Crust-mantle Boundary Research Articles

Neoproterozoic (750–711 Ma) Tectonics of the South Qinling Belt, Central China: New Insights from Geochemical, Zircon U‐Pb Geochronological, and Sr‐Nd Isotopic Data from the Niushan Complex

AbstractThe Xiejiaba and Fuqiangbei plutons form part of the newly identified Neoproterozoic Niushan complex, which is located in the southern South Qinling belt (SQB). The plutons are compositionally similar, were emplaced at 750–711 Ma, and provide insights into Neoproterozoic tectonism within the South Qinling belt. The Xiejiaba pluton contains diorite, quartz diorite, granodiorite, and granite phases, all of which are sub‐alkaline and have variable major element compositions with negative correlations between SiO2and MgO, TFe2O3, Al2O3, CaO, TiO2and P2O5. These rocks are enriched in light rare earth elements (LREEs) and large ion lithophile elements (LILEs) and have negative Nb, Ta, P and Ti anomalies, all of which are indicative of arc‐type magmatism. The Fuqiangbei pluton contains granitoids that are compositionally similar to the rocks in the Xiejiaba pluton. Samples from these plutons have similarɛNd(t) values (1.24–5.99) but very variable (87Sr/86Sr)ivalues (0.7010–0.7054). Combining these data with the geochemical data for these rocks suggests that the magmas that formed the Niushan complex were derived from the crust–mantle boundary. This, combined with the results of previous research, suggests that the transition from low pressure‐low temperature to low pressure‐high temperature conditions within a subduction zone caused melting during a period of late subduction and backarc extension. This constrain the timing of subduction within the South Qinling belt and the northern Yangtze Block (YB) to 750–711 Ma, with this Neoproterozoic subduction associated with an ocean to the north overprinting an existing continental rift‐type tectonic setting within the northern margin of the Yangtze Block and the South Qinling belt.

Spatial heterogeneity of the lithospheric destruction of the North China Craton: Evidence from an extended magnetotelluric sounding profile

To study the spatial heterogeneity of the North China Craton (NCC) destruction, this paper used a magnetotelluric sounding (MT) profile that passes through almost the entire NCC from west to east. Three-dimensional inversion is used to obtain a lithospheric resistivity model of the NCC. The results show that the upper crust of the Ordos Block is characterized by high resistivity. The lower crust to the upper mantle is characterized by low resistivity. The resistivity structure below the Trans-North China Orogen (TNCO) has stratification features; The Shanxi Graben shows high-low-high-low resistivity features from the upper crust to the asthenosphere; The lithosphere of the Lüliang and Taihang uplifts show high-resistivity features, and only some local relatively low-resistivity areas appear at the crust-mantle boundary. The upper crust on both sides of the Tan-Lu Fault Zone is characterized by high resistivity, but the resistivity structures of the lower crust and the lithospheric mantle are significantly different; The lower crust and the lithospheric mantle of the Sulu Orogenic Belt on the east are characterized by high resistivity; The Luxi Uplift on the west is represented by low resistivity. We propose that the mantle low-resistivity bodies (C1 and C4) of the Western and Eastern blocks may be related to the upwelling of partial melting materials along the ancient structurally weak zones in the lithosphere. The TNCO still has a typical Archean cratonic lithosphere, and the low-resistivity body C2 may be the remnant of the subducted oceanic crust. The Tan-Lu Fault Zone is structurally weak in the Eastern Block, while its western branch is a channel for the asthenospheric upwelling. We propose that the lithosphere of the northwestern Ordos Block and the Yinchuan-Hetao area is being destructed, and the TNCO is in the initial stage of being destructed. In contrast, the lithosphere of the Eastern Block has been severely destructed. In conclusion, affected by the subduction of the paleo-Pacific plate and the collision of the Indian and Eurasian plates, the ancient structures in the NCC were reactivated in the Mesozoic and Cenozoic, resulting in the spatial heterogeneity of the NCC destruction.

