Nappe Formation Research Articles

Thin-and thick-skinned nappes of the southern slope of the Georgian Greater Caucasus: indicators of syn-collisional A-type subduction

In the region of the Caucasus considered herein two large structural complexes have been identified: an autochthone, including the Gagra-Java zone (GJZ) of the Greater Caucasus fold-and-thrust belt, the Kura foreland basin (KFB), and an allochthone consisting of the Utsera-Pavleuri, Alisisgori-Chinchvelta, Sadzeguri- Shakhvetila, Zhinvali-Pkhoveli nappes and Ksani-Arkala parautochthone. The nappes are established on the basis of paleogeographic reconstructions, structural data, as well as drilling and geophysical data. The leading mechanism for the nappe formation is the advancement to the north and the underthrusting of the autochthone under the Greater Caucasus (A-type subduction). The nappes were formed mainly in the Late Alpine time (Late Eocene–Early Pliocene) and include only the sedimentary cover of the Earth’s crust (thin-skinned nappes). However the basal detachment (décollement) of the nappes, according to seismic data, penetrates deeply and cuts the pre-Jurassic crystalline basement, and even the entire Earth’s crust representing thick-skinned deformation. The total horizontal displacement of the flysch nappes of the southern slope of the Greater Caucasus in their eastern (Kakhetian) part is 90–100 km. While, considering the folding of the entire Greater Caucasus, the total transverse shortening of the Earth‘s crust within its limits is equal to 190–200 km.

Thin-and thick-skinned nappes of the southern slope of the Georgian Greater Caucasus: indicators of syn-collisional A-type subduction

In the region of the Caucasus considered herein two large structural complexes have been identified: an autochthone, including the Gagra-Java zone (GJZ) of the Greater Caucasus fold-and-thrust belt, the Kura foreland basin (KFB), and an allochthone consisting of the Utsera-Pavleuri, Alisisgori-Chinchvelta, Sadzeguri- Shakhvetila, Zhinvali-Pkhoveli nappes and Ksani-Arkala parautochthone. The nappes are established on the basis of paleogeographic reconstructions, structural data, as well as drilling and geophysical data. The leading mechanism for the nappe formation is the advancement to the north and the underthrusting of the autochthone under the Greater Caucasus (A-type subduction). The nappes were formed mainly in the Late Alpine time (Late Eocene–Early Pliocene) and include only the sedimentary cover of the Earth’s crust (thin-skinned nappes). However the basal detachment (décollement) of the nappes, according to seismic data, penetrates deeply and cuts the pre-Jurassic crystalline basement, and even the entire Earth’s crust representing thick-skinned deformation. The total horizontal displacement of the flysch nappes of the southern slope of the Greater Caucasus in their eastern (Kakhetian) part is 90–100 km. While, considering the folding of the entire Greater Caucasus, the total transverse shortening of the Earth‘s crust within its limits is equal to 190–200 km.

Influence of preexisting structures on salt structures in the Kuqa Depression, Tarim Basin, Western China: Insights from seismic data and numerical simulations

AbstractThe preexisting structures that developed in the basement and subsalt strata play a key role in the salt structural deformation in the Kuqa Depression, Tarim Basin. The characteristics of preexisting structures and their controls on the salt structure are investigated via the latest three‐dimensional seismic data and numerical modelling. The results show that the preexisting structures that developed in the Kuqa Depression mainly consist of basement faults, palaeouplifts, subsalt slopes and early passive salt diapirs. Basement faults are mainly distributed in the Kelasu and Qiulitag structural belts and control the position of development and deformation style of the Miocene compressive salt structure. The differences in styles and reactivation degrees of basement faults lead to great diversity in the salt structure. The palaeouplifts mainly include the Wensu, western Qiulitag, Xinhe and Yaha‐Luntai palaeouplifts. The original sedimentary range and later deformation space of the salt layer are limited by the palaeouplift, resulting in strong salt thrusting in the Awate sag in the western part of the Kuqa Depression. The heterogeneous spatial distribution of the palaeouplift promoted the development of regional strike‐slip transform belts. Subsalt slopes are located mainly on the northern edge of the western Qiulitag low uplift and block the southward flow of the salt, causing the salt to form salt domes; the size of these domes is closely related to the subsalt slope. Early passive salt diapirs mainly developed in the Quele and Bozidun areas of the western Kuqa Depression, and they were preferentially active during the compression period, inducing the formation of a piercement salt nappe. Numerical modelling revealed that the preexisting structure strongly controlled the stress–strain distribution during the deformation of the salt structure. The spatial distribution heterogeneity of the basement structure is an important factor in the structural zonation along the north–south strike and segmentation along the west–east strike in the Kuqa Depression, as well as an important inducer of the piercement salt structure.

