Crustal Thickness Research Articles

Significant variations in the structure and composition of the crust beneath northwestern India: Imprints of magmatism

Northwestern India was affected by the Neoproterozoic Malani and Erinpura magmatism, Cretaceous rifting, and magmatism associated with the Deccan precursors. Receiver function analysis was done using 1704 RFs from 18 stations to image and decipher the imprints of the magmatic events and comprehend the crustal modifications in terms of variations in structure and composition. The H-κ grid search and neighbourhood inversion techniques are used to retrieve the crustal structure and Vp/Vs ratios (κ). The study reveals significant variations in the crustal thickness (H = 34 km to 43 km), composition (κ=1.74 to 1.92) and shear wave velocity structure across northwestern India. The terrains encompassing the Erinpura granite and the Malani igneous suite in the vicinity of the Barmer rift are characterized by a thick (H ≥ 41 km) crust with felsic to intermediate composition (κ ≤ 1.81). The crust beneath the southern part of the Marwar basin is ≈ 38–40 km thick with mafic composition (κ> 1.81). The region around the Barmer rift has a thin crust (H = 34–36 km) with intermediate to mafic composition. The region hosting the Early Cretaceous to Paleogene alkaline complexes exhibits a high Vp/Vs ratio (κ= 1.91) that may be associated with mafic cumulates emplaced by magmatic events that overprint the signatures of the Malani event. The crust is heterogeneous with low/high velocity intracrustal layers that reflect the fractionation of magma at different depths. The mafic residue, together with magmatic intrusions, results in a mafic lowermost crust with high shear velocities of 3.9–4.0 km/s beneath most stations. Overall, northwestern India is characterized by a thick crust with intermediate crustal composition and intracrustal layering resulting from large scale magmatic events linked to the Neoproterozoic reorganization of plates and younger magmatic events.

Archean crustal growth and reworking in the Superior Province, Canada: Insights from whole-rock geochemistry and Nd isotopic data of the La Grande, Nemiscau and Opatica subprovinces

The southeastern Superior Province contains Meso- to Neoarchean (2952–2677 Ma) tonalite-trondhjemite-granodiorite (TTG) suites, volcanic (2773–2703 Ma) and sedimentary (<2724 Ma) supracrustal sequences, and granitoid intrusions (2708–2598 Ma). The region is a key crustal cross-section for understanding the growth, evolution, and stabilization of Archean cratons. Whole-rock geochemistry and Nd isotopic analyses have been performed on TTG suites, volcanic assemblages and granitoid plutons of the La Grande, Nemiscau and Opatica subprovinces. These analyses, combined with existing structural and U-Pb geochronological data, have been used to investigate the extend of the basement within the three subprovinces, their interaction, and the degree of crustal reworking in the southeastern Superior Province.Nd isotope data and zircon inheritance indicate that the TTG suites were generated from: (i) an old crustal domain with Nd model ages at 3.5–3.2 Ga that may represent the southern extension of the Langelier Complex, (ii) a ca. 3.0 Ga domain with isotopically juvenile signatures, and (iii) the reworking of < 3.0 Ga Nd model age TTG suites from the Opatica subprovince. The Nd juvenile signatures of the mafic to ultramafic volcanism at ca. 2773–2756 and ca. 2723–2703 Ma suggest continuous or multiple mantle plume events beneath the southeastern Superior Province, whereas interlayered felsic volcanic rocks were formed from anatectic melting at high pressure. Subsequent magmatism was related to sanukitoid suites (2704–2693 Ma) emplacement that record mantle-TTG melt interactions. Based on a common tectonometamorphic evolution of the three subprovinces from ca. 2756 Ma, previous studies argued that the Mesoarchean TTG suites exposed in the La Grande and Opatica subprovinces represent a single crustal block. The sedimentary belts of the area were deposited in sagduction-type basins formed at ca. 2724–2706 Ma and, within less than 15–20 myr, were buried and metamorphosed up to granulite facies at 2697–2685 Ma during dip-slip-dominated D2. The burial and the following exhumation of these rocks from ca. 2677 Ma may be attributed to the delamination of lower crustal residues, favoring interaction between mantle and TTG melts, and the formation of sanukitoids and anatectic S-type granites. Nd signatures of the 2646–2598 Ma I-type granitic intrusions indicate extensively reworking of TTGs due to crustal thickening during the D3-D4 events.Our study suggests that the Archean tectonic regime recorded in the southeastern Superior Province is the result of mantle plume tectonics. We conclude that the La Grande-Opatica boundary does not represent a major suture zone but rather but rather a fossil boundary within an older extensively reworked crustal block over which the metasedimentary rocks of the Nemiscau and La Grande subprovinces were deposited.

