Tectonic Style Research Articles

Neoarchean structural evolution of the Murchison Domain (Yilgarn Craton)

The progressive strengthening of the continental lithosphere triggered a secular transition in tectonic style during Neoarchean times, allowing the progressive establishment of Plate Tectonics on our planet. Structural investigations in Neoarchean cratons offer therefore a critical tool to constrain how such transition took place. The highly-mineralized Yilgarn Craton of Western Australia exposes one of the largest and best preserved portions of Archean continental crust worldwide. The Yilgarn crust was intensely deformed during the c. 2730–2650 Ma Neoarchean Yilgarn Orogeny, which was accompanied by widespread granitic magmatism, and was mostly unaffected by post-Archean pervasive tectonic events, representing therefore an ideal area for the study of Neoarchean tectono-magmatic events. Here, I combine new structural data with a re-examination of the recent regional structural literature, to offer an updated view of the structural evolution of the Murchison Domain, one of the best-exposed and mineralized portions of the Yilgarn Craton. The result is a new fourfold scheme for the structural evolution of the domain, supported by a detailed and structurally-controlled geochronological dataset. The preserved tectono-magmatic evolution of the domain started with a pre-orogenic period of dominant polydiapirism, during times of prevailing tectonic quiescence. This was followed by a sequence of orogenic deformation events that produced north-striking large-scale shear zones, along which syntectonic plutons, up to 100 km in size, were emplaced during three major tectonic pulses characterized by bulk east–west shortening, between 2730 and 2660 Ma. The deformation events identified here at the scale of the Murchison Domain are likely the local expression of Yilgarn-wide tectono-magmatic events, highlighting the importance of syntectonic granitic magmatism in shaping Neoarchean orogens.

Plate tectonics in relation to mantle temperatures and metamorphic properties

When plate tectonics emerged and how it has evolved over Earth history are two of the most fundamental challenges in Earth Sciences. These questions are tackled using a holistic approach to analyze tectonic styles in the history of Earth, giving rise to the interpretation of two styles of plate tectonics since the Archean. In these interpretations, there are different styles of deformation and metamorphism between early times dominated by warm subduction, and later times preferring cold subduction. The two styles of plate tectonics are recorded by different properties of regional metamorphism at convergent plate boundaries, which are linked to the differences in mantle temperature between the Archean and Phanerozoic. A transition to modern plate tectonics is recorded by the signature of blueschist facies metamorphism developed in the Neoproterozoic. This is consistent with geological evidence for the operation of ancient plate tectonics since the early Archean. The temporal cooling of the mantle explains the geochemical trends of mantle-derived melts, the likely change from numerous small plates to fewer but larger plates, changes in thickness and preservation of oceanic crust and lithosphere in accretionary and collisional orogens, and led to the oxygenation of the surface environment providing the environments needed to foster life.

The Mountain Front Flexure in the Lurestan region of the Zagros belt: Crustal architecture and role of structural inheritances

The Mountain Front Flexure is a major structure of the Zagros orogenic system, and is underlain by the deeply rooted and seismically active Mountain Front Fault system. These coupled structural features divide the belt from its foreland and their trace is sinuous, forming salients and recesses. The origin and tectonic significance of the Mountain Front Fault system and its sinuosity are still unclear, with most of hypotheses pointing to a strong structural control exerted by geological inheritances. In this work we combine interpretation of seismic reflection profiles, earthquake data, geomorphic analysis, and geological observations, to build a balanced cross section across the Mountain Front Flexure in the Lurestan region. Our data are suggestive of a hybrid tectonic style for the Lurestan region, characterised by a major and newly developed crustal ramp in the frontal portion of the belt (i.e the Mountain Front Fault) and by the reactivation of steeply dipping pre-existing basin-bounding faults, along with a minor amount of shortening, in the inner area. Specifically, the integration of our results with previous knowledge indicates that the Mountain Front Fault system developed ahead of an array of inverted Jurassic extensional faults, in a structural fashion which resembles that of a crustal-scale footwall shortcut. Within this structural context, the sinusoidal shape of the Mountain Front Flexure in the Lurestan area arises from the re-use of the original segmentation of the inverted Jurassic rift system.

