Wilson Cycle Research Articles

From divergent to convergent plate boundary: A ca. 200 Ma Wilson cycle recorded by ultrahigh-pressure eclogites in the Dora-Maira Massif, Western Alps

Sparse eclogite exposure in accretionary-to-collisional orogens cannot only reveal the sites of ancient subduction zones and plate boundaries, but also elucidate the multi-stage tectonic evolution in the Wilson cycle. The southern part of the Dora-Maira Massif in the Western Alps is well known for its ultrahigh-pressure (UHP) rocks and contains small occurrences of petrographically distinct types of eclogite. Due to the lack of detailed geochronological and geochemical data on these rocks, the nature of their protolith and related geodynamic setting has remained unknown. In the current study, 33 samples from 13 localities were studied by whole-rock major- and trace-element analysis, and a selection of the samples were studied using Sr-Nd-Hf isotopes, as well as high-resolution elemental mapping and U-Pb geochronology of zircon. According to macroscopic appearance, petrography, chemical composition, and principal-component analysis of multi-elements, two major types are distinguished. Light-colored eclogite is phengite-rich, commonly foliated, bears kyanite and quartz/coesite, shows relatively high MgO, K2O, SiO2, εNd(t) (−2.2 to +1.4), and εHf(t) (+3.5 to +7), with a protolith geochemically similar to an enriched-type mid-oceanic-ridge basalt (E-MORB). Dark eclogite generally is massive, rutile-rich, shows higher values of Ti, Fe, P, Nb, and Zr, εNd(t) values between −2.8 and −0.8, εHf(t) values between −6.1 and −3.2, with an oceanic island basalt (OIB)-type protolith composition. Magmatic zircon cores, which are characterized by steep heavy rare earth element (HREE) patterns and negative Eu anomalies, yield consistent protolith ages of ca. 253−252 Ma in both eclogite types. Metamorphic zircon domains with flat HREE patterns and insignificant Eu anomalies yield a younger mean age of ca. 34 Ma, which is the age commonly assigned to eclogite-facies UHP metamorphism. Considering the various geochemical signatures of E-MORB- to OIB-like eclogites and the regional tectonic evolution, their protolith is best explained as rocks, which crystallized from rift-related basaltic magma, associated with the break-up of the Pangea Supercontinent that eventually resulted in the birth of the Piemonte-Liguria Ocean (Alpine Tethys Ocean) between the future Eurasia and Adria plates. Therefore, the UHP eclogites in the Dora-Maira Massif likely fingerprint a multi-stage tectonic evolution from divergent (continental extension, rifting) to convergent (subduction zone) plate boundary, corresponding to the beginning and end of a Wilson cycle. Incidentally, they reveal that mafic rocks in the Alpine basement units may not be polymetamorphic, but may actually consist of post-Variscan products that underwent only Alpine metamorphism.

The Rheic riddle: ocean closed, but no orogen

Abstract Although the Rheic Ocean was the widest to be closed during the assembly of Pangaea, its suture zone is dominated by a younger (Rhenohercynian) Wilson Cycle. The present paper briefly summarizes the Rhenohercynian and reviews the relicts of the Rheic. The absence of a Rheic orogen is mainly due to bipolar subduction of the Rheic Ocean, which created two magmatic arcs and largely destroyed the passive margin prisms, thus depriving the collision zone of building material. Also, displacement of Variscan zones in England, France and Germany along a curvature of the dextral Bristol/Bray/Bohemia shear zone has effected their anticlockwise rotation. Younger, ENE-trending reverse faults have truncated the NE-trending structural boundaries and excised pre-Devonian, Rheic-related rocks west of the River Rhine, while the northern Rheic Arc and Cambrian–Silurian deposits of the Rheic Ocean have been preserved further east. Opening of the Rhenohercynian successor ocean to the Rheic put a brake on plate convergence and hindered formation of a large collisional belt. Scarce relicts of the southern Rheic Arc and rocks with Silurian deformation exist in the (largely eroded) upper plate of the Rhenohercynian collisional belt, mostly in the form of clasts and detrital zircons in Upper Devonian/lower Carboniferous flysch and in tectonic mélange at the base of the allochthon.