Model Variations of the Crust Thickness of Mars and Venus Using the Love Numbers Method

Based on topography and gravitational field data, model variations in the crust thickness of Mars and Venus were calculated using the Love numbers method. The method takes into account the adjustment of the planetary interior to loads on the surface and in the interior. Numerical modeling was carried out using the expansion in spherical harmonics of the topography and gravitational field data up to the 90th degree and order for Mars and up to the 70th degree and order for Venus. The topography of the crust–mantle boundary suggests partial Airy isostatic compensation. The model of the Martian crust is consistent with the interval of crustal thickness values under the site of the InSight station in the southwestern part of Elysium Planitia obtained from the results of a seismic experiment. The comparison with the available global models of the crust of Mars and Venus was carried out.

Tectonism of Late Noachian Mars: Surface Signatures from the Southern Highlands

Upwelling mantle plumes often instigate extensional stress within the continental crust of Earth. When stress exceeds crustal strength, extensional structures develop, reducing the effective stress and trigger magmatic processes at the crust–mantle boundary. However, such processes and their relationship to the formation of many surface structures remain poorly characterized on Mars. We identified a series of extensional structures in the southern highlands of Mars which collectively resemble continental rift zones on Earth. We further characterized these extensional structures and their surrounding region (area of ~1.8 M km2) by determining the surface mineralogy and bulk regional geochemistry of the terrain. In turn, this constrains their formation and yields a framework for their comparison with extensional structures on Earth. These terrains are notable for olivine and high-Ca pyroxene with a high abundance of potassium and calcium akin to alkali basalts. In the case of Mars, this Earth-like proto-plate tectonic scenario may be related to the plume-induced crustal stretching and considering their distribution and temporal relationship with the Hellas basin, we conclude that the plume is impact-induced. Overall, the findings of this work support the presence of mantle plume activity in the Noachian, as suggested by thermal evolution models of Mars.

Petrogenesis of Bonai volcanic rocks from Singhbhum Craton (Eastern India): Geophysical and geodynamic implications for pervasive plume-lithospheric interaction

• Petrogenesis and geodynamic evolution of Bonai volcanic, Singhbhum Craton. • Volcanic rocks generated by mantle plume within spinel lherzolite stability field. • Mantle potential temperature of 1300-1700°C and pressure 0.5-3.5 GPa. • Lower crustal exhumation, highly denudated lithospheric keel and magma underplating. • Crust-mantle thermal interaction, contamination and fractional crystallization. Singhbhum Craton of the Indian Shield is one of the oldest Eoarchean segments on the Earth, which contains several well-preserved Mesoarchean-Paleoproterozoic volcanic suites. In this study, we investigate petrogenesis of the Bonai suite of volcanic rocks located in the western part of the craton on the basis of detailed geological, geochemical and geophysical data, which provide a new understanding of its regional geodynamic evolution. These volcanics are generally fine to medium-grained, exhibit porphyritic texture and contain plagioclase and pyroxene as primary minerals and amphiboles, epidote, sericite and chlorite as secondary minerals. The accessory phases are mainly zircon, ilmenite, pyrite and chalcopyrite. They can be petrographically and geochemically classified as basalt and basaltic andesite of tholeiitic nature. They are characterized by moderate to high contents of SiO 2 (49.35-57.81 wt%), MgO (7.14-11.39 wt%), Al 2 O 3 (10.21-14.68 wt%), CaO (5.14-11.15 wt%), TiO 2 (0.48-2.03 wt%), Cr (32-483 ppm) and Ni (34-175 ppm). Their HFSE/REE ratios and REE patterns indicate their derivation from a relatively low melting of a similar mantle source within the spinel lherzolite stability field. They are formed as a result of decompressive melting of the mantle plume near the crust-mantle boundary, which underwent fractional crystallization and crustal contamination. The plume had an initial mantle potential temperature between 1300°C and 1700°C with corresponding pressure of 0.5 to 3.5 GPa. In Nb/Y-Zr/Y plot, almost all the samples fall in the field of oceanic plateau basalt with mantle plume affinity. This region is underplated by about 14 km thick magmatic rocks above the Moho, over which gravity field and regional elevation are consistently high and the lithosphere is extremely thin. These characteristics corroborate well with the mantle plume signatures. Subduction related geochemical signatures appear absent.

