Tectonic Environment Research Articles

A new anisotropic poroelasticity model to describe damage accumulation during cyclic triaxial loading of rock

SUMMARY Crustal rocks undergo repeated cycles of stress over time. In complex tectonic environments where stresses may evolve both spatially and temporally, such as volcanoes or active fault zones, these rocks may experience not only cyclic loading and unloading, but also rotation and/or reorientation of stresses. In such situations, any resulting crack distributions form sequentially and may therefore be highly anisotropic. Thus, the tectonic history of the crust as recorded in deformed rocks may include evidence for complex stress paths, encompassing different magnitudes and orientations. Despite this, the ways in which variations in principal stresses influence the evolution of anisotropic crack distributions remain poorly constrained. In this work, we build on the previous non-linear anisotropic damage rheology model by presenting a newly developed poroelastic rheological model which accounts for both coupled anisotropic damage and porosity evolution. The new model shares the main features of previously developed anisotropic damage and scalar poroelastic damage models, including the ability to simulate the entire yield curve through a single formulation. In the new model, the yield condition is defined in terms of invariants of the strain tensor, and so the new formulation operates with directional yield conditions (different values for each principal direction) depending on the damage tensor and triaxial loading conditions. This allows us to discern evolving yield conditions for each principal stress direction and fit the measured amounts of accumulated damage from previous loading cycles. Coupling between anisotropic damage and anisotropic compaction along with the damage-dependent yield condition produces a reasonable fit to the experimentally obtained stress–strain curves. Furthermore, the simulated time-dependent cumulative damage is well correlated with experimentally observed acoustic emissions during cyclic loading in different directions. As such, we are able to recreate many of the features of the experimentally observed directional 3-D Kaiser ‘damage memory’ effect.

Prospective and Retrospective Evaluation of the U.S. Geological Survey Public Aftershock Forecast for the 2019–2021 Southwest Puerto Rico Earthquake and Aftershocks

Abstract The Mw 6.4 Southwest Puerto Rico Earthquake of 7 January 2020 was accompanied by a robust fore- and aftershock sequence. The U.S. Geological Survey (USGS) has issued regular aftershock forecasts for more than a year since the mainshock, available on a public webpage. Forecasts were accompanied by interpretive and informational material, published in English and Spanish. Informational products included narrative “scenarios” for how the aftershock sequence could play out, infographics, and a report on the potential duration of the aftershock sequence through the next decade. Forecasts are based on the epidemic-type aftershock sequence (ETAS) model and generated using the USGS AftershockForecaster software—an interactive graphical user interface built on the OpenSHA platform (Field et al., 2003). The initial forecast is based on past sequences in similar tectonic environments; subsequent forecasts are tuned to the ongoing sequence via Bayesian model updating. Probabilistic aftershock forecasts for the next day, week, month, and year were publicly released and archived at a daily to monthly tempo, allowing for a truly prospective test of the forecast. Here, we evaluate the forecast over the first year of the recorded aftershocks. The ETAS-based forecast performed well overall, successfully capturing both the chance of having at least one earthquake of a given magnitude in a forecast interval as well as the non-Poissonian distribution of the total number of aftershocks within an interval. A retrospective analysis shows that the ETAS model is a substantial improvement over the existing Reasenberg and Jones (1989) forecast model. The exercise also reveals some limitations of the current model, in particular, with respect to nonstationarities in the aftershock magnitude distribution and model parameters throughout the evolving sequence.

Upper Oligocene–Lower Miocene Gangdese Conglomerate Along the Yarlung-Zangbo Suture Zone and its Implications for Palaeo-Yarlung-Zangbo Initiation

The upper Oligocene–lower Miocene Gangdese conglomerate is deposited along the Yarlung-Zangbo suture zone, which extends 1,500 km from west to east and is located in the core area of the India–Eurasia plate collision zone. The Gangdese conglomerate records richly uplift and denudation histories of the Lhasa terrane and Tethyan Himalaya on both sides of the suture zone, thus revealing the growth process of the southern Tibetan Plateau during the Late Oligocene–Early Miocene. In this study, we documented the detailed sedimentology and chronology of the Gangdese conglomerate. The Gangdese conglomerate is dominated by conglomerate and sandstone, with minor volumes of siltstone and tuff deposited in an alluvial fan and fluvial system. Based on sedimentology and structural relationships, we suggest that the Gangdese conglomerate was deposited in an extensional tectonic environment in the early period and an extrusion tectonic environment in the late period, which was controlled by Indian slab shearing and breakoff during the Late Oligocene–Miocene. According to the new magnetostratigraphy and detrital zircon U-Pb dating, the main depositional age of the Gangdese conglomerate was likely the Late Oligocene–Early Miocene (26–18 Ma), and it trended younger from west to east. Moreover, paleocurrent data from the Gangdese conglomerate showed westward axial sediment transport; thus, we inferred that a westward axial palaeo-Yarlung-Zangbo River occurred along the whole Yarlung-Zangbo suture zone during the Late Oligocene–Early Miocene, and its flow was opposite to that of the current Yarlung-Zangbo River.