Ultra-slow transverse waves during continental breakup

Continental rifting is one of the four fundamental geological processes of the Wilson cycle. Rifting results from the continuous stretching of continental lithosphere and involves mechanical, thermodynamic, and rheological processes. It may be followed by a catastrophic breakup stage, which determines cessation of extensional deformation and the final separation of a continent into two distinct tectonic plates that grow by accretion of oceanic lithosphere. To date, the transition to sea−floor spreading and the conditions for the development of a new ocean have not been fully understood. We propose that a consistent description of this process must consider the existence of long−term retarded elasticity in the mantle lithosphere, the superadiabatic conditions of this layer, and the combined action of such elastic forces with the localized buoyancy arising from thermal anomalies. We present a solution of the rheological equation for a nonlinear viscoelastic model of the lithosphere mantle and numerical experiments showing that transient thermal anomalies are generated during the extension, which lead to the formation of transverse waves having wavelengths of the order of hundreds to thousands km and periods of several tens kyrs. These waves induce oscillating topography and influence the relief. Therefore, they could be responsible for eustatic cycles both in the axial rift lacustrine system and in off−axis (dendritic) lakes placed in areas of reversed drainage. At sufficiently high extension rates, deformation localizes and these ultra-slow waves determine cyclic shear failure, with formation of X−shaped cross structures through the lithosphere that prelude to the final rupture.

The Variscan belts of North-West Africa: An African legacy to the Wilson Cycle concept

The concept of cyclic closure and opening of oceans along the same crustal scar was introduced by J.T. Wilson (1966) based on the example of the Atlantic Ocean and its continental borders. At that time, the only Variscan orogen cited south of Europe along the Atlantic coast of Africa was the Mauritanides of Mauritania. Here we report on the recent achievements in the Mauritanides from Mauritania to Morocco, and on the Moroccan Meseta orogen, which also records the Variscan orogeny. The Southern and Central Mauritanides are a poly-orogenic, Pan-African and Variscan orogen characterized by a thin-skinned tectonic style that mirrors the structure of the southern Appalachians, but with a different tectonic history. The Northern Mauritanides crops out in the Moroccan Oulad Dlim massif, northwest of the Reguibat Rise. This part of the belt compares with the southern part, but additionally exhibits in the west a Silurian-Devonian sector that shows possible affinities with Gondwana-derived Appalachian terranes. The Western Meseta is only affected by Variscan events, which were mild in the westernmost Meseta Coastal Block, while the Eastern Meseta was also affected by Eo-Variscan events. The along-strike change from the Mauritanides to the Meseta orogen is interpreted as a transition from a head-on collision south of the South Meseta transform fault (SMF, precursor of the South Atlas Fault, SAF) to a dextral, transpressional collision north of the SMF. South of the SAF, the Anti-Atlas and the Dhlou-Zemmour expose the foreland foldbelts of the Meseta and northernmost Mauritanides. The Coastal Block was likely displaced from the south-westernmost Anti-Atlas during the Early Carboniferous.The Wilson Cycle concept mostly applied in that the Atlantic Ocean opened where the prior Rheic Ocean had closed. Possible exceptions are the Sehoul Block north of Western Meseta and the Silurian-Devonian Sector of the Oulad Dlim massif, which may have separated from NW-Africa and re-amalgamated to it during the Variscan orogeny. Likewise, a NW-African fragment from the Anti-Atlas may have stranded in offshore Massachusetts in eastern North America. An early Ediacaran-Cambrian Wilson cycle nested in the classic cycle introduced by Wilson (1966) occurred along the margin of NW-Africa, where Cadomian terranes rifted off Africa, and some were transferred to Europe and some accreted back to NW-Africa. This early cycle likely controlled the localization of the subsequent Rheic rift, and that of the Atlantic rift along the Mauritanides after the Variscan collision.

Early Neoproterozoic magmatism and Caledonian metamorphism recorded by the Mårma terrane, Seve Nappe Complex, northern Swedish Caledonides