Wehrlite xenoliths and petrogenetic implications, Hosséré Do Guessa volcano, Adamawa plateau, Cameroon

Abstract Peridotite xenoliths of wehrlite composition, scarcely known in Adamawa plateau, Cameroon, were sampled by Mio-Pliocene basanites from Hosséré Do Guessa volcano. Their origins are discussed and elucidated trough petrography and mineral chemistry. Studied wehrlites exhibit poikilitic or protogranular textures and are composed of four main mantle phases (high Mg-olivine, augite, enstatite and Al-spinel). Petrographic and microprobe (Camebax SX100) chemical data (Fo90.8-91.4 olivine, Wo39.4-42.0 augite, En90.5-91.1 enstatite and Al-spinel) suggest a mantle origin for the Hosséré Do Guessa wehrlites. Hence, these rocks could not be considered cumulate. They have been equilibrated between 1,140 and 1,220°C, at pressures of 1.5–2.0 GPa, at 50–66 km deep, below the crust-mantle boundary. Wehrlites might result in reactions with carbonate/carbonatite melt, accompanying CO2 degassing and metasomatism by fluid phases. They suffered transpressional tectonics, during movement at Tertiary times of Pan-African strike-slip-faults, after solid-state tectonic relaxation.

Seismic anisotropy and crustal deformation in the Satluj valley and adjoining region of northwest Himalaya revealed by the splitting analysis of Moho converted Ps phases

• Significant crustal anisotropy has been detected in the Satluj valley, NW Himalaya. • Fast polarization directions are parallel or sub-parallel to surface geological features . • Strength of anisotropy suggests middle and lower crust as a primary source. • Extension tectonics in Tethyan Himalaya causes E-W or NW-SE anisotropy. • Structural anisotropy is predominant in Lesser and Higher Himalaya. Seismic anisotropy in the crust beneath the Satluj valley and the adjoining region of the northwest Himalaya has been studied with the help of shear wave splitting analysis of P -to- S or Ps converted phases originating at the crust-mantle boundary. A total of 144 splitting parameters (Φ, δ t ) have been computed from 130 teleseismic earthquakes recorded by 13 broadband seismological stations spanning from the Lesser Himalaya to Tethyan Himalaya passing across the Satluj valley region. The predominant NW-SE fast polarization directions (FPDs) in the Lesser and Higher Himalaya follow the strike of surface geological features suggesting structural anisotropy. The NW-SE oriented FPDs in the Tethyan Himalaya fairly coincide with the regional extensional strain. The presence of such extensional strain within the crust might cause Lattice Preferred Orientation (LPO) of anisotropic minerals resulting in observed anisotropy. The large strength of anisotropy (δ t : 0.15–0.80 s) suggests a primary contribution of anisotropy from the middle and lower crust. These observations support the assumption that the deep crust in the study region has undergone widespread and relatively uniform strain in response to crustal shortening and E-W extension.

Magmatism and tectonic setting of proto-Japan during the Early Carboniferous: Constraints from the geochemical characteristics of mafic volcanic rocks in the Hida Gaien Belt, SW Japan

• Early Carboniferous subduction-related basalts–andesites are found in SW Japan. • High-Al basalts were generated by arc tholeiitic magma near crust–mantle boundary. • Basaltic andesites were generated by fractional crystallization of the high-Al magma. • Early Carboniferous tectonic setting of proto-Japan was a primitive island arc . The Hida Gaien Belt in southwest Japan consists mainly of Paleozoic to Mesozoic shelf-facies rocks and records the tectonic evolution of East Asia. Here we report a study of the geochemistry of mafic volcanic rocks from the Early Carboniferous Arakigawa Formation of this belt. Two types of mafic volcanic rock are identified: (1) high-Al basaltic lavas, which were generated by island arc tholeiitic magma that formed under high-pressure conditions near the crust–mantle boundary and in which the nucleation of plagioclase was suppressed and delayed; and (2) basaltic andesitic lavas, which were generated by fractional crystallization of the high-Al tholeiitic magma. Combining our new results with previous geological findings, we consider that the tectonic setting of proto-Japan during the Early Carboniferous was a primitive island arc characterized by active island arc tholeiitic magmatism and the absence of a granitic batholith.

