Elsevier

Tectonophysics

Volume 602, 16 August 2013, Pages 69-77
Tectonophysics

The thermal structure of Israel and the Dead Sea Fault

https://doi.org/10.1016/j.tecto.2012.09.011Get rights and content

Abstract

In this paper we analyze temperature data from all the available oil and water wells in Israel and compare the results with seismicity depth and with heat flux estimation from xenoliths. We show that the average heat flux in Israel is 40–45 mW/m2, consistent with measurements of the Arabian Shield. A heat flux anomaly exists in Northern Israel and Jordan. This could be attributed to groundwater flow or young magmatic activity (~ 100,000 years) that is common in this area. A higher heat flux exists in Southern Israel and Jordan, probably reflecting the opening of the Red Sea and the Gulf of Eilat (Gulf of Aqaba) and does not represent the average value present in the Arabian Shield.

The temperature gradient at the Dead Sea basin is relatively low, resulting in low heat flux (< 40 mW/m2) and a relatively deep seismicity extending to lower crustal depths, in agreement with earthquake depths (< 25–30 km). Higher heat fluxes at the Sea of Galilee (70 mW/m2) and at the Gulf of Eilat (65 mW/m2) results with shallower seismicity (< 10–12 km). The steep geothermal gradients yielded by xenoliths (> 80 mW/m2) could be the result of local heating by magmas or by lithospheric necking and shear heating.

Highlights

► Temperature data from all the available oil and water wells in Israel are analyzed ► The average heat flux in Israel, 40–45 mW/m2, is compatible with deep seismicity ► Heat flux anomalies are attributed to groundwater flow or young magmatic activity. ► Elevated heat flux in the south reflects the opening of the Red Sea.

Introduction

Heat flux is a major factor that affects the rheology of the lithosphere, magmatism, and groundwater flow (Ranalli and Rybach, 2005). Different assumptions about the heat flux in Israel have led to different tectonic and seismological models (Aldersons et al., 2003, Al-Zoubi and ten Brink, 2002, Sobolev et al., 2005). The heat flux controls the thickness of the lithosphere, the type of deformation (brittle versus ductile), and the depth of the seismogenic zone (Jaupart and Mareschal, 2011, Ranalli, 1995). Sobolev et al. (2005) and Petrunin and Sobolev (2006) presented results of a three-dimensional thermo-mechanical model of a pull-apart basin, formed at left stepping segments of an active continental transform fault such as the Dead Sea (see Fig. 1 for location). Adopting the classical scheme of a pull-apart basin formation, they demonstrated that the major parameter controlling the basin structure and deformation pattern beneath the basin is the thickness of the brittle layer. Significant ductile deformation of the lower crust and the upper mantle associated with basin growth due to a pull-apart mechanism requires normal or elevated heat flux. The closest fit to the Dead Sea structure has been obtained with the model corresponding to a surface heat flow above 60 mW/m2. They also argued that a strong lower crust in a cold lithosphere with heat flow below 50 mW/m2 could not allow the opening of a pull-apart such as the Dead Sea basin.

Förster et al. (2007) determined the heat flux in five, up to 900 m deep boreholes in southern Jordan to be 60.3 mW/m2. Recently, Förster et al. (2010) analyzed a set of surface samples from the uppermost crust down to the lithospheric mantle underneath Jordan and assumed heat flux of 55–65 mW/m2 to construct a thermal model. Thermobarometric calculations, based on lower crustal and lithospheric mantle xenoliths, suggest even steeper geothermal gradients (e.g. Al-Mishwat and Nasir, 2004, McGuire, 1988, McGuire and Bohannon, 1989, Nasir, 1992, Stein et al., 1993), thus higher heat flux (> 80 mW/m2). The above considerations contradict the general view of the Arabian Shield as an anomalously cold terrain characterized by heat flux values below ~ 45 mW/m2 (Gettings and Showail, 1982). Davies and Davies (2010) showed that the heat flux along the Red Sea is very high (> 150 mW/m2), whereas the heat flux at the Arabian Shield is low (< 55 mW/m2). Measurements supporting low geothermal heat flux were published by Eckstein (1976), Eckstein and Simmons (1978), Levitte et al. (1984), and Eckstein and Maurath (1995) who measured thermal gradients and thermal conductivity in abandoned oil wells and unused water boreholes distributed over Israel. They calculated an average heat flux of 42 mW/m2. The mean value of the corrected heat data for the northern part of the Dead Sea basin is 38 mW/m2 (Ben-Avraham et al., 1978). Recent re-evaluation of the heat flow data for the Dead Sea basin (Shalev et al., 2007) confirmed these low values. Based on coal rank measurements, Bein and Feinstein (1988) showed that a low heat flux has prevailed in the Dead Sea area since the mid-Miocene period. Galanis et al. (1986) estimated a mean basal heat-flow value of 53 mW/m2 in Jordan. Local elevated heat flux values were shown to be associated with groundwater advection (Galanis et al., 1986, Gvirtzman et al., 1997, Kovach et al., 1990, Truesdell et al., 1983).

