Reinhard O. Greiling

A structural synthesis of the Proterozoic Arabian-Nubian Shield in Egypt

Detailed structural geological and related studies were carried out in a number of critical areas in the Proterozoic basement of eastern Egypt to resolve the structural pattern at a regional scale and to assess the general characteristics of tectonic evolution, orogeny and terrane boundaries. Following a brief account of the tectonostratigraphy and timing of the orogenic evolution, the major structural characteristics of the critical areas are presented. Collisional deformation of the terranes ended about 615-600 Ma ago. Subsequent extensional collapse probably occurred within a relatively narrow time span of about 20 Ma (575 – 595 Ma ago) over the Eastern Desert and was followed by a further period of about 50 Ma of late to post-tectonic activity. The regional structures originated mainly during post-collisional events, starting with those related to extensional collapse (molasse basin formation, normal faulting, generation of metamorphic core complexes). Subsequent NNW-SSE shortening is documented by large-scale thrusting (towards the NNW) and folding, distributed over the Eastern Desert, although with variable intensity. Thrusts are overprinted by transpression, which was localized to particular shear zones. Early transpression produced, for example, the Allaqi shear zone and final transpression is documented in the Najd and Wadi Kharit-Wadi Hodein zones. Two terrane boundaries can be defined, the Allaqi and South Hafafit Sutures, which are apparently linked by the high angle sinistral strike-slip Wadi Kharit-Wadi Hodein shear zone with a tectonic transport of about 300 km towards the W/NW. In general, the tectonic evolution shows that extensional collapse is not necessarily the final stage of orogeny, but may be followed by further compressional and transpressional tectonism. The late Pan-African high angle faults were reactivated during Red Sea tectonics both as Riedel shears and normal faults, where they were oriented favourably with respect to the actual stress regime.

Post-collisional shortening in the late Pan-African Hamisana high strain zone, SE Egypt: field and magnetic fabric evidence

Abstract The Hamisana zone (HZ) is one of the major high strain zones of the Pan-African (Neoproterozoic) Arabian–Nubian shield (ANS). It trends broadly N–S from northern Sudan into southeastern Egypt and meets the present Red Sea coast at ≈23°N. The HZ has been the subject of controversy with regard to its importance for the Pan-African structural evolution. Interpretations range from a suture zone, a regional shear zone, or a large-scale transpressional wrench fault system. In this study, we characterize the nature of the high strain deformation by applying the anisotropy of magnetic susceptibility method along with field and microstructural investigations. These investigations demonstrate that deformation in the HZ is dominated by pure shear under upper greenschist/amphibolite grade metamorphic conditions, producing E–W shortening, but with a strong N–S-extensional component. This deformation also led to folding of regional-scale thrusts (including the base of ophiolite nappes such as Gabal Gerf and Onib). Consequently, the high strain deformation is younger than ophiolite emplacement and suturing of terranes. A weak subsequent overprint was mostly non-coaxial. It took place under considerably lower temperature and led to a minor NE–SW-trending, dextral wrench fault. Although it is of only local importance this fault may be itself a conjugate relative to the prominent NW–SE-trending sinistral Najd faults in the northern ANS. Therefore, the HZ is dominated by late orogenic compressional deformation and cannot be related to either large-scale transpressional orogeny or major escape tectonics.

A Quantitative structural study of late pan-african compressional deformation in the central eastern desert (Egypt) during Gondwana assembly

