Orogenic Architecture Research Articles

Two key switches in regional stress field during multi-stage deformation in the Carboniferous–Triassic southernmost Altaids (Beishan, NW China): Response to orocline-related roll-back processes

AbstractThe orogenic architecture of the Altaids of Central Asia was created by multiple large-scale slab roll-back and oroclinal bending. However, no regional structural deformation related to roll-back processes has been described. In this paper, we report a structural study of the Beishan orogenic collage in the southernmost Altaids, which is located in the southern wing of the Tuva-Mongol Orocline. Our new field mapping and structural analysis integrated with an electron backscatter diffraction study, paleontology, U-Pb dating, 39Ar-40Ar dating, together with published isotopic ages enables us to construct a detailed deformation-time sequence: During D1 times many thrusts were propagated northwards. In D2 there was ductile sinistral shearing at 336–326 Ma. In D3 times there was top-to-W/WNW ductile thrusting at 303–289 Ma. Two phases of folding were defined as D4 and D5. Three stages of extensional events (E1–E3) separately occurred during D1–D5. Two switches of the regional stress field were identified in the Carboniferous to Early Permian (D1-E1-D2-D3-E2) and Late Permian to Early Triassic (D4-E3-D5). These two switches in the stress field were associated with formation of bimodal volcanic rocks, and an extensional interarc basin with deposition of Permian-Triassic sediments, which can be related to two stages of roll-back of the subduction zone on the Paleo-Asian oceanic margin. We demonstrate for the first time that two key stress field switches were responses to the formation of the Tuva-Mongol Orocline.

Imaging the Late Triassic lithospheric architecture of the Yidun Terrane, eastern Tibetan Plateau: Observations and interpretations

The present-day lithospheric architecture of modern and ancient orogens can be imaged by geophysical techniques. For ancient orogens, unravelling their architecture at the time of formation is hindered by later tectono-magmatic events. In this paper, we use spatial variations in radiogenic isotopic compositions of Late Triassic magmatism from the Yidun Terrane, eastern Tibetan Plateau, to establish its lithospheric architecture during the Triassic. Comprehensive geochemical and isotopic data of Late Triassic magmatic rocks from four transects across the Yidun Terrane document eastward enrichment in whole-rock Nd, Sr, and zircon Hf isotopic compositions. Mafic and felsic rocks of major plutons show coherent and nonlinear trends in the Zr and P2O5 systematics and have limited variation of isotopic compositions. This indicates that Late Triassic magmatic differentiation was dominated by fractionation of mantle-derived mafic magmas. The spatial isotopic trends result from changing mantle sources, including variable contributions of isotopically depleted asthenospheric mantle and isotopically enriched subcontinental lithospheric mantle (SCLM) to magma sources. The spatial variation of mantle sources suggests a westward thinning of the SCLM during the Triassic. We propose that this architecture is most likely associated with eastward subduction of oceanic lithosphere of the Jinshajiang Ocean located at the west of the Yidun Terrane, immediately prior to the Late Triassic magmatism.

Tectonic evolution of the Grenville Orogen in the central Appalachians

Remnants of the Grenville Orogeny—a globally pervasive tectonothermal event resulting from the amalgamation of the supercontinent Rodinia between ca. 1.3 and 1.0 Ga—are exposed throughout eastern North America for ca. 3000 km from Labrador to Alabama. Although tectonic models for the development of the Grenville have historically been based on observations from the large and well-exposed Canadian Grenville Province, recent investigations suggest that initial continental collision occurred proximal to the Appalachian Blue Ridge. Mesoproterozoic rocks within this portion of the orogen are exposed within a series of allochthonous terranes that were extensively reworked during Paleozoic orogenesis. To better understand the tectonic history of this critical region, we utilized garnet and monazite geochronology to reconstruct the metamorphic history of eleven samples of ortho- and paragneiss from the Appalachian Blue Ridge Province.The resultant dates, together with new results from trace element thermobarometry, define a prolonged record of high temperature metamorphism coincident with the timing of continent–continent collision as recognized in the northern portion of the orogen. Initial garnet and monazite growth commenced between 1100 and 1065 Ma with peak granulite-facies conditions (750–900 °C, 0.55–0.85 GPa) occurring from 1050 to 1010 Ma. Burial and heating of the paragneisses progressed rapidly with peak metamorphism occurring ∼20–30 Myr after deposition of the sedimentary protoliths. Local hydrothermal alteration followed peak metamorphism and occurred at 1000–990 Ma with later cooling through ∼660 °C between 980 and 935 Ma (3 ± 2 °C/Myr). The metamorphic history outlined here, together with insights from previous studies, is consistent with formation of the Blue Ridge rocks within a continental suture zone that had an orogenic architecture comparable to that of the Himalayan-Tibetan orogen.