Passive magmatism on Earth and Earth-like planets

Magmatism has occurred throughout Earth's history. From the early Earth to the modern plate-tectonic Earth, the amount of magmatism has varied, but it has always occurred on multiple scales, in various tectonic environments and at various depths in the crust and mantle. Magma compositions also vary. In this paper, we argue that the mechanism of magma emplacement has generally been passive at all stages of Earth evolution. We conclude that most magmatism related to subduction, rifting, mid-oceanic spreading, flood basalts and large igneous provinces and related to mantle upwellings, magma underplating, slab windows, orogenic collisions as well as Archean TTG formation are predominantly passive from the lithosphere-scale to the crystal-scale. Our results weigh against the view that magmatism drives plate motions. Most of the magmatism on other Earth-like planets is also passive regardless of the tectonic environments.

Archean versus Phanerozoic oceanic crust formation and tectonics: Ophiolites through time

• Global compilation of the salient features of ophiolites. • Lithological differences between Archean and Proterozoic/Phanerozoic ophiolites. • Subduction-dominant Archean oceanic crust with accretionary cycle plate tectonics. • For Proterozoic and Phanerozoic a combination of both accretionary cycle and Wilson cycles. A global compilation of structural, lithological, and geochemical data on a selection of Archean, Proterozoic and Phanerozoic magmatic complexes, interpreted as ophiolites, is presented. Ophiolites, based on Phanerozoic examples, can be classified into subduction-related and subduction unrelated categories. These categories can be further subdivided into several subtypes depending on the proximity to subduction zones, and on sequential development from rifting – drifting to seafloor spreading for the subduction-unrelated category. From bottom to top ophiolites exhibit a magmatic sequence of ultramafic rocks (upper mantle units), gabbros (layered and isotropic), basaltic dikes and lavas, as well as boninites and felsic dikes and lavas in the subduction-related types. Archean greenstone belts show large variations in their construction, but Eoarchean examples display identical structure and lithology to the Phanerozoic ophiolites, attesting to the operation of seafloor spreading and subduction zone processes in the early Earth's history. Lithological differences between the Archean and Proterozoic/Phanerozoic ophiolites are demonstrated in the common occurrence of komatiites and felsic rocks, and scarcity of sheeted dike complexes in the former, compared to the opposite situation in those ophiolites that are younger than ca. 2 Ga. Geochemically there is a concomitant decrease in the content of incompatible elements (e.g., Sr, Zr, Y, Nb) and an increase in the content of compatible elements (e.g., Mg, Cr, Ni). In terms of tectonic environment analyzes, the Archean ophiolites are more abundantly subduction-related than those of Proterozoic and Phanerozoic ages. The subduction-dominant Archean oceanic crust (ophiolites) was characterized by accretionary cycle plate tectonics, whereas for those of Proterozoic and Phanerozoic ages, a combination of both accretionary cycle and Wilson cycles plate tectonic processes was operative.

The Role of Tectonic Discontinuities in Triggering Large Seismic Landslides

Abstract Rock landslides are generally controlled by a series of discontinuous interfaces formed by damage to complex structural rock masses that is related to folds, faults, and shear zones, which provide potential failure boundaries. Based on a detailed case study, the authors illustrate that structural damage not only affects the 3D boundaries of landslides but also significantly controls the initiation mode. In this paper, six large landslides triggered by the 2008 Wenchuan earthquake (Ms 8.0) in the carbonate strata within two wings of an anticline are selected as examples. A detailed geological engineering investigation, rock mass testing, and rock mass quality assessment are performed. The results of the research show that landslides near the core of the anticline may be closely related to the geological strength index, while landslides far away from the core of the anticline are more likely to be controlled by the preexisting weak zone within the slope. In addition, the combination of structural planes and weak zones further determines the initiation mode. A comparison of earthquake-induced landslides with gravity-induced landslides reveals that strong earthquakes accelerate the process of geomorphic erosion by reducing the geological strength index (GSI) values of landslides. Finally, this study proposes that the damage to rock masses in complex tectonic environments is of great importance to assess the location, initiation mechanism, and mode of potential large-scale landslides caused by strong earthquakes.

