Hot Lithosphere Research Articles

A Two‐Stage Geodynamic Model for Post‐Collisional Potassic‐Ultrapotassic Magmatism in Southeast Tibet

AbstractPost‐collisional potassic‐ultrapotassic rocks can provide key clues to the change of the recycled material type and/or tectonic transition in subduction‐related zones. Despite continental materials widely recognized in their sources, it remains unclear whether such continental materials were contributed by former oceanic subduction or recent continental subduction. Here we address this issue by systematically investigating previously reported and our new chemical and Sr–Nd–Pb isotopic compositions of the post‐collisional K‐rich rocks in Southeast Tibet. Kink‐like compositional variations provide solid evidence for a primary control of fractional crystallization on the evolution of these K‐rich magmas. Their primary melts are demonstrated to have been produced by partial melting of phlogopite‐bearing peridotites in subcontinental lithospheric mantle (SCLM). The trace element and Sr–Nd–Pb isotopic signatures argue against involvement of the deeply subducted Indian continent but suggest a great contribution from sediments in oceanic slabs. A thinned (∼70–100 km) and hot (∼55–70 mW/m2) lithosphere is also unraveled beneath Southeast Tibet during the potassic‐ultrapotassic magmatism. Together with geophysical data, here we suggest a two‐stage geodynamic model for post‐collisional potassic‐ultrapotassic magmatism in Southeast Tibet: (a) Before the Indian‐Asia continental collision, phlogopite/K‐richterite‐bearing SCLM sources were formed through oceanic subduction‐related metasomatism; (b) After the Indian‐Asia continental collision, asthenosphere upwelling induced by post‐collisional tectonic extension or deep subduction of the Indian continental slab caused lithospheric thinning, partial melting of pre‐existing phlogopite/K‐richterite‐rich SCLM and thus K‐rich magmatism. This study provides new insights into the role of oceanic subduction and continental collision in post‐collisional potassic‐ultrapotassic magmatism.

Continental Thermal Structure and Carbonate Storage of Subducted Sedimentary Origin Control on Different Increases in Atmospheric CO2 in Late Ediacaran and Jurassic

AbstractCarbon release during continental rifting is thought to regulate atmospheric CO2 levels. Supercontinent dispersal‐induced extensional tectonics is similar during the Late Ediacaran and Jurassic, while they exhibit different increases in atmospheric CO2 concentration. The underlying mechanism of distinct CO2 emissions remains to be understood. Here, we conduct petrological‐thermomechanical modeling to show that metamorphic decarbonation and melting of carbonates that are derived from subducted sediments are ubiquitous during continental extension. We find that the hotter lithosphere and deeper storage of these carbonates cause more significant amounts of rift‐related CO2 release through volcanoes and faults. They may cause ∼12%–77% larger decarbonation efficiency, providing an efficient driving mechanism for a ∼31% larger increase in atmospheric CO2 levels during the Late Ediacaran than throughout the Jurassic. The rapid eruption and deposition of recycled carbonatite volcanic ash may contribute to the production of Late Ediacaran marine carbonates with the largest negative δ13C (−12‰).

Crustal Structure Across the Central Dead Sea Transform and Surrounding Areas: Insights Into Tectonic Processes in Continental Transforms

AbstractNew geophysical profiles across the central Dead Sea Transform (DST) near the Sea of Galilee, Israel, and surrounding highlands, augmented by static stress modeling, allow us to study continental transform plate deformation. The DST separates a ∼10 km thick sedimentary column above a thinned (16–23 km) crust to the west from a ∼7 km column above a ∼30‐km thick crust to the east. Crustal thinning starts under the DST, as observed also farther south, indicating that the DST is indeed located along the boundary between the Arabian plate and its continental margin. Moho step here is gradual. The DST's eastern shoulder dips westward toward the DST unlike the upward flexed shoulder observed farther south, perhaps delineating the northern limit of a thinner and hotter lithosphere. The shape of the Sea of Galilee is modeled as an asymmetric pull‐apart basin formed by a left‐lateral stepover of 2.6 km between slightly divergent and underlapping strike‐slip fault strands dipping 70° to the west. Reflection data indicate that these strands are not connected. Several fault traces within the Sea of Galilee have previously been suggested to carry part of the relative plate motion. However, given slip along the main DST faults, Coulomb stress will increase only on fault portions in the northern part of the lake, in accord with the geographical distribution of seismicity, suggesting that these faults are likely secondary. Mismatch between the DST strand locations in the geophysical profiles and the subsidence model, may reflect temporal changes in fault geometry.

