Application of Geomorphic Signatures in Relative Tectonic Activity Assessment of a Red Sea Coastal Basin between Al Farrah and Heelan, Saudi Arabia

Abstract

This work used an analysis of the geomorphic indices to effectively assess the relative tectonic activity of a Red Sea coastal region in Saudi Arabia between Al Farrah and Heelan. This approach is useful in examining topographical and geomorphological signatures in different landscapes. Through a detailed investigation of geomorphic indices, the study basin’s active and inactive characteristics may be observed and distinguished. The applied indices include a rock strength index, stream length gradient index, hypsometric integral index, drainage basin analysis index, mountain front sinuosity index, and valley floor width-to-valley floor height ratio index. The results obtained from this study are discussed and presented as a unique index of relative tectonic activity (Rta), which is divided into three different classes: low, moderate, and high tectonic activity. There have been few studies of active tectonics in the study basin along Saudi Arabia’s eastern Red Sea coast, making it an excellent choice to evaluate and simulate the relative activity based on large-scale basin analysis. The study basin exhibits variable classes of tectonic activity resulting from the Red Sea extension event. The idea that areas with anticipated relatively high rates of tectonic activity are coupled with indicators of Rta index values is examined in this study.

1. Introduction

Significant insights into recent seismic activity and tectonic evolution can be gained by examining drainage systems and sub-basins along major tectonic features [1]. Various studies were attempted by several researchers to investigate the evolution of drainage networks along faults, uplifting rocks, and active zones [2,3,4,5,6]. In some of these studies, mountain fronts were analyzed because of the interaction between erosional and tectonic processes [7], plateaus were created from mantle-driven uplift such as the Colorado Plateau in the US [8], and graben structures were mostly formed by active gravitational faults [9]. Understanding certain features of tectonic geomorphology allows for the observation and modeling of seismic signals, which are frequently encountered in active regions [10]. Recently, an integration of remote sensing including geospatial analysis and tectonic geomorphology applications has been widely used to investigate environmental risks such as seismic and flash flooding hazards [11]. Several geomorphological signatures such as incision and uplift rates for basins and active signals along active faults can be inferred through deep studies of geomorphological indexes and digital elevation models [12]. Geomorphological indexes are applied, including the valley floor width-to-valley floor height index (Vf), mountain front sinuosity index (Smf), stream length gradient index (SL), basin asymmetry (Af), drainage basin shape (Bs), and hypsometric integral and curves (Hi and Hc) [13,14,15]. Generally, studies on geomorphological signals, active mountain fronts, and drainage system development along major tectonic features are scarce [16]. Through an in-depth examination of fault segments and drainage analysis, seismic signatures can be observed and recorded [1,12]. Usually, regional seismic signals typically exhibit high frequencies associated with comparatively large earthquakes [3]. On a regional scale, it might be challenging to detect the rates of active tectonics or even know where to go in a particular region for quantitative investigations. In this study, we used the geomorphological indices of active tectonics research, which are known to be significant for comprehending changes in the tectonic signals of the study basin [6,17,18,19]. This quantitative model has been widely tested as a useful technique in several active tectonic regions, such as eastern Turkey [1], the southwestern US [20], northwestern Pakistan [19], and the eastern desert of Egypt [7]. Generally, the relationship between mountain front sinuosity (Smf) and valley floor width-to-valley floor height (Vf) indices provides three tectonic activity levels, assigning different uplift rates for every individual mountain front [3]. Generally, most geomorphic application papers have focused on specific geomorphic indices at specific locations, such as the mountain front and drainage basins [7]. Except for the stream length gradient index (SL) index, most of the indices are not spatially analyzed over the region [3,7].

The study basin is located between 26° and 27°30′ N and 36° and 38° E along the eastern Red Sea coast in Saudi Arabia, covering about 104,586.74 km² (Figure 1). It is delineated and shaped through several processes in ArcGIS 10.4 software employing hydrological tools. It extends between Al Faraah City in the north and Heelan City in the south. It covers several more cities including Alyutamah, Madinah, Al henakiyah, Khaybar, Al ais, Al mangor, Mogayra, and Al Ula (Figure 1). To give a unique indicator that can be used to designate the various tectonic activity classes of the research basin, the primary goal of this paper is to investigate and compute a number of effective geomorphological indicators of tectonic activity remarks and topographic characteristics.

