Crustal Zone Research Articles

Timing of the subduction initiation and ophiolite emplacement of the inner Tauride Ocean: Insight from the Pınarbaşı ophiolite in Central Türkiye

Late Cretaceous ophiolites are widespread in southern Türkiye, including the Tauride Belt Ophiolites and the Central Anatolian Ophiolites. Here, we report new geological, geochemical, geochronological, and zircon isotopic data from ophiolitic rocks of the Pınarbaşı ophiolite (Eastern Tauride Belt). The Pınarbaşı ophiolite forms incomplete sequences, with its mantle section dominated by serpentinized harzburgite and chromitites, while the crustal section comprises ultramafic and mafic cumulates. Whole-rock and mineral chemistry data suggest a supra-subduction zone origin. LA-ICP-MS zircon U-Pb age of 88.2 ± 0.4 Ma (MSWD = 0.84) was obtained from a plagiogranite dyke cutting serpentinized peridotite of the Pınarbaşı ophiolite. Another, U-Pb zircon crystallization age of 96.8 ± 0.6 Ma (MSWD = 1.53) was obtained from a plagiogranite dyke within the mélange of the Pınarbaşı ophiolite. Hafnium isotope compositions of these zircons (εHf(t) = +26.4 to +3.1) suggest a depleted to slightly enriched mantle source. The formation ages of the crustal rocks (gabbro, plagiogranite, and dolerite) of the Cretaceous ophiolites in the Tauride Belt from this and previous ophiolite occurrences, suggest that supra-subduction zone ophiolites in the Inner Tauride Ocean formed during the period ∼101–87 Ma. On the other hand, the peak metamorphism of the metamorphic sole rocks is reported to have occurred around ∼104 Ma. According to all available petrogenetic and geochronological data, it is considered that the supra-subduction zone crust developed shortly (∼ 3–4 Myr) after the initiation of intra-oceanic subduction and this lasted ∼10–15 Myr.

Importance of grain boundary processes for plasticity in the quartz-dominated crust: Implications for flow laws

When H2O is present along grain boundaries, the deformation processes responsible for plasticity in silicate mineral aggregates can deviate from what may be conventionally expected. Although a necessary component of understanding crustal deformation processes, there is no theoretical framework that incorporates grain boundary processes into polycrystalline quartz rheology. To address this issue, we carried out high-pressure and high-temperature deformation experiments on fine-grained quartz aggregates. Our study illustrates that grain boundary migration (GBM) through dissolution-precipitation (in the presence of an aqueous fluid) and grain boundary sliding (GBS) may act as accommodation mechanisms to prevent hardening from dislocation glide. GBM and GBS can relax incompatibilities resulting from an inadequate number of independent slip systems, plastic anisotropy between neighbouring grains, and non-planar grain boundaries together with grain boundary junctions. As demonstrated earlier in the literature, GBM may act as a recrystallization mechanism counteracting hardening, but also is a potential mechanism that allow H2O to enter in the quartz crystal (hydrolization) at the experimental time-scale. The above serial processes occur over a range of more than two orders of magnitude in grain size (∼3 to 200 μm) and explain a grain-size-insensitive stress exponent (n = 2) and low activation energy (Q = 110 kJ/mol). In the absence of a switch to grain size sensitive deformation mechanisms induced by grain size reduction, our results imply that only a modest weakening (∼5 times the strength of the protolith) is needed (or possible) to localize shear zones in the Earth's crust.

The North Sea rift basement records extensional collapse of the Caledonian orogen

Our knowledge of crystalline basement rocks flooring continental rift basins, rifted margin deposits, and foreland basins is poor, yet important for understanding basin evolution, crustal heat production, petroleum maturation, and for offshore mineral exploration. Using the northern North Sea rift as an example, we demonstrate how modern broadband seismic data can be utilized to unravel basement lithology and structure at a previously unprecedented level of detail. In our case study, we have discovered a pre-rift Caledonian basement with a folded structural pattern very similar to Devonian collapse-related structures onshore Western Norway. The data indicate that the collapse-generated Caledonian infrastructure extends >100 km into the northern North Sea, defining a wide zone of Caledonian crust that turned hot and mobilized vertically by upright folding and detachment faulting during the post-collisional stage. This extensive post-orogenic thermally induced collapse makes the south Scandinavian Caledonides unique among Phanerozoic orogens.

