Electrical conductivity in the mantle transition zone beneath eastern Central Asian Orogenic Belt revealed by geomagnetic signals

In the eastern segment of the Central Asian Orogenic Belt (CAOB), there is widespread volcanic magma activity. However, there is still considerable controversy over the formation mechanisms and material sources of these volcanoes. The mantle transition zone (MTZ), as a necessary channel for the upward and downward movement of mantle material and energy exchange may provide crucial constraints on the dynamic mechanisms of volcanic activity. This paper intends to obtain the deep structure beneath the eastern CAOB based on the geomagnetic depth sounding (GDS) method. First, the data of geomagnetic observatories in the study region are collected and processed, and the C-response curves are obtained by the bounded influence remote reference processing method (BIRRP). Then, the staggered grid finite difference method is used for forward modeling, and the finite memory quasi-Newton method based on L1-norm is used for three-dimensional (3-D) inversion. After that, 3-D inversion is carried out in spherical coordinates. Finally, the electrical conductivity model is obtained. The inversion model shows that there are two high conductivity anomalies in the MTZ beneath the Mongol-Okhotsk suture. Combined with the geological background of the structural domain, and constrained by the spatiotemporal variations in magmatism, we speculate that the high conductivity anomaly bodies are the stagnant oceanic crust material of the Okhotsk Ocean or the delaminated island arc accretionary wedge. The sinking slab or the detached lithosphere residual descending into the lower MTZ causes the upwelling of hot mantle material, forming widely distributed volcanic rocks on both sides of the Mongol-Okhotsk suture.

A Mantle Plume Beneath South China Revealed by Electrical Conductivity Obtained from Three-Dimensional Inversion of Geomagnetic Data

A three-dimensional electrical conductivity model of the mantle beneath South China is presented using the geomagnetic depth sounding method in this paper. The data misfit term in the inversion function is measured by the L1-norm to suppress the instability caused by large noises contained in the observed data. To properly correct the ocean effect in responses at coastal observatories, a high-resolution (1° × 1°) heterogeneous and fixed shell is included in inversion. The most striking feature of the obtained model is a continuous high-conductivity anomaly that is centered on ~(112° E, 27° N) in the mantle. The average conductivity of the anomaly appears to be two to four times higher than that of the global average models at the most sensitive depths (410-900 km) of geomagnetic depth sounding. Further analysis combining laboratory-measured conductivity models with the observed conductivity model shows that the anomaly implies excess temperature in the mantle. This suggests the existence of a mantle plume, corresponding to the Hainan plume, that originates in the lower mantle, passes through the mantle transition zone, and enters the upper mantle. Our electrical conductivity model provides convincing evidence for the mantle plume beneath South China.

Improved in situ analysis of lead isotopes in low-Pb melt inclusions using laser ablation-multi-collector-inductively coupled plasma-mass spectrometry

RATIONALE

In situ Pb isotope analyses of tiny melt inclusions using laser ablation-multi-collector-inductively coupled plasma-mass spectrometry (LA-MC-ICP-MS) are crucial for exploring the origins of mafic lavas. However, quantitative use of this technique with low-Pb (<10 ppm) melt inclusions is difficult due to their low ²⁰⁴ Pb content and ²⁰⁴ Hg interference.

METHODS

Pb isotopic ratios of various reference glasses and olivine-hosted melt inclusions were determined using LA-MC-ICP-MS. Multiple ion counters were used to simultaneously determine signal intensities of all Pb isotopes and ²⁰² Hg. An Hg signal-removal smoothing device reduced its signal in the gas blank by >80%. Instrumental mass bias was corrected using the standard-sample bracketing method.

RESULTS

With 24-90 μm diameter laser spots, 2-4 Hz repetition rates, and 2.5-4 J cm^-2 energy fluence, the analytical precisions of ^20x Pb/²⁰⁴ Pb ratios (x = 6, 7, 8) for standards BHVO-2G, ML3B-G, NIST 614, NKT-1G, T1-G, GOR132-G, and StHs6/80-G were <1.0% (2RSD) when ²⁰⁸ Pb signals >100 000 cps. The Wangjiadashan melt inclusions have ²⁰⁶ Pb/²⁰⁴ Pb = 17.14-18.44, ²⁰⁷ Pb/²⁰⁴ Pb = 15.28-15.66, and ²⁰⁸ Pb/²⁰⁴ Pb = 37.12-38.68.

