Continent-continent Collision Research Articles

The Cretaceous World: Plate Tectonics, Paleogeography, and Paleoclimate

Abstract The tectonics, geography, and climate of the Cretaceous world was a very different from the modern world. At the start of the Cretaceous, the supercontinent of Pangea had just begun to break apart and only a few small ocean basins separated Laurasia, West Gondwana, and East Gondwana. Unlike the modern world, there were no significant continent-continent collisions during the Cretaceous and the continents were low-lying and easily flooded. The transition from a Pangea-like configuration to a more dispersed continental arrangement had important effects on global sea level and climate. During the Early Cretaceous, as the continents rifted apart, the new continental rifts were transformed into young ocean basins. The oceanic lithosphere in these young ocean basins was thermally elevated, which boosted sea level. Sea level, on average, was ∼70 m higher than the present-day. Sea level was highest during the mid-Cretaceous (90 Ma – 80 Ma), with a subsidiary peak ∼ 120 million years ago (early Aptian). Overall, the Cretaceous was much warmer than the present-day (> 10˚C warmer). These very warm times produced oceanic anoxic events (OAEs) and high temperatures in equatorial regions sometimes made terrestrial and shallow marine ecosystems uninhabitable (temperatures > 40˚C). This is unlike anything we have seen in the last 35 million years and may presage the eventual results of man-made global warming. This mostly stable, hot climate regime endured for nearly 80 million years before dramatically terminating with the Chicxulub bolide impact 66 million years ago. Temperatures plummeted to icehouse levels in the “impact winter” resulting from sunlight-absorbing dust and aerosols. As a consequence of the collapse of the food chain, ∼75% of all species were wiped out (Sepkoski, 1996). The effect of this extinction event on global ecosystems was second only to the great Permo-Triassic Extinction (McGhee et al., 2013).

Large-volume and swift magmatic response to Late Cenozoic segmentation of the subducted Neotethyan oceanic slab: evidence from the Galatian Volcanic Province, northwestern Turkey

ABSTRACT The Miocene Galatian Volcanic Province (GVP) is one of the largest volcanic provinces in central-western Anatolia, with an extent of ~ 8,900 km2. The volcanic activity is extended from 22.5 to 7.5 Ma. The volcanic compositions straddle the alkaline-subalkaline fields, from basic to acid compositions and mostly transitional to sodic affinity. Major oxides show good correlation with SiO2 indicating prolonged effects of fractional crystallization. Primitive mantle-normalized multi-element patterns indicate overall similarities among the different samples of the three geographic sectors, sharing strong negative anomalies in Nb–Ta–Ti, strong positive peaks at Cs and K, coupled with a common, albeit not always present, positive anomaly at Pb. Mineral-melt geothermobarometric estimates indicates ~1070–1235°C and ~7–19 kbar for melting conditions of basaltic compositions and ~1000–1150°C and ~3–12 kbar for andesitic-dacitic rocks. The absence of correlation between radiogenic isotopes and SiO2 and MgO is here interpreted as consequence of assimilation-fractional-crystalization processes involving lower continental crust as contaminant. The GVP parental magmas are generated from ~2% to 10% partial melting of a lherzolitic mantle with high spinel/garnet ratio based on intra-REE fractionation constraints. The subduction-related metasomatism inferred for the GVP mantle sources based on their chemistry is interpreted to be linked to the northward subduction of the northern branch of the Neo-Tethys slab. Successive slab retreat resulted in extension for the critical stress distribution through the Cyprus slab, favouring magma propagation for the GVP volcanic region. The eventual break-off of the slab after the continent-continent collision of Arabian and Eurasian plates could have caused a toroidal mantle flow, favouring the widely distributed 15–16 Ma alkaline magmatism in the eastern GVP, associated with passive hot asthenospheric upwelling imaged by teleseismic P-wave tomography.

Structural expression of the frontal thrust of an active fold-and-thrust belt: The Holocene 123-km-long Kur fault, Greater Caucasus, Azerbaijan

