Isostasy of Iranian Plateau



Using global free air gravity and topography data, first we have calculated a Bouguer Anomaly (BA) map for Iranian plateau and then computed the residual isostasy anomaly map under the Airy-Heiskannen assumption. The term residual is used as to reflect the assumption of local isostatic compensation in contrast to a regional isostatic compensation. The value with which the gravity effect of the compensation mass (the root/anti-root in the Airy model) is calculated are chosen under careful considerations as to produce reliable results. The resulting residual isostasy map is then used to qualitatively interpret the isostatic highs and lows corresponding to crustal/lithospheric features of the Iranian plateau. The study area is a complex region as a result of its still active tectonics which is mostly driven by the continent-continent collision of the Arabian and Eurasian plates. The five most important tectonic settings in Iran are Zagros Mountains, an active belt formed as the result of the collision extending from south-west Iran along the Persian Gulf; Alborz Mountains, a young belt with an average topography of 3-5 km extending nearly in east-west direction, Makran, in south-east Iran, north of the Iran-Arabian plate boundary where an active subduction is taking place; Caspian Sea, with an oceanic crust covered with an average 15-20 km sediment layer at the Iran-Eurasia plate boundaries and Kopeh-Dagh mountains, an uplifted region as a consequence of converging continental plates. Our results indicate that the Zagros Mountains have reached an isostatic equilibrium but the scenario is slightly different for the Alborz chain. It seems that the isostatic equilibrium is not fully reached in the Alborz due to the observation of a continuous isostatic high (positive) anomaly which extends to north-west Iran, however, it may also be partly caused by a simple folding. In the southern Caspian region, there is an enormous isostatic low (negative), for the cause of which we have considered two possible reasons. First, the effect of the sediment layer on the gravity signal due to its negative density contrast. Second, we considered the deficiency in the rock mass at the base of the lithosphere due to an anti-plume or the downward flow of the lithospheric materials towards the mantle which may also explain the high depth of the southern Caspian Basin. Subduction zones are usually characterized with negative isostatic anomalies, but in the case of the subduction of the oceanic lithosphere of the Caspian under the continental crust of the Eurasia, there is no apparent negative isostatic anomaly in our map. We believe that this is probably due to the fact that the subduction is still young while in order to observe a negative effect on the residual isostasy anomaly map, the subducting slab must be in a deep position, in other words, be of older age. The subduction of the Makran, on the other hand, has caused a negative isostatic residual anomaly. This low anomaly is also partly due to the uplift of the Makran area. A high-low (positive-negative) residual isostasy anomaly pairs corresponds to suture zones. An example of which is seen for the Zagros-Bitlis suture zone which marks the continental collision of the Arabian-Eurasian plates. Our map also shows a negative residual isostatic anomaly in the Kopeh-Dagh Mountains, which we interpret as the uplift caused by the convergence of the Iranian and Eurasian plates. It must be noted that every high/low residual isostatic anomaly may not be interpreted as isostatically over/under compensated areas. on the contrary, it could be and usually is related to a geological feature of lithosphere/mantle scale.


Main Subjects

درویش‌زاده، ع.، 1370، زمین‌شناسی ایران، انتشارات امیرکبیر.
متقی، خ.، 1390، مطالعه ساختار لیتوسفر قاره‌ای در ناحیه برخوردی شمال شرق ایران. پایان‌نامه دکتری، پژوهشگاه بین المللی زلزله‌شناسی و مهندسی زلزله تهران.
Abbassi, A., Nasrabadi, A., Tatar, M., Yaminifard, F., Abbassi, M. R., Hatzfeld, D., & Priestley, K. 2010. Crustal velocity structure in the southern edge of the Central Alborz (Iran). Journal of Geodynamics, 49(2), 68-78.
Abdetedal, M., Shomali, Z. H. and Gheitanchi, M. R., 2014, Crust and upper mantle structures of the Makran subduction zone in south-east Iran by seismic ambient noise tomography, Solid Earth Discussions, 6(1), 1-34.
Al-Lazki, A. I., Seber, D., Sandvol, E., Turkelli, N., Mohamad, R. and Barazangi, M., 2003, Tomographic Pn velocity and anisotropy structure beneath the Anatolian plateau (eastern Turkey) and the surrounding regions. Geophys. Res. Lett., 30(24), 8043.
Angus, A., Wilson, D. C., Sandvol, E. and Ni, J. F., 2006, Lithospheric structure of the Arabian and Eurasian collision zone in eastern Turkey from S-wave receiver functions. Geophys. J. Int., 166(3), 1335-1346
Banerjee, P., 1998, Gravity measurements and terrain corrections using a digital terrain model in the NW Himalaya. Computers & Geosciences, 24(10), 1009-1020.
Berberian, M. and Yeats, R. S., 2000, Pattern of historical earthquake rupture in the Iranian plateau, B. Seismol. Soc. Am., 89, 120-139.
