Ancient Orogens Research Articles

The monazite record of (ultra)high-temperature metamorphism in southern Madagascar: Gondwana terrane correlations and structural trapping of hot middle crust

Ancient orogens eroded to midcrustal levels provide insight about strain accommodation, metamorphism, and melting in Himalaya-type continent-continent collisions. This study focuses on the Neoproterozoic–Cambrian Eastern-Africa / Kuunga orogen exposed in Madagascar, where uncertainty about the terrane correlations, and therefore structural framework, of the orogen persists. We present a comprehensive dataset of monazite petrochronology and thermobarometry across the southern Madagascar basement to quantify the regional and temporal variability of metamorphism. We argue that the ultrahigh-temperature Anosyen domain and associated Androyen domain have a shared geological history, recording two successive tectonic events at 630–600 Ma and 580–500 Ma. Other Madagascar domains record primarily the former (Vohibory domain to the west) or latter (all other domains to the northeast) event. From this inference, we discuss terrane correlations with Africa and India, then present a structural framework for the orogen in which the Anosyen–Androyen domain was structurally confined in a central, lithosphere-scale transpressional shear system between divergent, diachronous thrust belts. By limiting exhumation, extrusion, and collapse, the structural trapping of the Androyen–Anosyen domain facilitated longer-lasting, higher- T metamorphism than associated rocks in the adjacent nappe systems. Such structural trapping may be an important control on high- T metamorphism in the cores of Himalaya-type orogens in general.

Long-lived topography of the Oaxacan Complex (southern Mexico) after the assembly of Pangea: Evidence from apatite fission-track dating

Understanding the exhumation history of ancient orogens is crucial for unraveling the tectonic and climatic forces that shape Earth's landscapes. Relict topography from ancient orogenic belts can persist on the surface for tens of millions of years, providing detritus to surrounding areas even without significant uplift events. This study examines the exhumation history of the Oaxacan Complex in southern Mexico, a Proterozoic peri-Gondwanan continental block located at the periphery of western equatorial Pangea during the l ate Paleozoic. This complex supplied detritus to extensional basins in southern Mexico throughout the Mesozoic and Cenozoic. We reinterpret existing apatite fission track (AFT) data from in situ samples of the Oaxacan Complex and add a set of detrital AFT data from modern sediments of the Peñoles River, which drains the northern Oaxacan Complex. The detrital AFT age distribution is continuous, ranging from ∼180 Ma to ∼50 Ma, without a predominant component age and showing a positive age-elevation correlation, suggesting a slow cooling history of the basement exposed in the drained area. QTQt thermal history modeling of the detrital AFT ages corroborates this slow cooling. Moreover, reinterpretation of in-situ AFT data from the literature confirms that the cooling history of the crustal block exposed in the northern Oaxacan Complex is characterized by a prolonged residence time in the Partial Annealing Zone of the apatite fission-track thermochronometer, spanning at least from the Triassic to the Paleogene. These results suggest that the Oaxacan Complex did not undergo any significant exhumation events after the Late Paleozoic assembly of Pangea, neither during the Triassic–Early Cretaceous rifting associated with Pangea breakup nor during development of the Late Cretaceous–Eocene Mexican Orogen. According to this scenario, we suggest that the post-collision relict topography of the Oaxacan Complex was likely sustained by isostatic rebound and lithological resistance, supplying sediment for tens of millions of years to the Mesozoic and Cenozoic basins of southern Mexico.

Paleozoic Episodic Magmatism in Western Tianshan: Insight Into Assembling the Northeastern Pangea

AbstractThe late‐stage union of Pangea was associated with the convergence of Siberia with Laurussia, but the exact timing remains unclear. The orogenic duration of the Kazakhstan block can provide geochronological constraints as it connects Siberia, Baltica, and Tarim. Zircon petrochronology offers a reliable approach for ascertaining the lifespan of an ancient orogen. In this study, we explore three phases of magmatism recorded in detrital zircons from late Paleozoic and Mesozoic sandstone‐siltstones in Western Tianshan, that is, 400–470 Ma, 320–380 Ma, and 280–320 Ma. Based on their age‐propagated Hf isotopes, melt SiO2 contents, and crustal thicknesses, our findings suggest that the southern limb of Kazakhstan underwent the early Paleozoic amalgamation of microcontinents with arcs, the late Paleozoic maturation of an Andean‐like continental arc, and the late Carboniferous collision of Kazakhstan with the Junggar oceanic basin and the Tarim craton. Such characteristics manifest the long‐term orogenic progression of Kazakhstan. Combining published timelines and paleolatitudes of major orogens and blocks, we propose that the Kazakhstan block welded northeastern Pangea along with the cessation of these orogenic activities around it. Consequently, by docking Kazakhstan with surrounding cratons, the fundamental configuration of Pangea could have been established in the late Carboniferous.

