Compressional Tectonics Research Articles

To the features of the mechanism of ore formation

To date, there is no doubt that in the Paleozoic the crust of the Urals was formed in a setting of tangential compression as a result of manifestations of several tectonic cycles, each of which was due to the thrust of the oceanic crust on the continental base. As a result, the geochemical interaction of the compositions of volcanogenic-sedimentary formations, which evolved in accordance with the increase in tectonic compression from minimum to maximum values, was determined. Significantly increased tectonic loads lead to squeezing of eruption channels, cessation of magmatic activity, widespread development of flysch and olistostromal strata, massive thrusting and digging of previously formed formations, and the appearance of anatectic granite intrusions. The first half of the tectonic cycle, as a rule, evolutionary, led to the development of igneous rocks — the main carriers of ore components in a dispersed state. The second half is deformation, in addition to the listed processes, it contributed to the concentration of useful components in one way or another in the deposit. In accordance with the model of the formation of the earth’s crust, the epochs of ore formation are simultaneous with the thrusting and are genetically determined by it.

Structural Analysis of Kirthar Fold Belt, Lower Indus Basin, Balochistan, Pakistan; Implications from Compression and Inversion Tectonics

The current research work mainly focuses on tectonic evolution of the Central Kirthar Fold Belt. It also deals with the structural impact of the Late Cretaceous/Paleocene, Indian-Eurasian collision on the Central Kirthar Fold Belt which is situated in the western shear zone of Indian Plate. The Kirthar Fold Belt is different from the rest of western shear zone because it is mainly deformed by compressional tectonics which has created wide anticlines, narrow synclines and overturned folds as shown by geological mapping. It has been observed that thin-skinned deformation has played a major role in structural disharmony which is mainly seen in the Early Eocene Ghazij and Oligocene Nari formations. Seismic data shows thick-skinned deformation because the faults seem to penetrate deeper stratigraphic levels. Involvement of the basal decollement and the penetration of the interpreted faults to the level of basement rocks is also evident from the exposure of Jurassic/Cretaceous rocks at surface on the western part of the mapped area. Cross sections data shows that Kirthar Thrust Fault has a structural throw around 3-4 km in the Kirthar Folded Zone featuring deformational front while the seismic data and well data shows that this intensity decreases in the Kirthar Foredeep Zone towards east where the structural throw is about 2-3 km. Restoration of cross sections show an overall 17.3% shortening which shows that the area is under influence of compressional stresses.

Geochemical features and seismic imaging of the tectonic zone between the Tibetan Plateau and Ordos Block, central northern China

The Tibetan Plateau is growing by both vertical uplift and horizontal extension. It is a continuing debate how the Tibetan Plateau interacts with its surrounding plates and blocks. Due to intense tectonic activity, which produced catastrophic earthquakes, the tectonic zone between the northeast margin of the horizontal extending Tibetan Plateau and the stable Ordos Block has garnered considerable interest. This study investigated the spatial distribution of gas geochemical anomalies (e.g., high flux of CO2 in correspondence of the main faults) at regional scale together with the seismic tomography in correspondence of this tectonic zone with the aim to figure out the domain of convergent boundary between the Ordos block and Tibetan plateau, and trace the tectonic discontinuities which are able to transfer fluids through the crustal layers between the two main geological units. From northwest to southeast, obvious difference of spatial distributions of geochemical and geophysical features in the tectonic zone between the northeast margin of the Tibetan Plateau and the Ordos Block is inferred. The northeast area (Zone A) is dominated by thrust and strike-slip faults with clear velocity boundary underneath, where low crack density (ε), saturation rate (ξ) and Poisson’ ratio (σ) in the middle-lower crust coincided with the low values of heat flow and CO2 emissions, tectonic compression and regional locked-fault can be inducements. The southeast area (Zone C) is dominated by extensional tectonics with roughly E-W fast-velocity direction (FVD) of P-wave azimuthal anisotropy, where high permeability and porosity can be deduced from crustal high ε, ξ and relatively high σ anomalies, resulting in high heat flow, CO2 concentrations and fluxes at the surface, and predominantly crustal-derived gases. The intermediate area (Zone B) also dominated by thrust and strike-slip faults is an extraordinary zone, where intensely locked-fault were clearly revealed, while the predominant anisotropic FVDs in the middle crust changed obviously, more contribution of shallow gas component was detected, and CO2 flux, heat flow, and regional ε, ξ, and σ in the upper crust were higher, compared with those in Zone A, which indicated the regional crushing fragmentation underneath Zone B. The adopted multidisciplinary approach demonstrated that Zone B is the convergent boundary between the Tibetan Plateau and the Ordos Block.

