LATE PROTEROZOIC CRUSTAL THINNING IN THE ARABIAN SHIELD: geologic and metallogenic implications

A. Genna, P. Nehlig, I. Salpeteur and M. Shanti

A. G., P.N. and I.S. : BRGM 3, Avenue C. Guillemin, BP 6009, 45060 Orléans cedex 2, France.

M. S. : BRGM, P.O. Box 1492 Jeddah 21431, Saudi Arabia.

The Late Proterozoic formations of Saudi Arabia show evidence of crustal thinning, expressed by extensional deformation with contemporaneous bimodal magmatism, followed by a return to marine sedimentation. The volcanic activity, represented by the extrusive formations of the Shammar Group, was associated with various intrusive complexes and dike swarms dated between 530 and 590 Ma, whilst gravitational sliding in the uppermost part of the crust is reflected by listric or gently dipping faults. 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 the 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 fill deposits. This extensional episode ended in a marine transgression that is evidenced at various places in the Shield by the carbonate platforms of the Jibalah basins. Continuation of the thinning process may explain the deposition of the marine formations of the Paleozoic cover. The associated crustal warming instigated a major metallogenic event in the Arabian-Nubian Shield during the Late-Proterozoic extension.

Keywords: Saudi Arabia, extension, Proterozoic, Tectonism, dike.

1. Introduction

Extensional processes in mountain ranges may occur contemporaneously with convergence, or may reflect post-orogenic crustal thinning (Wernicke, 1981; Malavielle, 1993). Extension is governed by detachment faults and complex tectonic processes involving tilted fault blocks (Lister and Davis, 1989), and it initiates isostatic readjustment (Wernicke and Axen, 1988), thereby changing initial geometries. Significant magmatic activity develops when there is crustal warming (Lachenbruch and Morgan, 1990) and gold mineralization may be deposited during these processes (de Boorder et al., 1998). If thinning is sufficient, it ends in oceanization, and then becomes assimilated into a passive margin process.

By applying these concepts to the Proterozoic formations of the Arabian Shield, we can reinterpret the geodynamic context in which the sedimentary and intrusive formations were deposited between 590 and 530 Ma. There is, in fact, a complex of Late Proterozoic structures in the Shield that record a crustal thinning process (Fig. 1).

The thinning is evidenced by various kinds of extensional structure at all scales, whose basic kinematic processes are consistent within a single late-tectonic event (590-530 Ma) of the Panafrican compression (690-590 Ma). This event was contemporaneous with bimodal magmatic activity consisting of intrusions and dike and sill networks, and a number of associated volcanic formations collectively known as the Shammar Group. Strike-slip faults became active as transform faults in the thinning mechanism, and controlled the formation of the Jibalah basins. Normal faults are associated with the formation of conglomerate-filled basins, as represented by the Ar Rayyan and Jurdhawiyah formations (Fig. 2). They also governed deposition of the Fatima and Jibalah carbonate platform formations. Tangential block faulting is indicative of the gravitational processes initiated by stretching of the crust.

Apart from the ancient ocean and continental margin systems that were active before 700 Ma, earlier studies have rarely proposed extensional tectonic phases at Shield scale. A notable exception is the work of Bokhari and Forster (1988) who argued for extensional tectonism in an intracratonic domain between 700 and 540 Ma to which they attributed to a phase of continental sedimentation comprising the Murdama molasse formations, the Shammar volcanism and the Jibalah basins. According to our studies, however, the formation of the Murdama basins was associated with the emplacement of gneiss domes formed in a transpressional context contemporaneously with the Nabitah Belt (Genna et al., 1999 a), and extension began after these basins were formed. This chronology is also validated by a new understanding of how the Najd faults operated (Johnson 1998; Genna et al., 1999 b). The Najd faults are NW-SE striking and were active at two periods. The first was contemporaneous with the Murdama basins, and can be observed in areas of ductile deformation. The second was contemporaneous with the Shammar and Jibalah formations, and is evidenced in outcrops showing brittle deformation. We only consider the second period to be part of the extension phase in our interpretation, when the Najd faults operated as transform faults in the extensional system.

With this exception, we agree with Bokhari and Forster (1988) that extension was contemporaneous with bimodal magmatism, caldera formation, the Najd faults and the Jibalah basins. It has not been determined when extension began, but it was probably around 590 Ma. The final phase of extension is represented by the top of the Jibalah formations (540 Ma), although it has not been proven that it ended during deposition of the Paleozoic cover.