Abstract The Arabian-Nubian-Shield (ANS) is composed of a number of island arcs together with fragments of oceanic lithospere and minor continental terranes. The terranes collided with each other until c. 600 Ma ago. Subsequently, they were accreted onto West Gondwana, west of the present River Nile. Apart from widespread ophiolite nappe emplacement, collisional deformation and related lithospheric thickening appear to be relatively weak. Early post-collisional structures comprise not only extensional features such as fault-bounded (molasse) basins and metamorphic core complexes, but also major wrench fault systems, and thrusts and folds. The Hammamat Group was deposited in fault-bounded basins, which formed due to N-S to NW-SE directed extension. Hammamat Group sediments were intruded by late orogenic granites, dated as c. 595 Ma old. A NNW-SSE-oriented compression prevailed after the deposition of the Hammamat Sediments. This is documented by the presence of NW-verging folds and SE-dipping thrusts that were refolded and thrusted in the same direction. Restoration of a NNW-SSE- oriented balanced section across Wadi Queih indicates more than 25% of shortening. Transpressional wrenching related to the Najd Fault System followed this stage. The wrenching produced NW-SE sinistral faults associated with positive flower structures that comprise NE-verging folds and SW-dipping thrusts. Section restoration across these late structures indicates 15 17% shortening in the NE-SW direction. At a regional scale, the two post-Hammamat compressional phases produced an interference pattern with domes and basins. It can be shown that the Najd Fault System splays into a horsetail structure in the Wadi Queih area and loses displacement towards N and NW. The present study shows a distinct space and time relationship between deposition of Hammamat Group/late-Pan-African clastic sediments and late stages of Najd Fault wrench faulting: Hammamat deposition is followed by two episodes of compression, with the second episode being related to Najd Fault transpression. Therefore, the Hammamat sediments do not represent the latest tectonic feature of the Pan-African orogeny in the ANS. The latest orogenic episodes were the two successive phases of compression and transpression, respectively. It is speculated that extension during (Hammamat) basin formation was sufficiently effective to reduce the thickness of the orogenic lithosphere until it became gravitationally stable, and incapable of further gravitational deformation.

Tectonic evolution of the Aravalli orogen (NW India): an inverted Proterozoic rift basin?

The Aravalli mountain range (AMR) in the northwestern part of the Indian Peninsula consists of two main Proterozoic metasedimentary and metaigneous sequences, the Aravalli and Delhi Supergroups, respectively, which rest over the Archaean gneissic basement. A synthesis and reinterpretation of the available geological, geochronological and geophysical data, including results of own field work and geophysical interpretations pertaining to the AMR, indicate its origin as an inverted basin: rifting into granitoid basement began ca. 2.5; Ga ago with Aravalli passive rifting (ca. 2.5–2.0 Ga) and Delhi active rifting (ca. 1.9–1.6 Ga). Associated mafic igneous rocks show both continental and oceanic tholeiitic geochemistry and are comparable with Phanerozoic, rift-related magmatic products. Available data showed no conclusive evidence for oceanic lithoshere and island-arc/active margin magmatic activity in the AMR. Subsequent inversion and orogeny (Delhi orogeny, ca. 1.5-1.4 Ga) lead to complex deformation and metamorphism. Only in the western and central zones has the basement been involved in this mid-Proterozoic (Delhi) deformation, whereas it is unaffected in the eastern part, except for local shear zones mainly along the basement/cover interface. The grade of metamorphism increases from the greenschist facies in the east to the amphibolite facies in the west with local HP assemblages. These latter are explained by rapid burial and exhumation of thin and cool continental lithosphere. Subsequently, during a final, mild phase of inversion, the Vindhyan basins consisting mainly of sandstones, limestones and shales, flanking the AMR formed which are comparable to foreland basins. The tectonic evolution of the AMR is therefore interpreted as an example of a major inverted continental rift and of a Proterozoic intra-continental orogen.

Thrust-related very low grade metamorphism in the marginal part of an orogenic wedge, Scandinavian Caledonides

The pattern and characteristics of thrust-related very low grade metamorphism in the marginal part of an orogenic wedge have been investigated by combining a clay mineral crystallinity survey with detailed structural mapping of the thin-skinned foreland thrust belt along the external part of the Scandinavian Caledonides. This external part is composed of late Neoproterozoic to Ordovician sedimentary sequences of the autochthonous cover and Lower Allochthon, which are overlain by the higher Caledonian nappes (Middle and Upper Allochthonous units). Stages of the Scandian phase of thrust wedge development are described which are related to the very low grade metamorphic history. The initial stage involved the emplacement of cooled nappes belonging to the Middle and Upper Allochthons, with very low grade peak metamorphic conditions attained within the underlying Lower Allochthon (cover) sediments as they were progressively buried and deformed beneath the thrust wedge. During this initial emplacement the isotherms are considered to have been undisturbed and dipping parallel to the wedge surface. The following stages of wedge development consisted of extensive post-metamorphic imbrication of the underthrusted cover sediments, with a transition from basal accretion and uplift at the rear, to accretion and forward propagation at the wedges toe. During accretion into the wedge, the externally dipping isograd surfaces were extensively displaced from deeper levels toward higher tectonic horizons. The last stage of wedge development considered here was characterized by late out-of-sequence thrusting with enhanced (epizonal) metamorphic grades developed in the vicinity of the fault zones, which either resulted from further displacement of the isograds toward higher levels, or from localized heating via intense fluid activity. Overall, the pattern of metamorphic grade, fabric relationships, and physical calculations of heat transfer based on the geometry of the thrust wedge, suggest that neither inverted temperature gradients nor shear heating were likely causes of the metamorphism in this flat-lying part of the orogenic wedge. The description of inverted very low grade metamorphic isograds in other marginal parts of the Scandinavian Caledonides, which have been previously attributed to either the rapid emplacement of hot thrust nappes, or the effects of dissipative shear heating, are discussed in terms of variations in both the critical wedge geometry and its controlling boundary conditions.

