A Script-Driven Approach to Mapping Satellite-Derived Topography and Gravity Data Over the Zagros Fold-and-Thrust Belt, Iran

The Zagros Fold-and-Thrust Belt is one of the world’s largest petroleum provinces, which is a ca. 1,800-km long zone of deformed crustal rocks, formed as a result of the collision between the two tectonic plates: the Arabian Plate and the Eurasian Plate. The importance of the Zagros belt can be illustrated by the fact that it contains about 49% of the existing hydrocarbon reserves of the fold and thrust belts (Cooper, 2007). The Zagros Fold-and-Thrust Belt is formed along the line of the oblique convergence of the Arabian Plate, which moves northwards to the Eurasian Plate at a rate of ca. 3 cm/year (Talebian and Jackson, 2004).

The structural geologic complexity of Zagros belt results from the geologic deformation. It can be illustrated by the presence of the out-of-sequence, basement-rooted thrusts, incompetent cover strata or folds superimposed upon the pre-existing structures. It also includes a sequence of heterogeneous latest Neoproterozoic–Phanerozoic sedimentary cover 7–12 km thick strata overlying the Precambrian crystalline basement (Alavi, 2007). The Zagros suture, stretching between the Arabian and Eurasian plates, is directed along a line within southeast Turkey through northern Iraq and southern Iran. Southwest of this suture, the continental margin of the Arabian plate is deformed by folds, thrusts and strike-slip faults within the Zagros mountains. Nowadays, the geologic dynamics in the Zagros mountains continues with the deformation front lying roughly parallel to the Iranian shoreline of the Persian Gulf.

Regional tectonic movements can be illustrated by gradual growth of the Turkish–Iranian Plateau which incorporates the adjacent parts of the Zagros Fold-and-Thrust Belt (Allen et al., 2013).

The geologic stratigraphy of Zagros fold-thrust belt has recently been updated and classified as four groups of rocks:

Precambrian to Devonian group (the lowest), which consists of several subgroups:

Neoproterozoic to Cambrian rocks: evaporites, siliciclastic deposits and carbonates;

A notable part of the geomorphology of Zagros is presented by the Fars arc and thrust belt, propagated under the impact of the Oman Peninsula. The oblique convergence between the Arabic plate and the Iran block mostly extends along the Fars arc (Aubourg et al., 2008). The hydrocarbon production is notable for the Southwest Zagros where oil fields are concentrated. Among others, the Asmari Oligo-Miocene Formation is well-known since it contains a lot of the hydrocarbon reserves of this area (Ahmadhadi et al., 2008). Besides the petroleum resources of Zagros, southern Iran has a remarkable salt plugs segmented by salt diapiric provinces: Hormuz, Shiraz-Kazerun and Nyriz-Jahrum sub-basins, which is connected with the structure and evolution of the Zagros Fold-and-Thrust Belt faulting, as described in details (Bahroudi and Koyi, 2003; Jahani et al., 2009; Khodabakhshnezhad and Arian, 2016).

The tectonics of Zagros has a spatially diverse style with a general trend of northwest–southeast-oriented folds and thrusts, in the simple folded zone, and right-lateral strike-slip on the main recent fault. The fold geometries of Zagros indicate several décollement horizons including shale units, evaporites in the Neogene, Mesozoic, Lower Palaeozoic and upper Proterozoic successions (Blanc et al., 2003). A detailed stratigraphy of the tectonic sub-division of Zagros Suture Zone formed since Cretaceous was reported earlier (Ali et al., 2012, 2014; Authemayou et al., 2006).

The geologic stratigraphy of Zagros mirrors major steps in its evolution that can be briefly characterised as follows. The subduction of the Paleo-Tethys induced the formation of the Zagros Basin, which is caused by intense rifting, evolved in the Jurassic period. The main rocks of the Mesozoic and Cenozoic reservoirs (carbonate, shale and evaporite) are sourced from the continental marginal deposit between the Arabian Shield and Zagros belt, extending to over 1000 km southwards. Nowadays, such particular structural-geomorphological features and topographic fronts at the surface of Zagros are the main regions of earthquakes in Iran (Berberian, 1995). The thrusts, frontal asymmetric anticlines, escarpments and Quaternary deformation mark the topographic fronts on the relief surface of Zagros. They have vertically displaced geologic beds, which highlight a deep correlation between the topographic, geomorphological and geologic structure of Zagros Fold-and-Thrust Belt.

The proposed study is performed using a combination of methods using open source geoinformation tools such as console-based GMT, a graphical user interface (GUI) QGIS and programming language R. Full scripts used for mapping by R and GMT are publicly avaiilable on author’s GitHub repository for repeatability in similar studies: https://github.com/paulinelemenkova/Mapping_Iran_R_GMT_Scripts-. The datasets include openly available geospatial raster and vector layers as follows.

