Activation of Ad Damm shear zone, western Saudi Arabian margin, and its relation to the Red Sea rift system

1.1 Tectonic history of the Arabian shield

Most of rock units exposed on the flanks of the Red Sea are Neoproterozoic in age and recognized as part of the Arabian and Nubian shields (ANS) on the eastern and western margins of the Red Sea [3,4]. The ANS has been shaped by the amalgamation of different terranes and divided into nine terrains, which are separated by faults or suture zones [5] (Figure 1). Earliest terranes include Jeddah (870–740 Ma; [6]) and Asir (850–750 Ma; [7]). Middle age terrane covers Midayan (780–710 Ma; [8]) and younger terranes include Al-Ryan (>670 Ma; [9]) and Ad Dawadimi (674 ± 6 Ma; [10]). Most of these terranes are interpreted to have formed in supra-subduction zone environments and were subsequently amalgamated through collisional processes [5]. Although the radiometric-ages range is wide across the Arabian Shield (e.g. 870–674 Ma), there appears a general agreement that basement rocks are all Precambrian in age. Previous geochemical work indicates that the Arabian Shield has been subjected to two significant events of volcanic activities in response to the development of the Red Sea and Gulf of Aden rift system (Bohannon et al. [11]). The first major event occurred during late Oligocene, followed by the second event during Late-Miocene. Both episodes involve intense volcanic eruptions and uplifting. Although seafloor magnetic anomalies south of Arabian Shield is well observed, it does not correlate with north part of Arabian plate [12] (Stern and Johnson). The southern segment of the Red Sea is affected by the upwelling of Afar plume. However, the northern part of the Red Sea is influenced by its connection with the Dead Sea transform fault [13] (Lazar et al.).

Figure 1

Tectonic map of the western central part of the Arabian shield, displaying the boundaries and ages of several terranes (modified after Stoeser and Frost [18]). Focal mechanism solution recorded along ADSZ by an earlier study [14].

1.2 Geological and structural background

One of the large-scale, terrane bounding features in the Arabian Shield is the Ad Damm shear zone (ADSZ), located at the central-western part of Saudi Arabia and defined as NE-trending right-lateral strike-slip fault [3,4] (Figures 1 and 2). The ADSZ is thought to act as a tectonic boundary between Asir terrane to the South and Jeddah terrane to the North and is topographically expressed as a linear zone of mylonitic to highly sheared granite (e.g. Al-Saud [14]). North of the ADSZ, Jeddah terrane is characterized by heterogeneous composition. In contrast, south of the ADSZ exists the Asir terrane, which is composed of large silicic plutons and mainly of sheared to granite gneiss that believed to be a syntectonic intrusion during the deformation of the volcanic and the sedimentary rocks [15].

Figure 2

Simplified geological map and cross-section of the study area, display major geologic units and cross-cutting relationships (after an earlier study ref. [4]).

Structurally [3], simplified the deformation history in the ADSZ into three phases: NW-SE contractional regime (D1) which led to NE-trending thrust faults and tight-overturned folds, followed by NE-SW compressional episode (D2). The last phase of deformation (D3) represented by NE-SW dextral transcurrent shear zone which led to the formation of ADSZ. Tertiary and younger geologic activity was subjected to multiple generations of normal faults, with a NNW-NW strike, due to the opening of the Red Sea at approximately 25 Ma [5,16,17]. Furthermore, such recent faults are thought to be responsible for geographic relief differences between the northern and southern part of ADSZ. For instance, on land, the ADSZ bounds a large topographic escarpment perpendicular to the rift axis with higher elevation to the south. In this study, high-resolution field mapping coupled with petrographic and geochemical analyses are integrated to investigate whether ADSZ was solely a Neoproterozoic structure or if it has been reactivated during Cenozoic. The selected methods presented here help to constrain the effect of the Red Sea opening on the ADSZ by carefully characterize the Neoproterozoic structures from more recent Cenozoic-age structures.

3.1 Satellite image processing

Advanced spaceborne thermal emission and reflection radiometer (ASTER) satellite image was carefully selected to construct lithological discrimination map. In addition, four software packages were later used to process and layout satellite images (Arc map (Environmental System Research Institute, USA), ENVI (Environment for Visualising Images software, USA), ERDAS (Earth Resources Data Analysis System IMAGINE), and Global Mapper (Blue Marble Geographics, USA)). Processing of ASTER imagery included resampling the spatial resolution of the 30 m six-bands of short wave infrared to correlate with the three-bands of 15 m visible and near-infrared to produce nine bands with a spatial resolution of 15 m. Thereafter, the decorrelation stretch technique was applied to enhance differences in colour by stretching the image pixels. Next, the band ratio technique is applied using (7/6, 6/5, 6/4) ratios because it efficiently separates different geologic features and lithologic compositions (Figure 3). The Ad Damm narrow valley and other minor valleys are well defined in white colour due to the presence of quartz rich sand. In comparison, the dark blue colour represents amphibolite and metabasalt, which are more abundant in the Jeddah terrane and along the ADSZ. The dull green colour displays the unstrained granite (Asir terrane), and the reddish-brown colour symbolizes Jeddah terrane (heterogeneous granite). Basalt flows of the Harrat Ad Damm are clearly shown in a dark brown colour [4].

