Structure and evolution of the Dead Sea fault system in northwestern Syria (Al-Ghab Graben) from digital topography and Thematic Mapper imagery

ZAZA, Tarik, AL-IMAM, Anwar, SAWAF, Tarif, AL-YOUSSEF, Wasif (Syrian Petroleum Company, Ministry of Petroleum and Mineral Resources, Damascus, Syrian Arab Republic)

BARAZANGI, Muawia, LITAK, Robert, SEBER, Dogan (Institute for the Study of the Continents, Cornell University, Ithaca, New York 14853, USA)


The Al-Ghab pull-apart graben in northwestern Syria is the northern continuation of the Dead Sea "leaky" transform fault. Based on geological field observations, the left-lateral strike-slip motion of the Dead Sea was estimated at about 105 km south of the Sea of Galilee. About 65 km of this motion occurred in Miocene time, with the remaining 40 km during Pliocene and Quaternary time. However, only about 20 - 25 km of left-lateral strike-slip motion was previously estimated in the Al-Ghab pull-apart graben, resulting in a rate of motion ranging between 0.4 - 0.6 cm/yr.

The database for this study includes a digital elevation model, Thematic Mapper (TM) imagery, the geological map of Syria (1:200 000 scale), 250 km of seismic reflection data crosscutting the graben, the gravity map of Syria (1:500 000 scale), and all available auxiliary data. These data are used in an integrated interpretation to explain the tectonic and geologic structure of the area. Interpretation of the entire data base is presented through the following: (1) a new 1:200 000 scale tectonic map of the Al-Ghab region, (2) a new scheme for fault classification in the area, and determination of active faults along the northern portion of the Dead Sea transform fault, (3) an explanation for the deep structure of this graben/fault system.

Preliminary results indicate that the rift system in Syria was active during two phases. The first, in the Pliocene, was accompanied by continental deposition, and the second was reactivated in Quaternary along the previous faults and is still active. Large historical earthquakes have occurred along this fault system in western Syria; our results will be applied to studies of earthquake hazard assessment in the region. The thickest sediments are encountered near the southern end of the north-trending graben system. The basin shallows toward the north until it encounters an southwest-plunging horst-type subsurface structure. At this point, north- and northeast-trending secondary faults appear to be present between the two main rift- bounding faults. These secondary faults bound a series of small depressions, and form a right-stepping en- echelon geometry which progressively transfers deformation to the northeast. These secondary faults become less active toward the north. This interpretation is consistent with clockwise rotation of the northwestern part of the Arabian plate (Aleppo plateau). The western bounding fault of the Al-Ghab graben may be related to an earlier (Jurassic-Cretaceous) north-striking fault. Reconstructions of the southern part of the Al-Ghab graben indicate 24 - 28 km of left-lateral motion in the Pliocene and an additional 4 km during the Quaternary, for a total of 28 - 32 km.

In contrast to the southern part of the Dead Sea fault system, we find no evidence in the Al-Ghab area for substantial Miocene activity. Hence, the northern and southern parts of the Dead Sea fault system may have experienced considerably different histories.

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