Orogenic Core Research Articles

Supra-subduction thickening of a continental back-arc: Ediacaran–early Cambrian (Baikalian) metamorphism in the NE Baidrag block (Mongolian Collage)

In-situ monazite geochronology with P–T modelling of Barrovian-type metasediments was carried in the Burd Gol zone, NE Baidrag block. The P–T peak recorded by Grt−Sil migmatite is ∼780 °C, 7.5 kbar; by Grt–St–Ky gneiss ∼690 °C, 10 kbar and by Grt schist ∼590 °C, 8 kbar. Monazite ages related to metamorphic peaks indicate diachronous burial events, c. 570–565 Ma for the Grt–Sil migmatite, c. 565–560 Ma for the Grt–St–Ky gneiss, and at c. 538 Ma for the Grt schist. Tonian Grt–St schist recorded new monazite crystallization at c. 546–543 Ma. By c. 545–542 Ma, the Grt–Sil migmatite and Grt–St–Ky gneiss experienced exhumation to ∼5 kbar, while the Grt schist underwent burial. Further exhumation is dated by xenotime, rutile and monazite alteration down to c. 515 Ma. The data suggest a prolonged orogenic cycle initiated by burial of distal orogenic core to the depths of ∼35 km at c. 570–565 Ma while the proximal continental margin was buried at c. 545–538 Ma. The higher-grade core was subsequently exhumed to the middle crust at c. 550–540 Ma and the orogenic activity ceased at c. 515 Ma. The difference in metamorphic gradients around 570–560 Ma, with ∼27–28 °C/km for the Grt–Sil migmatite and ∼18–19 °C/km for the Grt–St–Ky gneiss suggests lateral variations as a result of localized juxtaposition of migmatite-magmatite lower-middle crust with colder regions. A new model shows the Burd Gol zone as a progressively thickened and exhumed continental back-arc system beneath the supracrustal 590–570 Ma continental back-arc represented by the adjacent Ulziit Gol unit. The data provide important constraint on the timing and mechanisms of orogenic Baikal cycle at the margin of the Siberian Craton.

The Grenville Province: revisiting the orogenic framework and integrating recent findings

The Grenville Province holds record of Mesoproterozoic orogenic processes at the Laurentian margin during the assembly of Rodinia. This contribution reviews key characteristics of the Province, integrates recently published data, and questions some earlier interpretations. Despite crustal reworking during the Grenvillian orogeny (1.09–0.98 Ga), contrasts between crustal-age domains across the Allochthon Boundary largely reflect differences between internal and external Laurentia, and allochthonous units were far-travelled in the west but not in the east. In addition, there is growing evidence towards a continuum between the Ottawan and Rigolet orogenic phases. Based on ages of metamorphism, the High-Pressure segments of the hinterland reflect localized thickening at the front of the Ottawan orogen and evidence for an orogenic plateau is limited to the western Grenville. Peak-metamorphic conditions in the Mid-P orogenic core, mostly within the confines of biotite dehydration melting (max ~900oC) for aluminous rocks, culminated in early Ottawan and were followed by protracted extension. In addition, high-temperature granitoid magmatism was active throughout the duration of the orogeny and localized in formerly pericratonic terranes accreted at ~1.4 Ga and 1.2–1.1 Ga, indicating an influence of inherited lithospheric structures on Grenvillian mantle dynamics. The overall orogenic pattern reveals considerable lateral variations in the hinterland potentially linked to the earlier configuration of external Lautentia and is consistent with weak lithosphere supporting a broad, hot, but not necessarily high-standing orogen for most of the Ottawan, in contrast to the Rigolet phase that marks the propagation of the orogen against Archean lithosphere of internal Laurentia.

Pan-African granitic magmatism of the Kaoko Belt: Tectonic perspective from its South American connection and insights into the crustal architecture of SW Gondwana