MECHANISM OF FORMATION OF INTERNAL AND SURFACE DEFECTS IN CASTING AND THEIR TRANSFORMATION INTO SURFACE DEFECTS OF SHEET

This study aimed to elucidate the formation mechanisms of micro-macrostructural heterogeneities in the ingot's cortical zone and their transformation into surface defects during heating, plastic deformation, and rolling processes. The methodology and results of studying the macro-and microstructural and chemical heterogeneities of ingots and finished rolled sheets are presented. A comparative method of structural-concentration analysis of metal at theend-to-end metallurgical processing stage of ingot -slab -rolled sheets was developed. The technique utilizes metallographic methods to study the structure and perform qualitative analysis of non-metallic inclusions. The research findings indicate that the quality of the rolled sheet surface is predominantly determined by the physical heterogeneity of the ingot's crust zone, while the internal defects in thin rolled sheets are largely influenced by the contamination of the metal with non-metallic inclusions. A novel mechanism is proposed for the transformation of defects in the casting cortical zone into surface and internal defects in rolled sheets. This study contributes to a deeper understanding of the formation and transformation of defects during metal processing, enabling the development of strategies for improving the quality of rolled sheets. Additionally, the study highlights the importance of controlling the physical heterogeneity of the ingot's crust zone and minimizing non-metallic inclusion contamination to achieve high-quality rolled sheets.

On the stability of the lithospheric geoelectrical structure resolution in Tarim – Tien Shan junction zone: application of various approaches to the magnetotelluric data inversion

For several decades, the intracontinental Tien Shan orogeny has served as a testing polygon for the study of modern geodynamic and seismic processes, carried out by Russian and foreign scientists at the Research Station RAS (Bishkek). The investigations have big practical significance for developing seismic hazzard zoning strategies. Magnetotelluric sounding, providing unique information on the lithosphere structures and rheology, plays an important role in the complex of applied geological and geophysical methods.
 The article describes the experience of studying the deep geoelectric structure of one of the most seismically active regions of the Tien Shan, the junction zone of the orogen and the Tarim plate. This experience was obtained during the interpretation of materials from the regional profile of MT soundings that cross this zone. Comparing the results of applying three different approaches to the inversion of magnetotelluric data, which differ not only in software implementations and algorithmic foundations, but also in model dimension, allows us to assess the reliability of the allocation and revealed properties of the main structural elements in the deep conductivity section, which help to clarify the seismotectonic pattern of the studied crustal zone.
 The objectives of the article also included support for new SinoProbe MT experiments.

High‐Resolution Imaging of the Alaska‐Aleutian Megathrust Using P‐to‐S Mode Conversions From Local In‐Slab Earthquakes

AbstractThe top of the subducting plate is a thick, complex zone where the heterogeneous structure likely controls earthquake rupture processes. Imaging this heterogeneous channel typically involves active‐source methods with limited depth penetration, or low‐resolution teleseismic methods. To access short wavelengths at greater depth, we use high‐frequency P‐to‐S (PS, 1–15 Hz) mode‐converted arrivals from nearby earthquakes >50 km deep to image the plate interface at vertical scales <1 km. We use 37 broadband stations in southcentral Alaska between 2007 and 2008 at 10–15 km spacing, spanning the great 1964 earthquake rupture zone and adjacent deeper slow slip and tremor regions. The central 21 stations record high‐amplitude PS arrivals converting from the megathrust region, at depths corresponding to the top of a prominent low‐velocity zone (LVZ) in receiver function images. The PS/P amplitude ratio (APS_P) varies along strike and with depth of the conversion point but is independent of earthquake location and varies slowly between adjacent stations. APS_P changes with slab depth, indicating changes in lithology or fluid content of the plate interface, consistent with transitions in slip behavior from locked to slow slip. High APS_P cannot be explained by a velocity step or a single low‐velocity, high Vp/Vs layer, but requires several alternating high and low‐velocity layers. These observations indicate that the LVZ is a highly heterogeneous channel at multiple scales, resembling a subduction channel or sheared zone of metasediment and altered crust as observed in many exhumed subduction zones.

