Crustal Domains Research Articles

P–T path reconstruction in a multi‐layered garnet‐bearing schist: Evidence for 2.2–2.1 Ga near‐isothermal loading in the northern margin of the Western Dharwar Craton (India), and regional implications

P–T paths were reconstructed using P–T pseudosection modelling from garnets in a finely laminated mylonitic hornblende‐biotite schist with four sub‐cm wide mineralogically distinct layers separated by poly‐crystalline quartz layers from the northern margin of the Western Dharwar Craton (WDC). The almandine‐grossular‐rich garnets, syn/post‐tectonic with respect to the mylonite foliation, exhibit considerable variations in the zoning patterns of divalent cations in the different layers. By contrast, the compositions of the matrix minerals dominated by biotite, chlorite, hornblende and plagioclase are tightly constrained within and across the layers. The varying abundances of major element oxides in the layers reflect variations in the abundances of the minerals, rather than the mineral compositions. P–T conditions estimated using mineral thermo‐barometry in the garnet‐muscovite‐biotite‐plagioclase assemblages are 520–575°C and 6–8 kbar; somewhat higher P–T values (615–665°C, 7–9 kbar) are obtained from hornblende‐garnet‐plagioclase‐quartz assemblages. The P–T paths obtained from the multi‐mineralic layers using P–T pseudosection modelling yield near‐isothermal loading (4–7 kbar) at 500–550°C in all layers. The near‐isothermal loading experienced by the multi‐layered rock suggests rapid crustal thickening involving overthrusting followed by nucleation of steeply dipping transpressional shear zones due to oblique convergence of the WDC with a crustal domain in the north. U‐Th‐total Pb chemical dating of metamorphic monazites indicates the crustal convergence occurred at 2.2–2.1 Ga. The event does not have known equivalents in the neighbouring crustal domains eastwards in India as well as in Madagascar in the west. This suggests the crustal domain may be an exotic block.

Continental Fragments in the South China Block: Constraints From Crustal Radial Anisotropy

AbstractThe lithospheric architecture of the South China Block (SCB) is crucial to understanding the formation and evolution of this distinctive and highly reworked continental lithosphere with over 3 billion years of tectonic history. However, due to a lack of high‐resolution geophysical datasets, a detailed picture of the SCB lithosphere is absent, and fundamental questions regarding its formation, assembly, and subsequent reworking processes are actively debated. Assuming that unique deformation patterns due to such tectonic processes can be mapped by seismic anisotropy, we present a new crustal radially anisotropic shear‐wave velocity model along a 1500‐km seismic transect that spans the major tectonic domains of the SCB to characterize the past deformation processes. The new seismic models show significant lateral variations in seismic anisotropy and velocity, suggesting that the SCB consists of several separated (micro)continental blocks or terranes that likely have different origins and have survived the prolonged deformation history since the early formation of these continental fragments. Combining available geophysical datasets, we link individual crustal domains of distinct anisotropy to constrain the multiphase deformation processes of the SCB, including the early formation of the Proto‐Yangtze and Cathaysia Blocks, the assembly of the SCB, and the subsequent reactivation of the interior and extensive deformation that have formed the Basin‐and‐Range style tectonics in the Cathaysia Block. We suggest that relict continental fragments have played critical roles in the formation and reactivation of the SCB lithosphere.

Feeding system and mantle sources of the southern and western sector of the Madagascar flood basalt province, and comparisons with Southwest Indian Ridge “anomalous” basalts

