Small-scale Mantle Convection Research Articles

Three-Dimensional Lithospheric Electrical Structure beneath the Handan-Xingtai District, North China: Implications for Tectonic Control of Skarn-Iron Mineralization

In this study, we determined the lithospheric electrical structure beneath the Handan-Xingtai district and its adjacent regions using magnetotelluric sounding data. To the west of the Handan-Xingtai district, the crust and upper mantle beneath the Taihang Mountains are mainly characterized by high resistivity (>1000 Ωm, which we interpreted to be the relic cratonic lithosphere. In contrast, the lithosphere beneath the North China Plain to the east shows high-conductivity features (<100 Ωm) overall, which may indicate that it has suffered significant modifications. Additionally, other geological and geophysical studies suggested that this district was located in a significant boundary zone where the lithospheric thickness, temperature and geochemistry properties sharply changed. Combined with our resistivity model, we attributed this to the different degrees of lithospheric modification. Specifically, since the late Mesozoic, the subduction, roll-back and dehydration of the Pacific slab caused an unsteady asthenospheric flow and upwelling; therefore, the deep-derived melts and fluids concentrated within the uppermost mantle had even underplated or intruded into the crust, while this process had a negligible effect on the Taihang Mountains. Small-scale mantle convection and upwelling are likely to occur in this kind of transfer zone of lithospherice properties, leading to mantle-derived melts and fluids transporting upwardly near the surface, which was confirmed by the significantly enhanced conductivity beneath the ore district in our resistivity model. During this process, Fe derived from mantle-source magma or relic Precambrian metamorphic basement beneath the Taihang Mountains was extracted and emplaced along with the Yanshanian magmatism.

Age and origin of the West Kimberley lamproites, Western Australia

New high precision 40Ar/39Ar ages are presented which more tightly constrain the eruption ages of the West Kimberley lamproites of northwestern Western Australia. The data show a southward progression in ages of the lamproites over a distance of ∼135 km, from ∼22.5 Ma in the northernmost lamproites of the Ellendale Field to ∼19 Ma in the Noonkanbah Field in the south, excluding the Walgidee Hill lamproite. The large (∼490 ha) zoned Walgidee Hills lamproite at the northern edge of the Noonkanbah Field is the youngest of the West Kimberley lamproites and represents a later pulse of lamproite magmatism at 17.4 Ma, >1 Ma after the main lamproite magmatism.The southward progression of lamproite ages extends across a wide swath from the Proterozoic King Leopold Orogen at the southern margin of the Kimberley Craton to the Phanerozoic Fitzroy Trough in the Canning Basin. The magmatism does not appear to be the result of the migration of a mantle plume, but more likely reflects small degrees of partial melting of old, thick, metasomatised lithosphere extending from the craton margin beneath the basin, with melting triggered by regional uplift and/or small-scale mantle convection at the craton margin. Emplacement of the lamproites likely exploited ancient trans-lithospheric structures associated with the Proterozoic orogen at the craton margin and structures associated with the formation of the Canning Basin in the Phanerozoic.

Late cretaceous magmatism in the Northern Lhasa subterrane, central Tibet: geodynamic implications