Hot lithosphere beneath the northeastern North China Craton detected by ambient noise tomography

In this study, we present a high-resolution, 3-D S wave velocity (Vs) model of the crust and uppermost mantle beneath the Liaodong area at the northeast margin of the North China Craton (NCC). We build the model based on ambient noise tomography. Ambient noise data were obtained from both permanent stations and two new broad-band station profiles covering the Dandong-Tongliao (NCISP-10) and Baishan-Kuandian (NCISP-11) areas of the Liaodong-jilin region from October 2016 to December 2019. Our model reveals that different sides of the Tanlu Fault exhibited different Vs anomalies in both the upper crust and lithospheric upper mantle. Low Vs anomalies are observed in the west of the Tanlu Fault, which are interpreted to represent compositional variations at the crustal level. On the eastern side, low Vs anomalies are imaged which appear to represent “heat” or “hot material” derived from the deep mantle. A high Vs anomaly imaged in the southwest of the Songliao basin spatially coincides with a geologic feature referred to as the southwest uplift, which divides the basin into two secondary depressions. The Vs anomaly is inferred to represent cold mafic magma produced by the subduction of the Pacific Plate during the Late Cretaceous. Upwelling of mantle materials provide the best explanation for a mass of hot lithosphere. They underplated at the base of the crust, which formed crustal reservoirs and was eventually emplaced as plutons or erupted as volcanic rocks observed around the study area.

Onset of Iberian-European plate convergence: Late Cretaceous flexural response of a hot lithosphere (Aquitaine Basin, France)

The Aquitaine Basin is closely linked to the evolution of the Pyrenees, providing precious evidence of the early stages of the Pyrenean orogeny in the Late Cretaceous. Although the timing and geometry of the Early Cretaceous rifting stage is well constrained in the Pyrenees and surrounding north Pyrenean basins, the subsequent early inversion stage has resisted characterization. In this study, we used well log correlations and interpretation of seismic reflection profiles to determine the Late Cretaceous evolution of the sedimentary record in the Aquitaine Basin, spanning the transition from the postrift stage to the beginning of convergence. We identified two main deformation stages during the Late Cretaceous with radically different basin geometries in the western and eastern portions of the Aquitaine Basin. The western part appears to remain under a thermal subsidence regime, inherited from the postrift stage initiated in the Turonian, with little to no deformation. In contrast, the eastern part records early inversion, as indicated by the presence of a well-defined Campanian flexural basin. Between them, Coniacian and Santonian strata indicate a sharply increasing distinction between a very slowly subsiding North Aquitaine Domain (condensed sedimentation) and a strongly subsiding South Aquitaine Domain. We interpret this development as a consequence of lithospheric buckling resulting from far-field compressive strain between the Eurasia and Africa plates. During the Campanian, northward convergence of the Iberia plate led to the formation of a flexural basin with a narrower, steeper geometry than the subsequent Eocene basin that formed in nearly the same region during the Pyrenean orogeny. We attribute this difference to the development of the foreland over a lithosphere that was not thermally reequilibrated in Campanian time, roughly 10 Myr after rifting. This thermal inheritance resulted in lithosphere of very low rigidity. Sedimentary basins formed during both the Coniacian-Santonian and Campanian to early Maastrichtian were thus accommodated by short-wavelength flexural deformation of the lithosphere, a distinctive deformation pattern that represents an immediate response to the onset of compressive strain.

