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High‐elevation, low‐relief continental plateaus are major topographic features and profoundly influence atmospheric circulation, sediment transport and storage, and biodiversity. Although orogenic surface‐uplift mechanisms for modern continental plateaus near known plate margins like Tibet are well‐characterized, they cannot account for examples in intracontinental settings like the Colorado Plateau. In contrast to canonical plate‐tectonic uplift mechanisms, broad‐scale hydration‐induced metasomatism of the lower crust has been suggested to reduce its density and increase its buoyancy sufficiently to contribute to isostatic uplift. However, the relationships between key petrophysical properties in these environments are not fully quantified, which limits application of this model. Here, we develop a series of petrological models that describe the petrological and topographic effects of fluid–rock interaction in non‐deforming continental crust of varying composition. We apply an open‐system petrological modelling framework that utilizes reactive‐transport calculations to determine the spatial and temporal scales over which mineralogic transformations take place compared with the magnitude of infiltration of aqueous fluids derived from devolatilization of subducting oceanic lithosphere. The buoyancy effect of hydration‐induced de‐densification is most significant for metabasic lower crust, intermediate for metapelitic crust, and minimal for granodioritic crust. We apply these results to a case study of the ~2 km‐high Colorado Plateau and demonstrate that under ideal conditions, hydration of its lower–middle crust by infiltrating aqueous fluids released by the Farallon slab during Cenozoic low‐angle subduction could have uplifted the plateau surface by a maximum of ~1 km over 16 Myr. However, realistically, although hydration likely has a measurable effect on surface tectonics, the uplift of orogenic plateaus is likely dominantly controlled by other factors, such as lithospheric delamination. [ABSTRACT FROM AUTHOR]

Magmatic periodicity is recognized in continental arcs worldwide, but the mechanism responsible for punctuated arc magmatism is controversial. Continental arcs in the Trans‐Himalayan orogenic system display episodic magmatism and the most voluminous flare‐up in this system was in early Eocene during the transition from subduction to collision. The close association of the flare‐up with collision is intriguing. Our study employs zircon Lu‐Hf and bulk rock Sr‐Nd isotopes, along with mineral geochemistry, to track the melt sources of the Nymo intrusive complex and the role of mantle magma during the early Eocene flare‐up of the Gangdese arc, Tibet. The Nymo intrusive complex is composed of gabbronorite, diorite, quartz diorite, and granodiorite which define an arc‐related calc‐alkaline suite. Zircon U‐Pb ages reveal that the complex was emplaced between ~50–47 Ma. Zircon Hf isotopes yield εHf(t) values of 8.2–13.1, while whole‐rock Sr and Nd isotopes yield εNd(t) values of 2.7–6.5 indicative of magmatism dominated by melting of a juvenile mantle source with only minor crustal assimilation (~15%–25%) as indicated by assimilation and fractional crystallization modeling. Together with published data, the early Eocene magmatic flare‐up was likely triggered by slab breakoff of subducted oceanic lithosphere at depths shallower than the overriding plate. The early Eocene magmatic flare‐up may have contributed to crustal thickening of the Gangdese arc. This study provides important insights into the magmatic flare‐up and its significant role in the generation of large batholiths during the transition from subduction to collision. [ABSTRACT FROM AUTHOR]