Plate‐Scale Imaging of Eastern US Reveals Ancient and Ongoing Continental Deformation

AbstractEastern North America was constructed over several Wilson cycles, culminating in the breakup of Pangea. Previous seismological imaging lacked the resolution to depict precisely how ancient tectonic boundaries manifest throughout the lithosphere, how continental breakup modified the plate, or how ongoing mantle dynamics shapes the continental margin. We present a high‐resolution, plate‐scale seismic tomography model of the eastern US by combining an unprecedented suite of complementary data sets in a Bayesian framework. These data provide detailed resolution from crust to asthenosphere, identifying the base of the lithosphere and mid‐lithospheric discontinuities. The plate thins in steps that align with ancient orogens. The lithospheric step at the Appalachian front is associated with cells of mantle upwellings, likely edge‐driven convection, that erode the base of the plate and shape modern Appalachian topography. Low‐velocity structures in the lithospheric‐mantle align with the Grenville front and may be remnants of Rodinia assembly.

Subduction erosion revealed by exhumed lower arc crustal rocks in an accretionary complex, northeastern China

Abstract Subduction erosion at convergent margins is a leading mechanism for the destruction (recycling and reworking) of continental crust. But because of the lack of direct evidence, it is not straightforward to identify erosive events and their intensities in fossil subduction zones. The Heilongjiang accretionary complex in northeastern China was formed during the early Mesozoic subduction of the Paleo-Pacific Ocean. We investigated amphibolites from this accretionary complex, whose protoliths (based on whole-rock trace elements and Sr-Nd-Hf isotopes) were mafic continental arc magmatic rocks (255–249 Ma; zircon core U-Pb ages) from the upper plate. Phase equilibria modeling constrained by mineral geochemistry indicates that the amphibolites and their wall rocks were first heated to low granulite facies (750–800 °C, ~7 kbar) at 251–244 Ma (zircon rim U-Pb ages) and then cooled to ~700 °C with increasing pressure (8–9 kbar) before 213–187 Ma (titanite and apatite U-Pb ages). To explain the occurrence of the lower arc crustal lithologies in the accretionary complex and their metamorphic history, we propose that the subducting plate strongly eroded the forearc crust, allowing the plate interface to advance landward and scrape the amphibolites and wall rocks formed under the old arc, which finally were exhumed along the subduction channel and became components of the complex. The case study exemplifies direct petrological evidence of strong subduction erosion occurring in an ancient orogen, thus implying that consumption of the entire forearc crust could occur within only ~50 m.y.

The myth of the Highland Cretaceous revealed by the art of palaeogeographic mapping

Abstract Since the term was coined in 1873, palaeogeography has provided a powerful map-based tool for synthesizing, visualizing, and testing diverse regional data and hypotheses, but there are problems in deducing the palaeogeography of areas with few data points. As an example, the Scottish Highlands are better mapped and more studied than almost any other ancient orogen, yet questions remain on the persistence of topography since the Caledonian orogeny and the possibility of Cretaceous marine inundation. This paper reviews the data available on Scottish Cretaceous outcrops using published geological maps and memoirs to extract data: outcrop length, stratigraphic thickness and component units, elevation range of the outcrop and subcrop formations. All Cretaceous rocks in western Scotland were deposited in shallow marine and shoreline environments; deposits thin northwards and appear to be close to a feather edge. Elevation data are used to construct a Cretaceous surface which intersects present-day topography at low to intermediate altitudes. It is concluded that the Scottish Highlands were largely emergent during the Late Cretaceous, confirming previous palaeogeographic studies.