Other, local, extensional processes have been recorded on a number of 1:250,000-scale maps, primarily in the northeastern part of the Shield. Cole's work (Cole, 1988) shows examples of this. He proposed a late extension for this part of the Shield, following erosion, weathering and peneplanation. The extension was N-S and contemporaneous with emplacement of the Abanat Suite (580-570 Ma) and its associated volcanic formations (the Humaliyah and Samra formations). Cole (op. sit.) considered that extension began at 640 Ma with emplacement of the Jurdhawiyah Group.

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 the Sinai (Blasband, 1999) and Egypt (Renno and Stanek, 1999). Their models are based primarily on studies of magmatism (magmatic core complexes) and structural analysis.

2. Geologic setting

The first overview of the geology of the Arabian Shield was provided by Karpoff (1958), who classified the Proterozoic rocks of Saudi Arabia into two distinct series: the older one, known as the Medina series, showing highly variable grades of metamorphism and being 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 main phases between 800 and 500 Ma, with 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.

More recently (Kröner, 1985) the Arabian Proterozoic basement has been considered in terms of an assemblage of microplates bounded by sutures associated with ophiolite formations (Shanti and Roobol, 1979); a model that has been extended to 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 the same lines, some authors (Camp 1984; Laval and Le Bel, 1986) have proposed a model of island arcs, subduction and collision for a number of volcanic belts. 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, involving 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 gabbroic plutonism giving way to calc-alkaline plutonism. Cratonization of the Arabian-Nubian Shield occurred by a juxtaposition of microplates between 720 and 640 Ma, and was followed by tectonic extension (700-540 Ma) marked by andesitic volcanism leading to molasse sedimentation. A parallel was established with the Tertiary extension of the Basin and Range province in the USA.

More recently, 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.

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

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.

Numerous radiometric age determinations have been undertaken for the Shield. A first comprehensive review of these was made in 1993 (Johnson et al., 1993), and stratigraphic implications were drawn later (Johnson, 1996), when three main geologic periods were proposed: a precratonic, oceanic assemblage evolving from 890 to 690 Ma; closure of the oceanic domain between 715 and 655 Ma; and the Nabitah Orogeny between 675 and 625 Ma, overlapping with the closure of the ocean. Post-tectonic magmatism occurred at intervals from 610 to 540 Ma. Fig. 3 summarizes our current understanding of the chronology of tectonic events in the Arabian-Nubian Shield, based on our study of the late tectonic evolution.

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., 1998 a, b). Two cartographic and structural syntheses were made for the Bidah and Shwas volcano-sedimentary belts (Donzeau and Béziat, 1989; Béziat and Donzeau, 1989).

3.1. Post-Panafrican intrusions

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 formed sills. The existence and general shape of concealed batholiths is evidenced by the dike swarms and ring dikes they produced. 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 intruded 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.

3.2. Dikes

Multiple generations of dikes (Figs. 4 and 5) 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 may have similar or different characteristics. They can also impart a ‘cracked glass’ pattern, 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, or the directions of opening in fault networks showing horsetail arrays. They also 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 been done. In the southern part of the Shield, the dikes were emplaced between 587 and 519 Ma (Greenwood, 1985 a, 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., 1998 a, 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 known in places. In the central part of the Shield, for example, they fed the Shammar volcanism (Delfour 1977).

3.3. Sills

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 an 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. 9) and could 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 formed above this fault. This is a feature that 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.

3.4. Sedimentary and volcanic formations of the post-Panafrican magmatism

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 northeast they also include the Qarfa (Vaslet 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).

3.5. The Najd fault network

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 rhyolite flows, 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. Four examples were studied in more detail: the Az Zamamil structure, the Bir' Sija Basin, the Umm Wazir structure and the Medina structure.

3.5.1. The Bir' Sija Basin

The Bir' Sija Jibalah basin is located north of Zalim on one of the NW-SE Najd faults (Letalenet, 1979). Fig. 6a shows its location on the Najd fault network. It is 20 km long and about 5 km wide, oriented NW-SE. Two boundary faults intruded by rhyolite dikes mark its northern and southern edges.

The basin fill 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 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.