2. Geological Setting of the Study Basin

A significant portion of the Arabian plate is occupied by Saudi Arabia, and to the west of it is the Red Sea. The geodynamic processes that have led to the Red Sea’s opening and present enlargement have a significant impact on and control over the geology and tectonics of western Arabia. The study basin is located along the eastern side of the Red Sea basin in northwestern Saudi Arabia. Geologically, major rock groups are assigned from Cambrian to Tertiary [21]. The lithology of the study basin includes several types of rock units (Figure 2). Syenogranite, syenite, monzogranite, and gabbro units were recorded as small units at the southern part of the study basin nearing Heelan (Figure 2). Within the area between Al henakiyah and Heelan, shale, siltstone, arenite, conglomerate, and felsic tuff lithologies were recorded. Also, a long strip of sand, silt, gravel, evaporites, and coral limestone is observed in the middle part of the study basin. Granite, granite gneiss, granodiorite gneiss mylonite, and gabbro were observed in several sites in the study basin (Figure 2). Basalt, andesite, dacite, rhyolite, felsic tuff, volcaniclastic arenite, and limestone cover most of the study basin, and they are mainly concentrated in Khaybar in the middle part. Alkali-feldspar granite and monzogranite are recorded at the northeastern part of the study basin. More lithological details are illustrated in Figure 2. According to certain geologists, the shield has been impacted by multiple cycles of metamorphism, tectonism, and plutonism of varying strengths, all of which have an impact on the study basin situated in western Arabia [22,23,24]. The NW and NE faults are the two main Tertiary tectonic features that dominate the western Arabian and Red Sea regions. The NE-trending faults in the Red Sea region can be divided into two groups: newly developed transverse faults associated with the opening of the Red Sea and ocean floor spreading, or faults controlled by older Precambrian ones that were reactivated during the Tertiary [22]. There is likely a connection between the grouping of NE-trending faults in the west-central Arabian region and the center Red Sea and preexisting Precambrian faults in this area [23].

The study basin offers a variety of morphological features because of the influence of tectonics and various types of rocks. Generally, variations in morphological features, such as variations in locations and altitudes, are what directly produce the variances in climatic conditions within the same season. In terms of topographic effects, the study basin landscape’s surface elevation recorded its highest values at the north, east, and southwestern parts by 2359 m a.s.l., whereas the eastern sections of the study basin show the lowest topographic values (Figure 3a). Consequently, the study basin’s overall surface elevation can be summed up as a significant landscape with a high elevation in the east that progressively descends towards the Red Sea (Figure 3a). Additionally, ArcGIS 10.4 is used to generate slope maps from the DEM. After analysis, the slope data produced values ranging from 0 to 36 (Figure 3b). The highest slope values are often found along the upstream basins, and the basin mouths are indicated by the lowest values [11]. As a result, topographical variation signals govern changes in the study basin’s overall slopes.

3. Material and Methods

Using a geospatial analysis technique, the geomorphic signatures over the study area basin between Al Faraah and Heelan along the eastern flank of the Red Sea were assigned. This method is applied by using digital elevation models (DEM) with a 30 m resolution obtained from a Shuttle Radar Topography Mission (STRM). The basin’s drainage systems were extracted using the hydrology tools in ArcGIS 10.4 software, which allowed for the classification of the basin into 42 sub-basins with various geometries. These 42 sub-basins were automatically delineated using ArcGIS software’s hydrology tools and watershed extraction method, considering the drainage systems that cover the entire area of the study basin. Figure 4 illustrates the different procedures and processes that the technique goes through. This study inferred geomorphological signatures using a variety of geomorphic indices to provide a model that suggests the relative tectonic activity of the study basin. Whereas some geomorphic indices are determined by analyzing the mountain fronts, others are calculated based on drainage analysis [7,19]. Geomorphic analysis studies may record different anomalies along mountain fronts or drainage streams [1,3,19,25]. These anomalies can be observed by local variations in relative tectonic activity that result in uplift or subsidence. In this study, several indices were applied to various sub-basins and based on the ranges of values for each indicator, and the sub-basins were classified into three distinct tectonic classes or levels. Following that, these indices were computed, averaged, and split arbitrary into three groups, each of which represented a different degree of relative tectonic activity (Rta). At the conclusion of the Methodology Section, a detailed explanation of the calculation of the Rta index is provided.

