Orogenic Lithosphere Research Articles

The Beginning of a Wilson Cycle in an Accretionary Orogen: The Mongol–Okhotsk Ocean Opened Assisted by a Devonian Mantle Plume

AbstractThe opening of oceans within accretionary orogens is important for understanding the Wilson cycle. The Mongol–Okhotsk Ocean (MOO) began opening within the early Paleozoic accretionary collage of the Central Asian Orogenic Belt (CAOB), representing a world‐class example to constrain the geodynamic history of ocean opening in accretionary orogens, but the kinematics and mechanisms associated to this process are highly debated. We report on a newly‐discovered bimodal volcanic suite and associated volcanic‐sediments that comprise part of the Altay‐Sayan Rift System, which indicate a widespread Early Devonian extensional event within the CAOB. This extension regime is attributed to a Devonian mantle plume, which is thought to have impinged upon and weakened the lithosphere of the Early Paleozoic collage, and drove the opening of the MOO. Opening of the MOO suggests continent breakup in accretionary orogens tends to focus along intervening weak orogenic lithosphere between the rigid microcontinents.

Syn- and post-collisional mafic igneous rocks from the East Kunlun Orogenic Belt: Implications for recycling of subducting continental crust during the Silurian-Devonian

Syn- to post-collisional mafic igneous rocks are widely distributed along continental collision orogenic belts and can be used to constrain the nature of the orogenic mantle and relevant chemical geodynamic processes during orogenesis. However, there is uncertainty about the mechanisms of recycling of continental crustal materials in continental subduction channels. We report chronological and geochemical data for syn- and post-collisional mafic dikes from the East Kunlun Orogenic Belt (EKOB), western China, which provide insights into the processes of geochemical transfer and geodynamic evolution of this fossil orogen. Zircon UPb dating of the mafic dikes yields ages of 433 Ma and 409–407 Ma, corresponding to syn- and post-collisional magmatism, respectively. The rocks exhibit typical arc-like trace-element characteristics and weakly enriched Sr–Nd–Hf isotopic compositions, as well as variable zircon O isotopic compositions. These geochemical characteristics are comparable to those of Neoproterozoic paragneiss influenced by Early Paleozoic metamorphism associated with high–ultrahigh-pressure eclogitization in the EKOB and indicate that paragneiss-derived hydrous melts were likely involved in the mantle source of the studied mafic igneous rocks during continental collision. This qualitative interpretation is supported by quantitative modeling of the geochemical transfer in the continental subduction zone. We propose that the syn-collisional mafic igneous rocks were formed during the continental collision between Central Kunlun and South Kunlun Belts, whereas the post-collisional mafic igneous rocks formed in an extensional regime associated with orogenic lithosphere thinning and asthenospheric upwelling.

The influence of lithospheric inheritance and Cenozoic magmatism on Phanerozoic rock cooling in the São Francisco Craton and adjacent orogens

The South American Platform encompasses the São Francisco Craton (SFC) and the Araçuaí-Ribeira orogenic system (AROS) located in the eastern Brazil. These geologic domains hold records of differential exhumation, sedimentation, volcanic activity, and post-rift reactivation of faults and shear zones. Therefore, these terranes provide outstanding opportunities for studying the dynamics of lithospheric processes, particularly in their interaction zones, characterized by the transition from cratonic to orogenic lithosphere. To investigate the processes involved in the contrasting behavior of the upper crust, we provide 23 new apatite fission track (AFT) ages and thermal histories from the northern segment of the AROS and from the eastern area of the SFC. The AFT ages range from 154.4 ± 20.1 and 37.1 ± 3.0 Ma and the mean track lengths range from 10.9 and 13.2 μm. We compiled thermal information from previous thermochronological analysis from 357 sites and constructed inverse distance weighted maps that depict the spatial distribution of AFT ages and the temperature evolution from 360 to 30 Ma. The results indicate that the SFC experienced its final exhumation within the apatite partial annealing zone (APAZ) at temperatures ranging from 110 °C to 60 °C during the late Paleozoic. In contrast, the AROS went through its last cooling within the APAZ during the Mesozoic or even in the Cenozoic. This suggests that reactivation and reheating, possibly associated with the South Atlantic rift, influenced the orogenic basement of the passive margin. We highlight that the reheating associated to the Abrolhos magmatic province in the northern segment of the AROS caused the youngest AFT ages of the study area. Our work demonstrates the importance of considering contrasting rheology when analyzing regional exhumation patterns, the structural fabric of orogens through reactivation of crustal discontinuities generating heating and exhumation, and how volcanism can disrupt the thermal record, inducing crustal reheating.

