Tectonic Regime Research Articles

Seismogenic model of the 2023 MW5.5 Pingyuan earthquake in North China Plain and its tectonic implications

The 6 August 2023 MW5.5 Pingyuan earthquake is the largest earthquake in the central North China Plain (NCP) over the past two decades. Due to the thick sedimentary cover, no corresponding active faults have been reported yet in the epicenter area. Thus, this earthquake presents a unique opportunity to delve into the buried active faults beneath the NCP. By integrating strong ground motion records, high-precision aftershock sequence relocation, and focal mechanism solutions, we gain insights into the seismotectonics of the Pingyuan earthquake. The aftershocks are clustered at depths ranging from 15 to 20 km and delineate a NE-SW trend, consistent with the distribution of ground motion records. A NE-SW nodal plane (226°) of the focal mechanism solutions is also derived from regional waveform inversion, suggesting that the mainshock was dominated by strike-slip motion with minor normal faulting component. Integrating regional geological data, we propose that an unrecognized fault between the NE-SW trending Gaotang and Lingxian-Yangxin faults is the seismogenic fault of this event. Based on the S-wave velocity structure beneath the NCP, this fault probably extends into the lower crust with a high angle. Considering the tectonic regime and stress state, we speculate that the interplay of shear strain between the Amurian and South China blocks and the hot upwelling magma from the subducted paleo Pacific flat slab significantly contributed to the generation of the Pingyuan earthquake.

Remote sensing analysis and geodynamic setting of magmatic spessartine-almandine-bearing leucogranites, Um Addebaa area, southeastern Desert, Egypt: Bulk rock and mineral chemistry

This study aims to incorporate optical multispectral remote sensing, petrography, geochemical, and field investigations in producing mapping and surface abundance of garnet mineralization within Um Addebaa area, southeastern Desert of Egypt. Datasets of Landsat-8 OLI/TIRS, ASTER and Sentinel-2A (S2A) were handled and scaled enhanced to discriminate the units of lithologic rock and garnet mineralization. The spectral mapping techniques of constrained energy minimization (CEM) and matched filtering (MF) are used to detect the garnet-rich zones which confined to the leucogranites masses. These granites are spessartine-almandine-rich, certainly along the interaction with the ophiolitic mélange. Notably, they have high total alkalis, SiO2, and Al2O3, Rb, 104Ga/Al, Y, Zr, Pb, Rb/Sr, Al2O3/TiO2, and FeO/MgO, and strong Ti, Sr, and Ba depletion as well as low CaO/Na2O ratio (<0.3). These geochemical parameters reflecting calc-alkaline, and strongly peraluminous magma of S-type. They are derived by two episodes of partial melting clay-rich metapelite crustal rocks followed by extreme Fe-Ti oxides and feldspar fractionation. These leucogranites formed within extension tectonic regime during post-collisional episode, resulting raising of asthenosphere and partial melting of the crustal metasedimentary rocks. Garnet occurs as, aggregates and/or vein like-shape, subhedral to rounded grains, homogenous and rarely reveal weak zonation. It’s cracked and free of inclusions. It contains appreciated concentrations of FeO and MnO and minor amounts of CaO and MgO, dominantly spessartine-almandine end-member (Alm52-59Sps 31-36 Grs 8-12 Prp0.5-0.8 Adr0). Garnet chemical composition reflect it is magmatic origin with MnO- FeO-rich magma that is produced from in situ nucleation from strongly peraluminous magmas during post- collision setting.

Semi-constant source radius and variable stress drop of repeating earthquakes: A fluid-invasion hypothesis for earthquake generation

