Arabian Shield: Overview and Recent Advances
by Peter R. Johnson
Overview:

The Arabian Shield comprises the Precambrian rocks within the Arabian Plate that are exposed in the western part of the Arabian Peninsula—in western Saudi Arabia, and to lesser extents, in Yemen and southern Jordan. The shield is part of a basement of variably metamorphosed and deformed Precambrian rocks that extends beneath the entire southern Arabian Peninsula. Phanerozoic sedimentary rocks, as much as 14 km thick, cover the Precambrian basement in central and eastern Saudi Arabia, but the basement rocks re-emerge in small exposures in Oman. The Arabian Shield is mostly Neoproterozoic and juvenile. However, enclaves of Archean to Paleoproterozoic rocks crop out in the eastern and southern parts of the shield in Saudi Arabia and Yemen. Inherited pre-Neoproterozoic zircon grains in plutonic and volcanic rocks, pre-Neoproterozoic detrital grains in sedimentary rocks, and locally strongly negative Nd and Hf values, measured from zircon in the plutonic rocks, indicate a variable but significant input from older crust in the formation of the Neoproterozoic crust.
The rocks of the shield record a geologic history of >2 billion years (>2 Ga), beginning with the formation of Archean-Paleoproterozoic continental crust, and continuing through the formation and accretion of Neoproterozoic oceanic and continental volcanic arcs, deposition in volcanic and sedimentary basins, and the intrusion of vast amounts of late Neoproterozoic granitoids. The Neoproterozoic rocks span a period of unprecedented tectonic, biological, biochemical, and climatic changes that accompanied the rifting and the reassembly of supercontinents, the formation and destruction of oceanic basins, worldwide glaciations, and the beginning of the proliferation of multicellular animal life forms that exploded in the Cambrian. The shield history ended with the creation of stable continental crust, 40–45 km thick. The uplift and erosion of the shield soon after its creation led to the development of a vast regional unconformity, the Angudan unconformity, dated ~525 Ma (Al-Husseini, 2014), on which the sandstones, siltstones, limestones, and evaporites of the Phanerozoic succession of the central, southern, and northern parts of the Arabian Peninsula were deposited.
 
The Arabian Plate, showing outcrops of Neoproterozoic and older rocks in the Arabian Shield and Oman, Cenozoic basalt associated with the opening of the Red Sea, and the plate boundaries—extensional in the south and west, convergent in the north and northeast, and transcurrent in the northwest and southeast.
 
Prior to the opening of the Red Sea at about 25 Ma (Bosworth, 2015), the rocks of the Arabian Shield were adjacent to similar rocks in northeast Africa, and together, they formed the Arabian-Nubian Shield. The Arabian-Nubian Shield extends from southern Jordan and Sinai to Kenya over a distance of 3,500 km N-S and 1,500 km E-W, and it underlies an area of about 2.7 million km2. The northwestern boundary of the Arabian-Nubian shield is a sheared contact (the Keraf suture) with the Saharan Metacraton (Abdelsalam and others, 1998, 2002). Its southwestern boundary is a west-verging thrust system with the Congo-Tanzania craton and reworked pre-Neoproterozoic rocks of the Mozambique Belt (Fritz and others, 2013). Its southeastern boundary is a poorly exposed contact in Kenya and Ethiopia with a pre-Neoproterozoic crustal block referred to as Azania (Collins and Pisarevsky, 2005). Farther northeast, the southeastern boundary is a poorly exposed contact, originating at a passive margin close to or part of the Indian craton (Cox and others, 2011; Denèle and others, 2012; Whitehouse and others, 2016). Its eastern boundary is a speculative contact in east-central Saudi Arabia and Oman.
 
Regional setting of the Arabian-Nubian Shield (after Fritz and others, 2013)
 
The Arabian-Nubian Shield and the Mozambique Belt form the East Africa Orogen (Stern, 1994), which originated as an axial region between converging Gondwana cratonic blocks during the culmination of the supercontinental cycle that began with the breakup of the supercontinent Rodinia and ended with the assembly of the supercontinent Gondwana (Stern, 1994). During this cycle, juvenile crust was produced between 870 Ma and 625 Ma by subduction and arc-related magmatism in the Mozambique Ocean that formed between the rifted blocks of Rodinia. The continental collision at the beginning of the assembly of Gondwana (~650–600 Ma) caused crustal and lithospheric reworking, resulting in a thickened and heated lower crust, brittle-ductile shearing and folding in the upper crust, metamorphism that reached its peak between 620 Ma and 585 Ma, and partial melting and crystallization in magma reservoirs in the lower crust that led to the intrusion of 635–580 Ma late- to post-tectonic granitoids (Stern, 1994; Fritz and others, 2013; Robinson and others, 2014).
 