The Kaoko Belt in northwestern Namibia formed during the assembly of Gondwana in the Neoproterozoic to Cambrian Pan-African orogenic cycle. The belt has a complex tectonic architecture, in which a variety of basement units and metasedimentary associations became juxtaposed along shear zones and experienced widespread intrusive magmatism, generally confined to its western domain. We present a large new dataset of whole-rock geochemical and isotope data, as well as U-Pb zircon ages, encompassing a wide variety of granitoids and subordinate basic rocks across the Kaoko Belt. This is complemented by whole-rock isotope analyses from various basement and metasedimentary units and by a compilation of data from the literature. The new dataset strengthens previous models that correlate the granitoids in the Kaoko Belt to the coeval Florianópolis Batholith in the South American Dom Feliciano Belt, which we use to shed new light on the tectonic organization and evolution of the Pan-African orogenic cycle in the Neoproterozoic. The similarities in the compositional range of granitoids from both sub-domains of the Western Kaoko Belt, namely the Orogen Core and the Coastal Terrane, suggest that the entire zone comprises a heterogeneous but comparable and consistently diverse association of rocks. The comparison to South America underscores that, while the granitoids have “crust-like” isotopic signatures, these are in sharp contrast with those of the mostly Archean to Paleoproterozoic basement rocks of both the Kaoko and Dom Felicano belts. This suggests that the magmatic sources of the intrusive magmatism of the Kaoko Belt are likely a mixture of autochthonous crustal melting with an important mantle-derived input at some point after the Paleoproterozoic. This input was probably driven by accretionary orogeny and/or rifting that took place in the Tonian, Ediacaran, or both. The presence of more juvenile associations in the western domain of the Kaoko Belt may indicate an isotopic boundary within the belt that mirrors the tectonic configuration of the Dom Feliciano Belt in South America. This isotopic border, which would correspond to the Puros Shear Zone, could have acted as a terrane boundary from the earliest stages of the tectonic evolution of the Kaoko Belt.

Late-orogenic potassic to ultrapotassic dykes from the central Grenvillian hinterland: Trace element and Sr-Nd-Pb-O isotopic perspective

New whole-rock Sr-Nd-Pb-O isotope data are combined with previously published results for Sr and Nd isotopes and major and trace element chemistry to examine the petrogenetic and tectonic environments of a suite of ca. 980 Ma potassic to ultrapotassic dykes (PUD) from the Canyon domain, central Grenville Province in Canada. The PUD suite can be divided into two geochemically distinct groups that exhibit characteristic source mineralogy, depth of partial melting, and multi-stage metasomatism of heterogeneous mantle sources. Major and trace element data suggest involvement of both fertile and depleted sources involving spinel and garnet peridotite, while trace element and isotope data indicate long-term LREE and LILE enrichments and HFSE depletions in ancient subduction settings. Low time-integrated Rb/Sr, Sm/Nd, and U/Pb ratios suggest enriched mantle EM-I metasomatized by ancient subduction of continental upper crust and/or carbonate sediments. A latest phase of fluid/melt metasomatism is attributed to a late-Grenvillian intracontinental subduction and limited lithospheric extension.The late-Grenvillian tectonic setting is proposed here to be characterized by shallow-angle underthrusting of the foreland beneath the Grenvillian hinterland, where SCLM, initially of Proterozoic age and eventually of Late Archean age, became transported beneath the orogenic core. The intracontinental collision was short-lived and probably terminated because of buoyancy constraints related to the refractory character of the subducted Archean lithospheric mantle.

Multiple Palaeozoic–Mesozoic orogenic events in the SE Yangtze Block, China: evidence from the petrogenesis and deformation of gneissic granites from the Nanwenhe metamorphic dome and regional correlation analysis

ABSTRACT The Nanwenhe metamorphic dome contains a rare Silurian granitic pluton that developed in the SE Yangtze Block during early Palaeozoic orogenesis, and this pluton comprises the Nanwenhe gneissic granites. The petrogenetic and structural deformation histories of the gneissic granites recorded the multi-stage interaction of the SE Yangtze Block with other blocks during the Palaeozoic and Mesozoic, including the Cathaysia, Indochina, and Paleo-Pacific blocks. Zircon U–Pb and Hf isotopes and geochemical data indicate that the Early Palaeozoic granites (433–420 Ma) are S-type granitic rocks, and they were derived from the partial melting of ancient continental crust with no contribution from mantle material. And the large variations and positive values of εHf(t) (−12.25 to +10.69) of the gneissic granites are ascribed to a heterogeneous source and disequilibrium melting. Here we compare early Palaeozoic granitoids in the SE Yangtze Block (foreland belt) with those in the orogenic core, and suggest that the SE Yangtze Block underwent limited syn-collisional crustal thickening and metamorphism, and following which post-collisional granitoids were formed through partial melting of continental crust. Structural analysis of the Nanwenhe gneissic granites allows two stages of deformation to be identified: northward detachment (D1) and northwestward thrusting (D2). 40Ar–39Ar isotope dating of muscovites from the gneissic granites yields a well-defined plateau age of 229.61 Ma, which is interpreted as the timing of Late Triassic structural overprinting associated with the formation of the Nanwenhe metamorphic dome. Integration of our new results with previous research findings from the dome and South China allows us to conclude that the detachment deformation (D1) represents Late Triassic post-collisional extension, and D2 represented Jurassic intracontinental orogeny associated with subduction of the Paleo-Pacific Block.

