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Extrusions of Hormuz Salt in Iran

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Extrusions of Hormuz Salt in Iran Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021 Extrusions of Hormuz salt in Iran CHRISTOPHER J. TALBOT Hans Ramberg Tectonic I_xtborator~; blstitute of Earth Sciences, Uppsala Universit3, S-752 36 Uppsala, Sweden Abstract: This work illustrates that Lyell's approach to inferring geological processes from field observations can be improved by using field measurements of their rates to scale dynamic models. A controversy in the 1970s about whether crystalline rock salt can flow over the surface echoed an earlier controversy resolved with Lyell's help about whether ice could flow and carry exotic blocks. This work argues that the external shapes, internal fabrics and structures and rates of flow of current salt extrusions in the Zagros Mountains are keys not only to understanding past and future extrusion of salt. but also to the extrusion of metamorphic cores of orogens. Salt is shown to extrude first in hemispherical domes, which later spread to the shapes of viscous fountains until they are isolated from their source, They then rapidly assume the shapes of viscous droplets, which they maintain until they degrade to heaps of residual soils. Salt emerges as a linear viscous Bingham fluid, and strain rate hardens downslope to a power-law fluid (n = 3) beneath a thickening carapace of brittle dilated salt. The velocities of salt constrained in emergent diapirs by measurements of extruding salt sheets in Iran are remarkably rapid compared with rates estimated for buried equivalents elsewhere in or under which it is planned to store nuclear waste or to extract hydrocarbons. Because halite (NaC1) is the product of evaporation porating more upright box folds with shorter exceeding the supply of sea water, beds of rock salt hinges- compared to adjoining thrust wedges not often occur near the base of sedimentary sequences underlain by salt, which are narrower and have a that accumulate in new basins. Diagenesis can steeper taper angle with constant structural render evaporated salt sequences essentially solid vergence (Davis & Engelder 1987). Laterally only tens of metres beneath the evaporative surface unsupported shelf sequence may collapse en masse (Lowenstein & Hardie 1985). As a result, beds of downslope into basins opened over salt deposited at rock salt soon attain a density close to the 2200 the rifting stage, as in the Gulf of Mexico (Wu et al. kg m -3 that they maintain as they are buried by 1990), Atlantic Ocean (Liro & Coen 1995) and Red clastic sediments that usually compact to greater Sea (Heaton et al. 1995). Gravity sliding and densities beyond a depth of about 1 kin. Super- spreading of open continental margins mimics the posed clastic sediments can sink into the salt, thin-skin effects of ocean-closure orogeny (Talbot leaving salt bodies near the depositional surface 1992). Progradation of the cover squeezes the which become the crests of salt diapirs downbuilt substrate into deep swells which break through the by flow during progressive burial of the surround- cover as giant nappes that laterally extend beneath ing source layer. Gravity can also upbuild diapirs the upper slopes as they spread towards or over fold into or through prehalokinetic overburdens that are belts of autochthonous cover rucked up beneath ductile (Ramberg 1981 ) or brittle (Schultz-Ela et al. them (Wu et al. 1990). 1993). Only recently has it been appreciated that The first workers who described the many the buoyancy of planar salt layers is sufficiently majestic extrusions of Hormuz salt in the Zagros low that most overburdens are so stiff that they Mountains of Iran (Fig. 1) appear to have appreci- have to be faulted and thinned before reactive ated that the salt in many of them is still flowing diapirs rise into or extrude through them (De B6ckh et al. 1929, Harrison 1930). However, (Vendeville & Jackson 1992a, b; Jackson & optimists advocating the storage of radioactive Vendeville 1994). waste in salt in the 1970s (e.g. Gera 1972), knowing As well as being buoyant, buried salt is also that the rock mechanics of the time could not weak enough to act as a lubricant along decol- account for salt flow at the surface (Carter & lements beneath basin sequences undergoing lateral Hansen 1983), interpreted extruded salt as having extension and/or shortening. Thrust prisms moving flowed over the surface only when it was hot. like over salt are wide with gentle taper angles incor- lavas (Gussfw 1966). This argument was rebutted TALBOT, C. J. 1998. Extrusions of Hormuz salt in Iran. In: BLUNDELL,D. J. & Scott, A. C. (eds) 315 Lvell: the Past is the Key to the Present. Geological Society, London, Special Publications, 143. 315-334. Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021 316 c.J. TALBOT Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021 EXTRUSIONS OF HORMUZ SALT 317 by good 'present is the key to past' arguments. Tall research programme on 'Zagros halokinesis' run extrusions of salt must still be active because some jointly by geologists from the Institute of Earth avalanche episodically, and supply must exceed Sciences at Uppsala University, Sweden, and the loss because many still rise a kilometre above the Institute for Earth Sciences at the Geological surrounding plains despite an annual rainfall Survey of Iran. By November 1997, field parties capable of dissolving them in a few tens of had spent five field seasons of a month each thousands of years (Kent 1966). Such rebuttals making new surveys and documenting the displace- were followed by remote (Wenkert 1979) and then ments, apparent to theodolites stationed on country field (Talbot & Rogers 1980) measurements rock, of hundreds of markers on ten mountains of demonstrating that some of the Zagros salt Hormuz salt. extrusions are still active. Measurements suggest- A brief introduction to the Hormuz salt and its ing that one of these salt mountains flowed faster history is followed by descriptions of the external when it rained (Talbot & Rogers 1980) were pre- shapes of salt extrusions at different stages and sented as a natural demonstration in bulk crystalline their internal fabrics and structures, both ductile salt of the long-known Joff6 effect, in which water and brittle. Recent measurements of the rates of one softens single crystals of halite (Kleinhamns 1914). of the salt extrusions are used to scale a non- Field measurements prompted laboratory experi- dimensional flow model and to constrain its ments (Urai et al. 1986) which led to the recog- viscosity and current extrusion velocity. nition that water-assisted dynamic recrystalization can account for the low-temperature flow of salt. The Hormuz Formation Arguments over the mobility of salt in the 1970s mirrored an earlier controversy that raged over the Beds of salt in the Hormuz sequence are estimated mobility of ice in the Alps, in which Lyell played a to have an original total thickness of about 1 km major role (see Boylan this volume, p. 157). (Kent 1958) and are divisible into two parts by red During studies of many of the 130 or so salt beds (probably of the Middle Cambrian Lalun intrusions emergent in the Zagros, Player (1969) Formation). Neoproterozoic salt consists of beds of made similar inferences for salt as those made multicoloured halite interlayered with beds of earlier by Lyell for ice. Thus Player used spreads of anhydrite, magnesium carbonates and fetid distinctive multicoloured insoluble Hormuz rocks limestone containing stromatolites. This cyclic among the honey-coloured country rocks to infer sequence probably accumulated in shallow Proto- the timings and magnitudes of past extrusions of Tethyan basins along the tropical margin of salt and thereby constrain rates of post-Triassic salt Gondwana from Iraq to Australia (Talbot & Alavi intrusion (1969; in Edgell 1996). 1996). Apart from dispersed anhydrite and blocks The remainder of this work treats the Zagros as a of bimodal volcanic rocks, Cambrian salt is more unique laboratory where salt extrusions not only pure and uniform and probably accumulated in mimic past and present salt extrusions elsewhere, graben that trended north-south, along the local but also represent smaller and faster natural grain of the underlying Pan-African (900-500 Ma) analogue models of the gravity spreading of other crystalline basement. The salt was buried by clastic tectonites extruded as piles of nappes in orogens. sediments in the Palaeozoic and platform This review of salt extrusions is based on visits to carbonates from Triassic until Miocene times. An at least 45 examples since 1993 as part of the overburden 2.5-3 km thick by Jurassic times Fig. 1. Photographs of profiles of salt extrusions at different stages in the Zagros. Young bun-shaped post-Zagros domes of Hormuz salt, (a) extruding through a collar of Cretaceous carbonates at Kuh-e-Gach (28 ~ 37' N, 52 ~ 18' E), reaches 956 m above sea level, but only about 400 m above the surrounding molasse plain (b) extruding through a dome of Eocene carbonates at Kuh-e-Charhal (dome 67. 28 ~ 00' N. 54 ~ 58' E) seen from the west; (c) Kuh-e-Charhal seen from the north. Salt fountains breaching anticlines at: (d) Angoreh (Dome 9, 27 ~ 20" N, 55 ~ 55' E) from the southwst; (e) Kuh-e-Namak (Dashti, 28 ~ 16' N, 51 ~ 42' E) from the northwest; and (f) Darbhast (dome 49, 26 ~ 57' N, 54 ~ 08" E) from the east-northeast. Salt fountains rising through molasse plains: (g) the 'fried egg" (dome 80, 28 ~ 00' N, 54 ~ 56" E) from east-southeast; (h) Kuh-e-namak (Feroosabad, 28 ~ 45' N, 51 ~ 22' E), from the south-southeast showing a dynamic fountainhead sinking into the plateau of a salt droplet. The southern reaches of namakiers at (i) Kuh-e-Namak (Feroosabad) and (j) Kuh-e-Jahani (28 ~ 37' N, 52 ~ 25' E) also have the profiles of viscous droplets.
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