Crustal Units Research Articles

The Neoproterozoic evolution of the western Siberian Craton margin: U-Pb-Hf isotopic records of detrital zircons from the Yenisey Ridge and the Prisayan Uplift

We present new LA ICP-MS U-Pb-Hf detrital zircon data from Neoproterozoic sedimentary basins located along the south-western margin of the Siberian Craton. In the Yenisey Ridge (the Taseeva basin), siliciclastic sedimentary units contain abundant 2.7–2.5 Ga, 2.0–1.75 Ga and 0.95–0.57 Ga detrital zircon populations, whereas for the Prisayan Uplift slightly different detrital zircon ages of 3.0–2.5 Ga, 2.2–1.8 Ga and 0.95–0.65 Ma are common. Mid Neoproterozoic sandstones were deposited in a rift setting or along a rifted cratonic margin facing a back-arc basin, whereas the immature to submature upper Ediacaran to Lower Cambrian fluvial successions were probably deposited in the foredeep setting, at the expense of proximal Neoproterozoic igneous suites and the underlying metasedimentary basement. A range of εHf values from −10 to +14 indicates intense mixing between juvenile and evolved magma components in a continental arc setting at ca. 950–570 Ma. Geological evidence, persistent input of detrital zircon material derived from ancient crustal units of western Siberian cratonic margin and evolution of Hf isotopic compositions suggest that the Neoproterozoic arc system of the Yenisey Ridge and, partly, Sayan orogens developed along the active margin of Siberia. In the Neoproterozoic, the arc system probably extended along the periphery of Rodinia into the Taimyr orogen of northern Siberia and Valhalla orogen of north-eastern Laurentia.

The Neoproterozoic-early Paleozoic evolution of the Jiamusi Block, NE China and its East Gondwana connection: Geochemical and zircon U–Pb–Hf isotopic constraints from the Mashan Complex

The Jiamusi Block is located in the easternmost Central Asian Orogenic Belt (CAOB) and its tectonic affinity and evolution is highly controversial. Here we describe newly discovered Neoproterozoic orthogneisses and early Paleozoic granitoids of the Mashan Complex from the Jiamusi Block. SIMS U-Pb dating for zircons from three high-grade orthogneisses yields protolith and metamorphic ages ranging from 757 to 751Ma and from 530 to 476Ma, respectively. LA-ICP-MS U-Pb dates for zircons from four gneissic granitoids define two stages of early Paleozoic magmatism (i.e., 530–502Ma, and 490–476Ma). These new data, together with recent published ages, indicate that the basement rocks of the Jiamusi Block record an important Neoproterozoic-early Paleozoic magmatic (ca. 895–476Ma) and metamorphic (ca. 563–476) event, and we accordingly infer that the granulite facies metamorphism might have occurred before 541–476Ma, and around 567–561Ma. Geochemical and zircon Hf isotopic data suggest that the ca. 752Ma rift-related gneisses were derived from dominantly juvenile crustal rocks with both the heat and material involvement of a depleted asthenosphere mantle at relatively low pressures, which then underwent an ancient wall-rock assimilation, whereas the ca. 490–476Ma post-collisional granitoids originated from reworking of an ancient crustal source and subsequently experienced intensive crystal fractionation. The good correlation on the ca. 755–115Ma geological events between the Jiamusi and Khanka blocks suggests they formed a contiguous crustal unit. The existing data indicate that this combined block cannot be directly derived from the Siberia, North China, South China, and Tarim cratons. Instead, it might be either a rifted portion of the Sayang-Baikal orogen or derived from a peri-Gondwana position. We evaluate the model that the Jiamusi-Khanka Block may have originated as part of the Late Neoproterozoic-Cambrian Kuunga orogen trapped between South India and East Antarctica-Australia. Sometime after ca. 470Ma, it rifted away from East Gondwana and drifted northward to collide with the Songliao Block of NE China in the Early Jurassic.

