The tectonic control on Cretaceous sedimentary basin in Isfahan region, Iran

Farzipour Saein A., Beygi S., Tajmir Riahi Z., Vatandust M.

Department of Geology, Faculty of Science, University of Isfahan, Isfahan, Iran

Asaein@gmail.com

The sediment accumulated in a sedimentary basin and their different thickness or facies depends on factors such as changes in global sea level and tectonic condition. This study includes the correlation of cretaceous litho-stratigraphic units in nine different sections around the Isfahan city. By studying the Lateral changes in the thickness and the facies of the sedimentary units we tried to understand the tectonic control on the cretaceous sedimentary basin in the area.

Isfahan is located in 51˚67́ E and 32˚66́. From the geological point of view it is located in the Sanandaj-Sirjan zone.

In this region, there is an angular unconformity between the Lower Jurassic (Liassic shale, equivalent to Shemshak Formation in Alborz area) and the Lower Cretaceous (Upper Barremian) sediments. This unconformity is created on the Early – Middle Cimmerian orogenic phase. The stratigraphic units during Malm, Dogger, Berriasian Valanginian and Hauterivian times are missed. The Lower Cretaceous basin was a shallow basin which was affected by the Orogenic phase of Early – Middle Cimmerian, and the igneous masses have penetrated the Jurassic shale of the Kolah-Ghazi and the Col Rokh regions. The basal conglomerate of the Lower Cretaceous were deposited on the angular unconformity demonstrates the expanding of the Cretaceous sea, because of the Early Cretaceous spreading Neo-Tethys Ocean between the Sanandaj-Sirjan structural zone and the Zagros. We saw the propagation of Cretaceous sea level rise can be followed from the South to the North -North West. Following this progress the sedimentary basin in the Isfahan contains most of the Cretaceous sediments outcrop. Most of this Upper Cretaceous outcrops in the region have been eroded by activation of the Laramidian orogenic phase (excluding the Kolah-Ghazi, Baharestan and Zefreh outcrops).

The main faults, which exist in this region, were studied in order to evaluate their control on the sedimentary basin during Cretaceous.

This study reviewed and updated 9 stratigraphic columns distributed in the region. The lithostratigraphic correlation chart was prepared and using tectonic studies the thickness or facies changes of the sediments evaluated.

Navab anticline: a contractional structure in the transpressive bend of Qom-Zefreh fault zone, west of central Iran

Kamali A.R., Nadimi A. and Farzipour Staefan A.

Department of Geology, Faculty of Science, University of Isfahan, Isfahan, Iran

alirezakamali110@yahoo.com

The contractional collision between the Arabian and Eurasian plates is deformed an area between the Aegean sea , the eastern part of Iran, the northern side of the Caucasus-Koppe Dagh mountains and the Persian Gulf. The collision, following the closure of the Neo-Tethys Ocean, resulted in the development of the Zagros orogen [3]. The timing of collision has been highly controversial, ranging from late Cretaceous to Miocene- upper Pliocene. Although there is a growing body of evidence in supports of late Eocene to Oligocene initial collision [1].

This research focuses on the north-eastern margin of Zagros orogen and western margin of central Iran. This part of central Iran is considered a shallow marine basin during the Oligocene and Miocene times. Some geologists believed that this basin was a back-arc environment during the Arabian and Eurasian convergence (e.g. [2]).

The study area (Navab anticline, SE Kashan) located in Qom-Zefreh fault zone that is one of the most important faults in central Iran. The fault developed during the collision and reactivated during strike-slip movements of Iranian plateaus in the late Pliocene to Quaternary times. This movement resulted from the change of the convergence direction of the Arabian and Eurasian plates and oblique collision.

The Qom-Zefreh fault zone with about 300 km length and NE-SE trend makes boundary between central Iran and Zagros orogen structures. The fault has dextral strike-slip and in some area has reverse dextral components of movement. There are several kinds of structures that formed related to dextral strike-slip motion of the fault. The structures are consist of duplex, transpression, transtension, and rotated blocks.

The Navab anticline area is surrounded by Qom-Zefreh fault and its branches faults (see figure). The Qom-Zefreh fault is divided in some smaller segments. Natanz fault is one of the segments that make southwestern boundary of the study area. The Natanz fault with dextral strike-slip and reverse dip-slip components of movements and its branches fault cut the Navab anticline. The Navab anticline is formed in Oligocene- Miocene sedimentary rocks and has sigmoidal shape. Eocene rhyolitic rocks have made core of the anticline.

The stages of tectonic evolutions of the Navab anticline in western margin of central Iran are interpreted as the product of three main events:

1) Formation of sedimentary basin in the Oligocene- Miocene ocean and deposition of shallow marine sedimentary rocks of Qom Formation and Lower Red and Upper Red formations.

2) Formation of contractional structures during convergence of Arabian and Eurasian plates. In this stage, during shortening, the basin closed and the sedimentary rocks were folded and thrusted.

3) Late Pliocene to Quaternary strike-slip movements that resulted from the oblique convergence, changed shape of structures. In this stage, the rock units sheared and several strike-slip faults created. During the strike-slip movements several contractional structures between the faults were formed.

Location of the studied area and the Zagros orogen structural zones are shown in small map. Location of the Navab anticline and shortening trajectories in the northeastern Qom-Zefreh fault zone are shown in big map.

References:

1. Agard, P., Omrani, J., Jolivet, L., and Mouthereau, F., (2005) Convergence history across Zagros (Iran): constraints from collisional and earlier deformation. International Journal of Earth Science, 94, 401–419.

2. Berberian, M. and King, G.C.P., (1981) Towards a paleogeography and tectonic evolution of Iran, Canadian Journal of Earth Sciences, 18, 210–265.

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