3.1. Rock Strength (Rs)

The rock strength index was designed using the scheme created by Ref. [26]. The definition of rock strength was provided by the author in Ref. [26], who linked the hardness levels to the cement and constituents that aid in the resistance against weathering and erosion processes [10,26]. He divided the strength of rocks into five categories: (a) very high strength level (gabbro, marble, granitoid, quartz schist, and gneiss), (b) high strength level (basalt), (c) moderate strength level (shale, sandy limestone of Eocene), (d) low strength level (sandstone, conglomerate), and (e) very low strength level (gypsum, alluvium, marl).

3.2. Stream Length Gradient (SL)

When rivers and streams flow across rocks and soils of varying strengths, erosional processes adjust to the terrain, resulting in topographic evolution [19]. Several studies have successfully used this useful measure to examine various topographic stream gradients along channels and rivers in different basins to determine the relationship between the erosional strength of rocks and relative tectonic activity [5,18,27,28]. The SL index can be calculated as follows:

SL = (∆H/∆L) × L

where ∆H/∆L provides signals about the stream slope of the basin reach of the mean basin, and the symbol L represents the length from the reach midpoint to the basin divide. Using SRTM DEM and GIS 10.4 software (Figure 4), we computed the SL index along streams and rivers and determined its standardized average value for each sub-basin (Figure 5).

3.3. Basin Asymmetric Factor (Af)

At the drainage basin scale, the basin asymmetric factor index (Af) can be used to assess the presence of tectonic activity. It is a metric for measuring and documenting tectonic tilting in relation to a basin’s size [7,29,30]. This index is generally applied to a relatively large region. The Af index is defined as follows:

Af = 100 (Ar/At)

The parameters of this formula can be defined as follows: Ar is the total of the basin to the right side of the main basin stream, and At measures the entire area of the basin. A change in inclination perpendicular to the direction of the stream might cause the Af index to change (Figure 6). An Af index significantly higher or lower than 50 indicates the influence of differential erosion, lithologic control, or active tectonics. One example of this would be the stream gradually eroding through bedding plains [3]. If the Af index is near 50, there is either not much or no tilting perpendicular to the trunk channel’s direction (Figure 6). The landforms in tectonically active terrain are distinguished by comparatively steep, mountainous sides and floors. The faults that cause the valley floor to move downward in relation to the surrounding margins or upward in relation to the floor are what cause the steep sides. With an arrow pointing in the direction of asymmetry, we expressed the Af index as the absolute value minus 50 to represent the tilting direction of the study sub-basins.

Af = |(Ar/At) × 100 − 50|

In other words, the values of Af represent Af − 50, or the difference between the neutral value of 50 and the observed values. Therefore, to evaluate the relative tectonic activity, the absolute difference is the most significant value.

3.4. Hypsometric Integral Index (HI)

The hypsometric integral index (HI) is an index that is independent of the basin area and is typically computed for a particular drainage basin. It is an indicator parameter that gives details on how elevations are distributed throughout a landscape study area [19,31,32]. The index represents the volume of a basin that has not been eroded and is defined as the region below the hypsometric curve [7]. The authors in Refs. [33,34,35] have applied an effective formula to illustrate this index. They use the following formula:

HI = E (mean) − E (min)/E (max) − E (min)

where E (mean) is the mean basin elevation, E (max) is the maximum elevation of the basin, and E (min) is the minimum elevation of the basin. This index and the SL index are comparable in that the value is influenced by other factors in addition to rock strength. High HI values typically indicate a reduced amount of erosion in the uplands, suggesting a more recent landscape, potentially resulting from active tectonics. A recent incision into a young geomorphic surface created by deposition could also be the cause of a high HI. According to our analysis of the HI, elevation along a fault may be related if a part of the HI is convex in the lower portion. Low values are associated with older landscapes that have been more degraded and less affected by recent active tectonics, whereas high values may be associated with young active tectonics (Figure 7).

3.5. Drainage Basin Shape (BS)

In regions of active tectonics, very young drainage basins typically have an extended shape parallel to the mountain’s topographic slope [19]. In tectonic geomorphology studies, most authors have stated that young and new basins in active regions generally show elongated shapes perpendicular to the slope of a mountain [3,6,9,19,30]. On the other hand, circular basin shapes are formed when tectonic activity starts to be reduced or ceases [3,6]. The reason for this transformation is that in tectonically active areas, where the stream’s energy has been directed primarily towards downcutting, the drainage basin widths are much narrower near the mountain front; in contrast, a lack of continuous rapid uplift permits widening of the basin upstream from the mountain front. This index is defined as follows:

Bs = Bi/Bw

where Bi is the distance from the mouth to the headwaters in each basin, and Bw represents the basin width projected at its widest ends. Extended basins are linked to elevated Bs values, which are typically indicative of heightened tectonic activity (Figure 8). Lower Bs values are indicative of a more circular-shaped basin, which is typically linked to less tectonic activity (Figure 8). Consequently, Bs might represent the rate of active tectonics [1,7].