Analysis of Regional and Residual Gravity Disturbance of Major Fault Belts in the Tarim Basin, Western China

Large fault belts often influence the paleo-geomorphic changes in basins and control hydrocarbon accumulation and distribution in basins. Based on the gravity field model European Improved Gravity model of the Earth via New techniques (EIGEN)-6C4, this study calculated the residual Bouguer gravity disturbance of the Tarim Basin by using the minimum curvature method and analyzed gravitational characteristics of major fault belts of the Tarim Basin. The residual Bouguer disturbance exhibits linear residual Bouguer disturbance zones in the Tianshan Mountains, West Kunlun, and the Altyn region, which is consistent with the spatial distribution of their related fault belts. The regional Bouguer disturbance is related to crust–mantle boundary depth, which can be used to roughly estimate crust thickness. Thus, we suggest that the crust–mantle boundary depth order of major faults from deep to shallow is the Altyn region, West Kunlun, and Tianshan Mountains. There is a discontinuity in the residual Bouguer disturbance of West Kunlun, which compares well with the fault belt of West Kunlun. Furthermore, the residual Bouguer disturbance of the Tarim Basin has a series of elliptical areas with a central positive disturbance located within the Bachu uplift and other uplift structures. The residual Bouguer disturbance also reflects the position and distribution of the major fault belts and the boundary of the Tarim Basin, which can provide guidance for dynamic evolution analysis of large basins.

The onset timing of plate tectonics: Controversies,progress, and prospects

<p indent="0mm">Plate tectonics is widely accepted as a fundamental theory in Earth science and is hailed as an important scientific discovery of the 20th century. However, the major challenge of this theory since its foundation for half a century now is the debate over when Earth first developed plate tectonics—4 billion years ago vs. later than 1 billion years ago—an uncertainty which spans nearly three-quarters of Earth history. At present, most researchers view the “earliest evidence of subduction” as the timing of onset of plate tectonics. However, it is debated which phenomena could be viewed as the “oldest evidence”. In this study, we argued that local subduction itself cannot be equated with plate tectonics, but rather, a global subduction network is the essence of plate tectonics. We first conducted comprehensive geological observations along the northern margin of the North China Craton. We recognized key information supporting significant horizontal movement and subduction of an oceanic plate at a convergent tectonic boundary nearly 2 billion years ago, and reconstructed the accurate position of North China in the Earth’s first supercontinent based on its iconic subduction record. According to the prediction of the new proposed geological model, there should be a relic of a subduction interface preserved beneath the North China Craton at the depth of the crust-mantle boundary. We then carried out a high-resolution seismic survey across the northern North China. The seismic profile indeed revealed the geophysical interface preserved by the ancient continental convergence. These features identified are identical to the surface and deep structure of the modern Himalayan orogenic belt, indicating that North China underwent a similar geological process at 2 billion years ago. Because we have assessed the geology and geophysics of the ancient Tethyan Realm, and concluded that subduction has been the main driver of plate motion over the last 500 million years, accordingly, the plate tectonics should work as a subduction network that was driven by subducting plates. Following this knowledge, we finally put the key findings from China in a global context, and proposed that Earth gave birth to its global plate subduction network 2 billion years ago, leading to the formation of Earth’s earliest supercontinent due to the pulling forces of subduction. Therefore, the supercontinent is the result of the global plate subduction network, which can be used as the signpost to identify the onset of plate tectonics. Only after the onset of the global tectonic network did the Earth start the cyclic assembly and breakup of supercontinents. Such active plate margins led to intensive mass and energy exchange between Earth’s surface and interior, which was an important driving force for maintaining and even enhancing planetary habitability. The onset of plate tectonics signifies the contrasting evolutionary paths among Earth and other planets. Future research involves quantitative analysis on why the plates can be subducted continuously. Innovative systems of data- and model-driven scientific research can help us better understand the Earth’s past and consequently predict its future.

