Physics of the Earth and Planetary Interiors最新文献

The Wudalianchi-Erkeshan-Keluo (WEK) volcanic belt is a significant component of intraplate volcanism in Northeast China and is composed of the Wudalianchi, Erkeshan, and Keluo volcanic clusters. Using joint inversion of receiver functions and ambient noise, we construct a high-resolution 3-D S-wave velocity model of the WEK volcanic belt and its adjacent region, taking advantage of a deployed dense seismic array around this volcanic belt. There is a prominent low-velocity anomaly at 8–15 km depth beneath the Wudalianchi volcanic cluster, suggesting the presence of a crustal magma chamber. Low-velocity anomalies are also observed at 30–35 km depth beneath the Erkeshan volcanic cluster and 30–40 km depth beneath the Keluo volcanic cluster, resulting in discontinuous velocity structures at the Moho discontinuity. We further find a distinct low-velocity anomaly in the uppermost mantle beneath the WEK volcanic belt. Combined with previous geophysical and geochemistry studies, we propose a magma system scenario for the WEK volcanic belt. The upwelling molten material from the asthenosphere accumulated in the uppermost mantle and the magma chamber was formed, which provided the same uppermost mantle magma sources for the WEK volcanic belt.

The origin of large igneous provinces (LIPs) is still an enigma but likely involves magma storage and pathways spread throughout the crust, requiring indirect methods for its study. Here, we present 3-D resistivity models derived from the inversion of broadband (∼0.0001–3000 s) magnetotelluric data with 9–13 km lateral spacing in the central Paraná Magmatic Province, an expressive Early Cretaceous LIP in South America. Our results map in greater detail the previously interpreted LIP magma conduit and support, in contrast with seismological models, significant magmatic underplating to explain the observed conductivity near the LIP central axis. The potential axial lava feeder appears as a pair of crustal conductors (5–15 km; >0.1 S/m) parallel to the region of maximum thickness of both pre-volcanic sedimentary rocks and erupted tholeiitic basalts along an extension of at least 800 km. We propose the high conductivity is due to graphite films of precipitated carbon during the ascension of carbon-bearing fluids released by crystallizing magmas underplated at the base of the crust. The association of high conductivity with underplating is supported by high Vp/Vs ratios close to the conductive lineament, by a lower-crustal zone of high P-wave velocities at the basin axis attributed to mafic intrusions, and by a residual gravity high interpreted as gabbros underplated/intruded in the lower crust. Moreover, the conductive lineament is spatially associated with intracrustal high densities inferred from geoid inversion and upper-crustal high P-wave velocities. Early CO₂ release during crystallization of underplated magma before eruption could explain the time gap between the Weissert ocean anoxic event and the volcanism. Our study advances in the controversial topic of magmatic intrusive components in the Paraná LIP with implications for LIP generation and paleo-climate studies.

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.

Remagnetization is a common yet notorious phenomenon that interferes with paleogeographic reconstruction. Classical paleomagnetic field tests are helpful in detecting remagnetization but their diagnostic power is limited: remagnetization may occur before folding, the tilting age may be ambiguous, or protracted remagnetization may yield dual polarities. Rock magnetic information can provide other constraints on our understanding of the origin of natural remanent magnetization (NRM). Here we focus on the rock magnetic properties of acknowledged remagnetized limestones and unremagnetized rocks of the Zaduo area in the Eastern Qiangtang Terrane, Tibetan Plateau (China). Chemical remanent magnetization is suggested as a more frequent mechanism than the thermoviscous resetting of the NRM. The secondary NRM resides in authigenic magnetite of stable single domain and superparamagnetic (SP) size which grew during post-depositional burial processes. Both high-field and low-field thermomagnetic runs reveal the alteration of existing iron sulfides to magnetite in the remagnetized limestones. NRM decay curves show that the maximum unblocking temperature of the remagnetized samples is significantly lower than that of the unremagnetized samples. Component analysis of acquisition curves of the isothermal remanent magnetization (IRM) reveals a hard component that represents SP magnetite in remagnetized limestones. This component is absent in unremagnetized rocks. End-member modelling reveals a convex curve in the coefficient of determination versus the number of end-members plot for the unremagnetized limestones, whereas the remagnetized rocks exhibit both near-linear and convex shapes. In addition, quantitative analysis of the hysteresis loop shape for different lithologies indicates its validity in detecting remagnetization. Furthermore, we show the differences in the hysteresis data distributions of the two rock types on the Day plot, the Néel diagram, the Borradaile diagram, and the Fabian diagram. Our research emphasizes that rock magnetic properties can serve as tools to diagnose remagnetization in magnetite-dominated rocks. We recommend a comprehensive rock magnetic study to discriminate remagnetization, involving the Day plot, Fabian diagram, thermal demagnetization curves, IRM component analysis and end member modelling.

