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Regional tectonic maps of the northern Arabian platform show how Syria is almost completely surrounded by active plate boundaries. The west of the country is cut by the 'leaky' left-lateral Dead Sea Fault system which extends from the Gulf of Aqaba in the south and into Turkey in the north. North of Syria lies the Bitlis suture which represents the collision zone between the Eurasian and north-moving Arabian plates. East and southeast of Syria the Neogene- Quaternary Zagros fold belt marks the collision between Iran and the Arabian plate.
It has been realized that the major structural deformation within Syria is a consequence of movement on these surrounding plate boundaries. Thus, through studying the deformation in Syria we can gain an understanding of the plate tectonics of the northern Arabian platform, and construct a picture of the tectonic evolution in the region.
The Palmyride fold and thrust belt extends for about 400 km approximately southwest to northeast across Syria and is about 100 km wide. It extends from the Dead Sea fault system in the west, and joins the Euphrates fault system in the east. The region has been studied in detail using seismic reflection data by McBride et al. (1990), Al-Saad et al. (1992), and Chaimov et al. (1992). Additionally, a '3-D cube' of data was examined by Chaimov et al. (1993) revealing the seismic fabric and 3-D structure of the southwestern Palmyrides.The Palmyrides resulted from Late Mesozoic and Cenozoic, mainly Neogene, inversion of a failed Early Mesozoic rift. This roughly linear, shallow marine trough had accumulated thick Triassic, Jurassic and especially Cretaceous carbonates. Minor evaporites were also deposited, mainly in the Late Triassic.
The inversion of the Palmyrides began in the Late Cretaceous in concert with uplift in the Sinai and Negev to the southwest. Much of the inversion took place in the Miocene, in response to stress transmitted across the Arabian plate due to accelerated convergence of the Arabian and Eurasian plates to the north. The initiation of inversion in the Palmyrides seems to have occurred at the same time as the abortion of rifting in the Euphrates fault system to the east.
Transpressive deformation of the Palmyrides continued through the Neogene and the Quaternary. Balanced cross-sections demonstrate that there has been a total of 20-25 km of shortening in the southwest of the Palmyrides where topographic relief approaches 2000 meters (Chaimov et al., 1990). The amount of shortening diminishes to the northeast and becomes only 1 or 2 km near the intersection of the Palmyrides with the Euphrates. The shortening within the Palmyrides is thus far too little to explain the discrepancy between the 105 km of offset observed along the southern portion of the Dead Sea fault system and the 25 km of offset along its northern segment.
Through gravity analysis Best et al. (1990) noted that there are fundamental differences in either the thickness or the density of crustal rocks to the north of the Palmyrides compared to those to the south. Refraction analysis by Seber et al. (1993) has also established significant differences in basement depth on adjacent sides of the Palmyrides. It has been hypothesized that these results could be explained by the accretion of two different terranes along the Palmyride trend. This suturing hypothesis is discussed further below.
The economically important Euphrates fault system in eastern Syria (over one billion barrels of proven reserves have been found in the Euphrates area since the mid-1980's) has been the subject of several investigations by the Cornell Syria project. Sawaf et al. (1993) analyzed the gross structure and stratigraphy of a transect in eastern Syria that crossed the Euphrates fault system. The system received more particular attention from Litak et al. (1997) who studied the timing of the Euphrates deformation and attempted to relate this to the regional tectonics of the northern Arabian platform.
The Euphrates fault system extends approximately southeast to northwest for about 160 km, from the Iraq border in the southeast toward the Turkish border to the northwest, changing significantly in character along this distance. The faulting is best developed in southeast Syria and consists of a complex network of grabens, half-grabens and flower structures. In central Syria, where the Euphrates trend meets with the Palmyrides, we see complex interaction between the structures (Alsdorf et al., 1995), and possibly also with the Abd el Aziz and Sinjar structures to the northeast. To the northwest the Euphrates faulting is less pronounced; thus, thick sedimentary accumulations found in the southeast are not observed. The fault system has an average width of around 90 km.
Seismic reflection interpretation has revealed that the style of faulting in the Euphrates is complex and movement is distributed amongst many faults instead of being concentrated on just a few. For the most part faults are steeply dipping, resulting in limited overall extension (about 6%). Whilst the Euphrates received only minor deposition during most of the Mesozoic, significant sinistral transtension commenced in the Senonian (late Cretaceous) resulting in appreciable Campanian and Maastrichtian deposition. This stretching was probably caused by regional plate boundary processes, and the Euphrates fault system may be the result of reactivation along a zone of crustal weakness. Rifting in the Euphrates was aborted in latest Maastrichtian time, probably due to collision along the northern margin of the Arabian plate. Late Cretaceous ophiolites found in Cyprus and Turkey could also attest to this collision. After the termination of stretching the Euphrates area underwent a long period of thermal subsidence resulting in a thick, relatively undeformed Paleogene section. However, Neogene deposition in the Euphrates seems to be more closely related to the Zagros continental collision and formation of the Mesopotamian foredeep to the east.