The orogenic wedge in the central Scandinavian Caledonides: Scandian structural evolution and possible influence on the foreland basin

Abstract The orogenic wedge of the central Scandinavian Caledonides - overlying the Baltic Shield without a substantial foreland basin - is built up of a stack of nappes, which are divided here into three mechanically distinct tiers: A: Lower Allochthon with exclusively Scandian (Middle to Late Silurian) structures and metamorphism, B: Middle Allochthon and Seve units of Upper Allochthon with traces of Finnmarkian (Early Ordovician) events, and C: overlying exotic terranes (Koli units of Upper Allochthon, Uppermost Allochthon). Detailed structural work documented shear criteria at the tier B-C (Seve-Koli) boundary, which consistently indicate a down-dip, top-to-the-west movement at a regional scale and immediately after peak PT conditions (c. Late Wenlockian). Subsequently, the Seve-Koli boundary was deformed by regional folds, caused by the stacking of thrust systems in the underlying Lower Allochthon. Due to lithology and early cooling, the tier B of the Caledonian orogenic wedge is dominated by strong ...

Subduction of continental margins and the uplift of high-pressure metamorphic rocks

Abstract The mechanism by which high-pressure metamorphosed continental material is emplaced at high structural levels is a major unsolved problem of collisional orogenesis. We suggest that the emplacement results from partial subduction of the continental margin which, because of its high flexural rigidity, produces a rapid change in the trajectory of the descending slab. We assume a two-fold increase in effective elastic thickness of the lithosphere as the continental margin approaches the subduction zone, and calculate the flexural profile of a thin plate for progressive downward migration of the zone of increased rigidity. We assess the effect of changes in the flexural profile on the overlying accretionary prism and mantle wedge as the continent approaches by estimating the extra stresses that are imposed on the wedge due to the bending moment exerted by the continental part of the plate. The wedges overlying the subduction zones, and the subducting slab itself, experience substantial extra compressional stress at depths of around 100 km, and extensional stress at shallower depths, as the continental margin passes through the zone of maximum curvature. The magnitudes of such extra stresses are probably adequate to effect significant deformation of the wedge and/or the descending plate, and are experienced in a time interval of less than 5 m.y. for typical subduction rates. The spatial variation of yield stresses in the region of the wedge and descending slab indicates that much of this deformation may be taken up in the crustal part of the descending slab, which is the weakest region in the deeper parts of the subduction zone. This may result in rapid upward migration of the crust of the partially subducted continental margin, against the flow of subduction. High-pressure metamorphosed terranes emplaced by the mechanism envisaged in this paper would be bounded by thrust faults below and normal faults above. Movement on the faults would have been coeval, and would have resulted in rapid unroofing of the high-pressure terranes, synchronous with arrival of the continental margin at the subduction zone and, therefore, relatively early in the history of a collisional orogen.

