This paper presents the role of tectonic and karstic processes in the formation and evolution of Barm Firuz Lake within the Zagros Mountains of Iran. This lake with elevation of 3 340 m from sea level is located in the crest of the Barm Firuz anticline. Results show that the structural evolution of the Barme Firuz Lake is related to both tectonic and karst processes. Therefore the term of tectonokarstic has been used for structural evolution of the lake. Structural studies especially on the stylolite structures revealed the occurrence of an important component of simple shear deformation in the study area. Based on structural and karstic evidence around the lake, the fault-dissolution sink model with oblique pure shear component is suggested as kinematic model of the Barm Firuz Lake evolution in the Zagros Mountains of Iran.
Sinkholes form by dissolution of carbonate formations by influence of surface and underground waters and rocks collapse due to gravity force (Filipponi et al., 2009). In many cases the effects of tectonics for increase of fractures density is very important for evolution of karst features. Therefore tectonics can be act as complementary phenomenon for formation of karst features. Sometimes a tectonic shock such as earthquake generates driving force for tumble down of the rocks. Anyway, the role of water in generate of karst cavities is the most important factor. So that in many cases, independent of the tectonic effects and only with operation of acidic water the formation of a karst sinkhole is possible. But in some cases the evidence of tectonic effects can be seen around the sinkholes (Neal, 2004). In such cases the terms of tectonokarstic sinkholes can be used. Determination of transposition of dissolution or tectonic operation in the formation of sinkholes is very difficult and sometimes simultaneous operation of these two phenomena forms a tectonokarstic sinkhole. There are many tectonokarstic structures in the world. San Gregorio Magno and Dasht Arjan in Italy and Iran are the examples of two known tectonokarstic features. This work investigates the importance of structural analyses in the characterizing of tectonic and karstic processes on the evolution of the Barm Firuz Lake.
1 REGIONAL GEOLOGICAL SETTING
The Zagros fold-thrust belt is part of the AlpineHimalayan orogenic belt (Berberian and King, 1981) and lies on the northeastern margin of the Arabian Plate. This belt contains 8–14 km thick Cambrian–recent succession, which lie on a Precambrian basement. These sediments were deposited on a platform that was relatively stable from the Cambrian until the collision between the Arabian and Iranian plates in the Late Cretaceous to Tertiary (Berberian and King, 1981; Falcon, 1974). Shortening across the Zagros fold-thrust belt in the Phanerozoic rock cover, which is estimated to be about 30–85 km (McQuarrie, 2004; Blanc et al., 2003; Falcon, 1974), occurred by thrusting and folding above various decollement horizons. Postcollisional crustal shortening is still active (Allen et al., 2004; Tatar et al., 2004; Talebian and Jackson, 2002; Jackson and McKenzie, 1984) with an N-S oriented convergence rate of approximately 20±2 mmyr-1 (Molinaro et al., 2005; Vernant et al., 2004). Shortening in the basement occurs dominantly by faulting. The thick Cambrian Hormuz Salt, at the base of the sedimentary succession, and other evaporite horizons (e.g., the Dashtak and Gachsaran formations) within the succession (Sepehr and Cosgrove, 2005; Talebian and Jackson, 2002), prevented these basement faults from rupturing the cover rocks and reaching the surface. As a result of these decoupling horizons, the deformation in the basement and the sedimentary cover occurred independently. This fold-thrust belt is approximately 1 800 km long and 200–300 km wide. It runs from eastern Turkey to the Strait of Hormuz, where it terminates against the Minab fault (Fig. 1), which separates the Zagros belt from the Makran accretionary prism (Molinaro et al., 2005). The study area is located in the Barm Firuz anticline near Ardekan Town 110 km northwestern of Shiraz. The Barm Firuz anticline such as many anticlines in the Zagros fold-thrust belt shows an axial trend with NW-SE orientation. The anticline shows doubly plunging geometry. The main part of this anticline consist of the Cretaceous Sarvak Formation composed mainly of carbonate rocks that is one of the main hydrocarbon reservoir rocks in Southwest Iran. In the study area, the Sarvak Formation consists of light to grey calcareous breccia and medium bedded limestone. Figures 2 and 3 show the oblique satellite image and the geological map of the study area.
2 STRUCTURAL AND KARSTIC EVIDENCE
Because of the unique position of the Barm Firuz Lake it is interest subject for many geologists. Several hydrogeological studies have been done on the lake (Eftekhari, 1993; Pezeshkour, 1991). These studies mainly concentrated on the hydrological relationships of the lake with the sprigs around the Barm Firuz anticline. In this research a possible kinematic model for the evolution of the Barm Firuz Lake is investigated using stylolite structures. Satellite images analyses and detection of linear elements show several main fault systems around the lake. It seems that two fault systems dominated in the area. Faults with parallel trend and faults with attitude of 30° to 40° in respect of the fold axis of the Barm Firuz Anticline (Fig. 4). Related to the faults vertical movement components along the long axis of the lake especially in the south rim, the lake area identified as a low elevation between the fault planes (Figs. 5a, 5b). Limestone scarp surfaces around the lake show numerous of dissolution cavities, karrens and fault slickensides features. In some cases the fault planes show a smooth surface with clear scratch-slip and in another places exhibit a rough and dissolution surface. Slickenfibres on the fault planes mainly show high plunge angles which confirm predominant of vertical movement on the fault planes. As mentioned above, karren features have formed on the carbonate surfaces around the lake.
These dissolution grooves mainly have vertical mode and no visible on the all of carbonate surfaces so it can be deduced that vertical faults and joints have played an important role to control of the water flow directions. Young fault movements have caused to cut karstic grooves and formed carbonate surfaces with both karren and slickenside features (Fig. 6).