Search

Topography in compressional mountain ranges represents an interface at which tectonic and climatic forces interact. Understanding the relative contribution of these two components to mountain formation has been at the forefront of research over the last two decades. The theory underlying the mechanics that govern these interactions has been built on Coulomb wedge mechanics, i.e., mechanical failure and rock uplift occur everywhere along the wedge and the orogen. Observed rock displacement along single, discrete fault planes, including the translation of uplifted topography laterally, appears to be counter to such mechanics. However, a critically tapered topography across fold-thrust belts still emerges. If a critically tapered topography along an orogenic wedge can be produced by the sequential evolution of the subsurface fault geometry and the associated motion of bedrock over discrete fault planes, then a mechanical failure everywhere is not required. Here, the geomorphic evolution of the fold-thrust belt in central Nepal since the Miocene is investigated using a numerical surface processes model whereby the structural geometry, location and magnitude of fault motion are prescribed and based on observations. In addition, end-member climatic scenarios are adopted, i.e., uniform precipitation and climatic change over geologic time as predicted by atmospheric general circulation models. The experiments reproduce the first-order topography of central Nepal. Our modelling results indicate a dynamic variability of erosional efficacy that promotes the interplay of two modes of orogenic wedge behaviour and are contrary to a mechanical failure everywhere along the wedge: (mode 1) phases of lateral translation of uplifted topography and in-sequence propagation of deformation fronts, and (mode 2) phases of hinterland incision during out-of-sequence fault activity. The successful replication of first-order geomorphic indices in central Nepal in our experiments confirms an unusually long-lasting Miocene to Pliocene activity of the Main Boundary Thrust in central Nepal. This period is followed by Late Pleistocene hinterland incision coeval with out-of-sequence fault activity prior to the onset of rock displacement along the Main Frontal Thrust during a time of increased precipitation relative to today.

The main site and timing of the final closure of the middle segment of the Paleo-Asian Ocean (PAO) has been an issue of hot debate, which hampers us from better understanding the late-stage tectonic evolution of the Central Asian Orogenic Belt (CAOB). Synthesizing the available geological records for the ophiolitic melanges in the Beishan Orogenic Belt (BOB), we regard the Liuyuan ophiolitic melange as the main site of the final closure of the middle segment of the PAO. To determine the final closure time of the middle segment of the PAO, this study mainly applied field-based, systematic zircon U-Pb-Hf isotopic analyses for the Carboniferous and Permian sedimentary successions on the northern and southern sides of the Liuyuan ophiolitic melange. Our results indicate that the late Carboniferous sedimentary successions north of the Liuyuan melange consisting mainly of interbedded sandstone and siltstone with minor conglomerate show primarily affinity with a local, single source, i.e. the constituent units of the BOB north of the Liuyuan melange. They were closely associated with the northward subduction of the middle segment of the PAO. By contrast, the unconformably overlying Permian clastic deposition on both sides of the Liuyuan ophiolitic melange shows comparable lithology that fines from a thick sequence of conglomerate at the base to thin-bedded turbidite sequences up section. These Permian units were probably deposited in a progressively deepening basin within an extensional post-collision regime after the disappearance of the middle segment of the PAO. All the

Many convergent orogens, such as the eastern European Alps, display an asymmetric doubly vergent wedge geometry. In doubly vergent orogens, deepest exhumation occurs above the retro‐wedge. Deep‐seismic interpretations depict the European plate dipping beneath the Adriatic, suggesting the pro‐wedge location on the north side of the orogen. Our new thermochronometer data across the Eastern Alps confirm distinct shifts in the locus of exhumation associated with orogen‐scale structural reorganizations. Most importantly, we find a general Mid‐Miocene shift in exhumation (in the Tauern Window and the Southern Alps) and focus of modern seismicity across the Southern Alps. Taken together, these observations suggest a subduction polarity reversal at least since the Mid‐Miocene such that the present‐day pro‐wedge is located on the south side of the Alps. We propose a transient tectonic state of a slow‐and‐ongoing slab reversal coeval with motion along the Tauern Ramp, consistent with a present‐day northward migration of drainage divides., Plain Language Summary: When tectonic plates collide, they bend downwards and form two lithospheric wedges dipping in opposite directions, such as in the Eastern Alps. We present new crustal cooling data along a transect in the Eastern Alps confirming that surface rocks across the central Tauern Window originated from the deepest structural levels along the transect. South of the Tauern Window rocks were exhumed from higher depths compared to those north of it and were exhumed more recently, while seismic activity is also focused across the Southern Alps. These observations suggest a subduction polarity reversal because they are inconsistent with the original southern and northern locations of overriding and subducting plates, respectively, >15 million years ago. This interpretation is contrary to lithosphere‐scale tomography that shows no change in subduction polarity. Therefore, we propose a transient tectonic state, that is, a slow‐and‐ongoing subduction polarity reversal that initiated when Tauern Window rocks began their steep ascent to the surface along a deep‐seated fault known as the Tauern Ramp. This study bridges observations in the mantle, crust and on the surface over geologic time., Key Points: Thermochronologic data in the Eastern Alps is consistent with a transient tectonic state toward complete slab reversal. The pro‐wedge has switched from north to south of the Periadriatic Fault along TRANSALP. Mid‐Miocene motion along the Tauern Ramp is the consequence of slab‐reversal., Deutsche Forschungsgemeinschaft (DFG) http://dx.doi.org/10.13039/501100001659