Crustal strength control on structures and metamorphism in collisional orogens

Although the upper crust is the most accessible among other crustal units, so far little attention was paid about its role on the continental collision style and on exhumation processes of metamorphic rocks. With this respect, several 2D thermo-mechanical numerical experiments of post-subduction continental collision were performed, which allowed investigating the interplay between surface processes, thermal history and rheology of the upper crust units. The viscous strength of upper and lower plate upper crust (U.U.C. and L.U.C.) was systematically changed, obtaining a reference set of experiments to which subsequently we applied enhanced erosion patterns and variable radiogenic heat productivity of the sediments.We obtained three classes of collisional style: 1) decoupled asymmetric; 2) coupled asymmetric; 3) coupled symmetric. In general, for high-rates of radiogenic heating, a strong U.U.C. leads to one-sided, asymmetric and coupled hot orogen, which shifts to two-sided symmetric for weaker U.U.C. Lower radiogenic heat productivities yield colder orogens, promoting more asymmetric orogens and eventually the decoupling between upper and lower plates. The enhanced erosion leads to narrower and colder orogens, affecting the collisional style only for a relatively strong L.U.C. rheology.Exhumation of high-grade metamorphic rocks mainly depends on the L.U.C. strength: a weak viscous rheology promotes pure shear thickening of the incoming crust in the orogenic front, which inhibits both burial and exhumation processes. Higher viscous strengths promote deeper subduction of the L.U.C., with subsequent heating, delamination, partial melting and exhumation of high-grade rocks via channel flow. Enhanced erosion, although not a necessary condition, favours the extrusion of the partially melted mid-crustal channel.Overall, the numerical experiments reproduce the first order structural and/or metamorphic patterns of several natural cases (Himalayas, Boehmian Massif, Eastern-Central Alps, Apennines) and further suggest that the upper crust rheology is one of the key parameters controlling the collisional style and exhumation processes.

Lateral displacement of crustal units relative to underlying mantle lithosphere: Example from the Bohemian Massif

We propose a mechanical model of deformation of the entire lithosphere of the Bohemian Massif (BM), whose core is formed by an asymmetric block of the Teplá-Barrandian (TB) unit in between the Saxothuringian (ST) and Moldanubian (MD) units. For the modelling, we have re-processed P-wave travel times recorded during the last two decades at dense networks of seismic stations installed in the BM during several passive seismic experiments. We also use previous results of anisotropic studies based on splitting of teleseismic shear waves. This allows us to refine estimates of the lithosphere thickness and delimit deep margins of the individual mantle lithosphere domains. The domains are rigid enough to preserve pre-orogenic olivine fabrics differently oriented in each of the units. Shapes and dips of the mantle boundaries, representing major zones of weakness inherited from the Variscan amalgamation of independent microplates, indicate that north-westward subductions beneath the TB unit dominated tectonic development of the core of the BM. Two mantle lithosphere domains with different fabric orientations, separated by a WSW-ENE striking shear zone, underlie the TB crust. The NW domain is the TB mantle lithosphere, while the SE domain is the MD mantle lithosphere thrust under the TB crust. Lithosphere of the north-western TB domain, compressed between early Variscan subductions of the ST continental lithosphere from the northwest and the MD continental lithosphere from the southeast, was pushed south-westward by about 50km. Though the crust of the south-westerly TB promontory is commonly attributed to the MD unit, apparently it preserves the TB mantle lithosphere. The shifted TB lithosphere provides compelling evidence in support of older views suggesting that the Zone Erbendorf-Vohenstrauss (ZEV) originally belonged to the tilted western rim of the TB unit. During the final phase of the assemblage of the BM, the rigid TB lithosphere was disrupted by the southward pushing ST lithosphere along the newly formed NW-SE striking Jáchymov Fault Zone (JFZ). This lithosphere-scale process most likely changed the tectonic regime, released subduction-related forces and started the gravity-dominated tectonics.

Paleoproterozoic (ca. 1.8 Ga) arc magmatism in the Lützow-Holm Complex, East Antarctica: Implications for crustal growth and terrane assembly in erstwhile Gondwana fragments