Here we present the main features of the frontal structure, known as Kur Fault, of the Plio-Quaternary Kura fold-and-thrust belt in the Greater Caucasus (Azerbaijan). The Kur Fault has been analysed thanks to geological-structural and geomorphological surveys of its whole length, integrated by a relocation of instrumental seismicity, data on historical seismicity, new focal mechanism solutions, and ambient vibration measurements across the fault trace. The in-depth study of the frontal structure can: i) provide insights into the shallow propagation of a regional reverse fault, ii) contribute to a better understanding of the earlier stage of development of a young continent-continent collision, and iii) have implications for seismic hazard assessment because the area is seismically active and hosts the most important infrastructure for energy production in the country. The results show that the fault deforms the surface for a total length of 123 km. The shallow expression is given by four main scarp segments, with a right-stepping arrangement, which have different structural significance; they are represented by an alternation of fault-propagation folds, folds with offset frontal limbs, and shallow faulting. Analyses of the age of deformed deposits and landforms suggest activity from Mid-Late Miocene times to the Holocene. The fault attitude and pure reverse kinematics are coherent with the Holocene and present-day state of stress, characterized by a N–S to NNE-SSW horizontal σ1, suggesting the capability for seismic reactivation. Calculation of potential Mw indicates values in the range 7.5–7.9 if we consider its entire fault length, 6.1–7.2 if we consider the single segments.

Cryogenian-Ediacaran crustal growth and evolution of the active margin of the Dahomeyide belt, Ghana

Abstract The study presents detailed petrographical, geophysical, structural and geochemical data of the internal nappes zone to establish the deformational history, origin and tectonic setting and constrain the crustal growth and evolution of the active margin of the Dahomeyide belt. Two main lithological units, (i) deformed meta-granitoids (migmatites and gneisses) and (ii) undeformed granitoids, dominate the internal nappes zone. The granitoids are generally I-type, metaluminous to weakly peraluminous, low-K tholeiite to high-K calc-alkaline and of tonalite, granodiorite and granite affinity. The overall trace element patterns of the studied granitoids characterized by the enriched LILE and depleted HFS, with negative peaks of Nb-Ta, Sr, P and Ti, are indications of arc-related magmatism. Structural analysis reveals four deformation phases (D1-D4). D1 represents Northwest-Southeast (NW-SE) Pan African shortening associated with a continent-continent collision, resulting in westward nappe stacking. Progressive NW-SE shortening resulted in D2 and D3 top-to-the-NW dextral and sinistral thrusting events during the Pan-African orogeny. D4 is an extensional event likely associated with the orogenic collapse phase. The gneisses and migmatites, with dominant axial planar foliations, point to their formation in a collisional setting or influence by the Pan-African collisional processes. Continental-arc signatures in these rocks imply continental subduction during their protolith formation. The intrusive granitoid and pegmatite are undeformed, meaning late- to post-orogenic emplacement. These findings suggest that the internal nappes zone archived the subduction-collision and post-collisional phase of the Pan-African orogeny and recorded large-scale migmatization and granitoid emplacement due to partial melting of thickened lower crust between Mid-Cryogenian and late Ediacaran.

A late Neoarchean continental arc system in the northeastern North China Craton: constrain from the Guanghua mélange in the Tonghua area, southern Jilin Province

ABSTRACT The late Neoarchean tonalite – trondhjemite – granodiorite (TTG) and mafic rocks record essential information regarding the early tectonic evolution of the North China Craton, which is crucial for understanding the onset and development of the plate tectonic regime. In this study, we present a comprehensive analysis of petrography, geochronology, and geochemistry of the Guanghua Mélange from the Tonghua area in NE China. The mélange comprises basalt, gabbro, pyroxene diorite, and trondhjemite, and is classified into three groups: basaltic – gabbroic rocks (Group #1), dioritic rocks (Group #2), and trondhjemites (Group #3). Zircon U-Pb analyses indicate that they formed at 2596–2550 Ma, 2531–2515 Ma, and 2502 Ma, respectively, with metamorphism occurring during ~ 2500–2450 Ma. Meanwhile, geochemical data indicate that the calc – alkaline basaltic rocks (Group #1) exhibit slight depletion of Nb or Ta with weakly negative Eu anomalies, suggesting derivation from the lithospheric mantle and formation within a continental arc environment. The pyroxene diorites (Group #2) display similar geochemical characteristics to Group #1, likely formed by partial melting of depleted mantle that underwent sgignificant metasomatism due to variable amounts of slab-derived fluids or melts. The tholeiitic granitoids (Group #3) exhibit strongly fractionated chondrite-normalized REE patterns with high (La/Yb)N and Sr contents, low Y contents, and depletion of Nb, Ta or Ti, resembling adakites generated in arc setting, indicating likely formation through partial melting of subducted oceanic crust. This process was controlled by the initial plate tectonics under a higher geothermal gradient. Consequently, when combined with previous findings, we conclude that an initial subduction of oceanic plate operated in the northeastern North China Craton during the late Neoarchean (>2.55 Ga). Subsequently, it transformed into modern-type plate tectonics during ~ 2.55–2.53 Ga, eventually followed by the continent-continent collision at ~ 2.50–2.45 Ga, resulting in the formation of the Ji – Liao – Ji (Jilin – Liaoning – Hebei) Collision Orogenic Belt.