The Central Asian Orogenic Belt (CAOB) was built up through protracted accretion and collision of a variety of terranes/micro-continents during Neoproterozoic–Mesozoic time. To understand potential links among Paleozoic subduction and accretionary processes that were operative during the development of the southeastern CAOB, we conducted a combined U-Pb and Hf-isotope analysis of detrital zircons from previously defined Devonian, Carboniferous and Early Permian strata in the Bengbatu area, Inner Mongolia. Detrital zircons from (meta-) sandstones in these strata commonly yield major Paleozoic age populations at ca. 300–261 Ma, 351–300 Ma and 517–419 Ma, and also give several Precambrian ages that range from 2687 Ma to 544 Ma. The youngest ages redefine the deposition of all these strata to be in the Middle Permian (Wordian–Capitanian) or later, much younger than previously considered. These ages, coupled with regional magmatic records, support an interpretation of most surrounding areas as possible detritus sources, including the Mongolian arcs to the north, the Northern Accretionary Orogen to the south, and the intervening Erenhot–Hegenshan Ophiolite Belt. Zircons with magmatic ages of ca. 500–350 Ma and ca. 300–261 Ma display a large range of εHf(t) values (−13.97 to +15.31), whereas ca. 350–300 Ma zircons are dominated by positive εHf(t) values (+0.14 to +16.00). These results support the occurrence of two significant shifts of the zircon εHf(t) values, which has tectonic implications for the understanding of the Carboniferous–Permian evolution of the southeastern CAOB. A marked shift from mixed to positive zircon εHf(t) values at 350–330 Ma likely manifests the incipient opening of the Hegenshan Ocean, due to the slab rollback of the subducting Paleo-Asian Oceanic lithosphere. Another shift from positive to mixed zircon εHf(t) values at ca. 300 Ma likely corresponds to a tectonic switch from syn-orogenic subduction-related to post-orogenic extensional setting, genetically related to the tectonic collapse of a formerly overthickened crust. Keywords: Detrital zircon, U-Pb dating, Hf isotope, Central Asian Orogenic Belt, Subduction, Back-arc basin

Many convergent orogens such as the eastern European Alps display an asymmetric doubly-vergent wedge geometry. Loci of deepest exhumation are located above the overriding retro-wedge, whereas increased fault activity occurs in the pro-wedge on the subducting plate. The main drainage divide separates steeper from more gently sloping topography on the two wedges of different critical taper. We performed apatite and zircon (U-Th)/He analyses densely spaced along the TRANSALP geophysical transect in combination with thermo-kinematic models based on cross-section balancing. Our new low temperature thermochronology data and thermo-kinematic model results underline (i) deepest levels of exhumation across the Tauern Window until the Pliocene and (ii) higher Late Neogene exhumation rates south of the Periadriatic Fault relative to the north, while seismic activity is focussed across the Southern Alps. Our proposed mantle-to-surface link positions the retro-wedge north of the Periadriatic Fault subsequent to subduction polarity reversal during continental collision. Present-day drainage divide migration trends and imaged locations of mantle-lithospheric slabs beneath TRANSALP suggest ongoing, slow slab reversal since Adriatic indentation in the Eastern Alps.

The location and tectonic nature of the Solonker Suture in East Asia and hence the late Palaeozoic to early Mesozoic closure of the Palaeo-Asian Ocean have notoriously been enigmatic in the past decades due to limited rock exposure and the absence of unambiguous collision-related regional features. Several tectonic models have been proposed since, but in many cases these models were derived only from single key exposures or methodologies, often being over-simplified, or -interpreted, leading to questionable extrapolations on regional and global scales despite the complexity of the Palaeozoic accretionary tectonic framework. Now, the regionally consistent availability of geochronological, geochemical, stratigraphic and palaeo-geographic data enables us to integrate these into a highly detailed and coherent Palaeozoic tectonic review of the suture. The region across the Solonker Suture can generally be subdivided into three major Palaeozoic tectonic provinces, e.g., (i) a Sino-Cratonic Province that resembles the active Palaeozoic northern margin of the North China Craton to the south of the suture, (ii) a Mongolian Province that comprises the south-eastern margin of the Mongolian Terrane to the north of the suture, and (iii) an East Asian Pre-Pacific Province in north-east China that accreted with the Late Permian initiation of Pacific Plate subduction. The Sino-Cratonic Province experienced episodic tectonic activity that includes the accretion of the initially ensialic Bainaimiao Arc onto the passive northern margin of the North China Craton at ~ 437–453 Ma, subsequent southward directed subduction activity beneath the amalgamated margin until at least ~ 400 Ma, followed by a temporary cessation of magmatic activity accompanied by a switch from north- to southward directed continental drift, and finally concluded by renewed subduction activity until the final closure of the Palaeo-Asian Ocean. The Mongolian Province is suggested to have been more closely associated with the Siberian Craton before the presumably Late Devonian opening of the Mongol-Okhotsk Ocean. The East Asian Pre-Pacific Province, originally rifted away from the eastern margin of Gondwana at ~ 600–750 Ma, recorded early Palaeozoic (~ 500 Ma) orogenic processes along the then northern margin of the Siberian Craton. The Baolidao arc system successively developed along the present-day south-eastern margin of the Mongolian Province due to continued subduction activity throughout the Palaeozoic. After the Middle Carboniferous opening of the Hegenshan back-arc basin renewed subduction beneath a matured Baolidao Arc during Carboniferous and Permian times led to the obduction of the Hegenshan ophiolite not earlier than ~ 270 Ma. Shortly after the Palaeo-Asian Ocean diachronously closed from west to east along the Solonker Suture in the Late Permian to Early Triassic. The East Asian Pre-Pacific Province re-attached itself by the mid-Mesozoic, followed by a change in the regional stress regime controlled by Palaeo-Pacific plate subduction. A divergent double-sided subduction model for the closure of the Palaeo-Asian Ocean provides, based on the reviewed data, a tectonic model that is not only able to reasonably well explain the absence of typical collision-related features in the region but also the abundance of Mesozoic A-type granitic magmatism in north-east China.

Situated between the South Tianshan suture zone to the west and the Solonker suture zone to the east, the Yagan and Zhusileng-Hangwula arcs (YZHAs) in western Inner Mongolia in China occupy...

In Northeast China, oceanic subduction between the Jiamusi and Songliao blocks remains topic of hot debate. The Heilongjiang Complex has been regarded as an accretionary belt resulting from the subduction of an intervening ocean between the two blocks. In this study, we carry out extensive geological, geochemical and geochronological investigations on the sedimentary rocks, amphibolites and blueschists from the Heilongjiang Complex. The detrital zircons from meta-sedimentary rocks yield U-Pb age spams ranging from 268 to 780 Ma. Whereas the interlayered amphibolites show negative Nb-Ta-Ti and positive Pb anomalies and have a protolithic age of 188.2 ± 1.0 Ma, suggesting a subduction zone or magmatic arc origin in the Jurassic. The elevated initial Sr isotopic ratios (0.708–0.711) and negative e Nd (t) values (− 4.3 to − 1.3) provide further evidence of the modification by upper continental crust during the magma ascending. LREE/HREE and MREE/HREE ratios suggest that the magma was likely derived from the mixing of lithospheric and asthenospheric melts. The presence of low Th/Ce (0.04–0.12) and Hf/Sm (0.35–0.55), but high Zr/Hf (35.4–43.4) and Pb/Ce (0.25–0.49) ratios strongly implies a contribution from subducted sediments. The identification of active continental margin type magmas of Early Jurassic age suggests that the subduction of the oceanic plate between the Jiamusi and Songliao blocks may have started around Early Jurassic time. In addition, the blueschists have a protolithic age of 186 ± 1.1 Ma and display geochemical affinities of oceanic island basalts, suggesting that the ocean between the Jiamusi and Songliao blocks closed sometime after 186 Ma. Mineral 40 Ar/ 39 Ar dating results from the Heilongjiang Complex further indicate that blueschist- to greenschist-facies metamorphism occurred in the Middle to Late Jurassic (158–175 Ma), marking the onset of termination of the oceanic subduction.