The Cretaceous of the Cameroon Atlantic Basin (Central Africa): sediment provenance, correlation, paleoenvironment and paleogeographic evolution of the Eastern Proto-Atlantic margin (Central Gondwana)

An integration of geochemistry, mineralogical and palynological data is used to depict the sediment provenance and distribution, the sedimentary environment and the paleogeographic evolution of the Cretaceous Atlantic Basin of Cameroon. The basin is located along the African western coast, in the heart of the Guinea Gulf, and includes the Campo, Douala, and Rio-del-Rey sub-basins from South to North. The sediments of the Campo sub-basin are derived from rocks of intermediate composition, those of the Douala sub-basin from intermediate to mafic rocks in the southern and felsic rocks in the northern parts, and those of the Rio-del-Rey sub-basin from felsic rocks with a contribution of rocks of intermediate composition. The paleoweathering index and Index of Chemical Variability values for Campo, Douala and Rio-del-Rey sub-basins range from 71.26 to 76.88 and 0.98 to 2.12, 67.15 to 99.39 and 0.37 to 0.90, and 73.17 to 92.90 and 0.44 to 10.03, respectively, indicating sub-maturity, high maturity and sub- to high maturity deposits. Al2O3 vs. V and Al2O3 vs. P2O5 plots, and palynologic data such as marine (e.g. dinoflagellates and microforaminifera) and continental (e.g. Classopollis sp., Ephedripites sp., Botryococcus sp.) species suggest a deposition in more or less deep lacustrine (Campo sub-basin), fluvio-deltaic to shallow marine (Douala sub-basin), and marginal to shallow marine (Rio-del-Rey sub-basin) environments. The paleoweathering index values, the clay minerals (kaolinite, illite and chlorite/vermiculite) distributions, and the palynological data point to arid to semi-arid climate in the Campo sub-basin and semi-arid to periodically humid conditions in the Douala and Rio-del-Rey sub-basins. The tectonic setting diagram suggests collision tectonic events that are inconsistent with the classic geological history of the Atlantic Basin. The situation is in fact related to Precambrian events that affected the basement rocks from which the studied sediments were derived, and probably to the compressional tectonics during the evolution of the South Atlantic Ocean. Although these sub-basins share the same geological history, there are some differences, such as the degree of sediment maturity, the diagenetic effects, the lithology of rock sources, the paleoenvironments of deposition and their fluctuation, depending on the moment the tectonic of Atlantic opening reached them (northward progression of the W-E extension) and the northern shearing along the southern area of the Benue through axis (Rio-del-Rey). The geological history of the Cameroon Atlantic Basin is similar to those of Gabonese and Nigerian basins, along the African western coast, and of those of the Brazilian basins along the eastern coast (Santos Basin to Pernambuco-Paraiba Basin).

Timing and Origin of Compressional Tectonism in Mare Tranquillitatis

AbstractThe lithosphere of the Moon has been deformed by tectonic processes for at least 4 billion years, resulting in a variety of tectonic surface features. Extensional large lunar graben formed during an early phase of net thermal expansion before 3.6 Ga. With the emplacement of mare basalts at ∼3.9–4.0 Ga, faulting and folding of the mare basalts initiated, and wrinkle ridges formed. Lunar wrinkle ridges exclusively occur within the lunar Maria and are thought to be the result of superisostatic loading by dense mare basalts. Since 3.6 Ga, the Moon is in a thermal state of net contraction, which led to the global formation of small lobate thrust faults called lobate scarps. Hence, lunar tectonism recorded changes in the global and regional stress fields and is therefore an important archive for the thermal evolution of the Moon. Here, we mapped tectonic features in the non‐mascon basin Mare Tranquillitatis and classified these features according to their respective erosional states. This classification aims to provide new insights into the timing of lunar tectonism and the associated stress fields. We found a wide time range of tectonic activity, ranging from ancient to recent (3.8 Ga to <50 Ma). Early wrinkle ridge formation seems to be closely related to subsidence and flexure. For the recent and ongoing growth of wrinkle ridges and lobate scarps, global contraction with a combination of recession stresses and diurnal tidal stresses, as well as with a combination of South Pole‐Aitken ejecta loading and true polar wander are likely.