Deep CO2 release and the carbon budget of the central Apennines modulated by geodynamics

Recent studies increasingly recognize the importance of critical-zone weathering during mountain building for long-term CO2 drawdown and release. However, the focus on near-surface weathering reactions commonly does not account for CO2 emissions from the crust, which could outstrip CO2 drawdown where carbonates melt and decarbonize during subduction and metamorphism. We analyse water chemistry from streams in Italy’s central Apennines that cross a gradient in heat flow and crustal thickness with relatively constant climatic conditions. We quantify the balance of inorganic carbon fluxes from near-surface weathering processes, metamorphism and the melting of carbonates. We find that, at the regional scale, carbon emissions from crustal sources outpace near-surface fluxes by two orders of magnitude above a tear in the subducting slab characterized by heat flow greater than 150 mW m–2 and crustal thickness of less than 25 km. By contrast, weathering processes dominate the carbon budget where crustal thickness exceeds 40 km and heat flow is lower than 30 mW m–2. The observed variation in metamorphic fluxes is one to two orders of magnitude larger than that of weathering fluxes. We therefore suggest that geodynamic modulations of metamorphic melting and decarbonation reactions are an efficient process by which tectonics can regulate the inorganic carbon cycle.

Crustal structure of the central and southern Kyushu-Palau Ridge: Implications for intra-oceanic arc evolution

As an intra-oceanic arc, the Kyushu–Palau Ridge (KPR) is an exceptional place for studying intra-oceanic arc evolution and related magmatism using its interior velocity structure. Two wide-angle seismic profiles along the KPR provide an opportunity to unveil the crustal structure of its central and southern parts. Strong along-ridge variations are found either in P-wave velocity or crustal thickness of the central and southern KPR. The crustal thickness is between 6 and 12 km, with the velocity increasing from 4.0 km/s to 7.0 km/s from top to bottom. The upper crust has a large velocity gradient of ∼1 s−1 and a thickness between 2 and 4 km, while the lower crust has a lower velocity gradient of <0.2 s−1 and is characterized by inhomogeneous velocity anomalies and a substantial lateral variation in thickness. By comparing with the mature arc in the Izu-Bonin-Mariana arc in the east, we suggest the central and southern KPR is a thickened oceanic crust rather than a mature arc crust. Partly thick crust, velocity anomalies in the lower crust and upper mantle imply the crust is in a transition stage from oceanic crust to mature arc one. Compared with the quantitatively calculated seamount volumes, the lower crustal thickness is generally correlated well with the distribution of the seamount volumes along the central and southern KPR. These observations imply that the central and southern parts of the KPR are at the juvenile stage of arc evolution, with crustal growth primarily attributed to the lower crust thickening.

Magma intrusion and migration under the continental large igneous provinces

Large igneous provinces (LIPs) are often the surface expressions of mantle plume process. Intensive magma could occur in the continental crust when a mantle plume penetrates the Moho. How magmas migrate in the crust, forming LIPs, remains debated. In this study, we employ 2D thermo‐mechanical numerical modelling to study the dynamics of magma intrusion and migration within the continental crust affected by a mantle plume. Our results suggest that (1) lateral magma migration dominates crustal deformation, promoting the possible formation of metamorphic core complexes; (2) two distinct crustal deformation modes are recognized regarding the presence or absence of crustal break‐up, affected by whether a significant vertical magma migration occurs; (3) the crustal thickness, Moho temperature and the length of the preset weak crust–mantle decoupling zone are the three key parameters controlling crustal magma migration. This study highlights the importance of lateral magma migration in the lower crust and provides physical mechanisms for the interpretation of magma migration under LIPs.

