Nappe Tectonics Research Articles

Timing and origin of gold mineralization in the Neoproterozoic Xixano Complex, Mozambique Belt, northeastern Mozambique: Case study of the Nanlia and Makorongo prospects

The metallogeny of gold in northeastern Mozambique is still debated, with the main point of contention being the timing of mineralization with respect to regional tectonism, metamorphism, and magmatism. In this study, we applied Re–Os sulfide geochronology to constrain the age of gold mineralization in the Nanlia and Makorongo prospects hosted by the Xixano Complex, in the southern part of the Mozambique Belt. In addition, chemical analyses of host rocks and sulfides complement the discussion on the genesis of mineralization. Single-mineral Re–Os model ages of pyrite and pyrrhotite in quartz veins constrain the occurrence of gold mineralization in the Nanlia and Makorongo prospects between ca. 580 and ca. 560 Ma. This mineralization age interval postdated the timing of peak metamorphism of the host rocks established at ca. 631–607 Ma and is contemporaneous with the metamorphism of basement rocks between ca. 607 and ca. 564 Ma, during the thrusting of tectonic nappes in the last stage of the Pan-African orogeny. In addition, the strike of the veins in the Nanlia and Makorongo prospects is parallel to the local orientation of the thrust fault that separated the overlying rocks from the basement rocks and the dominant S1 foliation of the host rocks. These geochronological and structural data indicate a close relationship between auriferous veins in the study area and the Pan-African thrust fault associated with the emplacement of the tectonic nappes. We propose a metamorphic model for the gold mineralization in the Nanlia and Makorongo prospects, in which the mineralizing fluids were sourced mainly from dehydration of basement rocks, as a result of prograde metamorphism triggered by the juxtaposition of the Neoproterozoic tectonic nappes, including the Xixano Complex, during the last stage of the Pan-African orogeny. The mineralizing fluids ascended through the Pan-African thrust faults and structures to the overlying rocks, where the gold was deposited along the S1 foliation.

Miocene evolution of the External Rif Zone (Morocco): Comparison with similar and lateral southern Mediterranean Tethyan margins

The Miocene evolution of the External Rif Zone (NW Africa Plate) was determined through multidisciplinary analysis of fourteen successions. The updated stratigraphic framework shows how Miocene sediments rest on the Cretaceous–Paleogene terrains through unconformity surfaces, whereas it rests with sedimentary continuity in two sectors. After recognition of lithofacies and three unconformities located near the Oligocene–Aquitanian, Aquitanian–Burdigalian and Serravallian–Tortonian boundaries, the Miocene sedimentary record was divided into three stratigraphic intervals representing deep to shallow marine deposits as Aquitanian–Burdigalian, Langhian and Upper Serravallian–Missinian. The two oldest unconformites are restricted to the central sector, while the upper one is generalized and probably related to the nappe tectonics registered in all sectors of the External Rif. Data from analysis of tectofacies, petrology, mineralogy, meaning and implications of unconformities, and subsidence indicate that: (i) mass flow deposits (turbidites, slumps, olistostromes) are common in all successions but more frequent during the Lower Miocene; (ii) petrology of the detrital components of the arenites indicates recycled orogen-derived sediments, with quartz coming from erosion of metamorphic rocks of the Atlas orogen and/or the African craton; (iii) mineralogy of mudstones suggests a complex erosional evolution of local emerged areas derived from a mixture of contributions coming from the erosion of Upper Jurassic to Paleogene suites, and especially from kaolinite-rich Albian–Cenomanian to Paleogene successions with absence of a clear unroofing. The conjunction of all these clues reinforces the idea of a synsedimentary tectonics affecting the margin/basin system during the Miocene. A thickness analysis of the studied sedimentary successions allows proposing the evolution of the orogenic front and main depozones (foredeep, bulges, wedge-top and intramontane sub-basins) integrated in a complex foreland system migrating from north to south with the Atlas-Mesetas area acting as foreland during Miocene. The orogenic front moved from the Internal Intrarif to Mesorif and later to Internal Prerif. The main wedge-top basin also migrated from the Internal Intrarif to External Intrarif. The foredeep migrated from the Mesorif to the Internal Prerif, while the main forebulge was located in the External Prerif and a secondary bulge developed in the External Intrarif. Intramontane basins developed behind the orogenic front in relative extensional conditions moving from the Internal Intrarif to External Intrarif. The reconstructed Miocene evolution was inserted into a 2D paleogeographic-geodynamic evolutionary model using GPlates software, and then compared to those reported in other external margins of the western Tethys (Betic Chain, Tunisian Tell, Sicilian Maghrebids and Apennines), revealing important similarities and local differences.

