. Man-made modifications and excavations of the ground surface some 700 m southeast of the Gabrabad fan provided important fault exposures in geologically young material (e in Fig. 5).
Fig. 2. Shaded 10-m-resolution topography model of the study area showing the major fault zones and geographical locations. Boxes show the regions covered by Figs. 3 and 4.
F. Jamali et al. / Journal of Asian Earth Sciences 40 (2011) 1015–1025
1017
Fig. 3. Simplified geological map of the southeast Kashan area (see Fig. 2 for location in the Kashan region) (adapted from Geological Survey of Iran, 1979). Active faults are shown by thicker lines. Right-lateral offset across the Qom-Zefreh fault is clear in the drainage. (1) rock units (2) active Neogene folds (3) older Quaternary terraces (4) younger Quaternary terraces (5) playa deposits (6) lake deposits (7) river beds (8) road (9) railroad (10) streams (11) anticline axis (12) syncline axis (13) village.
Fig. 4. Simplified geological map of the Natanz area (see Fig. 2 for location in the Kashan region) (adapted from Geological Survey of Iran, 1979). The box outlines the area in Fig. 6. Right-lateral offset across the Qom-Zefreh fault is clear in the drainage. (1) rock units (2) active Neogene folds (3) Quaternary deposits (4) alluvial plain deposits (5) river beds (6) road (7) railroad (8) streams (9) anticline axis (10) syncline axis (11) village.
1018
F. Jamali et al. / Journal of Asian Earth Sciences 40 (2011) 1015–1025
Here, late Quaternary well cemented debris is juxtaposed against severely altered volcanic rocks (tuffs and shales) of Neogene age across vertical faults. However, a subsidiary thrust cutting the volcanic rocks and its overlying Holocene soil of about 1 m thickness indicates recent activity along this segment of the fault. 2. The Hanjan locality includes two sections of the fault segment northwest of the village of Hanjan (Figs. 4 and 6). Fig. 6a shows the
satellite images of the two sections 1.5 km apart at this locality. Stream beds have been offset in a right-lateral sense along two parallel fault strands at one of these two sections (Fig. 6b). However, small gullies incised within the young alluvial fan are right-laterally offset by 21 m, indicating recent fault activity at this location (Fig. 6c). However, since the absolute age of the offset fan is not known, slip rate along this segment of the Kashan fault remains unknown.
Fig. 5. (a–c) Satellite images (Google Earth) of the Qom-Zefreh fault (Kashan segment) near Gabrabad. Rectangles in (a) enclose (b) and (c). Locations of (d) and (e) are indicated on (a). Filled arrows show the fault trace. (d) Line of Qanat is offset along the fault trace. (e) View of the Kashan fault in a man-made trench exposed the Quaternary and Holocene sediments.
F. Jamali et al. / Journal of Asian Earth Sciences 40 (2011) 1015–1025
1019
Fig. 6. Satellite images (Google Earth) of the Qom-Zefreh fault (Kashan segment) NW of Hanjan (see Fig. 4 for location). (a) Fault trace, indicated by black arrows, showing vertical component down to the NE, causing their SW side to be incised by drainage. Rectangles in (a) enclose (b) and (c). (b) Detail of the image showing two parallel fault trace, where right-lateral offsets is clear in the drainage delineated by white dashed lines. (c) Recent alluvial fan is offset for about 21 m.
Fig. 7. Cosmos satellite image of Natanz area. The Moghar antecedent stream incise deeply through the alluvial fan as it crosses the active growing folds and thrust.
1020
F. Jamali et al. / Journal of Asian Earth Sciences 40 (2011) 1015–1025
3. Active folding
3.2. The Navab folds
To the northeast of the Qom-Zefreh fault zone growing folds with axes trending NW–SE have developed parallel to the fault zone (Figs. 3 and 4). The deep incision of river beds, pattern of streams adjacent to the folds and presence of wind and water gaps are interpreted to indicate the direction of lateral propagation of the folds at the region between Kashan on the northeast and Ardestan to the southeast.
The Navab folds include a series of growing synclines and anticlines trending NW–SE and form the frontal structures of the Karkas Mountains some 20 km southeast of the city of Kashan (Figs. 2 and 3). The northeastern limbs of these folds are steeper than the southwestern limbs (Fig. 10). These asymmetric folds have probably formed above blind reverse faults dipping SW along the northeastern limb of the anticlines (Fig. 3). The overlying folds show doubly plunging terminations and a saddle in between suggesting that variation in displacement along the underlying reverse faults are reflected in the amplitude of folding at the surface (Fig. 3). Here, we apply the same criteria introduced by Keller et al. (1999) to the Navab folds to determine lateral propagation and infer activity of the underlying reverse faults. A topographic cross section along the plunging axis of the main Navab fold shows that the elevation of the topographic profile decreases outward from its center towards both the northwest and southeast (Fig. 11). Same decrease is seen when examining the drainage density and degree of dissection along the Navab folds (Fig. 12). The drainage density map for the Navab folds (Fig. 12down), which is defined by the ratio of the sum of total length of channels divided by drainage basin area, decreases outward from
3.1. East of Natanz Moghar River, as a persisting antecedent river, cuts across anticlines such as the Zardkuh and Kuh-e Charkheh anticlines east of the city of Natanz (Figs. 4 and 7). The northeastern limb of Kuh-e Charkheh anticline dips more steeply than the southwestern limb (Fig. 8a) due to displacement on the underlying SW-dipping thrust (Fig. 8b) as evidenced by bedrock exposure at the river channel (see also Fig. 7). The Moghar River incises through the growing folds (Figs. 4 and 7) suggesting that the river has sufficient power to erode enough through the uplifting structures. As a result, relief between the river channel and the surrounding topography has increased in the form of a notch (water gap) cut through the active folds. We investigated several sections of the Moghar River in the field to compare the depth of incision along the river (Fig. 9). Deep incision of the river begins where the thrust underlying the Kuh-e Charkheh anticline crosses the river (Fig. 7). The depth of incision from 7 to 10 m upstream (Fig. 9a) has increased to maximum 50 m a few hundred meter downstream (Fig. 9b). A few kilometers further downstream where the exposure of growing anticlines disappears the height of the river bluff decreases to few meters (Fig. 9c). Comparison of the Moghar River channel with other similar streams flowing from SW to NE across the Qom-Zefreh fault zone suggests that the growing Zardkuh and Kuh-e Charkheh anticlines have forced deep incision along this segment of the Moghar River.
Fig. 8. Photo of Charkheh anticline and reveres faulting east of Natanz city. (a) View of Kuh-e Charkheh asymmetric anticline, (b) reverse fault along northeastern limb of Kuh-e Charkheh anticline.
