Assessment of Relative Active Tectonics, Southwest Border of the Sierra Nevada (Southern Spain)
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Australian Journal of Basic and Applied Sciences, 4(6): 969-993, 2010 ISSN 1991-8178 Active Tectonics of the South Central Alborz (North Iran) 123Khavari, R. Arian, M. Ghorashi, M 1Islamic Azad University (IAU), Behbahan Branch, Behbahan, Iran 2Islamic Azad University, Sciences and research branch, Tehran, Iran 3Geological Survey of Iran, Tehran, Iran Abstract: The paper present a method for evaluating relative active tectonics based on geomorphic indices useful in evaluating morphology and topography. Indices used include: stream length-gradient index (SL), drainage basin asymmetry (Af), hypsometric integral (Hi), ratio of valley-floor width to valley height (Vf), index of drainage basin shape (Bs), and index of mountain front sinuosity (Smf). Results from the analysis are accumulated and expressed as an index of relative active tectonics (Iat), which we divide into four classes from relatively low to highest tectonic activity. The study area along the south flank of the central Alborz mountain range in north Iran is an ideal location to test the concept of an index to predict relative tectonic activity on a basis of area rather than a single valley or mountain front. The recent investigations show that neotectonism has played a key role in the geomorphic evolution of this part of the Alborz mountain range. Geomorphic indices indicate the presence of differential uplifting in the geological past. The area surrounding the Amirkabir lake shows very high relative tectonic activity. Key words: Tectonic geomorphology; Geomorphic indices of active tectonics; drainage basin; Central Alborz. INTRODUCTION Surrounding the south Caspian Basin, the narrow Alborz mountain of 100 km wide extends for about 2000 km from Lesser Caucasus in the northwest to the Paropamisus mountains in northern Afghanistan, to the east and shows strong tectonic activity (Berberian and Yeats, 2001). The regional seismic record is characterized by high frequency of relatively small magnitude earthquakes (less than magnitude 4) with infrequent, large, catastrophic earthquakes. The study of active tectonics, and in particular those areas with relatively high activity, in the Holocene and late Pleistocene is important to evaluate the earthquake hazard (Keller and Pinter, 2002). On a regional scale, obtaining rates of active tectonics is difficult or even knowing where to go in a particular region for quantitative studies to obtain rates. The approach of this paper is to provide a quantitative method to focus on areas for more detailed work to establish rates of active tectonics. We use geomorphic indices of active tectonics, known to be useful in active tectonic studies (Bull and McFadden, 1977; Azor et al., 2002; Keller and Pinter, 2002; Silva et al., 2003; Molin et al., 2004). This methodology has been previously tested as a valuable tool in different tectonically active areas, such as SW USA (Rockwell et al., 1985), the Pacific coast of Costa Rica (Wells et al., 1988), and the Mediterranean coast of Spain (Silva, 1994). In detail, the combination of the Smf and Vf indexes allows individual mountain fronts to be assigned different tectonic activity classes (Class 1 to Class 3) developed under decreasing uplift rates (Bull and McFadden, 1977; Rockwell et al., 1985). Most studies of geomorphic indices have concentrated on specific indices at specific sites, such as a drainage basin or mountain front. With the exception of the stream length-gradient index (SL), most of the indices are not spatially analyzed over a region. The objective of this paper is to quantify several geomorphic indices of relative active tectonic and topographic development to produce a single index that can be used to characterize relative active tectonics (Hamdouni et al., 2007). For this purpose, we will present the tectonic geomorphic analysis of indices followed by a discussion of the active tectonics based upon geomorphic analysis. Regional Setting of the Study Area: The Karaj drainage basin of the south Central Alborz in north Iran has an area about 1085 Km and a Corresponding Author: Khavari, R., Islamic Azad University (IAU), Behbahan Branch, Behbahan, Iran Fax: +986714220109 E-mail: [email protected] 969 Aust. J. Basic & Appl. Sci., 4(6): 969-993, 2010 length and width, 59 Km and 37 Km respectively (Figure 1). The Karaj river is drawing from the Gachsar at 2200 m over sea level to Amirkabir lake and continues southward to the Karaj alluvial fan close to 1360 m over sea level. According to Tchalenko et al (1974), study area extends over two units of the Alborz subdivision (Figure 2); the High Alborz, a complex folded zone of Precambrian, Paleozoic and Mesozoic sedimentary rocks thrust southwards over Eocene rocks in the Border Folds zone, forms the hanging-wall of the Mosha fault (Ehteshami Moinabadi and Yassaghi, 2006) and the Border Folds zone contains the Eocene Karaj formation which crops out in a series of synclines in the footwall of fault (Figure 1). The Palaeocene