Submission to Precambrian Research

A. Genna, P. Nehlig, E. Le Goff, C. Guerrot, M. Shanti

The structural synthesis presented in this paper is based on a re-evaluation of pre-existing results and the acquisition of new field data. The previous work includes 1:25,000-scale mapping of the Arabian Shield, comprehensive aeromagnetic mapping, geochemical data and geologic results from mineral exploration.

After a rapid overview of the various concepts concerning the structure of the Shield, the primary tectonic events are presented as they are currently understood. The pre-Panafrican structures (> 690 Ma), which are difficult to decipher due to the later phases of deformation, were essentially a result of the formation and collision of basins showing marine affinity and marginal volcanic arcs. The Panafrican tectonism (690-590 Ma) was marked by the formation of the Nabitah Belt and peripheral ranges punctuated by gneiss domes. Various sedimentary formations contemporaneous with this tectonism formed foreland or intracontinental molasse basins.

After the Panafrican tectonism, widespread extension (590-530 Ma) brought about crustal thinning, which generated bimodal magmatism and significant dike swarms; the associated volcanics form the Shammar Group. A marine transgression, associated with passive margin-type structures with tilted blocks, marked the end of the thinning. The platform facies from this transgression correspond to part of the Jibalah Formation. Other basins formed as deep continental structures along transform faults.

This new view of Arabian Proterozoic geodynamics has implications for the succession of mineralizing events in the Arabian-Nubian Shield.

Keywords: Tectonism, Proterozoic, Saudi Arabia, Panafrican orogeny, Arabian-Nubian Shield

The present structure of the Arabian Shield (Fig. 1) is the result of very diverse, polyphase geodynamic events. The first overview of the geology of this region was provided by Karpoff (1958), who classified the Proterozoic rocks of Saudi Arabia into two distinct series: an older Medina series, showing highly variable grades of metamorphism and cut by intrusives predating the discordant Wadi Fatima series. No general structural arrangement for the Shield was proposed at that time.

The Najd fault system was the subject of the first tectonic syntheses of the Proterozoic basement (Moore, 1979; Delfour, 1979a, 1980a). Delfour (1979a) considered the Najd orogenic cycle of the Arabian Shield to comprise four structural phases between 800 and 500 Ma. The main phase, dated at 550 Ma and ending in cratonization of the Shield, corresponded to the peak of the Panafrican orogeny. This idea was followed up by Caby (1982), who compared the geodynamics of the Arabian Shield to those of the Tuareg Shield, and considered the tectonic structures of Arabia to be similar to the Andean-type cordilleras. Ophiolite complexes were recognized by Shanti and Roobol (1979).

More recently, the Arabian Proterozoic basement has been considered in terms of an assemblage of microplates bounded by sutures that are in places associated with ophiolite formations (Kröner, 1985); this model was developed for the whole of the Arabian-Nubian Shield (Bentor, 1985). Stoesser and Camp (1985) compiled a tentative map of these boundaries and their associated orogenic zones for the Arabian Peninsula, and Vail (1985) extended this map to cover the entire Arabian-Nubian Shield.

Along these same lines, Laval and Le Bel (1986) set forth a model for the Al Amar Belt and the Abt Schists that involved subduction and collision of volcanic arcs. In order to clarify the relationships between mineralization (gold and base metals) and structural context, Bokhari and Forster (1988) posited a more complex tectonic evolution. They proposed the existence of a pre-Panafrican oceanic crust (>950 Ma) followed, from 950 to 720 Ma in a pre-cratonic context, by the development of volcanic arcs in a marine environment. This latter period was characterized by tholeiitic plutonism followed by calc-alkaline plutonism. Cratonization occurred through accretion of microplates between 720 and 640 Ma. Still according to Bokhari and Forster (1988), Arabian-Nubian cratonization was followed by tectonic extension (700-540 Ma) and marked by andesitic volcanism leading to molasse sedimentation. They established a parallel with the Tertiary extension of the Basin and Range province in the USA.

Quick (1991) also used Kröner's (1985) model. He considered the Nabitah Belt to be the main structure in the assemblage of terranes and proposed extending the model eastward, where he interpreted the Abt Schists and part of the Murdama formations as an accretionary wedge and fore-arc basin associated with west-plunging subduction. The southern extension of the Nabitah Belt has been identified in Yemen (Windley et al., 1996), where it is integrated within a succession of arcs and terranes correlating to those in Arabia and the Mozambique Belt.

Al-Saleh et al. (1998) believe that the eastern part of the Arabian Shield was subjected to two major orogenic events, 680 and 600 Ma, based on a metamorphic study and Ar⁴⁰/Ar³⁹ dating. Lescuyer et al. (1994) describe a right-lateral ductile fault in the western part of the Shield, which they attribute to the cratonization phase.

Numerous radiometric age determinations have been proposed for the Shield. A first comprehensive review of these was made by Johnson et al. (1993), and stratigraphic implications were drawn later (Johnson, 1996).

The Proterozoic sedimentary formations of the Arabian Shield have been the subject of many studies and classifications (Jackson and Ramsay, 1980; Delfour, 1980a). Hadley and Schmidt (1980) separated them into three depositional phases; a first phase composed only of volcanic formations and containing no plutons; a second phase consisting of sandstone formations, polygenetic conglomerate, meta-greywacke and marble; and a third phase of terrigenous and siliciclastic formations, as well as stromatolithic, limestone and dolomite formations. Phase 1 is represented by the Baish, Baha and Jiddah groups, Phase 2 by the Ablah, Halaban and Murdama groups and Phase 3 by the Shammar and Jubaylah groups.

In the context of mineral exploration, numerous gold and base-metal prospects have been the subject of detailed structural analysis (Koch-Mathian et al., 1994; Genna, 1996; Récoché et al., 1998a, b). Two cartographic and structural syntheses have been completed on the Bidah and Shwas volcano-sedimentary belts (Donzeau and Béziat, 1989; Béziat and Donzeau, 1989), and a composite gitological (metallogenic) map has been compiled by Béziat and Bache (1995). These studies also show local structural evolutions, which are addressed in our study.

In general, we note that the previous studies deal primarily with the ancient pre-Panafrican structures, and pay little attention to recent geodynamic events contemporary with or subsequent to the formation of the Nabitah Belt. Our study focuses on these later events. The main stages of the Shield's structural development, as established from our results, are summarized in Figure 2.

Before the formation of the Panafrican belt, Saudi Arabia was an area of sedimentary formations of primarily volcanic origin (Fig. 3) deposited in an oceanic or marginal marine environment and intruded by granite and diorite. These are represented by various major groups, the main ones being the Al Ays, Abt, Halaban, Jiddah, Bahah, Baish and Hali groups. Several authors (Hadley and Schmidt, 1980; Jackson and Ramsay, 1980; Delfour 1980a) have proposed classifications positing two or three major events for this first long period. The basement underlying these early sedimentary formations is either poorly or not at all defined. Sometimes dated as Early Proterozoic (Stacey and Hedge, 1984), certain authors merely considered it as an "older basement" of gneiss and migmatites or ophiolite formations (Delfour, 1980a; Jackson and Ramsay, 1980).