Fig. 9. Cross sections of the Moghar River showing (a) upstream, (b) midstream and (c) downstream. See Fig. 7 for locations.
F. Jamali et al. / Journal of Asian Earth Sciences 40 (2011) 1015–1025
1021
Fig. 10. Photo of the most northeastern asymmetric anticline of the Navab folds.
Fig. 11. (Up) Drainage map, fixed on topographic contour map of the Navab folds with 20 m contour intervals. (Down) Longitudinal section along the main Navab fold. Both horizontal and vertical axes have the same scale. Location of wind gaps (Ga) and water gaps (Gw) are shown along the section. Different levels of wind and water gaps are referred by (l = lower), (m = middle) and (u = upper) and numbered from NW to SE. Three stages of uplift in northwestern part of the fold are marked as separate levels. The AA0 , BB0 , CC0 and DD0 lines showing the seismic profiles location, described on Fig. 13.
the central part of the folds. The lower ratio of drainage density corresponds well to lower dissection (Fig. 12-up) and topography and relatively younger deposits and landforms. As it can be clearly seen from Fig. 3, several streams have eroded through the Navab folds. With time, most of these streams were defeated because of insufficient stream power and/or rapid fold uplift (Fig. 11). Thus, the defeated streams were forced to find a different path and deflect around the folds. The topographic cross section along the northwestern segment of the fold shows that all of the water gaps have become wind gaps due to several phases of
fold uplift. As a result, lake deposits accumulated in the catchments along the back limb of the fold (Fig. 3). The longitudinal topographic profile indicates three geomorphic levels each disturbed by 2–3 wind gaps along the NW segment of the main fold (Fig. 11). This can be interpreted to indicate that the Navab folds developed during three major phases of uplift and lateral propagation. Our measurements along the longitudinal section show topographic relief rising 15–20 m from the wind gap to the adjacent ridges of the lowest level. Similar measurements to that of the middle and top levels are 40–50 m and 70–80 m respectively.
1022
F. Jamali et al. / Journal of Asian Earth Sciences 40 (2011) 1015–1025
Fig. 12. (Up) Detailed drainage map of the Navab folds. The drainage pattern is changing along and across the fold system. Condensed drainage pattern defines the highest and simultaneously the oldest part of the fold system. The density of the drainage system decreases along the fold axes from the central part of the folds to either side. (Down) Drainage density map. Darker areas represent the higher drainage density. The areas with higher density conform with the higher magnitude of dissection.
These amounts are almost half of the topography elevation at each level. This implies that at each phase of the fold growths the antecedent river has incised across the Navab folds during the first half of the growth period. However, along the southeastern segment of the main fold only two wind gaps of the same level have formed and the only existing water gap has deflected. The absence of wind gaps along the southeastern segment of the fold is interpreted to indicate that the lateral propagation of the fold towards the southeast ceased sometime after the wind gap formed. The patterns of drainage system and topography of the Navab folds indicate lateral propagations on either end of the growing folds. Below we show that the lateral growth of folds at the surface is linked to fault growth in the subsurface.
4. Seismic reflection analyses Here we present four processed seismic sections across the Navab folds to gain subsurface information of the main structures (Fig. 13). Seismic sections presented here are plotted at 1:1 (no vertical exaggeration) assuming an average seismic P wave velocity of 3500 m/s, so that 1 s of two-way travel time corresponds to about 1750 m of depth. We also used unmigrated data which means that the reflections are plotted halfway between the source and the receiver. The AA0 profile (Fig. 13) crosses the northwestern part of the Navab folds (Fig. 11). Two synclines and an anticline can be easily detected along this profile. The NE syncline which is deep-seated and asymmetric with the steep southwest limb is overlain by
F. Jamali et al. / Journal of Asian Earth Sciences 40 (2011) 1015–1025
1023
Fig. 13. Seismic profiles across the Navab folds. Location of profiles are marked in Fig. 11 as the AA0 , BB0 , CC0 and DD0 lines. Vertical and horizontal scales in all seismic sections are the same, assuming an average seismic wave velocity of 3500 m/s that is 1 s of two-way travel time corresponds to about 1750 m of depth. Inferred faults are shown by thick lines.
growth strata of Pliocene age (Geological Survey of Iran, 1979). The southwestern syncline, constitute the highest topographic relief and is detached from the underlying anticline. The anticline is asymmetric with a relatively steep forelimb and much gentler backlimb. The forelimb of the anticline is cut by a low angle thrust (hereafter is referred to as Navab thrust) which appears to join to the Qom-Zefreh fault zone at depth. The Navab thrust is blind and truncated by the Pliocene deposits. Further southeast (Fig. 11), the BB0 profile shows a back thrust as well as the main blind thrust fault recognized on the AA0 profile (Fig. 13). This back thrust is dipping NE, parallel with the southwestern limb of the syncline and joins the tip of the Navab thrust. The fault geometry along this seismic profile can be interpreted as a wedge thrust responsible for producing the overlying anticline. With a wedge thrust underlying the fold, the syncline axial surface should be active (it migrates forward, toward NE, with respect to the strata above the thrust tip as suggested by Burbank and Anderson, 2001). The AA0 and BB0 seismic profiles show the Pliocene and older beds roll up from the basin through the synclinal axial surface. Also, the bedding geometry of the layers of the SW limb of the NE syncline are interpreted to indicate progressive deformation toward the NE. The CC0 and DD0 profiles show that by moving southeast, away from the older central part of the folds, less deformation occurs across the Navab folds as evidenced by wide gentle folds and small amount of displacement on the thrusts.
5. Discussion and conclusion In the previous sections we presented geomorphic evidence of active faulting and folding to indicate active deformation in the Kashan region. In the following discussion we describe the kinematics in west-central Iran.
Fig. 14. Schematic diagram illustrating the inferred slip partitioning along the Qom-Zefreh dextral fault and fold-thrust belt in Central Iran. Black arrows show the regional shortening direction.