Mesoproterozoic dyke swarms in foreland and nappes of the central Scandinavian Caledonides: structure, magnetic fabric, and geochemistry

As an example of microstructural and magnetic fabric evolution, and geochemistry of mafic dykes during a subsequent orogenic overprint, a major Mesoproterozoic dyke complex in Scandinavia, the Vasterbotten complex of the Central Scandinavian Dolerite Group, is traced westwards into the crystalline nappes of the early Phanerozoic Caledonian orogen. Using geophysical, field, microscopic, magnetic and geochemical information, dykes and sills are characterized, and their overprint during Caledonian orogeny documented. The Vasterbotten complex is composed of sets of dykes, trending NE–SW, NW–SE and WNW–ESE, respectively. Similar dykes are exposed in allochthonous positions (Lower and Middle allochthons) in the Caledonian fold-and-thrust belt. The autochthonous dykes are generally undeformed and retain both their primary texture and mineralogy. Chilled margins are well preserved. In the Caledonian Lower and Middle allochthons, similar dykes in crystalline basement rocks are progressively faulted and sheared when proceeding from the marginal to the interior parts of the orogen. Dyke margins are more likely to be sheared than the interior parts of dykes. In the Lower Allochthon, under very low- and low-grade metamorphic conditions, dykes are distinctly less competent than granitic host rocks. Thick dykes are more competent than gneisses; thin dykes do not show such competence contrasts. In the Middle Allochthon, metre-scale dykes with patches of altered plagioclase phenocrysts can still be discerned in low-strain domains. Highly sheared dykes are drawn out to thin layers of centimetre thickness. Dykes are deformed together with the crystalline country rocks under greenschist-grade metamorphic conditions without major competence contrasts. Magnetic fabrics show an evolution similar to the silicate mineral fabrics. The magnetic fabrics in the dykes are transformed successively from ferromagnetic–magmatic in the Autochthon to ferromagnetic deformational in the Lower Allochthon and, finally, paramagnetic deformational in the Middle Allochthon. As a consequence, the magnetic susceptibility decreases for several orders of magnitude. Geochemically, the dykes are dominantly sub-alkaline basalts typical for continental tholeiites and can be distinguished from the Neoproterozoic dykes in the Sarv-Nappe equivalents (highest part of the Middle Allochthon), which show a more MORB-like (E-MORB) magmatic signature. Preliminary age information from a dyke in the Lower Allochthon of the Borgefjell area and the Middle Allochthon is consistent with the assumption that these dykes are time equivalent with the Central Scandinavian Dolerite Group. Therefore, the studied dykes may represent an extension of the Vasterbotten complex or a new complex of the Central Scandinavian Dolerite Group. According to section restorations, the Caledonian allochthons were situated further WNW relative to their present position, and, originally, the mafic dykes cut across all of the Fennoscandian lithosphere, at least to the present Atlantic margin and the earlier passive margin of the Baltica terrane. As a consequence, these dykes may provide a link for pre-Caledonian and pre-Grenvillian plate reconstructions.

The pre-Pan-African deformed granite cycle of the Gabal El-Sibai swell, Eastern Desert, Egypt

Abstract The Gabal El-Sibai area is a structural high in which variably deformed granites and gneisses are structurally exposed underneath an overthrusted Pan-African ophiolitic melange and intruded by undeformed Pan-African granites. The deformed granites constitute a complete granite cycle starting with autochthonous to parautochthonous, calc-alkaline I-type granites and ending with post-tectonic, within-plate alkaline granites. They indicate that the Sibai infrastructure represents a pre-Pan-African continental crust incorporated in the Pan-African orogenic belt.

Early- to mid-Silurian extrusion wedge tectonics in the central Scandinavian Caledonides

We present evidence for an extrusion wedge in the Scandian fold-thrust belt of the central Scandinavian Caledonides (Seve nappe complex). Rb-Sr multimineral geochronology in synkinematic assemblages indicates simultaneous movements at the normal-sense roof shear zone and at the reverse-sense floor shear zone between 434 Ma and 429 Ma. A Sm-Nd age of 462 Ma from a mylonitic garnet mica schist documents prograde garnet growth and possible incipient subduction. Pressure-temperature pseudosection calculations provide evidence for eclogite facies metamorphic conditions and nearly isothermal decompression at ∼670 ± 50 °C from 17.5 to 14.5 kbar in garnet-kyanite mica schists during reverse-sense shearing, and from 15 to 11 kbar in garnet mica schists during normal-sense shearing. These data and the presence of decompression-related pegmatites dated at 434 Ma and 429 Ma indicate that the Seve nappes form a large-scale extrusion wedge. This wedge extends along strike for at least 150 km and marks an early stage of ultrahigh-pressure metamorphism, exhumation, and orogenic wedge formation in this part of the Scandinavian Caledonides predating the major, post–415 Ma ultrahigh-pressure exhumation processes in southwestern Norway.