Records of the accretionary, collisional and post-collisional evolution of western Gondwana in the high-grade core of the Araçuaí-Ribeira orogenic system, SE Brazil

High-grade metamorphism, extensive partial melting, and polyphase deformation often obliterate records of pre-collisional stages in ancient orogens, making it hard to unravel Precambrian accretionary events. In western Gondwana, the high-grade core of the Araçuaí-Ribeira Orogenic System (AROS) comprises pre-collisional rock-assemblages largely disrupted, overprinted, and remelted by collisional and post-collisional tectono-metamorphic and igneous events, hampering reconstructions of its accretionary history. Detailed studies reveled remnants of an Early Tonian (860–840 Ma) island arc, forming km-size roof-pendants and mega-xenoliths within the composite Caxixe batholith, in the boundary zone between the Araçuaí and Ribeira orogens. We further characterize the Caxixe batholith and surrounding plutons, showing that the main stage of magmatism is composed of pre-collisional orthogneisses, partially migmatized and crystallized between ca. 607–580 Ma, thus, much later than the Tonian island arc records. Those Ediacaran granitic rocks represent intrusions of calc-alkaline magmas with elemental geochemistry and isotopic (εHf(t): −6 to −10, whole-rock εNd(t): −4.9 to −9.5, and 87Sr/86Sr(i): 0.706–0.711) data correlated with the continental-margin Rio Doce arc. This contrasts with signatures indicative of a mantle source εHf(t) = +10 to + 14, εNd(t) = +0.9 to + 6.4, and 87Sr/86Sr(i) = 0.698–0.704) of the Early Tonian arc remnants. Intruding the pre-collisional granitic plutons, garnet-bearing leucogranites dated at ca. 575 Ma and a mafic intrusion of ca. 560 Ma mark the transition from the syn-collisional to late-collisional stages. Post-collisional, non-deformed, A-type granites with the most evolved isotopic signatures (εHf(t): −22; εNd(t): −5.6 to −13.2) intruded at ca. 500 Ma, ending the long-lasting orogenic history documented in the region. Our results demonstrate that a basement composed of an early Tonian juvenile arc docked with a continental margin and hosted several intrusions of an early Ediacaran continental-margin magmatic arc that, in turn, were intruded by collisional and post-collisional magmas from the Late Ediacaran to Cambrian, finally forming the main plutonic masses of the Araçuaí-Ribeira Orogenic System (AROS) core, during the complex history of western Gondwana assembly.

The consummate geoscientist: a celebration of the career of Maarten de Wit

Abstract This volume assembles a collection of papers on the application of plate-tectonic principles to the understanding of ancient orogens, in honour of Maarten de Wit, who was guided throughout his research career by such applications. There are several recurring themes including: (1) collisional orogenesis associated with supercontinent amalgamation; (2) accretionary orogenesis along supercontinent peripheries; and (3) intracontinental deformation as a consequence of stresses transmitted thousands of kilometres into the orogenic foreland. Very appropriately, given Maarten's huge influence on international cooperation among Earth scientists, 70 authors, from 15 different countries, from every continent except Antarctica, have contributed to the volume.

Effect of rheological heterogeneities on the lithospheric deformation of the Tibetan Plateau and neighbouring regions

The Tibetan Plateau, induced by the India-Eurasian collision, has the highest average altitude in the world. During its uplift vertically, the Tibetan Plateau has been considered to expand laterally. However, there are several strong and almost non-deformable cratons on its periphery, such as the Tarim, North China craton, and South China block. The present landform features show that these cratons limit the expansion of the Tibetan Plateau. However, there is still much controversy over whether the deformation can be transmitted to periphery orogens or reactivate ancient orogens in the cratons. This study used numerical models to investigate the effect of rheological heterogeneities on the lithospheric deformation of the Tibetan Plateau and its neighbouring regions. The results show that the lateral heterogeneities of the lithosphere have an important influence on the deformation or strain partitioning. Generally, during the lateral expansion of the Tibetan Plateau, its peripheral cratons can transmit the deformation or high strain to neighbouring weak orogens. This case can be used to understand the Tian Shan orogen, which was reactivated by the India-Eurasian collision. However, when the orogens inside the cratons have high lithospheric strength, high strain is difficult to distribute on them and the expanding Tibetan Plateau is constrained by its peripheral cratons. These results can be used to explain the ancient orogens that are not strongly deformed, such as the Jiangnan orogen in the South China block. Because these orogens formed at the same time as the cratons and have relatively high lithospheric strength. In addition, the large lithospheric thickness difference and low crustal rheological contrast favor high strain rates localized on the lithosphere of the ancient orogen in the craton, such as the Trans-North China orogen in the North China craton.