1024
F. Jamali et al. / Journal of Asian Earth Sciences 40 (2011) 1015–1025
The active tectonics of the Kashan region is dominated by the major Qom-Zefreh strike-slip fault, which accommodates right-lateral shear between the Sanandaj-Sirjan zone to the southwest and Alborz Mountains to the north. The North Tabriz right-lateral fault in NW, and the Dehshir right-lateral fault in SE of the Qom-Zefreh fault seem to play similar role in accommodating regional strain (Fig. 1 and see also Masson et al., 2006; Meyer et al., 2006). The rate of right-lateral motion decreases from NW along the North Tabriz fault (6–7 mm/yr) (Hessami et al., 2003; Masson et al., 2006) to SE along the Qom-Zefreh fault (2.2 mm/yr) as well as along the Dehshir fault (2 mm/yr). This decrease of motion from the North Tabriz fault on the northwest to the Qom-Zefreh fault on the southeast is due to the counterclockwise rotation of Arabia relative to Eurasia about the rotation pole at 27.9 N and 19.5 E (Vernant et al., 2004a). We also know from GPS that the rate of shortening between the Sanandaj-Sirjan zone and Alborz Mountains is about 3 mm/yr (Vernant et al., 2004b). As we indicated, a large component of shortening in the Kashan region is accommodated by shortening on parallel folds and blind thrusts while, so far, no Quaternary deformation has been described south of central Alborz. Therefore, the locus of the deformation between the Alborz and the Central Iran seems to lie along the northeastern side of the Urmieh-Dokhtar magmatic belt (Fig. 1) a possibility that Vernant et al. (2004b) have recently considered. In many examples around the world, it is shown that where overall convergence is oblique to the trend of deforming belt, the convergence is achieved by partitioning of the orthogonal strikeslip and shortening components on parallel faults (Lettis and Hanson, 1991; Angelier et al., 2003; Talebian and Jackson, 2004; Hollingsworth et al., 2008). The right-lateral offsets of geomorphic features along the Kashan segment indicate that slip vector is subparallel to the Qom-Zefreh fault (Fig. 14). However, slip vector seems to be roughly perpendicular to the trend of reverse faults and folds running parallel to the Qom-Zefreh fault zone. This would suggest that in this part of Central Iran the strike-slip and shortening components of the oblique N–S convergence of Arabia and Eurasia are partitioned. The identification of zones of active fault-related folding is important for the estimation of regional seismic hazard (Walker et al., 2003), as the historical record of destructive earthquakes and the very low level of instrumental seismicity in the region is not likely to represent the full hazard from earthquakes. The lack of seismic activity in this part of Central Iran prevents us from confirming the role of normal and tangential components in this partitioned phenomenon, nor the capability of the reverse faults for nucleating earthquakes independently. For a better understanding of the active tectonics of Central Iran a better knowledge of geodetic and geologic slip-rates is required. In the near future, we may have accurate dating of offset morphologic features along the Kashan fault segment. A dense GPS network across the whole region between the Sanandaj-Sirjan zone and the Alborz Mountains will also provide detailed information on the slip-rates of active structures across Central Iran. This in turn would help to retrieve the interseismic strain across individual faults. Acknowledgments The late Manoochehr Pedrami first identified the growing anticlines east of Natanz and drew our attention to their role on deep incision of the Moghar River. We would like to thank James Hollingsworth and Morteza Talebian for their thorough and constructive reviews that led to numerous improvements in the final manuscript. We also thank Esmaeil Shabanian for fruitful discussions and comments. The authors are grateful to the National
Iranian Oil Company for providing the seismic profiles. This work is part of the first author’s Ph.D. thesis at the International Institute of Earthquake Engineering and Seismology. The editors Bihong Fu and Richard Walker are acknowledged for help and handling the manuscript. References Ambraseys, N.N., Melville, C.P., 1982. A History of Persian Earthquakes. Cambridge University Press. pp. 219. Amidi, S.M., 1975. Contribution a Letude Stratigraphique Petrologique et Petrocimique des Roches Magmatiques de la Region Natanz-Nain-Surk (Iran Central). Thesis, Grenoble, pp. 316. Angelier, J., Lee, J.C., Hu, J.C., Chu, H.T., 2003. Three-dimensional deformation along the rupture trace of the September 21st, 1999, Taiwan earthquake: a case study in the Kuangfu School. Journal of Structural geology 25, 351–370. Berberian, M., King, G.C.P., 1981. Toward a paleogeography and tectonic evolution of Iran. Canadian Journal of Earth Science 18, 210–265. Burbank, D.W., Anderson, R.S., 2001. Tectonic Geomorphology. Blackwell Science. pp. 274. Fitch, T.J., 1972. Plate convergence, transcurrent faults, and internal deformation adjacent to Southeast Asia and Western Pacific. Journal of Geophysical Research 77, 4432–4460. Geological Survey of Iran, 1979. Geological Map of Kashan Quadrangle, No. F7, Scale: 1:250,000. Hessami, K., Nilforoushan, F., Talbot, C.J., 2006. Active deformation within the Zagros Mountains deduced from GPS measurements. Journal of the Geological Society 163, 143–148. Hessami, K., Pantosti, D., Tabasi, H., Shabanian, E., Abbasi, M.R., Feghhi, K., Soleymani, S., 2003. Paleoearthquakes and slip rates of the North Tabriz Fault, NW Iran: preliminary results. Annals of Geophysics 46 (5), 903–915. Hollingsworth, J., Jackson, J., Walker, R., Nazari, H., 2008. Extrusion tectonics and subduction in the eastern South Caspian region since 10 Ma. Geology 36 (10), 763–766. Jackson, J., McKenzie, D.P., 1988. The relationship between plate motions and seismic moment tensors, and the rates of active deformation in the Mediterranean and Middle East. Geophysical Journal 93, 45–73. Keller, E.A., Gurrola, L., Tierney, T.E., 1999. Geomorphic criteria to determine direction of lateral propagation of reverse faulting and folding. Geology 27, 515–518. Lee, J.C., Angelier, J., Chu, H.T., Yu, S.B., Hu, J.C., 1998. Plate-boundary strain partitioning along the sinistral collision suture of the Philippine and Eurasian plates: analysis of geodetic data and geological observation in southeastern Taiwan. Tectonics 17, 859–871. Lettis, W.R., Hanson, K.L., 1991. Crustal strain partitioning: implications for seismic hazard assessment in western California. Geology 19, 559–562. Masson, F., Djamour, Y., Van Gorp, S., Chéry, J., Tatar, M., Tavakoli, F., Nankali, H., Vernant, P., 2006. Extension in NW Iran driven by the motion of the South Caspian Basin. Earth and Planetary Science Letters 252, 180–188. Masson, F., Anvari, M., Djamour, Y., Walpersdorf, A., Tavakoli, F., Daignieres, M., Nankali, H., Van Gorp, S., 2007. Large-scale velocity field and strain tensor in Iran inferred from GPS measurements: new insight for the present-day deformation pattern within NE Iran. Geophysical Journal International 170, 436–440. doi:10.1111/j.1365-246X.2007.03477.x. McCaffrey, R., 1992. Oblique plate convergence, slip vectors, and forearc deformation. Journal of Geophysical Research 97, 8905–8915. Meyer, B., Mouthereau, F., Lacombe, O., Agard, P., 2006. Evidence of quaternary activity along the Dehshir fault: implication for the tertiary tectonics of Central Iran. Geophysical Journal International 164, 192–201. Nabavi, M.H., 1977. Introduction to the geology of Iran (in Farsi). Geological Survey of Iran. Nadji, M., 1972. Geologie und hydrogeology