Orogenic Domain Research Articles

Coupling of X-ray fluorescence and strontium isotopes to track the influence of brine in continental deposits: study of Oligocene sediments in the Digne area (SE basin, France)

It is often difficult to decipher past salt tectonics events in orogenic domains. The southwestern Alpine foreland in France presents a long and extensive salt tectonics history in the Mesozoic, which was later inverted during the Cenozoic Alpine Orogeny. Synorogenic Cenozoic salt-related deformations are difficult to identify due to the contemporaneous shortening experienced by the foreland. This study is based on sedimentological and geochemical (Sr concentrations and 87 Sr/ 86 Sr ratios) analyses of the Oligocene non-marine succession of St-Geniez (Digne region, France). Oligocene salt influences are highlighted by (1) high Sr concentrations (>1000 ppm) in most of the series, (2) the occurrence of halophytic gastropods and (3) the deposition of two gypsum beds. The 87 Sr/ 86 Sr ratios measured in the limestones and the gypsum beds confirm a Triassic origin for the Sr, originating from the Sorine diapir located to the SW, for the first part of the series and from the Authon thrust, located to the north, for the upper part of the series. The geochemistry also suggests the occurrence of two Triassic evaporite levels: one already known and attributed to the Carnian–Norian (Late Triassic) and one attributed to the Olenekian–Anisian (Early to Mid-Triassic). These results show that the combined use of field geology and geochemistry can provide information about previously erased salt tectonics events.

Rhyacian to Orosirian short-lived polyphase deformation and migmatization in northernmost São Francisco Craton, Brazil

The Minas-Bahia Orogen, which is Siderian-Orosirian age, is exposed in the northern sector of the São Francisco Craton and in its African counterpart, the Congo Craton. In this sector, two orogenic domains outcrops with geological evolution and terrains with different trends: Western and Eastern Bahia. The advance of scientific knowledge in the northeast of the West Bahia Orogenic Domain has revealed the existence of more tectonic terrains than those traditionally delimited, and reveal the presence of juvenile terranes and cratonic crust separated complex zones where they interacted. The region of interaction between the Gavião and Bom Jesus da Lapa Paleoplate has been the subject of debates about the tectonic significance of granulitic rocks with Meso-Neoarchean protoliths, which are intruded by Rhyacian - Orosirian granitoids. The set of geological data presented in this article demonstrates that the metatexite migmatites with felsic granulitic paleosome and charnockitic neosome were generated during the initial phases of the collision between these two paleoplates, between 2068 and 2058 Ma. The structural mass transport is from NW to SE. The youngest migmatization event, in high amphibolite facies, occurred during NNW-SSE trending sinistral strike-slip tectonics as suggested by the emplacement of Dn+3 neosomes controlled by S/C/C' structures, in between 2049–2000 Ma. The interpreted tectonic model is complex and involves the accretion and collision among Bom Jesus da Lapa, Gavião and Jequié paleoplates during the Rhyacian-Orosirian, contributing to the advancement of knowledge of tectonic pieces and their interactions that contributed to the formation of Columbia supercontinent.

Source‐To‐Sink Sedimentary Budget of the African Equatorial Atlantic Rifted Margin

AbstractDespite their very low relief and erosion rates, non‐orogenic (i.e., cratonic) continental domains account for over 60% of the Earth's exposed lands. Therefore, they contribute significantly to the clastic sediments and solutes exported to the ocean and should be accounted for in global studies. Nonetheless, they have been much less studied than orogenic domains. In this study, we establish the source‐to‐sink sedimentary budget of the sub‐saharan West African cratonic domain and its Equatorial Atlantic rifted margin using published low‐temperature thermochronological data to estimate onshore denudation and regional geological cross‐sections to estimate offshore accumulation. We show that during and immediately following rifting (130‐94 Ma), the build‐up and subsequent erosion of rift‐related relief resulted in a transient, 100–200 km wide strip along the margin recording high denudation rates (>50 m/Myr), while the inland domain underwent steady and very low denudation (<10 m/Myr). Afterward, the whole onshore domain underwent very low and steady denudation. Thus, the changes in post‐rift accumulation rates documented along the rifted margin were caused by changes in the climate and/or drainage network. During the Late Cretaceous, we document a regional rise in accumulation rates caused by the enlargement of drainage areas feeding the basins by a hinterlandward migration of the continental divide. During the Paleogene, we document a general drop in accumulation rates in all the basins of the African Atlantic margins caused by the global greenhouse climate, which enhanced the development of lateritic weathering mantles, storing clastic sediments on the continent and favoring solute exports to the ocean.