Geological interpretation of wide-angle seismic reflection/refraction profiles along the Scotian margin and across Nova Scotia, Canada

Two wide-angle seismic reflection/refraction profiles were acquired to determine the velocity characteristics of the Meguma terrane and adjacent Avalonia northeast of the Cobequid-Chedabucto Fault Zone (CCFZ) in eastern Canada. Line 99–1 is located along the Scotian margin whereas line 99–2 crosses the Scotian margin and onshore Nova Scotia and extends into the Gulf of St. Lawrence. Velocity models were derived by forward modelling of travel times. P-wave velocities of 5.5–6.0 km/s and 6.0–6.4 km/s are interpreted to be metasedimentary rocks of the Goldenville and Halifax groups and granitic rocks, respectively. This inference is based on comparison with laboratory velocities of a suite of rock samples, offshore sampling, links to onshore geology with multibeam bathymetry, and gravity modelling. On both lines a low velocity zone (LVZ) >350 km wide and up to 15 km thick is present at mid-crustal levels. The LVZ is not observed beneath onshore Nova Scotia or northeast of Orpheus Graben, the offshore extension of the CCFZ. The crustal thickness of the Meguma terrane varies between 31 and 38 km; in contrast, the Moho is at a depth of 41 km beneath Avalonia. The velocity of the lower crust on line 99–2 is 6.8–7.1 km/s beneath the LVZ and Avalonia, and 6.6 km/s beneath onshore Meguma terrane. Poisson's ratios calculated from P- and S-wave velocities are 0.19–0.23 in the Meguma terrane crust; in contrast, Poisson's ratios of 0.24–0.25 are indicated for Avalonian crust.

The Devonian clastic wedge and underlying strata of southwestern Canadian Arctic Archipelago: stratigraphic revisions

The Devonian clastic wedge (DCW) and underlying carbonate platforms and basinal mudrocks of the study area are re-examined using legacy seismic data and X-ray fluorescence surveys of borehole chip samples. The Ordovician–Devonian basinal succession of Melville Island is consolidated under the name Ibbett Bay Group within the Northwest Territories, whereas equivalent strata in Nunavut are grouped into the Cape Phillips Formation. The Kitson Formation black shale is correlative with the upper Ibbett Bay Group. Six horizons with high TOC and high gamma response are traced in the Ordovician–Devonian, with the fourth (4a) approximating the Silurian/Devonian boundary; the upper two (4b, 5) are Emsian and Eifelian. In the direction of progradation, the base of Kitson Formation rises stratigraphically from gamma horizon 4a to 5. The upper Kitson represents basinal toes of westward prograding DCW clinoforms. The Blackley and Cape de Bray formations of Embry and Klovan (Embry, A.F., and Klovan, J.E. 1976. The middle-upper devonian clastic wedge of the franklinian geosyncline. Bulletin of Canadian Petroleum Geology, 24: 485–639. https://pubs.geoscienceworld.org/cspg/bcpg/article/24/4/485/57135) are not traceable enough to warrant their formation rank. We revert to the original usage of Tozer and Thorsteinsson (Tozer, E.T., and Thorsteinsson, R. 1964. Western Queen Elizabeth Islands, Arctic Archipelago. Geological Survey of Canada, Memoir 332: 242 p. doi:10.4095/100556) where these units are members within the Weatherall Formation. The distinctive seismic character of the Cape de Bray in western and central Melville Island warrants its recognition as a formal member; elsewhere, it is informal as it cannot be consistently traced. The Blackey is treated as a formal member in an outcrop area of ∼2000 km 2 where it was defined; it is not recognized in the subsurface. Onset of the DCW is tentatively linked to flexural subsidence and crustal thickening caused by the Romanzof Orogeny in the hinterland.

Effect of Storage on the Antioxidant, Color and Sensory Properties of Local Bread

The aim of this study is to evaluate the effect of storage on antioxidant, color and sensory properties of bread, containing sourdough and potato puree, baked in stone oven in Afyonkarahisar which is city of Turkey. Sourdough was used as yeast for the production of bread. Potato puree was added to the formulation at the ratio of 30%. Breads were stored in sealed polyethylene bag at room temperature (20°C) for 1, 5 and 10 days. Bread characterization, color, β-carotene, phenolics, minerals were analyzed and a sensory evaluation was performed. Decrease in bread mass, height, width, crust thickness, total volume and specific volume were observed. β-carotene and phenolic compounds quantity of breads decreased with increasing of storage time. Storage caused a change in the crumb color of breads. Storage statistically didn’t affect the crust color values of breads. According to the panelist scores, it can be concluded that all breads stored 1, 5 and 10 days had a desired crust color, crumb texture, porosity, aroma and overall acceptance.

