Microcontinent Research Articles

The Formation of Continental Fragments in Subduction Settings: The Importance of Structural Inheritance and Subduction System Dynamics

AbstractMicrocontinents and continental fragments are pieces of continental lithosphere, formed by extension and breakup, followed by plate boundary relocations. Microcontinents or continental fragments affiliated with passive margins are well documented, but those close to active margins are less studied. We use dynamic two‐ and three‐dimensional numerical experiments to investigate how preexisting weaknesses within a continental upper plate affect extension and the possible formation of continental fragments. Our parametric study of the configuration (width and viscosity) of this imposed weakness indicates that stress localization and breakup of the upper plate are most efficient for narrow weak zones and a viscosity contrast between the weak zone and the surrounding crust of at least 1 order of magnitude. Moreover, upper plate extension and breakup occurs only if extension has a rotational component, here caused by the presence of a continental indenter on the downgoing plate. The width of the indenter relative to oceanic part of the downgoing plate controls differential slab pull that triggers trench retreat and upper plate deformation. A downgoing plate with a relatively large continental indenter yields large enough slab rotation to detach a continental block from the overriding plate and form wide back‐arc basins. Variations in the weak zone angle with respect to the trench result in different basin geometries. We successfully modeled the first step in breakup of active continental margins and determined the settings that may facilitate microcontinent formation in a subduction framework.

Wandering continents of the Indian Ocean

Wandering continents of the Indian Ocean

From Breakup of Nuna to Assembly of Rodinia: A Link Between the Chinese Central Tianshan Block and Fennoscandia

AbstractThe transition from breakup of Nuna (or Columbia, 2.0–1.6 Ga) to assembly of Rodinia (1.0–0.9 Ga) is investigated by means of U‐Pb and Lu‐Hf data of detrital zircons from three Neoproterozoic metasedimentary rocks in the Central Tianshan Block (CTB), NW China. These data yield six age peaks around 1.0, 1.13, 1.34, 1.4–1.6, 1.75, and 2.6 Ga. Few zircons are detected between 2.0 and 2.5 Ga. The Paleoproterozoic to Neoproterozoic detrital zircons have Hf isotopic compositions (−22.1 to +13.0) similar to those of coeval magmatic rocks in the CTB, indicating a proximal provenance. These results, together with the geological evidence and the presence of 1.4 Ga orogenic granitoids in the CTB, rule out most cratons as the CTB sources but support a Fennoscandia ancestry. Zircon U‐Pb ages and Hf isotopic compositions from the CTB and Fennoscandia suggest that from 1.8 to 1.4 Ga, the εHf(t) values increased toward more positive values, consistent with an exterior orogen characteristic that the lower crust was replaced by a juvenile arc crust. In contrast, from 1.4 to 0.9 Ga, zircon εHf(t) values decreased to more negative values, reflecting an interior orogen, characterized by enhanced contribution of recycled crustal material from collided continental fragments. This marked shift most likely reflected a transition from breakup of Nuna to assembly of Rodinia, accomplished by a transformation from an exterior orogen to an interior one.

Geodynamics of Anatolia: Lithosphere Thermal Structure and Thickness

AbstractWe present the first thermal model for the lithosphere in Turkey, which shows a highly heterogeneous pattern associated with mosaics of the Tethyan and modern subduction systems. We calculate a regionally average crustal density of 2.90 g/cm3 consistent with the presence of large volumes of mafic material. The Moho temperature with a regionally average value of 650–850 °C shows strong short‐wavelength variations. Lithosphere thinning to 50–75 km in most of western Anatolia may have developed in response to the Hellenic slab rollback, while the Neoproterozoic block in the Menderes Massif preserves a 150 km deep lithosphere root. In central Anatolia, the lithosphere thickness decreases southward from 100–150 to 50–60 km along a linear belt of young basaltic volcanism, followed by a belt of a 150 km thick lithosphere. We interpret this characteristic pattern by a SE dipping paleoslab beneath the western Taurides, which may cause the Cyprus subduction melting zone to deviate toward NW and NE. The Eastern Pontides‐Lesser Caucasus have 150–200 km thick lithosphere roots caused by collisional tectonics. The East Anatolian Plateau is underlain by a 80–140 km thick lithosphere, which suggests the presence of significant continental fragments; the patchy pattern of its thermal heterogeneity may be explained by teared and fragmented Tethyan slabs. A poor correlation between the lithosphere thermal structure, heat flux, the Neogene volcanic regions, and mantle seismic velocities implies that seismic anomalies are essentially controlled by heterogeneous mantle hydration by subduction systems of different ages and cannot be explained by temperature variations alone.

