Removal Of Lithosphere Research Articles

Rapid surface uplift and crustal flow in\xa0the Central Andes (southern Peru) controlled by lithospheric drip dynamics

The high flux magmatism, crustal shortening/extension and plateau formation in Cordilleran orogenic systems have been explained by removal of lithosphere (lower crust and the sub-arc mantle lithosphere) that develops beneath the magmatic arc and hinterland regions. However, the primary role of this process driving surface uplift, and crustal deformation is not well understood. Here, reconciling geodynamic model predictions with lithospheric structure and paleoelevation estimates, we suggest that viscous drip-type lithospheric removal from beneath the Central (Peruvian) Andes can explain several tectonic features: (1) “double humped” shaped/axisymmetric topographic profile and rapid surface rise (up to 1.2 km in ~ 4.31 Myrs); (2) thicker crust associated with the lower surface elevation of the Altiplano plateau (Lake Titicaca region) (negative residual topography) and higher topography and thinner crust of Western and Eastern Cordilleras (positive residual topography); and (3) faster wave speed (colder)/sub-Moho anomaly underlying the Altiplano, surrounded by slower speed anomalies on both western arc-forearc areas and parts of the eastern Cordillera and Sub-Andes. Our results emphasize the important role of lithospheric drip and associated mantle dynamics in the transient evolution of Andean orogeny controlling surface uplift and crustal flow and thickening.

Cenozoic stratigraphic chronology and sedimentary-tectonic evolution of the Qaidam Basin

<p indent="0mm">The Qaidam Basin hosts thick Cenozoic strata with an uninterrupted depositional time frame from early Cenozoic to the Quaternary, where remarkable number of mammalian fossils were discovered. These properties of the Cenozoic strata provide a unique chance to establish the basin-wide chronological framework by using palaeomagnetistratigraphy and biostratigraphy. Armed with the chronological framework, the sedimentary evolution and structural deformation of the Qaidam Basin and its adjacent region can be reconstructed to reveal the deformational processes and dynamic mechanism of the Tibetan Plateau expansion during the Cenozoic. According to the previous studies, the Cenozoic strata preserved in the Qaidam Basin, have been subdivided into seven primary progressively younger stratigraphic units: The Lulehe, Xia Ganchaigou, Shang Ganchaigou, Xia Youshashan, Shang Youshashan, Shizigou, and Qigequan formations. Despite the long study history of the Qaidam Basin, they are still controversial questions of basin-wide Cenozoic stratigraphic unit correlations and the time of formation of the Qaidam Basin. On the basis of the field observations, this paper summarizes relatively long magnetostratigraphic sections (thickness ≥ <sc>4500 m),</sc> such as the Huatugou, Lulehe, Honggou, Dahonggou, and Huaitoutala magnetostratigraphies in the Qaidam Basin from the west to east. The magnetostratigraphies/refined magnetostratigraphies dated the Lulehe Fm. to span from 31 to 23.7 Ma, Xia Ganchaigou Fm. from 23.7 to 17 Ma, Shang Ganchaigou Fm. from ca. 17 to ca. 12 Ma, Xia Youshashan Fm. from ca. 12 to ca. 10 Ma, Shang Youshashan Fm. from ca. 10 to ca. 7 Ma, and Shizigou Fm. is younger than ca. 7 Ma. Our interpretations suggest that the westernmost portion of the Qaidam Basin is probably formed in the Eocene, and the middle part of the basin began to accumulate sediments since the Oligocene (ca. 31–26 Ma). In the eastern part of the Qaidam Basin, Cenozoic subsidence was likely to initiate during the early to middle Miocene (ca. 16 Ma). Although onset of the Qaidam Basin expanded between Eocene and early Miocene from the west to east, significant portion of the basin seems to subsided after the Oligocene. This phenomenon indicates that the most profound episode of the tectonic deformation around the Qaidam Basin has occurred since the Oligocene. In the Qaidam Basin, the sediment accumulation rates over the basin had increased significantly during the middle Miocene (ca. 15 Ma). We rule out climate changes to drive the rapid middle-Miocene erosion and accumulation rates because of stepwise aridification of the northeastern Tibetan Plateau during the Neogene time interval. Thus, the increased accumulation rates imply direct relation to upward growth of the mountain ranges around the Qaidam Basin. There is abundant evidence demonstrates occurrences of the Oligocene (ca. 30 Ma) and middle-Miocene (10–15 Ma) stages of crustal shortening in many localities in and around the Tibetan Plateau, suggesting that both stages the deformation in ca. 30 and 10–15 Ma are synchronous and large-scale tectonic activation events. The regional synchronicity of plateau-wide deformational phases indicates significant changes in the dynamics of the plateau growths during the Oligocene and middle-Miocene. We suggest that ca. 30 and 10–15 Ma upward and outward growths of the Tibetan Plateau are likely to be driven by removal of mantel lithosphere beneath different portions of the Tibetan Plateau. Following the continuous convergence between the Indian and Eurasian continents, the lithosphere of the Tibetan Plateau was thickened. Removal of thickened, dense lithosphere could cause the uplift of the Tibetan Plateau, and then applied exert compressive stresses to the relatively lower relief region to trigger pulsed outward growth of the plateau to its recent margins.