Petrology, geochronology and bulk-rock chemistry are combined to investigate the early Neoproterozoic magmatism and Cambrian–Ordovician metamorphism in the northern Swedish Caledonides. This work includes several lithologies of the Mårma terrane in the Seve Nappe Complex exposed in the Kebnekaise region. U–Pb zircon geochronology yielded crystallization ages of 835 ± 8 Ma for a mylonitic orthogneiss, 864 ± 3 Ma for the Vistas Granite and 840 ± 7 Ma for an intruded granitic dyke, whereas a gabbro and a granodiorite intrusion gave U–Pb zircon crystallization ages of 856 ± 3 Ma and 850 ± 1 Ma, respectively. U–Pb monazite dating of the mylonitic orthogneiss gave an upper intercept age of 841 ± 7 Ma and a lower intercept age of 443 ± 20 Ma. Pressure–temperature estimates derived for the mylonitic orthogneiss reveal metamorphic peak-pressure and peak-temperature of 10.5–11.75 kbar and 550–610°C and 7.4–8.1 kbar at 615–675°C, respectively. Metamorphic pressure–temperature estimates for the Vistas Granite yield 6.5–7.5 kbar at 600–625°C. Whole-rock chemistry coupled with U–Pb geochronology indicates that bimodal magmatism was related to attempted rifting of Rodinia between 870 and 840 Ma. Supplementary material: Supplementary figures and tables are available at https://doi.org/10.6084/m9.figshare.c.6675075 Thematic collection: This article is part of the Caledonian Wilson cycle collection available at: https://www.lyellcollection.org/topic/collections/the-caledonian-wilson-cycle

Xenotime reveals Caledonian hydrothermal overprint on Neoproterozoic Cu mineralization, East Greenland

Constraining the age of many types of ore deposits remains challenging because of the lack of radiogenic isotopes incorporated into common ore-forming minerals. The timing of pre-Caledonian-hosted Cu mineralization along the entire c. 1200 km long East Greenland Caledonides remains virtually unknown, hampering our knowledge of ore deposit timing and genesis in a frontier exploration region. Here, automated mineral analysis of a series of nodular, disseminated and vein-hosted Cu- ± Pb-mineralized metasedimentary rocks in central East Greenland reveals detrital zircon and hydrothermal xenotime, both amenable to U–Pb geochronology. Detrital zircon geochronology of a co-deposited quartzite reveals an age distribution highly similar to the Cryogenian ( c. 700 Ma) upper Eleanore Bay Supergroup. Hydrothermal xenotime U–Pb analyses adjacent to nodular and disseminated chalcocite across three proximal samples have variable amounts of common Pb that together yield a well-defined single discordia with a lower concordia intercept of 438 ± 13 Ma (2 σ ). This age is within uncertainty of the onset of Caledonian regional metamorphism and granitoid production and clearly post-dates deposition of the upper Eleanore Bay Supergroup by several hundred million years. Considering a published chalcocite Pb–Pb isochron age of 680 ± 65 Ma, the hydrothermal xenotime U–Pb ages imply that Caledonian-driven fluid activity, sourced from metamorphic reactions or from granitoids, remobilized diagenetic Cu and Pb mineralization. Chalcocite Pb–Pb isotopes show that dissolved and reprecipitated portions are volumetrically minor, radiogenic and Pb-poor, implying that fluids stripped most of the Pb from the system. Thus, it is likely that remobilization was localized on the grain scale, although some Cu and Pb was transported away from diagenetic sites, perhaps into veins. Although Caledonian metamorphism and granitoid emplacement is widespread in central East Greenland, the full extent of their roles in upgrading Cu mineralization remains to be ascertained. Supplementary material: Supplementary figures and tables are available at https://doi.org/10.6084/m9.figshare.c.6675384 Thematic collection: This article is part of the Caledonian Wilson cycle collection available at: https://www.lyellcollection.org/topic/collections/the-caledonian-wilson-cycle

Continental Tip With Thinned Lithosphere and Thickened Crust Is a Favorable Location for Subduction Initiation

AbstractThe collapse of passive margins and subsequent subduction initiation (SI) are key steps in the Wilson Cycle. However, the occurrence and mechanism of SI at passive margins have long been questioned due to the lack of Cenozoic observations. In this study, we propose that the continental tip, where arc/plateau blocks with thinned lithosphere and thickened crust accrete beside the passive margin, is a favorable location for SI. Our three‐dimensional numerical model and force analysis demonstrate that the continental tip, with its reduced frictional resistance and increased gravitational instability, is better suited for SI than other parts of the passive margin. Upon completion of SI, the retreat of self‐sustained subduction can create a transform plate boundary by tearing the passive margin. Our model explains why SI can occur at continental tips such as the Scotia and the Caribbean, as well as locations with similar tectonic settings, while most parts of the Atlantic margin remain stable over long geological periods.