Palaeomagnetic assessment of tectonic rotation in Northeast Asia：implications for the coupling of intracontinental deformation and mantle convection

ABSTRACT Knowledge of the displacement history of the series of continental fragments in Northeast Asia is key to understanding the plate kinematic evolution of the western peri-Pacific tectonic domain. Palaeomagnetism is an effective approach for deciphering the horizontal movements of crustal elements, especially vertical-axis rotations. Here we present a set of new palaeomagnetic data from Lower Cretaceous volcanic rocks from the northeastern part of the North China Block (NCB), and from Lower Cretaceous sedimentary and volcaniclastic rocks from the Bureya-Jiamusi-Khanka terrane. The data suggest that the northeastern NCB crustal elements underwent a clockwise motion relative to the Eurasian continent after the Early Cretaceous, and in addition a substantial counterclockwise deformation component is observed in the Bureya-Jiamusi-Khanka terrane. Combined with the results of previous Cretaceous and Cenozoic palaeomagnetic studies, we suggest that the northeast part of the North China Block and the Korean Peninsula experienced ~20° of clockwise rotation during the Cenozoic. However, the Bureya-Jiamusi-Khanka terrane and its eastern neighbouring Pacific-related accretionary terranes, such as the Sikhote-Alin Orogenic belt, however, underwent a contemporaneous 40°counter-clockwise rotation. The Japanese Islands, with juvenile crust in the easternmost part of Asia, were divided into a pair of terranes, which rotated in opposite directions during the Middle Miocene. The opposing senses of rotation are attributed to mantle wedge convection. We suggest that tectonic rotations of parts of Northeast Asia are controlled by the subduction of the western Pacific Plate via lithospheric deep-shallow coupling. The two critical events were coeval with ridge subduction of the Izanagi-Pacific mid-ocean ridge under the continental margin during the Palaeocene, and the collision of the Izu-Bonin and Japanese Islands during the Middle Miocene, respectively.

Variable structural styles and tectonic evolution of an ancient backstop boundary: the Pieniny Klippen Belt of the Western Carpathians

The Pieniny Klippen Belt (PKB) and adjoining zones form a narrow, but lengthy belt that separates the Cretaceous nappe stack of the Central (Austroalpine) and the Cenozoic accretionary wedge of the External Western Carpathians (Flysch Belt). The PKB shares units and structures of both, in addition to the distinctive Oravic units, derived from a continental fragment in the Middle Penninic position. In map view, the northward-convex PKB consists of two branches—the western one striking roughly SW–NE and the eastern one oriented in the NW–SE direction. The western branch experienced a continuous NW–SE convergence and forward accretion of units derived from the foreland plate during the Late Cretaceous up to Oligocene. The developing accretionary wedge was supported by the backstop of the Central Carpathians. In contrast, the eastern branch originated by separation of PKB units and their dextral translation along the NE margin of the Central Carpathian block in the Late Eocene. During the Miocene reorganization of plate movements in the Carpathian area, the situation reversed. The eastern, formerly dextral transform margin was converted to the frontal backstop of the eastern part of the accretionary wedge. In contrast, the western, previously orthogonally convergent branch was affected by along-strike sinistral movements. Despite these considerable kinematic changes, the PKB remained fixed to both backstop edges and records deformation structures and associated sediments differentiated into several evolutionary stages.

Southeast Anatolian Orogenic Belt revisited (geology and evolution)

The Southeast Anatolian Orogenic Belt consists of the Arabian Platform, a zone of imbrication, and a nappe zone. The Arabian Platform is represented by a thick marine succession. The zone of imbrication is a narrow belt sandwiched between the Arabian Platform and the nappes. The nappes are the highest tectonic unit. They consist of two continental slivers separated by ophiolitic associations representing oceanic environments. They were involved in the orogenic development and formed two metamorphic belts. The oceanic environment survived by the end of Middle Eocene. A northward subduction began in this ocean and generated the Elbistan–Yüksekova arc built above the Göksun ophiolite. Development of the Southeastern Anatolian Orogenic Belt began in the north, where the Binboğa–Malatya metamorphic massif, collided with the Elbistan volcanic arc to the end of Early Eocene period. Later new tectonic entities were accreted to this progressively growing and southerly transporting nappe stack. In the lower plate, the southern continental sliver that was attached to the oceanic slab subducted together and underwent high-pressure metamorphism. The subducting oceanic slab retreated. Asthenospheric inflow caused high-temperature metamorphism, which superimposed on the previous high-pressure metamorphism. The oceanic and continental fragments formed the Bitlis Massif and the Berit metaophiolite when exhumed. A younger volcanic arc was built on the ocean floor to the south. Accretion of the volcanic arc to the nappe pile occurred during the Late Eocene period. The orogenic belt was formed when the nappes collided with the Arabian plate during the Late Miocene.