Magmatism at Passive Margins: Effects of Depth‐Dependent Wide Rifting and Lithospheric Counterflow

AbstractRifted passive margins exhibit a large variety in the timing, distribution, and amount of magmatism. The factors controlling magmatism during rifted margin formation, remain, however, incompletely understood, partly owing to the complex rifting styles. In this study, we use 2‐Dimensional numerical models to investigate the effects of depth‐dependent wide rifting and lithospheric counterflow on magmatism during rifted margin formation. Results show that a strong crust promotes narrow margins with a sharp transition to normal thickness oceanic crust whereas a weak crust promotes depth‐dependent wide rifting, with preferential removal of mantle lithosphere, leading to formation of wide margins with over‐thickened (&gt;18 km) igneous crust in the distal margin. Counterflow of depleted lithospheric mantle, in contrast, may delay syn‐rift magmatism, and result in exhumed a‐magmatic continental mantle at narrow margins. The combination of wide rifting and lithospheric counterflow results in magma‐poor wide margins, with in some cases a transition to excess magmatic activity at breakup. Our models provide an explanation for the contrasting magmatic productivity at narrow and wide rifted margins, such as observed in the Lofoten‐Vesterålen, Newfoundland, Kwanza Basin, and Orange Basin margins.

Pulsed rise and growth of the Tibetan Plateau to its northern margin since ca. 30 Ma

The onset of mountain building along margins of the Tibetan Plateau provides a key constraint on the processes by which the high topography in Eurasia formed. Although progressive expansion of thickened crust underpins most models, several studies suggest that the northern extent of the plateau was established early, soon after the collision between India and Eurasia at ca. 50 Ma. This inference relies heavily on the age and provenance of Cenozoic sediments preserved in the Qaidam basin. Here, we present evidence in the northern plateau for a considerably younger inception and evolution of the Qaidam basin, based on magnetostratigraphies combined with detrital apatite fission-track ages that date the basin fills to be from ca. 30 to 4.8 Ma. Detrital zircon-provenance analyses coupled with paleocurrents reveal that two-stage growth of the Qilian Shan in the northeastern margin of the Tibetan Plateau began at ca. 30 and at 10 Ma, respectively. Evidence for ca. 30 and 10 to 15 Ma widespread synchronous deformation throughout the Tibetan Plateau and its margins suggests that these two stages of outward growth may have resulted from the removal of mantle lithosphere beneath different portions of the Tibetan Plateau.