3.6. Mountain Front Sinousity (Smf)

In tectonic geomorphology, the mountain front sinuosity index (Smf) is used to determine whether mountain fronts are the result of erosional processes or tectonic forces [5,10,18]. Straight fronts with low Smf values are the result of uplift prevailing over erosional processes in active mountain fronts. Erosion processes will produce uneven or sinuous fronts, with high values of Smf along less active fronts. This index is computed as

Smf = Lmf/Ls

where the Lmf symbol measures the zigzag or un-straight line along the examined fronts, and Ls provides the length of the straight line of these given fronts. Using Lmf and Ls values obtained from the SRTM elevation model with a spatial resolution of 30 m, the values of Smf were computed for the recorded mountain fronts (Figure 9). Several researchers have used this index to assess the activity levels of structural lineaments, including fractures and faults. According to the value of this index, they classify the fronts into less, moderately, and highly active tectonic fronts as (1 < Smf ≤ 1.5); (1.5 < Smf ≤ 2.5); and (Smf > 2.5), respectively [3,10,12].

3.7. Valley Floor Width-to-Valley Floor Height Index (Vf)

The ratio of a valley’s width to its height inside a single valley is called the valley floor width-to-valley floor height index [10,17]. It is mainly an effective geomorphic index designed to distinguish between flat-topped valleys that are U-shaped and V-shaped (Figure 10). The values of this index are computed from the following formula:

Vf = 2Vfw/[(Eld − Esc) + (Erd − Esc)]

where Vfw is the measured width of the valley floor; Eld and Erd reflect the elevations of the valley divide on the left and right sides, respectively; and Esc is the averaged valley floor elevation. Since incision and uplift are related, this index is considered a stand-in for active tectonics, where higher rates of incision and uplift are linked to lower values of Vf (Figure 10). This index is calculated along several streams in each sub-basin, providing just one average value for every single sub-basin.

3.8. Relative Tectonic Activity Index (Rta)

By employing the technique designed by the authors in Ref. [3], the current study classified each of the geomorphic indices into three different tectonic levels (from 1 to 3). Following that, the study calculated an average index that we name the relative tectonic activity (Rta). Based on the Rta index, the authors defined the study basin into three classes: (a) class 1, which is high tectonic activity (1 < Rta ≤ 1.5); (b) class 2, which is moderate tectonic activity (1.5 < Rta ≤ 2); and (c) class 3, which is low tectonic activity (Rta > 2).

4. Results

4.1. Rock Strength (Rs)

The geological units of the study basin consist of different types of lithology. They include granite, gneiss, tonalite, gabbro, monzodiorite, basalt, andesite, dacite, conglomerate, sandstone, shale, siltstone, etc. For more details about the geological units, see Figure 2. The geological units were classified into three different categories: low, moderate, and high strength. Low-strength units are recorded in small parts at the north, south, and middle of the study basin. Moderate-strength units have bigger spaces than the low units and were dominant in the southern and northern parts, respectively. High-strength units occupy the majority of the study basin (Figure 11).

4.2. Stream Length Gradient (SL)

In this study, the SL index gives values between <100 and >200 along the main streams draining over the study basin. The streams with the lowest values (<100) mainly run over low-strength units in the middle and eastern parts. The streams with the lowest values run over some places with moderate- and high-strength units (Figure 11). Streams with moderate values were observed over all types of rock strengths (Figure 11), but they predominantly run over high-strength units. Streams with the highest values (>200) run over short incision passes over the study basin. They cut some faults/fractures at northern, southern, and eastern parts. In some places, they run over low- and moderate-strength rock units (Figure 11). Several anomalies were recorded in several parts of the study basin.

