Allaqi-Heiani Suture Research Articles

A reconnaissance study for tracing and ordering new mineralisation zones using integrated remote sensing, GIS, and aeromagnetic techniques, west Allaqi-Heiani-Suture,Egypt

ABSTRACT Reducing expenses is a critical aspect of natural resource explorations, especially in the reconnaissance stage. Accordingly, GIS algorithms using the free remote sensing data were used to rank the probable mineralised zones before any future implementations of the high-cost drilling and/or detailed geophysical programmes. Geological and tectonic settings, and being a historical mineral resource were motivations to choose the Allaqi-Heiani Suture (AHS), Upper Egypt, as a study area. Remote sensing data were processed for tracing the probably mineralised alteration zones, determining fractures and dominating structural systems, and mapping different geological units. Various alteration minerals including hydroxyls–carbonates, ferrous and ferric iron oxides were detected over different rock units using band ratio (BR) and Crosta techniques. Different geological units were distinguished using false colour composite (FCC) images. Moreover, an intense fractured system, demonstrating the area, was revealed by tracing lineaments using informative, spatially enhanced remote-sensed images. Fractures within the altered rocks represent 35% of the total fracturing system in the AHS to indicate a structural system controlling mineralisation occurrences. The processing of magnetic data suggested a deep/buried nature of these remotely detected lineaments/structures. Reduced to pole (RTP) and tilt derivative (TDR) maps detected the buried magnetic sources, whereas zero contours of TDR, Euler deconvolution, analytic signal and source parameter imaging (SPI) techniques traced buried contacts/structures and estimated their depths. A GIS strategy depending on the intensity of the fracturing system and the presence of more than one type of alteration was applied to potential-rank the prospective mineralised occurrences over AHS. The correlation between ranked zones with the old/known mines showed encouraging evaluation.

Neoproterozoic hybrid forearc – MOR ophiolite belts in the northern Arabian-Nubian Shield: no evidence for back-arc tectonic setting

ABSTRACT The Neoproterozoic Arabian-Nubian Shield (ANS) post-dates the Boring Billion (1.85−0.85 Ga), a transition dominated by extensive arc-magmatism, terrane accretion along active subduction zones. Understanding the nature of the oceanic crust fragments preserved in the ophiolite belts is essential in extrapolating the predominant style of plate tectonics during the Neoproterozoic Era. There are two ophiolite belts with controversial genetic models in the Central Eastern Desert (CED) of Egypt, namely Wadi Semna–Fawakhir–Um Gheig belt to the north, and Ghadir–Mubarak–Barramiya belt to the south. Geochemical database and magnetic data are herein integrated to envisage the tectonic setting of the dispersed ophiolite blocks within these two belts. The northern belt is dominated by NW obduction-related thrust faults and is intensely deformed by structures attributed to the Najd fault system (NFS). The southern belt is less affected by the Najd fault with NE obduction-related thrust faults. Based on various discrimination diagrams and REEs pattern, oceanic mafic-ultramafic rocks in these belts can be classified into three types, MOR, forearc and volcanic arc basalt, where the latter is MORB overprinted by arc affinity. Our proposed evolutionary model includes N-dipping subduction zones parallel to the ~ E-W Allaqi-Heiani suture with hybrid forearc-MOR affinities. The data synthesis presented here weighs with advancing accretionary orogeny along the intraoceanic subduction zones as the dominant tectonic style during the Neoproterozoic evolution of the CED rather than the retreating style of the modern western Pacific region.

Integrating aeromagnetic data and Landsat-8 imagery for detection of post-accretionary shear zones controlling hydrothermal alterations: The Allaqi-Heiani Suture zone, South Eastern Desert, Egypt