Near-source magnitude scaling of spectral accelerations: analysis and update of Kotha et al. (2020) model

Ground-motion models (GMMs) are often used to predict the random distribution of Spectral accelerations ( $$\mathrm{SAs}$$ ) at a site due to a nearby earthquake. In probabilistic seismic hazard and risk assessment, large earthquakes occurring close to a site are considered as critical scenarios. GMMs are expected to predict realistic $$\mathrm{SAs}$$ with low within-model uncertainty ( $${\upsigma }_{\upmu }$$ ) for such rare scenarios. However, the datasets used to regress GMMs are usually deficient of data from critical scenarios. The (Kotha et al., A Regionally Adaptable Ground-Motion Model for Shallow Crustal Earthquakes in Europe Bulletin of Earthquake Engineering 18:4091–4125, 2020) GMM developed from the Engineering strong motion (ESM) dataset was found to predict decreasing short-period $$\mathrm{SAs}$$ with increasing $${M}_{W}\ge {\mathrm{M}}_{\mathrm{h}}=6.2$$ , and with large $${\upsigma }_{\upmu }$$ at near-source distances $$\le 30\mathrm{km}$$ . In this study, we updated the parametrisation of the GMM based on analyses of ESM and the Near source strong motion (NESS) datasets. With $${\mathrm{M}}_{\mathrm{h}}=5.7$$ , we could rectify the $${M}_{W}$$ scaling issue, while also reducing $${\upsigma }_{\upmu }$$ at $${M}_{W}\ge {\mathrm{M}}_{\mathrm{h}}$$ . We then evaluated the GMM against NESS data, and found that the $$\mathrm{SAs}$$ from a few large, thrust-faulting events in California, New Zealand, Japan, and Mexico are significantly higher than GMM median predictions. However, recordings from these events were mostly made on soft-soil geology, and contain anisotropic pulse-like effects. A more thorough non-ergodic treatment of NESS was not possible because most sites sampled unique events in very diverse tectonic environments. We provide an updated set of GMM coefficients, $${\upsigma }_{\upmu }$$ , and heteroscedastic variance models; while also cautioning against its application for $${M}_{W}\le 4$$ in low-moderate seismicity regions without evaluating the homogeneity of $${M}_{W}$$ estimates between pan-European ESM and regional datasets.

Petrogenesis and tectonic environments for formation of peridotites and associated podiform chromite ore, southern part of Manipur Ophiolite Belt, Indo-Myanmar Orogenic Belt, NE India

Petrogenesis and tectonic environments for formation of peridotites and associated podiform chromite ore, southern part of Manipur Ophiolite Belt, Indo-Myanmar Orogenic Belt, NE India

Geochronology and geochemistry of the Triassic intrusive rocks in the Faku area, northern Liaoning, China: Constraints on the evolution of the Palaeo‐Asian Ocean

This study reports petrography, geochemistry, geochronology, and Lu–Hf isotopic analyses of Xiaofangshen, Lingshansibei, and Shidonggou plutons, which are located in the northern Liaoning Province, eastern segment of the northern margin of the North China Craton. In this study, we discuss their formation ages, petrogenesis, and tectonic environment. Petrographic characteristics suggest that these plutons are composed mainly of granitic rocks and are widely altered by later deformation. Zircon U–Pb dating results suggest formation ages of 248.2 ± 1.4 Ma, 245.1 ± 1.5 Ma, and 230.6 ± 2.5 Ma for the Xiaofangshen, Lingshansibei, and Shidonggou plutons, respectively. The geochemical characteristics indicate that both the Xiaofangshen and Lingshansibei plutons are metaluminous, high‐K calc‐alkaline‐shoshonitic granites that formed in a crustal thickening environment; the Shidonggou pluton is peraluminous, calc‐alkaline I‐type granite that formed in a post‐orogenic environment. All granitic plutons in the study area are enriched in large‐ion lithophile elements and light rare earth elements (REEs) and depleted in high‐field‐strength elements and heavy REEs. Combined with geochemical characteristics of the plutons and previous studies on the region, we conclude that the eastern part of the Palaeo‐Asian Ocean closed in the Late Permian to Early Triassic. The orogeny of the eastern segment of the northern margin of the North China Craton continued until the early Late Triassic.