The Metasomatized Mantle beneath the North Atlantic Craton: Insights from Peridotite Xenoliths of the Chidliak Kimberlite Province (NE Canada)

AbstractWe studied the petrography, mineralogy, thermobarometry and whole-rock chemistry of 120 peridotite and pyroxenite xenoliths collected from the 156–138 Ma Chidliak kimberlite province (Southern Baffin Island). Xenoliths from pipes CH-1, -6, -7 and -44 are divided into two garnet-bearing series, dunites–harzburgites–lherzolites and wehrlites–olivine pyroxenites. Both series show widely varying textures, from coarse to sheared, and textures of late formation of garnet and clinopyroxene. Some samples from the lherzolite series may contain spinel, whereas wehrlites may contain ilmenite. In CH-6, rare coarse samples of the lherzolite and wehrlite series were derived from P = 2·8 to 5·6 GPa, whereas predominant sheared and coarse samples of the lherzolite series coexist at P = 5·6–7·5 GPa. Kimberlites CH-1, -7, -44 sample mainly the deeper mantle, at P = 5·0–7·5 GPa, represented by coarse and sheared lherzolite and wehrlite series. The bulk of the pressure–temperature arrays defines a thermal state compatible with 35–39 mW m–2 surface heat flow, but a significant thermal disequilibrium was evident in the large isobaric thermal scatter, especially at depth, and in the low thermal gradients uncharacteristic of conduction. The whole-rock Si and Mg contents of the Chidliak xenoliths and their mineral chemistry reflect initial high levels of melt depletion typical of cratonic mantle and subsequent refertilization in Ca and Al. Unlike the more orthopyroxene-rich mantle of many other cratons, the Chidliak mantle is rich (∼83 vol%) in forsteritic olivine. We assign this to silicate–carbonate metasomatism, which triggered wehrlitization of the mantle. The Chidliak mantle resembles the Greenlandic part of the North Atlantic Craton, suggesting the former contiguous nature of their lithosphere before subsequent rifting into separate continental fragments. Another, more recent type of mantle metasomatism, which affected the Chidliak mantle, is characterized by elevated Ti in pyroxenes and garnet typical of all rock types from CH-1, -7 and -44. These metasomatic samples are largely absent from the CH-6 xenolith suite. The Ti imprint is most intense in xenoliths derived from depths equivalent to 5·5–6·5 GPa where it is associated with higher strain, the presence of sheared samples of the lherzolite series and higher temperatures varying isobarically by up to 200 °C. The horizontal scale of the thermal-metasomatic imprint is more ambiguous and could be as regional as tens of kilometers or as local as <1 km. The time-scale of this metasomatism relates to a conductive length-scale and could be as short as <1 Myr, shortly predating kimberlite formation. A complex protracted metasomatic history of the North Atlantic Craton reconstructed from Chidliak xenoliths matches emplacement patterns of deep CO2-rich and Ti-rich magmatism around the Labrador Sea prior to the craton rifting. The metasomatism may have played a pivotal role in thinning the North Atlantic Craton lithosphere adjacent to the Labrador Sea from ∼240 km in the Jurassic to ∼65 km in the Paleogene.

Petrogenesis and tectonic setting of Late Paleozoic to Late Mesozoic igneous rocks in Cambodia

Emplacement of igneous rocks from the Carboniferous to the Middle Jurassic of Cambodia was controlled by the Indosinian Orogeny, which was related to the sequence of suturing and collision between the continental fragments from Gondwana. To constrain magma genesis, tectonic setting, and relationship with igneous rocks in neighboring countries, we determined the magnetic susceptibility, chemical compositions, Rb-Sr isochron ages and Sr-Nd isotope ratios for igneous rock samples from Cambodia. Igneous rocks in Cambodia can be divided by the inferred Mae Ping fault into magnetite series in the NE region and ilmenite series in the SW region. The igneous rocks in the NE region showed I-type or A-type and metaluminous signatures, and strong involvement of mantle source materials. They are subdivided into older and younger adakitic rocks and non-adakitic rocks. The igneous rocks in the SW region showed non-adakitic and slightly metaluminous to peraluminous signatures. These have significant involvement of source materials with relatively uniform Nd isotope ratios but variable Sr isotope ratios, which may have originated from the lower to middle continental crust. The Late Triassic to Middle Jurassic igneous rocks were formed by the subduction of the Paleo-Tethys Ocean crust associated with the collision of the Sibumasu Terrane with the Indochina Terrane (the Indosinian Orogeny II). The younger granitic rocks showed A-type signature and are regarded as post-orogenic, within the plate or rifting zone that intruded in association with the subduction of the Paleo-Pacific Ocean crust beneath Indochina during the Cretaceous to the Paleogene.