CONCLUSIONS

The described method improves the precision and accuracy of in situ Pb isotope analysis in low-Pb melt inclusions using LA-MC-ICP-MS. The Pb isotopic compositions of the Wangjiadashan melt inclusions indicate the coexistence of LoMu and EMII+young HIMU components in the mantle source of weakly alkaline basalts.

Petrogenesis of the Early Cretaceous Hongshan Complex in the Southern Taihang Mountains: Constraints from Element Geochemistry, Zircon U-Pb Geochronology and Hf Isotopes

The Hongshan complex, located in the southern part of the Taihang Mountains in the central part of the North China Craton, consists of syenite stocks (including fine-grained biotite aegirine syenite, medium-grained aegirine gabbro syenite, coarse-grained aegirine gabbro syenite, syenite pegmatite, and biotite syenite porphyry), with monzo-diorite and monzo-gabbro dikes. This paper presents zircon U-Pb ages and Hf isotope data and whole-rock geochemical data from the Hongshan complex. LA–ICP-MS zircon U–Pb age from the fine-grained biotite aegirine syenite, monzo-diorite, and monzo-gabbro are 129.3 ± 2.0 Ma, 124.8 ± 1.3 Ma, and 124.1 ± 0.9 Ma, respectively, indicating their emplacement in the Early Cretaceous when the North China Craton was extensively reactivated. The monzo-diorite and monzo-gabbro have low SiO2 contents (48.94–57.75 wt%), total alkali contents (5.2–9.4 wt%), and εHf (t) values of −22.3 to −18.4 and are enriched in MgO (4.0–8.2 wt%), Al2O3 (14.3–15.8 wt%), light rare earth elements (LREEs) and large ion lithophile elements (LILEs). Interpretation of elemental and isotopic data suggests that the magma of monzo-diorite and monzo-gabbro were derived from partial melting of the enriched lithospheric mantle metasomatized by slab-derived hydrous fluids. Syenites with high alkali (K2O + Na2O = 9.4–13.0 wt%) and Sr contents (356–1737 ppm) and low Yb contents (0.94–2.65 ppm) are enriched in Al (Al2O3 = 16.4–19.1 wt%), but depleted in MgO (0.09–2.56 w%), Cr (Avg = 7.16 ppm), Co (Avg = 6.85 ppm) and Ni (Avg = 9.79 ppm), showing the geochemical features of adakitic rocks associated with thickened lower crust. Combining zircon 176Hf/177Hf ratios of 0.282176 to 0.282359, εHf(t) values of −18.3 to −11.8 and εNd (t) values of −11.1 to −8.2, we conclude that the syenite magma was derived from the mixing of the thickened lower crust and the enriched lithospheric mantle magma. These magma processes were controlled by Paleo-Pacific plate subduction and resulted in the destruction and thinning of the central North China Craton.

New Insights into Crust and upper Mantle Structure in Guangdong Province, China and Its Geothermal Implications

Southeast Asia contains significant natural geothermal resources. However, the mechanism for generating geothermal anomalies by the crust–mantle structure still needs to define. In this study, we focused on Guangdong Province, China. We conducted three magnetotelluric profiles to interpret the crust and upper mantle structure beneath the Guangdong Province and its geothermal implications. Based on data analysis results, a two-dimension inversion was conducted on the dataset. The inversion model revealed that there is a presence of upwelling channels, and some channels are connected with shallow crustal fault zone; the thickness of crust and lithosphere in Guangdong Province is relatively thin. Such a special crust and upper mantle structure form high surface heat flow. Merged with previous research, our results imply that massive Late Mesozoic granites, which contain high radioactive heat generating elements, are distributed on the surface and underground of Guangdong Province. Based on the correlation between high radioactive Late Mesozoic granites, crust-upper mantle structure, surface heat flow, and locations of natural hot springs, we established a geothermal conceptual model to visualize the origin of a current geophysical and geothermal anomaly in Guangdong Province.