Berberian, M., 1981, Active faulting and tectonics of Iran: Zagros-Hindukush-Himalaya geodynamic evolution Gupta, H. K. and Delany, F. M. (eds), Am. Geophys. Union and Geol. Soc. Am., Geodyn., 3, 33-69.
Blais, J. A. R. and Ferland, R., 1984, Optimization in gravimetric terrain corrections, Canadian Journal of Earth Sciences, 21, 505-515.
Byrne, D. E., Sykes, A. R. and Davis, D. M., 1992, Great thrust earthquakes and aseismic slip alongthe plate boundary of the Makran subduction zone, J. Geophys. Res., 97, 449-478.
Dehghani, G. and Makris, J., 1984, The gravity field and crustal structure of Iran, Neues Jahrb., Geol. Palaeontol. Abh., 168, 215-229.
Engdahl, E. R., Jackson, J. A., Myers, S. C., Bergman, E. A. and Priestley, K., 2006. Relocation and assessment of seismicity in the Iran region. Geophysical Journal International, 167(2),.761-778.
Farhoudi, G. and Karig, D. E., 1977, Makran of Iran and Pakistan as an active arc system, Geology, 5, 664-668.
Fullea, J., Fernandez, M. and Zeyen, H., 2008, FA2BOUG—A FORTRAN´ 90 code to compute Bouguer gravity anomalies from gridded free air anomalies: application to the Atlantic-Mediterranean transition zone, Comput. Geosci., 34, 1665-1681, doi:10.1016/j.cageo.2008.02.018
Guest, B., Guest, A. and Axen, G., 2007, Late Tertiary tectonic evolution of northern Iran: a case for simple crustal folding, Global Planet. Change, 58(1-4), 435-453, doi:10.1016/j.gloplacha.2007.02.014.
Hammer, S., 1939, Terrain corrections for gravimeter stations, Geophysics, 4(3), 184-194.
Jim´enez-Munt, Fern`andez, M., Saura, E., Verg´es, J. and Garcia-Castellanos, D., 2012, 3-D lithospheric structure and regional/residual Bouguer anomalies in the Arabia–Eurasia collision (Iran)., Geophys. J. Int., 190,1311-1324.
Karner, G. D. and Watts, A. B., 1982, On isostasy at Atlantic-type continental margins, doi: 10.1029/JB087iB04p02923
Knapp, C. C., Knapp, J. H. and Connor, J. A., 2004, Crustal‐scale structure of the South Caspian Basin revealed by deep seismic reflection profiling, Mar. Pet. Geol., 21(8), 1073-1081, doi:10.1016/j.marpetgeo.2003.04.002.
LaFehr, T. R., 1991, An exact solution for the gravity curvature (Bullard B) correction, Geophysics, 56, 1178-1184.
Maggi, A. and Priestley, K., 2005, Surface waveform tomography of the Turkish–Iranian plateau. Geophysical Journal International, 160, 1068-1080
Mallick, K. and Sharma, K. K., 1999, A finite element method for computation of the regional gravity anomaly, Geophysics, 64, 461-469.
Mohammadi, E., Sodoudi, F., Sadidkhouy, A. and Gheitanchi, M. R., 2012, Moho depth and VP/VS variations in the Kope Dagh region from analysis of teleseismic receiver functions, Journal of the Earth & Space Physics, 37(4), 1-12.
Molinaro, M., Zeyen, H. and Laurencin, X., 2005, Lithospheric structure beneath the southeastern Zagros Mountains, Iran: recent slab break-off? Terra Nova, 17, 1-6.
Motavalli-Anbaran, S.-H., Zeyen, H. and Ardestani, V. E., 2013, 3D joint inversion modeling of the lithospheric density structure based on gravity, geoid and topography data — Application to the Alborz Mountains (Iran) and South Caspian Basin region, Tectonophysics, 586, 192-205.
Motavalli-Anbaran, S. H., Zeyen, H. and Jamasb, A., 2016, 3D crustal and lithospheric model of the Arabia–Eurasia collision zone. J. of Asian Earth Sciences, 122, 158-167.
Motavalli-Anbaran, S.-H., Zeyen, H., Brunet, M.-F. and Ardestani, V. E., 2011, Crustal and lithospheric structure of the Alborz Mountains, Iran, and surrounding areas from integrated geophysical modeling, Tectonics, 30(5), TC5012, doi:10.1029/2011TC002934
Niazi, M., Shimamura, H. and Matsu'ura, M, 1980, Microearthquakes and crustal Sstructure of the Makran Coast of Iran, Geophys. Res.Lett., 7, 297-300.
Nilforoushan, F., Masson, F., Vernant, P., Vigny, C., Martinod, J., Abbassi, M., Nankali, H., Hatzfeld, D., Bayer, R., Tavakoli, F. and Ashtiani, A., 2003, GPS network monitors the Arabia-Eurasia collision deformation in Iran. Journal of Geodesy, 77(7-8), pp.411-422.