ABSTRACT Compositional changes of successively emplaced magmatites can be used to infer changes in their petrogenetic evolution and geodynamic setting. The central-northern Lhasa terrane hosts extensive Late Cretaceous magmatism, however, the mechanism that triggered the magmatism and its relationship to the directly preceding Late Cretaceous tectonic transition in the central Tibetan Plateau is still unresolved. Therefore, we conducted a detailed petrological, geochronological, geochemical, and isotopic study on recently identified Late Cretaceous doleritic and granodioritic rocks in the Xiongmei area, northern Lhasa subterrane. The potassic (K2O/Na2O ratios >1) dolerites (ca. 80 Ma) display a post-collisional basaltic shoshonitic signature with uniform moderately enriched whole-rock Sr–Nd and zircon–Hf isotopic compositions ((87Sr/86Sr)t = 0.7070 to 0.7095, whole-rock εNd(t) = – 2.4 to – 1.3, and zircon εHf(t) = – 5.9 to +1.9), suggesting that they evolved from a similar parental magma and were likely derived by partial melting of an enriched lithospheric mantle metasomatized by hydrous fluids. The granodiorites are geochemically divided into medium-K and high-K calc-alkaline rocks. The medium-K samples (ca. 80 Ma) are generally characterized by low K2O/Na2O ratios <1, whereas the high-K samples (78–76 Ma) display high K2O/Na2O ratios >1. The medium-K samples have positive zircon εHf(t) values from +4.8 to +6.5 and uniform whole-rock ɛNd(t) values from – 2.4 to – 1.3, which significantly differ from those of the high-K samples that have negative to positive zircon εHf(t) values from – 1.9 to +2.8 and negative whole-rock ɛNd(t) values from – 8.4 to – 5.1. The medium-K samples are characterized by high Sr contents and Sr/Y ratios, coupled with low Y and HREE contents, similar to adakitic compositions, and were interpreted to have been derived from deep partial melting of the foundering juvenile garnet-bearing mafic lower crust at relatively low temperatures. Compared to the medium-K samples, the high-K samples have lower Sr/Y and (La/Yb)N ratios, coupled with flat HREE patterns and distinct negative Eu, Sr and Ti anomalies and are most likely derived from shallower partial melting of the K-rich juvenile lower crust at relatively high temperatures. Considering the occurrence of variable magmatism in the central-northern Lhasa subterrane during the Late Cretaceous, the change in magma composition and lower crustal melting conditions (cold-deep vs. hot-shallow) suggest that the lithospheric mantle keel was partially detached by small-scale mantle convection in response to the low-angle Neo-Tethys-associated subduction between 85 and 70 Ma.

On the origin of the Canary Islands: Insights from mantle convection modelling

The Canary Islands hotspot consists of seven volcanic islands, mainly of Neogene age, rooted on oceanic Jurassic lithosphere. Its complex structure and geodynamic setting have led to different hypotheses about its origin and evolution, which is still a matter of a vivid debate. In addition to the classic mantle plume hypothesis, a mechanism of small-scale mantle convection at the edge of cratons (Edge Driven Convection, EDC) has been proposed due to the close proximity of the archipelago to the NW edge of the NW African Craton. A combination of mantle plume upwelling and EDC has also been hypothesized. In this study we evaluate these hypotheses quantitatively by means of numerical two-dimensional thermo-mechanical models. We find that models assuming only EDC require sharp edges of the craton and predict too narrow areas of partial melting. Models where the ascent of an upper-mantle plume is forced result in an asymmetric mantle flow pattern due to the interplay between the plume and the strongly heterogeneous lithosphere. The resulting thermal anomaly in the asthenosphere migrates laterally, in agreement with the overall westward decrease of the age of the islands. We suggest that laterally moving plumes related to strong lithospheric heterogeneities could explain the observed discrepancies between geochronologically estimated hotspot rates and plate velocities for many hotspots.

Process of lithospheric delamination beneath the Lhasa–Qiangtang collision orogen: Constraints from the geochronology and geochemistry of Late Cretaceous volcanic rocks in the Lhasa terrane, central Tibet

