Structural Features and Evolution of the Northwestern Sichuan Basin: Insights From Discrete Numerical Simulations

The northwestern Sichuan Basin has experienced Meso-Cenozoic intracontinental compressional tectonic processes and formed multi-detachment stratigraphic distribution of foreland basins and fold-thrust belts, which have caused complicated structural deformations in the deep buried layers. Rapid uplift with accelerated erosion and two sets of detachments in the Lower Triassic and Lower Cambrian controlled the multilevel deformation structure. We conducted discrete numerical simulations with double weak detachments and erosion under extrusion conditions in order to examine the mechanics and kinematics of the frontalpiedmont zones of the NW Sichuan Basin. The following findings were made. (1) With continuous compression, the weak detachments promoted the decoupled and ladder-like deformation of the thrust belt, where the deformation above the slip layer extended further than it did below it. Rapid uplift and erosion at the thrust front contributed to the formation of a passive roof fault and a monocline in the upper layer, a series of forward and backward thin-skinned thrust-buried structures in the middle layer sandwiched between two weak detachments and stacking structures in the lower layer. (2) Erosion effectively prevents the deformation from propagating above the upper detachment, but can advance a horizontal transition in the deformation style generated within the middle brittle layer: from oblique and tight fault propagation folds to symmetrical, wide, and gentle detachment folds. (3) The model results consistent with tectonic deformation in the NW Sichuan Basin indicate a possible evolutionary mechanism under compression. There is hierarchical deformation of uncoordinated contraction controlled by the Lower Triassic and Early Cambrian weak layers, with the characteristics of the shallow monocline, the middle thin-skinned thrusts, and the deeper basement-involved folds. Continuous compression contributed a sequential pattern of steps as a whole, from the frontalpiedmont zones to the foreland basin, autochthonous stacking thrusts, and the huge buried structure in the NW Sichuan Basin. During the Himalayan period, syntectonic erosion along with the uplifted thrust front maintained the development of a passive-roof duplex and a huge forward buried structure.

Reconstructing the Ediacaran to Ordovician history of the Baltoscandian Margin of continent Baltica

<p>In the Scandes, the lower thrust sheets of the Caledonian allochthons provide unambiguous stratigraphic evidence of correlation with the successions of the Baltoscandian platform. Cambrian successions, including the Alum Shale Formation, providing the footwall for the main Caledonian decollement in Scandinavia, can be followed at least 200 km westwards from the thrust front into the hinterland of the orogen. The overlying early Palaeozoic strata provide evidence of facies changes into foreland basin deposits in the mid Ordovician and early Silurian; also of Ediacaran and Cryogenian successions, including Marinoan tillites. The amount of internal shortening in the Lower Allochthon is not uncontroversial, but certainly amounts to more than 100 km, implying that all the overlying alllochthons in the Scandes were derived from west of the Norwegian coast.</p><p>The metamorphic grade of the units in the Lower Allochthon increases from low to high greenschist facies, from the thrust front westwards into the deep hinterland. Overlying thrust sheets of the Middle Allochthon are of higher metamorphic grade and more ductilely deformed. The basal parts are usually dominated by basement-derived units and Neoproterozoic sedimentary rocks. They are overthrust by dolerite dyke-intruded thrust sheets, the Särv Nappes, with host-rocks dominated by Cryogenian and Ediacaran sandstones, the former including subordinate limestones and Marinoan tillites. The Baltoscandian margin dolerite dyke swarms amount to up to c. 35% of these thrust sheets.</p><p>The overlying, highest tectonic units in the Middle Allochthon (the Seve Nappe Complex, SNC) are of amphibolite and higher metamorphic grade. They include a greater variety of lithologies, including some that are very similar to those in the underlying Särv Nappes (e,g. quartzites and eclogitized dolerites). The metasedimentay host rocks include a wide range of paragneisses and marbles. Abundant mafic rocks include metamorphosed gabbros, basalts and peridotites and, together with the dyke swarms, can totally dominate the composition of some thrust sheets. The similar geochemistry and early Ediacaran age (c. 600 Ma) of the mafic rocks in the Särv and Seve nappes define the Baltoscandian outermost margin and continent-ocean transition zone (COT). Iapetus Ocean terranes comprise the overlying thrust sheets of the Upper Allochthon (e.g. the Köli Nappe Complex).</p><p>The metamorphism of the different thrust sheets in the SNC provide clear evidence that some parts were subducted; others not. A wide range of isotope age data constrain the timing of subduction, with the earliest ages in the mid Cambrian (c. 505 Ma) to early Ordovician (c. 483 Ma). It has been suggested that the deposition of the Alum Shale Formation on the Baltscandian platform, was related to this early Caledonian subduction. A more probable interpretation is that subduction along the outermost edge of this highly extended COT did not influence the edge of the platform till the early Tremadoc.</p><p>Some authors have introduced cryptic sutures into the Baltoscandian outer margin, described above. They should reassess their data and better define the evidence for their conviction.</p>

3D geometry and kinematics of the Northern Variscan Thrust Front in Northern France: new insights based on reprocessing and interpretation of seismic reflection profiles.