Ordovician–Silurian deformation of the Neoproterozoic upper gneiss unit in the northern Seve Nappe Complex: implications for subduction of the Baltican margin

The upper gneiss unit is exposed in the northernmost Seve Nappe Complex (SNC) in the Scandinavian Caledonides. To investigate the Caledonian tectonic history of the unit, in situ white mica and biotite 40 Ar/ 39 Ar geochronology was applied to a leucogranite and two paragneisses. The leucogranite exhibits low-strain traits. Biotite porphyroblasts yielded a cooling age of 459 ± 2 Ma. White mica that replace biotite and plagioclase provided a crystallization age of 436 ± 5 Ma. White mica in both paragneisses exhibit high-strain characteristics associated with top-to-the-east sense of shear. One paragneiss provided dispersed late Cambrian to Late Ordovician dates with the youngest approximating deformation at 459 ± 2 Ma. The older dates are interpreted to reflect 40 Ar inherited from a previous metamorphic event. The second paragneiss yielded a deformation age of 434 ± 2 Ma. The collective dataset is interpreted to record exhumation of the upper gneiss unit at c. 459 Ma and deformation and fluid infiltration at c. 434 Ma during continental collision. The events closely resemble the deformation histories of other northern SNC terranes. Synthesizing these results with those for other northern SNC terranes suggests that the Baltican margin underwent oblique, south-to-north subduction during late Cambrian time. Supplementary material: Tables providing mineral chemistry of white mica and biotite and 40 Ar/ 39 Ar geochemistry results are available at https://doi.org/10.6084/m9.figshare.c.6639993 Thematic collection: This article is part of the Caledonian Wilson cycle collection available at: https://www.lyellcollection.org/topic/collections/the-caledonian-wilson-cycle

Three-dimensional magnetotelluric imaging of the Eastern Qinhang Belt between the Yangtze Block and Cathaysia Block: Implications for lithospheric architecture and associated metallogenesis

The Eastern Qinhang Belt represents the suture zone between the Yangtze and Cathaysia Blocks and controls the world-class Dexing porphyry copper (Cu) deposits. A three-dimensional three-dimensional structural interpretation based on magnetotelluric array data covering the Eastern Qinhang Belt was determined to provide new information on the lithospheric architecture of this mineral system. The regional electrical structure of the Eastern Qinhang Belt is consistent with the pattern of the Appalachian-style multi-terrane Wilson cycle, with the main electrical gradients corresponding to pre-existing lithospheric faults. It is possible that the central domain between the Huaiyu Terrane and Western Cathaysia Block preserved the slivers of the mafic lower crust generated by the modification of Neoproterozoic subduction. Partial melting of this juvenile mafic lower crust was triggered by the extension from the rollback of the Paleo-Pacific Plate during the Mid-Jurassic. The Northeast Jiangxi Fault represents the conduit for the rise of ore-bearing magmas capable of producing porphyry Cu mineralization.

Heterogeneity of the Ailaoshan–Song Ma ophiolitic mélange and its palaeogeographic implications for the evolution of Eastern Palaeo-Tethys

In comparison with ophiolite, ophiolitic mélange, especially its matrix, contains more information about the evolution of the relevant ocean. The evolution of the eastern Palaeo-Tethys recorded a whole Wilson cycle of numerous continental blocks such as the South China Block and Indochina Block, including rifting away from the Gondwana continent, subsequent northward migration and final collage with the Laurasia continent. Based on structural geology, detrital zircon, and related geochemical analyses, the Ailaoshan–Song Ma ophiolitic mélange can be divided into M1, M2, and M3 units with distinct detrital zircon age spectra and εHf(t) values suggesting for different provenances. These units are in faults-contact along the orogenic belt, indicating a strike–parallel heterogeneity of the ophiolitic mélange. To further understand the heterogeneity of these ophiolitic mélanges, five samples from the matrix of the Ailaoshan ophiolitic mélange were collected to be conducted detrital zircon UPb dating and LuHf isotope analyses. All samples share the same zircon age spectrum, with two obvious age peaks at 430 and 960 Ma, which is comparable with the M1 unit. These results imply that the northwestern Ailaoshan ophiolitic mélange segment can be subdivided into M1 in the interior and M2 in the exterior rather than the previous division, indicating a strike–perpendicular heterogeneity. Based on their geometries (NE-dipping) and depositional ages (M1, 310–270 Ma; M2, 260–240 Ma), the elder M1 unit was thrusted upon the younger M2 unit, indicating a tectonic inversion. This inversion, which is associated with the whole geometry of the Ailaoshan belt, is ascribed to the transpressive deformation of the Cenozoic Red River Fault. These spatial heterogeneities well match the different evolutionary stages of the eastern Palaeo-Tethys.