Seismogram Synthesis of Multi-Scale Layered Transversely Isotropic Saturated Half-Space Using a Revised Stiffness Matrix Method

Seismogram synthesis for a multi-scale transversely isotropic (TI) saturated layered half-space subjected to seismic dislocation source is studied by a revised stiffness matrix method in this paper. First, based on Biot’s theory of wave propagation in a fluid-saturated porous solid, the governing equations of motion in TI saturated medium are transformed by Fourier–Hankel integral transform. Next, the contribution of the seismic dislocation source can be imposed in the form of body forces. Then, the global stiffness matrix with well-conditioned properties is obtained by assembling each layer’s exact stiffness matrix, which can avoid accumulating errors in the matrix transmission. By extracting the divergence exponent terms of the stiffness matrix concerning thickness and wavenumber, the bottleneck of simulating the wave propagation process in multi-scale half-space, where the layer thickness and velocity vary sharply from the near-surface region to the crustal zone, could be overcome, and a fast and accurate seismogram synthesis could be achieved. The accuracy of the proposed method is verified by comparing the calculated results with those of published literature. Finally, the method is applied to the multi-scale system with realistic superficial fine layers. Numerical examples regarding thickness, TI properties and porosity are presented to illustrate the effects of superficial layers on ground motion. The numerical results show that the surface properties affect the wave propagation and energy distribution, leading to variable impacts on the surface response in different directions; this phenomenon is more pronounced in the case of high-frequency ground motions.

New heat flow data on the South Balearic margin: Evidence of regional fluid circulation system

Crustal heat loss processes in contexts other than mid-ocean ridge flanks are still poorly understood and described in the literature. In this work, we present 54 new surface heat flow measurements recorded along the South Balearic margin and across the Hannibal High area, in the Western Mediterranean Sea, away from spreading centres. The South Balearic margin is a narrow continental margin expressed on the seabed topography by the steep Emile Baudot Escarpment while the Hannibal High area, further south, is a presumed volcanic oceanic crust zone. Recent heat flow studies pointed out the presence of strong local thermal anomalies, imprinted in regional deep basin heat flow increasing from the eastern to the western Algerian basins (70–90 to 100–130 mW/m2). The new data in this study confirm local anomalies around the Emile Baudot seamounts and allow the delineation of some low and high heat zones. We show that (1) a correlation exists between the heat flow variability and the sedimentary cover thickness, and (2) the most pronounced anomaly is a low heat flow zone (30 km wide) observed in the mid-slope South Formentera basin, close to the basaltic basement of the Los Martines volcano. The correlation of heat flow with sedimentary thickness, the lateral extent of the heat flow anomalies, the flux peak magnitudes, and the co-existence of both low and high anomalies suggest the presence of an active fluid circulation system. Based on the arrangement of the anomalies in relation to the basaltic basement seamounts, we consider a syphon-type circulation system in the Formentera basin. Reduced heat flow suggests a recharge zone close to the Los Martines seamount, while local high heat flow located close to a smaller seamount could be a potential zone of fluid discharge. Although different buoyancy-driven circulations could occur in the South Balearic margin, we believe that a syphon-type hydrothermal circulation along permeable basaltic mounds and sealed basement conduits, similar to the off-axis oceanic crust, is best developed in the young marginal basin of the South Balearic margin.

New 2.75-D gravity modeling reveals the low-density nature of propagator wakes in the Juan de Fuca plate

Propagator wakes within the Juan de Fuca plate represent zones of oceanic crust affected by dynamic readjustments of the spreading centers. They are traditionally mapped from distortions of magnetic anomalies and have been previously interpreted as zones of denser than regular oceanic crust from gravity analysis along several seismic lines in 2-D approximation. In this paper, we utilized a 2.75-D modeling assumption that allowed us to incorporate the effects of the nearby seamounts that were not accounted for in the previous 2-D study. Our analysis reveals that the positive gravity effect of those geological objects was misinterpreted as a need for higher crustal density within propagator wake zones. When seamounts are included in the model, the crust of propagator wakes requires lower densities than the surrounding oceanic crust to explain the observed gravity field. We postulate that the lower densities of propagator wakes relate to faulting during the readjustment of the spreading centers, suggesting that they represent zones of weaker crust.