Vertical deformation partitioning across a collapsing large and hot orogen

AbstractFeaturing 3 000‐km‐long large and hot orogen, the Mantiqueira Province provides a rare opportunity to study the process of gravitational collapse at mid to deep crustal levels. Distinct but contemporary (~500 Ma) post‐collisional intrusions show structures and anisotropy of magnetic susceptibility (AMS) fabrics related to their emplacements, recording different flow patterns. In southern deep‐seated intrusions, ellipsoidal‐shaped roots with gabbroic‐to‐hybrid cores surrounded by granitic rocks show concentric patterns of AMS fabrics that cut across the NE‐trending regional foliation. In contrast, northern intrusions, exposed as the upper sections of batholith‐size bodies of coarse‐grained granite emplaced at the shallow to mid‐crust, show general NS‐trending magnetic fabrics roughly parallel to strike of the orogen and the regional foliation of host rocks. These contrasting magnetic patterns from shallow to deeper crust suggest vertical magma migration from the overthickened orogenic core to be emplaced across its thinner stretched flanks during the gravitational collapse of the orogenic edifice.

Hydration, melt production and rheological weakening within an intracontinental gneiss dome

The Entia Gneiss Complex (EGC) in central Australia represents a deeply exhumed Paleoproterozoic basement terrane that underwent fluid-catalysed transformation culminating in the formation of a migmatitic gneiss dome during the 450–300 Ma intracontinental Alice Springs Orogeny (ASO). Foremost amongst the changes the EGC experienced during this event was the regional-scale conversion of nominally anhydrous granulite and igneous crust to hydrous amphibolite facies migmatite, together with the emplacement of locally voluminous mica-bearing pegmatite swarms. LA–ICP–MS U–Pb dating of zircon, monazite and xenotime demonstrates that pegmatite emplacement dominantly occurred late in the Alice Springs Orogeny (c. 360–300 Ma). The oldest generation of pegmatites are transposed into the regional Palaeozoic gneissic fabric and formed at c. 1770 Ma, indicating they belong to protolith rocks of the EGC. Three subsequent generations of pegmatite have ages that range between 360 and 300 Ma. The oldest of these is parallel to the migmatitic gneissic fabric and dated at c. 360 Ma, whereas the younger two generations are largely undeformed, discordant and intruded throughout the period c. 350–300 Ma. The capability for a comparatively dry intracratonic orogenic core to repeatedly produce melt over an interval of c. 60 Myr requires explanation. We suggest crustal-scale duplex development during the latter stages of the ASO resulted in the juxtaposition of dry and mechanically strong Paleoproterozoic basement over deeply buried Neoproterozoic–Cambrian sedimentary sequences. Fluid derived from the metamorphic dehydration of these sequences led to hydration, partial melting and rheological weakening of the basement slab. Prior to its hydration at c. 360 Ma, the EGC was metamorphically unresponsive, accounting for its comparatively late timing of deformation and partial melting compared to the overall history of the Alice Springs Orogeny. The development of the crustal-scale duplex system, combined with hydrous melting of the low-density basement slab beneath high-density supracrustal rocks, resulted in a thermomechanically unstable architecture that facilitated the formation of an orogen-scale gneiss dome during the waning stages of intracontinental orogenesis.

Timescales of Partial Melting and Melt Crystallization in the Eastern Himalayan Orogen: Insights From Zircon Petrochronology

AbstractRevealing the timescales of metamorphic and anatectic processes is central to our understanding of tectonic evolution of collisional orogens. High‐temperature migmatites and leucogranites are well exposed in the Himalayan orogenic core, making it an ideal region to study the timing and duration of partial melting and melt crystallization of the orogen. Here, we report an integrated and comprehensive data set of petrography, U‐Pb age, and trace element data for zircon from a pelitic granulite and associated leucosomes of the Greater Himalayan Sequence (GHS) in the Yadong area, eastern Himalaya. Zircon grains with complex internal structure retain variable ages ranging from 32 Ma to 13 Ma that correlate systematically with changes in the concentrations of Y, Th, U, Hf, Nb, Ta, and HREE, and ratios of Th/U, Eu/Eu*, and Nb/Ta. Combined with petrologic analysis, we conclude that the granulite witnessed high‐temperature metamorphism, melting, and melt crystallization over ∼20 Myr. Prograde, simultaneous increases in pressure and temperature and associated dehydration melting began at least by ∼32 Ma and lasted until ∼24 Ma. Subsequent quasi‐isothermal decompression‐melting occurred between ∼22 and 19 Ma, and late melt crystallization spanned ∼19 to 13 Ma. Large volumes of melt generated during prograde metamorphism could have triggered exhumation of GHS rocks, increasing melt fraction through a positive feedback between exhumation and melting. More comprehensive analysis of different rock types led to more complete and different interpretations for the timing of exhumation and melt crystallization in the Yadong‐Sikkim region and might enable alternate interpretations elsewhere in the Himalayas.