Figure 3

ASTER satellite image using decolouration stretch and band ratio (7/6, 6/5, 6/4) in RGB. Field localities are also shown in red dots.

3.2 Field mapping

ADSZ is dominated by a right-lateral strike-slip mylonitic shear zone that strikes NE-SW and separates between the Asir to the south and Jeddah terranes to the north (Figure 2). The sense of shear and mylonitic fabrics have been identified in many localities and measured as a dextral slip (Figure 4a and b). North of ADSZ, the Jeddah terrane is characterized by heterogeneous magmatism (amphibolite to greenschist), intruded by several felsic to basaltic dikes) and North Northeast-Northeast striking foliation and mineral lineation. Such units are later folded in large-scale drag folds and overprinted by shear indicators (Figure 4c). In contrast, the Asir terrane is defined by massive granitic batholiths that include sheared to highly foliated granite, located adjacent to ADSZ, and granitic gneiss to granodiorite away from the shear zone. Subsequently, the western margin was intruded by a number of Eocene to Miocene mafic and silicic dikes, associated with NW to North Northwest (NNW)-striking faults that are interpreted to be related with the opening of the Red Sea (Figure 4d).

Figure 4

Displays some of prominent large-scale structures: (a) dextral sense of shear in mylonitic granite, (b) mylonitic fabrics of ADSZ rocks, (c) cross-section of large-scale syn-form drag folds in Jeddah terrane, and (d) NNW-NW Oblique normal faults cut across basement rocks.

3.3 Petrographic and microstructural analyses

The petrographic study shows Jeddah terrane composed of a mixture of quartz, K-feldspar (microcline), and plagioclase with more than 25% of biotite and hornblende (Figure 5a). Asir terrane contains abundant of quartz, K-feldspar (microcline), plagioclase, and occasionally biotite (Figure 5b and c). ADSZ rocks exhibit a high degree of recrystallization and are composed of quartz, K-feldspar, plagioclase feldspar, microcline, and occasionally biotite and hornblende (Figure 5d).

Figure 5

Petrographic analysis of mineral-forming rocks in the study area: (a) relative mineral compositions of Jeddah terrane, (b) relative mineral composition of Asir terrane, (c) eutectic minimum melt composition shows lower crystallization temperature in Asir terrane, and (d) dynamic recrystallization of mylonitic granite in ADSZ rocks.

The microstructural study indicates dynamic recrystallization in the mylonitic granitites of the ADSZ (Figure 6a). Dominant processes include sub-grain rotation recrystallization and grain boundary migration. Grain-size reduction and triple junction formation suggest high-temperature recrystallization (Figure 6a). A few kilometres away from ADSZ, the intensity of the dynamic recrystallization decreases, and samples are governed by recovery process, which involves the gradual evolution of sub-grains and deformation bands with undulose extinction (Figure 6b). About 10 km away from ADSZ, the crystals are very coarse in size, nearly strain-free, and display magmatic features suggesting an igneous origin (Figure 6c and d).

Figure 6

Microstructural analysis across ADSZ: (A) high-temperature dynamic recrystallization and grain-size reduction of quartz in ADSZ rocks, (B) gradual evolution of sub-grains and deformation bands with undulose extinction adjacent to ADSZ, (C and D) coarse crystalline, dominated by magmatic texture suggest plastic deformation, away from ADSZ.

3.4 Whole rock geochemistry

The main purpose of using whole rock geochemistry is to give a proper classification of rock units (using major elements; Table 1) and to determine whether or not the rock samples within the study area are arc-related and, therefore, potentially related to Neoproterozoic in age, or rift-related, and thus, most likely related to the Red Sea rift system during Cenozoic time (Table 2). The radiometric ages available for the rock units of the ADSZ and Jeddah/Asir terranes appears to be Precambrian in age. However, Asir and Jeddah terranes lithological units are distinctively older (e.g. 804 ± 8 to 895 ± 173 Ma; [19]) comparing to those collected from the ADSZ (e.g. 542 ± 23 Ma; [20]).