Characteristics structures of the mélange zones in the Ukrainian Carpathians

Mélange is characterized by the block-in-matrix fabric in which rigid blocks of different sizes, lithologies, and ages are distributed in a ductile matrix. Though mélange is a significant component of orogenic belts, in the Ukrainian Carpathians, mélanges of tectonic origin have not been reported. We have described the mélange zones widespread in the Pieniny Klippen Belt, Marmarosh Klippen Zone, and the inner nappes of the Outer Ukrainian Carpathians, as well as the typical deformation structures developed within them.
 The mélange matrix is characterized by a scaly fabric formed by cleavage surfaces and somewhere arranged in S-C structures. Lenticular clasts within the matrix show their long axis aligned to the tectonic foliation. Rotation of the boudins occurs against the shear direction following the formation of the S-C structures. The rigid clasts somewhere demonstrate sigma-type rotation structures. Some blocks within the mélange are highly fractured up to tectonic breccias.
 The study of tectonic slicken-sides and other deformation structures within the Pieniny belt demonstrates the presence of regular stress fields correlated with the geodynamics of the Carpatho-Pannonian region. The main stress field indicates the SW-NE regional compression related to the formation of Carpathian nappes and S-N — trending dextral strike-slip faults. Our study in the Priborzhava quarries records the Oash right-lateral strike-slip fault zone. In the Pieniny Klippen Belt some oblique normal faults of the Carpathian direction are related to the Transcarpathian Depression formation. The study showed that the mélange zones in the inner part of the Ukrainian Carpathians were formed largely due to strike-slip movements. In contrast, in the outer part of the Carpathian orogen, they formed mainly due to thrusting.

Miocene Volcanism in the Slovenský Raj Mountains: Magmatic, Space, and Time Relationships in the Western Carpathians

The Miocene volcanic-intrusive complex in the Slovenský Raj Mountains, middle Slovakia, comprises a swarm of subalkaline basalts and basaltic andesites with alkaline basalts, trachybasalts and basaltic trachyandesites. Basaltic to doleritic feeder dykes and sporadic hyaloclastite lavas are exposed in contact with the Triassic Bódvaszilas Formation of the Silica Nappe. The primary clinopyroxene, plagioclase, and Fe-Ti oxide assemblage also contains calcite spheroids inferred to represent carbonatitic melt. These spheroids are associated with subsolidus chlorite, actinolite, magnetite, titanite, calcite, and epidote. Micropoikilitic clinopyroxene, albite, and Ti-magnetite formed due to rapid quenching. There was an incorporation of host rock carbonate during the eruption. The erupted products are the result of magmatic differentiation of the parental basaltic tholeiitic magma with a redox of ∆QFM = +1 to +3, affected by varying degrees of 0%–50% fractionation and the assimilation of carbonate material in a shallow magmatic reservoir. REE geochemistry shows N-MORB-like type patterns with both LaN/YbN and LaN/SmN < 1 at near constant Eu/Eu* (~0.9). This is supported by εNd(t=13 Ma) values of +8.0 to +7.4 determined from the basaltic rocks. The REE values can be modeled by 1% fractional melting of garnet peridotite mixed with 7% melting of spinel peridotite of PM composition (1:9 proportions). SIMS and LA-ICP-MS U/Pb analysis of zircons yields a concordant age of 12.69 ± 0.24 Ma and a 13.3 ± 0.16 Ma intercept (Serravallian) age. The Middle Miocene volcanic activity was related to subduction-collision processes along the boundary of the Cenozoic ALCAPA (Alps–Carpathians–Pannonia) microplate and the southern margin of the European plate.