New cold seep sites on the continental slope southwest to Svalbard

We discovered seafloor features such as bacterial mats and carbonate crusts typical for active methane seeps on the continental slope southwest of Svalbard. These features are associated with two main northwest-southeast trending morphological structures that are oriented parallel to the regional continental slope. Both structures occur at c. 800 m water depth, at the boundary between the Storfjorden trough mouth fan to the south and the Hornsund trough mouth fan to the north, which suggests a loading related fluid seepage. The main structure displays depressions and ridges forming a crater in its center. Other occurring features include small sediment mounds, domes often covered by bacterial mats, and hummocky seafloor colonized by siboglinid tubeworms. Free gas bubbles were spotted close to the centre and plumes along the rims of the structure. Thick carbonate crusts indicate a long seepage history in the center of the structure and on top of the ridges. The sources of the seeps are likely to be Miocene old organic-rich deposits, or Paleocene hydrocarbon reservoirs.

Fingerprinting the geochemical signals of episodic arc activity in the Sierra Nevada batholith in space and time

Although subduction is a continuous process, arc system behavior is non-steady-state, leading to uncertainty surrounding the composite spatial and temporal evolution of transcrustal arc magma plumbing systems. This study integrates field, geochronologic, and geochemical data sets from the central Sierra Nevada arc section to investigate the extent to which spatial inheritance is recorded in arc geochemical compositions, and how these signals may be modified by dynamic arc behaviors through time, from arc-wide flare-ups, migration, and crustal thickening to regional magma focusing. Geochemical patterns across Mesozoic arc rocks characterize persistent spatial signals of inheritance, whereas geochemical trends during Cretaceous arc activity provide the temporal component of simultaneous dynamic processes. Distinct bulk-rock isotopic signals define each of the three Mesozoic magmatic flare-ups, which, during Cretaceous arc magmatism, is coupled with eastward arc migration. Additionally, Cretaceous magmatic and tectonic thickening doubled the thickness of arc crust, and magmatism was focused toward a central zone, culminating in the formation of the ∼1100 km2 Tuolumne Intrusive Complex. During magma focusing, temporal signals of magma mixing outweighed the previously pervasive signal of spatial inheritance. Distinct dynamic behaviors effectively primed the arc by the Late Cretaceous, generating transcrustal hot zones of increased magma mixing, recycling, long-term storage, and homogenization. Non-steady-state behavior in the Sierra Nevada resulted in mountain building and voluminous continental crust formation by transforming the physical, thermal, and chemical properties of the lithosphere over tens of millions of years.

Continental Residual Topography Extracted From Global Analysis of Crustal Structure

AbstractContinental topography is dominantly controlled by a combination of crustal thickness and density variations. Nevertheless, it is clear that some additional topographic component is supported by the buoyancy structure of the underlying lithospheric and convecting mantle. Isolating these secondary sources is not straightforward, but provides valuable information about mantle dynamics. Here, we estimate and correct for the component of topographic elevation that is crustally supported to obtain residual topographic anomalies for the major continents, excluding Antarctica. Crustal thickness variations are identified by assembling a global inventory of 26,725 continental crustal thickness estimates from local seismological data sets (e.g., wide‐angle/refraction surveys, calibrated reflection profiles, receiver functions). In order to convert crustal seismic velocity into density, we develop a parametrization that is based upon a database of 1,136 laboratory measurements of seismic velocity as a function of density and pressure. In this way, 4,120 new measurements of continental residual topography are obtained. Observed residual topography mostly varies between ±1 and 2 km on wavelengths of 1,000–5,000 km. Our results are generally consistent with the pattern of residual depth anomalies observed throughout the oceanic realm, with long‐wavelength free‐air gravity anomalies, and with the distribution of upper mantle seismic velocity anomalies. They are also corroborated by spot measurements of emergent marine strata and by the global distribution of intraplate magmatism that is younger than 10 Ma. We infer that a significant component of residual topography is generated and maintained by a combination of lithospheric thickness variation and sub‐plate mantle convection. Lithospheric composition could play an important secondary role, especially within cratonic regions.