Mesoarchean diamonds formed in thickened lithosphere, caused by slab-stacking

When and how Earth's ancient crust – the cratons – became underpinned by cool, thick lithospheric mantle roots capable of hosting diamonds are among the most controversial aspects of Archean geology. Alluvial diamonds in cratonic sedimentary cover rocks, whose minimum age is determined by detrital-zircon geochronology, provide a unique perspective on this topic. A new discovery of a diamond-bearing quartz-pebble conglomerate from the northern Slave craton, Canada contains detrital zircon with a restricted U-Pb age distribution that has a dominant peak at ∼2.94 Ga and depositional age of ∼2.83 Ga. Pressure-temperature constraints derived from an olivine-diamond host pair lie on a conductive Mesoarchean geotherm of ∼36–38 mW/m2, comparable to the coolest modern lithospheric geotherms. This result is at odds with a hotter geothermal gradient related to nearby Mesoarchean komatiites. We propose a model whereby early building blocks for cratons were small but with deep cool roots that formed by slab-stacking, and were subsequently juxtaposed with regions of thinner, hotter lithosphere. This heterogeneous initial architecture later amalgamated and thickened through lateral accretion forming the more uniformly thick cratonic lithosphere observed today. Thermal modelling indicates that stacking/thickening of cool initial lithosphere into a lithospheric keel thick enough to stabilise diamonds is the most likely way of generating the observed geotherm by Mesoarchean times.

Cenozoic tectono-sedimentary evolution of the northern Turkana Depression (East African Rift System) and its significance for continental rifts

Rifting is an emblematic facet of continental drift and its early stage (i.e., stretching phase) is pivotal to be fully characterized and understood because it then conditions all subsequent stages of the break-up cycle. Although the final architectures of rifted margins have been extensively documented and discussed over the past decades from a wide range of examples and from multiple surface and sub-surface data, the complete evolution of the stretching phase itself remains mostly illustrated from analogue and numerical models and more rarely from real case studies and field evidences. The northern Turkana Depression (Cenozoic East African Rift System) stands as a key natural example to investigate every stage of the evolution of a stretching phase, as it provides almost ∼30 Ma of rift evolution from its initiation to present-day. In this study, we combine field analyses and seismic reflection data to characterize every stage of this tectono-sedimentary evolution, with a focus on the, rarely observed, surface rupture initiation and tectonic decay. In particular, we show rifting initiates with the reactivation of non-optimal inherited structures, progresses with their abandonment during the development of long and wide grabens or half-grabens, and locally terminates with the development of a post-tectonic flexural sag, synchronously to the migration of the locus of brittle deformation to an adjacent area where starts a similar cycle. Our observations allow us to nuance commonly accepted generic rift evolution models, and to discuss possible geodynamic controls on fault evolution and strain migration. We propose a field-based alternate conceptual evolution model for rift systems sharing common characteristics with the northern Turkana Depression such as abundant inherited basement structures, thin and hot lithosphere and a low to moderate magmatism.

The Rosebel gold mining district (Trans-Amazonian belt, Suriname), a new structural framework

The Rosebel gold district belongs to the Paleoproterozoic Trans-Amazonian belt associated with sub-meridian crustal shortening. Here, we present new structural observations (cleavage, stretching lineations, veins, fault slip data, aeromagnetic map). The regional cleavage is steeply dipping and bears a steeply plunging stretching lineation. Finite strains are of flattening type. Fault slip data reveal a complex deformation history. The overall strain pattern in the area reflects vertical motions, a feature consistent with pop-down tectonics involving vertical stretch and burial of supracrustal deposits during horizontal shortening of a hot and weak continental lithosphere.