Post‐Subduction Tectonics of Sabah, Northern Borneo, Inferred From Surface Wave Tomography

AbstractWe use two‐plane‐wave tomography with a dense network of seismic stations across Sabah, northern Borneo, to image the shear wave velocity structure of the crust and upper mantle. Our model is used to estimate crustal thickness and the depth of the lithosphere‐asthenosphere boundary (LAB) beneath the region. Calculated crustal thickness ranges between 25 and 55 km and suggests extension in a NW‐SE direction, presumably due to back‐arc processes associated with subduction of the Celebes Sea. We estimate the β‐factor to be 1.3–2, well below the initiation of seafloor spreading. The LAB is, on average, at a depth of 100 km, which is inconsistent with models that ascribe Neogene uplift to wholescale removal of the mantle lithosphere. Instead, beneath a region of Plio‐Pleistocene volcanism in the southeast, we image a region 50–100 km across where the lithosphere has thinned to &lt;50 km, supporting recent suggestions of lower lithospheric removal through a Rayleigh‐Taylor instability.

Tectonic Control Over Shallow Crustal Exhumation Across the India‐Asia Convergent Margin

AbstractUnderstanding the processes and mechanisms responsible for exhumation are essential for determining the evolution of landscapes. Our knowledge of these issues in continental convergent margins is still incomplete despite decades of research. Here, we present a record of exhumation rates obtained from thermochronology across the collision zone of the Himalayan‐Tibetan orogen, which has been carved by the dextral Karakoram fault (KF) along its western margin. Our thermochronology results across the KF suggest that transpression controlled exhumation from ∼10 to 8 Ma. Exhumation rates at the margin of the Tibetan plateau, in the SE Karakoram range, to the north of the KF are twice as high as the Ladakh range, to the south of the KF, since at least middle Miocene. We hypothesize that rapid exhumation of the Tibetan margin was in response to topographic uplift and subsequent erosion due to the convective removal of the lower lithosphere, triggered by the rollback of the subducting Indian lower crust during the Oligocene‐Miocene in an early phase. Our thermochronology results document an additional two‐fold increase in the exhumation rates of the Tibetan margin since the late Mio‐Pliocene. We propose that the underthrusting of the Indian Plate below Tibet during the late Mio‐Pliocene was responsible for this additional increase of exhumation rates of the Tibetan margin in the late phase. These exhumation rates are in good agreement with the topographic slope, relief, and channel steepness data of these regions. Thus, the coupling of tectonics and topography dictated the exhumation patterns since late Mio‐Pliocene.

Morphotectonic configuration of the Podtatranská Kotlina Basin and its relationship to the origin of the Western Carpathians

In tectonically active regions, many aspects of the topography can provide clues to morphotectonic development. Morphological lineaments have varying degrees of reliability as indicators of neotectonic movement. Here, eleven lines of evidence are assessed and combined for the Podtatranská Kotlina Basin (PtK) between the Tatra and Low Tatra (Nízke Tatry) Mountains, the highest topography of the Western Carpathians. This permits identification of the configuration of structural-neotectonic blocks and documentation of their behaviour. Specifically we 1) evaluate the reliability of fault mapping results, 2) assess relative uplift and subsidence accommodated by the faults, 3) evaluate the intensity of fault movement and 4) explore the role of fault movement in the tilting of blocks. We define 358 structural-neotectonic blocks in the PtK region, providing an exceptionally detailed morphotectonic analysis. Distinction of two stages of neotectonic uplift provides explanation of the regional discrepancy between intensive neotectonic activity and limited recent seismicity with Mw < 5. The first stage, in the Pliocene-Early Pleistocene, was characterized by isostatic uplift linked to delamination and/or convective removal of an over-thickened lithosphere. The second stage during the Late Quaternary involved more complex uplift, including the probable influence of episodic mantle flow induced by the load of the Scandinavian Ice Sheet on northern Europe. This resulted in a change in the extension and configuration of structural-neotectonic blocks, the movements of the Tatra block now measured by GNSS, and the interaction of neotectonic activity with effects of glacial–interglacial transitions.