Tectonic fluid expulsion: U-Pb evidence for punctuated hydrothermal fluid flow and hydraulic fracturing during orogenesis

Tectonic fluid expulsion has been linked to fluid flow in permeable foreland basin sequences, but it is unclear how fluids migrate through underlying low-permeability successions. Here we report U-Pb data which indicate that fluids expelled during tectonic compression migrated through older compacted shales inboard of the deforming craton margin by fracture permeability and enhanced diffusion. From a ∼350 m drill-core interval of Neoarchean shale (drill-hole ABDP9), Hamersley region, Australia, we document abundant bedding-parallel veins indicating pervasive hydraulic fracturing. Thermometry of vein-filling chlorite indicates temperatures between 329-374°C, which is higher than estimates (175-280°C) from metamorphic assemblages in underlying basalts, implying the passage of hydrothermal fluids. New in situ secondary ion mass spectrometry (SIMS) U-Pb geochronology of monazite and xenotime in bedding-parallel veins verifies and expands earlier results, showing that microfracturing and hydrothermal fluid flow spanned ∼650 million years and was restricted to five short intervals: 2.30 Ga, 2.20 Ga, 2.10 Ga, 2.05 Ga and 1.65 Ga. Three of the five age populations coincide with known fabric-forming orogenic events (i.e. 2.30 Ga Sylvania orogeny; 2.22-2.15 Ga Ophthalmia orogeny; 1.68-1.65 Ga Mangaroon orogeny), which are focused along the southern Pilbara Craton margin. Our results show that the timing of veining was not random, but coeval with major orogenic events, linking fluid overpressure and hydraulic fracturing to the expulsion of heated orogenic fluids 100s of km inboard of the craton margin. The crack-seal veins indicate repeated cycles of fluid overpressure resulting in hydrofracturing and fluid drainage, followed by cavity collapse. The tectonic expulsion of pressurized hydrothermal fluids produced fracture and matrix permeability in Earth's upper crust, leading to episodic open-system behaviour with enhanced fluid-rock reactions and the large-scale transfer of heat and mass.

A comparison of hydrothermal events and petroleum migration between Ediacaran and lower Cambrian carbonates, Central Sichuan Basin

Hydrothermal minerals and associated organic matter have been frequently found in the Ediacaran and lower Cambrian carbonate reservoirs, separated by internal shale source rocks, in the Central Sichuan Basin. This work provides a thorough comparison of petrography, geochemical signatures, fluid inclusions and ages between the hydrothermal minerals in the Ediacaran Dengying Formation (Fm) and the lower Cambrian Longwangmiao Formation. This comparison leads to a better understanding of the behaviour and migration history of hydrothermal fluids and petroleum. During the Ediacaran–Cambrian transition, the flux of silicon-rich hydrothermal fluid triggered by seafloor-related volcanic events, resulted in the exclusive occurrence of silicification in the Dengying Fm. During P3–T3, two different hydrothermal systems were found in the Dengying and Longwangmiao formations. The hydrothermal fluids in the Dengying reservoirs were dominated by a mixture of basement-derived Mississippi Valley Type [MVT] ore fluids and shale-derived oil-bearing brines, while those in the Longwangmiao Fm were mainly the oil-bearing brines. The upwards migration of the MVT ore fluids, characterized by very hot, saline, radioactive and Eu-rich conditions, was promoted by the activity of the Emeishan mantle plume 259.4–248 Ma and then blocked by the lower Cambrian shales and trapped only within the underlying Dengying reservoirs. The expulsion of the warm, less saline and moderately radioactive brines and oil was induced by thermal maturation of the organic-rich shales, which might have occurred over a longer duration (261–218.7 Ma). During 170.7–125.8 Ma (J2–K2), the combined effects of the overmature source rocks and Yanshanian tectonic compression facilitated the migration of much hotter shale-derived brines, as well as gas hydrocarbons, across the Ediacaran to lower Cambrian carbonate reservoirs. This comparative study provides some insights into the behaviour of hydrothermal fluids and petroleum in carbonates with internal organic-rich shales.

