Tectonics of the Afar Depression: A review and synthesis

This article outlines geomorphological and tectonic elements of the Afar Depression, and discusses its evolution. A combination of far-field stress, due to the convergence of the Eurasian and Arabian plates along the Zagros Orogenic Front, and uplift of the Afar Dome due to a rising mantle plume reinforced each other to break the lithosphere of the Arabian–Nubian Shield. Thermal anomalies beneath the Arabian–Nubian Shield in the range of 150 °C–200 °C, induced by a rising plume that mechanically and thermally eroded the base of the mantle lithosphere and generated pulses of prodigious flood basalt since ∼30 Ma. Subsequent to the stretching and thinning the Afar Dome subsided to form the Afar Depression. The fragmentation of the Arabian–Nubian Shield led to the separation of the Nubian, Arabian and Somalian Plates along the Gulf of Aden, the Red Sea and the Main Ethiopian Rift. The rotation of the intervening Danakil, East-Central, and Ali-Sabieh Blocks defined major structural trends in the Afar Depression. The Danakil Block severed from the Nubian plate at ∼20 Ma, rotated anti-clockwise, translated from lower latitude and successively moved north, left-laterally with respect to Nubia. The westward propagating Gulf of Aden rift breached the Danakil Block from the Ali-Sabieh Block at ∼2 Ma and proceeded along the Gulf of Tajura into the Afar Depression. The propagation and overlap of the Red Sea and the Gulf of Aden along the Manda Hararo–Gobaad and Asal–Manda Inakir rifts caused clockwise rotation of the East-Central Block. Faulting and rifting in the southern Red Sea, western Gulf of Aden and northern Main Ethiopian Rift superimposed on Afar. The Afar Depression initiated as diffused extension due to far-field stress and area increase over a dome elevated by a rising plume. With time, the lithospheric extension intensified, nucleated in weak zones, and developed into incipient spreading centers.     

Magmatism-dominated intracontinental rifting in the Mesoproterozoic: The Ngaanyatjarra Rift,central Australia

The Late Mesoproterozoic (1085–1040 Ma) Ngaanyatjarra Rift, previously referred to as the Giles Event, is the dominant component of the Warakurna Large Igneous Province (LIP) that affected much of central and western Australia. This rift is well preserved and provides excellent examples of rift structure at a variety of crustal levels and times in the rift's evolution. Geological knowledge is integrated with geophysical interpretations and models to understand the crustal structure and evolution of this rift. Two phases are identified: an early rift stage (1085–1074 Ma) that is characterised by voluminous magmatism within the upper crust and relatively little tectonic deformation; and a late rift stage that is characterised by tectonic deformation, synchronous with the deposition of a thick pile of volcanic and sedimentary rocks (1074–1040 Ma). Compared to modern rift examples, this rift is unusual in that the crust was thickened by ~ 15 km and overall extension was very limited. However, its structure and evolution are very similar to the near-contemporaneous Midcontinent Rift, which shows the addition of a similar quantity of magmatic material as well as crustal thickening and limited extension. For these Mesoproterozoic rifts, we suggest that magmatism was the dominant process, and that the extension observed was a response to magmatism-induced crustal thickening and the gravitational collapse of the crustal column. Other Proterozoic rifts show similar characteristics (e.g. Transvaal Rift), whereas most Phanerozoic rifts are dissimilar, showing instead a dominance of extension, with magmatism largely a result of this extension. This change in the style of rifting from the Precambrian to the Phanerozoic may relate to the influence of a typically cooler and stronger lithosphere, which has caused stronger strain localisation and a greater role for extension as the controlling factor in rift evolution.     

Density structure of the cratonic mantle in southern Africa: 1. Implications for dynamic topography

The origin of high topography in southern Africa is enigmatic. By comparing topography in different cratons, we demonstrate that in southern Africa both the Archean and Proterozoic blocks have surface elevation 500–700 m higher than in any other craton worldwide, except for the Tanzanian Craton. An unusually high topography may be caused by a low density (high depletion) of the cratonic lithospheric mantle and/or by the dynamic support of the mantle with origin below the depth of isostatic compensation (assumed here to be at the lithosphere base). We use free-board constraints to examine the relative contributions of the both factors to surface topography in the cratons of southern Africa. Our analysis takes advantage of the SASE seismic experiment which provided high resolution regional models of the crustal thickness.We calculate the model of density structure of the lithospheric mantle in southern Africa and show that it has an overall agreement with xenolith-based data for lithospheric terranes of different ages. Density of lithospheric mantle has significant short-wavelength variations in all tectonic blocks of southern Africa and has typical SPT values of ca. 3.37–3.41 g/cm³ in the Cape Fold and Namaqua–Natal fold belts, ca. 3.34–3.35 g/cm³ in the Proterozoic Okwa block and the Bushveld Intrusion Complex, ca. 3.34–3.37 g/cm³ in the Limpopo Belt, and ca. 3.32–3.33 g/cm³ in the Kaapvaal and southern Zimbabwe cratons.The results indicate that 0.5–1.0 km of surface topography, with the most likely value of ca. 0.5 km, cannot be explained by the lithosphere structure within the petrologically permitted range of mantle densities and requires the dynamic (or static) contribution from the sublithospheric mantle. Given a low amplitude of regional free air gravity anomalies (ca. + 20 mGal on average), we propose that mantle residual (dynamic) topography may be associated with the low-density region below the depth of isostatic compensation. A possible candidate is the low velocity layer between the lithospheric base and the mantle transition zone, where a temperature anomaly of 100–200 °C in a ca. 100–150 km thick layer may explain the observed reduction in Vs velocity and may produce ca. 0.5–1.0 km to the regional topographic uplift.     

