TECTONICS, VOL. 7, NO. 6, PAGES 1291-1306, DECEMBER1988

EXTENSION OF THE NAJD SHEAR SYSTEM FROM SAUDI ARABIA TO THE CENTRAL EASTERN DESERT OF EGYPT BASED ON INTEGRATED FIELD AND LANDSAT OBSERVATIONS

Mohamed Sultan, Raymond E. Arvidson, and Ian J. Duncan

Department of Earth and Planetary Sciences, McDonnell Center for the Space Sciences, WashingtonUniversity, St. Louis, Missouri

Robert J. Stern

Programs in Geosciences,University of Texas at Dallas, Richardson, Texas

Baher El Kaliouby

Department of Geology, Faculty of Science, Ain Shams University, Abbassia, Cairo, Egypt

Abstract. The Najd Shear Systemin Saudi Arabia foliations, with local changesin attitude around large extends over 1200 km in a NW-SE direction and has a competent(e.g., granitic)bodies; and (4) lithologic width of approximately 300 km. A digital color mosaic, contactsthat are generallytectonic in nature and related compiled from seven Landsat thematic mapper scenes, to faulting. Thesefeatures are lesscommon to the north was used to delineate characteristic structural features of and south of both the Ajjaj Shear Zone and the central the Najd Systemin the Midyan region of Saudi Arabia Eastern Desert. Results are consistentwith the Najd and to search for similar features in the Egyptian Shear Systemextending into the Eastern Desert and Eastern Desert. The digital mosaicwas generatedusing dominating the structural patterns within the central ratios of Landsat thematic mapper bands(bands 5/4 x part of the Eastern Desert. 3/4, 5/1, 5/7) that are sensitive to the rock content of Fe- bearing aluminosilicates,spectrally opaque phases,and 1. INTRODUCTION hydroxyl-bearingor carbonateminerals, respectively. The mosaiccovers approximately 130,000 km 2 of late The Arabian Shield formed by the accretionof Proterozoic exposuresof the Arabian-Nubian Shield and ensimatic and ensialic island arcs between 715 and 630 has the Eastern Desert and the Midyan region placed in Ma [Stoeserand Camp, 1985](Figure 1). The Shield their approximate pre-Red Sea locations. The Ajjaj formed from at least five arcs, separated by four N-S or Shear Zone (AJZ) marks the termination of the Najd NE-SW trending ophiolite-bearingsuture zones [Stoeser Systemagainst the easternmargin of the Red Sea in the and Camp, 1985]. The collision-relatedtectonic fabric is Midyan region. The AJZ alignswith the central Eastern predominantlyoriented N-S or NE-SW [Moore, 1979]. Desert, based on analysisof pre-Red Sea locations. The Najd Shear System(NSS) modified the earlier Analysesof Landsatdata and field observationsshow structuresinto northwesterlyfabrics at about 530-630 that the Ajjaj Shear Zone and the central EasternDesert Ma [Staceyand Agar, 1985]. exhibit the following features in common: (1) outcrops The NSS of the Arabian Shield is the largest that are generally elongate in a NW-SE direction as a recognizedpre-Mesozoic transcurrent fault systemon result of folding, with fine-scalelithologic heterogeneity Earth [Stern, 1985], extendingin a NW-SE direction at the outcrop scale related to deformation associated over 1200 km in outcrop, with a width of approximately with faulting; (2) NW trending left-lateral faults and 300 km [Agar, 1987]. The NSS alignswith faults in the ductile shear zones; (3) subhorizontal, NW trending South Yemen coast and in the Arabian Sea bed to the mineral lineations, and variably dipping NW trending SE [Brown, 1972], making a potential total length in excessof 2000 km [Moore, 1979]. The main trends of the NSS terminate at the eastern margin of the Red Sea, Copyright 1988 between latitudes 26 ø and 27ø45'N [Brown, 1970]. by the American GeophysicalUnion. Along the zone occupiedby the NSS in Saudi Arabia brittle and ductile stylesof deformationprevail, Paper number 88TC03204. superimposeon, and obliterate earlier tectonicfeatures 0278-7407/88/88TC-03204510.00 related to the accretion of the Arabian Shield [Moore 1292 Sultan et al.: Najd Shear Systemin Arabia and Egypt

MEDITERRANEAN SEA DEAD SEA -- RIFT .•••• AJJAJ SHEAR

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PHANEROZOIC COVER NUBIAN COI I L• 20øN SHIELD •1 ARABIAN'" COl I .':'o.s.,.•..,. SHiELD•i:.::•:•..:..•:0 CENTER OFTM SCENE • '••"•) ..• 4/4&•/Y4'• ?•'"":•::'..:-";::?/i..".-"•:./ C),/ BYAREA EACH COVERED TM SCENE

i 30ON :55øE 40 ø E 45 ø E Fig. 1. Place map showinglocation of the studyareas in the EasternDesert, Egypt and Midyan region, and Saudi Arabia within the Arabian-NubianShield. Alsoshown are inferred boundaries between north Eastern Desert, central Eastern Desert, and south Eastern Desert from Stern and Hedge [1985]; the Ajjaj Shear Zone from Davies[1984]; distributionof suturezones and microplates from Stoeserand Camp [1985] and KrOner et al. [1987]; and outcrop traces of major faults of the Najd Systemin the Arabian Shield from Moore [1979]. In the upper right corner is a sketchmap showingthe relativedirections of movementfor the Arabian and Nubian Shieldalong the Gulfs of Aqaba and Suez due to openingof the Red Sea [from Garfunkel, 1981]. Full-resolutionimages for areas outlined by boxesA and B are shownin Figures3a and 3b, respectively.Locations of the centers of TM scenes used are numbered from I to 7. Scene I identification number (ID) 5015707420,path 174, row 42, latitude 26.01øN, longitude33.81øE, date August5, 1984, sun elevation 58 ø above horizon, and sun azimuth 100ø clockwisefrom north. Scene2, ID 5015007360, path 173, row 43, latitude 24.559øN,longitude 35.036øE, date July 29, 1984, sunelevation 59 ø, and sun azimuth 94 ø. Scene3, ID 5015007350, path 173, row 42, latitude 26.008øN, longitude 35.392øE,date July 29, 1984, sunelevation 59 ø, sunazimuth 97.0 ø. Scene4, ID 5015007363,path 173, row 44, latitude 23.125øN, longitude34.689øE, date July 29, 1984, sun elevation 59ø, and sun azimuth 92.0 ø. Scene5, ID 5016607355,path 173, row 41, latitude 27.451øN, longitude35.675øE, date August14, 1984,sun elevation 57 ø, sunazimuth 107 ø. Scene6, ID 5012707285,path 172, row 42, latitude25.995øN, longitude 37.011øE, date July6, 1984,sun elevation 60 ø, sunazimuth 89 ø. Scene7, ID 5008807223,path 171, row 43, latitude 24.548øN,longitude 38.194øE, date May 28 1984, sun elevation 61 ø, sun azimuth 89 ø.