From Tethyan subduction to Arabia-Eurasia continental collision: Multiple geo-thermochronological signals from granitoids in NW Iran

NW Iran, situated between the Arabian and Eurasian plates, carries a record of both Paleo-Tethyan and Neo-Tethyan tectonic evolution. During the Wilson Cycle of Tethys Ocean opening and closing, several episodes of magmatism from Late Paleozoic to latest Cenozoic generated a massive volume of intrusive rocks. These intrusives, which record cooling histories from high-temperatures to final exhumation, are ideal for multiple geo-thermochronological studies. Our new zircon and apatite U-Pb results suggest three regional magmatic events in the Late Carboniferous, mid-Cretaceous and middle Eocene, which could be related to Paleo-Tethys subduction, Neo-Tethys subduction and Neo-Tethyan ridge subduction, respectively. New apatite fission track and (U-Th)/He data reveal post-magmatic cooling and differential exhumation related to subduction and collision. By integrating published regional thermochronological data from northeastern part of the Middle East, a broader tectono-thermal framework is outlined as follows: 1) Cretaceous cooling signals are most pronounced in the Alborz and Caucasus, and reflect back-arc extension during Neo-Tethyan subduction; 2) diachronous Neo-Tethyan evolution led to the Paleocene assemblage of Anatolia in the west, and subduction associated with a late Paleocene-Eocene magmatic flare-up in Iran. Subsequent westward escape of Anatolia and extensive surface uplift were driven by propagation of Arabia-Eurasia collision in the Miocene; 3) Continuous northward indentation of Arabia into Eurasia triggered late Miocene-Pliocene fast exhumation of the Zagros, Alborz and the Caucasus.

Structural evolution of the reactivated Møre–Trøndelag Fault Complex, Fosen Peninsula, Norway

The ENE–WSW-trending Møre–Trøndelag Fault Complex (MTFC) in Central Norway is a 10–50 km-wide, steeply dipping reactivated fault zone. Onshore, it transects Devonian sedimentary rocks and a series of east to SE transported metamorphic nappes, which were emplaced during the Scandian (Silurian–Devonian) Orogeny. Offshore, the MTFC defines the southern margin of the Møre Basin and the northern margin of the Viking Graben, meaning that the fault complex played a major role in controlling the architecture of these Mesozoic basins. Onshore, the MTFC has had a prolonged and heterogeneous kinematic history. The complex comprises two major fault strands: the Hitra–Snåsa Fault (HSF) and the Verran Fault (VF). These two faults seem to have broadly initiated as part of a single system of sinistral ductile shear zones during the early Devonian ( c. 410 Ma). Sinistral transtensional reactivation (Permo-Carboniferous; 290 Ma) of the ENE–WSW-trending HSF and VF led to the development of cataclasites and pseudotachylites together with the formation of north–south-trending faults establishing the present-day brittle fault geometry of the MTFC. Later phases of Mesozoic reactivation focused along the Verran Fault Zone (VFZ) and north–south-linking structures were probably related to mid- to late Jurassic to early Cretaceous rifting and late Cretaceous to early Cenozoic opening of the North Atlantic. Supplementary material: Supplementary datasets are available at https://doi.org/10.6084/m9.figshare.c.6390518 Thematic collection: This article is part of the Caledonian Wilson cycle collection available at: https://www.lyellcollection.org/topic/collections/the-caledonian-wilson-cycle

Permian to Triassic protolith ages of type locality eclogites in the Eastern Alps: Implications for the opening of the Meliata back-arc basin