A more detailed section made along the road between Bir' Sija and Afif (Fig. 6b) seems to straddle two sequences 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 eastern part, 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.

3.5.2. The Az Zamamil structure

The Az Zamamil structure (Fig. 7) reveals a network of NW-SE faults (Najd faults) cutting across two late intrusions. These faults were intruded by dikes (Delfour, 1980b), and there is a large dike swarm oriented generally E-W surrounding them. This spatial arrangement (Fig. 7) is identical to the configuration of the structures associated with the Bir' Sija Basin (Fig. 6). We can thus assume that the context was equivalent (in age and kinematics) to that in the Bir' Sija Basin sector, knowing that the basin here was eroded (a postulated basin).

3.5.3. The Umm Wazir structure

A complex primary structure southeast of the Al Kibdi Basin (Delfour, 1980b) contains a curved dike swarm in a fault bifurcation zone within the Najd system (Fig. 8). 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 enclosing a large "S"-shaped dike swarm that continues to the southeast; the dikes have not been folded. Field observations led us to the conclusion that these dikes 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 be 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 the 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 late 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 near the relay zones of faults affected by changes in direction due to deviatoric stresses; and 3) within the basin boundary faults.

3.5.4. The Medina structure

North of Medina (Pellaton, 1981), a remarkably simple structure forms a series of nested south-pointing "V"s along a N-S axis (Fig. 9). 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 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 to 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.

3.6. Normal faults and gravitational slides

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) allowed us to locate and describe these faults. Two are within the late system (Fig. 10a), i.e. the Jabal Farasan Fault and the Wadi Fatima Fault, of which the former bounds Jabal Farasan to the north (Fig. 10b) and extends about 200 km to the northeast. The Jabal Farasan Fault appears as a corridor, several hundred metres wide, in which 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. 10c) 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. 14) 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., 1998a, 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 at all scales are observed in the large Murdama Basin. At map scale (Cole, 1988; Williams et al., 1986; Récoché et al., 1998 a, 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. 11). 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.

3.7. Other manifestations of the late extension

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 the 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. 12a); The 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) also provide an explicit example, as this ancient basin clearly shows the three main structural stages of the Shield, the chronology of which is given in Fig. 12 b. 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.

4. Discussion and proposed model

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. 13, 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 deep narrow 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 Paleozoic), 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 (Fig. 1): 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. 1) 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.

5. Metallogenic implications

Mineralization associated with mountain ranges generally occurs late in the collisional event (Boorder et al., 1998) and is related to late-orogenic extension processes. This concept is well documented in the Arabian Shield, where it has already been proposed (Bokhari and Forster, 1988). A significant thermal event (Brooijmans, 1999) accompanied extension and crustal thinning. This rise in heat was associated with emplacement of intrusions and large networks of sills and dikes between 590 and 530 Ma, and may have led to a major metallogenic event.

The Arabian Shield contains various kinds of precious- and base-metal mineralization (Béziat and Bache, 1995) which have been the subject of comprehensive studies. A number of ideas have been proposed concerning the relationships between mineral deposits and geotectonic contexts to explain mineral emplacement in the Late Proterozoic formations of Saudi Arabia. Following up on this work, our study reveals possible relationships between late extension and mineralization. Specifically we have observed that, whether mineralized or not, the large veins of late quartz in the Shield tend to be subparallel to dikes of the same age. This notion is illustrated clearly by the example in Fig. 7. The late mineralization emplaced in the flat-lying faults of gravitational slides in the Murdama Basin (Récoché et al., 1998 a, b) provide an example (Fig. 14). The faults are subhorizontal (Fig. 14a), marked by gold-bearing quartz veins (Fig. 14b) and dip NNE (Fig. 14c). Other mineralization in the same area is directly related to the late NNE-SSW microdiorite dikes (Récoché et al., 1999), which also intruded the NW-SE Najd faults. The currently active Sukhaybarat Mine may have derived from the same process, and the Al Wajh District (Lea Anderson et al., 1995) was probably formed in the same way.

As we have just seen, strike-slip faulting plays an important role in crustal thinning processes. This structural analysis raises questions about the mineralization associated with strike-slip faults in the Shield, because the faults governing them are not well dated. Thus the numerous prospects commonly associated with subhorizontal quartz veins or located in late normal faults in the central part of the Shield and the Al Amar structure may also have been emplaced during late extension.