The flood basalts of Cretaceous age in southwestern Madagascar range from moderately incompatible-element-enriched, tholeiitic, high-Ti ferrobasalt/basaltic andesite lavas (MgO = 4–5 wt%, Fe2O3t = 14–16 wt%; TiO2 = 3–3.6 wt%, Nb =16–22 ppm, Zr = 236–269 ppm; Lan/Ybn = 7–8) in the southernmost outcrops (Lavanono, Tulear), to incompatible element-poor basaltic dikes (MgO = 4.2–4.9 wt%, Fe2O3t = 12–13 wt%; TiO2 = 1.6 wt%, Nb = 8–9 ppm, Zr = 90–100 ppm; Lan/Ybn = 2.3) around Morondava. The two magma types, at the same degree of magmatic evolution, have markedly different Sr-Nd-Pb isotopic composition (e.g., 87Sr/86Sri = 0.7121–0.7123, 208Pb/204Pb = 40.9–41.3 for the Tulear-Lavanono lavas vs 87Sr/86Sri = 0.7033–0.7040, 208Pb/204Pb = 38.7 for the Morondava dikes), that suggest markedly different sources, magmatic history, and variable interplay between Madagascan Precambrian crust and mantle-derived melts, some with a marked enriched mid-ocean ridge basalt-like composition. The geochemical and isotopic data show no involvement of ancient basement crust in the genesis of the Morondava dikes. The Tulear-Lavanono lavas have high Ba/Nb and La/Nb ratios and Sr-Nd-Pb isotopic characteristic of magmas that have assimilated crustal material. The chemical and isotopic composition of the contaminated tholeiites of southern Madagascar matches some Late Cretaceous group B1 basalts of the Androy volcanic complex and tholeiites of the Ejeda-Bekily dike swarm, and therefore they could derive from the same feeding system or were fed by similar parental magmas. Basalts from the Southwest Indian Ridge between 39 and 41°E section (anomalous magmatic sectors) with low 206Pb/204Pb and 143Nd/144Nd have been interpreted to contain a component of either recycled lower continental crust and/or subcontinental lithospheric mantle. A detailed review of the isotopic and geochemical compositions of the Mesozoic -and Cenozoic- magmatism in Madagascar, and of the lower and upper crustal domains point out that the sources of the anomalous magmatic sectors of the Southwest Indian Ridge do not contain lower crustal (or lithospheric mantle) components of Madagascan origin.Comparison between the samples from the northern and southern parts of the Madagascar flood basalts province indicates that several different parental magmas and mantle sources were involved in the petrogenesis of the Madagascan basalts, and that chemical contributions from a “Marion” hotspot (as a magma source or source component) to the Madagascar primary magmas is not identifiable.

The Neoproterozoic Agudos Grandes granitic batholith, SE Brazil: Inferences on source areas from elemental and Sr-Nd-Pb isotope geochemistry

I-type, K-rich calc-alkaline hornblende-biotite granitoids (HKCA) suites are typical of both pre- and post-collisional tectonic settings, and their petrogenesis is challenging, since they are dominated by rocks with intermediate composition (e.g., 60–68 wt% SiO2; 10–20 vol % mafic mineral) that are neither typical products of crust nor of mantle partial melting. These granitoids are particularly abundant as the main components of Ediacaran batholiths intruding the Neoproterozoic Ribeira Fold Belt in SE Brazil, e.g. constituting ∼70% of the exposition of the Agudos Grandes Batholith, where they were emplaced at 620-610 Ma, as indicated by U–Pb zircon dating from our previous work and new data presented here. Two types of coeval granitoids comprise an expanded suite of mafic to felsic titanite and allanite-rich biotite granitoids (Itapevi-type) and a single occurrence of biotite-muscovite leucogranite (Turvo-type). Elemental and isotope whole-rock data indicate that different sources were involved in the genesis of their parent magmas, with crust with old residence predominating for the HKCA and Itapevi types, as indicated by very negative (−16 to −20) εNd(t) and low initial 206Pb/204Pb (16.4–16.7), and younger metasedimentary rocks as the source of the Turvo leucogranite (εNd(t) (∼-14); initial 87Sr/86Sr = 0.717). This “syn-orogenic” magmatism was immediately followed, as demonstrated by field and geochronological data, by ∼605-600 Ma circumscribed plutons dominated by more fractionated (±muscovite)-biotite granites that derive from melts generated in the middle crust from sources that are slightly younger than those of the HKCA granitoids, as indicated by slightly less negative (εNd(t) (similar to those of the Turvo leucogranite) and higher 206Pb/204Pb (17.1–17.3). After a ∼15 Myr lull, A-type plutons were emplaced in a post-orogenic setting, probably from mantle and crust sources. The syn-to late-orogenic (620-600 Ma) magmatism of the Agudos Grandes was produced by a thermal anomaly, probably associated with a continental arc. A combination of elemental and isotope geochemistry and inherited zircon data suggests that crustal melting was probably induced by ascension of mantle-derived basic magmas, and initiated in hot zones at the lower portions of a previously thickened crust, formed by old Paleoproterozoic to Archean material (the basement of the Apiaí-São Roque Domain of the Ribeira Belt?), and progressed to the middle portions of the crust, where it provoked partial melting of a different crustal domain with younger age and crustal residence (Embu Complex orthogneisses and metasediments).

