Structure and Evolution of the Petroliferous Euphrates Graben System, Southeast Syria1

Robert K. Litak,2 Muawia Barazangi,3 Graham Brew,3 Tarif Sawaf,4 Anwar Al-Imam,4 and Wasif Al-Youssef4

ABSTRACT Cenozoic of some structures also is evi- dent. Approximately 30 oil fields have been dis- The northwest-trending Euphrates graben sys- covered in the Euphrates graben system since tem is an aborted intracontinental of Late 1984. Recoverable reserves discovered to date age that has subsequently been hidden reportedly exceed 1 billion barrels of oil and less- by Cenozoic burial. Approximately 100 km wide, er amounts of gas. Light oil is primarily found in the system comprises an extensive network of Lower Cretaceous reservoirs juxtaposed grabens and half grabens extending some 160 km by normal faulting against Upper Cretaceous synrift from the Anah graben in western Iraq to the sources and seals. Palmyride fold belt in central Syria, where it becomes more subdued. The youngest prerift rocks are presently at a maximum depth of about 5 km. INTRODUCTION Based primarily on interpretation of 1500 km of seismic reflection profiles and data from 35 wells, Recent detailed studies on a number of continen- we mapped a complex network of numerous tal have shed considerable light on the archi- branching normal and strike-slip faults, generally tecture and evolution of these types of basins (e.g., striking northwest and west-northwest. Both Rosendahl, 1987; Morley, 1995). Continental rifts branched and single-strand linear normal faults of hosting major hydrocarbon accumulations include generally steep dip, as well as positive and negative the North Sea (e.g., Stewart et al., 1992), Gulf of flower structures, are manifest on seismic sections. Suez (e.g., Patton et al., 1994), and many others. No single rift-bounding fault is observed; instead, a The Euphrates graben system in southeastern Syria major flexure coupled with minor normal faulting is an aborted continental rift that also holds signifi- marks the southwestern edge of the basin, with cant petroleum reserves, but a comprehensive anal- considerable variation along strike. To the north- ysis of its development has been unavailable before east, deformation diminishes on the Rawda high now in the public literature. In this study, we near the Iraqi border. describe the structure of, and interpret the evolu- The Euphrates graben system likely formed in a tion of, the Euphrates graben system. transtensional regime, with active rifting primari- Much of the northern consists of a ly restricted to the Senonian and with an estimat- mosaic of relatively stable blocks, including the ed maximum extension of about 6 km. Minor Rutbah uplift, the Aleppo plateau, and the Rawda (Khleissia) and Mardin highs, bounded by zones of persistent, episodic instability (Figure 1). Deforma- ©Copyright 1998. The American Association of Petroleum Geologists. All rights reserved. tion in many of these unstable zones, including the 1Manuscript received July 22, 1996; revised manuscript received May 29, Palmyride fold belt, Euphrates/Anah fault system, 1997; final acceptance February 3, 1998. Azraq/Sirhan graben, and Sinjar and Abd El Aziz 2Institute for the Study of the Continents, Snee Hall, Cornell University, Ithaca, New York 14853. Present address: 218 Alden Avenue, New Haven, uplifts, can be traced back at least to the Connecticut 06515-2112. and possibly to the late Proterozoic (e.g., Best et 3Institute for the Study of the Continents, Snee Hall, Cornell University, al., 1993; Litak et al., 1997). Deformation within Ithaca, New York 14853. 4Syrian Petroleum Company, Ministry of Petroleum and Mineral the mobile zones has varied through space and Resources, Damascus, Syria. time, partly dependent upon the orientation of the Seismic and well data for this study were kindly provided by the Syrian Petroleum Company. This research is supported by Amoco, ARCO, Exxon, zones, and apparently has reflected tectonic Mobil, Unocal, and Conoco. D. Seber and W. Beauchamp reviewed a draft of events at nearby plate boundaries (e.g., Chaimov this manuscript. We greatly appreciate the comments of reviewers T. Patton, et al., 1992; Barazangi et al., 1993). Recent studies N. Gorur, P. Yilmaz, and former Elected Editor K. Biddle that helped to improve the manuscript. Institute for the Study of the Continents contribution of the Palmyrides have elucidated much of their number 230. tectonic history, from a possible Proterozoic suture

AAPG Bulletin, V. 82, No. 6 (June 1998), P. 1173Ð1190. 1173 1174 Euphrates Graben System

(A) (B)

Black Sea Caspian Sea

East Anatolian Fault Zagros Fold Belt o TURKEY 40 N Suture Bitli N Bitlis s S Mardin High E. Anatolian Flt. u yyyyyyyyyyyyyyyyyyytu re Turkey yyyyyyyyyyyyyyyyyyy

yyyyyyyyyyyyyyyyyyyZagros Thrust Zone

Mediterranean Sea yyyyyyyyyyyyyyyyyyy Sinjar-Abd El Aziz Uplift S Y R I A Euphrates R. yyyyyyyyyyyyyyyyyyyZagros Fold Belt IRAN Dead Sea Aleppo Rawda yyyyyyyyyyyyyyyyyyy Fault Fig. 2 Plateau ? High System yyyyyyyyyyyyyyyyyyyyyyyyyyyyyy Euphrates Depression yyyyyyyyyyyyyyyyyyyyyyyyyyyyyy o

yyyyyyyyyyyyyyyyyyyyyyyyyyyyyy 30 A yyyyyyyyyyyyyyyyyyyyyyyyyyyyyy ra Bishri F. Anah yyyyyyyyyyyyyyyyyyyyyyyyyyyyyybia Graben n Sea Palmyrides Abu Jir yyyyyyyyyyyyyyyyyyyyyyyyyyyyyyNajd Faults G Red ulf Mediterranean yyyyyyyyyyyyyyyyyyyyyyyyyyyyyy Rutbah

yyyyyyyyyyyArabian Uplift

yyyyyyyyyyy Fig. 2 Iraq yyyyyyyyyyyShield

yyyyyyyyyyy o A F R I C A Jordan yyyyyyyyyyy 20 Normal Fault

yyyyyyyyyyy

Sea yyyyyyyyyyy Thrust Fault

yyyyyyyyyyy

yyyyyyyyyyy Dead Sea Fault System 100 km 500 km Gulf of Aden Arabian Sea O O 40 50 E Figure 1—(A) Map showing simplified tectonic setting of the Arabian plate. (B) Map showing location of study region and nearby tectonic features. Location of the area shown in Figure 2 also is shown.

zone (Best et al., 1990) to a Mesozoic aulacogen 1995). In the context of both continental rifting (Ponikarov, 1967; McBride et al., 1990; Best et al., and regional tectonics, this study focuses on the 1993), to the Late Cretaceous and Cenozoic inver- structural and geologic evolution of the Euphrates sion and uplift (e.g., Al-Saad et al., 1992; Chaimov graben system and its subsequent burial and partial et al., 1992; Searle, 1994). inversion during the Cenozoic. Lovelock (1984) briefly described the Euphrates graben and “Al-Furat fault” in a review of regional tectonic elements. Sawaf et al. (1993) presented a DATABASE crustal-scale cross section across eastern Syria. In two recent papers, Alsdorf et al. (1995) described The principal data for this study include 1500 the junction between the Palmyrides and the km of seismic reflection profiles and information Euphrates depression, and Litak et al. (1997) dis- from 35 exploratory wells (Figure 2) covering an cussed the first-order structures and regional tec- area of approximately 13,000 km2. These data are a tonic framework along the length of the northwest- subset of a considerably larger database provided trending Euphrates (Al-Furat) fault system by the Syrian Petroleum Company as part of an throughout Syria. Brew et al. (1997) used seismic ongoing cooperative joint project with Cornell refraction and other data to determine depth to University. Seismic data were collected by a variety metamorphic basement and to examine upper of contractors throughout the 1980s, and most of crustal structure in eastern Syria. the lines are migrated. An array of vibrators provid- Late Cretaceous rift structures are best devel- ed the source for most of the data, although explo- oped in southeastern Syria, herein referred to as sive sources were used on several lines in the the Euphrates graben system. These structures, Euphrates River valley. Seismic data were interpret- now buried by up to 2.5 km of Cenozoic sedimen- ed in paper-record form. Stratigraphic tops were tary rocks, are largely responsible for the consider- available for all wells, and various logs and litholog- able oil reserves that have been discovered in the ic information were available for several wells, region during the past decade (de Ruiter et al., including sonic logs for six wells. The sonic logs Litak et al. 1175