Thermo-mechanical modeling of the obduction process based on the Oman Ophiolite case

Obduction emplaces regional-scale fragments of oceanic lithosphere (ophiolites) over continental lithosphere margins of much lower density. For this reason, the mechanisms responsible for obduction remain enigmatic in the framework of plate tectonics. We present two-dimensional (2D) thermo-mechanical models of obduction and investigate the possible dynamics and physical controls of this process. Model geometry and boundary conditions are based on available geological and geochronological data and numerical modeling results are validated against petrological and structural observations of the Oman (Semail) Ophiolite. Our model reproduces the stages of oceanic subduction initiation away from the Arabian margin, the emplacement of the Oman Ophiolite on top of it, and the domal exhumation of the metamorphosed margin through the ophiolitic nappe. A systematic study indicates that 350–400 km of bulk shortening provides the best fit for both maximum pressure–temperature conditions of the metamorphosed margin (1.5–2.5 GPa/450–600 °C) and the dimension of the ophiolitic nappe (~ 170 km width). Our results confirm that a thermal anomaly located close to the Arabian margin (~ 100 km) is needed to initiate obduction. We further suggest that a strong continental basement rheology is a prerequisite for ophiolite emplacement.     

Crustal and lithospheric thinning beneath the seismogenic Kachchh rift zone,Gujarat (India): Its implications toward the generation of the 2001 Bhuj earthquake sequence

A high-resolution passive seismic experiment in the Kachchh rift zone of the western India has produced an excellent dataset of several thousands teleseismic events. From this network, 500 good teleseismic events recorded at 14 mobile broadband sites are used to estimate receiver functions (for the 30–310° back-azimuth ranges), which show a positive phase at 4.5–6.1 s delay time and a strong negative phase at 8.0–11.0 s. These phases have been modeled by a velocity increase at Moho (i.e. 34–43 km) and a velocity decrease at 62–92 km depth. The estimation of crustal and lithospheric thicknesses using the inversion of stacked radial receiver functions led to the delineation of a marked thinning of 3–7 km in crustal thickness and 6–14 km in lithospheric thickness beneath the central rift zone relative to the surrounding un-rifted parts of the Kachchh rift zone. On an average, the Kachchh region is characterized by a thin lithosphere of 75.9 ± 5.9 km. The marked velocity decrease associated with the lithosphere–asthenoshere boundary (LAB), observed over an area of 120 km × 80 km, and the isotropic study of xenoliths from Kachchh provides evidence for local asthenospheric updoming with pockets of partial melts of CO₂ rich lherzolite beneath the Kachchh seismic zone that might have caused by rifting episode (at 88 Ma) and the associated Deccan thermal-plume interaction (at 65 Ma) episodes. Thus, the coincidence of the area of the major aftershock activity and the Moho as well as asthenospheric upwarping beneath the central Kachchh rift zone suggests that these pockets of CO₂-rich lherzolite partial melts could perhaps provide a high input of volatiles containing CO₂ into the lower crust, which might contribute significantly in the seismo-genesis of continued aftershock activity in the region. It is also inferred that large stresses in the denser and stronger lower crust (at 14–34 km depths) induced by ongoing Banni upliftment, crustal intrusive, marked lateral variation in crustal thickness and related sub-crustal thermal anomaly play a key role in nucleating the lower crustal earthquakes beneath the Kachchh seismic zone.     

Denudation history of the Bocaina Plateau,Serra do Mar,southeastern Brazil: Relationships to Gondwana breakup and passive margin development

The Bocaina Plateau, which is situated on the eastern flank of the continental rift of southeastern Brazil, is the highest part of the Serra do Mar. Topographic relief in this area is suggested to be closely related to its complex tectono-magmatic evolution since the breakup of Western Gondwana and opening of the South Atlantic Ocean. Apatite fission track ages and track length distributions from 27 basement outcrops were determined to assess these hypotheses and reconstruct the denudation history of the Bocaina Plateau. The ages range between 303 ± 32 and 46 ± 5 Ma, and are significantly younger than the stratigraphic ages. Mean track lengths vary from 13.44 ± 1.51 to 11.1 ± 1.48 μm, with standard deviations between 1.16 and 1.83 μm. Contrasting ages within a single plateau and similar ages at different altitudes indicate a complex regional tectonothermal evolution. The thermal histories inferred from these data imply three periods of accelerated cooling related to the Early Cretaceous continental breakup, Early Cretaceous alkaline magmatism, and the Paleogene evolution of the continental rift of southeastern Brazil. The oldest fission track ages (> 200 Ma) were obtained in the Serra do Mar region, suggesting that these areas were a long-lived source of sediments for the Paraná, Bauru, and Santos basins.     