In-situ mineralogical interpretation of the mantle geophysical signature of the Gangdese Cu-porphyry mineral system

The lithosphere provides a reservoir for ore-forming elements introduced by repeated fertilisation during asthenosphere-lithosphere interaction by the slab subduction. Here we use the Gangdese copper metallogenic belt to show how mineralogical study can help to interpret these mantle signatures. We present insights into the in-situ geochronological, geochemical and whole-rock isotopic analyses of minerals from mantle-derived lamprophyres that intruded the Bairong Cu-porphyry deposit in the Gangdese metallogenic belt. Zircon U-Pb dating yields weighted-mean concordant 206Pb/238U ages of 11.69 ± 0.27 Ma to 12.41 ± 0.83 Ma. These zircons show juvenile εHf (t) (+1.0 ∼ +10.9) and low δ18O (5.5‰ ∼ 7.7‰) values. Variable initial 87Sr/86Sr ratios (0.7073–0.7080), εNd(t) values (−6.3 ∼ −5.0) and initial 187Os/188Os ratios (0.1382–0.1591) suggest hybridization between lithosphere mantle and asthenospheric melts. Phlogopites in the lamprophyres reflect partial dissolution and incipient melting with low Mg# values but high Al2O3 contents in core. And these phlogopites are surrounded by a zone with high Mg# values. We interpret this as superheating and hybridization of refractory lithosphere mantle and asthenosphere. The 3-D magnetotelluric and seismic signature of the Gangdese mineral belt reflect a mantle source region comprising asthenosphere, old refractory lithosphere, ponded mafic melts near the crust-mantle boundary, and higher-level magma chambers where more extensive mixing between crust- and mantle-derived melts occurs. The asthenospheric upwelling provides a mechanism for heat transfer and the expulsion of volatile elements. It is generating a new young, fertile lithosphere beneath the southeast Lhasa block, which has driven the mineralization processes in the Gangdese metallogenic belt.

Grain-size-evolution controls on lithospheric weakening during continental rifting

Variation in the effective strength of the lithosphere allows for active plate tectonics and is permitted by different deformation mechanisms operating in the crust and upper mantle. The dominant mechanisms are debated, but geodynamic models often employ grain-size-independent mechanisms or evaluate a single grain size. However, observations from nature and rock deformation experiments suggest a transition to grain-size-dependent mechanisms due to a reduction in grain size can cause lithospheric weakening. Here, we employ a two-dimensional thermo-mechanical numerical model of the upper mantle to investigate the nature of deformation and grain-size evolution in a continental rift setting, on the basis of a recent growth law for polycrystalline olivine. We find that the average olivine grain size is greater in the asthenospheric mantle (centimetre-scale grains) than at the crust–mantle boundary (millimetre-scale grains). This grain-size distribution could result in dislocation creep being the dominant deformation mechanism in the upper mantle. However, we suggest that along lithospheric-scale shear zones, a reduction in grain sizes due to localized deformation causes a transition to diffusion creep as the dominant deformation mechanism, causing weakening of the lithosphere and facilitating the initiation of continental rifting.

Arclogite nature of the Colombian Andes magmatic arc root: A receiver-function approach

Arclogites are an important part of the continental arc root architecture. When overlooked, the crustal structure could be misinterpreted, assuming that the seismically defined Moho necessarily coincides with the crust-mantle boundary. Evaluating the possible presence of lower crustal rocks below the seismic Moho with physical properties resembling those of the underlying mantle, is therefore, of major importance. Through a multi-approach receiver function assessment, which includes shear wave velocity inversion, and determination of plausible crustal seismic velocity distributions from forward modeling, we present new constraints on the seismic structure of the arc root beneath the Colombian Andes magmatic arc. Results suggest a high-velocity arc root with a latitudinally variable thickness, from 13.5 to 20 km. The arc root is composed of a 4 to 6 km thick upper domain with seismic wave speeds 5% slower, and 8.5 to 14 km thick lower domain 5% faster, relative to the underlying uppermost mantle. This lower crustal structure, coupled with documented garnet pyroxenitic xenoliths, supports an arclogite nature for the Colombian Andes arc root.