Accretion, subduction erosion, and tectonic extrusion during late Paleozoic to Mesozoic orogenesis in NE China

The high-pressure Heilongjiang Complex records a long and complicated evolution from the late Paleozoic to Mesozoic in NE China, which is still subject to controversy. In this paper we present a new detailed study including structural, geochemical and geochronological analyses (Ar–Ar dating on metamorphic minerals, U-Pb SIMS dating on zircons). Two main periods of oceanward accretionary growth are identified between 305 Ma and 195 Ma and around 112–101 Ma. These periods of accretionary growth were associated with the emplacement of voluminous magmatism, slab roll-back with continental back-arc extension, and growth of a forearc accretionary prism. Between these periods of accretionary growth, a major phase of regional subduction erosion took place between 195 Ma and 142 Ma at the eastern edge of the Songliao Block. The main effects of this subduction erosion phase were that regional Jurassic-early Cretaceous extrusion overlapped with Mesozoic accretion and erosion in NE China, which disturbed the initial orogenic architecture. With a review of relevant zircon ages of this accretionary orogen, we unravel the complicated geodynamic history of accretion, subduction erosion and tectonic extrusion that shaped NE China from the late Paleozoic to Mesozoic.

Role of rift-inheritance and segmentation for orogenic evolution: example from the Pyrenean-Cantabrian system

The Basque-Cantabrian junction corresponds to an inverted rift accommodation zone at the limit between the former hyperextended Pyrenean and Cantabrian rift segments. The recognition of an inherited rift segment boundary allows to investigate the reactivation associated with large-scale rift segmentation in an orogenic system. We use criteria from published field observations and seismic data to propose a new map of rift domains for the Basque-Cantabrian junction. We also provide balanced cross-sections that allow to define the along-strike architecture associated with segmentation during rifting and subsequent Alpine reactivation. Based on these results, this study aims to characterize and identify reactivated and newly formed structures during inversion of two rift segments and its intermitted segment boundary. It also aims to describe the timing of thin-skinned and thick-skinned deformation associated with the inversion of segmented rift systems. During convergence, two phases have been recognized within the rift segment (eastern Mauléon basin). The Late Cretaceous to Paleocene underthrusting/subduction phase was mostly governed by thin-skinned deformation that reactivated the former hyperextended domains and the supra-salt sedimentary cover. The Eocene to Miocene collisional phase, controlled by thick-skinned deformation that took place once necking domains collided and formed an orogenic wedge. At the rift segment boundary, the underthrusting/subduction phase was already controlled by thick-skinned deformation due to the formation of shortcutting thrust faults at the termination of overlapping V-shaped rift segments. This led to the formation of a proto-wedge composed of the Basque massifs. We suggest that this proto-wedge is responsible for the preservation of pre-Alpine structures in the Basque massifs and for the emplacement of subcontinental mantle rocks at a crustal level beneath the western Mauléon basin. These results argue for a first order cylindrical orogenic architecture from the Central Pyrenean segment to the Cantabrian segment (up to the Santander transfer zone) despite rift segmentation. They also highlight the control of 3D rift-inheritance for the initial phase of orogenic evolution and for the local architecture of mountain belts.