The tectonics of introversion and extroversion: redefining interior and exterior oceans in the supercontinent cycle

Abstract Supercontinent amalgamation is described by the end-member kinematic processes of introversion – closure of interior oceans; extroversion – closure of exterior oceans; or orthoversion – amalgamation 90° from the centroid of the previous supercontinent. However, supercontinent formations are often ascribed to contradictory mechanisms; for example, Pangaea has been argued to have formed by introversion from Pannotia/Gondwana, and extroversion from Rodinia. Conflicting interpretations arise partly from attempting to define oceans as interior or exterior based on palaeogeography or the age of the oceanic lithosphere relative to the time of supercontinent breakup. We define interior and exterior oceans relative to the external subduction ring, and associated accretionary orogens that surround amalgamated supercontinents. All oceans within the continental dominated cell and internal to the subduction ring are interior oceans. The exterior ocean is separated from the interior oceans by the subduction ring and bordered by external accretionary orogens. Wilson cycle tectonics dominate the interior continental cell, conversely, subduction of the exterior ocean is doubly vergent and lacks continent–continent collision. For the exterior ocean to close, the subduction ring must collapse upon itself, leading to the collision of external accretionary orogens. Employing this definition, Rodinia formed by extroversion, but all other supercontinents formed by introversion.

Mapping of stress and structure controlled upper crustal anisotropy in Kumaon-Gharwal Himalaya

Shear wave splitting analysis of local earthquakes provides valuable insights into the structure and stress controlled upper crustal anisotropy signatures. We examine these signatures at the Kumaon-Garhwal Himalaya and provides an excellent example of anisotropic crustal structure in the ongoing continent-continent collision settings between Indian and the Eurasian plates. A total of 256 local earthquakes were selected for the analysis (1.0 ≤ M ≤ 5.4) between January 2017 and February 2021, recorded at a dense network of 51 broadband stations. The result indicates the observed crustal anisotropy is parallel to stress-aligned micro-cracks far from the major Himalayan fault zones and structure parallel near the fault zones. The dominant fast polarization directions (FPD) in the Inner Lesser Himalaya (ILH) and Higher Himalaya, are consistent with the maximum compressive horizontal stress directions (SHmax), essentially influenced by the local stress field. Whereas in the Outer Lesser Himalaya (OLH) and Sub Himalaya, fast directions are sub-parallel to the structural trends, suggesting that the anisotropy is associated with the shear fabric from recent deformation episodes related to structural induced anisotropy. The computed normalized delay times show a mean value of 1 ms/km, within the OLH and Sub-Himalaya, while in ILH and Higher Himalaya, this value increases up to 5.3 ms/km. The associated crack densities are 0.0038 and 0.0207, with shear wave velocity anisotropy of 0.38% and 2.07% respectively. Significant scatter within the depth range of 10–15 km is observed in normalized delay times, suggesting the source of anisotropy within the upper crust.

Intracontinental convergence accommodated by underthrusting of Tarim Basin beneath central Tian Shan, Central Asia: Insights from Cenozoic upper crust deformation across the western Kepingtage fold-and-thrust belt in the northern Tarim Basin

The way in which intracontinental convergence caused by remote continent-continent collision has been accommodated remains debatable, but it is attributed to either pure shear thickening or continental subduction/underthrusting. Quantification of the deformation architecture of the upper crust and its shortening is vital for testing these competing models. For this purpose, we conducted quantitative analyses of four seismic reflection profiles along the western Kepingtage fold-and-thrust belt of the intracontinental convergence zone between the northern Tarim Basin and the central Tian Shan, a result of the remote India-Eurasia collision, which capture the structures of the upper crust and constrain the amount of shortening in the region. The structural analyses show a thin-skinned style of deformation of the western Kepingtage fold-and-thrust belt, with minimum north−south shortening estimates of ∼71.1 km (∼46.6% strain), ∼76.3 km (∼51.4% strain), ∼67.1 km (∼56.1% strain), and ∼53.0 km (∼49.9% strain), respectively, from east to west. Assuming that coupling shortening occurred across the entire crust and an initial crustal thickness of ∼41−46 km, these shortening ratios would produce a ∼83−95-km-thick crust along the northern Tarim Basin, which is significantly larger than the actual thickness previously reported. This inconsistency leads us to rule out the pure-shear thickening model, which predicts crustal-scale coupling deformation. Alternatively, our results imply that the crust of the northern Tarim Basin was deformed in a decoupling style. These results, combined with those of previous studies, lend support to the intracontinental underthrusting model. We suggest that both the upper crustal fold-and-thrust belts and the lower crustal underthrusting of the northern Tarim Basin accommodated intracontinental convergence.