Growth of Collisional Orogens From Small and Cold to Large and Hot—Inferences From Geodynamic Models

AbstractIt is well documented that the interplay between crustal thickening and surface processes determines growth of continent‐continent collision orogens from small and cold to large and hot. Additionally, studies have demonstrated that the structural style of a mountain belt is strongly influenced by inherited (extensional) structures, the pattern of erosion and deposition, as well as the distribution of shallow detachment horizons. However, the factors controlling distribution of shortening and variable structural style as a function of convergence and surface process efficiency remain less explored. We use a 2D upper‐mantle scale plane‐strain thermo‐mechanical model (FANTOM) coupled to a planform, mass conserving surface‐process model (Fastscape), to investigate the long‐term evolution of mountain belts and the influence of lithospheric pull, extensional inheritance, surface processes efficiency, and decoupling between thin‐and thick‐skinned tectonics. We establish an evolutionary shortening distribution for orogenic growth from a mono‐vergent wedge to an orogenic plateau, and find that internal crustal loading is the main factor controlling the large scale evolution, while lithospheric pull modulates the plate driving force for orogenesis. Limited foreland‐basin filling and minor exhumation of the orogen core are characteristic for low surface‐process efficiency, while thick foreland‐basin fill, and profound exhumation of the orogen core are characteristic for high surface‐process efficiency. Utilizing a force balance analysis, we show how inherited structures, surface processes, and decoupling between thin‐and thick‐skinned deformation influence structural style during orogenic growth. Finally, we present a comparison of our generic modeling results with natural systems, with a particular focus on the Pyrenees, Alps, and Himalaya‐Tibet.

Rapid cooling during late-stage orogenesis and implications for the collapse of the Scandian retrowedge, northern Scotland

New 40 Ar/ 39 Ar thermochronological and deformation temperature analyses in the Scandian ( c. 435–420 Ma) orogenic retrowedge of northern Scotland demonstrate accelerated cooling during late syn- to post-orogenic exhumation of the high-grade orogenic core. Initial cooling rates of 10–30°C myr −1 immediately following peak orogenesis transitioned to rapid rates of 45–90°C myr −1 during final exhumation of the Naver thrust sheet in the orogenic core. The flanking ductile thrust sheets exhibit a similar, albeit less pronounced, acceleration of cooling, with rates increasing by c . 150–300% following peak orogenesis. Closer to the foreland, the Moine thrust sheet did not experience increased cooling rates. Calculated unroofing rates of 3.75 mm a −1 in the high-grade Naver thrust sheet suggest increasing, rapid exhumation in the orogenic core during a presumed collapse phase of orogenesis. This is contrary to the expectation of decreasing erosional efficiency as topography is diminished and is interpreted to suggest that unroofing of the Scottish Caledonides may have been partially enhanced by upper crustal extensional deformation during ductile flow of the infrastructure of the orogenic core. Similar processes have been interpreted in the East Greenland Caledonides, which form the northern extension of the Scandian retrowedge. Supplementary material: 40 Ar/ 39 Ar analytical data for muscovite (Supplementary Data Table 1), 40 Ar/ 39 Ar analytical data for amphibole (Supplementary Data Table 2), and electron microprobe analytical data for amphibole samples (Supplementary Data Table 3) is available at: https://doi.org/10.6084/m9.figshare.c.5087057

Cretaceous exhumation of the Triassic intracontinental Xuefengshan Belt: Delayed unroofing of an orogenic plateau across the South China Block?