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.

Effect of Orientation and Vegetation over the Embankment Crest for Energy Reduction at Downstream

Coastal embankments often collapse due to the tremendous destructive energy of an overtopping tsunami flow due to a deep scour by nappe flow. Hence, to clarify the nappe flow formation condition due to the overtopping, a series of tests were carried out within a laboratory flume with immobile settings by lowering the downstream surface angle of an embankment model while keeping the upstream surface slope constant (1:1) with five non-dimensional overtopping depths and six different crest conditions. The conditions imposed on the embankment crest in the flow direction were without vegetation; horizontal crest, (−)4% descending crest slope, (+)4% ascending crest slope, and adding vegetation model with three different densities across the horizontal crest to improve resistance to the flow. The increased resistance provided by the vegetation models were categorized based on the spacing ratio between cylinders to diameter: sparse, intermediate, and dense. Increased vegetation density above the crest results in a significant reduction of flow energy by approximately 30–50% at the downstream brink edge and 40–60% at the downstream plunge basin. In contrast, the maximum energy reduction was found to be by the dense vegetation model. Additionally, owing to the steep slope of the water surface profile and the increasing vegetation density, the impinging jet’s impact point moved closer to the toe of an embankment. This implies that vegetation covers a smaller area while increasing density to mitigate the destructive intensity of flood/tsunami movement. Meanwhile, the descending crest scenario results in a faster nappe flow formation. In contrast, the ascending crest scenario delays the nappe formation while reducing the downstream slope angle. It maintains the sub-critical flow at the crest, except near the downstream brink edge.

Dynamics of Early Neoproterozoic accretion, west-central India: I. Geochronology and Geochemistry

The Godhra-Chhota Udepur (GC) sector (west-central India) is the zone of convergence between two crustal-scale accretion zones, i.e. the N/NNE-striking Aravalli Delhi Fold Belt (ADFB) against the E-striking Central Indian Tectonic Zone (CITZ). In this study, we demonstrate the two orogens welded during the Early Neoproterozoic. In the GC sector, recumbently folded basement gneisses, shallow-dipping granitoid mylonites and a suite of allochthonous supracrustal rocks experienced top-to-the south thrusting and nappe formation (D2 deformation). The shallow-dipping crustal domain was modified by the superposition of a network of WNW/W-striking transpressional shear zones and related folds (D3 deformation). The D2-D3 deformations occurred due to oblique crustal convergence marked by the emplacement of pre-D2, post-D2 and syn-D3 granitoids. The post-D2, syn-D3 granite-granodiorites are weakly peraluminous, calc-alkalic, having ferroan to magnesian affinity, and characterised by LREE-enriched moderately fractionated REE patterns with variable negative Eu anomalies. Trace element geochemistry and whole rock Sr-Nd systematics suggest the granitoids were derived from dominantly meta-greywacke precursors. In South-GC, U-Pb zircons in the basement gneisses yield upper intercept/Concordia dates at 1.65–1.60 Ga for high-grade metamorphism. The lower intercept/Concordia dates in the gneisses coincide with the 0.95–0.93 Ga emplacement age of post-D2/syn-D3 granitoids throughout GC. However, the pre-D2 granitoids in South-GC are older, ~1.03 Ga. By contrast, pre-D2 granitoids in North-GC yield Late Neoarchean (2.5 Ga) Concordia/upper intercept emplacement ages, identical to the emplacement age of the Archean granites in the ADFB; the lower intercept/Concordia age is 0.95–0.93 Ga. The 2.5 Ga granites did not experience the 1.65–1.60 Ga high-grade metamorphism, but both the lithodemic units shared the 0.95–0.93 Ga D2-D3 deformation-metamorphism and the emplacement of post-D2 and syn-D3 0.95–0.93 Ga granitoids. The juxtaposition of the ~2.5 Ga granitoids and the 1.65–1.60 Ga gneisses is attributed to the 1.03–0.93 Ga ADFB-CITZ oblique accretion that involved contemporaneous emplacement of syn-collisional S-type granitoids.. • Godhra-Chhota Udepur (GC) is the western end of the Central Indian Tectonic Zone . • At GC, NNE-SSW shortening is contemporaneous with 0.95–0.93 Ga felsic emplacements . • In north GC, 2.5 Ga granite reworked at 0.95 Ga with no Paleoproterozoic overprint . • In south GC, no Archean inheritance noted in 1.6 Ga gneisses reworked at 0.95 Ga . • ~0.95 Ga accretion welded mutually exclusive Archean and Paleoproterozoic terrains .