Lithospheric structure of the xuefengshan belt, South China: Evidence from a seismic reflection profile

In this study, we analyse and interpret a new seismic reflection profile across the Xuefengshan Belt (XFSB), South China, to determine the deep structure of this belt and provide insights into collisional orogenic architecture. We identify a deep regional fault located beneath the XFSB, which cuts through the reflection Moho below the belt and is interpreted as representing the collisional boundary between the Yangtze and Cathaysia blocks along the southeastern margin of the belt. We identify structural evidence for collision between the two blocks, including two groups of layered narrow slip belts (∼30 km) in the middle crust, one distributed in the Yangtze Block and the other in the Cathaysia Block. A narrow velocity-gradient belt in the upper mantle at a depth of ∼90 km is revealed by numerous strong seismic reflectors, some of which are layered. The study provides important new information on the lithospheric structure of the South China Block at the interface between the Yangtze and Cathaysia blocks.

Mid-Paleozoic ridge subduction in the Central Beishan of the southern Altaids: evidence from geochemical, Sr–Nd and zircon U–Pb–Hf–O isotopic data of Gongpoquan volcanic rocks

The investigation of Mid-Paleozoic tectonics could constrain the relations of early and late Paleozoic subduction events in the southern Altaids and improve our understanding of the architecture of accretionary orogens. The Gongpoquan volcanic rocks are widespread in the Central Beishan and contain important information on the tectonics of the southern Altaids. Geochronological data constrain the eruptive time of the Gongpoquan volcanic rocks as early Silurian–early Devonian (434–411 Ma). The Gongpoquan andesites show similar geochemical characteristics to those of typical sanukitoids, including high MgO and compatible element concentrations, enrichments in large ion lithophile elements and light rare earth elements, and depleted isotopic compositions. The geochemical data show that the Gongpoquan andesites were generated through the partial melting of subducting sediments and subsequent melt–mantle peridotite interactions. The early Silurian basalts are Na-rich and exhibit high Nb/U and Nb/La ratios, an enrichment in high field strength elements and enriched isotopic compositions, making them akin to Nb-enriched basalts. Such Nb-enriched basalts should be derived from the melting of a mantle component similar to ocean island basalt in the mantle wedge. The early Devonian basalts are calc-alkaline, enriched in large ion lithophile elements and depleted in high field strength elements. They yield high Ba/Yb ratios and depleted isotopic compositions, indicating an origin via the partial melting of a depleted mantle wedge metasomatized by subduction-related fluids. Our data prove that the Paleo-Asian Ocean may still have been subducted during the Mid-Paleozoic. During this process, a spreading ridge was subducted beneath the Central Beishan, resulting in the opening of a slab window, which generated the observed 431–411 Ma sanukitoids and Nb-enriched basalts. Supplementary Material: Analytical methods, U–Pb geochronological data, zircon Lu–Hf-O isotopic compositions, whole-rock major and trace element data, detailed descriptions of the zircon U–Pb geochronology and Hf–O isotopic compositions for analysed samples, representative field photographs and photomicrographs of samples, and representative cathodoluminescence images of analysed zircon grains are available at: https://doi.org/10.6084/m9.figshare.c.4461062

Continuity and Episodicity in the Early Alpine Tectonic Evolution of the Western Carpathians: How Large‐Scale Processes Are Expressed by the Orogenic Architecture and Rock Record Data

Key Points Limited Cenozoic shortening of the Western Carpathians facilitates a comprehensive tectonic model of their pre‐Late Eocene evolution Western Carpathian orogeny was driven by pull of the Meliata oceanic slab and by the northward drift of Adria Multiple rock record data provide valuable constraints for evolutionary tectonic models of collisional orogens

The non-cylindrical crustal architecture of the Pyrenees

We exploit the data from five seismic transects deployed across the Pyrenees to characterize the deep architecture of this collisional orogen. We map the main seismic interfaces beneath each transect by depth migration of P-to-S converted phases. The migrated sections, combined with the results of recent tomographic studies and with maps of Bouguer and isostatic anomalies, provide a coherent crustal-scale picture of the belt. In the Western Pyrenees, beneath the North Pyrenean Zone, a continuous band of high density/velocity material is found at a very shallow level (~10 km) beneath the Mauleon basin and near Saint-Gaudens. In the Western Pyrenees, we also find evidence for northward continental subduction of Iberian crust, down to 50–70 km depth. In the Eastern Pyrenees, these main structural features are not observed. The boundary between these two domains is near longitude 1.3 °E, where geological field studies document a major change in the structure of the Cretaceous rift system, and possibly a shift of its polarity, suggesting that the deep orogenic architecture of the Pyrenees is largely controlled by structural inheritance.

