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Radon emanation along the Dead Sea transform (rift) in Jordan M.Y. Atallah á B.A. Al-Bataina á H. Mustafa

Abstract Radon emanations were sampled during summer and winter along the active faults of the Dead Sea Rift, which consists of three segments: the Wadi Araba in the south, the Dead Sea in the middle and the Jordan Valley in the north. In Wadi Araba, radon emanations were measured in three sites in gravel: two sites across the left lateral active strike slip fault zone and one site across the normal boundary fault. The highest values across the strike slip fault (maximum of 3,300 Bq/m3) were measured at the fault planes, while the lowest (minimum of 400 Bq/m3) were measured outside the fault zone. Across the normal boundary fault, higher concentrations (2,700 Bq/m3) were measured at the junction between the mountain front and the alluvial fan (where the eroded fault scarp is located). East of the Dead Sea, radon was measured in Quaternary evaporates and alluvial deposits. High concentrations were measured on the main topographic scarps, which formed as a result of normal faulting. In this area higher radon emanation indicates the presence of faults. In the area of the Jordan Valley, radon was sampled in highly fractured evaporates across the active strike slip fault. Higher concentrations (maximum of 1,940 Bq/m3) were measured at the fault planes, while lower concentrations (minimum of 570 Bq/m3) were measured at the nondeformed beds between the fault planes. Generally, summer readings are higher than winter readings due to the attenuation effect by pore water during the wet season. Keywords Radon emanation á Dead Sea rift á Seasonal effect

Received: 21 November 2000 / Accepted: 5 March 2001 Published online: 15 May 2001 ã Springer-Verlag 2001 M.Y. Atallah (&) á H. Mustafa Department of Earth and Environmental Sciences, Yarmouk University, Irbid, Jordan E-mail: [email protected] B.A. Al-Bataina Department of Physics, Yarmouk University, Irbid, Jordan

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Introduction Radon (Rn222), an alpha-emitting noble gas, is produced in the radioactive decay series of uranium (U238). It tends to migrate from its source mainly upwards. Soil-gas radon concentration has been correlated with factors such as geology, soil porosity and faults (Fleischer and MogroCampero 1978; Choubey and others 1994). Radon has been used as a pointer to buried uranium deposits, to trace moving air and groundwater masses, to indicate faults and as a possible tool for earthquake predictions (King 1980). Papastefanu and others (1989) and Ramola and others (1989) reported on radon anomalies preceding earthquakes. Choubey and Ramola (1997) discussed the relationship between geology and radon levels in groundwater, soil and indoor air. In Jordan, radon emanations were measured in soils, rocks and indoor atmospheres in different areas and cities. The present radon survey conducted in the Dead Sea transform (DST) was aimed at determining a possible connection between eventual radon anomalies and active faults. Radon was measured across the trace of the Dead Sea transform in Wadi Araba, the Dead Sea and the Jordan Valley. It was also measured along the boundary fault of the rift in both Wadi Araba and the Dead Sea.

Geological setting The Dead Sea transform (rift) is a major tectonic feature in the Middle East. It represents a plate boundary separating the Arabian micro-plate in the east from Sinai micro-plate in the west; it connects the Red Sea spreading zone in the south with the Taurus collision belt in the north. The left lateral strike slip movement along the transform is active. Geologic, geomorphologic and seismic evidences indicate sub-recent and present activities. The Dead Sea transform (rift) in Jordan consists of three morphotectonic segments: the Wadi Araba fault (WAF) in the south, the Dead Sea in the middle and the Jordan Valley fault (JVF) in the north. Many authors (e.g. Quennell 1959; Bender 1968a; Freund and others 1970; Atallah 1992) have described the formation of the rift. Its formation began in the Tertiary, in association with the opening of the Red Sea, and is still going on. The outcropping rock types in the topographic basin of the rift are mainly unconsolidated Quaternary gravel, sandstone, mud¯at and lake deposits. The major DOI. 10.1007/s002540100337

Cases and solutions

framework of the fault system in the rift area can be grouped into the following: 1. The Wadi Araba fault: this is a sinistral strike slip fault striking N15E. It extends from the northwestern side of the Gulf of Aqaba to the southeastern side of the Dead Sea. This fault shows many evidences of recent movement as the offset of streams, pressure ridges, sag ponds and recent fault scarps. 2. The Jordan Valley fault: this sinistral strike slip fault extends from the northwestern border of the Dead Sea, crossing the Jordan Valley diagonally to the eastern shores of Lake Tiberias. The above-mentioned two faults overlap in the Dead Sea area forming a pull-apart basin. 3. The eastern boundary normal fault: this has a large vertical throw. In some areas, it exposes the Precambrian basement (Saramuj conglomerate) as in the Sa® area southeast of the Dead Sea. In the Dhahal area (northern Wadi Araba), Upper Cretaceous rocks were upthrown relative to the Quaternary unconsolidated alluvium. The fault scarp shows triangular facets and well formed alluvial fans, in addition to a straight mountain front, which indicates active uplift.