Summary Earthquakes with very similar waveforms have been recognized as repeating earthquakes, or repeaters, which are interpreted as repeated ruptures of an isolated, frictionally locked asperity patch that are caused by stress loading due to aseismic slip in the surrounding stable regime of the fault. Ideally, the repeated ruptures of the same asperity result in the same source dimension and stress drop. However, significant variations in the stress drop have been observed during fluid-injection experiments even for a single repeater sequence. This study focuses repeater sequences in three different tectonic regimes beneath the Japanese Islands and investigates variations in the source dimension and stress drop in the same repeater sequences. The obtained results show that the variation in the source radius is mostly within 20–30% in individual repeater sequences but the stress drop is largely variable by one order of magnitude or more. We propose a fluid-invasion hypothesis to explain our observations. When pore-fluid pressures gradually invade into an asperity, the edge of the asperity tends to behave as aseismic because of the reduction of the effective normal stress to values below the critical value of effective normal stress, which makes the source radius somewhat smaller. Simultaneously, the amount of the fluid invasion into the asperity fluctuates the shear strength of the asperity, with the stress drop being smaller for higher pore-fluid pressures. The reduced source radius and stress drop expected from the fluid-invasion hypothesis can explain positive correlations between the source radius and seismic moment. We propose a conceptual model for earthquake generation by involving the triggering of aseismic slip by the enhancement of pore-fluid pressures and subsequent rupture of asperity by the stress loading due to the aseismic slip, which can be potentially applied to the genesis of repeaters and non-repeaters.

Paleoproterozoic sediment-derived magmas reveal a late orogenic stage in the southern São Francisco Craton, Brazil: Evidence from petrogenesis of the 2.0 Ga Cupim Pluton leucogranites

Sediment-derived granites (S-type) can provide detailed information about continental crust reworking processes and tectonic regimes through time. Their compositions reflect the nature of sedimentary rocks that melted to form them. Inherited accessory minerals in these granites can provide information regarding the age and nature of the crustal rocks that contributed to the formation of their sedimentary source rock. In addition, S-type granites typically form during collisional stages in modern orogens, and their Precambrian records can provide clues about ancient tectonics. In this contribution, we present a detailed petrogenesis of the Cupim Pluton, located in the Paleoproterozoic Mineiro Belt (Brazil). The Cupim pluton is composed of two-mica and garnet-bearing leucogranites that are strongly peraluminous (ASI > 1.1), high-K (K2O/Na2O > 1), low MgO + FeO + TiO2 (<2 wt%). Its composition is similar to that of melts experimentally produced by muscovite/biotite dehydration melting reactions. Rb/Sr and Rb/Ba ratios suggest that these leucogranites are formed by the melting of heterogeneous sedimentary source(s). Zircon crystals from the Cupim Pluton unveil a protracted crustal melting period (2.01–1.98 Ga) hitherto unknown in the Mineiro Belt, which itself discloses a late collisional orogenic stage in southern São Francisco Craton. Igneous zircon crystals exhibit εHf(t) between −16.4 and − 0.6, and δ30Si between −1.16 and − 0.26 ‰, which is typical for zircon from sediment-derived granites. Hafnium isotope variations reflect mixing between at least two crustal sources, which is in line with inherited zircon cores that evidence major Archean and minor Paleoproterozoic sediment contributions. The emplacement of the Cupim Pluton leucogranites ends a subduction-collision cycle, following a long period of TTG-sanukitoid magma production, and promoted the stabilization of the southern São Francisco Craton.

Lineaments Mapping Using Shaded Relief Images Derived from Landsat-9 Imagery at Diyala Government, NE Iraq

The Landsat-9 satellite images with a 30 m resolution at Diyala government was evaluated using the shaded relief technique to extract and map the surface lineaments. The processing of the azimuth and latitude method results in four shaded images (315\45, 200\50, 50\90, and 110\60). The optimal shaded relief (110\60) was used to extract the lineaments using the manual process. The results indicated that the primary structural trend of the lineaments was in the NW-SE direction, with a minor presence of NE-SW trends. It can be concluded that these structures are closely associated with the Makhoul-Hamrin trend in the Low Folded Zone (LFZ) influenced by the prevailing tectonic regime. In contrast, the lineaments structural trend of NE-SW would result from the neotectonics regime that linked to the listric fault system. On the other hand, the southeastern part of the study area shows no surface features due to the stress of the tectonic regime that affected the Tertiary deposits.

