Chah-e Deev: a model of sinkholes development in center of Iranian Plateau

Sohrabi A.¹, Nadimi A.² and Hajihashemi M.³

¹Zamin Pajuhan Kuhestan Geological Institute, Isfahan, Iran; ²Department of Geology, Faculty of Science, University of Isfahan, Isfahan, Iran; ³Islamic Azad University, Dolatabad Branch, Isfahan, Iran

[e-mail]

Researches in vertical caves and sinkholes are rare and usually restricted to speleological explorations because of difficult access. The systematic methodology of work in the caves and sinkholes has not been established. Scientific studies of these areas require speleological exploration, which can often include the discovery of new caves and passages, as well as careful documentation [2] such as cave surveys, exploration reports, photographs and morphological descriptions of the cavities. The exploration of large vertical caves and sinkholes in Europe began in the late 1970s with the publication of several speleological studies on the Alps, Slovenia, the Pyrenees and the Picosde Europa Mountains [1].

Three general types of sinkholes have been classified that are collapse, subsidence and solution sinkholes. Progression of the sinkholes is related to effects of water and structures in rocks. Groundwater percolating through the zone of aeration slowly dissolves the carbonate rock and enlarges its fractures and bedding planes. Gradually subsiding sinkholes commonly form where slow dissolution takes place, mostly along joints, fractures and faults in the rocks. Chah-e Deev sinkhole is one of the best examples of subsidence sinkholes that formed along brittle structures in the northeast of Isfahan in central part of Iranian plateau that has been introduced for the first time.

Iran provides one of the best examples of a youthful stage of continent-continent collisions in the world. The collision between the Arabian and Eurasian plates followed by the closure of the Neo-Tethys Ocean resulted in the development of Zagros orogen, which is the main deformation belt in this area [4]. The climax of the orogeny is indicated by the Zagros Mountains uplifts during the Late Neogene. This uplifting motion has created the present morphology of Iranian Plateau.

In vast areas of Iran, several caves and karstification phenomena have been recognized and described during recent years. These areas including Zagros, Alborz and central Iran mountains are partly made of karst stones, which are the main source of providing drinking water in arid and semi-arid states of the country. These phenomena have been studied in sedimentology, hydrogeology, hydrocarbon and touristic aspects. Researches that investigate the relationship between karst and tectonics are rare in Iran. Nadimi and Sohrabi [3] have studied Kalahrood cave in north Isfahan and showed that brittle deformation and karstification processes have created a suitable area for cave formation.

In this research, we have two goals. Firstly, we are going to introduce and describe a sinkhole in the north Isfahan city and secondly introduce role of tectonics in formation of Chah-e Deev sinkhole.

Chah-e Deev sinkhole area is located in southwestern margin of the Urumieh-Dokhtar Magmatic Arc (UDMA) and its distance is about 55km from Isfahan. The area is surrounded by high and uplifted mountains of UDMA to the northeast and the Sanandaj-Sirjan Zone flats to the southwest. The oldest exposed rocks in the Chah-e Deev area are yellowish dolomite of middle Triassic that have exposure in northeastern margin of the area and separated by a NW-SE-trending reverse fault. The area is covered by shales with intercalations of sandstones and ammonite limestones of Upper Triassic. Several generations of alluvial fans and vertically eroded recent alluvial deposits indicate uplifting of the area. In the Chah-e Deev area, the fault pattern consists of NW-SE and NE-SW- trending faults. These faults dissect older rocks and have made a brittle area that created a suitable area for forming of a sinkhole.

The Chah-e Deev sinkhole has 35m width and about 56m depth that is formed on top of a hill and is 2221m higher than sea level (see figure). Walls of the sinkhole are vertical and several sets of vertical joints that can be seen in the rocks. Two steeply dipping dextral strike-slip faults have conjugated each other on the surface and have made a crushed zone. A major fault plane was observed in the floor of the sinkhole that has made the northern wall of the hole and has sinistral strike-slip component of movement (see figure). The structural observations show that tectonic processes are the first and major factor for formation of Chah-e Deev sinkhole and also for determining its location.