de gebietes von Kashan/Iran. Geologische Mitteilung 11, 275–362. Neely, J.A., Wright, H.T., 1994. Early Settlement and Irrigation on the Deh Luran Plain; Village and Early State Societies in Southwestern Iran. Technical Report 26. University of Michigan Museum of Anthropology, Ann Arbor, MI. pp. 236. Nilforoushan, F., Masson, F., Vernant, P., Vigny, C., Martinod, J., Abbassi, M., Nankali, H., Hatzfeld, D., Bayer, R., Tavakoli, F., Ashtiani, A., Doerflinger, E., Daignieres, M., Collard, P., Chery, J., 2003. GPS network monitors the Arabia–Eurasia collision deformation in Iran. Journal of Geodesy 77, 411–422. Seeber, L., Pecher, A., 1998. Strain partitioning along the Himalayan arc and Nanga Parbat antiform. Geology 26, 791–794. Talebian, M., Jackson, J., 2004. A reappraisal of earthquake focal mechanisms and active shortening in the Zagros Mountains of Iran. Geophysical Journal International 156, 506–526. Vernant, P., Nilforoushan, F., Hatzfeld, D., Abassi, M., Vigny, C., Masson, F., Nankali, H., Martinod, J., Ghafory-Ashtiany, M., Bayer, R., Tavakoli, F., Chéry, J., 2004a. Present-day crustal deformation and plate kinematics in Middle East constrained by GPS measurements in Iran and Northern Oman. Geophysical Journal International 157, 381–398. Vernant, P., Nilforoushan, F., Che´ry, J., Bayer, R., Djamour, Y., Masson, F., Nankali, H., Ritz, J.-F., Sedighi, M., Tavakoli, F., 2004b. Deciphering oblique shortening of central Alborz in Iran using geodetic data. Earth and Planetary Science Letters 223, 177–185.
F. Jamali et al. / Journal of Asian Earth Sciences 40 (2011) 1015–1025 Walker, R., Jackson, J., Baker, C., 2003. Thrust faulting in eastern Iran: source parameters and surface deformation of the 1978 Tabas and 1968 Ferdows earthquake sequences. Geophysical Journal International 152, 749–765. Walker, R., Bergman, E., Jackson, J., Ghorashi, M., Talebian, M., 2005. The 2002 June 22 changureh (Avaj) earthquake in Qazvin province, Northwest Iran: epicentral
1025
relocation, source parameters, surface deformation and geomorphology. Geophysical Journal International 160, 707–720. Wilcox, R.E., Harding, T.P., Seely, D.R., 1973. Basic wrench tectonics. American Association of Petroleum Geologists Bulletin 57, 74–96.
Contents lists available at ScienceDirect
Journal of Asian Earth Sciences journal homepage: www.elsevier.com/locate/jseaes
Active tectonics and strain partitioning along dextral fault system in Central Iran: Analysis of geomorphological observations and geophysical data in the Kashan region Farshad Jamali a, Khaled Hessami a,*, Manoochehr Ghorashi b,c a b c
International Institute of Earthquake Engineering and Seismology, Tehran, Iran, P.O. Box 19395-3913, Tehran, Iran Geological Survey of Iran, PO Box 13185-1494, Tehran, Iran Islamic Azad University, North Tehran Branch, Tehran, Iran
a r t i c l e
i n f o
Article history: Available online 1 June 2010 Keywords: Strain partitioning Kashan Central Iran Active fold Active fault
a b s t r a c t This paper uses high-resolution images and field investigations, in conjunction with seismic reflection data, to constrain active structural deformation in the Kashan region of Central Iran. Offset stream beds and Qanats indicate right-lateral strike slip motion at a rate of about 2 mm/yr along the NW–SE trending Qom-Zefreh fault zone which has long been recognized as one of the major faults in Central Iran. However, the pattern of drainage systems across the active growing folds including deep incision of stream beds and deflected streams indicate uplift at depth on thrust faults dipping SW beneath the anticlines. Therefore, our studies in the Kashan region indicate that deformation occurs within Central Iran which is often considered to behave as a non-deforming block within the Arabia–Eurasia collision zone. The fact that the active Qom-Zefreh strike-slip fault runs parallel to the active folds, which overlie blind thrust faults, suggests that oblique motion of Arabia with respect to Eurasia is partitioned in this part of Central Iran. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction and tectonic setting The Kashan region is located in the southwestern part of Central Iran which is often considered to behave as a non-deforming block within the Arabia–Eurasia collision zone (Jackson and McKenzie, 1988; Vernant et al., 2004a) (Fig. 1). The motion of the Arabian plate with respect to Eurasia causes deformation along pre-existing structures striking oblique to the convergence direction in the Kashan region. Tectonic movement, oblique to plate boundaries is observed worldwide and can produce two different modes of tectonic regimes. The first and general case is wrench tectonics with oblique slip and relatively strong coupling along faults (Wilcox et al., 1973; Lee et al., 1998). The second mode was introduced first by Fitch (1972) as decoupling and then adopted by others as partitioning (e.g. Lettis and Hanson, 1991; McCaffrey, 1992). In this case, the oblique strain in the lower crust and/or lithospheric mantle partitions into nearly normal and tangential components in the brittle crust (Lettis and Hanson, 1991). In seismically active regions, the sort of tectonic regime could be recognized based on seismic activity (e.g. Seeber and Pecher, 1998), however, when seismic data is not available, or in aseismic regions,
* Corresponding author. Tel.: +98 21 22831116; fax: +98 21 22299479. E-mail addresses: [email protected], [email protected] (K. Hessami). 1367-9120/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jseaes.2010.05.013
tectonic geomorphology may help to find out the active mode of deformation. Central Iran is delineated by major boundary faults, which appear to be inherited from old geological times and limited by the Lut block to the east, the Alborz and Kopeh Dagh Mountains to the north, the Sanandaj-Sirjan zone and Zagros Mountains to the southwest, and Makran accretionary wedge to the southeast (Berberian and King, 1981) (Fig. 1). Recent GPS studies indicate that the actual motion of the Arabian plate relative to Eurasia is about 22 ± 2 mm/yr along the 51° east longitude (Nilforoushan et al., 2003; Vernant et al., 2004a; Masson et al., 2007). From this amount, about 7 ± 2 mm/yr of shortening occurs across the Zagros Mountains (Vernant et al., 2004a; Hessami et al., 2006), 5 ± 2 mm/ yr in Alborz Mountains, and 3 ± 2 in Central Iran, south of the Alborz and north of the Sanandaj-Sirjan zone (Vernant et al., 2004a,b). The rest of this motion of about 7 mm/yr is consumed in the south Caspian basin (Vernant et al., 2004b) (Fig. 1). Our goal is to determine where and how the 3 mm/yr convergence that occurs across Central Iran, between the Alborz Mountains and the Sanandaj-Sirjan zone, is accommodated. Relatively few historical earthquakes have occurred in the Kashan region (e.g. Kashan, 1755 and 1778, and Qohrud, 1844, see Ambraseys and Melville, 1982) with only minor seismicity in the instrumental period of observation. We combine observations from high-resolution images and field investigations, with seismic reflection data, to
1016
F. Jamali et al. / Journal of Asian Earth Sciences 40 (2011) 1015–1025
Fig. 1. Location map of Iran. Solid lines are active faults and thicker lines are rightlateral strike-slip faults that accommodate shear between Sanandaj-Sirjan zone and Central Iran; NTF: North Tabriz Fault, QZF: Qom-Zefreh Fault, DSF: Dehshir Fault. The box outlines the area in Fig. 2. Urmieh-Dokhtar magmatic arc running along the NE margin of the Sanandaj-Sirjan zone is marked in grey. The arrow at the bottom is the convergence direction between Arabia and Eurasia at this longitude (Vernant et al., 2004a).
