Eastern China provides a precious opportunity to explore how subduction drives evolution of the overlying continental lithosphere and to understand the fate of subducted plates. In this study, a ...synthesis of geochronological, whole-rock geochemical and zircon Hf isotopic data is used to examine temporal and spatial variations in distribution, composition and generation of Mesozoic magmas in the northern North China Craton. A compilation of age data reveals over 1000 km of inland-ward migration of a magmatic belt during 185– 145 Ma and then back again after 145– 140 Ma, coincident with the transition from contractional to extensional deformation regime in the very early Cretaceous. Distinct trends in lithologies, geochemistry and NdHf isotopes as a function of age and location are also observed in these magmas. The Mesozoic magmatism and deformation, as well as the lithospheric destruction, across the northern North China Craton is interpreted as the consequence of a change in subduction geodynamic regime of the Paleo-Pacific slab and its interaction with overlying continental lithosphere, which involves an active continental arc at Korean and Liaodong Peninsulas in the early-middle Jurassic, progressive shallowing of the subducting Paleo-Pacific plate in the middle-late Jurassic, and subsequent slab rollback in the early Cretaceous. Considering that trench retreat and slab-roll back are demonstrated as the pre-request of slab stagnation in the mantle transition zone, we further propose that the big mantle wedge structure in East Asia was probably initiated at 145– 140 Ma and was likely fully developed by ~120 Ma. Such a peculiar deep mantle structure governed the post-Cretaceous evolution of the Asian continental lithosphere by mediating the chemical and physical properties of upper mantle.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The North China Craton (NCC) was originally formed by the amalgamation of the eastern and western blocks along an orogenic belt at ∼1.9 Ga. After cratonization, the NCC was essentially stable until ...the Mesozoic, when intense felsic magmatism and related mineralization, deformation, pull-apart basins, and exhumation of the deep crust widely occurred, indicative of destruction or decratonization. Accompanying this destruction was significant removal of the cratonic keel and lithospheric transformation, whereby the thick (∼200 km) and refractory Archean lithosphere mantle was replaced by a thin (<80 km) juvenile one. The decratonization of the NCC was driven by flat slab subduction, followed by a rollback of the paleo-Pacific plate during the late Mesozoic. A global synthesis indicates that cratons are mainly destroyed by oceanic subduction, although mantle plumes might also trigger lithospheric thinning through thermal erosion. Widespread crust-derived felsic magmatism and large-scale ductile deformation can be regarded as petrological and structural indicators of craton destruction.
A craton, a kind of ancient continental block on Earth, was formed mostly in the early Precambrian (>1.8 Ga).
A craton is characterized by a rigid lithospheric root, which provides longevity and stability during its evolutionary history.
Some cratons, such as the North China Craton, can be destroyed by losing their stability, manifested by magmatism, deformation, earthquake, etc.
Three different conceptual models have been proposed for the Cenozoic subduction style in South Asia, including Greater India, Intra‐oceanic Arc, and Continental Terrane (or Greater Indian basin). ...Since these models imply distinctive origins for the Tethyan–Greater Himalayan (TGH) sequences, for example, as a relic of the subducted Greater India or Gondwana–affiliated continental terrane, quantitively reproducing the relic TGH crustal mass with numerical models could help further constrain the debated Cenozoic subduction history between India and Eurasia. Based on the modeling results, we show that the subducted plate since the Paleocene should consist of a significant oceanic portion that is, ∼1,000 km long for the Intra‐oceanic Arc model and up to 2,000 km long for the Terrane model. Our results do not support the existence of a continuous >3,000 km long continental Greater India before the early Eocene collision in South Asia.
Plain Language Summary
Approximately 55 million years ago, the Indian Subcontinent was in the Southern Hemisphere, >3,000 km away from Tibet. However, it remains unknown what tectonic units were between them. Previous studies suggest that this region was composed of either pure land or with parts being oceans. By now, this region has mostly foundered into the deep Earth, with its surface relics forming the Himalayan Mountains. Because land and ocean contribute differently to the relic materials, the mass of the Himalayan Mountain can provide essential information about this lost tectonic region. This study uses numerical models to replicate the relic Himalayan mass while evaluating the earlier proposed models. We found that to match the Himalayan mass, the region between Tibet and India ∼55 million years ago could not be purely continental and should include >1,000 km long oceanic plate.