The geochemistry of post-collisional granitoids can be used to reconstruct crustal thickness and constrain deep geodynamic processes. Two series of volcanic rocks of the Jiangba Formation in the Xiongmei area, central Tibet, record the process of lithospheric delamination in the Lhasa–Qiangtang collisional orogen. The andesites and dacites crystallized at c. 87 and 85 Ma, respectively, and have positive zircon εHf(t) values (2.7–5.4 and 2.7–7.1, respectively). They have calc-alkaline compositions, with relatively high Mg# (100 × molar Mg2+/[Mg2+ + Fe2+]; 39–66), Sr contents, and Sr/Y and (La/Yb)N ratios (subscript N signifies normalization to chondrite), low Y and Yb contents, and no clear negative Eu, Sr, or Ti anomalies, similar to adakites. The andesite–dacite series likely formed by partial melting of foundering juvenile mafic lower crust with residual garnet, followed by the interaction of this melt with peridotite. The rhyolite series was emplaced at c. 81 Ma, and has positive zircon εHf(t) values (1.0–3.5). The rhyolites are characterized by non-adakitic signatures and have high-K calc-alkaline to shoshonite compositions, with low MgO contents (Mg# = 21–34) and Sr/Y and (La/Yb)N ratios, coupled with flat HREE patterns and marked negative Eu, Sr, and Ti anomalies. Geochemical data indicate that these rhyolites were generated by partial melting of a much shallower crustal root, with a plagioclase-rich residue. Zircon U–Pb ages and different compositional trends for the two series indicate that the lithospheric mantle keel was partially removed by small-scale mantle convection that triggered the Late Cretaceous high-Mg# adakitic magmatism in the Lhasa–Qiangtang collisional orogen. Partial lithospheric delamination increased heat flow at the base of the crust and caused the thinned crust to melt, producing the low-Mg# non-adakitic rocks.

Edge-driven convection and thinning of craton lithosphere: Two-dimensional thermal-mechanical modeling

There are usually abrupt changes in lithospheric thickness at the boundaries between ancient cratons and adjacent young mobile belts. Lateral variations in temperature and density between the two can trigger small-scale mantle convection (edge-driven convection, EDC). Here, we use two-dimensional thermal-mechanical simulations to explore the EDC caused by the lithospheric step between a craton and a mobile belt, and its role in the thinning of the craton lithosphere. The results show that the impact of EDC on a craton depends on the properties of the craton lithosphere and on their contrasts with the adjacent mobile belt, given the same initial condition. When the craton lithospheric density is relatively large, a high-strength craton has strong ability to resist EDC, and craton lithospheric thinning is limited to the edge. In contrast, the ability of a low-strength craton to resist EDC is weak, and the craton lithosphere is gradually eroded by the downward flow, eventually leading to large-scale thinning of the cratonic lithosphere. When the craton lithospheric density is relatively small, regardless of the strength of the cratonic lithosphere, the craton can well withstand the impacts of EDC. In this case, upwelling flow drives cratonic lithosphere materials to the base of the mobile belt, and lithospheric thinning only occurs at the edge of the craton lithosphere. The Archean North China Craton (NCC) was surrounded by Phanerozoic mobile belts, and its lithospheric thinning first occurred on the northern and eastern margins, a process that lasted for a long time. We suggest that EDC has played an important role in lithospheric thinning of the NCC, in particular the initiation of lithospheric thinning, but we cannot rule out the additional contributions from other mechanisms.

Origin of ocean island basalts in the West African passive margin without mantle plume involvement

The geochemical variabilities in intraplate basalts (IB) from the West African passive margin (WAPM) region, have generally been employed to indicate the presence of recycled materials in an associated upwelling mantle plume. However, the absence of time-progressive linear hotspot tracks in WAPM-IB make it difficult to explain their genesis solely by the mantle plume hypothesis. Here, we show that the Sr–Nd–Hf–Pb isotopic variations in basalts from most of the WAPM-IB could have mainly attributed to the derivation from two types of fusible regions of the refertilized subcontinental lithospheric mantle (SCLM) and the sub-lithospheric mantle. The locations and magma genesis of WAPM-IB are strongly related to the distance from the Mesozoic rift axis and the structure of the rifted SCLM. The melting of the source region can possibly be attributed to small-scale mantle convection at the base of the SCLM without the involvement of a mantle plume.