The Arabian-Nubian Shield (ANS) underwent a regional carbonation event with CO2 fluxes derived from the mantle and circulated along post-accretionary shear zones, thus affecting the redistribution of gold mineralization. Formerly, the analyses of remotely-sensed data have suggested regions of gold mineralization linked to the post-accretionary structured framework, but the results were insufficient for detailed prospecting in the ANS. In this research, aeromagnetic data and Landsat-8 imagery were integrated for delineating new high potential zones of gold mineralization in the Allaqi-Heiani Suture (AHS) zone, the ANS, South Eastern Desert (SED) of Egypt. Aeromagnetic data were enhanced using the Center for exploration targeting (CET) grid enhancement technique to detect the main structures that control hydrothermal alterations in the study area. Principal Component Analysis (PCA) technique and ratios of spectral bands were applied to Landsat-8 data for mapping zones of hydrothermal alteration associated with gold mineralization. The traced structural elements from RTP, CET and Landsat-8 maps were statistically plotted and interpreted using rose diagrams. These diagrams indicated that the NW and NE trends are the most dominant in the RTP map and that the intersection zones obtained from the CET analysis are aligned along the NW-oriented trend. While the NNW, WNW and NE trends were the most dominant ones in Landsat maps. The integration of the results indicates several new high potential zones of gold and Cu/Ni mineralization types, which are mostly concentrated at the western part of the AHS zone. The hydrothermal alteration zones and associated gold mineralization are strongly linked to NW, NE, E-W and WNW trending lineaments. The approach used in this study can be applied to other parts of the ANS and other regions with similar geological conditions around the world.

Gold endowment in the evolution of the Allaqi-Heiani suture, Egypt: A synthesis of geological, structural, and space-borne imagery data

Field and structural studies supported by multi-sensor satellite-based imagery data are attempted to decipher the setting and controls of gold-bearing quartz veins in the central part of the greater Wadi Allaqi district in the South Eastern Desert of Egypt. Here, we assess the evolution of a complex part of the Neoproterozoic Allaqi-Heiani suture (∼730–700 Ma) with ample structures demonstrating the final Gondwanan collision. Abundant extensive N-trending folds, sinistral shear zones and oblate fold patterns and strain fabrics exemplifying an oblique convergence and torsion related to a regional ∼E-W compressional regime.In the Wadi Seiga area, the first order upright and overturned folds and thrusts (D1 structures) are mostly obliterated by intense NW-ward transpression, N-ward folding and transcurrent shearing assigned to as D2 and D3 structures. The domal crescentic and mushroom superimposed fold patterns suggest that Type 2 and Type 3 fold interference patterns were synchronous at some time during D2 or D3. Shear zones with abundant signs of plastic deformation and associated with quartz veins, within the axial planes of NNW- and NNE-trending F3 folds may imply gold mineralization during the third deformation stage (D3). The latter was supplemented or terminated by an episode of northward tectonic escape and slip reactivation of the pre-existing NW-trending faults.Integrated with field measurements, satellite-based radar and radiometer data abetted untangling the superimposed structures. Gold occurrences in the area are confined to domains of superimposed foliations, foliation deflection/steepening, foliation refraction and substantial strain partitioning, or at oppositely interfered plunging folds. The scattered small scale occurrences of ≤10 s g/t Au-bearing gold-quartz veins in such high strain zones, together with the lack of spatial association with a particular intrusive phase weigh with the metamorphic model for gold-only endowment in the area. The peak metamorphic conditions (amphibolite facies) in the central Allaqi-Heiani suture were attained not earlier than 630 Ma, during the collision of the Arabian-Nubian Shield with the Nile Craton to the west (provoked at 670–610 Ma). Gold-only endowment was complementing to the latest stages of deformation and terrane cooling between 610 and 585 Ma. Listvenitization of tectonized ophiolites and widespread sericite alteration of the island arc metavolcaniclastic rocks could have been promoted by the late-orogenic felsic intrusions (∼603 Ma) that filled oblate dilation spaces developed by structural transport to the north. The late orogenic timing of mineralization is one of many features making the Allaqi-Heiani suture comparable to well-documented global examples of accretionary metallogenic belts.