Ongoing shortening that was broadly orthogonal to the N-S trend of the Arabian-Nubian Shield (present day coordinates), northward tectonic escape, within-plate magmatism, and orogen collapse occurred between ~600 Ma and 540 Ma. The final assembly of Gondwana was achieved between 550 Ma and 520 Ma, constrained by a “hard-collision” between India and the Congo-Tanzania craton (Collins and Pisarevsky, 2005; Fritz and others, 2013) and the amalgamation of the South American continental nuclei with Africa during the Brasiliano orogeny (600–530 Ma) (Meert and Van der Voo, 1997). By 520 Ma, the East African Orogen formed an orogenic belt within the core of Gondwana. By 520 Ma also, the northern end of the East African Orogen (present-day coordinates) had undergone regional-scale exhumation. This exhumation event was associated with orogenic collapse, uplift, and denudation, and resulted in the formation of the regional Angudan unconformity (Al-Husseini, 2014) throughout North Africa and Arabia at the base of the vast Cambrian-Ordovician blanket of sandstone that was deposited on the stable crust of the Arabian-Nubian Shield. The East African Orogen is an accretionary orogen, one of the several end-Precambrian (Pan-African) belts of strongly deformed rocks that form a network of tectonized rocks that extend through Gondwana. The orogen represents the roots of a mountain belt that existed in Gondwana at the end of the Precambrian, similar to, although smaller than, the Alpine-Himalayan mountain belt that represents the present-day orogenic belt between the converging plates of Africa, Arabia, India, and Eurasia.
 
Cartoon showing the development of the Arabian-Nubian Shield and the East African Orogen within the context of the Rodinia-Gondwana supercontinental cycle (after Stern and Johnson, 2010).
 
Simplified maps (at the same scale), showing the comparison between the late Cryogenian-Ediacaran East African accretionary orogen and the Mesozoic-Cenozoic Alpine-Himalayan accretionary orogen.
 
The general lithologic, geochronologic, and structural characteristics and the tectonic setting of the Arabian Shield are well established (e.g., Nelig and others, 2002; Johnson and others, 2011; Johnson and Kattan, 2012; Fritz and others, 2013; Pease and Johnson, 2013). Lithologically and structurally, the shield is heterogeneous. The granodiorite and tonalite exposures form more than a quarter of the total area of the shield (25.7%), followed by the granites (16.9%). The volcanic and volcaniclastic rocks constitute 12.8% of the shield area, the mixed assemblages of volcanic and sedimentary rocks, 18.9%, and the sedimentary rocks, 15.5%. The ultramafic and ophiolitic rocks (peridotite, gabbro, and basalt) comprise less than 1% of the surface area of the shield, a small proportion of the overall shield rock types, but they are of considerable tectonic significance as markers of sutures.
 
Within this heterogeneity, the Tonian to late Cryogenian volcanic, volcaniclastic, sedimentary and dioritic, granodioritic, and tonalitic rocks constitute juvenile volcanic arc assemblages and are one of the main features of the Arabian Shield. They reflect much of its formation by magmatism at subduction zones in the Mozambique Ocean (Nelig and others, 2002; Johnson and others, 2011). Extensive areas of sedimentary and volcanic rocks represent depositional basins that periodically developed unconformably on the arc assemblages (Johnson and others, 2013). The plutons and the batholiths of relatively undeformed granitic rocks are among the youngest rocks in the shield, and they reflect late- to post-tectonic magmatism in an evolving and thickening continental crust.
 
Simplified geologic map of the Saudi Arabian part of the Arabian Shield (modified from Johnson and Kattan, 2012)
 
 
Crustal composition of the Arabian Shield. (A) Lithology of surface exposures expressed as percentage by area. (B) Vertical crustal structure of the Arabian Shield (after Stern and Johnson, 2010).
 
Difference in ages among the arc assemblages, varying isotopic characteristics of the volcanic and plutonic rocks, and contrasting structural trends divide the shield into tectonostratigraphic terranes (e.g., Stoeser and Camp, 1985; Nelig and others, 2002; Johnson and Woldehaimanot, 2003; Johnson and others, 2011). The ophiolite-decorated shear zones represent the sutures between the convergent terranes (Dilek and Ahmed, 2003; Johnson and others, 2004).
 