Petrogenetic Constraints of Early Cenozoic Mafic Rocks in the Southwest Songliao Basin, NE China: Implications for the Genesis of Sandstone-Hosted Qianjiadian Uranium Deposits

The tectonic inversion of the Songliao Basin during the Cenozoic may have played an important role in controlling the development of sandstone-type uranium deposits. The widely distributed mafic intrusions in the host sandstones of the Qianjiadian U ore deposits provided new insights to constrain the regional tectonic evolution and the genesis of the U mineralization. In this study, zircon U-Pb dating, whole-rock geochemistry, Sr-Nd-Pb isotope analysis, and mineral chemical compositions were presented for the mafic rocks from the Qianjiadian area. The mafic rocks display low SiO2 (44.91–52.05 wt.%), high TFe2O3 contents (9.97–16.46 wt.%), variable MgO (4.59–15.87 wt.%), and moderate K2O + Na2O (3.19–6.52 wt.%), and can be subdivided into AB group (including basanites and alkali olivine basaltic rocks) and TB group (mainly tholeiitic basaltic rocks). They are characterized by homogenous isotopic compositions (εNd (t) = 3.47–5.89 and 87Sr/86Sr = 0.7032–0.7042) and relatively high radiogenic 206Pb/204Pb (18.13–18.34) and Nb/U ratios (23.0–45.6), similar to the nearby Shuangliao basalts, suggesting a common asthenospheric origin enriched with slab-derived components prior to melting. Zircon U-Pb and previous Ar-Ar dating show that the AB group formed earlier (51–47 Ma) than the TB group (42–40 Ma). Compared to the TB group, the AB group has higher TiO2, Na2O, K2O, P2O5, Ce, and HREE contents and Ta/Yb and Sr/Yb ratios, which may have resulted from variable depth of partial melting in association with lithospheric thinning. Combined with previous research, the Songliao Basin experienced: (1) Eocene (~50–40 Ma) lithospheric thinning and crustal extension during which mafic rocks intruded into the host sandstones of the Qianjiadian deposit, (2) a tectonic inversion from extension to tectonic uplift attributed to the subduction of the Pacific Plate occurring at ~40 Ma, and (3) Oligo–Miocene (~40–10 Ma) tectonic uplift, which is temporally associated with U mineralization. Finally, the close spatial relation between mafic intrusions and the U mineralization, dike-related secondary reduction, and secondary oxidation of the mafic rocks in the Qianjiadian area suggest that Eocene mafic rocks and their alteration halo in the Songliao Basin may have played a role as a reducing barrier for the U mineralization.

Non-mantle-plume process caused the initial spreading of the South China Sea

The mantle plume process is thought to be the prevailing dynamic mechanism for the South China Sea opening, but controversy persists due to the lack of critical evidence of magma in the initial seafloor spreading. International Ocean Discovery Program (IODP) Expedition 367 successfully recovered at Site U1500 the mid-ocean ridge basalt (MORB) representing the magma activity of the initial spreading of the South China Sea during the earliest Oligocene. Here we present the whole-rock and olivine phenocryst geochemistry of the basalts to constrain the potential influence of the Hainan mantle plume on the evolution of the South China Sea. Major and trace elemental compositions indicate that the basalts were mainly influenced by fractional crystallization of olivine and formed by melting of a spinel peridotite source without any pyroxenite in mantle source. The calculated mantle potential temperature of those most primitive basalts is much lower than plume-related MORB of Iceland, but similar to normal MORB elsewhere. Both lithological composition and mantle potential temperature clearly contradict with the mantle plume model, signifying that the mantle plume didn't exist at the earliest Oligocene. Therefore, the initial spreading of the South China Sea should be caused by non-plume processes, most likely by the westward subduction of the Pacific Plate.

New Insights of Geomorphologic and Lithologic Features on Wudalianchi Volcanoes in the Northeastern China from the ASTER Multispectral Data

Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) imaging system onboard NASA’s (National Aeronautics and Space Administration’s) Terra satellite is capable of measuring multispectral reflectance of the earth’s surface targets in visible and infrared (VNIR) to shortwave infrared (SWIR) (until 2006) as well as multispectral thermal infrared (TIR) regions. ASTER VNIR stereo imaging technique can provide high-resolution digital elevation models (DEMs) data. The DEMs data, three-dimensional (3D) perspective, and ratio images produced from the ASTER multispectral data are employed to analyze the geomorphologic and lithologic features of Wudalianchi volcanoes in the northeastern China. Our results indicate that the 14 major conical volcanic craters of Wudalianchi volcanoes are arranged as three sub-parallel zones, extending in a NE (Northeast) direction, which is similar to the direction of regional fault system based on the ASTER DEMs data. Among the 14 volcanic craters in Wudalianchi, the Laoheishan, and Huoshaoshan lavas flows, after the historic eruptions, pouring down from the crater, partially blocked the Baihe River, which forms the Five Large Connected Pools, known as the Wudalianchi Lake. Lithologic mapping shows that ASTER multispectral ratio imagery, particularly, the Lava Flow Index (LFI) (LFI = B10/B12) imagery, can clearly distinguish different lava flow units, and at least four stages of volcanic eruptions are revealed in the Wudalianchi Quaternary volcano cluster. Thus, ASTER multispectral TIR data can be used to determine relative dating of Quaternary volcanoes in the semi-arid region. Moreover, ASTER 3D perspective image can present an excellent view for tracking the flow directions of different lavas of Wudalianchi Holocene volcanoes.

Nature and evolution of the lithospheric mantle revealed by water contents and He-Ar isotopes of peridotite xenoliths from Changbaishan and Longgang basalts in Northeast China

The nature and evolution of the lithospheric mantle underlying Northeast (NE) China were investigated by assessing the mineral chemistry, water contents, and noble gas (He-Ar) isotopes of peridotite xenoliths captured by Cenozoic basalts from the Changbaishan and Longgang regions. The xenoliths, which have 863-1141 °C equilibration temperatures, primarily comprise spinel lherzolites and rare spinel harzburgites. The Mg^# (Fo) values of olivine in the peridotite xenoliths vary from 86.9 to 91.3. The clinopyroxenes have high Ti/Eu and low (La/Yb)_N, and their chondrite-normalized rare earth elements (REEs) exhibit light REE-depletion to -enrichment patterns, indicating that the mantle underneath the investigated region was predominantly subjected to partial melting (1%-10%) and was metasomatized by silicate melts. The measured ³He/⁴He ratios of the Changbaishan xenoliths have a narrow range from 5.8 Ra to 8.4 Ra with an average of 7.4 Ra. The ³He/⁴He isotopic ratios of the Longgang xenoliths varied from 4.7 Ra to 8.1 Ra with an average of 5.9 Ra; slightly lower than the Changbaishan xenoliths. The whole-rock H₂O contents of the studied peridotite ranged from 9 to 132 ppm. The high H₂O contents in excess of 50 ppm (up to 132 ppm) might represent newly accreted and cooled asthenospheric materials, while those with H₂O contents lower than 50 ppm (as little as 9 ppm) may represent thinned, relic, ancient lithospheric mantle. These geochemical evidences, in combination with published data, indicated that the lithospheric mantle beneath the Changbaishan and Longgang in NE China is dominated by the younger and more fertile lithospheric mantle with a minor ancient and refractory keel. In addition, the lithospheric mantle of this area was metasomatized by melts related to the recent subduction event (e.g., Pacific oceanic plate). Therefore, the westward-dipping Pacific oceanic plate subduction had an important contribution to the transformation of the lithospheric mantle beneath NE China.

Slab morphology and deformation beneath Izu-Bonin

Seismic tomography provides unique constraints on the morphology, the deformation, and (indirectly) the rheology of subducting slabs. We use teleseismic double-difference P-wave tomography to image with unprecedented clarity the structural complexity of the Izu-Bonin slab. We resolve a tear in the slab in the mantle transition zone (MTZ) between 26.5° N and 28° N. North of the tear, the slab is folded in the MTZ. Immediately above the fold hinge, a zone of reduced P-wavespeed may result from viscous dissipation within an incipient shear zone. To the south of the tear, the slab overturns and lies flat at the base of the MTZ. The ~680 km deep 2015 Bonin earthquake (Mw~7.9) is located at the northernmost edge of the overturning part of the slab. The localised tearing, shearing and buckling of the Izu-Bonin slab indicates that it remains highly viscous throughout the upper mantle and transition zone.

In the 1000 km long Izu-Bonin subduction zone to the south of Tokyo, the Pacific Plate descends beneath the Philippine Sea Plate. Here the authors use teleseismic double-difference tomography to image the complex morphology of the Izu-Bonin slab, especially in the mantle transition zone.