40° 41° Figure 2—Database map of southeastern Syria showing location of oil fields, wells, and seismic profiles. See Figure 1 for

R. area location. Line AA′ denotes Euphrates line of cross section in Figure 5. Bold portions of seismic lines are ° shown in Figures 6, 7, 9, and 13. 35 R. Khabour A

Thayyem Rasin IRAQ Qahar Tayyani

SM 80-18 Dablan

AS-703

PS-417 Oil 34° fields PS-255 SYRIA Swab Wells: Named Unnamed A N 50 km Seismic lines

were digitized to produce synthetic seismograms correlatable event (McBride et al., 1990; Sawaf et and velocity information for depth-time conversion al., 1993), disappearing below 4 s beneath the to facilitate well ties to the seismic data. deepest part of the Euphrates graben system. Brew et al. (1997) determined basement depth beneath the Euphrates graben system to be around 9 km. Stratigraphy and Seismic Character On some seismic lines, the Burj marker is discor- dant with reflections observed below it, which The stratigraphy of southeastern Syria records a may represent Proterozoic sedimentary rocks [as clastic-dominated section more than 4 suggested by Seber et al.(1993)] beneath an km thick underlying Triassic–Neogene carbonates Eocambrian (Pan-African) . Good and interbedded with lesser amounts of regional reflections from the top of the Upper and sandstone (Figure 3). The stratigraphy of Ordovician Affendi and Lower Ordovician Swab eastern Syria is described in some detail by Sawaf et formations can be ascribed to sharp shale/sand- al. (1993), and lithostratigraphic charts have been stone and sandstone/shale discontinuities, respec- published by Lababidi and Hamdan (1985); there- tively. More than 3400 m of Ordovician section is fore, we concentrate on the major packages of rock penetrated in the Swab well (location shown in and their seismic signatures. Based on refraction Figure 2). The poorly reflective Silurian Tanf and wide-angle reflection data, Seber et al. (1993) Formation, up to 750 m thick, contains a sandstone and Brew et al. (1997) estimated the minimum unit underlying organic thought to represent thickness of the entire sedimentary section to be a good potential source rock (Beydoun, 1991). No over 8 km on the Rutbah uplift. The Paleozoic sec- Devonian rocks have been recognized in Syria, but tion is generally poorly reflective with a few promi- the unconformity (D on Figure 3) has little struc- nent exceptions (Al-Saad et al., 1992). A strong, tural relief, making its seismic identification prob- continuous, multicyclic reflection from the Middle lematic. Lower rocks of the Markada Cambrian Burj limestone is the deepest regionally Formation are penetrated in several wells. These 1176 Euphrates Graben System

Figure 3—Generalized stratigraphic column for Age Formation Maximum Description Notes eastern Syria. Thickness (m)

Quaternary conglomerate w/ Bakhtiary sandstone Pliocene 1000 Minor Upper Fars sandstone w/ shale reactivation; Lower Fars 700 anhydrite, gypsum, sh. transpression Jeribe 125 dolomitic lmst. w/ anhyd. Dibbane 250 anhydrite over halite

Tertiary Euphrates 200 limestone & marly ls. Mapped horizon Chilou 500 limestone & marly ls. on Figure 11 Jaddala 600 marly ls.& limestone Mapped horizon Kermav (Aliji) 600 marl & marly shale on Figure 10 Maastricht. Shiranish 1600 marl & argil. limestone A Main phase Soukhne/Massive 700 marl over cherty lmst. of rifting Derro 150 red congl. sandstone Upper Judea dolomitic ls.w/ anhydrite B 1200 Mapped horizon Cretaceous Lower Rutba 500 sst. over some on Figure 4 Sarjelu 700 dolomite w/ shale C Allan/Muss dolomite w/ shale Upper 150 Adaya 150 shale, anhyd. & dolomite Butma 300 dolomite w/ shale

Triassic Kurrachine Anhydrite anhydrite Middle 400 Kurrachine Dolomite 800 dolomite w/ some ls. Lower Amanous Shale 250 shale Amanous 750 sandstone Carboniferous Markada 1200 sst. w/ some dolomite Lower Silurian Tanf 750 shale w/ interbed. sltst D Affendi 1000 sandstone w/ shale Ordovician Swab 1000? shale w/ minor sst. Khanasser 1500? sandstone Sosink 750+ quartzitic sandstone Cambrian Burj 150+ dolomite limestone Zabuk ? quartzitic sandstone (?) rocks are up to 1 km thick and consist mainly of Bishri and Sinjar areas (and considerably more interbedded and shales; these rocks are along the Levantine margin), but are missing in the generally less reflective than the overlying section. Euphrates area, as is the Neocomian section. This Locally, however, a dolomite marker forms a promi- Early Cretaceous unconformity (C on Figure 3) is nent seismic horizon. The Upper Carboniferous, nearly concordant with underlying Triassic rocks, Permian, and Lower Triassic section is generally and may represent a regional uplift that eroded absent in the study area (Sawaf et al., 1993). and (partially) Triassic rocks, or may rep- The Triassic–Santonian section in eastern Syria resent simply a prolonged hiatus (Best et al., is dominated by carbonates and evaporites, with 1993). Several wells also encounter a thin section lesser amounts of shale, chert, and an areally of basalt in the Lower Cretaceous and lower restricted Lower Cretaceous sandstone. This sand- Upper Cretaceous. These volcanic rocks are found stone, the Rutba Formation, was most likely in a number of wells throughout Syria and proba- deposited in a deltaic environment and is an bly are related to regional extension rather than important hydrocarbon reservoir in the area. the onset of the Euphrates graben sensu stricto. Marked thinning of the Upper Triassic section onto Because of its lithologic heterogeneity, the entire the Rutbah uplift is noted on several seismic lines Triassic–Lower Cretaceous section often forms a and is attributed to the emergence of the uplift distinctive zone of high-amplitude reflections that during this time (Sawaf et al., 1993). Jurassic car- ranges in seismic thickness up to several hundred bonates attain a thickness of over 800 m in the milliseconds. Litak et al. 1177