Seismic data reveal a shield crustal thickness of 40–45 km, divisible into an upper heterogeneous, largely granitic layer and a lower mafic layer, each with ~20 km thickness (Gettings and others, 1986). The lithospheric mantle is 80–120 km thick (see discussion in Stern and Johnson, 2010).
 
The Arabian Shield is geologically complex, and despite more than 70 years of modern geologic investigation, profound questions about its genesis and tectonic setting remain. However, recent developments in three fields of geologic study are noteworthy in making significant contributions to improving the current understanding of the origins and the structural development of the shield. These are:
  1. The application of ion microprobe analysis to single zircons, thereby significantly strengthening the geochronologic model of the shield.
  2. The analyses and interpretations of single zircon Hf and O isotopes that augment the well-known Pb, Sr, and Nd isotope data sets and improve the petrogenetic modeling of the plutonic and volcanic rocks in the shield; and
  3. structural mapping and the petrologic studies of shear zones that have expanded knowledge about the dynamics of shearing on the sutures and transcurrent faults that form the dominant displacement structures in the shield.
 
Conventional tectonostratigraphic divisions of the terranes in the Saudi Arabian part of the Arabian Shield (after Johnson and Kattan, 2012). These divisions are not uniformly recognized, and other authors have proposed different numbers of terranes and differently located sutures.
 
 
 
Single-zircon geochronology
 
Geologic dating, in order to determine the formation (crystallization) ages of the volcanic and plutonic rocks, has been carried out in the Arabian Shield since the 1950s. Early works used K-Ar, Rb-Sr, and U-Pb methods on whole rocks, mineral separates, and groups of zircons. With the development of new analytic techniques for the analyses of trace elements and isotopes, it is now a standard practice in the study of Arabian Shield rocks to apply laser ablation and ion microprobe U-Pb analysis to single zircons. Coupled with cathodoluminescent imaging of zircons that guides the location of analytic spots, these methods yield results of great accuracy and reliability. The techniques applied to the Arabian Shield include LA-ICP-MS (laser ablation-inductively coupled plasma mass spectrometry), SIMS (secondary-ion mass spectrometry), and SHRIMP (sensitive high mass-resolution ion microprobe spectrometry). A separate method of dating, based on the analysis of lead isotopes released during the evaporation of individual zircon grains, referred to as the Pb-Pb evaporation method, has been applied in some studies.
 
To date, the single-zircon geochronologic dataset for the Arabian Shield contains more than 150 results, ranging from Archean (2,556 Ma for the granite gneiss in the Al Mafhid terrane in Yemen) (Whitehouse and others, 1998) to early Cambrian (526 Ma for the Mardabah syenite complex in northwestern Saudi Arabia) (Robinson and others, 2014). A frequency histogram of all data shows small peaks between 2,550 Ma and 2,560 Ma, denoting the Archean granite gneiss in the Al-Mahfid terrane and between 1,660 Ma and 1,680 Ma, denoting the Paleoproterozoic granite in the Khida terrane. The subset of Neoproterozoic data has peaks reflecting an abundance of Tonian magmatic activity (the formation of subduction-related volcanic island arc assemblages) at ~770–780 Ma and Ediacaran magmatic activity (mostly late- to post-tectonic granitic plutonism) at ~620 Ma. Unfortunately, the geochronologic data are not uniformly distributed, and reliable single-zircon ages are not available for the larger parts of the southern and northeastern shield. Furthermore, the samples in the present Neoproterozoic dataset are largely granitic in composition (alkali granite, granite, and granodiorite), so there is no comprehensive age dating of all the main rock types in the shield. Fewer than 12 single-zircon crystallization age are available for the mafic volcanic (basaltic and andesitic) and the mafic intrusive (gabbroic and dioritic) rocks, which means that most of the volcanic arc assemblages in the shield are not directly dated. In turn, this means that the history of subduction, ocean-basin convergence, arc magmatic and volcanic associated mineralization in the shield is not well constrained.
 
Single zircon geochronologic data for the Arabian Shield. The map shows the distribution of the currently available data, underscoring the lack of systematic coverage of the shield and the particular paucity of data for the southern and northeastern Arabian Shield. Insets are frequency distribution histograms for (A) the entire data set and (B) the Neoproterozoic subset.
 