Regional-scale polydiapirism predating the Neoarchean Yilgarn Orogeny

Gneiss domes occur in a wide variety of orogenic and anorogenic domains of the continental crust, and play a major role in lithosphere dynamics by allowing upwards transfer of heat and mass. They commonly contain a core of granites and/or migmatites, overlain by a mantle of lower-grade rocks. Evolutionary models of many gneiss domes are controversial.Here we use new structural and zircon isotopic data to unravel the tectono-magmatic evolution of the Yalgoo dome and surroundings, at the margins of the Neoarchean Yilgarn Orogen, Western Australia. The study area includes at least seven granite–migmatite domes (5–70 km in average diameter), and several subdomes within the larger domes, all showing comparable structural features and age. In each dome, a concentric domal foliation is concordant with granite–greenstone contacts, bearing a radial, outward-plunging lineation. The bulk of the structures in each dome reflect magmatic flow, with pervasive subsolidus fabrics occurring only along the outer dome margins, and reflecting dome-up kinematics. Narrow greenstone keels pinched between domes and subdomes display vertical constriction, in an area regionally dominated by flattening along the steep sides of the deeply-eroded domes, and along the flat-laying dome roofs.The regional structural pattern and the kinematics of the high-strain zones are best explained by sequential emplacement of nested diapirs, which delivered large volumes of granitic magma in a c. 20 Myr time-span. The dome-and-keel regional architecture and the internal structures of each dome resulted therefore from multiscale polydiapirism. These diapirs were later weakly overprinted by a tectonic fabric that developed during the Neoarchean Yilgarn Orogeny. The contrast between the Yalgoo region and the rest of the craton, which was strongly reworked by this orogeny, highlights the dichotomy of tectonic styles in Archean terranes, demonstrating that diapirism dominated in times of tectonic quiescence.

Polyphase Zircon Growth during Slow Cooling from Ultrahigh Temperature: an Example from the Archean Pikwitonei Granulite Domain

AbstractQuantifying the timescales of Archean ultrahigh temperature (UHT) metamorphism is essential for constraining the style of plate tectonics on the early Earth. However, such timescales can be difficult to quantify, due to the antiquity of Archean rocks and the extreme thermal conditions of UHT metamorphism. We constrain the timescales of Archean UHT metamorphic processes recorded by a single rock sample from the Pikwitonei granulite domain (northwestern Superior Province), through the integration of two U–Pb zircon petrochronologic techniques. In this study we combine: (1) high-spatial resolution laser ablation split-stream inductively coupled mass spectrometry (LASS) on in situ zircon (in thin section) and hand-picked zircon; and (2) high-precision isotope dilution thermal ionization mass spectrometry (ID-TIMS) analyses on microsampled fragments from the same hand-picked zircon analysed by LASS. Phase equilibria modelling and Zr-in-rutile thermometry suggest the rock followed a P–T path characterized by decompression at > 960 °C, followed by near-isobaric cooling at ∼0·8 GPa. In situ LASS zircon analyses could be interpreted to record zircon growth at broadly ∼2665 Ma, though the large uncertainties on isotopic dates make potentially distinct growth episodes difficult to distinguish. ID-TIMS U–Pb dates of zircon fragments reveal a polyphase zircon growth history over a 24 Ma duration, from 2673 to 2649 Ma. Zircon trace element compositions, textures, and microstructural relationships, as well as evaluation of zircon-garnet equilibrium, suggest zircon grew during melt crystallization, after UHT decompression and garnet resorption. Variable Ti concentrations within zircon domains indicate: (1) zircon crystallized through the temperature interval of ∼875 °C to ∼730 °C, potentially in isolated rock domains with variable zircon saturation temperature; and/or (2) zircon crystallized over a narrower temperature interval in isolated rock domains with variable aTiO2 and/or aSiO2. Collectively, the data suggest the west-central Pikwitonei granulite domain reached peak UHT conditions prior to 2673 Ma, after which suprasolidus conditions in the lower crust persisted for at least 24 Ma. Such an interpretation would be impossible if based on either the LASS or ID-TIMS zircon data alone, which highlights the utility of applying both techniques in tandem to constrain metamorphic timescales in ancient UHT terranes.