Juxtaposition of different-grade metamorphic rocks in an ancient orogen: Evidence from the Chengde Complex of the Trans-North China Orogen, North China Craton

Juxtaposition of different-grade metamorphic slices is a typical feature of Phanerozoic orogens but is relatively scarce in Precambrian orogens. Here, we focus on the Chengde Complex at the northern segment of the late Paleoproterozoic Trans-North China Orogen, the North China Craton as a case, to explore how different-grade orogeny-related metamorphism was juxtaposed in Precambrian orogens. The Trans-North China Orogen is a typical Precambrian collisional orogen and records abundant information about the Paleoproterozoic orogeny. High-pressure (HP) mafic granulite and two types of amphibolite samples were collected from the Chengde Complex, and we conducted a combined study involving litho-structural assemblage investigation, metamorphic petrology, and geochronology to decipher their metamorphic P-T−t history. HP mafic granulite and amphibolite occur as enclaves or dikes within felsic gneisses. HP mafic granulite records clockwise P-T paths with isothermal decompression (ITD) segments, and the peak metamorphic P-T conditions are 13.0−14.9 kbar/790−830 °C. Peak metamorphic P-T conditions retrieved from garnet-bearing amphibolite are 8.3 kbar/675 °C, whereas peak metamorphic P-T conditions retrieved from garnet-free amphibolite are 4.0−5.5 kbar/500−510 °C. Zircon U-Pb dating of HP granulite and amphibolite yield different prograde (1914−1871 Ma), peak (1869−1816 Ma), and retrograde (ca. 1830 Ma) metamorphic ages, and all of these data indicate that these metamorphic rocks were diachronously transferred to different depths in the subduction channel and subsequently exhumed at shallower crustal levels. Therefore, we conclude that the Chengde Complex is composed of imbricate rocks with different metamorphic grades and ages, and such an unordered juxtaposition of diverse metamorphism could also be a typical feature of Precambrian orogens.

Continental growth during migrating arc magmatism and terrane accretion at Sikhote-Alin (Russian Far East) and adjacent northeast Asia

Net additions to the continental landmasses, or continental growth, is a process that has irrevocably shaped our environment throughout the Earth history. Studies of ancient orogens suggest that continental growth can be facilitated by plate tectonics-driven arc magmatism and material accretion along convergent margins. However, such details are not well-understood due to the geological complexity of ancient orogens. Specifically, the role of the magmatic arc and its contributions to growing an accretionary margin remains unclear. This study explores an under-studied, long-lived magmatic arc along a 1500 km-wide swath of the NE Asian continental margin between 42° to 52°N (i.e., Sikhote-Alin and adjacent NE China, Sakhalin, and Hokkaido). We present new igneous rock zircon U-Pb ages (n = 93) and geochemistry (n = 61) from Sikhote-Alin that complement limited published data (n > 60). We synthesize these data to reveal four episodes of arc magmatism at Sikhote-Alin, including (1) 134 to 120 Ma S-type, (2) 110 to 100 Ma I-type, (3) 95 to 52 Ma highly-evolved I-type, and (4) <46 Ma I-type. We then combine our new results with published data (n > 600) and geology from adjacent NE Asia to analyze overall magmatic trends during a ∼ 1500 km trenchward migration of arc magmatism driven by Jurassic to Cenozoic juvenile terrane accretion. The young, growing continental margin was modified by arc magmatism with chemical compositions determined by: (1) generation from a mantle wedge mixed with 1.3– 1.4% sediment component from the downgoing slab; and, (2) silica-enrichment from crustal magmatic differentiation. Here, convergent margin continental growth that involved crustal accretion and trenchward migrating arc magmatism show magma compositions that evolve from initial mantle source mixing followed by crustal differentiation.

Why are the Appalachians high? New insights from detrital apatite laser ablation (U-Th-Sm)/He dating