Significance of weathering and regolith/landscape evolution for mineral exploration in the NE Albany-Fraser Orogen,Western Australia

The Albany-Fraser Orogen (AFO), southeast Western Australia, is an underexplored, deeply weathered regolith-dominated terrain that has undergone complex weathering associated with various superimposed climatic events. For effective geochemical exploration in the AFO, integrating landscape evolution with mineralogical and geochemical variations of regolith and bedrock provides fundamental understanding of mechanical and hydromorphic dispersion of ore and pathfinder elements associated with the different weathering processes.In the Neale tenement, northeast of the AFO, a residual weathering profile that is 20-55 m thick was developed under warm and humid climatic conditions over undulating Proterozoic sheared granitoids, gneisses, schists and Au-bearing mafic rocks. From the base, the typical weathering profile consists of saprock, lower ferruginous saprolite, upper kaolinitic saprolite and discontinuous silcrete duricrust or its laterally coeval lateritic residuum. These types of duricrusts change laterally into areas of poorly-cemented kaolinitic grits or loose lateritic pisoliths and nodules.Lateritic residuum probably formed on remnant plateaus and was transported mechanically under arid climatic conditions over short distances, filling valleys to the southeast. Erosion of lateritic residuum exposes the underlying saprolite and, together with dilution by aeolian sands, constitutes the transported overburden (2-25 m thick). The reworked lateritic materials cover the preserved silcrete duricrusts in valleys. The lower ferruginous saprolite and lateritic residuum are well developed over mafic and sulphide-bearing bedrocks, where weathering of ferromagnesian minerals and sulphides led to enrichment of Fe, Cu, Ni, Cr, Co, V and Zn in these units. Kaolinitic saprolite and the overlying pedogenic silcrete are best developed over alkali granites and quartzofeldspathic gneisses, which are barren in Au and transition elements, and enriched in silica, alumina, rare earth and high field strength elements.A residual Au anomaly is formed in the lower ferruginous saprolite above a Au -bearing mafic intrusion at the Hercules prospect, south of the Neale tenement, without any expression in the overlying soil (< 20 cm). Conversely, a Au anomaly is recorded in the transported cover, particularly in the uppermost 3 m at the Atlantis prospect, 5 km southwest of the Hercules prospect. No anomalies have been detected in soils using five different size fractions (> 2,000 μm, 2,000-250 μm, 250-53 μm, 53-2 μm and < 2 μm). Therefore, soil cannot be efficiently applied as a reliable sampling medium to target mineralization at the Neale tenement. This is because mechanical weathering was interrupted by seasonal periods of intensive leaching under the present-day surface conditions and/or dilution by recently deposited aeolian sediments which obscure any signature of a potential Au anomaly in soils. Therefore, surface soil sampling should extend deeper than 20 cm to avoid dilution by aeolian sands and seasonal leaching processes. Regolith mapping and the distinction between the residual and transported weathering products are extremely significant to follow the distal or proximal mineralization.     

Long wavelength gravity anomalies over India: Crustal and lithospheric structures and its flexure

Long wavelength gravity anomalies over India were obtained from terrestrial gravity data through two independent methods: (i) wavelength filtering and (ii) removing crustal effects. The gravity fields due to the lithospheric mantle obtained from two methods were quite comparable. The long wavelength gravity anomalies were interpreted in terms of variations in the depth of the lithosphere–asthenosphere boundary (LAB) and the Moho with appropriate densities, that are constrained from seismic results at certain points. Modeling of the long wavelength gravity anomaly along a N–S profile (77°E) suggest that the thickness of the lithosphere for a density contrast of 0.05 g/cm³ with the asthenosphere is maximum of ∼190 km along the Himalayan front that reduces to ∼155 km under the southern part of the Ganga and the Vindhyan basins increasing to ∼175 km south of the Satpura Mobile belt, reducing to ∼155–140 km under the Eastern Dharwar craton (EDC) and from there consistently decreasing south wards to ∼120 km under the southernmost part of India, known as Southern Granulite Terrain (SGT).The crustal model clearly shows three distinct terrains of different bulk densities, and thicknesses, north of the SMB under the Ganga and the Vindhyan basins, and south of it the Eastern Dharwar Craton (EDC) and the Southern Granulite Terrain (SGT) of bulk densities 2.87, 2.90 and 2.96 g/cm³, respectively. It is confirmed from the exposed rock types as the SGT is composed of high bulk density lower crustal rocks and mafic/ultramafic intrusives while the EDC represent typical granite/gneisses rocks and the basement under the Vindhyan and Ganga basins towards the north are composed of Bundelkhand granite massif of the lower density. The crustal thickness along this profile varies from ∼37–38 km under the EDC, increasing to ∼40–45 km under the SGT and ∼40–42 km under the northern part of the Ganga basin with a bulge up to ∼36 km under its southern part. Reduced lithospheric and crustal thicknesses under the Vindhyan and the Ganga basins are attributed to the lithospheric flexure of the Indian plate due to Himalaya. Crustal bulge due to lithospheric flexure is well reflected in isostatic Moho based on flexural model of average effective elastic thickness of ∼40 km. Lithospheric flexure causes high heat flow that is aided by large crustal scale fault system of mobile belts and their extensions northwards in this section, which may be responsible for lower crustal bulk density in the northern part. A low density and high thermal regime in north India north of the SMB compared to south India, however does not conform to the high S-wave velocity in the northern part and thus it is attributed to changes in composition between the northern and the southern parts indicating a reworked lithosphere. Some of the long wavelength gravity anomalies along the east and the west coasts of India are attributed to the intrusives that caused the breakup of India from Antarctica, and Africa, Madagascar and Seychelles along the east and the west coasts of India, respectively.     