Key Points
Different models of Cenozoic subduction history in South Asia imply distinctive origins for the Tethyan–Greater Himalayan sequences
Quantitively reproducing the Tethyan–Greater Himalayan mass helps constrain the Cenozoic subduction history in South Asia
Both the Intra‐oceanic Arc and Terrane models, but not the Greater India model, could have been operating during the Cenozoic subduction
Full text
Available for:
FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
The Early Permian magmatism in Tarim, NW China comprises diamondiferous kimberlites, lamprophyres, flood basalts, Fe–Ti oxide ore-bearing layered mafic–ultramafic intrusions, bi-modal dyke swarms, ...alkaline igneous complexes (including syenites and A-type granites), rhyolites and pyroclastic rocks. The extent of this intraplate magmatism exceeds 250,000km2, making it comparable to Large Igneous Provinces (LIPs). Screening of available radiometric ages reveals three main magmatic episodes in the Tarim LIP, with the first being marked by ~300Ma small-volume kimberlites, followed by two phases of bimodal magmatism at ~290Ma and at ~280Ma, respectively. This relatively long time interval of the Early Permian magmatism is consistent with a low eruption rate of the Tarim LIP and is supported by the intercalation of volcanic rocks with sediments in outcrops and drill holes. Although the spatial distribution of each magmatic episode in the Tarim LIP is far from assessed, it seems that the ~290Ma flood basalts are widespread across the province, whereas ~300Ma kimberlites and ~280Ma ultramafic–mafic–felsic intrusions and dyke swarms only occur in the Bachu Uplift and around the margins of the Tarim craton.
We propose that the ~300Ma kimberlites were derived from deep part of the metasomatized sub-continental lithospheric mantle (SCLM), while the ~290Ma flood basalts were likely formed as a result of mixing of plume-derived melts with SCLM-derived melts (e.g., lamproitic melt) as they rose through the SCLM. In contrast, the ~280Ma magmas were most likely derived from the convecting mantle. A plume incubation model is proposed to account for the temporo-spatial distribution of the Tarim LIP, in which different styles of plume–lithosphere interaction are recognized. In the first two episodes, the mantle plume incubating the base of the craton provides the heat that triggered melting of the enriched components in the SCLM. In contrast, adiabatic decompression melting within the plume produced the ~280Ma magmatic phase. Thermal modeling suggests that lithospheric thinning by thermal erosion might have been associated with the upwelling mantle plume, with the greatest thinning occurring in the Bachu area. Thinned spots and weak zones at the margins of cratons and mobile belts caused preferential channeling of plume flow and subsequent decompression melting. This explains the localized distribution of ~280Ma magmas in the Tarim LIP.
•Spatial distribution of igneous rocks in the Tarim large igneous province•Three magmatic episodes possess distinct rock association and geochemistry.•Lithospheric thinning in association with thermal erosion by upwelling mantle•Plume–lithosphere interaction in the formation of the Tarim LIP
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
The Cascadia margin is an unusual subduction zone characterized by the downdip movement of young and thin oceanic plates, where mantle flow and intraslab deformation are still unclear. Here we ...present new anisotropic tomography of the Cascadia subduction zone, in which the hexagonal symmetry axis of anisotropy is tilting rather than horizontal or vertical as assumed in previous studies of seismic anisotropy. Subduction‐induced entrained and toroidal flows under the Cascadia margin are discriminated well by the spatial relationship between tilting‐axis anisotropy and slab geometry. The obliquely entrained flow is trapped in a narrow zone (<100 km wide) above and below the subducting slab and reaches ∼200 km depth, which is surrounded by large‐scale sub‐horizontal toroidal flow. The intraslab anisotropy is trench‐normal above 80 km depth but changes to trench‐parallel at 100–400 km depths, which may reflect fossil anisotropy overprinted by deep deformation beneath the arc, or joint effect of serpentinization and hydrous faulting.
Plain Language Summary
The slab deformation and mantle flow under the Cascadia margin are controversial. We obtain new anisotropic tomography of the Cascadia subduction zone, which can reveal tilting 3‐D fast velocity directions (FVDs) and planes. Because the observed trench‐normal FVD in the Cascadia margin can be explained by both 3‐D toroidal and 2‐D entrained flows, we discriminate the two types of mantle flow with 3‐D FVDs that contain dip angle information. The 2‐D entrained flow above and below the subducting slab is trapped in a narrow zone of ∼100 km wide and is surrounded by large‐scale sub‐horizontal toroidal flow. The slab anisotropy above 80 km depth is consistent with that in the Juan de Fuca plate before subduction, but it is reshaped beneath the arc due to the slab deformation or phase transition.