Geochemistry of the Upper Cretaceous - Miocene strata in the Tekman-Karayazı Basin (Turkey): Tectonic processes, sedimentation patterns, and the source rock quality along the northern branch of the Neo-Tethys Basins and northeastern Arabian plate

Tekman-Karayazı Basin (TKB) is one of the Eastern Anatolia basins which is located in the south of the Izmir-Ankara-Erzincan Suture Zone and is characterized by sedimentary units ranging in age from Upper Cretaceous to Pliocene. The basin began to form in Upper Cretaceous (Maastrichtian) in a tectonic depression, which over time evolved into a collisional foreland basin. While the Jurassic and Cretaceous Oceanic Anoxic Event (OAE) rocks were preserved in the NE of the Arabian Plate (SW of Iran), they are represented by an ophiolitic mélange which was formed as a result of the closure of the northern branch in the TKB and other basins on the Anatolian microplate. Four tectono-sedimentary phases are recognized over the ophiolitic mélange: Phase I (Maastrichtian-Paleocene (?), after obduction) is comprised of submarine fan deposits. Phase II (Paleocene - Lower Eocene, obduction phase) is characterized by major changes in gradually depositional environments in the TKB (clastics-nummulitic limestone), and continental to ramp carbonate to slope facies in others (e.g., Yakacık-Orhaniye, Haymana-Polatlı). Phase III (Middle – Upper Eocene, ongoing collision phase) shows the development of turbiditic and continental facies show that both TKB and/or the coeval basins have poor source rock potential. Phase IV (Oligo-Miocene, ending collision) represents from eastern to central in eastern Anatolian basins alluvial-fluvial to lacustrine and marine units during Oligocene – Miocene, respectively. The geochemical correlation between the bitumen extracted from sandstone sample and the Ağçakoca shales suggests that this formation can be considered as the corresponding source rock unit; however, according to the Tmax values (364–444 °C, mean: 428 °C), this formation is mostly immature, a scenario supported by the dry wells in the region. The regional volcanic activities triggered in Miocene by continent-continent collision seems to have provided the heat which, in turn, caused the thermal cracking of kerogen in Ağcakoca shales and locally induced petroleum expulsion. Mixing with the underground water on its way to reach a reservoir rock, the generated oil formed the oil seep. Therefore, we propose that a speculative Mescitli-Haneşdüzü (?) petroleum system may exist in this basin. We suggest that the combination of factors such as the asymmetric closure of the Tethys Ocean and the escape tectonics of the Anatolian Plate gave rise to the deposition of Paleocene to Miocene organic matter-rich shales in the NE of the Arabian plate, whereas uplift, antecedent topography, and the global paleo-climatic events disturbed the favorable conditions and caused the studied basins to lack the aforementioned generative source units.

Magnetotelluric constraints on the tectonic evolution beneath the Luotian dome in the North Dabie complex zone, Central China

The Dabie orogen, formed by a Triassic continent-continent collision between the Yangtze and the North China craton, contains one of the largest ultrahigh-pressure (UHP) metamorphic terranes in the world. However, compared to the widespread UHP metamorphic rocks in the Central Dabie, little evidence shows the North Dabie Zone (NDZ) has experienced UHP metamorphism. To understand the cause of these variances and constrain the post-collision evolution of the NDZ, we conducted a magnetotelluric profile across the Luotian dome, southwestern NDZ. Our electrical resistivity model exhibits intriguing post-collision evolution-related characteristics. On both sides of the profile, the resistive features extend to ∼10 km, suggesting the UHP metamorphic or magmatic rocks are mainly concentrated in the upper crust; the underlying crustal high conductive features are interpreted to be the graphite from the mantle-derived or related magma, which were interconnected by the shearing. The middle resistive features extend deeper to ∼20 km, which may represent the magmatic rocks that contributed to the formation of the Luotian dome during the early Cretaceous. The lithosphere mantle generally behaves as high conductive features related to graphite, indicating a fertile mantle. Combining geological and geophysical observations previously, we suggest the Luotian region experienced doming in response to the hot asthenosphere upwelling resulting from the delamination of the lithospheric keel of the Dabie orogen during the early Cretaceous as triggered by synchronous tectonic events like the development of the Tan-Lu fault. These processes may fertilize the lithosphere, uplift and erode the Luotian or the NDZ, resulting in the observed resistivity structures and the lack of UHP metamorphic rocks.