Tectonic Factors Restricting Differences among Layers in Microstructure of Ultradeep Clastic Rock Reservoir: A Case Study on Cretaceous Bashijiqike Formation in Kuqa Depression

Abstract The Cretaceous Bashijiqike Formation in the Kelasu structural belt of Kuqa Depression is an ultradeep reservoir with burial depth of more than 6 km. Due to the influence of strong thrust nappe from the South Tianshan in the north, the burial depth gradually deepened from north to south in the Kelasu tectonic belt. With the increase of the burial depth, the influence of tectonic compression relatively weakens, and the formation mechanism that affects the microstructure of ultradeep clastic reservoir is changed. Comprehensive analysis of thin section and quantitative evaluation of minerals by scanning electron microscopy (QEMSCAN), scanning electron microscope (SEM), and micro-CT scanning are used to investigate the characteristics of the Cretaceous Bashijiqike Formation at the depth of 8000 m (TVD) in various stress sections. The result shows that, for the ultradeep reservoir with burial depth up to 8000 m, its reservoir characteristics and interlayer differences are not completely restricted by the neutral surface effect. (1) The porosity and permeability of various stress sections are different, and the reservoir physical properties of the tensile section are higher than that in the compressive section. (2) The pore types, pore radius, and pore throat connectivity of various stress sections are different, and the development of pores and throats in the tensile section is better than that in the compressive section. (3) As an ultradeep clastic rock reservoir, the vertical fracture development characteristics of Well Bozi9 at the depth of 8000 m differ from that in Kela-Keshen area at the depth of 6000 m in the model of strain neutral surface. The interlayer difference in the microstructure of ultradeep clastic rock reservoir is clarified, which has a positive effect to evaluate and predict the distribution of favorable deep reservoir.

Plumbing the depths of magma crystallization using 176Lu/177Hf in zircon as a pressure proxy

AbstractExtensional tectonics are marked by shallow magma crystallization depths, whereas compressional tectonics are associated with deeper crystallization depths. This implies that variations in crystallization depths can be used to track changes in Earth’s dominant tectonic regimes through time. We therefore developed a new “pressure of crystallization” proxy based on the variation of the 176Lu/177Hf ratio in zircon. This ratio is controlled by zircon fractionation and residual garnet, and it can be used to monitor the evolution of a crystallizing magma ascending within the crust. The secular evolution of the 176Lu/177Hf ratio in zircon is characterized by cyclical oscillations that are broadly in tune with the δ18O record in zircon and with periods of continental collision and supercontinent amalgamation. The apparent mean depth of crystallization of zircon-bearing igneous rocks has decreased with time over the last ~3.0 b.y. This can be linked to shallowing of the primary crystallization depths and/or to the effect of time-integrated erosion in the geologic record. Prior to ca. 3.0 Ga, crystallization depth maxima and oscillations in apparent depth are less clear, perhaps suggesting that the nature of tectonic interactions was different in the Mesoarchean and earlier.

Overpressure origins and evolution in deep-buried strata: A case study of the Jurassic Formation, central Junggar Basin, western China

Overpressure is significant to the exploration and exploitation of petroleum due to its influence on hydrocarbon accumulation and drilling strategies. The deep-burial hydrocarbon reservoirs of Jurassic strata in the central Junggar Basin are characterized by intensive overpressure, whose origins are complex and still unclear. In this study, Bowers’ method and sonic velocity-density crossplot method based on well logging data were used as a combination for overpressure judgements in geophysics. Furthermore, the corresponding geological processes were analysed in quality and quantity to provide a rational comprehension of the overpressure origins and the model of overpressure evolution and hydrocarbon accumulation processes. The results showed that hydrocarbon generation in the Jurassic source rocks led to overpressure in the mudstones, while hydrocarbon generation in Permian source rocks led to overpressure in the sandstone reservoirs in Jurassic strata by vertical pressure transfer. The burial and thermal history indicated that the aquathermal effect of pore fluids by temperature increase in deep strata is also an important origin of overpressure, while disequilibrium compaction may not be the dominant cause for the overpressure in deep-buried strata. Furthermore, the continuous tectonic compression in both the north–south and west-east trends from the Jurassic period to the present may also have enhanced the overpressure in deep strata. Meanwhile, the developed faults formed by intensive tectonic compression led to pressure transfer from source rocks to the Jurassic reservoirs. Overpressured geofluids with hydrocarbons migrated to sandstone reservoirs and aggravated the overpressure in the Jurassic strata. To conclude, the intensive overpressure in the central Junggar Basin is attributed to the combination of multiple mechanisms, including hydrocarbon generation, the aquathermal effect, tectonic compression and pressure transfer. Furthermore, the developed overpressure indicated hydrocarbon migration and accumulation processes and the potential of oil and gas reservoirs in deeply buried strata. We hope this study will provide a systematic research concept for overpressure origin analysis and provide guidance for petroleum exploration and exploitation in deep-buried strata.