Connection Between a Subcontinental Plume and the Mid‐Lithospheric Discontinuity Leads to Fast and Intense Craton Lithospheric Thinning

AbstractRemoval and thinning of cratonic lithosphere is believed to have occurred under different tectonic settings, for example, near subduction zones and above mantle plumes. Subduction‐induced cratonic modification has been widely discussed; however, the mechanisms and dynamic processes of plume‐induced lithospheric removal remain elusive and require further systematic investigation. In this study, we use a series of 2‐D thermo‐mechanical models to explore the dynamics of the removal and thinning of cratonic lithosphere due to the interaction between a mantle plume and a weak mid‐lithosphere discontinuity (MLD) layer. Our modeling results suggest that the interaction between a mantle plume and weak MLD layer can lead to a large‐scale removal of the cratonic lithosphere once a connection between the hot upwelling and weak MLD layer is established. The presence of a vertical lithospheric weak zone and its closeness to the plume center play critical roles in creating a connection between the weak MLD and hot plume/asthenosphere. Furthermore, delamination of cratonic lithosphere is favored by a larger plume radius/volume, a higher plume temperature anomaly, and a lower viscosity of the MLD layer. A systematic comparison between subduction‐induced and plume‐induced lithospheric thinning patterns was also conducted. We summarize their significant differences on the origin and migration of melt generation, the water content in melts, and topographic evolution. The combination of numerical models and geological/geophysical observations indicates that mantle plume‐MLD interactions may have played a crucial role in lithospheric removal beneath the South Indian, South American, and North Siberian Cratons.

Coupling Between Lithosphere Removal and Mantle Flow in the Central Andes

AbstractThe central Andes is part of a Cordilleran orogen formed through continent‐ocean convergence. In contrast to the thickened crust, the mantle lithosphere below much of the orogen is anomalously thin. Additionally, the surface is characterized by widespread backarc magmatism and transient ∼100‐km‐wide basins that developed the over last 30 million years, with basins located systematically seaward of major backarc ignimbrite centers and basin formation predating the late Miocene magma/ignimbrite flare‐up. Using numerical models, we propose a novel mechanism whereby lithosphere removal is coupled with mantle flow. First, a small area of high‐density eclogitized lower crust initiates a gravitational instability, causing a localized basin at the surface that subsides and then uplifts. Foundering crust and adjacent lithosphere are entrained by subduction‐induced mantle flow, driving regional lithosphere removal and magmatism. The models demonstrate that mantle flow can amplify a local lithosphere instability to orogen‐wide lithosphere removal, rapidly eliminating accumulated mass in the orogen.

Early Oligocene Surface Uplift in Southwestern Montana During the North American Cordilleran Extension

AbstractThe topographic history of the North American Cordillera holds the key to understanding the tectonic and geodynamic processes of orogenesis and post‐orogenic collapse. In southwestern Montana and its adjacent regions, previous studies have suggested that the early middle Eocene rollback of the Farallon flat‐slab caused the latest topographic growth, after which regional mean elevations were progressively lowered due to extensional collapse. In this study, we report 60 new Eocene‐Miocene stable hydrogen isotopic values (δD) of hydrated volcanic glass from southwestern Montana to examine regional topographic history during the extension. The δD values show a two‐stage negative shift of ∼35‰ during the late Paleogene, the timing of which is further refined by published ages and new zircon U‐Pb ages. About 19‰ of the negative shift occurred during the late Eocene‐earliest Oligocene (∼40–32 Ma) when global and regional climate experienced cooling, and the shift can be mostly accounted for by climate change. The remaining 16‰ negative shift occurred during the early Oligocene (∼32–30 Ma), which can be best explained by regional surface uplift of 0.9 ± 0.5 km. This inferred surface uplift occurred more than 17 Myr after the initiation of the Cordilleran extension in this region, and thus challenges the traditional assumption of progressive topographic lowering during the post‐orogenic collapse. This surface uplift, if occurred, may reflect lower mantle lithosphere removal in southwestern Montana.