Chah-e Deev sinkhole is the best and most unique example of sinkholes in central Iran. Based on this research in the Chah-e Deev sinkhole, we have considered two main stages for tectonic evolution and formation of the sinkhole. First, during the Zagros collision and start of strike-slip movements in Zagros collision zone after Pliocene, several fault sets were formed. Conjugation of these faults and their branches and joints has created a crushed zone in Chah-e Deev area, northeast Isfahan. Second, after formation of the faults and joint, uplifting of the area and activity of groundwater and solving of Upper Triassic limestones in the lower part of the sinkhole, created some karsts and small caves and these places were filled during subsidence the sinkhole.

Location of the studies area and Zagros orogen structural zones are shown in small map. A) Chah-e Deev Sinkhole on top of a hill. B) Cross section of the sinkhole. C) Plane and slickenlines of a sinistral strike-slip fault in bottom of the sinkhole. D) strike-slip faults of upper part of the sinkhole.

References:

1. Ballesteros, D., Jimenez-Sanchez, M., Garcia-Sansegundo, J., and Giralt, S., (2011) Geological methods applied to speleogenetical research in vertical caves: the example of Torca Teyera shaft (Picos de Europa, northern Spain), Carbonates Evaporites, 26, 29-40.

2. Kambesis, P., (2007) The importance of cave exploration to scientific research, Journal Cave and Karst stud. Nat. Speleol. Soc., 69, 46-58.

3. Nadimi, A., and Sohrabi, A., (2009) Role of Tectonics in Forming of the Kalahrood Cave, North Isfahan, Iran, European Geosciences Union, General Assembly, Vienna, Austria, Vol. 11, EGU-3936-1.

4. Stöcklin, J., (1968) Structural history and tectonics of Iran; a review, American Association of Petroleum Geologists Bulletin, 52, 7, 1229–1258.

Estimate earthquake risk to the Takht e Jamshid historical site (NE Shiraz, Fars, Iran)

Taheri F., Zamani A.

Department of Earth Sciences, College of Sciences, Shiraz University, Shiraz, Iran

[e-mail]

Geohazards and natural phenomena such as earthquakes, tsunami, and landslides are mainly caused by processes in the earth which are going on for a long time. Recently scientists used earthquake catalog and geodetic measurements for evaluating active tectonics. But the time span for these data doesn’t cover the whole cycle of deformation. To overcome this difficulty in the study of neotectonics, morphotectonic analysis of landforms is useful because they recorded history of deformation in about one million years.

Slip and creep behavior of Faults may lead to uplift or subsidence and changes attitude of the geological structure and related geological hazards will be accrued. Detecting state of active tectonic and relative activity in structures is the first step to identify any possible geological hazards that related to tectonic. Geological hazards threaten human life and also their structures. Some of these structures are important because of the historical and cultural concepts such as archeological sites.

Iran is one of the high seismic active countries in the world. The occurrence of big and destructive earthquake in this country resulted in the destruction of buildings and economic institutions, cultural and historical sites. The major damage of. Arg e Bam one of the UNESCO’s world heritage sites in Iran. This is destructed in December 26, 2003 by Bam earthquake. During the history, many of Iranian archeological sites are destroyed or damaged by geological hazards the attention of government officials and experts. Many of these sites placed in Fars province. One of the most important sites is Persepolis (Takht e Jamsid).

The Study area is bounded between East longitude 52 ^o 50' and 53 ^o 10' and North latitude 29 ^o 45' to 29 ^o 55.' This is situated in 70 Km northeast of Shiraz in Rahmat anticline. This area registered as a UNESCO’s world heritage site in 1979. On the other hand, this site is located in simple fold zagros (Zagros orogenic belt, one of the active tectonic regions in the world). This incline included faults with combined reverse and strike–slip movements.