The Lützow-Holm Complex (LHC) of East Antarctica forms a part of the latest Neoproterozoic–Cambrian high-grade metamorphic segment of the East African–Antarctic Orogen. Here we present new petrological, geochemical, and zircon U–Pb geochronological data on meta-igneous rocks from four localities (Austhovde, Telen, Skallevikshalsen, and Skallen) in the LHC, and evaluate the regional Paleoproterozoic (ca. 1.8Ga) arc magmatism in this terrane for the first time. The geochemical features reveal a volcanic-arc affinity for most of the meta-igneous rocks from Austhovde and Telen, suggesting that the protoliths of these rocks were derived from felsic to mafic arc magmatic rocks. The protoliths of two mafic granulites from Austhovde are inferred as non-volcanic-arc basalt such as E-MORB, suggesting the accretion of remnant oceanic lithosphere together with the volcanic-arc components during the subduction–collision events. The weighted mean 206Pb/238U ages of the dominant population of magmatic zircons in felsic orthogneisses from Austhovde and Telen show 1819±19Ma and 1830±10Ma, respectively, corresponding to Paleoproterozoic magmatic event. The magmatic zircons in orthogneisses from other two localities yield upper intercept ages of 1837±54Ma (Skallevikshalsen), and 1856±37Ma and 1854±45Ma (Skallen), which also support Paleoproterozoic magmatism. The earlier thermal events during Neoarchean to Early Paleoproterozoic are also traced by 206Pb/238U ages of xenocrystic zircons in the felsic orthogneisses from Austhovde (2517±17Ma and 2495±15Ma) and Telen (2126±16Ma), suggesting partial reworking of the basement of a 2.5Ga microcontinent during ca. 1.8Ga continental-arc magmatism. The timing of peak metamorphism is inferred to be in the range of 645.6±10.4 to 521.4±12.0Ma based on 206Pb/238U weighted mean ages of metamorphic zircon grains. The results of this study, together with the available magmatic ages as well as geophysical and lithological data from the region, suggest that the LHC can be divided into three units: Neoarchean (ca. 2.5Ga) unit in the southern LHC (Shirase Orthogneiss or “Shirase microcontinent”), Neoproterozoic (ca. 1.0Ga) unit in the northern LHC, and supracrustal unit in the central LHC with fragments of Paleoproterozoic (ca. 1.8Ga) and minor Neoarchean (ca. 2.5Ga) and Neoproterozoic (ca. 1.0Ga) magmatic arcs. The 1.8Ga arc magmatism inferred in this study has also been reported from adjacent Gondwana fragments such as the Highland Complex in Sri Lanka, and the Trivandrum and Nagercoil Blocks in southern India. Although the ca. 1.8Ga arc-magmatic event is coeval in these regions, the Paleoproterozoic supracrustal unit in the central LHC may not be contiguous with those in the Highland Complex of Sri Lanka because recent studies have shown that the Vijayan Complex in Sri Lanka and the ca. 1.0Ga northern LHC possibly were part of a single crustal unit (northern Lützow-Holm–Vijayan Complex) within the Kalahari Block. The supracrustal unit possibly marks part of a discrete suture formed by the collision of the ca. 2.5Ga southern LHC (Shirase microcontinent) and the ca. 1.0Ga northern Lützow-Holm–Vijayan Complex during the latest Neoproterozoic–Cambrian Gondwana amalgamation, which might be coeval with the collision of the Vijayan and Wanni Complexes and the formation of the Highland Complex in Sri Lanka. Our study provides new insights on crustal growth and terrane assembly in the ancient continental blocks of Gondwana.

Possible mechanism of horizontal overpressure generation of the Khibiny, Lovozero, and Kovdor ore clusters on the Kola Peninsula

The paper discusses questions related to the generation of increasing crustal horizontal compressive stresses compared to the idea of the standard gravitational state at the elastic stage or even from the prevalence of horizontal compression over vertical stress equal to the lithostatic pressure. We consider a variant of superfluous horizontal compression related to internal lithospheric processes occurrin in the crust of orogens, shields, and plates. The vertical ascending movements caused by these motions at the sole of the crust or the lithosphere pertain to these and the concomitant exogenic processes giving rise to denudation and, in particular, to erosion of the surfaces of forming rises. The residual stresses of the gravitational stressed state at the upper crust of the Kola Peninsula have been estimated for the first time. These calculations are based on the volume of sediments that have been deposited in Arctic seas beginning from the Mesozoic. The data speak to the possible level of residual horizontal compressive stresses up to 90 MPa in near-surface crustal units. This estimate is consistent with the results of in situ measurements that have been carried out at the Mining Institute of the Kola Science Center, Russian Academy of Sciences (RAS), for over 40 years. It is possible to forecast the horizontal stress gradient based on depth using our concept on the genesis of horizontal overpressure, and this forecasting is important for studying the formation of endogenic deposits.

Crustal basement controls granitoid magmatism, and implications for generation of continental crust in subduction zones: A Sr–Nd–Hf–O isotopic study from the Paleozoic Tongbai orogen, central China