The Heilongjiang Complex is a blueschist facies metamorphic belt located within the Zhangguangcailing Orogen between the Jiamusi and Songliao blocks in Northeast China. This complex has been regarded as an accretionary belt related to the subduction of an intervening oceanic domain between the two blocks. However, the timing of ocean closure and final amalgamation has not been well constrained, with different models arguing for a period of 210–180 Ma or sometime after ~ 140 Ma. This work reports in-situ detrital zircon U–Pb and Hf isotopic analyses of meta-sedimentary rocks from the Heilongjiang Complex. Detrital zircons from seven meta-sedimentary rocks samples yield U–Pb ages spanning from 1690 to 167 Ma, with main populations matching those of multi-phase magmatism in the Jiamusi and Songliao blocks. Several Precambrian age groups (600 Ma, 700 Ma, 900 Ma, 960 Ma, 1200 Ma, and 1300 Ma) are consistent with the inherited zircons from the mafic rocks in the Heilongjiang Complex. A comparison with compiled data of magmatic rocks suggests that the two blocks may have been connected to each other during Permian time. Detrital zircon dating of all siliciclastic rocks yielded the youngest age component of ~ 170 Ma, suggesting that the latest deposition of the mica schists happened at some time after ~ 170 Ma. We propose that the Jiamusi and Songliao blocks once existed as a single block around Late Permian, which underwent a rifting event in the Permian to form a rifting basin that was subsequently evolved into an oceanic domain (Heilongjiang Ocean). The closure of the Heilongjiang Ocean occurred after ~ 170 Ma.

This article reports new geochemical data from Permian sandstones, most probably deposited during a period of postcollisional extension, from the Yamansu, Shaquanzi, and Xingxingxia areas in the Eastern Tianshan, northwestern China, to determine and characterize their provenance and source nature. The medium- to coarse-grained sandstones are composed mainly of angular to subangular quartz and volcanic fragments, with minor plagioclase and/or microcline, suggesting proximal deposition. Although the samples show relatively large variations in SiO2 (59.7–70.8 wt%), Al2O3 (9.0–14.9 wt%), and Fe2O3T + MgO (2.3–5.2 wt%) contents, they yield low chemical index of alteration values (41–55) but high index of compositional variability values (1.06–1.78), suggestive of their derivation mainly from compositionally immature sources with weak chemical weathering. Their relatively low Zr concentrations (99–225 ppm) and GdN/YbN ratios (1.20–1.53) manifest insignificant zircon and monazite enrichments. Most of the...

Connecting the North China Craton to the east and the Tarim Craton to the west, the Alxa Terrane is a key place in investigating the timing of the final closure of the Paleo-Asian Ocean (PAO). New LA-ICPMS zircon U–Pb dating results reveal ca. 300–268 Ma gabbros and diorites in the Bayan Nuru area in the eastern part of the Alxa Terrane. The ~ 300 Ma gabbros show plagioclase accumulations with anorthite compositions, arc-like geochemical affinities with relative enrichment in large ionic lithophile elements and depletion in high field strength elements (e.g., Ti, Nb and Ta), as well as negative e Hf (t) and e Nd (t) values and high initial 87 Sr/ 86 Sr ratios. These features indicate a magma source of an enriched lithospheric mantle metasomatized by high fluid activities. In comparison, the 280–268 Ma gabbros and diorites also have arc-like geochemical affinities but show increasingly evolved isotope compositions, implying more sediment inputs. Compiled zircon e Hf (t) and whole-rock e Nd (t) values of the magmatic rocks in the Alxa Terrane decrease from the Late Carboniferous to the Early Permian, and increase from the Middle Permian to the Triassic. The significantly large variation in zircon e Hf (t) and whole-rock e Nd (t) values at ca. 280–265 Ma likely reflects a tectonic switch from a subduction setting to a post-collisional setting, corresponding to the timing of the final closure of the PAO in the Alxa Terrane. Thus, the PAO progressively closed from west to east along the northern margin of the Tarim Craton, the Alxa Terrane, and then the northern margin of the North China Craton during Late Carboniferous to Middle Triassic time.

The Heilongjiang complex, extending along a suture zone between the Jiamusi and Songliao blocks in Northeast China, is composed mainly of blueschists, greenschists, meta-ultramafic rocks, quartzites, muscovite–albite schists and two-mica schists. Controversy has long surrounded the ages and tectonic settings of mafic rocks from the complex, which are crucial part of the complex. The lithological associations and their major and trace element compositions indicate that the mafic protoliths of the Yilan greenschists can be subdivided into alkali and tholeiitic basalts, which were derived from partial melting of a garnet-bearing and spinel-bearing mixed source, whereas the protoliths of the amphibolites are tholeiitic and were generated from the partial melting of spinel peridotite. Magmatic zircons from a tholeiitic amphibolite sample yielded a 206Pb/238U age of 256 ± 2 Ma, interpreted as its protolithic age. The sample also contains small amounts of older inherited zircons up to 344 Ma, which, together with its origin from shallow lithospheric mantle, indicate that the tholeiitic rocks were generated in a continental rift. The geochemical data suggest that further rifting led to the formation of an ocean between the Jiamusi and Songliao blocks, in which some oceanic islands developed, represented by the alkali basaltic protoliths of the Yilan greenschists. Magmatic zircons from an alkaline greenschist sample yielded a 206Pb/238U age of 162 ± 3.9 Ma, which, together with protolithic age of 141.8 ± 1 Ma previously obtained for the Yilian blueschist, support the model that the ocean between the Jiamusi and Songliao blocks closed at some time after ~ 141 Ma, not earlier at 210–180 Ma as previously considered.