Genetic Relationship between Subduction of Slab Topographic Anomalies and Porphyry Deposit Formation: Insight from the Source and Evolution of Rio Blanco Magmas

AbstractThe Rio Blanco deposit, which is one of the largest porphyry Cu–Mo deposits in northern Peru, formed coevally with the subduction of the Inca Oceanic Plateau at 12–10 Ma. However, the genetic relationship between the subduction of oceanic plateaus and the porphyry deposit formation remains unclear. Igneous rocks emplaced at 23–12 Ma in northern Peru, including the Portachuela batholith (which hosts the Rio Blanco porphyry complex), are normal calc-alkaline to weakly adakitic. In comparison, the 12–8 Ma igneous rocks, including the ore-related Rio Blanco porphyry complex, have typical adakitic signatures, such as high Sr/Y ratios (up to 180) and LaN/YbN ratios (up to 32). The Rio Blanco igneous rocks (Portachuela batholith and Rio Blanco porphyry complex) have uniform zircon εHf(t) values (+0.3 ± 1.2) and δ18O values (6.5 ± 0.14‰). These geochemical characteristics indicate that the Rio Blanco igneous rocks evolved from mantle-derived parental melts in a long-lived, stable, homogeneous isotopic reservoir at the crust–mantle boundary. However, whereas both the Portachuela batholith and the Rio Blanco porphyry complex formed from hydrous parental magmas (&amp;gt;5 wt %; based on plagioclase hygrometry), the ones of the Rio Blanco porphyry complex seem to be more oxidized, hydrous, and sulfur-rich compared with the older batholitic rocks. Reverse zoning in plagioclase phenocrysts, with a systematic core–mantle–rim variation in An (anorthite) and Fe (total iron) contents, are common in the intermineralization rocks. The An content of the mantles of the plagioclase phenocrysts correlates positively with the Fe content, but in the rims, the An contents significantly decrease while Fe remains constant. The apatite inclusions in the mantles are richer in S (0.24 ± 0.06 wt %) and Cl (1.42 ± 0.32 wt %) than those in the phenocryst cores (S: 0.09 ± 0.07 wt %; Cl: 1.03 ± 0.56 wt %) and rims (S: 0.14 ± 0.09 wt %; Cl: 0.83 ± 0. 35 wt %). These systemic geochemical variations in the plagioclase phenocrysts suggest recharge by S- and Cl-rich melts followed by fluid exsolution. This magma recharge and subsequent fluid exsolution may have triggered porphyry Cu mineralization at Rio Blanco. The coincidence of timing between the geochemical transition and collision (initial subduction) of the Inca Oceanic Plateau with the South American plate may indicate a change in the tectonic regime to a compressional state of stress and a thickening of the crust during the collision. The tectonic transition would have facilitated the fractionation of mantle-derived magma in a deep crustal hot zone, resulting in oxidized, volatile-rich residual melts. Replenishment of the upper-crustal magma chamber by such volatile-rich magmas and the subsequent discharge of fluids are interpreted to be fundamental for porphyry Cu mineralization at Rio Blanco and plausibly for the formation of Late Miocene porphyry ore deposits in northern Peru in general.

Lithosphere beneath the Evolving Tianshan Orogen: Constraints from Xenoliths

Abstract The Ortosuu and Uchkuduk regions of the Tianshan orogen contain a volumetrically small series of basaltic rocks erupted primarily during the late Mesozoic-Paleogene. Petrology, chemical composition, and P-T geotherm data from xenoliths within the basalts characterize the nature of the lithospheric mantle beneath this orogenic belt. Two groups of clinopyroxene can be identified from the studied xenoliths based on their Mg# and trace element patterns. Group 1, primitive clinopyroxenes, has lower Mg# (86–90) and LREE-depleted patterns than group 2, depleted clinopyroxenes, which are characterized by a relatively high Mg#, 91–92, and LREE-enriched patterns. The REE distribution in group 1 clinopyroxenes suggests that they were controlled by partial melting, whereas group 2 clinopyroxenes are far more complex involving partial melting degrees of 6–11%, and later metasomatism by carbonatite and/or silicate melts. Coupled P-T estimations from geothermobarometry indicate that the more fertile group 1 xenoliths were probably derived from the uppermost mantle, and the more depleted group 2 xenoliths were likely derived from a depth close to the crust mantle boundary.

