Hot Mantle Research Articles

Petrogenesis of the > 2.90 Ga komatiite in the Sujiagou region of the North China Craton: Implications for Archean mantle source and geodynamic setting

Komatiite, characterized by high MgO content and low H2O concentration, is crucial to understanding the Archean magma processes and crust–mantle differentiation. Although well preserved in most cratons, Archean komatiites are scarce in the North China Craton (NCC). The Sujiagou komatiites, as the most typical Archean komatiites in China, were exposed in the Luxi terrane, but their geochronology and petrogenesis types remain controversial. In this study, our new Re–Os data indicate the Sujiagou komatiites originated before 2.90 Ga. The felsic zircons discovered in the komatiites, coupled with Nb–Th–La simulation, suggest ∼ 2 % crustal contamination of the komatiites. Thus, the least-altered, spinifex- and massive-texture samples that underwent unconspicuous crustal contamination were selected as proximate proxies for the primary magma composition. The Al/Ti ratios of the Sujiagou komatiites are comparable to or slightly exceed those of chondrite, suggesting they were aluminium-undepleted komatiites and formed at ∼ 8 GPa. The primitive mantle-normalized rare earth element (REE) patterns show depleted light REE and flat heavy REE, suggesting approximately 50 % partial melting based on REE simulation. Additionally, the komatiites exhibit slightly depleted Os isotope ratios, with an average γOs(t) value of -0.3, indicating the magma source is comparable to or slightly depleted to chondrite. Simulation of the relevant Nb/Zr and La/SmPM ratios further suggests the mantle source became slightly depleted due to ca. 1 % melt extraction. Furthermore, the Mg isotopic composition of the Sujiagou komatiites (δ26Mg = -0.23 ± 0.03 ‰) is consistent with that of the primitive mantle, indicating the absence of recycled carbonate material in the mantle source. Both Re–Os and Mg isotopes reveal limited involvement of crustal or carbonate material in the mantle source, implying the absence of oceanic slab subduction in the Luxi terrane during this time. The formation of the Sujiagou komatiites can be attributed to a hot mantle plume, resulting from high potential and eruption temperature. The identification of felsic zircons in the Sujiagou komatiites indicates the presence of an ancient continental nucleus in the Luxi terrane. Combined with the spatially adjacent terrigenous clastic sedimentary rocks, we suggest that the Sujiagou komatiites erupted most likely in a continental margin of the old terrane.

Crust and uppermost mantle S-wave velocity structure of the Wudalianchi volcanic belt from direct surface-wave tomography

The Wudalianchi volcanic belt is located at the junction among the Greater Xing’an range, the Lesser Xing’an range, and the Songliao basin. Investigation of detailed crustal velocity structure under the Wudalianchi volcanic belt is of great significance to better understand the deep geodynamics of Northeast Asia. Applying the direct surface-wave tomography method to ambient noise records of our newly deployed dense WAVESArray stations from 2015 to 2019, we aim to determine a high-resolution three-dimensional (3-D) S-wave velocity (Vs) model of the crust and uppermost mantle down to 45 km depth beneath the Wudalianchi volcanic belt and surrounding areas. The WAVESArray stations have good azimuthal coverage around the Wudalianchi volcano. Our high-resolution tomographic model reveals low-Vs anomalies in the upper crust and the uppermost mantle beneath the Wudalianchi volcano and high-Vs anomalies in the mid-lower crust, indicating the presence of hot mantle upwelling and lower crust cooling solidification. This upwelling process is related to the complex dynamics of the big mantle wedge that has developed above the subducted Pacific slab that is stagnant in the mantle transition zone beneath East Asia. In addition, low-Vs anomalies are visible beneath the Erkeshan, Wudalianchi, Keluo, and Xunke volcanoes in the upper crust and they are connected with a low-Vs layer in the mid-lower crust, which provides a new piece of seismological evidence for the homology of these volcanoes, suggesting that there might be exchanges of material and energy between these volcanoes.