Development of the Arabian-Nubian Shield along the Marsa Alam-Idfu transect, Central-Eastern Desert, Egypt: geochemical implementation of zircon U-Pb geochronology

The magmatic complex along the Marsa Alam-Idfu transect, Central-Eastern Desert of Egypt, represents the northern segment of the Arabian–Nubian Shield (ANS), which developed within the framework of the East African Orogen. The basement rocks of the Arabian-Nubian Shield have been developed through three distinct phases of magmatic activity: the island-arc, the syn-orogenic, and the post-orogenic phases. Transitioning of the magmatic phases from the syn-orogenic to the post-orogenic, identifies changing the tectonic regime from a compressional to an extensional setting. The scarcity of comprehensive regional geochronological data that rely on precise isochron methods, such as the zircon U-Pb technique, could limit the comprehensive understanding of this region’s geological and tectonic history. That would raise a number of uncertainties ranging from the timing of the different magmatic activities and timing of changes in the tectonic regime to the existence of the pre-Pan-African crust in the CED. Our study provides new insights into the aforementioned uncertainties through zircon U-Pb dating of different rock units along the Marsa Alam-Idfu transect, CED, Egypt. The resulting ages ranged from 729 ± 3 Ma to 570 ± 2 Ma, constraining the temporal evolution of the ANS in the studied region into (1) the island-arc phase, represented by a metamorphic sample with an age of 729 ± 3 Ma. (2) the syn-orogenic phase, represented by calc-alkaline and alkaline granitic samples with ages ranging from 699 ± 4 Ma to 646 ± 2 Ma. These two phases indicate initiation of the compressional subduction regime in the CED since 729 ± 3 Ma and being dominated till 646 ± 2 Ma. (3) the post-orogenic phase, represented by metavolcanics, volcanic rocks, and alkaline plutonic samples with ages ranging from 623 ± 3 Ma to 570 ± 2 Ma. This phase suggests dominance of the compressional-to-extensional tectonic transition setting from 623 ± 3 Ma to 600 ± 1 Ma along with the Dokhan volcanism and activation of post-collision tensional regime activated at 582 ± 3 Ma. Our findings discourage the proposed dominance of the island-arc and syn-orogenic phases in the CED and the classical restriction of older magmatic activity to calc-alkaline granitic rocks and younger magmatic activity to alkaline granitic rocks. Additionally, we identified evidence of local magmatic sources by dating five grains with Mesoproterozoic (pre-Arabian–Nubian Shield) xenocrysts with ages ranging from 1549 ± 4 to 1095 ± 25 Ma.

Investigation of Horst-Graben Structures around Gülşehir (Nevşehir-Central Anatolia) with PSInSAR and Lidar

This study examined horst-graben structures in the study area using the Interferometric Synthetic Aperture Radar (PSInSAR) method, and the movements in the line of sight (LOS) were determined. Both field and PSInSAR studies were evaluated together. Accordingly, the uplift observed in the horst-graben structures, and the descending movements observed in the grabens continue today. It has been observed that the faults forming the Horst and Graben structures are active. The structures determined by the field studies are in agreement with the data obtained as a result of the PSInSAR studies. It has been determined that these structures continue to develop even today. The region continues to expand under the extensional tectonic regime.

Tectonic Inversion in Sediment-Hosted Copper Deposits: The Luangu Area, West Congo Basin, Republic of the Congo

Complex Neoproterozoic tectonic processes greatly affected the West Congo Basin, resulting in a series of dispersed copper deposits in the Niari Sub-basin, Republic of the Congo. Structural observation and analysis can help in understanding both the transportation pathways for copper accumulation and the detailed tectonic evolution processes. This study examines cases from four copper mine sites in the Luangu region of the Niari Basin, using a set of codes that consider the three regional tectonic regimes (extension, extrusion, and contraction) and three deformation criteria (maximum effective moment criterion, tensile fracture criterion, and the Coulomb criterion). By combining these two aspects, nine new codes are introduced: the extension maximum effective moment criterion (EM), extension tensile fracture criterion (ET), extension Coulomb criterion (EC), strike-slip maximum effective moment criterion (SM), strike-slip tensile fracture criterion (ST), strike-slip Coulomb criterion (SC), compression maximum effective moment criterion (CM), compression tensile fracture criterion (CT), and compression Coulomb criterion (CC). By analyzing and applying these codes to the selected sites, we show that the new codes can present a geometric coordination catering to an exhumation-related inversion process from extension, strike-slipping, to contraction. The existence of SM- and CM-related structures that occurred during regional extrusional and contractional events may indicate a deeper level of exhumation for layers related to copper deposits in the field sites. A new tectonic evolution model is presented, considering the hypothesis of vertical principal stress changes while the two horizontal principal stresses remain relatively constant during copper mineralization affected by the Western Congo Orogen. The application of the nine codes facilitates the determination of interrelations between different tectonic regimes.