and AI-Shanti, 1980; Davies, 1984]. Displacement deformation is commonlymore pronouncedalong associatedwith the NSS was accommodatedthrough anastomosingshear zonesof variable lengthsand ductile stretchingand shortening,arcuate folding, and widths. Davies [1984] documentedup to 300% brittle !eft-lateral faulting [Davies, 1984]. Ductile stretchingdeformation along a ductileshear zone over

Sultan eta!.: Najd Shear Systemin Arabia and Egypt 1295

100 km in strike and 20 km in width within the Ajjaj Red Sea. Reflectance values were selected for a number Shear Zone of the Midyan region (Figure 1). of control points in areas of overlap between the control Moore [1979], Abuzied [1984], and Stern [1985] scene and the adjacent scenes. The reflectancesfor proposedthat the NSS projects into the Eastern Desert each band in the adjacentscenes were then recomputed of Egypt. However, deformation in the Eastern Desert is to match thoseof the controlscene, using a linear commonly attributed to obduction-accretiontectonics brightnesstransformation derived from the controlpoint involving island arcs [e.g., Shackletonet al., 1980; Ries data. The calibrated scenes were then used as control et al., 1983; El Ramly et al., 1984]. The main objective scenes to calibrate other scenes. Scenes with overall of this paper is to demonstratethrough integration of reflectancefor deep waters similar to the control scene field and Landsat data that structural patterns in part of (within 3%) were not altered. the Eastern Desert, specificallythe central Eastern Reflectance ratios have been found to suppress Desert, are similar to structural patterns within the NSS spectral variations due to topographyand grain size in Saudi Arabia and to suggestthat Najd-related differences [Rowan et al., 1974; Abrams et al., 1977]. deformation causedmany of the structuresseen in the Certain TM band ratios also maximize discrimination of central Eastern Desert. rock typesfound in the Eastern Desert [Sultan et al., Our approach is to generate Landsat productsthat 1986, 1987]. Specifically,Sultan and coworkersfound emphasize the lithologic and structural characteristicsof that (1) increasingamounts of spectrallyopaque phases the NSS near its termination at the Red Sea coast and with low, fiat spectral reflectances(e.g., magnetite, over its postulated extension in the Eastern Desert. We ilmenite, and chromite) decreases the ratio of band 5 then characterize structural patterns for areas in the (1.55-1.75 lam) to band I (0.45-0.52 lam); (2) increasing Midyan region where Najd-related deformation has amountsof hydroxyl-bearingor carbonateminerals (e.g., occurred and determine if similar structures can be serpentine and calcite) with vibrational absorptionsin found in the Eastern Desert. Field observations are used TM band 7 wavelength region (2.08-2.35 lam) increases to check inferencesdrawn from analysisof the Landsat the ratio of band 5 to band 7; and (3) increasing data. amountsof Fe-bearingaluminosilicates (e.g., hornblende)that absorbin the band 4 wavelengthregion 2. DIGITAL LANDSAT THEMATIC MAPPER (0.76-0.9 lam) increasesthe productof the followingtwo MOSAICS TM ratios: band 5 to band 4 and band 3 (0.63-0.69 lam) to band 4. The standard Landsat thematic mapper (TM) scene The reflectance ratio data were integrated into one consistsof seven images, each 185 by 185 km, registered color digital mosaic for the seven scenes(Plate la) and to one another in a space oblique Mercator projection. an interpretation map was produced (Plate lb). The Six images are acquired with broadband passesin the mosaic showsdata presented at an approximate scale of 0.4-2.34 lam wavelengthregion, and one in the thermal 1:1,000,000 and reducedin resolutionby approximately infrared (10.4-12.5 lam). Visible and reflected infrared a factor of 2. In generating the interpretation map we image elements are 30 m across. In this study the used full-resolutionhard copy imagesgenerated at a visible and reflected infrared data for seven digital scale of 1:150,000. scenesare used, covering most of the Midyan region in In Plate la the 5/4 x 3/4 image is assignedthe blue northern Saudi Arabia and much of the Eastern Desert component,the 5/1 image the green component,and the of Egypt (Figure 1). Data processinginvolved two 5/7 image the red component. Serpentiniteswith a high stages: (1) processingto emphasize lithologic variations, abundanceof hydroxyl-and/or carbonate-bearing and (2) processingto correct for the spatial displacement minerals, opaque phases, and a low content of Fe- resultingfrom the opening of the Red Sea. bearing aluminosilicatesappear pure red. For example, the Gebel El Rubshi serpentinites(SI on Plate lb) have 2.1. Processingto EmphasizeLithologic Information approximately 10% by volume magnetite, chromite, ilmenite, and other opaque phasesand 90% by volume The proceduresused were those applied by Sultan serpentine minerals, talc, chlorite, and carbonate. et al. [1986, 1987] in the Eastern Desert. Data for each Serpentinitessurrounding Gebel Gerf in the south scenewere first converted to brightnessvalues in Eastern Desert and within the NE trendingYanbu suture proportion to bidirectional reflectance, using nominal zone in the Arabian Shield are also red (S2 and S3 on gain and offset parameters associatedwith ancillary data Plate lb). Granitic rocks with low abundance of opaque files for each scene. Differences in absolutebrightness phases, hydroxyl-bearingphases, and Fe-bearing of up to approximately 15% were noticed between aluminosilicatesappear green. For example, the granite adjacent sceneswhere they overlapped. These gneissin the Meatiq dome area (G in Plate lb), has variations are due to a combination of variations in approximately 5% by volume amphibolesand lessthan extent of atmosphericscattering and absorptionand to 1% by volume magnetite. Similarly, post tectonic calibration drift not accountedfor in the gain, offset granites show as green outcrops. These intrusions are data. Thus the data were normalized to the reflectance outlined in Plate lb. Mafic rocksthat are generally rich of the scene with the least amount of scattering and in Fe-bearing aluminosilicatesand spectrallyopaque absorption. That scene was identified on the basis of the phaseswill have a blue color since the ratio 5/4 x 3/4 lowest overall spectral reflectance over deep water of the will be high [Sultan et al., 1987]. For example, massive 1296 Sultan et al.: Najd Shear Systemin Arabia and Egypt