The Upper Mantle Structure of Northwestern Canada From Teleseismic Body Wave Tomography

AbstractThe Northern Canadian Cordillera (NCC) is an actively deforming orogenic belt in northwestern Canada. Geochemical and geophysical data show that the NCC is underlain by a thin and hot lithosphere, in contrast with the adjacent cold and thick cratonic lithosphere to the east. This juxtaposition of cold/hot and thick/thin lithosphere across a narrow transition zone has important implications for regional geodynamics. The recent deployment of USArray Transportable Array and other seismic stations across Alaska, USA, and northwestern Canada allows us to image lithosphere and upper mantle three‐dimensional seismic velocity structure at significantly improved resolution. Our model reveals a broad high‐velocity anomaly across northern Yukon and Northwest Territories, which is interpreted as buried cratonic lithosphere and which we refer to as the Mackenzie craton. Another prominent high‐velocity anomaly is imaged beneath northeastern British Columbia and is interpreted to indicate cratonic lithosphere beneath the Northern Rocky Mountains. These two mechanically strong lithospheric blocks, also suggested by regional magnetic data, are interpreted to buttress the ends of the Mackenzie Mountains fold and thrust belt, guiding intervening cordilleran mantle flow toward the Canadian Shield and controlling the arcuate geometry of the Mackenzie Mountains fold and thrust belt. Both and wave models also reveal the signature of a northward dipping, subducting Wrangell slab across the southern region of the Alaska/Yukon border. Strong and wave velocity contrasts across the Tintina Fault suggest that it is a lithosphere‐scale shear zone that extends into the upper mantle beneath the NCC and demarcates distinct regions of lithospheric mantle.

Radiogenic trigger and driver for continental rifting and initial ocean spreading

AbstractClosure and opening of oceans on time‐scales of a few hundred million years is a fundamental tectonic process on Earth, typically referred to as a “Wilson cycle”. Subduction of oceanic and continental crust leading up to and during continent–continent collision can refertilize and enrich the orogenic continental lithospheric mantle in heat‐producing elements. The resulting thermal anomaly weakens the lithosphere and, along with structural weaknesses (e.g. sutures), make this orogenic lithosphere more prone to rifting given an extensional stress field. Thermal modelling shows that anomalously hot lithosphere can focus asthenospheric upwellings over time‐scales of a few hundred million years. Processes related to closure of oceans thus provide a mechanism for later localization of rifting and an extensional driving force.

U–Pb ages of Miocene near-trench granitic rocks of the Southwest Japan arc: implications for magmatism related to hot subduction

AbstractWe present a new dataset of zircon U–Pb ages that document igneous activity in the SW Japan arc during middle Miocene time and discuss its relationship with the opening of the Japan Sea, Philippine Sea plate migration, and subduction of the young hot lithosphere of the Shikoku Basin. Precursory magmatism, characterized by dike and stock intrusions, startedc. 15.6 Ma in both Kyushu and the Kii Peninsula. Most plutonism occurred between 15.5 and 13.5 Ma in an area 600 km long and 150 km wide. No along-arc trend was recognized in the U–Pb ages of igneous activity near the trench. Our data indicate that all near-trench middle Miocene igneous activity occurred immediately after the opening of the Japan Sea ceased, i.e. after 16 Ma, implying that melt extraction and the emplacement of granites in the near-trench region had some influence on the back-arc opening. Our data also imply that the trench–trench–trench-type triple junction between the Japan arc and the Izu–Bonin–Mariana arc must have reached the east side of the Kii Peninsula by 15.6 Ma. The wide distribution of contemporaneous magmatic activity along the arc requires a trench-parallel heat source, such as the subduction of a trench-parallel ridge or a young and highly segmented ridge–fracture zone system in addition to the hot wedge mantle condition related to the opening of Japan Sea.