Miocene sedimentary provenance and paleogeography of the Hengchun Peninsula, southern Taiwan: Implications for tectonic development of the Taiwan orogen

The pre-collisional sedimentary provenance of southern Taiwan not only can offer important information about the paleogeography of the region prior to the Taiwan orogeny, but may also provide significant constraint for understanding the origin of the Central Range of Taiwan and the history of the orogen. Although there have been previous studies on sedimentary provenance of the Hengchun Peninsula, the southernmost part of the Taiwan mountain belt, these studies appear to have limited sample coverage. Therefore, we performed detrital zircon U-Pb geochronology and hafnium isotopic analysis of samples covering all three Mid-Late Miocene stratigraphic units of the peninsula. Our results suggest that, whereas the Minjiang (Min River) and the Jiulongjiang (Jiulong River) in SE China are the primary sediment sources for the Shihmen and Lilungshan Formations, the Loshui Formation was likely deposited in a deep sea fan originated from a rifted continental fragment in the south. The developing accretionary prism and proto-Taiwan may have also provided sediments to the area. The proposed presence of a continental fragment that provided sediments in the Loshui Formation supports the hypothesis that the Taiwan orogen may have formed as a double collision that incorporated not only the Luzon Arc and the Eurasian continental margin, but also rifted continental blocks in between.

Neoarchean arc magmatism and Paleoproterozoic granulite‐facies metamorphism in the Bhavani Suture Zone, South India

The Bhavani Suture Zone in the Southern Granulite Terrane marks the zone of amalgamation of the Neoarchean Nilgiri Block and the northwestern Madurai Block in southern India. Here, we report detailed petrological, geochemical, and geochronological data on the Mettupalayam mafic–ultramafic complex within this suture zone with a view to evaluate the tectonothermal history of the Bhavani Suture Zone and adjoining crustal blocks. The metamorphosed complex includes charnockite, hornblende‐biotite gneiss, mafic granulite, amphibolite, garnet‐bearing mafic granulite, and dioritic gneiss along with metamorphosed banded iron formation. The mafic granulite and the dioritic gneiss occur as concordant layers of varying thickness within the hornblende‐biotite gneiss. The salient geochemical features of the mafic granulite and the dioritic gneiss including the enrichment of large‐ion lithophile elements and depletion of high‐field‐strength elements suggest a subduction‐related arc magmatic setting. However, the amphibolites show MORB‐related affinity, suggesting its formation from a N‐MORB‐related source and their accretion together with the overlying banded iron formation. The peak metamorphic conditions of the garnet‐bearing mafic granulite were estimated using conventional geothermobarometers as 800–820 °C/8.5–9.2 kbar, which we further confirm through phase equilibrium modelling in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 (NCKFMASHTO) system. Magmatic zircons with high Th/U ratio from the amphibolite display a well‐defined discordia with an upper intercept age of 2,600 ± 38 Ma. Zircon grains from the dioritic gneiss show weighted mean 206Pb/207Pb age of 2,524 ± 6 Ma from concordant zircon spots and a comparable upper intercept age of 2,562 ± 34 Ma from discordant zircon spots, indicating protolith emplacement related to Neoarchean arc magmatism as inferred from our geochemical data. The thin overgrowth rims around the magmatic zircon grains in the amphibolite yielded an upper intercept age of 2,520 ± 20 Ma, which is comparable with the protolith crystallization age of the dioritic gneiss. Slightly younger weighted mean 207Pb/208Pb ages of 2,463 ± 27 Ma (from dioritic gneiss) and 2,422 ± 30 Ma (from amphibolite) are also obtained from the metamorphic zircon rims. These ages are correlated to the timing of high‐grade metamorphism associated with final collision of the Nilgiri Block and the northwestern Madurai Block. Similar Neoarchean–Paleoproterozoic magmatism and high‐grade metamorphism were reported from many localities south of the Dharwar Craton. Our study further confirms the previous tectonic model that envisages multiple subduction and collision of magmatic arcs and continental fragments towards the Dharwar Craton during the Archean–Paleoproterozoic transition.