Geochemistry and Zircon U–Pb Chronology of Jinchanshan Gold-Hosted Granitoids, Inner Mongolia: Implications for Petrogenesis and Geodynamic Evolution

Jinchanshan is a medium-sized, granitoid-hosted gold deposit located in the Kalaqin area of Inner Mongolia. Mineralization predominantly occurs in the contact zone between biotite granites and quartz porphyry rocks, associated with the Jinchanshan minor intrusion, suggesting a genetic link to the granitoid-hosted gold deposit. In this study, the petrography, geochemistry, and LA-ICP-MS zircon U–Pb chronology of these two granitoid samples were studied. The results indicate that the zircon U–Pb age of the biotite granites is 127.9 ± 3.0 Ma, while that of the quartz porphyry is 121.4 ± 1.5 Ma, both dating back to the Early Cretaceous. The average SiO2 content of the granites is 66.64%, and the rocks have high total alkali (K2O + Na2O) content, averaging 9.13%. The average K2O content is 4.39%, with a K2O/Na2O ratio of 0.93. The quartz porphyry rocks are enriched in SiO2 (74.41%–76.85%) and have high Na2O + K2O content (8.67%–9.59%), but are low in MgO (0.03%–0.09%), CaO (0.44%–1.02 %), and TiO2 (0.08%–0.12%). Most samples of the biotite granite and the quartz porphyry rocks exhibit high-K peraluminous and medium-K calc-alkaline characteristics, respectively. Both rock types are enriched in Rb, Th, U, K, Zr, Hf, and Gd and relatively depleted in Ba, Sr, P, Ti, Nb, Ta, and Eu, with a pronounced negative Eu anomaly. The biotite granites show high ∑LREE/∑HREE ratios (6.1–6.9), while the quartz porphyry rocks exhibit lower ratios (2.0–4.2). Both granitoid types have elevated FeOT content and FeOT/(FeOT + MgO) ratios, indicating that the Jinchanshan granitoids are A-type granites. The zircon U–Pb ages, combined with the regional tectonic settings, suggest that these granitoids formed during large-scale metallogenic events in the Early Cretaceous, within the Yanshanian post-orogenic extensional tectonic regime. This is consistent with the lithospheric thinning and extensional processes in Eastern China during this period.

Geochronological assessment of the Arabian-Nubian Shield plutonic intrusions in the arc assemblages along the Qift-Quseir transect, Central Eastern Desert of Egypt

The ophiolitic mélange and granitic intrusions along the Qift‒Quseir transect in the Central Eastern Desert (CED) of Egypt are parts of the Egyptian Nubian Shield (ENS), which is the northern segment of the East African Orogeny (EAO). The Arabian-Nubian Shield (ANS) is the longest Neoproterozoic belt on Earth, and it was formed during the East and West Gondwanaland collisions within the framework of the EAO. The ANS basement rocks were developed during three distinct phases of magmatic activity: the island arc and syn-collisional phases, identifying a compressional tectonic regime, and a post-collisional phase that identifies changing the tectonic regime into an extensional type. The geochronological assessment of these magmatic activities is essential for understanding the regional geology and tectonic development of the ANS. In our study, we dated different rock units along the Qift‒Quseir transect and revealed ages ranging from the Late Tonian (820 ± 8 Ma) to the Late Ediacaran (563 ± 4 Ma). These ages were associated with three different magmatic pulses: (1) a seafloor spreading and island arc phase (ophiolite and related rocks), represented by sample QQ05, which was dated from 820 ± 8 Ma; (2) a syn-collisional phase, represented by samples QQ08 and QQ10, dated from 733 ± 10 Ma and 729 ± 10 Ma, respectively; and (3) a post-collisional phase, represented by all the other samples, dated from the Ediacaran at 603 ± 9 Ma to 563 ± 5 Ma. These results showed that the post-collisional phase was dominant, especially in terms of the alkali-feldspar granites, relative to ophiolitic rocks, and the syn-collisional granites in the CED. Initiation of the Dokhan Volcanic eruptions at 639 ± 2 Ma gave us the date of the compressional-to-extensional tectonic transition setting, and the post-collisional tensional regime was activated at 603 ± 9 Ma. Additionally, we identified evidence of local magmatic sources by dating 11 grains of Paleo-to Meso-Proterozoic xenocrysts with ages ranging from 1876 ± 18 to 1070 ± 13 Ma (i.e., components of the pre-Arabian-Nubian Shield).