The Central Asian Orogenic Belt (CAOB), as one of the largest accretionary orogens in the world, was built up through protracted accretion and collision of a variety of terranes due to the subduction and closure of the Paleo-Asian Ocean in the Neoproterozoic to Early Mesozoic. Located in the Uliastai continental margin of the southeastern CAOB, the Chagan Obo Temple area is essential for understanding the tectonic evolution of the southeastern part of the CAOB and its relation with the “Hegenshan Ocean”. In this study, detrital zircon U-Pb geochronology coupled with Hf isotopic analysis was performed on Paleozoic sedimentary strata in this area. Most detrital zircons from the studied samples possess oscillatory zoning and have Th/U ratios of 0.4-1.73, indicative of an igneous origin. Detrital zircons from the Ordovician to Devonian sedimentary strata yield a predominant age group at 511-490 Ma and subordinate age groups at 982-891 Ma, 834-790 Ma and ~ 574 Ma, and have a large spread of eHf(t) values (-20.77 to + 16.94). Carboniferous and Early Permian samples yield zircon U-Pb ages peaking at ~ 410 Ma and ~ 336 Ma, and have dominantly positive eHf(t) values (+ 1.30 to + 14.86). Such age populations and Hf isotopic signatures match those of magmatic rocks in the Northern Accretionary Orogen and the Mongolian arcs. A marked shift of provenance terranes from multiple sources to a single source and Hf isotope compositions from mixed to positive values occurred at some time in the Carboniferous. Such a shift implies that the Northern Accretionary Orogen was no longer a contributor of detritus in the Carboniferous to Early Permian, due to the opening of the “Hegenshan Ocean” possibly induced by the slab rollback of the subducting Paleo-Asian Ocean.

The maturity of geochronological techniques in recent years, e.g. (semi-)automated fission-track analysis [1] and laser-ablation systems [2], made large data sets more effectively available in shorter analyses times at an acceptable cost of precision and accuracy. Complex geological systems harbouring multiple age populations, typical in sedimentary provenance studies, instinctively require statistically robust large numbers of single geochronological analyses per sample in order to approximate the ideal age distribution.

The sedimentary provenance analysis of Palaeozoic arc basins across the Solonker Suture Zone can provide important insights into the role of major tectonic units as sedimentary provenance terranes during the final closure of the Palaeo–Asian Ocean. Their relative contributions can be identified and defined by their characteristic age spectrum [1, 2]. Their palaeo-geographic locations, as an indirect measure for the direction of regional sediment transport toward the arc basins, may also serve as an indicator for subduction polarities in convergent tectonic settings.

The study area is situated in the southeastern segment of the Central Asian Orogenic Belt (CAOB) across the more than 500 km wide accretionary collision zone between the Mongolian Arcs to the north and the North China Craton to the south (Figs. 1.3 and 2.1). Its central position provides an ideal opportunity to ascertain the exact location and timing of the formation of the cryptic Solonker Suture Zone, and evaluate existing suggestions [23].

The Alxa Terrane is situated in a key area between the North China and Tarim cratons. Paleozoic magmatic records in this terrane place important constraints on the subduction processes of the southern Paleo-Asian Ocean. New data of zircon U-Pb ages and whole-rock elemental and isotopic data reveal two groups of intermediate to felsic plutons in the Alxa Terrane. One group consists of diorites and granitoids that were emplaced at ca. 460–440 Ma and characterized by lower Al 2 O 3 /TiO 2 ratios and higher TiO 2 contents, implying high temperature–low pressure crystallization conditions and a shallow source region. The second group is dominated by granitoids aged at ca. 420–407 Ma and displays high Sr and Ba, low Y and high rare earth elements, with very high Sr/Y ratios and mostly positive Eu anomalies. These characteristics imply low temperature–high pressure crystallization conditions and source regions at deep crustal levels where garnet is stable in the residual phase. Both of the two groups are mostly calc-alkaline to high-K calc-alkaline, depleted in Nb, Ta and Ti and enriched in Ba, K and Sr, indicative of an arc affinity most likely related to the southward subduction of the Paleo-Asian Ocean. Zircon e Hf (t) and whole-rock e Nd (t) values of these magmatic rocks decrease from 458 Ma to 440 Ma and increase from 417 Ma to 407 Ma, whereas whole-rock initial 87 Sr/ 86 Sr ratios display an opposite trend. Such an isotopic change suggests a tectonic switch from an advancing to a retreating subduction regime at ~ 407 Ma. Synthesized data from this and previous studies suggest that the 460–400 Ma magmatic arc in the Alxa Terrane represented the western extension of the Paleozoic arc belt on the northern margin of the North China Craton.

The closure of the Junggar Ocean between the Central Tianshan and Junggar terranes is essential in understanding the final assembly of the southern Central Asian Orogenic Belt. This study presents new whole-rock geochemical, Sr–Nd and zircon U–Pb–Hf isotopic data for mafic–intermediate intrusions from the Central Tianshan block to provide robust constraints on the final closure of the ocean. LA-ICP-MS U–Pb dating on magmatic-type zircons yields weighted mean 206Pb/238U ages of ca. 310 Ma and ca. 290 Ma, which are interpreted as the crystallization ages of the intrusions. Petrographic and geochemical analyses of the Late Carboniferous mafic–intermediate rocks, characterized by typical subduction-related signatures, low Sm/Yb (< 2.0) but high Lu/Hf (≥ 0.2) ratios and positive eNd(t) (+ 1.2 to + 3.1) and zircon eHf(t) (+ 4.1 to + 7.8) values, suggest that their parental magmas were most likely generated by the partial melting of a metasomatized lithospheric mantle wedge in the spinel stability field and emplaced in a continental arc setting. This consideration is consistent with the occurrence of Carboniferous ophiolitic and arc-related granitoids in the region, probably as a result of the southward subduction of the Junggar oceanic plate. In contrast, the Early Permian mafic magmatism exhibits typical within-plate basalt affinities, such as high TiO2 (2.7–3.2 wt.%) contents, elevated Ti/V (86.0–115.1) and Zr/Y (4.9–9.3) ratios, OIB-like trace element patterns and high eNd(t) (+ 1.1 to + 4.5) and zircon eHf(t) (+ 3.0 to + 9.8) values. In association with previous investigations, we suggest that their protoliths were most probably derived from the partial melting of an asthenospheric mantle source in the garnet stability field, plausibly induced by asthenosphere upwelling during the slab break-off of the Junggar oceanic plate, which agrees well with the linear distributions of Permian mafic–ultramafic rocks in the Eastern Tianshan. Collectively, our data pinpoint a tectonic transition from oceanic subduction to post-collisional extension during Late Carboniferous to Early Permian time, probably triggered by the closure of the Junggar Ocean and subsequent arc–continent collision between the Central Tianshan and Junggar terranes that gave rise to the final assembly of the Eastern Tianshan.