In this research in order to study active structures, morphotectonic analysis using geomorphic indices of mountain fronts and related fluvial systems such as mountain front sinuosity index, hypsometric integral, stream, Valley floor width – height ratio and Drainage Basin Shape Index. These indices have been measured with digital elevation model (DEM) with the use of GIS environment.

1- Mountain front sinuosity index: The S mf index reflects a balance between the tendency of stream and slope processes to produce an irregular (sinuous) mountain front and vertical active tectonics to produce a prominent straight front. The L mf is the length of mountain front along the mountain–piedmont junction and L s is the straight-linelength of the front [1, 2].

2- Valley floor width – height ratio and Drainage: This study transverse valley profiles were located 0·5km upstream from the mountain front in smaller drainage basins, V fw is the width of valley floor, E ld and E rd are the respective elevations of the left and right valley divides and E sc is the elevation of the valley floor.

3- Drainage Basin Shape Index: B l is the length of the basin. The B w is the width of the basin. The typical basin of a tectonically active mountain range is elongate, and basin shapes become progressively more circular with time after cessation of mountain uplift.

4- Hypsometric integral: In this study Hmin, Hmax, Hmean were defined elevation sub basin [3].

Result:

Results indicate that Rahmat fault zone which is in southeast of study area has a higher level of activity Naghshe Rostam and Takhte Jamshid are located close to this fault. In order to prevent serious damage to these historical sites evaluation of the seismic risk in the study area plays an important role in mitigation of earthquake damages.

References:

1- Keller, E.A., (1986), Investigation of active tectonics: use of surficial Earth processes, In Wallace (ed.), Active Tectonics, Studies in Geophysics, National Academy Press, Washington, DC, p 136–147.

2- Maria Teresa Ramirez-Herrera (1997) Geomorphic assessment of active tectonics in the acambey graben, Mexican volcanic belt P 317-332

3- Strahler, A.N. (1952)”Hypsometric (area-altitude) analysis of,” erosionaltopography Geological Society of America Bulletin 63p 1117–1142.

Fluid inclusion planes as tectonic and ore formation indicators: an example the Antei uranium deposit (SE Transbaikalia)

Ustinov S.A., Petrov V.A., Poluektov V.V.

The Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry RAS, Moscow, Russia

[e-mail]

Migration of the dissolved substances or colloidal particles in fluid flows under the influence of gravitation, stress and temperature is carried out by faults and cracks, from which substances and particles both penetrate to surround volume of rocks and migrate back to discontinuities. Paleofluid pathways can be observed not only at macro- but also at microscale by studying microcracks of various generations and types.

Cracks in rocks can be considered as Open Cracks (O.C.: not filled with secondary mineral phases) or Filled Cracks (F.C: filled with secondary mineral phases or Fluid Inclusions). F.C. are often partially filled or reopened after a first time of filling. The best records of fluid percolation are paleofluids trapped as fluid inclusions in healed microcracks of the rock-forming minerals [2]. Usually such fluid inclusions with liquid, vapor and solid phases form differently oriented systems, known as fluid inclusion planes (FIPs). FIPs result from the healing of former open cracks and appear to be fossilized fluid pathways [3]. FIPs are totally sealed and do not present secondary opening.

Microcracks should provide valuable information about the local stress in rocks and can be assumed to be (σ₁σ₂)

planes [4]. The FIP are mode I cracks that occur in sets with a predominant orientation perpendicular to the least principal compressive stress axis σ₃. These mode I cracks propagate in the direction which favors the maximum decrease in the total energy of the system. They do not disrupt the mechanical continuity of mineral grains and do not exhibit evidence of shear displacement like mode II and III cracks. The FIP are usually observed and characterized in minerals which crack according to the regional stress field, independently of their crystallographic properties, and may easily trap fluids as fluid inclusions when healing. In some minerals (carbonates, feldspars), the fluids are not always preserved due to alteration or dissolution and cracks display more complex patterns resulting from the presence of cleavages, subgrain boundaries or twin planes. The rate of healing is rapid in quartz (compared to geological times), so it is more informative for studying FIP systems [1].

