Thrust Nappes Research Articles

Controls on deep-water slope channel complex fill, propagation, stacking, and orientation in the Middle Eocene-Oligocene Kırkgeçit Formation, Elazığ, eastern Turkey

Deep-water channels are well-exposed along the northern margin of the east-west oriented Late Paleocene to Oligocene Elazığ Basin of eastern Turkey. The deep-water slope channel complexes are characterised by their association with prominent normal faults, where they seem to have controlled their geometries at the northern margin. The channels were fed by coarse-grained sediment shed by subaqueous gravity flows such as high and low-density turbidity currents, and both cohesive and noncohesive debris flows, from elevated hinterland across narrow shelves and through canyons and gullies into the channels. The faulting and subsequent folding along the northern basin margin created a significantly irregular deep-water slope, and these gravity flows were deflected around, and ponded against this topography. The nature of the impact of seafloor topography, created both before and during channel activity, was examined to create a scheme that documents the range of such effects. Detailed field mapping in this semidesert area shows that these slope channel complexes were strongly controlled by block fault topography generated during early basin evolution in the Late Paleocene, and again during fold development from thrust nappe emplacement during basin filling in the Late Middle Eocene when the Hazar- Maden Basin closed in the south. Four styles of slope channel complex-fold interaction have been recognised: deflection, blocking, diversion, and confinement. In each style of interaction, folding and faulting controlled both channel complex planform geometry and architectural style of the channel fills. In addition to seafloor topography inherited from these earlier phases of compression, subsequent gravitational collapse on the northern basin margin also created normal fault-bound blocks that locally controlled turbidity current pathways and channel complex orientation. It has also been shown that the orientation of the slope channel complexes within mudprone slope sequences has later controlled the orientation of younger fold axis in the Middle Eocene. The study illustrates very well-exposed examples of complex seafloor topography generated by compressional and extensional tectonics, and the impact this has had on slope channel complex evolution.

The Characteristics and Tectonic Evolution of Deep Buried Structures in Manas in the Southern Margin of Junggar Basin

The Manas anticline belongs to the second tectonic belt in front of the southern margin of Junggar Basin. Due to the multi-stage tectonic compression activity of the Indian plate and the Eurasian plate since the Cenozoic era, the tectonic activity is frequent and the deep tectonic style is complex, so the deep tectonic style of the quasi-south has not been clearly understood. For the study of the deep concealed characteristics of Manas anticline structure, in this paper, we will use the theory of fault-related fold, the geometric analysis and the balanced section forward simulation to construct the detailed structural analysis and simulation of kinematics of structural deformation process of the type section of the Manas anticline based on wide line seismic profile. The study shows that the vertical and upward parts of the Manas anticline are fault-propagated folds and thrust nappe structures while the deep parts are mainly composed of complex structural triangular wedges, burst structures and secondary fold regulating faults. The deformation process is the early low-amplitude fault-turning fold, which is blocked at the front of thrust and forms complex triangular wedge and burst structure. In the late neogene, large thrust faults and fault-propagated folds were developed along the bottom of Anji Haihe formation under the action of strong thrust nappe in the piedmont.

Structural modeling and evolution of the piedmont zone in north margin of Qaidam Basin, northeastern Tibetan Plateau

The analysis and interpretation of the Dachaidan area, Qaidam Basin, is difficult, owing to the co-location of two groups of thrust faults (N–E faults and N–W faults) there and the area’s complicated structural deformation history. To address this problem, field geological investigation, seismic study, well logging, and drilling data were used to identify the key fault systems and their distribution patterns through the area. By integrating surface and subsurface structural features and seismic and non-seismic data, we carried out studies using structural modeling and analysis of the Dachaidan area. Study results identified two systems of thrust faults (N–W faults and W–E faults). We found that these faults could be categorized into three systems: a basin-margin thrust system, an intro-basin thrust system, and an intro-basin compression and strike-slip fault system. These systems showed different features in different areas and zones. We also constructed interpretation models of different deformation mechanisms in the basin and on basin margins. Three tectonic systems (compression, extension, and strike-slip) were identified, which were further divided into eight structural domains. We also established structure coexistence and distribution patterns. The overall structural character of the area was summarized as the northern and southern parts belonging to different zones, with the western and eastern parts belonging to different systems. By analyzing the SW–NE tectonic evolution sections, we defined the back-propagation structural evolution sequences of thrust nappes (on the basin margin or in the basin) and back-thrust structures (in the basin) as well as their influence on the residual Mesozoic strata.