determine the anomalies across the faults. The mean, standard deviation and range of the radon emanation at each site are presented in Table 1. The Wadi Araba fault (WAF) In this area the WAF forms a distinguished topographic line with typical physiographic features indicating active strike slip movement as fault scarps, sag ponds, pressure ridges and stream offsets. In this part of the transform, three bulldozer trenches were excavated across the active fault zone (Niemi and others 1998) to study the paleo-

Sampling The locations of sampling sites are given in Figs. 1 and 2. These sites were chosen because they represent different fault styles of the Dead Sea rift. The ®rst stage of sampling was in April 1998 across the Wadi Araba fault in Dhahal area in two sites. The ®rst site was in a trench and the second was in a borrow pit. The primary results show some sort of anomaly in the fault zone. A second stage of sampling was in June 1998 (summer sampling). In addition to the previous two places, sampling was performed across the boundary fault in the Dhahal area, across the boundary fault east of the Dead Sea and across the Jordan Valley fault. Another sampling period took place in the ®ve sites in March 1999 (winter sampling).

Methods Square pieces (1.5´1.5 cm) of solid state nuclear track detectors (CR-39) were mounted in the internal bottom of cylindrical plastic cans. The lid of each can is punched with few holes and these holes are then covered with thin (0.5 cm) piece of sponge to ®lter out Rn220. The resulted dosimeters were previously calibrated. Each dosimeter was put upside down in a hole dug at 50 cm depth in all sampling sites.

Results The radon survey was carried out in ®ve geographical sites. The summer readings are considered the base to

Fig. 1 Location map of the sampling sites

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outside the fault zone is 750 Bq/m3, while within the fault zone it is 927 Bq/m3. The ®rst ®gure is more than the measured ®gure for the same rock type on the western side earthquake history of the area. Radon was measured in of the Dead Sea, where the average radon concentration for two sites: across one of these trenches (the southern one) the recent alluvium is 300 Bq/m3 (Shirav and Vulkan and across an open burrow pit. The excavations show that 1997). It is noteworthy that the difference between radon the fault zone is a 9-m-wide ¯ower structure. concentrations outside and within the fault zone is not big. This can be explained due to the high porosity of the alThe southern trench (site 1) luvial sediments which allow the radon to migrate from its This trench is 19 m long and located in a modern wadi source, so the effect of the faults is not clear if the rocks are deposits. Radon detectors were placed across the fault hard and have low porosity. zone (inside the trench), and also on both sides of the fault zone (east and west of the trench). The distance between the detectors varies; outside the trench the distance is Table 1 17 m, but inside the trench the detectors are located on the Radon concentrations at sampling sites major fractures; the distance ranges between 1.5 and 2 m. Site Number of Mean (Bq/m3) Standard Range (Bq/m3) The samples were located in a recent alluvium and are samples deviation derived from Upper Cretaceous rocks which have no phosphates in their constituents. 1 18 866 256 500±1,400 Table 1 shows the range of radon concentrations. The 2 18 1,111 688 400±3,300 9 1,300 623 800±2,700 samples outside the fault zone have low radon emanation, 3 4 28 918 214 570±1,520 while the samples located at the fracture planes have 5 13 1,146 448 570±1,940 higher radon emanation. The mean radon concentration Fig. 2 Sampling map in Dhahal area (Wadi Araba)

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Fig. 4 Sampling map, cross section and radon results east of the Dead Sea (site 4) Fig. 3 Dhahal area: radon results. a Radon concentration in site 1. b Radon because they are measured on the bedrock. Outside the concentration in site 2. c Radon concentration and cross section in borrow pit, one sample has high concentration 3 site 3