The Central Indian Tectonic Zone: A Rodinia supercontinent-forming collisional zone and analogy with the Grenville and Sveconorwegian orogens

Abstract In the paleogeographic reconstructions of the Rodinia supercontinent, the circum-global 1.1–0.9 Ga collisional belt is speculated to skirt the SE coast of India, incorporating the Rodinian-age Eastern Ghats Province. But the Eastern Ghats Province may not have welded with the Indian landmass until 550–500 Ma. Instead, the ~1500-km-long, E-striking Central Indian Tectonic Zone provides an alternate option for linking the 1.1–0.9 Ga circum-global collisional belt through India. The highly tectonized Central Indian Tectonic Zone formed due to the early Neoproterozoic collision of the North India and the South India blocks. Based on a summary of the recent findings in the different crustal domains within the Central Indian Tectonic Zone, we demonstrate that the 1.03–0.93 Ga collision involved thrusting that resulted in the emplacement of low-grade metamorphosed allochthonous units above the high-grade basement rocks; the development of crustal-scale, steeply dipping, orogen-parallel transpressional shear zones; syn-collisional felsic magmatism; and the degeneration of orogenesis by extensional exhumation. The features are analogous to those reported in the broadly coeval Grenville and Sveconorwegian orogens. We suggest that the 1.1–0.9 Ga circum-global collisional belt in Rodinia swings westward from the Australo-Antarctic landmass and passes centrally through the Greater India landmass, which for the most part welded at 1.0–0.9 Ga. It follows that the paleogeographic positions of India obtained from paleomagnetic data older than 1.1–0.9 Ga are likely to correspond to the positions of the North and South India blocks, respectively, and not to the Greater India landmass in its entirety.

Crustal Imaging of Portugal Mainland Using Magnetotelluric Data

AbstractThe comprehensive mapping of resistivities in depth and its interpretation is of great importance to unravel the electrical properties of deep‐seated rocks, providing significant insights into the structure of the crust. The first 3D resistivity model for Portugal mainland is here reported using data from 31 broadband MT soundings spaced 50 × 50 km apart. The model shows large crustal volumes with contrasting resistivity values. The central and northern regions are characterized by a high resistivity crustal domain spreading in‐depth (103–105 Ω.m) that correlates well with voluminous granitoid bodies and high‐grade metamorphic rocks. To the west and the south, roughly corresponding to the Portuguese Western Shore and the South Portuguese Zone (SPZ) respectively, the crustal resistivity tends to decrease unevenly to values between 1 and 103 Ω.m. Interconnected graphite in structural discontinuities, possibly forming a regional mid‐crustal décollement at ≈13–15 km depth, are highlighted by multiple low resistivity crustal domains (1–100 Ω.m). Similarly, the model reveals a very large E‐W low resistivity crustal domain in SPZ possibly representing a deep‐seated major décollement. The present study aims to contribute to a better understanding of the physical properties characterizing the concealed crust in Portugal mainland, providing also indirect information on its composition and structure.

Crustal architectural controls on critical metal ore systems in South China based on Hf isotopic mapping

Abstract There is increasing demand for critical metals (e.g., W-Sn, Li-Be-Nb-Ta, and rare earth elements [REEs]) to sustain the transition to green energy, yet it is unclear what controls the formation of such critical metal ore systems. Here, we focus on South China, which is well endowed with critical metals, and imaged its crustal architecture by zircon Hf isotopic mapping using 1096 zircon Hf isotope data sets for 1457 samples of Mesozoic granitoids and silicic volcanic rocks. We demonstrate that the crust is isotopically heterogeneous, characterized by spatial juxtaposition of ancient, reworked, and juvenile crustal domains. The granite-related W-Sn-Nb-Ta and REE deposits occur mainly in reworked crustal domains, where multiple stages of reworking and Mesozoic melting events likely resulted in the release of these metals into crust-derived magmas. Compared with W-Sn deposits, REE deposits occur mostly in strongly reworked crustal blocks with a juvenile input. The porphyry Cu-Au deposits are spatially confined to Cu-fertilized juvenile crustal domains, whereas U and Ag-Pb-Zn deposits occur predominantly in old crustal domains and at their margins. This study demonstrates the importance of isotopic mapping as a tool for characterizing crustal architecture and processes that lead to the formation of metal ore systems.

Toward More Accurate GIC Estimations in the Portuguese Power Network

AbstractNew geomagnetically induced current (GIC) computations for mainland Portugal include the entire power network, with network parameters and topology provided by the transmission grid operator for all the high voltage lines (150, 220, and 400 kV). The first 3D conductivity model for the west region of the Iberian Peninsula, based on 31 broadband magnetotelluric soundings, is used in calculations, revealing the effect of different crustal domains in GIC distribution. Geomagnetic field variations are taken from Coimbra or San Fernando magnetic observatories, according to the Nearest Neighbor method, and used together with surface impedance values predicted from the new conductivity model to calculate the induced electric field on a regular grid. The global distribution of GICs over the power network is characterized based on results derived for the eight most significant storms registered in the Iberia during solar cycle 24. Substations susceptible to the highest GICs are found near the transition between the granitic geotectonic unit of Central Iberian Zone and the Lusitanian Basin. A prototype of a Hall effect sensor has been installed at a substation and is active since the end of August 2021. In order to validate our GIC model, recent measurements are compared with simulations. GIC computation is prone to uncertainties from various sources, possibly contributing with different weights to the final error in computed values. Here, we evaluate the contribution of substation earthing resistance and nonuniqueness of the conductivity model to the final GIC uncertainties.