Control of 3-D tectonic inheritance on fold-and-thrust belts: insights from 3-D numerical models and application to the Helvetic nappe system

Abstract. We apply three-dimensional (3-D) thermo-mechanical numerical simulations of the shortening of the upper crustal region of a passive margin in order to investigate the control of 3-D laterally variable inherited structures on fold-and-thrust belt evolution and associated nappe formation. We consider tectonic inheritance by employing an initial model configuration with basement horst and graben structures having laterally variable geometry and with sedimentary layers having different mechanical strength. We use a visco-plastic rheology with a temperature-dependent flow law and a Drucker–Prager yield criterion. The models show the folding, detachment (shearing off) and horizontal transport of sedimentary units, which resemble structures of fold and thrust nappes. The models further show the stacking of nappes. The detachment of nappe-like structures is controlled by the initial basement and sedimentary layer geometry. Significant horizontal transport is facilitated by weak sedimentary units below these nappes. The initial half-graben geometry has a strong impact on the basement and sediment deformation. Generally, deeper half-grabens generate thicker nappes and stronger deformation of the neighbouring horst, while shallower half-grabens generate thinner nappes and less deformation in the horst. Horizontally continuous strong sediment layers, which are not restricted to initial graben structures, cause detachment (décollement) folding and not overthrusting. The amplitude of the detachment folds is controlled by the underlying graben geometry. A mechanically weaker basement favours the formation of fold nappes, while stronger basement favours thrust sheets. The model configuration is motivated by applying the 3-D model to the Helvetic nappe system of the Central Alps of France and Switzerland. Our model reproduces several first-order features of this nappe system, namely (1) closure of a half-graben and associated formation of the Morcles and Doldenhorn nappes, (2) overthrusting of a nappe resembling the Wildhorn and Glarus nappes, and (3) formation of a nappe pile resembling the Helvetic nappes resting above the Infrahelvetic complex. Furthermore, the finite strain pattern, temperature distribution and timing of the 3-D model is in broad agreement with data from the Helvetic nappe system. Our model, hence, provides a 3-D reconstruction of the first-order tectonic evolution of the Helvetic nappe system. Moreover, we do not apply any strain softening mechanisms. Strain localization, folding and nappe transport are controlled by initial geometrical and mechanical heterogeneities showing the fundamental importance of tectonic inheritance on fold-and-thrust belt evolution.

Patterns of Silurian deformation and magmatism during sinistral oblique convergence, northern Scottish Caledonides

Regional ductile thrusting and syn-kinematic granitic magmatism within the Caledonides of northern Scotland occurred within a sinistrally oblique convergent tectonic setting during the Silurian closure of the Iapetus Ocean. The highest thrust nappes are dominated by structures of probable Grampian (Ordovician) age, and Scandian (Silurian) deformation dominates the underlying thrust nappes. Deformation was overall foreland-propagating but the nappe stack was modified by out-of-sequence thrusting and probable synchronous development of thrusts at different structural levels. Localized dextrally transpressive deformation is related to an inferred lateral ramp located offshore. New U–Pb zircon ages from syn-tectonic granites indicate that the internal Naver Thrust was active between c. 432 and c. 426 Ma. This is consistent with other data sets that indicate that contractional deformation and high-grade metamorphism, and by implication displacements in the Moine Thrust Zone, may have lasted until c. 420–415 Ma. The synchroneity of thrusting and strike-slip movements along the Great Glen Fault implies that partitioning of transpressional strain occurred above a regional basal decollement. The short duration of the Scandian orogen in Scotland ( c. 437–415 Ma?) is consistent with only moderate crustal thickening and a location on the periphery of the main Laurentia–Baltica collision further north. Supplementary material: Details of analytical procedures, complete U-Pb isotopic data and methods of U-Pb age calculation and error reporting are available at https://doi.org/10.6084/m9.figshare.c.4962251

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.