The recent work of Robinson and others (2014) recognizes four discrete magmatic events in the Arabian Shield: the ~845 Ma island arc activity, the ~710 Ma syncollisional activity, the ~620 Ma post-tectonic activity, and the ~525 Ma anorogenic activity. However, these events are very generalized and do not take into account the differences in the geologic history of the different parts of the shield that a detailed geochronology will reveal. A relevant example is that dating of the arc-related volcanic, sedimentary, and intrusive rocks in the Eastern Desert of Egypt and Sinai suggest that as many as five arcs are present in the northern part of the Nubian Shield, ranging from the Feiran-Elat arc (~870–740 Ma) in northern Sinai (Stern and Manton, 1987; Eyal and others, 2014) to the Aswan arc (~625 Ma) in the vicinity of Aswan (Finger and others, 2008; Sultan and others, 1994). Furthermore, the SIMS dating of single zircons in the metasedimentary, metavolcanic, and plutonic rocks in Sinai suggest an even older late Mesoproterozoic-early Tonian arc event (~1.03-0.93 Ga) that is present in northernmost Sinai (Sa’al arc; Be’eri-Shlevin and others, 2009a; 2012; Eyal and others, 2014). In contrast, the currently oldest rock, dated by single-zircon methods in the Midyan terrane, is the tonalitic gneiss in southern Jordan that has a U-Pb SIMS crystallization age of 787±3 Ma (Jarrar and others, 2013). Farther south, the Midyan terrane has indications of arc magmatism between ~750 Ma and 700 Ma (Kozdrój and others, 2017). To determine whether the Midyan terrane contains more than one arc or indications of a late Mesoproterozoic event, in the manner of the Eastern Desert and northern Sinai, will require extensive, detailed sampling, and the dating of the volcanic, volcaniclastic, and arc-related intrusive rocks.
 
Cathodoluminescent imaging of zircons is an indispensable step prior to microprobing for dating purposes, because the imaging reveals the internal structure of the zircon grains. This allows the recognition of inherited cores and metamorphic overgrowths, in addition to the sectors of magmatic zircon, and allows the collection of microprobe age data for the crystallization stage of geologic history as defined by each zircon grain, as well as earlier pre-magmatic and the later metamorphic stages. Interestingly, the single-zircon dating and the cathodoluminescent imaging programs reveal that the mafic rocks in the Arabian Shield contain a considerable amount of inherited zircon (Hargrove, 2006; Stern and others, 2010). In the west-central part of the Arabian Shield, such inherited zircon has early Neoproterozoic and Mesoproterozoic, and to a lesser degree, Paleoproterozoic and Archean ages (Hargrove and others, 2006). The source of such xenocrystic zircons is not fully understood. Pre-Neoproterozoic zircon grains may have been incorporated into the Neoproterozic magmas from the sediment shed from the continental margins that flank the Arabian-Nubian Shield, and may have been transported fluvially or by glaciers into the Arabian-Nubian Shield oceanic basin; or from the assimilation by melting of a cryptic older basement that may underlie parts of the “juvenile” core of the ANS (Hargrove and others, 2006); or inherited from the mantle source region (Stern and others, 2010).
 
 
 
Isotope analyses
The importance of analyzing the isotopic composition of the rocks and mineral samples in pursuit of understanding the tectonic setting and the evolution of the Arabian Shield is well demonstrated by the work on Pb, Sr, Nd, and O isotopes. Stoeser and Frost (2006) identified isotopically defined.
  1. arc terranes of oceanic affinity in the western part of the shield;
  2. eastern terranes that contain igneous rocks, sourced from a less depleted mantle than that in the west or derived from or composed of Neoproterozoic material that was mixed with a small amount of cratonic sourced material, or both; and
  3. the Khida terrane, underlain by pre-Neoproterozoic continental crust.
 
Neodymium isotopes are particularly important in such tectonic studies, making tectonic inferences on the basis of the differences between the isotopic composition of a sample and the isotopic composition of the mantle at the same time (expressed by the Nd notation) and the degree of concordance between the model age of the sample and the crystallization age of the host rock. By this means, terranes in the Arabian Shield are clearly differentiated as being juvenile, characterized by positive Nd values close to the expected values of the depleted mantle at the time of their formation and their Nd model ages close to their U-Pb crystallization ages, or as having older, more evolved continental sources that are characterized by negative Nd values and their Nd model ages considerably older than their crystallization ages. Such data indicate that most of the Arabian-Nubian Shield is juvenile, withNd(t) values that plot between the line representing the CHUR value and the depleted mantle growth curves. The Khida, Abas, and Al-Mahfid terranes, in contrast, have Nd values that plot below the chondritic line.
 
