Ammar Amin

Karst Hazard Assessment of Eastern Saudi Arabia

Karst phenomena exist in areas in the eastern part of Saudi Arabia, forming solution features such as sinkholes, collapsed dolines and solution caverns, as a result of the chemical leaching of the carbonate and evaporite formations by percolating water. The instability of these karst phenomena could produce land subsidence problems. This paper reviews the geology of documented karstic rock units in Saudi Arabia and proposes a simple engineering classification of the solution features characteristic of limestone. Two case histories in the Dhahran area, eastern Saudi Arabia, will be used as examples for the application of a modified engineering classification.

Causes of Land Subsidence in the Kingdom of Saudi Arabia

The occurrence of land subsidence in the Kingdom Saudi Arabia is either natural or man-made. Natural land subsidence occurs due to the development of subterranean voids by a solution of host rocks in carbonate and evaporite terrains, over many areas of Saudi Arabia. Man-induced land subsidence is either due to the removal of groundwater in the agricultural areas or to wetting of unstable soils. Therefore, earth fissures and a lowering of the ground surface in unconsolidated sediments took place in alluvial plains and volcanic vent terrains. Unstable soils include Sabkha soils and loess sediments. These types of soils occur in coastal plains, desert areas and volcanic terrains. When this soil is wetted either during agricultural activities, waste disposal or even during a rain storm, subsidence takes place due to either the removal of salts from the Sabkha soil or the rearrangement of soil particles in loess sediments.

Geotechnical Hazards Associated With Desert Environment

The aridity of the Arabian Peninsulas deserts ranges between arid to hyperarid with hot dry climate, scarce precipitation and sparse vegetation. These harsh environmental conditions enhance some geomorphologic processes more than others, cause specific geotechnical problems, and increase desertification.From west to east, the general physiography of Saudi Arabia shows the Red Sea coastal plains and the escarpment foothills called Tihama followed by the Arabian Shield mountains, the Arabian Shelf plateau and finally the Arabian Gulf coastal plains. Sand moves by wind either as drifting sand or migrating dunes in four major sand seas, over the Arabian Shelf, and in the inter-mountain valleys, in the Arabian Shield causing problems of erosion and deposition. Human activities in the deserts may cause more instability to the sand bodies, enlarging the magnitude of the problem. Fine silty soil particles also move by wind, depositing loess mainly in selected areas downwind in the Tihama. These loess deposits subside and may form earth fissures by the process of hydrocompaction upon wetting. The addition of water can be either natural through storms or man-made through human agricultural or civil activities. Extensive sabkhas exist along the coastal plains of both the Red Sea and Arabian Gulf. The sabkha soil may also heave by salt re-crystallization or collapse by wetting. The shallow groundwater brines present in sabkhas also attack and corrode civil structures. Urbanization and excessive groundwater pumping may also deplete the fresh groundwater resources and may cause subsidence, ground fissuring and surface faulting as observed in some locations in the Arabian Shield. Although the average annual precipitation is very low, rain usually falls in the form of torrential storms, collected by dry valley basins and causing floods to unprotected downstream areas on the coastal plains of the Red Sea.The desert environment, being a fragile echo system, needs to be treated with care. Intercommunications between different national and international agencies and education of the layman should help to keep the system balanced and reduce the resulting environmental hazards. In addition, any suggested remedial measures should be planned with nature and engineered with natural materials.

Lithosphere Dynamics and Sedimentary Basins of the Arabian Plate and Surrounding Areas

The Arabian Shield and Red Sea region is considered one of only a few places in the world undergoing active continental rifting and formation of new oceanic lithosphere. We determined the seismic velocity structure of the crust and upper mantle beneath this region using broadband seismic waveform data. We estimated teleseismic receiver functions from high-quality waveform data. The raw data for RF analysis consist of 3-component broadband velocity seismograms for earthquakes with magnitudes Mw > 5.8 and epicentral distances between 30° and 90°. We performed several state-of-the-art seismic analyses of the KACST and SGS data. Teleseismic Pand S-wave travel time tomography provides an image of upper mantle compressional and shear velocities related to thermal variations. We present a multi-step procedure for jointly fitting surface-wave group-velocity dispersion curves (from 7 to 100 s for Rayleigh and 20 to 70 s for Love waves) and teleseismic receiver functions for lithospheric velocity structure. The method relies on an initial grid search for a simple crustal structure, followed by a formal iterative inversion, an additional grid search for shear wave velocity in the mantle and finally forward modeling of transverse isotropy to resolve surface-wave dispersion discrepancy. Inversions of receiver functions have poor sensitivity to absolute velocities. To overcome this shortcoming we have applied the method of Julia et al. (Geophys J Int 143:99–112, 2000), which combines surface-wave group velocities with receiver functions in formal inversions for crustal and uppermost mantle velocities. The resulting velocity models provide new constraints on crustal and upper mantle structure in the Arabian Peninsula. While crustal thickness and average crustal velocities are consistent with many previous studies, the results for detailed mantle structure are completely new. Finally, teleseismic shear-wave splitting was measured to estimate upper mantle anisotropy. These analyses indicate that stations near theGulf ofAqabah display fast orientations that are aligned parallel to the Dead Sea Transform Fault, most likely related to the strike-slip motion between Africa and Arabia. The remaining stations across Saudi Arabia yield statistically the same result, showing a consistent pattern of north-south oriented fast directions with delay times averaging about 1.4 s. The uniform anisotropic signature across Saudi Arabia is best explained by a combination of plate and density driven flow in the asthenosphere. By combining the northeast oriented flow associated with absolute plate motion with the northwest oriented flow associated with the channelized Afar plume along the Red Sea, we A. Al Amri (&) K. Abdelrahman M.O. Andreae M. Al-Dabbagh Geology and Geophysics Department, King Saud University, Riyadh, Saudi Arabia e-mail: alamri.geo@gmail.com M.O. Andreae Biogeochemistry Department, Max Planck Institute for Chemistry, Mainz, Germany

Arabian plate: lithosphere dynamics, sedimentary basins, and geo-resources

This topical issue of the Arabian Journal of Geosciences focusing on the Arabian plate, its lithosphere dynamics, sedimentary basins, and geo-resources is an initiative of the Task Force 6 of the International Lithosphere Program, an international network dedicated to the study of sedimentary basins (Roure et al. 2010a, b). Summary of former activities of the ILP Task Force on sedimentary basins and perspectives