<p>In NW Europe, the Upper Carboniferous Variscan collision between Avalonia and the Armorica-Gondwana accretion complex led to the progressive tectonic inversion of the southern Avalonian margin and the development of a crustal-scale north-vergent thrust system propagating outward from the Late Mississippian to the Middle Pennsylvanian (330-305 Ma). The northern Variscan thrust front spreads over 2,000 km across NW Europe. In the Nord-Pas-de-Calais (NPC) coal district area (northern France), its 3D geometry and kinematics have been investigated through the reprocessing and interpretation of 532 km in length of industrial seismic reflection profiles acquired in the 1980s. The seismic interpretations point out the major compressional and extensional tectonic features affecting this fossil, deeply eroded, mountain front, highlighting its very atypical structure and kinematics.</p><p>The deformation front is characterized by a main frontal thrust zone localizing most of the northward displacement (i.e. several tens of kilometers) of the Ardennes Allochthonous Unit above the slightly-deformed part of the Avalonian margin, referred to as the Brabant Para-autochthonous Unit. This large displacement induced the underthrusting of the molassic foreland basin (NPC coal basin) over nearly 20 km and was associated to the out-of-sequence dislocation of the mountain front. The underthrust Brabant Para-autochthonous Unit, made of both the Namurian-Westphalian (330-305 Ma) molassic foreland basin and the underlying Mid-Upper Devonian (390-360 Ma) and Dinantian (360-330 Ma) carbonate platform, is deformed by a series of second-order north-vergent thrust faults, often associated with ramp-related folds. These thrust faults are rooted in décollement zones located either at the transition between the Namurian shales and the Dinantian carbonates or in the Famennian shales.</p><p>The 3D integration of the seismic interpretations led to the characterization of a major lateral ramp oriented NW-SE, affecting both the main frontal thrust zone and the basal thrust of some Overturned Thrust Sheets developed at its footwall. This lateral ramp represents a major zone of relay along the thrust front, in between two major segments, oriented respectively ENE-WSW to the east and WNW-ESE to the west. At the base of the underthrust Brabant Para-autochthonous Unit, the Mid-Upper Devonian platform is shown to be structured by synsedimentary normal faults responsible for the southward deepening and thickening of the southern Avalonian margin. These faults are oriented along two main directions i.e. N060-080° and N110-130°, that is the general orientation of the future Variscan structures. Overall, the results indicate that the Devonian pre-structuration of the southern Avalonian margin exerted a primary control on the dynamics and segmentation of the Northern Variscan Front in northern France by localizing both the frontal and lateral ramps within the thrust wedge.</p>

STRATIGRAPHIC AND STRUCTURAL RELATIONSHIPS THROUGH GEOLOGICAL AND GAMMAESPECTROMETRY ANALYSIS IN THE INTERFACE BETWEEN THE SÃO FRANCISCO CRATON AND THE ARAÇUAÍ OROGEN (SANTA RITA ANTICLINE, MINAS GERAIS, BRAZIL)

Geological mapping (1:25.000 scale), supported by gammaespectrometric analysis, was conducted in the Santa Rita Anticline region, a regional structure located in the contact between the Araçuaí Orogen external belt and the São Francisco Craton. The work aimed to detail the region's structural and stratigraphic relationships between the rocks of Espinhaço and São Francisco supergroups, which contact is not characterized by a thrust front, as it is in a large part of the Espinhaço Meridional Range (EMR). Five units from Espinhaço were mapped: Galho do Miguel Formation and Conselheiro Mata Group, represented by the Santa Rita, Córrego dos Borges, Córrego Bandeira and Córrego Pereira formations. By its turn, three units from the São Francisco Supergroup were mapped: undivided Macaúbas Group and Bambuí Group, represented by the Serra de Santa Helena and Lagoa do Jacaré formations. Restricted to the Espinhaço units, the Santa Rita Anticline consists in a structure with a south dipping axis and inflections that form saddle structures. The analysis of the gammaespectrometric responses provided clues that were confirmed in the field, highlighting an angular unconformity between the Córrego Pereira Formation and the Macaúbas Group and the local presence of isoclinal folds in Serra de Santa Helena rocks, promoting relevant questions regarding possible intermediary deformational events between the Statherian and Tonian tafrogenesis. The study contributes by raising questions about the evolution of the Araçuaí Belt in its contact with the São Francisco Craton and possible implications imposed by the Paramirim Keywords: Aerogeophysics, Litho-structural framework, Fold belt and craton transition, Espinhaço Meridional Range

Tectonic inversion of the Dnieper-Donets basin. Part 1. Collizion tectonics of the Western-Donets graben

