BREW, GRAHAM, Cornell University; and MUAWIA BARAZANGI, Tarif Sawaf Syrian Petroleum Company, Damascus, Syria

Abstract: A Tectonic Model for Syria: Hydrocarbon Implications

Introduction

We present a new tectonic model for Syria based on over ten years of ongoing investigations concerning mobile zones within the northern Arabian platform.All the tectonic zones studied have proven hydrocarbon reserves or significant hydrocarbon potential.We have set our findings into a broad plate tectonic context, thus making the results relevant to both hydrocarbon exploration and regional investigations.

Investigated Areas

Our interpretations are based on integrated analyses of seismic reflection data, well records, seismic refraction profiles, potential field data, geologic maps, topographic and remote sensing imagery, and other relevant geoscience data sets. Maps generated from seismic and well interpretations are correlated with surface data and potential field data to yield models fully consistent with all available information.The investigations of four tectonic zones (Figure 1), discussed briefly below, have formed the basis for our tectonic model.

The Palmyride fold and thrust belt was the focus of many Cornell University studies (e.g., Best et al., 1990; Chaimov et al., 1990; Chaimov et al., 1992). Preferential deposition in the Palmyride basin is apparent since the Carboniferous, with the Late Permian being a time of significant rifting and subsidence (Sawaf et al., 1999).This rifting and deposition continued episodically throughout the Early Mesozoic.After a widespread unconformity during the Late Jurassic and Early Cretaceous and more subsidence in the Cretaceous, extension within the Palmyrides ceased in the Latest Cretaceous, and the region was subjected to compression. The main phase of compression, however, has occurred since the Late Miocene. This has been manifest in the reactivation of some Mesozoic normal faults, uplift and the consequent formation of up to 1500 m of elevation within the southwestern Palmyrides. Hydrocarbon interest in the Palmyride fold and thrust belt mainly concerns gas reserves found in Middle Triassic strata, sealed by Triassic salts and anhydrites, and sourced by Triassic and Carboniferous shales.

The tectonic evolution of northeast Syria was discussed most extensively by Brew et al. (1999).They found that during Late Paleozoic and Early Mesozoic time, northeast Syria was the site of the northeastern continuation of the southwest-northeast--trending Palmyride basin. However, the most significant tectonic event in northeast Syria occurred near the end of the Cretaceous. Late Campanian and Maastrichtian formations show fault-related thickening of well over 1.5 km in the Sinjar area, with lesser fault throws in the Abd el Aziz region.These extensional faults strike east - west, across the structural grain of the Late Paleozoic and Early Mesozoic sedimentary trough (Figure 2). After abrupt cessation of the extensional event at the end of the Cretaceous, the Paleogene was a time of quiescence in northeast Syria. Since mid to late Pliocene, the region has been subjected to significant compression and consequent structural inversion through reactivation of the latest Cretaceous normal faults.This has led to over 1000 m of topography on the Sinjar uplift relative to the surrounding plains.At shallow levels, this structural inversion led to the formation of fault-propagation anticlines that form the traps AAPG International Conference and Exhibition 79.for many oil fields in the area.

The Euphrates fault system, roughly 100 km wide and located beneath the Euphrates River in central and southeast Syria (Figure 1), runs almost orthogonal to the Palmyride / Sinjar trend. Over a billion barrels of oil have been found within the Euphrates fault system since the early 1980's.Workers at Cornell (e.g., Litak et al., 1997; Litak et al., 1998) have concluded that the Euphrates fault system is predominantly a Late Cretaceous transtensional feature, that experienced minor transpression in the Late Tertiary.Total amounts of extension were not in excess of ~6 km. Most of the hydrocarbons in the area are sourced from the syn-tectonic Campanian and Maastrichtian units that matured after being down-thrown into the graben. Fault-trapped reservoirs are formed predominately in Lower Cretaceous sandstones.Abrupt abortion of the rifting at the end of Cretaceous time was followed by broad thermal sag above the rift. Minor right-lateral transpressional reactivation of the Late Cretaceous normal faults in the Late Cenozoic did little to affect trap integrity in the Euphrates fault system.