From dome to duplex: Convergent gravitational collapse explains coeval intracratonic doming and nappe tectonics, central Australia

Abstract In central Australia, an apparently coeval gneiss dome (Entia Dome) developed adjacent to a thrust belt (Arltunga Nappe Complex) within an intracratonic setting. Here we employ a combination of fieldwork, geochronology, and numerical modeling to investigate the structure and tectonic evolution of these features. We present a structural model linking an extensional domain comprising the Entia Dome, across a transitional zone containing the Bruna décollement zone and the Illogwa shear zone, into a contractional zone comprising thrusts and duplexes of the Arltunga Nappe Complex. Supported by numerical modeling, we propose a tectonic model in which the dome and nappe complex formed synchronously because of the convergent gravitational collapse of the 30–40-km-deep Paleozoic Harts Range rift.

Early Cretaceous Thrust and Nappe Tectonics in North Qilian Shan, Northern Tibetan Plateau: Evidence from Field Mapping, Geochronology, and Deep Structural Analysis

AbstractThe North Qilian Shan fold and thrust belt, located at the northern Tibetan Plateau and southern margin of the Hexi Corridor, is a key tectonic unit to decode the formation and expansion of the plateau. Previous studies emphasize the Cenozoic deformation due to the far‐field response to the Indo–Asian collision, but the Mesozoic deformations are poorly constrained in this area. We conducted detailed field mapping, structural analysis, geochronology, and structural interpretation of deep seismic reflectional profiling and magnetotelluric (MT) sounding, to address the superposed results of the Mesozoic and Cenozoic deformation. The results recognized the North Qilian thrust and nappe system (NQTS), the root and the frontal belt are the North Qilian thrust (NQT), and the Yumu Shan klippe (YK), respectively. The middle belt is located between the NQT and the YK. Monzonitic granite zircon U‐Pb dating from the middle belt yields an age of ca. 415 Ma, which is similar to south NQT. The thrusting displacement is estimated at ca. 48 km by structural interpretation of deep profiles. The timing is constrained in the early stage of the Early Cretaceous by the formation of simultaneous growth strata. We suggest that the NQTS has resulted from the far‐field effect of the Lhasa–Qiangtang collision, and the Yumu Shan is uplifted by the superposed Cenozoic deformation.

The Alpi Apuane and their surroundings: a tale of the origins of modern Italian geological maps and of a missed ‘early recognition’ of nappes in the Apennines

Abstract The northernmost coastal sector of the Italian peninsula from the La Spezia Gulf to Monte Pisano, including the Alpi Apuane mountain range, represents a special morpho-structural domain of the inner northwestern Apennines. Described by naturalists since the Roman age, its location on the land and sea track of the Italian Grand Tour makes it a special zone in the Italian peninsula that was visited by some of the most eminent European geologists of the nineteenth century, including Brongniart, Buckland, De la Beche, Hoffman, Escher von der Linth, Murchison and Lyell. The area has been also the homeland of naturalists and scientists who played a significant role during the nineteenth century in the advancement of geological studies in Italy. Thanks to Capellini's ‘Geological map of the La Spezia Gulf and surroundings’ (1863) and Zaccagna's ‘Geological maps of the Alpi Apuane’ (1879–97), the area became central in the history of the foundations of modern Italian geological maps. However, the opportunities provided by early mapping and palaeontological discoveries for developing tectonic concepts were squandered by Italian Apennines geologists, who remained stuck on explanations of autochthonism, thus missing an early recognition of nappe tectonics that was only accepted in the middle of the twentieth century.

Fossil divergent double-subduction zone in the Great Xing’an Range, NE China: Evidence from a deep seismic reflection profile

Divergent double-subduction is an important component of plate tectonics and continental growth at modern active convergent margins. However, such a process is rarely identified by deep seismic reflection (DSR) profiles, which casts doubt on its past significance. We report here a 416 km DSR profile of large dynamite shots across the Great Xing’an Range (NE China), which shows multiple arc-shaped reflections in the upper-middle crust of the Xing’an terrane, and densely layered reflection structures suggesting tectonic nappes in the lower crust. This new evidence, together with recent geological studies, indicates that the Xing’an terrane may not be a micro-continental block as previously thought, but a Paleozoic accretionary complex. In addition, there is a strong west-dipping reflection at the boundary between the Xing’an and Erguna terranes in the western DSR profile extending from the lower crust to the lithospheric mantle, suggesting that this fossil subduction zone is most likely the Toudaoqiao-Xinlin suture, which separates the Xing’an and Erguna terranes. In addition, an east-dipping strong reflection was also found in the eastern profile between the Xing’an and Songliao terranes, corresponding to the Hegenshan-Nenjiang suture at the surface. Given the newly identified crust-mantle structure, the opposite subduction zone, and other geological evidence, we propose a new model that the Xing’an terrane was developed by divergent double-subduction of the Paleo-Xing’an Ocean (part of the Paleo-Asian Ocean in the area) from ca. 500 Ma to ca. 300 Ma. Our study provides the first DSR evidence for a divergent double-subduction in the Paleozoic, which emphasizes its importance in past accretionary orogens.