Synchronous climate anomalies in the western North Pacific and North Atlantic regions during the last 14,000 years

A peat cellulose δ¹⁸O record spanning around 14,000 years from the Hani peat mire in northeastern China reveals several abrupt temperature anomalies in the period from the last deglaciation through the Holocene. The timing of these anomalies coincides well with the notable cooling events recorded respectively using the GISP2 ice core and ice-rafted sediment of the North Atlantic Ocean, such as the Older Dryas, Inter-Allerød, Younger Dryas, and the nine ice-rafted debris events. The results demonstrate that this repeating pattern of abrupt temperature deterioration is not limited to the North Atlantic area at high latitude but also exists in the western North Pacific region at middle latitude. The synchronous temperature anomalies possibly are resulted from the joint effects of meltwater discharge into the North Atlantic Ocean and reduced solar activity. In the period from around 8600 to 8200 cal. yrs BP the Hani peat record shows a broad δ¹⁸O peak that may reflect compound climate signals resulting from the two kinds of forcing factors: the temperature drop related to reduced solar activity at around 8600–8250 cal. yrs BP, and the temperature anomaly attributed to the meltwater effect at around 8220 ± 70 cal. yrs BP. This result may provide palaeo-temperature evidence for existence of the sharp “8.2 k” event in the western North Pacific region. In addition, our results have revealed that in the period from the last deglaciation through the Holocene the synchronous temperature anomalies before and after the “8.2 k” event seem to be related to meltwater outflow and reduced solar activity, respectively. It is important that the all temperature anomalies—whether because of reduced solar activity in the late Holocene or from meltwater discharge in the early Holocene—are accompanied by an abrupt decline in the Indian Ocean summer monsoon and abrupt strengthening of the East Asian summer monsoon. It is likely that reduced solar activity and meltwater outflow appear to modulate Earth system changes in the same direction. The influences could be compounded. Reduced solar activity and meltwater outburst both appear to act as triggers for occurrence of the El Niño phenomenon in the equatorial Pacific Ocean, which may result in broad teleconnections between the temperature anomaly in the Northern Hemisphere and abrupt variation of the Asian monsoon.     

Influence of pre-existing fabrics on fault kinematics and rift geometry of interacting segments: Analogue models based on the Albertine Rift (Uganda), Western Branch-East African Rift System

This study aims at showing how far pre-existing crustal weaknesses left behind by Proterozoic mobile belts, that pass around cratonic Archean shields (Tanzania Craton to the southeast and Congo Craton to the northwest), control the geometry of the Albertine Rift. Focus is laid on the development of the Lake Albert and Lake Edward/George sub-segments and between them the greatly uplifted Rwenzori Mountains, a horst block located within the rift and whose highest peak rises to >5000 m above mean sea level. In particular we study how the southward propagating Lake Albert sub-segment to the north interacts with the northward propagating Lake Edward/George sub-segment south of it, and how this interaction produces the structures and geometry observed in this section of the western branch of the East African Rift, especially within and around the Rwenzori horst. We simulate behaviour of the upper crust by conducting sandbox analogue experiments in which pre-cut rubber strips of varying overstep/overlap connected to a basal sheet and oriented oblique and/or orthogonal to the extension vector, are placed below the sand-pack. The points of connection present velocity discontinuities to localise deformation, while the rubber strips represent ductile domain affected by older mobile belts. From fault geometry of developing rift segments in plan view and section cuts, we study kinematics resulting from a given set of boundary conditions, and results are compared with the natural scenario. Three different basal model-configurations are used to simulate two parallel rifts that propagate towards each other and interact. Wider overstep (model SbR3) produces an oblique transfer zone with deep grabens (max. 7.0 km) in the adjoining segments. Smaller overlap (model SbR4) ends in offset rift segments without oblique transfer faults to join the two, and produces moderately deep grabens (max. 4.6 km). When overlap doubles the overstep (model SbR5), rifts propagate sub-orthogonal to the extension direction and form shallow valleys (max. 2.9 km). Relative ratios of overlap/overstep between rift segments dictate the kind of transition zone that develops and whether or not a block (like the Rwenzoris) is captured and rotates; hence determining the end-member geometry. Rotation direction is controlled by pre-existing fabrics. Fault orientation, fault kinematics, and block rotation (once in play) reinforce each other; and depending on the local kinematics, different parts of a captured block may rotate with variable velocities but in the same general direction. Mechanical strength anisotropy of pre-structured crust only initially centres fault nucleation and propagation parallel to the grain of weakness of the basement, but at later stages of a protracted period of crustal extension, such boundaries are locally defied.     