Early Devonian Post‐collisional Granitic Magmatism in the North Qilian Orogenic Belt, Western China: Insights into Lithospheric Delamination and Orogenic Collapse

AbstractPost‐collisional magmatism contains important clues for understanding the reworking and growth of continental crust, as well as lithospheric delamination and orogenic collapse. Early Devonian magmatism has been identified in the North Qilian Orogenic Belt (NQOB). This paper reports an integrated study of petrology, whole‐rock geochemistry, Sm‐Nd isotope and zircon U‐Pb dating, as well as Lu‐Hf isotopic data, for two Early Devonian intrusive plutons. The Yongchang and Chijin granites yield zircon U‐Pb ages of 394–407 Ma and 414 Ma, respectively. Both of them are characterized by weakly peraluminous to metaluminous without typical aluminium‐rich minerals, LREE‐enriched patterns with negative Eu anomalies and a negative correlation between P2O5 and SiO2 contents, consistent with geochemical features of I‐type granitoids. Zircons from the studied granites display negative to weak positive εHf(t) values (–5.7 to 2.1), which agree well with those of negative εNd(t) values (−6.4 to −2.9) for the whole‐rock samples, indicating that they were derived from the partial melting of Mesoproterozoic crust. Furthermore, low Sr/Y ratios (1.13–21.28) and high zircon saturation temperatures (745°C to 839°C, with the majority being &gt;800°C) demonstrated a relatively shallow depth level below the garnet stability field and an additional heat source. Taken together, the Early Devonian granitic magmatism could have been produced by the partial melting of ancient crustal materials heated by mantle‐derived magmas at high‐temperature and low‐pressure conditions during post‐collisional extensional collapse. The data obtained in this study, when viewed in conjunction with previous studies, provides more information about the tectonic processes that followed the closure of the North Qilian Ocean. The tectonic transition from continental collision to post‐collisional delamination could be constrained to ∼430 Ma, which is provided by the sudden decrease of Sr/Y and La/Yb ratios and an increase in zircon εHf(t) values for granitoids. A two‐stage tectonic evolution model from continental collision to post‐collisional extensional collapse for the NQOB includes (a) continental collision and crustal thickening during ca. 455–430 Ma, characterized by granulite‐facies metamorphism and widespread low‐Mg adakitic magmatism; (b) post‐collisional delamination of thickened continental crust and extensional collapse of orogen during ca. 430–390 Ma, provided by coeval high‐Mg adakitic magmatism, A‐type granites and I‐type granitoids with low Sr‐Y ratios.

Study on crustal thickness and the prediction of prolific depressions: the Bohai Basin as an example

Study on crustal thickness and the prediction of prolific depressions: the Bohai Basin as an example

Quantifying pigment content in crustose coralline algae using hyperspectral imaging: A case study with Tethysphytum antarcticum (Ross Sea, Antarctica).

Crustose coralline algae (CCA) are a highly diverse group of habitat-forming, calcifying red macroalgae (Rhodophyta) with unique adaptations to diverse irradiance regimes. A distinctive CCA phenotype adaptation, which allows them to maximize photosynthetic performance in low light, is their content of a specific group of light-harvesting pigments called phycobilins. In this study, we assessed the potential of noninvasive hyperspectral imaging (HSI) in the visible spectrum (400-800 nm) to describe the phenotypic variability in phycobilin content of an Antarctic coralline, Tethysphytum antarcticum (Hapalidiales), from two distinct locations. We validated our measurements with pigment extractions and spectrophotometry analysis, in addition to DNA barcoding using the psbA marker. Targeted spectral indices were developed and correlated with phycobilin content using linear mixed models (R2 = 0.64-0.7). Once applied to the HSI, the models revealed the distinct phycoerythrin spatial distribution in the two site-specific CCA phenotypes, with thin and thick crusts, respectively. This study advances the capabilities of hyperspectral imaging as a tool to quantitatively study CCA pigmentation in relation to their phenotypic plasticity, which can be applied in laboratory studies and potentially insitu surveys using underwater hyperspectral imaging systems.