Implications of New Geological Mapping and U‐Pb Zircon Dating for the Barrovian Tectono‐Metamorphic Evolution of the Lepontine Dome (Central European Alps)

AbstractThe Barrovian metamorphism of the Lepontine dome is manifested by isograds that cross‐cut tectonic nappe contacts, which is commonly interpreted as metamorphism that occurred after nappe emplacement. However, the pervasive mineral and stretching lineation in amphibolite facies, associated with top‐to‐foreland shearing, suggests that peak Barrovian conditions are coeval with nappe‐overthrusting. Here, we combine mapping and U‐Pb zircon dating to better constrain the relation between metamorphism and overthrusting. Metamorphic zircon rims show two age populations at 31–33 and 22–24 Ma. The younger population is locally observed in post‐foliation dikes (and associated metasomatism) likely sourced from deep‐migmatites exhuming along the Alpine backstop. The older population occurs regionally and is found in syn‐kinematic migmatites which occur along a crustal‐scale shear zone. Below this shear zone, magmatic and detrital zircon cores suggest that the Cima Lunga unit, previously interpreted as a tectonic mélange with Mesozoic fragments, was a pre‐Variscan metasedimentary sequence intruded by Permian granitic sills, now orthogneisses. This unit was strongly sheared along the top of the Simano nappe during overthrusting of a rock pile here‐termed Maggia‐Adula nappe. This large‐scale nappe emplacement imprinted the regional lineation and peak temperatures until 31–33 Ma. Péclet (1–10) and Brinkman (0.002–1.8) numbers, estimated for the overthrusting, suggest an advection‐dominated heat transfer caused by rock exhumation, with some diffusion (conduction) during nappe emplacement. Diffusion contributed to Barrovian isograds discordant to the thrust. Shear heating was important if stress times shearing rate >∼5·10−6 W·m−3 within the nappe. The thermal evolution after overthrusting was spatially heterogeneous until ca. 22 Ma.

Controls on anomalously high porosity/permeability of Middle Jurassic deeply buried tight sandstones in the Taibei Sag, Turpan-Hami Basin, northwestern China: Implications for reservoir quality prediction

The lower member of the Middle Jurassic Xishanyao Formation (J2x1+2) sandstones are significant exploratory targets for hydrocarbon resources in the Taibei Sag of the Turpan-Hami Basin, northwestern China. Formation of anomalously high porosity/permeability in deeply buried J2x1+2 sandstones and reservoir quality prediction were investigated using a variety of petrographic analyses. These results show that the J2x1+2 sandstones are mostly feldspathic litharenites and litharenites, which are characterized by moderate-to good sorting, silty to medium granularity, and point- to long grain contacts. The J2x1+2 sandstone has low porosity (avg 6.0%) and permeability (avg 1.12 mD), but shows anomalously high porosity/permeability at depth interval of 3850–4050 m. There is a noticeable variation in pore types and sizes from intergranular-intragranular dissolution pores with a size mode of 2.0 μm in anomalously high porosity (AHP) sandstones, to major intragranular dissolution pores with a size mode of 0.5 μm in normally high porosity (NHP) sandstones, to most micropores with a size mode of 0.25 μm occurred in the normally low porosity (NLP) sandstones. The compaction is the main cause of porosity destruction, resulting in an average porosity loss of 89.6%. However, the authigenic minerals have relatively little influence on reservoir quality. The combination of nappe tectonics and well-sorted particles alleviated the compaction and thus preserved more intergranular pores in the AHP sandstones. Dissolution further increases the porosity and eventually forms the AHP sandstones. The strong carbonate-cemented facies (SCC), tightly compacted facies (TC), moderately compacted with moderate dissolution facies (MCMD), and weakly compacted with strong dissolution facies (WCSD) are determined in the J2x1+2 sandstones. The sandstones with SCC and TC are recognized as the NLP reservoirs. The MCMD and WCSD correspond to the NHP and AHP reservoirs, respectively. The reservoir quality predicted using the constructed diagenetic facies charts is in good agreement with the photomicrograph observations and physical property tests. Additionally, the AHP reservoirs are still developed in the deeply buried sandstones with depths larger than 4000 m.

Triassic evaporites and the structural architecture of the External Hellenides and Albanides (SE Europe): controls on the petroleum and geoenergy systems of Greece and Albania