A large plateau can be produced by crustal thickening in convergent zones such as continental collision belts and Andean-type subduction zones, but the life cycles of such plateaux are not well-understood. In particular, it is not clear how long they persist after construction, before other tectonic processes or erosion reduce crustal thickness and elevation to near-normal levels. Triassic subduction- and collision-tectonics produced intense deformation, magmatism and metamorphism across the entire South China Block. This large-scale crustal shortening created a broad orogenic belt, uplifted most parts of the South China Block, and probably initiated the growth of an orogenic plateau. Our study presents low-temperature thermochronology data from the Xuefengshan Belt in the interior of the South China Block. There was along-strike variation in exhumation. The north orogenic core was subjected to Triassic (~245–210 Ma), and Late Cretaceous (~100–80 Ma) exhumation, whereas the cooling path of the south orogenic core reflects a two stage Cretaceous evolution. The variable exhumation pattern reflects non-uniform tectonics in different regions, but both regions were subject to Late Cretaceous extension. We tentatively reconstruct the original plateau paleo-elevation to be ~1.5 km above sea level, based on the amount of exhumation (~10 km) and the present crustal thickness (~35 km). The T-t trajectories of the Xuefengshan Belt and other Triassic belts highlight the significance of Cretaceous extension and exhumation in shaping the tectonic configuration of the South China Block. Large-scale extension was probably triggered by rollback of the Paleo-Pacific subduction zone.

Major lithologies of the high-grade Zhoutan terrane within the Cathaysia Block and their tectonic implications for the Neoproterozoic - Paleozoic South China

Pre-Devonian crystalline basement of the Cathaysia Block was united through the Wuyi-Yunkai orogeny. The uplifted orogenic core is dominantly composed of Paleozoic high-grade terranes occurring in a northeast-southwest trending belt. The Zhoutan terrane in the middle part of this belt is one of the key terranes to reveal the tectonic architecture of the South China Block. However, how to correlate it with other high-grade terranes, what kind of rock records are preserved in this terrane and their tectonic implications for the Wuyi-Yunkai orogen are still unresolved. This paper carries out an integrated study on the petrology, zircon U--Pb geochronology, geochemistry and zircon Lu--Hf isotopic compositions of the main rock types in the Zhoutan terrane, with a purpose of better understanding the Wuyi-Yunkai orogeny. Mafic granulite (noritic) of high-T and low-P conditions occurs as lenses within the Zhoutan Formation. The zircon U--Pb geochronological data give magmatic and metamorphic ages of 460–450 Ma and ~ 430 Ma, respectively. Neoproterozoic and Paleoproterozoic (1.89 Ga) xenocrysts also occur. Meta-pelites of the Zhoutan Formation experienced high T/P amphibolite facies metamorphism, and ages of their detrital zircons are dominantly Neoproterozoic (810–820 Ma) with some subordinate Paleo- to Mesoproterozoic and Archean signatures. Amphibole gneiss layers of this formation give a Paleozoic metamorphic age of ~450 Ma, consistent with the high-P amphibolite facies metamorphism of the neighboring Longyou terrane. Fine-grained garnet mica-schist of the Wanyuan Formation consists of greenschist to amphibolite facies mineral assemblage and detrital zircons of these meta-sediments display both Neoproterozoic and early Precambrian ages at ~770 Ma, ~820 Ma, ~850 Ma, ~1.84 Ga and 2.48 Ga. Metamorphosed calc-silicates of the Wanyuan Formation show similar detrital zircon age peaks of 624 Ma, 780–790 Ma, ~810 Ma, 840–850 Ma, 1.8–1.98 Ga and ~ 2.48 Ga, and a Paleozoic metamorphic age of ~447 Ma. The Cizhu pluton is a composite granitoid body intruding the Zhoutan and Wanyuan formations, with emplacement ages ranging at least from 485 Ma to 446 Ma. εHf(t) values of zircons from the mafic granulite range from −10.9 to +8.5, indicating its protolith derived from enriched mantle sources with or without crustal contaminations. The Cizhu magmatic event is a crustal reworking event as indicated by the coherently negative zircon εHf(t) values and ancient Hf crustal model ages of 1530–1830 Ma. Zircon εHf(t) values of the metamorphosed calc-silicates show large variations, ranging from −21.2 to +11, consistent with its complex provenances. The deposition of the sedimentary rocks of the Zhoutan and Wanyuan formations is consistent with the development of the Neoproterozoic-Paleozoic Nanhua Rift. The development of continental margin sequences like the Zhoutan and Wanyuan formations, the evolution of the Paleozoic metamorphism from early higher P/T metamorphic stage to later lower P/T stage, as well as the generation of the Paleozoic mafic-ultramafic rocks and the intermediate Cizhu pluton, are best explained by collision to post-collision settings. We, therefore, conclude that the Zhuotan terrane (as well as other high-grade terranes) witnessed a whole geological cycle, from early rifting to the closure of a rifted basin (Nanhua Basin), and the final lithospheric delamination following the rejoining of continental blocks flanking the rift.