High-magnesium igneous associations record final-stage geodynamic process of the southeastern segment of the Central Asian Orogenic Belt

Much controversy exists regarding the evolution and termination of accretionary orogens where typical continent-continent collision features are lacking. The Central Asian Orogenic Belt provides a key opportunity to explore the genetic relationships between the formation of high-Mg igneous associations and the evolution of accretionary orogens. In this study, two-stage Triassic magmatic events were identified in the southeastern segment of the Central Asian Orogenic Belt (including ca. 247 Ma andesites and ca. 222 Ma diorites), through precise secondary ion mass spectrometry (SIMS) and laser ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS) zircon dating. The Olenekian andesites show typical sanukitoid affinities with high TiO2, MgO, Ni, and Cr contents, and enrichment in large ion lithophile elements (LILEs; K, Ba, and Sr). They yield depleted Hf isotopic compositions and high δ18O values, which along with Hf-O isotope modeling results indicate that they originated from the partial melting of mantle wedge peridotites with input of 20% terrigenous sediments. The Norian diorites have moderate SiO2, relatively high MgO, Cr, and Ni contents, Mg# values of 63−65, high Sr, low Y and Yb contents, relevant high La/Yb ratios, and negligible Eu anomalies, which makes them comparable to high-Mg andesites with adakitic affinity. Their depleted isotopic compositions are similar to coeval A-type granites that resulted from lithospheric delamination after the final collision. In addition, with high K2O/Na2O ratios and incompatible element ratios similar to those of the continental crust (e.g., Nb/U, Ce/Pb, and Nd/Sm), they conform to an origin in which partial melting of delaminated mafic lower crust subsequently interacted with mantle materials. These two-stage abnormal Triassic high-Mg rocks archive a hot slab window triggered by the break-off of the Paleo-Asian oceanic slab and lower crustal delamination related to collapse of the southeastern segment of the Central Asian Orogenic Belt. These findings provide beneficial information for the reconstruction of the East Asian blocks in Pangea.

Spatial-temporal distribution, metallogenic mechanisms and genetic types of nephrite jade deposits in China

The nephrite jade deposits of different tectonic units in China exert significant commercial quality, which have attracted wide attention. However, these deposits have not been systematically summarized to date. Here, we investigate the major nephrite jade deposits in China to decipher their gemological and mineralogical characteristics, spatial-temporal distribution, and mineralization processes as well as to identify their geological settings and gemological properties, via integrating published geochronology, major and trace elements as well as H-O isotopes. The compiled data suggest that the major nephrite jade deposits in China can also be generally divided into green jade-type (GJ-type) and white jade-type (WJ-type) which covers different species, but most of them belong to the latter. The GJ-type nephrite deposits predominantly occur in ophiolite/ophiolitic mélange suites or are embedded into ultramafic serpentine jade orebodies. This type of nephrite jade mainly formed through the late auto-metamorphic metasomatism of serpentine or the spontaneous crystallization/precipitation along suture/shear zones that acted as pathways to migrate Ca-rich fluids during orogeny. In contrast, the orebodies of WJ-type nephrite jade are usually hosted at the contact zones between the dolomitic marbles and intermediate-felsic or mafic intrusive rocks, which were produced through metamorphism-metasomatism during post continent-continent collision, such as the Kunlun orogenic belt with about ∼1,300 km Hetian nephrite belt, and the eastern nephrite jade deposits along the Pacific Ocean. Our study highlights that the nephrite jade deposits in China originally formed during multiple tectonic stages related to the interactions between hydrothermal fluids and metamorphism under subduction accretion- and collision orogenesis-related settings. In addition, this study also provides insights into the genetic discrimination, mineral exploration, and occurrence characteristics of nephrite jade deposits as well as the evaluation and identification of nephrite jade quality.

Magmatic records of subduction and closure of the Meso-Tethys Ocean in the northern-central Tibetan Plateau