Slab‐Plume Interactions Beneath Australia and New Zealand: New Insight From Whole‐Mantle Tomography

AbstractAustralia, New Zealand, and the surrounding regions have experienced complex plate interactions with significant seismic and volcanic activities. The Taupo volcano on the North Island of New Zealand has experienced multiple catastrophic eruptions. Although Australia is known as a stable landmass with low seismic and volcanic activity, intraplate volcanoes along its eastern coast are considered to be caused by hot mantle plumes. To better understand the seismic and volcanic activities in the region, it is necessary to study the detailed 3‐D structure of the crust and mantle. Here we apply a well‐established global tomography method to reveal the 3‐D P‐wave velocity () structure of the whole mantle beneath this region. We used ∼7 million P, pP, PP, PcP, and Pdiff wave arrival times of 23,666 earthquakes recorded at 14,181 seismograph stations worldwide. The resulting tomography clearly shows high‐ subducted slabs, and low‐ anomalies above and below the slabs, which may reflect corner flow in the mantle wedge and subslab hot mantle upwelling (SHMU), respectively. A slab window is revealed beneath the North Island of New Zealand. Given the development of SHMU beneath this region, the catastrophic eruptions of the Taupo volcano might be powered by a mixture of island arc magma and SHMU through the slab window. Beneath the intraplate volcanoes along the eastern coast of Australia and the Tasman Sea, a thin low‐ zone exists and extends down to the core‐mantle boundary, suggesting that the intraplate volcanoes might be, at least partially, fed by a plume rising from the lower mantle.

Tectonic evolution of the Early Paleozoic proto-East Asian continental margin: Inference from Paleozoic metamorphic rocks in the South Kitakami belt, northeast Japan

To constrain the incipient Pacific-type orogeny and tectonic processes in the Early Paleozoic proto-East Asian continental margin, the Motai–Matsugadaira–Yamagami (MMY) metamorphic rocks in the South Kitakami belt, northeast Japan were investigated. They are divided into two different types: amphibolite-facies rocks associated with serpentinite and blueschist-facies rocks associated with pelitic and psammitic schists. Three geochemical groups are identified from the MMY metamorphic rocks. Groups 1 and 2 resemble geochemical characteristics of mid-ocean ridge basalt and continental arc rocks, respectively. Group 3 exhibits considerable depletion of highly incompatible elements, which is caused by the high degree of partial melting of a hot mantle plume. The zircon U–Pb ages of Group 1 indicate that the protoliths experienced amphibolite-facies metamorphism soon after their formation in the Early Ordovician. Group 2 exhibits a coeval zircon U–Pb age with Group 1. The age distribution of detrital zircons in the MMY psammitic schists shows a peak of 500–400 Ma, the presence of Archean to Neoproterozoic zircons, and the youngest Late Devonian zircon. The following model is proposed for the tectonic evolution of the proto-East Asian continental margin: (1) the formation of an arc in the eastern margin of the South China craton in the Cambrian to Ordovician; (2) the subduction of a spreading axis and an oceanic plateau at the same time as the continental arc formation; (3) the consumption and subduction of arc materials by tectonic erosion; and (4) the formation of the Carboniferous accretionary complex and high-pressure metamorphic rocks under steady oceanic plate subduction. The proposed tectonic evolution model may also be applicable to equivalent Early Paleozoic rocks in southwest Japan.

Petrogenesis of flood basalts and shield basanite from Mertolemariam- Abamineos area, northwestern Ethiopian plateau: Assessments for mantle source variations