Consistent detachment of supracrustal rocks from a fixed subduction depth in the Cyclades

While subduction zones recycle Earth's crust back into the mantle, high pressure metamorphic rocks exposed at the surface testify to occasional crustal subduction failure. Balancing the budget of Earth's recycling factory requires a fundamental comprehension of how and when some crustal rocks are returned to the surface. Here, we constrain the timing and conditions for rocks that were subducted, exhumed and now exposed on Syros and Sifnos islands (Cyclades, Greece). Detailed petrochronology on domains of supracrustal oceanic, passive margin and continental provenance shows that progressive subduction and exhumation has resulted in imbrication of a former continental margin. The consistency of age groups in 2–4 Myr intervals across two islands suggests episodic triggering of exhumation of coherent slices, similar to nappe stacking in the upper levels of orogens. Similar maximum burial conditions across the domains indicate that their highly variable material properties have not exerted a control on the depth from which they were recovered, and supracrustal exhumation was instead triggered by a steady state mechanism operating at ∼65–70km depth in the subduction zone. We propose that for mechanically strong rocks, consistent exhumation may have been triggered by thermal weakening at the depth of coupling between the subducting slab and mantle wedge corner flow in an extended back-arc. For weakly coupled supracrustal rocks, consistent exhumation may have been triggered by a stepped buoyancy increase for subducting rocks at the crust mantle boundary of a thickened overlying plate. These models help to understand the mechanisms and conditions by which supracrustal rocks may be systematically detached from a downgoing plate.

The crustal configuration of the West and Central African Rift System from gravity and seismic data analysis

SUMMARYThe West and Central African Rift System (WCARS) is the only stable continental geological structure on Earth that is formed by large-scale topographic massifs (swells). However, knowledge of the origin and evolution of the WCARS remains limited mainly due to the scarcity of high-resolution geophysical observations. To better understand the crustal structure beneath the entire WCARS, we use the XGM2016 global gravitational model and the ETOPO1 global topographic-bathymetric model to determine a gravimetric Moho (crust-mantle boundary) model constrained by seismic Moho depth estimates at 41 seismological stations distributed irregularly within the study area. The result reveals a regional Moho deepening to ∼40 km beneath the Hoggar, Aïr and Tibesti Massifs. The largest Moho deepening to ∼46 km is detected beneath the Archean Congo Craton, while the Moho depth under the Adamawa Plateau reaches 42 km. The Moho geometry beneath the Chad Basin, the Chad Lineament and the Termit Basin is relatively even with the Moho depth mostly within 24–26 km. A significant Moho deepening as well as large Moho depth variations within 32–45 km beneath the Saharan Metacraton and the Congo Craton (especially under its northern margin) reflect the metacratonization processes that occurred during the Neoproterozoic. The Niger Delta and the Benue Trough are characterized by a very thin continental crust with the Moho depth varying from ∼20 km in the south along the Atlantic coastline to ∼24 km in the northeastern branch of the Cretaceous Benue Trough around the Garoua-Yola Rift.

Episodic emplacement of the Lingshan Granitic Complex and related two-stage molybdenum mineralization in the Dabie orogenic belt

The Lingshan Granitic Complex consists of a granitic batholith and its proximal Mushan and Xiaofan porphyry stocks, and hosts stockwork-type Mo mineralization within the porphyry stocks and the fine-grained monzogranite, and several vein-type Mo occurrences in the coarse-grained porphyritic monzogranite. All these rocks show similar values of the whole-rock initial 87Sr/86Sr (0.70442–0.70987), εNd(t) (–17.2 to –11.0), and zircon εHf(t) (–27.2 to –9.3), indicating their derivation from partial melting of the Paleoproterozoic-Neoarchean recycled continental crust induced by basaltic underplating at the crust-mantle boundary. The zircon U-Pb and molybdenite Re-Os geochronological results indicate that the Lingshan Granitic Complex was formed by at least two pulses of magmatism at 140–137 Ma and 128–125 Ma, accompanied by two periods of Mo mineralization, which is the epitome of the peak granitic magmatism and related Mo mineralization in the whole Dabie orogenic belt. Combined with previous data, it can be concluded that the discontinuous and episodic nature of magma emplacement in the whole Dabie Mo metallogenic belt has significant consequences for large-scale, multi-staged Mo mineralization. Although Mo mineralization can occur in various lithofacies of the Lingshan Granitic Complex, the industrial Mo enrichment mainly lies within the small shallow emplacement granite porphyry stocks and subordinately within the fine-grained phase of the pluton. These features indicate that the Mo industrial enrichment is not only related to the highly evolved magma, but also more closely related to mechanism of magma emplacement. Rapid decompression induced by rapid ascent of the Mo-bearing granite porphyry stocks, and rapid crystallization of magma induced by quickly cooling due to conductive heat loss to the sidewalls of the fine-grained phase of the outer zone of the granite batholith, are favorable for the ore-fluid exsolution responsible for the magmatic-hydrothermal Mo mineralization.