The evolution of the Tethys Ocean has received much research attention; however, the timing and subduction mechanisms involved in the closure of the Bangong-Nujiang Meso-Tethys Ocean remain poorly constrained. In this study, we present geological, geochronological, geochemical, and zircon isotopic data from Cretaceous magmatic rocks and combine these with previously published data within the central Bangong-Nujiang suture zone in the north-central Tibetan Plateau. The integrated data update the regional tectonic framework and enable a comprehensive geodynamic model to be developed for the subduction and ocean closure events. In detail, temporal and spatial variations in the Jurassic−Cretaceous magmatism reflect the influence of the northward subduction of the Dongqiao-Amdo oceanic basin and the bidirectional subduction of the Bangong-Nujiang Meso-Tethys oceanic crust. The evolution of the Bangong-Nujiang Meso-Tethys Ocean further involved initial intra-ocean subduction, slab rollback, and flat subduction, as evidenced by two phases of north-south migration of magmatism at ca. 190−160 and 160−130 Ma and a magmatic hiatus at 160−140 Ma. The ocean closed during a two-stage process, including the closure of the Dongqiao-Amdo oceanic basin to the north and the Bangong-Nujiang Meso-Tethys Ocean to the south. The Dongqiao-Amdo oceanic basin closed soon after ca. 180 Ma, accompanied by continent-continent collision between the Amdo microcontinent and the South Qiangtang terrane. The Bangong-Nujiang Meso-Tethys Ocean closed at 130−120 Ma, corresponding to a period of waning magmatism. This closure represented complete oceanic closure and caused an arc-continent collisional event involving the Baingoin magmatic arc, the Amdo microcontinent, and the Lhasa terrane. The Lower Cretaceous terrestrial strata and their basal unconformity mark the final closure of the ocean, and the Early Cretaceous ocean islands might have formed before ca. 130 Ma.

Structures, deformation history and dynamic background of the Qianyingzi Coal Mine in the Huaibei Coalfield, eastern China

The Qianyingzi Coal Mine is located in the west of the Suxian Mining District of the Huaibei Coalfield, eastern China. The study on structural development patterns and genetic mechanisms in this mine lays an important foundation for safe and efficiently underground mining, and is also the key to understanding the regional tectonic evolution. In this study, based on the analysis of three-dimensional seismic, drilling and underground measured data and regional tectonic correlation, the structures, evolution history and dynamic background of the Qianyingzi Coal Mine are discussed. The Carboniferous-Permian coal measure strata in the mine are generally a gentle syncline with a NNE-trending axis, and cut by a series of faults. The faults developed in this mine are mainly medium- and small-sized with a throw of less than 20 m, and the number of reverse faults is significantly greater than that of normal faults. The strikes of reverse and normal faults are both mainly NE, followed by NNE and nearly N‒S. According to the characteristics of structural geometry, tectonic association, fault property and cross-cutting relation, the structural deformation of coal measure strata in the Qianyingzi Coal Mine can be divided into five stages, and the corresponding tectonic stress fields are NWW‒SEE compressive stress, nearly E‒W compressive stress, NW‒SE compressive stress, nearly E‒W and NW‒SE extensional stresses, respectively. It developed the Fengjia Syncline with a NNE-trending axis in the first stage and nearly N‒S-striking reverse faults in the second stage, which were the results of foreland deformation and subsequent continent-continent collision during the convergence of the North China Craton and South China Plate in the Indosinian period. The NNE-striking reverse sinistral faults and NE-striking reverse faults developed in the third stage is related to the rapid oblique subduction of the Izanagi Plate toward the East Asian continental margin at the beginning of the Early Cretaceous in the western Pacific region. Later, the fourth and fifth stages of the nearly N‒S- and NE-SW-striking normal faults were developed under the backarc extensional background in eastern China during the Early Cretaceous. These new results can be used to guide the rational arrangement for underground mining and also provide a new understanding for regional tectonic evolution of the Huaibei Coalfield.

Azimuthal Investigation of a Fractured Carbonate Reservoir

Summary Oil production and enhanced oil recovery in carbonate reservoirs in Abu Dhabi, UAE, are largely affected by fracture systems that control the fluid path and the permeability of reservoirs. Most fracture properties, such as fracture orientations and density, are obtained by interpreting petrophysical data acquired at the wellbores, whereas fracture properties between wells are typically derived from nonzero offset seismic data. However, deriving fracture properties from seismic data is challenging, as it requires a robust methodology and a careful seismic processing procedure. In the current case study, we used the azimuthal amplitude vs. offset (AVAz) method on 3D seismic data acquired in onshore Abu Dhabi, to generate maps of fracture orientation and density in a carbonate reservoir. A sophisticated processing series was carefully performed to increase signal-to-noise ratio (SNR) and preserve seismic amplitudes. The main parameters controlling the AVAz method were investigated and optimized before being applied to the 3D seismic data. The reservoir has a high fracture density in the lower regions, but a low fracture density in the upper parts, indicating a weaker anisotropy. The resulting dominant fracture directions span from north-northwest/south-southwest to north-northeast/south-southwest, as well as from northwest/southeast to east/west, which is consistent with the primary fracture orientations determined from the interpretation of fullbore formation microimager (FMI) data acquired at well locations. These fracture systems are the result of the Late Cretaceous obduction of the Semail ophiolite, which was oriented east/west and northeast/southwest, followed by the south/north to southwest/northeast trending Late Oligocene-Miocene continent-continent collision of the Arabian and Central Iran plates along the Zagros orogenic front.