Frequently, FIP form well defined networks which allow the determination of a chronology. After a first generation of FIP, a second crack family can be formed with the trapping of different fluid. This second FIP generation commonly cross-cuts the first one. Thus, FIPs provide good records of successive episodes of crack initiation and fluid migration.

So orientation of FIPs is defined by reorganization of the local stress field and it is possible to use them as geostructural markers for reconstruction of porosity and paleopermeability of rocks, geometry of fluids migration pathways, reconstruction of fluid migration stages and for studying dynamics of change of PT, physical and chemical conditions at various events of deformation of geological objects and ore formation.

FIP generations were studied at the Antei uranium deposit. It is located in Eastern Transbaikalia within the Streltsovskaya caldera, generated in process of late-Mesozoic tectonic and magmatic activation of the region. Samples were taken from all fault zones – from the central part (core), the zone of its dynamic effect (failure), and undestroyed wall rock (protolith) at the9th to 11th horizons. Also for microfissural mapping of natural cracks and allocation of FIP systems, samples must be oriented in space, north direction must be fixed. The chronology of FIP generations and their spatial parameters (extension, dip angle) can be established by means of the classical microstructural analysis (Fedorov’s stage), or by method of statistical analysis of 2D and 3D digital images of thin sections by means of the special software [2]. Besides, this analysis allows quantifying paleofluid flow porosities and permeabilities by the reconstruction of the crack network consisting of cracks described as discs using the geometry of the crack network. So for each FIP system, one can determine dip direction, length, thickness, porosity and paleopermeability using microstructural analysis. The data on composition and properties of fluid inclusions trapped in the cracks (temperature, pressure, salinity, phase content) to separate different sets of FIP was found out using microthermometry and Raman spectroscopy.

As the main ore component of the Antei deposit is uranium, we can use sufficiently effective method for reconstruction fluid filtration processes and stages of intraore tectonics during the past geological events – fission-track radiography (FTR) of thin sections. As detector lavsan fibre was used. Thin section and lavsan were placed into reactor and irradiated with a stream of slow neutrons. By means of this method it is possible to establish characteristics of uranium distribution at microscale, and also to calculate its concentration in mineral phases, microcracks and pores. The thin sections for FTR were the same, as for microstructural analysis. So it allows to define spatial distribution of uranium concentrations, their relation to already revealed systems and types of microcracks and other microstructures, and also to allocate various stages of ore formation.

The comparative characteristic of orientation of open microcracks, fluid inclusion planes and linear distributions of uranium concentrations was carried out by construction of roses-diagrams for each type of objects. It allowed to define spatial parameters of the chosen linear objects, allocate stages of fluid migration and to confirm conception about polystage development of intraore tectonic processes.

After getting parameters of each FIP generation, making their interpretation and finding out distribution of uranium ore the conclusion can be drawn:

1) Ore forming process at the Antei uranium deposit took place through several (minimum 2) stages of fluid migration;

2) FIPs change their orientation from the north-northeast to the east-northeast on a vertical interval from 9th to 11th horizon, this fact confirms that reorientation (about 30 degrees) of the horizontal paleostress axis take place at this interval;

3) This approach helps us to model paleopermeability of crystalline massifs (tectonics, fluid flow pathways, fluid chemical composition, etc.) as a function of stressed-strained and temperature state in space-time context.

This work was financially supported by the Russian Foundation of Basic Researches (grant № 12-05-00504).

References:

1. Lespinasse M. Are fluid inclusion planes useful in structural geology? // J. Struct. Geol. 1999. N 21. P. 1237-1243.

2. Lespinasse M., Désindes L., Fratczak P., Petrov V. Microfissural mapping of natural cracks in rocks: implications on fluid transfers quantification in the crust // Chemical Geology Spec. Issue, 223, 170-178 (2005).

3. Roedder E. Fluid Inclusions. Review of Mineralogy, 12 (1984).

4. Tuttle O. F. Structural petrology of planes of liquid inclusions. J. The journal of geology. 1949. N 57. P. 331-356.

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