A combination of well data, seismic information on thrusting and tectonic shortening, plus analyses of the nature and depth of the Mesozoic units related to main detachment horizons (Triassic evaporites, flysch), are used to review, and further update the complex structural styles of the External Hellenides and Albanides Orogenic Belts, SE Europe. In the study area, the late Alpine orogenic evolution resulted in a structural architecture characterised by the successive westward thrusting of tectonic nappes (Gavrovo/Kruja, Ionian), with an imbricate tectonic style prevailing in all external zones. In the internal and central Ionian zone, imbricate thrusts and duplex structures are recorded, especially where about 24 km of stacked Mesozoic–Tertiary successions have been formed in Albania. Triassic evaporites, up to 3.5 km thick, acted as a detachment horizon for the internal deformation of the Ionian and Pre-Apulia zones (1.7–1.8 km thick in Greece), whereas the flysch unit of Upper Eocene–Oligocene (External Hellenides) and Oligocene–Aquitanian (Albanides) contributed to the deformation of the Ionian zone. In the Ionian zone, Triassic evaporites are laterally continuous and act as an effective seal unit when thrusted above Mesozoic carbonate/Tertiary units. Most of the oil and gas fields, oil shows and surface seeps have been developed in association with the relative more complex structure styles in the internal and the central Ionian zone of Albania and Greece. The Triassic evaporite detachment is detected at depths of 8–22 km in Albania, and about 5 km to 12–13 km in Greece. Thin and probably combination of thin- and thick-skinned deformation (SW Greece) of the Meso-Cenozoic succession above the Triassic evaporite and Permian sequences better depicts the complex structural architecture of the External Hellenides and Albanides, later affected by strike-slip tectonics and extensional deformation active since the Early Pliocene times. The main source rock levels, Lower Cretaceous shale/carbonate, Toarcian Posidonia and Triassic shales, are common in the Albanides and the Hellenides forelands. The thickness and Total Organic Carbon of these levels significantly increase in Albania, whereas trapping mechanism is almost the same and the Triassic evaporites play a significant role in both forelands. The hydrocarbon migration primarily followed developed thrust faults and important halokinesis, to charge fractured Cretaceous–Eocene carbonate reservoirs in the overthrust unit (Ionian zone), likely beneath the thrusted anticlinal belts. These sub-thrust structural models are related to the evaporites. Sub-thrust plays, referred to the autochthonous units (Apulia, Pre-Apulia/Paxos, Sazani zones), may also present reservoir potential in Albania and Western Greece. Upper Miocene and Pliocene deposits (sands) record reservoir potential in stratigraphic traps and good caprock characteristics in the Peri-Adriatic depression (Albania), whereas Messinian evaporites and clays may document seal potential for Upper Miocene sands in the southern areas of the Ionian Sea (Kyparissiakos Gulf, SW Kythira).

TURBIDITES IN THE MIDDLE PALEOCENE VARIEGATED FORMATIONS OF THE SKYBA NAPPE BETWEEN THE RIVERS OPIR AND SVICHA OF THE UKRAINIAN CARPATHIANS

The paper presents the results of the study of sections of the Middle Paleocene variegated the Yaremche horizon in the skybas of Parashka, Skolivska, Orivska Skiba tectonic nappe. The work is based on the results of geological observations between the rivers Opir and Svicha and laboratory studies. The Yaremche horizon is characterized by rhythmic-cyclic interlayering of greenish-gray siltpsammitic and siltpelitic turbidites and cherry-red hemipelagites and pelagites. The purpose of the paper is to study the features of horizon sections and to establish the features of paleogeodynamic and paleogeographic modes of their formation. Rhythmostratigraphic study of the Yaremche horizon, allows it to be included in the sequence of the first order, which contains sequences of higher, second and third orders. Terrigenous and carbonate turbidites take part in the construction of sections of the Yaremche horizon. For turbidite sequences, both terrigenous and carbonate composition, are characterized by elements of textures A. Bouma Tdce. However, there are terrigenous layers that have a disordered texture with intervals (bottom up) of dish (plate) structure, which contain fluide escape “pipes”, convoluted limination with gravitational intralayer deformations (in flat top). These layers have an unsorted, multi-grained, pudding structure. The lower contact is erosive, the upper – smooth with hidden traces of erosion. These layers are parallelized by the mechanism of formation with deposits of fluidized (and/or liquefied) flows that occur at the final stage of turbidity currents. The study of petrographic features of petrotypes of turbidite sequences makes it possible to isolate chlidolites (pattums, mixstolites), which belong to greywackes in terms of the material composition of the fragments. The content of cement in them is not less than 10–25 % and indicates the turbulent flow nature of the clay-sand mass from which the deposits were formed. Among the greywackes, a separate group is occupied by fragmentary carbonate rocks – calcarenite-calcillite turbidites (Parashka skyba, Oryava river). They were formed during the redeposition of shelf carbonate material and/or intra-oceanic uplift material to the deep-water sedimentary basin of the continental shelf, which at the time of their accumulation had depths above the CCD. The formation of the Yaremche horizon took place at the foot of the continental slope of the passive outskirts of Eurasia, on the border of the deep-water plain of the External Carpathian flysch paleobasin during the temporary stabilization of the tectonic regime. The continental foothills pulsatingly changed their bathymetric parameters, due to convergent events between Eurasia and the microcontinents of the Tethys Ocean. They controlled the periodic approach and formation of flysch nappe units of accretion prisms on the active continental margin. The scientific results of the study will contribute to a better understanding of the geological structure of the Skiba zone of the Ukrainian Carpathians. The practical results of the study will contribute to the study of geological bodies promising for hydrocarbons.