Considerable heating during clockwise decompression of a long-lived Paleoproterozoic hot orogen: Evidence from the Xuanhua Complex, North China Craton

Granulites comprise the root of collisional orogens, and decoding their evolution may provide key information on orogenic processes from collisional thickening to post-collision extensional thinning. However, the records of collisional orogeny in most cases have been obscured or even erased by later high-/ultrahigh-temperature (HT/UHT) overprinting. Here, we report new petrological, geochemical and geochronological data for granulite-facies rocks in the Xuanhua Complex of the northern Trans-North China Orogen (TNCO). We reveal the complex metamorphic features and tectonic evolution of a long-lived Paleoproterozoic hot orogen as well as their link with Phanerozoic orogens. The rocks are composed mainly of garnet + clinopyroxene + orthopyroxene ± amphibole + plagioclase + quartz + rutile + ilmenite with symplectitic intergrowths of clinopyroxene + orthopyroxene ± amphibole + plagioclase ± quartz. Pseudosection modeling and thermobarometric calculations suggest that the high-pressure (HP) mafic granulites record clockwise pressure–temperature (P-T) paths, from the prograde stage (M1) to the pressure-peak stage (M2; 13.5–15.0 kbar and 850–875 °C), followed by considerable heating during decompression to the temperature-peak stage (M3a; 8.0–9.6 kbar and 910–950 °C) and final cooling to the M3b stage. Magmatic zircon U-Pb dating reveals c. 2.1–2.0 Ga protolith ages for the granulites. Metamorphic zircon and titanite U-Pb results, and trace elements in zircons indicate that the age of c. 1.92 Ga is possibly close to the lower limit of HP granulite-facies pressure-peak, and c. 1.84–1.81 Ga ages are related to the timing of the final cooling stage. Hence, by comparison with existing geological data, the synthesized metamorphic P-T-t data of the Xuanhua granulites indicate two stages of tectonic evolution, including: (1) crustal thickening by continental collision between the Western and Eastern Blocks at c. 1.95–1.92 Ga, resulting in an early HP granulite-facies metamorphism at relatively low thermal gradients of 16–19 °C/km; and (2) the collision-thickened orogenic lithosphere was thinned controlled by continental extension and gravitational instability, which might provide additional heat in the interval c. 1.90–1.84 Ga, resulting in a later UHT overprinting at reduced pressures and higher gradients of 30–36 °C/km. In addition, the post-collisional extension accompanying exhumation decompression shows a long-lived timescale of > 80 Myr, corresponding roughly to a slow exhumation rate of < 0.3 mm/yr. To sum up, these features of long-lived tectono-thermal evolution (tectonic transition from crustal thickening to lithospheric thinning, and prolonged orogeny and exhumation), slow exhumation rates, and the occurrence of UHT overprinting, are becoming more commonly identified in Precambrian hot orogens like the TNCO.

Thermodynamics of continental deformation

Continental deformation is known to be controlled by the interplay between tectonic and gravitational forces modulated by thermal relaxation-controlled lithospheric strength leading to oscillations around an equilibrium state, or to runaway extension. Using data-driven thermomechanical modelling of the Alpine Himalayan Collision Zone, we demonstrate how deviations from an equilibrium between mantle dynamics, plate-boundary forces, and the thermochemical configuration of the lithosphere control continental deformation. We quantify such balance between the internal energy of the plate and tectonic forces in terms of a critical crustal thickness, that match the global average of present-day continental crust. It follows that thicker intraplate domains than the critical crust (orogens) must undergo weakening due to their increased internal energy, and, in doing so, they dissipate the acquired energy within a diffused zone of deformation, unlike the localized deformation seen along plate boundaries. This evolution is controlled by a dissipative thermodynamic feedback loop between thermal and mechanical relaxation of the driving energy in the orogenic lithosphere. Exponentially growing energy states, leading to runaway extension are efficiently dampened by enhanced dissipation from radioactive heat sources. This ultimately drives orogens with their thickened radiogenic crust towards a final equilibrium state. Our results suggest a genetic link between the thermochemical state of the crust and the tectonic evolution of silicate Earth-like planets.