amphibolites(MI in Plate lb) to the NW of the Meatiq 3. STRUCTURAL CONSTRAINTS ON THE EXTENSION dome have approximately 60% by volume amphiboles OF THE NAJD SYSTEM INTO THE EASTERN DESERT and 3% by volume magnetite and appear blue. FROM TM-BASED LITHOLOGIC MAPS, FIELD OBSERVATIONS, AND AVAILABLE GEOLOGIC MAPS

2.2. Processingto Removethe Spatial The NSS trends NW-SE and enters the Red Sea rift DisplacementAssociated With the as the Ajjaj Shear Zone between26 ø and 27ø45'N Red Sea Rifting latitude; the most conspicuoustrend intersectsthe Red Sea coast at 27ø20'N latitude [Brown, 1970]. The Ajjaj Shear Zone has an exposedstrike length of at least The openingof the Red Sea at 25 Ma [Cochran, 200 km and a width of approximately100 km (Figure 1) 1983] was associatedwith left-lateral displacementalong [Davies, 1984]. the Gulf of Aqaba and the Dead Sea Rift and extension Inspection of Plate la showsthat the outcrop in the Gulf of Suez. The directions of displacement patterns of the the Ajjaj Shear Zone and the central causedby the Red Sea rifting are shownin the upper Eastern Desert are similar, as are those in the region right cornerof Figure1. The extensionin the Gulf of south of the Ajjaj Shear Zone and in the southEastern Suezis approximately30 km [Steckler,1985], and left- Desert. The Ajjaj Shear Zone and central Eastern Desert lateral motionalong the Gulf of Aqaba is approximately outcropsare predominantly NW elongated and show 110 km [Freund et al., 1970]. These estimates were fine-scale !ithological diversity. In contrast, outcrop basedprimarily on the onshoregeology and were usedto patterns in the region south of the Ajjaj Shear Zone and restore Africa and the Arabian Peninsula to their the south Eastern Desert are relatively equant, with prerifting positions[Freund, 1970; Girdler and lithologicvariability occurringon a large scale. For Darracott, 1972; Cochran, 1981]. These reconstructions example, serpentinitesin the central Eastern Desert predict that continentalcrust underlies most of the show a different pattern from those in the south Eastern current Red Sea rift and that the restored Red Sea Desert (red outcrops in Plate la). On the basis of the coastsare widelyseparated (approximately 80-100 km Red Sea reconstruction (section 2.2) and examination of apart). Alternatively,studies based on seismicrefraction outcrop patterns discussedabove, we conclude that there data [Bohannon, 1986a, b], magneticdata [Girdler and is strongevidence that the Ajjaj Shear Zone extendsinto Styles,1974], plate kinematics [McKenzie et al., 1970], the central Eastern Desert. and Gemini and Apollo photographs[Abdel-Gawad, We developed criteria for identifying Najd-related 1970]suggest that oceaniccrust underlies most of the folding, sinistral faults and ductile shear zones, and current Red Sea rift and that the separation between the areas of high and low strain from TM data coveringthe restored Red Sea coastsis negligible. Midyan region in Saudi Arabia. Thesestructural We used the conservative estimates of the elements are shown in the interpretation map (Plate lb). displacementmagnitudes and directionsoutlined above We then searched for similar features in the central to translate the TM scenes for the Eastern Desert and Eastern Desert to test the hypothesisthat NSS projects the Midyan region to their approximateprerift relative into the central Eastern Desert. Field data in Saudi positions(Plates la and lb). Examinationof Platesla Arabia and the Eastern Desert were used to test our and lb show that the northwesterlyextension of the interpretations. We also used single-bandTM imagesin Ajjaj Shear Zone regionwould be generallyinto the central Eastern Desert. Note that the Red Sea shores in conjunctionwith the TM color mosaicwhen additional topographicinformation was needed to help delineate this reconstructionare approximately80-100 km apart. particular structural elementsbecause the mosaic If we were to increase the closure of the Red Sea so that (Plate l a) de-emphasize• spectral variations related to the shoresare approximately20-40 km apart, the Duwi topographicrelief. Areas to the north of the Ajjaj Shear and Hafafit wrench fault zones in the Eastern Desert Zone and the central Eastern Desert do not exhibit wouldalign with the Abu Masariband Alhamdwrench major NW-SE trendingstructures characteristic of the fault zones in Saudi Arabia (Plates la and lb), Najd Shear Systemin Arabia [Brown,1970; Stern, respectively,as predictedby Abdel-Gawad[1970]. 1985], and thus these areas were not examined. Given the limited number of crosscuttingfeatures in the studyarea, and the uncertaintyin their strikein between the Red Sea coasts,we are unable to resolve 3.1. AreasAffected by NW trendingFolding thesecontroversial issues regarding the nature of the crustunderlying the Red Sea and the amountof In the Midyan region the areas occupiedby the NSS separationbetween the restoredRed Sea coasts. are characterizedby a general NW trendingoutcrop However, even if we used the conservativeestimates, the pattern predominantly controlledby Najd-related central Eastern Desert will still mark the extension of the folding. This conclusionis based on comparisonof the Najd ShearSystem into the EasternDesert. Thuswe spatial congruencybetween NW trendingoutcrops in conclude that the central Eastern Desert is the most Plate la and areas occupied by Najd-related folds likely area to searchfor structuressimilar in styleto extractedfrom geologicmaps (Figure 2). Similar thosefound in the Ajjaj Shear Zone. outcrop patterns and fold traces have been reported in Sultan et al.: Najd Shear Systemin Arabia and Egypt 1.297