The transition from ancient to modern-style tectonics: Insights from lithosphere dynamics modelling in compressional regimes

Orogens are traditionally classified according to their tectonic style. Paleoproterozoic tectonics is referred to as “ancient-style tectonics” while Proterozoic tectonics is referred to as “modern-style tectonics”. Ancient-style tectonics is characterised by distributed vertical structures and low topography gradients, often associated with diapirism and partial melting. In contrast, modern-style tectonics involve prominent strain localisation and the formation of thrusts, nappes and high topographic gradients. However, the parameters controlling the transition from ancient to modern-style tectonics are poorly understood. To quantify this transition, a combination of 1D and 2D high resolution lithospheric-scale thermo-mechanical models was conducted. The parameters controlling the strength of the lithosphere (i.e., Moho temperature, strain rate, crustal rheology, crustal radiogenic heat production and role of shear heating) were investigated in detail. Our results show that tectonic style is controlled by the maximum of crustal strength (shear stress). Modern-style tectonics is observed to occur when the maximum of crustal strength is greater than 300 MPa. At the opposite, a maximum crustal strength lower than 300 MPa leads to ancient style tectonic structures. Therefore, crustal rheology, temperature and background strain rate significantly influence the transition from ancient to modern-style tectonics. Shear heating remains a key factor in promoting strain localisation in modern-style tectonics. Crustal radiogenic heat production has a moderate influence by increasing/decreasing the tendency for faulting within the crust. This crustal strength criterion also provides an excellent fit for a second potential proxy: a localisation criterion of ca. 225°C. These two proposed proxies can be used interchangeably to predict the transition from ancient to modern-style tectonics.

Evolving temperature field in a fossil subduction channel during the transition from subduction to collision (Tauern Window, Eastern Alps)