Retracing the Africa–Eurasia nascent convergent boundary in the western Mediterranean based on earthquake and GNSS data

In the western Mediterranean, following the intervening continent-continent collision, the subduction of the Tethyan ocean has progressively come to an end or almost in large sectors. Compressional deformation connected with the ongoing Africa–Eurasia convergence has therefore progressively resumed mostly along the southern passive margins of the Mediterranean back-arc basins. The aim of this paper is to trace this nascent boundary and constrain its kinematics through geodetic and seismological data recorded between the Ionian Sea and Gulf of Cadiz, and through pre-existing tectonic data. Based on these data, the nascent plate boundary is drawn, kinematically defined, and compared with the previously identified boundaries in the same region. The nascent boundary is weaving and formed by variably oriented inherited structures. It is characterized by a discrepancy between the general motion of Africa with respect to Eurasia and the local contractional/compressive axes deduced from geodetic and seismic data. The oblique convergence along the nascent boundary matches that recorded in other instances of subduction initiation elsewhere; however, the average convergence rate (∼5mm/yr) in the Mediterranean seems currently too small for such a subduction initiation. Based on the assumption of a future northward tectonic vergence (i.e., Eurasian foreland), the Tyrrhenian, Algerian, and Betic salients, the Oran and Fès recesses, and the Ionian, Trans-Alboran, and Gibraltar transfer zones are identified along the nascent boundary. The latter zones connect salients and recesses through strike-slip displacements. The Algerian offshore hosts a long segment of the boundary characterized by locally increased seismic rate and actual northward vergence that would suggest this area being the first nucleus of subduction initiation in the western Mediterranean, as was previously proposed.

Deciphering the role of late Quaternary sea level fluctuations in controlling the sedimentation in the Brahmaputra Plains

Continent-continent collision between Eurasian and the Indian plate during the Cenozoic period lead to the formation of the Himalayan Mountain chain and the development of the Indus-Ganga-Brahmaputra foreland basin to the south. Complex climate-tectonic interactions in this orogenic belt are responsible for the rapid erosion and filling of the Indus-Ganga-Brahmaputra foreland basin with the eroded materials. This study based on geomorphic mapping, lithofacies analysis, and geochemical (Strontium-Neodymium i.e., SrNd analysis) provenance characterization, as well as optically stimulated luminescence (OSL) ages, provides a dated sedimentation framework for the western Assam lowland areas. The dated relict fan surface lies ~40 m above mean sea level (msl), is incised and forms a regional valley terrace T1 composed of meandering channel deposits. Modern braided rivers flow on the T0 surface. The findings suggest that the alluvial fan is composed of three distinct lithofacies associations and aggraded during 27 to 3 ka. The bottom-most gravelly-sandy facies indicates progradation of the fan during the last glacial maximum (LGM), owing to the increased gradient of the Himalaya bound rivers. The middle facies is a sheet flood deposit which formed during the Latest Pleostocene-early Holocene period with rising sea level and increasing precipitation. During the Mid-Late Holocene, the uppermost facies is deposited as rivers lost their gradient in response to high sea level stand, resulting in inland sedimentation within muddy meandering channels. Our analysis found that falling sea level during the late Holocene was associated with greater precipitation and allowed the river to incise, to form gullies over the fan surface and form the valley terrace T1. The SrNd isotope fingerprints have been used to identify varying fan sediment sources in the Himalaya's southern front (i.e., Lesser and Higher Himalaya) as a function of changing monsoon conditions.

India-Antarctica collisional ridge structure below Nellore Schist and Eastern Ghats Belts as revealed by seismic studies