Does stress transmission in forelands depend on structural style? Distinctive stress magnitudes during Sevier thin‐skinned and Laramide thick‐skinned layer‐parallel shortening in the Bighorn Basin (USA) revealed by stylolite and calcite twinning paleopiezometry

AbstractThe Sheep Mountain‐Little Sheep Mountain Anticlines, Bighorn Basin (USA) formed as basement‐cored Laramide structures in the formerly undeformed foreland of the thin‐skinned Sevier orogen. We take advantage of the well‐constrained microstructural network there to reconstruct differential stress magnitudes that prevailed during both Sevier and Laramide layer‐parallel shortening (LPS), before the onset of large‐scale folding. Differential stress magnitudes determined from tectonic stylolites are compared and combined to previous stress estimates from calcite twinning paleopiezometry in the same formations. During stress loading related to LPS, differential stress magnitudes transmitted from the distant Sevier thin‐skinned orogen into the sedimentary cover of the Bighorn basin (11–43 MPa) are higher than the differential stress magnitudes accompanying the early stage of LPS related to the thick‐skinned Laramide deformation (2–19 MPa). This study illustrates that the tectonic style of an orogen affects the transmission of early orogenic stress into the stable continental interior.

Analogue modeling of the northern Longmen Shan thrust belt (eastern margin of the Tibetan plateau) and strain analysis based on Particle Image Velocimetry

In response to the multistage convergences across the western margin of the Sichuan Basin, a complex thrust belt developed along the Longmen Shan thrust belt (LTB). Compressional analogue modeling with double weak detachments is carried out to shed light on the development and kinematics of the triangle zone at the front of the northern LTB. (1) Two weak detachments create an essential configuration for the development of duplexes. Each detachment displays different mechanical behaviors during the evolution of the thrust belt and decouples deformation at different depths. (2) Regular erosion at the thrust front can effectively localize strain and promote the development of the back-thrust along the upper detachment to generate a sustained passive-roof duplex in the front. The similarity of the tectonic deformation between the analogue modeling and northern LTB implies a possible evolutionary mechanism of the Shuangyushi Structure. The Middle Triassic and early Cambrian weak layers act as two pivotal detachments, decoupling the tectonic styles at different depths. During the Indosinian and Yanshanian period, continuous compression contributed to the establishment of the main tectonic framework of a passive-roof duplex. In the following Himalayan period, the eastward compression complicated the middle structural layer and uplifted the thrust front, and was accompanied by regular erosion, contributing to the development of a passive-roof duplex.

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

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

Dawn of metazoans: to what extent was this influenced by the onset of “modern-type plate tectonics”?

The appearance of complex megascopic multicellular eukaryotes in the Ediacaran occurred just when the dynamics of a cooling Earth allowed establishment of a new style of global tectonics that continues to the present as “modern-type plate tectonics”. The advent of this style was first registered in 620 Ma-old coesite-bearing Ultra-High Pressure eclogites within the Transbrasiliano-Kandi mega-shear zone along the site of the West Gondwana Orogeny (WGO). These eclogites comprise the oldest evidence of slab-pull deep subduction capable of inducing continental collisions and producing high-relief Himalayan-type mega-mountains. Life, prior to this time, was essentially microscopic. Yet with increasing Neoproterozoic oxygenation and intensified influx of nutrients to Ediacaran oceans, resulting from the erosion of these mountains, complex macroscopic heterotrophic eukaryotes arose and diversified, taking the biosphere to a new evolutionary threshold. The repeated elevation of Himalayan-type mega-mountains ever since then has continued to play a fundamental role in nutrient supply and biosphere evolution. Other authors have alluded to the influence of Gondwana mountain-building upon Ediacaran evolution, however we claim here to have identified when and where it began.