Petrographic and geochemical data (major and trace elements) are presented for Oligocene flood basalts and Miocene shield lavas from the Mertolemariam-Abamineos area, northwestern Ethiopian Plateau to examine the petrogenesis of the erupted magmas and the nature of mantle source compositions. The Mertolemariam flood basalts have mainly aphyric and plagioclase phyric textures. The Abamineos shield lavas have sparsely clinopyroxene phyric to highly plagioclase-clinopyroxene phyric textures. Geochemical classification shows that the Mertolemariam Oligocene flood basalts are sub-alkaline in composition, whereas the Abamineos Miocene shield lavas are highly alkaline in composition. The Abamineos shield has a unique composition (i.e., basanites) compared with all other shield basalts (transitional to alkaline) in the northwestern Ethiopian Plateau. Major and trace element compositions display two distinct trends between Mertolemariam flood basalts and Abamineos shield basanites. Fractional crystallization, partial melting, and crustal contamination of a homogeneous mantle source cannot explain the compositional variations between the Mertolemariam flood basalts and the Abamineos shield basanites. The trace element composition suggests that the Mertolemariam Oligocene flood basalts were more likely generated from the mixing of OIB (mantle plume) and E-MORB (enriched asthenosphere) mantle components. The Abamineos Miocene shield basanites were derived from the mantle plume (OIB) component. LREE/MREE and LREE/HREE indicate that both groups possibly originated from a mantle source within the stability field of spinel and garnet. In comparison, the Mertolemariam flood basalts were formed by a higher degree of partial melting from relatively shallow depths than the Abamineos shield basanites. We propose a scenario that explains the magmatic genesis in the northwestern Ethiopian Plateau: volcanism initiated by the initial arrival of the mantle plume (OIB-like) beneath the lithosphere, which comes across a geochemically fertile and enriched MORB (E-MORB) mantle component in the upper asthenosphere. The hot mantle plume triggered melting of the fertile and enriched MORB, and then melting occurred in the plume (OIB-like) at depth in the stability field of spinel and garnet. Melts from the mantle plume (OIB-like) and E-MORB components mixed to produce sub-alkaline flood basalts during the Oligocene in the northwestern Ethiopian Plateau Subsequently, melts from the advanced upwelling mantle plume (OIB-like) produced Miocene shield lavas in the northwestern Ethiopian Plateau.

The Dynamic Crust of Northern Afar and Adjacent Rift Margins: New Evidence From Receiver Function Analysis in Eritrea and Ethiopia

AbstractAfar is undergoing the final stages of continental rifting and hosts the triple junction between the Red Sea, Gulf of Aden, and Main Ethiopian rifts. To better understand the nature of the crust and continental breakup in the region, we calculate teleseismic receiver functions across northeastern Afar and the Danakil microplate, using new data from a regional deployment in Eritrea. We estimate the Moho depth and bulk crustal VP/VS ratio using the H‐κ stacking method. The heterogeneity of our crustal thickness estimates (∼19–35 km) indicates that the Danakil microplate has undergone stretching and crustal thinning. By investigating the relationship between crustal thickness and topographic elevation, we estimate the regional crustal bulk density as ρc ≈ 2,850 ± 20 kg m−3, which is higher than expected, given the crustal thickness of the region. We show that topography is 1.5 ± 0.4 km higher than would be expected due to crustal isostasy alone. We propose that this topography is supported by the same hot mantle upwelling suggested to be responsible for the onset of rifting in East Africa. Uplift is generated due to the presence of a hot thermal anomaly beneath the plate and by thinning of the lithospheric mantle. Our results are consistent with a number of independent constraints on the thermal structure of the asthenospheric and lithospheric mantle. Evidence of melt within the crust is provided by anomalously high VP/VS ratios of >1.9, demonstrating that magma‐assisted extension continues to be important in the final stages of continental breakup.

Unexpectedly High Magma Productivity Inferred From Crustal Roughness and Residual Bathymetry on the Eastern Part of the Ultra‐Slow Spreading Gakkel Ridge Since ∼45 Ma, Eurasian Basin, Arctic Ocean

AbstractThe Gakkel Ridge in the Eurasian Basin has the slowest seafloor spreading worldwide. The western Gakkel Ridge (3°W–85°E; 14–11 mm/a) alternate between magmatic and sparsely magmatic zones, while the eastern Gakkel Ridge (85–126°E; 11–6 mm/a) appears to be dominated by magmatic zones despite ultraslow spreading. Little is known about the seafloor spreading conditions in the past along the entire ridge. Here, we exploit the residual bathymetry and basement roughness to assess the crustal accretion process of the Gakkel Ridge over time using 23 published regional multichannel seismic reflection profiles. Full seafloor spreading rates were faster (20–24 mm/a) up to ∼45 Ma, and residual bathymetry for the older crust is deeper than the world average in the entire Eurasian Basin. There is a sharp transition to 300–400 m shallower residual bathymetry for seafloor <45 Ma in the eastern Eurasian Basin. The crustal roughness versus spreading rate of the western Eurasian Basin is on the global trend, while that of the eastern is significantly below. Both low roughness and shallow residual bathymetry of the eastern Eurasian Basin is close to that of oceanic crust for spreading rates above 30 mm/a, demonstrating increased magmatic production of the eastern Gakkel Ridge since ∼45 Ma. A recent mantle tomography model predicts partial melting in the upper mantle based on the low Vs anomaly underneath. The sedimentary pattern toward the Lomonosov Ridge indicates that this hot mantle anomaly started to cause dynamic uplift of the area at ∼45 Ma.