Origin and magmatic evolution of the Quaternary syn-collision alkali basalts and related rocks from Salmas, northwestern Iran

Quaternary alkali basalts and related rocks are located in the northern portion of the Sanandaj–Sirjan zone in Iran (SaSZ). These rocks, also known as the Salmas Quaternary volcanic rocks (SQVR), consist primarily of basalts and trachy basalts (mafic parts) and basaltic trachy andesites and trachy andesites (intermediate parts), which are highly sodic alkaline and high-K calc-alkaline in character, respectively. These mafic to intermediate samples contain variable proportions of pyroxene, olivine and plagioclase, alongside ubiquitous accessory levels of biotite, amphibolte and oxides. Based on the negative Nb, Ta, Zr, and Hf, and also positive Th and U anomalies in some samples, the SQVR formed in a widely-recognized syn-collision environment following the termination of oblique Neotethys subduction beneath the Central Iranian microplate. Following slab break-off of the remnants of the oceanic lithosphere under the Arabia-Eurasia collision zone, a combination of asthenospheric upwelling, small-scale mantle convection, and localized crustal extension promoted magmatism. We model that decompression melting occurred on the boundary of the lithosphere and asthenosphere at around 65–70 km. The magma of the SQVR resulted from relatively small degrees of partial melting (<5%) of a metasomatized amphibole-bearing garnet-spinel lherzolite. Fractional crystallization, combined with a small amount of assimilation and fractional crystallization occurred during transfer to the surface and resulted in the evolution of basaltic parent magmas towards intermediate compositions.

Application of Geoid Anomalies to the Tectonic Research in the East Asian Continental Margin

In this paper, we calculated multi-scale residual geoid anomalies with the method of geoid separation processing, according to EGM2008 ultra-high order gravity field model, remove-restore technique and Stokes integral. The East Asian continental margin was selected as the study area. The residual geoid anomalies have been calculated by programming. On the basis of residual geoid anomalies at various orders, the interlayer geoid anomalies at different depths were calculated to depict the spatial distribution characteristics of the residual geoid. Finally, we conducted a detailed geophysical interpretation for the study area according to the geoid anomalies in combination with other geophysical datasets. Four conclusions can be outlined as follows: 1) it is impracticable that geoid anomalies are used in the interpretation of the shallow objects due to the influence of the terrain; 2) the anomalies of residual geoid can reflect the intensity of small-scale mantle convection in the asthenosphere; 3) the interlayer geoid anomalies can reflect the magmatic activities associated with the mantle convection and mantle plume in different scales; 4) the study of the geoid may provide an approach for the research of the subduction zone, mantle convection and mantle plume.

Clustering of arc volcanoes caused by temperature perturbations in the back-arc mantle

Clustering of arc volcanoes in subduction zones indicates along-arc variation in the physical condition of the underlying mantle where majority of arc magmas are generated. The sub-arc mantle is brought in from the back-arc largely by slab-driven mantle wedge flow. Dynamic processes in the back-arc, such as small-scale mantle convection, are likely to cause lateral variations in the back-arc mantle temperature. Here we use a simple three-dimensional numerical model to quantify the effects of back-arc temperature perturbations on the mantle wedge flow pattern and sub-arc mantle temperature. Our model calculations show that relatively small temperature perturbations in the back-arc result in vigorous inflow of hotter mantle and subdued inflow of colder mantle beneath the arc due to the temperature dependence of the mantle viscosity. This causes a three-dimensional mantle flow pattern that amplifies the along-arc variations in the sub-arc mantle temperature, providing a simple mechanism for volcano clustering.

