u pb zircon and 40 ar 39 ar geochronology of sericite from hydrothermal alteration zones new constraints for the timing of ediacaran gold mineralization in the sukhaybarat area western afif terrane saudi arabia the sukhaybarat east and red hill deposits in the northeastern part of the arabian shield are mesothermal vein type gold deposits hosted by late cryogenian ediacaran intrusive rocks of the idah suites diorite tonalite granodiorite and at sukhaybarat east also by ediacaran metasedimentary rocks gold mineralization comprises quartz arsenopyrite veins sukhaybarat east quartz carbonate pyrite veins red hill and subordinate gold base metal sulfide veins in the red hill deposit alteration is complicated due to multiple overprinting hydrothermal events and is characteristically affected by pervasive pink quartz k feldspar hematite alteration which is overprinted by potassic alteration characterized by a quartz biotite carbonate muscovite sericite rutile apatite assemblage this assemblage is associated with molybdenite veins which appear to form late in the paragenetic sequence and may represent either evolution of the ore fluid composition or a later unrelated mineralized fluids hydrothermal alteration at the sukhaybarat east deposit is dominated by quartz carbonate sericite arsenopyrite assemblages zircon from ore hosting tonalite at sukhaybarat east yields a u pb age of 629 6 ma and biotite from the same rock gives an 40ar 39ar age of 622 23 ma the 40ar 39ar age is within the uncertainty range for the u pb age of the host intrusion and is interpreted as a minimally disturbed cooling age for the tonalite in the red hill area granodiorite was emplaced at 615 5 ma whereas muscovite sericite separated from a mineralized sample of a quartz carbonate pyrite vein that was overprinted by molybdenite bearing veinlets yields an 40ar 39ar age of 597 8 ma we interpreted this age to represent the maximum age of the molybdenite mineralization and the probable minimum age of gold mineralization in the red hill deposit

a complex seismogenic shear zone the duzce segment of north anatolian fault turkey abstract this work highlights how surface geological data can integrate and complete a fault characterization including its seismogenic depth we investigated the area struck by the november 1999 mw 7 1 earthquake that ruptured the duzce segment of the north anatolian fault zone nafz after the august 1999 mw 7 4 izmit earthquake to explore the persistency or evolution of the active fault setting at the surface we compared detailed coseismic surface expressions with the long term morphology and structural architecture of the duzce zone we identified a simple 1999 coseismic fault trace cft and an older complex fault system rfs involving a wider zone of deformation two different parts of the duzce segment were recognized a western section presents a staircase trajectory of the coseismic fault trace that reactivated part of the older fault system an eastern section presents a straight coseismic fault trace that cross cuts the older and complex fault system the western duzce fault segment splays out from the izmit counterpart of the nafz forming a releasing fault junction by comparing a model of its coseismic surface deformation field with the long term morphology we observed that the present topography of this western section results from the nafz activity by repeating coseismic deformations like those in 1999 the mechanical interaction in the release fault junction produces local transtension across the western duzce fault and justifies its stable complexity it does not evolve into a straight trace the surface boundary between the duzce fault sections also separates at depth two fault sections one with low slip to the west and another with a big asperity to the east thus the izmit duzce interaction 1 may produce an unfavorable setting for a build up of asperities in the western duzce segment and 2 could delay the propagation of the izmit rupture along the duzce segment that nucleates mainshocks only on its eastern section in other words the arrangement of the fault junction may permanently control how the two sections of the duzce fault behave and rupture

an integrated approach for identifying aquifers in transcurrent fault systems the najd shear system of the arabian nubian shield groundwater potentials were assessed for major transcurrent fault systems in basement complex terrains using the najd transcurrent shear system of the arabian nubian shield as a test case spatial analysis within a geographical information system environment of relevant data sets field remote sensing geophysical and digital elevation revealed a spatial correlation between existing well locations and distribution of najd transcurrent shear system structural elements i e faults shear zones melange dykes within highly fractured domains suggesting a causal relationship specifically at 1 intersection of najd brittle shear zones and fault systems 2 intersections of two or more fault systems 3 intersections of najd related dykes with the main streams within catchment areas and 4 within tectonic melange for each of the former aquifer types aquifer specific field and satellite based characteristics were identified and used in conjunction with complementary data e g watershed morphology precipitation soil moisture to target aquifers of similar settings postulated well locations were then successfully tested and refined using very low frequency technologies results could potentially highlight the important role of major transcurrent fault systems in providing settings conducive for hosting and transmitting groundwater and could potentially serve as a replicable model for regional assessment of groundwater potentials in similar tectonic and hydrologic settings