Finally, we have used a network of seismic reflection data, digitally reprocessed and interpreted, combined with well and gravity data to analyze the history of the Ghab basin, a pull-apart structure on the Dead Sea fault system (Figure 1). Our workstation-based seismic reflection interpretations allowed a full analysis of the styles of deformation within the basin. These show an extensive series of faults in the Mesozoic basement beneath the basin, trending parallel and oblique to the bounding Dead Sea fault system.The syn-tectonic sedimentary fill is only slightly deformed.The maximum thickness of the Plio-Quaternary fill is ~3000 m in the south of the basin.

Regional Model

We have combined the above interpretations into a unified model of northern Arabian plate deformation since the Late Paleozoic.This model illustrates many of the contempora-neous tectonics events that can be correlated across the Arabian platform. In general, it illustrates that the episodes of tectonic deformation in the northern Arabian platform have been controlled in time and space by activity on nearby plate boundaries, and the locations of zones of weakness within the platform. Although the model is not pictorially shown here, the relative timing of events is illustrated in Figure 3.

The northern Arabian platform is hypothesized to consist of a series of relative stable blocks (the Rutbah uplift, the Aleppo plateau, the Mardin high, see Figure 1), bounded by more mobile weak zones.These weak zones, possibly suture zones relic from the Pan African accretion of the Arabian plate, have been repeatedly reactivated since their formation, and still form the locus of deformation and tectonic activity in Syria today. Although Early Paleozoic tectonics are difficult to interpret with the available data, an unconformity of late Silurian and Devonian age is observed regionally, and could be related to the formation of the PaleoTethys ocean. Since the Carboniferous, the northern Arabian platform underwent a series of extensional periods associated with subsidence and rifting, resulting in large sedimentary accumulations. The Palmyride / Sinjar trough formed during Carboniferous time, possibly as a result of stress reorganizations on the Laurussian margins. The more significant rifting in the Palmyrides in Permian time was coincident with the separation of the Cimmerian block from Gondwana. This separation eventually led to the demise of the PaleoTethys ocean, as the NeoTethys formed through accretion off the northeastern margin of Gondwana. The formation and expansion of the NeoTethys in the Permian and Early Mesozoic was concurrent with the early stages of rifting in the Eastern Mediterranean and rifting in the Palmyrides. We hypothesize that at this time the Palmyride trend could have been an aulacogen, formed during the opening of the Eastern Mediterranean, during a period of regional extension.The growth of the Mediterranean basin and NeoTethys continued episodically throughout the Early Mesozoic, and rifting and subsidence in the Palmyride and Sinjar areas accompanied this.

Consumption of the NeoTethys started in the mid-Mesozoic, and by mid-Cretaceous time a subduction zone had formed near the northeastern margin of Arabia.Widespread extension in the Sinjar and Euphrates areas in the latest Cretaceous is thought to be a consequence of reorganized subduction in the NeoTethys.This extension ceased abruptly at the end of the Cretaceous as the Arabian peninsula reached the trench of the subduction zone.This was accompanied by minor uplift in the Palmyrides. Terminal closure of the NeoTethys and suturing of Arabia to Eurasia in the Miocene led to the main phase of uplift in the Palmyrides and minor transpression in the Euphrates, in concert with the opening of the Red Sea. Further reorganization in the Pliocene resulted in uplift in northeast Syria.

Acknowledgements

The authors would like to thank the Syrian Petroleum Company (SPC) for the generous provision of data.This work was funded though collaboration with Exxon,Arco, Conoco and AEC International oil companies. Corresponding author: G. Brew, Snee Hall, Cornell University, Ithaca, NY 14853, USA, [email protected].

References

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