Thermophysical properties and geochemical characteristics of granites in the Tengchong area: indications for resource potential of hot dry rocks

Ascertaining regional radiogeochemical characteristics and lithospheric thermal structure characteristics is paramount for identifying the resource potential of hot dry rocks. This study conducted thermal conductivity testing and whole-rock analyses of outcrop granites of different formation epochs from the Tengchong area. Moreover, combined with the previously published whole-rock data and the data related to radioactive heat production rates, this study conducted geological, geophysical and geothermal analyses. Key findings are: (1) Granites in the Tengchong area exhibit an average radioactive heat generation rate of 5.54 μ W m −3 . Among them, the Late Cretaceous granites have an average radioactive heat generation rate of 8.08 μ W m −3 , suggesting high heat production granites. (2) Granites in the Tengchong area present an average thermal conductivity of 3.135 W.m −1 K −1 , relatively akin to the average values of granitic rocks worldwide. Compared to granites of other epochs, the Late Cretaceous granites manifest significantly higher thermal conductivities, with an average of 3.28 W.m −1 K −1 . (3) There is a negative correlation between the mica content and the thermal conductivity of the granites in the Tengchong area. (4) The Tengchong area holds potential for hot dry rocks exploitation and is inferred to have a lithospheric structure characterized by hot mantle and cold crust.

Onset of Plate Motion in the Presence of Chemical Heterogeneities in the Mantle and the Effect of Mantle Temperature

AbstractChemical heterogeneities of various origins have been observed in the Earth's mantle. Their assumed higher density acts on the style of mantle convection and therefore, as tectonic plates form the highly viscous upper boundary layer of mantle convection, the onset of surface motion is also affected by chemical heterogeneities. We perform 2D basally heated thermochemical mantle convection models which initially include layers of dense material at different mantle depths. In comparison to purely thermal convection models, the onset of first plate motion is delayed. This delay is even more pronounced, the deeper the location of dense material, the larger its volume and the higher its density. Additionally, we varied the starting temperature of our models. In an initially hot mantle, a strong temperature‐dependent viscosity contrast between the mantle and cold surface leads to a cold, highly viscous plate (stagnant lid). For an initially cold mantle, rising plumes first need to transport basal heat upwards to create a sufficient viscosity contrast. Consequently, the formation and subsequent breaking of the stagnant lid is delayed. Considering a hotter mantle after Earth's magma ocean phase, we find that plate motion can occur within approximately the first 0.5 Gyr of solid‐state convection if no chemical structures are present or for dense material situated at the surface. Deep chemical heterogeneities delay the onset by at least one order of magnitude. Furthermore, the early surface motion will have been more erratic than todays stable plate motion, probably with a few single subduction events.

Plume-lithosphere interaction in the Comei Large Igneous Province: Evidence from two types of mafic dykes in Gyangze, south Tibet, China