Rapid late Pleistocene/Holocene uplift and coastal evolution of the southern Arabian (Persian) Gulf

The coastline along the southern Arabian Gulf between Al Jubail, Kingdom of Saudi Arabia, and Dubai, UAE, appears to have risen at least 125 m in the last 18,000 years. Dating and topographic surveying of paleo-dunes (43–53 ka), paleo-marine terraces (17–30 ka), and paleo-marine shorelines (3.3–5.5 ka) document a rapid, > 1 mm/a subsidence, followed by a 6 mm/a uplift that is decreasing with time. The mechanism causing this movement remains elusive but may be related to the translation of the coastal area through the backbasin to forebulge hinge line movement of the Arabian plate or, alternatively, by movement of the underlying Infracambrian-age Hormuz salt in response to sea-level changes associated with continental glaciation. Independent of the mechanism, rapid and episodic uplift may impact the design of engineering projects such as nuclear power plants, airports, and artificial islands as well as the interpretation of sedimentation and archeology of the area.     

Sea-level change and paleogeographic reconstructions,southern Vancouver Island,British Columbia,Canada

Forty-eight new and previously published radiocarbon ages constrain deglacial and postglacial sea levels on southern Vancouver Island, British Columbia. Sea level fell rapidly from its high stand of about +75 m elevation just before 14 000 cal BP (12 000 radiocarbon yrs BP) to below the present shoreline by 13 200 cal BP (11 400 radiocarbon years BP). The sea fell below its present level 1000 years later in the central Strait of Georgia and 2000 years later in the northern Strait of Georgia, reflecting regional differences in ice sheet retreat and downwasting. Direct observations only constrain the low stand to be below ?11 m and above ?40 m. Analysis of the crustal isostatic depression with equations utilizing exponential decay functions appropriate to the Cascadia subduction zone, however, places the low stand at ?30 ± 5 m at about 11 200 cal BP (9800 BP). The inferred low stand for southern Vancouver Island, when compared to the sea-level curve previously derived for the central Strait of Georgia to the northwest, generates differential isostatic depression that is consistent with the expected crustal response between the two regions. Morphologic and sub-bottom features previously interpreted to indicate a low stand of ?50 to ?65 m are re-evaluated and found to be consistent with a low stand of ?30 ± 5 m. Submarine banks in eastern Juan de Fuca Strait were emergent at the time of the low stand, but marine passages persisted between southern Vancouver Island and the mainland. The crustal uplift presently occurring in response to the Late Pleistocene collapse of the southwestern sector of the Cordilleran Ice Sheet amounts to about 0.1 mm/yr. The small glacial isostatic adjustment rate is a consequence of low-viscosity mantle in this tectonically active region.     

The seismic b-value and its correlation with Bouguer gravity anomaly over the Shillong Plateau area: Tectonic implications

Clues to the understanding of intra- and inter-plate variations in strength or stress state of the crust can be achieved through different lines of evidence and their mutual relationships. Among these parameters Bouguer gravity anomalies and seismic b-values have been widely accepted over several decades for evaluating the crustal character and stress regime. The present study attempts a multivariate analysis for the Shillong Plateau using the Bouguer gravity anomaly and the earthquake database, and establishes a causal relationship between these parameters. Four seismic zones (Zones I–IV), with widely varying b-values, are delineated and an excellent correlation between the seismic b-value and the Bouguer gravity anomaly has been established for the plateau. Low b-values characterize the southwestern part (Zone IV) and a zone (Zone III) of intermediate b-values separates the eastern and western parts of the plateau (Zones I and II) which have high b-values. Positive Bouguer anomaly values as high as +40 mgal, a steep gradient in the Bouguer anomaly map and low b-values in the southwestern part of the plateau are interpreted as indicating a thinner crustal root, uplifted Moho and higher concentration of stress. In comparison, the negative Bouguer anomaly values, flat regional gradient in the Bouguer anomaly map and intermediate to high b-values in the northern part of the plateau are consistent with a comparatively thicker crustal root and lower concentration of stress, with intermittent dissipation of energy through earthquake shocks. Further, depth wise variation in the b-value for different seismic zones, delineated under this study, allowed an appreciation of intra-plateau variation in crustal thickness from ∼30 km in its southern part to ∼38 km in the northern part. The high b-values associated with the depth, coinciding with lower crust, indicate that the Shillong Plateau is supported by a strong lithosphere.     