0 50 IOOkm

CENTEROFTM SCENE 24ø1•

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Fig. 2. Structural sketchmap showingthe major faults and folds for the area occupiedby the examined TM scenesand surroundingareas in Saudi Arabia. Map compiledfrom Smith [1979], Dhellemmes and Dellour [1980], Clark [1981], Kemp [1981], Pellaton [1979, 1981, 1982a, b] Davies [1980, 1983], Johnson[1983], and Daviesand Grainger [1985].

many other areas occupiedby the NSS elsewherein the Figure 2). Examples of these outcropsare labeled NS2 Arabian Shield [Delfour, 1979; Letalenet, 1979; Moore, on Plate lb. These N-S to NNE-SSW trends are 1979; Kemp et al., 1982]. The identification of controlled by folds, as indicated by examination of individual folds from TM images is hindered by (1) the detailed geologicmaps [Pellaton, 1979; Kemp, 1981]. compositionalsimilarities of folded units, and/or (2) the Similar structural observations to those observed in tight nature of many Najd-related folds. Both factors the Ajjaj Shear Zone and to the south of the Ajjaj Shear render the identification of closures in folded units Zone are reported from the central Eastern Desert and difficult. Thus no attempt is made to map individual south Eastern Desert, respectively. Locally, NW folds. Instead, we mapped domains affected by NW trending outcrop patterns in the central Eastern Desert trending folding (Plate lb). In the southAjjaj Shear have been shown to be controlled by NW trending open Zone the prominent trend is N-S to NNE-SSW; the Najd to tight folds [El Shazly, 1964; Abdel-Khalek, 1980; NW trend is not conspicuous(Plates la and lb and Stern, 1981; Abuzied,1984; Greene, 1984; Greiling 1298 Sultan et al.: Najd Shear Systemin Arabia and Egypt