The Khida terrane contains small exposures of intact Paleoproterozoic rock but otherwise comprises Neoproterozoic volcanosedimentary and vast amount of Neoproterozoic granitic rocks. However, the Neoproterozoic granites yield strongly negative Nd values, indicating a major input from older continental crust. Similarly, the granitic rocks in the Abas terrane yields Neoproterozoic crystallization ages (~790–725 Ma granitic gneisses and ~625–590 Ma post-tectonic granite) but have negativeNd values (Yeshanew and others, 2015) and Archean to Paleoproterozoic Nd model ages (Whitehouse and others, 2001). These results indicate that the terrane is a Neoproterozoic addition to the crust, but magma that formed the granitic rocks of the terrane incorporated a large amount of older crustal material.
 
More recently, analysis of single-zircon hafnium isotopes has been added to the isotope toolkit for Arabian Shield studies. Hf isotopes, together with the single-zircon crystallization ages and the Nd and O isotope analyses, provide important information about the magmatic sources and refine the petrogenetic models. A caveat is that the Hf data used in such modeling constrains the origin of zircon, which is one step away from the technique of modeling magma composition by calculating the Sm-Nd whole-rock data model ages. Currently, Hf isotope data are only available for some parts of the Arabian Shield.
 
Most Hf values are consistently close to, but below, the expected depleted mantle values, and are compatible with the mostly juvenile origin as inferred from the Nd data (Robinson and others, 2014). Nevertheless, the Hf data may give insights into the magmatic origins that are not provided by the Nd data alone, and raise questions that caution against a too simple interpretation of magma origins. A case in point is the Hadb adh Dayheen granite ring complex, a post-tectonic granite intrusion in the central part of the Arabian Shield. It has U-Pb zircon crystallization ages of 625–612 Ma, and Nd model ages between 0.71 Ga and 0.81 Ga, implying that it crystallized from a magma sourced from juvenile material, such as already existing juvenile Neoproterozoic crust (Ali and others, 2014), a source that is commonly proposed for granitic rocks elsewhere in the Arabian-Nubian Shield (Liégeois and Stern, 2010; Stern and others, 2010). A juvenile source is in agreement with the absence of inherited zircon cores in the complex. The granite of the complex also yields 0.81 to 1.1 Ga Hf depleted-mantle model ages and +4.5 to +8.4Hf values, which likewise indicate a juvenile source for the granite magma (Ali and others, 2014). But some of the zircons from the complex have pre-Neoproterozoic Hf model ages, ranging from 1.1 Ga to 1.3 Ga. Even though the model ages were obtained from merely a few zircons in contrast to the Neoproterozoic model age range that is indicated by most zircons from the complex, they suggest that the Hadb adh Dayheen magma was not entirely juvenile, but rather incorporated material from older sources. This implies that the petrogenetic story of the Hadb adh Dayheen complex is more complicated than what is indicated by the Nd and most of the Hf isotope data alone.
 
 
(A) Epsilon Nd(t) versus zircon crystallization age diagram for parts of the Arabian-Nubian Shield showing the positive, juvenile character of most of the Nd data apart from the Abas and Khida terranes that have negative values indicative of significant input from older crust. The depleted mantle growth curves are from Goldstein and others (1984) and DePaolo (1981). (B) Epsilon Hf(t) versus zircon crystallization age diagram for the Arabian Shield plotted against the data field for the northern Nubian Shield, indicating a juvenile affinity for the crust in both areas. The depleted mantle (DM) growth curves are for a chondritic uniform reservoir (CHUR) value at the Earth's formation (4.56 billion years ago) to εHf = 17 at the present time (Vervoort and Blichert-Toft, 1999) and εHf=13.2 for the average mean of 13 modern island arcs (Dhuime and others, 2011).
 
Another study by Robinson and others (2014) of intrusive rocks across the Arabian Shield, using U-Pb zircon ages and Lu-Hf isotope data, identifies the four discrete magmatic cycles mentioned above (~845 Ma island arc, ~710 Ma syncollisional, ~620 Ma post-tectonic, and ~525 Ma anorogenic magmatic activities). Furthermore, a set of 227 Hf isotopic analyses for the granitoid rock samples used in the study yieldsHf values that, regardless of their age or spatial relationships, are >+5 to +10, indicative of an origin from juvenile melts. However, subtle changes in the isotopic signatures of the samples suggest changes in the granite source melts between 850 and 600 Ma that Robinson and others (2014) interpret as a transition from an initial development of a subduction-related basaltic crustal underplate and a limited interaction with Paleoproterozoic crustal sources to a later episode of slab tear, lithospheric delamination, and (or) subduction rollback.
 