Compressional tectonics and volcanism: the Miocene-Quaternary evolution of the Western Cordillera (24–26°S), Central Andes

Compressional tectonics and volcanism: the Miocene-Quaternary evolution of the Western Cordillera (24–26°S), Central Andes

Thrust Faults Bound an Elevated Mantle Plug Beneath Several Lunar Basins

AbstractThe lunar maria are large expanses of basalt that infill antecedent impact basins and show evidence for postemplacement deformation. Landforms within many of these basins suggest a period of compressive tectonics, although their formation mechanism has yet to be established. Previous work for Mare Crisium demonstrated that basin‐circumferential wrinkle ridges, which typically demarcate the inner edge of an annulus of elevated terrain, are the result of deep‐seated thrust faults that preferentially form along the boundary of an elevated superisostatic portion of mantle and a thick, subisostatic collar of crustal material. Here, we show that a similar fault architecture exists for several other mascon‐bearing basins, including Maria Serenitatis, Nectaris, Moscoviense, and, to a lesser degree, Humorum and Imbrium. These deeply penetrating basin‐circumferential thrust faults, as for Mare Crisium, form a (partial) outward‐dipping ring‐fault system that bounds the elevated mantle plug beneath each basin as a geometric consequence of mascon evolution. If this geometric arrangement is unique to the Moon, then some characteristic(s) of lunar mascon evolution enables the formation of such mascon‐bounding faults. Despite the ubiquitous nature of mascon‐bound thrust ring faults at several lunar basins, whether such structures exist at mascon basins on other terrestrial worlds remains an open question.

Variations of Pore Structure and Methane Adsorption of Continental Deformed Shales from Small-Scale Anticline and Syncline: Two Cases Study of the Triassic Yanchang Formation, Ordos Basin and Jurassic Yaojie Formation, Minhe Basin.

In order to investigate the effect of tectonic compression on pore structures and methane adsorption capacity, the continental deformed shales are collected from the Rujigou section in the Ordos Basin and the Hongshawan section in the Minhe Basin as research objects. The porosity, N2 and CO2 adsorption, field emission scanning electron microscopy (FE-SEM), and methane isothermal adsorption experiments are used to investigate the pore structure and methane adsorption of continuous deformation shales. Combined with formation curvature, the quantitative correlations between tectonic compression deformation and reservoir structure parameters and methane adsorption are analyzed. The interlayer pore, intergranular pore, dissolution pore, and microfracture are developed, and the amounts of microfractures are obviously higher in anticlinal cores than that of the anticlinal wings. The porosities for the Rujigou and Hongshawan sections are 2.1-4.73% (avg 3.21%) and 1.49-9.05% (avg 4.80%), respectively. The corresponding mesopore volume, specific surface area, and average diameter for the Rujigou section are 0.022-0.038 cm3/g, 12.35-14.30 m2/g, and 8.76-11.21 nm and for the Hongshawan section are 0.0033-0.0052 cm3/g, 0.376-0.875 m2/g and 21.61-36.37 nm, respectively. The corresponding micropore specific surface area and pore volume for the Rujigou section are 11.301-16.068 m2/g and 0.0033-0.0051 cm3/g and for the Hongshawan section are 10.951-15.912 m2/g and 0.0043-0.0058 cm3/g, respectively. The methane adsorption capacities for the Rujigou and Hongshawan sections are 2.76-3.82 mg/g and 1.62-2.135 mg/g, respectively. The porosity, mesopore and micropore structure parameters, and methane adsorption capacity in synclinal core are lower than those in synclinal wings. However, the above tendency is reversed for the anticline, whose adsorption capacity in the anticlinal core is higher than that in anticlinal wings. There exist high correlations between formation curvature and pore structure, physical property, and methane adsorption, indicating that formation curvature has a significant effect on the shale reservoir. By combining theoretical maximum adsorbed gas and free gas results, it implies that the anticlinal core undergoing moderate structural compression deformation can provide more methane adsorption sites and reservoir space, which is the shale gas favorable accumulation area. This study could provide beneficial references for continental shale gas exploration and evaluation in complex structure areas.