Ascertaining the petrogenesis of granitoid rocks in subduction zones holds the key for understanding the processes of how continental crust is produced. The synchronous Taoyuan and Huanggang plutons occur in two different geological units of the Paleozoic Tongbai orogen of central China. They provide an optimal opportunity for a study to address the role of the crustal basement in generating voluminous granitoid magmatism in subduction zones. The Taoyuan and Huanggang plutons have identical U–Pb zircon crystallization ages of 440–444Ma, which are temporally related to northward subduction of the Paleotethyan Ocean. The Taoyuan samples show high SiO2 (73.36–79.16%) and low Al2O3 (12.00–13.45%) contents, Mg numbers (20.6–38.2), and Sr/Y (2.04–10.1) and (La/Yb)N (2.34–7.32) ratios with negative Eu anomalies (Eu/Eu*=0.33–0.93). They yielded positive εNd(t) (+3.0 to +6.7) and εHf(t) (+11.8 to +13.2) values, elevated initial Sr isotopic ratios (0.7040–0.7057) and relatively low zircon δ18O values of 4.62–5.39‰. These suggest that they were produced through partial melting of hydrothermally altered lower crust of the accreted Erlangping oceanic arc. In contrast, the Huanggang samples exhibit variable whole-rock geochemical and isotopic compositions with SiO2 contents of 57.01–64.42wt.%, initial Sr isotopic ratios of 0.7065–0.7078, and εNd(t) values of −5.7 to −9.4. Additionally, they have high zircon δ18O values of 7.57–8.45‰ and strongly negative zircon εHf(t) values of −14.4 to −10.5. They were suggested to have been mainly derived from ancient continental crust of the Kuanping crustal unit with the addition of 20–40% juvenile, mantle-derived material. Accordingly, the granitoids in both oceanic and continental arcs are likely to be mainly derived from intracrustal melting of their crustal basement. It is revealed by the Huanggang pluton that little net continental crust growth occurs in continental arcs, and addition of new volume of continental crust may be balanced by the loss of old mafic lower continental crust. Continental growth was likely achieved through the accretion and subsequent differentiation of oceanic arcs, as manifested by the Taoyuan pluton.

Basement structure of the Paleozoic Platform in Poland

The Paleozoic Platform comprises the southwestern half of the Polish territory, separated from the Precambrian East European Platform (EEP) by a NW–SE trending subvertical lithospheric-scale discontinuity – the Teisseyre-Tornquist Zone. In the present study, the Paleozoic Platform basement is subdivided based on geological and geophysical evidence acquired during the last decades, including deep seismic refraction and reflection results, as well as gravity, magnetic and magnetotelluric data. The units adjacent to the EEP, the Mid-Polish Domain (comprising the Łysogory Block) and the Pomeranian Block, are characterized by a thinned three-layer EEP-type crystalline crust (age 1.7–3.7 Gy). Their oldest platform cover is probably composed of thick Ediacaran syn-rift sedimentary and volcanic rocks. Their western extension in the contiguous German and Baltic area can be traced as far as the East Elbian Massif and Thor Suture based mainly on magnetic anomaly patterns. The Upper Silesian and Malopolska blocks located in SE Poland are characterized by Cadomian (660–600 My) basement overlain by the thick deformed Ediacaran foredeep deposits. Whereas the units with the EEP-type basement are interpreted as proximal terranes displaced along the EEP margin during the late Early Paleozoic, the blocks with a Cadomian basement are conceived as exotic terranes of a possible Gondwanan provenance. All these terranes were accreted ultimately during the final, latest Silurian stages of the Caledonian deformation although the exact scenario of the amalgamation remains hypothetical. The Sudetic Domain in SW Poland is a collage of different crustal units juxtaposed ultimately in the Pennsylvanian (ca. 310 Ma) during the final episodes of the Variscan Orogeny. Their northern boundary corresponds to the Rheic Suture traceable in magnetic and gravity anomalies pattern along the northern margin of the Wolsztyn–Leszno High, and continuing westwards along the Mid-German Crystalline High. The arcuate trace of the suture in the NE is dextrally displaced along the Odra Fault, and continues southwards where it is mapped as the Moravian–Silesian Suture.

Geology of the Gobi Altai and Tseel terranes in the central part of the Sagsai River Watershed, SE Mongolian Altai

ABSTRACTA geological map is an indispensable tool for understanding the structure of the Earth’s crust but high-quality geological maps are usually lacking in remote areas of mountainous Central Asia covered by vast deserts. The progress in remote sensing and geographical information system (GIS), as well as the advancement in analytical methods, have generated new challenges in producing modern geological maps in such regions. The presented 1:50,000 geological map along the Sagsai River summarizes new and more accurate geological data from the geologically interesting region at the contact of the supracrustal and deep crustal Tugrug and Tseel units forming the metamorphosed accretionary wedge on the S and SW slopes of the Mongolian Altai. These geological units are formed by the lower Palaeozoic volcano-sedimentary sequences affected by Devonian Barrovian metamorphism ranging from lower greenschist to granulite facies. This metamorphic basement was subsequently intruded by the post-orogenic late Carboniferous Sagsai Pluton. The presented map shows complex relationships between different crustal levels of the metamorphosed accretionary wedge and post-tectonic intrusion evaluated using a GIS, numerical processing of remote sensing data as well as field documentation and laboratory studies.