et al., 1984]. In the south Eastern Desert, N-S to NNE- north: (1) ductile stretchingstrains of up to 300% of the SSW trending structural trends and outcrop patterns are original dimension, (2) subhorizontallinear penetrative observed(e.g., areas labeled NSI on Plate lb) and NW fabrics, and (3) congruencybetween axial surfacesof the trending outcropsare absentexcept for a minor NW folds, slaty cleavage, penetrative linear fabrics, and the trending zone (Plate la) labeled F2 in Plate lb. walls of the shear zone [Kemp, 1981; Davies, 1983]. Outcrop patterns within parts (wadi Beitan area, Criteria 3 is the presenceof subparalleltopographic Plate lb) of this minor zone are controlled by NW ridges within the inferred ductile shear zones, seen in trending folds [Abdel-Khalek, 1980]. individual band TM images. These ridges are probably We mapped areas affected by NW trending folding in related to differential weathering of compositionally the Midyan region and the Eastern Desert, and the different, subparallel lithologic units within the shear results are shown in the upper right corner of Plate lb. zones. For example, the band 3 image (Figure 4) for the These areas are characterized by NW trending outcrop area covered by Figures3b and 3d showssubparallel patterns and structural trends as seen in TM data. Note ridges within the ductile shear zone outlined in that both the Ajjaj Shear Zone and the central Eastern Figure 3d. Desert showNW trending folds. Similar structural observations to those described above can be found in the central Eastern Desert. For 3.2. Strike-SlipFaults and DuctileShear Zones example, Figures3a and 3c showthe following: 1. Outcrop patterns for rock units within the shear Numerous left-lateral faults and ductile shear zones zone are elongated as compared to lithologically similar are reported along the Ajjaj Shear Zone [Davies, 1984]. rocks outside the zone. For example, note differences in By examining full-resolution TM images over these faults outcrop patterns of serpentinitesand amphibolites within and shear zones, we developed criteria to identify Najd- as opposed to outside the shear zone. related faults and shear zones and to determine the 2. Trails of serpentinitesare observedalong fault nature (brittle versus ductile) of deformation and sense traces. For example, numerous stretched out of displacement. Figures3a and 3b are full-resolution serpentinite outcropsalong faults A-A' and B-B' imagesfor the areas coveredby boxesA and B, (Figure 3c). Faults have been observedelsewhere to be respectively,in Figure I and Plate lb. These figures preferentially localized along serpentinite outcropssince illustrate criteria for identification of Najd-related faults these rocks accommodatemovement efficiently and at and shear zonesfrom TM data. Figures3c and 3d are low temperatures through ductile deformation [Raleigh TM-based interpretation maps for Figures3a and 3b, and Paterson, 1965]. respectively, drawn using three criteria defined in 3. Outcrop patternsand structuraltrends change ensuing paragraphs. their orientation to align with inferred shear zones and Criteria 1 is the presenceof NW trendinglithologic fault traces. An example of this is the N-S trending discontinuities that are tens of meters (faults) to serpentinite outcropssurrounding the Bir Fawakhir hundreds of meters (shear zones) wide. Both faults and granite that change their orientation to align with the shear zones show left-lateral offsets. The sense of NW trending shear zone. displacementwas commonlydetermined from the The inferred ductile style of deformation for the changesin directionsof structuraltrends and outcrop shear zonesseen in Figure 3c was confirmed through our patterns of distinctive lithologies, as they approach the reconnaissance field work in the Fawakhir area, as well inferred fault or ductile shear zone trace and less as field work by Ries et al. [1983] and Bennet and commonly determined by observing displacement of Mosley [1987]. The followingfield observationsfrom distinctiveiithologies acrossthe discontinuity. Figures these areas are pertinent to confirming a ductile style of 3b and 3d showN-S trending outcroppatterns and deformation: (1) NW trending subhorizontalmineral structural trends subtended between a shear zone to the and stretching lineations, (2) NW trending foliation, and south and strike-slipfaults to the north that changetheir (3) congruencybetween axial traces of folds, penetrative orientation to align with the bounding NW trending planar and linear fabrics described above, and the faults and shear zones, developingsigmoidal patterns. inferred walls of the shear zone. The sigmoidal patterns indicate left-lateral displacement Using some or all of the above mentioned criteria, we along the boundingfaults and shear zone. The inferred mapped the major faults and shear zones (Plate lb). left-lateral faults and shear zone were confirmed Smaller and less obvious ones are not shown. through comparisonwith available geologicmaps [Kemp, 1981; Pellaton, 1982b; Davies, 1983]. 3.3. Areasof Medium to High Strain (Melanges?) Criteria 2 is the presenceof stretchedoutcrops, especially serpentinites, within the inferred shear zones Examination of Plate la showsthat in the Ajjaj and along some of the fault traces, implying that Shear Zone there are areas that show considerable fine- deformation was partly accommodatedthrough ductile scale heterogeneity on the outcrop scale (tens to deformation. The inferred ductile nature of the shear hundreds of meters). Some areas that show this zone shownin Figure 3b and outlined in Figure 3d was character distinctly are located between the Najd-related confirmed through the followingfield observationsalong faults and the ductile shear zone shownin Figures3b the shear zone and its extension some 30 km to the and 3d. The spatial correlation with shear zones and Sultan et al.: Najd Shear Systemin Arabia and Egypt 1299 faults suggestsa causal relationship. For example, these Criteria I is the presenceof large rock bodiesthat highly complex areas might lie within ductile shear are composedpredominantly of competent rock units zones, at their terminations, or within adjacent faults such as granite, diorite, and gabbro. Competent rocks and/or shear zones. of granitic compositioncan be readily mapped from TM Similar areas were identified in the Eastern Desert images (refer to section2.1), but more mafic types such (Plates la and lb). Two examples are the areas located as gabbroscannot be differentiated from chemically to the south of the Meatiq dome (DI in Plate lb) and similar, yet mineralogically different, incompetent rocks the wadi Ghadir and surroundingareas (D2 on such as greenschists. Thus additional information Plate lb). On the basis of our field observations and regarding the rock type of the inferred buttresseswas those of EI-Sharkawy and EI-Bayoumi [1979], Ries et al. also extracted from available geologicmaps (refer to [1983], El Bayoumi [1984], and Habib et al. [1985] the caption of Plate lb). fine-scale heterogeneity seen in TM data is causedby Criteria 2 is the presenceof rock bodies that deflect lithological heterogeneity. Highly dismembered the orientation of strike-slip faults and ductile shear ophiolitic rock associationsare found in a melange zones as they approach the inferred buttresses,giving composedpredominantly of fragmented, serpentinized rise to thrust faults. The identification of thrust faults ultramafic rocks, sheeteddikes, pillow basalt, layered from TM images is not as straightforwardas strike-slip and massivegabbro, and sedimentaryrocks of oceanic faults and shear zones. Unlike strike-slip fault traces facies. Individual lithologicallydistinct fragments that generally appear as long linear features on TM average tens of meter in size and range from cobbleto images, thrust fault traces are highly irregular, mountain size. All lithologiccontacts are tectonic and rendering their identificationfrom TM data difficult. not stratigraphic. We did not observea pervasive Thus thrust faults shown on Plate lb are based on matrix in between the rock fragments, as describedby published geologicmaps. the above mentioned workers. For example, in the wadi Criteria 3 is the presenceof conformable boundaries Ghadir area, contactsbetween lithologic units are sharp. between the inferred buttressesand their surroundings. Only locally,along highly tectonized shear zones, do This criterion allows separation of buttressesfrom post competentrocks appear to be embeddedin a finer- tectonic intrusionsthat crosscuttheir surroundings(e.g., grainedmatrix. This observationis importantto the granite intrusionsurrounding G. Um Had in Figures3a tectonicanalysis discussed in the next section. and 3c). We have mapped areas of extensivedeformation Criteria 4 is the presence of rock bodies that possess based on the followingTM-based characteristicsand highly sheared, commonlymylonitized margins, with the field observations: (1) fine-scale heterogeneityon the intensity of deformation decreasingaway from the scale of tens to hundreds of meters seen on TM data, marginstoward the interior of the body seenin the field. and (2) similar fine-scale lithologic heterogeneitiesseen We have mapped a number of areas in the Eastern in outcrop or reported in literature. Resultsare shown Desert that show the characteristics outlined above. For in Plate lb. example, note that the buttressmarked B5 (Plate lb) is composedprimarily of granites and granodiorites. 3.4. Areas of Low Strain (Buttresses) WNW-ESE trending strike-slipfaults and the ductile shear zone (labeled F1 in Plate lb) intersect the NE Plate la shows that the shear zones and faults have a margin of the buttress, an area known as the Hafafit NW trend but abruptly change strike near large bodies Culmination [El Ramly eta!., 1984]. At the intersection of competent rocks such as granite. The NW trending we observethe following: (1) the faults and shear zones ductile shear zone (SZI) bifurcates into E-W and N-S are reoriented along a NW trend and develop a thrust trending shear zones around a large granitic body (B7 in component (Plate lb), (2) subhorizontalNW trending Plate lb). As the ductile shear zones change their mineral-stretchinglineations [Greiling, 1987; Greiling orientation, they develop a thrust component that et al., 1988] and subverticalfoliations [El Ramly et al., depends on the extent to which the shear zone has 1984; Greiling et al., 1984] predominate. Bennett and changed in strike and dip. Thus thrust faults frequently Mosley [1987] explain the above mentionedfeatures in occur at the intersectionof shear zonesand large the Hafafit Culmination and other structura!ly similar competentbodies. The B7 granite in Plate la is areas (e.g., Meatiq dome), as resultingfrom Najd- structurally concordantwith the surroundings,indicating related thrusting in a duplex system. a pretectonicor syntectonicmode of eraplacement. Davies [1983] describesthis pluton and others belonging 4. DISCUSSION AND IMPLICATIONS OF THE to the Marabit Complexfound within the Najd-related EXTENSION OF THE NAJD SYSTEM INTO THE ductile shear zones, as deformed lensoid diapirs, EASTERN DESERT confirming the mode of emplacementinferred from TM products. The NSS was first recognizedsome 20 years ago in Large rock bodiessuch as B7, showingsome or all of the central and southernparts of the Arabian Shield. the followingcriteria from TM and field data, are For example, Brown [1972] documenteda cumulative mapped as buttressesin Plate lb and are labeled with sinistraldisplacement of 240 km throughcorrelation of "B" designation: displacedN-S trendingophiolitic belts (Figure 1). Stern ,.•