Tectono-sedimentary evolution of a rift system controlled by Permian post-orogenic extension and metamorphic core complex formation (Bidarray Basin and Ursuya dome, Western Pyrenees)

This study documents the sedimentary and structural response of continental crust in relatively hot lithosphere that is subjected to extension. We focus on the Permian rift system in the Western Pyrenees, where the narrow, post-orogenic intracontinental extensional Bidarray Basin is in contact with late Variscan granulites of the Ursuya massif. The western margin of the N-S trending Bidarray Basin preserves alluvial fans dominated by hyperconcentrated flows and interdigitating eastward into a N-S trending fluvial system. Structural analysis of the Ursuya granulites shows that they underwent orogen-parallel mid-crustal flow and were exhumed owing to strain localization during retrogressive metamorphism within an extensional shear zone flanking an E-W elongated domal structure. We show that the Bidarray Basin formed during Permian time on the hanging wall of a south-vergent detachment system that developed in response to the formation of an immature “a-type” metamorphic core complex (the Ursuya massif) under regional E-W extension, resulting in homogeneous thinning of the hot crust. This core complex was later exposed by denudation during Cenomanian time. The preservation of the Permian and Triassic paleogeography and structure indicates that there has been no lateral motion between Iberia and Europe in the study area. The Cretaceous Pamplona transfer zone, responsible for the shift of the Mesozoic rift axis, reactivated a N-S trending Permian crustal heterogeneity.

Super-reducing conditions in ancient and modern volcanic systems: sources and behaviour of carbon-rich fluids in the lithospheric mantle

Oxygen fugacity (ƒO2) is a key parameter of Earth’s mantle, because it controls the speciation of the fluids migrating at depth; a major question is whether the sublithospheric mantle is metal-saturated, keeping ƒO2 near the Iron-Wustite (IW) buffer reaction. Cretaceous basaltic pyroclastic rocks on Mt. Carmel, Israel erupted in an intraplate environment with a thin, hot lithosphere. They contain abundant aggregates of hopper-shaped crystals of Ti-rich corundum, which have trapped melts with phenocryst assemblages (Ti2O3, SiC, TiC, silicides, native V) requiring extremely low ƒO2. These assemblages are interpreted to reflect interaction between basaltic melts and mantle-derived fluids dominated by CH4 + H2. Similar highly reduced assemblages are found associated with volcanism in a range of tectonic situations including subduction zones, major continental collisions, intraplate settings, craton margins and the cratons sampled by kimberlites. This distribution, and the worldwide similarity of δ13C in mantle-derived SiC and associated diamonds, suggest a widespread process, involving similar sources and independent of tectonic setting. We suggest that the common factor is the ascent of abiotic (CH4 + H2) fluids from the sublithospheric mantle; this would imply that much of the mantle is metal-saturated, consistent with observations of metallic inclusions in sublithospheric diamonds (e.g. Smith et al. 2016). Such fluids, perhaps carried in rapidly ascending deep-seated magmas, could penetrate high up into a depleted cratonic root, establishing the observed trend of decreasing ƒO2 with depth (e.g. Yaxley et al. in Lithos 140:142–151, 2012). However, repeated metasomatism (associated with the intrusion of silicate melts) will raise the FeO content near the base of the craton over time, developing a carapace of oxidizing material that would prevent the rise of CH4-rich fluids into higher levels of the subcontinental lithospheric mantle (SCLM). Oxidation of these fluids would release CO2 and H2O to drive metasomatism and low-degree melting both in the carapace and higher in the SCLM. This model can explain the genesis of cratonic diamonds from both reduced and oxidized fluids, the existence of SiC as inclusions in diamonds, and the abundance of SiC in some kimberlites. It should encourage further study of the fine fractions of heavy-mineral concentrates from all types of explosive volcanism.