4.3. Basin Asymmetric Factor (Af)

The basin asymmetry factor index was applied on 42 sub-basins of the study basin. It defines four different types of sub-basins according to the degree of tilting of individual basins. Symmetrical or near-symmetrical sub-basins were recognized in several areas of the study basin by 10 sub-basins (6, 8, 16, 18, 19, 20, 36, 38, 40, and 41) (Figure 12). Sub-basins with a basin asymmetry factor of class 1 were recorded mainly in the southern part of the study basin. This class also occupies some sub-basins in the northen part of the study basin. Class 2 of the basin asymmetry factor was recorded by only six sub-basins (1, 7, 14, 17, 26, and 35) (Figure 12). It was recognized as a less frequent class over the study basin. Finally, class 3 of the asymmetry factor was observed for adjacent sub-basins in the western part of the study basin (3, 4, 5, and 13) (Figure 12).

4.4. Hypsometric Integral Index (HI)

In this study, the hypsometric index values vary from 0.26 (sub-basin 32) to 0.48 (sub-basin 35). The values demonstrate that the lowest and highest values of the HI index are observed close to each other at the southern portion of the study basin (Figure 13). The HI values within the study basin are distributed over 40 different sub-basins. Hypsometric curves vary between concave and S-shaped curves in this study (Figure 13).

4.5. Drainage Basin Shape (Bs)

In this study, the drainage basin index presents values between 0.45 (sub-basin 11) and 3.52 (sub-basin 7) (Figure 14). Similar to the hypsometric integral analysis that has been conducted in this study, sub-basins with the lowest and highest Bs index values are observed to be connected. The rest of the sub-basins vary between circular and elongated sub-basins (Figure 14).

4.6. Mountain Front Sinousity (Smf) and Valley Floor Width-to-Valley Floor Height (Vf) Indices

In this study, the mountain front sinuosity index (Smf) and the valley floor width-to-valley floor height index were calculated for 42 sub-basins in the study basin. The Smf results were extracted along the faults and fractures that were mapped, and the Vf values were recorded across the main channels and streams for each individual sub-basin. The Smf values vary from 1.01 (sub-basin 18) to 1.17 (sub-basin 22) at the northern and middle parts of the study area, respectively (

Table 1. Smf and Vf values of the studied sub-basins.

Basins	Smf	Vf	Basins	Smf	Vf
1	1.14	0.86	22	1.17	0.8
2	1.05	0.40	23	1.03	1.6
3	1.03	0.30	24	1.03	0.14
4	1.15	0.21	25	-	0.44
5	1.03	0.32	26	-	0.20
6	-	2.96	27	1.00	2.92
7	1.48	1.22	28	1.03	0.42
8	1.06	0.12	29	1.02	2.66
9	1.01	0.35	30	-	2.9
10	1.05	0.47	31	-	3.2
11	1.14	0.67	32	-	5.5
12	1.11	2.62	33	-	3.7
13	1.00	1.66	34	-	2
14	1.00	4.41	35	-	-
15	1.07	0.82	36	-	3.71
16	1.07	0.63	37	-	6.28
17	1.03	1.02	38	1.12	-
18	1.01	0.34	39	-	-
19	1.12	2.43	40	-	0.76
20	1.15	4	41	-	3.2
21	1.04	0.73	42	-	1.9

). The Vf values vary from 0.2 (sub-basin 26) to 6.28 (sub-basin 37) (Table 1). Several sub-basins did not show any structural elements, so they do not provide Smf values (Table 1).

4.7. Relative Tectonic Activity (Rta)

Generally, Rta (S/N) values fall between 1.2 and 2.4. The study basin’s northern, eastern, and southern sections are mostly home to class 1 Rta values (high tectonic activity), although the middle sections of the eastern and western regions show class 1 to class 2 Rta values (moderate to high tectonic activity). Just six sub-basins (3, 4, 5, 12, 17, and 22) had a Rta class of 3, indicating comparatively little tectonic activity (Table 1). Rta values showed 61.68% of the area in class 1, 31.23% in class 2, and 7.09% in class 3 (

Table 2. Classes of Rta (relative tectonic activity index) in 42 sub-basins in Al Farrah and Heelan basin (Af: basin asymmetry factor; HI: hypsometric integral; Bs: drainage basin shape; Smf: mountain front sinuosity; Vf: valley floor width-to-valley floor height ratio).

