Middle East Tectonics: Applications of Geographic Information Systems (GIS)
Dogan Seber, Marisa Vallve, Eric Sandvol, David
and Muawia Barazangi
Institute for the Study of the Continents, Cornell University, Snee Hall, Ithaca, NY 14853-1504. e-mail: email@example.com
ABSTRACT The Middle East region incorporates all known types of major plate boundaries in its territory as well as significant active intraplate deformation. Until recently, understanding the tectonics in this complex region has been hindered by a relative lack of data and the complexity of the geologic and tectonic problems. Even with the increase in the amount of data in the past decade or so, the complexities of the region require multidisciplinary approach to understand the geology and tectonics. In order to handle large, multidisciplinary data sets with varying quality and resolution, we have adopted a Geographic Information System (GIS) approach for construction of a multi-purpose database to look at these problems in a comprehensive and unconventional way. Here, we present new compilation maps of surficial tectonic features and of depth to the Moho for the Middle East, and describe a cross-section tool to work with data of this type in a GIS format. Interested parties can access these maps at our web site at http://atlas.geo.cornell.edu.
At present, the earth sciences are undergoing a revolution. Evidence comes from collection to analysis of data, interpretation and publication. Classical approaches are being increasingly supplemented by digital techniques, i.e., analog maps by digital counterparts, air photos by high resolution satellite imagery, hand collection of field data by GPS receivers and lap top computers, simple modeling by computer modeling using sophisticated software, and electronic publication of results. This development is an inevitable outcome of modern technology. However, the revolution is not without problems. Already somewhat chaotically organized databases are appearing in the "digital world" owing to problems like data accessibility and formats. Geographic Information Systems (GIS) provide a means to eliminate these problems and to keep data in an organized and centralized system (see also Walker et al., 1996 in March 1996 issue of GSA TODAY). Structured properly, well engineered databases are easy to use, update, modify, manage, distribute and exchange. One of the common misconceptions about GIS database development is that it is primarily a mapping tool. Although the outputs are often in map form, the main use of GIS is to analyze, search, manipulate, and select databases for a specific purpose. The use of GIS systems opens new avenues for comprehensive studies and solving complex problems related to integrated and dynamic earth systems. Earth sciences, by their very nature, are among the most suitable disciplines for GIS applications.
BUILDING A COMPREHENSIVE DATABASE FOR THE MIDDLE EAST
In this paper, we apply GIS technology to regional scale tectonic problems of the Middle East. To do this, we are developing a comprehensive database at a resolution of 1:1,000,000 scale which can be used as both a scientific and educational tool. Developing such a database system for multiple users is most advantageous if easy to use tools for accessing and manipulating data sets are built into the system so that scientists can use the database in innovative ways to make research advances. The principal reasons for constructing this database in the Middle East are to help in the recently signed global Comprehensive Test Ban Treaty (CTBT), especially in monitoring and verification efforts (see Barazangi et al. 1996), and in studying the complex tectonic/geologic problems of the region with multidisciplinary approaches. Such databases will also have impact in natural hazard evaluation, such as understanding the earthquake occurrences in the region and seismic risk assessment. These data sets can also be used in classrooms as educational tools. Below, we present some examples of uses of our databases.