U–Pb and Lu–Hf isotopic data of detrital zircons from late Paleozoic and Mesozoic strata along the southern flank of the Chinese western Tianshan enable to identify provenance changes and reconstruct early stage uplift and denudation history of the Tianshan range. Detrital zircons from Permian and Early–Middle Triassic siliciclastic rocks show two prominent age populations at 500–390 Ma and 310–260 Ma, and subordinate Precambrian ages at ~ 2.5 Ga, 2.0–1.7 Ga, 1.2–0.9 Ga and 900–600 Ma, with rare ages between 390 and 310 Ma. These characteristics and zircon eHf(t) data consistently suggest a sediment source predominantly from the Tarim Craton, rather than the Central Tianshan–Yili Block. In contrast, Late Triassic to Cretaceous strata additionally contain abundant 390–310 Ma and 260–220 Ma detrital zircons, implying multiple source regions from the Central Tianshan–Yili Block, Tarim Craton, and Western Kunlun Orogen. A significant switch of sedimentary provenances occurred in the mid-Triassic and is consistent with contemporaneous change of paleocurrent directions and the onset of intense tectonothermal events in the broad region of the Chinese western Tianshan and Kyrgyz Tianshan. These data collectively indicate that the significant surface uplift and denudation of the Tianshan range were probably initiated in the mid-Triassic (~ 240 Ma) after the assembly of the southwestern Central Asian Orogenic Belt. This uplifting event represents an intracontinental orogeny most likely in response to the collision between the Qiangtang Block and southern Eurasia, following the closure of the western part of the Paleo-Tethys Ocean.

Late Ordovician (ca. 451 Ma) calc-alkaline adakitic granodiorite and granite, associated with coeval S-type granites, were discovered in the Central Tianshan block, NW China. They are characterized by high Sr (526−620 ppm), but low Y (6.8−8.6 ppm) and heavy rare earth element (HREE) contents (e.g., Yb = 0.7−0.8 ppm), and thus have high Sr/Y (66−86) ratios. The granodiorite has SiO 2 of 69.3 wt%, MgO of 1.4 wt%, and Mg# of 48, exhibiting light (L) REE−enriched patterns ([La/Yb] N = 15.82, [Dy/Yb] N = 1.25) with strong positive Eu (δEu = 1.65) but weak negative Nb, Ta, and Ti anomalies. Its low Cr (9.0 ppm) and Ni (6.2 ppm) concentrations, low negative e Hf ( t ) and e Nd ( t ) values (−11.3 to −5.0 and −4.8, respectively), old one-stage Nd model age (ca. 1.4 Ga), and high initial 87 Sr/ 86 Sr ratio (0.7129) are characteristics of derivation from the partial melting of thickened ancient lower crust, most likely in the eclogite facies. The adakitic granite samples contain slightly higher SiO 2 (72.4−73.0 wt%), but lower MgO (0.6−0.7 wt%) and Mg# (37−40), with low Cr (7.9−12.1 ppm) and Ni (5.6−7.4 ppm), displaying concave-upward REE patterns ([La/Yb] N = 19.25−30.01, [Dy/Yb] N = 0.85−1.01) with negligible Eu (δEu = 0.87−1.03) and pronounced negative Nb, Ta, and Ti anomalies. Their positive e Hf ( t ) (+4.3 to +7.1) and e Nd ( t ) (+1.6 to +3.2) values, young one-stage Nd model ages (0.9−0.8 Ga), and low initial 87 Sr/ 86 Sr ratios (0.7055−0.7060) indicate generation by the partial melting of relatively juvenile lower crust, probably in the garnet-bearing amphibolite facies. Combined with previous investigations, we suggest that the Central Tianshan developed as a continental arc along northern Tarim, facing the southward subduction of the Junggar Ocean during Ordovician time. Lower-crustal melting was likely induced by asthenosphere upwelling in response to rollback of the Junggar oceanic slab, accompanied by crustal extension and incipient opening of the South Tianshan back-arc basin along northern Tarim and separation of the Central Tianshan as a microcontinent. The heat brought by the ascending adakitic magmas possibly caused anatexis of the basement rocks, forming the coeval pelite-derived S-type granites. The eastern segment of the South Tianshan Ocean was most probably closed prior to ca. 380 Ma. This study provides ample evidence that adakitic rocks can be produced by partial melting of (thickened) lower crust during incipient opening of a back-arc basin.

The provenance of late Paleozoic siliciclastic sedimentary strata on the northern margin of the Tarim Craton and the South Tianshan Orogenic Belt provides important insights into subduction and collision processes during the formation of the southern Central Asian Orogenic Belt. Detrital zircons from Carboniferous and Permian sedimentary rocks in the South Tianshan belt show two predominant age populations of 500–400 and 305–270 Ma, and three subordinate clusters around ~ 2.5 Ga, 2.0–1.7 Ga, and 1.2–0.6 Ma. Such age patterns are similar to major magmatic episodes in the Tarim Craton but are distinct from those in the Central Tianshan–Yili Block, implying that the Carboniferous–Permian strata in the South Tianshan belt were deposited on the northern margin of the Tarim Craton. These data, in combination with Carboniferous passive margin deposition along the South Tianshan and northern Tarim regions and intense arc magmatism in the Central Tianshan area, support the northward subduction of the South Tianshan oceanic crust. The abrupt decrease of zircon e Hf ( t ) values at ~ 310 Ma indicates the tectonic transformation from oceanic subduction to continental collision. However, syn- and post-collisional sediments in the South Tianshan and northern Tarim regions did not contain detritus from the Central Tianshan–Yili Block, as evidenced by the absence/paucity of 380–310 Ma detrital zircons in the late Paleozoic strata and by the stratigraphic record in the regions. Insignificant surface erosion and uplifting during the collision and (ultra-)high-pressure rock exhumation require divergence in the frontal wedge. We propose that the switch from a convergent to a divergent regime was triggered by the arrival of the Tarim mantle plume in the latest Carboniferous, which possibly had profound effects on regional sedimentation and exhumation of (ultra-)high-pressure rocks in the orogenic belt.