Unpaired ophiolite belts in the Neoproterozoic Allaqi-Heiani Suture, the Arabian-Nubian Shield: Evidences from magnetic data

Under oblique convergence, the intraoceanic subduction zones tend to have a tectonically-escaped accretionary prism, incorporates exotic materials from different sources and oceanic crust fragments. The western part of the Allaqi-Heiani suture (AHS) is the type locality for a significant accretionary prism extends for ⁓ 200 km; this prism is mainly metavolcanic, and metasedimentary rocks have interwoven serpentinite, marble blocks and intruded with different phases of plutonism. Geologic studies of the western part of the AHS have suggested the allochthonous origin for this complex, and terrane accretion happened earlier than ⁓ 700 Ma and subsequently thrusted over Gabgaba terrane to the south. Magnetic data was applied to follow the deep-seated structures and the connectivity between ophiolite belts in the AHS.Magnetic data unravelled the presence of a deep-seated strike-slip faults aligned along N–S and NNE trends underneath the northern belt, and this fault zone was erased at the surface by a major zone of the post-accretionary left-lateral strike-slip faults. In the southern part, E-W and WNW-ESE faults are dominant at the edge of the autochthon Gabgaba terrane. At greater depths, thrusts related to the emplacement of the northern belt disappeared, while those to the south aligned with the suture zone becomes significant. In conclusion, there are two ophiolite belts are located in the western part of the AHS; the southern belt is extended along the edge of the Gabgaba terrane to the south and the Eastern Desert (ED) to the north. This south belt is the western extension of a major arc-arc suture zone that extends to the east into the Arabian shield. The northern ophiolite belt was emplaced onto the southern part of the ED ⁓ 70 km away from the arc-arc to the south.

ASTER-based mapping of ophiolitic rocks: examples from the Allaqi–Heiani suture, SE Egypt

ABSTRACTThe Allaqi–Heiani suture is an ~250 km-long ophiolite-associated fold-and-thrust belt in the South Eastern Desert of Egypt. It comprises imbricate thrust sheets of ophiolites (serpentinite, amphibolite, metagabbro, and metabasalt) and island-arc metavolcanic/metasedimentary rocks. Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER)-based mafic–ultramafic indices are combined with band ratioing (BR) and relative absorption band-depth techniques to comprehensively map the ophiolitic rocks along the eastern part of the Allaqi–Heiani suture. The applied techniques are found efficient in delineating the ophiolitic rocks in the study area, despite their highly tectonized and variably carbonate-altered character. Identification and mapping of the exposed mafic–ultramafic rocks were possible using shortwave infrared (SWIR) band ratio 4/8 and thermal infrared (TIR) band ratio 12/14. The refined mafic index (MI) provides an efficient discrimination of the ophiolitic mafic rocks, while the mafic-ultramafic indices (M1 and M2) show high sensitivity to ophiolitic ultramafic units. The mafic–ultramafic indices enabled an efficient discrimination of mafic ophiolites from mafic island-arc rocks. Combining mafic indices and SWIR band ratio 4/8 allowed an enhanced delineation of the ophiolitic mafic–ultramafic rocks from other rock units in the area. Detailed mapping of the ophiolitic units in the ophiolitic terranes was obtained using the relative absorption band depth (RBD8) together with the MI and band ratio 4/8. This approach offers a quick method to discriminate among ophiolitic rock units and to map their structural contacts, i.e. in such hitherto poorly studied and hardly accessible areas.

Serpentinized Peridotites at the North Part of the Wadi Allaqi District (Egypt): Implications for the Tectono-Magmatic Evolution of Fore-arc Crust

The Neoproterozoic Allaqi-Heiani suture (800–700 Ma) in the south Eastern Desert of Egypt is the northernmost linear ophiolitic belt that defines an arc-arc suture in the Arabian-Nubian shield (ANS). The Neoproterozoic serpentinized peridotites represent a distinct lithology of dismembered ophiolites along the Allaqi-Heiani suture zone. The alteration of peridotites varies, some contain relicts of primary minerals (Cr-spinel and olivine) and others are extremely altered, especially along thrusts and shear zones, with development of talc, talc-carbonate and quartz-carbonate. The fresh cores of the chromian spinels are rimmed by ferritchromite and Cr-magnetite. The fresh chromian spinels have high Cr# (0.62 to 0.79), while Mg# shows wider variation (0.35–0.59). High Cr# in the relict chromian spinels and Fo content in the primary olivines indicate that they are residual peridotites after extensive partial melting. The studied ophiolitic upper mantle peridotites are highly depleted and most probably underwent high degrees of partial melting at a supra-subduction zone setting. They can be produced by up to ∼20%–22% dynamic melting of a primitive mantle source. The mineralogical and geochemical features of the studied rocks reflect that the mantle peridotites of the north part of the Wadi Allaqi district are similar to the fore-arc peridotites of a supra-subduction zone.