crustal accretion in the pan african nd and sr isotope evidence from the arabian shield abstract nd and sr isotope determinations on late precambrian to early palaeozoic igneous and sedimentary rocks from the arabian shield are used to investigate the proportion of reworked older crust and the rate at which new crust was generated during the pan african event eight rb sr whole rock isochrons on igneous suites yield ages in the range 770 590 ma and initial 87 sr 86 sr ratios of 0 7038 0 7023 these data confirm that magmatism in this area was largely restricted to the period 850 550 ma and the initial ratios are sufficiently low to preclude significant contributions from a long lived upper crustal source the initial 143 nd 144 nd ratios of a variety of lithologies including several samples of possible basement are all higher than the contemporaneous values for chur e nd 1 6 to 6 9 suggesting that many were derived directly from the upper mantle and that any inferred crustal source regions for the remainder could not have separated from likely lree depleted mantle reservoirs before 1200 ma the arabian shield therefore provides an example of rapid crustal growth during the late proterozoic and contrasts with the damara intracratonic belt of namibia where nd and sr isotopes provide strong evidence for extensive reworking of older continental crust during the same period

the bani ghayy group sedimentation and volcanism in pull apart grabens of the najd strike slip orogen saudi arabian shield abstract the cratonisation of the arabian shield involved the accretion of ensimatic arc terranes in the west and south followed by continental collisions along the nabitah and al amar sutures and the onset of the najd strike slip orogen volcanosedimentary successions which post date these collisions have been proposed as the products of magmatism in transitional volcanic arcs one such succession the bani ghayy group lies unconformably above a series of volcanosedimentary rocks associated with an older active continental margin and was deposited in three separate elongate fault bounded grabens each graben contains coarse fanglomerate and fan delta deposits along the margin and proximal greywackes toward the centre reef limestones occur locally whereas volcanogenic rocks are widespread the latter are geochemically bimodal and with the sediments are typical of continental rifting environments although deformed ultramafic rocks within internal reverse faults suggests that the grabens were floored in part by primitive mantle derived crust deposition of the bani ghayy group post dates incipient motions along fractures of the najd fault system the orientation of the bani ghayy grabens and the geometry of the boundary faults are consistent with development in a regime of northwest trending dextral shear however the bani ghayy grabens are displaced and deformed by later movements of the same faults but in a sinistral sense other successions correlated with the bani ghayy also preserve features consistent with deposition in continental grabens and the possibility that they represent arc volcanism is discounted

emplacement in an extensional setting of the mont lozere borne granitic complex se france inferred from comprehensive ams structural and gravity studies the emplacement mode and setting of the late hercynian mont lozere borne granitic complex french massif central which consists of several plutons is investigated structural and anisotropy of magnetic susceptibility ams studies have been carried out to characterize the internal fabrics of the granitic plutons throughout the pont de montvert borne pluton an e w trending magnetic lineation is well developed in the host rock and the thermal aureole a conspicuous e w trending lineation is interpreted as an evidence of a late orogenic extensional event to the east of the pluton the ams fabric is characterized by values of anisotropy degree p around 4 5 with a prolate ellipsoid and subsolidus structures whereas to the west the p parameter is weaker with an oblate ellipsoid and purely magmatic microstructures a gravity investigation allows determination of the 3d shape of the pluton the western part of the granitic complex is thicker than the eastern one and is interpreted as the feeder zone this suggests an eastward spreading of the magma the consistency between regional stretching and directions of ams lineations in the pluton and the shape of the complex deduced from gravity strongly argues that the emplacement mode of the complex was influenced by the regional extensional tectonic setting during the collapse of the hercynian belt