Once situated between the Central Tianshan and Junggar terranes during Paleozoic time, the Junggar Ocean was a major southern branch of the Paleo-Asian Ocean. Thus, when and how it was closed are essential in understanding the final assembly of the Central Asian Orogenic Belt. However, the exact closure time of the Junggar Ocean remains unresolved due to the lack of reliable timing of collision-related regional metamorphism. This paper reports whole-rock geochemical and zircon U–Pb and Lu–Hf isotopic data for granitic gneisses from the northern margin of the Central Tianshan block, which can provide crucial constraints on the final closure of the Junggar Ocean. Mineral assemblages and geochemical suggest that the protoliths of the Central Tianshan gneisses are weakly peraluminous high-K calc-alkaline I-type granites, possessing typical subduction-related features such as strong enrichment in LREE and LILE and depletion in HFSE. Negative Eu anomalies (δEu = 0.46–0.81) and highly variable zircon Hf isotope compositions indicate various amounts of residual plagioclase in the source and crustal contamination during magma formation. LA-ICPMS U–Pb dating on magmatic-type zircons (72%), characterized by euhedral to subhedral shapes, concentric oscillatory zoning, high Th/U ratios (0.30–2.05) and large ranges of e Hf (t) values (− 3.4 to + 8.7; up to 6 epsilon units in each sample), yields consistent weighted mean 206 Pb/ 238 U ages of ca. 325–320 Ma, interpreted as the crystallization ages of the granitic protoliths. Geochemical signatures and dominantly positive zircon e Hf (t) values reveal that the protoliths were emplaced in a continental arc setting, pinpointing the development of a late Early to early Late Carboniferous continental arc system on the northern margin of the Central Tianshan block, probably related to the southward subduction of the Junggar oceanic plate. Meanwhile, younger ages at ca. 303–301 Ma were obtained on recrystallized zircon-rims and unzoned/homogeneous zircons (25%) with narrow ranges of e Hf (t) values (− 1.8 to + 7.2

The Junggar ocean, once situated north (present coordinate) of the Yili and Central Tianshan blocks during early Paleozoic to late Carboniferous time, was a major southern branch of the Paleo–Asian Ocean, the opening, expansion, and final closure of which led to the development of the Central Asian Orogenic Belt between Eastern Europe–Siberia and Tarim–North China. However, the detailed evolution of the Junggar ocean has not been well constrained. This paper reports U-Pb and Lu-Hf isotopic data of detrital zircons from sandstones in the North Tianshan belt, which can provide new insights into understanding the Paleozoic development of the Junggar ocean. Most detrital zircons exhibit oscillatory zoning and high Th/U ratios, typical of igneous origin. The predominant Paleozoic zircons yield major age populations at ca. 294, 313–327, 338–375, 440–455, and 474–502 Ma and are interpreted to have been derived from the long-lived volcanic- and island-arc systems formed by the southward subduction of the Junggar ocean and subsequent collisional and postcollisional magmatism. The minor Precambrian zircons yield ages scattering at ca. 550, 680–765, 890, 970–990, 1160–1250, 1500, 1690–1750, 1840–1970, 2440–2500, and 2615–2700 Ma, which are nearly, but not fully, congruent with those from the adjacent Beishan and Kuluketage Precambrian terranes. Therefore, our results indicate that the Central Tianshan block was once part of the Tarim block during Precambrian time. Most of the 541–440 Ma zircons possess low negative eHf(t) values, while the

As a major Precambrian microcontinent in the southernmost Central Asian Orogenic Belt (CAOB), the Central Tianshan block (CTS) in the Chinese Tianshan is essential for understanding the final assembly of the southern CAOB. It experienced multistage Paleozoic magmatism and metamorphism, but the detailed processes are still controversial and far from being completely understood. This paper reports coupled U–Pb and Lu–Hf isotopic data of detrital zircons from late Paleozoic (meta-)sedimentary strata in the South Tianshan belt, which can provide new insight into deciphering the Paleozoic evolution of the eastern segment of the CTS block. Characterized by typical oscillatory zoning and high Th/U ratios (> 0.2), detrital zircons in the Permian sedimentary samples yield dominant age populations at ca. 505–490 Ma, 475–440 Ma, 430–400 Ma and 340–250 Ma, pinpointing the development of four episodes of magmatism in the eastern CTS block. Particularly, Ordovician–Silurian (475–440 Ma) zircons, possessing low negative eHf(t) values, predominate in sedimentary strata in and surrounding the CTS block, indicating that the 475–440 Ma magmatic rocks probably constitute the main body of the CTS block. The origin of this (early Paleozoic) episode of magmatism was most likely related to the southward subduction of the Junggar Ocean beneath the CTS block. Carboniferous–Triassic (340–250 Ma) zircons have dominantly positive eHf(t) values, probably derived from the post-collisional juvenile rocks in the CTS block. Combined with previous studies, our data suggest that the single source terrane for the sampled strata was the CTS block, which had been a topographic high providing substantial detritus to the surrounding areas at least since the Early Permian. In the metasedimentary sample, detrital zircons mostly show partially/fully recrystallized internal textures and low Th/U ratios (