The pre-Panafrican formations were subjected to highly varied degrees of deformation before deposition of the Panafrican molasse. For example, the Al Ays Formation in the northwest (Kemp, 1981) was not deformed, whereas the Jiddah Formation in the south was intensely foliated before the Panafrican basin molasse was deposited.

Among the Panafrican basin deposits, the Abt Schists constitute a unit that is exceptional for its very thick turbidite succession (Delfour, 1979b; Delfour et al., 1982). The facies are more distal and siliciclastic than the schist facies in the other basins which are commonly composed of tuff and cinerite. This difference led some authors to interpret the Abt Schists as accretionary wedges (Laval and Le Bel, 1986).

Volcanic arcs typical of a subduction context indicate that convergence occurred in an oceanic environment (Camp, 1984; Laval and Le Bel, 1986; Pallister et al., 1987). However, we have few kinematic elements for this first period, and they cannot be extrapolated to the scale of the whole Shield. The earliest faults of regional importance that have been observed seem to be younger than the basins, and are probably indicative of the ocean closure mechanism.

The precratonic elements were brought together during a collision that occurred about 690 Ma (Johnson, 1996; Fig. 4). The ocean closure mechanisms are still poorly understood, but the resultant sutures are marked by ophiolite formations such as Jabal Ess (Shanti and Roobol, 1979), or by ultramafic formations that are not clearly defined as ophiolitic. Close analysis of these sutures has provided a few keys to understanding the phenomenon.

The first deformation to affect the basin sedimentary series was characterized by a very steeply dipping lineation concentrated in kilometre- to multi-kilometre-wide corridors whose regional extent is not everywhere clearly defined (Fig. 5). A number of faults are attributable to this phase of deformation, such as the Ad Dafinah Fault, the fault bounding the Abt Schists to the east, the Nabitah fault, and various transverse faults in the Al Amar Belt, and many other shear zones that are visible on satellite photos probably belong to the same family. Overall, we can recognize two types of major fault: the first demarcate large blocks and correspond to major sutures; the second are faults that developed within a block, like those in the Al Amar Belt, and are accompanied by folds with vertical axes (Ad Dafinah Fault), quartz exhudations in the form of boudins (Nabitah Fault) or tension gashes that are variably boudinaged (Abt Schists). This first deformation was also accompanied by a remobilization of sulfide mineralization along shear zones in the Al Amar Belt, as demonstrated by the Khnaiguiyah prospect; structural analysis of this prospect provides an example of the elementary kinematics operating in these shear zones (Fig. 6).

In Saudi Arabia, the Panafrican orogeny is represented by a complex web of orogenic zones arranged in an anastomosing network of primarily left-lateral strike-slip faults. It includes the Nabitah Belt (Quick, 1991), which is the central part of the structure, and a number of peripheral ranges. Note that this distribution (Fig. 7) does not correspond completely to the configuration of orogenic zones proposed by Stoesser and Camp (1985) based on the distribution of ultramafic complexes.

The Nabitah Belt (Quick, 1991) is oriented generally N-S (Fig. 7) and divides the Shield in two. For descriptive purposes, the belt can be divided into an inner zone and an outer zone with distinct structural features. The inner zone is composed of large, pre-existing intrusions and sedimentary formations that have undergone compressional deformation. It is characterized by sigmoid fish-shaped shear zones whose geometry is clearly visible on satellite photos. The outer zone is characterized by lateral slip along the margin, reflected by the presence of gneiss domes.

The intrusive complexes of the inner zone of the Nabitah Belt are primarily batholiths that acquired a sigmoidal shape during the Panafrican deformation. The most striking on the satellite photos are the Furayhah Batholith (Kemp et al., 1982) and the Al Bara Batholith (Letalenet, 1979). The An Nimas Batholith in the southern part of the Shield (Prinz 1983, Greenwood et al., 1986) shows a more complex evolution, with subsequent deformation.

The large Murdama Basin, located east of the Nabitah Belt, displays increasing degrees of deformation and metamorphism from east to west. To the west, it is bounded by gneiss domes with foliation folds that have curved axes and merge progressively southwestward into the Nabitah structures and northeastward into the folds of the sedimentary basin.

One of the domes, Jabal Kirsh (Figs 8, 9 and 10) has been extensively studied from the standpoint of both geologic mapping (Delfour, 1979b; Delfour, 1980b) and mineral exploration (Genna, 1996). Figure 8 shows the general structure of the dome within its context on the belt margin. The gneiss is boudinaged at all scales (Fig. 9). Figure 10 shows the influence of lithology on the spatial arrangement of the deformation.

The Nabitah Belt is cut by a number of anastomosing faults, the largest being the Jabal Tin Fault (Fig. 11). This is marked by a band of NW-SE-trending gneiss whose width varies from 25 km in the northwest to 10 km in the southeast. The Jabal Tin exposures (Fig. 11b, c, d) illustrate the transpressional context of the deformation, with partial melting and the emplacement of leucogranite veins as shear planes during the last stages of deformation. As in the northwestern part of the Shield, the relationships between foliation and microshearing (Fig. 11e) indicate a left-lateral sense of shear for the whole of the Tin Complex.

The peripheral ranges are punctuated with gneiss domes, which are common in the Proterozoic basement of Saudi Arabia; for example, Johnson (1997) shows six major gneiss structures in his tectonic map of Saudi Arabia. The domes exhibit left-lateral transcurrent tectonism along the so-called "Najd" faults and right-lateral movement along conjugate structures. The whole unit forms a network of anastomosing faults within the major orogenic zone known as the Nabitah Belt (Quick, 1991) and its adjacent structures.

Analysis of a number of anticlinal gneiss structures shows that they are composed of ortho- or para-derived formations. One sector that has been thoroughly studied is the northwestern part of the Shield (Grainger and Rashad Hanif, 1989; Davies, 1985; Hadley, 1987; Pellaton, 1982a; Kemp, 1981; Pellaton, 1979; Fig. 12) where satellite imagery reveals a unique structural arrangement comprising an anastomosing network of planar structures that demarcate large fish-shaped units (Fig. 12b). Various measurement points have revealed the existence of a network of ductile left-lateral transcurrent faults with occasional right-lateral faults. The network is marked by numerous domes, four of which have been studied in detail.

The Hamadat anticlinorium is a huge gneiss dome (Pellaton, 1982a; Kemp, 1981; Fig. 12) stretching 100 km in a NW-SE direction. Its primary structural feature consists of second-order folds that are very pinched to the north and more open to the south. The entire structure is marked by a well-developed stretching lineation parallel to the fold axes; it is subhorizontal over most of the dome and plunges southward in the southern part, at the same angle as the dome itself. The kinematic criteria of the deformation, based on S/C plane relationships, indicate left-lateral shear at all the observation points.

The Wajiyah anticlinorium (Kemp, 1981; Fig. 12) bounds the Hadiyah Basin to the north. It exhibits the same structural characteristics as the Hamadat anticlinorium, but with less clear-cut boundaries and no obvious direction of plunge. The shear direction is left-lateral. Relationships with the sedimentary formations of the Hadiyah Basin are clear and it is possible to determine that the intense, constrictive deformation advanced through the basin from north to south.