In Columbia, Rodinia and Gondwana supercontinental assembly periods, eastern margin of India lay quite close to the east Antarctica. During these periods, this region underwent prolonged sedimentation, magmatism, oceanic subduction, multiple accretion and continent-continent collision which have left their imprints in the geological and geophysical observations. In the present study, we have analyzed crustal seismic velocity structure along the DSS profiles located in the eastern part of the Indian shield that cuts across the Proterozoic Cuddapah Basin, Godavari and Mahanadi Gondwana grabens. The study revealed presence of a prominent high velocity (6.2-6.5 km/s) hidden ridge structure, which is 30 to 50 km wide and located at a shallow subsurface depth beneath the Nellore Schist Belt and Eastern Ghats Belt. It underlies below Kaligiri-Duttaluru along the Kavali-Parnapalle profile, Darsi-Addanki along the Alampur-Koniki-Ganapeswaram profile and Sattupalli-Ashwaraopet in Kallur-Polavaram profile. This ridge structure is conspicuously characterized by high gravity anomaly gradients from -90 to -15 mGal. Gravity patterns over these regions further indicate that this ridge which appears to continue from the north of Chennai to Bhavanipatna region in Bastar Craton, may have been formed due to the collision of East Antarctic terrain with eastern Indian Shield after the cessation of supra-subduction at the end of Mesoproterozoic. Further, this ridge structure is flanked by Proterozoic sediments on either side; in Cuddapah Basin on its western side and East Coast Sedimentary Basin on its eastern side, which together with the possible occurrence of Proterozoic sediments in other basins, would indicate that the extent of Proterozoic sedimentation was much larger than hitherto known. We suggest that this region was rifting during Paleo-Mesoproterozoic period and possibly a shallow marine basin existed, when it was an integral part of the Columbia supercontinent assembly. It underwent another rifting phase and denudation during the Gondwana period as indicated by thin patches of Gondwana sediments which are lined up all along the east coast. Quite likely, the rifting between India and Antarctica during the Gondwana super assembly period initiated in the Permo-Triassic Gondwana period itself, with final separation taking place much later during early Cretaceous period.

4D reconstruction of the Doldenhorn nappe-basement system in the Aar massif: Insights into late-stage continent-continent collision in the Swiss Alps

The inversion of passive margins and their transportation into fold-and-thrust belts is a critical stage of mountain-building processes. In this study, we selected the Doldenhorn Nappe and Aar/Gastern Massifs (Central Swiss Alps) system as an ideal laboratory to document the impact of inherited structures, their along-strike variations, and basement tectonics on the evolution of a fold-and-thrust belt. Three-dimensional geological modelling and cross-section restoration allowed us to reconstruct the 4D evolution of the investigated area during the late-stage Alpine orogeny (30 to 0 Ma). Our results demonstrate that: (i) the Doldenhorn Nappe is the product of the inversion of an asymmetric half-graben basin; (ii) variations in incipient basin sediment thicknesses correlate directly with the along-strike variation of the deformation of the Doldenhorn Nappe; and (iii) the multiphase thick-skinned deformation that overprinted the Doldenhorn Nappe from 22 Ma until today changed the shape of the Doldenhorn Nappe and Aar/Gastern Massifs. This reconstruction shows how thin-skinned nappe formation mechanisms and the nappe geometries are controlled by the initial basement geometry and by the rheological strength contrasts between basement and cover sediments. Basement-involved uplift and shortening controls then the late-stage collisional 3D overprint and mechanics of the fold-and-thrust belts.

Orogenic belt resulting from ocean-continent collision

Abstract Orogenic belts have been thought to form through plate convergence, involving subduction of oceanic lithosphere at continental margins (accretionary orogens), which may ultimately lead to ocean closure and continent-continent collision (collisional orogens). Intraplate orogens away from plate margins have been known, but the mechanisms controlling their evolution are poorly understood. The South China craton, including the Yangtze and Cathaysia blocks, underwent a Paleozoic orogenesis that formed a >500-km-wide orogenic belt with widespread granitoids that are unconformably overlain by Devonian cover sequences. The pre-Devonian basement rocks were subjected to strong deformation and greenschist-to amphibolite-facies metamorphism at 460–400 Ma. Paleozoic magmatism was characterized by voluminous crustally derived Silurian granitoids associated with incorporation of ancient crustal materials at 450–440 Ma and addition of juvenile mantle-derived melts at 420–410 Ma. Based on the absence of arc-like magmatism and the existence of ophiolites in the West Cathaysia terrane, geochemical evidence that oceanic crust existed beneath the East Cathaysia terrane, and geophysical evidence of contrasting lithospheres on both sides and two discrete slabs beneath their fault boundary, we propose that this Paleozoic orogenic belt was formed by collision between the two terranes that was driven by far-field forces during the assembly of Gondwana, and the East Cathaysia terrane represents oceanic lithosphere that was overthrusted by the continental crustal materials of the West Cathaysia terrane. Numerical modeling shows that this type of collision can explain the dynamics of the Paleozoic orogenesis in the South China craton and may be a mechanism for some orogens in which subduction-related igneous and metamorphic rocks are lacking.