Late Paleozoic sedimentary and tectonic evolution of the Linxi basin in the eastern Inner Mongolia, eastern Central Asian Orogenic Belt

The eastern Central Asian Orogenic Belt (CAOB) encompasses a vast region from northeast China to Inner Mongolia, where there were many Late Paleozoic sedimentary basins that can provide key constraints to tectonic evolution of the eastern CAOB. Analysis of sedimentary sequence and environment of eight measured sections from the Linxi basin, Inner Mongolia indicates that there were shallow marine clastic-carbonate sedimentary facies, delta and littoral facies and terrestrial facies during the Early, Middle and Late Permian, respectively, and no oceanic sedimentary facies was found. Sedimentary sequence from the initial clastic-carbonate rocks to marine volcanic rocks reveals a transgression process during 300–270 Ma, while clastic and carbonate rocks with conglomerates and plant fossils represent a regression process during 270–250 Ma. According to the detrital zircon (DZ) age distribution of twenty-two samples (1610 zircons), the northern and southern provenances of the Permian sedimentary strata have been recognized, respectively. These DZ samples express the dispersed polymodal age populations and are attributed to a divergent dominated tectonic setting, implying the Linxi basin is an intracontinental extension basin during the Permian. This is consistent with an extensional process revealed by the syn-depositional deformed structures, coarse clastic sediments and olistostrome successions in the Linxi basin during the Early to Middle Permian and also similar to the speculation that there was an extension setting based on studies of volcanic rocks and intrusions in the Linxi basin. Combined with previous studies on the West Ujimqin basin, the Late Carboniferous to Permian tectonic frame of the eastern Inner Mongolia expresses extensional tectonics characterized by alternating combination of basins and uplifts superimposed on a pre-existing Early-Middle Paleozoic orogenic lithosphere and Precambrian basement, rather than a single ocean basin of Paleo Asian Ocean (PAO), which provides constraints for understanding sedimentary and tectonic evolution of the eastern CAOB during the Late Paleozoic.

Mid-Ordovician stratigraphy and volcanism in the Hølonda area, Scandinavian Caledonides: complex tectonomagmatic development following arc–continent collision near the Laurentian margin of Iapetus

The Hølonda area of the central Scandinavian Caledonides is a key for models of the Caledonian orogen owing to its Ordovician fauna of Laurentian affinity, now stranded on the Baltic side during opening of the North Atlantic. Here, we present a revised stratigraphic and tectonomagmatic model based on remapping, sedimentology, igneous geochemistry, Nd and Sr isotopes and geochronology. The Hølonda Group ( c. 470–461 Ma) reflects a transition from subaerial and shallow-marine deposition on a continental shelf to deeper-water sedimentation along a subsiding slope. Adakitic and mid-ocean ridge basalt type magmatism at c. 468 Ma was succeeded by benmoreitic–rhyolitic, shoshonitic, calc-alkaline and ultra-alkaline volcanism at c. 467–465 Ma. The complex magmatism followed arc–continent collision along a microcontinent outboard of Laurentia, associated with subduction polarity flip and slab rollback. This led to rifting and opening of a wide basin and its adjoining shelf on thickened orogenic lithosphere. Associated mantle upwelling and partial melting of depleted and variably metasomatized mantle occurred in a tectonomagmatic setting comparable with that of the central Mediterranean. The Hølonda–Ilfjellet setting is unique along the Caledonian–Appalachian orogen, possibly reflecting interaction with Laurentia-derived continental terranes at the northeastern end of the Taconian–Grampian orogenic tract. Supplementary material: Analytical data and supplementary figures are available at https://doi.org/10.6084/m9.figshare.c.6368922 Thematic collection: This article is part of the Caledonian Wilson cycle collection available at: https://www.lyellcollection.org/topic/collections/the-caledonian-wilson-cycle

Metamorphic patterns and zircon U–Pb dating of the Xilingol complex in Inner Mongolia, China: Implications for rifting metamorphism and tectonic evolution in eastern Central Asian Orogenic Belt