Destruction of the Wyoming craton: Seismic evidence and geodynamic processes

Cratons are old and strong continental cores where the lithosphere is thick and remains largely undeformed for 2–3 b.y. Unlike typical cratons, the Wyoming craton underwent pervasive deformation ca. 80–55 Ma during the Laramide orogeny in the west-central United States, and has been subsequently encroached upon by the Yellowstone hotspot since 2.0 Ma. However, the mechanism for the deformation and the craton-hotspot interaction are not well understood. We present here a three-dimensional shear wave velocity model beneath the Wyoming craton constrained from Rayleigh wave data, which reveal new details about the cratonic lithosphere. The average lithosphere thickness beneath the craton is ∼150 km, significantly thinner than a normal cratonic root (>200 km). Continuous low velocities are observed beneath the Yellowstone hotspot and the Cheyenne belt. A low-velocity column is also present in the central-eastern craton at depths of 115–250 km. These low velocities can be explained by hot temperature and partial melting, implying mantle upwelling. A high-velocity anomaly with a dripping shape in central Wyoming extends to 200–250 km depth, indicating mantle downwelling and lithosphere erosion. Our model provides the first seismic evidence for complex small-scale mantle convection beneath the Wyoming craton. The convection probably developed during the subduction of the Farallon plate and has been reinforced by the Yellowstone hotspot. We propose that the combination of flat-slab subduction, small-scale convection, and hotspot activity can lead to massive destruction of a cratonic lithosphere.

Crustal structure north of the Taiping Island (Itu Aba Island), southern margin of the South China Sea

Based on the multi-channel seismic (MCS) and gravity data offshore north of Taiping Island (Itu Aba Island) in the Spratly Islands (Nansha Islands), we revisited the crustal structures in the northern part of the southern margin of the Southwest (SW) Sub-basin of the South China Sea (SCS). The MCS data suggest that the basement structural highs in the southwest margin of the SCS are dominated by both fault blocks and volcanic basement structures that probably formed along with the basement faults. The gravity modeling results reveal that these volcanic basement structures were probably associated with the high velocity or density lower crust (HVDLC), suggesting that the southern margin of the SW Sub-basin serves as an intermediate margin. Based on the tectonic features in the SCS margins, including (1) the transition between the magma-poor margins of the Northwest (NW) Sub-basin and intermediate margins of the East and SW Sub-basins; (2) the cessation of the seafloor spreading in the NW Sub-basin being succeeded by a major ridge jump event, which was very likely to reflect an active mantle upwelling event; (3) the extensive distribution of the HVDLC beneath in the well-investigated northern margin of the East Sub-basin and as explored in the SW Sub-basin in this study; and (4) basement faulting and volcanism shown in this study that may be related to the HVDLC and hyper-extension, we proposed a conceptual model to interpret the origin of the HVDLC in the southern margin of the SW Sub-basin. The margin of the SCS basin was magma-poor at the time it occurred. An active mantle upwelling event caused by small-scale mantle convection, possibly related to the influence of the nascent Hainan mantle plume, was formed subsequently, resulting in a southward ridge jump and the HVDLC beneath the current margins of the East and SW Sub-basins. The East and SW Sub-basins continued to spread, latitudinally dividing their HVDLCs as the margins separated accordingly. Afterwards, the hyper-extension in the distal margins may also be responsible for a crust weakening zone wherein the subsequent volcanic events could occur.

Regimes of subduction and lithospheric dynamics in the Precambrian: 3D thermomechanical modelling

Comparing the early Earth to the present day, geological–geochemical evidence points towards higher mantle potential temperature and a different type of tectonics. In order to investigate possible changes in Precambrian tectonic styles, we conduct 3D high-resolution petrological–thermomechanical numerical modelling experiments for oceanic plate subduction under an active continental margin at a wide range of mantle potential temperature TP (∆TP=0−250K, compared to present day conditions). At present day mantle temperatures (∆TP=0K), results of numerical experiments correspond to modern-style subduction, whereas at higher temperature conditions important systematic changes in the styles of both lithospheric deformation and mantle convection occur. For ∆TP=50−100K a regime of dripping subduction emerges which is still very similar to present day subduction but is characterised by frequent dripping from the slab tip and a loss of coherence of the slab, which suggests a close relationship between dripping subduction and episodic subduction. At further increasing ∆TP=150−200K dripping subduction is observed together with unstable dripping lithosphere, which corresponds to a transitional regime. For ∆TP=250K, presumably equivalent to early Archean, the dominating tectonic style is characterised by small-scale mantle convection, unstable dripping lithosphere, thick basaltic crust and small plates. Even though the initial setup is still defined by present day subduction, this final regime shows many characteristics of plume-lid tectonics. Transition between the two end-members, plume-lid tectonics and plate tectonics, happens gradually and at intermediate temperatures elements of both tectonic regimes are present. We conclude, therefore, that most likely no abrupt geodynamic regime transition point can be specified in the Earth's history and its global geodynamic regime gradually evolved over time from plume-lid tectonics into modern style plate tectonics.