An Improved Tectonic Map of the Middle East Among the data sets available to us is a high resolution (~ 90m ) digital topographic map of the Middle East obtained from the Defense Mapping Agency (DMA). Digital Elevation Models (DEM) like these provide highly accurate elevation information that can be used as a guide in defining boundaries of tectonic units, especially those related to young (i.e., Quaternary) deformation. Using the digital topography data, we redefined the boundaries of the previously outlined main tectonic units, such as the North and East Anatolian faults. We also defined a new boundary for the Turkish - Iranian plateau (see Figures 1-3). These main tectonic units can be seen by comparing the GIS based map in Figure 1 with the tectonic map in Figure 2. As shown, the Middle East region incorporates all known types of major plate boundaries around the borders of the Arabian plate (e.g., Dewey and Sengor, 1979). To the south along the Red Sea and the Gulf of Aden new oceans are opening (see Cochran, 1983; Le Pichon and Francheteau, 1978). To the north and east, continental collision is occurring along the Bitlis suture zone in southern Turkey (Sengor and Kid, 1979; Sengor et al., 1985) and the Zagros suture zone in western Iran (Snyder and Barazangi, 1986; Ni and Barazangi, 1986). The current counter-clockwise rotation and northward motion of the Arabian plate relative to Eurasia is accommodated along these collision zones. Well developed arc volcanoes and a foreland basin along the entire Zagros mountain system indicate Neogene subduction in this region. Although a similar volcanic arc and foreland basin are not easily identified along the Bitlis suture, a Neogene subduction zone is also inferred in this region, especially in southeast Turkey. To the northwest, the Dead Sea fault system manifests itself as a left-lateral strike-slip plate boundary that extends approximately 900 km along the boundary between the Arabian and African/Levantine plates (Garfunkel, 1981; Girdler, 1990; and Chaimov et al., 1990). Other major strike-slip zones are the right-lateral North Anatolian in northern Turkey and the left-lateral East Anatolian fault in eastern Turkey, which respectively form the northern and eastern boundaries of the Anatolian block. These faults developed to accommodate the escape of the Anatolian block towards the west in response to the collision of Arabia and Asia (Sengor et al., 1985). A consequence of the collision between the Arabian and Eurasian plates is the development of the high plateau in eastern Turkey and northwestern Iran (Turkish and Iranian Plateau) which covers a wide zone behind the main Zagros and Bitlis suture zones. We used the high resolution topographic data to map the boundaries of this plateau by following the 1500 m elevation contour. This value represents the maximum elevation or base level of the plateau that defines a continuos elevated surface covering the entire area. Although the mechanism supporting this high plateau is not well understood, extensive volcanism and strong seismic shear wave attenuation in the mantle lithosphere beneath the plateau (e.g., Kadinsky-Cade et al., 1981) do suggest a thermal component. Further work is needed to fully understand the crustal and upper mantle structures of this region. In the western part of the Arabian platfrom, east of the Red Sea, lies a shield area where Precambrian age of crystalline rocks are exposed tocover a large area (Figure 2). Although this region is called the Arabian Shield, it certainly has disctinct features that do not exist in other shield areas. The Arabian Shield has been subjected to rifting that formed the Red Sea. The rifting still continues today. The Shield certainly does not fit in the general definition of shied regions which are identified as regions of long term tectonic stability. These variations from a typical shield structure manifests itself in geophysical data as well. This shield region, in contrast to other shield regions, is characterized by very low seismic crustal Q values (e.g., Seber and Mitchell, 1992), which are usually interpreted to result from high tectonic activity and/or eleveted temperature. High Cenozoic volcanic activity within the shield area is also an indication of departure from a typical shield environment (e.g., Camp and Roobol, 1992). In addition to the boundaries of the major tectonic units, the map in Figure 1 also has other types of digitized data. In particular, secondary faults, volcanic rock distributions, ophiolites, basement outcrops, and basin locations have been compiled from several tectonic and geologic maps, such as the Seismotectonic Map of the Middle East published by the Geological Survey of Iran, Geologic Map of Syria of the Syrian Arab Republic's Ministrity of Petroleum and Mineral Resources, and the Active Fault Map of Turkey of the General Directorate of Mineral Research and Exploration of Turkey. These geologic features have all been assigned attributes defining their properties. Using this GIS data system, one can then, for example, select faults longer or shorter than any given length, display faults relative to any spatial data in the database or choose active faults and volcanoes from a given region to study spatial correlation among data sets. In this way, a first order correlation between topography, faults, and seismicity has been found that suggests that most of the faults shown in Figure 1 are still active and that the topography is in large part shaped by active tectonic processes. High resolution satellite imagery and field geology data can also be incorporated in the database system to study the effects of erosion on topography. Other prominent problems for study in the region are volcanic activity in both tectonically active and platform-like environments, the complex patterns of seismicity, and the variations in crustal structure.