The seismic data recorded at 48 broadband stations on an 1,800‐km‐long linear array have been used to image the deep structure and deformation in the intra‐continental collision between the Tarim Basin and the Northern Tibetan Plateau (NTP). Common Conversion Point (CCP) stacking imaging along the linear array and joint inversion of receiver function and surface wave dispersion defined the crustal and upper mantle structure, indicating that the Tarim block underthrusts the Altyn Tagh Range–Qaidam Basin. Whereas Moho is flattened, the lithospheric mantle beneath the Qaidam Basin has unusually low velocity, which is thought to be the consequence of the delamination in the lower crust and mantle lithosphere. Strong positive phase occurring at depths of 150–200 km on the CCP stacking imaging is likely to be associated to the remnant subducted oceanic lithospheric slab or the underthrusting Tianshan lithospheric slab beneath the collision zone. The removal of the Altyn Tagh Range–Qaidam Basin orogenic root may be due to convection‐driven delamination underneath it and subsequent underthrusting of the Tarim block. The spatial variation of the SKS splitting is manifested as the large‐scale pattern of lithospheric deformation and local abrupt changes, transitioning from pure shear on the NTP to simple shear in the Altyn Tagh Range. A comprehensive analysis of SKS splitting and GPS data reveals a simple shear pattern of vertical coherent deformation in the Altyn Tagh Range and its adjacent areas, which is evidence of the lithospheric shear zones. Plain Language Summary: The seismic wave data received by 48 broadband stations on the newly 1,800 km long linear array were used to determine the deep structure and deformation of the crust and upper mantle caused by the intra‐continental collision between the Tarim block and the Northern Tibetan Plateau by the joint inversion of the receiver function and the surface wave dispersion method. The results of this study reveal that the Tarim block underthrusts the Altyn Tagh Range–Qaidam Basin, and the Moho discontinuity below it appears nearly flat, indicating that the underlying orogenic roots have been removed. The mantle lithosphere below the Qaidam Basin is a low‐velocity anomaly zone which is thought to be evidence of the delamination of the lower crust and upper mantle. Convection‐driven delamination underneath the Qaidam Basin and subsequent underthrusting of the Tarim block resulted in the removal of the orogenic roots below the Altyn Tagh Range–Qaidam Basin. The delamination and uplift resulted in the shortening and thickening of the Altyn Tagh Range–Qaidam Basin. A comprehensive analysis of SKS splitting and GPS data reveals the vertical coherent deformation of the Altyn Tagh Range. The consistency between the coherent SKS fast direction and the strike of the crustal faults is considered evidence of simple shear of the Altyn Tagh Range. Key Points: Flat Moho and anomalous low‐velocity lithospheric mantle beneath the Qaidam Basin are thought to be the consequence of delaminationStrong positive phases of receiver function at depth of 150–200 km may be related to the remnant subducted Tarim oceanic lithospheric slabSimple shear pattern of vertical coherent deformation resulting from SKS splitting suggests that the Altyn Tagh Range is a lithospheric shear zone [ABSTRACT FROM AUTHOR]

The subduction and demise of an ocean plate are generally recognized as essential processes that result in the reworking and maturation of the continental crust. The northern Lhasa Terrane in central Tibet represents the forefront of the Lhasa-Qiang-tang collision belt following the closure of the Bangong-Nujiang Ocean. Thus, it is a pivotal location to study the transition processes from oceanic lithosphere subduction to continental collision as well as pertinent crustal growth mechanisms. Here, we present zircon U-Pb dating, whole-rock major and trace element and Sr-Nd isotope, and zircon Hf isotope and trace element data of the Mendang igneous complex, Baingoin County, northern Lhasa Terrane. Geochro-nological results show that the granodiorites, trachydacites, and rhyolites in the Mendang igneous complex formed at ca. 122-116 Ma, 97 Ma, and 73 Ma, respectively. The Early Cretaceous granodiorite samples are peraluminous with high SiO2, Al2O3, and K2O contents, and moderate A/CNK (molar ratio of Al2O3/[CaO + Na2O + K2O]) values, which are similar to those of typical felsic peraluminous I-type granites. The granodiorites are characterized by enrichment in light rare earth elements and large ion lithophile elements (e.g., Rb) and depletion in high field strength elements (e.g., Nb, Ta). They also show the most enriched whole-rock Sr-Nd [(87Sr/86Sr)i = 0.7072-0.7078; eNd(t) = -7.60 to -5.08] and zircon Hf [eHf(t) = -4.46 to +1.02] isotope compositions, indicating that the Early Cretaceous granodiorites were likely derived from an ancient basement under a subduction setting. The trachy-dacites have uniform SiO2, high Al2O3, Sr contents, and Sr/Y values, and low Y and Yb contents, belonging to adakitic rocks. They show more depleted whole-rock Sr-Nd [(87Sr/86Sr)i = 0.7065-0.7066; eNd(t) = -0.56 to -0.22] and zircon Hf [eHf(t) = 4.36-7.84] isotopes than the granodiorites, suggesting that the trachydacites may have generated from partial melting of the juvenile thickened lower continental crust. The rhyolites have the highest SiO2 and K2O contents in the Mendang igneous complex, and significant depletion of Ba, Sr, Eu, P, and Ti. They have slighter more enriched whole-rock Nd [eNd(t) = -3.71 to -1.16] and zircon Hf [eHf(t) = 1.03-4.31] isotope compositions than the trachydacites. These features suggest that the rhyolites were highly fractionated products of the crustal melts. Wholerock Sr-Nd and zircon Hf isotopes of the Mendang igneous complex show a kink trend from enrichment to depletion and then transfer to enrichment again, signifying an increased contribution of juvenile materials in the northern Lhasa Terrane toward progressively replacing the ancient lower crust and accumulating to newly formed crust. The estimated crustal thickness beneath the northern Lhasa Terrane shows a sharp increase from the Early to Late Cretaceous and peak at ca. 97 Ma, whereas it largely decreases in the Late Cretaceous. Integrated with previous studies, we propose that the formation of the Mendang igneous complex (122-73 Ma) elaborately documents the regional tectonic transition from oceanic lithosphere subduction to demise as well as continental crustal differentiation and maturation. The rollback and breakoff of the southward subducted Bangong-Nujiang oceanic slab in the Early Cretaceous initiated diverse magmatism in the northern Lhasa Terrane. During the early Late Cretaceous, the widespread ada-kitic and Mg-rich magmatism was attributed to the delamination of the thickened lithosphere following the final amalgamation of the Lhasa and Qiangtang Terranes. In the late Late Cretaceous, post-collisional extension induced the formation of the rhyolites in the northern Lhasa Terrane. The growth and destruction of the continental crust had certain tempos from the oceanic lithosphere subduction to continental post-collision stage. [ABSTRACT FROM AUTHOR]

