Isostasy of Iranian Plateau

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Abstract

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.

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References

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Journal of the Earth and Space Physics
Volume 42, Issue 3
December 2016
Pages 549-562

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