The Xilingol complex is one of the maximal outcropped metamorphic terranes in the eastern segment of the Central Asian Orogenic Belt. Detailed petrological and geochronological study of the Xilingol complex will provide valuable insights into the orogenic process and closure of the Paleo-Asian Ocean. We present a comprehensive study of petrology, pseudosection modelling, and zircon U-Pb geochronology of representative rocks collected in Daqing Pasture, including two-mica quartz schist, epidote amphibolite, biotite plagioclase gneiss and quartzite. The Xilingol complex in Daqing Pasture can be preliminarily divided into the north and south segments based on field and petrological observations. The north segment comprises two-mica quartz schist, biotite quartz schist, amphibolite and minor epidote amphibolite, whereas the south segment consists mainly of biotite gneiss, amphibolite and quartzite, with varying degrees of migmatization developed. Combined pseudosection modelling and composition isopleths of garnet and biotite constrain a peak P–T condition of ~660 °C, ~5.5 kbar for the garnet-bearing two-mica quartz schist in the north segment, corresponding to low amphibolite facies metamorphism. The garnet-bearing biotite plagioclase gneisses in the south segment are interpreted to experience high amphibolite facies metamorphism at a peak condition of ~750–790 °C, ~5.5–8.0 kbar, followed by nearly isobaric cooling. The variations of peak P–T estimates within Daqing Pasture, coupled with the spatial heterogeneities of lithological associations between different outcropped terranes, reveal an overall northward decreasing metamorphic grade for the Xilingol complex. Metamorphic zircons from three biotite plagioclase gneisses in the south segment yield apparent 206 Pb/ 238 U age spreads of 397–348 Ma, 363–294 Ma and 341–301 Ma, respectively. These partially overlapping zircon ages are an indication of discrete metamorphic events involving localized thermal metamorphism at depth in the Middle Devonian and long-lasting Buchan-type metamorphism in the Carboniferous, with intermediate data probably related to a mixture of multiple growth domains. Five metamorphic zircons with low Th/U ratios from the epidote amphibolite in the north segment yield a concordia age of 284.8 ± 1.9 Ma, reflecting the diachronism of the Late Paleozoic Buchan-type metamorphism. Summarizing all U-Pb data of inherited magmatic zircons and metamorphic zircons, a distinct U-Pb age continuum is retrieved to elucidate a successive process from sedimentary diagenesis of forearc sedimentary formations to deep burial in the Early Devonian, and then to subsequent multi-stage metamorphic overprinting from the Middle Devonian to the Permian. The isograd patterns, P–T–t paths (clockwise paths and long-term duration) and spatial distribution of metamorphic terranes of the Xilingol complex are more compatible with an active continent rift setting rather than paired metamorphic belts in the arc-basin system, revealing post-collisional extension of the Early-Middle Paleozoic orogenic lithosphere of eastern CAOB in the Late Paleozoic. • The Xilingol complex underwent long-lasting Buchan-type metamorphism in the Carboniferous to Permian. • Metamorphic patterns of the Xilingol complex are compatible with continent rift setting. • Zircon U-Pb age continuum reveals a successive process from diagenesis to deep burial to multiple metamorphic episodes.

Post-UHP tectonic evolution of the East Kunlun orogenic belt, northern tibetan plateau: insight from the regional-scale crustal anatexis

ABSTRACT Extensive crustal anatexis is common during the decompressional exhumation of deeply subducted continental crust and the delamination of a collision‐thickened orogenic lithosphere, indicating that determining the timing of partial melting can provide insights into the possible tectonic evolution during orogenic processes. An integrated study of petrology, zirconology and whole-rock geochemical compositions from migmatites and pegmatites in the Nageng, Langmuri and Wulonggou area of the East Kunlun orogenic belt, is presented in this study. The protoliths of these migmatites include Tonian sedimentary rocks and ca. 440 Ma arc-related dioritic rock. Together with previous studies, an extensive crustal anatexis event recorded by anatectic zircons occurred after ca. 410 Ma. In addition, hafnium isotope analyses indicate that the anatectic zircons may or may not inherit their Hf isotope compositions from protolith zircons during partial melting, depending on whether or not the garnet Hf contributes to the Hf budget of anatectic melt. Thus, the most reasonable method to trace the source of granite is to compare the Hf isotope composition of anatectic zircon in migmatite and magmatic zircon in contemporaneous granite. By such method, anatectic melt produced by migmatites in this study could account for both penecontemporaneous A- and I-type granites with high magmatic zircon εHf(t) values and S-type granites with low magmatic zircon εHf(t) values. As the extensive crustal anatexis is later than both metamorphic peak and amphibole-facies retrogression of high-pressure and ultrahigh-pressure (HP-UHP) metamorphic rocks, it is reasonable that post-collision orogenic collapse of the collision‐thickened orogenic lithosphere as a result of delamination and asthenospheric upwelling occurred after ca. 410 Ma, which is also supported by contemporaneous granulite-facies metamorphism, molasse deposition, and magmatic events including Xiarihamu mafic-ultramafic intrusions, A-type granites, and mafic dykes formed in the continental rifting setting.