Abstract The Austroalpine domain contains the type locality of eclogites, but their protolith age is unknown except for a Permian metagabbro. Therefore, we studied the non-gabbroic eclogites from the Saualpe-Koralpe Complex (SKC) representing meta-basalts within a coherent continental rift fragment subducted during the Cretaceous. Zircon laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) U-Pb dating revealed protolith ages of 283 ± 5 Ma, 255 ± 3 Ma, 251 ± 3 Ma, and 241 ± 3 Ma. Magmatic zircons exhibit 176Hf/177Hf ratios of 0.282935–0.283090 and juvenile εHf(t) values of +10 to +17.4. Typical normal mid-oceanic ridge basalt (N-MORB) geochemistry is established. The SKC host metasedimentary rocks are interpreted as Variscan continental crust close to the margin of the oceanic Meliata basin and were affected by Permian metamorphism. Metamorphic zircon of one SKC eclogite yields an age of 87–93 Ma. These results combined with previous data are used to present a new model for the tectonic evolution of the distal Austroalpine unit associated with the Meliata Ocean in a Wilson cycle: The SKC represents a distal continental margin during Permian to Middle Triassic rifting, which was intruded by few gabbro bodies and numerous basaltic sills and mostly Permian pegmatites. In contrast, structurally separated thick Triassic sedimentary cover successions of the Austroalpine domain lack any magmatism, excluding SKC as Triassic basement of the sedimentary cover successions. The present eclogite-bearing piece of continental crust adjacent to the Meliata oceanic lithosphere was subducted to mantle depth during Late Cretaceous and then exhumed.

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

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

Early cretaceous bimodal volcanic rocks in Wuga Co area, central tibet: The first identification of direct products derived from slab sinking in the Bangong-Nujiang suture zone

Introduction: The Bangong-Nujiang Suture Zone (BNSZ) in central Tibet is a remnant of the Bangong-Nujiang Ocean that records its entire Wilson Cycle. The model of divergent double-sided subduction (DDS) is crucial for elucidating the evolution of tectonomagmatic activity on both sides of the BNSZ and for understanding why no high-pressure metamorphic rocks occur in the BNSZ. However, the DDS geodynamics remain poorly constrained. In particular, there is a lack of reports on magmatic rocks directly associated with slab sinking in the DDS terminal stage.Methods: This study presents new geochronological, geochemical, and isotopic data for the Early Cretaceous bimodal volcanic rocks around the Wuga Co area.Results: The bimodal volcanic rocks are divided into the Wuga Co rhyolites (SiO2 = 77.0–79.0 wt%) and the Wuga Co basaltic andesites (SiO2 = 53.9–55.5 wt%). The isotopic values of the Wuga Co basaltic andesites with low (87Sr/86Sr)i values (+0.7040 to +0.7044) and high εNd(t) values (+3.8 to +4.1) lie among three endmembers (the BNO sediments in accretionary wedge, depleted mantle and the BNO slab).Discussion: These values indicate the partial melting of a mantle peridotite that interacted with the subducted slab and sediment in the accretionary wedge, which was caused by the sinking of the Bangong-Nujiang oceanic slab. The Wuga Co rhyolites (108 Ma) have low (87Sr/86Sr)i values (+0.703 to +0.706), high εNd(t) values (+2.25 to +2.49), and high εHf(t) values (+5.6 to +10.0). These values indicate that the rhyolite formed by partial melting of juvenile basaltic crust. This study also collected Hf isotope data from both sides of the BNSZ to constrain its evolution. Our results show that the εHf(t) values of magma on both sides of the BNSZ were elevated simultaneously at 130 Ma, which may be caused by the Bangong-Nujiang oceanic slab rupture. Based on these new data, we propose that the Bangong-Nujiang oceanic slab ruptured from the two overlying terranes at approximately 130 Ma and subsequently sank into the mantle at approximately 108 Ma.

Evidence, or not, for the late Tonian break-up of Rodinia? The Dalradian Supergroup, Scotland

The Tonian–Cambrian Dalradian Supergroup in Scotland is a siliciclastic–carbonate succession that can be up to 10 km thick. The consensus view is that its lower part, the mid- to late Tonian Grampian and Appin groups, formed in rift basins: the deep marine turbidites of the Grampian Group infilled rift depocentres, whereas the shallow marine strata of the Appin Group mark basin-bounding palaeohighs. This scenario is used as a key line of evidence to infer the onset of the break-up of Rodinia between Laurentia and Baltica. However, deformation during the mid-Ordovician Caledonian Orogeny obscured the original depositional frameworks. Reconstructing these frameworks (and hypothesized rift basins) has relied on the trace and major element log-ratio geochemistry of minor carbonate rocks to assign the units to either the Grampian or Appin group – that is, to rift depocentres or basin-bounding palaeohighs, respectively. We report new carbon and oxygen isotope and geochemical data and use these to create a revised stratigraphic framework for the Grampian and Appin groups. Our findings show that the previous geochemical-based correlations are unreliable and that there is no evidence for palaeohighs or rift basins. Instead, the Grampian–Appin groups are a deeper marine flysch to a shallower marine molasse succession formed in response to the mid-Tonian Knoydartian Orogeny. From a Scottish perspective, evidence for the break-up of Rodinia is recorded higher in the Dalradian succession during the deposition of the early Cryogenian Argyll Group. Supplementary material: Geochemical analysis of samples is available at https://doi.org/10.6084/m9.figshare.c.6317830 Thematic collection: This article is part of the Caledonian Wilson cycle collection available at: https://www.lyellcollection.org/topic/collections/the-caledonian-wilson-cycle