Seismic behaviour of mountain belts controlled by plate convergence rate

The relative contribution of tectonic and kinematic processes to seismic behaviour of mountain belts is still controversial. To understand the partitioning between these processes we developed a model that simulates both tectonic and seismic processes in a continental collision setting. These 2D seismo-thermo-mechanical (STM) models obtain a Gutenberg–Richter frequency–magnitude distribution due to spontaneous events occurring throughout the orogen. Our simulations suggest that both the corresponding slope (b value) and maximum earthquake magnitude (MWmax) correlate linearly with plate convergence rate. By analyzing 1D rheological profiles and isotherm depths we demonstrate that plate convergence rate controls the brittle strength through a rheological feedback with temperature and strain rate. Faster convergence leads to cooler temperatures and also results in more larger seismogenic domains, thereby increasing both MWmax and the relative number of large earthquakes (decreasing b value). This mechanism also predicts a more seismogenic lower crust, which is confirmed by a transition from uni- to bi-modal hypocentre depth distributions in our models. This transition and a linear relation between convergence rate and b value and MWmax is supported by our comparison of earthquakes recorded across the Alps, Apennines, Zagros and Himalaya. These results imply that deformation in the Alps occurs in a more ductile manner compared to the Himalayas, thereby reducing its seismic hazard. Furthermore, a second set of experiments with higher temperature and different orogenic architecture shows the same linear relation with convergence rate, suggesting that large-scale tectonic structure plays a subordinate role. We thus propose that plate convergence rate, which also controls the average differential stress of the orogen and its linear relation to the b value, is the first-order parameter controlling seismic hazard of mountain belts.

Climatic and glacial impact on erosion patterns and sediment provenance in the Himalayan rain shadow, Zanskar River, NW India

Erosion is a key step in the destruction and recycling of the continental crust, yet its primary drivers continue to be debated. The relative balance between climatic and solid Earth forces in determining erosion patterns and rates, and in turn orogenic architecture, is unresolved. The monsoon-dominated frontal Himalaya is a classic example of how surface processes may drive focused denudation and potentially control structural evolution. We investigate whether there is a clear relationship between climate and erosion in the drier Himalayan rain shadow on the periphery of the Tibetan Plateau, where a coupled climate-erosion relationship is less clear. We present a new integrated data set combining bulk petrography, geomorphometric analysis, detrital U-Pb zircon geochronology, and bulk Nd and Sr isotope geochemistry from modern river sediments that provides constraints on spatial patterns of sediment production and transport in the Zanskar River. Zanskar River sands are dominated by Greater Himalayan detritus sourced from the glaciated Stod River catchment, which represents only 13% of the total basin area. Prevalent zircon peaks from Cambrian-Ordovician (440-500 Ma) and Mississippian- Permian (245-380 Ma) units indicate more abundant pre-Himalayan granitoids in the northwest Indian Himalaya than in the central and eastern Himalaya. Erosion from the widely exposed Tethyan Himalaya, however, appears modest. Spatial patterns of erosion do not correlate with highest channel steep-ness. Our data demonstrate that Zanskar differs from the monsoon-soaked frontal Himalaya and the arid, extremely slow-eroding Tibetan orogenic interior in that focused erosion and sediment production are driven by glaciers. Subsequent remobilization of glacially derived sediments is likely controlled by monsoonal rainfall, and we suggest sediment reworking plays an important role. These data support a strong climatic control on modern orogenic erosion in the Himalayan rain shadow on the periphery of the Tibetan Plateau.

Defining diagnostic criteria to describe the role of rift inheritance in collisional orogens: the case of the Err-Platta nappes (Switzerland)

