Salt Diapirs of the Great Kavir, Central Iran

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Abstract
More than 50 salt diapirs are exposed on a pediment swell in the northern Great Kavir, the largest salt desert in Iran. The diapirs’ large sizes (as much as 10 km wide), abundance, spectacular degree of exposure, and correlatable stratigraphy within them constitute a remarkable and unrivaled array of features. Hence, the Kavir diapirs provide an exceptional opportunity to investigate at the surface the style and mechanics of diapiric intrusion. Maps of the internal and external structure of the diapirs constructed from field data and remotely sensed data are interpreted in three dimensions with the aid of centrifuge and analytical modeling. Three stratigraphic units are present; the proportion of evaporites decreases upward, and the density increases. A relatively pure, gypsum-bearing Older Salt (with minor mafic flows and sills in the north) accumulated under marine conditions in the Eocene and Oligocene. The original thickness of the Older Salt is estimated to have been between 1 km and more than 2 km. The variegated Younger Salt consists of a cyclic repetition of salt, gyprock, gypsum marl, saliferous mudstone, and claystone deposited under playa and lacustrine conditions in the Miocene and possibly Oligocene. The original thickness of the Younger Salt was about 1.5 km. The youngest unit consists of at least 3 km of Miocene saliferous red shale and interbedded gyprock and fine red sand-stone, together forming the M2 and M3 Members of the Upper Red Formation (URF). Sedimentation, regional tectonics, and salt tectonics were strongly linked in the Great Kavir basin. Three late Alpine anticlines in the Elburz foreland cross the diapir province; the outer ones bound the diapir province, and the central one (Kuh-e-Gugird) divides the province into two subbasins. The West Dulasian anticline in the north is cored by an incipient salt wall 52 km long. Evidence for the relative age of regional folding and salt diapirism is equivocal. Both folding and diapirism are still active. Alternative hypotheses are presented: in the first, syndepositional regional growth folding controlled the locations of diapirs; in the second, salt diapirism controlled the site of postedpositional regional folds. To the east the pillow province consists of large, widely spaced, regional late Alpine folds superimposed on a pattern of smaller domes and basins apparently cored by pillows of Younger Salt. Twelve diapirs spaced 5.9 km apart have coalesced laterally to form a continuous mass of evaporites 40 km wide known as a salt canopy, a remarkable new class of salt structure. The canopy and a less tightly spaced cluster of discrete diapirs to the east are flanked by 30 diapirs, more erratically spaced and apparently localized along regional faults. Away from faults, diapir planforms are circular, elliptical, or egg-like—the latter indicating tilted or asymmetric diapirs with bulbs (now removed by erosion) overdeveloped above the narrow end of the egg-like planform. Second-order withdrawal basins appear to partially encircle most of the diapirs, and first-order withdrawal basins apparently enclose groups of diapirs. Diapirs are surrounded by a ductile-strain aureole whose width is 5 to 25 percent of the diapir diameter; diapir-induced brittle faulting is remarkably rare. The inner part of this aureole is a concordant shear zone comprising an updragged collar of URF overburden 20 to 600 m wide. The innermost margin of this concentric shear zone is a steep gypsum rim 20 to 200 m wide, which is all that remains of a cap rock draping the subsurface flanks of the domes. A younger cap rock is forming subaerially as a discontinuous mat of residual gypsum soil over much of the Older Salt in the diapirs. Although most of the domes are still rising, extrusive (glacial) salt is apparently restricted to small flows in two domes. Some of the Kavir diapirs have a simple structure comprising a core of Older Salt and an envelope of overlying Younger Salt. But other diapirs appear to display erosional sections through mushroom-shaped bulbs. Such bulbs have a core of Older Salt separated from a narrow, downward-facing (synformal anticline), skirtlike rim of Older Salt by a crescentic or circular, downward-facing (antiformal syncline) infold of Younger Salt. Mushroom bulbs with skirts are apparently rare in nature. But even rarer complexities than mushroom bulbs are also present in the Kavir: in at least three domes the peripheral skirt of Older Salt coils inward to form a vortex structure. Centrifuged models based on the Kavir stratigraphy simulate the exposed large-scale structure of the Kavir salt domes (though not the striking small-scale deformation) and show that mushroom diapirs are merely the more mature equivalents of the simple diapirs. Centrifuged models indicate that the salt canopy was formed by coalescence of laterally spreading, mushroom-shaped diapiric bulbs. The canopy thus represents a particularly mature type of diapiric structure. Model diapirs coalesced to form the canopy by spreading below the upper free surface (air), as we believe was the case for the Kavir canopy. Model canopies evolved from initially circular diapirs that became polygonal through mutual interference before joining to form a canopy. At this final stage the underlying diapiric stems became constricted and convoluted to form buttressed stems. The buttresses pass downward into polygonal ridges connecting adjoining diapirs; similar features appear to be present in at least part of the Kavir salt canopy. Analytical modeling of the Rayleigh-Taylor instability satisfactorily explains why the salt canopy diapirs are spaced anomalously closely. We tested the validity of linear analysis to predict the spacing of mature diapirs against the centrifuged models, whose properties and kinematics are precisely known. The analysis correctly reproduced the observed diapiric spacing in the centrifuged models. In the Kavir the observed spacing of 5.9 km was locked in when the Older Salt was overlain only by 1.5 km of Younger Salt. The narrow spacing was perpetuated during subsequent deposition of the URF over-burden because the growing salt structures were large enough to continue rising even though their close spacing was no longer the most efficient mode of growth. A viscosity ratio close to unity between the Younger and Older Salt is required to explain both the close spacing of the diapirs and the evolution of mushroom-shaped diapiric bulbs with skirts. Thermal convection can also account for the clustering of diapirs if the viscosity of both salt layers was less than 1016 Pa s. If so, thermal effects could be superimposed on Rayleigh-Taylor instability, providing a combination that would enhance the mushroom shape and vorticity of the diapirs. In contrast to the Kavir, viscosity ratios of more than 100 between overburden and source layer have typically been estimated for other diapir provinces. For example, the 28-km observed spacing in the Zagros diapir province of southern Iran can only be modeled if the viscosity of the largely carbonate overburden is at least 100- and probably 1,000-fold greater than that of the Hormuz salt forming the Zagros diapirs. In contrast to the important effects of viscosity and of total stratigraphic thickness, the number and thickness of layers in the analytical models have much less influence on diapiric spacing. Analytical modeling of the Kavir pillow province explains the apparent absence of diapirs there as being due to slow growth of the Younger Salt pillows because the Older Salt is missing. As a result, the simple pattern of diapiric upwellings has been disrupted and overwhelmed by superposed regional folding in the foreland of the Elburz orogen.

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