Metamorphic evolution of Palaeoproterozoic anatectic migmatites in the eastern part of the Aravalli–Delhi Fold Belt, India: constraints from thermodynamic modelling and monazite dating

AbstractGarnetiferous pelitic to psammopelitic migmatites widespread across the central and eastern part of the Aravalli–Delhi Fold Belt in NW India record two distinct orogenies, e.g. the Aravalli Orogeny (1.7–1.6 Ga) and the Delhi Orogeny (1.0 Ga). In this study, we integrate field geological studies with textural and mineral–chemical analyses,P–Tpseudosection modelling andin situmonazite dating in anatectic migmatites in the Aravalli Supergroup occurring along the Deoli–Shahpura segment. The study reveals formation of peak assemblages of garnet + sillimanite + biotite + K-feldspar + melt and garnet + muscovite + K-feldspar + melt in two anatectic migmatite samples.P–Tpseudosection modelling suggests that anatexis in the gneisses occurred at ~8 kbar and 700–800°C along a tight-loop clockwiseP–Tpath. Monazite ages from the migmatites indicate that the anatexis occurred at ~1.73–1.74 Ga. This age is similar to the Palaeoproterozoic anatexis (at 7–8 kbar) and charnockite emplacement in the Sandmata and the Mangalwar complexes, the subsolidus amphibolite-facies metamorphism in the Rajpura–Dariba and Pur–Banera supracrustal belts, and the A-type granite magmatism in the North Delhi Fold Belt. We propose that the Palaeoproterozoic migmatites in central and eastern Rajasthan are part of the one crustal unit that underwent anatexis during an accretion event along the NE–SW-trending Aravalli orogenic belt.

Collision of Manihiki Plateau fragments to accretional margins of northern Andes and Antarctic Peninsula

The Manihiki Plateau, a large igneous province (LIP), was emplaced in the Early Cretaceous as a single plateau together with the Ontong Java Plateau and the Hikurangi Plateau. Additional to the present Manihiki Plateau, fragments to its northeast and east have been formed. Plate kinematic reconstructions suggest the capturing of these fragments by the Farallon Plate and the Phoenix Plate, respectively. By tracing these fragments, we report a Paleocene collision of the northeastern Manihiki Plateau fragment with the northern South American craton. The northern Andes exhibit multiple terranes of LIP origin. We infer, based on geophysical, petrological, and geochemical data, that the Pinon formation consists of crustal units of the former Manihiki Plateau. An Early Cretaceous collision of the eastern Manihiki Plateau fragment is reconstructed for West Antarctica. The subduction of this fragment in the Palmer Land region can be associated with the so-called Palmer Land Event and a flattening of the subduction slab. By reconstructing the dispersal of the fragments of the Manihiki Plateau, we provide a deeper insight in the possible subduction scenarios and buildup of the accretional margins of the northern Andes and the Antarctic Peninsula.

Crustal exhumation during ongoing compression in the Variscan Maures-Tanneron Massif, France-Geological and thermo-mechanical aspects

The Maures-Tanneron Massif (MTM), together with Corsica and Sardinia, represent the Southeastern branch of the huge European Variscan belt. A continuous evolution from continental collision to exhumation is described from ca. 350Ma to ca. 320Ma, based on an extended compilation of available geological and geochronological data. This geological compilation is complemented with thermo-mechanical modelling that tests the conditions under which deep-seated HP units can melt and exhume massively during continued convergence. We simulate in two-dimensions the development of gravitational instabilities in a partially melting crust, depending on rheological layering and heat sources, and accounting for temperature dependent elasto-visco-plastic rheologies. In order to reproduce PT exhumation patterns in the MTM, over the appropriate time-scale (>20My) and spatial extent (>100km), a best fit was obtained with a convergence rate of 0.5cm/yr and moderate diffusive surface processes. The contribution of radiogenic heating in the pre-stacked felsic crustal units is crucial. A lateral alternation with mafic units is also required in order to prevent lateral spreading of the orogen. A low viscosity partially molten crust (1020Pa·s, thus rather felsic) accompanies crustal exhumation in a decoupled mode from the deeper mantle. A shallow asthenosphere below the orogen (LAB at ~70km depth, e.g. shallow slab break-off) produces too warm and sudden exhumation as opposed to a competent mantle lithosphere (120km depth LAB) that restrains it. The MTM witnesses the typical competition between far-field plate convergence and internal body forces, and our study pleads for a progressive evolution of transpression towards perpendicular extension from ca. 320Ma.