It is commonly accepted that collisional orogens involve the reactivation of former rifted margins. While it remains debated how rift inheritance can be identified and how it controls the architecture of orogens this case study analyses the importance of rift-inheritance during reactivation of a passive margin. The study analyses complex, non-layer cake rift structures within the well-exposed Err and Platta nappes (SE Switzerland), representing the former distal Adriatic margin of the Alpine Tethys. Diagnostic criteria for rift inheritance include: (1) typical fault rocks with a mantle derived fluid signature, and (2) tectono-sedimentary breccias made of reworked exhumed basement and grading upwards into late syn- and post-rift sediments. Based on the study of “recognisable” features, a methodology is etablished, which enables to (1) map rift-related detachment faults and (2) to analyse their role during reactivation and formation of a thrust stack. First, second and third order thrust systems are defined. First order thrust systems juxtapose different rift domains (proximal, necking, and distal). Second order systems are dominantly made up of basement sheets sampling the former footwall of an extensional detachment fault. Third order systems mainly consist of the former hanging wall of an extensional detachment fault. A major result of this study is that thrust faults commonly reactivate former extensional detachment faults, especially in the exhumed mantle domain (Platta nappe), while in the hyperextended domain (Err nappe) reactivation of rift-inherited structures is more complex and often incomplete. The results of this study may help to better identify remnants of former distal margins and to define and analyse their complex stacking patterns observed in many internal parts of collisional orogens.

The backbone of Alpine-type orogens: assessing the role of rift inheritance

Assessing the relative role of rift-inherited hyper-extension and subduction/collisional dynamics in establishing the lithostratigraphic associations and overall architecture of orogens has key implications for constraining convergent margin dynamics. Giorgio Vittorio Dal Piaz, with his seminal papers across the late '90's and early 2000's, first suggested that seemingly unrelated Jurassic ophiolites, Paleozoic continental basement and Triassic shelf sediment from the Zermatt-Saas Zone were already part of a Jurassic ocean-continent transition zone, prior to undergoing eclogite facies metamorphism during the Alpine orogeny. Subsequent studies, performed in other parts of the Western Alps and Alpine Corsica provided further evidence that a large part of the apparent complexity of the axial zone of Alpine-type orogens may be a result of rift inheritance.

Geodynamics and Metallogeny of the Eastern Tethyan Metallogenic Domain

Abstract The Tethyside orogen, a direct consequence of the separation of the Gondwanaland and the accretion of Eurasia, is a huge composite orogenic system that was generated during Paleozoic–Mesozoic Tethyan accretionary and Cenozoic continent–continent collisional orogenesis within the Tethyan domain. The Tethyside orogenic system consists of a group of diverse Tethyan blocks, including the Istanbul, Sakarya, Anatolide–Taurides, Central Iran, Afghanistan, Songpan–Ganzi, Eastern Qiangtang, Western Qiangtang, Lhasa, Indochina, Sibumasu, and Western Burma blocks, which were separated from Gondwana, drifted northwards, and accreted to the Eurasian continent by opening and closing of two successive Tethyan oceanic basins (Paleo-Tethyan and Neo-Tethyan), and subsequent continental collision. The Tethyan domain represents a metallogenic amalgamation across diverse geodynamic settings, and is the best endowed of all large orogenic systems, such as those associated with the Cordilleran and Variscan orogenies. The ore deposits within the Tethyan domain include porphyry Cu–Mo–Au, granite-related Sn–W, podiform chromite, sediment-hosted Pb–Zn deposits, volcanogenic massive sulfide (VMS) Cu–Pb–Zn deposits, epithermal and orogenic Au polymetallic deposits, as well as skarn Fe polymetallic deposits. At least two metallogenic supergroups have been identified within the eastern Tethyan metallogenic domain (ETMD): (1) metallogenesis related to the accretionary orogen, including the Zhongdian, Bangonghu, and Pontides porphyry Cu belts, the Pontides, Sanandaj–Sirjan, and Sanjiang VMS belts, the Lasbela–Khuzdar sedimentary exhalative-type (SEDEX) Pb–Zn deposits, and podiform chromite deposits along the Tethyan ophiolite zone; and (2) metallogenesis related to continental collision, including the Gangdese, Yulong, Arasbaran–Kerman and Chagai porphyry Cu belts, the Taurus, Sanandaj–Sirjan, and Sanjiang Mississippi Valley-type (MVT) Pb–Zn belts, the Southeast Asia and Tengchong–Lianghe Sn–W belts or districts, the Himalayan epithermal Sb–Au–Pb–Zn belt, the Piranshahr–Saqez–Sardasht and Ailaoshan orogenic Au belts, and the northwest Iran and northeastern Gangdese skarn Fe polymetallic belts. Mineral deposits that are generated with tectonic evolution of the Tethys form in specific settings, such as accretionary wedges, magmatic arcs, backarcs, and passive continental margins within accretionary orogens, and the foreland basins, foreland thrust zones, collisional sutures, collisional magmatic zones, and collisional deformation zones within collisional orogens. Synthesizing the architecture and tectonic evolution of collisional orogens within the ETMD and comparisons with other collisional orogenic systems have led to the identification of four basic types of collision: orthogonal and asymmetric (e.g., the Tibetan collision), orthogonal and symmetric (Pyrenees), oblique and symmetric (Alpine), and oblique and asymmetric (Zagros). The tectonic evolution of collisional orogens typically includes three major processes: (1) syn-collisional continental convergence, (2) late-collisional tectonic transform, and (3) post-collisional crustal extension, each forming distinct types of ore deposits in specific settings. The resulting synthesis leads us to propose a new conceptual framework for the collision-related metallogenic systems, which may aid in deciphering relationships among ore types in other comparable collisional orogens. Three significant processes, such as breaking-off of subducted Tethyan slab, large-scale strike-slip faulting, shearing and thrusting, and delamination (or broken-off) of lithosphere, developed in syn-, late- and post-collisional periods, repsectively, were proposed to act as major driving forces, resulting in the formation of the collision-related metallogenic systems. Widespread appearance of juvenile crust and intense inteaction between mantle and crust within the Himalayan–Zagros orogens indicate that collisional orogens have great potential for the discovery of large or giant mineral deposits.