Differential exhumation of cratonic and non-cratonic lithosphere revealed by apatite fission-track thermochronology along the edge of the S\xe3o Francisco craton, eastern Brazil

Lithosphere of cratons and orogens generally reacts differently to tectonic events. Although these differences are mostly clear during the orogenic phases, understanding how they respond to tectonic reactivation is still challenging. Here, we report the first detailed apatite fission-track (AFT) study pinpointing the gradual transition between cratonic and orogenic lithosphere, using the case study of the São Francisco craton (SFC) and the adjacent Araçuaí-West Congo Orogen (AWCO), eastern Brazil. The collision that built the AWCO partially affected the inherited rift structures of the Paramirim Aulacogen, embedded in the São Francisco-Congo paleocontinent. Our data reveal a differential Phanerozoic exhumation between closely interspaced areas affected and not affected by the AWCO deformation. Samples from the SFC present slow and protracted basement cooling during the Phanerozoic, while samples from the orogen display rapid exhumation since the Eocene. An intermediate ~ N–S zone of c.40 km shows lower magnitude basement cooling during the Cenozoic, possibly because the propagation of AWCO deformation decreases towards the craton interior. Within the orogen, the Rio Pardo salient is the main reactive structure and probably results from the deformation of a master fault, inherited from its precursor rift. Here, we show how the magnitude of Phanerozoic denudation may be deeply associated with previous events of lithosphere weakening.

Thermal history along the Araçuaí Orogen and São Francisco Craton border, eastern Brazilian continental margin, based on low-temperature thermochronologic data

Passive margins form in response to rifting affecting a continental area. Rifting evolution is dependent on structural and rheological inheritance, generally controlling differential denudation, and deposition. The Brazilian continental margin developed over a complex basement, composed of the orogenic lithosphere of the Brasiliano-Pan-African orogenic belts (e.g., the Araçuaí Orogen), and the cratonic lithosphere of the São Francisco Craton. In this work, we focus on the transitional zone between the warm and cold lithosphere of the northern segment of the Araçuaí Orogen to the São Francisco Craton. This study provides the first suite of two low-temperature thermochronometers, i.e., zircon and apatite (UTh)/He, for the region. Using this approach, we examine the main cooling events, the geodynamic forces acting upon the lithosphere, the denudation rates affecting this region since West Gondwana amalgamation, and the correlation between thermochronology and structural inheritance. Our results indicate that ZHe corrected ages range from 483.1 ± 38.6 to 93.8 ± 7.5 Ma and eU concentrations vary from 10.91 to 632.51 mg/g. AHe corrected ages span from 129.9 ± 7.79 to 32.7 ± 1.96 Ma and eU concentrations range from 5.6 to 62.8 mg/g. Both ZHe and AHe ages display a positive correlation with eU concentration. Based on this dataset, we identify three cooling episodes, constrained from Lower Devonian to Carboniferous, Triassic/Jurassic to the present day, and Late Cretaceous/Paleocene to the present day. Additionally, two reheating episodes are constrained from Middle Jurassic to Early Cretaceous and from Late Cretaceous to Paleocene. Furthermore, we analyzed lineament directions in the orogenic, cratonic, and Cenozoic sedimentary cover domains to understand how structural inheritance affects the thermochronological record. Our observations reveal that reactivation of the Brasiliano NW-SE inherited structures during continental breakup up until the Paleocene possibly kept the AHe system open, resulting in the youngest AHe ages of our data set.

Orogenic lithosphere and slabs in the greater Alpine area – interpretations based on teleseismic P-wave tomography