Lithospheric thermal evolution and dynamic mechanism of destruction of the North China Craton

The dynamic mechanism for destruction of the North China Craton (NCC) has been extensively discussed. Numerical simulation is used in this paper to discuss the effect of mantle upward throughflow (MUT) on the lithospheric heat flux of the NCC. Our results yield a three-stage destruction of the NCC lithosphere as a consequence of MUT variation. (1) In Late Paleozoic, the elevation of MUT, which was probably caused by southward and northward subduction of the paleo-Asian and paleo-Tethyan oceans, respectively, became a prelude to the NCC destruction. The geological consequences include a limited decrease of the lithospheric thickness, an increase of heat flux, and a gradual enhancement of the crustal activity. But the tectonic attribute of the NCC maintained a stable craton. (2) During Late Jurassic-Early Cretaceous, the initial velocity of the MUT became much faster probably in response to subduction of the Pacific Ocean; the conductive heat flux at the base of the NCC lithosphere gradually increased from west to east; and the lithospheric thickness was significantly decreased. During this stage, the heat flux distribution was characterized by zonation and partition, with nearly horizontal layering in the lithosphere and vertical layering in the underlying asthenosphere. Continuous destruction of the NCC lithosphere was associated with the intense tectono-magmatic activity. (3) From Late Cretaceous to Paleogene, the velocity of MUT became slower due to the retreat of the subducting Pacific slab; the conductive heat flux at the base of lithosphere was increased from west to east; the distribution of heat flux was no longer layered. The crust of the western NCC is relatively hotter than the mantle, so-called as a ‘hot crust but cold mantle’ structure. At the eastern NCC, the crust and the mantle characterized by a ‘cold crust but hot mantle.’ The western NCC (e.g., the Ordos Basin) had a tectonically stable crust with low thermal gradients in the lithosphere; whereas the eastern NCC was active with a hot lithosphere. The numerical results show that the MUT is the main driving force for the NCC destruction, whereas the complex interaction of surrounding plates lit a fuse for the lithospheric thinning.

Symmetry during the syn- and post-rift evolution of extensional back-arc basins: The role of inherited orogenic structures

Rheological heterogeneities in the lithosphere have first order control on the topographical expression of tectonic processes. Pre-existing orogenic suture zones localise extensional deformation resulting in asymmetric basins. Such crustal geometries are often in contrast with the more symmetric regional lithospheric structure observed beneath extensional basins. We study such (a)symmetries and their controlling parameters by conducting a series of 2D thermo-mechanical numerical experiments of the extension of an overthickened, hot lithosphere that contains a weakness zone. The modelling shows that syn-rift subsidence is low to moderate creating asymmetric half grabens where extension migrates in space and time, grouped in an overall symmetrical appearance on a larger scale. The initial lithospheric mantle asymmetry is attenuated by the lateral heat conduction and further dynamic evolution of the thermal anomaly during the “post-rift” phase, resulting in differential vertical movements of the crust including additional 2–3 km subsidence in the basin centre. The modelling shows that the initial crustal and lithospheric thicknesses, rate of extension and surface processes strongly control the thermo-mechanical evolution of the extensional system. The numerical modelling yields new insights into the mechanics of coupling between near-surface kinematics and the evolution of deep lithospheric structure in the Pannonian basin and the Aegean, two of Europe's largest back-arc systems.

Contribution of slab-derived fluid and sedimentary melt in the incipient arc magmas with development of the paleo-arc in the Oman Ophiolite

Major-element and trace-element geochemistry of fresh volcanic glass, and the whole-rock Hf and Nd isotopic compositions of volcanic rocks from the Oman Ophiolite were used to understand the extent to which slab-derived fluids and melts were involved in magma generation during incipient arc development. The spreading stage (V1) samples have trace-element characteristics similar to those of mid-ocean-ridge basalts, with Hf and Nd isotopic compositions of the Indian mantle domain. The incipient arc stage (V2) volcanic glasses have lower abundances of high field strength elements (HFSEs) and middle and heavy rare earth elements (MREEs and HREEs), and higher abundances of large ion lithophile elements (LILEs) than the V1 glasses. Progressive increases in B, Pb, and LILEs in the V2 stage glasses with age indicate an increasing contribution of slab-derived fluids from earlier arc tholeiite (LV2) to later boninite (UV2). The LV2 was generated by the contribution of amphibolite-derived fluid. The UV2 is subdivided into low-Si UV2 and high-Si UV2 magmas, with the former having lower SiO2, and LILE, and higher Na2O, HFSE and REE concentrations than the latter. The low-Si UV2 magma was generated with the involvement of high-temperature slab-derived fluid. In contrast, the high-Si UV2 shows a spoon-shaped trace-element pattern and Hf and Nd isotope chemistry, indicating the involvement of sediment melt in its magma genesis. Decreasing HFSEs and HREEs in the magmas with age indicate progressive depletion of the source mantle during the V2 arc magmatism, suggesting an absence of convection in the mantle wedge that resulted from subduction of a hot, buoyant slab beneath young, hot lithosphere.