geochemistry and sr and nd isotopic study on rare metal bearing granitic rocks central eastern desert egypt abstract three post orogenic granitic plutons exposed in the egyptian basement complex namely homr akarem homrit mukbid and nuweibi were selected for geochemical and nd sr isotopic studies based on petrographic and geochemical considerations the granitic rocks are divided into medium grained pink granites and albite rich granites both types exhibit peraluminous character with many chemical similarities to the high y nb a subtype granites owing to the extreme fractionated nature and complex magma source of these granitic rocks in addition to the high volatile activity their compositions resemble within plate magma originated in attenuated crustal lithosphere whole rock rb sr isochron ages of the medium grained pink granites from homr akarem and homrit mukbid plutons are 541 4 ma and 527 26 ma respectively the nuweibi albite granite nag represents the youngest granite phase and yields a rb sr age of 490 27 ma the medium grained pink granites show a moderate and narrow range of initial 87 sr 86 sr ratio 0 7064 0 7081 and negative t nd values 1 31 to 6 68 the nag has an initial 87 sr 86 sr 0 7071 and t nd 3 26 to 4 72 comparable to those of the medium grained pink granites the sm nd data yield model ages t dm of 0 96 to 1 70 ga similar nd model ages 1 2 1 8 ga have been reported for other post orogenic granites from southwest egypt and northern sudan the nd and sr isotopic compositions of the studied mineralized granites suggest the involvement of old lree enriched continental crust of pre pan african age and lower crust or mantle derived magmas the nd and sr isotopic variations can be explained by a mixing hypothesis

a pan african age for the hp ht granulite gneisses of zabargad island implications for the early stages of the red sea rifting abstract up to now the age of granulite gneisses intruded by the zabargad mantle diapir has been an unsolved problem these gneisses may represent either a part of the adjacent continental crust primarily differentiated during the pan african orogeny or new crust composed of miocene clastic sediments deposited in a developing rift crosscut by a diabase dike swarm and gabbroic intrusions and finally metamorphosed and deformed by the mantle diapir previous geochronological results obtained on zabargad island and al lith and tihama asir complexes saudi arabia suggest an early miocene age of emplacement for the zabargad mantle diapir during the early opening of the red sea rift in contrast sm nd and rb sr internal isochrons yield pan african dates for felsic and basic granulites collected 500 600 m from the contact zone with the peridotites devoid of evidence for retrograde metamorphic minerals from a felsic granulite provide well defined rb sr and sm nd dates of 655 8 and 699 34 ma for the hp ht metamorphic event 10 kbar 850 c the thermal event related to the diapir emplacement is not recorded in the sm nd and rb sr systems of the studied gneisses in contrast the development of a retrograde amphibolite metamorphic paragenesis strongly disturbed the rb sr isotopic system of the mafic granulite the initial 143 nd 144 nd ratio of the felsic granulite is higher than the contemporaneous value for chur and is in agreement with other nd isotopic data for samples of upper crust from the arabian shield this result suggests that source rocks of the felsic granulite were derived at 1 0 to 1 2 ga from either an average morb type mantle or a local 2 2 ga lree depleted mantle zabargad gneisses represent a part of the disrupted lower continental crust of the pan african afro arabian shield during early stages of the red sea rifting in the miocene these precambrian granulites were intruded and dragged upwards by a rising peridotite diapir