The Baladiyah Complex, located southeast of the Thalba Basin (Davies, 1985; Fig. 12), is exposed over 20 km along a N-S axis. It has a very regular geometry that is particularly apparent on aerial photos. Most of the structure is marked by a subvertical to very steeply dipping foliation that, at its northern end, closes into a north-plunging dome. The complex also displays a well-developed subhorizontal stretching lineation parallel to the dome axis, again north-plunging at the northern end of the structure. Amphibolite-facies metamorphism affected the central part of the dome (Davies, 1985), which also exhibits a loss of foliation in favour of a more developed lineation, indicating a primarily constrictive environment of the deformation.

The Qazaz complex, located east of the Thalbah Basin (Davies, 1985; Fig. 12) is generally antiform with several second-order folds. In the northern part the foliation forms an arch with a NW-SE axis. The upper part of the structure consists of orthogneiss marked by a well-developed stretching lineation parallel to the dome axis. The core of the structure comprises mica schist (amphibolite facies) with two foliations: a first foliation that has been folded isoclinally along subhorizontal axial planes and a second foliation forming the anticlinal geometry of the dome. The "tail" of the structure, at its northern end, is composed of amphibolite-facies gneiss (Davies, 1985) marked by left-lateral shear.

A comparative analysis of these domes, which reflect transpression in a domain of ductile deformation, has enabled us to develop a general genetic model for this type of structure, as summarized in Figure 13. Under this model, the lower part of the structure would show subvertical metamorphic foliation and display a slight horizontal lineation, the central part would be a zone of intense constriction, and the top part would show subhorizontal foliation and a poorly developed subhorizontal lineation. The consequences we would expect from this model are, for example, that the kyanite-bearing quartzite at Jabal Kirsh represents the exhumation of deep metamorphic facies and that the Hamadat anticlinorium represents the onset of gravitational processes lateral to the dome.

A number of molasse basins collected the sediments derived from the progressive erosion of the Nabitah Belt (Fig. 14). Among these are the large foreland basin and the intramontane basins associated with the peripheral ranges.

The foreland basin is the large N-S-trending Murdama Basin lying to the east of the Nabitah Belt (Letalenet, 1979; Delfour, 1979b; Pellaton, 1985; Fig. 14f) and from which only the marginal sedimentary deposits were caught up in the deformation. The basin is cut by granitic intrusions whose ages vary from 650 to 530 Ma (Béziat and Bache, 1985).

The sedimentary formations of the Murdama Group consist primarily of sandstone from the underlying Afif Formation. At the basin’s western margin (Letalenet, 1979), the base of the series is marked locally by rhyolite flows and polygenetic conglomerate; argillaceous or conglomeratic intercalations and greywacke are present in the succession. At the eastern margin (Delfour, 1979b) the series begins with a conglomerate, more than 200 m thick (the Hibshi Formation) overlain by a complex formation (the Farida Formation) of carbonates, sandstone, conglomerate and stromatolithic rocks. A 10,000-m-thick sandstone formation occupies the central part of the basin.

The Thalbah Basin (Davies, 1985) consists of two main structural basins in the south (Fig. 14d) and a single basin in the north (Fig. 14c) that are filled with deposits from three distinct formations (Hashim, Ridam and Zhufar) separated by discontinuities. The oldest of these formations, the Hashim Formation, comprises a 50-300-m-thick basal conglomerate, overlain by a 1000-m-thick succession of sandstone, siltstone and intraformational conglomerate. This was transgressed from east to west by the Ridam Formation which, in the centre of the basin, is as much as 1000 m thick with a thick basal conglomerate. The Zhufar Formation is discordant on the Ridam Formation and ranges in thickness from 600 m in the west to 1400 m in the east.

These sedimentary formations also show significant structural and metamorphic variation from west to east. In the west, the succesion shows little folding and is monoclinal with a westward dip of 5° to 15°. In the east, in contact with the gneiss massifs, the succession is intensely folded, almost vertical and displays a subhorizontal stretching lineation; it also shows significant vertical variation in its deformation. A left-lateral shear zone with characteristics identical to those of the Panafrican shear zones is observed in the basement rocks of the southeastern part of the basin (Johnson and Offield, 1994). This fault is reflected by a simple fold in the Thalbah sedimentary formations.

The Hadiyah Basin (Pellaton, 1979; Kemp, 1981), at the eastern margin of the Al Ays range (Fig. 12), consists of a single main structural basin that is folded and overturned to the east (Fig. 14b). The fill comprises three elementary megasequences, the first (the Siqam Formation) being primarily volcanic and the other two (the Tura'ah and Aghrad formations) being entirely sedimentary. They each have a thick basal conglomerate and show significant variations in thickness; the maximum thickness of the three formations taken together is about 7000 m. As with the Thalbah Basin, the deposits of the Hadiyah Basin shows significant variations in deformation style and metamorphic grade. In the west, folds are upright and fairly compressed, whereas in the east, they are less closely spaced and east-verging. Similarly, from south to north, the sediments pass from undeformed, with perfectly preserved sedimentary structures, to intensely deformed in the greenschist facies, with a well-developed subhorizontal stretching lineation identical to that found in the neighbouring gneiss massif.

The molasse basins in the northwestern part of the Shield show varied degrees of deformation. Those to the west reveal no significant deformation and were never buried (no vertical or burial foliation, and incomplete lithification in places). Those to the east are metamorphosed (greenschist facies or higher), intensely deformed, and display the same well-developed horizontal lineation as the basement and nearby gneiss domes. These basins thus appear to be post-tectonic in the west and pre-tectonic in the east. This observation, which is supported by a geometric analysis of their sedimentary fill, leads us to consider them as syntectonic and as reflecting the topographic variations that accompanied this stage of deformation.

The southwestern part of the Shield is characterized by the sedimentary molasse formations of the Ablah Basin (Greenwood, 1985a, b; Prinz, 1983; Carter and Johnson, 1987), a deformed sedimentary basin between the granites of the Shwas Pluton and the Thurat Batholith. In its northern part, the basin is generally monoclinal with an easterly dip, and has a fold train parallel to its axis; it shows little deformation and no metamorphism. In the east, the basal deposits comprise a coarse conglomerate, several hundred metres thick and in places dipping almost vertically along a subvertical boundary fault. In the west, the basal series is composed of gently dipping tuff and basic lava lying discordantly on the Thurat Pluton. The asymmetry marked by the difference in basal facies to the east and west indicates that the basin was initiated by an eastward tilt of its substratum.

Above the basal deposits, the fill consists of siltstone, sandstone, and conglomerate, with interbedded volcanic flows. The folded rocks show a general eastward dip, reflecting the progressive tilting of the basin during its formation. This configuration is similar to that of the Thalbah Basin (Davies, 1985), which developed along a left-lateral strike-slip fault of the Najd system during the Panafrican tectonism in the northwestern part of the Arabian Shield (Delfour, 1979a).

Deformation in the Ablah Basin intensifies southward as the basin deposits thicken. A vertical foliation appears in the fold hinges, at first in the carbonate layers at the top of the succession and then in the other facies. The folds become tighter and their axes become sigmoidal. Still farther south, the deformation intensifies and the metamorphic grade reaches amphibolite facies conditions (Greenwood, 1985a, b), so it is difficult to determine the initial structure of sedimentary facies.