Transcarpathian Depression: Study of Low-Velocity Zones in the Earth’s Crust Based on the Seismic Regional Profiles Data

Transcarpathian depression (TD) is located in the junction zone of the eastern margin of ALCAPA terrain and the northern part of Tisza-Dacia one, buried under thick Neogene molasses. The Earth’s crust structure of the TD is not clearly understood.
 The purpose of this work is to refine the TD crust structure and identify low-velocity zones by interpreting wave images obtained from WARR PANCAKE profile data using the finite-difference reflection/refraction migration method.
 The crustal domain beneath the TD is interpreted to be limited from southwest by a fault gently southwest dipping, traced from Pieniny Klippen Belt on the surface. Within domain two suture zones being the Alpine Tethys ocean remnants and cemented the European plate and ALCAPA microplate, were distinguished: Piemont-Liguria and PKB ones, as well as a rootless fragment of ALCAPA terrain. The Outer Carpathians thrust belt of 13 km depth borders the TD by subvertical Transcarpathian fault. The crust structure of the Pannonian segment is interpreted to be a pile of thick- and thin-skinned basement nappes of the Tisza terrain and cover nappes with superimposed younger extensional structures.
 High-reflectivity and low-velocity zone at depths of 10­—20 km is identified. The zone follows the pattern of isotherms in the temperature range of 300—500°. On the deep seismic sounding (DSS) Chop—Velykiy Bychkiv profile, running along TD and crossing the PANCAKE line, two low-velocity zones were also distinguished. Published data on numerical and physical modeling, the deep well cores, as well as fault zones in natural outcrops suggest that the low-velocity zones have increased porosity, fracturing, and fluid saturation. Our results suggest a high hydrocarbon potential of the TD, associated with the low-velocity zones.

Heat flow modelling of the Punta del Este Basin (offshore Uruguay) and its correlation with structural crustal domains

The Punta del Este Basin is located at the offshore of Uruguay, and it is considered an underexplored basin with a high potential for holding hydrocarbon plays. Geologically, it constitutes an aborted rift related to the Gondwana fragmentation process, and subsequent opening of the Atlantic Ocean (Upper Jurassic - Lower Cretaceous). The stratigraphic and tectonic evolution comprises four phases from the Paleozoic pre-rift stage to present-day post-rift stage (Neogene/Quaternary). Due to the estimated hydrocarbon potential of this basin, exploratory activities have been intensified in the last decades, based on the acquisition of a significant amount of geophysical data which allowed a better comprehension of the basin's geology. Previous studies related to this basin were mainly oriented to the stratigraphy, structural framework, and petroleum systems comprehension. This research is focused on the thermal evolution of the Punta del Este Basin and its correlation with the crustal geological structure. For this purpose, a heat flow model was developed in order to understand thermal evolution through geological time. A 2D heat flow modelling based on pseudo wells was carried out to research and reconstruct the paleo heat flow. Bottom-hole temperatures (BHTs), seabed heat flow measurements, and Moho discontinuity depth interpretations have been employed in the reconstruction of heat flow evolution through time. A structural analysis based on seismic interpretation allowed to identify the Moho discontinuity and define four crustal domains in the study area: proximal (unattenuated crust), necking, hyperextended (stretched and thinned transition crust), and oceanic crust. The thermal model shows a strong correlation between heat flow and crustal thickness. In this way, the proximal domain develops low heat flow values while the distal sector shows higher values, due to the strong influence of the rifting event in crust thinning.

South and East African fracture zones: a long lifespan since the breakup of Gondwana

Abstract Gondwana started to split up during the Early Jurassic ( c. 180 Ma) with the separation of Antarctica and Madagascar from Africa, followed by the separation of South America and Africa during the Early Cretaceous. Thanks to recent seismic profiles, the architecture of rifted margins and the transform fault zones, which developed as a result of the relative motion between tectonic plates, have been recently evidenced and studied along the whole eastern and southeastern Africa margins (i.e. in the Western Somali Basin, the Mozambique Basin, the Natal Basin and the Outeniqua Basin). But, the structure and overall kinematic evolution of the three major transform fault zones – such as the Agulhas, the Davie and the Limpopo fracture zones (FZ) – that together control the opening of major oceanic basins (Antarctic Ocean, Weddell Sea and Austral South Atlantic) remain poorly studied. The interpretation of an extensive regional multi-channel seismic dataset coupled with recent studies allows us to propose a detailed regional synthesis of the crustal domains and major structural elements of the rifted margins along the whole eastern and southeastern Africa. We provide new constraints on the structure and evolution of these three transform systems. Although our findings indicate common features in transform style (e.g. a right-lateral transform system, a wide sheared corridor), the deformation and magmatism along these systems appear quite different. In particular, our results show that the Davie and Agulhas transform faults postdate the development of the rift zone-controlling faults, whereas the Limpopo margin seems to be a simple intra-continental transform. Moreover, the Davie and Agulhas FZ recorded spectacular inversions during the transform stage, whereas transtensional deformation is developed along the Limpopo FZ. This different style of deformation may be explained by two main forcing parameters: (i) the far-field forces that may induce a rapid change of regional tectonic stress and (ii) the magmatic additions that modify mainly the crustal rheology. In the post-drift history, several reactivations of transform fault zones are recorded, implying that some transform margins are excellent recorders of large plate kinematic changes. Such reactivations can serve also as drains for magmatic fluids in the vicinity of hotspots emplacement.