Evidence of intraplate magmatism and subduction magmatism during the formation of Nagaland–Manipur Ophiolites, Indo–Myanmar Orogenic Belt, north‐east India

Mafic extrusive rocks (basalts) and intrusive rocks (gabbros) from the Nagaland–Manipur Ophiolite (NMO) of the Indo–Myanmar Orogenic Belt (IMOB), north‐east India, are investigated to understand their magmatic evolution in diverse tectonic environments. Basalts are distinguished into two types: basalt‐I and basalt‐II. Basalt‐I type shows the sub‐alkaline character with Nb/Y < 0.50, low Nb/Th (2.36–7.94), and low to moderate La/Sm (1.00–4.12) indicating derivation from a slightly enriched mantle source and also supported by their enriched LREE pattern with flat HREE. They are depleted in HFSEs (Nb and Ti) but enriched in U and Pb, which is indicative of a typical subduction origin derived from an MORB‐type mantle source. Investigated samples of basalt‐II and gabbros have an equal composition with alkaline characteristics. They have Nb/Y > 0.50, high Nb/Th (8.38–13.37), and highly enriched LREE (La/Sm = 4.41–6.35) pattern. They show typical Ocean Island Basalt (OIB) characters of a plume source. The two sets of basalts and gabbros found in this study have no sign of genetic relationship, and therefore, it strongly suggests that they were derived from two different mantle sources of a plume and a subduction zone mantle wedge. Our study supports the theory that the NMO has records of different magmatic episodes produced ranging from plume‐related magmatism, to divergent and convergent plate magmatism that were generated at diverse tectonic settings.

ПРОГНОЗУВАННЯ ПОКЛАДІВ ВУГЛЕВОДНІВ З ВИКОРИСТАННЯМ КОМПЛЕКСУ АТМОГЕОХІМІЧНИХ І ДИСТАНЦІЙНИХ МЕТОДІВ

Using the technology of structural, thermic, and atmogeochemical investigations a number of the studies within the Piatyhorivka area of the DnieperDonets Depression were performed. Among the studies there were the geostructural and morphostructural analyses, the structural interpretation of satellite data, field works on the emanation and thermometric surveys, laboratory chromatographic analyses of gas samples collected from the undersoil layer, integrated interpretation of atmogeochemical and remote sensing data, predicting oil-gas occurrence for the Piatyhorivka area. It is found that this area is located in the complicated structural and tectonic environment. Within its boundaries the relative "autonomic" northern and southern zones of the anomalous gas elimination are detected and five local plots of the difficult configuration in a plane projected for the further exploration of oil and gas deposits are identified.

Late Devonian magmatic event in the South Beishan orogenic belt, Gansu: Constraints from zircon U-Pb chronology, geochemistry and Sr-Nd-Hf isotopes

北山造山带处于中亚造山带南缘，研究其岩浆事件对于推演中亚造山带地质构造演化具有重要意义。在甘肃北山南带的双鹰山-花牛山岛弧带中，分布有大量花岗质岩体。本文选取甘肃北山南带双峰山南复式岩体中的花岗闪长岩和二长花岗岩，开展LA-ICP-MS锆石年代学、Hf同位素、全岩地球化学和Sr-Nd同位素研究。研究结果表明该复式岩体成岩时代、地球化学与同位素特征基本一致，具体如下：（1）锆石年代学结果显示二长花岗岩年龄为367.5±1.7Ma、368.0±1.8Ma，花岗闪长岩年龄为366.3±2.2Ma，均为晚泥盆世；（2）复式岩体SiO₂含量为64.39%~74.95%、碱含量为5.01%~9.21%，属准铝质（A/CNK=0.88~1.00），P₂O₅含量低（0.02%~0.17%），且与SiO₂呈负相关，具有典型的I型花岗岩特征；（3）样品总体富集轻稀土元素（（La/Yb）_N=6.23~23.0），具有较强烈的Eu负异常（δEu=0.31~0.55），富集Rb、Th、U等大离子亲石元素，亏损Nb、Ti、P等高场强元素；（4）锆石ε_Hf（t）值为-3.1~+6.0，t_DM2为928~1327Ma，全岩（⁸⁷Sr/⁸⁶Sr）_i在0.705607~0.708523之间，ε_Nd（t）在-4.1~-1.9之间。结合区域构造环境，实验数据表明，双峰山南晚泥盆世复式岩体是在活动大陆边缘的环境下，由幔源岩浆底侵加热下地壳使之部分熔融，并与之混合形成的，代表了晚泥盆世柳园洋向北俯冲的构造事件。;Beishan orogenic belt (BOB) is located on the southern margin of the Central Asian Orogenic Belt (CAOB). The study of magmatic events in BOB is of great significance for understanding the tectonic evolution of the CAOB. A large number of granitiod plutons are distributed in the south of BOB, where the Shuangyingshan and Huaniushan arcs are located. In this study, we focus on granodiorite and monzogranite from the south Shuangfengshan complex in the southern BOB. We present whole rock geochemistry, Sr-Nd isotopic composition, in situ U-Pb dating and Hf isotopic composition of zircons from this complex, from which the following results are obtained:(1) The granodiorites and monzogranites have similar zircon U-Pb chronology, whole rock geochemistry and isotopic composition. (2) Zircon LA-ICP-MS U-Pb dating yields ages of 367.5±1.7Ma and 368.0±1.8Ma for the monzogranite, and 366.3±2.2Ma for the granodiorite, suggesting they were formed in the Late Devonian. (3) The SiO₂ contents from samples collected in the complex are 64.39%~74.95%, with alkali contents of 5.01%~9.21%, suggesting it belongs to metaluminous (A/CNK=0.88~1.00); the measured samples have low P₂O₅ contents (0.02%~0.17%), which are negatively correlated to SiO₂. These features indicated they are of typical I-type granite characteristics. (4) For the REE compositions, this complex is characterized by enriched light rare earth elements ((La/Yb)_N=6.23~23.0), negative Eu anomaly (δEu=0.31~0.55), enriched large ion lithophile elements, such as Rb, Th, U, and depleted high field strength elements such as Nb, Ti, and P. (5) The complex has ε_Hf(t) values of -3.1~+6.0, two stage model ages (t_DM2) of 928~1327Ma, (⁸⁷Sr/⁸⁶Sr)_i values of 0.705607~0.708523, and ε_Nd(t) values of -4.1~-1.9. Combined with the regional geology and tectonic environment, these data reveals that the south Shuangfengshan complex was formed in an active continental margin in Late Devonian, and during its evolution, the lower crust was partially melted by the underplating of mantle-derived magma and mixing with mantle-derived materials, which represented a northward subduction of the Liuyuan Ocean plate in the Late Devonian.