Formulation of the problem. New results of tectonophysical studies of the structural-kinematic evolution of the Earth's crust of Dnieper-Donets paleorift at the collision stage are presented in the paper. The subject of research is a complex of deformation structures that complicate the sedimentary cover in the transitional zone with Donets folded structure. Review of previous publications and studies. According to new mapping data, tectonic deformations of the sedimentary cover were controlled by systems of faults of the north, northwest, and southeast vergence. The lattices of the Hercynian, Lamaric, and Attic tectonites determine the specific “cross-thrust” structure of pushing. Due to the heterogeneous distribution of deformations of different ages, the stratigraphic volume of the sedimentary sequence varies in area from the Pliocene-Anthropogenic to the Mesozoic-Cenozoic. Overthrusts and linear folding of three generations permeate the sedimentary sequence of the transition zone from east to west for hundreds of kilometers within the eastern part of Izyumsky paleorift segment. Methods. Using the original method of tectonic deformation fields’ reconstruction and tectonophysics analysis of structures, collision deformations of the platform sedimentary cover of the southeastern part of Dnieper-Donets depression are studied. The materials of geological mapping of the transition zone and instrumental definitions of the vergence of the Hercynian, Laramian, and Attic faults constituted the analytical base of tectonophysical studies. Results. In the transition zone, West-Donets segment of the tectonic wedging, which completely destroys the riftogenic structure in the southeast of the basin was identified. It was formed as a result of a thrust on the syneclise autochthon repeatedly deformed, crumpled into the folds of geomass from the western slopes of Donets folded structure. It consists of structural ensembles of linear thrust folds, tectonic covers of transverse, pushing of sedimentary geomas from axial to onboard zones, and folded covers of longitudinal thrust. The thrust covers and folds of allochthon end in the depression with scaly compression fans formed at the ends of the main thrusts. In the thrust front of West Donets segment, a geodynamic discharge strip was diagnosed, where structural zones of displacement of geomass were formed. On the north-eastern flank of the thrust front they are represented by large linear anticlinal zones of the so-called “open Paleozoic structures”, such as Torsko-Drobishevsk, North-Donets, Matrossko-Toshkovsk zones, and on the southwestern - Petrovsky-Novotroitsk zone of reverse-folds. The structural peak of the West-Donets segment is the tectonic node of thrusts and strike-slips of three generations, localized in the joint area of the central and southern structural branches of the axial salt-dome folding. Scientific novelty and practical significance. The current regional tectonic scheme has been revised on the conceptual foundations of dynamic geotectonics. It is shown that the riftogenic structure of the transition zone at the collision stage of evolution was completely destroyed by deformations of three generations. Lattice of tectonites, thrust cover and uplift folding zones of the Hercynian, Laramian, and Attic generations together form West-Donets cover-fold region within it. The main tectonic element of the region is the segment of the tectonic wedging of the same name. By the nature of the structure in its northern part, Lugansk-Kamyshevakhsky district is distinguished by a link echelon of linear folding, and in the south by Kalmius-Toretsky region of tectonic thrust cover. West Donets segment is an important component of the structural-tectonic frame of the sedimentary cover, which determines the modern transverse tectonic segmentation of the paleorift.

Control of increased sedimentation on orogenic fold-and-thrust belt structure – insights into the evolution of the Western Alps

We use two-dimensional thermomechanical models to investigate the potential role of rapid filling of foreland basins in the development of orogenic foreland fold-and-thrust belts. We focus on the extensively studied example of the Western European Alps, where a sudden increase in foreland sedimentation rate during the mid-Oligocene is well documented. Our model results indicate that such an increase in sedimentation rate will temporarily disrupt the formation of an otherwise regular, outward-propagating basement thrust-sheet sequence. The frontal basement thrust active at the time of a sudden increase in sedimentation rate remains active for a longer time and accommodates more shortening than the previous thrusts. As the propagation of deformation into the foreland fold-and-thrust belt is strongly connected to basement deformation, this transient phase appears as a period of slow migration of the distal edge of foreland deformation. The predicted pattern of foreland-basin and basement thrust-front propagation is strikingly similar to that observed in the North Alpine Foreland Basin and provides an explanation for the coeval mid-Oligocene filling of the Swiss Molasse Basin, due to increased sediment input from the Alpine orogen, and a marked decrease in thrust-front propagation rate. We also compare our results to predictions from critical-taper theory, and we conclude that they are broadly consistent even though critical-taper theory cannot be used to predict the timing and location of the formation of new basement thrusts when sedimentation is included. The evolution scenario explored here is common in orogenic foreland basins; hence, our results have broad implications for orogenic belts other than the Western Alps.

Afghanistan-US Relations

Afghanistan has twice been thrust front and center of US national security concerns in the past half-century: first, during the Soviet-Afghan War, when Afghanistan served as a proxy for American efforts to combat Soviet influence; and second, as the frontline state and host for America’s global response to al-Qaida’s terrorist attacks of 2001. In both instances, American involvement swung from intensive investment and engagement to withdrawal and neglect. In both cases, American involvement reflected US concerns more than Afghan realities. And both episodes resulted in short-term successes for American security with long-term consequences for Afghanistan and its people. The signing of a strategic partnership agreement between the two countries in 2012 and a bilateral security agreement in 2013 created the possibility of a steadier and more forward-looking relationship—albeit one that the American and Afghan people may be less inclined to pursue as America’s longest war continues to grind on.