Tectonic Evolution of the Thrust-Nappe Belt in the Southwestern Ordos Basin (China): New Constraints from Exploration Seismic Data

The thrust-nappe belt in the Western Ordos Basin is located in the northern segment of the North‒South structural belt in the mainland China. It is the junction of different tectonic units. The structural evolution of the region is significant to the knowledge of regional geology as well as to oil and gas exploration initiatives. However, due to insubstantial data, the structural evolution of the region is still controversial. In our study, the stratigraphic age was constrained via new drilling data, which allowed newly acquired 2D high-precision seismic data to be interpreted, revealing the tectonic evolution history of the thrust-nappe belt on the Southwestern Ordos Basin. Our results show that the thrust-nappe belt originated from the south‒north trending aulacogen developed during the Meso-Neoproterozoic age. The main fault in the aulacogen experienced two stages of compressional inversion deformation at the end of Triassic and in the Late Jurassic. At the Late Triassic, pre-existing extensional faults experienced the low intensity compressional inversion, and the low-amplitude fold deformation occurred on the western margin of the basin, which was strong in the south and it was weak in the north. The Late Jurassic compression deformation resulted in the large-scale inversion and thrust-decollement of preexisting main faults, and produced a large number of new thrust faults and several thrust nappes, resulting in the strong uplift and denudation on the front of the nappes. Comparing the two stages of compression, we reveal that the Late Jurassic compression was the main tectonic event that created the thrust-nappe system and the present basin-mountain system in the southwestern Ordos Basin.

Cenozoic tectonic patterns and their controls on growth strata in the northern Tianshan fold and thrust belt, northwest China

Field-based mapping and subsurface interpretations of high-resolution seismic images, as well as other geophysical data, have been integrated to investigate tectonic patterns and growth strata in the northern Tianshan fold and thrust belt (NTFTB). The NTFTB formed as a result of intense deformation and uplift of the northern Tianshan region that was triggered by the far field effects of the Indo-Eurasia plate collision. The structural styles of the NTFTB vary, and are primarily classified as thick-skinned thrust nappes that involve basement rocks and thin-skinned fault-related fold structures with multiple décollement layers in the sedimentary strata. The overall NTFTB can be divided into three segments based on its structural geometry. The western segment is characterized by an intense backthrust in the northern Tianshan piedmont, with little displacement transfer into the southern Junggar Basin. In the central segment, the emergence of three rows of anticlines reveals a progressive deformation sequence and displacement transfer into the foreland through multiple décollements. In the eastern segment, the tectonic pattern is characterized by steep faults that branch forward in a narrow zone to generate intense uplift. Evidences indicate that major basement faults, paleo-uplifts, and inheritance of former structures play key roles in driving the segmentation of the NTFTB, and that secondary fault assemblages may control regional tectonic styles within each tectonic unit. Cenozoic growth strata have been observed in the NTFTB, the geometry and deformation mechanism of which are subject to the structural style in a given structure. Growth strata can also provide constraints for time of deformation in the northern Tianshan, and the Dushanzi Formation has been shown to represent clear growth in the piedmont, which indicates rapid uplift of the northern Tianshan as well as intense tectonic activity in the NTFTB in the late Miocene. The growth strata become younger within structures to the north, towards the foreland. In general, the Cenozoic evolutional process has been relatively uniform in the northern Tianshan, generally showing a forward propagation of fault splays into the foreland, and the geometry and distribution of sedimentary strata have been controlled by tectonics.