The Ablah basin sediments are dated at 642 + 1 Ma (Genna et al., written comm.), i.e. contemporaneous with the Murdama basins in the north of the Shield, and lie discordantly over the Jiddah Group, which was affected by subvertical foliation before their deposition. According to Donzeau and Béziat (1989), the basin underwent two phases of deformation; the first with right-lateral shearing, and the second with left-lateral shearing.

The Junaynah Fault corresponds to an intramontane trough in our model. It is marked by the sedimentary formations of the Junaynah Belt (Greene, 1993), which are of Murdama age (Simons, 1988). Most likely highly eroded, they were folded and foliated (Fig. 14e) during a single phase of deformation. These formations display a subvertical foliation that is axial plane with respect to the single large fold that forms each part of the basin. Relationships between the foliation planes and shear planes clearly indicate a left-lateral sense of shear with a subhorizontal stretching lineation. Note that this shear occurs on a N-S fault, and that the faults with this orientation in the Shield are generally right-lateral. We can thus assume that the regional direction of shortening either varied from the north to the south of the belt, or varied over time. If it varied over time, then did the deformation propagate from north to south or from south to north?

The Murdama formations of the Junaynah Basin are discordant over the volcanic and volcano-sedimentary succession of the Halaban Group to the east and also over some older granitic formations. The base of the basin fill is a conglomerate that varies from a few to 300 metres in thickness and that generally contains clasts of its immediate country rock, providing evidence of correlative tilting of the basement during the filling of the basin. The entire succession attains local thicknesses of 800 m and consists primarily of sandstone and siltstone, in sequences several metres thick, with thin intermediate arkose and conglomerate. An E-W cross section through the west branch of the basin (Fig. 14e) reveals a very asymmetric syncline. There is no boundary fault to the west, and the faults along the eastern margin are reverse strike-slip. Deformation was single phased. The presence of a subhorizontal stretching lineation suggests, as with the Murdama basins in the northern part of the Shield, a left-lateral transpressional mode during basin development.

The gneiss domes we have described fit into a left-lateral kinematic context of deformation (Figs. 8 to 12). They are contained within the Panafrican Belt in Saudi Arabia, whose axial zone is represented by the Nabitah Belt – a belt consisting primarily of granite and granodiorite batholiths (Johnson, 1983) whose geometry, generally sigmoidal, is consistent with the formation kinematics of the domes.

The left-lateral faults of this belt are oriented NW-SE, with conjugate N-S to NNE-SSW right-lateral faults also being observed. In the northwest of the Shield, they are evidenced by a gneiss shear zone with a well-developed stretching lineation (Fig. 12), where the kinematic criteria of the S/C planes indicate right-lateral movement. In the Samran Belt (Genna, 1994; Bellivier et al, 1998), right-lateral faults that developed in the ductile-brittle transition were synchronous with the emplacement of gold mineralization, and the Nabitah Belt, the N-S trend of the pre-Murdama batholith margins correlates with the same structural position. In the southwest of the Shield, the main ancient volcano-sedimentary belts and the large batholiths underwent right-lateral transcurrent tectonism, the sedimentary markers of which are the Ablah formations (Donzeau and Béziat, 1989).

Obvious relationships exist between the gneiss domes we have described and the adjacent sedimentary formations of Murdama age. Sedimentological and structural analyses of these basins (Davies, 1985; Kemp, 1981; Pellaton, 1979; Camp, 1986) reveal that they were formed during a period of ductile tectonism, with the basins occupying the synclinal depressions and the domes representing the anticlines of the same fold train. These kinematic processes were accompanied by the deposition of sedimentary megasequences in the basins, with large wedges and internal unconformities, which sealed the successive movements of the basement. The total thickness of the basin sediments was about 10,000 m.

The end-Proterozoic formations in Saudi Arabia indicate that the crustal thickening brought about by the formation of the Panafrican Belt was followed by an episode of crustal thinning (Fig. 15; Genna et al., written comm.). This was expressed by extensional deformation with contemporaneous bimodal magmatism, later followed by a return to marine sedimentation. The volcanic activity was associated with various intrusive complexes and dike swarms dated between 530 and 590 Ma. The extrusive formations are known collectively as the Shammar Group. Listric or gently dipping faults reflect gravitational sliding in the uppermost part of the crust. The geometry of these normal fault networks and dike swarms indicates extension in multiple directions. The subsidence is also indicative of the more complex geometry of inferred basements of the intracontinental or marine basins induced by this deformation.

Thinning was governed by a system of transform faults, known as the "Najd faults", which also controlled the formation of the Jibalah basins in which the Shammar generally makes up the basal formations of the sedimentary fill. This extensional episode ended in a marine transgression, evidenced by the carbonate platforms of the Jibalah basins at various places in the Shield. Continuation of the thinning process may explain the deposition of the marine formations of the Paleozoic cover. This crustal warming instigated a major metallogenic event in the Arabian-Nubian Shield during the late-Proterozoic extension.

Our results are consistent with recent studies by a number of authors proposing late-Panafrican extension mechanisms in the Arabian-Nubian Shield based on observations made in Sinai (Blasband, 1999) and Egypt (Renno and Stanek, 1999). Their models are based primarily on studies of the magmatism (magmatic core complexes) and structures of the terranes.

A significant phase of peneplanation subsequently eroded the belt and revealed a variety of structural layers before deposition of the discordant Ar Rayyan and Jurdhawiah conglomerates.

The post-Panafrican intrusive complexes show a great variety of compositions and shapes. The felsic intrusions are circular, elongate, or raindrop-shaped, whereas the mafic intrusions generally form sills. Moreover, the existence of concealed batholiths is evidenced by the dike swarms and ring dikes they produced, which give an indication of their general shape. Ring complexes and calderas are commonly associated with the batholiths.

Late sills are also present; they developed particularly in the northern part of the Shield and near the Paleozoic cover. This magmatic activity was bimodal, being represented mainly by granite and gabbro in the plutonic bodies, and by rhyolite and basalt in the extrusive formations of the Shammar Group. The mafic igneous rocks are mainly concentrated in the northern part of the Shield. In the northwest (Pellaton, 1979; Kemp, 1981), gabbro and diorite cut the Hadiyah Group molasse deposits, which are the local representatives of the Panafrican molasse deposition. In the northeast, gabbro cuts the Jurdhawiyah Formation (Cole, 1988) and listwaenite is present in the thrusts that we believe correspond to listric faults of gravitational detachments that were tilted by late isostatic phenomena, as in the model set forth by Lister and Davis (1989) for the Tertiary structures of the USA.

Multiple generations of dikes (Figs. 16 and 17) were emplaced in the Arabian Shield after the Panafrican compression. They tend to occur in linear or curved swarms and vary from a few centimetres to several decametres in thickness and from several metres to several tens of kilometres in length. They can also form unidirectional or bidirectional (perpendicular) swarms. In the latter scenario, the two directions correspond either to the same lithology or to different lithologies. They can also have a ‘cracked glass’ appearance, and some swarms are temporally superposed. The distribution of the dikes relative to the fault systems is not random. We have observed that they tend to parallel the late faults and in places indicate deviatoric stresses in bifurcations or fault relay zones. They also indicate the directions of opening in fault networks showing horsetail arrays. They form a circular pattern around late intrusions, calderas and the Shammar basins in the northern part of the Shield.