Abstract. Based on recent results of AlpArray, we propose a new model of Alpine collision that involves subduction and detachment of thick (∼ 180 km) European lithosphere. Our approach combines teleseismic P-wave tomography and existing local earthquake tomography (LET), allowing us to image the Alpine slabs and their connections with the overlying orogenic lithosphere at an unprecedented resolution. The images call into question the conventional notion that downward-moving lithosphere and slabs comprise only seismically fast lithosphere. We propose that the European lithosphere is heterogeneous, locally containing layered positive and negative Vp anomalies of up to 5 %–6 %. We attribute this layered heterogeneity to seismic anisotropy and/or compositional differences inherited from the Variscan and pre-Variscan orogenic cycles rather than to thermal anomalies. The lithosphere–asthenosphere boundary (LAB) of the European Plate therefore lies below the conventionally defined seismological LAB. In contrast, the lithosphere of the Adriatic Plate is thinner and has a lower boundary approximately at the base of strong positive Vp anomalies at 100–120 km. Horizontal and vertical tomographic slices reveal that beneath the central and western Alps, the European slab dips steeply to the south and southeast and is only locally still attached to the Alpine lithosphere. However, in the eastern Alps and Carpathians, this slab is completely detached from the orogenic crust and dips steeply to the north to northeast. This along-strike change in attachment coincides with an abrupt decrease in Moho depth below the Tauern Window, the Moho being underlain by a pronounced negative Vp anomaly that reaches eastward into the Pannonian Basin area. This negative Vp anomaly is interpreted as representing hot upwelling asthenosphere that heated the overlying crust, allowing it to accommodate Neogene orogen-parallel lateral extrusion and thinning of the ALCAPA tectonic unit (upper plate crustal edifice of Alps and Carpathians) to the east. A European origin of the northward-dipping, detached slab segment beneath the eastern Alps is likely since its down-dip length matches estimated Tertiary shortening in the eastern Alps accommodated by originally south-dipping subduction of European lithosphere. A slab anomaly beneath the Dinarides is of Adriatic origin and dips to the northeast. There is no evidence that this slab dips beneath the Alps. The slab anomaly beneath the Northern Apennines, also of Adriatic origin, hangs subvertically and is detached from the Apenninic orogenic crust and foreland. Except for its northernmost segment where it locally overlies the southern end of the European slab of the Alps, this slab is clearly separated from the latter by a broad zone of low Vp velocities located south of the Alpine slab beneath the Po Basin. Considered as a whole, the slabs of the Alpine chain are interpreted as highly attenuated, largely detached sheets of continental margin and Alpine Tethyan oceanic lithosphere that locally reach down to a slab graveyard in the mantle transition zone (MTZ).

Differential Phanerozoic evolution of cratonic and non-cratonic lithosphere from a thermochronological perspective: São Francisco Craton and marginal orogens (Brazil)

The São Francisco Craton (SFC) and its marginal Araçuaí and Brasília orogens exhibit a significant diversity in their lithospheric architecture. These orogens were shaped during the Neoproterozoic–Cambrian amalgamation of West Gondwana. The rigid cratonic lithosphere of the SFC and the relatively weak lithosphere of the Araçuaí Orogen were disrupted during the Cretaceous opening of the South Atlantic Ocean, whereas the Brasília Orogen remained in the continental hinterland. In earlier research, the thermal effects of the Phanerozoic reactivations in the shallow crust of the Araçuaí Orogen have been revealed by low-temperature thermochronology, mainly by apatite fission track (AFT) analysis. However, analyses from the continental interior are scarce. Here we present new AFT data from forty-three samples from the Brasília Orogen, the SFC and the Araçuaí Orogen, far from the passive margin of the Atlantic coast (~150 to 800 km). Three main periods of basement exhumation were identified: (i) Paleozoic, recorded both by samples from the SFC and Brasília Orogen; (ii) Early Cretaceous to Cenomanian, recorded by samples from the Araçuaí Orogen; and (iii) Late Cretaceous to Paleocene, inferred in samples from all domains. We compare the differential exhumation pattern of the different geotectonic provinces with their lithospheric strengths. We suggest that the SFC likely concentrated the Meso-Cenozoic reactivations in narrow weak zones while the Araçuaí Orogen displayed a far-reaching Meso-Cenozoic deformation. The Brasília Orogen seems to be an example of a stronger orogenic lithosphere, inhibiting reworking, confirmed by our new AFT data. Understanding the role of the lithosphere rigidity may be decisive to comprehend the processes of differential denudation and the tectonic–morphological evolution over Phanerozoic events.

Weak orogenic lithosphere guides the pattern of plume-triggered supercontinent break-up

The importance of nonrigid geological features (such as orogens) inside tectonic plates on Earth’s dynamic evolution lacks thorough investigation. In particular, the influence of continent-spanning orogens on (super)continental break-up remains unclear. Here we reconstruct global orogens and model their controlling effects on Pangea break-up. We show that while loci of Pangea break-up are linked to mantle plumes, development of continental rifts is guided by orogens. Rifting at Central Atlantic is driven by the modelled plume responsible for the Central Atlantic Magmatic Province (CAMP) within Pangea-forming orogens. South Atlantic rifting is controlled by necking between Pangea- and Gondwana-forming orogens with the assistance of plume-induced lithospheric weakening. Without CAMP-induced weakening, South Atlantic rifting fails between the West African and Amazonian cratons, but occurs between the West African and Saharan cratons instead. Our modeling on Pangea break-up is able to recreate present-day continental geometry through the combined effect of orogens and plume center-locations.