Another characteristic of the Dead Sea fault is the significant variations in the depth of the seismogenic zone. In the central part (the Dead Sea basin) the seismicity is anomalously deep extending almost to the mantle (Aldersons et al., 2003, Braeuer et al., 2010, Shamir, 2006). Sixty percent of well-constrained micro-earthquakes (ML  3.2) in the Dead Sea basin during the period 1984–1997 were located at depths up to 25–30 km. The seismogenic zone becomes shallower to the south, toward the Gulf of Eilat, where most earthquakes are located at depths shallower than 10 km (the European-Mediterranean Seismological Centre catalog, 2011). Recent study of the seismicity in the northern part of the Dead Sea fault (Navon, 2011) shows that at the Sea of Galilee area, the seismogenic zone is also shallow (~ 12 km) and that it deepens northwards.

The two opposing opinions of high versus low heat flux suffer from some limitations. The analysis of Förster et al. (2007) is based on a very detailed study of just five closely located boreholes in southern Jordan, which they extrapolated on the entire region. On the other hand, the analysis of Eckstein and Simmons (1978), which suggested low heat flux, does not include many new measurements that were collected in recent years.

The purpose of this paper is to re-examine the geothermal heat data collected in Israel for the past 50 years and to determine the average geothermal heat flux in this area. Subsequently, we compare the depth of the seismogenic zone along the Dead Sea Fault with the depth of the isotherm corresponding to the brittle–ductile transition.

Section snippets

Geological setting

The crust of the Arabian Shield is composed of Phanerozoic sedimentary rocks overlying a Late Proterozoic crystalline basement (Garfunkel, 1988, Ginzburg and Gvirtzman, 1979). The basement consists of metamorphic and plutonic rocks (mainly granite and diorite compositions). In Israel, these old rocks (older than 550 Ma) are exposed only in the Eilat area and are covered by thick sedimentary sequences throughout most of Israel (Fig. 2). The sedimentary cover thickens toward the northwest and

Temperature data

Temperature was measured by three major methods: temperature logging, drill stem test, and bottom-hole temperature. Most of the data was published by Shalev et al. (2008).

Data interpolation

A geothermal gradient was calculated for each well with reliable newly generated temperature data. The temperature distribution in the homogeneous layer with the thermal conductivity, K1 and the radiogenic heat production H1 was calculated using the analytical solution of the equation for the heat transfer which has a parabolic form:Tz=T0+zqs/K1z2H1/2K1where z is the depth beneath the surface, T0 is the temperature at the surface, qs is the surface heat flux. The thermal conductivity was

Discussion

The average geothermal heat flux throughout Israel is low (40–45 mW/m2), compatible with the previous studies of the region (Ben-Avraham et al., 1978, Eckstein and Simmons, 1978). Such low heat flux is also consistent with the measurements at the Arabian Shield (Gettings and Showail, 1982). In southern Israel, the relatively high heat flux values are similar to the flux found by Förster et al., 2007, Förster et al., 2010 in this area, which probably represent the effects of the opening of the

Conclusions

Re-examination of temperature data confirms that the common geothermal heat flux throughout Israel is low (40–45 mW/m2). The heat flux is higher in southern Israel as a result of the opening of the Red Sea. In northeastern Israel, the relatively high heat flux is attributed to groundwater flow or to the young magmatic activity (~ 100,000 years) that is common in this area. The steeper geothermal gradients implied by data from xenoliths could be the result of local heating either by the magmatic

Acknowledgments

We would like to thank the editor S. Cloetingh for his constructive remarks. We appreciate the thorough evaluations of our manuscript by Dr. Gideon Rosenbaum and two additional anonymous reviewers. This project was funded by Ormat Systems, Inc. V. Lyakhovsky acknowledges support from the Israel Science Foundation (grant ISF 753/08).

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