For our structural analysis, we drew up a composite map of the dikes and used them to reconstruct the stress states during the phase of extension, knowing that the orientation of the opening plane indicates the minor principal stress component sigma 3. Other structural elements thought to be contemporaneous with the dikes, such as the displacement of the associated faults and the geometries of the correlative extrusive formations, were also used to constrain the basic kinematic processes.

The dikes are generally subvertical with very diverse orientations. They are commonly rhyolitic, and are also found in swarms of bimodal composition (Camp, 1986). Their role in the successive tectonic events has never been clearly stated, even though they were very thoroughly surveyed during mapping for the 1:250,000-scale coverage of the Shield. However, the dikes that were mapped are generally the youngest ones (530-590 Ma), as they are not folded, are less weathered, do not blend with the country rocks through metamorphism, and are more easily discernable on aerial photos. The oldest proposed ages tend to depend on the age of the country rocks, but direct dating has also been done. For example, in the southern part of the Shield, the dikes were emplaced between 519 and 587 Ma (Greenwood, 1985a, b).

Various kinds of relationship can be established between the dikes and the intrusions. In the northeastern part of the Shield (Williams et al., 1986; Ekren et al., 1987; Vaslet et al., 1987) the intrusions and the large felsic dikes have NE-SW orientations, whereas most of the dikes are small and oriented WNW-ESE. These smaller dikes are perpendicular to the direction of gravitational sliding revealed by mineral exploration surveys (Réchoché et al., 1988a, b). The intrusions are perpendicular to the Najd faults, which can be interpreted as transform. It would seem, then, that the intrusions and large dikes indicate the direction of crustal extension (opening), whereas the smaller dikes indicate shallower, gravitational phenomena. This conclusion is confirmed by the fact that the dike swarms in this zone developed largely above the main level of detachment located at the base of the Murdama Basin.

In the Samran Belt (Ramsay, 1986), the dikes are oblique to the NE-SW intrusions; they form a curve that trends N-S at its northern end. Here again, the intrusions record the deeper opening process, while the dikes mark the shallower gravitational effects and the geometry of the basins which, farther north (Camp, 1986), are represented by the Ar Rayyan conglomerates.

The relationships between the dikes and the correlative extrusive formations are also known in places. In the central part of the Shield, for example, they fed the Shammar volcanism (Delfour, 1977).

Large sills have been identified in the northern (Ekren et al., 1987) and central (Pellaton, 1981) parts of the Shield, and also in the Murdama Basin (Cole, 1988). They are locally associated with gold-bearing quartz veins (Récoché et al., 1998b). The Furayh Formation (Pellaton, 1981) is cut by a 80-km-long subhorizontal sill of monzonite, diorite and gabbro. Its source is unknown, but it may be rooted in a significant magnetic anomaly located immediately north of this zone (Asfirane et al., 1999). The sill is located geometrically above the Medina structure (see Fig. 21) and may be contemporaneous with the crustal thinning; this observation is reminiscent of the listwaenite formations of the flat-lying faults in the Jurdhawiyah Formation (Cole, 1988). Large sills may have developed into gravitational slides during the deformation.

Thus the injection of multiple dikes, both felsic and mafic, in the upper crust probably created detachment horizons that became preferred zones for the emplacement of mafic and intermediate intrusions originating from major crustal faults. We can also consider the location and the structural role of the dikes relative to the detachment faults. We see that in the northern part of the Murdama Basin (Cole, 1988; Williams et al., 1986), a detachment affects the base of the sedimentary formations. A dense, curved dike swarm occurs above this fault. This feature occurs twice in this part of the basin, following the evolution of the detachment. It is possible that these dikes only intruded the upper compartment of the fault, originating from material injected along the fault. Identical features are common throughout the Shield, and probably indicate the presence of deep, unexposed detachments. This finding is supported by the fact that the slide directions inferred from these assumed faults are consistent with the geometry of the basins' postulated basement as inferred from microtectonics and the aeromagnetic map.

A number of geologic maps show volcanic formations attributed to post-Panafrican magmatism, or with an assigned age between 530 and 590 Ma. These are mainly Shammar formations scattered throughout the Shield and generally concentrated near the Paleozoic cover. In the north, they are the Minaweh (Clark, 1987), Meddan and Farra'ah (Grainger and Rashad Hanif, 1989) formations of the Shammar, and in the northwest they also include the Qarfa (Vaset et al., 1987), Humaliyah and Samra (Cole, 1988) formations.

In the central part of the Shield, a narrow N-S basin contains the Qettann formations composed of sandstone and conglomerate, limestone with algal laminations, and various mafic and felsic volcanic formations (Ziab and Ramsay, 1986). These can be correlated with the Shammar Group, as can the Hima rhyolite that composes the local basement of the Wajid Sandstone in the south of the Shield (Greenwood, 1985b).

The Arabian Shield was cut by a late system of faults striking NW-SE (Fig. 1). Known as the "Najd faults" (Moore, 1979), they are left-lateral strike-slip faults that either followed or cut across the margins of the Panafrican structures. They also controlled the formation of the Jibalah basins (Delfour, 1970). The boundary faults of these basins are in places intruded by dikes that fed the Shammar volcanism, depositing material both inside and outside the basins (Letalenet, 1979). This NW-SE fault system is conjugate to a fault system that is much less well developed, but which also controlled the deposition of the Shammar and Jibalah formations (Dhellemmes and Delfour, 1980).

A comparative geometric analysis of the dike swarms, the late faults and the Shammar formations (both inside and outside the Jibalah basins) shows that they are contemporaneous. Thus the Ar Rika fault zone (Fig. 18a) reveals the relationships between the Bir' Sija (Figs 18b and 19) and Al Kibdi (Fig. 18c) basins and the Shammar volcanism that emanated from a system of late dikes (analysed from the Umm Wazir structure; Fig. 18d). The Medina structure (Fig. 20) demonstrates the relationships between the NW-SE faults and the dike swarms.

The Bir' Sija (Fig. 18b) and Al Kibdi (Fig. 18c) basins are located on the late Najd fault system. They were formed on relay zones or bifurcations of the Najd faults, and consequently we can tentatively identify them as pull-apart basins. Rhyolite flows forming part of the Bir' Sija Basin fill (Letalenet, 1979) were fed by a dike swarm that rose along the basin's boundary faults. The Al Kibdi Basin, whose fill contains andesitic formations (Delfour, 1980b), straddles a bifurcation zone within the same fault system.

The fill in the Bir' Sija Basin (Fig. 19a) consists of two main formations – the Shammar Rhyolites, which were fed by the boundary dikes and also extended beyond the tectonic trough formed by the boundary faults, and the Jibalah Sandstones (Delfour, 1970). The two formations are generally considered as stratigraphically separate, but we have observed that, in actuality, they are vertical and horizontal variations within the same sedimentary formation. The sandstone formations are contained within the main trough throughout the basin in its present state, and only the volcanic formations extend beyond the trough, primarily to the southeast.

Overall, the basin's fill comprises rhyolite flows at the base, overlain by siliciclastic deposits comprising three main sedimentary bodies that are distinguishable on aerial photos by the sedimentary wedges separating them. About 30 elementary sequences make up the three megasequences and progradation occurred from east to west (Fig. 19d).