Evaluating the Rheological Controls on Topography Development During Craton Stabilization: Objective Approaches to Comparing Geodynamic Models

AbstractSurface topography is an important yet largely neglected aspect of the early evolution of cratons. The lateral accretion of cratonic nuclei inevitably forms orogenic belts that subsequently provide a sediment source for large, resource‐rich intracratonic basins, but to date, geodynamic models have focused exclusively on lithospheric root processes. Here we use two‐dimensional thermal‐mechanical models to study the topography and lithospheric deformation during 50 Myr of compression of a cratonic nucleus, to simulate the lateral accretion phase of craton growth in the Neoarchean. Although the cratonic nucleus thickens slightly during the compression phase, most of the deformation occurs in the regions adjacent to the nucleus that have weaker lithosphere. Here, crustal thickness triples developing high topography in excess of 10 km without active erosion. Models with different initial rheological parameters will have different final topography and lithosphere geometry, but in general it is difficult to shorten and deform the depleted cratonic nucleus, unless there are significantly weak heterogeneities in the mantle lithosphere. We apply two quantitative analysis techniques to objectively evaluate a multitude of model outputs. Cross‐correlation clustering (CCC) measures the degree of similarity between topography profiles and categorizes models based on the general topographic character. Six different topography families are possible in the context of our models and crustal strength is the most important parameter affecting the shape. From principal component analysis (PCA) we identify four dominant lithosphere geometries. When used together, these two methods provide distinct yet complementary information about the surface and subsurface deformation features in our models.

New Numerically Derived Scaling Relationships for Impact Basins on Mars

AbstractMost impact basins are believed to have formed during the early epochs of planetary evolution. The planet's gravity, internal structure, and thermal regime have the strongest control over their formation. Because of this, we can use the geophysical constraints on Mars' interior composition, structure, and geophysical evolution derived from the InSight mission to better understand the formation of impact basins on the planet. To achieve this, we performed numerical simulations of large impacts using the iSALE shock physics code. We investigated the effects of temperature and crustal thickness variations on impact basin size and morphology. Our scaling relationships indicate that: (a) basins formed in a warmer crust have larger final diameters in comparison to basins formed in a colder crust, a difference that is further accentuated as basin size gets bigger; and (b) the largest impact basins on Mars were created by impactors ranging from 35 to 680 km in diameter, up to ∼32% larger than estimates based on classical scaling. Our results expand the current understanding of the extent of early and large impact bombardment on Mars and provide a more comprehensive knowledge of impact basin formation on planetary surfaces.

Crustal‐Scale Pop‐Up Structure at the Junction of Two Continental‐Scale Deformation Zones in the Southern Baltic Sea

AbstractThe southern Baltic Sea is a peculiar area, where the Sorgenfrei‐Tornquist Zone (STZ), stretching from Bornholm into the North Sea, connects to the Teisseyre‐Tornquist Zone (TTZ) that continues SE up to the Black Sea. In this study, we show the structure and evolution of this controversially debated area, both on crustal and basin scale, by using three seismic reflection profiles combined with 2‐D potential field data. The results demonstrate that the southern Baltic Sea is underlain by a thick crust of the East European Craton with a Moho depth in the range of 38–42 km. The overall crustal architecture is shaped by three phases of localized stretching in the early Paleozoic, Devonian‐Carboniferous, and Permian‐Mesozoic. The most spectacular feature of the southern Baltic Sea is a zone of thick‐skinned compressional deformation produced by Late Cretaceous‐early Paleogene inversion, including a system of thrusts and back thrusts penetrating the entire crust in an 80–90 km wide inversion zone. ENE‐vergent thrusts are traced from the top of the Cretaceous down to the Moho and they are accompanied by back thrusts of opposite vergence, also reaching the Moho. Inversion tectonics resulted in the uplift of a block of cratonic crust as a pop‐up structure, bounded by thrusts and back thrusts, and the displacement of the Moho within the STZ and TTZ. The similar mechanism of intra‐cratonic inversion was recognized for the Donbas Foldbelt in eastern Ukraine, and it may be characteristic of rigid cratons, where deformation is localized in a few preexisting zones of weakness.