u pb zircon geochronology and geological evolution of the halaban al amar region of the eastern arabian shield kingdom of saudi arabia u pb zircon model ages for eleven major units from the halaban al amar region of the eastern arabian shield indicate three stages of evolution 1 plate convergence 2 plate collision and 3 post orogenic intracratonic activity convergence occurred between the western afif and eastern ar rayn plates that were separated by oceanic crust remnants of oceanic crust now comprise the ophiolitic complexes of the urd group the oldest plutonic unit in the study is from one of these complexes and gave an age of 694 8 ma detrital zircons from the sedimentary abt formation of the urd group which is intercalated with the ophiolitic rocks were derived from source rocks with a mean age of 710 ma the abt formation may be an accretionary wedge on the western margin of the ar rayn plate plate convergence was terminated by collision of the afif and ar rayn plates during the al amar orogeny which began about 670 ma during collision the urd group rocks were deformed and in part obducted on to one or both plates synorogenic leucogranitoid rocks were intruded from 670 to 640 ma from about 640 to 630 ma widespread unfoliated dioritic plutons were emplaced in the ar rayn block and represent the end of orogenesis related to collision there is no definitive evidence for a significantly older basement beneath the study region

the tectono metallogenic evolution of the arabian shield abstract the arabian shield comprises a group of diverse terranes accreted during the late proterozoic their boundaries are suture zones characterised by tectonised ultramafic bodies aligned within distinct orogenic belts the asir hijaz and midyan terranes consist of a series of late proterozoic immature island arcs and together form the western half of the shield in the east however the afif and ar rayn terranes have a continental affinity with the afif terrane being much older at least early proterozoic and possibly archaean metallogeny within the arabian shield is also diverse and includes cu ni and cr mineralisation associated with proterozoic oceanic crust or ophiolite arc type volcanogenic base and precious metal deposits associated with formerly active plate margins shear zone hosted mesothermal gold mineralisation related to accretionary and strike slip tectonics as well as sn w ta mo ree mineralisation associated with post cratonisation alkali granite magmatism of late proterozic to early phanerozoic age it is possible to trace a systematic evolution in metallogeny both within each terrane and throughout the craton as a whole as a function of crustal development and tectonics although the age range of rocks exposed in the shield is from early proterozoic to tertiary all known styles of primary metallogeny are associated with its cratonisation in the late proterozoic

distribution of oceanic and continental leads in the arabian nubian shield new common lead data for feldspar whole rock and galena samples from the arabian nubian shield together with data from previous work can be divided into two main groups group i leads have oceanic mantle characteristics whereas group ii leads have incorporated a continental crustal component of at least early proterozoic age the group i leads are found in rocks from the red sea hills of egypt and the western and southern parts of the arabian shield group ii leads are found in rocks from the northeastern and eastern parts of the arabian shield as well as from the southeastern shield near najran they are also found in rocks to the south in yemen to the east in oman and to the west at aswan egypt this distribution of data suggests that the arabian nubian shield has an oceanic core flanked by rocks that have developed at least in part from older continental material two mechanisms are suggested by which this older lead component could have been incorporated into the late proterozoic rocks and each may have operated in different parts of the shield the older lead component either was derived directly from an underlying early proterozoic basement or was incorporated from subducted pelagic sediments or sediments derived from an adjacent continent

tectonic and deposition model of late precambrian cambrian arabian and adjoining plates during the late precambrian the terranes of the arabian and adjoining plates were fused along the northeastern flank of the african plate in gondwanaland this phase which ended approximately 640 to 620 ma was followed by continental failure 620 to 580 ma and intracontinental extension 600 to approximately 550 ma during the infracambrian extensional phase a triple junction may have evolved near the sinai peninsula and may have consisted of the 1 jordan valley and dead sea rift branch 2 sinai and north egypt rift branch and 3 the najd wrench rift branch the najd hawasina and zagros fault systems may have been transverse faults that accompanied rifting in the arabian gulf and zagros mountains southern oman pakistan and kerman in central iran while the area was extending and subsiding the tethys ocean flooded the eastern side of the arabian plate and iran and deposited calcareous clastics carbonates and evaporites including the hormuz and ara halites this transgression extended into the western part of the arabian plate via the najd rift system the termination of the extensional phase during the late early cambrian was accompanied by a major regression and terrestrial conditions on the arabian peninsula however by the early ordovician as sea level peaked to a highstand the arabian plate was blanketed with marginal marine sediments