The topographic map (Figure 1) was based on GEBCO/SRTM (GEBCO Compilation Group, 2020), a high-resolution (15 arc-second) raster grid of the Earth relief. The GEBCO grid has been widely used in geoscience papers where its advantages were used (Lemenkova, 2020a,d, 2021e; Lindh and Lemenkova, 2021a). GEBCO presents a combination of land topography with partially measured and estimated bathymetry using remote sensing data.

Figure 2.

Figure 3.

Figure 4.

Figure 5.

Figure 6.

Figure 7.

Figure 8.

Figure 9.

Figure 10.

Figure 11.

3.1.

Mapping by GMT

The maps in GMT are performed in several stages using several GMT modules (Wessel et al., 2019) using occasional data processing (converting, projection, retrieving data information) by GDAL (GDAL/OGR, 2021). The general steps are given as follows. Image capture and visualisation by the code 1:

Listing 1.

The topographic image was uploaded to GMT and visualised using the coordinate extent by using the ‘grdimage’ module. Adding cartographic grid by the code 2 using the ‘psbasemap’ module of GMT:

Listing 2.

Text annotations are plotted using the code in 3 (abbreviated example with selected names), where the ‘56.7 26.0’ in the line 3 signify the longitude/latitude coordinates of the location and the ‘+a-80’ flag means the rotation of the text. Similar logic has been used throughout:

Listing 3.

The insert globe map was added on the main image using the code 4:

Listing 4.

Similar logic of the modular-based cartographic plotting was applied for maps in Figures 1, 2, 5 and 6. The PostScript files were converted into JPG format by ‘psconvert’ using the code 5:

Listing 5.

The 3D modelling in Figure 5 (3D model of Zagros) was performed using the code in Listing 6:

Listing 6.

3.3.

Plotting by R language

The maps in Figures 7–11 are plotted using R language in RStudio integrated development environment, which combines a console for coding and a graphical menu for visualisation of plots. The main approach by ‘raster’ package of R consists in visualising geomorphometric models using DEM. The data were captured using the following code 7:

Listing 7.

To extract the relief characteristics of the Iranian Plate, the ‘raster’ package of R was used. The results were visualized as several thematic maps Figures 7 to 11 presenting a series of morphometric analyses, modelled in ‘raster’ package with a default visualisation and then adjusted in ‘tmap’ package for a better cartographic setting: 1) slope analysis, Figures 7 and 8; 2) aspect maps, Figure 9; 3) hill shade map, Figure 10; 4) DEM elevation map, Figure 11.

The ‘raster’ package has a straightforward syntax and rather simplified manner of cartographic data processing. The code for modelling is as follows: slope = terrain(alt, opt = ”slope”), where the ‘opt’ function was changed for ‘aspect’ and ‘alt’ aimed to visualize aspect and elevation. The hillshade was visualised using the theoretical terrain computations described in Horn (1981). The value of hillshade modelling consists in mapping the surface topography, which includes visual depth cue. The visualisation is theoretically based on the dependence of the elevation depicted on a map on the tone of gray or colour level, as well as the compass orientation of the topographic surface elements. The hillshade was visualised by the code 8:

Listing 8.

The example of the code for cartographic adjustments is as follows, Listing 9:

Listing 9.

The default visualization of the ‘raster’ package is illustrated in Figure 7, which shows a slope map of Iran. After the initial models were created, the data for each map in Figures 8–11 were imported to the ‘tmap’ package where cartographic processing is more sophisticated. For instance, the cartographic elements are adjusted using special flags in the options of the function.

The ’tmap’ functions ’tm scale bar’, ’tm raster’ and ’tm graticules’ were used for plotting self-explaining cartographic elements. The main adjustments of the graphical output were set up in the ’tm layout’ function where the following elements were visualized (colour, size, locations of the elements on the map): legend, panel annotations, title and labels.

The relationship between the geomorphological elements, such as slope, aspect, elevation (Figures 8–11), 3D topographic model (Figure 5), lithologic and geologic structure of Iran (Figures 2 and 3) was used to analyse the variations of the homogeneity of the relief, i.e., if the area has an increasingly higher variations in slope and steepness in Zagros Mountains or the dominating landforms are the stable low-level lowlands mostly located in the basin of tributaries of Tigris River in the south-western part of Iran. Then the histograms for each of the morphometric maps were created using the ’legend.hist = T’ option in the ’tm raster’ function based on the automatically analysed data distribution.