Due to the presence of several , nearest available seismic line by considering seis- the completeness of the Cretaceous section differs mic character and structural trends. Data quality greatly from well to well. The late Campanian– varies, with mapping generally most reliable in the Shiranish Formation is present southwestern part of the study area, and less reli- throughout the area, although its thickness varies able in the deeper parts of the graben system. dramatically. This formation primarily consists of Where strike lines are sparse, fault correlations are marl having poor seismic reflectivity; strong local made on the basis of similar seismic expression and reflections within the Shiranish interval are likely consistency of structural levels. due to interbedded limestone or sandstone. The Deformation is distributed over a wide area Shiranish Formation overlies either the Upper among many closely spaced faults; only the larger Cretaceous Soukhne or Judea, the Lower Cretaceous or more continuous faults are shown. Faults gener- Rutba, the Triassic Serjelu (Mollussa F), or the ally trend northwest to west-northwest, with only Carboniferous Markada formations (Figure 3). The minor branches and transfer zones of differing ori- Coniacian Derro Formation, unconformably bound- entations. The rift axis generally coincides with the ed at both top and base, is present in only eight Euphrates River valley, with the deepest part just of the available wells. The base Coniacian unconfor- north of the junction with the Khabour River mity (B on Figure 3) probably signals the onset (Figure 4). This map documents two sets of fault of rifting in the Euphrates graben (Lovelock, 1984), domains. In the north and northwest, faults strike and generally exhibits the most pronounced discor- west-northwest, with several large normal faults dance of any Cretaceous unconformity in the study typical of a classical graben. In the south and south- area. In addition, a seismic discordance of lower east, however, faults strike more northwest, with relief, but considerable extent, is apparent in sever- more diffuse deformation and near-vertical flower- al areas within the Shiranish itself. This upper type structures suggesting strike-slip motion (e.g., unconformity (A on Figure 3) appears to be espe- Harding, 1985). Deformation is transferred in a step cially notable in the northwestern part of the study wise fashion toward the southeast, with down-to- area, and is here interpreted as the synrift/postrift the-north fault motion transferred from the Deir boundary. Azzor fault to the Thayyem fault, and eventually to rocks (Chilou, Jaddala, and Kermav the El Ward fault (Figure 4). The El Ward fault formations) grade upward from marl and calcareous exhibits progressively less throw to the southeast, shale to limestone, and underlie the lower and mid- becoming more of a hinge or flexure. Several faults dle Miocene evaporites of the Dibbane, Jeribe, and (notably the El Ward fault) exhibit a convex shape Lower Fars formations (Figure 3), deposited in open in map view. This pattern is unusual in continental or sometimes restricted seas (Sawaf et al., 1993). rifts, with the “ideal” half graben of Rosendahl Less than about 400 m of upper Neogene and (1987) being of concave arcuate shape; however, Quaternary continental clastics (Upper Fars and similar faults have been reported in the Suez rift by Bakhtiari formations) are exposed at the surface Patton et al. (1994). We found no evidence for over much of the study area. The low-amplitude, major transverse structures such as those reported discontinuous seismic reflections of these units in the (e.g., Rosendahl et al., 1986). contrast sharply with the strong continuous reflec- The general structure of the Euphrates graben tions of the underlying Miocene evaporites. system is illustrated by a seismically derived cross section (Figure 5). Relatively steep (>70°), planar normal faults are approximately symmetrically dis- STRUCTURAL DEVELOPMENT OF THE tributed around a central graben. Many of these EUPHRATES GRABEN SYSTEM faults clearly offset or disrupt the Cambrian Burj Formation between 3 and 4 s, corresponding to Seismic Interpretation approximately 5–8 km (Figures 5, 6), indicating that they penetrate deep into the section, probably A time-structure map on the base of the Upper into basement. Basement depth along Cretaceous (the Rutba Formation over most of the this cross section is between approximately 6 and study area) illuminates the extent and style of defor- 8.5+ km (Brew et al., 1997). Faults are spaced irreg- mation of the Euphrates graben system (Figure 4). ularly about every 5 km, and are mostly depicted Sonic logs were available for at least portions of six here as single faults with occasional synthetic or wells; these logs were digitized to produce synthet- antithetic faults; however, only major faults are ic seismograms that were tied to the nearest seis- shown, and observed disruption of seismic data mic lines. The resulting velocity information was suggests numerous additional minor faults. With used to tie in stratigraphic tops from the additional few exceptions, faults do not extend upward into 29 wells in the study area. Where necessary, wells the Tertiary section, constraining the main phase of were projected up to several kilometers to the active extension to the Late Cretaceous. From the 1178 Euphrates Graben System

Euphrates 40° 41°

R.

R. 1.5 F. 1.5 1.5 35° Khabour « Al-Bishri A S. Deir 2.0 Azzor 2.0 F.

2.0

2.0 Thayyem F. 2.5 1.5

1.5 2.5 2.0

2.0 1.5 1.5

2.0

2.0 1.0 1.5 2.0 2.0 2.0

El 1.5 1.5 Ward

1.0 F.

1.5 N 1.5

1.5 contour interval 100 ms 1.5 34°

> 2.0 s 1.0 Al Faydat > 2.5 s F. 20 km IRAQ SYRIA A

Figure 4—Time-structure map of the base Upper Cretaceous horizon, in seconds two-way traveltime. Large west- northwest–striking normal faults (Deir Azzor, Thayyem) dominate in the northwestern part of the area, whereas numerous northwest-striking normal and strike-slip faults abound near the Iraqi border, with no single fault domi- nant. Seismic and well control shown in Figure 2. AA′ denotes line of cross section shown in Figure 5. relatively undeformed section of the Rutbah uplift the Euphrates graben system is sometimes referred to the southwest, minor down-to-the-northeast to as the El Madabe step (Sawaf et al., 1993). The faulting is apparent in front of a flexural feature deepest part of the main graben occurs just east of where Upper Cretaceous and Tertiary rocks begin the Euphrates River, where the dominant fault dip to thicken fairly abruptly (Figure 5). This “edge” of changes from northeast to southwest. This central Rutbah SW Uplift Euphrates R. NE A Swab Dablan Tayyani Qahar Rasin A' 0

  2 ? ?

4

6 Depth (km)

 

8

0 20 40 60 80 100 120 140 160 Distance (km)

Neogene (Ng) Lower Cretaceous (Kl) Ordovician - U. Cambrian VERTICAL EXAGGERATION 3 x

 Paleogene (Pg) Triassic (Tr) Pre Upper Cambrian

Upper Cretaceous (Ku) Carboniferous - Silurian (Pzu)

Figure 5—Structural cross section across the Euphrates graben system based on seismic interpretation constrained by well data. Location shown in Figures 2 and 4. 1180 Euphrates Graben System

SW PS-255 NE Figure 6—Seismic example 0 of southwestern edge of Euphrates graben system. Neogene Pzu = rocks of Paleozoic age younger than Paleogene Ordovician. See Figure 2 1 for location. Abbreviations Ku are explained in Figure 5. Kl

(s) Tr Pzu time

2 Travel

Ord Two-Way

3

C

4 2 km graben area approximately coincides with the the Euphrates River and the Rawda high; however, main oil-producing trend of the Euphrates play Paleogene rocks continue to thin over this sec- (Figure 2). The northeastern portion of the tran- ondary graben, in contrast to the central part of the sect displays a series of southwest-dipping half Euphrates graben system. A southwest-dipping grabens bounded by northeast-dipping planar nor- monocline can be dated to the Neogene (Figure 5). mal faults (Figure 5). A local structural high at The monocline approximately coincides spatially Lower Cretaceous levels occurs near the Rasin with the underlying half grabens, suggesting minor well. Based on limited data (see Figure 2), we inter- Neogene reactivation of the earlier structure. pret the area northeast of this local high as another In the vicinity of the El Madabe step the Con- zone of Late Cretaceous normal faulting between iacian unconformity (B on Figure 3) is most clearly

SW PS-417 NE Figure 7—Seismic example 0 of Coniacian unconformity (B on Figure 3) truncating Lower Cretaceous rocks at Neogene transition from Rutbah uplift to Euphrates graben system. Pzu = rocks of

(s) Paleozoic age younger than Ordovician. See Figure 2 for

time Paleogene 1 line location. Abbreviations are explained in Figure 5. Travel Ku

Kl Tr Two-Way

Pzu

2

Ord?