Cenozoic thermal-rheological evolution of the coastal cathaysia block of East Asia: geodynamic implications

ABSTRACT The thermal-rheological structure of the continental lithosphere is an important indicator of geodynamic processes. Tectonic compression and extension may change thermal-rheological structure of lithosphere. However, the thermal-rheological evolution over geological time may be difficult to determine because of overprints of subsequent geological events. Mantle xenolith temperature and pressure conditions can differentiate the lithospheric thermal structure over geological time and indicate the lithospheric rheological strength. Chronological and temperature-pressure condition data from 11 mantle xenoliths in Cenozoic basalts from the Wuyishan terrane (WYT) and coastal terrane (CT) on both sides of the Zhenghe-Dapu Fault (ZDF) in the Cathaysia Block indicate that these terranes have experienced distinct thermal-rheological evolution since the Eocene. Moreover, the thermal-rheological evolution of the CT and WTY were differently influenced by the West Pacific tectonic domain, respectively, because the ZDF has been relatively active since 23 Ma and thus has partly accommodated and thus reduced the tectonic impact of Eurasia-Pacific convergence. Therefore, we classified the thermal-rheological structural evolution of the Cathaysia Block into three stages (35.5–20 Ma, 20–6.5 Ma, and 6.5–0 Ma) that sequentially recorded the opening and closing dynamics of the northeastern margin of the South China Sea and uplift of Taiwan Island on the eastern margin of Eurasia.

Overpressure and gas charging in tight sandstone: Xujiahe Formation, northeastern Sichuan Basin

Overpressure is a key factor for oil and gas charging in tight reservoirs, but it is still a challenge to evaluate the overpressure evolution and its control on oil and gas charging. Taking Xujiahe Formation in the northeastern Sichuan Basin as an example, this paper presented a method for evaluating overpressure and its effect on natural gas charging in tight sandstone in compressional basin. The abnormally high pressure and its causes were analyzed by measured data and logging evaluation. Theoretical calculation and PVT simulation were used to investigate the amounts of overpressure resulted from hydrocarbon generation and tectonic compression, respectively. Then the source rock - reservoir pressure differences were calculated and the characteristics of natural gas charging during the natural gas charging periods were analyzed. It was revealed that hydrocarbon generation and tectonic compression were the main causes of the overpressure. The overpressure of both source rocks and reservoir exhibited a gradually increasing trend from Middle Jurassic to Early Cretaceous (J2-K1), then decreased since Later Cretaceous (K2), and some of that preserved to now. The contributions of the hydrocarbon generation and tectonic compression to overpressure were different in different periods. The residual pressure difference between the source rocks and the reservoir is the major driving force for tight sandstone gas charging. The main hydrocarbon generating area of the source rocks and the area of high driving force were major natural gas enrichment areas, and the driving force determined the natural gas charging space in the pore throat system of the reservoir. This research helps evaluate the overpressure and pressure difference between source rocks and reservoir in compressed basin, as well as investigate the effective pore throat space of tight gas charging by the driver of overpressure.

Application of anisotropy of magnetic susceptibility to construct the palaeocurrent direction using slender fossil orientations

Oriented samples were collected in the southern Mongolia Plateau and north‐eastern Tibetan Plateau to detect links between microstructural features, which are inferred from magnetic fabrics, and outcrop‐scale sedimentary structures that include slender fossil orientations. Palaeocurrent imprints in particles and their recognition in magnetic fabrics depend on the kinetics of current and any post‐depositional reworking. Here, we restored the flooding palaeocurrent direction through the statistical orientation of slender fossils and imprints indicated by magnetic fabric patterns in the sediments. We found that the tilt response of the minimum susceptibility axes preserves more links to the dynamic orientation of fossil deposits, while the maximum susceptibility axes may be easily reworked by a weak but long‐term tectonic compression. Comparing the two methods reveals the correlation between fossil distribution and magnetic fabric. The dynamic identification of anisotropy of magnetic susceptibility on large quantitative samples contributes to provide an alternative way for further statistics of fine‐grained sediments and fabric arrangement.