Petrogenesis of the 2115 Ma Haicheng mafic sills from the Eastern North China Craton: Implications for an intra-continental rifting

The Rhyacian (2300–2050 Ma) is a special era of the Paleoproterozoic represented by large layered intrusions in many cratons. It is well known that there are widespread igneous events at ~ 2100 Ma in the Eastern North China Craton; however, their tectonic environments are under debate: whether they were related to an intra-continental rifting or an arc/back-arc setting along a continental margin. These ~ 2100 Ma igneous events comprise several mafic dykes/sills, with some coeval A-type granites and volcanic events in several rifts; among them, the Haicheng mafic sills in the Liaohe rift are unique as their host rock, the Liaohe Group, bears the world's largest magnesium deposit. Most of the mafic sills are E-W-elongated at present coordinates. Exclusive of superimposition caused by deformation, the widths of the individuals are tens to hundreds of meters and the lengths are hundreds to thousands of meters. They have metamorphosed to an assemblage of plagioclase and hornblende, with minor quartz and accessory chlorite, epidote, apatite, ilmenite, and magnetite. However, relic gabbro and ophitic textures with mainly plagioclase and clinopyroxene are well-preserved. SIMS Pb–Pb dating on baddeleyites from one ~ 1000 m thick sill near Xialiulinzi village yields an average ²⁰⁷Pb/²⁰⁶Pb age of 2115 ± 3 Ma (n = 15, MSWD = 2.3), representing the timing of crystallization. SIMS U–Pb dating on zircon yields a similar forming age. They are tholeiitic in composition (MgO: 4.36–8.88 wt.%; SiO₂: 45.76–53.39 wt.%), enriched in light rare earth elements ((La/Yb)_N = 1.72–4.37) and large ion lithophile elements (i.e., Cs, Rb, Sr, and K) but depleted in high field strength elements (i.e., Nb, Ta, and Ti). These features were unlikely caused by crustal contamination during their emplacement, as there are little variations in Nb/La and Th/Nb. The rocks have experienced significant plagioclase-plus clinopyroxene-dominating fractional crystallization. Their enriched Sr–Nd isotope characteristics (⁸⁷Sr/⁸⁶Sr_t = 0.703 ~ 0.705, εNd_t = − 1.9 ~ 0.6) and trace element patterns indicate that their source(s) could be the ancient subcontinental lithospheric mantle; and this source is similar to those coeval sills from other parts of the craton. Their arc-like trace element features could be inherited from their source regions formed via a subduction process at the late Archean rather than at the middle-late Paleoproterozoic. These sill swarms, throughout the craton, might have developed in an integrated intra-continental rift system at ~ 2100 Ma.     

Neoproterozoic–Early Cambrian tectono-magmatic evolution of the Central Iranian terrane,northern margin of Gondwana: Constraints from detrital zircon U–Pb and Hf–O isotope studies

U–Pb dating and oxygen and Lu–Hf isotope analyses are applied to ~ 400 detrital zircon grains from the Neoproterozoic–Cambrian Kahar, Bayandor and Zaigun sandstones. The results reveal the evolutionary history of the Central Iranian continental crust in the northern margin of Gondwana during the Neoproterozoic–Cambrian. The U–Pb dating produces major peaks of crystallization ages at 0.5–0.7 Ga and minor peaks around the Tonian, Paleoproterozoic and Neoarchean. The zircon population in the Zaigun sandstone is dominated by long-transported grains and exhibits slightly different zircon distribution patterns than those from the older Kahar and Bayandor units. The zircon population ages and Hf isotopes of the Zaigun sample are very similar to the Neoproterozoic–Early Palaeozoic siliciclastic units in the Arabian Nubian shield (ANS) and Turkey, which suggests the late to post–Pan-African unroofing of the Afro–Arabia realm as the main process for detritus accumulation in Central Iran during the early Palaeozoic. A significant proportion of the Tonian-aged zircons (~ 64%) in the Kahar and Bayandor samples show positive εHf_(t) values, whereas those with late Cryogenian–Ediacaran ages have high δ¹⁸O and variable εHf_(t) values (~− 30‰ to + 17‰), suggesting that the crustal evolution of provenance of the Tonian-aged zircons commenced in an island arc setting and continued in an active continental margin. All the samples contain pre-Neoproterozoic zircons that are ca 1.9–2.3 Ga or 2.5–3.2 Ga, which are much older than the known Neoproterozoic igneous rocks in Iran and are more consistent with pre-Neoproterozoic igneous-metamorphic rocks in the eastern ANS and northern Africa. These ages support the eastern sector of the Afro–Arabia margin as a provenance for the detrital zircons in the oldest sedimentary sequences of Iran during the late Neoproterozoic–Cambrian. The Hf model ages of zircons with mantle-like δ¹⁸O values suggest that a significant amount of continental crust in the provenance of the detrital zircons was generated at around 1.0–2.0 and 3.0–3.5 Ga, likely by mantle-derived mafic magmas, and subsequently reworked during crustal differentiation into younger, more felsic crust with varying crustal residence times.     