1 km Litak et al. 1181

40° 41°E well control indicates that the pre-Coniacian Judea Formation is thicker on top of this structure Euphrates R. than in the downthrown fault block, signifying 100 that the Judea has not been cut out by the uncon- R. ° Khabour formity. Thus, we interpret this unconformity (A 35 on Figure 3) to occur within the Shiranish (Late 0 Campanian–Maastrichtian) rather than the Coniacian. In fact, the Santonian–early Campanian Soukhne Formation is thinner on the structure IRAQ 100 than elsewhere, consistent with a post–early Campanian age for the unconformity. The major 100 0 0 thickening, from 200 to 500 ms, occurs in the Campanian–Maastrichtian Shiranish Formation, dat- ing the main phase of extension to that time. Occurring near the top of the Shiranish Formation as seen in Figure 9, the unconformity is likely of late Maastrichtian age (A on Figure 3), and may rep- 34° N resent the boundary between the synrift and SYRIA postrift sequences (Litak et al., 1997). Note also the slight rollover of Neogene reflections, with a north- 50 km western (counterregional) dip suggesting minor inversion of this structure. The overall postrift structure is illustrated by a generalized depth map to the top of Cretaceous Figure 8—Isopach maps of the Lower Cretaceous Rutba (thin lines) and Upper Cretaceous Judea (heavy lines) rocks (Figure 10). This map, based primarily on formations (see Figure 3). The subcrop pattern suggests well tops and guided by seismic data, shows a subsequent erosion along the graben flanks. Contour broad depression centered just east of the junction interval 50 m. between the Euphrates and Khabour rivers, with a maximum depth exceeding 2600 m BSL. At the Oligocene level, the basin is much broader, with evident. This unconformity progressively cuts out Neogene deposits (primarily the upper Miocene Lower Cretaceous and then Triassic rocks to the Lower Fars Formation) extending over much of east- southwest as it merges with base Cretaceous and ern Syria (Figure 11). Subtracting these two maps Upper Triassic unconformities. In wells drilled far- yields an isopach map of Paleogene rocks, inferred ther to the southwest, the Rutba Formation, the to represent the postrift sequence (Figure 12). Note principal reservoir in the area, is missing. This that the Paleogene and Neogene depocenters are off- unconformity can be mapped on seismic reflection set somewhat. This offset may indicate that, data in the area (Figure 6, 7). An isopach map of the although the existence of the Euphrates weak zone Rutba Formation based on seismic and well data likely had some influence on the location of indicates thickening along the axis of the graben sys- Neogene sedimentation, Neogene deposition may tem (Figure 8). At least part of this thickening likely have been primarily controlled by other factors, is due to erosion. The Cenomanian–Turonian Judea which may include the Zagros collision and develop- Formation pinches out closer to the graben axis than ment of the Mesopotamian foredeep. the underlying Rutba Formation (Figure 8), a sub- In the easternmost part of the study area, both crop pattern consistent with postdepositional ero- positive and negative flower structures (Harding, sion rather than onlap. This updip pinch-out of the 1985) are interpreted as indicative of strike-slip reservoir raises the possibility of a combination faulting (e.g., Figure 13). These faults also appear to stratigraphic/structural play in this area, a concept have been active during the Late Cretaceous. that, to our knowledge, has not been tested. Reactivation of these structures is commonplace An excellent example of Euphrates structure is and appears to be diachronous on different struc- shown on line SM 80-18 (Figure 9). A major normal tures. Figure 13 depicts subtle deformation of fault system drops Lower Cretaceous rocks approx- Paleogene reflections; other structures evince imately 500 ms (∼1 km) down to the northeast. growth during the (e.g., Figure 12) Footwall uplift, perhaps including a component of and Pliocene (Quaternary?). These structures bear strike-slip faulting, is apparent beneath a clear considerable similarity to mild inversion structures unconformity. Because a sonic log is not available noted in east Africa by Morley (1995). Cenozoic for the Thayyem well, the precise age of this inversion in the Euphrates graben area is minor and unconformity is difficult to ascertain; however, results in minimal shortening. 1182 Euphrates Graben System

Figure 9—Seismic example SM 80-18 showing Thayyem field SW Thayyem 103 NE 0 and fault, an example of a major normal fault into Neogene a half graben. Note the marked unconformity 1 Paleogene within the Upper Cretaceous. Pzu = rocks of Ku

Paleozoic age younger than veltime (s) ra

Ordovician. See Figure 2 T 2 Kl y Tr for location. Abbreviations a are explained in Figure 5. o-W w

T Pzu ? 3 Ord ?

4 5 km

Magnitude of Extension where ys = the synrift sedimentary thickness, the first term on the right = the combined synrift and The cross section in Figure 5 can be used to pro- postrift thickness; the second term = the postrift γ vide simple estimates of extension across the accumulation. We wish to solve for C, the crustal γ Euphrates graben system. Two methods are applied thinning factor, and L, the lithospheric thinning here, and both methods yield estimates of not more factor. Absent evidence to the contrary, these terms than 6 km total extension. First, line-length balancing are assumed to be equal. For many of the variables at the base of the Upper Cretaceous results in an esti- used in these terms, we have taken these common- mate of only about 0.9 km; however, line-length bal- ρ ly used values: m = the density of the mantle (3300 ancing commonly gives lower estimates of extension 3 ρ 3 kg/m ), c = the density of the crust (2750 kg/m ), than do other methods. In this instance, the low esti- α m = the coefficient of thermal expansivity of typi- mate of extension primarily is due to the steepness of × –5 cal rocks (3 10 /K), Tm – T0 = the difference in the faults, typically more than 70°, but their true atti- temperature between the surface and the mantle tude is poorly constrained by the seismic data (e.g., (1250 K), and yL0 = the initial lithospheric thick- see Figure 10). Because the extension along a given ness (assumed to be 180 km). fault varies as the cosine of the dip angle, slight The initial crustal thickness, yC0, is taken as 37 changes in fault dip lead to significant differences in km based on a seismic and gravity transect in bed length measurements. Also, in presenting Figure southwestern Iraq that passes close to southeast- 5 we did not consider many minor faults and splays ern Syria (Alsinawi and Al-Banna, 1990), and on that may accommodate additional extension, so by the gravity model of Sawaf et al. (1993) and Khair necessity these results represent a minimum esti- mate. Marrett and Allmendinger (1991) argued that et al. (1997) in eastern Syria. The area of the syn- small faults can account for a significant amount of rift (Late Cretaceous below the postrift unconfor- mity; A on Figure 3) section was measured from the total strain across a fault system. 2 An alternative method is to calculate the extension Figure 5, and found to be 50 km . For a rift zone directly from the excess sediment area. This method 100 km wide, the measured areas correspond to assumes that isostatic equilibrium was maintained average sediment thickness of 500 m. For the throughout rifting, and that low-density synrift and postrift (primarily Paleogene) thickness we also postrift sediments are compensated by thinning of subtracted the average thickness of postrift-age the crust and lithosphere. The isostatic balance is sediments observed outside of the rifted area. summarized by Turcotte (1983) in the following This method yields a value of 625 m. The average ρ equation: density of the synrift and postrift sediments ( s) is derived from the density log of the El Madabe ρ − ρ ( m c ) NW-101 (about 15 km northwest of the cross sec- y = y (1 − γ ) − s ρ − ρ C0 C tion). The synrift density is 2.43 kg/m3, and ( m s ) (1) postrift density is 2.22 kg/m3. An average value ρ α − m m (Tm T0 ) for the combined synrift and postrift sections, 0.5 γ (1 − γ ) (ρ − ρ ) L0 L weighted in proportion to their thicknesses, is m s thus 2.313 kg/m3. Litak et al. 1183

40° 41° 40° 41°

-1500 -500 Euphrates Euphrates R. -1000 R. -500 R. R. Khabour ° 35° 35 -500 0 -2000 Khabour

0 0 IRAQ IRAQ

-1500 -1000 -500 -500

0 0 ° 34° 34 N N SYRIA SYRIA 50 km 50 km

Figure 10—Generalized structure map, top of Cretaceous Figure 11—Generalized structure map, top of Oligocene (see Figure 3). Contours in meters below sea level. (see Figure 3). Contours in meters below sea level.

