Sawaf, T., Al-Saad, D., Gebran, A., Barazangi, M., Best, J. A., and Chaimov, T.

Stratigraphy and structure of eastern Syria across the Euphrates depression

Tectonophysics, 220, 267-281, 1993

Abstract

A crustal-scale geotransect across the northern Arabian platform, oriented north-south in eastern Syria, reveals an alternating series of basement uplifts and basins separated by predominantly transpressional fault zones above an effectively uniform crust. Four major tectonic provinces are crossed along a 325 x 100 km corridor that extends from the Iraqi border in the south to the Turkish border in the north: the Rutbah uplift, the Euphrates depression, the Abd el Aziz structural zone, and the Qamichli uplift. These features are the manifestations of reactivated Pre-Cenozoic structures that responded to forces acting along nearby Arabian plate boundaries, particularly Cenozoic convergence and collision along the margins of the northern Arabian platform, i.e., the Bitlis suture and the East Anatolian fault in southern Turkey and the Zagros suture in Iran and Iraq.

The database for this study consists of 3000 km of industry seismic reflection data, 28 exploratory wells, geologic, and Bouguer gravity maps. The deep crustal structure and, in part, the basement geometry along this transect are inferred from two-dimensional modeling of Bouguer gravity, whereas the shallow (about 8 km) structure is constrained primarily by well and seismic data.

Features of the geotransect reveal:

1. A relatively uniform crustal column approximately 37 km thick with only minor crustal thinning beneath the Euphrates. Crustal thinning may be slightly more pronounced beneath the Euphrates (about 35 km) to the southeast of the transect where the Bouguer gravity anomaly is slightly higher.

2. Along the Euphrates depression, ongoing subsidence, which began during Late Cretaceous time, resulted in the deposition of at least 3 km of Late Cretaceous and Cenozoic rocks. The structural complexity of the Paleozoic and most of the Mesozoic sedimentary sections along the transect contrasts markedly with a relatively simple, flat-lying Cenozoic section along most of the transect. A notable exception is the Abd el Aziz uplift, where Cenozoic rocks are strongly deformed.

3. While Euphrates subsidence continued throughout the Cenozoic, the east-west trending Abd el Aziz structure began to be inverted into a fault-bounded tilted block since the Miocene, perhaps as a response to the last episode of intense Miocene collision along the nearby Bitlis and Zagros suture zones.

Key Figures and Captions

Figure 1. Example of the seismic reflection character of the Paleozoic section on a portion of line PS 417A.

Figure 2. Example of seismic reflection data across Abd el Aziz mountain, line UN 310. Note undeformed sedimentary basin rocks to the north.

Figure 3. Gravity model used to constrain the structure of the mid to deep crust along the transect. Gravity values are in units of g/cm³. The observed Bouguer gravity data can be modeled by a simple crustal model with very minor crustal thinning beneath the Euphrates depression. "Major" anomaly variations can be explained by changes in basement depth, i.e., sediment thickness. See text for discussion of other data used to constrain this model.

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