Two-step Gravity Inversion Reveals Variable Architecture of African Cratons

The lithospheric build-up of the African continent is still to a large extent unexplored. In this contribution, we present a new Moho depth model to discuss the architecture of the three main African cratonic units, which are: West African Craton, Congo Craton, and Kalahari Craton. Our model is based on a two-step gravity inversion approach that allows variable density contrasts across the Moho depth. In the first step, the density contrasts are varied for all non-cratonic units, in the second step for the three cratons individually. The lateral extension of the tectonic units is defined by a regionalization map, which is calculated from a recent continental seismic tomography model. Our Moho depth is independently constrained by pointwise active seismics and receiver functions. Treating the constraints separately reveals a variable range of density contrasts and different trends in the estimated Moho depth for the three cratons. Some of the estimated density contrasts vary substantially, caused by sparse data coverage of the seismic constraints. With a density contrast of Δ ρ = 200 kg/m3 the Congo Craton features a cool and undisturbed lithosphere with smooth density contrasts across the Moho. The estimated Moho depth shows a bimodal pattern with average Moho depth of 39–40 km for the Kalahari and Congo Cratons and 33–34 km for the West African Craton. We link our estimated Moho depth with the cratonic extensions, imaged by seismic tomography, and with topographic patterns. The results indicate that cratonic lithosphere is not necessarily accompanied by thick crust. For the West African Craton, the estimated thin crust, i.e. shallow Moho, contrasts to thick lithosphere. This discrepancy remains enigmatic and requires further studies.

Geomorphology, Mineralogy, and Geochronology of Mare Basalts and Non-Mare Materials around the Lunar Crisium Basin

The Nectarian-aged Crisium basin exhibits an extremely thin crust and complicated lunar geological history. This large multi-ring impact basin is characterized by prolonged lunar volcanism ranging from the Imbrian age to the Eratosthenian period, forming the high-Ti mare unit, low-Ti mare basalts, and very low-Ti mare unit. We produced an updated geological map of the Crisium basin and defined four mare units (Im1: 3.74 Ga; Im2: 3.49 Ga; Im3: 3.56 Ga; EIm: 2.49 Ga) in terms of distinct composition and mineralogy. Olivine was widely determined in the Ti-rich Im1, implying the hybridization source in the lunar mantle with the occurrence of small-scale convective overturn. The major phase of low-Ti basaltic volcanism occurred c.a. 3.5 Ga, forming Im2 and Im3 in the western area. The youngest mare unit (EIm) has slight variations of pyroxene compositions, implying a decrease of calcic content of basaltic volcanisms with time. Later, distal material transports from large impact events in highlands could complicate the mixing of local mare basalts in the Copernicus age, especially the Im3 unit. The identified olivine-bearing outcrops and widely Mg-rich materials (Mg# > 70, where Mg# = molar 100 × Mg/(Mg + Fe)) in the western highlands, assumed to be the occurrence of the Mg-suite candidates, require future lunar exploration missions to validate.

Extinct Style of Plate Tectonics Explains Early Earth’s Flat Mountains

The geologic record suggests that despite Earth’s hot, thin crust during the Proterozoic, mountains were still able to form thanks to an extinct style of crustal deformation.