(mean=1,700 Bq/m ; Fig. 3B). This point is located on an east-facing scarp, indicating a subsurface fault. The borrow pit (site 2) Within the fault zone, radon concentration varies for each This site is 200 m north of the previous site. This area was fracture (Fig. 3A, B), it can be attributed if the fracture is a excavated across the DST fault in a pressure ridge. Upper major one, or if it is connected with the main zone of the Cretaceous limestone and Lisan marl were exposed due to ¯ower structure. this excavation. The DST fault at this point forms a ¯ower structure with clear fractures. Radon detectors were placed inside the pit across the fault zone on the main fractures The boundary fault in Dhahal area (site 3) with distances ranging from 1 to 1.5 m. Outside the fault This site is 200 m east of the above-mentioned pit. The zone, three detectors are located on both sides with a 17 m N±S striking boundary fault separates Upper Cretaceous distance. Radon concentrations range between 400 Bq/m3 limestone from the alluvial fans west of it. The radon outside the fault zone and 3,300 Bq/m3 inside the fault detectors were arranged in one row across the fault with a zone (Table 1). The mean concentration inside the fault 17 m distance. Two rows (three detectors in each row) zone is 1,528 Bq/m3, while the mean concentration outside were placed at the base of the fault scarp; the distance the fault zone is 850 Bq/m3. Radon concentration across between the rows is 3 m and the distance between the the fault zone in this site is more than that for site 1, detectors is 10 m. Radon concentration ranges between Environmental Geology (2001) 40:1440±1446

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Fig. 5 alluvial deposits (Fig. 4), which may indicate that these Sampling map, cross section and radon results in Damya area, Jordan scarps were formed due to normal faults. Valley (site 5)

800 Bq/m3 west of the fault scarp and 2,700 Bq/m3 at the junction of the mountain front and the alluvial fan.

The DST in Damya area, Jordan Valley (site 5) This site is located 40 km north of the Dead Sea, close to the Jordan River. The outcropping rocks in this site are the Lisan Formation which is composed mainly of laminated evaporates overlain by the 5-m-thick Damya Formation, which is composed mainly of clay layers. The DST exhibits a highly fractured zone, highly tilted beds, ¯exures and vertical offset faults. A straight topographic fault scarp represents the western boundary of the fault zone (Fig. 5). The radon detectors were located at the main fractures, at the edge of the tilted blocks, and along the scarp, in addition to the horizontal non-deformed Lisan beds. The mean radon concentration on the fractures and faults is 1,454 Bq/m3 and on the non-deformed Lisan beds is 790 Bq/m3.

The boundary fault east of the potash factory (site 4) This site is located along the mountain front bordering the Dead Sea. In this site the Precambrian Saramuj conglomerate was uplifted against the unconsolidated Quaternary Lisan marl and alluvium. The Saramuj conglomerate is composed of particles from different igneous rocks. The alluvial deposits are composed of different particles mainly of igneous rocks and sandstone. The fault trace is indicated by more than one topographic scarp. The sampling area is very steep. The detectors were placed in a row across the fault scarps with a 17 m distance. Another row of detectors was placed at the junction between the bedrock and the alluvium. The mean radon concentration Seasonal effect ranges between 570 Bq/m3 and 1,520 Bq/m3 (Table 1). The lower values are mainly measured in the Lisan evaporates, Temporal variation in radon emanation from soil and rock while the higher ones are measured on the topographic is a well known phenomena (Hutter 1995; Shirav and scarps and along the junction between the bedrock and the Vulkan 1997), generally attributed to an attenuation effect 1444

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Fig. 6 Seasonal variation of radon measurements in some sites. a site 1, b Site 2, c Site 3 and d site 5

by pore water during the wet season. In order to evaluate the seasonal effect on the radon emanation in the Dead Sea Rift, four sites were measured at the end of the winter season, in addition to the normal summer readings. The results are summarized in Figure 6, showing lower radon emanations for the winter readings.

Conclusions Radon emanations were measured across different active segments of the Dead Sea transform (rift) in ®ve sites. In Wadi Araba fault, radon concentrations were measured in two trenches crossing the active strike slip fault. In these two areas, radon concentration across the fault zone is larger than that of the surrounding alluvial sediments, but the difference is less than expected due to the high porosity of both the fault zone and the alluvium, which allows the radon to migrate upwards. Radon concentrations are varied within the fault zone depending on the connection of the fractures with the main fault. In the boundary fault in Dhahal area (Wadi Araba), the higher radon concentration is found at the junction between the mountain front and the alluvial fans. At the boundary faults east of the Dead Sea,

the higher values are measured at the topographic scarps, which may indicate fault scarps. In this case radon can be used to detect hidden faults. In the Damya area (Jordan Valley), radon was measured across the Jordan Valley strike slip fault. In this area radon concentrations at the faults, fractures and tilted blocks are higher than the nondeformed Lisan marl beds. Generally the measured radon emanation during winter is less than during summer due to the effect of pore water during the wet season. Acknowledgements This work was supported by the Yarmouk University. Thanks to Mr. Musa Jad Hussein for helping in editing the paper.

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