Crustal growth of the Eastern Dharwar Craton: a Neoarchean collisional orogeny?

Abstract The Eastern Dharwar Craton (EDC) is predominantly made of Neoarchean potassic granitoids with subordinate linear greenstone belts. Available geochemical and isotopic systematics of these granitoids suggest variations in the source and petrogenetic mechanisms. By compiling the available geochemical data, these granitoids can be classified into four groups, namely: TTGs (tonalite–trondhjemite–granodiorite); sanukitoids; biotite and two-mica granites; and hybrid granites. This classification scheme is in line with the global classification of Neoarchean granites, and enables the sources and petrogenetic mechanisms of these variants to be distinguished. Available geochemical, isotopic and geochronological datasets of these granitoids are integrated and the existing tectonic models for the Neoarchean EDC are reviewed. The variability of the EDC granitoids is ascribed to crustal reworking associated with the collision of two continental blocks. The tectonomagmatic evolution of the EDC is analogous to the development of the Himalayan Orogeny. Based on the evolutionary history of the Dharwar Craton, it can be concluded that convergent margin tectonics were operational in the Indian Shield from at least c. 3.3 Ga and continued into the Phanerozoic. However, the nature and style of plate tectonics could be different with time.

КОЛІЗІЙНІ ДЕФОРМАЦІЇ ДНІПРОВСЬКО-ДОНЕЦЬКОЇ ЗАПАДИНИ Стаття 1. Тектоніка зони зчленування з Донецькою складчастою спорудою

The article is the first part of a trilogy devoted to the study of post-rift deformations of the riftogenic structure of the Dnieper-Donets paleorift. The mechanisms of collision warping of the horizons of the sedimentary cover of the southeastern part of the Dnieper-Donets depression are considered. According to the previous mapping data, the tectonic deformations of the sedimentary cover were controlled by systems of faults of the north, north-west, and south-east vergence. The lattices of tectonites of the Hercynian, Lamaric, and Attic generations determine the specific “cross-thrust” structure of pushing. 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. The analytical base of the research was the materials of geological mapping of the zone of the junction of the depression with the Donets fold structure. Using field definitions of the tectonite vergency of the Hercynian, Laramide and Attic phases of tectogenesis, the original method of reconstruction of tectonic deformation fields and tectonophysics analysis of structures, collision deformations of the sedimentary cover of the south-eastern part of the Dnieper-Donets paleorift are studied. The tectonophysical analysis of tectonites of different ages indicates that together they control the cover-thrust and folded deformations of the riftogenic structure. Overthrusts and linear reverse-folding of three generations form the West-Donetsk integumentary-folding region, within which a segment of the same name tectonic thrust is distinguished. By pushing the system of repeatedly deformed, crushed into folds of geomass sedimentary rocks on weakly deployed syneclise deposits, the riftogenic structure of the south-eastern part of the basin is completely destroyed. The structural-tectonic framework of the allochthone, pushed from the side of the Donets structure, is composed of dynamically conjugated lattices of Hercynian, Laramide, and Attic tectonites. They control the echelon backstage of linear reverse-folds, tectonic plate-covers of transverse extrusion of sedimentary geomass from axial to airborne zones and folded covers of longitudinal thrust from the south-east. The riftogenic structure of the transition zone between the Dnieper-Donets basin and the Donets folded structure was completely destroyed by deformations of three generations of platform activation. The dynamically coupled tectonite lattice, the overlays, and the folded zones of the Hercynian, Laramide, and Attic generations jointly form the West-Donets fold-fold region within its boundaries. The main tectonic element of the area is the eponymous subregional tectonic thrust segment. The central structural zone is Veliko-Kamyshevakhskaya, Novotroitskaya, Druzhkovsko-Konstantinovskaya and Main anticlines. The central zone divides the body of the segment into two tectonic regions according to the tectonic style and intensity of deformation of the sedimentary sequence. The northern part is occupied by the Luhansk-Kamyshevakhsky region of the rocky-layered linear folding of the thrust, and the southern part is the Kalmius-Toretsky region of scaly tectonic covers.