Scaling laws for stagnant-lid convection with a buoyant crust

SUMMARY Stagnant-lid convection, where subduction and surface plate motion is absent, is common among the rocky planets and moons in our solar system, and likely among rocky exoplanets as well. How stagnant-lid planets thermally evolve is an important issue, dictating not just their interior evolution but also the evolution of their atmospheres via volcanic degassing. On stagnant-lid planets, the crust is not recycled by subduction and can potentially grow thick enough to significantly impact convection beneath the stagnant lid. We perform numerical models of stagnant-lid convection to determine new scaling laws for convective heat flux that specifically account for the presence of a buoyant crustal layer. We systematically vary the crustal layer thickness, crustal layer density, Rayleigh number and Frank–Kamenetskii parameter for viscosity to map out system behaviour and determine the new scaling laws. We find two end-member regimes of behaviour: a ‘thin crust limit’, where convection is largely unaffected by the presence of the crust, and the thickness of the lithosphere is approximately the same as it would be if the crust were absent; and a ‘thick crust limit’, where the crustal thickness itself determines the lithospheric thickness and heat flux. Scaling laws for both limits are developed and fit the numerical model results well. Applying these scaling laws to rocky stagnant-lid planets, we find that the crustal thickness needed for convection to enter the thick crust limit decreases with increasing mantle temperature and decreasing mantle reference viscosity. Moreover, if crustal thickness is limited by the formation of dense eclogite, and foundering of this dense lower crust, then smaller planets are more likely to enter the thick crust limit because their crusts can grow thicker before reaching the pressure where eclogite forms. When convection is in the thick crust limit, mantle heat flux is suppressed. As a result, mantle temperatures can be elevated by 100 s of degrees K for up to a few Gyr in comparison to a planet with a thin crust. Whether convection enters the thick crust limit during a planet’s thermal evolution also depends on the initial mantle temperature, so a thick, buoyant crust additionally acts to preserve the influence of initial conditions on stagnant-lid planets for far longer than previous thermal evolution models, which ignore the effects of a thick crust, have found.

Chronological Paradoxes of Mohammed Shah Bey’ Turbe in the Bahchisaray Town

According to the memorial inscription without date, which is above the entrance, someone Muhammad Shah Bey ordered to build a turbe for his mother – Bey Yude Sultan. Traditionally, it is considered one of the earliest Muslim memorial buildings in Crimea and usually dates back to the 14th–15th centuries. The building is constantly cited as an example as the closest analogy during considering a some group of Golden Horde monuments – the Eastern and Northern mausoleums of Bolgar, the mausoleum of Tura Khan in Bashkiria. According to the results of excavations in 1991, the building was dated to the end of the 18th century (near 1778). These conclusions are based on numismatic material, a number of specifically signs and, mainly, stratigraphy. Scurf of the building horizon was overlapped by a layer of destroyed build mortar. This top layer is associated with the destruction and restoration of the turbe. Between themselves, these two stratigraphic periods are separated only by thin crust of trampled soil. This gives reason to think that the mausoleum was built shortly before the damage, which caused the loss of the masonry of the upper part of the main facade and roof. Otherwise, if dated the building by the 14th–15th centuries, then during the next three or four centuries a wide cultural layer would have to accumulate near its walls, above the foundation. But archaeological excavations have not revealed such layer.

Integration of gravity, magnetic, and seismic data for subsalt modeling in the Northern Red Sea

Rifts and rifted passive margins are often associated with thick evaporite layers, which challenge seismic reflection imaging in the subsalt domain. This makes understanding the basin evolution and crustal architecture difficult. An integrative, multidisciplinary workflow has been developed using the exploration well, gravity and magnetics data, together with seismic reflection and refraction data sets to build a comprehensive 3D subsurface model of the Egyptian Red Sea. Using a 2D iterative workflow first, we have constructed cross sections using the available well penetrations and seismic refraction data as preliminary constraints. The 2D forward model uses regional gravity and magnetic data to investigate the regional crustal structure. The final models are refined using enhanced gravity and magnetic data and geologic interpretations. This process reduces uncertainties in basement interpretation and magmatic body identification. Euler depth estimates are used to point out the edges of high-susceptibility bodies. We achieved further refinement by initiating a 3D gravity inversion. The resultant 3D gravity model increases precision in crustal geometries and lateral density variations within the crust and the presalt sediments. Along the Egyptian margin, where data inputs are more robust, basement lows are observed and interpreted as basins. Basement lows correspond with thin crust ([Formula: see text]), indicating that the evolution of these basins is closely related to the thinning or necking process. In fact, the Egyptian Northern Red Sea is typified by dramatic crustal thinning or necking that is occurring over very short distances of approximately 30 km, very proximal to the present-day coastline. The integrated 2D and 3D modeling reveals the presence of high-density magnetic bodies that are located along the margin. The location of the present-day Zabargad transform fault zone is very well delineated in the computed crustal thickness maps, suggesting that it is associated with thin crust and shallow mantle.