Prolonged Partial Melting of Garnet Amphibolite from the Eastern Himalayan Syntaxis: Implications for the Tectonic Evolution of Large Hot Orogens

AbstractThe Himalayan orogen, which formed due to collision of the Indian and Asian continents during the Early Tertiary, is a prime example of a large, hot collisional orogen. Despite decades of study, the duration of partial melting of migmatitic rocks exposed in the Himalayan orogenic core remains highly controversial. As such, we have performed detailed petrological and geochronological analyses of garnet amphibolite from the eastern Himalayan syntaxis in order to reveal the thermal conditions, tectonometamorphic mechanisms, and timing and duration of anatexis in metabasic rocks that form a major component of the thickened lower crust of the eastern Himalayan orogen. Phase equilibrium modeling and geothermobarometry show that the studied sample underwent high‐pressure granulite‐facies metamorphism and partial melting at 15−17 kbar and 805−840°C. Dehydration melting of amphibole during prograde metamorphism generated up to 20 vol. % partial melt with a granitic composition, which thus represents a potential source for Himalayan syn‐ to post‐orogenic crustal‐derived granites. Zircon U‐Pb geochronology shows that the garnet amphibolite witnessed a long‐lived anatectic and melt crystallization process lasting more than 30 Myr. Prolonged anatexis from ca. 40 to ca. 20 Ma predates initiation of the extrusion of Himalayan metamorphic core by up to 15 Myr, indicating that the thick and weak crust of the Himalayan‐Tibetan orogen must have remained stationary for this length of time, despite the ongoing continental collision. This study thus provides new insight into the tectonic evolution of hot orogens.

Three-dimensional strain accumulation and partitioning in an arcuate orogenic wedge: An example from the Himalaya

AbstractIn this study, we use published geologic maps and cross-sections to construct a three-dimensional geologic model of major shear zones that make up the Himalayan orogenic wedge. The model incorporates microseismicity, megathrust coupling, and various derivatives of the topography to address several questions regarding observed crustal strain patterns and how they are expressed in the landscape. These questions include: (1) How does vertical thickening vary along strike of the orogen? (2) What is the role of oblique convergence in contributing to along-strike thickness variations and the style of deformation? (3) How do variations in the coupling along the megathrust affect the overlying structural style? (4) Do lateral ramps exist along the megathrust? (5) What structural styles underlie and are possibly responsible for the generation of high-elevation, low-relief landscapes? Our model shows that the orogenic core of the western and central Himalaya displays significant along-strike variation in its thickness, from ∼25–26 km in the western Himalaya to ∼34–42 km in the central Himalaya. The thickness of the orogenic core changes abruptly across the western bounding shear zone of the Gurla Mandhata metamorphic core complex, demonstrating a change in the style of strain there. Pressure-temperature-time results indicate that the thickness of the orogenic core at 37 Ma is 17 km. Assuming this is constant along strike from 81°E to 85°E indicates that, the western and central Nepal Himalaya have been thickened by 0.5 and 1–1.5 times, respectively. West of Gurla Mandhata the orogenic core is significantly thinner and underlies a large 11,000 km2 Neogene basin (Zhada). A broad, thick orogenic core associated with thrust duplexing is collocated with an 8500 km2 high-elevation, low-relief surface in the Mugu-Dolpa region of west Nepal. We propose that these results can be explained by oblique convergence along a megathrust with an along-strike and down-dip heterogeneous coupling pattern influenced by frontal and oblique ramps along the megathrust.

Salt décollement and rift inheritance controls on crustal deformation in orogens

AbstractWe investigate the factors that control the shortening distribution and its evolution through time in orogenic belts using numerical models. We present self‐consistent high‐resolution numerical models that simulate the inversion of a rift to generate an upper crustal antiformal stack, a wide outer pro‐wedge fold‐and‐thrust belt, characterised by a two‐phase evolution with early symmetric inversion followed by formation of an asymmetric doubly‐vergent orogen. We show that a weak viscous salt décollement promotes gravitational collapse of the cover. When combined with efficient erosion of the orogenic core and sedimentation in adjacent forelands, it ensures the thick‐skinned pro‐wedge taper remains subcritical, promoting formation of an upper crustal antiformal stack. Rift inheritance promotes a two‐phase shortening distribution evolution regardless of the shallow structure and other factors. Comparison to the Pyrenees strongly suggests that this combination of factors led to a very similar evolution and structural style.