Zones of Weakness Within the Juan de Fuca Plate Mapped From the Integration of Multiple Geophysical Data and Their Relation to Observed Seismicity

AbstractThis study aims to explain the nonuniform earthquake pattern along the Cascadia Subduction Zone. In particular, we investigate the relationship between the tectonic features of the subducting oceanic Juan de Fuca slab and the onshore seismicity pattern. We have integrated multiple geophysical data sets toward three general objectives. The first study intends to study variations in physical properties along three 2‐dimensional models through regions of different seismicities that combine public gravity, magnetic, and seismic data sets. These models reveal multiple zones of decreased crustal density that we interpret as regions of weaker oceanic crust. The second objective is to delineate major tectonic features by performing spatial analysis of potential fields. The overall methodology comprises gravity and magnetic data filtering, followed by lineaments mapping and cross‐referencing interpretation with available seismic reflection data. This process allows delineating zones of crustal weakness by extrapolating outside our three 2‐D models. We also map multiple seamounts that appear to cluster along identified zones of weaker crust. Third, we investigate the relationship between the mapped tectonic elements, namely the zones of weak crust with accompanying seamounts, and the observed seismicity trends within the subducted slab. The alignment between those suggests that mapped weak crust zones and associated seamounts may have an influence on the overall subduction process. As more of these structures are heading toward the Washington portion of the margin than to the Oregon portion, more earthquakes are observed in the north than in the south.

The development of rifting and magmatism in the multiply rifted Turkana Depression, East Africa: Evidence from surface-wave analysis of crustal and uppermost mantle structure

The Turkana Depression separates the uplifted Ethiopian and East African Plateaus. It was the site, in Mesozoic times, of a failed episode of NE–SW-oriented rifting (the Anza Rift), but now hosts E–W-oriented Nubia–Somalia separation at the junction between the Main Ethiopian Rift in the north and the Eastern Rift to the south. However, the time-integrated effect of these rifting phases on crustal and lithospheric mantle architecture and thermal structure is poorly understood. Utilising data from new seismograph networks in the Turkana Depression and northern Tanzania Craton, we produce a detailed anisotropic crustal and uppermost mantle shear-wave velocity model of the region. Within the Tanzania Craton, slightly lower uppermost mantle wavespeeds (4.4–4.5 km/s) compared to neighbouring regions, and coincident rift-parallel crustal anisotropy, imply the Nyanza Rift developed in relatively weak mobile belt lithosphere between two refractory Archean blocks. At upper-crustal (≲10 km) depths in the Turkana Depression, the slowest velocities (≲3.2 km/s) are attributed to thick Mesozoic-age sedimentary basins. Nowhere within the Depression is the mid-to-lower crust or lithospheric mantle associated with wavespeeds as slow, or seismic anisotropy as strong, as that observed below the melt-rich central and northern Main Ethiopian Rift (MER) and Ethiopian Plateau further north. High upper mantle wavespeeds (≳4.5 km/s), coinciding with the broadening of MER-rifting into southern Ethiopia, confirm the presence of refractory Proterozoic lithosphere acting as a rheological boundary to rift development. Thinned crustal zones associated with failed Mesozoic Anza rifting are also underlain by fast wavespeed (>4.5 km/s) mantle lithosphere, implying this area has resisted significant thermomechanical modification from Miocene-Recent extension and magmatism. Pre-existing crustal thin zones do not, therefore, necessarily represent zones of plate-weakness where subsequent phases of rifting will develop.

Assessment of Groundwater Flow for the Western Side of Al-Kut City, Southern Iraq

Groundwater is the quantity of water captured in voids of the saturated zone of the Earth's crust, it transports and stores in pores of fractures existed in the joints of hard or sedimentary rocks. The study area is located on the western side of the city of Kut between Tigris and Al-Gharraf rivers, so the groundwater levels in the area are high. The GMS program was used to model the movement of groundwater in the Study area, Groundwater levels were measured daily for six months, from 26 December2021 to 30 June 2022, in five piezometric wells to a depth of 8.5 meters on the western edge of the Kut City. the model of groundwater movement was made in the Study area There was an agreement between measured data and data obtained from the model, where the average of the lowest value of the daily levels was 17.9 and the highest value of 18.88, and the values of the grades obtained when calibrating the model in the steady state ranged between 15.2- 18.8. After altering the hydraulic conductivity to a negative value, the model's sensitivity analysis was run. The hydraulic conductivity values that give acceptable results are close to their values in the model, where the lowest value is 0.0134 and the highest value is 0.0136 and the hydraulic conductivity values in the model are equal to 0.0135. Through the modflow program, the values of Tigris and Al-Gharraf river flood levels were entered for the western side of Kut city, where Tigris river flood level reached 18.40, and Al- Gharraf river flood level reached 18.20. The modeling that was conducted to the area helped know the distribution of groundwater in the area and the extent of the hydraulic conductivity of the area, the aquifer, and the flow to/from the aquifer.