Review on geology and mineralization of North Korea (II: Phanerozoic)

AbstractIn this review, we attempted to summarize and interpret the Phanerozoic geology and mineralization following the article (vol. 71 in Resource Geology) which reviewed the Precambrian geology and mineralization of North Korea. The basement of the Korean peninsula was built during the Precambrian and gradually became more evolved and complex during the Phanerozoic. The northern part of the peninsula was in the active and passive continental margins of the Korea–China platform during the Phanerozoic. In the tectonic environments of the Phanerozoic, the tectonic provinces of North Korea comprise several Paleozoic–Mesozoic intracontinental and rift basins as well as Cenozoic ocean border and rift basins. During the Paleozoic, the sedimentary strata were formed within the marine‐to‐nonmarine intracontinental basins, such as the largest Pyeongnam and Hyesan–Iwon basin, Imjingang rift basin, and several small structural basins, and the sedimentary type of limestone, dolomite, and coal deposits were formed. Mesozoic orogenic events in the peninsula were the most overwhelming geologic event causing block movements through collision and subduction of the paleo‐Pacific plate. Songrim orogeny (late Permian to early Triassic) might be caused by continental collision associated with strong deformation, metamorphism, and granite intrusion. Daebo orogeny (Jurassic) resumed the crustal deformation under a contractional setting during the subduction and caused dextral ductile shearing, magmatism, and metamorphism in the entire peninsula. Amnokgang orogeny, corresponding to Bulguksa orogeny in South Korea, during the Cretaceous formed several pull‐apart or transtensional basins probably because of oblique subduction of the Izanagi plate, and nonmarine sedimentary and pyroclastic sequences were deposited in the basin. The mineralization related to the Mesozoic plutonism was the most dominant in the peninsula. The considerable mineral deposits are ultramafic related magmatic Ni deposits owing to rift magmatism and granite related hydrothermal Au–Ag, Cu, Pb–Zn, Fe, W, and Mo deposits caused by the subduction of paleo‐Pacific plate. During the Tertiary, several structural basins under the extensional regime were overprinted on the Mesozoic basins in the northern part of the peninsula. They contain marine and nonmarine sedimentary rocks and felsic–mafic extrusives. Some sedimentary deposits, such as coal, kaolin, bentonite, diatomite, and zeolite, were formed. The Tertiary Pohang basin in South Korea shows basin geology, magmatism, and mineralization similar to those of North Korea. The Phanerozoic geotectonics of the peninsula are characterized by the evolution of the structural basins and violent magmatism. The peninsula is located on the Amur plate, bounded by the Philippine plate, Pacific plate, and Eurasian plate. Since Phanerozoic, the peninsula's geographic position created an important tectonic link between northeastern China and the Japanese Islands throughout geotectonic evolution.