AbstractWe investigate the evolution of the three‐dimensional thermal structure of a palaeo‐subduction channel exposed in the Penninic units of the central Tauern Window (Eastern Alps). Structural and petrological observations reveal a sheath fold with an amplitude of some 20 km that formed under high‐P conditions (~2 GPa). The fold is a composite structure that isoclinally folded the thrust of an ophiolitic nappe derived from Alpine Tethys Ocean onto a unit of the distal European continental margin, also affected by the high‐P conditions. This structural assemblage is preserved between two younger domes at either end of the Tauern Window. The domes deform isograds of the T‐dominated Barrovian metamorphism that itself overprints the high‐P metamorphism partly preserved in the sheath fold. Using Raman spectroscopy on carbonaceous material (RSCM), we are able to distinguish peak‐temperature domains related to the original subduction metamorphism from domains associated with the later temperature‐dominated (Barrovian) metamorphism. The distribution of RSCM temperatures in the Barrovian domain indicates a lateral and vertical decrease of peak temperature with increasing distance from the centres of the thermal domes. This represents a downward increase of palaeo‐temperature, in line with previous studies. However, we observe the opposite palaeo‐temperature trend in the lower limb of the sheath fold, namely an upward increase. We interpret this inverted palaeo‐temperature domain as the relic of a subduction‐related temperature field. Towards the central part of the sheath fold's upper limb, RSCM temperatures increase to a maximum of ~520°C. Further upsection in the hangingwall of the sheath fold, palaeo‐peak temperatures decrease to where they are indistinguishable from the peak temperatures of the overprinting Barrovian metamorphism. Peak‐temperature contours of the subduction‐related metamorphism are oriented roughly parallel to the folded nappe contacts and lithological layering. The contours close towards the northern, western and eastern parts of the fold, resulting in an eye‐shaped, concentric pattern in cross‐section. The temperature contour geometry therefore mimics the fold geometry itself, indicating that these contours were also folded in a sheath‐like manner. We propose that this sheath‐like pattern is the result of a two‐stage process that reflects a change of the mode of nappe formation in the subduction zone from thrusting to fold nappe formation. First, thrusting of a hot oceanic nappe onto a colder continental nappe created an inverted peak‐thermal gradient. Second, sheath folding of this composite nappe structure together with the previously established peak‐temperature pattern during exhumation. This pattern was preserved because temperatures decreased during retrograde exhumation metamorphism and remained less than the subduction‐related peak temperatures during the later Barrovian overprint. The fold ascended with diapir‐like kinematics in the subduction channel.

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.

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.

ГЕОЛОГІЧНІ/ГЕОТУРИСТИЧНІ ОБ'ЄКТИ ЗАКАРПАТСЬКОЇ ОБЛАСТІ ЯК ВІДОБРАЖЕННЯ ГЕОЛОГІЧНОЇ ІСТОРІЇ КАРПАТ

Consideration is given to description of important geological objects (observation points) of the southern slope of the Carpathians as reflections of sedimentary, paleogeographic and paleogeodynamic history of the Carpathians formation. Method. Standard method of geological description of outcrops, elements of sedimentological analysis of structural and textural features of rocks, and micropaleontological method are used. Results. A number of key geological objects that reflect the important stages of the geological evolution of the Carpathians in the Transcarpathian region near highways is described. These objects (the observation points in the article) can be represented as geotourist ones. In the first location which is in Yablunytsa Pass, an outcrop of Oligocene "striped" Holovets Limestone; further, along the Trostyanets Stream – Jurrassic pillow-lavas in the front of the Burkut Nappe; along the Kamyanyi Potik Stream – Jurassic- Early Cretaceous volcanogenic-sedimentary succession of the Kamyanyi Potik Nappe; along the Kosivka River – Eocene marls of the Marmarosh Massif sedimentary cover and the Inner Carpathian flysch of the Monastyrets Nappe; in the area of Novoselytsya village in the basin of the Luzhanka River – the Jurassic-Paleogene deposits of the Pieniny Klippen Belt; along the riverbed Tereblya in Zabrid village – Cretaceous-Paleogene deposits of the Marmarosh Klippen Zone are traced. Scientific novelty. The characteristic of a number of geological objects of the Transcarpathian region is supplemented and detailed and their brief description as reflections of sedimentary, paleogeographic and paleogeodynamic history of the Carpathians is presented. These objects reflect subsequent geological events: the birth of the Carpathian sedimentary basin; the formation of sub-oceanic and oceanic crust of the Outer Carpathian Flysch Basin; the formation of Inner Eastern Carpathian nappes and their destruction due to the formation of Early Cretaceous olistostrome of the Maramorosh Klippen Zone; ForeMarmarosh and Pieniny Klippen Belt accretionary prisms growing; and the final Oligocene stage of the Outer Carpathian Basin development. In addition, the range of Paleocene-Eocene paleogeographic elements such as (from the East to the West): (a) the passive margin of the Tisza-Dacia – (b) the deep-water Inner Carpathian Flysch Basin – (c) the Pieniny active edge of the Alcapa Terrane is demonstrated. The route of the geological tour described in the article is recommended to get acquainted with the main features of Geology, Geodynamics and processes of formation of the Carpathians. It can be used for further development of geotourism in the Carpathians.