Knowledge of the evolution of the Bangong-Nujiang Tethyan Ocean is crucial for reconstructing the paleography of the Tethyan Realm, given its significance as a key component of the eastern Tethys. Nonetheless, there has been uncertainty regarding both the timing and the processes involved in the closure of this ocean. This study focused on a 110-106 Ma igneous complex comprising basalts-basaltic andesites, trachyandesites, and granodiorites from the Sumxi area in the western part of the Qiangtang terrane of west-central Tibet. The basalts-basaltic andesites have SiO2 contents of 52.5-58.7 wt% and MgO contents of 2.89-4.63 wt%, and exhibit some arc-like geochemical signatures. However, these rocks also have elevated Nb contents (>10 ppm) and Nb/La ratios (>0.5), as well as enriched Sr-Nd isotopic composition [eNd(Z) = -7.40 to -6.00], implying that they are products of a mantle source metasomatized by adakitic melts. The trachyandesites are characterized by intermediate compositions (SiO2 = 63.6-65.2 wt%), high Mg number (40-60), and more enriched eNd(t) values (-8.37 to -7.49). Comparing their geochemical composition to that of mélange rocks, it is postulated that these trachyandesites were formed through the partial melting of a mantle source including mélange matrix rocks within a subduction zone. The granodiorites exhibit adakitic geochemical features (Sr = 830.14-1032.70 ppm, Y = 14.8615.37 ppm, Sr/Y = 54-68), indicating that they originated from the partial melting of a thickened lower crust in a continental arc setting. Our results, in combination with a synthesis of tectonomagmatism along the Bangong-Nujiang suture zone, provide con-vincing evidence for subduction of an oceanic plateau and subsequent slab roll-back. The Sumxi igneous complex, with its clear arc affinity, suggests that the Bangong-Nujiang Tethyan Ocean, or at least its western part, remained open until the late Early Cretaceous (ca. 106 Ma). [ABSTRACT FROM AUTHOR]

The post-collisional evolution of the Tibetan lithosphere is of paramount significance to our understanding of collisional orogeny. It is generally postulated that the Lhasa lithospheric mantle was horizontally shortened and thickened coherently with the overlying crust to form a physical barrier, preventing Indian subduction beneath Tibet until the thickened mantle root was foundered during the Miocene. This study first identifies post-collisional oceanic-island basalt (OIB)-type magmatism in the Lhasa Block (LB), as attested by zircon U-Pb age (ca. 58 Ma) and geochemistry--positive Nb-Ta anomalies, high La/Yb, and depleted bulkrock Sr-Nd and zircon Hf isotopes, of diabase in the northern (inboard relative to Indus Suture) part of this block. Coupled with extensive early Paleogene arc-type magmatism in the southern-central LB and thermodynamic modeling, we suggest that these diabases were formed by partially molten upwelling asthenosphere near the base of continental crust, where much of the underlying lithospheric mantle had been removed due to Neo-Tethyan slab rollback and lithospheric delamination. Compared to OIB-type magmatism worldwide, the diabases investigated here were emplaced peculiarly in a region where the continental crust was under horizontal compression and shortening by coeval thrusting. Our study thus implies a decoupled deformation between the crust and mantle of the LB during the early Indian-Asian collision. [ABSTRACT FROM AUTHOR]