Two suites of mafic dykes, T1193-A and T1194-A, outcrop in Gyangze area, southeast Tibet. They are in the area of Comei LIP and have indistinguishable field occurrences with two other dykes in Gyangze, T0902 dyke with 137.7±1.3 Ma zircon age and T0907 dyke with 142±1.4 Ma zircon age reported by Wang YY et al. (2016), indicating coeval formation time. Taking all the four diabase dykes into consideration, two different types, OIB-type and weak enriched-type, can be summarized. The “OIB-type” samples, including T1193-A and T0907 dykes, show OIB-like geochemical features and have initial Sr-Nd isotopic values similar with most mafic products in Comei Large Igneous Provinces (LIP), suggesting that they represent melts directly generated from the Kerguelen mantle plume. The “weak enriched-type ” samples, including T1194-A and T0902 dykes, have REEs and trace element patterns showing within-plate affinity but have obvious Nb-Ta-Ti negative anomalies. They show uniform lower εNd(t) values (–6––2) and higher 87Sr/86Sr(t) values (0.706–0.709) independent of their MgO variation, indicating one enriched mantle source. Considering their closely spatial and temporal relationship with the widespread Comei LIP magmatic products in Tethyan Himalaya, these “weak enriched-type” samples are consistent with mixing of melts from mantle plume and the above ancient Tethyan Himalaya subcontinental lithospheric mantle (SCLM) in different proportions. These weak enriched mafic rocks in Comei LIP form one special rock group and most likely suggest large scale hot mantle plume-continental lithosphere interaction. This process may lead to strong modification of the Tethyan Himalaya lithosphere in the Early Cretaceous.©2024 China Geology Editorial Office.

Big Mantle Wedge and Intraplate Volcanism in Alaska: Insight From Anisotropic Tomography

AbstractWe determine high‐resolution tomographic models of isotropic P‐wave velocity (Vp) and tilting‐axis anisotropy of the Alaska subduction zone using a large number of local and teleseismic data recorded at many portable and permanent network stations in and around Alaska. We find a flat high‐Vp slab in the mantle transition zone (410–670 km depths) beneath western Alaska, which is connected with the subducting Pacific slab at 0–410 km depths, suggesting that a big mantle wedge has formed under western Alaska. Our tilting‐axis anisotropy model reveals complex mantle flows in the asthenosphere. Corner flow in the mantle wedge above the subducting Pacific slab and toroidal flow in the big mantle wedge are revealed, which may cause the Cenozoic intraplate volcanoes in western Alaska and the Bering Sea. In central Alaska, the mantle wedge beneath the Denali volcanic gap is characterized by high‐Vp and subhorizontal fast velocity directions normal to the volcanic arc, which may reflect a remnant of the subducted Yakutat slab. In SE Alaska, the shallow subduction of the Wrangell slab is visible above 150 km depth, and hot mantle upwelling through the Wrangell‐Yakutat slab gap may contribute to the Wrangell volcanic field.

Tracing the Iceland plume and North East Atlantic breakup in the lithosphere

Plumes are domains where hotter material rises through Earth´s mantle, heating also the moving lithospheric plates that may experience thinning or even continental breakup. In particular, the Iceland plume in the NE Atlantic (NEA) could have been instrumental in facilitating the breakup between Europe and Laurentia in the earliest Eocene. Here we present an open access three-dimensional density model of the NEA crust and uppermost mantle that is consistent with previously un-integrated available data. We propose that high-density anomalies in the crust represent the preserved modifications of the lithosphere in consequence of the plate’s journey over the hot mantle plume. Besides, low-density anomalies in the uppermost mantle would represent the present-day effect of the mantle plume and its interaction with the mid-ocean ridges. Overall, the model indicates that the presence of the plume together with the pre-existing crustal configuration controlled the timing, mechanisms and localization of the NEA breakup.

Olivine microstructure and thermometry in olivine‐phyric shergottites Sayh al Uhaymir 005 and Dar al Gani 476