Key Points
Tilting‐axis anisotropy can reconcile contradictory assumptions of azimuthal and radial anisotropies
Entrained flow occurs within ∼100 km depth above and below the subducting slab and is surrounded by toroidal flow
Fossil anisotropy in the slab is overprinted beneath the arc due to intraslab deformation or phase transition
Full text
Available for:
FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
In subduction zones with slab‐slab interactions, the pattern of mantle convection is very complex and still unclear. In this study, we jointly invert a large number of P and S wave arrival time data ...of local earthquakes for 3‐D isotropic and anisotropic velocity structures of the Banda subduction zone. Along the curved Banda arc, the subducting Indo‐Australian slab is detected clearly as a high‐velocity zone, and its azimuthal anisotropy changes along the arc strike, representing fossil anisotropy within the slab and modified anisotropy by the subduction processes. Around the northern edge of the Banda slab, a semi‐toroidal pattern of anisotropy appears in low‐velocity anomalies, representing mantle flow extruded from the Banda arc and escaped from a gap of the Banda‐Molucca slab toward the northeast. Our 3‐D anisotropic tomography uncovers the mantle convection pattern induced by the slab‐slab interactions, shedding new light on the complex dynamical processes in this curved subduction zone.
Plain Language Summary
The study of mantle convection pattern can improve our understanding of plate tectonics and geodynamics. The mantle convection pattern is affected by several factors, including the subducting slab geometry and slab‐slab interactions. The tectonics in the Banda area is rendered particularly complex due to the subduction and interactions of multiple slabs. Hence, this region is an ideal natural laboratory to study the mantle flow pattern associated with slab‐slab interactions. Here we invert both P and S wave arrival times of local earthquakes for 3‐D anisotropic structure of the upper mantle beneath the Banda region. Our results show that in this curved subduction zone, the mantle convection pattern is different from that in normal subduction zones. Trench‐parallel anisotropy is revealed beneath Banda, which reflects extruded material flow in the curved subduction zone. Our results provide new insight into subduction dynamics and mantle convection.
Key Points
The first P and S wave anisotropic tomography of the Banda subduction zone is obtained
Trench‐parallel anisotropy along the Banda arc reflects lateral mantle flow caused by the highly curved Banda slab
Semi‐toroidal anisotropy around the northern edge of the Banda arc reflects extruded mantle flow from the arc
Full text
Available for:
FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
The Permian Emeishan large igneous province (LIP) in Southwestern China is recognized as a remnant of mantle plume activity. Its crust composition has a significant bearing on understanding the ...magmatic plumbing system in a LIP and the volume of magmas generated, which is critical to the viability of the mantle plume model and its potential environmental impact but is difficult to constrain. Here, a SiO2‐VP/VS‐VS relationship, compiled from existing laboratory measurements, is used to estimate the crustal SiO2 content of the Emeishan LIP based on the joint constraint of the crustal bulk VP/VS ratio and average VS. The crust underneath the northeastern inner zone of the Emeishan LIP has a SiO2 content of ∼55.7 wt%, in contrast with the bulk continental crust's SiO2 content of 62.1 ± 2.1 wt% but largely overlapping with the exposed mafic‐ultramafic intrusions. Such an unusually “mafic” crust is attributable to the intensive magmatic underplating and intraplating associated with the Emeishan mantle plume. It further allows a minimum estimate of the volume of ∼1.45 × 106 km3 for erupted magmas and ∼2.39 × 106 km3 for total emplaced magmas, which is comparable to the volume of typical plume‐generated LIPs.
Plain Language Summary
The Permian Emeishan flood basalts in the Southwestern China have been recognized as a mafic large igneous province (LIP), which is temporally coincident with end‐Guadalupian mass extinction. However, the original volume of this LIP remains poorly constrained due to severe post‐emplacement erosion and tectonic disruption, which makes it difficult to elucidate the effect and mechanism of volcanism on the biosphere. Magmas solidified in the deep crust could survive in the post‐formation processes and thus preserve information about the original volume. Seismic waves travel through the deep crust at different velocities greatly depending on the crustal composition. Laboratory measurements show that the SiO2 content correlates with the shear wave velocity (VS) and its ratio to compressional one (VP/VS ratio). These relationships allow us to use the crustal bulk VP/VS ratios and average VS to estimate the crustal SiO2 content of this LIP. We find an extremely mafic crust (∼55.7 wt% SiO2) in the northeastern inner zone of the Emeishan LIP, that most likely records the intensive magmatic underplating and intraplating associated with the Emeishan mantle plume. Consequently, the minimum volumes of erupted and total emplaced magmas in the Emeishan LIP are estimated to be ∼1.45 × 106 km3 and ∼2.39 × 106 km3, respectively.