Compositional variations and tectonic settings of podiform chromitites and associated ultramafic rocks of the Neoproterozoic ophiolite at Wadi Al Hwanet, northwestern Saudi Arabia

Wadi Al Hwanet area in NW of Saudi Arabia is part of the Jebel Ess ophiolite constituting the northeastern part of the ∼700Ma Yanbu–Sol Hamed–Onib–Allaqi–Heiani suture of the northern Arabian–Nubian Shield. The mantle section of Wadi Al Hwanet ophiolite consists mainly of voluminous harzburgites overlain by thick, massive transition-zone dunites, and small-scale chromitite pods. The harzburgites and massive dunites are exceptionally fresh; primary magmatic textures and silicate minerals are still preserved. Two modes of podiform chromitites exist; small lensoidal pods (group I), and relatively large dike-like pods (group II). Geochemically, the former chromitite type contains chromian spinels with high Cr# (0.79–0.81) and displays a PGE-poor character, with steep negatively-sloped PGE distribution patterns, whereas the latter chromitite type contains chromian spinels with relatively lower Cr# (0.61–0.71) and is PGE-rich (up to 1000ppb), with positively-sloped PGE distribution patterns. The group II chromitites have much higher sulfide content than the group I suite. Parental melt compositions, in equilibrium with podiform chromitites, vary in Al2O3, FeO*/MgO and TiO2 contents from group I to group II chromitites, although both of them are in the range of the boninitic melts. The differences in the chromitites chemistry are most probably due to variable degrees of partial melting of the involved melts. Two stages of a magmatic activity were inferred for the chromitites genesis. The group I chromitites, of high Cr# of chromian spinels and PGE-poor negatively-sloped patterns, were precipitated in the first stage from a boninitic melt produced by a high degree of partial melting at a supra-subduction zone setting. The second chromitite-forming stage involves a relatively low degree of partial melting under high activities of sulfur and oxygen to produce the group II chromitites with enrichment in sulfides and PGE contents, possibly in a supra-subduction zone setting. In contrast to the chromitites, the harzburgites have low PGE contents, with characteristic unfractionated patterns, and low Cr# (0.46–0.57) of the chromian spinels suggesting mantle residues after low degrees of mantle melting beneath a mid-ocean ridge setting. Together with the entire plotting within the olivine-spinel mantle array, the similarity of olivine and spinel chemistry of dunites with those of harzburgites suggests a replacement origin for the dunites by the consumption of pyroxenes. It is likely that Wadi Al Hwanet mantle section was initially derived from a mid-ocean ridge environment and modified later, under a supra-subduction zone regime, to form podiform chromitites.

Syn- and post-accretionary structures in the Neoproterozoic Central Allaqi-Heiani suture zone, Southeastern Egypt

The Allaqi-Heiani suture (AHS) is the western part of the main Allaqi-Heiani-Gerf-Onib-Sol Hamed-Yanbu suture and represents one of the Neoproterozoic, arc–arc sutures in the Arabian-Nubian Shield (ANS). It separates the ca. 750Ma South Eastern Desert terrane in the north from the ca. 830–720Ma Gabgaba terrane in the south. The AHS is a deformed belt of ophiolitic rocks, syn-tectonic granitoids and metasediments. The central AHS zone is divided into three structural domains. The western domain (I) is characterized by NNE dipping thrusts and SSW-vergent folds. The central domain (II) includes upright tight to isoclinal NNW–SSE oriented folds and transpressional faults. The eastern domain (III) shows NNW–SSE oriented open folds. Structural analysis indicates that the area has a poly-phase deformation history involving at least two events. Event D1 was an N–S to NNE–SSW regional shortening generating the SSW-verging folds and the NNE dipping thrusts. Event D2 was an ENE–WSW shortening producing NNW–SSE oriented folds in the central and eastern parts of the study area and reactivating older thrusts with oblique-slip reverse fault movement. The tectonic evolution of the area involves two episodes of collision: an early collision between the South Eastern Desert terrane and the Gabgaba terrane along the AHS after the consumption of a basin floored by oceanic crust above a north-dipping subduction zone; and a later collision between East- and West-Gondwanas at ca. 750–650Ma, leading to the closure of the Mozambique Ocean. This collision deformed the AHS along N–S trending shortening zones and produced NW–SE and NE–SW oriented sinistral and dextral transpressional faults, respectively. The early collision episode is related to the terrane accretion during the early Pan-African orogen, while the later phase is related to a late Pan-African or Najd orogen.