An appraisal of the geochemistry and geotectonic settings of the pan african rocks around akure and are-ekiti and environments, Southwestern, Nigeria

The geochemical characteristics of the Pan-African rocks from Akure and Are –Ekiti and Environs show a minor variations in the contents of silica (SiO2 50.18-98.92 wt%,), alumina, Al2O3 (0.26-17.26wt%), potash (K2O, 0.07-5.98wt%), and iron F2O3 (0.52-10.77wt%) respectively. The rocks suites from both areas are also characterized by high alumina, high silica, and presence of biotite, microcline, monzogranite, syenogranites, diorite, granodiorite as well as granites. The rocks from both studied areas plotted into the S-Type and I-Type granites suggestive of their derivation from mixed sources. The presence of shoshonitic series suggests rock association characterized by k-rich, high Na2O + K2O and low TiO2. The granites from b Akure and Are Ekiti and environs are characteristically potassic with metaluminous I-Type and peraluminous S-Type affinity and elong to high-K calcalkaline to shoshonitic series. The rocks are strongly enriched in Ba-Sr with pronounced negative Eu anomaly, belonging to syn-to post collisional tectonic setting. However, the plots of Rb versus Y+Nb, Rb versus Ta + Yb, Ta versus Yb, Nb versus Y, as well as Ta versusYb respectively showed that the granitic rocks of Akure and Are Ekiti and environs were formed in the tectonic environments of volcanic arcs, syn-collisional and post collisional as well as within plate settings, suggesting their derivation from fractional crystallization and partial melting.

Geochemistry and Sr–Nd isotopes of the Late Mesozoic lamprophyres in the Chaohu area, eastern China: Petrogenesis and tectonic implications

AbstractLate Mesozoic lamprophyres are widespread in the Chaohu area of the Lower Yangtze region, eastern China. These mantle‐derived rocks are of great significance for understanding the characteristics of mantle source and tectonic environment of eastern China during the Late Mesozoic. The lamprophyres from the Chaohu area can be divided into two types. The type‐I lamprophyres have low SiO2 (43.8–44.5 wt.%) and TiO2 (1.06–1.14 wt.%), high MgO (13.5–17.5 wt.%) and high contents of compatible elements, and are enriched in LREE and LILE but depleted in HFSE. They have high initial 87Sr/86Sr (0.70767–0.70904) and negative εNd (t) (−4.8 to −4.4). The type‐II lamprophyres are characterized by low contents of SiO2 (43.0–45.4 wt.%), high TiO2 (1.74–2.55 wt.%), MgO (9.17–15.4 wt.%), and compatible elements. They are also enriched in LREE and LILE but depleted in HFSE. They show enriched Sr‐Nd isotopic compositions with initial 87Sr/86Sr values (0.70773–0.70886) and εNd (t) values (−8.6 to −7.8). The type‐I lamprophyres show higher Mg# and MgO contents but significant lower TiO2 than the type‐II lamprophyres. In addition, the type‐I lamprophyres have relatively similar initial 87Sr/86Sr ratios but higher εNd(t) values than the type‐II lamprophyres. The difference of geochemical characteristics between the type‐I and type‐II lamprophyres may be caused by different degrees of partial melting of an amphibole‐bearing garnet lherzolite mantle. The lithospheric mantle was metasomatised by the fluids caused by the rollback of the Paleo‐Pacific plate subduction during the Late Cretaceous.

Geochemistry of Proterozoic and Cambrian granites from Meghalaya Plateau, north‐east India: Implication on petrogenesis of post‐collisional, transitional from I‐type to A‐type felsic magmatism