Geodynamic Modeling of Lithospheric Removal and Surface Deformation: Application to Intraplate Uplift in Central Mongolia

AbstractIntraplate surface deformation is enigmatic and the underlying mechanisms responsible are not fully understood. We use thermo‐mechanical numerical modeling to explore the conditions under which lithospheric removal processes can occur and investigate the timing and amplitude of consequent surface deformation. We allow lithospheric removal to develop dynamically and self‐consistently by applying a phase transition and density jump that is hypothesized to be a consequence of metamorphic eclogitization in a thickened crust, rather than simply imposing an initial dense block. By systematically varying parameters, we test their influence and control on lithospheric removal. We confirm that a weak and dense lower crust, a hot crust‐mantle boundary, and a convergent regime are critical requirements for delamination‐style removal. We apply these results to central Mongolia, which is an ideal natural laboratory for studying intraplate surface uplift because of its high topography and location in the continental interior. We determine that the physical parameters and structure inferred in this region match the conditions for delamination. Thus, model outputs are evaluated against the available observational evidence. We find that the removal of the lithosphere by delamination can explain: the dome‐shaped topographic pattern and elevated surface; the thin lithosphere and its structure; the elevated temperature at the crust‐mantle boundary. Additionally, it is hypothesized that delamination leads to magmatism due to mantle decompression melting, consistent with intraplate volcanism. Ultimately, the results suggest that lithospheric removal by delamination is a physically plausible mechanism and a potential explanation for intraplate uplift.

High‐Resolution Crustal and Uppermost Mantle Structure Beneath Central Mongolia From Rayleigh Waves and Receiver Functions

AbstractIn this study, I present a high‐resolution 3‐D Vs model of the crust and uppermost mantle beneath central Mongolia, based on seismic data recorded at the Central Mongolia Seismic Experiment array and some other networks. The model is constructed by a Bayesian Monte Carlo inversion that jointly interprets Rayleigh wave phase speeds and receiver functions. The Rayleigh wave dispersion measurements are derived from both ambient noise and teleseismic earthquake data. To improve ambient noise data coverage, a recently developed three‐station interferometry (Zhang et al., 2020, https://doi.org/10.1093/gji/ggaa046) technique is applied to produce high‐quality three‐station interferograms that augment the ambient noise data base. There are several prominent geological and tectonic features captured by the model, including (1) the Khövsgöl rift system is imaged as a high‐speed anomaly in the upper crust, whose southern edge follows the Bulnay fault. (2) The estimated crustal thickness map identifies relatively thick crust beneath the Khövsgöl rift system, the Hangay Dome, and the Gobi‐Altai Mountains. (3) A crustal high‐velocity anomaly is imaged beneath the Taryatu‐Chulutu volcanic field, which is interpreted as a solidified intrusive magma body. (4) Dispersed low‐velocity anomalies in the mantle identify locations with possible partial melting. The melting zones could be related to small‐scale asthenospheric upwelling and convective removal of lithosphere, which produces uplift of the Hangay Dome.

Nature of the pre-collisional lithospheric mantle in Central Tibet: Insights to Tibetan Plateau uplift

The properties of accreted terranes in a large collisional orogen determine, in large part, the nature and subsequent development of the orogen. In this paper we present a detailed study of Late Mesozoic sodic mafic magmatic rocks from Central Tibet. These sodic mafic rocks from Bandatso provide an excellent opportunity to assess the nature and evolution of pre-collisional lithospheric mantle in the Himalayan–Tibetan region, and will thus aid our understanding of the evolution of the orogeny. The Bandatso sodic mafic rocks are characterised by marked oceanic island basalt (OIB)-like enrichment in incompatible elements, such as Nb (up to 139 ppm) and light rare-earth elements. This Late Mesozoic (ca. 148 Ma and 90 Ma) mafic magmatism is likely to be the result of two-stage mantle evolution in the northern Qiangtang. The late Jurassic rocks were derived from carbonated lithosphere metasomatized by subducted oceanic sediments during flat subduction. Whereas the late Cretaceous basanites were likely derived from interaction of lithosphere with upwelling asthenosphere induced by removal of lithosphere that had previously been thickened by relamination. Our study suggests the presence of an inherited carbonated lithospheric mantle beneath the northern Qiangtang prior to continental collision. The conclusions from this work suggest that the enrichment of the mantle source of Cenozoic potassic rock closely related to plateau uplift, more likely result from continental subduction after the late Cretaceous (ca. 90 Ma). These conclusions will help our understanding of the Cenozoic geology of this multi-terrane collision system and uplift mechanism of the Tibetan Plateau.