Late Cretaceous short-lived magmatism and related metallogenesis in the Carpathian area (Romania): connections with Balkans

<p>The first magmatic event that post-dates the Meso-Cretaceous orogeny in the Carpatho-Balkan area took place in the Upper Cretaceous at the same time and after the formation of Gosau-type molasses basins, the whole being controlled by an extensional tectonic transpressive-transtensive type (Schuller, 2004; Schuller et al., 2009; Drew, 2006; Georgiev et al., 2009). This tectonic regime controlled the spatial and temporal distribution of both magmatites and metallogenesis associated with the main feature discontinuity.</p><p>This aspect is suggested by gravimetry and magnetism studies (Andrei et al., 1989), and also structural studies (Schuller et al., 2009; Drew, 2006; Georgiev et al., 2009).</p><p>The age data attest to the temporal sequentially of Upper Cretaceous magmatism's evolution in the Carpathians and the Balkans. The most accurate age data (using geochronometers of zircon U-Pb and molybdenite Re-Os) suggest a very narrow evolutionary range (70.2-83.98 Ma, after Nicolescu et al., 1999; Galhofer, 2015 and 72.36-80.63 Ma, after Ciobanu et al., 2002; Zimmerman et al., 2008), which is characteristic to short-lived magmatism. In contrast, the same magmatism exists between 84-86 Ma in Serbia (Bor-Madjanpek district) and between 86-92 Ma and 67-70 Ma in Bulgaria (Srednogorie massif) in the Rhodope massif (von Quadt et al., 2007).</p><p>The magma volumes have been significant several times, so much so that we have circumstances such as that in Vlǎdeasa (Apuseni Mts), and not only, in which sedimentary deposits of the Gosau type are "suspended" at high altitude, "behind" the granodiorite intrusions. According to Lin & Wang (2006), there are two approaches to explain this situation in the Carpathians during Upper Cretaceous: (1) mechanical convective ablation of the lithosphere, as suggested by Bird (1979) for North American mountain ranges, or (2) detachment of a large piece of the lithospheric mantle, as suggested by Houseman et al. (1981). The thin crust can be explained in an extensional context, regardless of the adopted model, which facilitates rapid ascents of magmas induced by adiabatic detente at the base of the lithosphere and/or in the asthenosphere.</p><p>Irregular variations in La<sub>N</sub>/Yb<sub>N</sub>, Eu/Eu*, Ce/Ce*, and initial <sup>87</sup>Sr/<sup>86</sup>Sr, and <sup>143</sup>Nd/<sup>144</sup>Nd ratios that are in the range between 0.704957-0.706774 and 0.512456-0.512538 respectively, suggest that the banatites were generated by partial melting of the LCC, with the involvement of mantle-derived magmas.</p><p>The metallogenesis associated with banatitic magmatism is characterized by a great typological variety of metalliferous accumulations forming mineral deposits with main commodities of Fe, Cu, Pb, Zn, ± Au, Ag, W, Mo, B, Mg, Te, Bi, Sb, spatially dominated by transpressive-transtensive tectonics. The most common forms of mineralization is skarn, porphyry copper, massive sulfide, and veins. These mineral deposits exibit complex paragenesis of more than 200 minerals, some of which were first described: ludwigite, szaibelyite, dognacskaite, rezbanyite, veszelyite and csiklovaite. The main mineral deposits associated with the Romanian banatites are Baita Bihor (Mo-Bi-W-Cu-U-Pb-Zn-B), Baisoara (Fe-Zn-Pb), Ocna de Fier-Dognecea (Fe-Cu-Pb-Zn-Bi), Moldova Noua (porphyry Cu±Au-Ag-Mo), Oravita-Ciclova (Cu-Mo-W-Bi) and Sasca (Cu-Mo).</p><p> </p><p> </p><p>Acknowledgments<br>This work was supported by two PNCDI III grants of the Romanian Ministry of Research and Innovation, PN-III-P1-1.2-PCCDI-2017-0346/29 and PN-III-P4-ID-PCCF-2016-4-0014.</p><p> </p>