Research on the fracture structure and activity of the Qinling Mountains thrust nappe system in western Hubei

The Western Hubei section of the Liangyun fault is an important structural belt of the southern Qinling Mountains thrust nappe system. As the significant activity of the Liangyun fault since the Quaternary has led to high seismic risk in the surrounding area, the research on the characteristics and activity of fault structure is of great significance for deepening the construction of a seismic safety system in this area. In this study, we conducted a field geology survey combined with quartz optical stimulated luminescence dating, scanning electron microscopy dating, and thermoluminescence dating results and comprehensive application of shallow seismic reflection and high-resolution refraction) to analyze the activities of the Liangyun fault in the Quaternary period. Sediment optical stimulated luminescence dating results of samples from the breakpoint were 134.99 + 15.52 and 160.95 + 16.88 ka. Combined with the seismic profile, outcrop observation, and previous dating results, we conclude that the new era is in fault activities in the early Pleistocene to late Pleistocene (Q2–Q3). The combined application of shallow seismic reflection and high-resolution refraction method can confirm each other’s measured results, providing more parameters for the interpretation of seismic data under complex conditions and ensuring the accuracy of data interpretation at the same time. At present, the seismic experiment scheme is less used in the field of active fault detection, since its good detection effect and the application of the trial to shallow geophysical exploration has a certain application value and global scalability.

Low Temperature History of the Tiegelongnan Porphyry–Epithermal Cu (Au) Deposit in the Duolong Ore District of Northwest Tibet, China

AbstractThe Tiegelongnan is the first discovered porphyry–epithermal Cu (Au) deposit of the Duolong ore district in Tibet, China. In order to constrain the thermal history of this economically valuable deposit and the rocks that host it, eight samples were collected to perform a low‐temperature thermochronology analysis including apatite fission track, apatite, and zircon (U‐Th)/He. Apatite fission track ages of all samples are between 34 ± 3 and 67 ± 5 Ma. Mean apatite (U‐Th)/He ages show wide distribution, ranging from 29.3 ± 2.5 to 56.4 ± 9.1 Ma. Mean zircon (U‐Th)/He ages range from 79.5 ± 12.0 to 97.9 ± 4.4 Ma. The exhumation rate of the Tiegelongnan deposit was 0.086 km m.y.−1 between 98 and 47 Ma and decreased to 0.039 km m.y.−1 since 47 Ma. The mineralized intrusion was emplaced at a depth of about 1400 m in the Tiegelongnan deposit. Six cooling stages were determined through HeFTy software according to low‐temperature thermochronology and geochronology data: (i) fast cooling stage between 120 and 117 Ma, (ii) fast cooling stage between 117 and 100 Ma, (iii) slow cooling stage between100 and 80 Ma, (iv) fast cooling stage between 80 and 45 Ma, (v) slow cooling stage between 45 and 30 Ma, and (vi) slow cooling stage (<30 Ma). Cooling stages between 120 and 100 Ma are mainly caused by magmatic–hydrothermal evolution, whereas cooling stages after 100 Ma are mainly caused by low‐temperature thermal–tectonic evolution. The Bangong–Nujiang Ocean subduction led to the formation of the Tiegelongnan ore deposit, which was buried by the Meiriqiecuo Formation andesite lava and thrust nappe structure; then, the Tiegelongnan deposit experienced uplift and exhumation caused by the India–Asia collision.

Indentation tectonics of the Fanshi Block in the Trans-North China Orogen

The Trans-North China Orogen (TNCO) is a tectonic zone that records significant magmatic, metamorphic, and tectonic events during the Paleoproterozoic. The central part of the TNCO comprises several Precambrian blocks (the Hengshan Block, the Wutai Block, the Fuping Block, and the Fanshi Block) that are separated by three Paleoproterozoic ductile shear zones as tectonic boundaries. The Hengshan Block and the Fuping Block are represented by a group of large-scale gneissic domes, whereas the Wutai Block characterized by the ENE–WSW-trending synclinorium, and the framework of the Fanshi Block is dominated by strike-slip ductile shear zones superimposed on tight folds. We present detailed structural analysis of the Zhujiafang ductile shear zone in the central Hengshan, the Wangjiahui ductile shear zone at the northern margin of the Wutaishan, and the metamorphic complex between these two shear zones. The macro- and microstructures indicate that the ductile shear zones show different kinematics. The Zhujiafang ductile shear zone is E–W-trending and record sinistral strike-slip shearing, and the Wangjiahui ductile shear zone is WSW–ENE-trending and record dextral strike-slip shearing. Both of shear zones have similar deformation ages according to the 40Ar/39Ar dating results, and they cut through all the gneissosity and the fold developed in these metamorphic rocks. The kinematic difference between the Zhujiafang and the Wangjiahui ductile shear zones clearly define the Fanshi Block sandwiched between them, which is characterized by movement from west to east. The Fanshi block differs from the Hengshan Block and the Wutai Block in terms of deformation history, metamorphism, and magmatism. In addition, a series of NE-trending ductile–brittle thrust nappes inside the Wutai Block under the influence of the indention of the Fanshi Block inside. Therefore, we propose that the Fanshi Block was indented to the TNCO from west to east, accompanied by strong strike-slip ductile shearing and thrusting during the late Paleoproterozoic.