Nowell, D. A. G., 1999, Gravity terrain corrections-an overview, Journal of Applied Geophysics, 42, 117-134.
Paul, A., Hatzfeld, D., Kaviani, A., Tatar, M. and Péquegnat, C., 2010, Seismic imaging of the lithospheric structure of the Zagros mountain belt (Iran), Geol. Soc. Spec. Publ., 330, 5-18, doi:10.1144/SP330.2.
Priestley, K., McKenzie, D., Barron, J., Tatar, M. and Debayle, E., 2012, The Zagros core: deformation of the continental lithospheric mantle, Geochem. Geophys. Geosyst., 13, Q11014, doi:10.1029/2012GC004435.
Radjaee, A. H., Rham, D., Mokhtari, M., Tatar, M., Priestley, K. and Hatzfeld, D., 2010, Variation of Moho depth in the central part of the Alborz Mountains, northern Iran, Geophys. J. Int., 181(1), 173-184, doi:10.1111/j.1365-246X.2010.04518.x.
Ritz, J.‐F., Nazari, H. B., Ghassemi, A., Salamati, R., Shafei, A., Solaymani, S. and Vernant, P., 2006, Active transtension inside central Alborz: a new insight into northern Iran‐southern Caspian geodynamics, Geology, 34(6), 477-490, doi:10.1130/G22319.1
Rogozhin, E. A., 1995, Tectonic position and geologic manifestations of the Ashkhabad earthquake, Institute of Physics of the Earth, Russian Academy of Sciences.
Sandwell, D. T. and Smith, W. H. F., 1997,. Marine gravity anomalies from GEOSAT and ERS-1 satellite altimetry. J. Geophys. Res., 102(B5), 10039-10054.
Sengor, A. M. C., Altiner, D., Cin, A. and Ustaomer, T., 1988, Origin and assembly of the Tethysideorogenic collage at the expens of Gondwana Land, in: Gondwana and Tethys, edited by:Audley Charles, M. G. and Flallam, A., Geol. Soc. Spec. Publ., 37, 119-181.
Sengör, A. M. C., Özeren, S., Genc, T. and Zor, E., 2003, East Anatolian high plateau as a mantle-supported, north-south shortened domal structure, Geophys. Res. Lett., 30(24), 8045.
Shad Manaman, N. M., Shomali, Z. H. and Koyi, H., 2011, New constraints on upper-mantle Svelocity structure and crustal thickness of the Iranian plateau using partitioned wave forminversion, Geophys. J. Int., 184, 247-267.
Sharkov, E., Lebedev, V., Chugaev, A., Zabarinskaya, L., Rodnikov, A., Sergeeva, N. and Safonova, I., 2015, The Caucasian-Arabian segment of the Alpine-Himalayan collisional belt, Geology, volcanism and neotectonics, Geoscience Frontiers, in press.
Simpson, R. W., Jachens, R. C., Blakely, R. J. and Saltus, R. W., 1986, A new isostatic residual gravity map of the conterminous United States with a discussion on the significance of isostatic residual anomalies, Journal of Geophysical Research, 91(B8), 8348-8372.
Tchalenko, J. S., 1975, Seismicity and structure of Kopet Dagh (Iran, U.S.S.R.), Phil. Tranc .Roy. Soc., Lond., 278(1275), 1-28.
Uchupi, E., Swift, S. A. and Ross, D. A., 2002, Tectonic geomorphologyof the Gulf of Oman Basin, in Clift, P. D., Gaedicke, C., Koon, D. and Craig, J. (eds.), Tectonic and Climate Evolution of theArabian Sea Region., Geol. Soc. London, Special Publication, 195, 37-69.
Vernant, P., Nilforoushan, F., Hatzfeld, D., Abbassi, M. R., Vigny, C., Masson, F., Nankali, H., Martinod, J., Ashtiani, A., Bayer, R. and Tavakoli, F., 2004. Present-day crustal deformation and plate kinematics in the Middle East constrained by GPS measurements in Iran and northern Oman. Geophysical Journal International, 157(1), 381-398.
Walker, R. and Jackson, J., 2002, Offset and evolution of the Gowk fault, SE Iran; a major intra-continental strike-slip system, J. Struct.

Geol., 24, 1677-1698.
White, R. S. and Ross., D. A., 1979, Tectonics of the western Gulf of Oman, J. Geophys. Res.-Sol. Ea., 84, 3479-3489, doi:10.1029/JB084iB07p03479.
White, R. S. and Klitgord, K., 1976, Sediment deformation and plate tectonics in the Gulf of Oman, Earth Planet. Sci. Lett., 32, 199-209.
Yamini-Fard, F. and Hatzfeld, D., 2008, Seismic structure beneath ZagrosMakran transition zone (Iran) from teleseismic study: seismologicalevidence for understanding and buckling of the Arabian plate beneathcentral Iran, JSEE, 10(1), 11-24.