The origin of diversity in insects: Speciation, adaptation and the Earth dynamics

The diversity of insects is often explained as a product of major radiations, triggered by remarkable adaptations that allowed them to exploit different environments and to accompany the rise of flowering plants. A significant part of insect diversity results however from allopatric speciation with niche conservatism that exposed species to different conditions and stimulated adaptive divergence. We demonstrated that for several groups of Insects (cockroaches, crickets and grasshoppers) in the island of New Caledonia. Since Darwin and Wallace, islands are considered as laboratories of evolution, being closed systems of manageable size. New Caledonia is especially interesting in this respect, according to unique characteristics. Being the oldest oceanic island in the world and quite isolated from continents, biological evolution occurred there in situ during long periods. As a tropical and medium-sized island harboring a very rich biodiversity, it also offered facilities to study many different evolutionary questions. New Caledonia (hereafter NC) has thus the best characteristics as a model system, allowing testing very diverse evolutionary assumptions with limited efforts and infrastructures. Unfortunately, these amazing geographical characteristics have also constrained the scientific study of NC until now, especially because of its isolation from large academic centers. Establishing NC as a modern oceanic model system resulted however from our work, later than the 1990s. Before, NC had the reputation of an amazingly old Gondwanan place and was intuitively considered as a continental fragment. If so, its usefulness as a model system to study evolution would have been limited since the biota of continental islands (e.g., Madagascar or New Zealand) are a complicated mix of organisms dating back to the separation with neighboring continental territories or having colonized the island by dispersal. In most of these cases, it happened difficult to distinguish between both these different components and to examine evolutionary assumptions that require a reference dating point. The reasons why NC was traditionally considered a continental territory were the age of its deep geological basement and the local occurrence of relict species. In agreement with this common assumption, the NC geological deep basement is ancient and predates Gondwanan breakup. But this basement has been submitted to important environmental disturbances because of the island location at the limit between two tectonic plates. Actually, the island has been submerged twice for a long time at Paleocene and Eocene epochs and only emerged around 37 ± 3 Ma. This geological background was in need of independent testing with biological studies. We thus built multiple molecular phylogenetic trees of different groups of organisms, dated with probabilistic methods and external calibration points. A first review [1] and a more recent meta-analysis [2] of these studies showed that most groups colonized NC and diversified just after 37 My, confirming the geological scenario of recent terrestrial emergence. However, a few local species belong to groups that were dated much older than 37 My, representing true relicts, i.e. recent species that remained from old clades [3]. These old clades can only be assumed to have been ancestrally present in the region, colonizing NC after its emergence and going extinct in other territories, either drowned islands or mutated continental ecosystems. These different results strongly indicated that NC is an oceanic island, even if very old–actually the oldest one in the world–and that its local biota dated back to island emergence. Relicts are fascinating species, unfortunately too sparsely known after the major extinction of their relatives to support any robust biogeographic scenario. Fossil data are absolutely needed to assess the evolutionary history of these taxa without speculation. Setting up this biogeographic scenery has allowed one to properly conduct speciation studies by inferring the age and the ancestral phenotypic characters of local lineages. We did that in several groups of insects, cockroaches, crickets, and grasshoppers, and we showed that speciation occurred mainly in relation to orography, in allopatry and with niche conservatism. Closely related species occurred as narrow endemics in the same habitats of neighboring areas, mainly on adjacent small mountains. Sympatry, when observed, was inferred to be secondarily caused by the increase of distribution area of older species. Even if most speciation events seemed to have occurred without any important evolutionary change, some adaptive shifts have been detected in several cases, from different food plants on metalliferous soils to different habitats in the forest understory. The major conclusion that can be drawn from high speciation rate and narrow endemicity in New Caledonia is that speciation went before, not always together with adaptive divergence. Here we offer both a new and powerful natural laboratory of evolution, calibrated in space and time, and a model of speciation where niche conservatism is the engine for adaptive divergence.