magnetically inferred basement structure in central saudi arabia abstract a compilation of magnetic data acquired during the past three decades for a region in central saudi arabia where precambrian basement is partly exposed on the arabian shield and partly concealed by overlying phanerozoic strata shows a central sector of conspicuous n s trending anomalies a heterogeneous western sector of short wavelength high intensity anomalies and an eastern sector of low to moderate intensity broad wavelength anomalies anomalies in the western and central sectors correlate with neoproterozoic metavolcanic metasedimentary and intrusive rocks of the arabian shield and are interpreted as delineating extensions of shield type rocks down dip beneath phanerozoic cover these rocks constitute terranes making up part of a neoproterozoic orogenic belt that underlies northeast africa and western arabia and it is proposed that their magnetically indicated easternmost extent marks the concealed eastern edge of the orogenic belt in central arabia the flat magnetic signature of the eastern sector not entirely accounted for as an effect of deep burial may reflect the presence of a crustal block different in character to the terranes of the orogenic belt and speculatively may outline a continental block that according to some tectonic models of the region collided with the neoproterozoic terranes and thereby caused their deformation and tectonic accretion

stratigraphy and palaeogeography of the siham group direct evidence for a late proterozoic continental microplate and active continental margin in the saudi arabian shield the presence of ancient continental crust in the afif terrane of the arabian shield has been indicated by radiometric data detailed mapping in the central shield reveals a volcano sedimentary succession older than one of the final cratonizing events of the shield and unconformable upon high grade paragneisses providing direct evidence for a western continental margin to the afif terrane in late proterozoic times the basement gneisses were metamorphosed to almandine sillimanite amphibolite facies and preserve evidence of a deformation not seen in any other rocks of the area the siham group unconformable upon this basement was metamorphosed to upper greenschist facies it has been divided into six formations which provide an e w section through the group the lower members are volcanic and are absent in the easternmost formation their clastic content increases eastward a carbonaceous shale with exhalative chert and marble tuffs and associated volcanogenic massive sulphides is associated with the volcanics it is absent in the east where shallow marine marbles and quartzites lie unconformably upon basement rhyolite and conglomerate complete the succession in the west the base of the group is not seen but the shale is associated with numerous sheet like bodies of mafic and ultramafic rock emplaced in an extensional tectonic setting other plutonic rocks are calc alkaline and are emplaced mainly in the west deep water oceanic conditions prevailed in the west and a shallow marine continental shelf environment existed in the east with deposition of the group across this continental margin the stratigraphy of the siham group is particularly akin to the mesozoic of peru magmatic arc and marginal basin were coincident in space and time and the marginal basin was probably extensional there is some limited evldence that it was ensialic the siham group is bounded to the west by a n s suture zone formed in one of the final collisional events in the cratonization of the shield this event caused the deformation of the siham group and marked the termination of the magmatic are

late proterozoic evolution of the northern part of the hamisana zone northeast sudan constraints on pan african accretionary tectonics new structural data from the red sea hills northeast sudan add constraints to accretionary tectonic processes that acted in the arabian nubian shield during the late proterozoic geometric constraints provided by field data are used together with landsat thematic mapper imagery to interpret the regional distribution and structural relations of spectrally and lithologically distinctive rock assemblages in contrast to interpretations reported elsewhere these studies indicate that the hamisana zone a prominent structural feature in the northern red sea hills post dated terrane accretion it is characterized by east west crustal shortening and by steep folds and thrust faults calc alkaline magmatism was contemporaneous with and outlasted main phase deformation thus deformation occurred in an intra arc setting thrust faults were reactivated after east west contraction compromising their usefulness in delineating 9sutures9 or as piercing points across older faults pan african accretionary processes may have been analogous to phanerozoic ophiolite and island arc accretion in the western north american cordillera where penetrative deformation occurred in response to periodic intra plate shortening events rather than an ultimate collision of unrelated crustal fragments such an analogy implies a mechanism for crustal thickening by structural as well as igneous processes during evolution of an accretionary belt in a long lived convergent margin setting consistent with the long standing observation that pan african magmatism became more evolved with time this view emphasizes that an interplay of deformation and magmatism resulted in protracted accretion and crustal thickening in contrast to models where crustal shortening results directly from collision of a volcanic arc microcontinent or other buoyant features