The timing of surface uplift across the Appalachian Mountains, Eastern USA, remains a controversial topic. Thermal history modelling, geomorphological analysis, stratigraphic analysis, and river profile analysis have all suggested the region experienced surface uplift during the late Cenozoic. This is in contrast with many low-temperature thermochronology studies, which consistently present Mesozoic dates and typically infer the landscape to be the remnant of a much older mountain belt that has been slowly exhumed following the end of rifting in the Late Triassic-Early Jurassic. Resolving a regional exhumation history of the Appalachians through traditional low-temperature thermochronology methods is difficult due to the spatial constraints of bedrock sampling, while traditional detrital river sampling has limitations underpinned by analytical issues and grain selection bias. We aim to resolve the exhumation histories of large areas across the Appalachians using detrital laser ablation apatite (U-Th-Sm)/He dating (LA-AHe). LA-AHe is a new dating approach that produces a high number of apatite (U-Th-Sm)/He ages from modern river sands to infer the low-temperature (70–40°C) exhumation history of the bedrock across a catchment. Supplementary apatite and zircon U-Pb dates were obtained to study sediment transport in each catchment and their respective bedrock geological histories. LA-AHe results show most dates to be Cretaceous (69%), with only a small Cenozoic fraction (6%), while thermo-kinematic modelling of data from four catchments can be explained by protracted Mesozoic-Cenozoic exhumation with rates ∼31–20 m/Myr. The lack of Cenozoic dates does imply the magnitude of exhumation has not been significant enough to greatly affect the apatite (U-Th-Sm)/He system, though models incorporating enhanced late Cenozoic exhumation between 2.1–1.4 km are consistent with data from three catchments. These results have implications for the study of elevated topography across passive margins and ancient orogens and highlight how laser ablation apatite (U-Th-Sm)/He dating can be effectively used to study the change of landscapes over geological timescales.

Lithological control on topographic relief evolution in a slow tectonic setting (Anti-Atlas, Morocco)

Topographic relief results from the complex interactions between tectonics and erosional surface processes, which are primarily mediated by bedrock erodibility and climatic conditions. Ancient orogens offer a favourable setting to isolate the contribution of lithology, as tectonically driven rock uplift is typically negligible and rock strength variability can exert a critical role on the evolution of the topography. The Anti-Atlas in NW Africa is a late Paleozoic orogen comprising a well-preserved, elevated, relict landscape delimited by non-lithological knickpoints, that was uplifted during a regional late Cenozoic phase of topographic rejuvenation. Here, we combine a geomorphic analysis with 10Be-derived denudation rates to quantify bedrock erodibility and get insight into the surface evolution of the Anti-Atlas and the adjacent Siroua Massif. Specifically, we show that 10Be basin-wide denudation rates for the relict landscape are rather uniform and range from 5 to 12 m/Myr. These rates agree with long-term rates estimated from different methods suggesting that the relict topography archives erosional quasi-erosional steady-state conditions at least since the latest Cretaceous. The uniformly low 10Be denudation rates in the relict topography are consistent despite the variability in channel steepness and topographic relief that correlates with changes in rock type. The expansion of this analysis to the denudation rates of the downstream portion of the landscape, allows to demonstrate a linear relationship between denudation and channel steepness for catchments draining quartz bearing lithologies. This provides the chance to constrain a narrow range of bedrock erodibility values for different rock-types (quartzite, granitic and sedimentary rocks). These values are comparable with estimates from other slowly deforming settings. Specifically, our compilation from tectonically inactive to slow tectonic regions indicates that bedrock erodibility does not change significantly across different climatic zones and precipitation regimes. This highlights the critical role of lithology in controlling the production of topographic relief in post-orogenic/slow tectonic settings. Finally, we calculate the predicted denudation rates for the steeper portions of the landscape that adjusted to the new uplift rates based on the linear correlation between erosion rates and normalized steepness indices. These rates range from 20 to 50 m/Myr and agree with the direct measurements from two catchments.

Chemical Mohometry: Assessing Crustal Thickness of Ancient Orogens Using Geochemical and Isotopic Data.

Convergent plate boundaries are key sites for continental crustal formation and recycling. Quantifying the evolution of crustal thickness and paleoelevation along ancient convergent margins represents a major goal in orogenic system analyses. Chemical and in some cases isotopic compositions of igneous rocks formed in modern supra-subduction arcs and collisional belts are sensitive to Moho depths at the location of magmatism, implying that igneous suites from fossil orogens carry information about crustal thickness from the time they formed. Several whole-rock chemical parameters correlate with crustal thickness, some of which were calibrated to serve as "mohometers," that is, quantitative proxies of paleo-Moho depths. Based on mineral-melt partition coefficients, this concept has been extended to detrital zircons, such that combined chemical and geochronological information extracted from these minerals allows us to reconstruct the crustal thickness evolution using the detrital archive. We discuss here the mohometric potential of a variety of chemical and isotopic parameters and show that their combined usage improves paleocrustal thickness estimates. Using a MATLAB® app developed for the underlying computations, we present examples from the modern and the deeper time geologic record to illustrate the promises and pitfalls of the technique. Since arcs are in isostatic equilibrium, mohometers are useful in reconstructing orogenic paleoelevation as well. Our analysis suggests that many global-scale correlations between magma composition and crustal thickness used in mohometry originate in the sub-arc mantle; additional effects resulting from intracrustal igneous differentiation depend on the compatible or incompatible behavior of the involved parameters.