Some seismic evidence suggests that the mantle transition zone (MTZ) may become highly hydrous and anisotropic, particularly in the vicinity of subduction zones. The two-dimensional path-integrated anisotropy from the upper mantle to the MTZ has been well established beneath the northwestern region of South America. However, explicit details of azimuthal anisotropy on the MTZ boundaries remains ambiguous. Therefore, we attempted to constrain the azimuthal anisotropy on the MTZ boundaries by implementing the P-to-S anisotropic receiver function analysis. We detected significant seismic evidence of azimuthal anisotropy on the 410-km discontinuity, but weak anisotropy on the 660-km discontinuity. The synthetic waveform modeling indicated the fast symmetry axis of anisotropy trends 50° from the north and plunges 40° downwards from horizontal with an anisotropy strength of 4.0% near 410 km depth. The direction of anisotropy suggests the mantle material moves downwards and towards the subducting Nazca slab near the depth of 410 km. The increased anisotropy strength around the 410 km suggests the hydrous wadsleyite may attribute to anisotropy in the upper MTZ. The lack of detectable seismic anisotropy near the depth of 660 km could be caused by the insufficient amount of aligned anisotropic minerals, even though the mantle material continues moving downwards.

We perform an adjoint inversion by using antipodal PKPab phases to estimate the heterogeneous Vp structure at the base of the lowermost mantle. We have carefully examined antipodal stations with high S/N ratios during the past 30 years and selected 20 source-receiver pairs with the epicentral distances >179.0 degree and the Mw <7.0. We have used the spectral-element method on the Earth Simulator of JAMSTEC to calculate the synthetic seismograms for a heterogeneous mantle Vp model with the accuracy of period about 8 s (110 km wavelength at the CMB). We have set up the time window to retrieve the PKPab phase from the vertical component of both observed and computed seismograms to calculate the adjoint source to obtain the sensitivity kernels of Vp in the mantle. The computed Vp sensitivity kernel for each event shows the characteristic annulus pattern in the lowermost mantle, which covers a large area of the CMB. The twenty PKPab earthquake-station pairs in this antipodal study contribute the equivalent of about 3140 measurements at the CMB—compared with 1871 previously studied—and provide new data. Therefore, although the number of individual source-receiver pairs is not large, the summed sensitivity kernels of the PKPab phase for Vp structure at the base of the mantle may be sufficient to model heterogeneity of the Vp structure at the CMB. We summed each event kernel to set up a sensitivity kernel of Vp in the lowermost mantle and iterated the inversion to estimate a heterogeneous structure in D″. Although we have iterations that dominantly affect both South America and the South Pacific, the summary final model shows features within South of Africa, South Pacific, SE Australia, and Central & South America. Keeping the Vs model fixed, we map the CMB Vp heterogeneity measured by the parameter R_s,p = dlnVs/dlnVp and find qualitative, proximal correspondence with the degree-2 pattern of Large Low Velocity Shear Provinces observed in shear tomographic models: in the Pacific and Africa R_s,p > 2.0, whereas the surrounding edges of the edges of the Pacific show R_s,p < 2.0.

Seismic clusters in background seismicity have been associated with high stress levels and can be an important precursor to large earthquakes, but there is not a unanimous concept of cluster and most cluster identification methods are cumbersome and involve a priori assumptions. We propose a simple definition of seismic cluster and a straightforward method of identification involving a minimum of parameters that can be objectively determined in a data-driven way according to a principle of low random occurrence. As an illustration, definition and method were applied to the identification of cluster activity from October 1979 to March 2010 in northern Baja California, Mexico, between 118°W to 113°W and 30°N to 33°N, a tectonically complex seismic region with several fault systems. Twenty-one clusters were identified, of which 17 located around the places at the northeastern corner of the study area that would be ruptured on April 4, 2010 by the El Mayor-Cucapah M_w 7.1 earthquake, the largest recorded earthquake in Baja California, Mexico, and the four others occurred within 9 km from its epicenter. Clustering also became slightly more frequent as the time of the earthquake approached, so that if the clustering survey had been carried out before the whole northern Baja California area, the clustering might have identified the future epicentral region as a region of interest to be closely monitored (this earthquake featured foreshock activity starting some 15 days before the main event). Although the reliability of clusters as precursors to large earthquakes is still to be studied, it is certainly useful to have a reliable and simple method to identify and characterize them.