Ultrahigh-Temperature Mafic Granulites in the Rauer Group, East Antarctica: Evidence from Conventional Thermobarometry, Phase Equilibria Modeling, and Rare Earth Element Thermometry

AbstractThe Rauer Group in East Antarctica is a typical high- to ultrahigh-temperature (HT–UHT) granulite-facies terrane. As UHT metamorphism has been recognized only in Mg–Al-rich pelitic granulites from the Mather Paragneiss, the regional extent of UHT metamorphism remains uncertain, which has hindered our understanding of the genesis and tectonic setting of UHT metamorphism in the Rauer Group. In this study, representative samples of mafic granulite were selected from Archean crustal domains to constrain the peak metamorphic conditions and P–T path and to assess the regional extent of UHT metamorphism in the Rauer Group. Integrated results from mineral reaction histories, thermobarometry, and phase equilibria modeling indicate a multi-stage clockwise P–T evolution for mafic granulites involving pre-peak compression, heating to UHT peak conditions, post-peak near-isothermal decompression under UHT conditions, and subsequent decompressional cooling. The pre-peak prograde history is based mainly on the inclusion assemblage of clinopyroxene + plagioclase + amphibole + quartz + ilmenite ± orthopyroxene ± k-feldspar within porphyroblastic garnet and clinopyroxene and records the transformation from a quartz-present to quartz-absent system. The UHT peak conditions are well constrained at 930°C–1030°C and 10.6–12.8 kbar on the basis of the stability field of the observed peak assemblage of (orthopyroxene–quartz)-free garnet + clinopyroxene + plagioclase + amphibole + ilmenite + melt, as well as measured mineral compositions, including the high Ti content in amphibole (Ti = 0.38–0.42 p.f.u.), the anorthite content of coarse-grained plagioclase cores (XAn = 0.35–0.42), and the grossular content in garnet (XGrs = ~0.21) in P–T pseudosections. The peak T conditions are consistent with thermometric estimates in the range of 930°C–1030°C obtained from garnet–clinopyroxene, garnet–orthopyroxene, and Ti-in-amphibole thermometers, and are slightly lower than estimates (1020°C–1120°C) obtained from thermometers based on rare earth elements. The near-isothermal decompression under UHT conditions can be divided into two stages. The early stage is recorded by coronae of orthopyroxene + plagioclase around clinopyroxene and core–mantle/rim anorthite-increasing zoning in plagioclase. The late stage is identified from symplectites of orthopyroxene + plagioclase ± amphibole around porphyroblastic garnet, which were formed at the expense of garnet at 915°C–950°C and 7.6–8.2 kbar as inferred from the amphibole–plagioclase thermometer. The subsequent decompressional cooling to fluid-absent solidus conditions (~875°C and ~6.5 kbar) is indicated by the growth of biotite, which formed at the expense of symplectic minerals, reflecting back-reaction of melt with symplectite minerals. The peak UHT metamorphic conditions and clockwise P–T path of the studied mafic granulites from the Archean crustal domains are similar to those of Mg–Al-rich pelitic UHT granulites from the Mather Paragneiss. The UHT conditions recorded by the mafic granulites, combined with previously identified isolated UHT localities in the Rauer Group, imply that UHT metamorphism in the Rauer Group occurred over a much wider region than previously thought and probably extends over the whole Archean crustal domain. Our findings have general significance in understanding the regional extent of other UHT granulite-facies terranes worldwide.