Geometry and development of a hybrid thrust belt in an inner forearc setting: Insights from the Potrerillos Belt in the Central Andes, northern Chile

Thick- and thin-skinned “hybrid” thrust belts are typically developed in areas where crustal shortening was preceded by extension and normal faulting. They have been recognized in many places within the Central Andean backarc regions of Perú, Argentina, and Bolivia. In contrast, these structures have not been broadly studied in forearc regions. To understand the geometry and kinematics of this style of tectonism in a forearc setting related to Andean-type subduction zones, we used field and industrial 2D seismic data to study the Potrerillos thrust and fold belt and the Salar de Pedernales Basin in northern Chile (26°S). Our results indicate that the structure in this region consists of a dominantly east-verging contracting system composed of large basement thrusts, Mesozoic inverted normal faults, and shallow thrust-related folds. Large basement thrust ramps and imbricated wedges represent the most prominent basement structures, which accommodated major crustal shortening. Reactivated and tectonically inverted Mesozoic extensional structures also constitute an important structural component, especially beneath the Salar de Pedernales Basin. These exhibit asymmetrical inverted anticlines with arrowhead shapes on the hanging walls of the inverted normal faults. The subsidiary shallow thrust-related folds commonly display fault bend and propagation folds that primarily accommodated the shortening of the Mesozoic and Cenozoic stratified deposits overlying the basement thrusts and cored anticlines. The age of the contraction in this region is poorly constrained; however, many of the oldest sequences accumulated in the top of the syn-rift sequences and related to synorogenic deposits, can be correlated with those identified in neighboring regions (e.g., Salar de Atacama, Salar de Punta Negra basins and Frontal Cordillera, among others), which have reported Upper Cretaceous ages. Based on this stratigraphic correlation we suggest that orogenesis may have begun in the Late Cretaceous with basin inversion, and then continued during the Cenozoic with basement thrusting.

Large landslides and deep-seated gravitational slope deformations in the Czech Flysch Carpathians: New LiDAR-based inventory

Moderate-relief landscapes, such as the Czech Flysch Outer Western Carpathians (COWC), in comparison with alpine regions are rarely subject to extensive landslide inventory mapping. An understanding of landslides in such a landscape is needed, because densely populated hilly landscapes in temperate zones are usually of major socio-economic importance. In this study, we performed the first LiDAR-based landslide mapping for the entire COWC area (~7539 km2), one of the most landslide-prone regions in Europe. By calculating various landscape and landslide metrics, we infer the distribution, frequency-area relationships, kinematics and controls of mass movements with special attention on large landslides (≥0.1 km2) and deep-seated gravitational slope deformations (DSGSDs). We mapped a total of 13,611 landslides, of which 1357 failures are large landslides and DSGSDs. Whereas the lower and more subdued areas in the southern part of COWC are hotspots in terms of the total number of landslides, the higher and more topographically pronounced areas in the northeast are affected predominantly by large landslides and DSGSDs. However, landslides with ≥0.1 km2 are widespread throughout the whole territory of COWC. A discrepancy also exists in the spatial distribution of different types of landslides. Rock slides and DSGSDs are dominant in the north-east, while flow-type landslides are dominant in the southern lower topographic relief with claystone-dominated flysch. We conclude that 1) distinct geological units (nappes) produce landslide populations with different frequency-area distributions; 2) stratigraphic composition alongside the tectonic style of flysch formations control the type of landslides; 3) DSGSDs affect mainly slopes formed by rigid rocks sitting atop soft formations; and 4) geological conditions, rather than topography, control distribution of large landslides and DSGSDs in COWC.