The Eastern Iranian Mountain Ranges (EIR) emerged as a consequence of the Late Cretaceous collision between the Afghan and Lut blocks. However, the response of the uppermost mantle to this collision remains enigmatic. Additionally, although petrological evidence suggests that post-collisional delamination is possible, it has not been conclusively identified in prior regional seismic imagery. This observation leads us to further explore this possibility using a dense seismic network. To gain insight into the geodynamic implications for eastern Iran and address knowledge gaps, we extensively investigated the seismic structure of the uppermost mantle beneath the EIR using a dense seismic network of 34 temporary stations, complemented by data from nine additional local permanent stations. By meticulously analyzing 6589 relative arrival time residuals from teleseismic records with favorable signal-to-noise ratios, we applied a non-linear tomography method to map P-wave velocity perturbations in a relative sense. Our tomographic images unveiled distinct instances of rapid high-velocity anomalies beneath low-velocity regions in the shallow mantle, suggesting the potential occurrence of lithospheric dripping, followed by subsequent asthenospheric upwelling. This observation offers a plausible explanation for the observed post-collisional magmatism over the Lut Block. Furthermore, to maintain the approximately 1.5-km positive residual topography across the EIR, beyond the influence of crustal properties, additional support from the hot and buoyant asthenosphere becomes crucial, particularly in the absence of a substantial lithospheric mantle.

The hyper-oblique indentation of the Indian plate beneath the Burmese sliver gives rise to the Indo-Burma Ranges (IBR) in the eastern part of the Indian subcontinent. This geological formation encompasses one of the enigmatic hotspots and includes the densely populated regions of India and Myanmar. Harmonic Decomposition (HD) of the receiver functions, derived from the Multi-Taper Correlation (MTC) technique, is used to model seismic anisotropy and morphological crustal deformation caused by subduction underneath IBR. We used teleseismic earthquake data from eleven broadband seismic stations installed within the IBR and its foredeep region. The findings indicate that the attitude of the fast symmetry axis or dipping direction of the interface is influenced by the trend of regional geological features and absolute plate motion, with the IBR exhibiting NNE-SSW and N-S directions and the Himalayan region showing NE-SW and E-W directions. Our results reveal that the coupling of the Indian plate with the Burmese and Eurasian plates induces lithospheric fabrics that align perpendicular to the coupling direction, resulting in anisotropy in the brittle upper crust. Directional analysis of the HD model for the interfaces at the middle or lower crust reveals the strike of the fast symmetry axis in the NNE-SSW direction, which suggests the alignment of minerals and partial melt in the direction of the major shear stress. The interface across the Moho reflects four-lobed periodicity, that is, 90^o ambiguity in the strike direction of the fast symmetry axis, varying from the E-W to the N-S directions. The ambiguity indicates the possibility of the 2D-induced entrained mantle flow along the subducting Indian plate and the 3D toroidal flow parallel to the trend of the IBR.

Fennoscandia is continuously uplifting in response to past deglaciation, termed glacial isostatic adjustment or GIA, and its mantle viscosity is well constrained from ice sheet and sea level data. Here, we compare those GIA-constrained viscosities for the Fennoscandian upper mantle with geophysically-constrained viscosities. We construct the upper mantle viscosity structure of Fennoscandia by inferring temperature and water content from seismic and magnetotelluric (MT) data. Using a 1-D MT model for Fennoscandian cratons together with a global seismic model, we infer an upper mantle viscosity (below 250 km) of ∼10^21±2 Pa·s, which encompasses the GIA-constrained viscosities of 10²⁰ − 10²¹ Pa·s. The GIA viscosities are better matched if the Fennoscandian upper mantle is a wet harzburgite or a dry pyrolite, where pyrolite is ∼10 times more viscous than harzburgite. Using the average temperatures and water contents for harzburgitic upper mantle, the GIA viscosities require 1–4 mm grain sizes indicating a diffusion creep regime. In northwestern Fennoscandia, where a high-resolution 2-D resistivity model is available, greater inferred mantle water content implies viscosities that are 10–100 times lower than those for the Fennoscandian Craton. Our work suggests that the combination of seismic and MT observations can improve upper mantle viscosity estimates, especially for regions with laterally-varying viscosity structures or where GIA constraints are not available. Although our method represents an important step forward, viscosity uncertainty can be further reduced by incorporating additional constraints on rock composition, grain size and mantle stress, as well as more accurate geophysical data, into the viscosity calculation.