Basins	Af Class	HI Class	Bs Class	Smf Class	Vf Class	S/N	Rta Class
1	2	3	3	1	1	2	2
2	1	3	2	1	1	1.6	2
3	3	3	3	1	1	2.2	3
4	3	3	3	2	1	2.4	3
5	3	3	3	1	1	2.2	3
6	-	3	1	-	3	1.4	1
7	2	2	1	1	2	1.6	2
8	1	2	1	1	1	1.2	1
9	1	2	3	2	1	1.8	2
10	1	3	3	1	1	1.8	2
11	1	3	3	2	1	2	2
12	1	3	3	1	3	2.2	3
13	3	2	2	1	2	2	2
14	2	2	1	2	3	2	2
15	3	2	2	1	1	1.8	2
16	-	2	2	2	1	1.4	1
17	2	2	2	3	2	2.2	3
18	-	2	2	1	1	1.2	1
19	-	2	2	2	2	1.6	2
20	-	2	3	1	3	1.8	2
21	1	2	2	1	1	1.4	1
22	3	3	3	2	1	2.4	3
23	1	2	3	1	2	1.8	2
24	3	3	2	1	1	2	2
25	3	2	1	-	1	1.4	1
26	2	3	3	-	1	1.8	2
27	3	2	1	1	3	2	2
28	1	2	1	2	1	1.4	1
29	1	2	2	1	3	1.8	2
30	1	3	3	-	3	2	2
31	1	3	3	-	3	2	2
32	1	3	1	-	3	1.6	2
33	1	2	3	-	3	1.8	2
34	-	3	2	-	2	1.4	1
35	2	2	2	-	-	1.2	1
36	-	2	1	-	3	1.2	1
37	3	2	2	-	3	2	2
38	-	2	1	1	-	0.8	1
39	3	2	1	-	-	1.2	1
40	1	3	3	-	1	1.6	2
41	-	2	3	-	3	1.6	2
42	-	2	2	-	2	1.2	1

). The geomorphic index classes of the relative active tectonics or Rta are compiled in Table 2.

5. Discussion

5.1. Tectonic Geomorphology Techniques

Several scientists have divided the analyzed landforms into varying degrees of relative tectonic activity using the average approach [3,7]. With the aid of this technique, it is possible to record and understand the morphological signatures of several landforms with great efficiency [36,37,38]. In addition, many researchers have tried to investigate the tectonic signals, because distinct mountain fronts had been found [10,25,39]. In certain research, morphometric analysis involves creating frequency distribution diagrams and other diagrams that display the distribution of the Vf and Smf values along streams and along the mountain front [10,12,39]. In order to identify three distinct classes and provide a relative degree of tectonic activity, the Vf and Smf values are presented together in the same diagram [3,17,18]. Based on the analysis performed by theses authors, “active fronts” are defined as having low Smf < 1.6 and Vf < 0.5 index values. They are also distinguished by the presence of steep, untrenched fans that receive Holocene sediments at the apex, which are caused by uplift rates ranging from 1 to 5 m/ka. According to Ref. [20], active fronts (class 1) were produced at lower uplift rates of 0.4–0.5 m/ka, which were adequate to maintain Smf values below 1.4 and Vf values below 1. Furthermore, it was noted by Ref. [20] that proximally trenched fans may form along active fronts. The authors focused on the relationship between mountain front sinuosity and valley floor width-to-valley floor height indexes through modeling an effective chart in order to assign the tectonic signatures of each mountain.

5.2. Geomorphic Indices

Even though the study basin makes up a sizable portion of the eastern Red Sea coastal region, there are not many related tectonic studies conducted there. In this study, we investigate and calculate the geomorphic index that has been utilized in previous research to assess the landscape for possible tectonic activity. This could be a very useful strategy to test the relative tectonic activity of a large basin extending from Al Farrah and Heelan along the Red Sea coast in Saudi Arabia. Many geomorphic indexes may be utilized to model topography signatures as well as tectonic activity signals [3,19]. Results from individual indexes are based on examining mountain fronts or drainage systems. The geomorphic indices are a great tool for identifying tectonic structures and geomorphic signatures associated with erosive actions and depositional processes, such as stream courses and valley morphology. Relatively active tectonic indices may show certain abnormalities because of localized tectonic process modifications brought on by uplifting or subsidence. The current study’s design involved analyzing many indices in 42 sub-basins and using each index’s value to categorize the sub-basins into distinct tectonic classes.