Using the values in equation 1 yields a thinning estimates are not possible with the current data set; factor of 0.946, corresponding to a stretching fac- however, comparing the number, continuity, and tor (β value of McKenzie, 1978) of 1.06. Thus, 6 km apparent seismic disruption of the normal faults of stretching has taken place, thinning the initial 37 and faults interpreted to involve some strike-slip km crust to 35 km, and thinning the lithosphere motion suggests that the magnitude of strike-slip approximately 10 km, from 180 km to 170 km. The displacement is not greater than the extensional second term of equation 1 also provides an inde- displacement. Mapping of Lower Cretaceous reflec- pendent assessment of the postrift thickness based tions provides little evidence for significant strike- on thermal subsidence after the end of rifting, slip movement. In particular, the Euphrates fault which here yields an average postrift thickness of system probably did not accommodate the nearly 604 m, compared to the observed value of 625 m. 60 km of offset necessary for the Abd El Aziz and These calculations, although necessarily nondefini- Sinjar zones to represent the northeastward contin- tive because they are based on certain assumptions uation of the Palmyrides (see Figure 1B), as suggest- (lithospheric thickness, mantle temperature, etc.), ed by Lovelock (1984). thus suggest that the Euphrates graben system underwent stretching of about 6 km, and subse- quent thermal subsidence appears to have extend- DISCUSSION ed throughout the Paleogene. The fact that lower and middle Miocene forma- Hydrocarbon Habitat tions are thin or absent also suggests that postrift sedimentation effectively ceased before the The trend of major producing oil fields in the Neogene. In addition, the late Miocene depocenter Euphrates graben system occurs in a fairway is offset somewhat to the northeast from the locus approximately 40 km wide (Figure 2). This fairway of rifting (compare Figures 10 and 11). Thus, the is centered northeast of the Euphrates River near upper Miocene section probably is dominated by Iraq and trends northwest, becoming more subsidence that may be associated with develop- west–northwesterly near the Khabour River. ment of the Mesopotamian foredeep and the Inspection of the structure map (Figure 4) reveals Zagros continental collision. that this productive trend follows the fault trends The amount of strike-slip along the Euphrates discussed, as well as the rift axis. In addition, the fault system is more difficult to quantify, and reliable larger fields generally are located near the deepest 1184 Euphrates Graben System

40° 41° relatively undeformed sealing lithology that exists

1000 above good reservoir rocks. Trap integrity is some- Euphrates R. what of a concern only in the areas that have expe- rienced significant reactivation in the northwestern

R. part of the Euphrates fault system. In addition to 35° Khabour 500 the Shiranish Formation, the shaly Derro clastics have been proven to be good seals, and several 1000 reservoir/seal pairs have been documented in the Triassic carbonates and evaporites (Beydoun, 1000 IRAQ 1991). Although the Rutba sandstone is the most prolific reservoir in the graben, other reservoir- 500 quality rocks have been documented in the Triassic and Carboniferous, although more drilling is need- 1000 ed to fully assess Paleozoic lithology. Sealing of a Carboniferous reservoir might have to rely on interbedded shales or on being upthrown against ° the Upper Cretaceous carbonates. Proven recover- 34 able reserves in the Euphrates area are estimated at N well over 1 billion barrels of light, sweet oil and 50 km SYRIA much lesser amounts of gas. Current production from the Euphrates is around 450,000 barrels per day (OAPEC Bulletin, 1996).

Figure 12—Isopach map of Paleogene rocks (see Figure 3), contours in meters. Cenozoic Reactivation In contrast to the Palmyrides (e.g., Chaimov et al., 1990), Abd El Aziz-Sinjar uplift (Sawaf et al., part of the basin. This correlation suggests that the 1993), and even the northwestern part of the location of productive fields is largely governed by Euphrates fault system (Litak et al., 1997), the the thickness and maturity of synrift source rocks. Euphrates graben system in southeastern Syria has The source rocks are principally the Soukhne and experienced relatively little inversion associated Shiranish formations, but others are documented in with the Cenozoic collision along the northern the Silurian Tanf formation (Figure 3), and possibly edge of the Arabian plate. Near the junction with within the Carboniferous (de Ruiter et al., 1995). the Palmyrides, several Late Cretaceous extensional The Thayyem oil field (Figure 9) is exemplary of structures appear to have been uplifted during the oil-producing structures in the Euphrates graben Neogene. In particular, an anticline along the system, albeit most structures are considerably sub- Euphrates River near the main graben axis may be tler. The Thayyem was the first field discovered in associated with reactivation of a Cretaceous normal the Euphrates play in 1985 (Beydoun, 1988; de fault (at approximately 34°45′N, 40°10′E on Figure Ruiter et al., 1995), and remains one of the most 11), and probably accommodates some of the dex- prolific fields. Primary production is from the tral transpression along the South Al-Bishri (Figures Rutba Formation, with additional reserves in 1B, 4) and related faults of the Palmyrides [see Miocene carbonates trapped in the inverted Alsdorf et al. (1995) for more discussion of this rollover structure. The Rutba reservoir is charged area]. In contrast to the South Al-Bishri fault, how- by Upper Cretaceous source rocks, principally the ever, there is no seismicity or topographic evidence Soukhne and Shiranish formations, downthrown to for Quaternary inversion of this structure. the normal fault system. The thick Shiranish sec- Shortening associated with this structure is limited tion also provides the seal both above and laterally, to a maximum of a few hundred meters, and proba- with closure achieved against the normal faults. bly less. Most oil fields are similarly located on structural In the northwestern part of the Euphrates fault highs associated with normal faults and rely on system, Litak et al. (1997) documented significant upthrown closure against those faults. The Rutba uplifts and clear instances of true inversion of nor- sandstone is juxtaposed against the marly shales of mal faults. Such features are not apparent in south- the Shiranish Formation by even relatively small eastern Syria. Instead, Late Cretaceous faults seem faults. Many fields of limited areal extent, but signif- to have been reactivated as positive flower struc- icant productivity, are due to the extremely numer- tures or uplifted blocks (see Figure 13). These ous minor faults of the Euphrates system and the structures may be related to their proximity to the Litak et al. 1185

SW AS-703 NE Figure 13—Seismic example 0 showing uplifted block exhibiting apparent Neogene strike-slip faulting adjacent to half-graben (to Paleogene 1 northeast). Note slight upwarping of Paleogene Ku reflections signifying (s) minor Cenozoic

time Kl&Tr reactivation. This structure 2 is the site of a reported Pzu

Travel 1994 oil and gas discovery. Ord Pzu = rocks of Paleozoic age younger than

Two-Way 3 Ordovician. See Figure 2 for location.