:. '•

b Sultan et al.: Najd Shear Systemin Arabia and Egypt 1301

.... 26øN

33o30'E 37o30'E

I SERPENTINITEPHANEROZOICSEDIMENT :i• POSTGRANITE TECTONIC PRETECTONICGRANITE -• MAFICUNITS ROCK

DUCTILEZONE SHEAR STRIKE-SLIPFAULT -• STRUCTURALTRENDS Fig. 3 (a) and (b) Full-resolutionimages for the area coveredby boxesA and B in Plate lb, respectively. Reproducedby permissionof Earth ObservationSatellite Company,Lanham, Maryland. (c) and (d) Interpretation maps showingNajd-related faults and shear zonesand their inferred senseof displacementfor Figures3a and 3b, respectively.MI denotesselected amphibolite outcropswithin and outsidethe ductile shear zone. Selectedoutcrops were outlined within and outsidethe shear zonesto demonstratethe stretchingand the dismemberingof outcropsin areas of medium to high strain.

[1985] argued that the dimensionsand orientationsof Stern [1985]. Abuzied documented deformation the NSS is better understood in central and southern characteristic of the NSS In the Hamrawin area, Saudi Arabia, as compared to the areas in the north approximately10 km NW of Quseir (Figure 1). Stern (Midyan region and Eastern Desert). He cited the [1985] reinterpretedthe observedNW trend commonly following reasons: (1) the abundanceof granitic bodies reported in the central Eastern Desert for fault traces, intruded through the Najd lines of weaknessin the pencil lineations,and pebbleand mineral stretching north, and (2) the predominanceof ductile deformation lineations,as resultingfrom movementalong a segment in the north as opposed to brittle deformation ill the of the NSS and not due to thrusting,as had previously south. In addition, there are no obviousdisplaced units been suggested. However, the extensionof the NSS into (e.g., linear ophiolitic belts crosscuttingthe NSS in Egypt has remained unsubstantiatedin the absenceof central Saudi Arabia) in the north. However, it became publisheddetailed structuralmaps on a regionalscale. clear that the Ajjaj Shear Zone structuresare Najd- Using TM data, we have mapped numerous NW related, given the continuity of structural patterns with trending left-lateral faults and ductile shear zonesin the Najd-related features located to the SE and the Ajjaj Shear Zone and the central Eastern Desert dominance of left-lateral displacementsin the Ajjaj (Plate lb). Most of these left-lateral faults and ductile Shear Zone. shear zonesin the Eastern Desert are not reported in Following the recognition of the NSS in Saudi published geologicmaps. The presumed absenceof such Arabia, Moore [1979] speculated that the NW faults and ductile shear zones has led workers to assume termination of the NSS should be in the Eastern Desert the absenceof the Najd Systemin the central Eastern of Egypt. The probable extensionof the NSS into the Desert and to relate the tectonic features observed in the Eastern Desert was first illustrated locally by Abuzied Eastern Desert solelyto the accretionof the Nubian [1984], and its regional extent was first suggestedby Shield. 1302 Sultaneta!.: Najd ShearSystem in Arabia and Egypt.