Tectono-magmatic evolution during the extensional phase of a Wilson Cycle: a review of the Alpine Tethys case and implications for Atlantic- type margins

Despite the recent progress in seismic imaging and dynamic modelling, the tectono-magmatic evolution during late orogenic extension and rifting is at present little understood. Answering questions such as when and where magma forms, how it percolates and how it is emplaced requires access to magmatic rocks from different crustal and mantle levels, which is impossible in modern rift systems. An alternative, presented in this study, is to use remnants of fossil extensional systems exposed in collisional orogens. Here we review the Permian to Early Cretaceous tectono-magmatic evolution recorded in the Alpine realm of Western Europe. While most of the previous studies reviewed the geochemical and petrological characteristics of the magmatic rocks, here we rely on a holistic approach to link the magmatic evolution to extensional processes on a large scale. We synthesize our observations on the timing and location of magma production in a Wheeler diagram. Our study shows that two major magmatic events can be defined: the first one took place during the Permian and was linked to post- orogenic extension of the Variscan belt. It affected a large portion of Western Europe, where it was responsible for significant sub-aerial volcanic activity, for the emplacement of massive magmatic additions at the base and within the crust, and for the depletion of the lower subcontinental lithospheric mantle. The second included several pulses of magmatic activity related to the multiphase rifting event that led to the successive opening of the Meliata-Vardar, Alpine Tethys and southern North Atlantic (proto-)oceanic domains. In the Alpine Tethys, the first rift-related magmatic activity was linked to melt infiltration within the subcontinental lithospheric mantle during the so-called necking stage. Basalts were emplaced only later, during and after mantle exhumation. Their chemical signature reflects the interaction between melts derived from Depleted Mid-oceanic ridge basalt Mantle (DMM) and the previously refertilized/infiltrated subcontinental mantle. Late alkaline magmatic activity post- dated breakup. A main result of this study is that the tectono-magmatic evolution of extensional systems cannot be explained by simple models using ß-factors or mantle temperatures alone but also needs to integrate the inheritance acquired during previous tectonic phases.

The role of V-shaped oceans and ribbon continents in the Brasiliano/PanAfrican assembly of western Gondwana

Western Gondwana amalgamated by collision of continental blocks that did not form prior conjugated margins (extroversion), and by typical Wilson cycles, when continental blocks that rifted away giving birth to new oceans were subsequently re-joined in approximately the same position (introversion). The introverted systems are characterized by the opening of V-shaped basins through rifting and hyperextension of various continental pieces (micro- and ribbon continents) from a former Central African Block. These continental fragments lost substantial parts of their mantle lithosphere and became decratonized while drifting towards the external Goiás-Pharusian ocean. Protracted seafloor spreading and consumption through subduction of the internal and external oceans, respectively, ultimately led to multiple, diachronous collisions with other continental blocks detached from Rodinia (Amazonian, West Africa, Embu, etc.). These collisions pushed the ribbon continents back and closed the introverted basins, squeezing and incorporating the reworked basement tracts between the main colliding blocks and the rigid remainder of the Central African Block (the São Francisco-Congo craton). Continental extrusion and lateral escape tectonics ensued, generating thousands-of-km long networks of anastomosing directional shear zones (keirogens), as a consequence of both the accretionary systems developed between the involved blocks and the highly deformable nature of the decratonized ribbon continents.