Pliocene adakite-like accent of andesites and dacites from the Orlov volcanic field (Sakhalin Island)

Adakite-like geochemical signature (high Sr/Y ratio at a low Y concentration) is recognized in andesites and dacites, associated with intraplate basalts in the Orlov volcanic field of Sakhalin Island. These rocks denote the final (Pliocene) accent of intraplate volcanism in the Lesogorsk zone, which began in the Middle Miocene in an area of its junction with the Chekhov zone of the preceded (Oligocene-Early Miocene) suprasubduction one. The adakite-like accent was related to the Sakhalin folding phase that accompanied the general structural reorganization in the back-side region in the Japan arc system. Such a geological environment differed from the one of classical adakites generation resulted from melting of a young slab in the Aleutian island arc. It is supposed, that the Sakhalin adakite-like magmas were produced in deep-seated sources of the crust-mantle transition displayed in the Sakhalin-Hokkaido-Japan Sea zone of hot transtension due to drastic change of tectonic deformations from the thin crust of the South Tatar Basin to the thicker one of its northeastern extremity.

Post-collisional crustal thickening and plateau uplift of southern Tibet: Insights from Cenozoic magmatism in the Wuyu area of the eastern Lhasa block

The nature and timing of post-collisional crustal thickening and its link to surface uplift in the eastern Lhasa block of the southern Tibetan plateau remain controversial. Here we report on Cenozoic magmatism in the Wuyu area of the eastern Lhasa block. The Eocene (ca. 46 Ma) trachyandesites and trachydacites show slight fractionation of rare earth elements (REE), slightly negative Eu and Sr anomalies, and relatively homogeneous Sr-Nd and zircon Hf isotopes (87Sr/86Sr(i) = 0.7050−0.7063, εNd(t) = −0.92 to −0.03, εHf(t) = +2.6 to +4.8). Previous studies have suggested Neo-Tethys oceanic slab break-off at 50−45 Ma; thus, the Wuyu Eocene magmatism could represent a magmatic response to this slab break-off and originate from relatively juvenile Lhasa crust. The Miocene (ca. 15−12 Ma) dacites and rhyolites have adakitic affinities, e.g., high Sr (average 588 ppm), Sr/Y (29−136), and La/Yb (30−76) values, low Y (4−12 ppm) and Yb (0.4−0.9 ppm) contents, and variable Sr-Nd and zircon Hf isotopes (87Sr/86Sr(i) = 0.7064−0.7142, εNd(t) = −11.7 to −3.7, εHf(t) = −3.2 to +4.5). Their more enriched Sr-Nd-Hf isotopes relative to the Eocene lavas indicate that they should be derived from mixed Lhasa lower crust comprising juvenile crust, ultrapotassic rocks, and probably Indian lower crust-derived rocks. This study has also revealed the transformation from Eocene juvenile and thin crust with a thickness of <40 km to Miocene mixed and thickened crust with a thickness of >50 km. Combined with published tectonic data, we suggest that both lithospheric shortening and magma underplating contributed to eastern Lhasa block post-collisional crustal thickening. Given the spatial-temporal distribution of eastern Lhasa block magmatism and regional geology, we invoke a post-collisional tectonic model of steep subduction of the Indian plate and subsequent westward-propagating plate break-off beneath the eastern Lhasa block, which caused the surface uplift.