Early Eocene clinoenstatite boninite and boninite-series dikes of the ophiolite of New Caledonia; a witness of slab-derived enrichment of the mantle wedge in a nascent volcanic arc

Clinoenstatite-bearing boninites (CE-boninite) from the serpentinite sole of the Cenozoic ophiolite of New Caledonia near Nepoui have been dated by the 40Ar/39Ar method, yielding two plateau ages of 47.4±0.9Ma and 50.4±1.3Ma. Coarser grained, geochemically similar boninite-series felsic dikes consistently yielded U–Pb zircon ages of ca. 54Ma.Nepoui CE-boninites display whole rock geochemical features similar to that of Cape Vogel boninites (Papua-New Guinea). They similarly have been generated by low degree hydrous melting of depleted peridotite. High contents in LILE and LREE, and some elemental ratios suggest source enrichment by subduction-derived fluids and melts. However, unlike the Cape Vogel boninite, moderately depleted MORB-like isotopic signatures (εNd50=7.9) rule out the role of OIB-like, or E-MORB component that might account for the relatively high LREE and LILE contents measured in the rocks. Nd isotopic ratios and positive anomalies in Zr and Hf are closely similar to that of the slightly older felsic dikes (55–50Ma) that crosscut the peridotite from the ophiolite in New Caledonia. Most of these magmas have been generated by slab melting during the early stages of intra-oceanic subduction. The Early Eocene subduction started at or near the “oceanic” ridge and involved young and hot lithosphere; therefore, slab-derived melts may have reacted locally with hot depleted peridotites. Finally, water influx into the mantle wedge during the subduction of slightly older (cooler and hydrated) lithosphere initiated a low degree partial melting event in the mantle wedge and generated the CE-boninite magma.Geochemical modeling of hydrous melting of a depleted mantle re-enriched by slab melts suggest that the additional slab melt component was derived from the partial melting of a BABB-like barroisite-bearing eclogite, similar to some elements of the Eocene HP–LT Pouebo terrane. This potential magma source is similar to the BABB-like HT amphibolites of the metamorphic sole of the ophiolite, which have the same origin. Geochemical modeling also suggests that CE-boninite magma may have been in equilibrium with the enstatite-bearing gabbro cumulates that crop out in several places of the Massif du Sud.However, modeling fails in establishing that harzburgite of the same massif simply corresponds to the melting residue of this process. It appears that ultra-depleted supra-subduction peridotites of the Massif du Sud are probably not directly related to the overlying gabbro cumulates.

Lithospheric strength and elastic thickness of the Barents Sea and Kara Sea region

Interpretation of tomography data indicates that the Barents Sea region has an asymmetric lithospheric structure characterized by a thin and hot lithosphere in the west and a thick and cold lithosphere in the east. This suggests that the lithosphere is stronger in the east than in the west. This asymmetric lithosphere strength structure may have a strong control on the lithosphere response to tectonic and surface processes. In this paper, we present computed strength and effective elastic thickness maps of the lithosphere of the Barents Sea and Kara Sea region. Those are estimated using physical parameters from a 3D lithospheric model of the Barents Sea and Kara Sea region. The lithospheric strength is computed assuming a temperature-dependent ductile and brittle rheology for sediments, crust and mantle lithosphere. Results show that lithospheric strength and elastic thickness are mostly controlled by the lithosphere thickness. The model generally predicts much larger lithospheric strength and elastic thickness for the Proterozoic parts of the East Barents Sea and Kara Sea. Locally, the thickness and lithology of the continental crust disturb this general trend. At last, the gravitational potential energy (GPE) is computed. Our results show that the difference in GPE between the Barents Sea and the Mid-Atlantic Ridge provides a net horizontal force large enough to cause contraction in the western and central Barents Sea.