ГЕОЛОГІЯ СУБСІЛЕЗЬКОГО ПОКРИВУ В БАСЕЙНІ Р. РІКА (УКРАЇНСЬКІ ЗОВНІШНІ КАРПАТИ, ГОЛЯТИНСЬКА СТРУКТУРА)

The author's detailed geological mapping works suggest that the Holyatyn Structure (located near the villages of Holyatyn and Maidan in the Rika River basin, Ukrainian Carpathians, Transcarpathian administrative region) is a destructured fragment of the Sub-Silesian Nappe located in the Outer Carpathians between the Silesian and Skyba nappes. Within this structure, Lower Cretaceous–Oligocene deposits are developed among the Oligocene flysch of neighboring tectonic units. The stratigraphic succession of these deposits is represented by: dark to black flysch (Shypot Formation, Barremian–Albian); green shales with cherts; red and green shales and marls (Holyatyn Beds, Upper Cretaceous–Paleocene); greenish and dark to black flysch (Soimy Formation, Eocene); black shales, cherts and sandstones (Menilite Formation, Oligocene) and gray flysch with layers of black shales (Verets or Transitional Formation, Oligocene). The limestone blocks (unknown age) is developed in front of the thrust-sheet. According to sedimentological features (typical hemipelagites), age and microfauna content, the Holyatyn Beds correspond to the Weglowka Marls of the Subsilesian Unit in the Polish Carpathians, while the Weglowka Marls are a “diagnostic lithofacies” for the Subsilesian Unit. The Holyatyn Structure is probably an “extruded lens”, the anticline core of which is composed of deformed Lower Cretaceous flysch and ductile Upper Cretaceous clay-marly Holyatyn Beds. This extrusion apparently occurred after nappe structure forming and was caused by transpressive movements. In result, the relatively thin and ductile Subsilesian Nappe was stretched into separate tectonic lenses (large boudins ?) placed between the rigid flysch of neighboring tectonic nappes. One such lens is the Holyatyn Structure, which is similar to the transpressive “flower structure”. A ductile-type melange is exposed in the thrust zone of the Silesian/Sub-Silesian nappes. It suggests that the initial thrust stage occured in poorly consolidated water-saturated sediments.

Extension and early orogenic inversion along the basal detachment of a hyper-extended rifted margin: an example from the Central Pyrenees (France)

The North Pyrenean Zone inverts remnants of an Aptian–Cenomanian rifting during which subcontinental mantle was exhumed. These remnants contain a synrift HT–LP metamorphic domain, the Internal Metamorphic Zone. New field data, Raman spectroscopy on carbonaceous material data and structural cross-sections constrain the structural and metamorphic relationships between the Internal Metamorphic Zone and the underlying low-grade North Pyrenean Zone. The Internal Metamorphic Zone is a tectonic nappe that overthrusts the European margin along the 3M Fault. Along this contact, the Tuc de Haurades peridotite is surrounded by tectonic breccia composed of ductilely deformed carbonate and sparse lherzolite clasts that passes upward into foliated marbles. Marbles contain top-to-south ductile shear, recording continuing extensional deformation that marks the onset of HT metamorphism. During Early Cretaceous rifting, European Mesozoic sedimentary cover metamorphosed and its base brecciated as it slid basinward on Triassic salt onto exhumed mantle. As the exhumed mantle domain closed during early convergence, the detached metamorphosed cover was transported northward and thrust onto the distal European margin, sampling lherzolite tectonic lenses. This triggered the first tectonic loading on the European plate. This study highlights the role of the Internal Metamorphic Zone in the early Pyrenean orogenic phase and gives new insights into the east–west diversity of structural setting of the North Pyrenean Zone peridotites. Supplementary material : A table with Raman spectroscopy on carbonaceous material temperatures and original and high-quality photographs of the samples are available at https://doi.org/10.6084/m9.figshare.c.5539260

Variations of lacustrine shale reservoirs in different deformation zones of Mohe Basin, northeastern China: Insights into the impact of thrust nappe structure on shale gas preservation