constrain the active tectonics of this region. We show that active tectonics in the study region is manifested by both active faulting and folding. This information leads us to a partitioned model along this part of Central Iran rather than a wrench tectonic model.
2. Active faulting The southwestern margin of Central Iran, in the Kashan region, is marked by the Qom-Zefreh fault zone (Nabavi, 1977) which cuts
across the overall structure and morphology of the region (Fig. 2). Although, Nadji (1972) and Amidi (1975) studied two different locations of this long fault, Nabavi (1977) first identified this fault zone as a major right-lateral strike-slip fault. Occurrences of historical and recent earthquakes along the Qom-Zefreh fault zone and its NW continuation (Ambraseys and Melville, 1982; Walker et al., 2005) highlight the active deformation accommodated along the NE boundary of the Urmieh-Dokhtar magmatic arc (i.e. southwestern margin of Central Iran). The Qom-Zefreh fault zone forms the topographic boundary between Eocene volcanic rocks on the southwest and elongated depressions to the NE (Fig. 2). This fault zone is made of several segments trending NW–SE, along which systematic right-lateral offsets of stream beds and alluvial fans are evident (Figs. 3–6). The SW block is up-thrown relative to the northeastern one as a result of a reverse component of motion on the fault segments which dip southwest. Here we present the surface exposures and horizontal offsets of geomorphic features in two localities to document recent activity and the sense of movement along the Kashan segment of the Qom-Zefreh fault zone. 1. The Gabrabad locality is named after the Gabrabad village (Fig. 3) situated on an alluvial fan which is clearly offset in a right-lateral sense along the Kashan fault trace (Fig. 5a). The alluvial fan, incised by drainages, show a 570 ± 30 m horizontal offset and a 25 ± 5 m vertical displacement. Offsets of several stream beds elsewhere along this section of the fault also contain evidence for cumulative displacements by several individual offset events, however without quantitative age constraints, these offsets are difficult to interpret (a, b and c in Fig. 5). The most spectacular feature along this section of the Kashan fault is a line of Qanat (underground water tunnels, marked by lines of access shafts) displaced in a right-lateral sense by 11 ± 1 m (d in Fig. 5). Since Qanats in the Iranian plateau have been constructed since 5000 yr ago (Neely and Wright, 1994), it implies a minimum horizontal slip rate of about 2.2 mm/yr along this section of the Kashan fault segment. Man-made modifications and excavations of the ground surface some 700 m southeast of the Gabrabad fan provided important fault exposures in geologically young material (e in Fig. 5).
Fig. 2. Shaded 10-m-resolution topography model of the study area showing the major fault zones and geographical locations. Boxes show the regions covered by Figs. 3 and 4.
F. Jamali et al. / Journal of Asian Earth Sciences 40 (2011) 1015–1025
1017
Fig. 3. Simplified geological map of the southeast Kashan area (see Fig. 2 for location in the Kashan region) (adapted from Geological Survey of Iran, 1979). Active faults are shown by thicker lines. Right-lateral offset across the Qom-Zefreh fault is clear in the drainage. (1) rock units (2) active Neogene folds (3) older Quaternary terraces (4) younger Quaternary terraces (5) playa deposits (6) lake deposits (7) river beds (8) road (9) railroad (10) streams (11) anticline axis (12) syncline axis (13) village.
Fig. 4. Simplified geological map of the Natanz area (see Fig. 2 for location in the Kashan region) (adapted from Geological Survey of Iran, 1979). The box outlines the area in Fig. 6. Right-lateral offset across the Qom-Zefreh fault is clear in the drainage. (1) rock units (2) active Neogene folds (3) Quaternary deposits (4) alluvial plain deposits (5) river beds (6) road (7) railroad (8) streams (9) anticline axis (10) syncline axis (11) village.
1018
F. Jamali et al. / Journal of Asian Earth Sciences 40 (2011) 1015–1025
Here, late Quaternary well cemented debris is juxtaposed against severely altered volcanic rocks (tuffs and shales) of Neogene age across vertical faults. However, a subsidiary thrust cutting the volcanic rocks and its overlying Holocene soil of about 1 m thickness indicates recent activity along this segment of the fault. 2. The Hanjan locality includes two sections of the fault segment northwest of the village of Hanjan (Figs. 4 and 6). Fig. 6a shows the
satellite images of the two sections 1.5 km apart at this locality. Stream beds have been offset in a right-lateral sense along two parallel fault strands at one of these two sections (Fig. 6b). However, small gullies incised within the young alluvial fan are right-laterally offset by 21 m, indicating recent fault activity at this location (Fig. 6c). However, since the absolute age of the offset fan is not known, slip rate along this segment of the Kashan fault remains unknown.
Fig. 5. (a–c) Satellite images (Google Earth) of the Qom-Zefreh fault (Kashan segment) near Gabrabad. Rectangles in (a) enclose (b) and (c). Locations of (d) and (e) are indicated on (a). Filled arrows show the fault trace. (d) Line of Qanat is offset along the fault trace. (e) View of the Kashan fault in a man-made trench exposed the Quaternary and Holocene sediments.