Triassic crust–mantle interaction in the Eastern Tianshan, southern Altaids: Insights from microgranular enclaves and their host Tianhu granitoids

Triassic magmatism has been increasingly identified in the Eastern Tianshan and played a crucial role in the tectonic evolution of the Tianshan Orogenic Belt. The petrogenesis and geodynamic setting of the Triassic magmatism, however, are poorly constrained. In this study, we integrate new field and petrological observations, zircon UPb ages and Hf isotopic compositions, and whole-rock geochemical data for the microgranular enclaves (MEs) and their host Triassic Tianhu granitoids to investigate their origin and the tectonic regime of the Eastern Tianshan during the Triassic. Based on zircon UPb geochronology, the Tianhu granitoids and MEs have indistinguishable Early Triassic crystallization ages of 247.1 ± 1.8 Ma, 249.5 ± 2.0 Ma, and 251.0 ± 3.2 Ma. The MEs in the Tianhu granitoids are typically sub-rounded to ellipsoidal in shape. They contain abundant acicular apatite, plagioclase that has been partially resorbed, and megacrysts of feldspar and quartz. They are characterized by high Mg# (50–52), positive εHf(t) values of 3.41–5.83, and young one-stage Hf model ages, all of which are indicative of crystallization from mantle-derived melts. The host Tianhu granitoids, which belong to I-type granites, contain higher SiO2 contents (68.3–69.8 wt%) and lower Mg# (38.3–45.4) than the MEs, are metaluminous (A/CNK = 0.97–0.98), and are characterized by high εHf(t) values of 2.26–5.46 and young two-stage Hf model ages. These geochemical characteristics indicate that the Tianhu granitoids and the MEs likely formed through mixing between mafic and felsic magmas. The mafic magma was originally derived from partial melting of a depleted mantle source, and subsequently evolved via fractional crystallization and modification by the felsic magma. In contrast, the felsic magma was generated by partial melting of the juvenile continental lower crust. The Tianhu granitoids would likely form in an intracontinental extensional environment, in which upwelling of the asthenospheric mantle led to partial melting of the lithospheric mantle to form the mafic magmas. And the felsic magmas were produced by partial melting of the deep juvenile crust with a heat source either from the ascending mafic magmas or the upwelling asthenospheric mantle along the thinned orogenic lithosphere. In either case, there would be significant Triassic crust–mantle interaction in the Eastern Tianshan and its adjacent Beishan area. This interaction was responsible for the formation of widespread Triassic granites and related ore deposits in this intracontinental region.

First Record of Circa 970 Ma Post-Collisional A-Type Magmatism in the Sendra Granitoid Suite, Central Aravalli Orogen, Northwest India

This study provides the first record for the emplacement of post-collisional A-type granites in extensional regime during the late Grenvillian period in northwest India. The ca. 970 Ma granites of the Sendra Granitoid Suite (Chang pluton) intrude calc-silicate rocks of the South Delhi Supergroup in the central Aravalli orogen. The Chang pluton is composed of granite sensu stricto ; the granites are metaluminous, ferroan, calc-alkalic, and are characterized by high Ga/Al (>2.5), Nb + Y (>60 ppm), Ta + Yb (>6 ppm), REE, HFSE and zircon saturation temperatures, typical of A-type granites. The Y/Nb >1.2 further classified the rocks as A2-subtype, signifying their derivation from crustal sources in a post-collisional setting. The crustal source is also supported by their high LILE (Rb, K and Ba), and Pb, Th and REE. The geochronological data and tectonics of the region indicate that the granites were emplaced about 30 Myr after the Grenvillian collisional orogeny. This scenario likely resulted due to delamination of the lower part of the thickened orogenic lithosphere. These results are expected to have significant implications for the assembly tectonics of the Rodinia supercontinent.

Granulites record the tectonic evolution from collisional thickening to extensional thinning of the Tongbai orogen in central China

AbstractA combined study of petrology and geochemistry was carried out for granulites from the Tongbai orogen in central China. The results reveal the tectonic evolution from collisional thickening to extensional thinning of the lithosphere at the convergent plate boundary. Petrographic observations, zircon U–Pb dating, and pseudosection calculations indicate that the granulites underwent four metamorphic stages, which are categorized into two cycles. The first cycle occurred at 490–450 Ma and involves high‐P (HP) metamorphism (M1) at 785–815°C and 10–14 kbar followed by decompressional heating to 840–880°C and 8–9 kbar for medium‐pressure granulite facies metamorphism (M2), defining a clockwise P–T path. The high pressure is indicated by the occurrence of inclusions of rutile+kyanite+K‐feldspar in the garnet mantle. The second cycle occurred at c. 440 Ma and shows an anticlockwise P–T path with continuous heating to ultrahigh‐temperature (UHT) metamorphism (M3) at 890–980°C and 9–11 kbar, followed by decompressional cooling to 740–880°C and 7–9 kbar (M4) till 405 Ma. The HP metamorphism is synchronous with the ultrahigh‐pressure eclogite facies metamorphism in the Qinling orogen, indicating its relevance to the continental collision in the Cambrian. The UHT metamorphism took place at reduced pressures, indicating thinning of the collision‐thickened orogenic lithosphere. Therefore, the Tongbai orogen was initially thickened by the collisional orogeny and then thinned, possibly as a result of foundering of the orogenic root. Such tectonic evolution may be common in collisional orogens where compression during continental collision switched to extension during continental rifting.