Thermal evolution of the Scandian hinterland, Naver nappe, northern Scotland

The Northern Highlands Terrane of Scotland hosts several thrust nappes that were deformed and metamorphosed during the Silurian Scandian orogeny. Quantitative petrological analysis of metamorphic assemblages indicates that the hinterland-positioned Naver nappe experienced decompression heating from 8–9 kbar and 600°C to 6–7 kbar and 700°C. Monazite–xenotime thermometry and geochronology delineate a detailed temperature–time history for the Naver nappe. Monazite often exhibits compositional zoning, which is used to establish multiple temperature–time points in several samples. These data indicate that the Naver nappe experienced relatively fast heating ( c. 50°C myr −1 ) and relatively slow cooling (15–20°C myr −1 ), with peak temperatures occurring at c. 425 Ma. This temperature–time evolution is compatible with the early Emsian (407–403 Ma) deposition of unmetamorphosed conglomerates that rest on high-grade metamorphic rocks in the Naver nappe, but requires an acceleration in the cooling rate to 40–50°C myr −1 at 420–410 Ma. Geochronological constraints from this study and previous work suggest that deformation and metamorphism in the hinterland of the Scandian orogen in northern mainland Scotland are younger than the c. 430 Ma deformation in the foreland-positioned Moine thrust zone. We postulate that heat from pervasive granitic intrusions in the Naver nappe weakened the crust, allowing deformation to retreat to the hinterland of the orogen. Supplementary material: A description of our analytical methods, all U–Pb-trace element data, additional figures explaining our petrological analysis and other relevant data are available at: https://doi.org/10.6084/m9.figshare.c.4458041

Mesozoic evolution of the eastern Pamir

AbstractWe present field and analytical results from the Tashkurgan and Waqia valleys in the southeastern Pamir that shed new light on the tectonic evolution and terrane architecture of the region. Field mapping of metasedimentary and igneous units along the Tashkurgan and Waqia valleys in the Southeast Pamir, integrated with metamorphic petrology, garnet-biotite thermometry, and zircon U/Pb isotopic analysis, help identify major structures and terrane boundaries in the region, as well as compare structural units across the Miocene Muztaghata gneiss dome. South of the Muztaghata dome, the gently northwest-plunging synformal Torbashi thrust klippe juxtaposes amphibolite facies Triassic Karakul-Mazar terrane schist and gneiss structurally above (1) greenschist facies Triassic Karakul-Mazar terrane metasedimentary rock in the north, and (2) lower-amphibolite facies schist in the south that are interpreted to be Gondwanan-derived crust (Central or South Pamir terrane). Farther south, the Rouluke thrust fault imbricates the Gondwanan crust, placing early Paleozoic schists over Permian marble and slate. Exposure of the Torbashi thrust sheet terminates in the southeast, and with it the surface exposure of the Triassic Karakul-Mazar terrane, leaving the Paleozoic Kunlun terrane juxtaposed directly against Gondwanan terrane crust. Based on lithologic and isotopic similarities of units north and south of the Muztaghata gneiss dome, we document the existence of a regionally extensive thrust nappe that stretched across the northern and eastern Pamir, prior to being cut by Miocene exhumation of the Muztaghata dome. The thrust nappe links the Torbashi thrust in the southeast Pamir with the Tanymas thrust in the northern Pamir, and documents regionally extensive exposure of lithologically continuous units across the northeast Pamir. While timing of emplacement of the Torbashi thrust klippe and displacement on the Rouluke fault to the south is not well constrained, we interpret shortening to be Cretaceous in age based on previously published cooling ages. However, a component of Cenozoic shortening cannot be ruled out.A key observation from our mapping results is that the surface exposures of the Karakul–Mazar–Songpan Ganzi terrane are not continuous between western Tibet and the Pamir, which indicates tectonic and/or erosional removal, likely sometime in the Mesozoic. Furthermore, our documentation of the Jinsha suture in the southeast Pamir on the eastern side of the Karakoram fault shows deflections of terranes across the Himalayan-Tibetan orogen were not primarily accommodated along discrete, large displacement faults (>400 km) faults. Instead, oroclinal bending of the northern Pamir, and dextral shear along the Pamir margins, may be largely responsible for the northward deflection of terranes.