Metamorphism and the evolution of plate tectonics.

Earth's mantle convection, which facilitates planetary heat loss, is manifested at the surface as present-day plate tectonics1. When plate tectonics emerged and how it has evolved through time are two of the most fundamental and challenging questions in Earth science1-4. Metamorphic rocks-rocks that have experienced solid-state mineral transformations due to changes in pressure (P) and temperature (T)-record periods of burial, heating, exhumation and cooling that reflect the tectonic environments in which they formed5,6. Changes in the global distribution of metamorphic (P, T) conditions in the continental crust through time might therefore reflect the secular evolution of Earth's tectonic processes. On modern Earth, convergent plate margins are characterized by metamorphic rocks that show a bimodal distribution of apparent thermal gradients (temperature change with depth; parameterized here as metamorphic T/P) in the form of paired metamorphic belts5, which is attributed to metamorphism near (low T/P) and away from (high T/P) subduction zones5,6. Here we show that Earth's modern plate tectonic regime has developed gradually with secular cooling of the mantle since the Neoarchaean era, 2.5 billion years ago. We evaluate the emergence of bimodal metamorphism (as a proxy for secular change in plate tectonics) using a statistical evaluation of the distributions of metamorphic T/P through time. We find that the distribution of metamorphic T/P has gradually become wider and more distinctly bimodal from the Neoarchaean era to the present day, and the average metamorphic T/P has decreased since the Palaeoproterozoic era. Our results contrast with studies that inferred an abrupt transition in tectonic style in the Neoproterozoic era (about 0.7 billion years ago1,7,8) or that suggested that modern plate tectonics has operated since the Palaeoproterozoic era (about two billion years ago9-12) at the latest.

The nature of the Paleoproterozoic orogen in the Jacobina Range and adjacent areas, northern São Francisco Craton, Brazil, based on structural geology and gravimetric modeling

Defintion of the geometry and tectonic style of Paleoproterozoic mobile belts at the Paleoarchean cratonic margin of the Gavião Block brings new insights into the early evolution of the São Francisco Craton. This paper presents the regional structural framework and tectonic stratigraphic 3D models made from geological mapping and gravimetric modeling of the Jacobina Range. The Jacobina Range is composed of an Archean greenstone belt, Au-U detrital pyrite-bearing sedimentary rocks and Paleoproterozoic granite-gneissic terranes deformed between 2.1 and 1.9 Ga as the result of collision of the Archean Gavião, Serrinha, and Jequié blocks. Two deformational phases and three subdomains were described in the field area. The deformational phases include (i) a compressional phase with a westward tectonic convergence that built an imbricated fault system and (ii) a sinistral transpressional phase with a northwestward tectonic convergence, which forms a subvertical fault system with sinistral-oblique to lateral displacement. The southern subdomain is characterized by an imbricated tectonic system overprinted by transpressional faults. The central subdomain contains western-verging meso- to megascopic, slightly asymmetric folds, preserved as evidence of high strain. The northern subdomain is characterized as a fold and thrust system that was tilted by the intrusion of late granitic bodies. The modeling of two gravimetric sections identified deep structures that suggest sutures between the Gavião and Serrinha crustal blocks. Here, we propose a model for the Archean evolution of the Mundo Novo Greenstone Belt and the subsequent deposition of sediments in the Jacobina Basin, followed by intrusion of mafic-ultramafic sills and dikes and Paleoproterozoic deformation.