Strato-structural evolution of the deep-water Orange Basin: constraints from 3D reflection seismic data

Abstract. Deep-water fold-and-thrust belt (DWFTB) systems are gravity-driven collapse structures often found in passive margin settings and are comprised of a linked up-dip extensional domain, central transitional/translational domain, and down-dip compressional domain. Many Late Cretaceous DWFTB systems occur along the SW African passive margin with multiple, over-pressurized, seaward-dipping shale detachment surfaces accommodating gravitational slip. In this study we use 3D reflection seismic data to constrain the strato-structural evolution of the translational and compressional domains of a Late Cretaceous DWFTB system and the overlying Cenozoic deposits in the Orange Basin, South Africa. The stratigraphy and structure of the Late Cretaceous DWFTB system is shown to have controlled fundamental sedimentary processes and the stability of the evolving margin. The compressional domain exhibits large-scale landward-dipping DWFTBs with thrust faults detaching the main Turonian shale detachment surface at depth and terminating at the early Campanian surface. A major ∼ 7 km wide seafloor slump scar reflecting margin instability occurs directly above a syncline of the same width from the buried DWFTB system's compressional domain. The translational domain is imaged as a complex region displaying overprinted features of both extensional and compressional tectonics with the downslope translation of sediment comprising listric normal and then thrust and oblique-slip faults distally. Thrust sheets are segmented along strike by extensive oblique-slip faults which extend from the translational domain into the down-dip compressional domain. Smaller, localized fold-and-thrust belts are found directly below the kilometre-scale DWFTB system in the down-dip compressional domain detaching a lower, Albian shale detachment surface which corresponds to an older gravitational collapse. The upward propagation of normal and oblique-slip faults with progressive sedimentation is hindered by the Oligocene or Miocene stratigraphic markers corresponding to mass erosional processes in the Cenozoic. A large (∼ 2.3 km wide), roughly slope-perpendicular Oligocene submarine canyon formed by turbidity currents is attributed to a major sea-level fall at ∼ 30 Ma. Oceanographic circulation is shown to have held a significant control on the deposition of mid-Miocene to present-day sedimentary sequences. Between 1200 to 1500 m water depths along the upper continental slope well-preserved extensive slope-parallel, sinusoidal channel-like features occur on the Miocene stratigraphic marker. The channels are confined within a ∼ 14 km wide zone at the interface of the upper northward-flowing Antarctic Intermediate Water (AAIW) and deeper southward-flowing North Atlantic Deep Water (NADW) currents. The erosive interaction of these oppositely flowing bottom currents combined with the effects of the Benguela Upwelling System (BUS), all of which formed or intensified at ∼ 11 Ma, are responsible for the creation and preservation of the extensive slope-parallel channels. This study shows the difference in structural styles of the translational and compressional domains of a Late Cretaceous DWFTB system and the processes responsible for mass-scale erosion in the Cenozoic.

The Geomorphological and Geological Structure of the Samaria Gorge, Crete, Greece—Geological Models Comprehensive Review and the Link with the Geomorphological Evolution

The Samaria Gorge is a dominant geomorphological and geological structure on Crete Island and it is one of the national parks established in Greece. Due to the complex tectonics and the stratigraphic ambiguities imprinted in the geological formations of the area, a comprehensive review of the geological models referring to the geological evolution of the area is essential in order to clarify its geomorphological evolution. In particular, the study area is geologically structured by the Gigilos formation, the Plattenkalk series and the Trypali unit. Regarding lithology, the Gigilos formation predominantly includes phyllites and slates, while the Plattenkalk series and the Trypali unit are mainly structured by metacarbonate rocks; the Plattenkalk series metacarbonate rocks include cherts, while the corresponding ones of the Trypali unit do not. Furthermore, the wider region was subjected to compressional tectonics, resulting in folding occurrences and intense faulting, accompanied by high dip angles of the formations, causing similar differentiations in the relief. Significant lithological differentiations are documented among them, which are further analyzed in relation to stratigraphy, the tectonics, and the erosion rate that changes, due to differentiations of the lithological composition. In addition, the existing hydrological conditions are decisive for further geomorphological evolution.