Mineral fabrics unravel syn-magmatic deformation and constrain Permian and Jurassic tectonic regimes in arc plutons from the southern Colombian Andes

ABSTRACT Mineral fabric analyses of Permian La Plata and Jurassic Paez arc plutons provide information about the interplay of tectonic deformation and magma emplacement in the southern Colombian Andes. We combined field mapping, microstructural data, AMS fabrics, rock magnetic properties, vorticity analysis, and thermobarometry to study the internal fabrics of the plutons. Our results indicate that the studied plutons have well-developed NE magmatic and mylonitic foliations, with the latter being distinctive for the Permian pluton. These foliations are consistent with the N65°E/63°W Permian and N43°E/82°W Jurassic AMS magnetic fabrics. Magnetic lineations record subhorizontal (Permian) and vertical (Jurassic) magmas upwards in the NE–SW trajectories. The orientations of all the plutonic fabrics indicate structurally controlled magma emplacement consistent with compressional tectonics and bulk NW–SE crustal shortening. Microstructural features recorded in the plutons reveal magmatic to sub-magmatic and solid-state deformation developed at high- to low-temperatures during their syn-tectonic emplacement history. Thermobarometric calculations indicate similar shallow emplacement conditions (~10 km depth) in the La Plata and Paez plutons. The application of vorticity analysis shows that the magnetic fabrics and syn-emplacement of the Permian pluton developed by simple-shear dominated dextral transpression, whereas the AMS fabrics for the Jurassic syn-tectonic intrusion may have been associated with pure-shear contractional deformation.

Seismic reflection data interpretation and petrophysical evaluation of the Meyal area, the Potwar Basin, Pakistan

Despite extensive exploration efforts in the Potwar Basin, a thorough understanding of the structural framework and reservoir characteristics within the Meyal Field remains critical for pinpointing potential hydrocarbon zones. This study aims to integrate geology and geophysics to emphasize the structural and stratigraphic interpretation of the Meyal Field, including reservoir characterization using seismic and well-log data. The seismic interpretation of six seismic lines focuses on four key reflectors: the Kamlial, Chorgali, Sakesar, and Salt Range Formations. The Eocene Chorgali and Sakesar Formations are of primary importance for exploration and production. Structural analysis revealed that the Meyal anticline is a plunging structure bounded by a back thrust to the north and a fore thrust to the south, suggesting a favorable location for hydrocarbon accumulation. Time-depth contour maps derived from seismic data further delineate potential sites for future investigations. The well correlation indicated an uplift toward the Meyal-2 compared with Meyal-5, signifying a shallowing trend attributed to thrust tectonics. This tectonic regime has rendered the area highly prospective for hydrocarbon exploration, with thrusting and oblique-slip faulting enhancing the reservoir qualities of Eocene Formations. The petrophysical evaluation revealed favorable reservoir characteristics, including an average porosity ranging from 0% to 12% with an effective porosity of approximately 7.5%, water saturation up to 42%, and hydrocarbon saturation reaching 58% within the pay zone of the reservoir. These findings suggest that the Sakesar and Chorgali Formations within the Meyal oil field hold promise as productive hydrocarbon reservoirs. The integrated study provides valuable insights into the structural framework and reservoir characterization, highlighting the potential of the Eocene Formations as productive hydrocarbon reservoirs supported by favorable structural configurations and petrophysical properties.