Eburnean deformation pattern of Burkina Faso and the tectonic significance of shear zones in the West African craton

Shear zones of the Paleoproterozoic Eburnean accretionary Orogen (West African craton) are investigated by means of large-scale structural mapping. Regional scale (10-100 km) mapping was based on the aeromagnetic survey of Burkina Faso and craton-scale (1000 km) mapping on a compilation of fabric data. At both scales, shear zones are arranged as an anastomosed transpressional network that accommodated distributed shortening and lateral flow of the orogenic lithosphere between the converging Kénéma-Man and Congo Archean provinces. Structural interference patterns at both scales were due to three-dimensional partitioning of progressive transpressional deformation and interactions among shear zones that absorbed heterogeneities in the regional flow patterns while maintaining the connectivity of the shear zone network. Such orogen-scale kinematic patterns call for caution in using the deformation phase approach without considering the “bigger structural picture” and interpreting displacement history of individual shear zones in terms of plate kinematics. The West African shear zone pattern is linked to that of the Guiana shield through a new transatlantic correlation to produce an integrated kinematic model of the Eburnean-Transamazonian orogen.

The role of accreted continental crust in the formation of granites within the Alpine style continental collision zone: Geochemical and geochronological constrains from leucogranites in the Çataldağ Metamorphic Core Complex (NW Turkey)

The recently discovered Çataldağ Metamorphic Core Complex (ÇMCC) of Eo-Oligocene age is situated in NW Turkey, which is a part of the Alpine collisional belt. The footwall of the ÇMCC consists of a granite-gneiss-migmatite complex (GGMC) with a domal structure. Here, we document new major-trace element geochemistry, Sr-Nd-Pb isotope characteristics and Ti-in-zircon thermometry data from granites and syn-plutonic mafic dykes that are connected to the gneiss-migmatite dome of ÇMCC, and discuss granitic melt evolution within the context of the syn-to post-collisional evolution of the western Anatolian orogenic crust. Granites of the GGMC include both peraluminous garnet-bearing leucogranite and two-mica granite that emplaced from 32 to 35 Ma. Both types of leucogranite are enriched in light rare earth elements (Rb, U, K, Pb) and depleted in HFSE (Nb, Ta, Zr, Ti). Their 87Sr/86Sr, 206Pb/204Pb and 207Pb/204Pb initial isotope values range from 0.7094 to 0.7113, 18.79 to 18.91, and 15.71 to 15.73, respectively, and εNd(33) values vary between −5.13 and − 7.79. On the other hand, mafic dykes show similar isotopic characteristics (87Sr/86Sr(33) = 0.7055, εNd(33) = −1.8 and 206Pb/204Pb = 18.8) to enriched mantle melts. Geochemical modelling shows that the granites have dominant crustal melt component (85–70%) and a minor mantle component (<30%). According to partial melting modelling and Ti-in-zircon thermometer calculations, granitic melt was formed by water-absent muscovite dehydration melting of a micaschist source (a melt fraction of max. 35%) at ≥7–10 kb and 739–840 °C. Whole rock geochemistry, isotopic characteristics and inherited zircon chronology, combined with the geology of the region, indicate that granitic melts were derived by partial melting of Anatolide-Tauride crustal units that were underthrusted below the Sakarya Continent along the İzmir-Ankara Suture Zone. On the other hand, the source of the syn plutonic mafic dykes emplaced in the core of the ÇMCC is inferred to derived from enriched mantle (EM II) that lay beneath the western Anatolian orogenic crust. We infer that partial melting and melt migration to produce leucogranites are most probably related to thermal weakening and partial removal of western Anatolian young orogenic lithosphere during the transitional phase between the latest phase of collision and the earliest phase of extension, in the Eo-Oligocene.

Radiogenic trigger and driver for continental rifting and initial ocean spreading

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