5.3. Geomorphic Indices Distribution and Analysis

In this study, to comprehend the differences in tectonic signatures of the study basin, it is recommended to take into consideration the relative tectonic analysis based on each unique geomorphic index. This study applies a technique for assessing an index (Rat) that shows the relative tectonic activity across a region. The different indices are arbitrarily divided into three classes: class 1 represents high activity, class 2 shows moderate activity, and class 3 represents low activity. For our purposes here, we have selected the boundaries of the various classes that generally agree with changes in the range of the values of the various indices. The limits of the various classes vary depending on which index is being examined. The limits of the various classes that we have chosen for our purposes here are those that, for the most part, correspond to shifts in the range of values of the various indices. Depending on the index being looked at, different classes have different restrictions. Rock strength and SL indexes were examined over the entire basin [3,10,25]. The study basin’s rock strength affects the SL index. Throughout the study basin, several anomalies were observed in various locations. The influence of lithology on the computed SL index may be reduced because of the strong rocks and the abundance of geological anomalies (Figure 11). The Af index can assist in understanding landscape developments [40]. Sub-basins that have developed under relatively stable conditions with no tilting give values near 50. Due to the change in tilting perpendicular to the main stream direction, Af values below or above 50 indicate whether basin tilting results from lithological structures or active tectonic processes [41]. In other words, the Af index was defined as the absolute value minus 50. This index applies to the tilting direction of the catchments in the study basin. An arrow was used to indicate the direction of asymmetry. The Af values depicted in Figure 12 provide Af-50, which is the difference between the observed value and the neutral value of 50. In order to use this index to aid in evaluating the relative tectonic activity, the absolute difference is required. The Af-50 values vary from 0.5 to 46. The orientation of bedding could be a significant factor in basin asymmetry development [19]. An asymmetric valley is created when the valley migrates due to the inclination of the bedding. In the study basin, 10 sub-basins were observed to provide symmetrical positions, and these were not included in the evaluation of the relative tectonic activity of the Af index. Generally, the most asymmetric sub-basins are recorded in the southern part of the study basin (Figure 12). The calculation of the hypsometric integral index is obtained from digital elevation models. The HI values do not directly indicate relative tectonic activities. Like the SL index, these values depend on the study basin’s rock strength in addition to other factors that alter the various results. In general, high Hi values present young landforms and erosion signals [42]. They could be produced by incision over recent geomorphic landscapes created by deposition [7]. In the current study, we include the fact whether the hypsometric curves are convex in the upper parts, convex to cocave, or convex in the lower parts, as well as the HI index values. This study indicates that hypsometric curves with convex curvature in the lower part indicate uplifting rates with tectonic signals [3,6,7,19]. Alternately, low values of the HI index are caused by old landscapes with erosion signatures and minimal tectonic signals. In general, hypsometric integral values greater than 0.5 represent convex curves, whereas values less than 0.4 represent curves with concave shapes. Hypsometric integral intermediate values (0.4–0.5) indicate straight or concave-convex curves. In the current study, analysis of this index was based on information extracted from digital elevation models of all sub-basins greater than the fifth order. Figure 13 shows the results of this index. The drainage basin shape index plays a significant role in most tectonic geomorphology studies. Young basins in active regions tend to have elongated shapes perpendicular to the relief slope of mountains [15,43,44]. With less tectonic activity, the elongation basins tend to become circular in shape [17,19]. Many authors have used this index to assess the relative tectonic activity for several regions [45,46]. They suggested that high Bs values are observed from elongated basins with relatively high tectonic activity levels. Furthermore, low values of this index are associated with more circular basins, resulting in low tectonic activity. In the current study, the Bs index was calculated for 42 different sub-basins, and the results are illustrated in Figure 14. The values of this index vary from 0.45 to 3.52, representing elongated, semi-circular, and circular basins [10,12,17,18,20,47].

5.4. Geomorphic Indices and Relative Tectonic Activity Index

In the present study, we examined processed data on geomorphic indices that have been used in several previous studies to evaluate the basin landscape in relation to indications of tectonic activity. Previous research evaluating relative active tectonics using geomorphic indices has tended to concentrate on a single mountain front or a limited area [2,11,14,17,20]. The authors of this work applied an effective technique for determining an averaged index over the study basin that yields a relative tectonic activity (Rta) index. The different indexes were classified arbitrarily into three different tectonic levels: low, moderate, and high tectonic activity (Table 2). Many geomorphic studies have been performed along several regions, but limited studies have attempted to examine the relative tectonic activity along the eastern flank of the Red Sea coast. In this study, we developed an areal index (Rta). This index is assigned by the average of the geomorphic index classes (S/N) and is classified into three different classes. Class 1 assigns high-activity signatures with values of S/N < 1.5; class 2 assigns moderate-activity signatures (S/N between 1.5 and 2); and class 3 assigns low-tectonic-activity signatures (S/N > 2). Table 2 presents the averaged geomorphic indexes of the relative activity S/N and Rta values for 52 sub-basins in the study basin. The lower class of sub-basin values (high tectonic activity) is distributed in the northern, eastern, and southern parts of the study basin, mainly in Al Farrah, Heelan, and Al henakiyah cities, respectively. The high class of sub-basin values (low tectonic activity) is represented by six sub-basins (3, 4, 5, 12, 17, and 22) that are found mainly in the middle part of the study basin around Al mangor city. The rest of the basin is coverd by moderate tectonic signatures (Figure 15).