late proterozoic transpression on the nabitah fault system implications for the assembly of the arabian shield abstract the longest proposed suture zone in saudi arabia the nabitah suture can be traced as a string of ophiolite complexes for 1200 km along the north south axis of the arabian shield results of a field study in the north central shield between 23 and 26 n indicate that the nabitah suture is indeed a major crustal discontinuity across which hundreds of kilometers of displacement may have occurred on north south trending subvertical faults of the nabitah fault system although not a unique solution many structures within and near these faults can be reconciled with transpression i e convergent strike slip and syntectonic emplacement of calc alkaline plutonic rocks transcurrent motion on the nabitah fault system appears to have began prior to 710 ma was active circa 680 ma and terminated prior to significant left lateral strike slip on the najd fault system which began sometime after 650 ma northwest directed subduction in the eastern shield could have produced the observed association of calc alkaline magmatism and left lateral transpressive strike slip and is consistent with interpretation of the abt schist and sedimentary rocks of the murdama group as relics of the associated accretionary wedge and fore arc basin

two linear granite belts in the central western north china craton and their implication for late neoarchaean palaeoproterozoic continental evolution abstract precambrian granitoid rocks are widely distributed in the north china craton ncc the dominant granitic rocks in the central western ncc are trondhjemitic tonalitic and granodioritic ttg gneisses as well as some calc alkaline granitic gneisses and minor charnockites two linear granite belts in this area are different from other early precambrian granitic rocks in the ncc in terms of their occurrence and chemistry the liang tu lt garnet granite belt and the sanggan sg granite belt the lt belt occurs along two large scale ductile shear zones in metamorphosed al rich sedimentary country rocks khondalitic series in the central part of the lt belt are metagabbroic granulite lenses that were originally mafic dykes in the outer part of the belt garnet granites grade into khondalitic rocks lt belt granites have geochemical characteristics of s type granites the sg granite belt is 400 km long and composed of many deformed gneissic granitic bodies and granitic sills and veins with without gneissosity they contain various lenses including high pressure granulites and retrograded eclogites the sg granite belt trends east northeast parallel to the regional gneissosity of the country rocks major and trace element geochemistry of these granites shows characteristics of syn collisional and post collisional granite the country rocks beside the sanggan granite belt are different including the huai an fengzhen terrain hft to the northwest and the wutai fuping terrain wft to the southeast the lt and sg granite belts formed within an old continental craton and an old orogenic zone between two blocks respectively

the arabian infracambrian extensional system abstract during the late precambrian and early cambrian about 600 540 ma extensive left lateral faulting along the complex nw se trending najd fault system cut across the arabian shield this tectonic episode was accompanied by nw se directed extension in northern egypt and the sinai peninsula based on these relationships stern 1985 interpreted the najd fault system as a set of transform faults emanating from a rift basin in ne egypt contemporaneous to this system in the western part of the arabian plate and northern egypt extensive evaporitic basins developed in northern pakistan southern oman and the arabian gulf and zagros mountains these basins are generally bounded by nw trending faults such as the najd hawasina and zagros faults and ne trending faults such as the ghudun khasfah and dibba faults these evaporitic basins are interpreted as rift basins such that stern s 1985 extensional model in northern egypt may be expanded regionally to include the eastern part of the arabian plate and the indian subcontinent this broader interpretation implies that after the e w directed collisional tectonics of the late precambrian in western arabia and africa which terminated around 640 600 ma the arabian plate experienced nw se directed crustal extension and continental break up