Recent oil discoveries in the Euphrates are largely in Lower Cretaceous sandstones which has been charged by down-faulted Upper Cretaceous, syn-rift sediments. Most of the reservoirs are located in the deepest parts of the rift, suggesting that maturity of the Synrift sediments is a crucial control in field productivity. The discovery, through seismic refraction interpretation by Brew et al. (1997) of a thick Paleozoic sequence in eastern Syria also demonstrates the possibility of future Paleozoic hydrocarbon plays. The presence of source and reservoir rocks within the Paleozoic has previously been established and Paleozoic discoveries could form an appreciable component of future production from the region. A detailed study of the petroliferous southeast part of the Euphrates system was the focus of Litak et al. (1998).
Expressed by two topographic highs, northeast Syria contains the oldest producing hydrocarbon fields in the country. Caught between the Alpine / Zagros collision in the northeast, and the Palmyrides and Euphrates in the southwest, this was a compelling target for investigation. As much of the area is covered with thick Cenozoic cover, geophysical data provided the bulk of our observations.
Our findings from northeast Syria have been presented by Brew et al. (1999). It has been documented that the during Late Paleozoic and Early Mesozoic time northeast Syria was a sedimentary trough, probably an extension of the Palmyride trough to the southwest. Subsidence in the northeast took place across a broad zone, and was only moderately fault-controlled.
Regional emergence and consequent erosion during Late Jurassic / Early Cretaceous time left Jurassic sediments preserved in only the deepest part of the trough. Subsidence continued following this erosion.
A marked change took place toward the end of the Cretaceous. East-west trending faults formed throughout northeast Syria in response to regional extension. Large thicknesses of Maastrichtian age marly limestones were thus deposited. This extensional episode appears to have ended abruptly at the end of Cretaceous time, in concert with the abortion of rifting in the Euphrates and other regional tectonic events, suggesting a common cause.
In Late Pliocene to Recent time northeast Syria has experienced contractional tectonics induced by the Alpine collision the Arabian and Eurasian plates. This has led to structural inversion in northeast Syria. In particular, the latest Cretaceous east-west trending normal faults have been reactivated in a reverse sense. This caused the formation of fault-propagation folds above the older normal faults. These anticlines form the trapping mechanism for many of the oil fields in northeast Syria.
In recently completed work, we have studied the Ghab depression, a pull-apart basin which lies along the Dead Sea fault system in the northwest of the Syria. This work was abstracted by (Al-Imam et al., (1996); Zaza et al., (1996) and our work extended these preliminary observations.
The history of movement on the Dead Sea Fault system in Syria in controversial. The active tectonics of the Ghab Basin appear to attest to recent movement on the fault. The Ghab Basin has formed at a complex step-over in the fault system and is filled with mainly Pliocene sediments. Two depocenters are found in the basin. The southern, larger one, is over 3 km deep. A subsurface ridge cuts across the basin separating the two depocenters. Both depocenters display asymmetry relating to the movement on the fault.
The Syrian Coastal Ranges, directly to the west of the Ghab Basin, form a prominent topographic and structural high. Analysis of this structure suggests that the uplift is a direct consequence of movement on the Dead Sea Fault System. All of these findings, and more, are discussed in Brew et al. (2001).
In related work, Khair et al. (1993) interpreted Bouguer gravity observations from across the Dead Sea transform fault and adjacent areas in Lebanon.
Several summaries of Syria Project work, and regional syntheses based on our results, have been written. An early example by Barazangi et al., (1993) concentrated mainly on western Syria. This is complimented by a more sedimentologically based study by Best et al. (1993).
More recently, Sawaf et al. (2001) have written a comprehensive synthesis and integration of conclusions. This work summarizes findings from past studies of the Palmyride and Euphrates areas, and combines them with significant new results concerning the stratigraphy and subsidence history of the Palmyrides. This work sets the deformation with the northern Arabian platform in the context of closure of the NeoTethys Ocean.
The current work of the Syria Project involves the construction of tectonic and structural maps for the whole country. This will included analysis of Landsat imagery as well as subsurface data.
As mentioned previously many of the tectonic episodes in Syria are found to be contemporaneous in the different tectonic zones. A preliminary regional tectonic model illustrating these connections was presented by Brew et al. (1999). The current regional mapping work at Cornell aims to expand and improve upon this.
On a regional scale, it has been discovered through refraction
interpretation that metamorphic basement depth to the southwest of the Euphrates
trend is greater by at least 2.5 km than that to the northeast. These findings
are very similar to the deeper basement trends discovered south
of the Palmyrides compared to basement depth to the north of the mountain
belt. Gravity interpretation across the Euphrates trend has also established
that crustal rocks to the southwest of the Euphrates are of lower density than
those to the northeast. Again this is similar to results from gravity modeling
over the Palmyrides and adjacent areas which were discussed earlier. These discoveries
have led to speculation of a system of suture zones which extend beneath the
thick sedimentary cover of the northern Arabian platform. It is well known that
the Arabian shield accreted from several microplates and island arcs during
the late Proterozoic. It is possible that a similar pattern of accretion in
the northern Arabian platform could explain the findings of variations in basement
depth and crustal characteristics. Thus the Palmyride mountains and Euphrates
fault system may have formed along suture zones. Hence the northern Arabian
platform could be considered as a series a structurally high and relatively
undeformed regions (the Rutbah uplift, Aleppo plateau, Mardin high and Rawda
high) bounded by weak, mobile zones that have been repeatedly reactivated by
movement along nearly plate boundaries.
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For questions or comments, please contact Muawia Barazangi: email@example.com.
Last updated: August 2006