The deep thermal field of the Upper Rhine Graben

The Upper Rhine Graben has a significant socioeconomic relevance as it provides a great potential for geothermal energy production. The key for the utilisation of this energy resource is to understand the controlling factors of the thermal field in this area. We have therefore built a data-based lithospheric-scale 3D structural model of the Upper Rhine Graben and its adjacent areas. In addition, 3D gravity modelling was performed to constrain the internal structure of the crystalline crust consistent with seismic information. Based on this lithosphere scale 3D structural model the present-day conductive thermal field was calculated and compared to measured temperatures. Our results show that the regional thermal field is mainly controlled by the configuration of the upper crust, which has different thermal properties characteristic for the Variscan and Alpine domains. Temperature maxima are predicted for the Upper Rhine Graben where thick insulating Cenozoic sediments cause a thermal blanketing effect and where the underlying crustal units are characterised by high radiogenic heat production. The comparison of calculated and measured temperatures overall shows a reasonable fit, while locally occuring model deviations indicate where a larger influence of groundwater flow may be expected.

Re-evaluation of polyphase kinematic and 40Ar/39Ar cooling history of Moldanubian hot nappe at the eastern margin of the Bohemian Massif

A structural and geochronological 40Ar/39Ar study was performed in kilometre-scale middle and lower crustal lens-shaped domains dominated by a preserved subvertical foliation, surrounded by horizontally foliated migmatites. These domains occur within the Moldanubian nappe overlying the Brunia microcontinent at the eastern margin of the European Variscides. Three main deformation phases were recognized: subvertical S2 fabric trending NW–SE in lower crustal rocks and NE–SW in mid-crustal rocks. It is reworked by HT/MT horizontal fabric S3 along margins of crustal domains and in surrounding migmatites. S3 bears a prolate NE lineation parallel to the S2–S3 intersection in the lower crustal domain. In the middle crustal units, L3 is weak, connected to oblate strain and trends NE–SW parallel to the S2–S3 intersection. D4 non-coaxial shear deformation is mainly localized at the boundary between the Moldanubian nappe and Brunia and bears strong top to the NNE shear criteria. In order to constrain kinematics of the D3 deformation, strain modelling was performed to show that the Moldanubian hot nappe was frontally thrust over the Brunia indentor. The renewed D4 tangential movement only heterogeneously reactivates the horizontal S3. This evolution is recorded in 40Ar/39Ar amphibole cooling ages, which show two statistically significant Carboniferous peaks at ~342 and 332 Ma, which are also reflected by published detrital muscovite 40Ar/39Ar ages in the adjacent foreland basin. This geochronological record is correlated with progressive erosion of the topographically elevated upper crustal part of the Moldanubian nappe during D3 frontal thrusting, followed by greenschist facies D4 transpressive reactivation and subsequent erosion of high-grade parts of the nappe.

Mesoarchean and Paleoproterozoic history of the Nimrod Complex, central Transantarctic Mountains, Antarctica: Stratigraphic revisions and relation to the Mawson Continent in East Gondwana