Tracking basement cross-strike discontinuities in the Indian crust beneath the Himalayan orogen using gravity data – relationship to upper crustal faults

The Himalaya is the result of the on-going convergence and collision of India and Asia. The internal configuration and processes that govern the rise of the Himalayan Mountains and Tibetan Plateau are crucial to understand continental collision zones. However, knowledge of the prior configuration of the colliding plates is equally important, since inherited (pre-orogenic/basement) structures can undeniably influence the development of the orogenic architecture throughout the orogen's cycle of collision and eventual collapse. Three northeast-trending palaeotopographic ridges of faulted Precambrian Indian basement underlie the Ganga basin south of the Himalaya. Our paper illustrates a crustal-scale fault origin for these ridges and succeeds in determining how far north beneath the Himalayan system they extend and how they ultimately govern the location of upper crustal faults in southern Tibet. Spectrally filtered EGM2008 Bouguer gravity data and edges in its horizontal gradient at different source depths ('gravity worms') over northern Peninsular India, the Himalaya and southern Tibet reveal several continuous Himalayan cross-strike discontinuities interpreted to represent crustal faults. Gravity lineaments in Peninsular India coincide with edges of the Precambrian basement ridges and megakinks up to 100 km wide develop in foreland cover sequences between the interpreted basement faults. The interpreted basement faults project northward beneath the Himalayan system and southern Tibet. Our results suggest that several active Himalayan cross-strike faults, such as the ones related to many graben in southern Tibet, are rooted in the underplated Indian lower crust or step en echelon along interpreted basement faults. Our interpretation thus suggests that south Tibet graben are spatially related to deep-seated crustal-scale faults rooted in the underplated Indian crust. These major discontinuities partition the Himalayan range into distinct zones, and could ultimately contribute to lateral variability in tectonic evolution along the orogen's strike.

The role of rift‐inherited hyper‐extension in Alpine‐type orogens

AbstractAlpine‐type orogens are interpreted to result from the collision of former rifted margins. As many present‐day rifted margins consist of hyper‐extended domains floored by thinned continental crust (<10 km) and/or exhumed mantle, this study explores the influence of rift inheritance on the architecture and final evolution of Alpine‐type orogens. We propose that rift‐related necking zones, separating weakly thinned 25‐ to 30‐km‐thick crust from hyper‐extended domains, may act as buttresses during the transition from subduction to collision. As a result, former necking zones may now be found at the boundary between a highly deformed and overthickened nappe stack, made of relics of hyper‐extended domains, and an external, weakly deformed fold‐and‐thrust belt, which largely escaped significant rift‐related crustal thinning and orogeny‐related thickening. Therefore, the role of rift inheritance is of critical importance and is largely underestimated in controlling the architecture and evolution of Alpine‐type orogens.

