The Complexity of Geology of Lebanon

Lebanon is located at the intersection of the African, Arabian, and Eurasian tectonic plates. Because of the intersection of the three plates happens here, the region has a complex history and an interesting geography. Lebanon has three major mountain ranges: Mount Lebanon, the Bekaa, and the Anti-Lebanon range. Lebanon also has a western flexure and the lower regions of the Palmyrides and the Syrian arc. These features of the region all have important geological significance.

Mount Lebanon is the first of a few NNE-SSW aligned ranges (Walley, 1998). Jurassic and Cretaceous period rocks form the largest mountain range in Lebanon. Being both the largest and the closest to the coast makes Mount Lebanon a major geological feature. The range has been separated into the northern and southern zones, both of which have been useful in studying Lebanon’s geological history considering the area is hard to study and most things known about its history come from tectonic plate models (Walley, 1998). The northern zone of Mount Lebanon is mostly over one thousand meters in elevation despite its deep-cut valleys. The range has clearly been influenced by the East-West fault zones I the immediate region of the mountain range. The southern zone of the range is closer to the shoreline and is subsequently lower in elevation. Magnetic and gravitational occurrences in this region suggest that the crust is thinner. The size of the range and proximity to the coast and populated areas leads to runoff and erosion, which is one contemporary issue the geography causes for the area (Khawlie, 2002). The erosion goes back hoistoricaly as well, the Levant margin in the Miocene period was eroded because of the evolution of the Levant Facture System combined with the continued collision of Arabian with Eurasian plates (Hawie, 2013).

In addition, the elevated basin of the Bekaa is another significant mountain range in Lebanon. This, along with the Anti-Lebanon range, are the two more inland mountain ranges, encompassing Mount Hermon at the southern end of the Anti-Lebanon range. Despite differences in age and history, both follow the same general North-South direction along East-West faults. The Bekaa is interesting because it contains more deposits from the Quaternary. There are basically two major crustal uplifting that have happened within the past six thousand years affecting these ranges (Morhange, 2006). Elevated fossil benches between Tripoli and Beirut called the “Tabarjan” were first reported on the coast 40 years ago. These features ascribed to historical tectonic movements, lasting from the second century BC to the second or third centuries AD.

Although there is no current volcanic activity in the region, the geological makeup of these features shows there was a significant and notable amount of volcanic activity within the last ten million years. The Golan volcanism in particular seems to have died out very recently, likely within the last ten thousand years. Older volcanic artifacts can be seen existing within a significant number of Late Jurassic volcanic vents (Fig. #1) There is a good one at Aintoura on the Dour Choueir-Zahle road. The Western Lebanon Flexure links the region to the Israel/Palestine region along with the eastern Mediterranean region (Walley, 1998). As a feature of Mount Lebanon, the NNE–SSW monoclinal flexure zone has dips ranging between 45° to 90°. It contains both outward-dipping faults and inward-dipping high-angle reverse faults, spanning for over 100 km. There is a notable diffence on the east and west sides of the flexure: the east side carbonates show shallow-marine sediments whereas the westward side has chalkier sediments, demonstrating a historical environmental difference (Walley, 1998). The indication is clear of a basin from the Mesozic, referred to as the Lebanon hinge line.

This hinge line runs more or less along a NNE-SSW trend from eastern Sinai, around the coastline of southern Israel/Palestine. Once again we see this Jurassic and Cretaceous period development carries over into the Carmel-Sinai hinge that continues westward. Parallelism between the present coastline, the flexure, the Mount Lebanon, Bekaa and Anti-Lebanon mesostructures, and the Yammouneh Fault strongly suggests a common control on these features. The structure is likely the early Mesozoic development leading to the opening of the Levant branch into the NeoTethyan Ocean. The structure had probably formed by end Early Miocene period based on the unconformity dated surface. The Palmyride ranges of Syria is connected to Lebanon in Cenozoic history. The NE-SW-trending range has a max elevation at only 1385 meters and is flanked by Rutbah and Aleppo, to the south and north respectively (Wally, 4). The Palmyrides seem to correspond to Mount Hermon Massif of the Anti-Lebanon, but the exact relationship between the Palmyrides and the Lebanon geology is not clear.

The Palmyride Fold Belt can be discussed as its northern and southern zones. The southern zone is the major section, which has smaller, asymmetrical folds. The northern zone has larger folds that go as far back as the Paleozoic and Precambrian times. The Mount Lebanon Massif feeds into the northern zone through Aleppo. In how they compare to the southern section of Mount Lebanon, both are topographically lower lying with respect to their norther counter parts. A demonstrated bias in elevation happens in the northern area of the region. Sedimentary facies variations suggest this higher elevation around Mount Lebanon and the northern region existed from the Late Burdigalian period until present day (Hawie, 2013). The quick build-up in the Mediterranean during the Early Pliocene led to deposition of deposits that added to existing structures. Likewise, the absence of such deposits or a hinge line in the southern area of the Lebanon region corelates to connections in the southern geography, providing a historical trend of development between northern and southern structures.

The Syrian arc is a fold of belts between Egypt and Sinai through the Palmyra folds of Syria (Wally, 5). The concept originally referred to the continuous northwestern facing arc but now refers to the belt of folds that as a whole is southeastern facing. It used to be thought to be a product of the Dead Sea fault, but based on the age and expansiveness of the Syrian arc, that is now widely disbelieved. Three distinct segments exist within the Syrian Arc Fold Belt: the western, central, and eastern. The western segment is aligned NE–SW in Sinai, the central one is aligned NNE–SSW to almost N–S (a noticeable shift in curvature) in Israel/Palestine and Lebanon and the eastern segment trends NE–SW into the Palmyrides. Chronologically, the main part of the first Syrian Arc appears to have formed in early Senonian period. There is a widespread break in deposition in Israel/Palestine, which separates the Judea and Mount Scopus Groups, dated as late as the Coniacian period, making an early Senonian age seem reasonable. In Syria the timming of deformation is still up for reasonable debate and likely post-Turonian and pre-late Maastrichtian. What is known is that the first major opening of the Euphrates Graben, believed to be related to the Palmyride compression, started in the Coniacian or Santonian time. The process of formation seems to last all the way through the late Campanian into the early Maastrichtian periods.

Understanding these issues in Lebanon is an integral part of informing our knowledge of history. For example, a palaeoseismic trench investigation exposed a complex fault zone as a potential source of the double shock earthquake of in 1956 on March 16 (Nemer, 2006). The epicenter of the 1956 earthquake is right near the prohatic fault.

To summarize, the geology of Lebanon is more complex than it may at first seem and is greatly influenced by a variety of faults (the African, Arabian, and Eurasian tectonic plates) that converge on the area and have created different structures over time. The Mount Lebanon, the Bekaa, and the Anti-Lebanon ranges are the parallel structures that show most of Lebanon’s geographical history and create an impact to this day. The Western Flexure and Syrian Arc are among other significant structures in the surrounding region. The deposits and erosions that create the highs and lows of the area goes back well into the Miocene period and beyond. A great deal of similarities are seen both laterally between parallel structures and between regional trends in north and south zones. As features are assessed, such as the Syrian Arc, there continue to be new opinions on these issues, such as the dismissal of the Dead Sea fault theory for the creation of the arc.