u pb isotopic evidence for the accretion of a continental microplate in the zalm region of the saudi arabian shield the area covered by this work includes three of the main tectonic units of the arabian shield the afif continental terrain the nabitah suture with its associated mobile belt and the asir ensimatic arc terrain the geology of the zalm area is well understood and this geochronologic and isotopic study confirms that the southern afif terrain was a continental microplate in the late proterozoic the study also provides a time frame for the crustal evolution of this part of the shield the kabid formation is the oldest in the region and comprises pelitic arkosic and felsic high grade paragneisses u pb zircon data from a pelitic garnet sillimanite gneiss show that this part of the continental basement in the southern afif terrain may be as old as 1770 ma isotopic nalyses indicate that lead from the kabid gneiss resided in the upper continental crust for a long period before 1770 ma and consequently archaean source rocks may be present within the southern afif terrain pb and rb sr isotopic data in the zalm region reveal a change in the nature of the underlying crust from continental basement in the northeast to less radiogenic marginal arc rocks in the southwest this change is coincident with both aeromagnetic data and a facies change within a pre collision marginal basin miogeosynclinal continental shelf facies of the siham group lie unconformably over the kabid formation and are in the area of continental lead signatures eugeosynclinal deep water sediments and volcanics in association with ultramafic rocks occur in the area of marginal arc signatures u pb zircon age determinations show that this 9andean9 continental margin developed before about 720 ma and emplacement of calc alkaline plutonic rocks continued until about 690 ma during the period 685 640 ma the continental afif microplate collided with the asir terrain as part of the nabitah orogeny at approximately 640 ma ago the najd strike slip faulting commenced with a dextral phase that controlled the emplacement of granitic plutons as well as the development of a series of large pull apart grabens some of these grabens were floored by new oceanic crust and were filled with volcanosedimentary rocks of the bani ghayy group subsequently the najd fault system changed to sinistral strike slip motion at about 620 ma ago

extension of the najd shear system from saudi arabia to the central eastern desert of egypt based on integrated field and landsat observations the najd shear system in saudi arabia extends over 1200 km in a nw se direction and has a width of approximately 300 km a digital color mosaic compiled from seven landsat thematic mapper scenes was used to delineate characteristic structural features of the najd system in the midyan region of saudi arabia and to search for similar features in the egyptian eastern desert the digital mosaic was generated using ratios of landsat thematic mapper bands bands 5 4 3 4 5 1 5 7 that are sensitive to the rock content of fe bearing aluminosilicates spectrally opaque phases and hydroxyl bearing or carbonate minerals respectively the mosaic covers approximately 130 000 km of late proterozoic exposures of the arabian nubian shield and has the eastern desert and the midyan region placed in their approximate pre red sea locations the ajjaj shear zone ajz marks the termination of the najd system against the eastern margin of the red sea in the midyan region the ajz aligns with the central eastern desert based on analysis of pre red sea locations analyses of landsat data and field observations show that the ajjaj shear zone and the central eastern desert exhibit the following features in common 1 outcrops that are generally elongate in a nw se direction as a result of folding with fine scale lithologic heterogeneity at the outcrop scale related to deformation associated with faulting 2 nw trending left lateral faults and ductile shear zones 3 subhorizontal nw trending mineral lineations and variably dipping nw trending foliations with local changes in attitude around large competent e g granitic bodies and 4 lithologic contacts that are generally tectonic in nature and related to faulting these features are less common to the north and south of both the ajjaj shear zone and the central eastern desert results are consistent with the najd shear system extending into the eastern desert and dominating the structural patterns within the central part of the eastern desert

geochronology of the late precambrian hamisana shear zone red sea hills sudan and egypt the hamisana shear zone hsz is a broad zone of deformation approximately 50 km wide and at least 300 km long making it one of the largest basement structures in ne africa it has been interpreted as a precambrian suture as a zone of strike slip displacement or as a zone of crustal shortening the results of new rb sr and u pb zircon geochronological studies indicate that the northern hsz was thermally active during the pan african event until c 550 mg ago initiation of the structure may have begun 40 110 ma earlier all units have low initial 87 sr 86 sr ratios indicating juvenile derivation the timing of activity in the hsz is 50 150 ma younger than collisional suturing and terrane assembly in the arabian nubian shield but is similar to the 655 540 ma najd fault system of egypt and arabia deformation and metamorphism along the hsz clearly post date terrane accretion and probably are closely related to the najd tectonic cycle the most important deformation of the hsz is unrelated to suturing and at least one late precambrian suture must extend west from arabia into the interior of n africa