Formation of Eocene−Miocene felsic magmatic rocks along N-S−trending Yardoi-Kongbugang mountain ranges in the eastern Himalaya: New insights into surface uplift and the initiation of E-W extension in southern Tibet

Large-scale N-S shortening induced by India-Asia convergence caused the formation of numerous E-W−trending mountain ranges in Tibet. However, the mechanism(s) of formation of N-S−trending mountain ranges remains elusive. We report on a felsic magmatic belt located along the N-S−trending Yardoi-Kongbugang mountain ranges on the flank of the Cona rift in the eastern Tethyan Himalaya. Zircon and monazite geochronology and whole-rock geochemistry revealed three epochs of middle- to lower-crustal anatexis beneath the Cona rift at ca. 47−42 Ma, 35−34 Ma, and 24−14 Ma. The mid-Eocene and early Oligocene granitoids show adakitic signatures indicating continuous crustal thickening, while the formation of Miocene leucogranites and N-S−trending dacitic dikes was related to ductile crustal extension. Silicic melts were exposed along the whole rift since the early Oligocene, suggesting that the early Oligocene could be regarded as a transitional epoch from tectonic compression to orogen-parallel extension. Widespread mid-Eocene and Miocene magmatism in the Himalaya, together with coeval metamorphic anatexis, represents two phases of crustal weakening. The weakened crustal zones under continued India-Asia convergence may have favored uplift and subsequent lateral flow of the weak zones, which initiated E-W extension. Finally, significant upwelling of the weak zones evolved into magma extrusion and formed the N-S−trending mountain ranges. This study provides new insights into the mechanisms of surface uplift and E-W extension and challenges the common view of initiation of E-W extension in southern Tibet not earlier than the early Miocene.

Crustal velocity images of northwestern Türkiye along the North Anatolian Fault Zone from transdimensional Bayesian ambient seismic noise tomography

SUMMARY Türkiye poses a complex crustal structure and tectonic settings owing to the northward convergence of the Arabian and African plates with respect to the Anatolian and Eurasian plates. A reliable 3-D crustal structure of the unruptured segment of the North Anatolian Fault Zone (NAFZ) in the Sea of Marmara is thus of utmost importance for seismic hazard assessments considering that the megacity Istanbul—with more than 15 million habitants—is close to this seismic gap. This study provides high-resolution shear wave velocity images of northwestern Türkiye, including the NAFZ, revealed from ambient seismic noise tomography. We extract over 20 000 Green’s functions from seismic ambient noise cross-correlations and then construct group velocity perturbation maps from the measured group delays with a transdimensional Bayesian tomographic method. We further perform an S-wave velocity inversion to image depth-varying velocity structures. Our high-resolution data allowed us to image S-wave velocities down to 15 km depth and reveal weak crustal zones along the NAFZ, as indicated by low shear wave velocities. We find a low-velocity zone along the Main Marmara Fault, linked with aseismic slip and a deep creep mode. Furthermore, we identify a high-velocity anomaly associated with the unruptured section that defines the boundaries of the locked zone in the crust, which can potentially trigger a destructive earthquake in the future.

Foundations of the theory of magmatic ore formation in works by M. N. Godlevsky and their modern development

On the occasion of the 120th anniversary of the birth of Mikhail Nikolayevich Godlevsky, an outstanding geologist and researcher of magmatic sulfide deposits, a book about the life of this remarkable man “From the depths of the Norilsk ores. Mikhail Godlevsky” is being prepared at the Central Research Geological Prospecting Institute for Base and Precious Metals, where he worked in 1961-1984. The material published below is based on the article “The theory of magmatic ore formation in works by M. N. Godlevsky and its modern aspects” from the anniversary edition and contains minor editorial changes. The article considers the principal theoretical provisions of the magmatic genesis of the Norilsk platinum-copper-nickel deposits, that were developed by M. N. Godlevsky in his works. Particular attention is paid to his still unpublished doctoral dissertation that covers all aspects of the genesis of the unique Norilsk ores: from the formation conditions of the ore-bearing magmas in the northwest Siberian Platform and their crystallization in the upper zones of the earth's crust, to the formation of disseminated and veined ores of the Norilsk 1 and Zub-Marksheiderskoe deposits. Modern views are presented on the origin of ore deposits of the Norilsk region, which were characterized in the works by M. N. Godlevsky.