AbstractOlivine‐phyric shergottites are relatively young Martian meteorites that resemble primitive mantle‐derived melts, so offer insight into the causes of recent magmatism on Mars. The Al‐in‐olivine geothermometer offers the potential to examine (near‐)liquidus melt temperatures. However, the ubiquitous shock features in most Martian meteorites, caused by high‐energy impacts, can change the structure and composition of olivine crystals, making the applicability of mineral geothermometry methods uncertain. This study examines microstructure and mineral chemistry in two shocked primitive, depleted olivine‐phyric shergottites, Sayh al Uhaymir (SaU) 005 and Dar al Gani (DaG) 476. DaG 476 is unsuitable for Al‐in‐olivine thermometry because of the presence of difficult‐to‐observe but pervasive networks of undulating veins in olivine down to sub‐micron sizes, caused by melting and providing pathways for cation diffusion. In contrast, SaU 005 can be used for Al‐in‐olivine thermometry despite the presence of conjugate shear and fracture sets and micron‐scale cpx‐spinel exsolution. The average crystallization temperature of Fo>70 olivine in SaU 005, 1380°C, is near‐identical to the average temperature of new and published Fo>70 data from all olivine‐phyric shergottites. When corrected for equilibrium with mantle olivine (Fo80) this corresponds to a mantle temperature of approximately 1500°C, 130°C hotter than ambient Martian mantle when shergottites formed. Shergottites were generated by melting within a moderately hot mantle plume or thermal anomaly, in support of other evidence that the Martian mantle is actively convecting. However, it does not support the extremely high potential temperatures estimated for the shergottite source by a whole‐rock petrological method.

Revisiting the geodynamics of the Middle East region from an integrated geophysical perspective

A long-standing question in geodynamics is whether mantle flow is driven by the plate motion alone, or mantle upwelling makes a significant contribution to it. Subducting slabs and lateral variations of the continental lithosphere can further influence the asthenospheric flow and control its direction. The Middle East region (MER) is a complex continental setting where different processes such as rifting, break-up, plate collision, and tectonic escape kinematically interact with each other. In this context, the role that lithospheric structure, mantle flow, and active upwellings may play is debated. Tomographic images provide a snapshot of the current thermal conditions of a region and seismic anisotropy can also help resolve mantle convection. Here, we synthesize shear-wave splitting observations together with up-to-date tomography models of the mantle structure beneath the MER and other geophysical data. Low-velocity anomalies are seen at asthenospheric depths beneath W Arabia, NW Iran, and Anatolia, suggesting a spreading zone of warm mantle. Two deep low-velocity bodies in Afar and Levant –interpreted as hot mantle plumes– are the sources of this shallower mantle flow. Where low velocities are imaged, we observe predominantly NE–SW oriented anisotropy, anomalously high topography, and abundant basaltic volcanism. The integrated analysis suggests that a horizontal component associated with active upwelling is present in the upper-mantle flow field. The large-scale circulation flow fed by the Afar and Levant Plumes, aided by the subduction-induced forces, facilitates the lateral motion of the Anatolian microplate and affects the dynamic evolution of the Zagros orogen. The proposed scenario demonstrates that the interplay between plate-tectonic events and mantle dynamics controls the kinematics of the region and can explain the general patterns of deformation observed at the surface.

Deep geodynamics and metallogenic mechanism of the South China block: New insight from mantle tomography

The South China block (SCB) is located at the southeastern margin of the Eurasian plate, where intense Mesozoic magmatism and metallogenic belts occurred. The mechanism of mineralization is still unclear and debated. To clarify this issue, we determine a robust 3-D model of P-wave tomography of the upper mantle beneath the SCB by using 77,969 high-quality travel-time data of 934 teleseismic events recorded at 315 permanent and 72 portable stations in the SCB. Our results show that high-velocity (high-V) zones exist in the upper mantle under the Yangtze block and in the mantle transition zone under the eastern segment of the Jiangnan orogen. The former reflects thicker cratonic lithosphere, whereas the latter may reflect the deeply subducted paleo-Pacific plate. Low-velocity (low-V) anomalies are revealed in the upper mantle beneath the Cathaysia block, which are located above the high-V anomaly in the mantle transition zone. These low-V anomalies may reflect hot upwelling flows in the upper mantle, which might be related to the large-scale magmatic activities in the late Mesozoic. Our results suggest that the low-V hot mantle materials beneath the Cathaysia block have risen to the shallow lithosphere along magmatic channels such as fault zones, which eroded the nearby cratonic root of the Yangtze block and the overlying lithosphere and destroyed the lithosphere beneath the eastern SCB. The hot upwelling materials mineralized in the shallow part and so formed many metallogenic belts in the SCB.