Seismic discontinuities beneath the southwestern United States from S receiver functions

S-receiver functions along the Colorado Plateau-Rio Grande Rift-Great Plains Transect known as LA RISTRA in the southwestern United States have been utilized to map seismic discontinuities beneath this tectonically active region. Individual receiver functions were stacked according to ray piercing points with moveout corrections in order to improve the signal-to-noise ratio of the converted S-to-P phases. A mantle discontinuity, which is interpreted as the lithosphere-asthenosphere boundary (LAB), is observed along the profile with depth ranging from 80km beneath the Rio Grande Rift (RGR) to 100km beneath the Great Plains (GP) and 120–180km beneath the Colorado Plateau (CP). The shallow LAB beneath the Rio Grande Rift is indicative of lithosphere extension and asthenosphere upwarp. The LAB deepens sharply at the RGR-CP and RGR-GP boundaries, providing evidence for edge-driven, small-scale mantle convection beneath LA RISTRA. Two local discontinuities beneath the southeastern Colorado Plateau are imaged at ~250km and ~300km and could be the top and base of the eroded lithosphere, respectively. The S receiver function images suggest that edge-driven, small-scale convection is probably the mantle source for recent extension and uplift in the Rio Grande Rift and the Colorado Plateau.

Melt evolution beneath a rifted craton edge: 40Ar/39Ar geochronology and Sr–Nd–Hf–Pb isotope systematics of primitive alkaline basalts and lamprophyres from the SW Baltic Shield

A new high-precision 40Ar/39Ar anorthoclase feldspar age of 176.7±0.5Ma (2-sigma) reveals that small-volume alkaline basaltic magmatism occurred at the rifted SW margin of the Baltic Shield in Scania (southern Sweden), at a time of global plate reorganization associated with the inception of Pangea supercontinent break-up. Our combined elemental and Sr–Nd–Hf–Pb isotope dataset for representative basanite and nephelinite samples (>8wt.% MgO) from 16 subvolcanic necks of the 30 by 40km large Jurassic volcanic field suggests magma derivation from a moderately depleted mantle source (87Sr/86Sri=0.7034–0.7048; εNdi=+4.4 to +5.2; εHfi=+4.7 to +8.1; 206Pb/204Pbi=18.8–19.5). The mafic alkaline melts segregated from mixed peridotite–pyroxenite mantle with a potential temperature of ∼1400°C at 2.7–4.2GPa (∼90–120km depths), which places ultimate melt generation within the convecting upper mantle, provided that the lithosphere–asthenosphere boundary beneath the southern Baltic Shield margin was at ⩽100km depth during Mesozoic–Cenozoic rifting. Isotopic shifts and incompatible element enrichment relative to Depleted Mantle reflect involvement of at least 20% recycled oceanic lithosphere component (i.e., pyroxenite) with some minor continent-derived sediment during partial melting of well-stirred convecting upper mantle peridotite. Although pargasitic amphibole-rich metasomatized lithospheric mantle is excluded as the main source of the Jurassic magmas from Scania, hydrous ultramafic veins (i.e., hornblendite) may have caused subtle modifications to the compositions of passing sublithospheric melts. For example, modeling suggests that the more radiogenic Hf (εHfi=+6.3 to +8.1) and Pb (206Pb/204Pbi=18.9–19.5) isotopic compositions of the more sodic and H2O-rich nephelinites, compared with relatively homogenous basanites (εHfi=+4.7 to +6.1; 206Pb/204Pbi=18.8–18.9), originate from minor interactions between rising asthenospheric melts and amphibole-rich metasomatic components. The metasomatic components were likely introduced to the lithospheric mantle beneath the southern Baltic Shield margin during extensive Permo-Carboniferous magmatic activity, a scenario that is supported by the geochemical and isotope compositions of ca. 286Ma lamprophyres from Scania (87Sr/86Sri=0.7040–0.7054; εNdi=+2.0 to +3.1; εHfi=+6.1 to +9.0; 206Pb/204Pbi=17.8–18.2).Strong variations in lithosphere thickness and thermal structure across the southern Baltic Shield margin may have caused transient small-scale mantle convection. This resulted in relatively fast and focused upwellings and lateral flow beneath the thinned lithosphere, where mafic alkaline magmas formed by low degrees of decompression melting of sublithospheric mantle. Such a geodynamic scenario would allow for enriched recycled components with low melting points to be preferentially sampled from the more depleted and refractory convecting upper mantle when channeled along a destabilizing craton edge. Similar to the ‘lid effect’ in oceanic island volcanic provinces, lithospheric architecture may exert strong control on the mantle melting regime, and thus offer a simple explanation for the geochemical resemblance of continental and oceanic intraplate mafic alkaline magmas of high Na/K affinity.