Growth of mountain belts in central Asia triggers a new collision zone in central India

Several unusual strong earthquakes occurred in central India along the Narmada-Son Lineament (NSL) zone, far from active plate boundaries. To understand the role of collisional processes in the origin of this seismicity, we develop a numerical thermomechanical model of shortening between the Indian Plate and Asia. We show that at the final stage of collision, the shortening rate of the high mountain areas slows. The continuing convergence of India and Asia triggers the initiation of a new collision zone in continental part of India. Various geological and geophysical observations indicate that the NSL is a weakest zone with northward thrusting of the thinner central Indian lithosphere underneath the thicker northern part of the Indian Plate. We hypothesize that the NSL was reactivated during the final stage of the India Asia convergence and it will possibly form a new mountain belt within the Indian continent.

Hainan mantle plume produced late Cenozoic basaltic rocks in Thailand, Southeast Asia

Intraplate volcanism initiated shortly after the cessation of Cenozoic seafloor spreading in the South China Sea (SCS) region, but the full extent of its influence on the Indochina block has not been well constrained. Here we present major and trace element data and Sr-Nd-Pb-Hf isotope ratios of late Cenozoic basaltic lavas from the Khorat plateau and some volcanic centers in the Paleozoic Sukhothai arc terrane in Thailand. These volcanic rocks are mainly trachybasalts and basaltic trachyandesites. Trace element patterns and Sr-Nd-Pb-Hf isotopic compositions show that these alkaline volcanic lavas exhibit oceanic island basalt (OIB)-like characteristics with enrichments in both large-ion lithophile elements (LILE) and high field strength elements (HFSEs). Their mantle source is a mixture between a depleted Indian MORB-type mantle and an enriched mantle type 2 (EMII). We suggest that the post-spreading intraplate volcanism in the SCS region was induced by a Hainan mantle plume which spread westwards to the Paleozoic Sukhothai arc terrane.

Distribution, cycling and impact of water in the Earth's interior

The Earth's deep interior is a hidden water reservoir on a par with the hydrosphere that is crucial for keeping the Earth as a habitable planet. In particular, nominally anhydrous minerals (NAMs) in the silicate Earth host a significant amount of water by accommodating H point defects in their crystal lattices. Water distribution in the silicate Earth is highly heterogeneous, and the mantle transition zone may contain more water than the upper and lower mantles. Plate subduction transports surface water to various depths, with a series of hydrous minerals and NAMs serving as water carriers. Dehydration of the subducting slab produces liquid phases such as aqueous solutions and hydrous melts as a metasomatic agent of the mantle. Partial melting of the metasomatic mantle domains sparks off arc volcanism, which, along with the volcanism at mid-ocean ridges and hotspots, returns water to the surface and completes the deep water cycle. There appears to have been a steady balance between hydration and dehydration of the mantle at least since the Phanerozoic. Earth's water probably originates from a primordial portion that survived the Moon-forming giant impact, with later delivery by asteroids and comets. Water could play a critical role in initiating plate tectonics. In the modern Earth, the storage and cycling of water profoundly modulates a variety of properties and processes of the Earth's interior, with impacts on surface environments. Notable examples include the hydrolytic weakening effect on mantle convection and plate motion, influences on phase transitions (on the solidus of mantle peridotite in particular) and dehydration embrittlement triggering intermediate- to deep-focus earthquakes. Water can reduce seismic velocity and enhance electrical conductivity, providing remote sensing methods for water distribution in the Earth's interior. Many unresolved issues around the deep water cycle require an integrated approach and concerted efforts from multiple disciplines.

Deep carbon cycles constrained by a large-scale mantle Mg isotope anomaly in eastern China

Although deep carbon recycling plays an important role in the atmospheric CO2 budget and climate changes through geological time, the precise mechanisms remain poorly understood. Since recycled sedimentary carbonate through plate subduction is the main light-δ26Mg reservoir within deep-Earth, Mg isotope variation in mantle-derived melts provides a novel perspective when investigating deep carbon cycling. Here, we show that the Late Cretaceous and Cenozoic continental basalts from 13 regions covering the whole of eastern China have low δ26Mg isotopic compositions, while the Early Cretaceous basalts from the same area and the island arc basalts from circum-Pacific subduction zones have mantle-like or heavy Mg isotopic characteristics. Thus, a large-scale mantle low δ26Mg anomaly in eastern China has been delineated, suggesting the contribution of sedimentary carbonates recycled into the upper mantle, but limited into the lower mantle. This large-scale spatial and temporal variation of Mg isotopes in the mantle places severe constraints on deep carbon recycling via oceanic subduction.