Large variations in the lithospheric thickness of northwestern India: Imprints of collisional and thermal reworking

The fold belts, which border the cratons, are the building blocks for understanding the origin and modification of older continents. The Aravalli-Delhi Fold Belt (ADFB) in northwestern India provides evidence of crustal block accretion due to continental collision, whereas Malani Igneous Suite (MIS) modified with widespread magmatism. Imaging of upper mantle structure and lithospheric modifications, if any, of such regions has been intricate owing to reworking by subsequent superimposed tectono-magmatic processes. We applied a 2D modelling approach to model the lithospheric architecture along a 1000-km long WNW-SSE geotransect across northwestern India, which has been deformed in the past. Our modelling technique combines terrestrial gravity anomaly, heat flow data, satellite-based geoid, and topographic datasets using the basic premise of thermal steady-state and local isostasy. The overall 38 to 40 km thick crustal geometry underneath the Marwar Block had the maximum lithological heterogeneity. The region surrounding the MIS is characterised by 8–10 km thick high-density (2.78 g/cm3) sills deposited in the upper crust down to 9 km depth and another 10–15 km thick high-density (3.05 g/cm3) mafic mantle material near the Moho. About 42 km thick crust, including an 8 to 10-km thick high-density (3.05 g/cm3) underplated layer at its bottom, characterises the high-relief ADFB. The Vindhyan region of Bundelkhand craton is defined by a ∼ 1 km thick trap, having Moho extending at a depth of ∼40 km. The lithospheric thickness varies substantially from ∼143–168 km underneath the Marwar block, which thins to ∼135 km under the ADFB and thickens gradually to ∼150–165 km beneath the Vindhyan region. Substantial crustal density differences in distinct crustal domains, when integrated with the thin lithosphere, reinforce the concept that tectono-magmatic processes might have modified the lithosphere in NW India.

Seismic probing of buried ancient terrane boundaries - insight into Fennoscandia’s Palaeoproterozoic continental formation

Terrane boundaries are usually mapped from structures, lithologies and ages of the bedrock geology. Geophysical studies occasionally used to image terrane boundaries at depth typically yield localised information. In this contribution we demonstrate that it is possible to map Palaeoproterozoic terrane boundaries over large areas in northwest Fennoscandia through modelling of the crustal structure using receiver function inversion. Our new model suggests the existence of three “arms” of thick crust interpreted as deep expressions of Palaeoproterozoic terrane boundaries. We also observe previously unknown high velocity lower crust that we relate to Palaeoproterozoic rifting. The deep structural information shows that crustal structures can be preserved over billions of years and yields a three-dimensional crustal terrane model. This model improves our understanding of Palaeoproterozoic tectonics and continent formation processes.

Origin of a high-velocity layer: Insights from seismic reflection imaging (South China Sea)

Magmatism and crustal structural characteristics are well-known phenomena that support understanding the marginal sea's tectonic evolution. The South China Sea (SCS) has become a natural laboratory for researchers to investigate the evolution of the marginal sea because of its complex tectonic background and multiple phases of magmatic activities. However, due to limitations in resolution and depth of observations, the evolutionary process of the SCS is still controversial. We use the latest processed multi-channel seismic reflection profile Line AB to evaluate the sedimentary sequences, crustal tectonics, and the Cenozoic magmatism in the southern SCS. The gravity modeling results indicate a reduction in crustal thickness from 18 km in the proximal domain to 7 km in the Continent-Ocean Transition (COT). The COT is roughly 35 km wide, representing a rapid break up process of the SCS. A high-velocity layer (HVL) of about 3 km thickness was formed in the continental slope crust, interpreted as post-spreading magmatic underplating. In addition, four sedimentary sequences, post-spreading magmatic intrusion, and the possible magma conduits were identified in the profile. Combining with previous ocean bottom seismometer (OBS) data, multi-channel seismic data, fault system data, and petrologic observations, we indicate the deep mantle magmatic material upwelled to the crust and further intruded into sediments along the hyper-extended crust or detachment faults during the post-spreading stage. We contend that cooling and heat shrinking within the SCS oceanic lithosphere have the potential to initiate decompression melt deformation in the continental slope. In summary, we construct a model of the origin and migration mechanism of extensive post-spreading magmatism in the SCS. Our research reveals the Cenozoic evolution and the HVL of the southern SCS, and provides a possible genesis for the anomalously young magmatism observed at the continental margin.