Robert Litak

Tectonic evolution of the Northern Arabian Plate in Western Syria

The primary geologic structures of the northern Arabian plate in western Syria include the intracontinental Palmyride mountain belt and the interplate boundary of the Dead Sea transform fault system. The Palmyride belt strikes NE and is sandwiched between two relatively stable crustal blocks of the Arabian platform: the Aleppo plateau in the north and the Rutbah uplift in the south. The Palmyrides were the site of an early Mesozoic aulacogen-type depression that was linked to the Levantine rifted continental margin in the eastern Mediterranean. The location of this postulated aulacogen may be genetically associated with a crustal zone of weakness, possibly a Proterozoic suture and/or shear zone, between the Aleppo and Rutbah crustal blocks. Uplift of the intraplate Palmyride depression initiated in the Late Cretaceous, penecontemporaneous with emplacement of ophiolites along the nearby Arabian plate boundaries in southern Turkey and western Iran. More intense episodes of shortening during the Cenozoic also appear to be temporally related to collision along nearby plate boundaries, implying that stresses have been transmitted hundreds of kilometers across the northern Arabian platform. The style and intensity of the inversion process vary considerably along the strike of the Palmyrides and involves both shortening by folding and reverse faulting as well as translation and rotation along numerous strike-slip faults. Such folds and faults clearly define at least three structurally distinct small crustal blocks within the Palmyrides. Shortening of about 20% in the southwest Palmyrides near Lebanon gradually dies out to the northeast near the intersection of the Palmyrides with the NW-trending Euphrates depression. Depth to metamorphic basement beneath the Palmyra mountain belt increases from 9 km in the northeast to 11 km in the southwest, compared with a basement depth of about 6–8 km in the adjacent Arabian platform, indicating that shortening along the Palmyrides has been insufficient to invert the previously extended basement morphology. Finally, slip measurements along the Dead Sea fault and estimates of crustal shortening in the Palmyride belt indicate that the northern segment of the seismogenic active Dead Sea fault in Lebanon and Syria is considerably younger (Pliocene) than the southern part (Miocene).

Mesozoic-Cenozoic evolution of the intraplate Euphrates fault system, Syria: implications for regional tectonics

A lack of dramatic surface geological structures along the Euphrates River in Syria belie a complex tectonic history revealed by newly released seismic reflection and well data. We document the intraplate Euphrates fault system, characterize the variation in structural style along its 350 km length in Syria, and infer its Mesozoic-Cenozoic tectonic and deformational history. We then relate the deformation of the Euphrates system and other proximate intraplate structures to nearby Arabian plate boundary processes in order to develop a new model for the tectonic evolution of the northern Arabian plate. Throughout most of Mesozoic time, the Euphrates area experienced minor deposition compared to the Palmyride trough to its southwest, and the Sinjar trough to its northeast. During latest Cretaceous time, however, significant sinistral transtension occurred along the length of the Euphrates fault system in Syria, with graben formation especially noteworthy in southeastern Syria. This episode was probably related to events at nearby plate boundaries, and may have reactivated a zone of weakness formed during Pan-African accretion of the Arabian plate. A Palaeogene sag basin formed over the graben system in southeastern Syria. Neogene continental collision along the northern and eastern Arabian plate boundaries caused minor reactivation of the Euphrates fault system in a dextral transpressional sense, in concert with significant inversion and the main phase of uplift of the nearby Palmyride and Sinjar mountains.