Loess landslides on the South Jingyang Platform in Shaanxi Province, China

Continuous use of diversion irrigation has been associated with an increase in the frequency of loess landslides on the South Jingyang Platform in Shaanxi Province, China. Continued irrigation results in increased water content, which in turn leads to liquefaction, causing flowslides characterized by high speed and long runout distance. After the first flowslide occurs in an area, the seepage field and stress field of the slopes surrounding the flowslide change, resulting in a decrease in slope stability. This, in turn, leads to the occurrence of retrograde loess slides in the same position as earlier flowslides. These failures lead to evolution of the platform. Shear liquefaction occurs in the terrace sand layers, resulting in liquefied sand layer flows that occur in a non-uniform and irregular manner. This leads to the formation of a thrust nappe structure over a long distance at the distal part of the deposits.

A revised geodynamic model for the Lapland-Kola Orogen

The Paleoproterozoic Lapland-Kola Orogen in Fennoscandia has been studied for decades and several plate tectonic models have been proposed including one-sided subduction zone, either towards SW or NW, or two opposite-verging subduction zones before the collision. Based on new structural and isotope data from Finland and recently published data from Russia, we propose a revised tectonic model for the Paleoproterozoic Lapland-Kola Orogen. The main components are foreland in the NE followed by cryptic suture, Inari arc, retro-arc basin and retro-arc foreland in the SW. The latter three constitute the Inari Orocline.Subduction towards present SW and subsequent arc magmatism (Inari arc) started at ca. 1.98 Ga followed by voluminous sedimentation in the deepening retro-arc basin. Underplating of a mid-ocean ridge caused flat subduction and magmatic flare at 1.92 Ga over a broad distance in the retro-arc basin. Rapid heating led to melting of the retro-arc basin sediments and voluminous amounts of granulite-facies diatexites formed. During collision (D1) at 1915–1910 Ma, large thrust nappes formed on the foreland. Deformation in the retro-arc basin is seen as recumbent folding and shearing of diatexites in the lower parts of the basin and thrusting of metatextite-diatexite packages in the upper parts. A post-collisional stage is seen as 1904 Ma appinites and decompression derived granites at 1.90–1.89 Ga. Renewed shortening (D2), due to far-field effects in SW at 1.88–1.87 Ga, led to thick-skin shortening of the Archean middle crust, large-scale crustal duplexing of already cooled granulites towards the retro-arc foreland and inclined upright folding of granulites in the opposite direction towards the Inari arc. A switch in the stress field from NE-SW to NW-SE led to orogen-parallel contraction and buckling started along a dextral strike-slip fault zone to form the Inari Orocline. Buckling is seen in the bending of pre-orocline fabrics and formation of syn-orocline fabrics: radial conical folds (D3), radial fractures, a strike-slip fault zone and thrusting at the hinge zone. The end-result is a mega-scale parallel multi-layer fold composed of the Inari arc, retro-arc basin and possibly also the heated retro-arc foreland.