Analysis of the upper mantle plumes spatial distribution in the inner part of the Sunda arc shows a number of plume bodies interrupting the stagnant slab framed from the south by the sinking slab of the Sunda arc. Possible mechanisms resulting in this structure of the mantle are (i) W-E-trending toroidal mantle flow through a gap in a flat slab and (ii) rollback capable of forming a gap in a flat slab and launching upper mantle plumes in it without deep (>1000 km) roots. The space above the top of the slab consists of local hot mantle bodies, which are considered secondary plumes and often form local rift segments. The 3D visualization of velocity variations δVp in the Tibet and Central Asia region demonstrates structural patterns similar to those in the Sunda arc area. One can observe a region of subhorizontal slab fragments and a gap, which is recorded by plume anomalies of deep and secondary origin. The motion vectors of the rock mass along the fault planes of the Sunda arc, detected from seismic events, are directed outward from its center of curvature, in which secondary upper mantle plumes are concentrated. This indicates the development of thrust processes at the arc front unassociated with the subducting plate. The arc thrusting is accompanied by a small number of displacements along antithetic thrust faults. The displacement azimuths along the Himalayas are fan-shaped, oriented toward Hindustan. This shows that the main indicator of tectonic activity (seismic events) has a direction of rock mass displacement to the south from the extensional back-arc basin within Tibet with the development of thrust deformations during movements along the detachment planes. Two directions of seismic movements are distinguished in the Himalayan arc, as well as in the Sunda arc. The first direction follows the Indian Plate subduction model. The second direction includes variable azimuth rock mass movements along the thrust planes onto the Indian Plate. [ABSTRACT FROM AUTHOR]

The distribution of Oligo--Miocene magmatic rocks from southern Tibet in space and time yields critical information on the geometry and deformation of the subducted Indian lithosphere which impacts on plateau growth following the India and Eurasia collision. A growing body of geophysical evidence has shown that the subducted Indian lithosphere beneath the Tibetan Plateau has been torn apart. However, the spatiotemporal distribution and cause of the tearing remain enigmatic. Timing of the post-collisional magmatic rocks in southern Tibet exhibits four patterns of decreasing ages; magmatism began earlier in the west and east Himalayan syntaxis and evolved to two age undulations in the central southern Tibet. Seismic images show that regions of slab window (both 90°E and 84°E) and flattened subducted lithosphere (both 86°E and 81°E) are present at depth of 135 km. Correspondingly, increasing mineral crystallization temperatures (absolute value of 50 °C) were recorded in the Oligo--Miocene ultrapotassic-potassic rocks at 90°E and 84°E, while opposing trends were shown by coeval ultrapotassic-potassic rocks at 86°E and 81°E. Besides, the melting depth of the Oligo--Miocene ultrapotassic-potassic primitive melts decreases from nearly 100 km to 70 km between 81°E and 90°E, probably indicating progressive rising of the lithosphere-asthenosphere boundary. Such variations were possibly the results of the focused flow and upwelling of asthenosphere, which advanced rapidly but diachronously through weakened and torn sectors within the overlying Indian slab. The upwellings probably induced diachronously upward bending of the residual Indian slab and its flattening, which accelerated the tearing of the Indian lithosphere during continental subduction. [ABSTRACT FROM AUTHOR]