The Beishan Orogenic Belt (BSOB) in China plays a pivotal role in understanding the tectonic evolution of the southern Central Asian Orogenic Belt. Despite numerous studies primarily emphasizing on Phanerozoic events, the existence and significance of Precambrian rocks in the BSOB still remain controversial. Here we report new geochemical and zircon U–Pb–Hf isotopic data for the Gubaoquan ortho- and paragneisses in the BSOB and the comparable Xingxingxia orthogneisses in the Central Tianshan Orogenic Belt to constrain their age, petrogenesis and tectonic implications. New LA-ICPMS U–Pb zircon dating results of zircon reveal that the protoliths of the Gubaoquan orthogneisses crystallized at 905–871 Ma, while the Gubaoquan paragneisses yielded an age peak at 1040–910 Ma with a maximum depositional age of ∼871 Ma. They are the oldest rocks discovered in the BSOB, mainly distributed in the southern units and interpreted as primary sources for the local Phanerozoic igneous and sedimentary rocks. The intrusion age of the Xingxingxia orthogneisses was ∼1014 Ma. All the Gubaoquan and Xingxingxia orthogneisses show low A/CNK, Rb/Sr and Rb/Ba ratios and similar depletion of Nb, Ta and Ti, akin to I-type granitoids related to subduction. However, the former have much lower Sr contents and Sr/Y ratios, lower light to heavy rare earth element fractionation (LREE/HREE = 4.43–10.12), and weaker negative Eu anomalies, related to a back-arc setting at low pressures, whereas the latter formed at high pressures in a continental arc setting. Zircon Hf-isotope compositions in the Gubaoquan ortho- and paragneisses exhibit a large range of ɛ Hf ( t ) values from −3.77 to +5.29, and two-stage model ages ( T C DM ) of 1.98–1.44 Ga, while those in the Xingxingxia orthogneisses are clustered with slightly higher ɛ Hf ( t ) values (+4.02 to +8.53) and younger T C DM ages (1.60–1. 34 Ga). The protoliths of both the Gubaoquan and Xingxingxia orthogneisses were derived from the mixing of basaltic magmas and those derived from the partial melting of meta-greywackes. Based on a review of Neoproterozoic events, the BSOB tends to closely link with the Chinese Tianshan Orogenic Belt and Tarim Craton. They were most likely located in the periphery of the already assembled Rodinia supercontinent.

The Solonker Suture Zone is commonly recognised as the location of the Late Permian to Early Triassic closure of the Palaeo-Asian Ocean in the southeastern segment of the Central Asian Orogenic Belt. However, the absence of typical suture-related features, as a consequence of uncommon collisional geometries, gave it a cryptic nature. Thus, the tectonic setting, which led to suturing, still remains enigmatic. A geochemical characterisation of Permian sedimentary and volcanic rocks across the suture was carried out. Supplemented with Hf and Nd isotopic analyses, this approach enables not only a better definition of such regional suture, but also estimates on the long-controversial issue of net crustal growth in accretionary tectonic environments. The results indicate short sedimentary transport distances between the arc basins and their provenances, of which the studied volcanic rocks were a major contributor. Similar enrichment and depletion patterns with respect to N-MORB and average continental crust further corroborate a close source–sediment relationship. Immobile element provenance analyses indicate that the active continental northern margin of the North China Craton was a major source for arc basins to the south of the Solonker Suture Zone. To its north, arc basins are interpreted to be sourced by a more complex mixture of provenances, e.g., the Baolidao volcanic arc suite and the heterogenous Precambrian basement of southern Mongolia. An overall collisional tectonic setting across the suture is recognised. The geochemical signature of sedimentary rocks to the south of the suture points at an active continental arc setting, whereas the bimodal geochemical distribution of the samples to the north shows a contemporaneous active oceanic island arc as well as a passive margin environment. These features favour a double-sided subduction of the Palaeo-Asian Ocean beneath the North China Craton and the Mongolian Arcs throughout the Palaeozoic, including back-arc basin opening north of the suture in the Permian. Analysis of Hf and Nd isotopic compositions revealed that magmas to the south were produced involving significant crustal contamination, thus having less radiogenic compositions. North of the suture, however, isotopic compositions tend to be more radiogenic, implying a more juvenile contribution. However, the bulk isotopic compositions are close to present-day CHUR, suggesting that crustal reworking appears to be equally balanced by juvenile addition during the Palaeozoic closure of the Palaeo-Asian Ocean across the Solonker Suture Zone.

The Central Asian orogenic belt formed by accretion subsequent to the contraction of the paleo–Asian Ocean, and its southeastern segment terminated along the Solonker suture zone, amalgamating the Mongolian arcs and the North China craton by the end of the Early Triassic. Since typical regional collisional features are absent, its tectonic evolution remains speculative. An investigation into the variability of detrital zircon U-Pb ages and Hf ratios in Permian strata along a southeast-northwest transect from the Mongolian arcs to the North China craton reveals distinct differences: Northern basins carry a broad Mesoproterozoic to latest Precambrian age signature, and their sedimentary source terranes are of mixed juvenile to crustal magmatic origin. In contrast, southern basins contain detritus from the North China craton, and their sources are of dominantly crustal contaminated magmatic origin. Provenance analysis suggests that in the early Paleozoic (ca. 429 Ma), the paleo–Asian Ocean was consumed along the Uliastai arc and the North China craton, initiating the formation of the northern and southern accretionary orogens, respectively. By the end of the middle Carboniferous, the Mongolian arcs were consolidated after the accretion of the Uliastai arc. In the late Carboniferous (ca. 314 Ma), the Hegenshan back-arc basin opened, detaching the northern accretionary orogen. While subduction continued there, it may have temporarily ceased in the south after the accretion of a microcontinent (ca. 300 Ma). By the end of the Middle Permian (ca. 269 Ma), back-arc basin closure led to the formation and obduction of the suprasubduction zone Hegenshan ophiolite. Eventually, the paleo–Asian Ocean closed by double-sided subduction. Such accretionary wedge-wedge collision would not involve continental deep subduction, unlike other continental collisions. The early stages of the sequence likely resembled a Pacific-type scenario dominated by the subduction of oceanic lithosphere, including a Japanese Sea–type back-arc basin opening. The late stages involved the accretion of large-scale continental blocks, ultimately leading to final ocean closure, which may be similar to the archipelago-type setting of present-day Southeast Asia.

The geological attraction of the Heilongjiang Complex, extending along a suture zone between the Jiamusi and Songliao blocks in Northeast China, resides in excellent exposures of blueschists, which occur in association with granitic gneisses, marbles, meta-ultramafic rocks, greenschists, quartzites, muscovite–albite schists and two-mica schists from the complex. However, controversy has long surrounded the ages and tectonic settings of mafic protoliths of the blueschists from the complex. The lithological association and major and trace element compositions indicate that the ultramafic–mafic protoliths of the Yilan blueschists can be subdivided into the tholeiitic and alkali groups, both of which were derived from partial melting of garnet-facies peridotites, but at different degrees of melting. Magmatic zircons from a tholeiitic blueschist sample yield a 206 Pb/ 238 U age of 275 ± 2 Ma, interpreted as its protolithic age. The sample also contains large amounts of older inherited zircons up to 1200 Ma, which, together with the geochemical features of the sample, indicate that the tholeiitic basalts were generated in a continental rift. The further development of the rift led to the formation of an ocean between the Jiamusi and Songliao blocks, in which some ocean islands developed, represented by the alkali mafic protoliths of the Yilan blueschists, as supported by geochemical data. Magmatic zircons from an alkaline mafic blueschist sample yield a 206 Pb/ 238 U age of 141.8 ± 1 Ma, indicating that the ocean between the Jiamusi and Songliao blocks had not been closed by ~ 141 Ma, not at 210–180 Ma as previously considered.