Age constraints on the formation and emplacement of Neoproterozoic ophiolites along the Allaqi–Heiani Suture, South Eastern Desert of Egypt

Ophiolites are key components of the Neoproterozoic Arabian–Nubian Shield (ANS). Understanding when they formed and were emplaced is crucial for understanding the evolution of the ANS because their ages tell when seafloor spreading and terrane accretion occurred. The Yanbu–Onib–Sol Hamed–Gerf–Allaqi–Heiani (YOSHGAH) suture and ophiolite belt can be traced ∼ 600 km across the Nubian and Arabian shields. We report five new SHRIMP U–Pb zircon ages from igneous rocks along the Allaqi segment of the YOSHGAH suture in southernmost Egypt and use these data in conjunction with other age constraints to evaluate YOSHGAH suture evolution. Ophiolitic layered gabbro gave a concordia age of 730 ± 6 Ma, and a metadacite from overlying arc-type metavolcanic rocks yielded a weighted mean 206Pb/ 238U age of 733 ± 7 Ma, indicating ophiolite formation at ∼ 730 Ma. Ophiolite emplacement is also constrained by intrusive bodies: a gabbro yielded a concordia age of 697 ± 5 Ma, and a quartz-diorite yielded a concordia age of 709 ± 4 Ma. Cessation of deformation is constrained by syn- to post-tectonic granite with a concordia age of 629 ± 5 Ma. These new data, combined with published zircon ages for ophiolites and stitching plutons from the YOSHGAH suture zone, suggest a 2-stage evolution for the YOSHGAH ophiolite belt (∼ 810–780 Ma and ∼ 730–750 Ma) and indicate that accretion between the Gabgaba–Gebeit–Hijaz terranes to the south and the SE Desert–Midyan terranes to the north occurred as early as 730 Ma and no later than 709 ± 4 Ma.

The tectono-metamorphic evolution of the central part of the Neoproterozoic Allaqi–Heiani suture, south Eastern Desert of Egypt

The Precambrian basement of the Um El Tuyor area represents the central part of the ca. 750–720 Ma Allaqi–Heiani suture in the extreme south of the Egyptian Eastern Desert. It is composed of allochtonous ophiolitic thrust slices and detached sheets, island arc metavolcanic–volcanoclastic and metasedimentary assemblages, and syn- to post-orogenic granitoid intrusions. These rocks are collectively referred to as a fold-and-thrust belt in view of their deformational styles, and as ophiolites and their lithologic association are concerned. Structural analysis reveals three major deformation events, namely: (a) D1: an early thrust-and-fold system, through which overturned to recumbent folds and major thrusts were developed and later overprinted by a sinistral transpression, (b) D2: a NE–SW-compressional regime that deformed the D1 structures by major folds (F 2) and penetrative axial planar NW–SE foliations (S 2) and associated major NW–SE sinistral faults and shear zone, and (c) D3: a late ENE–WSW compressional strain increment expressed by a series of NNW–SSE striking folds and crenulations (F3). Related right-lateral NNW–SSE strike slip faults and discrete brittle–ductile shear zones indicate a later transcurrent shear which is best developed in the metasedimentary successions. Mineral associations and porphyroblast-matrix fabrics in the pelitic metasediments indicate three phases of regional metamorphism (M1 → M3), syn-kinematic with D1, D2 and D3 respectively. Based on the petrographic and microprobe data, the peak metamorphic conditions were attained at 534°–561 °C/5.26–6.20 kbar, under which garnet, staurolite and sillimanite coexisted during D2. Although associated with the most significant metamorphic event, D2 corresponds roughly to a late collisional stage (650–620 Ma) during which the accreted Pan-African terrains attached to the East Saharan Craton. This is further confirmed by the D2-related sinistral strike slip faults that affiliate to the ca. 630–540 Ma Najd strike slip fault system in the Arabian–Nubian Shield. Criteria including an anticipated fold–thrust vergence generally to northwesterly directions, discrete mesoscopic duplexes, shear bands, and minor folds and thrust zones all indicate that the ophiolites were emplaced roughly from northwest to southeast. However, deformation of the ophiolite basal contacts and associated fabrics argue that the northwestward rooting of the ophiolitic massif in the study area cannot be considered as a criterion for the original direction of tectonic transport, which was likely from north to south if the entire Allaqi–Heiani belt is considered. Such deviation is best attributed to rotation through deformation by the N–S Hamisana shear zone. This principally agrees with the tectonic evolution of the Arabian–Nubian Shield in south Egypt proposed previously, which includes an early N–S arc–arc collision followed by a stage of E–W crustal shortening.