Magmatic expressions of continental lithosphere removal

AbstractGravitational lithosphere removal in continental interior has been inferred from various observations, including anomalous surface deflections and magmatism. We use numerical models and a simplified theoretical analysis to investigate how lithosphere removal can be recognized in the magmatic record. One style of removal is a Rayleigh‐Taylor‐type instability, where removal occurs through dripping. The associated magmatism depends on the lithosphere thermal structure. Four types of magmatism are predicted: (1) For relatively hot lithosphere (e.g., back arcs), the lithosphere can be conductively heated and melted during removal, while the asthenosphere upwells and undergoes decompression melting. If removal causes significant lithospheric thinning, the deep crust may be heated and melted. (2) For moderately warm lithosphere (e.g., average Phanerozoic lithosphere) in which the lithosphere root has a low density, only the lithosphere may melt. (3) If the lithosphere root has a high density in moderately warm lithosphere, only asthenosphere melt is predicted. (4) For cold lithosphere (e.g., cratons), no magmatism is induced. An alternate style of removal is delamination, where dense lithosphere peels along Moho. In most cases, the lithosphere sinks too rapidly to melt. However, asthenosphere can upwell to the base of the crust, resulting in asthenospheric and crustal melts. In delamination, magmatism migrates laterally with the detachment point; in contrast, magmatism in Rayleigh‐Taylor‐type instability has a symmetric shape and converges toward the drip center. The models may explain the diversity of magmatism observed in areas with inferred lithosphere removal, including the Puna Plateau and the southern Sierra Nevada.

Lithospheric structure of southern Indian shield and adjoining oceans: integrated modelling of topography, gravity, geoid and heat flow data

For the present 2-D lithospheric density modelling, we selected three geotransects of more than 1000 km in length each crossing the southern Indian shield, south of 16°N, in N–S and E–W directions. The model is based on the assumption of local isostatic equilibrium and is constrained by the topography, gravity and geoid anomalies, by geothermal data, and where available by seismic data. Our integrated modelling approach reveals a crustal configuration with the Moho depth varying from ∼40 km beneath the Dharwar Craton, and ∼39 km beneath the Southern Granulite Terrane to about 15–20 km beneath the adjoining oceans. The lithospheric thickness varies significantly along the three profiles from ∼70–100 km under the adjoining oceans to ∼130–135 km under the southern block of Southern Granulite Terrane including Sri Lanka and increasing gradually to ∼165–180 km beneath the northern block of Southern Granulite Terrane and the Dharwar Craton. This step-like lithosphere–asthenosphere boundary (LAB) structure indicates a normal lithospheric thickness beneath the adjoining oceans, the northern block of Southern Granulite Terrane and the Dharwar Craton. The thin lithosphere below the southern block of Southern Granulite Terrane including Sri Lanka is, however, atypical considering its age. Our results suggest that the southern Indian shield as a whole cannot be supported isostatically only by thickened crust; a thin and hot lithosphere beneath the southern block of Southern Granulite Terrane including Sri Lanka is required to explain the high topography, gravity, geoid and crustal temperatures. The widespread thermal perturbation during Pan-African (550 Ma) metamorphism and the breakup of Gondwana during late Cretaceous are proposed as twin cause mechanism for the stretching and/or convective removal of the lower part of lithospheric mantle and its replacement by hotter and lighter asthenosphere in the southern block of Southern Granulite Terrane including Sri Lanka. Unusually thinned LAB beneath the region near Bangalore apparently indicates the preserved tectonocompositional effect of late Proterozoic rifted margin of Dharwar Craton.