Mesozoic intraplate granitic magmatism in the Altai accretionary orogen, NW China: Implications for the orogenic architecture and crustal growth

The Central Asian Orogenic Belt (CAOB) is the world9s largest Phanerozoic accretionary orogen and is the most important site for juvenile crustal growth in the Phanerozoic. In this work, we employed U-Pb zircon geochronology to identify the early and middle Mesozoic intraplate granitic intrusive events in the Chinese Altai segment of the southern CAOB in order to better understand the crustal architecture of the CAOB. We also used whole-rock geochemical, Sr-Nd isotopic and zircon Hf isotopic data to constrain the generation for these granitic rocks and to evaluate the implications for vertical crustal growth in this region. The Early Mesozoic granitic intrusions were emplaced between 220 and 200 Ma in the central Altai “microcontinental terrane” (also widely referred to as Units 2 and 3). The granites have shoshonitic and high-K calc-alkaline affinities and show the characteristics of differentiated I-type granite. The whole-rock initial ⁸⁷Sr/⁸⁶Sr ratios (0.7058-0.7128) and ε_Nd(210) values (−0.6 to −4.3), as well as the zircon ε_Hf(t) values (−4.0 to +5.0) and two-stage Hf model ages (0.94-1.52 Ga), suggest that the granitic magmas were produced from a mixed source with both mantle-derived and recycled crustal components. The middle Mesozoic granites were emplaced at ∼150 Ma in the southern Altai “accretionary terrane” (Units 4 and 5). They show A-type characteristics with the REE tetrad effect and have positive ε_Nd(151) whole-rock values of +1.0 to +5.2 and two-stage Nd model ages (T_DM2) of 0.6 to 1.0 Ga. Zircon Hf data show positive zircon ε_Hf(151) values of +1 to +8 and two-stage Hf model ages of 0.6 to 1.2 Ga. The Nd-Hf isotopic data suggest that the granitic magmas were derived from short-lived juvenile mantle-derived materials. Thus, the isotopic signatures of all the Mesozoic granites from the central (old terrane) and southern (young accretional terrane) Altai suggest that the basement of both terranes has retained its original nature. The data further imply that the Altai orogen has kept its original architecture of Paleozoic horizontal accretion during Mesozoic time, as commonly observed in accretionary orogens where horizontal tectonics are dominant. All the early Mesozoic intrusions in the Altai were emplaced in an intraplate anorogenic setting; hence are distinguished from the contemporaneous syn- or post-orogenic magmatism in the eastern CAOB. We conclude that the early Mesozoic granites in the CAOB were emplaced in a variety of tectonic settings.

Geon 12 crustal extension in the central Grenville Province, implications for the orogenic architecture, and potential influence on the emplacement of anorthosites

Remnants of a ca. 1.24 Ga old volcanic belt formed in a within-plate setting (Manicouagan Crustal Extension Belt) are exposed over several tens of kilometres in the hinterland of the central Grenville Province, and can be recognized despite granulite-facies Grenvillian metamorphism and deformation. This belt mainly consists of layered felsic–mafic rock units, some of which are documented here for the first time. In the vicinity of the 1.24 Ga belt, a Geon 14 arc-related mafic suite and a Geon 15 metasedimentary package were pervasively injected by felsic (and mafic) material, show evidence of a Geon 12 thermal event, and therefore may represent remnants of rifted crust. Following Geon 12 crustal extension, lithospheric-scale magmatic activity in the immediate region continued intermittently, producing mafic dykes at 1.17 Ga and anorthosite at 1.16 and 1.05 Ga. A close association to 1.16 Ga or younger anorthosite is conspicuous to several Geon 12 rock units known in the central and eastern Grenville Province. We therefore suggest that subsequent anorthositic magmatism was focused on zones of lithospheric weakness inherited from Geon 12 crustal extension and remained active intermittently until the end of the Grenvillian orogeny.