Outline of Structural Evolution of the Thaumasian Back-Arc Trap Province of Mars, and Related Rim -Orogenic Arcs: A Speculative Hypothesis Based on Earth Geology

The Thaumasian lava plateau developed within an already deformed domain, consisting of composite orogenic belts. A crude scheme of the deformation history of the Thaumasian plateau summarizing the most significant events of its polycyclic development is presented here. It is suggested that the region evolved in four tectonism phases: (1) alpinotype regional compression, formation of basement nappes (early and middle Noachian orogen); (2) late Noachian regional, domal uplift, and denudation of the internal fold belt; (3) late Noachian-early Hesperian supracrustal deformation, as décollement nappes; and (4) Hesperian crustal foundering and great effusion of trap basalts flooding of the inner, alreadypeneplain mountain belts, balanced by uplifting of peripheral arcuate rim-highlands (swells and mountain belts). The two orogenic arcs of the Coprates and Thaumasia -Claritas belts exhibit thrusting vergence and orogenic migration, E- and SW-directed, indicating the presence of topographic doming associated with plume activity, which was probably positioned, in the late Noachian, upon the modern Solis planum lowland. There is a substantial possibility that the two arcs followed different tectonic evolutions. The lateral intrusion of hot, mafic fluids in interconnected fractures, near the crust-mantle boundary, is supposed to have been an important driving mechanism of tectonism beneath the plateau region. The thick bodies of diapir-like, moving aqueous fluid were probably related to ascending deepmantle plumes.

Research on High Density Oil-Based Drilling Fluid and Application in the Southern Edge of Junggar Basin, China

The southern edge of the Junggar basin has broad exploration potential. However, high-frequency complex drilling accidents in this area lead to the reduction of the average drilling speed and the extension of the drilling cycle. The main geological reason is that this area was affected by the process and developed high and steep geological structure. The nappe formation with a large dip angle and poor stability developed along faults. The major rock type of this E2-3a formation is mud-stone which has strong water sensitivity and can be hydrated and dispersed easily, which leads to the wellbore shrinkage and the borehole collapse. In order to improve the drilling speed in the southern edge of the Junggar basin and accelerate the exploration and development process, the drilling fluids associated with drilling problems in this area were investigated in this paper by combing the geological information and historical drilling records. Because of the special characteristics of this formation, it is concluded that the water-based drilling fluids previously used in this area are not suitable to solve the complex drilling conditions; a high density oil-based drilling fluid system has been developed for this area by conducting studies in the laboratory initially and being tested in the field later. In addition, its rheology, stability and pollution resistance performances were evaluated. Results show that, the rheology, the stability and pollution resistance capacities of the drilling fluid satisfy the drilling requirements. The fluid was applied in well H101 and the results show that the ROP of section 311.1 mm of the well was increased by 763% compared to the wells drilled in the same area and the total drilling time of well H101 was reduced by more than 80%, which was a historic breakthrough of the drilling speed in the southern edge.

