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Peace and Science in the Middle East


The subsurface structure of the Dead Sea rift valley from gravity field anomalies

      The gravity field is indicative of density variations of subsurface rocks. Subsurface basins, generated as a result of the relative plate motion and fault geometry are detected in the gravity field because of the low density of their fill relative to the surrounding rocks. The location and shape of small basins, together with the seismic reflection data can be used to delineate fault segments that are the result of the vertical motions, which accompany and accommodate lateral (strike-slip) motion.

      Over a period of many years, Israel and Jordan have collected their own gravity data and have kept them in their respective archives. However to have a full understanding of the subsurface structure of the Dead Sea Transform, these data sets had to be combined. A single profile was extracted in 1988 from paper copies of the gravity maps of Jordan and Israel, to study the deep structure of the Dead Sea plate boundary (ten Brink et al., 1990). A marine gravity survey was carried out by Tel Aviv University and Lamont-Doherty Earth Observatory of Columbia University in 1988 in the Dead Sea. Data from this survey were combined by the USGS Woods Hole Science Center with data from the archives of the Natural Resources Authority, Jordan and the Geophysical Institute of Israel. Combining all these data constituted a problem due to the different methods in which they were processed and the different tie points used to process the data. Analysis and interpretation of that data revealed for the first time the dimensions and great depth of the Dead Sea Basin (ten Brink et al., 1993).

      As part of the US-AID project, common gravity stations were measured at the Bet She'an/Sheikh Hussein Bridge and Elat/Aqaba border crossings to bring the gravity measurements to a common datum. The Israel gravity database was recalculated using a similar formula to the one used for the Jordanian database and its reference value was shifted to the Jordanian database. This allowed both databases to be merged. A combined database for Jordan and Israel was established and is residing in the national archives of both Jordan and Israel. This data set continues to be updated by direct data exchange between Natural Resources Authority of Jordan and Geophysical Institute of Israel as new measurements are collected. All the new gravity data stations were checked and reduced according to conventional procedures. These include calculation of instrument drift and tidal attraction, the conversion of field measurements to gravity values, the subtraction of the International Gravity Formula, free-air and Bouguer corrections, and Terrain correction. The figure shows the combined Bouguer gravity anomaly field of Israel and Jordan. This map is based on approximately 40,000 gravity stations including several thousands of new stations measured as part of this project.

      The qualitative analysis of the Bouguer gravity anomaly map, overlain on a shaded relief topography provides a wealth of structural information of the Dead Sea. The maps show that the Dead Sea fault system is broken into many fault segments. Some segments parallel each other, others are arranged en-echelon and others are simply aligned. Some of these faults are still active, and some are buried and are presumably inactive.

The basins of the Dead Sea region

      The valley along the Jordan - Israel border is typically divided into five basins: Wadi Araba (Arava Valley), Dead Sea, Jordan Valley, Sea of Galilee and Hula. The gravity map indicates that there are more basins than the above mentioned. These basins are from south to north: Aqaba/Elat, Timna/Qa-At-Taba, Gharandal, Dead Sea, Jericho, Damia, Bet-Shean, Kinarot/Baqura, Sea of Galilee (southern and northern basins) and Hula basins. Several of these basins do not occupy the entire width of the rift valley. Some of these basins, especially in the Arava/Araba Valley, probably contain groundwater aquifers.

      Our interpretation of fault locations based on the gravity and on seismic reflection data indicates that the motion along the 420-km long Dead Sea tectonic plate boundary is accommodated by at least 15 separate fault segments with segment lengths varying between 25 and 55 km. More fault segments are shown buried on seismic reflection profiles and do not offset or perturb the upper sedimentary section, indicating that they are no longer active. Strike-slip faults tend to smooth themselves as they mature. By comparison with other plate boundaries worldwide, the cumulative offset of 105 km along the Dead Sea plate boundary is expected to be accommodated by a single smoothed 420-km-long fault segment (Stirling et al., 1996; Wesnousky, 1988). The observations of a large number of basins and of numerous and overlapping fault segments along the Dead Sea plate boundary suggest a continuous adjustment to changes in relative plate motion.

References

  • Stirling, M.W., S.G. Wesnousky, and K. Shimazaki, Fault trace complexity, cumulative slip, and the shape of the magnitude-frequency distribution for strike-slip faults; a global survey, Geophysical Journal International, v. 124, pp. 833-86, 1996.
  • ten Brink, U.S., N. Schoenberg, R. L. Kovack, and Z. Ben-Avraham, Uplift and a possible Moho offset across the Dead Sea transform, Tectonophysics, v. 180, pp. 71-85, 1990. PDF
  • ten Brink, U.S., Z. Ben-Avraham, R.E. Bell, M. Hassouneh, D. F. Coleman, G. Andreasen, G. Tibor and B. Coakley, Structure of the Dead Sea Pull-Apart Basin From Gravity Analyses, J. Geophys. Res., v. 98, pp. 21,877 - 21,894, 1993. PDF
  • Wesnousky, S.G., Seismological and structural evolution of strike-slip faults, Nature, V. 335. p. 340-343, doi:10.1038/335340a0, 1998.

      (The reader is directed to USGS Open-File Reports 00-0341 and 01-216 for more information and maps of the Dead Sea gravity data and USGS Fact Sheet FS-239-95 for a thorough explanation of a gravity survey.)

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