Protracted (>60 Myrs) thermal evolution of a Neoproterozoic metasedimentary sequence from eastern Borborema Province (NE Brazil): Thermal and rheological implications for orogenic development

The thermal evolution of orogenic belts is quite varied. While some show rapid cooling, others record high temperatures for tens of millions of years. Here, we report the latter situation based on U-Pb zircon and monazite dating of paragneiss and micaschist from the Surubim Complex (eastern Borborema Province, NE Brazil). The Borborema Province and its African counterparts (Cameroon belt and Nigerian Shield) formed during amalgamation of West Gondwana in the Late Neoproterozoic (Brasiliano/Pan-African Orogeny). The location of the Surubim Complex in the central core of this large orogenic system offers the opportunity to study tectonic processes at work in the inner regions of ancient orogens. Basement-involved thick-skinned tectonics, regional low-angle foliation related to NW-directed thrusting, transcurrent shear zones and intrusion of granitic plutons characterize the Brasiliano Orogeny in the study area. Zircon data indicate provenance of the Surubim Complex mainly from proximal sources, predominantly 2.2–2.0 Ga-old basement rocks (including an orthogneiss here dated at 2132 ± 11 Ma), and from Neoproterozoic extension-related granitoids. The ages of the youngest detrital zircon grains range from 690 to 650 Ma. The weighted mean age (647 ± 4 Ma) of low Th/U zircon overgrowths in one sample is tentatively interpreted as dating syn-extensional metamorphism given the small age difference to the youngest detrital zircon population. A time lag of c. 17 Myrs, between 647 Ma and 630 Ma, suggests a temperature drop due to contractional deformation. The ages of the oldest magmatic grains from a leucosome sample and of monazite grains from a paragneiss sample, together with data from the literature, show that temperatures high enough to promote local anatexis during development of the regional low-angle foliation were reached around 630 Ma. Afterwards, as indicated by zircon and monazite growth and/or recrystallization, temperatures remained relatively elevated until the transition to the transcurrent regime, at c. 590 Ma, and persisted so up to c. 563 Ma. The results outline a protracted thermal history spanning more than 60 Myrs. This evolution is comparable to the tectonothermal evolution of long-lived hot orogens, where persistence of high temperature conditions for a long time has been related to the formation of orogenic plateaus.

Magmatic, hydrothermal and ore element transfer processes of the southeastern Archean Superior Province implied from electrical resistivity structure

Although magmatic- and metamorphic- derived fluids are widely recognized mineralizing agents, the role of crustal architecture in defining source and sink zones within the middle to lower crust and upper mantle of ancient orogens remains enigmatic. The globally largest and best-preserved Archean greenstone belts lie in the Superior Province, Canada. They provide an ideal location to investigate the influence of igneous construction and subsequent syn-deformational plutonism and metamorphism on the localization of metal-rich melts and fluids throughout the crustal column. Integration of three-dimensional magnetotelluric modelling and seismic reflection sections across the Abitibi subprovince reveals details of a 'whole-of-crust' magmatic and hydrothermal system. East-west low resistivity structures broadly underlie the surface traces of the major deformation zones that are host to significant gold endowment (>200 Moz), while mid-crustal cross trends suggest mineralized fluids flowed along 'pipes' within fault planes. Most low resistivity structures are inferred to represent domains containing interconnected zones of graphite and/or sulfide. These delineate relict mantle source/transit domains and crustal pathways enriched by the flow of magmas or metamorphic fluids genetically related to a late-stage pulse of ore-bearing magmatism, possibly as a result of slab break-off or delamination. Using the combined MT and seismic data, we develop a 3-D crustal-scale model which highlights how evolving orogenic architecture-controlled mass transfer and metallogenic processes developed.