A detrital zircon U-Pb and Lu-Hf isotopic study was carried out in the Middle Silurian to Late Carboniferous sedimentary strata of the northwestern Tarim Craton in order to understand accretionary processes in the southern part of the Central Asian Orogenic Belt. Detrital zircons from these strata yielded U-Pb ages clustering around 2.8–2.3 Ga, 2.0–1.7 Ga, 1.3–0.9 Ga, 880–600 Ma, and 500–400 Ma, with age populations and Hf isotopic signatures matching those of magmatic rocks in the Tarim Craton and the Central Tianshan Block. Abundant 500–400 Ma detrital zircons most likely reflect deposition in a retroarc foreland basin inboard of an Andean-type magmatic arc to the north, supporting the northern Tarim-Central Tianshan connection during early Paleozoic time. The absence of 380–310 Ma zircon population in the Carboniferous siliciclastic rocks suggests that the Central Tianshan Block may have been separated from the Tarim Craton in the Early Devonian, caused by the interarc/back-arc opening of the South Tianshan Ocean. We propose an accretionary orogenic model switching from advancing to retreating mode during Paleozoic time in the southwestern part of the Paleo-Asian Ocean. This transition most likely occurred coevally with the rifting of Southeast Asian blocks from the northeastern margin of Gondwana.

In this study, reconstructions of a balanced geologic cross section in the Himalayan fold–thrust belt of eastern Bhutan are used in flexural–kinematic and thermokinematic models to understand the sensitivity of predicted cooling ages to changes in fault kinematics, geometry, topography, and radiogenic heat production. The kinematics for each scenario are created by sequentially deforming the cross section with ∼ 10 km deformation steps while applying flexural loading and erosional unloading at each step to develop a high-resolution evolution of deformation, erosion, and burial over time. By assigning ages to each increment of displacement, we create a suite of modeled scenarios that are input into a 2-D thermokinematic model to predict cooling ages. Comparison of model-predicted cooling ages to published thermochronometer data reveals that cooling ages are most sensitive to (1) the location and size of fault ramps, (2) the variable shortening rates between 68 and 6.4 mm yr−1, and (3) the timing and magnitude of out-of-sequence faulting. The predicted ages are less sensitive to (4) radiogenic heat production and (5) estimates of topographic evolution. We used the observed misfit of predicted to measured cooling ages to revise the cross section geometry and separate one large ramp previously proposed for the modern décollement into two smaller ramps. The revised geometry results in an improved fit to observed ages, particularly young AFT ages (2–6 Ma) located north of the Main Central Thrust. This study presents a successful approach for using thermochronometer data to test the viability of a proposed cross section geometry and kinematics and describes a viable approach to estimating the first-order topographic evolution of a compressional orogen.

As a pivotal junction between the North China and Tarim cratons, the Alxa Terrane provides an ideal window to constrain the final closure of the middle segment of the Paleo-Asian Ocean (PAO). This study carried out petrological, whole-rock geochemical, and zircon U-Pb-Hf isotopic investigations on four major granitic plutonic complexes in the Alxa Terrane. The Bayan Nuru and Yabulai plutonic complexes are I-type granitoids that yield crystallization ages of 281–268 Ma and 277–270 Ma, respectively, with negative zircon e Hf (t) values (-11.5 to -3.2), primarily sourced from the Neoproterozoic rocks in the region. The Nuergai granitoids yield crystallization ages of 281–268 Ma and show I-type affinities and positive zircon e Hf (t) values (+1.6 to +6.4), indicating an origin related to magma mixing. Emplaced at ca. 241 Ma, the Oliji granitoids display varying zircon e Hf (t) values from -0.9 to +11.5, which necessitate a predominant source of mantle-derived materials. A compilation of zircon e Hf (t) and whole-rock e Nd (t) values of the magmatic rocks in the Alxa Terrane illustrates a decreasing trend from the Late Carboniferous to the Early Permian and an increasing trend during Middle Permian to Triassic time. The marked shift with a large variation of zircon e Hf (t) and whole-rock e Nd (t) values at 280–265 Ma indicates a tectonic switch from subduction to post-collision tectonic regimes in the Alxa Terrane, marking the final closure of the middle segment of the PAO. Comparable isotope variations are also identified from 260 to 245 Ma magmatic counterparts on the northern margin of the North China Craton, hence suggesting a progressively eastward closure of the PAO.

Bulk-rock major and trace element, Sr-Nd isotopic, and zircon U-Pb-Hf isotopic data are reported for Jurassic igneous rocks from a north-south traverse through the Zhangguangcailing orogen, which formed by the subduction of an ocean and final collision between the Jiamusi and Songliao blocks in northeastern China. These results provide new insights into tectonic processes involving the subduction and collision to form the Zhangguangcailing orogen and final amalgamation of the Jiamusi and Songliao blocks. Our new results, together with data from the literature, indicate that the Jurassic granitoid rocks in the Zhangguangcailing orogen were emplaced in the period from ca. 191 to 163 Ma, with a magmatic flare-up ca. 180 Ma accompanied by mafic magmatism. These granitoid rocks are metaluminous and I-type in composition, and enriched in K, Rb, Th, and U, and depleted in Ba, Nb, Ta, Sr, P, and Ti, with constant initial 87 Sr/ 86 Sr (0.704338–0.705349), and positive e Nd (t) (+1.6 to +3.1) and zircon e Hf (t) (+0.83 to +10.51) values. These geochemical features indicate that the Jurassic I-type granitoid rocks were probably generated as a consequence of variable degrees of interaction between continental crust and mantle-derived melts, which resulted from the subduction of oceanic lithosphere (Heilongjiang ocean), and led to an active continental margin during Early to Middle Jurassic time.