Cyclical one-way continental rupture-drift in the Tethyan evolution: Subduction-driven plate tectonics

Numerous continents have rifted and drifted away from Gondwana to repeatedly open ocean basins over the past ~500 million years. These Gondwana-derived continents drifted towards and collided with components of the Eurasian continent to successively close the preexisting oceans between the two. Plate tectonics satisfactorily describes the continental drift from Gondwana to Eurasia but does not define the geodynamic mechanism of continuously rifting to collisions of continents in the Tethyan Realm. After reappraisal of geological records of the rift, collision and subduction initiation from the surface and various geophysical observations from depth, we propose that Eurasia-directed subducting oceanic slabs would have driven Tethyan system in the Phanerozoic. The Eurasia-directed subduction would have dragged the passive Gondwana margin to rift and drift northwards, giving birth to new oceans since the Paleozoic. The closure of preexisting oceans between the Gondwana-derived continents and Eurasia led to continental collisions, which would have induced the initiation of oceanic subduction in the Tethyan Realm. Multiple episodic switches between collision-subduction-rift repeatedly led to the separation of continental fragments from Gondwana and dragged them to drift towards Eurasia. The final disappearance of Neo-Tethys would have induced collision of the Gondwana-derived continents with the Eurasian continent, giving rise to the Cenozoic Alpine-Zagros-Himalayan collisional system. Therefore, the Eurasia-directed oceanic subduction would have acted as a ‘one-way train’ that successively transferred the ruptured Gondwana continental fragments in the south, into the terminal in the north. In this regard, the engine of this “Tethyan one-way train” is the negative buoyancy of subducting oceanic slabs.

Zircon U–Pb ages and Hf isotope compositions of the Neoproterozoic magmatic rocks in the Helan Mountains, North China

AbstractThe periodic dispersal and assembly of continental fragments has been an inherent feature of the continental crust. Based on the discovery of large-scale supercontinent cycle and the theory of plate tectonics, several supercontinents have been identified, such as Columbia/Nuna, Rodinia, Gondwana and Pangaea. Neoproterozoic magmatic events related to the break-up of Rodinia are globally well preserved. Although Neoproterozoic magmatic events were very weak in the North China Craton (NCC), they are crucial in reconstructing the geometries of the NCC and could facilitate the completion of the Neoproterozoic configuration of the supercontinent. In this study, c. 853–835 Ma magmatic rocks are identified in the western margin of the NCC. Precise zircon U–Pb age determination yields 206Pb/238U average ages of 835.5 ± 5.3 Ma (HL-39) and 853.7 ± 4.5 Ma (HL-30). In situ zircon Hf isotope compositions of the samples reveal that their parental magma was formed by the reworking of ancient crust evolved from Mesoproterozoic mantle. In summary, the discovery of Neoproterozoic magmatic rocks in the western margin of the NCC, and reported synchronous rocks in other parts of the NCC indicate that the NCC might be conjoined with the supercontinent Rodinia during the Neoproterozoic. This discovery is of significant help in unravelling the early Neoproterozoic history of the NCC and the evolution of the supercontinent Rodinia.

The uppermost mantle seismic velocity structure of West Antarctica from Rayleigh wave tomography: Insights into tectonic structure and geothermal heat flow

We present a shear wave model of the West Antarctic upper mantle to ∼200km depth with enhanced regional resolution from the 2016-2018 UK Antarctic Seismic Network. The model is constructed from the combination of fundamental mode Rayleigh wave phase velocities extracted from ambient noise (periods 8-25s) and earthquake data by two-plane wave analysis (periods 20-143s). We seek to (i) image and interpret structures against the tectonic evolution of West Antarctica, and (ii) extract information from the seismic model that can serve as boundary conditions in ice sheet and glacial isostatic adjustment modelling efforts. The distribution of low velocity anomalies in the uppermost mantle suggests that recent tectonism in the West Antarctic Rift System (WARS) is mainly concentrated beneath the rift margins and largely confined to the uppermost mantle (<180km). On the northern margin of the WARS, a pronounced low velocity anomaly extends eastward from beneath the Marie Byrd Land dome toward Pine Island Bay, underlying Thwaites Glacier, but not Pine Island Glacier. If of plume-related thermal origin, the velocity contrast of ∼5% between this anomaly and the inner WARS translates to a temperature difference of ∼125-200C∘. However, the strike of the anomaly parallels the paleo-Pacific convergent margin of Gondwana, so it may reflect subduction-related melt and volatiles rather than anomalously elevated temperatures, or a combination thereof. Motivated by xenolith analyses, we speculate that high velocity zones imaged south of the Marie Byrd Land dome and in the eastern Ross Sea Embayment might reflect the compositional signature of ancient continental fragments. A pronounced low velocity anomaly underlying the southern Transantarctic Mountains (TAM) is consistent with a published lithospheric foundering hypothesis. Taken together with a magnetotelluric study advocating flexural support of the central TAM by thick, stable lithosphere, this points to along-strike variation in the tectonic history of the TAM. A high velocity anomaly located in the southern Weddell Sea Rift System might reflect depleted mantle lithosphere following the extraction of voluminous melt related to Gondwana fragmentation. Lithospheric thickness estimates extracted from 1D shear wave velocity profiles representative of tectonic domains in West Antarctica indicate an average lithospheric thickness of ∼85km for the WARS, Marie Byrd Land, and Thurston Island block. This increases to ∼96km in the Ellsworth Mountains. A surface heat flow of ∼60mW/m2 and attendant geotherm best explains lithospheric mantle shear wave velocities in the central WARS and in the Thurston Island block adjacent to Pine Island Glacier; a ∼50mW/m2 geotherm best explains the velocities in the Ellsworth Mountains, and a ∼60mW/m2 geotherm best explains a less well-constrained velocity profile on the southern Antarctic Peninsula. We emphasise that these are regional average (many hundreds of km) heat flow estimates constrained by seismic data with limited sensitivity to upper crustal composition.