F. Jamali et al. / Journal of Asian Earth Sciences 40 (2011) 1015–1025
1019
Fig. 6. Satellite images (Google Earth) of the Qom-Zefreh fault (Kashan segment) NW of Hanjan (see Fig. 4 for location). (a) Fault trace, indicated by black arrows, showing vertical component down to the NE, causing their SW side to be incised by drainage. Rectangles in (a) enclose (b) and (c). (b) Detail of the image showing two parallel fault trace, where right-lateral offsets is clear in the drainage delineated by white dashed lines. (c) Recent alluvial fan is offset for about 21 m.
Fig. 7. Cosmos satellite image of Natanz area. The Moghar antecedent stream incise deeply through the alluvial fan as it crosses the active growing folds and thrust.
1020
F. Jamali et al. / Journal of Asian Earth Sciences 40 (2011) 1015–1025
3. Active folding
3.2. The Navab folds
To the northeast of the Qom-Zefreh fault zone growing folds with axes trending NW–SE have developed parallel to the fault zone (Figs. 3 and 4). The deep incision of river beds, pattern of streams adjacent to the folds and presence of wind and water gaps are interpreted to indicate the direction of lateral propagation of the folds at the region between Kashan on the northeast and Ardestan to the southeast.
The Navab folds include a series of growing synclines and anticlines trending NW–SE and form the frontal structures of the Karkas Mountains some 20 km southeast of the city of Kashan (Figs. 2 and 3). The northeastern limbs of these folds are steeper than the southwestern limbs (Fig. 10). These asymmetric folds have probably formed above blind reverse faults dipping SW along the northeastern limb of the anticlines (Fig. 3). The overlying folds show doubly plunging terminations and a saddle in between suggesting that variation in displacement along the underlying reverse faults are reflected in the amplitude of folding at the surface (Fig. 3). Here, we apply the same criteria introduced by Keller et al. (1999) to the Navab folds to determine lateral propagation and infer activity of the underlying reverse faults. A topographic cross section along the plunging axis of the main Navab fold shows that the elevation of the topographic profile decreases outward from its center towards both the northwest and southeast (Fig. 11). Same decrease is seen when examining the drainage density and degree of dissection along the Navab folds (Fig. 12). The drainage density map for the Navab folds (Fig. 12down), which is defined by the ratio of the sum of total length of channels divided by drainage basin area, decreases outward from
3.1. East of Natanz Moghar River, as a persisting antecedent river, cuts across anticlines such as the Zardkuh and Kuh-e Charkheh anticlines east of the city of Natanz (Figs. 4 and 7). The northeastern limb of Kuh-e Charkheh anticline dips more steeply than the southwestern limb (Fig. 8a) due to displacement on the underlying SW-dipping thrust (Fig. 8b) as evidenced by bedrock exposure at the river channel (see also Fig. 7). The Moghar River incises through the growing folds (Figs. 4 and 7) suggesting that the river has sufficient power to erode enough through the uplifting structures. As a result, relief between the river channel and the surrounding topography has increased in the form of a notch (water gap) cut through the active folds. We investigated several sections of the Moghar River in the field to compare the depth of incision along the river (Fig. 9). Deep incision of the river begins where the thrust underlying the Kuh-e Charkheh anticline crosses the river (Fig. 7). The depth of incision from 7 to 10 m upstream (Fig. 9a) has increased to maximum 50 m a few hundred meter downstream (Fig. 9b). A few kilometers further downstream where the exposure of growing anticlines disappears the height of the river bluff decreases to few meters (Fig. 9c). Comparison of the Moghar River channel with other similar streams flowing from SW to NE across the Qom-Zefreh fault zone suggests that the growing Zardkuh and Kuh-e Charkheh anticlines have forced deep incision along this segment of the Moghar River.
Fig. 8. Photo of Charkheh anticline and reveres faulting east of Natanz city. (a) View of Kuh-e Charkheh asymmetric anticline, (b) reverse fault along northeastern limb of Kuh-e Charkheh anticline.
Fig. 9. Cross sections of the Moghar River showing (a) upstream, (b) midstream and (c) downstream. See Fig. 7 for locations.
F. Jamali et al. / Journal of Asian Earth Sciences 40 (2011) 1015–1025
1021
Fig. 10. Photo of the most northeastern asymmetric anticline of the Navab folds.
Fig. 11. (Up) Drainage map, fixed on topographic contour map of the Navab folds with 20 m contour intervals. (Down) Longitudinal section along the main Navab fold. Both horizontal and vertical axes have the same scale. Location of wind gaps (Ga) and water gaps (Gw) are shown along the section. Different levels of wind and water gaps are referred by (l = lower), (m = middle) and (u = upper) and numbered from NW to SE. Three stages of uplift in northwestern part of the fold are marked as separate levels. The AA0 , BB0 , CC0 and DD0 lines showing the seismic profiles location, described on Fig. 13.
the central part of the folds. The lower ratio of drainage density corresponds well to lower dissection (Fig. 12-up) and topography and relatively younger deposits and landforms. As it can be clearly seen from Fig. 3, several streams have eroded through the Navab folds. With time, most of these streams were defeated because of insufficient stream power and/or rapid fold uplift (Fig. 11). Thus, the defeated streams were forced to find a different path and deflect around the folds. The topographic cross section along the northwestern segment of the fold shows that all of the water gaps have become wind gaps due to several phases of
fold uplift. As a result, lake deposits accumulated in the catchments along the back limb of the fold (Fig. 3). The longitudinal topographic profile indicates three geomorphic levels each disturbed by 2–3 wind gaps along the NW segment of the main fold (Fig. 11). This can be interpreted to indicate that the Navab folds developed during three major phases of uplift and lateral propagation. Our measurements along the longitudinal section show topographic relief rising 15–20 m from the wind gap to the adjacent ridges of the lowest level. Similar measurements to that of the middle and top levels are 40–50 m and 70–80 m respectively.
1022
F. Jamali et al. / Journal of Asian Earth Sciences 40 (2011) 1015–1025
Fig. 12. (Up) Detailed drainage map of the Navab folds. The drainage pattern is changing along and across the fold system. Condensed drainage pattern defines the highest and simultaneously the oldest part of the fold system. The density of the drainage system decreases along the fold axes from the central part of the folds to either side. (Down) Drainage density map. Darker areas represent the higher drainage density. The areas with higher density conform with the higher magnitude of dissection.
These amounts are almost half of the topography elevation at each level. This implies that at each phase of the fold growths the antecedent river has incised across the Navab folds during the first half of the growth period. However, along the southeastern segment of the main fold only two wind gaps of the same level have formed and the only existing water gap has deflected. The absence of wind gaps along the southeastern segment of the fold is interpreted to indicate that the lateral propagation of the fold towards the southeast ceased sometime after the wind gap formed. The patterns of drainage system and topography of the Navab folds indicate lateral propagations on either end of the growing folds. Below we show that the lateral growth of folds at the surface is linked to fault growth in the subsurface.