'.::-• ß ...•...... ß ...... '""•••½-',•.:.:.,.•.:, .• ...... • .. -•?•'"::"•-:•::•:.....½:•.:.:--.-,.•.:.-...... ":.:•.....'•.•:- •. •*.::".... ,•:•:.:-:•:'•:•-.-,-•:•:'---• ....'"',•7•L•t•-:•;.:•:-"•. ...,:.-•..• ...... •**-.::::•,;•-...... ;'•'• ..... ,•..',.-,..,.....:• '•.:,- •.:..-•-.-..:.••'...... ',. .-..%...... •:....--:•::•; ..•.:. :..:•:...... -.:•- ½,*:•%.:..•-.•.'• •:' •:½'•...... •.,....•.::'.•...'•c.... •-'--'---'•,...... •. ;•%s.,,:•-•:..-...... --:..,:, '; ':-.:-..-:•-:...... •.•..• ...... "*-"-;:•'*•'•:-.::+---.•,•....•..•_," ••. .'.'r.:...... ß '".-•:(-'.....E•'"'•::':•::-:;:•;:-•----::•:%; • "::::'"::;.'%...... •"•:::':"•:--.• .... • •... ' ';. "';;...... '" ;•'½-•..•--•"::".•::.;•:.,<,, :..: -•'--'" '.• ;:--;e..:l; .... ":-;•,...' ...... '-,..""%::.:.L';;;;-•:.. ...•.... ' •" • . •.•. .•'• ...... -:::....::'.'...... '•:..':.:•.•L•.:::•:,,.•-•x•,. '%•:: ...... '.::.-::,•;...... :% --•:•...... -'•%-,:..:?•-•'-.. .:•:..•[•...:.'-% -.... --.....•.---..:;..; •:-.••....::;•;:..:.• ...;-,: ...... :::.... .-- •?•-.....,-...:.?...,.•:.,;..;.: .:•.:..•-- ...... c..;-..:...... ::...... ?; :.;:.:. .•:.... C::-•;...:;..:...... :,• :::..:...... •.... -.: . :•...... •.... :½" ,:•:;.::.:,•;::;•::..:•..."•-.:-:.....-?•.• .-':-•::::..'...?-?-?,:.•,..•L:•-•:-:,::-.•..:':..::.•..E.•;?:• ...... :*":':•:2':'*':½:::"%? ...... •"::'"*'•"•:,'";•'"•::"...... ::. -.•:-.:..... •;;'•';:.:;,';:.•:.::'".•:.;• ...... '::':"'½:' ::.:-''" :•:•:--?;; ..?:::...... •,.:-.::::•"':•"::'•: :•'-:...7:•.:?:'":½?'•,:;•...?--•:•;::-,-:,.•.•...... :. .• .?' '•::....:•:'-',' ;:;:--::•.. •:,. •. ;..:•-. , .•,• .. .:•..:...•,.,.•.?...... • ...... :.:...•:.•..:•:... .:..½...... •. •:...... :,•.½4•%•.. •::•...... :•:..•:.•:.•;.:...... :..::...... :...... •: ...... •.....•:.,.•..:...... • •.,. • ..•-.' .%•:;:::"...... -: ,: --..•," '"':',-:' .'•':""::•:•'•::: ,...... :.½.:•...... • -•-%:,.?,-...;:[" :i:•:.,...,?•.::•.;:•:;-::::--•.:,.;•*---:•:,,.- -...... :.:..•;..,.'': '•;•-.".L. ;---": ?::":'•'"::? ..... •"•---....?•;'...... :--: ...... , ...... '.':'"?•:::::?-•'•½;.%;;..•::'"•...... :-•;•.... •...... •;;.,:: %':•:-.;:'•: -:•.• ...... •7.... ,•• "%::...... :: ...... ,..•...•....,..% •;•: -,*'":'....:.. -?: %•..•::•:;•L;;"%:-•-•:?::•, •::.-.:.,,.-•;,.•...... ;•..- ::•:;::•:..•...... -;::.,-,,::.-.•:•...-::. ":?'::•E:;';.....*"•.."•?'•"-:•;::.:..' •..•:•.-•. •::•;•::'-...... •% ...... •...... •a...... , ...... •-.•':"•::':':'"-:;•:•:.•'.':::'(?;-::::; •;;:• '... ;,-..•• .:.....:• •.-:;,::'•'•:••:::.•....•.• •...•:-:'-.•.... .:•...;.-...... ••.<;., .::.?•:,.-... , • , ....• ::"•'•?,...,'•:...... •::;;•4•;•...... •:.:...-..:..;½• •;•;:;.... .:.:•:•::•..::.•... :: ;•': :.:...... •.:•:...... •.:,:.-• ..... •*•:.%, • '.-:•--:•u;,•,...... :• ..½•:•....•: ""•..-?.....--•/•;• :7 '•'?••{• :.;:-.;;:;:?• '...... •:..;:•:....•.:"•:'•½:•4.... '--:--:,;•....'??;•-•;•;•.. .%•,' •.•...... , ... ':":(:...' . ,';...... :-•-:-•....•:.:•,..:-..,::•'½½?•?":½'?•'• .•.•:•.:,,.'•:•%.;•-.., •::.;•:•:•:•.....:;;":..•..:..., '"':• :, ,.2:::. ' ½%. • "•. • - ß•:. •-'>'.:":•-'• ,•:':•.:. '%. • ':•:::'. '. ":;•-:::'•%•:::;•:;:::::::::""';.... ::::•"'-•i•;:•:•::: •::':"•' . . ,•;:::,.'•:: ...:•.::.½•:•;.•..';•;:* , : ' *'-. '.'7;:•:•...;'.-: '.•.,:'•.. ' -' - '..-•C..... :•.•.. :•"s'-..,'.: ' .--•ß •Z•½•:...•;•;:; •: ...•.....,.½:Ef:;:;?..::..?•.:-•::::•. ,:.., ¾•e½:-•;.'--.:...:-•:• '.;:;;...... •, : .:... .•:-• :.• ...... •;•%•::•.... ,•--..c:•['•).. .%:.

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Fig. 4. LandsatTM band 3 image for the area coveredby Figures3b and 3d showingsubparallel topographicridges within the ductile shear zone outlined in Figure 3d. Reproducedby permissionof Earth ObservationSatellite Company, Lanham, Maryland.