Depth variations of P-wave azimuthal anisotropy beneath Mainland China

A high-resolution model of P-wave anisotropic tomography beneath Mainland China and surrounding regions is determined using a large number of arrival-time data recorded by the China seismic network, the International Seismological Centre (ISC) and temporary seismic arrays deployed on the Tibetan Plateau. Our results provide important new insights into the subducted Indian plate and mantle dynamics in East Asia. Our tomographic images show that the northern limit of the subducting Indian plate has reached the Jinsha River suture in eastern Tibet. A striking variation of P-wave azimuthal anisotropy is revealed in the Indian lithosphere: the fast velocity direction (FVD) is NE-SW beneath the Indian continent, whereas the FVD is arc parallel beneath the Himalaya and Tibetan Plateau, which may reflect re-orientation of minerals due to lithospheric extension, in response to the India-Eurasia collision. There are multiple anisotropic layers with variable FVDs in some parts of the Tibetan Plateau, which may be the cause of the dominant null splitting measurements in these regions. A circular pattern of FVDs is revealed around the Philippine Sea slab beneath SE China, which reflects asthenospheric strain caused by toroidal mantle flow around the edge of the subducting slab.

Orogens in the evolving Earth: from surface continents to ‘lost continents’ at the core–mantle boundary

Orogens and their posthumous traces are the basic elements that can be used to understand the material circulation within the Earth. Although information preserved in the rocks on the surface ranging in age from 4.4 Ga to the present has been used to characterize orogens, it is important to understand orogens on a whole-Earth scale to evaluate global material circulation through time. In this paper, we synthesize the general concepts and characteristics of orogens and orogenic belts. The collision type and accretionary type constitute the two end-member types of orogens, both sharing similar structural features of subhorizontal disposition, bounded above and below by paired faults. Their exhumation generally occurs in two steps: first by wedge extrusion to form a sandwich structure with subhorizontal boundaries, which is followed by domal uplift of all the units. In the accretionary type, oceanic lithosphere subducts under the continental margin, and in the collision type, buoyant continents collide with each other. Of the various types of subduction and collision processes, arc–arc collision orogeny may have been widespread in the Archaean, although most of the intra-oceanic arc crust must have been destroyed and dragged down to the Archaean core–mantle boundary (CMB). Here we propose a broad two-fold classification of orogens and their subducted remnants, based on (1) their thermal history and (2) temporal constraints. Based on their thermal history, orogens are grouped into three types: cold orogens, hot orogens and ultra-hot orogens. Two extreme situations, which are anomalous and unlikely to occur on Earth, termed here super-cold and super-hot orogens, are also proposed. We discuss the characteristics of each of these subtypes. Based on temporal constraints, we group orogens into Modern and Ancient, where in both cases regional metamorphic belts occupy the orogenic core. In both groups, the overlying and underlying units of the regional metamorphic belts are weakly metamorphosed or unmetamorphosed, and are either accretionary complex in origin (Pacific type) or continental basement and cover (collision type). Major structures are subhorizontal with oceanward vergence of deformation, for both types. Orogens in the Modern Earth are grouped into four sub-categories: (1) deeply subducted orogens that are taken down to mantle depths and never return to the surface, termed here ‘ghost orogens’; (2) those that are subducted to deep crustal levels, undergo melting and are recycled back to the surface, forming resurrected and temporarily ‘arrested orogens’; (3) ‘extant orogens’, which are partly returned to the surface after deep subduction; (4) ‘concealed orogens’, which have been deeply subducted and only the traces of which are represented on the surface by mantle xenoliths carried by younger magmas. The preservation of orogens on the surface of the Earth occurred through an unusual return process from their natural course of total destruction, a phenomenon that operated more efficiently in the Phanerozoic through exhumation from ultra-deep domains against the slab-pull force of the plate, aided by fluids derived by dehydration of subducted lithosphere. Orogens at present represented on the surface of the Earth constitute only a fraction of the total volume formed in Earth history. Traces of the deeply subducted ‘lost orogens’ are sometimes returned to the surface in the form of melt or mantle xenoliths through combined processes of plume and plate tectonics. From a synthesis of the processes associated with the various categories of orogens proposed in this study, we trace the time-dependent transformations of orogens in relation to the history of the evolving Earth.