The Meghalaya Plateau including the Mikir Hills represents the northeastern extension of the Precambrian Indian Shield and mainly comprises the Proterozoic basement granite gneisses, granites (sensu lato), granulites, metasediments, Cambrian granites, and Mesozoic‐Tertiary lithounits. A new whole‐ rock geochemical dataset of Proterozoic and Cambrian granites is presented and investigated to decipher the petrogenesis of these granites with its implications on understanding the crustal growth history of Meghalaya Plateau. Cambrian granites commonly intrude the Proterozoic basement granite gneisses and Shillong Group of rocks. Both the Proterozoic and Cambrian granites exhibit similar mineral assemblages (Bt ± Amp‐Pl‐Kf‐Qz‐Zrn‐Mag‐Ttn‐Ap ± Ilm), but they are texturally distinct. Microgranular enclaves are ubiquitous in Cambrian plutons but are devoid or rare in the Proterozoic granites. Cambrian granites are medium‐ to coarse‐grained, inequigranular, hypidiomorphic, frequently porphyritic, and undeformed to mildly deformed, whereas Proterozoic granites are medium‐ to coarse‐grained, less frequent porphyritic, and mildly to strongly deformed. Geochemically, the Proterozoic (molar Al2O3/CaO + Na2O + K2O (A/CNK) = 0.86–1.15; FeOt/FeOt + MgO = 0.60–0.93) and Cambrian (A/CNK = 0.77–1.20; FeOt/FeOt + MgO = 0.56–0.90) granites are nearly identical showing strongly metaluminous to moderate peraluminous, magnesian to ferroan, and alkali‐calcic to calc‐alkaline and transitional character between I‐type and A‐type oxidized granites formed in a post‐collision tectonic environment. Comparison of studied granites with experimental melt compositions derived from various protoliths suggests that they are sourced from metabasics to tonalites. Harker bivariate plots demonstrate fractional differentiation as a dominant process in the evolution of these granites. However, occurrence of hybrid microgranular enclaves and geochemical features manifest that the mixing and fractionation of coeval mafic and felsic magmas have also played a key role in the evolution of some Cambrian plutons. A slightly higher zircon saturation temperatures for Cambrian (700–950°C) than the Proterozoic (675–900°C) granites characterize a relatively higher‐T melting regime for the generation of Cambrian than the Proterozoic granites. Petrogenetic modelling constrains that the parental magma to the Proterozoic granites can be generated by about 25.5% melting of heterogeneous lower crustal sources with low maficity, which subsequently had undergone fractional crystallization involving bt‐amp‐pl‐Kf‐qz assemblage. However, parental to Cambrian granites can be produced by about 32% melting of the amphibolitic lower crust that subsequently evolved through synchronous fractional crystallization and mixing with a mantle‐derived mafic melt. The chronological records and the present petrogenetic findings on Proterozoic granites propound a viable geodynamic model that the assembly and growth (thickening) of the Columbia Supercontinent during ca. 1,800–1,600 Ma caused a high‐temperature regime that induced the melting of the lower crust and formed the Proterozoic granites. On the other hand, the delamination‐induced decompression melting of lower (amphibolite) crust during the break‐up (ca. 550–500 Ma) of Pan‐African‐Brasiliano orogeny‐related Gondwana Supercontinent led to the generation of felsic magmas in batches that interacted, at least in some cases, with lithospheric mantle‐derived mafic magma and formed the Cambrian plutons of Meghalaya Plateau.

Low‐temperaturethermochronological insights into the Mesozoic–Cenozoic exhumation history of the Taihang–Lvliangshan region: A review

The Taihang–Lvliangshan region is located in the central part of the North China Craton (NCC), which has undergone significant reactivation and reconstruction during the Mesozoic–Cenozoic period. Documenting its long‐term uplift and exhumation history is critical for accurately characterizing the reformation extent of the NCC. In order to further reveal the Mesozoic and Cenozoic exhumation stages and spatial exhumation pattern of the Taihang–Lvliangshan, we present a comprehensive low‐temperature thermochronometric datasets from the region. The compilation revealed five distinct rapid exhumation episodes within the Taihang–Lvliangshan, including the Middle‐Late Jurassic (~160–150 Ma), Early Cretaceous (~120–110 Ma), Late Cretaceous (~100–60 Ma), Eocene (~50–40 Ma), and Late Oligocene–Early Miocene (~30–20 Ma). During the Middle–Late Jurassic, the Taihang–Lvliangshan underwent a strong thrusting deformation caused by the synthesis of northwestward subduction of the Palaeo‐Pacific Plate and southward subduction/collision of the Mongol–Okhotsk suture zone. Subsequently, rapid exhumation under an extensional tectonic environment occurred during the Early Cretaceous primarily owing to the subduction of the Palaeo‐Pacific Plate. During the Late Cretaceous, the collision between the Okhotomorsk Block and East Asia resulted in the regional northwest–southeast shortening and uplift of the Taihang–Lvliangshan. During the Eocene, normal fault activations were occurring throughout the range, likely owing to the change in the convergence rate and direction between the Palaeo‐Pacific Plate and East Asia. Another important exhumation event occurred during the Late Oligocene–Early Miocene period, which was likely attributed to the result of combined effects from both the India–Eurasia collision and Pacific Plate subduction. Furthermore, differential exhumation of the Taihang–Lvliangshan were clearly revealed according to the five separate E–W elevation–distance profiles of the Taihang–Lvliangshan. They displayed greater exhumation thicknesses at the southern regions than those along the northern regions, which can be explained by the development of the Shanxi Rift that began to form following the Oligocene–Miocene period.