The structural preservation conditions are crucial for shale gas accumulation and preservation. In this study, the Emuerhe thrust nappe structural belt in the Mohe Basin is taken as an example to investigate the influence of the thrust nappe structure on shale reservoir quality. According to the rock metamorphic characteristics and deformation degree, the thrust nappe belt is divided into the root zone, middle zone, and front zone. Based on the observations of field outcrops, cores, and thin sections, the hydrocarbon generation capacity and reservoir properties of shale in different units were investigated through an integrated analysis of organic geochemical data, X-ray diffraction, low-pressure CO2 and N2 adsorption, and helium porosity measurements. Furthermore, implications for shale gas preservation are discussed. Owing to the influence of high temperature and pressure caused by dynamic metamorphism, the Ro value of shales in the root zone exceeds 5.5%, which is far beyond the theoretical upper limit of hydrocarbon generation potential. Thus, the hydrocarbon generation potential of shales exhibits an opposite trend to the deformation strength of the Emuerhe thrust nappe belt. The S1+S2 value of shales within the root zone and middle zone is generally lower than 0.3. The primary pore structure of shales in the root zone was changed or destroyed by ductile shearing and mylonitization. Furthermore, dynamic metamorphism led to a significant decrease in the adsorption capacity of shales. Both the micropores and mesopores in these shales are very limited, and the storage space is relatively low. The strong compaction caused by the thrust nappe tectonic led to a reduction in the pore size. Thus, pores with diameters <10 nm dominated the main reservoir space, and the porosity of shales in the middle zone was lower than that in the root zone. The influence of thrust nappe tectonics on the front zone was relatively weak. Overall, the shales exhibited good hydrocarbon generation potential, high specific surface area, and pore volume. The pore–fracture system consists of relatively large pores, and the extensional fractures are favorable for enhancing the physical properties of shale reservoirs in the front zone. The strong thrust nappe resulted in caprock that experiences broad uplift-erosion and fault failure. The original structure of the shale can be changed or destroyed by ductile shearing and strong compaction. Heating-up caused by tectonics could also promote the rapid desorption of shale gas, which can lead to poor self-sealing performance of shale. The hydrocarbon generation capacity, reservoir property, and preservation conditions of shale display an improving trend along the thrust direction of the Emuerhe thrust nappe structural belt.

Gliding and overthrust nappe tectonics of the Barberton Greenstone Belt revisited: A review of deformation styles and processes

AbstractInterpretations of the structural/tectonic evolution of the Barberton Greenstone Belt (BGB) and its surrounding granitoid rocks remain controversial, with proponents for both horizontal thrust-accretion (plate tectonic) and partial convective overturn (vertical tectonic) models. Here, an area of complex folds that was used to support the operation of plate tectonic-derived gliding and overthrust nappe tectonics is re-investigated in detail and placed within the broader structural development of the BGB and surrounding granitoid domains via a re-analysis of structures, and geochronological, stratigraphic and metamorphic data across the whole of this important geological terrain.The results of detailed field mapping show that the complex folds, which occur on the northern limb of the 20 km wavelength, vertically plunging, Onverwacht Anticline, do not represent a re-folded, originally recumbent, isoclinal fold, as previously interpreted. Instead, the folds represent a moderately shallow east-plunging fold train that formed from a single episode of deformation. Fold asymmetry is consistent with formation during originally north-side-up reverse shear on bounding faults, consistent with the offset direction required to explain the fault-repeated slices of Mendon Formation + Fig Tree Group rocks that uniquely occur across the northern limb of the Onverwacht Anticline.More broadly, a review of the BGB and surrounding granitoid rocks show that formation was likely through two discrete, ~120 Ma long, episodes of mantle upwelling, or plume, magmatism, each of which led to crustal melting and partial convective overturn (PCO), a tectonic mechanism that arises from the gravity-driven interaction between dense, upper crustal greenstones and partially melted, more buoyant, granitoid-dominated middle crust.The first mantle upwelling episode, at 3 530 to 3 410 Ma, commenced with long-lived eruption of ultramafic-mafic lavas of the Sandspruit, Theespruit, Komati, and lower Hooggenoeg formations (3 530 to 3 470 Ma). Heat from this magmatic event gave rise to partial melting of the crust that, combined with fractionation of mafic magma chambers produced widespread felsic magmatism at 3 470 to 3 410 Ma (upper Hooggenoeg Formation and Buck Reef Chert), the latter parts of which were accompanied by the formation of D1 dome-and-keel structures via PCO in deeper-levels of the crust represented by the Stolzburg Domain in the far southwest part of the belt.The second mantle upwelling, or plume, episode commenced at 3 334 to 3 215 Ma with the eruption of ultramafic-mafic lavas of the Kromberg, Mendon and Weltevreden formations. Heat from this magmatic event gave rise to renewed partial melting of the crust that, combined with fractionation of mafic magma chambers, produced widespread felsic magmatism at 3 290 to 3 215 Ma. A second, longer-lived and more complex, multi-stage episode of PCO (D2-D4) accompanied deposition of the Fig Tree and Moodies groups from 3 250 to 3 215 Ma. Late D5 deformation accompanied emplacement of the Mpulizi and Piggs Peak batholiths at ca. 3.01 Ga, as previously identified.The Inyoka and Kromberg faults, which separate domains with distinct structural styles, represent neither terrane boundaries nor suture zones, but rather axial faults that separate deformed but generally inward-facing greenstone panels that sank inwards off rising granitoid domains that surround the BGB.