 
Shear-zone dynamics
Structurally, the Arabian Shield is dominated by late Cryogenian-Ediacaran shear zones that trend N-S in the southern shield and variably NW-SE and NE-SW in the central and northern shield. The effective boundary between these regions is a line along the Ruwah and Ad Damm fault zones that marks the transition between the northern and the southern crustal domains in the Arabian-Nubian Shield recognized by Fritz and others (2013).
 
These domains are coherent belts with distinct late tectonic, structural, magmatic, and sedimentary assemblages (Fritz and others 2013). The northern domain is characterized by the transcurrent faults of the Najd fault system, comprising NW-trending, dominantly sinistral shears and a small number of conjugate NE-trending dextral shears that resulted from late Cryogenian-Ediacaran E-W transpressional bulk shortening and northerly directed tectonic extension or orogenic escape. The Najd fault system extends from the Arabian into the Nubian Shield and is part of the largest pre-Mesozoic transcurrent fault zone on Earth (Stern, 1985; Stüwe and others, 2014). The southern domain is characterized by N-trending shears, fold axes, and bedding strikes. It is divisible into 1) a northern section that evolved through orthogonal E-W convergence, resulting into N-trending shortening zones and numerous sinistral and dextral N-trending shear zones, and 2) a southern section that was deformed by NW-SE to W-E extension induced by gravitational collapse but also contains many N-trending shears.
 
Major shears zones in the Arabian Shield, showing their spatial association with bodies of exhumed high-grade crustal rocks (gneiss and schist)
 
The origin, age, and significance of the individual shear zones in the Arabian-Nubian Shield are debated. But overall, they are components of the late Cryogenian-Ediacaran terminal orogeny that included periods of crustal shortening, transcurrent faulting, orogenic collapse, and escape (Fritz and others, 2013, Johnson and others, 2011). Importantly, recent studies elucidate the dynamic and tectonic significance of several shear zones in the Arabian Shield.
 
The Nabitah fault zone is a dextral shear zone in the southeastern Arabian Shield. It is located in the southern part of the Nabitah mobile belt (Stoeser and Stacey, 1988), a region of deformation and metamorphism that extends from the north to the south across the entire shield. This mobile belt encompasses the Hulayfah, Ad Dafinah, and Nabitah faults, and in its southern part, it is commonly referred to as a suture. However, recent geochronology and isotope studies challenge this interpretation. The U-Pb zircon ion-microprobe geochronology, whole-rock geochemistry, and feldspar Pb and Sm-Nd isotopic analyses of the granitoid samples, collected from an E-W transect across the fault zone, identifies the domains of juvenile oceanic arc crust on either side that differ in age and geochemical character. The domain to the east, conventionally referred to as the Tathlith-Malahah terrane, consists of an older volcanic and younger sedimentary succession, referred to as the Tathlith arc, and has plutonic rocks that formed between ~700–670 Ma (Flowerdew and others, 2013). The arc rocks west of the fault, referred to as the Tarib arc, are intruded by a suite of plutons with ages ranging from 750–720 Ma. Both sets of intrusions yield positive Nd values and have feldspar Pb isotope signatures that plot within the Type I and Type II fields of Stoeser and Stacey (1988), indicating that they are juvenile crustal additions. However, Flowerdew and others (2013) see little significant isotopic difference between the arcs. They envisage that they are related to a single subduction system with the Tathlith arc that developed on the Tarib forearc because of subduction rollback. If this model is correct, the arcs on either side of the Nabitah fault zone share a geologic history, and the fault zone is, therefore, not a suture.
Age dating and Hf isotope data form the basis of a separate proposal that subsequent extension above the Tathlith subduction zone, caused by slab rollback and tear, resulted in continued magmatism across the fault zone and the emplacement of A-type granite plutons (Robinson and others, 2015). An early pulse at ~636 Ma represents the initial melting of the lower crust by the upwelling of hot mantle and mafic material with N-MORB and IAT affinities. A ~50 million-year period of melting, assimilation, storage, and homogenization, or MASH zone, ensued, in which melting and fractionation resulted in granitic magmatic pulses at ~630–614 Ma and ~618 Ma, followed by further melting and fractionation and the production of ~610 Ma and ~610–594 Ma granites.
 