Early Cretaceous mafic–intermediate dykes in the Dabie orogen as indicators for post-collisional lithosphere removal

The lithospheric root underneath the Dabie orogen, which is the key part of the Central Orogen of China, experienced a cycle of thickening during Triassic continent-continent collision with subsequent post-collisional collapse during the Early Cretaceous. However, the process and mechanism of how thickened lithospheric roots are removed generally remain controversial. U–Pb ages and oxygen isotopes of zircons along with whole-rock geochemistry of post-collisional mafic–intermediate dykes in the region were used to track magmatic contributions from the lower crust, and constrain magma provenance from compositionally variable reservoirs in the upper mantle. Relict zircons show evidence of recycling of subducted continental crust of South China Block that not only has the Neoproterozoic ages and experienced the Triassic metamorphism, but also contains both 18O-depleted and enriched components. A first generation of hypersthene-normative tholeiitic dykes and nepheline-normative alkaline dykes were emplaced at ca. 130–120 Ma. They are overall characterized by arc-like geochemical patterns with εNd values from −21.6 to −4.6 and initial 87Sr/86Sr from 0.7056 to 0.7094. Coeval alkaline dykes are shifted towards more depleted-mantle like compositions, indicating simultaneous melt extraction from different mantle domains and depths. In contrast, second-generation hypersthene-normative mafic dykes (ca. 115–110 Ma) show OIB-like characteristics with εNd values from −1.5 to +4.8 and initial 87Sr/86Sr ratios from 0.7039 to 0.7049, indicating an increasing contribution from upwelling asthenospheric mantle during lithospheric thinning. The temporal evolution of the geochemical characteristics of mafic melts derived from the upper mantle in the Dabie orogen is consistent with foundering of thickened lithosphere and subsequent influx of asthenospheric mantle. The removal of thickened lithosphere root is suggested to be triggered by subduction of the Paleo-Pacific plate and lasted from ca. 130–115 Ma as constrained by the intrusive ages of two distinct types of tholeiitic dykes.

Age and mantle sources of Quaternary basalts associated with “leaky” transform faults of the migrating Anatolia-Arabia-Africa triple junction