A more detailed section made along the road between Bir' Sija and Afif (Fig. 19b) seems to straddle two sequences (Fig. 19c) that make up elementary landforms. The southern part of the section shows the unbroken sequence of lithologic and sedimentary variations at the edge of the basin and reveals the integration of Shammar volcanic activity during the sedimentary evolution of the Jibalah basin. This example demonstrates that dike emplacement, the Najd faulting and the Jibalah basin all resulted from the same tectono-sedimentary event. It also shows that the Shammar volcanism was fed by the rhyolite dikes. This volcanic activity is found at various places in the Shield. In the east, for example, it is represented by the Dab Formation (Manivit et al., 1985), which lies parallel to one of the Najd faults at the contact with the Paleozoic cover formations.

Southeast of the Al Kibdi Basin, the complex primary Umm Wazir structure (Delfour, 1980b) contains a curved dike swarm in a fault bifurcation zone within the Najd system (Fig. 18d). One of the Najd faults, oriented NW-SE, has a "Y"-shaped bifurcation in which the secondary fault forms the northern boundary of a pointed, triangular-shaped wedge. A large ‘S’-shaped dike swarm occurs within this wedge, and to the southeast; the dikes have not been folded. Field observations led us to the conclusion that they formed in a porphyritic granite with no folding. The tightest curve (the apparent hinge) of this feature, which makes up the lower part of the "S", shows that the structures, which could interpreted as a single dike from aerial photos, are actually composed of several dikes, each occupying a limb of the apparent fold, and intersecting in the hinge. This observation confirms that the structure is not the result of folding. To explain this unusual geometry, we propose that the "S" shape formed by the dike system represents the trajectory of the major principal stress (sigma 1) during the formation of the dikes. This interpretation implies that a) dike emplacement was contemporaneous with the Najd faulting, and b) part of the dike system intruding the blocks bounded by the Najd fault system was also contemporaneous with the fault activity. Thus, the dikes that accompanied formation of the Jibalah basins could possibly occupy three different positions relative to the fault system: 1) generally E-W dikes forming fairly dense swarms in the compartments delineated by the faults; 2) in or the near relay zones of faults affected by changes in direction due to deviatoric stresses; and 3) within the basin boundary faults.

North of Medina (Pellaton, 1981), a remarkably simple structure forms a series of nested south-pointing "V"s along a N-S axis (Fig. 20). The "V"s are formed by two different kinds of structural feature – faults and dikes. Those formed by the dikes have an angle of about 120° and are more open than the those formed by the faults, where the angle is about 90°. In the western half of the structure, the dikes are both felsic and mafic, whereas in the eastern half, they are exclusively felsic. The NW-SE faults in the western half of the structure have a left-lateral slip component consistent with the movement of the late Najd faults in the Shield. The NE-SW faults in the eastern half of the structure appear to be conjugate to the former, but there is no overlap between the two fault systems.

The dike swarms in this "V" structure have been dated (Pellaton, 1981) as younger than the rest of the intrusions in the area, with the exception of the circular Jabal al Bayda alkaline granite massif, which represents the last intrusional event before the deposition of the Paleozoic cover. They postdate the sedimentary formations of the Furayh Group, which locally represent the Panafrican molasse, and they predate the Cambrian-Ordovician Paleozoic cover. They are neither metamorphosed nor foliated.

The Shammar volcanic formations and the Jibalah sedimentary formations are not represented in this area. However, in the area immediately to the north (Hadley, 1987), dike swarms with the same orientation represent the feeder network of the Shammar volcanism. It is therefore possible to interpret all of the preceding structural elements as part of a single tectono-sedimentary event. By this interpretation, the NW-SE and NE-SW faults form conjugate sets of right-lateral and left-lateral strike-slip faults that were contemporaneous with the formation of the Jibalah basins, in which the fill started with the Shammar rhyolites. This phenomenon is well known within the Nabitah Belt, which was cut by these late faults. There the dike swarms fed the eruptive processes during a crustal thinning event governed by strike-slip faulting. Thinning took place primarily on a N-S axis that passed immediately east of Medina. This axis is the southward extension of the basement axis of the Tabuk Basin in the north, which originally extended southward the Ar Rayyan Formation (Camp, 1986). The Ar Rayyan Formation probably made up the basal fill of the basin as it was formed and thus, according to this hypothesis, is the lateral equivalent of the Shammar and Jibalah formations.

Despite the fact that various authors mention extensional phases in the structural evolution of the basement (Camp, 1986; Bokhari and Forster, 1988), not many normal faults have been described in the Arabian Shield. Field observations of lineaments revealed through aeromagnetic surveys (Asfirane et al., 1999) allow us to locate and describe these faults. Two have been described within the late system (Fig. 21a), i.e. the Jabal Farasan Fault and the Wadi Fatima Fault, of which the former bounds Jabal Farasan to the north (Fig. 21b) and extends about 200 km to the northeast. The Jabal Farasan Fault appears as a corridor several hundred metres wide where the pre-existing foliation is refolded. The axial planes of these structures are horizontal. Shear planes that developed in a brittle environment dip gently (about 30°) to the southeast and display striations with an azimuth of 160°. The Wadi Fatima Fault (Fig. 21c) also strikes NE-SW, and dips about 50° NW. Drag folds and tension gashes perpendicular to the striations reveal normal slip.

The largest detachment fault attributable to identified gravitational sliding is located at the base of the Murdama Basin in the northeastern part of the Shield (Williams et al., 1986; Cole, 1988). Above this fault, various low-angle normal and reverse faults cut the Murdama molasse (see Fig. 25) with a general north-northeasterly shear. These are primarily faults lined with quartz veins that were identified during mineral exploration in the large Murdama Basin (Récoché et al., 1988a, b). Other faults of the same kind have been identified in older terranes (Genna, 1994). They are all late faults, and have the same structural characteristics. The kinematic processes involved in these faults have been compared to the distribution of the Najd faults and dike swarms.

Tangential faults on all scales are observed in the large Murdama Basin. At map scale (Cole, 1988; Williams et al., 1986; Récoché et al., 1988a, b), reverse and normal faults dipping gently northeast cut the Jurdhawiyah Formation, which postdates the Murdama molasse formations. These are the Raha, Ata and Lughfiyah faults in which the fault planes are marked by mafic rocks (metagabbro and listwaenite). They are traditionally interpreted as lateral faults of the Najd strike-slip system, with the whole unit forming a positive flower structure as defined by Lowell (1972). However, the Najd faults do not exhibit transpressional fault geometries, and the tangential faults have nowhere been found to be rooted in the strike-slip faults. Of these three tangential faults, the Ata fault is a normal, N-dipping fault (Cole, 1988). However, the geometry of the dikes associated with the late complexes indicates NE-SW extension. For this reason, the flat to low-angle faults are interpreted as basal faults of gravitational slide blocks or as normal detachment faults that experienced late tilting from isostatic adjustment, following the model proposed by Lister and Davis (1989) (Fig. 22). This interpretation is supported by the fact that the youngest detrital sedimentary formation (the Jurdhawiyah) is located at the back of the assumed detachment, and may correspond to a Lister and Davis (op. cit.) "half-graben complex".