Collisional orogeny between the Siberian and Sino-Korean paleoplates: Evidence from geochronology and metamorphism of the metamorphic ophiolite mélange in the Banlashan area near the Xar Moron River in the southeastern Central Asian orogenic belt

As the largest Phanerozoic accretionary orogenic belt, whether the Central Asian Orogenic Belt (CAOB) experienced a strong collisional orogenic process after Paleo-Asian Ocean (PAO) closure remains controversial. The age and P-T conditions recorded by metamorphic rocks could be used to study the thermal and tectonic regimes during orogenic processes, providing constraints for orogenic evolution. In this paper, the geochronology and metamorphism of a newly discovered metamorphic ophiolite mélange in the Banlashan area near the Xar Moron River in the southeastern CAOB were studied. The association and age of the metamorphic ophiolite mélange protoliths are similar to those of the Xar Moron River ophiolite belt to the north. The metamorphic ages of different ophiolite mélange lithologies are all late Permian (250.2–256.5 Ma), indicating that the Siberian and Sino-Korean paleoplates collided during the late Permian. The metamorphic pressure and temperature recorded by the garnet amphibolite block in the ophiolite mélange could reach 8.5–8.8 kbar and 700–725 ℃. The core-to-rim zoning of garnet indicates that the garnet amphibolite underwent a clockwise P–T path, which is the typical metamorphic feature of collisional orogens. The ophiolite mélange metamorphic features, combined with regional magmatic, metamorphic, sedimentary and tectonic deformation data, indicate that a strong collisional orogeny occurred rather than a soft orogeny between the Siberian and Sino-Korean paleoplates after the PAO closure.

Оценка удароопасности массива горных пород Стрежанского месторождения

The article considers the aspects of assessment of the rock burst hazard of the rocks of the Strezhan deposit using various methods: visual inspection, fracturing of the massif, core disking, and lab tests for extreme deformation. In 2023, ore samples and host rocks were selected at the Strezhan deposit to determine their deformation and strength properties. The studies revealed their high strength. In the same year, the components of the stress tensor were measured at the mine using the well hydraulic fracturing method at two measuring stations at a horizon of +750 m (at a depth of approximately 240 m from the daytime surface). It has been established that the natural field of the Strezhan deposit is characterized by a tectonic regime with horizontal tectonic stresses as maximum stresses, whereas vertical stresses are minimal. The azimuth of the maximum horizontal stress was 140 ± 10° in the northwest-southeast direction. As a result of field measurements of the natural stress field at the horizon +750 m at the Strezhan deposit, the values of the highest horizontal stresses in the rock mass (ore) were established as equal to 15.2 MPa. This means that the level of stress of the rock mass at the horizon +750 m (mining horizon) is insufficient to destroy mine workings. Calculations established the critical depth of mining operations as 500 m for the rock burst condition. In addition, ores prone to rock bursts have been detected in the deposit: solid copper-zinc and sulfur-pyrite. The assessment of the structural disturbance of the deposit rocks via the rock quality index RQD = 60.26 % enabled the classification of the deposit rock mass mainly as moderately fractured, where dynamic manifestations of rock pressure are not registered and not forecasted within the designed mining depths. As a result of the studies, it has been established that the rock mass of the Strezhan deposit located at a depth of 500 m from the daytime surface can be classified as non-hazardous in terms of rock bursts on the condition of observation of the mining technology adopted in the design.

40Ar/39Ar Dating of Hydrothermal Processes in Large Gold Deposits of the Kochkar Anticlinorium (South Urals, Russia)

The age restrictions for productive mineral associations of the two largest gold deposits of the Southern Urals—Svetlinsk and Kochkar, which are located in the East Uralian megazone—are discussed. The 40Ar/39Ar dating method was performed for the first time for potassium-containing hydrothermal minerals (micas and amphiboles) from the ore veins and ore-host alteration, as well as from marbles. The age estimates obtained are in the range of 290–276 Ma; the weighted average value for the Svetlinsk deposit is 276 ± 2 Ma; for the Kochkar field, 284 ± 2 Ma. It is supposed that the studied mineral assemblages of the Svetlinsk and Kochkar deposits were formed in the beginning of the postcollision stage corresponding to the tectonic relaxation regime. Age of hydrothermal mineral formation in the gold fields of the Kochkar anticlinorium are consistent with the time of post-tectonic plutonic activity, which was expressed in the Middle–Southern Urals in large-scale granitization (~300 Ma). The ore-bearing alteration of a femic profile (of the same age for two deposits), which are most likely basificates, are near-synchronous with the granitization.