Deep structure of the Southern Apennines as imaged by active and passive seismic data along the CROP-04 (crustal) reflection seismic profile

We combine active and passive seismic data to reconstruct a coherent picture of the deep crust beneath the Southern Apennines orogen. The Southern Apennines collisional setting is characterized by deep structural complexities, which have been here investigated through different seismic data from both active and passive surveys. Several geological reconstructions have been carried out in the last two decades to interpret the detected substructures, but failed to provide a univocal interpretation of the Apennine roots. At present, strong disagreements exist among geologists about the tectonic style of the chain in that portion of the Italian peninsula. In this study, we first combine five passive seismic datasets from the overall available literature and we then project them at relative depths on a co-located active seismic profile (CROP-04). The general aim of the study is to shed light on the Southern Apennines structure at depths greater than 25 km, where, through the adopted approach, an approximate shape of the lower crust and upper mantle structure can be delineated. Our results indicate the following: (1) during the Adriatic slab retreat, the crust along the Tyrrhenian margin was only marginally thinned (Moho depth about 28–30 km), much less than along the Tyrrhenian coast of the Northern Apennines and Calabria (where Moho depth has been found at 20–25 km); (2) the subduction process has locally been stationary for a long time, leading to the complete eclogitization of the subducted crust and total regeneration of the Tyrrhenian Moho; (3) subduction fluids underwent a complete migration out of the mantle wedge, as testified by the high velocities found in the upper mantle below the Moho in the Tyrrhenian region (observed nowhere else in the Apennine subduction setting). Thematic collection: This article is part of the ‘Apennines and Tyrrhenian system’ collection available at: https://www.lyellcollection.org/cc/the-apennines-tyrrhenian-system

Plio-Quaternary transpressive tectonics: a key factor in the structural evolution of the outer Apennine–Adriatic system, Italy

We used data from seismic reflection profiles and well logs from both onshore and offshore locations along the Adriatic coast to investigate the structural style and recent active deformation in the external area of the northern Apennines adjoining the Po Plain. We prepared four NE–SW sub-parallel transects based on a detailed interpretation of the available subsurface data, supported by surface geological data. Through analysis of the well logs, we identified two significant seismo-stratigraphic markers, together with some unconformities within the Middle Pliocene/Quaternary succession. We propose a detailed structural scheme for the area that identifies and interprets both the outcropping and buried Apennine structures. NW/NNW–SE/SSE high-angle transpressive or flower structures, referred to the recent and actual deformation history, may be responsible for surface features and features at shallow depths, with possible implications for active tectonics. We show the continuity of some of these structures towards the Conero area to the south, suggesting a wider and more regional meaning for the proposed tectonic style. These Apenninic structures close against the active NNE–SSW Cattolica fault system in the northern offshore part of the area to form an asymmetrical arch. This study contributes to the interpretation of the deformation style of the outer northern Apennines. Supplementary material: High resolution seismic lines not interpreted are available at https://doi.org/10.6084/m9.figshare.c.4497542 Thematic collection: This article is part of the ‘Apennines and Tyrrhenian system’ collection available at: https://www.lyellcollection.org/cc/the-apennines-tyrrhenian-system

Spatial-temporal Distribution Characteristics of Global Seismic Clusters and Associated Spatial Factors

Earthquakes exhibit clear clustering on the earth. It is important to explore the spatial-temporal characteristics of seismicity clusters and their spatial heterogeneity. We analyze effects of plate space, tectonic style, and their interaction on characteristic of cluster. Based on data of earthquakes not less than moment magnitude (MW) 5.6 from 1960 to 2014, this study used the spatial-temporal scan method to identify earthquake clusters. The results indicate that seismic spatial-temporal clusters can be classified into two types based on duration: persistent clusters and burst clusters. Finally, we analysed the spatial heterogeneity of the two types. The main conclusions are as follows: 1) Ninety percent of the persistent clusters last for 22–38 yr and show a high clustering likelihood; ninety percent of the burst clusters last for 1–1.78 yr and show a high relative risk. 2) The persistent clusters are mainly distributed in interplate zones, especially along the western margin of the Pacific Ocean. The burst clusters are distributed in both intraplate and interplate zones, slightly concentrated in the India-Eurasia interaction zone. 3) For the persistent type, plate interaction plays an important role in the distribution of the clusters’ likelihood and relative risk. In addition, the tectonic style further enhances the spatial heterogeneity. 4) For the burst type, neither plate activity nor tectonic style has an obvious effect on the distribution of the clusters’ likelihood and relative risk. Nevertheless, interaction between these two spatial factors enhances the spatial heterogeneity, especially in terms of relative risk.