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

The northwest-trending Euphrates graben system is an aborted intracontinental rift of Late Cretaceous age that has subsequently been hidden by Cenozoic burial. Approximately 100 km wide, the system comprises an extensive network of grabens and half grabens extending some 160 km from the Anah graben in western Iraq to the 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. Based primarily on interpretation of 1500 km of seismic reflection profiles and data from 35 wells, we mapped a complex network of numerous branching normal and strike-slip faults, generally striking northwest and west-northwest. Both branched and single-strand linear normal faults of generally steep dip, as well as positive and negative flower structures, are manifest on seismic sections. No single rift-bounding fault is observed; instead, a major flexure coupled with minor normal faulting marks the southwestern edge of the basin, with considerable variation along strike. To the northeast, deformation diminishes on the Rawda high near the Iraqi border. The Euphrates graben system likely formed in a transtensional regime, with active rifting primarily restricted to the Senonian and with an estimated maximum extension of about 6 km. Minor Cenozoic inversion of some structures also is evident. Approximately 30 oil fields have been discovered in the Euphrates graben system since 1984. Recoverable reserves discovered to date reportedly exceed 1 billion barrels of oil and lesser amounts of gas. Light oil is primarily found in Lower Cretaceous sandstone reservoirs juxtaposed by normal faulting against Upper Cretaceous synrift sources and seals.

The Bagdad Reflection Sequence as tabular mafic intrusions: Evidence from seismic modeling of mapped exposures

Two-dimensional seismic modeling of exposed rocks implicates subhorizontal, tabular mafic intrusions as the likely cause of the unusually prominent reflections that constitute the Bagdad Reflection Sequence (BRS). These basement reflections, from depths of ∼3-15 km, are observed on COCORP and other data to extend for some 100 km across the Basin and Range/Colorado Plateau Transition Zone in west-central Arizona. The nearby Buck Mountains fortuitously expose an upended upper-crustal block, which is used in this study as the basis for a 12 x 6 km seismic model. Ray tracing through this model produces synthetic reflections from the numerous Proterozoic diabase intrusions occurring in the Buck Mountains. These synthetic reflections approximate the scale, geometry, and character of reflections observed on the COCORP data, indicating that the intrusions in the Buck Mountains furnish a viable geologic analogy for the BRS. Several lines of evidence argue that the major competing hypothesis—that Tertiary detachment surfaces cause the BRS—is less satisfactory. Although we cannot rule out the possibility that the BRS is related to hypothetical Tertiary intrusions of similar distribution and structure, geologic evidence more directly implies that these reflectors are Proterozoic in age. If so, the Bagdad reflections serve as strain markers and can be used to constrain subsequent deformation in the Arizona Transition Zone. Their relatively continuous nature thus argues against widespread disruption of the upper crust in the Transition Zone during Tertiary extension. Moreover, identification of these reflectors as subhorizontal mafic intrusions may have profound consequences for interpretation of similar reflections observed elsewhere by bolstering the contention that mafic intrusions are an important cause of basement reflections in the continental crust.

A modern perspective on the Conrad Discontinuity

V. Conrad in 1925 postulated that seismic energy propagated in a lower crustal layer with a velocity intermediate between that of the upper crust and mantel. That suggestion led to the idea of a universal or at least pervasive midcrustal velocity discontinuity, a concept that soon became widely, albeit not universally, accepted. As early studies of earthquake records were supplanted by data from controlled-source refraction surveys, references to the “Conrad discontinuity” in the continental crust became common in theliterature. As more and increasingly detailed data have been collected, the nature and even the existence of this feature has been called into question. However, some results of deep seismic reflection profiling seem to indicate a distinction between upper and lower crust, which suggests that a midcrustal boundary may indeed exist in some areas. In light of these developments and the continuing debate over the nature of the continental crust, it is appropriate at this time to examine the historical development of the concept of the Conrad discontinuity and the objections and responses and how recent data affect perspectives on the Conrad discontinuity.

Summary of the geological evolution of Syria through geophysical interpretation: Implications for hydrocarbon exploration

This paper was published in the journal The Leading Edge by the Society of Exploration Geophysicists. SEG retains the copyright to this paper. See also: http://www.edge-online.org/; http://atlas.geo.cornell.edu/syria/brew_tle_1997.html

Determination of Basement Depth And Paleozoic Thickness Using an Integrated Geophysical Approach In the Northern Arabian Platform, Eastern Syria

Basement depth and Paleozoic thickness in eastern Syria are found to be much greater than previously supposed and deep-seated faulting in the Euphrates depression is documented. Data from a high density 300 km long reversed refraction profile, with offsets up to 54 km, are analyzed and interpreted in conjunction with data from other coincident sources including seismic reflection data, various well logs and potential field data.