Amount of Asian lithospheric mantle subducted during the India/Asia collision

Body wave seismic tomography is a successful technique for mapping lithospheric material sinking into the mantle. Focusing on the India/Asia collision zone, we postulate the existence of several Asian continental slabs, based on seismic global tomography. We observe a lower mantle positive anomaly between 1100 and 900 km depths, that we interpret as the signature of a past subduction process of Asian lithosphere, based on the anomaly position relative to positive anomalies related to Indian continental slab. We propose that this anomaly provides evidence for south dipping subduction of North Tibet lithospheric mantle, occurring along 3000 km parallel to the Southern Asian margin, and beginning soon after the 45 Ma break-off that detached the Tethys oceanic slab from the Indian continent. We estimate the maximum length of the slab related to the anomaly to be 400 km. Adding 200 km of presently Asian subducting slab beneath Central Tibet, the amount of Asian lithospheric mantle absorbed by continental subduction during the collision is at most 600 km. Using global seismic tomography to resolve the geometry of Asian continent at the onset of collision, we estimate that the convergence absorbed by Asia during the indentation process is ~ 1300 km. We conclude that Asian continental subduction could accommodate at most 45% of the Asian convergence. The rest of the convergence could have been accommodated by a combination of extrusion and shallow subduction/underthrusting processes. Continental subduction is therefore a major lithospheric process involved in intraplate tectonics of a supercontinent like Eurasia.     

U–Pb zircon chronology,geochemistry and isotopes of the Changyi banded iron formation in the eastern Shandong Province: Constraints on BIF genesis and implications for Paleoproterozoic tectonic evolution of the North China Craton

The Changyi banded iron formation (BIF) in the eastern North China Craton (NCC) occurs within the Paleoproterozoic Fenzishan Group. The BIF shows alternating quartz-rich light and magnetite-rich dark bands with magnetite (15–65 vol.%), quartz (25–65 vol.%) and amphibole (15–30 vol.%) constituting the major minerals. Minor garnet, epidote, chlorite, calcite, biotite and pyrite occur locally. The BIF bands are interlayered with amphibolite, hornblende gneiss, biotite quartz schist, garnet biotite schist, biotite gneiss and leptynite, and are intruded by granites. LA-ICP-MS U–Pb dating on zircons separated from the BIF bands and the wallrocks constrains the depositional age as 2240–2193 Ma and metamorphic age as ~ 1864 Ma. The dominant composition of SiO₂ + Fe₂O₃^T (average value of 92.3 wt.%) of the BIF bands suggests their formation mainly through chemical precipitation. However, the widely varying contents of major elements such as Al₂O₃ (0.58–6.99 wt.%), MgO (1.00–3.86 wt.%), CaO (0.22–4.19 wt.%) and trace elements such as Rb (2.06–40.4 ppm), Sr (9.36–42.5 ppm), Zr (0.91–23.6 ppm), Hf (0.04–0.75 ppm), Cr (89.1–341 ppm), Co (2.94–30.4 ppm), and Ni (1.43–52.0 ppm) clearly indicate the incorporation of clastics, especially continental felsic clastics, as also confirmed by the presence of ancient detrital zircons in the BIF bands. When normalized against Post Archean Average Shale (PAAS), the seawater-like signatures of REE distribution patterns, such as LREE depletion, positive La and Y anomalies, and superchondritic Y/Ho ratios (average value of 36.3), support the deposition in seawater. Strong positive Eu anomalies (Eu/Eu*_PAAS = 1.14–2.86) also suggest the participation of hydrothermal fluids. In addition, the sympathetic correlation between Cr, Co and Ni as well as the Co + Ni + Cu vs. ∑ REE and the Al₂O₃ vs. SiO₂ relations further indicates that the iron and silica mainly originated from hydrothermal fluids. Combined with regional geological investigation and protolith restoration of the wallrocks, a continental rift environment is suggested for the Changyi BIF deposition. The appearance of negative Ce_PAAS anomalies might suggest the influence of the Great Oxidation Event at the time of deposition. The Changyi BIF witnessed the major Paleoproterozoic rifting–collision events in the NCC and their unique distribution in the NCC contrasts with other examples elsewhere in the world.     

Adakite-like signature of porphyry granitoid stocks in the Meiduk and Parkam porphyry copper deposits,NE of Shahr-e-Babak,Kerman, Iran: Constrains on geochemistry

The Middle Miocene porphyry granitoid stocks of Meiduk and Parkam porphyry copper deposits are intruded in the north-western part of the Dehaj-Sarduiyeh volcano-sedimentary belt in the south-eastern extension of the Urumieh-Dukhtar Magmatic Arc (UDMA) in Iran. The porphyritic to microgranular granitoids are mainly consist of quartz diorite, granodiorite and diorite. The whole rock geochemical analyses of these rocks reveals sub-alkaline, calc-alkaline, meta-peraluminous and I-type characteristics. Their geochemical characteristics such as Al₂O₃ content of 13.51–17.05 wt%, high Sr concentration (mostly >400 ppm), low Yb (an average of 0.74 ppm) and Y (an average of 9.02 ppm) contents, strongly differentiated REE patterns (La/Yb ≥ 20), lack of Eu anomaly (Eu/Eu¹ ≥ 1) are indicative of adakitic signature. Their enrichment in low field strength elements (LFSE) and conspicuous negative anomalies for Nb, Ta and Ti are typical of subduction related magmas. Detailed petrological studies and geochemical data indicated that Meiduk and Parkam porphyry granitoids were derived from amphibole fractionation of hydrous melts at a depth of >40 km in a post-collisional tectonic setting.     

