The Dead Sea Fault and Its Effect on Civilization

The Dead Sea Fault and its Effect on Civilization Zvi Ben-Avraham, Michael Lazar, Uri Schattner, Shmuel Marco Department of Geophysics and Planetary Sciences, Tel Aviv University, Israel Abstract The Dead Sea fault (DSF) is the most impressive tectonic feature in the Middle East. It is a plate boundary, which transfers sea floor spreading in the Red Sea to the Taurus collision zone in Turkey. The DSF has influenced many aspects of this region, including seismicity and ground water availability. It may have even affected the course of human evolution. Numerous geophysical and geological studies of the Dead Sea fault pro- vide insight into its structure and evolution. Crustal structure studies have shown that the crust at the fault zone is slightly thinner than that of the regions west and east of it. A transition zone between the lower crust and the Moho under the fault was mapped. The region has a remarkable paleoseismic record going back to about 70 ka years. Several earthquakes, such as the one that occurred in the Dead Sea region on 31 BC, may have even influenced the course of history of this region. The confusion and fear inflicted by the earthquake paved the way for the expansion of Herod’s kingdom. Places such as Jericho, the oldest city in the world, which are located within the valley formed by the fault, were affected immensely by seismic activity. The DSF is an important part of the corridor through which hominids set off out of Africa. Remains of the earliest hominids are found in several sites along the Dead Sea fault, including Erk-el-Ahmar, Ubediya and Ge- sher Benot Ya’aqov. It is interesting to note that acceleration in the vertical motion along the Dead Sea fault, which produced its present physiography, began slightly before man had started his way out of Africa northwards. 148 The Dead Sea Fault and its Effect on Civilization 1. Introduction The Dead Sea fault (DSF) is the most impressive tectonic feature in the Middle East (Fig. 1). It is a left lateral transform plate boundary, separating the Arabian plate and the Sinai sub-plate. The transform has been active since the Miocene (Garfunkel, 1981; Garfunkel and Ben-Avraham, 1996) with movement continuing today. Motion is transferred along the fault from the opening of the Red Sea in the south, to the Taurus-Zagros collision zone in Turkey and Iran to the north. It is one of the most seismically active regions in the Middle East. The region has a remarkable historical and geological record of seismicity going back to about 70 ka. Several historical earthquakes have caused extensive damage in the area. Places such as Jericho, the oldest city in the world, or Bet She’an (Fig. 2), one of the largest cities in the region in Ro- man time, were greatly affected by seismic activity. Crustal structure studies have shown that the crust directly under the fault valley is somewhat different from that on the sides. These differences in crustal structure may have controlled the evolution of physiography in the region. Since the Miocene the margins of the transform were uplifted in several stages and the rift floor subsided, creating the present-day physiography. A large part of the transform is situated below sea level. The lowest place along the transform (and on Earth) is the Dead Sea basin (Ben-Avraham, 2001). The physiography of the Dead Sea fault is a result of vertical motion, which caused subsidence of the floor of the rift and uplift of its shoulders. Acceleration in the vertical motion began shortly before man started his way out of Africa. Remains of the most ancient hominids outside Africa are found along the DSF, which actually formed a corridor through which hominids set off out of Africa. The geological evolution of the DSF and the active tectonic processes occurring along its length, thus, may have affected the course of human history. 2. Crustal Structure “In his hand are the deep places of the earth: and the strength of the hills is his also. The sea is his and he made it: and his hands formed the dry land” Psalm 95:4-5 Seismic refraction studies along the DSF indicate that the crust directly under the fault valley is thinner than that on both sides. It is 33 km thick at 2. Crustal Structure 149 Fig. 1. Digital Terrain Model (DTM) of the Middle East. Inset: tectonic setting 150 The Dead Sea Fault and its Effect on Civilization Fig. 2. Sites along the Dead Sea fault mentioned in the text (modified after Horowitz, 2001) 2. Crustal Structure 151 Fig. 2 A composite section along the Dead Sea Fault from the Sea of Galilee to the Red Sea showing the calculated crustal model and the relative Bouguer anom- aly (after Ginzburg, 1981). The locations of Elat, Mt. Sodom, and En Gedi are shown in Fig. 2 152 The