Comparison of extensometric results measured in the Vyhne Tidal Station (Slovakia) and in the Sopronbánfalva Geodynamic Observatory (Hungary)

Two extensometer stations have been set up at the margin of the Pannonian Basin to monitor tectonic movements as well as Earth tides and related phenomena. Because the Sopronbánfalva Geodynamic Observatory (SGO) in Hungary and the Vyhne Tidal Station (VTS) in Slovakia are located in different geological, topographic, and tectonic environments, the analysis and comparison of the extensometer data measured here provides a useful opportunity to interpret the observed data. The tectonic deformation at the SGO shows an average contraction of: −2.94 μstr y−1 (1 μstr is 10−6 relative deformation) which can be explained by the uplift of the Alps and the anticlockwise motion of the Adria microplate, causing compression in the Eastern Alps. At the VTS an average compression of −14.8 nstr y−1 (1 nstr is 10−9 relative deformation) was measured which can be explained by the NW compression direction in this area. The measured deformations in both observatories show a good agreement with the results of GPS measurements. The deformation at the VTS is characterized by small dilatation anomalies caused by the different topographic, tectonic environment and probably by the high heat flow in the area of the station. At this station the calculated amplitude factors for O1, P1, K1, M2 are 1.01482, 1.21691, 0.83173, 1.09392 and the ocean load corrected values are 1.10817, 1.35717, 0.92809, 1.28812, respectively. At the SGO the calculated amplitude factors for the same tidal components are 0.58776, 0.38967, 0.41548, 1.00564 and the ocean load corrected values are 0.98893, 1.89117, 1.00430, 1.04962, respectively. These results show that the effect of the ocean tide loading is greater at Sopronbánfalva, than at Vyhne. Based on the comparison, we can say that the result of the local strain measurement can be considered realistic.

Automated Detection of Clipping in Broadband Earthquake Records

Abstract Because the amount of available ground-motion data has increased over the last decades, the need for automated processing algorithms has also increased. One difficulty with automated processing is to screen clipped records. Clipping occurs when the ground-motion amplitude exceeds the dynamic range of the linear response of the instrument. Clipped records in which the amplitude exceeds the dynamic range are relatively easy to identify visually yet challenging for automated algorithms. In this article, we seek to identify a reliable and fully automated clipping detection algorithm tailored to near-real-time earthquake response needs. We consider multiple alternative algorithms, including (1) an algorithm based on the percentage difference in adjacent data points, (2) the standard deviation of the data within a moving window, (3) the shape of the histogram of the recorded amplitudes, (4) the second derivative of the data, and (5) the amplitude of the data. To quantitatively compare these algorithms, we construct development and holdout datasets from earthquakes across a range of geographic regions, tectonic environments, and instrument types. We manually classify each record for the presence of clipping and use the classified records. We then develop an artificial neural network model that combines all the individual algorithms. Testing on the holdout dataset, the standard deviation and histogram approaches are the most accurate individual algorithms, with an overall accuracy of about 93%. The combined artificial neural network method yields an overall accuracy of 95%, and the choice of classification threshold can balance precision and recall.

Neoarchean reworking of Mesoarchean and Paleoarchean crust (3.4 ∼ 3.0 Ga) within the North China Craton: Constraints from zircon U-Pb geochronology and Lu-Hf isotopes from the basement of the Bohai Bay Basin

The Bohai Bay Basin basement is mainly composed of Archean granitoid gneisses with minor supracrustal rocks and is the largest basin in the Eastern Block of the North China Craton. Due to a cover of Mesozoic and Cenozoic strata, little is known about the age and crustal evolution of this basement. In this study we report new zircon SHRIMP and LA-ICP-MS U-Pb and Lu-Hf isotope data for drill core samples, including TTGs (granodiorite, tonalite gneiss, trondhjemite gneiss), granites (monzogranite, syenogranite) and a leptite, with the aim of revealing the Archean crustal evolutionary history of the Bohai Bay Basin basement. The U-Pb age of magmatic zircons from these granitoids reveals that basement rocks were mainly generated by two-stage events at ∼3.1 Ga and ∼2.5 Ga. The εHf(t) values of ∼3.1 Ga magmatic zircons vary from +0.56 to +8.27, and their corresponding single-stage model ages range from 3.3 Ga to 3.0 Ga. The εHf(t) values of ∼2.5 Ga magmatic zircons range from −12.87 to −0.07, their corresponding two-stage model ages range from 3.8 Ga to 2.9 Ga with most ages from 3.4 Ga to 3.0 Ga. The Hf isotopic characteristics show that the crustal growth of basement beneath Bohai Bay Basin occurred mainly between 3.4 Ga and 3.0 Ga, different from crustal accretion ages of 2.9–2.7 Ga on the periphery of the Bohai Bay Basin. However, both areas were reworked by the ∼2.5 Ga tectono-thermal event. Integration of this new data from the basin basement with previous data, indicates that the Eastern Block of the North China Craton may be controlled by a mantle plume during the ∼2.5 Ga period. The results from this study are significant in assessing the tectonic environment of the eastern basement in the North China Craton.