- 4 Mid C

2 km

east-trending Anah graben in Iraq (Dunnington, “Abu Jir” trend. Kader and Marouf (1994) discussed 1958; Ibrahim, 1979) and also accommodate mini- parallel structures farther east along the Tigris River mal shortening. Upwarping of Pliocene reflectors that appear to have a remarkably similar history to can be noted on several structures. Strike-slip the Euphrates graben system. These workers deformation also may occur along the northern described flower structures, normal faulting, and edge of the Euphrates graben system where seismic graben formation active during the Campanian– control is relatively poor. (This end of the Maastrichtian with minor late Neogene dextral Euphrates graben and its relationship to the Abd El transpression. Litak et al. (1997) attempted to fur- Aziz and Sinjar uplifts will be further explored in ther link the Najd fabric to the Sirhan graben and future research.) The presence of several linear seg- overlying basalt field in Jordan, and also suggested ments of the Euphrates River and the general coin- that the Euphrates fault system may represent a cidence of its present course with the mapped fault Proterozoic suture between the Rutbah and Rawda trends suggest some Quaternary activity along blocks. This conjecture is supported by the gravity those faults, especially in southeastern Syria modeling of Brew et al. (1997). (Ponikarov, 1967), but no significant Quaternary The fault trends mapped in this study generally offsets are observed on the few seismic lines that are consistent with the exposed Najd faults, lend- cross the Euphrates River valley. ing credence to those interpretations; however, as discussed, two fault populations of different char- acteristics and slightly different strikes can be dis- Regional Relationships tinguished. The northwest-striking faults in south- easternmost Syria are more nearly parallel to those Northwest-trending structures are apparent all of the Najd trend. Because they also appear to over the northern Arabian plate, from the Red Sea evince more strike-slip movement than the graben- to the Zagros Mountains. Beydoun (1991) suggest- type normal faults farther northwest, one interpreta- ed that these structures might all be related to the tion is that these northwest-striking faults were late Proterozoic Najd fault system exposed in Saudi oblique to the Late Cretaceous extension direction, Arabia (e.g., Stoesser and Camp, 1985; Agar, 1987; whereas the graben-type normal faults may have Husseini, 1989). The Najd deformation may have been more nearly orthogonal to extension. This sce- imparted a deep-seated regional fabric that con- nario implies that the west-northwest–striking faults trolled the locus of later episodes of deformation. initially may have formed in response to Late Lovelock (1984) conjectured that the Euphrates Cretaceous stresses rather than being reactivated fault system, after an eastward step at the Anah Proterozoic faults. That is, Late Cretaceous intraplate graben, continued along the southeast-striking strain may have been initially accommodated by the 1186 Euphrates Graben System

Figure 14—Series of (A) schematic cross sections End Early Cretaceous illustrating conceptual SW NE Lower Cretaceous evolution of the Euphrates graben system. Triassic Darkest shaded area Paleozoic represents most recent (B) sedimentation in Coniacian each case. SW NE

Triassic Paleozoic ? ?

(C) SW Maastrichtian NE

Triassic Paleozoic ? ?

(D) SW End Paleogene NE

Upper Cretaceous Triassic Paleozoic

Lower Cret.