Seismic Wave Attenuation Characteristics from the Ground Motion Spectral Analysis around the Kanto Basin

In order to study the seismic wave attenuation characteristics of complex plate tectonics in and around the Kanto Basin, based on the focal mechanism and Slab1.0 model, the research area is divided into four regions. The one-step non-parametric generalized inversion technique was used to analyze the seismic wave attenuation characteristics of each region separately. The results show that the seismic path attenuation of earthquakes occurring in the shallow crust (Reg.1) is weak, and the seismic wave refraction at the crust–mantle boundary leads to almost no attenuation over a long hypocentral distance (&gt;60 km), the frequency–dependent inelastic attenuation is also weak with the 0.5–20 Hz quality factor Q = 92.33f1.87. The seismic path attenuation of the upper mantle earthquakes occurring in the Kanto Basin (Reg.2) is strong, and the attenuation curve decreases with the increase of hypocentral distance, which is approximately parallel to the geometric diffusion R−2.0, the frequency–dependent inelastic attenuation is stronger with the quality factor Q = 27.75f1.08. The seismic path attenuation of subduction zone earthquakes (Reg.3 and Reg.4) is more obvious in the high–frequency band and has a frequency correlation, indicating that the attenuation of subduction zone earthquakes includes more inelastic attenuation. The frequency–dependent inelastic attenuation Q of Reg.3 and Reg.4 are 52.58f0.95 and 58.07f0.89, respectively.

Seismology on Mars: An analysis of direct, reflected, and converted seismic body waves with implications for interior structure

The InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) mission has been collecting high-quality seismic data on Mars since early 2019 that provide the first direct observations of its interior structure. Here we report on a complete analysis of the part of the marsquakes known as the low-frequency seismic events (main energy below 1 Hz) that are sensitive to the deep interior. To identify body-wave arrivals in the highly-scattered martian seismograms, we employ four complementary approaches: 1) time-domain envelopes; 2) polarised waveforms and their time-domain envelopes; 3) polarisation analysis; and 4) waveform matching. Through careful application of this processing scheme to each marsquake, we are able to significantly increase the number of phase picks relative to earlier analyses (from 41 to 76), including body-wave arrivals from direct (P and S), reflected (PP, SS, PPP, SSS, and ScS), and converted (Ps and Sp) phases. To constrain the depth of the marsquakes, we also identify depth phases (pP and sS). Following this, we invert an initial set of phase picks for models of interior structure, event distance, and depth, while predicting travel times for seismic phases not identified at the outset. Based on the predictions, we repick (every pick is subject to our processing scheme), thereby enlarging our differential travel-time data set (all picks are relative to the main P-wave arrival), and subsequently re-invert for an updated set of interior structure models, distances, and depths. Proceeding thus, we present updated radial seismic velocity models of the crust, mantle, and core. We observe crustal interfaces at average depths of 10, 25, and 45 km, respectively, of which the former two are interpreted as intra-crustal interfaces and the latter as the crust-mantle boundary. We find an upper mantle structure consistent with a low-velocity zone associated with a thermal lithosphere and a thermal gradient in the range 2.4–2.9 K/km that extends to a depth of ~450 km. The thermal structure of the Martian mantle indicates potential and core-mantle-boundary temperatures in the ranges 1650–1750 K and 1900–2100 K, respectively, implying an entirely liquid core at present. Based on the identification of ScS phases, we obtain an improved estimate of the Martian core radius (1820–1870 km) and mean core density (6–6.2 g/cm3).