High-grade metamorphic and igneous rocks originally mapped as the Nimrod Group represent the only known crystalline basement of the East Antarctic shield exposed in the Transantarctic Mountains. SHRIMP U–Pb age data from zircon show that this assemblage preserves multiple geologic events spanning 2.5b.y. of Archean to early Paleozoic history, culminating in thermomechanical reworking and active-margin magmatism during the Ross Orogeny. New age data from igneous and metamorphic rocks, as well as from detrital zircons in metasedimentary quartzites, refine the Mesoarchean to Paleoproterozoic history and indicate the need to abandon the stratigraphic term Nimrod Group and its five formations. In its place, we redefine only the igneous and high-grade gneissic parts of the assemblage as the Nimrod Complex, and all other metasedimentary rocks exposed in the Miller and Geologists ranges are included in a newly defined Argosy Schist. Within the Nimrod Complex, the oldest layered gneisses represent magmatic Mesoarchean crust formed between about 3150 and 3050Ma. Correspondence of U–Pb zircon crystallization ages with whole-rock Sm–Nd model ages indicates formation from juvenile mantle melts. Magmatism at ca. 3100Ma was followed closely by high-temperature metamorphism, recorded by zircon crystallization at 2955–2900Ma, likely due to high advective heat transfer and/or thermal insulation of newly stabilized crust. Metaigneous units with ages of ca. 2500Ma indicate a late Neoarchean period of anatexis and/or magmatism, although the geologic context for these events is uncertain. Gneissic, eclogitic and metaigneous rocks record an important period of deep-crustal metamorphism, thickening and magmatism between 1730 and 1700Ma, referred to as the Nimrod Orogeny, that may have played an early role in assembly of East Gondwana cratons. A new lithodemic unit, Argosy Schist, consists of interlayered mica schist, quartzite, amphibolite, and calc-silicate schist. Quartzites from the schist unit have two distinctive detrital-zircon provenance characteristics; one type has populations that correlate with dated gneissic and metaigneous units of the Nimrod Complex, indicating a proximal provenance, whereas a second type has detrital zircon ages resembling those from Neoproterozoic Beardmore Group sandstones, indicating a common broader East Antarctic shield provenance. Their maximum depositional ages range widely from about 2000 to 900Ma, leaving unresolved their age and geologic relationship to either the Nimrod Complex or Beardmore Group, yet they comprise a metasedimentary assemblage that is distinctive from the Nimrod Complex. Recent findings of Mesoarchean gneisses in the Gawler Craton, geochronology from Terre Adélie, and our new age data from the Nimrod Complex thus highlight the importance of ca. 3.1, 2.5 and 1.7Ga events in formation of the composite East Antarctic shield adjacent to the modern Transantarctic Mountains. Correlative crustal units may extend to the southern Prince Charles Mountains and Shackleton Range. Collectively, this extensive crustal province, referred to as the Mawson Continent, represents an elongate belt of primary Mesoarchean crust that experienced major reworking at ca. 2.5 and 1.7Ga. The latter period includes both high- and low-P/T petrologic signatures, providing evidence of crustal thickening and magmatism that may signify collisional processes related to Paleoproterozoic cratonal amalgamation during formation of the Nuna supercontinent.

Detrital zircon record of Meso- and Neoproterozoic sedimentary basins in northern part of the Siberian Craton: Characterizing buried crust of the basement

We present new LA ICP-MS detrital zircon data from Meso- and Neoproterozoic sedimentary basins located in the northern and western parts of the Siberian Craton. Along the western cratonic margin (Turukhansk Uplift, northern Yenisei Ridge), the basins accumulated predominantly 2.6–2.5Ga and 1.9–1.85Ga erosional products, while the main sources for the fill of intracratonic basin to the northeast near the Anabar Shield (East Anabar basin) were 2.9–2.7Ga and 2.1–1.95Ga old igneous rocks. The studied Meso- to Early Neoproterozoic sandstones were deposited in rift-related or passive margin settings, and underwent major craton-wide recycling to produce late Ediacaran silicilastic successions. Meso- to Early Neoproterozoic sandstones are immature to submature pointing to erosion of proximal Archean and Paleoproterozoic crustal units. The unique detrital age spectra for the northeast and western basins indicate provinciality and provide a basis for unravelling the age of buried domains of the Siberian Craton basement.

Ternary feldspar thermometry of Paleoproterozoic granulites from In-Ouzzal terrane (Western Hoggar, southern Algeria)

The In Ouzzal terrane in western Hoggar (Southern Algeria) preserves evidence of ultrahigh temperature (UHT) crustal metamorphism. It consists in Archean crustal units, composed of orthogneissic domes and greenstone belts, strongly remobilized during the Paleoproterozoic orogeny which was recognized as an UHT event (peak T > 1000 °C and P ≈ 9–12 kbar). This metamorphism was essentially defined locally in Al-Mg granulites, Al-Fe granulites and quartzites outcropping in the Northern part of the In Ouzzal terrane (IOT).In order to test and verify the regional spread of the UHT metamorphism in this terrane, ternary feldspar thermometry on varied rock types (Metanorite, Granulite Al-Mg and Orthogneiss) and samples that crop out in different zones of the In Ouzzal terrane. These rocks contain either perthitic, antiperthitic or mesoperthitic parageneses. Ternary feldspars used in this study have clearly a metamorphic origin.The obtained results combined with previous works show that this UHT metamorphism (>900 °C) affected the whole In Ouzzal crustal block. This is of major importance as for future discussion on the geodynamic context responsible for this regional UHT metamorphism.