Paleomagnetism of the Chuar Group and evaluation of the late Tonian Laurentian apparent polar wander path with implications for the makeup and breakup of Rodinia

AbstractPaleogeographic models commonly assume that the supercontinent Rodinia was long-lived, with a static geometry involving Mesoproterozoic links that developed during assembly and persisted until Neoproterozoic rifting. However, Rodinian paleogeography and dynamics of continental separation around its centerpiece, Laurentia, remain poorly constrained. On the western Laurentian margin, geological and geochronological data suggest that breakup did not occur until after 720 Ma. Thus, late Tonian (ca. 780–720 Ma) paleomagnetic data are critical for reconstructing paleogeography prior to dispersal and assessing the proposed stasis of Rodinia. Here, we report new paleomagnetic data from the late Tonian Chuar Group in the Grand Canyon, Arizona. We combined this new data set with reanalyzed existing data to obtain a new paleopole preserved in hematite, the reliability of which is supported by six of the seven (Q1–Q6) Van der Voo reliability quality criteria. In addition, we identified pervasive mid- to high-temperature overprints. This new paleomagnetic pole was incorporated with recent high-precision geochronological data and existing paleomagnetic data to present a new late Tonian Laurentian apparent polar wander path (APWP). Having examined the paleomagnetic data of other cratons, global reconstructions for 775 Ma, 751 Ma, and 716 Ma are presented. These reconstructions are consistent with Australia located near the present southern margin of Laurentia. However, a stringent analysis of the global data set does not support a good match between any major craton and the rifted conjugate margin to western Laurentia. Breakup on the western Laurentian margin may have involved rifting of a continental fragment or a craton with uncertainties in its late Tonian geochronologic and paleomagnetic constraints. Our revised Laurentian APWP will allow for more robust tests of paleogeography and evaluation of the proposed supercontinent Rodinia.

Precambrian basement, provenance implication, and tectonic evolution of the Gargan block of the Tuva-Mongolia terranes, Central Asian Orogenic Belt

Although the Phanerozoic geology of the Central Asian Orogenic Belt is extensively studied, there is still much to reveal about the initiation and early stages of tectonism. The Gargan block of the Tuva-Mongolia terranes at the Mongolian-Russian border is one of the Archean-Proterozoic continental crustal fragments that record the earliest evolutionary history of the belt. To better constrain the age of its basement, extent, and provenance in regional tectonic framework, we present new zircon UPb geochronologic data from previously undated basement gneisses and the overlying Neoproterozoic meta-sedimentary rocks of southern part of the Gargan block. Comparison of available basement ages and Precambrian detrital zircon spectra from nearby Archean-Proterozoic continental crustal fragments show complex relationship amongst them. We propose that the Gargan block travelled by itself in most of the Proterozoic or was derived from an unknown continental block. Obtained age confirms the existence of Neoarchean-Neoproterozoic basement in the region; however, for the first time, the 2749–814 Ma Salig Complex reveals multiple stages of Neoproterozoic metamorphic events within the Central Asian Orogenic Belt. Older of which, at ∼1 Ga, may relate to an assembly of several of the pre-existing continental crustal fragments in Mongolia.

Dating Gondwanan continental crust at the Rio Grande Rise, South Atlantic

AbstractCrystalline continental rocks and associated crust‐contaminated basaltic rocks were unexpectedly dredged on the crest and at seamounts of the Rio Grande Rise, South Atlantic. Zircon U–Pb ages of one gabbro (ca. 2,200 Ma) and four granitoids (between ca. 1,430–480 Ma) indicate that the breakup of SW Gondwana left behind continental fragments of dominantly African age. These rocks may have been incorporated into the oceanic lithosphere by complex processes including rifting and interaction of the Tristan‐Gough mantle plume with hyperextended continental margins. Until ca. 80–70 Ma, the Rio Grande Rise and an old portion of the Walvis Ridge formed a conjugate pair of aseismic ridges, and the Tristan‐Gough plume was positioned at the Mid‐Atlantic Ridge. The finding of continental rock fragments in one of these conjugate pairs opens new perspectives on the mechanisms of continental break‐up, the nature of this conjugate pair, and the geodynamic evolution of rifted Gondwana margins in the South Atlantic.

