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.Keywords: Syria, Arabian Plate, intraplate processes, seismic profiles, deformation.The northern Arabian plate comprises diverse structural elements and a variety of temporally and spatially differing structural styles. Although located in a primarily intraplate setting, the tectonic history of Syria has been profoundly affected by events along nearby plate boundaries (Fig. 1). The northern boundary of the Arabian plate represents a collision zone in southern Turkey often referred to as the Bitlis suture (e.g., Sengör & Yilmaz 1981; Hempton 1985). The northwesttrending Zagros collision zone is essentially an eastward continuation of the Bitlis zone, produced by continental collision between Arabia and Iran (e.g., Sengör & Kidd 1979; Berberian & Berberian 1981; Berberian & King 1981; Barazangi 1989). The emplacement of ophiolites during Late Cretaceous time along the northwestern, northern, and eastern margins of the Arabian plate indicates an initial episode of convergence at that time, the extent and nature of which remain controversial (e.g., Beydoun 1991). However, Tertiary island-arc-type volcanism in Iran implies that subduction of oceanic crust continued until Miocene time, when full-fledged continental collision began (e.g., Coleman-Saad 1978). Debate continues concerning the Cenozoic history of the Arabian/Anatolian boundary, with various workers favoring either continued convergence, subduction, and shortening (Yilmaz 1993) or strike-slip motion with periods including both extension and converg...

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Basement depth in the Arabian plate beneath eastern Syria is found t o be much deeper than previously supposed. Deep-seated faulting in the Euphrates fault system is also documented. Data from a detailed 300 k m long reversed refraction profile, with offsets up to 54 km, are analysed and interpreted, yielding a velocity model for the upper 9 km of continental crust. The interpretation integrates the refraction data with seismicreflection profiles, well logs and potential field data, such that the results are consistent with all available information. A model of sedimentary thicknesses and seismic velocities throughout the region is established. Basement depth o n the north side of the Euphrates is interpreted to be around 6 km, whilst south of the Euphrates basement depth is at least 8.5 km. Consequently, the potentially hydrocarbon-rich pre-Mesozoic section is shown, in places, t o be at least 7 km thick. The dramatic difference in basement depth on adjacent sides of the Euphrates graben system may suggest that the Euphrates system is a suture/shear zone, possibly inherited from Late Proterozoic accretion of the Arabian plate. Gravity modelling across the southeast Euphrates system tends to support this hypothesis. Incorporation of previous results allows us to establish the first-order trends in basement depth throughout Syria

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.

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