(E) Late Neogene SW NE

Paleogene Upper Cretaceous Lower Cret. Triassic Paleozoic

? ? Not to scale

Najd faults, but later broke through a new set of Comparison With Other Rifts more favorably oriented structures. This interpreta- tion would suggest that Late Cretaceous motion of At about 160 × 90 km, the scale of the Euphrates the Rawda block relative to the Rutbah block was graben system in southeast Syria is roughly compa- north–northeast rather than north as implied by rable to both the North and South Viking grabens Lovelock (1984) and Litak et al. (1997). Alternately, (e.g., Beach et al., 1987), and to the half grabens of the change in fault orientation may reflect a change northern Lake Tanganyika (Rosendahl et al., 1986). in the preexisting structures at the “corner” of the The Euphrates graben system is slightly larger than Rutbah block formed by the intersection of the the largest basins in the Rio Grande rift (e.g., Euphrates and Palmyrides trends. Russell and Snelson, 1994), and about the same Litak et al. 1187 width, but one-half the length, of the Suez rift (e.g., Although not ubiquitous, high-angle faults and Chénet et al., 1987). Thus, the Euphrates system deep-water sedimentation were argued by Morley probably would be classified as a rift unit in the (1995) to be common features at early stages of con- nomenclature of Rosendahl (1987), although its tinental extension. Their presence here again sug- aspect ratio is approximately one-half that of typi- gests that the Euphrates rift aborted early on. On the cal rifts. In this nomenclature, the combination of northwestern margin of the Española basin in New the Euphrates graben system and the Abu Jir zone, Mexico, Baldridge et al. (1994) also noted numerous described by Lovelock (1984) as a transtensional high-angle, planar normal faults accommodating structure, may represent a rift zone. The offset minimal extension and distributed over a broad along the Anah graben might then form an accom- zone. Baldridge et al. (1994) interpreted that zone as modation zone between these two. Litak et al. an aborted rift boundary and suggested that this (1997) demonstrated that Senonian extension, style of deformation may characterize the initial for- albeit much less pronounced, is also present along mation of rift basins, an inference consistent with the Euphrates trend northwest of the Palmyrides, our results. The cessation of rifting in the Euphrates occurring at least as far north as the Turkish border. graben may have been due to a change in the state of Our extension estimate of about 6% (β = 1.06) is stress in the Arabian plate related to the Late lower than that reported in the literature for most Cretaceous convergence associated with ophiolite other continental rifts. Kusznir and Park (1987) emplacement along the Arabian plate margin. compiled β values ranging from 1.1 to 1.3 in the Rhine graben to 1.55 to 1.9 in the North Sea central graben, with a representative average of 1.4–1.5. CONCLUSIONS Using a variety of techniques, several workers have estimated extension in the ranging The Euphrates graben system in southeast Syria is from 1.1 in the northern part (Colletta et al., 1988) an aborted continental rift that was primarily active to 1.65–1.7 in the southern part (Angelier, 1985) in the Late Cretaceous. Following Lower Cretaceous [see Patton et al. (1994) for compilation]. Bosworth deposition of the Rutba Formation (Figure 14A), (1995) recently estimated β for the central basin of extension began during the Coniacian with block the southern Gulf of Suez rift at 1.9–2.0, placing it faulting, development of a regional unconformity, as perhaps the most highly extended of failed conti- and limited deposition of continental clastics (Figure nental rifts. The relatively low value for extension 14B); however, the main phase of deformation in the Euphrates graben system signifies that it was occurred during the Campanian–Maastrichtian arrested at an early of development. (Figure 14C), with extensive normal faulting and In contrast to the “classical” rift structure as graben formation. Faulting essentially ceased by the seen in the East Africa rift (Rosendahl, 1987), the Paleocene; a Paleogene thermal sag basin (White and Euphrates graben system does not exhibit sharp McKenzie, 1988) overlies the graben system (Figure edges characterized by major listric bounding 14D). An additional phase of deposition in the upper faults. Instead, the southwestern edge of the Miocene overlies part of the Euphrates graben, but Euphrates system is marked by several faults that probably is primarily related to the Mesopotamian become progressively smaller onto the edge of the foredeep associated with the nearby Zagros conti- Rutbah uplift (Figure 5). The northwestern part of nental collision. Minor late Neogene transpression the rift (near the Palmyrides) in particular has the reactivated some of the structures in response to the gross morphology of a full graben; farther south- Zagros-Bitlis continental collision (Figure 14E). The east, the southwestern margin is manifested by a area appears to be tectonically inactive at present. steep-sided flexure with minimal throw across the The total amount of extension is minimal, proba- associated fault (Figures 6, 7). This feature has the bly not more than 6 km, but deformation is extreme- appearance of the updip end of a half graben, but ly widespread and complex considering the amount does not correspond to a major southwest-dip- of extension. This may be partly due to the presence ping normal fault. Major transverse structures, of a preexisting zone of weakness that served to dis- such as those reported in the East African rift tribute deformation across numerous preexisting (Rosendahl, 1987), are not observed. Chénet et al. faults. The amount of strike-slip displacement also is (1987) postulated that transverse faults in the inferred to be minimal. Two distinct fault popula- Suez rift followed the trend of a preexisting tions are noted: west-northwest–striking normal crustal fabric. If Najd-type faults underlie the faults with relatively large throws in the northwest- Euphrates graben system, the preexisting fabric is ern part of the study area, and steeply dipping, nearly orthogonal to the extension direction, per- northwest-striking flexures and strike-slip faults haps explaining the lack of transverse structures. nearer to the Iraqi border. Listric faults are only clearly observed in the deep- Light oil and minor gas accumulations are found est part of the graben. primarily in Lower Cretaceous clastic reservoirs 1188 Euphrates Graben System upthrown to both large and small normal faults. Chénet, P. Y., B. Colletta, J. Letouzey, G. Desforges, E. Ousset, and The charge is believed to emanate from late E. A. Zaghloul, 1987, Structures associated with extensional tectonics in the Suez rift, in M. P. Coward, J. F. Dewey, and Senonian synrift source rocks downthrown to the P. L. Hancock, eds., Continental extensional tectonics: trapping faults. Larger fields occur in the deepest Geological Society Special Publication 28, p. 551–558. part of the basin, probably due to greater thickness Colletta, B., P. Le Quellec, J. Letouzey, and I. Moretti, 1988, and increased maturity of source rocks. Longitudinal variation of the Suez rift structure (Egypt): Tectonophysics, v. 153, p. 221–233. de Ruiter, R. S. C., P. E. R. Lovelock, and N. Nabulsi, 1995, The Euphrates graben of eastern Syria: a new petroleum province REFERENCES CITED in the northern Middle East, in M. I. Al-Husseini, ed., GEO ’94: The Middle East petroleum geosciences, v. 1: Manama, Bahrain, Agar, R. A., 1987, The Najd fault system revisited; a two-way strike- Gulf PetroLink, p. 357–368. slip orogen in the Saudi Arabian : Journal of Structural Dunnington, H. V., 1958, Generation, migration, accumulation, Geology, v. 9, p. 41–48. and dissipation of oil in northern Iraq, in L. G. Weeks, ed., Al-Saad, D., T. Sawaf, A. Gebran, M. Barazangi, J. Best, and Habitat of oil: Tulsa, Oklahoma, AAPG, p. 1194–1251. T. Chaimov, 1992, Crustal structure of central Syria: the intra- Harding, T. P., 1985, Seismic characteristics and identification of continental Palmyride mountain belt: Tectonophysics, v. 207, negative flower structures, positive flower structures, and posi- p. 345–358. tive structural inversion: AAPG Bulletin, v. 69, p. 582–600. Alsdorf, D., M. Barazangi, R. Litak, D. Seber, T. Sawaf, and D. Al-Saad, Husseini, M. I., 1989, Tectonic and depositional model of late 1995, The intraplate Euphrates depression–Palmyrides moun- Precambrian–Cambrian Arabian and adjoining plates: AAPG tain belt junction and relationship to Arabian plate boundary Bulletin, v. 73, p. 1117–1131. tectonics: Annali di Geofisica, v. 38, p. 385–397. Ibrahim, M. W., 1979, Shifting depositional axes of Iraq: an out- Alsinawi, A. S., and A. S. Al-Banna, 1990, An E-W transect section line of geosynclinal history: Journal of Petroleum Geology, through central Iraq: Basement Tectonics, v. 9, p. 191–196. v. 2, p. 181–197. Angelier, J., 1985, Extension and rifting: the Zeit region, Gulf of Kader, A. B. S., and N. Z. Marouf, 1994, Tigris structures in Iraq, Suez: Journal of Structural Geology, v. 7, p. 605–612. and example of fault rejuvenation and its control of sedimen- Baldridge, W. S., J. F. Ferguson, L. W. Braile, B. Wang, K. Eckhardt, tation and hydrocarbon accumulation (abs.): Book of D. Evans, C. Schultz, B. Gilpin, G. R. Jiracek, and S. Biehler, Abstracts, Fifth Jordanian Geological Conference and Third 1994, The western margin of the Rio Grande rift in northern Geological Conference on the Middle East (GEOCOM III), New Mexico: an aborted boundary?: Geological Society of p. 165–166. America Bulletin, v. 105, p. 1538–1551. Khair, K., G. N. Tsokas, and T. Sawaf, 1997, Crustal structure of Barazangi, M., D. Seber, T. Chaimov, J. Best, R. Litak, D. Al-Saad, the northern Levant region: multiple source Werner deconvo- and T. Sawaf, 1993, Tectonic evolution of the northern Arabian lution estimates for Bouguer gravity anomalies: Geophysical plate in western Syria, in E. Mantovani, A. Morelli, and Journal International, v. 128, p. 605–616. E. Boschi, eds., Recent evolution and seismicity of the Kusznir, N. J., and R. G. Park, 1987, The extensional strength of the Mediterranean region: Netherlands, Kluwer Academic, NATO continental lithosphere: its dependence on geothermal gradient, ASI Series, p. 117–140. and crustal composition and thickness, in M. P. Coward, J. F. Beach, A., T. Bird, and A. Gibbs, 1987, Extensional tectonics and Dewey, and P. L. Hancock, eds., Continental extensional tecton- crustal structure: deep seismic reflection data from the north- ics: Geological Society Special Publication 28, p. 33–52. ern North Sea Viking graben, in M. P. Coward, J. F. Dewey, and Lababidi, M. M., and A. N. Hamdan, 1985, Preliminary lithostrati- P. L. Hancock, eds., Continental extensional tectonics: graphic correlation study in OAPEC member countries: Kuwait, Geological Society Special Publication 28, p. 467–476. Organization of Arab Petroleum Exporting Countries, 171 p. Best, J. A., M. Barazangi, D. Al-Saad, T. Sawaf, and A. Gebran, 1990, Litak, R. K., M. Barazangi, W. Beauchamp, D. Seber, G. Brew, Bouguer gravity trends and crustal structure of the Palmyride T. Sawaf, and W. Al-Youssef, 1997, Mesozoic–Cenozoic evolu- Mountain belt and surrounding northern Arabian platform in tion of the Euphrates fault system, Syria: implications for Syria: Geology, v. 18, p. 1235–1239. regional tectonics: Journal of the Geological Society of London, Best, J. A., M. Barazangi, D. Al-Saad, T. Sawaf, and A. Gebran, 1993, v. 154, p. 653–666. Continental margin evolution of the northern Arabian platform Lovelock, P. E. R., 1984, A review of the tectonics of the northern in Syria: AAPG Bulletin, v. 77, p. 173–193. Middle East region: Geological Magazine, v. 121, p. 577–587. Beydoun, Z. R., 1988, The Middle East: regional geology and Marrett, R. A., and R. W. Allmendinger, 1991, Estimates of strain petroleum resources: Beaconsfield, Scientific Press, 292 p. due to brittle faulting: sampling of fault populations: Journal of Beydoun, Z. R., 1991, Arabian plate hydrocarbon geology and Structural Geology, v. 13, p. 735–738. potential—a plate tectonic approach: AAPG Studies in Geology McBride, J. H., M. Barazangi, J. Best, D. Al-Saad, T. Sawaf, M. Al- 33, 77 p. Otri, and A. Gebran, 1990, Seismic reflection structure of Bosworth, W., 1995, A high-strain rift model for the southern Gulf of intracratonic Palmyride fold-thrust belt and surrounding Suez (Egypt), in J. J. Lambiase, ed., Hydrocarbon habitat in rift Arabian platform, Syria: AAPG Bulletin, v. 74, p. 238–259. basins: Geological Society Special Publication 80, p. 75–102. McKenzie, D., 1978, Some remarks on the development of sedi- Brew, G. E., R. K. Litak, D. Seber, M. Barazangi, A. Al-Imam, and mentary basins: Earth and Planetary Science Letters, v. 40, T. Sawaf, 1997, Basement depth and sedimentary velocity p. 25–32. structure in the northern Arabian platform, eastern Syria: Morley, C. K., 1995, Developments in the structural geology of Geophysical Journal International, v. 128, p. 617–631. rifts over the last decade and their impact on hydrocarbon Chaimov, T., M. Barazangi, D. Al-Saad, T. Sawaf, and A. Gebran, exploration, in J. J. Lambiase, ed., Hydrocarbon habitat in rift 1990, Crustal shortening in the Palmyride fold belt, Syria, and basins: Geological Society Special Publication 80, p. 1–32. implications for movement along the Dead Sea fault system: OAPEC Bulletin, 1996, News of member countries, Syria: v. 22, Tectonics, v. 9, p. 1369–1386. p. 13. Chaimov, T., M. Barazangi, D. Al-Saad, T. Sawaf, and A. Gebran, Patton, T. L., A. R. Moustafa, R. A. Nelson, and S. A. Abdine, 1994, 1992, Mesozoic and Cenozoic deformation inferred from seis- Tectonic evolution and structural setting of the Suez rift, in mic stratigraphy in the southwestern intracontinental S. M. Landon, ed., Interior rift basins: AAPG Memoir 39, p. 9–55. Palmyride fold-thrust belt, Syria: Geological Society of Ponikarov, V. P., ed., 1967, The geology of Syria: explanatory America Bulletin, v. 104, p. 704–715. notes on the geological map of Syria, scale 1:500 000, part I: Litak et al. 1189