Thermal, rheological and kinematic conditions for channelized lower crustal flow in a threshold example

Lower crustal flow zones occur in large and hot orogens and rifts, where they occur in association with large areas possessing high gravitational potential energy (GPE) and low lower crust viscosity. Lower crustal flow zones are also known from regions where neither the rheological nor GPE conditions are sufficiently well developed for widespread flow to occur. For these examples, the conditions required to form crustal flow zones are less well defined. One such case occurs within the ca. 600–530 Ma Petermann Orogeny in central Australia. This work investigates the conditions under which this flow zone developed, considering differences with the adjacent regions where flow is not observed. A transect of crustal structure in the central Petermann Orogen is developed that takes into account geological and geophysical data, including time-constrained pressure and temperature estimates for the Petermann Orogeny. The deformation of this region is reconstructed, showing that lower crustal flow was driven by high amplitude crustal-scale folding. The folding extruded the lower crust towards the foreland, causing crustal thickening and buoyant uplift of the orogenic core. Channelized lower crustal flow is not known from either the eastern or western parts of the orogen, and a comparison with these regions suggests three main conditions are required: Firstly, the lower crust must be sufficiently hot and volatile-rich to allow melt-weakening. Secondly, crustal rheology must allow ductile deformation of the mid to lower crust without high-strength layers. Finally, a high degree of lateral confinement is required to form pressure-gradients capable of driving flow. These three conditions were met only in the central Petermann Orogen, and not in the east or the west. This example of a lower crustal flow zone illustrates the combined influence of thermal, rheological and kinematic threshold conditions on the initiation and development of channelized lower crustal flow in orogens.

Tectonometamorphic evolution of the tip of the Himalayan metamorphic core in the Jajarkot klippe, west Nepal

AbstractNew phase equilibrium modelling, combined with U–Th/Pb petrochronology on monazite and xenotime, and 40Ar/39Ar geochronology on white mica, reveal the style of deformation and metamorphism near the southern tip of the extruded Himalayan metamorphic core (HMC). In the Jajarkot klippe, west Nepal foreland, greenschist to lower amphibolite facies metamorphism is entirely constrained to the Cenozoic Himalayan orogeny, in contrast with findings from other foreland klippen in the central Himalaya. HMC rocks exposed in the Jajarkot klippe yield short‐lived, hairpin pressure–temperature–time–deformation paths that peaked at 550–600°C and 750–1,200 MPa at 25 Ma. The Main Central thrust (MCT) and the South Tibetan detachment (STD) bound the base and the top of the HMC, respectively, and were active simultaneously for at least part of their deformation history. The STD was active at c. 27–26 Ma and possibly as late as c. 19 Ma, while the MCT may have been active as early as 27 Ma and was still active at c. 22 Ma. The tectonometamorphic conditions in the Jajarkot klippe are characteristic of crustal thickening and footwall accretion of new material at the tip of the extruding metamorphic orogenic core. Our new results reveal that collisional processes active in the middle to late Miocene at the base of the HMC now exposed in the hinterland were also active earlier, during the Oligocene, at the tip of the southward‐extruding middle crust.

Greenschist Facies Metamorphic Zircon Overgrowths as a Constraint on Exhumation of the Brooks Range Metamorphic Core, Alaska

AbstractLike many other mountain belts, the metamorphic core or hinterland of the Brooks Range fold and thrust belt in Arctic Alaska is characterized by multiply deformed and polymetamorphosed rocks whose histories have been challenging to decipher and thus difficult to relate to the supracrustal history of the orogen. The multiple greenschist and blueschist facies metamorphic events have been particularly difficult to resolve. This study provides petrologic context for recently identified low‐temperature metamorphic zircon overgrowths at two localities across some 200 km of orogenic strike that offer a unique and precise constraint on the timing of events recorded in the Brooks Range hinterland. In consideration of microstructural context, graphite thermometry, metamorphic mineral inclusions, and zircon trace element and Lu‐Hf‐isotope data, metamorphic zircon growth at 114 ± 5 Ma in the southern Brooks Range is interpreted to coincide with greenschist facies metamorphism, most probably linked to decompression and/or increased temperatures within the orogenic core. Their age coincides with a proposed pulse of extension within a >600‐km shear zone along the southern flank of the Brooks Range and a regional flare‐up in magmatism and pronounced subsidence within hinterland depocenters (Yukon‐Koyukuk Basin). These regional events are consistent with subduction retreat/rollback in mid‐Cretaceous time. This study adds to a growing body of literature demonstrating the importance of searching for and characterizing metamorphic zircon growth in low‐ to medium‐grade metamorphic terranes to provide better constraints on otherwise cryptic tectonic events.