Structure and tectonic evolution of the southwestern Trinidad dome, Escambray complex, Central Cuba: Insights into deformation in an accretionary wedge

The Trinidad dome, Escambray complex, Cuba, forms part of an accretionary wedge built during intra-oceanic subduction in the Caribbean from the Late Cretaceous to Cenozoic. The structure reflects syn-subduction exhumation during thickening of the wedge, followed by extension. Field mapping, metamorphic and structural analysis constrain the tectonic evolution into five stages. Three ductile deformation events (D1, D2 and D3) are related to metamorphism in a compressional setting and formation of several nappes. D1 subduction fabrics are only preserved as relict S1 foliation and rootless isoclinal folds strongly overprinted by the main S2 foliation. The S2 foliation is parallel to sheared serpentinised lenses that define tectonic contacts, suggesting thrust stacks and underthrusting at mantle depths. Thrusting caused an inverted metamorphic structure with higher-grade on top of lower-grade nappes. Exhumation started during D2 when the units were incorporated into the growing accretionary wedge along NNE-directed thrust faults and was accompanied by substantial decompression and cooling. Folding and thrusting continued during D3 and marks the transition from ductile to brittle-ductile conditions at shallower crustal levels. The D4–5 events are related to extension and contributed to the final exhumation (likely as a core complex). D4 is associated with a regional spaced S4 cleavage, late open folds, and numerous extension veins, whereas D5 is recorded by normal and strike-slip faults affecting all nappes. The P-t path shows rapid exhumation during D2 and slower rates during D3 when the units were progressively incorporated into the accretionary prism. The domal shape formed in response to tectonic denudation assisted by normal faulting and erosion at the surface during the final stages of structural development. These results support tectonic models of SW subduction of the Proto-Caribbean crust under the Caribbean plate during the latest Cretaceous and provide insights into the tectonic evolution of accretionary wedges in an intra-arc setting.

Tectonic inheritance and kinematic strain localization as trigger for the formation of the Helvetic nappes, Switzerland

The Helvetic nappes in Switzerland consist of sediments, which have been sheared off and thrust over the crystalline basement of the European passive continental margin during Alpine orogeny. Their basal shear zones usually root above the external crystalline massifs. However, the mechanisms that initiated the shear zones and the associated nappe formation are still debated. We perform two-dimensional numerical simulations of the shearing of linear viscous fluids above a linear viscous fluid with considerably higher viscosity (quasi-undeformable). The boundary between the fluid, mimicking the sediments, and the quasi-undeformable fluid, mimicking the basement, exhibits geometrical perturbations, mimicking half-grabens. These geometrical perturbations can trigger significant strain localization and the formation of shear zones within the linear viscous fluid although no rheological softening mechanism is active. This kinematic, ductile strain localization is caused by the half-grabens and the viscosity ratio between basement and sediments. The viscosity ratio has a strong control on the kinematics of strain localization, whereas the depth of the half-grabens has a weak control. For sediment viscosities in the order of 1021 Pas and typical half-graben geometries of 5 km depth and 25 km width the localization generates (a) low-angle shear zones at the basement-sediment interface, but also entirely within the sediments, (b) horizontal transport >10 km associated with the shear zones, (c) shear zones with thickness in the order of 100 m, (d) an ordered stacking of model nappes and (e) shear zones that root above the basement. The results suggest that tectonic inheritance in the form of half-grabens and associated kinematic strain localization could have been the triggering mechanism for Helvetic nappe formation, and not rheological softening mechanisms, which might, however, have subsequently further intensified shear localization significantly.

A coupled phase–field and volume-of-fluid method for accurate representation of limiting water wave deformation

A coupled phase–field and volume-of-fluid method is developed to study the sensitive behavior of water waves during breaking. The THINC model is employed to solve the volume-of-fluid function over the entire domain covered by a relatively coarse grid while the phase–field model based on Allen–Cahn equation is applied over the fine grid. A special algorithm that takes into account the sharpness of the diffuse–interface is introduced to correlate the order parameter obtained on the fine grid and the volume-of-fluid function obtained on the coarse grid. The coupled model is then applied to the study of water waves generated by moving pressures on the free surface. The deformation process of the wave crest during the initial stage of breaking is discussed in details. It is shown that there is a significant variation of the free nappe developed at the front side of the wave crest as the wave steepness differs. It is of a plunging type at large wave steepness while of a spilling type at small wave steepness. The numerical results also indicate that breaking occurs later and the duration of breaking is shorter for waves of smaller steepness and vice versa. Neglecting the capillary effect leads to wave breaking with a sharper nappe and a more dynamic plunging process. The surface tension also has an effect to prevent the formation of a free nappe at the front side of the wave crest in some cases.