Within the study basin, about 61.68% of the area (64,509 km²) represents class 2 of tectonic activity as assigned by the Rta index; 31,23% presents spaces of high tectonic activity (32,666 km²) (class 1); and finally, 7.09% (7411 km²) of the total area of the study basin shows low-tectonic-activity signatures (class 3) based on Rta index values. Consequently, nearly one-third of the study basin is assigned into class 1, whereas the other two-thirds are assigned into the other two classes (class 2 and class 3) (Figure 15). In other tectonic regions and environments, the geomorphic indexes could vary, as could their value ranges [3]. The Rta index could also display different values, as would the classes of tectonic activity boundaries.

5.5. Validity of the Relative Tectonic Activity Index

This specific relative tectonic activity assessment technique has been used by several authors to evaluate the activity signatures in various contexts, and it has proven to be quite effective. It helps in determining different classes of tectonic activity in addition to a number of tectonic anomalies in the southwest border of the Sierra Nevada (southern Spain) [3]. Also, the Hindu Kush region in NW Pakistan and NE Afghanistan was classified into low, moderate, and high tectonic activity based on an analysis of the relative tectonic technique [19]. Additionally, in eastern Turkey, the strike slip adıyaman fault was classified into different segments with low, moderate, and high tectonic signatures using a similar Rta technique [12]. On the other hand, this technique was also applied successfully along the Red Sea coast. In addition to classifying the Abu Dabbab region along the western coast of the Red Sea into different areas of tectonic activity, the authors in Ref. [7] located the origin of the seismicity that has been recorded in this region using the Rta technique. The Fatima suture zone in the western Red Sea coast was also examined with the same technique, providing different signatures of relative tectonic activity [47]. This implies that this method can be used elsewhere for other large faults, especially ones whose tectonic activity and uplift rates are not well defined. Our understanding of the geomorphological evolution of the study basin landscape and its relationship to seismic activity will be greatly enhanced by future surface and subsurface research.

6. Conclusions

An effective and powerful method for examining the impact of active tectonics is to use geomorphic indices. The benefit of using these indices as a reconnaissance tool to find geomorphic abnormalities potentially associated with active tectonics is that they may be computed by GIS and remote-sensing packages over vast regions. The authors of the current study document a complete and comprehensive dataset that includes digital elevation models, structural characteristics, and geology. The authors tested the concept of relative tectonic activity signatures using geospatial techniques and seven geomorphological indexes—including the rock strength index (Rs), stream length gradient index (SL), hypsometric integral index (HI), drainage basin analysis index (Bs), mountain front sinuosity index (Smf), and valley floor width-to-valley floor height ratio index (Vf)—along the Red Sea coastal basin between Al Farrah and Heelan for the first time. Based on the results obtained from the analysis of the geomorphological indexes, the authors developed an index based on the average of the geomorphic index classes (S/N) and divided it into three different classes. Class 1 assigns high-activity signatures with values of S/N < 1.5; class 2 assigns moderate-activity signatures (S/N between 1.5 and 2); and class 3 assigns low tectonic activity (S/N > 2). The results present the averaged geomorphic indexes of the relative tectonic activity (Rta) values for 52 sub-basins in the study basin. Nearly two-thirds of the basins are assigned into the second two classes (class 2 and class 3), whereas class 1 sub-basins only cover the last third of the study basin. Rock strength and stream length gradient indexes show several geological and tectonic anomalies in the study basin; thus, the effect of these two indexes was minimal. Finally, an analysis of geomorphological indexes based on multiple sub-basins provided us with significant information on tectonic signals and the evolution of landforms. This means that this analysis can therefore be applied to other large faults in other places, particularly those with poorly defined tectonic activity and uplift rates. Finally, upcoming surface and subsurface research will significantly advance our understanding of the study basin area’s geomorphological evolution and its relationship to seismic activity.