Earthquake Magnitude Estimation through Mixed-Effects Ground-Motion Modeling of Early P-Wave Arrivals

Abstract A regional earthquake early warning system (EEWS) warrants potential predictive models to accurately extract earthquake parameters like magnitude and intensity from the first few seconds of a P-wave arrival. In this study, a maiden predictive model depicting the relationship between peak displacement amplitude (Pd) and magnitude (ML) is proposed for the western Himalayan region through a mixed-effects regression and compared with those from similar tectonic regimes. This model for EEWS is derived from the vertical-component waveforms with a high signal-to-noise ratio, using three different time-window lengths (Td) of 1, 2, and 3 s, just after the P onset. Waveforms from 83 earthquakes in the magnitude (ML) range of 3 and 5.5 registered at 27 strong motion seismic stations are used for this purpose. The hypocentral distance range varies between 5 and 264 km. A comparative analysis between the models obtained through linear and linear mixed-effects (lme) regression reveals that the latter is robust. It is observed that the intra-event uncertainties are significantly reduced after site corrections and contribute more toward the total variabilities, compared to the inter-event uncertainties. Based on the results from this study, it is emphasized that the local site effects should be incorporated while developing the predictive models for EEWS. Importantly, the displacement magnitude (Mpd) derived from Pd values, accurately matches with ML, even for the data not used to derive the model, lending credence to the final model. A scaling relation between the peak ground velocities (PGV) and Pd values is also established to evaluate the seismic hazard levels. Advocating that the adapted models should be calibrated for a targeted region, the dissimilarities among different models and the implications from epistemic uncertainties are also discussed in the present study.

Zircon geochemistry from early evolved terranes records coeval stagnant- and mobile-lid tectonic regimes

Determining the mechanisms by which the earliest continental crust was generated and reworked is important for constraining the evolution of Earth's geodynamic, surface, and atmospheric conditions. However, the details of early plate tectonic settings often remain obscured by the intervening ~4 Ga of crustal recycling. Covariations of U, Nb, Sc, and Yb in zircon have been shown to faithfully reflect Phanerozoic whole-rock-based plate-tectonic discriminators and are therefore useful in distinguishing zircons crystallized in ridge, plume, and arc-like environments, both in the present and in deep time. However, application of these proxies to deciphering tectonic settings on the early Earth has thus far been limited to select portions of the detrital zircon record. Here, we present in situ trace-element and oxygen isotope compositions for magmatic zircons from crystalline crustal rocks of the Acasta Gneiss Complex and the Saglek-Hebron Complex, Canada. Integrated with information from whole-rock geochemistry and zircon U-Pb, Hf, and O isotopes, our zircon U-Nb-Sc-Yb results reveal that melting of hydrated basalt was not restricted to a single tectonomagmatic process during the Archean but was operative during the reworking of Hadean protocrust and the generation of juvenile crust within two cratons, as early as 3.9 Ga. We observe zircon trace-element compositions indicative of hydrous melting in settings that otherwise host seemingly differing whole-rock geochemistry, zircon Hf, and zircon O isotopes, suggesting contemporaneous operation of stagnant-lid (oceanic plateau) and mobile-lid (arc-like) regimes in the early Archean.

Zircon U-Pb Ages of the Granitoids in Shanxi and Its Significance for Tectonic Evolution of North China Craton in Mesozoic

Mesozoic granitoid formations offer crucial insights into the tectonic history of the North China Craton. New zircon U-Pb ages of two Mesozoic granitoids in the Huai’an terrane yield ages of 226.4 ± 1.1 Ma for the Yihe (YH) granite and 156.3 ± 2.9 Ma for the Zhujiagou (ZJG) granodiorite. The negative Nb, Ta, and Ti anomalies; high Nb/Ta ratios (20.4 to 24.1); high (La/Yb)N (30–84); low initial 87Sr/86Sr ratios (0.707725–0.708188); and negative ƐNd(t) values of the Yihe complex suggest that it originated from the partial melting of the lower crust and lithospheric mantle. However, the geochemical and Sr-Nd isotopic results of the ZJG granodiorite are characterized by I-type granites: Na2O + K2O values of 7.27 to 7.94 wt%, negative Nb anomalies, positive Pb anomalies, higher initial 87Sr/86Sr ratios (0.710979–0.714841), and much lower ƐNd(t) values (−27.1 to −30.1). The Late Jurassic Zhujiagou complex was derived from partial melting of a thickened low crust, and during the Late Triassic, magmatic rocks were formed under a post-collisional extensional regime. Multiple upwellings of the asthenosphere facilitated the mixing of magmas derived from partial melting of the lithospheric mantle and lower crust. These mixed magmas then ascended to the upper crust after undergoing fractional crystallization, leading to the formation of the YH complex. In the Late Jurassic, the tectonic regime of the NCC shifted from compression to extension. The Late Jurassic intrusion identified in this study developed within a compressional setting linked to the subduction of the Paleo-Pacific Ocean.

