The geology of Lebanon
The oldest exposed rocks in Lebanon belong to the Jurassic period. The Jurassic aged rocks have a thickness of more than 1,500 m dominated mainly by limestone rocks while the total thickness of the Cretaceous rocks is approximately 1,800m dominated also by limestone rocks. The early stages of the Cenozoic period witnessed stable conditions continuing from the Cretaceous period with marly limestone as major deposits reaching a thickness of up to 900m, until the late Eocene in which the first stages of uplift of Lebanon occured. The formations from Late Eocene to Mid Miocene are missing and in the Mid Miocene some deposition occurred however, tectonism resumed after.
The deformation in Lebanon has created the major geomorphological components of Lebanon that we see today. The NNE-SSW trending mountain ranges with their steep slopes on both sides and the high altitude Bekaa plane and the narrow costal plane are all imprints of the structural deformation that has occurred starting late Eocene.
The present structural framework of Lebanon is mainly shaped by the activity of the Dead Sea Transform fault (DSTF) that connects the ongoing opening of the Red Sea and Gulf of Aqaba to the Tauros-Zagros subduction zone. When the DSTF enters the Hula depression in south Lebanon it changes orientation from N-S to NNE-SSW giving rise to a 50km broad right stepping restraining bend. In that area the DSTF also branches out into five main faults. These are the Yammouneh, Hasbaya, Rachaya, Serghaya and Roum faults and the sense and amount of motion on them is difficult to determine due to the lack of marker horizons and the fact that those that do exist cut the fault at a low angle.
The deformation along the Syrian arc from the Late Cretaceous to the present times have also shaped the structures of Lebanon but those structural elements are not clear and maybe overprinted during the earlier stages of deformation by the DSTF.
The deformation in Lebanon is not only observed as fault but also observed in several major and minor folds. Those include large in-line anticlines such as the folds of Mount Lebanon and Anit-Lebanon ranges and the syncline of the Bekaa Valley, and other minor folds that disturb the monotonous flat plane of the Bekaa valley.
Lebanon is at the active boundary between the Levant Plate and the Arabian Plate and the tectonic activity is not only documented through the tectonic framework of this area but felt through small and large earthquakes that shake the earth to the present days.
By: Issam Bou Jaoude
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