Geodynamic regime of the southeastern North China Craton before the Tectono-Magmatic Lull: evidence from ca. 2510 Ma and 2430 Ma igneous rocks in the Xuhuai micro-block

ABSTRACT The Tectono-Magmatic Lull (TML) refers to the period between 2.37 and 2.24 Ga when magmatic activity declined globally, the cause of which remains debated. The North China Craton (NCC), which was formed by assembling several micro-blocks in Neoarchean to Paleoproterozoic, preserves the most records of magmatism across the TML. In this contribution, we report petrological, geochemical, and zircon U-Pb-Hf isotopic data of Paleoproterozoic basement rocks from the Xuhuai (XH) micro-block in the southeastern part of the NCC, and use them to constrain the geodynamic regime at the onset of the TML. Our data reveal that the central part of the XH micro-block is mainly composed of biotite granodioritic gneiss and plagioclase amphibolite, which is divided into a ca. 2510 Ma suite of low- and medium-pressure TTGs and a ca. 2430 Ma bimodal igneous suite including high-pressure TTG and plagioclase amphibolite. Variable magmatic zircon εHf(t) values (1.9 to 7.2), Nb, Ta and Ti anomalies for the 2510 Ma TTGs indicate that the parental magma, which was involved with mantle materials, was generated in an active subduction environment in this pre-TML era. The felsic component of bimodal igneous rocks with high Na/Ta ratios (27 to 30) and stable Mg# (41 to 44) suggests their magma source region was as deep as the rutile stability field, and the delamination-derived asthenosphere mantle ascending. The influx of hot mantle induces thermal weakening and mechanical stretching of the overlying crust, leading to back-arc extension. The results of this study support the hypotheses that the NCC formed from micro-block amalgamation and the trigger of the TML could be the large-scale delamination of the lower crust into the mantle.

Stress regime analysis for the transition to a stagnant-lid convection regime in the terrestrial mantle

A series of numerical simulations of terrestrial mantle convection with temperature-dependent viscosity in a three-dimensional spherical geometry was performed to investigate the thermal structure of the mantle interior and the mechanical condition of the lithosphere. The common “sluggish-lid” convection regime has the thermal structure of the mantle interior with a slowly mobile lid under a moderately temperature-dependent viscosity of mantle rocks, whereas the “stagnant-lid” convection regime has a convection pattern in which the entire surface is covered by a highly viscous lid due to the strongly temperature-dependent viscosity. This study focused not only on the thermal structure of the mantle but also on the mechanical conditions in the lithosphere on an intermediate, transitional convection regime between these end-member convections both under the standard- and the extended-Boussinesq approximations. In this “quasi-stagnant-lid” convection regime, the entire surface of the planet is covered by a highly viscous stagnant-lid that moves slowly, whereas the mantle interior is dominated by a long-wavelength (i.e., degree-one) thermal structure. The stress regime analyses revealed that the strike-slip regime is highly restricted spatially in the lid. If the constitutive laws allow the formation of faults in the future numerical model, the time-dependent formation of weak faults that can initiate plate tectonics may differ among the three convection regimes (i.e., degree-one, quasi-stagnant-lid, and stagnant-lid convection regimes). The range of viscosity contrast of the lid required to realize the “quasi-stagnant-lid” convection regime in the model under the extended-Boussinesq approximation was wider than under the standard Boussinesq approximation, because the adiabatic heating of the mantle increased the mantle temperature with the depth and enhanced the formation of stagnant-lid owing to the strong mechanical decoupling between the cold lid and the underlying hot mantle.