Studying local earthquakes in the area Baltic-Bothnia Megashear using the data of the POLENET/LAPNET temporary array

Abstract. Earthquakes in areas within continental plates are still not completely understood, and progress on understanding intraplate seismicity is slow due to a short history of instrumental seismology and sparse regional seismic networks in seismically non-active areas. However, knowledge about position and depth of seismogenic structures in such areas is necessary in order to estimate seismic hazard for such critical facilities such as nuclear power plants and nuclear waste deposits. In the present paper we address the problem of seismicity in the intraplate area of northern Fennoscandia using the information on local events recorded by the POLENET/LAPNET (Polar Earth Observing Network) temporary seismic array during the International Polar Year 2007–2009. We relocate the seismic events using the program HYPOELLIPS (a computer program for determining local earthquake hypocentral parameters) and grid search method. We use the first arrivals of P waves of local events in order to calculate a 3-D tomographic P wave velocity model of the uppermost crust (down to 20 km) for a selected region inside the study area and show that the velocity heterogeneities in the upper crust correlate well with known tectonic units. We compare the position of the velocity heterogeneities with the seismogenic structures delineated by epicentres of relocated events and demonstrate that these structures generally do not correlate with the crustal units formed as a result of crustal evolution in the Archaean and Palaeoproterozoic. On the contrary, they correlate well with the postglacial faults located in the area of the Baltic-Bothnia Megashear (BBMS). Hypocentres of local events have depths down to 30 km. We also obtain the focal mechanism of a selected event with good data quality. The focal mechanism is of oblique type with strike-slip prevailing. Our results demonstrate that the Baltic-Bothnia Megashear is an important large-scale, reactivated tectonic structure that has to be taken into account when estimating seismic hazard in northern Fennoscandia.

Insights into the crustal structure of the transition between Nares Strait and Baffin Bay

The crustal structure and continental margin between southern Nares Strait and northern Baffin Bay were studied based on seismic refraction and gravity data acquired in 2010. We present the resulting P wave velocity, density and geological models of the crustal structure of a profile, which extends from the Greenlandic margin of the Nares Strait into the deep basin of central northern Baffin Bay. For the first time, the crustal structure of the continent-ocean transition of the very northern part of Baffin Bay could be imaged. We divide the profile into three parts: continental, thin oceanic, and transitional crust. On top of the three-layered continental crust, a low-velocity zone characterizes the lowermost layer of the three-layered Thule Supergroup underneath Steensby Basin. The 4.3–6.3km thick oceanic crust in the southern part of the profile can be divided into a northern and southern section, more or less separated by a fracture zone. The oceanic crust adjacent to the continent-ocean transition is composed of 3 layers and characterized by oceanic layer 3 velocities of 6.7–7.3km/s. Toward the south only two oceanic crustal layers are necessary to model the travel time curves. Here, the lower oceanic crust has lower seismic velocities (6.4–6.8km/s) than in the north. Rather low velocities of 7.7km/s characterize the upper mantle underneath the oceanic crust, which we interpret as an indication for the presence of upper mantle serpentinization. In the continent-ocean transition zone, the velocities are lower than in the adjacent continental and oceanic crustal units. There are no signs for massive magmatism or the existence of a transform margin in our study area.

3-D Inversion of MT Data for Imaging Deformation Fronts in NW Poland

The Pomerania region (northwest part of Poland) occupies a significant position, where the largest European tectonic boundary is situated. This is the area of the contact between the East European Craton (EEC) and the Paleozoic Platform (PP) and it is known as the Trans-European Suture Zone (TESZ). The TESZ was formed during Paleozoic time as a consequence of the collision of several crustal units and it extends from the Black Sea in the southeast to the British Isles in the northwest. It is a region of key importance for our understanding of the tectonic history of Europe. Previous magnetotelluric (MT) results, based on 2-D inverse modeling, show that the contact zone is of lithospheric discontinuity character and there are distinct differences in geoelectric structures between the Precambrian EEC, transitional zone (TESZ), and the younger PP. The presence of a significant conductor at mid and lower crustal depths was also shown. Thus, the main aim of the research presented here was to obtain detailed, 3-D images of electrical conductivity in the crust and upper mantle and its regional distribution below the TESZ in the northwest part of Poland. To accomplish this task we applied the latest 3-D inversion codes, which allowed us to get more realistic model geometries. Additionally, to confirm and complement the study, the Horizontal Magnetic Tensor (HMT) analysis was realized. This method gives us an opportunity to efficiently locate the position of well-conducting structures. As a result we obtain a clearer, three-dimensional model of conductivity distribution, where highly conductive rock complexes appear which we tentatively connected to deformation fronts.