Upper mantle structure of the Saharan Metacraton

The ∼500,000 km² Saharan Metacraton in northern Africa (metacraton refers to a craton that has been mobilized during an orogenic event but that is still recognisable through its rheological, geochronological and isotopic characteristics) is an Archean–Paleoproterozoic cratonic lithosphere that has been destabilized during the Neoproterozoic. It extends from the Arabian–Nubian Shield in the east to the Trans-Saharan Belt in the west, and from the Oubanguides Orogenic Belt in the south to the Phanerozoic cover of North Africa. Here, we show that there are high S-wave velocity anomalies in the upper 100 km of the mantle beneath the metacraton typical of cratonic lithosphere, but that the S-wave velocity anomalies in the 175–250 km depth are much lower than those typical of other cratons. Cratons have possitive S-wave velocity anomalies throughout the uppermost 250 km reflecting the presence of well-developed cratonic root. The anomalous upper mantle structure of the Saharan Metacraton might be due to partial loss of its cratonic root. Possible causes of such modification include mantle delamination or convective removal of the cratonic root during the Neoproterozoic due to collision-related deformation. Partial loss of the cratonic root resulted in regional destabilization, most notably in the form of emplacement of high-K calc-alkaline granitoids. We hope that this work will stimulate future multi-national research to better understand this part of the African Precambrian. Specifically, we call for efforts to conduct systematic geochronological, geochemical, and isotopic sampling, deploy a reasonably-dense seismic broadband seismic network, and conduct systematic mantle xenoliths studies.     

Geochemistry,provenance, and tectonic setting of Neoproterozoic metavolcanic and metasedimentary units,Werri area,Northern Ethiopia

Major, trace and rare earth element (REE) compositions of upper Proterozoic metavolcanic and metasedimentary rocks from the Tsaliet and Tembien Groups in the Werri district of northern Ethiopia were determined to examine their tectonic setting of eruption, provenance and source area weathering conditions. Tsaliet Group metavolcanic rocks in the Werri area have sub-alkaline chemistry characterized by low to intermediate SiO₂ contents, high Al₂O₃, low MgO and very low Cr and Ni. High field strength element (HFSE) abundances are highly variable. ∑REE abundances vary from 66.7 to 161.3 ppm, and chondrite-normalized REE patterns are moderately fractionated, with La_N/Yb_N values of between 3.1 and 9.0. Europium anomalies are variable (Eu/Eu* 0.80–1.21) but are generally positive (average Eu/Eu* 1.06). On tectonic discrimination diagrams, most samples have either volcanic-arc chemistry or fall in the overlap field with mid-oceanic ridge basalt (MORB). However, primitive mantle-normalized trace element abundances are comparable with sub-alkaline basalts from developed island arcs. ¹⁴⁷Sm/¹⁴⁴Nd ratios range from 0.1167 to 0.1269 (n = 3), yielding initial εNd_(800 Ma) of +3.8 to +4.9 and mean T_DM model age of 0.96 Ga, indicative of derivation from juvenile Neoproterozoic mantle. Metasediments from three locations (Werri1, Werri2 and Tsedia) in the Werri and Tsedia Slates have similar Al₂O₃, TiO₂ and HFSE contents but variable and low Na₂O, CaO and K₂O. Cr and Ni are slightly enriched in the Werri2 and Tsedia suites. SiO₂ is very variable, with average values of 70.75, 72.2 and 66.4 wt.% in the Werri1, Werri2 and Tsedia suites, respectively. ∑REE abundances in the metasediments (14.74–108.1) are lower than in the metavolcanics, and are slightly less fractionated, with La_N/Yb_N ratios of 0.8–5.9. Europium anomalies vary (Eu/Eu* 0.80–1.21) but are insignificant on average (Eu/Eu* 0.96). High values for the Chemical Index of Alteration (generally 70–90), and Plagioclase Index of Alteration (>75) in the Werri metasediments indicate moderate to severe chemical weathering in their source. Average major and trace element compositions of the metasediments and their REE patterns are comparable with the metavolcanics. ¹⁴⁷Sm/¹⁴⁴Nd ratios of the metasediments range from 0.1056 to 0.1398 (n = 4), with initial εNd_(800 Ma) of +3.4 to +5.0 and mean T_DM model age of 0.97 Ga, indicating derivation from juvenile Neoproterozoic crust similar to the underlying metavolcanics, with minimal (4–10%) contribution from older crust. The most sensitive tectonic setting discriminators indicate the Werri metasediments represent developed oceanic island arc sediments. The chemical similarity of the Werri metavolcanics to the nearby Adwa metavolcanics, Nakfa terrane in Eritrea, and volcanic units in central Saudi Arabia imply that juvenile Neoproterozoic Arabian Nubian Shield crust extended south at least as far as the Werri area of northern Ethiopia. The comparable geochemistry of the metasediments and their underlying lithologies attests to their derivation from this juvenile crustal material.