Abstract The Anatolia (Eurasia), Arabia, and Africa tectonic plates intersect in southeast Turkey, near the Gulf of İskenderun, forming a tectonically active and unstable triple junction (the A3 triple junction). The plate boundaries are marked by broad zones of major, dominantly left-lateral transform faults including the East Anatolian fault zone (the Anatolia-Arabia boundary) and the Dead Sea fault zone (the Arabia-Africa boundary). Quaternary basalts occur locally within these “leaky” transform fault zones (similar to those observed within oceanic transform faults), providing evidence that mantle melting, basalt genesis, and eruption are linked to crustal deformation and faulting that extends into the upper mantle. We investigated samples of alkaline basalt (including basanite) from the Toprakkale and Karasu volcanic fields within a broad zone of transtension associated with these plate-boundary faults near the İskenderun and Amik Basins, respectively. Toprakkale basalts and basanites have 40Ar/39Ar plateau ages ranging from 810 ± 60 ka to 46 ± 13 ka, and Karasu volcanic field basalts have 40Ar/39Ar plateau ages ranging from 2.63 ± 0.17 Ma to 52 ± 16 ka. Two basanite samples within the Toprakkale volcanic field have isotopic characteristics of a depleted mantle source, with 87Sr/86Sr of 0.703070 and 0.703136, 143Nd/144Nd of 0.512931 and 0.512893, 176Hf/177Hf of 0.283019 and 0.282995, 206Pb/204Pb of 19.087 and 19.155, and 208Pb/204Pb of 38.861 and 38.915. The 176Hf/177Hf ratios of Toprakkale basalts (0.282966–0.283019) are more radiogenic than Karasu basalts (0.282837–0.282965), with some overlap in 143Nd/144Nd ratios (0.512781–0.512866 vs. 0.512648–0.512806). Toprakkale 206Pb/204Pb ratios (19.025 ± 0.001) exhibit less variation than that observed for Karasu basalts (18.800–19.324), and 208Pb/204Pb values for Toprakkale basalts (38.978–39.103) are slightly lower than values for Karasu basalts (39.100–39.219). Melting depths estimated for the basalts from both volcanic fields generally cluster between 60 and 70 km, whereas the basanites record melting depths of ∼90 km. Depth estimates for the basalts largely correspond to the base of a thin lithosphere (∼60 km) observed by seismic imaging. We interpret the combined radiogenic isotope data (Sr, Nd, Hf, Pb) from all alkaline basalts to reflect partial melting at the base of the lithospheric mantle. In contrast, seismic imaging indicates a much thicker (&amp;gt;100 km) lithosphere beneath southern Anatolia, a substantial part of which is likely subducted African lithosphere. This thicker lithosphere is adjacent to the surface locations of the basanites. Thus, the greater melting depths inferred for the basanites may include partial melt contributions either from the lithospheric mantle of the attached and subducting African (Cyprean) slab, or from partial melting of detached blocks that foundered due to convective removal of the Anatolian lithosphere and that subsequently melted at ∼90 km depth within the asthenosphere. The Quaternary basalts studied here are restricted to a broad zone of transtension formed in response to the development of the A3 triple junction, with an earliest erupted age of 2.63 Ma. This indicates that the triple junction was well established by this time. While the current position of the A3 triple junction is near the Amik Basin, faults and topographic expressions indicate that inception of the triple junction began as early as 5 Ma in a position farther to the northeast of the erupted basalts. Therefore, the position of the A3 triple junction appears to have migrated to the southwest since the beginning of the Pliocene as the Anatolia-Africa plate boundary has adjusted to extrusion (tectonic escape) of the Anatolia plate. Establishment of the triple junction over the past 5 m.y. was synchronous with rollback of the African slab beneath Anatolia and associated trench retreat, consistent with Pliocene uplift in Cyprus and with the current positions of plate boundaries. The A3 triple junction is considered to be unstable and likely to continue migrating to the southwest for the foreseeable geologic future.

On the destructive tendencies of cratons

AbstractWe propose that subducting slabs may cause lithospheric removal by directing mantle flow along the craton margin. This process could carve and shape the cratons, leading to conditions that impact the overall (in)stability of the lithosphere. We use three-dimensional geodynamic models to investigate how subduction-driven directed flow interacts with cratonic lithosphere of differing shape, concluding that the margin shape controls both channelization of flow around the craton as well as the potential for destruction. While the simulations show that all craton shapes aid in channelization, the cratons with straight vertical margins are the most resistant to deformation, and the cratons with gradually thickening margins are less resistant to deformation. The dependence on shape could contribute to the progressive removal of cratonic lithosphere along its margin in a runaway process until a more stable vertical margin shape evolves.