Geology of the Eastern Anatolian Plateau (Turkey): a synthesis

The Eastern Anatolian Plateau (EAP), approximately 2000 m above sea level, is located between the Eastern Pontides to the north, the Arabian Platform to the south, and the Iranian Plateau to the east. It is characterized by approximately 6 km-thick Maastrichtian to Quaternary volcano-sedimentary cover which unconformably overlies continental and oceanic basement units. Overall, the outcrops of the pre-Maastrichtian basement are rare and include both continental and oceanic units. This led to drastically different interpretations of the nature of the pre-Maastrichtian basement as (i) the oceanic accretionary complex or (ii) continental crust and overlying ophiolitic mélange. This synthesis deals with the relationships between continental and oceanic units in light of the recent geological, geophysical, and geochemical studies. Geophysical studies consistently indicate the presence of a spatially thickened continental crust with a lateral variation ranging from 38 to 52 km. Seismological models estimate lithospheric thicknesses to be in the range of 70-80 km, suggesting the presence of a rather thinned lithosphere. The pre-Maastrichtian continental units include late Cretaceous high-T/low-P metamorphic rocks, which are intruded by late Cretaceous basic to acidic intrusions at the base. Protoliths of the high-T/low-P metamorphic rocks can be closely correlated with those of the Anatolide-Tauride Block, probably representing the metamorphosed equivalents of the Anatolide-Tauride Block. The continental crustal nature is also testified by the presence of metasyenite to -granite with igneous crystallization ages of 430-440 Ma. The Late Cretaceous ophiolitic mélanges with locally intact tracks of ophiolite and overlying forearc deposits tectonically sit over the Late Cretaceous high-T/low-P metamorphic rocks. These ophiolitic mélanges probably form part of the North Anatolian ophiolitic belt, related to the İzmir-Ankara-Erzincan suture. Maastrichtian to Quaternary volcano-sedimentary rocks overlie both the continental crustal and tectonically overlying oceanic units, representing probably collisional and postcollisional basin fills. Available geological, geochemical, and geophysical data suggest a pre- Maastrichtian basement that comprises a continental crustal domain and an overlying ophiolitic mélange beneath the Masstrichtian to Quaternary cover.

Tectonic affinity and significance of the Qilian Block: Evidence from river sediments in the Central Qilian Belt

The tectonic affinity of the Qilian Block is crucial to reconstructing the ocean-continent configuration of Rodinia. This study integrates previous studies with new whole-sample Nd and zircon U-Pb-Hf isotopic data from modern river sediments to identify the origin of the Qilian Block and its tectonic role in the Rodinia supercontinent cycle. Zircon U-Pb-Hf data in combination with whole-sample/whole-rock Nd and Pb isotopic data indicate that the Central Qilian Belt (CQB) shows close tectonic affinity with the Hualong and North Wulan terranes and together constituted a coherent Qilian Block crustal domain, whereas the Quanji Block likely underwent an independent evolution prior to the early Paleozoic orogeny. A comprehensive assessment of geochronological and whole-rock geochemical-isotopic data reveal that the Precambrian basement of the Qilian and Alxa blocks experienced similar historical evolutions and possibly formed a coherent Qaidam-Altyn-Qilian-Alxa continent before the breakup of Rodinia. Moreover, the Qaidam-Altyn-Qilian-Alxa continent was probably once part of the Yangtze Block, laying along the periphery of the supercontinent Rodinia. Therefore, the northwestern boundary of the Yangtze-affinity block can extend to the Alxa Block and the Qilian Ocean is a branch of the proto-Tethys Ocean. A micro-block can serve as a window into reconstructing the ocean-continent configurations of supercontinents during certain periods.

Empirical correlations between an FAS non-ergodic ground motion model and a GIT derived model for Central Italy

SUMMARY In this study, we investigate the correlation between the residuals of a neGMM (non-ergodic Ground Motion Model) and the physics-based parameters obtained using a non-parametric GIT approach (Generalized Inversion Technique) to lay the groundwork for the implementation of an ad-hoc FAS (Fourier Amplitude Spectra) neGMM for the Central Italy region. This region is particularly suitable for data-driven methodologies as those applied in this work because of the large amount of available data due to the recent multiple main shock–aftershock sequences occurred in this area. Both neGMM and GIT models are developed for Fourier spectra in the frequency range between 0.5 and 25 Hz and using the same reference sites. The comparison of the non-ergodic terms with the source, path and site spectral parameters provides interesting results. First, we find a strong correlation between the source parameters, stress drop Δσ and decay ${k}_{\mathrm{ source}}$ and the source neGMM corrective terms (the combination of the between-event δBe and the location-to-location terms δL2L). This correlation is frequency dependent and, at high frequency, is remarkably positive for Δσ and negative for ${k}_{\mathrm{ source}}$. Concerning the attenuation terms, the path-to-path residuals (δP2P) are clearly associated with the deviations from the regional Q estimates obtained from GIT analysis. This indicates that the neGMM properly captures the properties of the anelastic attenuation and that the corrective terms δP2P can be used to account for differences in travel paths across different crustal domains. Finally, adopting the same reference sites for neGMM and non-parametric GIT, we observe that the systematic site terms (δS2Ss) and the GIT-derived amplification functions are in good agreement. The next step for an appropriate modelling is to identify the physical parameters (e.g. VS,30 and ${k}_0$) describing the empirical amplification curves to be introduced as explanatory variables in the ground motion model.