4. Seismic reflection analyses Here we present four processed seismic sections across the Navab folds to gain subsurface information of the main structures (Fig. 13). Seismic sections presented here are plotted at 1:1 (no vertical exaggeration) assuming an average seismic P wave velocity of 3500 m/s, so that 1 s of two-way travel time corresponds to about 1750 m of depth. We also used unmigrated data which means that the reflections are plotted halfway between the source and the receiver. The AA0 profile (Fig. 13) crosses the northwestern part of the Navab folds (Fig. 11). Two synclines and an anticline can be easily detected along this profile. The NE syncline which is deep-seated and asymmetric with the steep southwest limb is overlain by
F. Jamali et al. / Journal of Asian Earth Sciences 40 (2011) 1015–1025
1023
Fig. 13. Seismic profiles across the Navab folds. Location of profiles are marked in Fig. 11 as the AA0 , BB0 , CC0 and DD0 lines. Vertical and horizontal scales in all seismic sections are the same, assuming an average seismic wave velocity of 3500 m/s that is 1 s of two-way travel time corresponds to about 1750 m of depth. Inferred faults are shown by thick lines.
growth strata of Pliocene age (Geological Survey of Iran, 1979). The southwestern syncline, constitute the highest topographic relief and is detached from the underlying anticline. The anticline is asymmetric with a relatively steep forelimb and much gentler backlimb. The forelimb of the anticline is cut by a low angle thrust (hereafter is referred to as Navab thrust) which appears to join to the Qom-Zefreh fault zone at depth. The Navab thrust is blind and truncated by the Pliocene deposits. Further southeast (Fig. 11), the BB0 profile shows a back thrust as well as the main blind thrust fault recognized on the AA0 profile (Fig. 13). This back thrust is dipping NE, parallel with the southwestern limb of the syncline and joins the tip of the Navab thrust. The fault geometry along this seismic profile can be interpreted as a wedge thrust responsible for producing the overlying anticline. With a wedge thrust underlying the fold, the syncline axial surface should be active (it migrates forward, toward NE, with respect to the strata above the thrust tip as suggested by Burbank and Anderson, 2001). The AA0 and BB0 seismic profiles show the Pliocene and older beds roll up from the basin through the synclinal axial surface. Also, the bedding geometry of the layers of the SW limb of the NE syncline are interpreted to indicate progressive deformation toward the NE. The CC0 and DD0 profiles show that by moving southeast, away from the older central part of the folds, less deformation occurs across the Navab folds as evidenced by wide gentle folds and small amount of displacement on the thrusts.
5. Discussion and conclusion In the previous sections we presented geomorphic evidence of active faulting and folding to indicate active deformation in the Kashan region. In the following discussion we describe the kinematics in west-central Iran.
Fig. 14. Schematic diagram illustrating the inferred slip partitioning along the Qom-Zefreh dextral fault and fold-thrust belt in Central Iran. Black arrows show the regional shortening direction.
1024
F. Jamali et al. / Journal of Asian Earth Sciences 40 (2011) 1015–1025
The active tectonics of the Kashan region is dominated by the major Qom-Zefreh strike-slip fault, which accommodates right-lateral shear between the Sanandaj-Sirjan zone to the southwest and Alborz Mountains to the north. The North Tabriz right-lateral fault in NW, and the Dehshir right-lateral fault in SE of the Qom-Zefreh fault seem to play similar role in accommodating regional strain (Fig. 1 and see also Masson et al., 2006; Meyer et al., 2006). The rate of right-lateral motion decreases from NW along the North Tabriz fault (6–7 mm/yr) (Hessami et al., 2003; Masson et al., 2006) to SE along the Qom-Zefreh fault (2.2 mm/yr) as well as along the Dehshir fault (2 mm/yr). This decrease of motion from the North Tabriz fault on the northwest to the Qom-Zefreh fault on the southeast is due to the counterclockwise rotation of Arabia relative to Eurasia about the rotation pole at 27.9 N and 19.5 E (Vernant et al., 2004a). We also know from GPS that the rate of shortening between the Sanandaj-Sirjan zone and Alborz Mountains is about 3 mm/yr (Vernant et al., 2004b). As we indicated, a large component of shortening in the Kashan region is accommodated by shortening on parallel folds and blind thrusts while, so far, no Quaternary deformation has been described south of central Alborz. Therefore, the locus of the deformation between the Alborz and the Central Iran seems to lie along the northeastern side of the Urmieh-Dokhtar magmatic belt (Fig. 1) a possibility that Vernant et al. (2004b) have recently considered. In many examples around the world, it is shown that where overall convergence is oblique to the trend of deforming belt, the convergence is achieved by partitioning of the orthogonal strikeslip and shortening components on parallel faults (Lettis and Hanson, 1991; Angelier et al., 2003; Talebian and Jackson, 2004; Hollingsworth et al., 2008). The right-lateral offsets of geomorphic features along the Kashan segment indicate that slip vector is subparallel to the Qom-Zefreh fault (Fig. 14). However, slip vector seems to be roughly perpendicular to the trend of reverse faults and folds running parallel to the Qom-Zefreh fault zone. This would suggest that in this part of Central Iran the strike-slip and shortening components of the oblique N–S convergence of Arabia and Eurasia are partitioned. The identification of zones of active fault-related folding is important for the estimation of regional seismic hazard (Walker et al., 2003), as the historical record of destructive earthquakes and the very low level of instrumental seismicity in the region is not likely to represent the full hazard from earthquakes. The lack of seismic activity in this part of Central Iran prevents us from confirming the role of normal and tangential components in this partitioned phenomenon, nor the capability of the reverse faults for nucleating earthquakes independently. For a better understanding of the active tectonics of Central Iran a better knowledge of geodetic and geologic slip-rates is required. In the near future, we may have accurate dating of offset morphologic features along the Kashan fault segment. A dense GPS network across the whole region between the Sanandaj-Sirjan zone and the Alborz Mountains will also provide detailed information on the slip-rates of active structures across Central Iran. This in turn would help to retrieve the interseismic strain across individual faults. Acknowledgments The late Manoochehr Pedrami first identified the growing anticlines east of Natanz and drew our attention to their role on deep incision of the Moghar River. We would like to thank James Hollingsworth and Morteza Talebian for their thorough and constructive reviews that led to numerous improvements in the final manuscript. We also thank Esmaeil Shabanian for fruitful discussions and comments. The authors are grateful to the National