3.4.1 Major elements

Total Alkaline Silicate (TAS) chart [21] indicates that the granitoid bodies of the Jeddah terrane, Asir terrane, and ADSZ rocks have similar and high concentration of SiO2 vs K₂O, and Na₂O (Figure 7). Eocene–Miocene Ad Damm dikes complex is mainly basaltic in composition with SiO2 concentrations range between 50 and 45 wt% (Figure 7). The K₂O vs SiO₂ classification diagram of an earlier study [21] illustrates that most of the granitic rock units of Asir, Jeddah, ADSZ, and folded silicic dikes of Ad Damm drag folds are characterized as high-K calc-alkaline in composition (Figure 8). Several samples of the Jeddah terrane are characterized as medium-K calc-alkaline, which reflects the heterogeneous composition of Jeddah terrane. The mafic group, which includes meta-basalt, amphibolite (Jeddah terrane host rock), Eocene–Miocene Ad Damm dikes complex basalt, folded basaltic dikes, plots in the low to medium-K calc-alkaline fields. Finally, Harker variation diagrams (Figure 9) show a strong correlation between most of the major elements of the different geologic units versus SiO₂. CaO, MgO, MnO, FeO, Fe₂O₃, TiO₂, and P₂O₅ decrease with increase SiO₂ whereas K₂O and the sum of K₂O and Na₂O increase dramatically with the increase of SiO₂. Such observations indicate a clear fractionation pattern. The percentage of Al₂O₃ remains almost constant with a slight increase during the formation of the basaltic dikes.

Figure 7

TAS chart of Jeddah and Asir terrane, ADSZ rocks, folded silicic/basaltic dykes, Eocene–Miocene basaltic-dykes, and southern basaltic flow [21].

Figure 8

SiO₂ vs K₂O classification of Jeddah and Asir terrane, ADSZ rocks, folded silicic/basaltic dykes, Eocene–Miocene basaltic-dykes, and southern basaltic flow [21].

Figure 9

Harker variation diagram of major elements from Jeddah and Asir terrane, ADSZ rocks, folded silicic/basaltic dykes, Eocene–Miocene basaltic-dykes, and southern basaltic flow.

3.5 Trace elements

Samples are divided on the basis of their lithological difference and field relation into six main groups as follows: Jeddah terrane, Asir terrane, ADSZ rocks, Eocene–Miocene basaltic dikes, southern basalt flow, and folded Silicic-basaltic dikes (in the Ad Damm drag folds; Table 2). Multi-element diagrams show that all six groups are defined by high abundance of large ionic lithophile elements (LILEs; Cs, Rb, K, Ba, and Sr) compared to the high field strength elements (HFSEs; Th, U, Ce, Ti, Nb, Ta, Zr, and Hf) and heavy rare earth elements (HREEs). However, the Neoproterozoic rocks are characterized by a large negative Nb–Ta anomaly, suggesting an arc origin consistent with that of an earlier study [22] (Figure 10). In contrast, the Cenozoic groups (Ad Damm dike complex and the southern basaltic flow) exhibit significantly reduced negative Nb-Ta anomaly that is consistent with a rift origin (Figure 11). In detail, the spider diagrams suggest that both the Jeddah and Asir terranes, and ADSZ rocks have broadly similar trace element patterns, which suggests that they may have originated in similar tectonic environments. Normalized multi-element diagrams indicate that LILEs are elevated in composition. In contrast, HFSE are depleted. The plutonic and volcanic groups listed above are characterized by an arc-related signature (Figure 10). The rare earth element pattern is defined by sharp slop where the average of light rare earth elements to HREEs (Ce/Yb) n ranging between 2 and 28 and the average of LILE to HFSE ranges between (K/Ta) 15 and 8. The highest LREE/HREE and LILE/HFSE average content belongs to Asir terrane where (Ce/Yb) n = 28 and (K/Ta) n = 20. Lower trace element ratios and LILE/HFSE values are found in the folded basaltic and silicic dikes and amphibolite of Jeddah terrane where (Ce/Yb) n = 2 and (K/Ta) n = 8. Overall, the trace element signature in both the Jeddah and Asir terranes is consistent with a volcanic arc, subduction-related origin. In contrast, Eocene–Miocene basaltic dikes and southern basaltic flow are distinguished from the other groups by a significantly reduced Nb-Ta anomaly, which is consistent with a rift-related environment. The rare earth element pattern is defined by the slop between LREE/HREE (Ce/Yb) n ranging between 2.1 and 2.3 and the average of LILE to HFSE fluctuated (K/Ta) between 1.1 and 2.2. LIL elements have a slight enrichment content (Cs, Rb, Ba, K, and Sr) relative to the older groups; whereas, HFS elements have relatively less depletion in their content (Nb, Ta, U, Zr, Hf, and Ti).

Figure 10

Multi-element diagram of average samples of Jeddah terrane (granitic plutons, silicic and basaltic folded dikes, and amphibolite/meta-basalt), Asir terrane, ADSZ rock. Data normalized to Normal Mid-Ocean Ridge Basalt (N-MORB) [23].

Figure 11

Multi-element diagram of average samples of Eocene–Miocene Ad Damm Dykes complex and southern Basaltic flow. Data normalized to N-MORB [23].