The northwestern boundary between the northern and southern crustal domains in the Arabian Shield, shown in the inset to is the Ad Damm Fault, a NE- (to NNE-) trending shear zone that separates the Jiddah and Asir terranes (Hamimi and others, 2013). It has abundant dextral transcurrent shear-sense indicators, moderately to steeply NW-plunging stretching lineations, and mainly developed under amphibolite- to greenschist-facies metamorphic conditions. The age of shearing is not well established, but the presence of 620 Ma mylonitic granite in the Numan Complex suggests that the deformation occurred during the middle part of the Ediacaran, on which basis the Ad Damm fault is considered to be a shear conjugate to the NW- to NNW-trending sinistral Najd fault system (Hamimi and others, 2013). In detail, the Ad Damm fault deformation history is complicated and polyphased and involved the overprinting of earlier phases by later phases.
 
The D1 and D2 phases were contractional––the NW-SE compression during D1 resulted in NE-trending low-angle thrusts and tight-overturned folds and the NE-SW stress during D2 resulted in open folding. The D3, the most conspicuous deformation phase of the NE-SW trending shear zone, resulted from intensive dextral transcurrent brittle-ductile shearing.
 
Eighty km north of the Ad Damm fault, another NE-trending shear zone occurs along Wadi Fatima. Recent geologic mapping of the Fatima shear zone indicates that it is also structurally complex and has no simple interpretation (Hamimi and others, 2012; Abd-Allah and others, 2014). Overall, it has the form of a fold-and-thrust belt, in which the unmetamorphosed to slightly metamorphosed sedimentary rocks (Fatima Group) to the northwest are in contact to the southeast with orthoamphibolites, ortho- and paraschists, and two granites, one dated 773±16 Ma and the other with a younger, but unknown, age. The mapping by Hamimi and others (2012) shows that the metamorphic rocks form a local basement in the footwall and the Fatima group in the hanging wall of a NW- to NNW-vergent fold and thrust duplex system. The metamorphic rocks contain tight isoclinal folds, sheared out hinges, a NE-striking penetrative foliation, and subhorizontal stretched and mineral lineations that are related to an episode of dextral shearing that predated the folding and thrusting. Hamimi and others (2012) envisage that this early episode of deformation in the basement was followed by the granite intrusion and the unconformable deposition of the Fatima group on the basement. Both the basement and the Fatima group were subsequently folded and thrusted. An alternative interpretation by Hamimi and others (2012) is that the deformation of the Fatima group resulted from gravitational soft sediment slumping and deformation above the pre-existing faults in the basement. A third interpretation of the Fatima shear zone by Abd-Allah and others (2014) involves the shifting of the boundary of the Asir terrane from the Ad Damm fault zone north to the Fatima shear zone, and treating this shear zone as the suture between the juvenile arcs of the Asir and the Jiddah terranes. The preserved bodies of deformed ophiolitic rocks along the shear zone are mapped as a type of flower structure that contains opposite-verging overturned folds and together with the thrust faults are treated as evidence for suturing. Abd-Allah and others (2014) suggest that the Fatima group was deposited unconformably on the amalgamated Asir and Jiddah terranes and the entire area was subsequently affected by NW-SE shortening, which inverted the Fatima basin and caused the formation of NW-verging thrust faults and folds.
 
A notable feature of the Nubian Shield is structural highs or domes of high-grade schist and gneiss that, in some cases, appear to constitute extensional metamorphic core complexes (e.g., Blasband and others, 1997, 2000; Fritz and others, 2002; Abd El-Naby and others, 2008; Abu-Alam and Stüwe, 2009). The high-grade rocks have protolith ages between 744 Ma and 630 Ma (Ali and others, 2012, 2015; Abu El-Enen and others, 2016; Andresen and others, 2009), and their Sr, Nd, and Pb isotope characteristics indicate mostly a juvenile origin (Liégeois and Stern, 2010; Ali and others, 2012). Peak metamorphism occurred between 620 Ma and 585 Ma at depths of as much as ~25 km (Neumayr and others, 1998; Fritz and others, 2002; Abd El-Naby and others, 2008; Eliwa and others, 2008; Abu El-Enen and Whitehouse, 2013; Abu El-Enen and others, 2016), and 40Ar/39Ar cooling ages of 587 Ma and 579 Ma for hornblende and white mica, respectively, indicate rapid exhumation and dome emplacement soon after metamorphism.
 