Dead Sea Fault and its Effect on Civilization the Dead Sea basin and shows a slight thickening towards Elat. From Elat south the crust thins gradually from a thickness of 35 km to a thickness of 27 km, 160 km south of Elat (Ginzburg et al., 1979). A discontinuity between the upper and lower crust was also observed (Fig. 3). The seismic refraction data also show a 5 km thick velocity transition zone within the lower crust above the crust-mantle boundary. In other areas adjacent to the fault the crust-mantle boundary is manifested as a sharp velocity discontinuity. However, many questions still remain as to the detailed crustal structure along the DSF (DESERT group, 2000). The Dead Sea basin itself provides evidence for dramatic activity during the Plio-Pleistocene. Crustal studies indicate that a thick sedimentary fill characterizes the two basins of the Dead Sea graben. Depth to basement is about 6 km in the northern basin and more than 12 km in the southern basin (Fig. 4) (Ginzburg and Ben-Avraham, 1997). Local earthquake data indicate the presence of lower crustal seismicity under the transform. In the Dead Sea basin, hypocenters are located almost as deep as the Moho discontinuity (Aldersons, in preparation). The changes in crustal thickness across the transform implies that it is a relatively narrow zone of deformation, which penetrates the entire crust (ten Brink et al., 1990). This unique crustal structure allowed the dramatic uplift of the transform margins and subsidence of the floor of the fault since the Miocene until present. 3. Evolution of Physiography “Every valley shall be exalted, and every mountain and hill shall be made low; and the crooked shall be made straight, and the rough places plain” Isaiah 40:4. The prominent morphotectonic expression of the DSF (Fig. 1) is character- ized by the deepest continental depressions in the world, flanked by up to ~3-km-high margins (Fig. 1). In general, the eastern shoulder rises gently towards the west, reaching its highest elevations near the transform and drops abruptly into the median valley. The elevation of the eastern shoulder is usually higher than that of the western shoulder, reflecting broad re- gional uplift along the rift (Wdowinski and Zilberman, 1997). Present-day physiography of the DSF formed as a result of continental breakup processes took place since the beginning of the Miocene through- out the Present. In the Plio-Pleistocene, a major tectonic phase uplifted the margins. During this period the Arava rift valley subsided and the Negev was uplifted and tilted (Avni, 1998). The accelerated uplift at the end of 3. Evolution of Physiography 153 Fig. 3 Velocity-depth section along Dead Sea from north of the northern basin, to the southern basin. The 2.0 km/s velocity represents the Pleistocene fill of the basin. The 4.2 km/s velocity is associated with Pliocene evaporates. The 6.0 km/s velocity represents the top of the crystalline basement, while the overlying 3.0-3.8 km/s is associated with the Tertiary to pre-Cretaceous sediments (after Ginzburg, 1997) the Pliocene and the Pleistocene affected river drainage patterns in the Negev and a massive rise in topography. Consequently large fresh water lakes developed in the southern Negev (such as Nahal Zihor, Avni, 1998; Ginat, 1996) and within the Arava valley, indicating a more humid environment (Avni, 1998). At the same time 200 m arching occurred in the Galilee (40-60 km wavelengths) dated at 1.8 Ma using basalt clasts con- straint (Matmon et al., 1999). Subsidence of the Dead Sea graben began in the early Miocene. Thick sequence of Pliocene and probably also Late Miocene evaporites, with Pleistocene sediments represent acceleration of subsidence rate (Fig. 5) (ten Brink and Ben-Avraham, 1989). Sedimentological evidence also sup- ports a low topographic relief during the Pliocene, but a high relief during the Pleistocene (Sa'ar, 1985). A detailed study of subsidence rate in the southern Dead Sea basin was carried out by Larsen et al. (2002). It has been suggested (Ginzburg and Ben-Avraham, 1997) that during subsidence, normal faults between the northern and southern basins (Ben- Avraham, 1997) induced large vertical displacements, which in turn may have caused a greater deepening of the southern Dead Sea basin. The tectonic processes that have modified the crustal structure along the Dead Sea fault are responsible for the creation of a morphological valley where a unique microclimate could develop. These ideal conditions would create a friendly environment, which would allow the migration of flora and fauna (and even man) from Africa northwards, within a wider corridor, often referred to as the Levantine Corridor (Bar-Yosef and Belfer-Cohen, 2001).

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