Modes of Mantle Convection, Their Stability, and What Controls Their Existence

AbstractThe motion of Earth's tectonic plates is the surface expression of mantle convection beneath. Analytical convection models have attempted to relate observables, such as plate velocities and surface heat flow, with the thermo‐mechanical state of the mantle, and remain deeply influential in global geophysics. While such models tend to focus on describing the mantle's behavior today, there is evidence that suggests they may not be the best description for an earlier, hotter mantle. Early in Earth's history, higher temperatures may have led to different convective regimes that include active‐lid (today's form of convection), sluggish‐lid, and stagnant‐lid convection. In this study, we adopt and extend the analytical theory laid out by Crowley & O'Connell (2012, https://doi.org/10.1111/j.1365-246X.2011.05254.x) that self‐consistently characterizes the first two of these convection regimes. For a given thermo‐mechanical state, the theory predicts one to multiple solutions that each represent distinct modes of mantle convection. Here, we derive new scaling laws that connect these modes to the mantle's Rayleigh number and identify a new fundamental, dimensionless number, which we term the O’Connell number (named after Richard “Rick” J. O'Connell (1941–2015)), that describes a plate's resistance in context of the overall convection of the system. In addition, we identify two key bifurcations that bracket the Rayleigh‐O'Connell number space within which multiple solutions may exist. Finally, we remove the assumption of steady‐state in their original framework in order to perform a linear stability analysis to characterize the relative stability of each convection mode.

Electrical Response of a Reactivated Ancient Suture Revealed by Three‐Dimensional Magnetotelluric Inversion Across the Jinsha River Suture, Northern Tibetan Plateau

AbstractThe northern Tibetan Plateau offers an excellent opportunity to investigate continental collision involving the Indian and Asian plates in this remote, high‐altitude region. However, the deep structure of the region remains incomplete and controversial, which limits our understanding of the processes of continental convergence. This study presents a new 350‐km‐long magnetotelluric (MT) profile across the Northern Qiangtang and Hoh Xil terranes in the western part of the northern Tibetan Plateau to elucidate the lithospheric structure of the reactivated Jinsha River suture. We carry out a three‐dimensional inversion to acquire the electrical structure beneath the Jinsha River suture, and reveal the presence of large‐scale conductive anomalies in the middle‐lower crust of the Northern Qiangtang and Hoh Xil terranes. The high‐conductivity anomaly beneath the Jinsha River suture extends to >80 km depth and is coincident with a zone of high melt fraction (>5%) in the middle‐lower crust beneath the Jinsha River suture, thereby indicating that this suture has become a rheological channel. The convergence of Indian and Asian lithosphere beneath the northern Tibetan Plateau may be the driving mechanism for the reactivation of this ancient suture. Continental subduction leads to enhanced convection in the mantle. This reactivated suture, which is a tectonically weak zone, provides a conduit for the upwelling of hot mantle material to result in the subsequent melting of the middle‐lower crust. Moreover, our findings provide valuable insights into the geodynamic evolution of the northern Tibetan Plateau, as well as contribute to our understanding of continental convergence processes.

Syn‐Rift Magmatism and Sequential Melting of Fertile Lithologies in the Lithosphere and Asthenosphere

AbstractThe timing and rate of decompression melting of a compositionally heterogeneous mantle during continental rifting are assessed with a new one‐dimensional geodynamic code, MELT1D. MELT1D computes pressure and temperature in the extending lithosphere and rising asthenosphere and calculates the resulting melt fraction and eruption rates for different lithologies. A series of models simulate syn‐rift melt production from (a) dry and wet depleted lherzolite similar to the mantle that underlies most mid‐ocean ridges, (b) dry and wet relatively fertile ultramafic compositions representing plume or primitive mantle material, (c) pyroxenite representing recycled ultramafic oceanic crust or magmatic metasomes, and (d) basalt representing recycled mafic crust or metasomes. The models predict sequential melting of the different compositions that is broadly consistent with basalt eruption histories in many Phanerozoic rifts. Results show a progressive transition in magma sources as the lithosphere thins, beginning with melting of wet mantle and compositionally fertile mafic components near the lithosphere‐asthenosphere boundary during the earliest stages of extension. This transitions to magmatism dominated by melting of relatively fertile ultramafic components (pyrolitic and pyroxenitic compositions) as extension progresses, and finally to melting of ambient lherzolite asthenosphere as lithosphere thinning approaches breakup. Mantle composition, pre‐rift lithosphere thickness, and mantle temperature exert the greatest controls on the timing and volumes of magmas produced from each lithology. In general, a cool or thick lithosphere has a greater capacity to sequester fertile lithologies than thin or warm lithosphere, and thus has a greater capacity to produce early syn‐rift magmas without requiring a hot mantle plume.