The Borborema Strike-Slip Shear Zone System (NE Brazil): Large-Scale Intracontinental Strain Localization in a Heterogeneous Plate

Abstract Large-scale strike-slip faults are fundamental tectonic elements of the continental lithosphere. They constitute plate boundaries (continental transforms), separate terranes with contrasting geological histories within accretionary orogens, or accommodate heterogeneous deformation in intracontinental settings. In ancient orogens, where deeper levels of the crust are exposed, these faults are expressed as shear zones materialized by up to tens of km-wide mylonitic belts. The Borborema shear zone system in northeastern Brazil is one of the largest and best-exposed intracontinental strike-slip shear zone systems in the world, cropping out over 250,000 km2. Here, we review its main geophysical, structural, petrologic, and geochronologic characteristics and discuss the factors controlling its development. This complex continental scale shear zone system is composed of a set of NE- to NNE-trending dextral shear zones from which there are two major E-trending dextral shear zones with horse-tail terminations into the transpressional belt branch, as well as several smaller E-trending dextral and NE-trending dextral and sinistral shear zones. The major shear zones are marked by extensive linear or curvilinear magnetic gradients, implying their continuation at depth. The major shear zones are materialized by migmatite to amphibolite-facies mylonites, but the entire system shows evidence of late deformation at lower temperatures. The system developed during the late stages of the Neoproterozoic Brasiliano (Pan-African) orogeny (mainly from 590 to 560 Ma), postdating by more than 20 Ma the main stage of contractional deformation. Localization of strike-slip shearing in this intraplate setting was controlled by rheological contrasts between blocks with distinct Paleoproterozoic histories, the presence of preorogenic Neoproterozoic rifts, the craton geometry, and zones of enhanced magmatic activity, highlighting the importance of rheological heterogeneity in controlling shear zone nucleation and evolution.

Tracing tectonic processes from oceanic subduction to continental collision through detrital zircon U-Pb and Lu-Hf isotope data: An example from the Chinese West Tianshan Orogen

Combining detrital zircon (DZ) U-Pb age and Lu-Hf isotope data is important in revealing the tectonic evolution of ancient orogens. Here, we conducted DZ U-Pb and Lu-Hf isotope research on 7 modern river sand samples collected from the northern Chinese West Tianshan Orogen (CWTO). The DZs from modern river sands yield ages 340 to 320 Ma and reflect Mesozoic magmatism in the northern Yili Block (YB). In addition, the DZs reflected disproportionate contributions from the source area, which might be affected by the zircon fertility and erodibility of the source rocks. These newly obtained data, together with compiled DZ U-Pb and Lu-Hf isotope data from other modern river sands and ancient sedimentary rocks in the CWTO, are used to constrain the subduction and closure history of the southwestern part of the Paleo-Asian Ocean. The εHf(t) values of 500 to 450 Ma zircons from the southern YB and Central Tianshan Block display a fanning trend (changing from a narrow to a wide range over time), indicating a crustal thickening event at ∼ 450 Ma. The closure of the North Tianshan Ocean was completed when the YB and an immature/nascent island arc collided in the latest Carboniferous. The South Tianshan Ocean (STO) subducted bidirectionally beneath the Yili–Central Tianshan Block (YCTB) and the Tarim Craton starting at 460 Ma. Slab rollback or retreat of the STO in the northern Tarim Craton starting at 440 Ma caused the opening of several small diachronous back-arc basins. The fanning trend of εHf(t) values indicates that collision between the YCTB and the Tarim Craton occurred at 320 to 310 Ma.

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.

Long-lived seamount subduction in ancient orogens: Evidence from the Paleozoic South Tianshan

AbstractAlong the present-day circum-Pacific subduction girdle, seamount subduction is known to have significant effects on subduction dynamics including on seismicity and arc magmatism. Because seamount subduction should have occurred throughout much of Earth history, its effects on orogenesis in the overriding plate should be identifiable in ancient orogens. In this study, we investigate the Paleozoic South Tianshan orogen of Central Asia, for which abundant evidence of seamount subduction exists, further bolstered by the continuation of a long-lived plume-induced large igneous province on the subsequently accreted Tarim craton. We find that semi-continuous seamount subduction from ca. 400 to 330 Ma temporarily shut down arc magmatism, and once the seamounts were completely subducted, then arc magmatism resumed and eclogites were quickly exhumed. If such an orogenic fingerprint of seamount subduction can be identified in ancient orogens, a much more complete picture of plume-subduction interaction and its influence on both crustal and mantle processes can be developed.