High-grade schist and gneiss also occur along some of the Najd faults in the Arabian Shield. Recent studies have been made of these rocks at either end of the most conspicuous belt of the Najd faulting in the shield that extends virtually continuously from the Ar Rika fault zone to the Qazaz-Ajjaj shear zones. At Jabal Kirsh, the high-grade rocks crop out as a dome of orthogneiss and migmatite, enclosed by an envelope of paragneiss and quartz kyanite schist. It is located between two left-stepping strands of the Ar Rika fault zone and occupies the resulting extensional zone (Al-Saleh, 2010). The Kirsh dome is juxtaposed with the relatively unmetamorphosed rocks of the Murdama group on the north and the unmetamorphosed sedimentary rocks of the Jibalah group on the south. The pegmatite in orthogneiss has a single zircon U-Pb SHRIMP age of 637±2 Ma (Kennedy and others, 2005), suggesting a protolith age for the gneiss of >637 Ma. An 40Ar/39Ar age of 557±15 Ma for the biotite paragneiss indicates cooling below the biotite closure temperature between 637 Ma and 557 Ma.
 
This constrains the age of exhumation and dome emplacement to some time prior to or about 557 Ma, which is comparable to the timing of the exhumation and the cooling of the Eastern Desert gneiss domes. Al-Saleh (2010) suggests that movement within zone may have caused decompressional melting of granitic precursor material in the middle crust, followed by diapiric emplacement of the Kirsh rocks in their present position along the Ar Rika fault zone facilitated by strike-slip dilatancy pumping.
 
At the northwestern end of the Ar Rika-Qazaz/Ajjaj belt of the Najd fault, a range of U-Pb crystallization ages reveal a complex history of magmatism along the Ajjaj shear zone (Hassan and others, 2016a, b). These include an age of 696±6 Ma for weakly deformed coarse-grained diorite; ages of 747±12 Ma–668±8 Ma obtained from the two samples of strongly deformed granodiorite-tonalite intrusives at the border of the Ajjaj shear zone; and ages of 601±3 Ma–584±3 Ma obtained from mylonitic granites. An undeformed, discordant granite that crosscuts the shear foliation of the Ajjaj shear zone has an age of 581±4 Ma. The age data reported by Hassan and others (2016a,b) constrain the movement on the Ajjaj shear zone to between 604 Ma and 581 Ma, which is consistent with the cessation of movement separately indicated by a 573±5 Ma lamprophyre dike that crosscuts the Ajjaj shear zone foliation, an undeformed 569±15 Ma alkali-feldspar granite in the Hanabiq shear zone, a branch of the Ajjaj shear zone, and a merely weakly foliated 575±10 Ma granite on the edge of the Ajjaj shear zone (Kozdrój and others, 2017). The peak metamorphism along the Ajjaj shear zone occurred under conditions of 505°–700°C and pressure ranges of 8 to 14.5±2 kbar, suggesting that the deformed rocks were exhumed from crustal depths of as much as 58 km (Hassan and others, 2016a).
 
A further illustration of the dynamics of exhumation of the gneiss is given by the study of a dome of medium- to high-grade gneiss along the Qazaz shear zone (Meyer and others, 2014), 75 km north of the Ajjaj shear zone study area. The Qazaz gneiss is enclosed by low-grade mylonite zones and very low-grade metapelite, conglomerate, and volcanic rocks, belonging to an arc assemblage and a younger sedimentary basin (Thalbah group) in the Midyan terrane.
 
The contact between the gneiss dome and these surrounding rocks is a low-angle, south-dipping detachment on the south and steeply dipping shears on the southwest and the northeast. Across the southern detachment, which includes the Qazaz gneiss and the metasediments of the Thalbah group, the gentle southerly dip of the mylonitic foliation in the gneiss increases to as much as 40° and becomes steeply south dipping. The strain indicators in the footwall of this detachment zone show a dominant downdip, top-to-the-south sense of shear. A similar sense of shear is given by S-C shear bands and chlorite veins in low-grade rocks in the hanging wall of the detachment that overprint the footwall high-grade shear indicators, suggesting that the exhumation of the gneiss was synkinematic (Meyer and others, 2014). The southwestern and northeastern margins of the gneiss dome are strike-slip shear zones, in which sinistral shear fabric overprints the mylonitic fabric of the gneiss. Overall, it appears that the Qazaz metamorphic core complex was exhumed along a crustal-scale strike-slip fault system that accommodated crustal shortening (Meyer and others, 2014). Its specific location was controlled by a gently dipping jog on the Qazaz shear zone, accommodating 30 km lateral motion accompanied by more than 25 km exhumation of the lower crustal rocks. The core complex was subsequently cut by strike-slip faults that accommodated another 30 km of lateral motion. Such a mechanism is different from the conventional mechanism of core-complex emplacement, namely regional crustal extension and thinning, and the exhumation of deep crustal material along gently dipping normal shear zones oblique to the regional extension direction. The Qazaz study is an illustration of the important contribution that Arabian Shield geologic investigations can make to the modeling of geologic processes.
 
 
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