Fault — Dissolution front relations and the Dead Sea sinkhole problem
Introduction
Evaporite rocks, mostly salt and gypsum, are globally widespread (Ford and Williams, 2007, p. 3). Circulating water can dissolve them with consequences for life and infrastructure (Cooper and Gutierrez, 2013, Frumkin, 2013). The most hazardous zones often occur along dissolution fronts at the near-surface edge of tilted evaporites of either ancient or modern age. This dynamic hazard is aggravated where groundwater levels are modified by human interference. For example, in Teesside, England, the Permian east-dipping halite and sulfate deposits are bordered by a dissolution front demonstrated by subsidence features which developed on the overlying beds (Cooper, 2002). Dissolution fronts often migrate laterally down-dip or towards the center of the basin (Warren, 2006). Over the geological timescale a dissolution front in a cratonic setting can propagate over distances greater than 100 km, as reported on salt and gypsum of Elk Point Formation in Canada (Ford and Williams, 2007, p. 386). Within a geological section, abrupt termination or wedging out of the salt layer, giving way to stratiform brecciated remains of overlying formations, commonly mark the location of the dissolution front (Johnson, 2003, Frumkin, 2013). Modern lakes and deviations of fluvial streams start to occur along dissolution fronts (Anderson and Hinds, 1997). The global distribution of dissolution fronts and sinkholes over evaporites and their associated hazards underscore the need to understand these features further. Yet, the dynamics of dissolution fronts are still poorly understood. The Dead Sea (DS) sinkhole hazard zone provides a unique opportunity to study a highly dynamic dissolution front using geophysical methods.
During the last thirty years thousands of sinkholes have appeared along the Dead Sea shoreline in both Israel and Jordan (Fig. 1) (Arkin and Gilat, 2000, Frumkin and Raz, 2001, Yechieli et al., 2006, Closson and Abu Karaki, 2009). The process began in the southern part of the DS coast and slowly spread northward along the Israeli coast and localized in the northern and southern parts on the Jordanian side.
There are two main hypotheses that provide an explanation for the location of sinkholes in the DS area. The first hypothesis is based on a visual correspondence between sinkhole lineaments and western and eastern fault escarpments (Abelson et al., 2003, Closson, 2005, Abelson et al., 2006, Closson and Abu Karaki, 2009). Based on this similarity, a structural control was assumed and a corresponding numerical model of fault-induced sinkholes was developed (Shalev et al., 2006). The other hypothesis proposes that sinkholes form along a dissolution front over the buried edge of a tilted salt layer (western edge in Israel and eastern edge in Jordan). This hypothesis is based on seismic refraction surveys carried out mainly along the western DS shore (Ezersky, 2006, Ezersky et al., 2010) and in Ghor Al-Haditha (El-Isa et al., 1995, Abueladas and Al-Zoubi, 2004).
Numerous faults widely distributed along the DS coastal area have been reported (Garfunkel, 1981, Gardosh et al., 1990, Ben-Avraham et al., 1993, Garfunkel and Ben-Avraham, 1996, Sagi et al., 2003, Shamir, 2006), whereas sinkholes are developed in a narrow 50–100 m-wide strip which hardly follows the fault lines in detail (Fig. 2c). However, there are also some arguments supporting the structural hypothesis: some sinkholes are associated with artesian groundwater, most probably emerging from the Early Cretaceous aquifer. This water is (1) hydrothermal (up to 42 °C) and (2) charged with hydrogen sulfide and iron (Charrach, 2011).
The goal of this paper is to analyze the above two hypotheses and search the relations between them.
Section snippets
General
The DS is the terminal lake of the Jordan River system in the Dead Sea Transform (Fig. 2a). The DS is the deepest subaerial point on Earth (Neev and Hall, 1979), located in an extremely arid environment with annual precipitation of 50–100 mm. The DS lies within a depression 150 km long and 15–17 km wide in its center, divided into two sub-basins (Fig. 2): The deepest point in the northern sub-basin is ~ 730 m b.s.l. (maximum water depth of ~ 300 m). The southern basin is dry (with artificial
Tectonic hypothesis
The tectonic hypothesis of sinkhole formation is based on an observation that there is a visual similarity between sinkhole lineaments and the western and eastern fault escarpments (Abelson et al., 2003, Closson, 2005). The following arguments are used to link between faults and sinkholes. (1) The trends of sinkhole lineaments, exposed faults, and zig-zagging segments of the rift escarpments show a striking similarity. All features have a predominantly bimodal distribution with NNE and NNW
Structural hypothesis
We have analyzed the sinkhole distribution along the western DS shore in relation to the DS contour line at − 400 m (b.s.l.) and the main margin faults (Fig. 2c). From Fig. 2c one can see that, in general, the western major faults, DS shoreline and sinkholes are oriented roughly along the same N–S direction. From Fig. 2 it is also evident that the sinkholes deviate from faults to shoreline with an amplitude of hundreds of meters. Some segments of sinkhole clusters can be approximated by straight
Conclusions
Results of a geophysical study of sinkhole development sites in the Dead Sea coastal area in Israel and Jordan are presented. Relations between sinkhole lineaments, salt edge and faults have been studied using new imaging methodology. It is shown that sinkhole lineaments are arranged along the salt layer edge acting as a dynamic dissolution front. We suggest that superficial coastal basins where salt formed could be generated by sub-vertical displacements along faults. This reconciles the two
Acknowledgments
The study has been performed within the framework of MERC Project M27-050, sponsored by the USAID fund (the opinions expressed in this study are those of the authors and do not necessary reflect the views of the USAID). The authors thank the geophysical team at UPMC (Université Pierre et Marie Curie, Paris, France), which provided the seismic data of Ghor Al-Haditha, originally collected during an integrated geophysical survey carried out in the framework of the NATO Science for Peace Program
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