We have demonstratedthat the Ajjaj Shear Zone [El Ramly et al., 1984]. The open NW trendingfolds in and the central EasternDesert share the followingfield the central EasternDesert have beenthought to be and TM-basedcharacteristics: (1) NW trendingfolds, indicative of a NE-SW directed compressionalevent (2) thrust faults and local modificationsin penetrative [Sturchioet al., 1983]. The subhorizontalNW trending deformation developingas strike-slipfaults and ductile mineral lineations commonlyreported in the central shear zonesapproach buttresses, (3) penetrative Eastern Desert (and rarely reported in the southEastern subhorizontalNW mineral lineations,(4) variably Desert) have been thought to indicate the directionof dipping NW trending foliations, and (5) predominanceof tectonic transport of nappes [Ries et al., 1983]. Local tectonic contacts. In the Ajjaj Shear Zone thesefeatures modificationsin the general NW trending penetrative have been interpreted to be a consequenceof NSS deformation around large domains composedof deformation. However, in the central Eastern Desert competentrock typesremain unaccountedfor in many these features have previouslybeen assignedother areas, for example, the E-W trending cleavage in the origins. For example, the thrust faults reported to the Barramyia area [Shukri and Lotfi, 1955]. We propose NE of the B5 buttressare presumedto be related to an that the NW trending ductile shear zone and left-lateral overthrustingevent during continent/arccollision faults running parallel to the Hahafit Culmination Sultan eta!.: Najd Shear Systemin Arabia and Egypt •L303 change their orientation to align with the E-W trending mafic/ultramafic sequencesare all in tectonic contact buttress (labeled B4 on Plate lb). This body is [Shackleton eta!., 1980]. In contrast, mafic/ultramafic composed predominantly of metagabbro, diorite, and sills with intrusive contacts were reported in the south epidiorite [El Ramly, 1972; E! Ramly and Hermina, Eastern Desert [Dixon, 1981]. Thus we conclude that 1978]. records of the accretionary history of the Arabian- The predominanceof highly deformed rocksshowing Nubian Shield are probably better preserved in the south ophiolitic affinities in the central Eastern Desert has Ajjaj Shear Zone and the south Eastern Desert been interpreted as an extensiveophiolitic melange [Ries compared to the Ajjaj Shear Zone and the central eta!., 1983; Shackletonet al., 1980]. It is widely Eastern Desert. thought that the ophiolitic melange in the Eastern Desert is partly trench and partly tectonic melange, perhaps 5. SUMMARY related to the Sol Hamed-Onib suture some 300-500 km to the south [Ries eta!., 1983](Figure 1). Shackleton 1. A digital color mosaiccompiled from seven et al. [1980] concluded that the ophiolites are Landsat the•natic mapper sceneswas used to delineate a!!ochthonous,transported by gravity sliding in a trench, characteristic structural features for the NSS in the followed by thrusting through nappe eraplacement. Midyan region of Saudi Arabia and to search for similar We favor the hypothesisthat Najd-related features in the Egyptian Eastern Desert. The digital deformation is responsiblefor the dismemberment of the mosaic was generated using ratios of Landsat thematic ophiolitic sequenceand production of the melange for mapper bands (5/4 x 3/4, 5/1, and 5/7). These ratios are the following reasons: sensitive to the rock content of Fe-bearing 1. Extensive melanges are absent in the Midyan aluminosilicates, spectrally opaque phases, and hydroxyl- region, despite the continuationof the Sol Hamed-Onib bearing or carbonate minerals of the outcrops, suture in the Midyan region as the Yanbu suture zone respectively. The mosaiccovers approximately 130,000 (Figure 1 and Plate lb). If the melanges of the Eastern km:z of late Proterozoicexposures of the Arabian-Nubian Desert are related to the Sol Hamed-Onib-Yanbu suture, Shield and has the Eastern Desert and the Midyan we would expect a similar melange to be present in the region placed in their approximate pre-Red Sea Midyan region. locations. Approximately 30 km of extensionwithin the 2. There is a close spatial correlation between the Gulf of Suez and 110 km of !eft-lateral motion along the distribution of so-called melangeswith the postulated Gulf of Aqaba were used to define displacement extension of the NSS in the Eastern Desert. The magnitudesand directionsfor correctionsto pre-Red Sea melanges as describedearlier are clearly affected by locations. Najd-related NW-SE structures. 2. The Ajjaj shear zone that marks the termination 3. Melanges of the central Eastern Desert are not of the Najd Systemagainst the eastern margin of the typical trench melangesbecause they lack the pervasive Red Sea aligns with the central Eastern Desert, based on matrix on a regional scale that is typical of such analysisof pre-Red Sea locations. melanges reported elsewhere. Typical melanges are 3. We have mapped for the first time left-lateral formed of "blocks in a matrix" [Raymond, 1984]. The faults and ductile shear zones in the Eastern Desert matrix in melanges in other regionsof the world is characteristicof the Najd Systemin Saudi Arabia. The pervasive; only locally is the matrix missing[e.g., Yilmaz presumed absence of such features has led workers to and Maxwell, 1984]. assume that the NSS is absent in the Eastern Desert and 4. Finally, Shackletonet al. [1980] also realized that to relate deformation in the Eastern Desert entirely to their model had shortcomings,especially the lack of obduction/accretion tectonics. evidence for extensive tectonic transport that would be 4. Analyses of Landsat data and field observations required. show that the Ajjaj Shear Zone and the central Eastern NW trending folds, structural trends, !eft-lateral Desert exhibit the following features in common: faults and ductile shear zones, and penetrative planar (1) outcropsthat are generally elongate in a NW-SE and linear fabrics are uncommon in the area south of direction due to folding, with fine-scalelithologic the Ajjaj Shear Zone and in the south Eastern Desert, heterogeneity at the outcrop scale due to deformation based on analysisof TM data and published geologic associatedwith faulting, (2) NW trending left-lateral maps. Instead, the dominant tectonic fabric in these faults and ductile shear zones, (3) subhorizontal, NW areas is N-S to NNE-SSW (Plate lb). These trends trending mineral !ineations, (4) variably dipping NW represent the dominant tectonic grain in the Arabian trending foliations, with local changesin attitude around Shield [Brown and Coleman, 1972; Moore, 1979]. competent (e.g., granitic) rocks, and (5) !ithologic Stoeser and Camp [1985] have shown that the suture contactsthat are generally tectonic in nature and related zones, predating the Najd System,have a N-S to NE-SW to faulting. These features are lesscommon to the north trend (Figure 1). These suturesprobably control the N-S and south of both the Ajjaj Shear Zone and the central to NNE-SSW structures discussed above. Modes of Eastern Desert. We propose that these features are emplacement of mafic/ultramafic sequencesin the largely controlled by Najd deformation and are not the central Eastern Desert and south Eastern Desert are result of obduction/accretiontectonics as currently different. In the central Eastern Desert, accepted. 1304 Sultan et al.: Najd Shear Systemin Arabia and Egypt

5. We proposethat the dismemberingof the Bennett, J.D., and P.N. Mosley, Tiered-tectonicsand ophiolitic sequencesof the Eastern Desert are related evolution, Eastern Desert and Sinai, Egypt, in Current largely to Najd deformation. This hypothesisdiffers Research in African Earth Sciences, edited by from interpretations that explain the dismemberingof G. Mathels and H. Schandelmeier, pp. 79-82, the ophiolitic sequencesin the Eastern Desert through Balkema, Rotterdam, Netherlands, 1987. gravity slidingin a trench and/or thrustingthrough Bohannon, R.G., How much divergence has occurred nappe emplacementaccompanying arc/arc or between Africa and Arabia as a result of the opening arc/continent collision. of the Red Sea?, Geology, 14, 510-513, 1986a. 6. Recordsof the pre-Najd accretionaryhistory of Bohannon, R.G., Tectonic configuration of the western the Eastern Desert are probably better preserved in the Arabian continental margin, southernRed Sea, south Eastern Desert where the Najd deformation was Tectonics,_5, 477-499, 1986b. apparently less pronounced. Brown, G.F., Eastern margin of the Red Sea and the 7. Geologistsinvolved in the tectonicsynthesis of coastal structures in Saudi Arabia, Philos. Trans. R. large domainsof the lesserknown continentssuch as the Soc. London, Ser. A, 267, 75-87, 1970. Arabian-Nubian Shield are often faced by the absenceof Brown, G.F., Tectonic map of the Arabian Peninsula, (1) detailed geologicmaps on a regionalscale, scale 1:4,000,000, Map Ap-2, Saudi Arabia Dir. Gen. (2) reliable databasesfor geochronologicand of Miner. Resour., Jiddah, 1972. geochemicaldata, and (3) integratedstudies between Brown, G. F., and R.G. 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