Key Points
Crustal SiO2 contents of the Emeishan large igneous province (LIP) are jointly constrained by the crustal bulk VP/VS ratios and VS
Extremely mafic crust (∼55.7 wt% SiO2) in the inner zone of the Emeishan LIP hints at intensive magmatic underplating and intraplating
Minimum volumes of ∼1.45 × 106 km3 for erupted magmas and ∼2.39 × 106 km3 for total emplaced magmas are estimated for the Emeishan LIP
Full text
Available for:
FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Whether Mn carbonates can be used as a proxy for the oxygenation event is debated. Here we examined the Early Cretaceous lacustrine Mn carbonates from North China, which contain abundant microbial ...fossils. The extremely positive δ13C (up to +15‰ relative to Vienna Peedee belemnite) and micro‐area enrichment of Ni strongly indicate a methanogenic archaea origin of these microorganisms. Transmission electron microscope and electron energy loss spectroscopy show the nanoscale transformation of Mn‐oxides (Mg‐exchanged phyllomanganate) to Mn carbonates (kutnohorite), on extracellular polymeric substances. The reaction of the Mn oxides with organic matter resulted in increasing pH and alkalinity, together with the fluctuating pH, offering a suitable micro‐environment for the transformation processes. These Mn carbonates are therefore indicative of an oxidized, sulfate‐absent environment. The depicted scenario serves as a reference to ocean of the early Earth and provides a referable Mn oxide tracer for determining the emergence of the Great Oxidation Event.
Plain Language Summary
Microbial‐mineral interactions have played an important role in the evolution of the Earth. Active methanogenesis exists in submerged sediments, and the biological activity of methanogen is directly or indirectly involved in the process of geochemical cycling and mineral precipitation. In modern ocean environments, the methanogenesis zone and the manganese reduction zone are respectively, located in the lower and upper layers of a chemical zone, so it is difficult for them to interact directly with each other. This study documents evidence for the direct reaction between them in the Early Cretaceous lacustrine sediment cores from north China. Precipitation of Mn carbonates was likely mediated by methanogenesis in couple with Mn oxide reduction. Such an environment resembles the oxygen oases in the ocean before the Great Oxidation Event and thus provides new insights into the marine environment at that time.
Key Points
Abundant methanogen microfossils in Mn carbonates with positive δ13C
Nanoscale transformation from Mn oxides to Mn carbonates
pH fluctuations due to changes in redox conditions promote the formation of kutnohorite
Full text
Available for:
FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
We determine high‐resolution 3‐D tomographic images of P‐wave isotropic velocity, radial anisotropy and azimuthal anisotropy beneath NE Asia down to 800 km depth. Our results show negative radial ...anisotropy (i.e., Vhorizontal < Vvertical) in the asthenosphere of the big mantle wedge (BMW), which may reflect mineral alignment caused by vertical flow in the asthenosphere. Across the Tanlu fault zone (TLF), the western and eastern parts of the BMW exhibit high and low P‐wave velocities, respectively. Combining our tomographic results with surface geological features, we speculate that convection in the BMW includes upwelling asthenosphere beneath the Japan Sea and the Korean Peninsula and downwelling asthenosphere beneath the Songliao and North China basins. The downwelling asthenosphere beneath the two basins is associated with diminishing volcanism and anomalous tectonic subsidence since ∼110 Ma. The great TLF is an important boundary for the BMW structure and dynamics.
Plain Language Summary
The subducting Pacific plate becomes flat in the lower part of the mantle transition zone beneath NE Asia, and a big mantle wedge (BMW) has formed above the flat slab. The BMW controls tectonic and geological activities in NE Asia, which are characterized by large‐scale sedimentary basins, lithospheric thinning, large strike‐slip faults, deep earthquakes and intraplate volcanism. However, the convection pattern in the BMW is still unclear. We determine the first 3‐D P‐wave radial anisotropy model down to 800 km depth beneath NE Asia, as well as high‐resolution tomographic images of isotropic P‐wave velocity and azimuthal anisotropy. We find predominant negative radial anisotropy in the BMW and east‐west variations of velocity structure. Combining the tomographic results with surface geological features, we speculate that convection in the BMW may include upwelling flows beneath the SW Japan Sea and the southern Korean Peninsula and downwelling flows beneath the Songliao and North China basins.
Key Points
The first 3‐D radial anisotropy model beneath NE Asia is obtained
East‐west structural variations in the big mantle wedge (BMW) are revealed
A possible convection pattern in the BMW is proposed
Full text
Available for:
FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Abstract
How serpentinites in the forearc mantle and subducted lithosphere become involved in enriching the subarc mantle source of arc magmas is controversial. Here we report molybdenum isotopes for ...primitive submarine lavas and serpentinites from active volcanoes and serpentinite mud volcanoes in the Mariana arc. These data, in combination with radiogenic isotopes and elemental ratios, allow development of a model whereby shallow, partially serpentinized and subducted forearc mantle transfers fluid and melt from the subducted slab into the subarc mantle. These entrained forearc mantle fragments are further metasomatized by slab fluids/melts derived from the dehydration of serpentinites in the subducted lithospheric slab. Multistage breakdown of serpentinites in the subduction channel ultimately releases fluids/melts that trigger Mariana volcanic front volcanism. Serpentinites dragged down from the forearc mantle are likely exhausted at >200 km depth, after which slab-derived serpentinites are responsible for generating slab melts.
PDF