Geochronological and geochemical constraints on magmatic evolution and mineralization of the northeast Ke’eryin pluton and the newly discovered Jiada pegmatite-type lithium deposit, Western China

The Ke’eryin ore field in the Songpan-Garze Fold Belt (SGFB), western China, is an important area of pegmatite-type lithium accumulation. However, the petrogenesis of the pegmatites and their genetic relationships with the neighboring granitic pluton are still a topic of debate. In this work, LA-MC-ICP-MS zircon and columbite-tantalite U-Pb dating, whole-rock and mica geochemical analysis, and lithium isotopic analysis of the Ke’eryin pluton and pegmatites in the newly discovered Jiada deposit were performed. Zircon U-Pb dating of biotite K-feldspar granite (BG), fine-grained two-mica monzogranite (FTG), and medium-grained two-mica monzogranite (MTG) of the Ke’eryin composite pluton yielded crystallization ages of 219.5 ± 0.7 Ma, 212.8 ± 1.1 Ma, and 209.1 ± 0.7 Ma, respectively. Columbite-tantalite U-Pb dating of albite-spodumene type pegmatite (ASP) from the Jiada deposit yielded crystallization ages of 204.7 ± 1.0 Ma. The results show that the lithium mineralization in the Jiada deposit was temporally closest to the MTG emplacement. Combined with the regional magmatic evolution sequence, it is suggested that the Ke’eryin composite pluton and the Jiada deposit were emplaced during the period of transition from tectonic compression to extension under the effect of lithospheric delamination during post-collision. Geochemically, the compositional variation in the major elements of mica minerals indicate that BG, FTG, and MTG of the Ke’eryin composite pluton and the AP (albite type pegmatite) and ASP in the Jiada deposit were formed via the same continuous magmatic evolution process. The increasing Li, Rb, Cs, Nb, Ta concentrations and decreasing K/Rb, K/Cs, Nb/Ta ratios in mica minerals show that the degree of magmatic differentiation increased in the order BG-FTG-MTG-AP-ASP, indicating that MTG could be the parent rock of the Jiada pegmatites. The increase in the δ7Li values in muscovites from MTG through AP to ASP provides new evidence to support the fact that the fractional crystallization played a significant role in the formation of pegmatites in the Jiada deposit. Alkali metal elements (e.g., K, Rb, Cs) in the muscovites of MTG, AP, and ASP followed the Rayleigh fractionation law, indicating that a Rayleigh-type fractionation process occurred. Rayleigh fractional crystallization modelling of these alkali metal elements showed that the MTG could produce a magma melt of ASP through extreme fractional crystallization (∼98–99 % fractionation). Taken together with associated geological evidence, the aforementioned features indicate that the favorable tectonic setting and the prolonged and extreme fractional crystallization of the granitic melt jointly contributed to the formation of the Jiada pegmatite-type lithium deposit.

Mineralization of MVT Pb-Zn Deposits in the Process of Hydrocarbon Accumulation and Destruction in the Strong Structural Deformation Area of Eastern Sichuan, South China

The Mississippi Valley-type (MVT) Pb-Zn deposits and hydrocarbon reservoirs coexist around the margin of the eastern Sichuan basin. This study examined the fluid inclusions, Sr isotope and systematic Rb-Sr, Sm-Nd geochronology for the distinct ore and gangue minerals of four orebodies from two MVT Pb-Zn deposits in the margin of the eastern Sichuan basin, combined with the existing research foundation of oil and gas accumulation and evolution, which was designed to understand the internal relationship between oil and gas accumulation and the involvement of organic matter in metal mineralization. High-density methane, moderate temperature, and salinity inclusions were discovered in the studied MVT Pb-Zn deposits, combined with relatively higher 87Sr/86Sr ratios (0.71088~0.714749), indicating that the ore-forming fluids were derived largely from the hydrocarbon associated basinal brines. Rb-Sr isochron of paragenetic sphalerites and pyrites and Sm-Nd isochron of paragenetic fluorites and calcite from the studied MVT Pb-Zn deposits define isochron ages of 144.5 Ma~138.5 Ma, and 147.6 Ma, respectively, indicating that the Late Jurassic to Early Cretaceous Yanshanian orogeny was an important metallogenic event in the margin of the eastern Sichuan basin. The close temporal and spatial relationship between the MVT mineralization and hydrocarbon accumulation and destruction in the strong structural deformation area of the margin of the eastern Sichuan basin allows us to propose a possible model in which the Yanshanian compressional tectonics drove a large-scale flow of metal- and sulfate- bearing basinal fluids passed under or through methane-bearing carbonate reservoirs, resulting in the MVT mineralization and hydrocarbon accumulation and destruction.