Strain analysis of the Xuefengshan Belt, South China: From internal strain variation to formation of the orogenic curvature

The Early Mesozoic Xuefengshan Belt, located in the central part of the South China Block, East Asia, developed as an intracontinental belt during the Middle Triassic. Analyses of structural and strain patterns in Neoproterozoic conglomerates constrain the internal deformation and three-dimensional kinematic evolution of this fold-and-thrust belt. Finite strain data (RXZ) range from 1.1 to 3.2 and generally increase eastwards towards the deep units of the orogenic core. Our results also show that at basal décollement zone the D1 schistosity-forming deformation has a gradient of stronger fabrics, higher strain ratios, and a dominant thrust-normal flattening mechanism. High tectonic levels exhibit lower strain ratios and later structural overprinting that locally modified the D1 strain patterns. Parallel to the belt, enhanced strain and tectonic uplifting along the northern and southern sections of the belt indicate an along-strike structural variation that corresponds to the formation of the orogenic curvature. During the thrusting propagation, large igneous intrusions escaped deformation due to their stiffness and facilitated strain accumulation on the peripheries, whereas stress in the central part migrated progressively and formed a tectonic salient towards the west without significant strain localization.

Petrogenesis of granitoids and associated xenoliths in the early Paleozoic Baoxu and Enping plutons, South China: Implications for the evolution of the Wuyi-Yunkai intracontinental orogen

The early Paleozoic Wuyi-Yunkai orogen was associated with extensive felsic magmatic activities and the orogenic core was mainly distributed in the Yunkai and Wugong domains located in the western Cathaysia block and in the Wuyi domain located in the central part of the Cathaysia block. In order to investigate the evolution of the Wuyi-Yunkai orogen, elemental and Sr-Nd isotopic analyses were performed for granites from the Baoxu pluton in the Yunkai domain and from the Enping pluton in the central part of the Cathaysia block. The Baoxu pluton consists of biotite granite with abundant xenoliths of gneissic granite, granodiorite and diorite, and the Enping pluton is mainly composed of massive granodiorite. Biotite granites (441 ± 5 Ma) and gneissic granite xenolith (443 ± 4 Ma) of the Baoxu pluton are all weakly peraluminous (A/CNK = 1.05–1.10). They show high Sr/Y and La/Yb ratios and have negative bulk-rock εNd(t) values (−7.0 to −4.4), which are similar to coeval gneissic S-type granites in the Yunkai domain and were probably derived from dehydration melting of a sedimentary source with garnet residue in the source. Granodiorites (429 ± 3 Ma) from Enping and granodiorite xenolith (442 ± 4 Ma) from Baoxu are metaluminous and have REE patterns with enriched light REE and flat middle to heavy REE, possibly generated by the dehydration melting of an igneous basement at middle to lower crustal level. Diorite xenolith from Baoxu is ultrapotassic (K2O = 4.9 wt%), has high contents of MgO (7.0 wt%), Cr (379 ppm) and Ni (171 ppm) and shows pronounced negative Nb, Ta and Ti anomalies. This xenolith also has negative εNd(t) value (−3.6) and low Rb/Ba and high Ba/Sr ratios, and is thus interpreted to be derived from an enriched lithospheric mantle with the breakdown of phlogopite. Early Paleozoic I- and S-type granites in the Wuyi-Yunkai orogen mostly have negative εNd(t) values and do not have juvenile components, consistent with genesis by an intracontinental orogenic event. These early Paleozoic granites occur near the ancient suture zone between the Yangtze and Cathaysia blocks and have high La/Yb and Sr/Y ratios, likely due to the existence of residual garnet in the source, suggesting the thickened crust at ca. 440 Ma. The 450–440 Ma gneissic S-type granites near the suture zone are earlier than those in the central part of the Cathaysia block (∼430 Ma). The crustal thickening along the ancient suture zone at 440 Ma propagated into the central part of the Cathaysia block as evidenced by the 430 Ma granites. Early Paleozoic I-type granites near the suture zone clearly show involvement of significant mantle-derived materials, in contrast to granites in the central part of the Cathaysia block. The ancient suture zone may have acted as channels for the emplacement of mafic magmas during the collapse of an intracontinental orogen.