The Mid-Albian unconformity, a key to understand the geodynamics of the North Pyrenean Trough

The Pyrenees are the result of the structural inversion of a rift of general direction N110E, established at the articulation of the European and Iberian plates. This rift was created by left-lateral transtension with severe crustal thinning during the latest Early Cretaceous and Mid-Cretaceous. Its inversion, which led to the uplifting of the Pyrenean chain, took place gradually, by transpression from east to west, during the Late Cretaceous followed by an Eocene collision. From historical fieldwork, this note mainly focuses on the rifting period at the scale of the Pyrenean domain. It clearly shows the fundamental role of the stratigraphic approach in structural interpretation. Although synchronous at the scale of the Pyrenees, the Cretaceous extension is more important in the western part of the chain, near the Bay of Biscay. It is therefore at this location, within the Tardets-Mauléon Basin, that the various stages of creation and evolution of the rift could be clearly recognized with, successively: (1) an epirogenic pre-rifting stage, during which the facies lines, submeridian during the Jurassic, gradually change into N110E during the Neocomian before giving way, in the Early Cretaceous (Barremian-Gargasian), to a moderate acceleration of subsidence and, consequently, to the gentle uplift of a southern and western margin, future High Primary Belt (also often called Axial Zone) and Landes Plateau, initiating the next stage; (2) a major, brittle, newly identified rifting stage in the Latest Aptian (Clansayesian) and Early Albian. It is characterized by the installation of elongated lows on the downthrown side of transverse N20 normal faults, active diapirism of the Triassic evaporites and central detachment of the post-Triassic cover. During the Mid-Albian, a major surface of stratigraphic unconformity, often associated to uplift, erosion, and local centripetal gliding of the basin borders, marks a new structural organization, a rapid mantle ascent and the end of this episode. Although obscured by the later Pyrenean compression, the reconstructed basin geometry is interpreted as a “pull-apart” basin associated to a modest left-lateral motion; (3) a late rifting stage characterized by the collapse of the internal zones by rapid mantle cooling. The erosion of the coeval uplift of the southern High Primary Belt and the Landes Plateau feeds the accumulation of the thick, deep and transgressive unconformable formation of the Black Flysch of Mid-Albian to Early Cenomanian age in a larger coalescing basin. Faulting associated with a simple NS distension is only active on the last major border faults; (4) finally, a post-rift stage of Late Cretaceous age illustrated by the establishment of a passively subsiding basin, devoid of major deformations and whose heterogeneity of the previous depocenters is gradually erased. The Pyrenean transpression, later and less marked than in the eastern and central Pyrenees, allowed the conservation of most of these Cretaceous transtensive structures. Within the Tardets-Mauléon Basin, the Pernes and Col d’Urdach sector, the only Pyrenean location where the Mid-cretaceous transtension led to a proven underwater mantle exhumation, illustrates from outcrops the various stages that have marked the history of the Pyrenean Trough. Towards the east, along the Internal Metamorphic Zone, the Lourdes-Bagnères, Baronnies, Ballongue, Aulus and Saint-Paul de Fenouillet-Boucheville Basins, or those, further north like the Camarade and Quillan Basins, successively examined, testify to the same sedimentary and structural evolution. As in the case of the Tardets-Mauléon Basin, we deal here again with “pull-apart” type basins established on a thinned crust, arranged in transtensive or locally transpressive relays and separated by zones of thicker crust and shallower facies successions. The Mid-Albian unconformity is always present, heralding a general drowning of the North Pyrenean Trough and cessation of the extensional activity. The Pyrenean deformation, however, constitutes an obstacle for a precise geodynamic reconstruction, in particular in the eastern and central Pyrénées. Towards the west and north-west, in the South Aquitaine, Parentis and Basque-Cantabrian Basins, where Pyrenean deformation has far less obscured the rifting events, a very comparable Cretaceous geodynamic evolution is observed thus strengthening the present interpretation.

Plume‐Induced Rifting of Thickened Crust: 2D Numerical Model and Implications for N–S Rifts in Southern Tibet

AbstractN–S rifting is one of the most typical tectonics in southern Tibet, but its formation mechanism remains controversial. Geophysical observations indicated spatial correlations between rifts and lithospheric mantle anomalies, presumably caused by asthenospheric upwelling. Here, we investigate possible plume‐induced rifting via a series of 2‐D thermomechanical models of plume interactions with a heterogeneous lithosphere. The numerical results indicate that a small‐scale mantle plume could promote the formation of a single giant rift throughout the whole thickened crust. In addition, presence of a weak mid‐crustal zone facilitates the rifting development in the upper crust while inhibiting the formation of a giant crustal rift. Instead, multiple rifts develop in the upper crust and thickened crust, jointly controlled by a heterogeneous weak crustal zone and mantle plume. Our numerical results thus emphasize the distinct roles of the weak mid‐crustal zone and small‐scale mantle plume in promoting N‒S rifting in southern Tibet.