Double layer anisotropy beneath the New Madrid seismic zone and adjacent areas: insights from teleseismic shear wave splitting

A total of 93 well-defined PKS, 54 SKKS, and 126 SKS shear-wave splitting parameters are determined at 25 broadband seismic stations in an approximately 1000 by 1000 km&lt;sup&gt;2&lt;/sup&gt; area centered at the New Madrid seismic zone (NMSZ) in order to test the existence of two anisotropic layers and to map the direction and strength of mantle fabrics. The individual splitting parameters suggest a significant and systematic spatial and azimuthal variation in the splitting parameters. The azimuthal variations at most stations can be explained as the results of present SW ward asthenospheric flow and NNE trending lithospheric fabrics formed during past orogenic events. In the NMSZ, rift-parallel fast directions (potentially related to a long-rift flow) and rift-orthogonal fast directions from small-scale mantle convection are not observed. In addition, reduction in splitting times as a result of vertical asthenospheric flow is not observed.

小尺度地幔对流及其与层序地层学的相关性

小尺度地幔对流理念的提出距今已有20多年，人们用它来解释一些较异常的地质现象，如青藏高原、天山的快速隆起等。这些构造的沉降与隆升直接影响着层序界面的划分，其形成时间也影响着层序级次的划分。根据Petersen的数值模拟实验显示，小尺度地幔对流的作用周期为2~20个百万年，与三级层序的周期范围最为吻合，对于区域构造的升降及海平面变化具有不可忽略的影响。虽然小尺度地幔对流对海底地形影响不大，但对于被动和主动大陆边缘，内陆塑、刚性岩石圈均有影响。笔者认为在进行层序划分时应考虑小尺度地幔对流的存在。 The concept of small-scale mantle convection has been proposed for more than 20 years. We use it to explain some of the more unusual geological phenomena, such as the rapid uplift of the Tibetan Plateau and the Tianshan etc. These tectonic subsidence and uplift are a direct impact on the division of sequence boundaries as well as the formation time. According to Petersen’s numerical simulations, it shows that small-scale convection driven stratigraphic sequences occur at periods of 2 to 20 million years. By correlating the third-order sequences, it is very important for the regional tectonic movements and sea level changes. Although small-scale mantle convection has little effect on the seafloor to- pography, it affected the passive and active continental margins, inland plastic and rigid lithosphere very differently. The author holds that the existence of small-scale mantle convection should be considered when dividing the sequences.