Recycling of granulitic lower crust into the mantle

Foundering of eclogitic lower crust into the convecting mantle is a well-recognized process invoked to explain the relatively evolved chemical composition of Earth's continental crust. However, it remains unclear the extent to which the foundering could control the newly-made lowermost crust, and whether mafic granulite in a thickened eclogitic-granulitic lower continental section is effectively removable. Here we report a geochemical study of zoned clinopyroxene phenocrysts in Early Cretaceous high-Mg, Eu-depleted andesites (adakite-like intermediate lavas with high La/Yb and Sr/Y ratios and low heavy rare-earth element and Y contents) from the eastern North China craton (NCC), which suggests crust-to-mantle recycling of plagioclase-rich lithologies. Three types of zoned clinopyroxenes are recognized. The type I and II zoned clinopyroxenes show normal- and reverse-zonation, respectively, and both are featured by clear negative Eu anomalies in their cores. The type III zoned clinopyroxene is poikilitic with a high-Mg# core but no Eu anomaly, and enclosing low-Mg#, partially resorbed olivine grain. We therefore propose that these high-Mg andesites were derived from the partial melting of recycled plagioclase- and garnet-bearing granulitic lower crust, which have interacted with refractory peridotites at spinel-facies depth. Owning to mafic plagioclase- and garnet-bearing granulite with density higher and viscosity lower than refractory peridotite at the same temperature, the subcontinental granulitic lower crust could be preferentially recycled into the mantle through delamination. This correlates well with the continental Moho depth, as lower crustal xenolithes and geophysical studies in the eastern NCC revealed the existence of a granulite-facies rather than eclogite-facies lowermost crust residue (present-day ~30-km-thickness) with a pronounced Eu depletion (Eu/Eu* = ~0.8). Consequently, our preferred model has important implications for the compositional evolution of the Earth's continental crust, and offers new constraints on the scale and timing of lithosphere removal beneath the eastern NCC.

New apatite fission track evidence from the northern Qiangtang terrane reveal two‐phase evolution of central Tibet

AbstractThe growth of central Tibet remains elusory, albeit important in evaluating different topographic growth models accounting for the Tibetan Plateau development. Thermochronological records in the northern Qiangtang terrane (QT) provide valuable information for investigating the cooling and exhumation history in central Tibet. New apatite fission track data, assisted by inverse thermal modelling, reveal two stages of accelerated cooling. The Early Cretaceous cooling is related with refrigeration of the QT and exhumation probably induced by crustal shortening. The Eocene‐Oligocene renewed cooling reflects the far‐field contraction after the onset of the India‐Asia collision and Cenozoic crustal shortening deformation in the QT, coupled with thermal relaxation and transient lithospheric removal. Our data support models indicating that Cretaceous crustal shortening produced a thickened crust in the QT, whereas the present‐day elevation was established during Eocene‐Oligocene due to crustal shortening, continental subduction and lithospheric delamination.

A Geothermal Heat Flux Map of Antarctica Empirically Constrained by Seismic Structure

AbstractThe geothermal heat flux (GHF) is an important boundary condition for modeling the movement of the Antarctic ice sheet but is difficult to measure systematically at a continental scale. Earlier GHF maps suffer from low resolution and possibly biased assumptions in tectonism and crustal heat generation, resulting in significant uncertainty. We present a new GHF map for Antarctica constructed by empirically relating the upper mantle structure to known GHF in the continental United States. The new map, compared with previously seismologically determined one, has improved resolution and lower uncertainties. New features in this map include high GHF in the southern Transantarctic Mountains where warmer uppermost mantle is introduced by lithospheric removal and in the Thwaites Glacier region. Additionally, a modest GHF in the central West Antarctic Rift system near the Siple Coast and an absence of large‐scale regions with GHF greater than 90 mW/m2 are found.

Early‐Stage Lithospheric Foundering Beneath the Eastern Tibetan Plateau Revealed by Full‐Wave Pn Tomography

AbstractThe west‐east contrast of magmatism in northern Tibet suggests that the lithospheric root has been removed in the west, following continental collision that led to lithospheric thickening and removal, but not in the east where paradoxically larger convergence occurred. Here we show a full‐wave Pn tomography model for the eastern Tibetan Plateau, which reveals a high‐velocity layer beneath the Moho extending to 150‐km depth. The anomalously high velocities and its northward dipping top surface suggest a very depleted and cold mantle consistent with an underthrusted Precambrian Lhasa lithosphere. A high‐velocity column connects this layer to another high‐velocity layer below 190‐km depth, representing early‐stage removal of the Tibetan mantle lithosphere and its interaction with the underthrusted Indian lithosphere. The west‐east contrast is thus attributed to different stages of lithospheric removal, which may be controlled by varying angles of Indian subduction from the west to the east.