Iranian Oil Company for providing the seismic profiles. This work is part of the first author’s Ph.D. thesis at the International Institute of Earthquake Engineering and Seismology. The editors Bihong Fu and Richard Walker are acknowledged for help and handling the manuscript. References Ambraseys, N.N., Melville, C.P., 1982. A History of Persian Earthquakes. Cambridge University Press. pp. 219. Amidi, S.M., 1975. Contribution a Letude Stratigraphique Petrologique et Petrocimique des Roches Magmatiques de la Region Natanz-Nain-Surk (Iran Central). Thesis, Grenoble, pp. 316. Angelier, J., Lee, J.C., Hu, J.C., Chu, H.T., 2003. Three-dimensional deformation along the rupture trace of the September 21st, 1999, Taiwan earthquake: a case study in the Kuangfu School. Journal of Structural geology 25, 351–370. Berberian, M., King, G.C.P., 1981. Toward a paleogeography and tectonic evolution of Iran. Canadian Journal of Earth Science 18, 210–265. Burbank, D.W., Anderson, R.S., 2001. Tectonic Geomorphology. Blackwell Science. pp. 274. Fitch, T.J., 1972. Plate convergence, transcurrent faults, and internal deformation adjacent to Southeast Asia and Western Pacific. Journal of Geophysical Research 77, 4432–4460. Geological Survey of Iran, 1979. Geological Map of Kashan Quadrangle, No. F7, Scale: 1:250,000. Hessami, K., Nilforoushan, F., Talbot, C.J., 2006. Active deformation within the Zagros Mountains deduced from GPS measurements. Journal of the Geological Society 163, 143–148. Hessami, K., Pantosti, D., Tabasi, H., Shabanian, E., Abbasi, M.R., Feghhi, K., Soleymani, S., 2003. Paleoearthquakes and slip rates of the North Tabriz Fault, NW Iran: preliminary results. Annals of Geophysics 46 (5), 903–915. Hollingsworth, J., Jackson, J., Walker, R., Nazari, H., 2008. Extrusion tectonics and subduction in the eastern South Caspian region since 10 Ma. Geology 36 (10), 763–766. Jackson, J., McKenzie, D.P., 1988. The relationship between plate motions and seismic moment tensors, and the rates of active deformation in the Mediterranean and Middle East. Geophysical Journal 93, 45–73. Keller, E.A., Gurrola, L., Tierney, T.E., 1999. Geomorphic criteria to determine direction of lateral propagation of reverse faulting and folding. Geology 27, 515–518. Lee, J.C., Angelier, J., Chu, H.T., Yu, S.B., Hu, J.C., 1998. Plate-boundary strain partitioning along the sinistral collision suture of the Philippine and Eurasian plates: analysis of geodetic data and geological observation in southeastern Taiwan. Tectonics 17, 859–871. Lettis, W.R., Hanson, K.L., 1991. Crustal strain partitioning: implications for seismic hazard assessment in western California. Geology 19, 559–562. Masson, F., Djamour, Y., Van Gorp, S., Chéry, J., Tatar, M., Tavakoli, F., Nankali, H., Vernant, P., 2006. Extension in NW Iran driven by the motion of the South Caspian Basin. Earth and Planetary Science Letters 252, 180–188. Masson, F., Anvari, M., Djamour, Y., Walpersdorf, A., Tavakoli, F., Daignieres, M., Nankali, H., Van Gorp, S., 2007. Large-scale velocity field and strain tensor in Iran inferred from GPS measurements: new insight for the present-day deformation pattern within NE Iran. Geophysical Journal International 170, 436–440. doi:10.1111/j.1365-246X.2007.03477.x. McCaffrey, R., 1992. Oblique plate convergence, slip vectors, and forearc deformation. Journal of Geophysical Research 97, 8905–8915. Meyer, B., Mouthereau, F., Lacombe, O., Agard, P., 2006. Evidence of quaternary activity along the Dehshir fault: implication for the tertiary tectonics of Central Iran. Geophysical Journal International 164, 192–201. Nabavi, M.H., 1977. Introduction to the geology of Iran (in Farsi). Geological Survey of Iran. Nadji, M., 1972. Geologie und hydrogeology de gebietes von Kashan/Iran. Geologische Mitteilung 11, 275–362. Neely, J.A., Wright, H.T., 1994. Early Settlement and Irrigation on the Deh Luran Plain; Village and Early State Societies in Southwestern Iran. Technical Report 26. University of Michigan Museum of Anthropology, Ann Arbor, MI. pp. 236. Nilforoushan, F., Masson, F., Vernant, P., Vigny, C., Martinod, J., Abbassi, M., Nankali, H., Hatzfeld, D., Bayer, R., Tavakoli, F., Ashtiani, A., Doerflinger, E., Daignieres, M., Collard, P., Chery, J., 2003. GPS network monitors the Arabia–Eurasia collision deformation in Iran. Journal of Geodesy 77, 411–422. Seeber, L., Pecher, A., 1998. Strain partitioning along the Himalayan arc and Nanga Parbat antiform. Geology 26, 791–794. Talebian, M., Jackson, J., 2004. A reappraisal of earthquake focal mechanisms and active shortening in the Zagros Mountains of Iran. Geophysical Journal International 156, 506–526. Vernant, P., Nilforoushan, F., Hatzfeld, D., Abassi, M., Vigny, C., Masson, F., Nankali, H., Martinod, J., Ghafory-Ashtiany, M., Bayer, R., Tavakoli, F., Chéry, J., 2004a. Present-day crustal deformation and plate kinematics in Middle East constrained by GPS measurements in Iran and Northern Oman. Geophysical Journal International 157, 381–398. Vernant, P., Nilforoushan, F., Che´ry, J., Bayer, R., Djamour, Y., Masson, F., Nankali, H., Ritz, J.-F., Sedighi, M., Tavakoli, F., 2004b. Deciphering oblique shortening of central Alborz in Iran using geodetic data. Earth and Planetary Science Letters 223, 177–185.
F. Jamali et al. / Journal of Asian Earth Sciences 40 (2011) 1015–1025 Walker, R., Jackson, J., Baker, C., 2003. Thrust faulting in eastern Iran: source parameters and surface deformation of the 1978 Tabas and 1968 Ferdows earthquake sequences. Geophysical Journal International 152, 749–765. Walker, R., Bergman, E., Jackson, J., Ghorashi, M., Talebian, M., 2005. The 2002 June 22 changureh (Avaj) earthquake in Qazvin province, Northwest Iran: epicentral
1025
relocation, source parameters, surface deformation and geomorphology. Geophysical Journal International 160, 707–720. Wilcox, R.E., Harding, T.P., Seely, D.R., 1973. Basic wrench tectonics. American Association of Petroleum Geologists Bulletin 57, 74–96.