Tracing along-strike structural continuity in the Neoproterozoic Allaqi-Heiani Suture, southern Egypt using principal component analysis (PCA), fast Fourier transform (FFT), and redundant wavelet transform (RWT) of ASTER data

We have used the visible and near infrared (VNIR) and short wave infrared (SWIR) bands of the advanced space-borne thermal emission and reflectance radiometer (ASTER) to trace along-strike structures in the ophiolite-decorated Neoproterozoic Allaqi-Heiani Suture in southern Egypt. The suture is E-trending in the west, changes to N-trending in its central part, and E-trending further east. The western part of the suture is defined by a fold and thrust belt dominated by an autochthon in the south followed by a southern, central and northern allochthons that were emplaced top-to-the-south. Tracing along-strike structures in the Allaqi-Heiani Suture has been a challenging task due to structural complexity and change of structural style. Principal component analysis (PCA), fast Fourier transform (FFT), and redundant wavelet transform (RWT) are used to identify ophiolite components such as talc carbonate schist, gabbro, and serpentinite as well as Neoproterozoic ductile structures to trace along-strike continuation in the Allaqi-Heiani Suture. (1) The PCA is performed on the 9 × 9 covariance matrix of the ASTER VNIR and SWIR bands. PC5 is selected because the ophiolite components and the Neoproterozoic ductile structures are best emphasized. (2) The FFT is performed on PC5 to reduce noises and mosaicking boundaries of the ASTER scenes. (3) The RWT is performed to enhance the contrast between ophiolite components and the surrounding rock types. This work helped in tracing structures along the Allaqi-Heiani Suture and revealed structural features that have not been identified before: (1) Some folds associated with the S-verging fold and thrust belts are periclinal folds that might be associated with nappe emplacement. (2) Truncation of structures associated with the S-verging fold and thrust belts occurs along NNW-trending sinistral strike-slip shear zones that swing at their extreme ends to become parallel to the truncated structures. (3) Presence of N-trending folds close to the NNW-trending sinistral strike-slip shear zones. (4) Presence of N- and NE-trending brittle faults younger than the Neoproterozoic ductile structures. Nappes identified in the west are traced into the central part of the suture allowing for subsequent modification by younger structures. The difference in structural styles along the suture might be due to development of frontal and lateral ramps along the traces of the orogen front. This was followed by superimposition of E–W shortening to produce N-trending folds that are localized close to the lateral ramps. Subsequently, N- and NE-trending brittle faults were developed and deformed early Neoproterozoic ductile structures.

Spectral analysis of ASTER data covering part of the Neoproterozoic Allaqi-Heiani suture, Southern Egypt

This paper examines the potential of ASTER data in mapping the geology of an arid area in Africa using spectral analysis techniques. Equipped with a relatively broad spectral range, fine spatial resolution, and a large number of bands, ASTER data are especially attractive to many geological researchers in Africa, due to their open availability and associated minimal costs for academic use. The spectral analysis of an ASTER dataset covering part of the Neoproterozoic Allaqi-Heiani suture, Southern Egypt has demonstrated it to be more effective in detecting lithological units than traditional multispectral analysis procedures, such as the maximum likelihood classifier. The effectiveness of these spectral analysis techniques used in this study lies in their ability to compare a pixel spectrum with the spectra of known pure materials, extracted from the spectral endmember selection procedures, including minimum noise factor (MNF), pixel purity index (PPI) and n-dimensional visualization. Among the three spectral analysis algorithms employed, spectral angle mapping (SAM) and spectral feature fitting (SFF), produced accurate classifications that were close to the ground reference data. Linear spectral unmixing (LSU), although failing to achieve a satisfactory overall classification accuracy, demonstrated potential in depicting intercalation of layered rocks. It is concluded that these spectral analysis techniques have proven to be useful tools in geological mapping of the arid regions in Africa, with their abilities to capitalize on the enhanced spectral information of ASTER data.