3-D numerical models of viscous flow applied to fold nappes and the Rawil depression in the Helvetic nappe system (western Switzerland)

The Helvetic nappe system exhibits three-dimensional (3-D) features such as the lateral variation in geometry between the Morcles and Doldenhorn fold nappes or the Rawil depression. We perform 3-D finite element simulations of linear and power-law viscous flow to investigate fold nappe formation during shortening of a half graben with laterally varying thickness. 3-D ellipsoids and corresponding 2-D intersection ellipses are used to quantify finite strain. Fold nappes which formed above a thicker graben have (i) larger amplitudes, (ii) a less sheared and thinned overturned limb, and (iii) a larger thickness than fold nappes formed above a thinner graben. These results agree with observations for the Morcles and Doldenhorn nappes. We also perform 3-D simulations for a tectonic scenario suggested for the evolution of the Rawil depression. The basement is shortened and extended laterally and includes a graben which is oblique to the shortening direction and acts as mechanical weak zone. The graben causes laterally varying basement uplift generating a depression whose amplitude depends on the graben orientation and the stress exponent of basement and sediments. The axial plunge of the depression is smaller (approximately 10°) than the observed plunge (approximately 30°) indicating that additional processes are required to explain the geometry of the Rawil depression.

Petrology of Late Jurassic allochthonous ultramafic lamprophyres within the Batain Nappes, Northeastern Oman

ABSTRACTLate Jurassic ultramafic lamprophyre (UML) sills and dikes occur as 3 km-long intrusions within the allochthonous Whara Formation of the Batain nappes, eastern Oman. The sills and dikes comprise macrocrystic phlogopite and spinel-bearing aillikite and damtjernite. Aillikite is a light grey, massive fine-grained tuffaceous rock with euhedral laths of mica, while damtjernite is a dark grey, medium- to coarse-grained rock with abundant pelletal lapilli and globular segregationary textures. Both lithologies are composed of calcite, phlogopite, apatite, magnetite, spinel, diopside, and richterite. Orthoclase occurs only within damtjernite. The rocks are strongly silica undersaturated (17.6–33.7 wt.% SiO2), with low MgO (4.7–10.2 wt. %) and high Al2O3 (3.5–8.6 wt.%). The aillikites are distinguished from the damtjernites by their lower SiO2, Al2O3, and Na2O abundances, and their higher MgO, CaO, and P2O5 contents. The rare earth element (REE) patterns of both rock types are similar and show strong light REE (LREE) enrichment. Both are enriched in Ba, Th, U, Nb, and Ta, with normalized concentrations of up to 1000 times those of primitive mantle. Relative depletions are apparent for high REE (HREE), K, Rb, Pb, Sr, P, Zr, and Hf. The rocks have initial 87Sr/86Sr ratios of 0.70435–0.70646, whereas initial 143Nd/144Nd ratios vary between 0 · 512603 and 0 · 512716 (εNdi 2.6–3.2). Pb isotopic ratios are more varied among the aillikites and damtjernites: 208Pb/204Pbi = 38.97–39.39 and 207Pb/204Pbi = 15.35–15.58, 206Pb/204Pbi = 18.08–18.96. The abundance of phlogopite, apatite, and rutile and enrichment in LREEs, Ba, Th, U, Nb, and Ta in the Sal UMLs suggest metasomatic enrichment of these rocks following a low degree of partial melting of a depleted source region. Ar–Ar age dating of phlogopite macrocrysts from the aillikites and damtjernites (154 and162 Ma, respectively) correlates with large-scale tectonic events recorded in the proto-Indian Ocean at 140–160 Ma.