Deformable plate tectonic models of the southern North Atlantic

Significant, poly-phase deformation occurred prior to, simultaneous with, and after the opening of the North Atlantic Ocean. Understanding this deformation history is essential for understanding the regional development and the mechanisms controlling rifting and subsequent failure or breakup. Here, we primarily use published constraints to construct deformable plate tectonic models for the southern North Atlantic from 200 Ma to present using GPlates. The aim of this work is to test both the capability of the GPlates deformable modelling approach and the reliability of published plate reconstructions. Overall, modelled crustal thickness values at 0 Ma produced from the deformable models show general, regional-scale, similarities with values derived from the inversion of gravity data for crustal thickness. However, the deformable models typically underestimate thinning in marginal basins and overestimate crustal thickness in continental fragments compared to values from gravity inversion. This is possibly due to: 1) thinning occurring earlier than the 200 Ma start time modelled, 2) variations in the original crustal thickness, 3) depth-dependent stretching, 4) rigid blocks undergoing some degree of thinning, and 5) variations in the mesh density of the models. The results demonstrate that inclusion of micro-continental fragments, and locally defined limits of continental crust, generally produce results more akin to observations. One exception is the Grand Banks where global GPlates models produce more realistic deformation, likely due to the inclusion of the exhumed domains continent-ward of the transition zone boundary. Results also indicate that Flemish Cap rotation is required to provide a reasonable fit between North America and Iberia, with the palaeo-position of the Flemish Cap likely to be the proto-Orphan sub-basins. Moreover, the East and West Orphan sub-basins formed separately due to the respective rotations of the Flemish Cap and the Orphan Knoll, which was likely associated with other continental fragments that subsequently contributed to the thicker crust forming the boundary between the East and West Orphan basins. The results also suggest a link between tectonic and magmatic processes. For example, the inclusion of an Orphan Knoll micro-continental block results in greater extension (higher beta factors) in the northern West Orphan Basin near the termination of the Charlie-Gibbs Fracture Zone, and the site of the Charlie-Gibbs Volcanic Province (CGVP). Thus, we infer that the CGVP was likely influenced by plate tectonic processes through the concentration of strain resulting from interaction in proximity to the transform system. Finally, marginal basins that were considered to be conjugate and thus related, may only appear conjugate through later rotation of micro-continental blocks, and thus their genesis is not directly related.

Structural evolution of the Eastern Anatolian Basins: an example from collisional to postcollisional tectonic processes, Turkey

The Eastern Anatolian Basins (EABs) started to develop on a mosaic, made up of forearc deposits of the Eastern Pontide Arc (EPA), continental metamorphic rocks of the Anatolian-Iranian Platform (AIP), and obducted ophiolitic units of the North Anatolian Ophiolitic Belt (NAOB) and Southeastern Anatolian Ophiolitic Belt (SAOB). The basement rocks of the East Anatolian Region represent a continental fragment positioned between the EPA to the north and the Arabian Platform to the south. In appearance, it can be said that there are many different basins in the Eastern Anatolian Region. This paper reports on three of them that were studied as a part of the work, defining the setting and structural evolution of the EABs as a whole. These are, from north to south, the Oltu-Balkaya Basin, the Tekman-Karayazi Basin, and the Mus Basin. The EABs started to develop in the beginning of the Maastrichtian age with tectonic subsidence. They then evolved as collisional foreland basins on the AIP and ophiolitic units of the NAOB and SAOB during the Paleocene and Early Eocene period, as well. The fill of these basins is represented by basal conglomerate, shallow marine limestone, and clastic rock units. During deposition of this fill, syncollisional Paleocene peraluminous leucogranitic magmatism developed along the Eastern Pontides. From the Middle Eocene to the Quaternary age, different superimposed postcollisional basins developed over the collisional foreland basin fill. In this time interval, a shallow marine-continental molasse deposition was accompanied with high-K calc-alkaline volcanism, which may be attributed to postcollisional magmatism in the Eastern Anatolian Region. As a result, for the structural evolution of EABs, it can be said that the EABs represent a foreland basin in the Maastrichtian-Early Eocene and superimposed postcollisional basins in the Middle Eocene-Quaternary interval.