A New Moho Map of the Middle East One of the least known geological features in the Middle East region is the thickness of the continental crust , that is the depth to the crust-mantle boundary or Moho. To constrain Moho depth in this region, we have digitized over 50 interpreted crustal scale refraction and gravity profiles (Figure 4) from the published literature. All boundaries have been assigned specific attribute names like basement and Moho. To this database, we have added Moho depth estimations obtained using a single station technique (e.g., Sandvol et al., 1996). Moho depth values from surface wave tomographic studies (Ghalib, 1992) and interpretations of Bouguer gravity data in Iran (Dehghani, 1981) were also incorporated. The new, more accurate Moho map in Figure 4 was made by selecting and gridding all of the depth to Moho values in the database. In regions where data are limited, Moho depth has been determined by interpolation from the nearest data points. Examination of the map in Figure 4 points to a number of first order characteristics of the crust surrounding and within the Arabian plate. First, the thickest crust occurs beneath the Zagros mountains in Iran where continental collision is taking place. Second, the thinnest crust occurs beneath the southern Red Sea where new oceanic crust is forming. Third, the crustal thickness beneath the Arabian shield is about 40-45 km. The thickning beneath the Arabian Shield is constrained by a single profile, and hence this result should be taken cautiously. New seismological data have been collected from this region and the results of which will provide additional constraints in Moho depth in this region. Fourth, the crust is very thin (~ 8 km) beneath the Afar triangle of Africa, just west of the southern Red Sea. This region is thought to be underlain by either an oceanic type crust or a new igneous crust (Soriot and Brun, 1992)
The Profile Maker A tool called Profile Maker that we have developed to be used with gridded databases aids in detailed studies of these crustal variations. This tool extracts and draws a crustal scale cross section between any two points within the area of data coverage. Any combination of topographic, metamorphic basement, crustal thickness, seismic velocity, gravity or any other available data can be incorporated in a two-D cross-section. These profiles then can be used, for example, for seismic waveform or gravity modeling, or for other research and teaching efforts. A cross-section using this tool that incorporates topography, depth to metamorphic basement, and crustal thickness is shown in Figure 5. The cross section shows the thin crust in the Red Sea and the thick crust beneath the Zagros mountains in comparison with the rest of the Arabian platform. The Mesopotamian foredeep is also identifiable by the thickening of sediments toward the Zagros collision zone.
CONCLUSIONS As we progress into the digital technology age, proficient ways of capturing, storing, organizing, manipulating, and updating data sets have to be developed before we are overwhelmed by the amount, diversity, and heterogeneity of the data. Clearly, GIS provides a convenient platform for data collection, organization and research with multidisciplinary data sets. As more groups adopt GIS applications, the earth sciences community will be in a position to prepare a unified global database to conduct more efficient, productive and rewarding scientific research. Such a database platform will significantly impact the way we conduct research, teach and educate future generations of earth scientists. This study shows that there are significant scientific return on the efforts that are spent on putting together such a GIS database. Using our Middle East GIS database we now have a better understanding of the tectonics and crustal structure in this region. Further information about our efforts on the Middle East database as well as access to mentioned databases/tools is provided on the Internet at http://atlas.geo.cornell.edu.
ACKNOWLEDGMENTS Research reported in this paper is supported by the Department of Energy (DOE) contract F19628-95-C-0092. Institute for the Study of the Continents (INSTOC) contribution No. 234. We thank the Science Editor, Suzanne M. Kay, and reviewers T. Pavlis, and R. Reilinger for constructive comments and suggestions.
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Figure 1. New map of the Middle East region showing locations of major faults and ophiolites, regions of basement outcrop, and principal areas of volcanism. Map is compiled from GIS data set that includes tectonic and geologic maps of various scales from the different countries in the region. See Figure 2 for tectonic interpretation. Background image is the shaded relief map of topography shown in more detail in Figure 3. A clear correlation between topographic features and faults suggests that most of these faults are still active.
Figure 2. Simplified tectonic map of the Middle East showing types of plate boudaries surrounding the Arabian Plate plate and location of profile A-B in Figure 5. The extend of the plateau, plate boundaries and faults have been modifed from earlier maps by incorporating topographic information in Figure 3.
Figure 3. Shaded relief map showing topography of the Middle East created using data obtained from the Defense Mapping Agency and the USGS's 1 km global elevation model. Elevations range from sea level to over 6000 meters in some localized peaks. Refer to Figure 2 for definitions of tectonic units and to Figure 4 for geographic names..
Figure 4. New Moho map of the Middle East compiled by merging more than 50 crustal scale profiles (see inset map) and additional information. The deepest Moho is found beneath western Iran, whereas the shallowest Moho is under the southern part of the Red Sea. Moho depth in most of the Arabian plate is a little over 40 km. The inset map shows locations of published profiles used in the compilation.
Figure 5. Crustal scale section showing how the crust-mantle boundary and the thickness of sedimentary cover over basement changes on a southwest to northeast profile across the Red Sea, the Arabian plate, and the Zagros suture. Profile is along line A-B in Figure 2 and incorporates basement depths from our Middle East GIS data base as well as information shown on Figures 3 and 4.. Basement depths defined here is based on seismic velocity information. The boundary at which seismic P-wave velocities reach 6 km/s is interpreted to represent the contact between the sedimentary rocks and crytalline rocks. Note the effects of rifting and thinning of the crust in the Red Sea in contrast with the thickening of foredeep sediments and crust beneath the Zagros collision zone.