From shallow-water carbonate ramp to hemipelagic deep-marine carbonate deposition: Part 1. General characteristics, microfacies and depositional history of the Middle to Late Anisian Bulog sedimentary succession in the Inner Dinarides (SW Serbia)

The opening of the Neo?Tethys started in the Middle Anisian and is recorded in the drowning succession of the shallow?water Ravni/Steinalm Carbonate Ramp and the subsequent deposition of deep?marine limestones, e.g., the red nodular limestones of the Bulog Group and equivalents. In the Inner Dinarides of southwest Serbia the continental break?up of the Neo? Tethys Ocean is characterized by the formation of a horst?and?graben topography. The change from deposition of shallow?water carbonates formed in an epicontinental sea (graben stage of the Neo?Tethys Wilson Cycle) to red nodular deep?marine limestones (Bulog Limestone) in the late Pelsonian (Middle Anisian) is relatively abrupt due to the rapid decrease of carbonate production. The deeper?water Bulog Limestone, deposited in the early stage of the passive continental margin evolution of the western Neo?Tethys can be dated by conodonts and in rare cases also by ammonoids quite exactly, and therefore it is possible to reconstruct the Pelsonian to Illyrian sedimentological evolution precisely: 1) The late Pelsonian is characterized by the drowning of the shallow?water Ravni Carbonate Ramp and a rapid deepening of the depositional realm. Extension led to the formation of neptunian dikes in the shallow?water Ravni Formation, filled with deeper?marine red micrite, and the formation of a horst?and?graben morphology. Whereas some of the horsts uplifted and emerged in the grabens near to the newly formed escarpments thick breccia successions were deposited with a fining?upward trend during the early?middle Illyrian. On top of other horsts, the grabens, or the newly formed gentle slopes red nodular limestones were deposited. In cases layers with enriched ammonoids formed (Fossillagerst?tten). 2) The early?middle Illyrian ongoing subsidence resulted in the deposition of more and more condensed red nodular limestones with hardground formation. 3) Around the middle/late Illyrian boundary a new pulse of tectonic motions resulted in the tilting of blocks, the formation of new escarpments and again mobilization of mass transport deposits. In addition, a second generation of neptunian dikes was formed. They crosscut the late Pelsonian to middle Illyrian Bulog Limestone, the Pelsonian Ravni Formation, and the older generation of neptunian dikes in the shallow?water Ravni Formation. All formerly emerged horsts flooded and red nodular limestones were deposited on the karstified shallow?water Ravni Formation after a gap. This second pulse of tectonic motions is related to the widespread volcanism in the Dinarides as visible in the appearance of mm?sized biotite clasts in the late Illyrian Bulog Limestone. In contrast to the Outer Dinarides, where thick volcanics are intercalated in the Illyrian sedimentary succession, in the Bulog Limestone successions of the Inner Dinarides volcanics are missing. In general the Pelsonian?Illyrian sedimentary succession is characterized by a stepwise deepening of the depositional realm.

Ocean-continent subduction cannot be initiated without preceding intra-oceanic subduction!

The formation of new subduction zones is a key element of plate tectonics and the Wilson cycle, and many different controlling mechanisms have been proposed to initiate subduction. Here, we provide a brief overview of the known scenarios of subduction initiation in intra-oceanic and ocean-continent tectonic settings. Intra-oceanic subduction is most commonly associated with mechanical heterogeneities within the oceanic lithosphere, such as pre-existing fracture zones, spreading ridges, and transform faults. Numerous and well-recognized examples of new active subduction zones formed in intra-oceanic environments during the Cenozoic, suggesting that the initiation of ocean-ocean subduction must be a routine process that occurs “easily and frequently” in the mode of plate tectonics currently operating on Earth. On the contrary, the most traditional mechanisms for the establishment of classic self-sustaining ocean-continent subduction—passive margin collapse and subduction transference—are surprisingly rare in observations and difficult to reproduce in numerical models. Two alternative scenarios—polarity reversal and lateral propagation-induced subduction initiation—are in contrast much better documented in nature and experimentally. However, switching of subduction polarity due to arc-continent collision and lateral transmission of subducting plate boundaries are both inextricably linked to pre-existing intra-oceanic convergence. We, therefore, conclude that the onset of classic ocean-continent subduction zones is possible only through the transition from a former intra-oceanic subduction system. This transition is likely facilitated by the ductile damage accumulation and stress concentration across the aging continental margin. From this perspective, the future closure of the Atlantic Ocean can be viewed as an archetypal example of the role of transitional process between intra-oceanic subduction (Lesser Antilles) and the development of a new subduction zone at a passive continental margin (eastern North America).