Characteristics of buried structures in northern Longmenshan mountains and its significance to oil and gas exploration in the Sichuan Basin

Since 2014, great gas discoveries have been achieved in the Upper Paleozoic Shuangyushi zone in the piedmont zone of the Longmenshan mountains, northwestern Sichuan Basin, where multiple gas wells with industrial flows have been drilled and completed successively in the Middle Permian Qixia Fm and Middle Devonian Guanwushan Fm, etc. Along with the steady progress of exploration there, to make a further in-depth study on the characteristics of the frontal–piedmont zones in the Longmenshan thrust–nappe structures will be of great significance to both a better understanding of Upper Paleozoic gas reservoirs in this study area and the expansion of oil and gas exploration field. In view of this, based on the newly deployed 3D seismic surveys, gravity–magnetic–electronic data, and practical drilling information from exploration wells like Well ST 9, the characteristics of the frontal–piedmont zones in the Longmenshan thrust–nappe structures are analyzed as well as the features of the buried structures, gas reservoir forming conditions and exploration prospect there. The following findings were obtained: (1) A huge buried structure belt is developed underlying the Longmenshan front fault belt No. 1 (LSFFB 1) and marine strata are thus formed below the Lower Triassic Jialingjiang Fm due to the roles of compressive folding, extrusion and deformation; (2) The detachment horizons from LSFFB 1 go from shallower in the south to deeper in the north, turning from Cambrian slipping in the south to basement-involved in the north. The Paleozoic and its overlying strata were overall uplifted with a higher degree northward and so did the strata between the up-thrown and down-thrown sides of the faults with a greater displacement distance and higher folding degree, resulting in that the Paleozoic strata in the north are 1500 higher uplifted than those in the south; (3) In the western Jiange and Guangyuan areas, the eastern LSFFB 1 presents a tectonic pattern of “three steps” as a whole. In conclusion, there are good gas reservoir forming conditions in LSFFB 1, the down-thrown sides of which cover an area of 1800 km2, where the Guanwushan Fm and Qixia Fm provide good accumulation and preservation conditions for gas reservoirs, which will become favorable gas exploration targets in the deep marine strata in NW Sichuan Basin.

Deformation patterns and timing of the thrust‐nappe structures in the Mohe Formation in Mohe Basin, Northeast China: Implication of the closure timing of Mongol–Okhotsk Ocean

The Mohe Basin, to the south of the eastern Mongol–Okhotsk suture belt, contains important stratigraphic records and thrust‐nappe structures for understanding the closure of the eastern Mongol–Okhotsk Ocean. Meso‐micro structures in the Mohe Formation indicate a low‐temperature deformation/metamorphism (ca. 400°C) under an N‐S compression related to the southward thrusting faulting. The S‐L‐tectonites record strain similar or close to flattening/compressional strain and denote sinistral shearing in analogously N‐S‐trending thrusting. Our new detrital zircon U–Pb ages exhibit that maximum depositional ages of the Mohe Formation were conservatively estimated as latest Middle Jurassic (ca. 165 Ma). The Northeast China and Erguna Block to the south of the Mohe Basin, and Siberian Block to the north were the sources for the Mohe foreland basin's sediments throughout the deposition of the Mohe Formation, which derived from the basement uplift where erosion cuts deep into the dissected arcs. In the latest Jurassic to earliest Cretaceous, the southward collision and indentation of the Siberian Block (final closure of Mongol–Okhotsk Ocean) led to intense thrust nappe structures, syn‐collisional folds, and low‐temperature metamorphism occurred along the northwestern margin of Mohe Basin.

Genesis of Xinzhai Sandstone-Type Copper Deposit in Northern Laos: Geological and Geochemical Evidences