Many tangential faults have been described within the mineral prospects (Récoché et al., 1998 a, b). They are reverse or normal, and are defined by quartz veins lying parallel or subvertical to the faults. Steeply dipping normal faults are also present in the prospects. Displacement occurred to the north and northeast, consistent with the map-scale structures.

The post-Panafrican extension that we have just described is generally observed at a structurally shallow level, above the mylonitic front. However, some sedimentary facies display a primitive subhorizontal foliation attributable to this extension phase and postdating all the structural events. This was observed in the pre-Nabitah Bani Ghayy Formation, where a late subhorizontal foliation is superposed on the Panafrican compressional deformation (Fig. 23a). This area also contains subvertical quartz veins offset by SW-dipping tangential faults. These two observations are compatible with a late extension affecting the margin of the inferred substratum of the Jeddah Basin.

The Abt Schists (Delfour, 1979b; Delfour et al., 1982) is also an explicit example, as this ancient basin clearly shows the three main structural stages of the Shield. Fig. 23b shows the chronology of the events. Phase 1 deformation (S0-S1, L1), which probably marked the collision and closing of the ocean, preceded the Nabitah Orogeny (L2, P2), which is expressed in this basin as drag folds with subhorizontal to NW-plunging axes. The last phase (S3) is marked by the development of a subhorizontal foliation, which is found very locally in the basin.

In the south of the Shield, the late tectonism was primarily transcurrent (vertical foliation and vertical kink axes). Movement was left-lateral on the main N-S fault at Wadi Bidah (Koch-Mathian et al., 1994), which splays into a horsetail structure to the northwest. The secondary faults display the same kinematic indicators (Coumoul et al., 1989). The faults observed in the brittle deformation zone conformed to earlier structures of the previous ductile deformation, after significant uplift and several thousand metres of erosion. As in the northern part of the Shield, the previous structure consisted of batholiths and hard sigmoidal cores separating narrow, foliated corridors of sedimentary terrane. In this context, the main late faults developed in the sedimentary troughs, with lateral secondary fault networks; the whole unit forms horsetail structures. Thus we observe kinematic reversals of fault movement, which are particularly apparent on the 1:250,000-scale maps of the southwestern part of the Shield. The reversals were also revealed in mineral exploration studies (Donzeau and Béziat, 1989). The observed kinematic indicators reveal right-lateral movement of the NW-SE faults that were originally left-lateral faults during the Panafrican, and a left-lateral movement of the NE-SW faults that were right-lateral during the Panafrican.

The Fatima Formation, of Early Cambrian age (Basahel et al., 1984), is located east of Jeddah. It is characteristic of the Jibalah carbonate platforms deposited under tropical or subtropical conditions. However, in contrast to the platform formations (Jibalah) in the northern part of the Shield, the Fatima Formation contains a NE-SW fold train with an axial-plane cleavage, which developed in a transcurrent shear corridor. Thus, it is possible that the deformation of the carbonates in this formation reflects the same transcurrent deformation found in the southern part of the Shield, which would make it of Cambrian age.

Our observations on the Arabian Shield must be compared with the studies made on the evolution of the deformation and metamorphic conditions in the Proterozoic formations of the Sinai (Blasband, 1999; Brooijmans, 1999), which comprise the northern extension of the Arabian-Nubian Shield. These studies revealed a NW-SE extension (Blasband, 1999) associated with granites and dikes (590-530 Ma) and a HT-LP metamorphism (Brooijmans, 1999) between 600 and 530 Ma. Farther south, in the Egyptian desert near Marsa Alam (Renno and Stanek, 1999), circular post-orogenic intrusive complexes exhibit bimodal compositions in a structural compartment bounded by two of the Najd faults.

A simple geologic evolution model can be developed that accounts for all the structural elements described above. In this model, summarized in Fig. 24, thinning was penetrative on a crustal scale and was accompanied by bimodal magmatism instigated by crustal melting and influxes of mantle-derived material that could have been transported via major faults. This resulted in the emplacement of complex intrusive suites and associated dike swarms. Magmatism was controlled by subvertical transform faults (Najd faults) that initiated the formation of narrow, deep basins, and which were also injected by dikes. The dikes fed the flows that were deposited in the bottom of the basins. Dikes and mafic sills were emplaced at various levels in the structure and initiated or intruded local gravitational slides.

Note that these phenomena, which continued up until 530 Ma (i.e. into the Paeozoic), led to the transgression of the Jibalah basins. The platforms (Basahel et al., 1984) were undoubtedly the precursors of the initial geometry of the cover basins. Three large structures were preserved through the Paleozoic and may represent the continuation of this extensional event: the Tabuk and Widyan basins in the north, with the Saq Formation sandstones at the base, and the Rub al Khali Basin to the south, which contains sandstone formations (Wajid Sandstone) and Eocambrian salt formations (Faqira and Al-Hauwaj, 1998). The "Jeddah Basin" (Fig. 15) probably constituted another unit of this assemblage, marked by the Fatima carbonate formation (Basahel et al., 1984); its extension is found on the African continent.

A structural analysis of Saudi Arabia's various Proterozoic sedimentary formations and their substrata provides a new tectonic synthesis whose principal events are outlined in Table 1.

The first discernable events in the Shield were the development of marginal basins and volcanic arcs typical of the pre-Panafrican environment. The exact kinematic processes involved in their formation cannot be discerned at present because of the deformation they have undergone; the mechanisms that closed the oceanic domains remain unknown. It is thought that these events were followed by a phase of erosion that probably was not homogeneous throughout the Shield. Next came the Panafrican tectonism, the major cratonization event of the Shield that produced gneiss domes and molasse basins over a system of intracontinental shear zones. The resultant belt was then subjected to tangential events (late-orogenic extension and crustal thinning) controlled by the Najd transform faults – faults that gave rise to the Shammar volcanic formations and the Jibalah detrital formations. Widespread erosion brought about gradual peneplanation of the Shield and crustal thinning instigated a late marine transgression represented by the Jibalah carbonate platforms.

This new description of the chronology of geodynamic events in the Arabian Shield has significant implications for our understanding of the mineralizing events that produced the gold and base-metal prospects and deposits in the Shield. We can distinguish three types of mineralization, corresponding to three main phases of structural development: (i) mineralization emplaced before the formation of the Nabitah Belt, (ii) mineralization contemporaneous with the Nabitah fault activity, and (iii) mineralization postdating the belt, which was emplaced during crustal thinning. Figure 25 gives an example of each type of mineralization.

This synthesis, which was accomplished using all of the currently available geologic data, is a new overall vision of the tectonic evolution of the Arabian Shield. It has direct implications for metallogenic models dealing with the emplacement of the stratiform or discordant sulfide mineralization (gold and base metals) because it proposes a new chronology of mineralizing events.

The consequences we foresee from this study are primarily an understanding concerning the genesis of the gneiss domes and the exhumation of high-grade metamorphic facies, as well as of the associated intracontinental sedimentary basins. This paper is a contribution to the overall understanding of the Arabian-Nubian Shield and the Panafrican Belt.

BRGM Contribution No. ???

Fig. 1: General structural framework of the Arabian Shield (colour).

Fig. 2: Chronology of structural events in the Arabian Shield, relationships with the main types of structure.