Crustal and Thermal Heterogeneities Across the Fram Strait and the Svalbard Margin

AbstractThe lithospheric structure of the Fram Strait and the extent from the Knipovich Ridge to the Barents Sea shelf and Svalbard are poorly understood. Several multi‐geophysical investigations from various campaigns since the 90s along the Western Barents Sea margin and the Northeast Greenland margin resulted in insufficient and contradicting interpretations of the crustal and upper mantle settings in the oceanic and continental domains. New airborne magnetic data across the Knipovich Ridge and west of Svalbard provide new insights, reveal the complexity of the seafloor spreading history of the Arctic Atlantic Ocean, and indicate a European‐Eurasian continent‐ocean boundary located ∼150 km farther west than previously suggested. This new location of the continent‐ocean boundary prompted to revise the existing 2‐D seismic interpretations in terms of crustal domains and tectono‐stratigraphic setting. This is tested using joint 2‐D gravity and magnetic field modeling to derive an improved crust‐mantle model of the study. One recently acquired combined 2‐D controlled source electromagnetic/magneto‐telluric (CSEM/MT) profile across the Mohns Ridge was also modeled with potential field data and provided new insights into the tectonic settings of the crust and the mantle thermal anomalies. This study proposes to unify the various seismic and CSEM/MT interpretations using the new aeromagnetic compilation.

Dynamics of Early Neoproterozoic accretion, west-central India: I. Geochronology and Geochemistry

The Godhra-Chhota Udepur (GC) sector (west-central India) is the zone of convergence between two crustal-scale accretion zones, i.e. the N/NNE-striking Aravalli Delhi Fold Belt (ADFB) against the E-striking Central Indian Tectonic Zone (CITZ). In this study, we demonstrate the two orogens welded during the Early Neoproterozoic. In the GC sector, recumbently folded basement gneisses, shallow-dipping granitoid mylonites and a suite of allochthonous supracrustal rocks experienced top-to-the south thrusting and nappe formation (D2 deformation). The shallow-dipping crustal domain was modified by the superposition of a network of WNW/W-striking transpressional shear zones and related folds (D3 deformation). The D2-D3 deformations occurred due to oblique crustal convergence marked by the emplacement of pre-D2, post-D2 and syn-D3 granitoids. The post-D2, syn-D3 granite-granodiorites are weakly peraluminous, calc-alkalic, having ferroan to magnesian affinity, and characterised by LREE-enriched moderately fractionated REE patterns with variable negative Eu anomalies. Trace element geochemistry and whole rock Sr-Nd systematics suggest the granitoids were derived from dominantly meta-greywacke precursors. In South-GC, U-Pb zircons in the basement gneisses yield upper intercept/Concordia dates at 1.65–1.60 Ga for high-grade metamorphism. The lower intercept/Concordia dates in the gneisses coincide with the 0.95–0.93 Ga emplacement age of post-D2/syn-D3 granitoids throughout GC. However, the pre-D2 granitoids in South-GC are older, ~1.03 Ga. By contrast, pre-D2 granitoids in North-GC yield Late Neoarchean (2.5 Ga) Concordia/upper intercept emplacement ages, identical to the emplacement age of the Archean granites in the ADFB; the lower intercept/Concordia age is 0.95–0.93 Ga. The 2.5 Ga granites did not experience the 1.65–1.60 Ga high-grade metamorphism, but both the lithodemic units shared the 0.95–0.93 Ga D2-D3 deformation-metamorphism and the emplacement of post-D2 and syn-D3 0.95–0.93 Ga granitoids. The juxtaposition of the ~2.5 Ga granitoids and the 1.65–1.60 Ga gneisses is attributed to the 1.03–0.93 Ga ADFB-CITZ oblique accretion that involved contemporaneous emplacement of syn-collisional S-type granitoids.. • Godhra-Chhota Udepur (GC) is the western end of the Central Indian Tectonic Zone . • At GC, NNE-SSW shortening is contemporaneous with 0.95–0.93 Ga felsic emplacements . • In north GC, 2.5 Ga granite reworked at 0.95 Ga with no Paleoproterozoic overprint . • In south GC, no Archean inheritance noted in 1.6 Ga gneisses reworked at 0.95 Ga . • ~0.95 Ga accretion welded mutually exclusive Archean and Paleoproterozoic terrains .