stratigraphy, igneous rocks, and tectonics: Damascus, Syrian fold belt, Syria: Geological Society of America Bulletin, v. 106, Arab Republic, Ministry of Industry, 229 p. p. 1332–1350. Rosendahl, B. R., 1987, Architecture of continental rifts with spe- Seber, D., M. Barazangi, T. A. Chaimov, D. Al-Saad, T. Sawaf, and cial reference to east Africa: Annual Review of Earth and M. Khaddour, 1993, Upper crustal velocity structure and base- Planetary Science, v. 15, p. 445–503. ment morphology beneath the intracontinental Palmyride fold- Rosendahl, B. R., D. J. Reynolds, P. M. Lorber, C. F. Burgess, thrust belt and north Arabian platform in Syria: Geophysical J. McGill, D. Scott, J. J. Lambiase, and S. J. Derksen, 1986, Journal International, v. 113, p. 752–766. Structural expressions of rifting: lessons from Lake Tanganyika, Stewart, I. J., R. P. Rattey, and I. R. Vann, 1992, Structural style and Africa, in L. E. Frostick, R. W. Renout, I. Reid, and J. J. the habitat of hydrocarbons in the North Sea, in R. M. Larsen, Tiercelin, eds., Sedimentation in the African rifts: Geological H. Brekke, B. T. Larsen, and E. Talleraas, eds., Structural and Society Special Publication 25, p. 29–43. tectonic modeling and its application to petroleum geology: Russell, L. R., and S. Snelson, 1994, Structural style and tectonic NPF Special Publication, Elsevier, p. 197–220. evolution of the Albuquerque basin segment of the Rio Grande Stoesser, D. B., and V. E. Camp, 1985, Pan-African microplate rift, New Mexico, U.S.A., in S. M. Landon, ed., Interior rift accretion of the Arabian shield: Geological Society of America basins: AAPG Memoir 39, p. 205–258. Bulletin, v. 96, p. 817–826. Sawaf, T., D. Al-Saad, A. Gebran, M. Barazangi, J. A. Best, and Turcotte, D. L., 1983, Mechanisms of crustal deformation: Journal T. Chaimov, 1993, Structure and stratigraphy of eastern Syria of the Geological Society of London, v. 140, p. 701–724. across the Euphrates depression: Tectonophysics, v. 220, White, N., and D. McKenzie, 1988, Formation of the “steer’s head” p. 267–281. geometry of sedimentary basins by differential stretching of the Searle, M. P., 1994, Structure of the intraplate eastern Palmyride crust and mantle: Geology, v. 16, p. 250–253.

ABOUT THE AUTHORS

Robert Litak Graham Brew Bob Litak received his bachelor’s Graham Brew graduated from degree in geology from Michigan University College London, United State University in 1982. He then Kingdom, with a bachelor’s degree worked for ARCO as a geophysicist in geophysics in 1995. During his until 1986 before returning to grad- years as an undergraduate he uate school at Cornell. There, he worked for a time with RTZ Mining gained his master’s degree in 1990, and Exploration in Santiago, Chile. and Ph.D. in 1993 before joining He is currently studying for his the Cornell Syria Project as a Ph.D. in geophysics at Cornell research associate under the direc- University under the supervision of tion of M. Barazangi. His interests M. Barazangi. His interests include include regional tectonics of the Middle East and seismic seismic interpretation, regional tectonics, and potential interpretation. field methods.

Muawia Barazangi Tarif Sawaf Muawia Barazangi is a senior sci- Tarif Sawaf is a senior geologist entist and faculty member in the and associate head of the regional Department of Geological Sciences geologic department, Syrian Petro- and Institute for the Study of the leum Company. His academic Continents (INSTOC), Cornell background includes a B.S. degree University. He is the associate in applied geology from Damascus director of INSTOC, and leader and University, Syria. His professional coordinator of the Syria Project at experience includes seven years Cornell University. His academic employment as a petroleum geolo- background includes a B.S. degree gist with Sonatrach, Algeria, and he in physics and geology from currently specializes in the subsur- Damascus University, Syria, an M.S. degree in geo- face and structural geology of Syria. physics from the University of Minnesota, and a Ph.D. in seismology from Columbia University’s Lamont-Doherty Geological Observatory. His professional experience includes global tectonics, seismotectonics of continental collision zones, tectonics of the Middle East, and struc- ture of intracontinental mountain belts. 1190 Euphrates Graben System

Anwar Al-Imam Wasif Al-Youssef Anwar Al-Imam obtained both a Wasif Al-Youssef was the director B.S. degree in 1965 and a Ph.D. in of the Exploration Department, 1974 in applied geophysics from Syrian Petroleum Company, 1988– Moscow State University, Russia. 1997. Since 1972, he has held the He has worked as a lecturer in positions of director, Service- Damascus University, as director of Contract Department; director, the General Geological Survey of Petroleum Studies Department; Syria, and as head of Geophysical and chief of Petroleum Reservoir Interpretation for the Syrian Engineering. Wasif received his Petroleum Company. He also diploma-engineering degree in worked as a professor in the petroleum geology in 1972, and his Institute of Earth Sciences in Oran in Algeria, doctorate in petroleum engineering in 1993 from the 1990–1992. His interests include seismic interpretation Institute for Geology of Energy and Exploitation of and modeling. Combustible Resources, Russia. Wasif has participated in numerous energy-related conferences as a speaker.