Geochemistry of clumped isotopologues of CH4 within fluid inclusions in Alpine tectonic quartz fissures

Petroleum-bearing fluid inclusions are small encapsulations of oil and gas that persist in the rock record long after their parent fluids have moved on. The chemical and isotopic compositions of inclusions offer a unique way to investigate hydrocarbon formation pathways in deep Earth environments and in ancient geological times. Here we use stable isotope compositions of methane, including clumped isotopologues 13CH3D and 12CH2D2, to evaluate the formation conditions of methane within fluid inclusions in Alpine tectonic quartz fissures. We observe that this CH4 is relatively 13C and 2H enriched (δ13C = −26 to −39‰; δ2H = −126 to −146‰), consistent with the formation by advanced catagenesis of organic matter, and has clumped isotope compositions indicating intramolecular isotopic equilibrium at catagenetic to low-grade (sub-greenschist) metamorphic temperatures (TΔ13CH3D: 120-300°C; TΔ12CH2D=2120-260°C). Our findings, combined with an analysis of the geographic distributions of inclusion-bearing fissures and fluid inclusion PVT behaviors, provide direct evidence of reservoirs of thermogenic methane trapped beneath the Alps during Mid-Miocene tectonic nappe emplacement (c.a. 25 to 15 Ma). The gas was sequestered in a concentrated form that likely favored the accumulation of up to 50,000-150,000 Megatons (Mt) of CH4 across the Helvetic domain in Switzerland. Following the subsequent tectonic uplift during the period 17 to 10 Ma, we suggest that the trapped methane-rich fluids migrated to shallower depths along faults, leading to the release of Alpine metamorphic methane into near-surface aquifers and the atmosphere at rates of 0.01 to 0.03 Mt⋅y−1.

Multistage Thrust and Nappe Tectonics in the Southeastern Part of East Sayan and Its Role in the Formation of Large Gold Deposits

Abstract ––Comprehensive studies of structural geology and metallogeny, taking into account the authors’ previous works started as early as the last century, have shown that the southeastern part of East Sayan formed mainly in the Neoproterozoic–early Paleozoic in the settings of multistage thrust and nappe tectonics and tectonomagmatic restructuring of autochthonous and overthrust allochthonous oceanic (ophiolitic), island arc, and ocean-marginal terranes as well as amalgamation of accretion–collision and postcollisional igneous complexes that formed during the opening and subsequent closure of the Paleoasian Ocean marginal structures. In the middle and late Paleozoic, active intraplate volcanic and plutonic processes continued in the thrust/overthrust fault setting, which led to the formation of new dome-shaped nappe structures and the redistribution of ore matter (gold etc.) in large mineral deposits. The final structure of the East Sayan region formed during the late Cenozoic as a result of mountain uplifting and volcanic eruptions, including those in the valley of the Zhombolok River.

Upper crustal velocity and seismogenic environment of the &lt;em&gt;M&lt;/em&gt;7.0 Jiuzhaigou earthquake region in Sichuan, China

On August 8, 2017, a magnitude 7.0 earthquake occurred in Jiuzhaigou County, Sichuan Province, China. The deep seismogenic environment and potential seismic risk in the eastern margin of Tibetan Plateau have once again attracted the close attention of seismologists and scholars at home and abroad. The post-earthquake scientific investigation could not identify noticeable surface rupture zones in the affected area; the complex tectonic background and the reason(s) for the frequent seismicity in the Jiuzhaigou earthquake region are unclear. In order to reveal the characteristics of the deep medium and the seismogenic environment of the <i>M</i>7.0 Jiuzhaigou earthquake region, and to interpret the tectonic background and genesis of the seismicity comprehensively, in this paper, we have reviewed all available observation data recorded by the regional digital seismic networks and large-scale, dense mobile seismic array (China Array) for the northern section of the North–South Seismic Belt around Jiuzhaigou earthquake region. Using double-difference seismic tomography method to invert the three-dimensional P-wave velocity structure characteristics of the upper crust around the Jiuzhaigou earthquake region, we have analyzed and discussed such scientific questions as the relationship between the velocity structure characteristics and seismicity in the Jiuzhaigou earthquake region, its deep tectonic environment, and the ongoing seismic risk in this region. We report that: the P-wave velocity structure of the upper crust around the Jiuzhaigoug earthquake region exhibits obvious lateral inhomogeneity; the distribution characteristics of the shallow P-wave velocity structure are closely related to surface geological structure and formation lithology; the <i>M</i>7.0 Jiuzhaigou earthquake sequence is closely related to the velocity structure of the upper crust; the mainshock of the <i>M</i>7.0 earthquake occurred in the upper crust; the inhomogeneous variation of the velocity structure of the Jiuzhaigou earthquake area and its surrounding medium appears to be the deep structural factor controlling the spatial distribution of the mainshock and its sequence. The 3D P-wave velocity structure also suggests that the crustal low-velocity layer of northeastern SGB (Songpan–Garzê Block) stretches into MSM (Minshan Mountain), and migrates to the northeast, but the tendency to emerge as a shallow layer is impeded by the high-velocity zone of Nanping Nappe tectonics and the Bikou Block. Our results reveal an uneven distribution of high- and low-velocity structures around the Tazang segment of the East Kunlun fault zone. Given that the rupture caused by the Jiuzhaigou earthquake has enhanced the stress fields at both ends of the seismogenic fault, it is very important to stay vigilant to possible seismic hazards in the large seismic gap at the Maqu–Maqên segment of the East Kunlun fault zone.