Neoproterozoic High-K calc-alkaline granites forming the large Agudos Grandes and Socorro batholiths, SE Brazil: Crystallization conditions and tectonic implications

I-type, high-K calc-alkaline (HKCA) granites are the main components of two large Neoproterozoic batholiths that occur in different tectonic domains in SE Brazil (Agudos Grandes (emplaced in the central Ribeira Fold Belt) and Socorro (emplaced in the Socorro-Guaxupé Nappe). We investigated representative samples from both batholiths using detailed petrography by optical and electronic microscopy and chemical analysis of main minerals (amphibole, biotite and plagioclase) by EPMA aiming to obtain reliable estimates of magmatic temperatures and pressures. Granites from both batholiths show broadly similar whole-rock compositions and crystallized from oxidized magmas at ∼ NNO+1 to +2. However, they were emplaced at different depths (Agudos Grandes: 3–5 kbar; Socorro: 5–6 kbar; Al-in-hornblende geobarometry) and crystallized at slightly, but consistently different temperatures (respectively, lower and greater than 800 °C). The lower temperatures of the Agudos Grandes granites may account for the presence of titanite as a main accessory mineral, in contrast with its absence, or occurrence as thin rims in ilmenite in the Socorro granites. The pressure estimates are consistent with the metamorphic conditions of the country rocks and suggest that magma emplacement was influenced by a combination of density control and evolving tectonic stress regimes, with the dominantly more mafic rocks of Socorro emplaced at greater depths. On the other hand, higher Sr/Y ratios and REE fractionation shown by the Agudos Grandes whole-rocks may result from magma equilibration in a thicker crust (>60 km, as contrasted with ∼50 km for Socorro) at the time of magma generation. However, the possible presence of cumulate components (as suggested, for instance, by high apatite and hornblende saturation temperatures) requires that these contrasts are tested in the future by proxies that are independent of whole-rock composition (e.g., trace-element chemistry of minerals).

Interplay and feedback between tectonic regime, faulting, sealing horizons, and fluid flow in a hydrocarbon-hosting extensional basin: The Val d'Agri Basin case, Southern Italy

Understanding the factors that govern past fluid circulation in tectonically active and/or hydrocarbon-rich basins is crucial for elucidating present-day fluid-flow scenarios. We investigate the circulation of paleo-fluids in the extensional-transtensional Val d'Agri Basin (southern Italy), home to a giant oil field and significantly affected by both natural and human-induced seismicity. Our aim is to understand how faulting and the variable thickness of the clay-rich tectonic mélange, which constitutes the seal of the hydrocarbon reservoir, influenced past fluid flow under different tectonic regimes. To achieve this, we combined multiscale structural observations with isotope (C, O, clumped, and Sr) and Rare Earth and Yttrium (REY) analyses of fault-related calcite mineralizations. Using analytical methodologies that allow the analysis of sub-milligram samples for carbonate clumped isotopes, we provided a detailed characterization of the variability in precipitation temperatures and composition of parental fluids in both space and time. Our results reveal five main types of parental fluids, ranging from meteoric to intraformational and deep crustal, which were differently involved in the tectonic evolution of the Val d'Agri Basin. During orogenic shortening, vertical fluid circulation was mostly limited and compartmentalized, whereas post-orogenic extensional faulting promoted the ascent of deep fluids. Our findings indicate that the sealing properties of the mélange were likely enhanced locally by increased thickness but were also compromised by fault activity and associated seismic events. Fluid circulation in the study area has been influenced by the prevailing tectonic regime (compressive vs. extensional), stratigraphic-structural architecture, properties of impermeable horizons, and seismic events. The model proposed for the Val d'Agri Basin elucidates past processes that are useful for understanding current fluid circulation in the basin itself and can be applied to other basins where fluid circulation is partly manipulated by human activities.