Role of rift structural inheritance in orogeny highlighted by the Western Pyrenees case-study

It is commonly accepted that many orogens form by contractional reactivation of earlier continental rifts or rifted margins. Therefore, to better understand orogenesis, it is important to also understand how rift domains and their associated structures are incorporated into orogens.We investigate the role of rift structural inheritance during orogeny using the Western Pyrenees as a case-study. To achieve our aim, we use a kinematic forward lithosphere deformation model (RIFTER) to produce flexural isostatically compensated as well as balanced cross-sections showing the structural and stratigraphic development of both the rift and orogenic stages of the Western Pyrenees. The cross-section produced extends from the Northern to the Southern Foreland Basins and crosses the Mauléon-Arzacq Basin.Our modelling results show how rift-domains and their faults are sequentially reactivated and incorporated into the present-day Western Pyrenees architecture. Based on the results from our case-study, we identify a sequence of tectonic stages separated by critical events that record the transition between different tectonic styles by which lithosphere is deformed. The pre-orogenic extensional stage is characterized by a hyper-extended rift system that eventually led to exhumed mantle. This constitutes the pre-orogenic template. The subsequent contractional tectonics consists of two stages: (i) the inversion of the hyper-extended rift system reactivating extensional structures and (ii) the crustal shortening of the southern proximal rift domain.Results of the Western Pyrenees case-study may be used to understand the development of other Alpine-type collisional systems involving the sequential reactivation and inversion of former hyper-extended rift systems.

The evolution of quartz veins during the tectonometamorphic development of the Brusque Metamorphic Complex, Brazil

Microstructural analysis and characterization of quartz veins hosted in rocks from the Brusque Metamorphic Complex in the region of Brusque, State of Santa Catarina, have been performed to establish the relations between fluid regimes, tectonic styles and deformation/recrystallization mechanisms. The analysis was based on observations of structural overprinting, spatial structural relations, the origin of the quartz veins and microtectonics studies. Five types of veins were identified: meter-long veins parallel to the regional foliation (V1-veins); massive, meterwide veins present in thermal aureoles (V2-veins); millimeter-wide, erratic veins, also present in thermal aureoles (V3-veins); tabular, undeformed and NW striking (V4-veins); millimeter-wide erratic veins restricted to brittle reactivation of strike-slip shear zones (V5-veins). The regional foliation, developed under garnet zone metamorphic conditions, was more effective in vein production when compared to the steeply dipping mylonitic foliation corridors developed under chlorite zone conditions. Pressure solution was the main deformation mechanism during the regional foliation development. However, granoblastic and decussate textures in hornfels reveal an influence of grain boundary area reduction mechanism. The granoblastic texture in hornfels would have inhibited fluid circulation during dehydration reactions, increasing fluid pressure and promoting massive (V2-veins) and erratic hydraulic fracture-related (V3) veins. In dextral NEtrending strike-slip shear zones, tabular quartz veins (V4-veins) are parallel to tension gashes. Reactivation of strike-slip shear zones under brittle conditions has produced local brecciated millimeter-wide quartz veins (V5-veins). This study underscores the important role of fluids during orogenic evolution near the brittle-plastic transition of the crust, and demonstrates how combined vein and microtextural analysis can reveal the tectonometamorphic history of lowgrade metamorphic rocks.