Damage stratigraphy as a controlling mechanism of earthquakes distribution and fault patterns

Decollement zones are sub-horizontal weak zones in the earth's crust that strongly affect tectonic deformation. The weakness of a decollement zone is commonly attributed to thermal and composition layering that is relatively static in time. Here, we present evidence for damage stratigraphy in which a stratigraphic unit evolves dynamically into a weak decollement zone. The analysis is focused on the internal structure of a deep, tectonically active, decollement zone in the Sichuan Basin, China. The analyzed zone is about a 1 km thick sequence of Triassic rocks that extends across most of the Sichuan Basin at a depth range of 5.8–6.8 km. This sequence is composed of evaporites (mostly anhydrite) and carbonates (dolomite and limestone). We used three-dimensional seismic surveys, borehole observations, and rock-mechanics experiments to document pervasive internal damage revealed by fractures and flow features within this unit. As this damaged unit is defined by its stratigraphic position, we term it a Damaged Stratigraphic Unit (DSU). The damage-induced weakness of the DSU controls two active tectonic features in the western Sichuan Basin. First, large, intra-basin, frontal thrust faults terminate against the DSU from above and from below. Second, regional earthquakes display a profound decrease in frequency and seismic moment at the depth interval of 5.8–6.8 km that coincides with the DSU. We argue that this earthquake distribution may pose an increased geohazard to the Chengdu area by shallow, moderate earthquakes.

A hybrid GNA instability

A semi-analytic admixed model formalism to study the stability effects of the inner crust regions against the local collective perturbations in non-rotating neutron stars is proposed. It consists of the viscoelastic heavy neutron-rich nuclei, superfluid neutrons, and degenerate quantum electrons. A normal spherical mode analysis yields a generalized linear dispersion relation multiparametrically mimicking the inner crust features of neutron stars. A hybrid gravito-nucleo-acoustic (GNA) instability mode is found to be excited. It is demonstrated that the electron density and the inner crust curvature act as its accelerating and antidispersive agents. In contrast, the heavy neutron-rich nucleus and neutron densities act as decelerating factors. The heavy nucleus density, electron density, and geometric curvature act as its destabilizers. It is only the neutron density that acts as the GNA stabilizing agent. The heavy neutron-rich nucleus and neutron densities are found to act as dispersive broadening factors to it. The high-K regions are the more unstable spectral windows indicating that the GNA mode plays a dominant role in the inner crust zone towards the local stability. Its fair reliability is indicated in light of the recent astronomic observed scenarios. It could be useful to explore acoustic mode signatures in non-rotating neutron stars and similar other compact astroobjects.

Prospects for searching for hydrocarbon deposits in the Paleozoic complex of the South Torgai basin

Intensive development of discovered oil and gas fields since the early 90s.of the last century, mainly associated with structural traps in Jurassic-Cretaceous complex, to date has led to a reduction in proven hydrocarbon reserves in the South Torgai sedimentary basin. The prospective potential for an increase in the resource base of the studied region can be focused within the deposits of pre-Mesozoic complex.
 Historically it was believed that the South Torgai sedimentary basin has a two-deck structure: a sedimentary cover of the Mesozoic-Cenozoic age and an underlying crystalline-folded basement of the Paleozoic-Proterozoic complex. The oil and gas potential of the Paleozoic complex in the region has been proven by obtaining direct signs of oil and gas in the core, oil and gas shows, up to industrial oil flows on a limited number of areas within the weathering crust zones.
 Analytical studies of recent years indicate a significant heterogeneity of the Paleozoic-Proterozoic complex, where promising structures of the Upper Paleozoic quasi-platform complex, represented by carbonate rocks, may be present. This article is focused on the analysis of a new and previously unexplored potential oil and gas complex of deeper-lying Paleozoic sediments that are of interest for setting up geological exploration.
 Due to the confidentiality policy, the names of wells, fields, coordinates and other detailed information are hidden. The article was formed to demonstrate the concept of the prospects of the heterogeneous Paleozoic-Proterozoic complex.