Fig. 3: Pre-Panafrican volcano-sedimentary formations of the Arabian Shield.

Fig. 4: General structure of the Shield divided into terranes according to Johnson (1997).

Fig. 5: Structural pattern of the currently recognized major faults attributable to the collisional phase in the Arabian Shield.

Fig. 6: The Khnaiguiyah prospect (Cu, Zn). a: general structural setting of the study zone (showing the mineralized zone in black and the foliation trace), b: stereographic projection of the stretching lineation (Schmidt, lower hemisphere), c: kinematic interpretation.

Fig. 7: Main structures attributable to the Nabitah deformation phase. Molasse basins: T: Thalbah, H: Hadiyah, F & G: Furayh and Ghamr, M: Murdama, A: Ablah, J: Junaynah, L: Lasasah.

Fig. 8: The Jabal Kirsh gneiss dome. a: general location, b: NE-SW section, c: stereographic projection of S/C (Foliation/Shear) relationships in the Jabal Kirsh gneiss (Schmidt, lower hemisphere), d: general organization of second-order folds in the axial part of the dome near the Jabal Kirsh prospect (kyanite).

Fig. 9: Modes of deformation in the Jabal Kirsh Dome (boudinage). a: outcrop drawing, b: composite block diagram at the scale of a second-order fold, c: stereographic projection of the stretching lineation (Schmidt, lower hemisphere).

Fig. 10: Jabal Kirsh Dome, influence of lithology on modes of deformation. a: initial layers, b: deformation mechanism (stretching-shear), c: general configuration of quartzite boudins, d: strain ellipsoid, e: final geometry of a quatzite boudin.

Fig. 11: Jabal Tin example. a: general structure of the Tin Complex, b: E-W section of the Tin Complex, c: outcrop drawing (map view showing left-lateral shearing of the foliation, contemporaneous with emplacement of leucogranite veins), d: outcrop drawing (seen in cross section, showing the compressional aspect of the Jabal Tin Dome), e: composite stereograph of the S/C (Foliation/Shear) relationships and the elongation lineation (L) in the Jabal Tin Dome (Schmidt, lower-hemisphere), 1: Tertiary basalt, 2: Bani Ghayy Group, 3: gneiss of the Tin Complex, 4: pre-Bani Ghayy formations, 5: fault, 6: thrust.

Fig. 12: Panafrican structural development in the northwestern part of the Shield. a: general structural pattern, b: satellite photo survey of the metamorphic and kinematic foliation trace of the major faults (stereographic projection of S/C relationships, Schmidt, lower hemisphere); 1: molasse basin, 2: metamorphic foliation trace, 3: wedge effect in the Al Ays region, 4: synclinal axis, 5: anticlinal axis.

Fig. 13: Theoretical block diagram showing the vertical evolution of the strain ellipsoid at the core of a gneiss dome.

Fig. 14: The molasse basins of the Nabitah Orogeny.

a: cross section of the Talbah Basin

b: cross section of the Talbah Basin

c: cross section of the Hadiya Basin

d: cross section of the great Murdama Basin

e: E-W cross section of the Murdama formations in the Junaynah Belt.

Fig. 15: Composite structural framework of the post-Nabitah extension in the Arabian Shield. a: map showing the general location of the Arabian Shield, b: structural pattern, T: axis of the Tabuk Basin, J: substratum axis of the Jeddah Basin, W: axis of the Widyan Basin, RK: axis of the Rub al Khali Basin. 1: normal fault, 2: strike-slip fault, 3: gravitational slide, 4: presumed boundary of the basin substratum, 5: axis of the inferred basin substratum.

Fig. 16: Map showing the general distribution of dike swarms and late intrusions in the Shield. 1: Recent formations, 2: Tertiary basalt, 3: mean trajectory of the dike swarms, 4: fault, 5: dike swarm.

Fig. 17: Rose diagram showing the orientation of the dikes. a: examples of rose diagrams of the inferred substrata of the Tabuk and Jeddah basins, b: rose diagram of all of the survey data on the Arabian Shield.

Fig. 18: Ar Rika fault zone.

a: general structure of the Ar Rika fault zone,

b: general structure of the vicinity around the Bir' Sija Basin

c: Al Kibdi Basin and Jabal Kirsh Dome,

d: Umm Wazir structure,

1: Jibalah Formation in the Bir' Sija Basin, 2: Shammar Formation, 3: dikes in ancient host rock, 4: post-Murdama intrusions, 5: Panafrican molasse, 6: pre-Murdama formations, 7: gneiss domes, 8: Panafrican molasse of the Murdama Basin, 9: dikes in granitic host rock.

Fig. 19: Cross section of the Bir' Sija Basin.

a: general interpretive cross section; b: detailed cross section of the southern edge of the basin, 1: rhyolite, 2: tuff and vitric flows, 3: tuff and cinerite, 4: sandstone slabs, 5: interbedded siltstone, sandstone and tuff, 6: predominant sandstone, 7: microconglomerate (channel), 8: boundary fault zone; c: elementary depositional sequence; d: northwestward progradation of successive sedimentary sequences.

Fig. 20: The Medina structure, example. 1: Proterozoic formations, 2: alkaline granite, 3: microgranite, 5: Tertiary basalt and Quaternary formations, 5: dike, 6: fault.

Fig. 21: Normal faults of the Jeddah region.

a: general location.

b: Jabal Farasan Fault, interpretive block diagram of a fold train with subhorizontal axes and network of gently dipping secondary normal faults, stereographic projection of the fault plane solution and direction of slip (Schmidt, lower hemisphere).

c: Wadi Fatima Fault, interpretive block diagram with: 1: open gashes, 2: stretched and boudinaged quartz, 3: drag fold, 4: secondary fault, stereographic projection of the fault plane solution and direction of slip (Schmidt, lower hemisphere).

Fig. 22: Conceptual model of crustal thinning with the faults and post-Murdama sedimentary formations, northeastern part of the Arabian Shield.

Fig. 23: Other manifestations of late extension. a: example, Bani Ghayy Formation, 1: stereographic projection of striations observed on quartz veins (Schmidt, lower hemisphere), 2: drawing of outcrop, S1: Panafrican foliation, S2: foliation due to late extension; b: chronology of deformation observed in the Abt Schists. S0: stratification, S1: foliation 1, L1: lineation 1, P2 : phase 2 folds, L2: phase 2 lineation, S3: foliation 3.

Fig. 24: Composite model of post-Panafrican crustal thinning in the Arabian-Nubian Shield. 1: mafic magmatism, 2: felsic magmatism, 3: dikes, 4: syn-rift deposits, 5: post-rift deposits.

Fig. 25: Examples of mineralization (gold and base metals) emplaced during the main structural events of the Arabian Shield. a: Rabathan sedimentary massive sulfides, emplaced prior to formation of the Nabitah Belt (Wadi Bidah Belt; Koch-Mathian et al. 1994), b: Shayban hydrothermal gold mineralization, emplaced during the Panafrican Orogeny (Samran Belt; Genna 1994), c: gold mineralization emplaced in subhorizontal faults of the Mohsiniyah prospect during crustal thinning (Silsilah District; Récoché et al. 1998).

Table 1: Composite table showing relationships between the tectonic phases and sedimentary formations.