Active Horsetail Splay Structure in the Cenozoic Magmatic arc of Iran

Beygi S., Nadimi A., Safaei H. and Farzipour Sraifan A.

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

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Collision between the Arabian and Eurasian plates following the closure of the Neo-Tethys Ocean resulted in the development of the Zagros orogen, which is the main deformation belt in Iran (e.g. [2]). Time of the collision is reported from ~65 to ~5 Ma. Based on the absence of sedimentation during Late Eocene-Oligocene in western part of the Central Iran posited that the collision started between 35-25 Ma [1]. The Zagros orogen is subdivided into several tectonic units that include: the Zagros simply folded belt, the Zagros imbricate belt, the Sanandaj-Sirjan zone, and the Urumieh-Dokhtar magmatic arc (UDMA) (e.g. [4]). The study area is located in the central part of the UDMA.

The UDMA is considered as an Andean-type magmatic arc, with an almost continuous calc-alkaline magmatic activity from the Eocene till present (e.g. [2]), which peaked during the Oligocene–Miocene. It comprises various lithological units including gabbro, diorite, granodiorite and granite bodies of different size. The UDMA with NW-SE trend is located in southwestern margin of the Central Iranian Microcontinent and northeastern margin of the Arabian Plate. One of the major structural characteristic of the UDMA is presence of numerous strike-slip faults and their associated structures that are stretched with NW-SE-direction and displaced the magmatic rocks in different scales. The major fault of the study area is Qom-Zefreh fault that cut and dextrally displaced the arc.

The figure attached below shows location of the study area and its major faults. In the area, a complex fault pattern was analysed. Among the different fault sets many active faults with different components of movements on their fault planes were described.

In the central part of the UDMA, the fault pattern consists of major NW-SE, N-S, NE-SW and E-W–trending faults. These faults dissect older sedimentary rocks of Paleozoic and Cenozoic magmatic rocks, and in some places, Pliocene to Quaternary rocks and sediments. Most of these faults are still active. Along the faults horizontal offsets of ridges, alluvial deposits, stream channels, and tectonic structures associated with the strike-slip displacement were observed. Elongate outcrops parallel to the major faults were helped us to determine fault-plane dips, which generally ranged from 60° to 90°.

The NW-SE–trending faults are parallel to the UDMA. The fault set, developed in upper Cenozoic-Quaternary, is part of a system of NW-SE–trending dextral strike-slip faults that cut the upper continental crust of the central part of the Iranian Plateau and the Zagros orogen [3]. This set consists of the Qom-Zefreh and Marbin-Rengan faults and have dextral oblique thrusting component prevails on the fault planes. The Qom-Zefreh fault (QZF) with 90 km length in the study area is stretched from northwest Milajerd village to southeast Zefreh. The fault direction is changed from N45W in the northwest Milajerd village to N20W between Milajerd and Zefreh and then to N50W direction in the southeast Zefreh. The Marbin-Rengan fault has 32 km length and with N70W direction is linked to the QZF. It is considered a branch fault of the QZF.

The N-S-trending Abbas-Abad fault is the longest and major fault of this set. This fault has 50 km length and is linked to the QZF in south Milajerd village. It is a dextral strike-slip fault and is considered a branch of the QZF.

The NE-SW–trending faults in the study area are perpendicular to the UDMA. The faults have a major normal dip-slip component of motion. The Bar-Gohar fault with 20 km length is the longest fault of this set. The fault trend is changed from N80W in the west to N75E in the east. Most faults of this set linked to the QZF and show curvilinear shape.

The E-W-trending Kacho-Mesqal fault with 40 km length is the longest fault of E-W-trending faults. It has dextral strike-slip component of movement. The fault is linked to the QZF in south Milajerd village and is considered one of the QZF branches.

The faults of the study area are commonly divided into smaller segments. In the southeast Milajerd the QZF suddenly changes its direction from NW-SE to NNW-SSE trend and several branches faults are separated from the QZF and created a dextral horsetail splay structure in the southeastern termination of the QZF. Several hot water springs are formed along some of the faults of the splay structures. Addition to springs, concentration of earthquakes epicentres along the active strike-slip faults of the structures and displaced Quaternary sediments along the faults are confirmed that the faults and structures are active.

The following conclusions can be drawn from the study:

The described fault pattern of central part of UDMA consists of NW-SE, N-S, NE-SW and E-W-trending fault sets. Most of the faults sets formed and/or reactivated during dextral strike-slip movements of the Zagros Orogenic structures after Pliocene. The dextral strike-slip movements along the faults created a horsetail splay structure in southeastern termination of the QZF. Activity of the faults is confirmed by distribution of earthquakes epicentres along the faults, hot water springs and displaced Quaternary deposits.

Location of the studied area and the Zagros orogen structural zones are shown in small map. Active fault patterns and earthquakes distributions in central part of the UDMA 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. Nadimi, A., (2010) Active strike-slip faults in the central part of the Sanandaj-Sirjan Zone of Zagros Orogen (Iran), Ph.D. thesis, University of Warsaw, Poland, 121pp.

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

Дата добавления: 2015-10-29