Xinzhai sandstone-type copper deposit located in northern Laos lies in the Jiangcheng-Phongsaly-Phrae Mesozoic basin (JPMB), which is regarded as southern extension of the Lanping-Simao Mesozoic basin in China. The copper deposit belt is bounded by the Ailaoshan-Heishui River fault and the Dian Bien Phu-Luang Prabang fault at the east and Lancang River-Bannankan faults at the west. Two types of orebodies are identified in the Xinzhai area based on geological investigation. One is lamellar copper orebody hosted by the fine lithic feldspar sandstones and feldspar lithic sandstones; another is vein-type orebody. The sulfur isotopic compositions of the chalcopyrite and tetrahedrite are from -11.6‰ to -1.8‰, indicating that sulfur is derived from bacterial sulfate reduction (BSR). δD values of fluid inclusions in ore-bearing quartz samples are from -99‰ to -78‰. The calculated δ18OH2O values of ore-forming fluid vary from -2.3‰ to 0.4‰ using the quartz-water fractionation equations and the mineralization temperature. Oxygen and hydrogen isotopic compositions show that the ore-forming fluid was derived from basin fluid. Rock-mineral identifications show that both of the mineral grain maturity and the structural maturity are high in the Jurassic Huakaizuo Formation, reflecting a far-source accumulation and lake facies sedimentatary environment. Based on tectonic determination diagram of the Al2O3/SiO2-TFe+MgO, the sandstone samples collected from the Huakaizuo Formation were plotted in the passive continental margin. The collision of the Indian and Eurasian blocks during the Cenozoic formed large-scale strike-slips and thrust nappe structures in margin of the basin. With the tectonic movement, Cu-rich basin fluid from the basement of basin migrated upward along the contemporaneous fault and into the high porosity strata. At the same time, in organic matter-riched condition, bacterial sulfate reduction (BSR) has been triggered, forming a large number of S2- ions, and then precipitation of sulfide started. This mechanism describes the process of copper mineral deposition in the Xinzhai deposit.

Gravity-driven structures and deposits resulting from slope collapse in the margin of a carbonate platform (Pennsylvanian, Cantabrian Zone, NW Iberia)

Lower-slope to basin-floor deposits associated to isolated microbial carbonate platforms of the Valdeteja Formation (Carboniferous foreland basin, Cantabrian Zone, NW Spain), containing large slid blocks and debrites, exhibit a combination of contractional and extensional structures, which include thrusts, backthrusts, folds, extensional faults and joints, and boudinaged beds. Several lines of evidence indicate that deformation took place in poorly lithified conditions, such as thickened hinges, hydroplastic fractures, fold axes scatter or folding of microbial limestones.Once restored, all structures indicate a transport direction towards the NNE-ENE. This azimuth coincides with the direction towards which the thrust nappes of the southern branch of the Cantabrian Zone were emplaced during the Variscan Orogeny. However, the interpretation of the deposits and structures as gravity-related, associated to the collapse of the carbonate platform slope, is supported by features such as the rootless character of thrusts, the parallelism between transport direction and paleocurrent orientation, the concomitant development of contractional and extensional structures, the development of normal listric faults affecting the Valdeteja platform, the sedimentary environment (lower slope to basin floor) and the geological context of the Valdeteja carbonate platforms, which predate the Variscan deformation in the study area.

Evidence for an Early Silurian Synorogenic Basin Within the Metamorphic Hinterland of the North Atlantic Caledonides: Insights From the U‐Pb Zircon Geochronology of the Funzie Conglomerate, Shetland, Scotland

AbstractThe spectacular kilometer‐thick Funzie Conglomerate is a deformed and metamorphosed high‐energy sedimentary deposit that occurs in Shetland (Scottish Caledonides). Its age and tectonic significance in relation to Early Ordovician (485–475 Ma) and Late Ordovician (450–445 Ma) accretionary orogenic events and the culminating Silurian (430–425 Ma) continental collision and closure of the Iapetus Ocean have hitherto been unknown. U‐Pb zircon dating indicates that the clast population of the conglomerate was derived from easterly sources comprising Neoproterozoic metasedimentary rocks and the northern extension of the Ordovician‐Silurian Midland Valley Arc that formed during Iapetus closure. A lower age limit of 440 Ma is indicated by the ages of the youngest matrix detrital zircon grains and the youngest granitic clast. Deformation and metamorphism of the conglomerate is attributed to sinistrally oblique extension at ca. 430 Ma that excised at least 10 km of crustal section. Sedimentation of the conglomerate is therefore constrained to a 10‐myr time span (440–430 Ma). The tectonic setting is interpreted as a synorogenic basin that received detritus from, and was buried by, advancing thrust nappes during the Early Silurian collision of Laurentia and Baltica. Tectonic burial was followed rapidly by ductile deformation during oblique extensional unroofing of the metamorphic pile and accounts for preservation of the Funzie Conglomerate as a rare example of a synorogenic basin within the metamorphic hinterland of the North Atlantic Caledonides.