Tectonometamorphic evolution of the Trivandrum and Southern Madurai blocks in the Southern Granulite Terrane, south India: correlation with south-central Madagascar

AbstractDetailed structural investigations across the Achankovil terrane boundary shear zone (AKSZ) system show distinct differences in the geometry of superposed fold structures between the Trivandrum (TB) and Southern Madurai (SMB) blocks separated by the AKSZ. The metamorphic temperatures and pressures estimated from the SMB and TB have a similar range, that is, 600–880°C and 5–8 kbar. The similar clockwiseP-Tpaths retrieved by phase equilibrium modelling from both the blocks represent the last deformation and metamorphism shared between them during their accretion along the AKSZ. The distinct evolutionary history of the SMB and TB prior to their amalgamation is supported by the contrasting structural, metamorphic and chronological patterns, particularly the lack of prominent middle Neoproterozoic ages and the presence of Palaeoproterozoic ages in the TB, and vice versa in the SMB. The prominent 600–500 Ma monazite ages in the TB, SMB and AKSZ attest to their timing of accretion along the AKSZ. The study corroborates the S-directed subduction model proposed for terrane accretion along the AKSZ, and provides further insight into the subduction–accretion–collision tectonics associated with the late Neoproterozoic – Cambrian evolutionary history of this region.The tectonothermal histories and geochronology of the SMB and TB are compared to the once adjoined crustal domains of Madagascar within Gondwanaland. It is suggested that the TB is equivalent to the Androyan and Anosyan domains of southern Madagascar, the SMB is equivalent to the Antananarivo and Itremo–Ikalamavony blocks of the central Madagascar, and the sinistral AKSZ is contiguous with the Ranotsara Shear Zone in southern Madagascar.

Continental-scale structural heritage from rift extension to postrift inversion: Implications for the central Brazilian Equatorial Margin evolution

The link between major basement anisotropies and basin evolution is crucial for understanding the role of tectonic inheritance on continental margins. However, it is still a matter of debate how and why it happens, especially in the Equatorial Atlantic, where the interplay between Precambrian fabric and the shearing of major oceanic fractures has been studied. Here, we investigate the offshore Ceará Basin and its basement in the central part of the Brazilian Equatorial Margin to (1) map basement anisotropies underneath marginal basins and show how Precambrian crustal domains influenced basin evolution; (2) determine the deformation style of reactivated basement faults; and (3) assess the interplay between reactivated faults and the oceanic Romanche Fracture Zone. The combined geological and geophysical data show that reactivation of NE-SW to ENE-WSW continental-scale ductile shear zones that acted as crustal weakness zones started in the Early Cretaceous under a normal stress field. Stress field inversion occurred in the Barremian during the continental breakup when the South American plate started to became an independent intraplate area under a strike-slip stress regimes. The shear zones arrest at the E-W-striking Romanche Fracture Zone, which marks the continental-oceanic boundary. The major fault offsets occurred along the Transbrasiliano Shear Zone, which represented a collisional boundary between paleocontinents in the Neoproterozoic. Fault reactivation also occurred in the eastern boundary of the Ceará Terrace marginal ridge along the Transbrasiliano Shear Zone, uplifted during tectonic inversion. We conclude that fault reactivation mainly occurred between contrasting Precambrian terrains separated by ductile shear zones. Additionally, the folds associated with the Romanche Fracture Zone were mainly generated along reactivated faults that affected syn- to postrift units on the Equatorial Margin.

Hydrogeologic and Thermal Effects of Glaciations on the Intracontinental Basins in Central and Northern Europe

We use a fully coupled hydro-thermal model (TH) to quantify changes in the pore pressure and temperature distribution following the Last Glacial Maximum (LGM) in the intracontinental basins in Central and Northern Europe. We demonstrate that even without considering a direct mechanical coupling from the visco-elastic lithosphere rebound, the system is, at present-day, in a state of hydrogeologic and thermal disequilibrium as a result of the past ice sheet dynamics. We find that the local geology exerts an additional control on the subsurface response to imposed glacial loading, as evidenced by a contrasting thermal and pore pressure configuration in time and space. Highest rates of pore pressure dissipation are restricted to crustal domains that underwent substantial glacial loading, while the majority of the sedimentary sub-basins show a prominent signature of hydraulic disequilibrium (overpressure) at present. Groundwater-driven convective cooling and heating during the advance and retreat of the ice cap occurred mainly within sedimentary rocks, domains where thermal equilibration is ongoing. The spatial correlation between modeled pore pressure dissipation rates and postglacial uplift rates is indicative of a complex and transient hydrogeological system structurally connected to the viscous tail of the ongoing isostatic adjustment after the LGM, with important implications for assessing the long-term mechanical stability of this intraplate setting.