Fajan continental conglomerates throughout northern Iran marks the base of the Eocene Karaj formation (Stocklin and Setudehnia, 1977). The North Tehran fault upthrusts the rocks of the Karaj formation southwards over the alluvial deposits of the Pediment zone (Figure 2). Alluvial deposits are rare in the Karaj river valley, so it flows over a channel of exposed bedrock over much of its length. This shows that the Karaj river is a bed rock river in that driving forces tend to be greater than resisting forces, and most of deposits supplied transport away. Morphometric Analysis and Geomorphic Indices of Active Tectonics: Several geomorphic indices may be used to analyze topography as well as relative tectonic activity. Individual indices are based on analysis of the drainage network or mountain fronts. The indices represent a quantitative approach to differential geomorphic analysis related to erosion and depositional processes that include the river channel, long profile, and valley morphology as well as tectonically derived features, such as fault scarps. Indices of active tectonics may detect anomalies in the fluvial system or along mountain fronts. These anomalies may be produced by local changes from tectonic activity resulting from uplift or subsidence (Hamdouni et al., 2007). The research design is to analyze several different indices in subbasins of the Karaj River basin (Figure 3) and break them into tectonic classes based upon the range of values of individual indices. These are then summed and averaged and arbitrarily divided into classes of relative tectonic activity over the study area (Hamdouni et al, 2007). Stream Length-gradient Index (SL): Development of topography results from an adjustment between processes of erosion such as streams and rivers flow over rocks and soils of variable strength (Hack, 1973). The adjustment eventually reaches a dynamic equilibrium. The stream length-gradient index (SL) was defined by Hack (1973) in a study of the role of rock resistance in streams of the Appalachian Mountains of the southeastern United States. The SL index is defined as: SL = (Äh/Äl)l (1) where Äh/Äl is the local slope of the channel segment being evaluated and l is the channel length from the divide to the midpoint of the channel reach for which the index is calculated. The SL index can be used to evaluate relative tectonic activity. The SL index will increase in value as rivers and streams flow over active uplifts and may have lesser values when flowing parallel to features such as valleys produced by strike–slip faulting (Keller and Pinter, 2002). Values of the SL index over the study area, determined from digital elevation models and geographic information system (GIS), are shown on Figure 4. According to Hamdouni et al 2007, in order to discriminate values at the index related to rock resistance, different levels of average rock strength were defined (by rock type and field observation) from very low strength (alluvial deposits), low strength (slope deposits), moderate strength (siltstone, shale), high strength (sandstone, conglomerate, limestone, and tuff), and very high strength (quartzite, and monzodiorit-monzogabbro). The observed SL index anomalies were then plotted on the map of the relative strength of materials (Figure 5), and their relation to rock strength were analyzed utilizing GIS applications (Figures 6, 7, 8 and 9). Based upon the quantitative SL indices linked to relative rock resistance described above with field observations suggest that: (i)Along the N border of the Karaj drainage basin, values of the SL index show a variable distribution. Along the Varangehrood and Velayatrood rivers, Vashkestanak and Sutak creeks on the eastern part of the Karaj river high indices are not associated with particularly resistant rocks, and so we interpret these anomalies in the SL index to be a tectonic signal (Hamdouni et al 2007) (Figure 6). Several locations along the western part of the Karaj river also have anomalously high SL index values on relatively resistance rocks. Along the Azadbar creek, indices increase where it crosses the mountain front with the Taleghan fault. Along the 970 Aust. J. Basic & Appl. Sci., 4(6): 969-993, 2010 Hasanakdar creek, the highest values of the indices results from active folding of a Pleistocene alluvial fan (Figure 6). Another anomaly, farther downstream along the Hasanakdar creek, exists where the river crosses the hard limestone. (ii)Along the south and southwestern portions of the study area, SL indices tend to be relatively higher than the N border. Anomalous values of the indices along the all rivers and creeks in this part are related to outcrops of the resistant rocks found along the rivers and creeks profiles as well as tectonic activity (Figure 7). (iii)Along the southeastern portion of the Karaj drainage basin the values of the SL index are very high. The highest and perhaps most anomalous values of the index are along the Shahrestanak river on the eastern part of the Karaj river (Figure 8)These high indices are related to changes in rock resistance not associated with tectonic activity.