Structural evolution of the Neoproterozoic Western Allaqi–Heiani suture, southeastern Egypt

The Neoproterozic Allaqi–Heiani suture in southeastern Egypt is the western extension of the Allaqi–Heiani–Onib–Sol Hamed–Yanbu suture that represents one of arc–arc sutures in the Arabian–Nubian Shield. It extends for more than 250 km from the N-trending Hamisana Shear Zone in the east to Lake Nasser in the west. It separates the 750-Ma-old Southeastern Desert terrane in the north from the 830–720-Ma-old Gabgaba terrane in the south. Structural studies supported by remote sensing investigations including Landsat Thematic Mapper (TM) images show that the western part of Allaqi–Heiani suture zone constitutes three S- to SW-verging nappes in the north overriding an autochthonous block to the southwest. SW-verging, low-angle thrust sheets and folds, forming a 10-km wide imbrication fan, dominate the northern upper nappe (northern allochthon). These folds and thrusts deform shelf metasedimentary rocks including psammitic metasediments, marble and subordinate conglomerate. Volcanic rocks including rhyolites and felsic tuffs dominate the upper part of the northern allochthon. The contacts between the metasedimentary rocks on the one hand and the rhyolites and felsic tuffs on the other hand are extrusive. This allochthon overrides an internally deformed nappe (central allochthon) dominated by arc and ophiolitic assemblages now preserved as felsic and mafic schist, talc schist, serpentinites, and metagabbros. This allochthon is characterized by NW-trending, upright folds, which deform the earlier sub-horizontal structures. The structurally lower nappe (southern allochthon) is dominated by NNE-trending folds which deform amphibolite facies schistose metavolcanic and metavolcanoclastic rocks. The NNE-trending folds deform earlier NW-trending folds to produce crescentic dome interference pattern with well-developed NE-trending axial planar cleavage, consistent with ESE–WNW bulk shortening. The southernmost structural unit is an autochthonous block dominated by arc-related volcanic and volcanoclastic rocks. This block has suffered only minor deformation compared to the nappes to its north. The consistent SW-vergence of the structures indicates tectonic transport from northeast to southwest, followed by ESE–WNW shortening similar to that found in the Hamisana Shear Zone, further east. Collision between the Gabgaba–Gebeit terrane and the Southeastern Desert terrane along the Allaqi–Heiani suture, after the consumption of a marginal basin probably over an N-dipping subduction zone, led to the formation of EW- to NW-trending folds and thrusts. This was followed by ESE–WNW tectonic shortening to form NNE-trending folds, which are found to be overprinting the earlier structures. This latest shortening might be due to collision between the Arabian–Nubian Shield and the Saharan Metacraton along an N-trending arc–continent suture represented farther south by the Keraf suture.

Orientation of late Precambrian sutures in the Arabian-Nubian shield

Recent tectonic models have resulted in conflicting descriptions of how the late Precambrian sutures of the Arabian-Nubian shield extend into northeast Africa. The Hamisana shear zone in northeastern Sudan is critical to this discussion because it truncates and disrupts two sutures, the Allaqi-Heiani and the Onib-Sol Hamed. Analysis of field structural data, Thematic Mapper imagery, and Rb-Sr and U-Pb geochronology suggests that the Allaqi-Heiani suture is the western extension of the Onib-Sol Hamed suture and that both make up the exposed parts of a far-traveled, polydeformed ophiolitic nappe complex. Subsequent deformation localized in the Hamisana shear zone disrupted this nappe and displaced the suture between 660 and 550 Ma during regional deformation associated with the Najd fault system. These results indicate that at least one suture extends westward into the interior of northern Africa.