Thermochronology of the Mont Laurier terrane, southern Canadian Grenville Province, and its bearing on defining orogenic architecture

Convergence during pre-Grenvillian accretion led to crustal thickening and juxtaposition of shallowly southeast-dipping imbricated terranes of the Central Metasedimentary Belt in Ontario and its equivalent to the north in Québec known as the Mont Laurier terrane (MLT). This investigation focuses on the Otter Lake region of the southern MLT, which is composed of a Mesoproterozoic back-arc sequence metamorphosed to upper amphibolite/granulite facies (at conditions of 6±1kbar and 650–700°C). Notable peak assemblages include Fo-bearing marbles and Sil+Kfs migmatites, with little to no retrogression. The terrane is divided into the Marble (west) and Quartzite (east) domains, separated by the east-dipping Heney shear zone (HSZ). LA-ICPMS U-Pb geochronology on select migmatites from both domains yielded bimodal U-Pb zircon and monazite age populations at ca. 1230Ma and 1180–1140Ma, with mineral ages from the leucosomal portions of the migmatites populating the younger age bracket, and implying that the metamorphism and anatexis took place during the accretionary Shawinigan orogeny. Directly east of the HSZ, 40Ar/39Ar hornblende and phlogopite ages are ca. 1145Ma and ca. 1150Ma, respectively, whereas to the west in the Marble Domain hornblende ages are markedly younger (ca. 980–950Ma). Rapid cooling of the Quartzite Domain at the end of the Shawinigan orogeny together with the lack of evidence for any metamorphic overprinting in the southern MLT during the long-lived Grenvillian orogeny (1095–980Ma) is consistent with this region being part of an “orogenic lid” or suprastructure. Post-Grenvillian cooling of Marble Domain rocks through 500°C nearly 200m.y. after cooling of the Quartzite Domain suggests a protracted middle crustal residence of the Marble domain under thermally elevated conditions, implying that portions of the orogenic lid remained warm (above 500°C) during the Grenville orogeny. Moreover, the disparate cooling ages on either side of the HSZ provide evidence for important late displacement across the HSZ comparable to that proposed for terrane-bounding shear zones such as the Robertson Lake and Bancroft shear zones. These structures may be an expression of vertical orogenic collapse or lateral stretching of the orogenic lid. Biotite cooling ages are uniformly <900Ma and are similar to the majority of biotite 40Ar/39Ar ages reported across the southern Grenville Province, indicative of orogen-wide thermal stabilization by ca. 900Ma.

Architecture and evolution of accretionary orogens in the Altaids collage: The early Paleozoic West Junggar (NW China)

Understanding the development and evolution of accretionary orogens is crucial for characterizing continental crust growth in time and space. In the Altaids tectonic collage of Central Asia, conflicting geodynamic models have been proposed about the Paleozoic evolution, during which juvenile continental crust has been extensively formed. This study focuses on West Junggar (NW China), a key region that has not been extensively studied yet. A multidisciplinary approach, including detrital zircon provenance study, geochemistry, and field structural analysis, provides new constraints on the evolution of three Early Paleozoic units of West Junggar. Oceanic subduction predominates in Early Paleozoic time, as indicated by ophiolitic melanges, fore-arc volcaniclastic turbidites and magmatic arc suites. However, the development of an olistostrome supplied by a sub-contemporaneous carbonate platform, the occurrence of continental molasse, and the chronology and geochemistry of magmatic events reveal interruptions of subduction. Discrete collisions of volcanic arcs and micro-continent are inferred from the tectonic structure of the belt, which displays allochthonous units rooted in the suture zone. Early Paleozoic magmatic and sedimentary rocks of West Junggar bear some resemblance to contemporaneous analogues in Eastern Kazakhstan, and suggest a lateral connection. On the basis of these new results, we propose a geodynamic evolution in four steps: 1) Early to Middle Ordovician intra-oceanic subduction forming island arcs, 2) Late Ordovician tectonic accretion of island arcs against the Kazakhstan continental margin, 3) Early to Middle Silurian resumption of subduction and active continental margin magmatism, and 4) Late Silurian-Early Devonian micro-continent collision. This scenario, consistent with the formation of the Kazakhstan Block, supports a model of multiple episodic accretions for the Altaids tectonic collage rather than a continuous continental margin evolution. These results also suggest that, in Central Asia, continental crust growth results from alternating vertical and horizontal growth episodes throughout the Paleozoic.