Numerical modeling of extensional sedimentary basin formation with MATLAB: Application to the northern margin of the South China Sea

The coupled simple-shear/pure-shear model (CSSPSM) with broad application for studying the evolution of continental extensional sedimentary basins was proposed by Kusznir and co-workers. It can be used to determine the geometry of sedimentary basins and their crustal structure by integration of the rheological, thermal and isostatic response of lithosphere to various loads caused by lithosphere extension. We developed a MATLAB code, MODBAS, to model extensional sedimentary basin formation based on the CSSPSM. The validity of the code was tested with a single listric fault model and a multiple-fault model. The application of the code in the Pearl River Mouth Basin (PRMB) on the northern margin of the South China Sea demonstrates (1) the effective elastic thickness of the lithosphere beneath the PRMB is very low (<5km), which may support the idea of a very weak continental crust beneath the northern margin of the South China Sea; and (2) there are significant misfits between predicted and observed basements beneath the two high-standing areas on the profile, which was interpreted as an indication of substantial dynamic support from small-scale mantle convection.

Seismic rays and traveltime tomography of strongly heterogeneous mantle structure: application to the Central Mediterranean

SUMMARY The structure of the Earth’s upper mantle near convergent plate margins, such as along the Nubia‐Eurasia collision zone in the Mediterranean, involves strong seismic wave speed contrasts associated with subducting lithospheric slabs and opening backarc basins. In this environment, seismic wave propagation is strongly influenced by heterogeneity, and requires appropriate modelling practice. Although accurate numerical methods are often used to model seismic traveltimes in the crust, only approximate techniques have been used for the mantle, on the assumption that speed contrasts are weaker. We devise, optimize, and test a method aimed at recovering strongly heterogeneous mantle structures using a finite-difference scheme to calculate first-arrival traveltimes and trace seismic rays with high accuracy even in the presence of strong gradients. We adapt this forward scheme—successfully used in local-scale tomography—to spherical geometry through source-specific Earth flattening approximations, and we split calculations in meshes with different step size to model optimally the crust (with a 2 km step) and the mantle (6 km step). We then use an iterative non-linear inversion approach, starting from a simple 1-D prior model. We test the ability of this procedure to reconstruct sample structures, devised to be illustrative for the Mediterranean region, using synthetic data calculated on the real distribution of sources and stations reported by the Bulletins of the International Seismological Centre (ISC). Besides regular checkerboard patterns, we also reconstruct a more representative model. Different strategies are used and compared in linear and non-linear inversion. We find that a linear approach, by which rays are only traced once in the background model, may result in an illusory fit to data. Realistic upper-mantle structures strongly deflect seismic rays, and correct paths can only be found after a few iterations. Although linear inversion seems able to identify the main features quite well, we verify how non-linear inversion and 3-D ray tracing significantly improve the results, especially when we attempt to reconstruct a realistic structure. We also apply the finite-difference, non-linear, traveltime tomography to data from the ISC to retrieve upper-mantle structure in the Central Mediterranean. We verify that the non-linear inversion is able to reveal sharpened velocity contrasts and thinner bodies than linear inversion. Clear differentiation found in the nonlinear result, between signatures of northern and southern Dinarides—showing lithosphere subducting only beneath the southern sector—is more coherent with the regional geodynamic framework. Such improvements due to non-linear mantle tomography may contribute to the general picture of slab detachment and small-scale mantle convection in the Mediterranean region, and therefore, significantly impact on geodynamic implications of resulting models.

Small-Scale Mantle Convection Produces Stratigraphic Sequences in Sedimentary Basins

Cyclic sedimentary deposits link stratigraphic sequences that are now geographically distant but were once part of the same depositional environment. Some of these sequences occur at periods of 2 to 20 million years, and eustatic sea-level variations or regional tectonic events are likely causes of their formation. Using numerical modeling, we demonstrate that small-scale mantle convection can also cause the development of stratigraphic sequences through recurrent local and regional vertical surface movements. Small-scale convection-driven stratigraphic sequences occur at periods of 2 to 20 million years and correlate only at distances up to a few hundred kilometers. These results suggest that previous sequence stratigraphic analyses may contain erroneous conclusions regarding eustatic sea-level variations.