The comparisons of the topographic, geological, geomorphological and geophysical maps as well as the values of general morphometric elements (slope, aspect, hillshade and elevation) with the relief map revealed that the regions of the Zagros fold and thrust zones often corresponds to differing morphometric properties. For instance, higher values of Elburz and Zagros ranges, selected ranges of Kopet Dag and Makran slopes (Figure 1) shown as bright red colours in Figure 8 are associated with the extreme steep slopes, while major deserts of Iran, such as Dasht-e Kavir, Dasht-e Lut and Great Salt Desert (Figure 1), are mostly associated with the slopes having the lowest steepness (grey colours in Figure 8).

Three distinct Fold Belts shown in Figure 3 – the Kopet Dag, Elburz and Zagros – have the dense locality of major faults (shown red in Figure 3) that are associated with the geologic formation controlled by the tectonic constraints on the evolution (e.g., salt-bearing Neogene sediments are folded and cut by faults in Alborz Mountains), as it is also reported in the existing works (Baikpour et al., 2010; Burberry et al., 2010). Variations in the fold patterns, geologic characteristics and deformation style can be seen by comparison of the Figures 2 and 4 showing the three-dimensional and plain map visualizations of topography and geology.

The variations in the relief changing according to various areas of Zagros belt are also reported in previous publications (Casciello et al., 2009; Hessami et al., 2001; Jiménez-Munt et al., 2012). The geologic structure of Zagros Fold-and-Thrust Belt, including visualized geologic provinces, major faults, surficial lithologic boundaries and bedrock geology also affects both the local shape of geomorphic landforms of Iran and the slope steepness.

The geophysical discontinuities (Figure 5)) and free-air gravity anomalies (Figure 6) were demonstrated by the two GMT-based maps showing a general increase of the geoid undulation values in southeast–northwest direction. That is, the Indian geopotential low-level geoid anomalies can be clearly seen (Figure 5) as low values (below 60 m, coloured blue to deep blue in Figure 5 while Zagros fol and thrust belt are mostly covered by the slightly positive values (above minus 10 to 5: yellow to orange colours in south-east Zagros area), followed by the strongly positive values of above 5 m (bright red colours in Figure 5), which shows an increase of the geoid heights along the Zagros mountains chain.

The basin of Persian Gulf demonstrates mostly negative values from minus 65 to minus 40 m (light blue colours in Figure 5) with slightly negative values in the Tigris and Euphrates rivers (aquamarine colours, from minus 40 to minus 20 m). The general geophysical properties, such as free-air gravity (Figure 6), also correspond to the geologic rock types and lithology of the region. This can be seen by the comparison of Figures 3 and 6 where isolines of gravity correspond to the lithological rock types distribution, which points to the strong correlation between the geophysical and geological setting due to the rock density, porosity and structure.

The geomorphometry of Iran in general and the Zagros mountain in particular, as its most remarkable landform and relief feature (e.g., Elburz, Kopet Dag mountain chains) is visualized in the set of figures plotted in R. The specifics of the Elburz Mountains includes alluvial fans, which are particularly well developed along their southern margins, as also noted in previous studies (Beaumont, 1972) The evaluation of slope (Figures 7 and 8), aspect orientation of slopes (Figure 9), hillshade with artificial illumination highlighting the relief (Figure 10) and DEM-based visualization of the geomorphological properties better highlight the geologic structure of Zagros and the surrounding relief (Figure 11). These models were plotted to show complex landforms along the Zagros Fold-and-Thrust Belt and in other regions of country.

The wide river valleys (e.g., tributaries of the Tigris River) have a gentler, almost flat relief with low-leveled, smoothed and gentle slopes. On the contrary, mountainous areas have steeper slope gradients, suggesting that the geologic evolution of the Zagros Fold-and-Thrust Belt formation created a complex alteration between the large and gentle landforms. At the same time, local development of the flattened relief in the Dasht-e Kavir, Dasht-e Lut and Great Salt Desert is evident by the comparative analysis between the maps in Figure 1 with Figures 7 and 8 (slope), as well as Figure 9 (aspect), with Figures 10 and 11 (relief).

This paper addressed the issues of cartographic data processing and visualization using scripts and traditional GIS. Specifically, the use of QGIS and GMT was presented and proposed an efficient scheme of 2D and 3D modelling of the multi-source spatial data. Different from traditional GIS, the GMT can not only process geospatial data in an accurate and rapid way effectively, but also enables advanced 3D modelling. The performance and functionality of both GMT and QGIS were evaluated comprehensively on 11 datasets used for plotting a series of maps. Moreover, the geologic analysis and the comparison of maps were performed using new maps and the available literature.