Tectonic inheritance controls nappe detachment, transport and stacking in the Helvetic nappe system, Switzerland: insights from thermomechanical simulations

Abstract. Tectonic nappes have been investigated for more than a hundred years. Although geological studies often refer to a “nappe theory”, the physical mechanisms of nappe formation are still disputed. We apply two-dimensional numerical simulations of shortening of a passive margin to investigate the thermomechanical processes of detachment (or shearing off), transport and stacking of nappes. We use a visco-elasto-plastic model with standard creep flow laws, Drucker–Prager and von Mises yield criteria. We consider tectonic inheritance with two initial mechanical heterogeneities: (1) lateral heterogeneity of the basement–cover interface due to half-grabens and horsts and (2) vertical heterogeneities due to layering of mechanically strong and weak sedimentary units. The model shows detachment and horizontal transport of a thrust nappe that gets stacked on a fold nappe. The detachment of the thrust sheet is triggered by stress concentrations around the sediment–basement contact and the resulting brittle–plastic shear band that shears off the sedimentary units from the sediment–basement contact. Horizontal transport is facilitated by a basal shear zone just above the basement–cover contact, composed of thin, weak sediments that act as a décollement. Fold nappe formation occurs by a dominantly ductile closure of a half-graben and the associated extrusion of the half-graben fill. We apply our model to the Helvetic nappe system in western Switzerland, which is characterized by stacking of the Wildhorn thrust nappe above the Morcles fold nappe. The modeled structures, the deformation rates and the temperature field agree with data from the Helvetic nappe system. Mechanical heterogeneities must locally generate effective viscosity (i.e., ratio of stress to viscoplastic strain rate) contrast of about 3 orders of magnitude to model nappe structures similar to the ones of the Helvetic nappe system. Our results indicate that the structural evolution of the Helvetic nappe system was controlled by tectonic inheritance and that material softening mechanisms are not essential to reproduce the first-order nappe structures.

Finite strain analysis for deformed Neoproterozoic rock associations of the Al Amar fault zone, Eastern Arabian Shield, Saudi Arabia

The Al Amar area has played important role to effect the Al Amar–Idsas fault zone, which consists of a sequence of volcanic rocks and sedimentary detrital. Also, it is characterized by a difference in facies with intruded granitoid bodies. The present work has used the finite strain techniques to investigate the relationship of the different lithologies and nappe contacts, which recorded in the Al Amar area. Furthermore, the microstructural analysis has investigated the Abt schist, deformed granitoids, and metavolcanic-sedimentary rocks, which represented the Al Amar area. The axial ratio results average from 1.55 to 2.87 for the Rf/φ method and 1.43 to 7.51 for the Fry method in the XZ sections for metamorphic rocks. In the finite strain data, the long axes direction (X) displays grouping along a NW–SE trend with shallow plunging. The direction of short axes (Z) displays subvertical and accompanied with a subhorizontal foliation. Our strain measurement reveals that strain symmetry shows an oblate shape (flattening) and the strain magnitudes do not display any valuable increasing towards the tectonic nappe contacts. Also, it is indicated that the volume change in the different rock units was unassociated with the accumulation of finite strain. In the investigated area, the subhorizontal foliation in the different lithologies was parallel to thrusting. Furthermore, it shows mainly the similar attitudes for tectonic contact nappes. In the present work, it is suggested that the ductile strain is accumulated during thrusting and involves as vertical shortening component with the subhorizontal foliation. The field observations with finite stain data are not sufficient to decide that the tectonic contact in the orogeny is formed as the simple-shear deformation.

Oligocene molasse sedimentation in Central Taurides: Records of the onset of extensional tectonic regime

The lacustrine units have been deposited in the Ermenek, Bucakkisla, Korucuk and Camliyayla basins in the Central Taurides since early Oligocene. In this study, structural features and stratigraphical properties of Oligocene basins have been described in detail and the lacustrine units have been dated. Regional geological interpretations have been made by using tectono-stratigraphic and age data of the basins. Lacustrine Oligocene sedimentation in these basins, which were opened on the tectonic units of Taurides due to normal faults, constitute the first records of the sedimentation occurring under the extensional tectonic regime after the orogeny of the Central Taurides. All the pre-Oligocene units that had been emplaced in the region due to a north-south compressional movement in the Central Taurides, continued to be compressed until the end of Eocene, and completed their orogenic development as napped structures. The bedrock units, reaching the maximum elevation, have been subjected to “orogenic collapse” due to the strain or gravity forces as a result of an interruption or termination of the compression which lasted until the end of Eocene. Thus, “orogenic collapse” basins have started to be formed in the Central Taurides since Oligocene. The orogeny and the compressional tectonic regime forming the Taurides lasted until late Miocene in the Eastern Taurides. On the other hand, extensional basin formation since early Oligocene on the Taurus units, which had been emplaced in the region due to nappe tectonics, indicates that a new tectonic period has started in the Central Taurides.