Differentiating between advancing and retreating subduction zones and delineating the timing and processes for switching between these end-member types are critical factors in understanding orogenic evolution. In this study, we used temporal composition variations and spatiotemporal distribution of igneous rocks to constrain the types and directions of the subduction zone beneath the Lhasa terrane in southern Tibet during the mid-Mesozoic. Comprehensive geochronological and geochemical data from mid- Mesozoic magmatic rocks in five areas of the central Lhasa terrane document two compositionally distinct magmatic suites: older (ca. 172-150 Ma) high-Mg diorite-granodiorite and younger (ca. 150-130 Ma) low-Mg granodiorite-syenogranite. The high-Mg rocks with enriched isotopes were likely generated by the mixing of Lhasa terrane basement- derived magmas with mantle-derived magmas that incorporated extensive contributions from subducted sediments. The low- Mg rocks with relatively depleted isotopes are interpreted as derived from a mixed lowercrust source (ancient basement and juvenile crust) with minor mantle contributions followed by crystal fractionation. Both high-Mg and low-Mg rocks show arc signatures and formed the 1200 km Shiquan-Zhegu-Menba arc. A combination of the Mesozoic geological and geochemical data suggests that both the Neo-Tethyan and Bangong Tethyan slabs were likely subducting beneath the Lhasa terrane, creating two opposing subduction zones. The change from high-Mg to low- Mg magmatism within the Shiquan-Zhegu- Menba arc likely records a tectonic switch from advancing to retreating subduction of the south-dipping Bangong Tethyan slab. Our results provide a good example for identification of subduction polarities and types of ancient orogenic belts through spatiotemporal distributions and changes in geochemical compositions of igneous rocks. [ABSTRACT FROM AUTHOR]

Slab breakoff can explain the postcollisional igneous and metamorphic events in many collisional orogens and triggers the transition from initial continental collision to deep subduction. However, the relationship between the deep-seated geometry of slab breakoff and shallow tectono-magmatic events remains unclear. In this study, we report new geochronological, geochemical, and Hf isotope data for granitic rocks from the Qiangtang block. Three granitic samples yielded zircon U-Pb ages of 226-214 Ma and mostly gave negative zircon εHf(t) values of -20.1 to +0.57. These granitoids are K-rich, calc-alkaline, magnesian, and metaluminous to weakly peraluminous. We propose that the granites were generated by partial melting of compositionally heterogeneous lower crust in the Southern Qiangtang block. Our new data, along with previously published data for igneous and metamorphic rocks in the Qiangtang block, suggest that the Longmu Co--Shuanghu suture zone represents a remnant of the closure of the Paleo-Tethys Ocean, and that the Late Triassic tectono-magmatic evolution was controlled by slab breakoff after continental collision. The contrasting magmatic and metamorphic ages show that the two stages of tectono-magmatic activity in the Late Triassic were due to a change in the subduction angle of the continental lithosphere from steep (225-215 Ma) to shallow (215-205 Ma) during slab breakoff. Furthermore, we infer that slab breakoff of the oceanic lithosphere caused extensive crustal melting and reworking and initiated far-field subduction. Our results provide new insights into the slab breakoff processes that control the geometry of subducting continental lithosphere and related postcollisional magmatic and metamorphic events in collision zones. [ABSTRACT FROM AUTHOR]

The geodynamic evolution of the Himalayan–Tibetan orogen during the transition from Neo-Tethyan oceanic to Indian continental subduction remains unclear. Therefore, we conducted a new geochronological, geochemical, and Sr–Nd–Pb–Hf isotopic study of post-collisional diabase dikes in the central segment of the southern Lhasa subterrane. The diabase dike samples yield zircon U–Pb ages of 51–49 Myr, consistent with the timing of the Gangdese magmatic flare-up (ca. 53–50 Myr). In addition, the diabase dikes exhibit a geochemical intraplate affinity and depleted Sr–Nd–Pb isotopic compositions. Their variable Nb contents allow, the dikes to be divided into low-Nb (<8 ppm) and high-Nb (>9 ppm) types, which represent the melting products of slab fluid-modified Gangdese lithospheric mantle and a mixed mantle source comprising Gangdese lithospheric and deep asthenospheric mantle, respectively. In addition, the wide variation in εHf(t) values (−10.0 to +12.3) of zircons from these diabase dikes is consistent with the Gangdese zircon Hf isotopic 'pull-down' at ca. 50 Ma. Integrating our new data with published results suggests that the zircon Hf isotopic 'pull-down' was a direct consequence of the involvement of subducted Indian continental components in the formation of the Gangdese continental lithosphere. The combined data further suggest that upwelling of the asthenosphere, triggered by breakoff of the Neo-Tethyan oceanic slab, resulted in extensive mantle and crustal melting in the Lhasa terrane and the Tethyan Himalaya in southern Tibet. • The diabase dikes in the Pengcuolin–Dazhuqu region were emplaced at ca. 50 Ma. • Indian continental components were involved in the Gangdese lithosphere at ca. 50 Ma. • The breakoff of the Neo-Tethyan slab is believed to have occurred at ca. 53–50 Ma. [ABSTRACT FROM AUTHOR]