This study presented a script-based mapping of the geomorphology of Zagros Fold-and-Thrust Belt using a combination of GMT, R and QGIS tools for 2D and 3D modelling. The 2D and 3D visualization of geomorphology of Zagros Basin, together with mapping of the geophysical fields, helped to analyse the tectonic-geologic structure of this region formed under the strong influence of complex interaction between the Arabian and Eurasian tectonic plates. The integrated use of GIS and GMT presented a well-balanced combination of technical cartographic tools for 2D and 3D visualization of the topographic, geological and geophysical data.

The actuality and technical novelty of this research consists in the scripting used for mapping. Besides, the integration of several tools was applied for interpretation of the geomorphology of Iran, which is largely affected by geophysical and geodynamic setting. This study is based on using multi-source data from the open source datasets: topographic (GEBCO/SRTM, ETOPO), geophysical (Faye’s gravity, geoid EGM-2008), geological and tectonic mapping, 2D and 3D data models, lithological maps from the UNESCO datasets and DEM embedded in R library ‘raster’. The data were integrated to construct a regional project on the Iranian geomorphology and to develop a series of maps covering Zagros area and the surroundings. The integration of the high-resolution datasets resulted in the 11 new maps presented in this study. Moreover, the study was supported and complemented by the extended analysis of the literature resources on the Zagros Fold-and-Thrust Belt and the geomorphology of the Iranian Plate.

The presented multi-source thematic geodata modelling helped to describe the geomorphic variations in Zagros Fold-and-Thrust Belt and assist in the monitoring of its mineral resources. The main results define the significant geomorphological variations along the Zagros mountains belt and within Iran. In addition, using the geomorphometric analysis by means of R language, this paper provided a numerical evaluation and visual presentation of the slope, aspect and hillshade relief visualization illustrating the topographic structure of the Zagros basin.

Using scripting technical approach by a combination of GMT and R supported by the GIS mapping in QGIS, the multi-source datasets combined from the open data sources on Iran have been compiled and visualized in conjunction with new tools of cartographic data analysis with the aim of providing new interpretations of the geomorphology, geology and topography of Iran. Thus, this paper provides a better cartographic picture of the geologic structures, geophysical anomaly fields, 2D and 3D models and tectonic deformation lineaments of Iran with a particular focus on Zagros mountains as a major geomorphological structure of the country. Besides Zagros mountains, the Elburz and Kopet Dag belts were visualized and analysed together with Dasht-e Kavir, Dasht-e Lut and Great Salt Desert as the significants geomorphological structures of Iran.

These geomorphological zones were analysed by means of R geomorphometric analysis and quantitative maps were presented for the slope and aspect maps showing histograms of data frequency for each class of the slope steepness and aspect orientation by the compass direction (major West, East, South and North and the derivative ones), which divide the country into the zones with different geomorphological and topographic patterns well correlating with geophysical setting, and geological and lithological structure. These variations mirror different geological structural characteristics developing along the Zagros Fold-and-Thrust Belt during the tectonic plate collision (Arabian and Eurasian) as a result of the geologic evolution (Schoenbohm, 2022).

The automation in cartographic analysis has been the point of discussion in various papers (Ebi, 1995; Gilliot et al., 1993; Kazemi et al., 2005; Lemenkova, 2019a, 2022c,d,e; Lopez-Ornelas and Sedes, 2008) and widely used recently along with rapid progress of the machine learning and development of the scripting languages. The presented integrated combination of various scripting mapping tools in one project, as demonstrated in this paper, has shown the effectiveness of the computer-based methods in cartographic workflow with a case of the geomorphology of Iran with a special focus on Zagros Fold-and-Thrust Belt. Thus, this study hybridizes the traditional mapping presented by the QGIS and the scripting cartographic method of GMT and two special packages of R (’raster’ and ’tmap’) used to visualze 2D and 3D topographic, geophysical and geological maps of Iran.

The importance of the Zagros Fold-and-Thrust Belt as a study area is explained by its unique geologic and geomorphological structure, which contains vast mineral resources of Iran (Motaghi, Shabanian and Kalvandi, 2017; Regard et al., 2004; Saura et al., 2015; Sepehr and Cosgrove, 2004). At the same time, this region is subject to seismic activity and earthquakes, as reported in various papers (Nissen et al., 2011; Paul et al., 2006; Talebian and Jackson, 2004; Tavakoli et al., 2008; Yamini-Fard et al., 2006). Besides, the Zagros mountain chain significantly influences the surrounding nature, environmental and climate conditions of Iran as a major distinct topographic landmark of the country controlling the habitats, vegetation and landscape distribution, and serving as a key geomorphological structure of Iran.