Overall, we believe that the significant metallogenic events of this extension and its associated magmatism, of which the dikes are simply the most visible manifestation, are to be found three levels in the post-Murdama crust:

a) At deep levels where intrusive subvolcanic calc-alkaline domes may contain significant porphyritic mineralization.

b) At shallower levels where the dikes fed highly explosive rhyolitic and dacitic volcanic centres (ignimbrites, caldera breccias), which are likely sites for the development of epithermal mineralization (Au, Ag, U, Mn, Ba, F and so on). Several occurrences of this kind have been identified in the central and northern part of the Shield (Paupy, 1986).

c) At the base of the sedimentary basins where convective circulation of water originating from the dehydration of clays and the outgassing of magmas may have caused numerous low-temperature vein-type mineralization (barite, fluorite, galena, etc.) to be deposited in the structural traps of the basement and cover.

A structural re-evaluation of late mineralization associated with normal, tangential and strike-slip faults would help clarify how it was emplaced in its post-orogenic context.

6. Conclusion

Between 590 and 530 Ma, the Arabian Shield underwent an episode of crustal thinning that produced a number of extension-related structures. Thinning instigated partial melting of the crust, which fed a suite of granitic intrusions and associated dike swarms. Influxes of mantle-derived material generated large sills and dike swarms. The extrusive formations produced by this magmatic activity comprise the bimodal Shammar volcanic formations. Normal detachment faults and isostatic readjustments governed this deformation. Conjugate arrays of dextral and sinistral faults were integrated into the context as transform faults.

Bringing this phenomenon to light may have significant implications relative to the Late Proterozoic geology of Arabia and East Africa. The metallogenic evolution of the Arabian-Nubian Shield during this period can be understood more clearly by applying a new concept concerning the relationships between the intrusive formations (the source of mineralization) and the contemporaneous fault networks (structural traps).

ACKNOWLEDGMENTS

The authors are grateful to Dr M.A. Tawfiq, Deputy Minister, Directorate General Resources and Saudi Geological Survey and Dr F. Le Lann (Senior Vice President Director BRGM Saudi Arabia) for their help and permission to publish this paper.

We thanks Dr. M. Sahl, M. Eberlé, J.M. Leistel, Ignace Salpeteur, D. Thiéblemont for reviews and constructives discussions. BRGM Scientific Contribution No 43.

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FIGURES

Fig. 1: Simplified structural framework of the Late Proterozoic 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. 2: Map showing the locations of the main examples given in the text.

Fig. 3: Chronology of structural events in the Arabian-Nubian Shield.

Fig. 4: 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. 5: Rose diagrams showing the orientation of the dikes. a: examples of rose diagrams for the inferred substrata of the Tabuk and Jeddah basins, b: rose diagram for all of the survey data on the Arabian Shield.

Fig. 6: Bir' Sija. a: general structure, 1: dike, 2: fault, 3: Shammar and Jibalah formations, 4: late intrusion, 5: Proterozoic formations; b: detailed cross section, 1: sandstone slabs, 2: tuff and cinerite, 3: tuff and vitric flows, 4: rhyolite, 5: microconglomerate (channel), 6: predominant sandstone, 7: interbedded siltstone, sandstone and tuff, 8: boundary fault zone; c: general cross section of the basin; d: elementary depositional sequence; e: northwestward progradation of successive sedimentary sequences.

Fig. 7: Az Zamamil structure. 1: dike, 2: quartz vein, 3: fault, 4: shallow formations, 5: late intrusion, 6: Proterozoic formations.

Fig. 8: Umm Wazir, 1: Nabitah Belt, 2: Murdama sandstone and conglomerate, 3: granite, 5: dikes.

Fig. 9: The Medina structure. 1: Proterozoic formations, 2: alkaline granite, 3: microgranite, 4: Tertiary basalt and Quaternary formations, 5: dike, 6: fault.

Fig. 10: 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. 11: Conceptual model of crustal thinning with the faults and post-Murdama sedimentary formations, northeastern part of the Arabian Shield.

Fig. 12: Other manifestations of late extension. a: 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. 13: 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. 14: The Al Mohsiniyah Prospect (Au), Murdama Basin. a: N-S section of a mineralized subhorizontal fault, b: detail from a, c: stereographic projection of the striations measured on the normal faults (Schmidt, lower hemisphere).