Refined Tectonic Evolution of the Betic‐Rif Orogen Through Integrated 3‐D Microstructural Analysis and Sm‐Nd Dating of Garnet Porphyroblasts

AbstractHigh‐resolution microstructural analysis of porphyroblast inclusion trails integrated with Sm‐Nd garnet geochronology has provided new insight into the tectonic history of the Betic‐Rif orogen. Three principal age groups of porphyroblasts are demonstrated with distinctly oriented inclusion‐trails. Inclusion‐trail curvature axes or “FIA” (Foliation Inflexion/Intersection Axes) are shown to represent “fossilized” crenulation axes from which a succession of different crustal shortening directions can be deduced. The regional consistency of microstructural orientations and their geometric relationship with multiple sets of macroscopic folds reveal the composite character of the Gibraltar Arc formed by a superposition of different folding directions and associated lineations. Bulk‐garnet ages of 35–22 Ma obtained from five micaschist samples of the Alpujarride‐Sebtide complex (ASC) and of 35–13 Ma from four micaschists of the Nevado‐Filabride complex (NFC) allow to deduce NNE‐SSW directed shortening in the Late Eocene changing to NW‐SE shortening in the early Oligocene, alternating with suborthogonal NE‐SW shortening during the Miocene. These directions can be related to a major swing in the direction of relative Africa‐Iberia plate‐motion known from kinematic modeling of magnetic seafloor anomalies, and subsequent dynamic interference between plate convergence and suborthogonal “tectonic escape” of the Alboran Domain. Coupled to previously established P‐T‐t paths, the new garnet ages support a common tectono‐metamorphic evolution of the ASC and NFC as laterally equivalent orogenic domains until, in the Miocene, the second became re‐buried under the first.

Together but separate: decoupled Variscan (late Carboniferous) and Alpine (Late Cretaceous–Paleogene) inversion tectonics in NW Poland

Abstract. In Europe, formation of the Palaeozoic Variscan orogenic belt, and the Mesozoic–Cenozoic Alpine–Carpathian orogenic belt led to a widespread inversion events within forelands of both orogenic domains. We used legacy 2-D seismic data together with the newly acquired 3-D seismic data that, for the first time, precisely imaged sub-Zechstein (i.e. sub-evaporitic) upper Palaeozoic successions in NW Poland in order to develop a quantitative, balanced 2-D model of the late Palaeozoic–recent evolution of this area, characterised by a complex pattern of repeated extension and inversion. Four main tectonic phases have been determined: (1) Late Devonian–early Carboniferous extension and subsidence possibly related to extensional reactivation of Caledonian thrusts, (2) late Carboniferous inversion caused by the Variscan orogeny, (3) Permo-Mesozoic subsidence related to the development of the Polish Basin and (4) its Late Cretaceous–Paleogene inversion. Variscan and Alpine structures form a superimposed multilayer inversion system, mechanically decoupled by the Zechstein evaporites.

Folded joints at first glance

The tectonic evolution of orogenic domains may involve the superposition of ductile and brittle deformation. In some cases, banded or foliated rocks that transiently behaved mechanically brittle, developing sets of variably oriented joints, can be subjected to later ductile deformation. The result can be a complex structural pattern of folded and sheared fractures and rock fabrics. With this work we aim to draw attention to the occurrence of what appears to be folded joints. Some examples of folded joints located in the Variscan basement of the Pyrenees are briefly described by first time, and insight is given into the identification and significance of these structures that have been so far little studied.

Towards an integrated tectonic model for the interaction between the Bambuí basin and the adjoining orogenic belts: Evidences from the detrital zircon record of syn-orogenic units

The São Francisco craton and its surrounding orogens are some of the most important tectonic elements of South America, and key regions to understanding the assembly of Western Gondwana in the Proterozoic-Palaeozoic transition. During this major tectonic event, diachronic collisions between small continents developed an intricated orogenic system, where several orogens evolved in unique paths through time and space. In such collisional settings, there are some tectonic processes that promote subsidence, thus controlling the formation of orogenic-related sedimentary basins. Furthermore, the tectonic activity in collisional orogens generates dynamic landscapes that usually favor increased erosion and sediment generation to feed these basins, making them key places to seek for clues about the tectonic evolution of their surroundings. On the São Francisco craton, the Bambuí Group records a complex foreland system, which evolved in response to the lithospheric overload exerted by the uplift of both Brasília belt and Araçuaí orogen. In turn, both Salinas Formation and Ibiá Group comprise orogenic deposits resting within the Araçuaí orogen and Brasília belt, respectively, whose tectonic significance is still under debate. Here we present new U–Pb (LA-ICP-MS and SHRIMP) and Lu–Hf isotopic analyses on a great amount of detrital zircon grains extracted from the rocks of the Bambuí Group and Salinas Formation, together with a thorough data compilation from the literature. The two units present similar provenance patterns, sharing the major detrital zircon age peaks (550–650 Ma, 950–1050 Ma, 1750–2000 Ma, 2600–2800 Ma) and maximum depositional age in c. 550 Ma. The Ediacaran zircons recovered from the Bambuí Group show a wide range of the εHf(t), ranging from c. −17 to +15, which suggest the existence of multiple late Neoproterozoic sources, some of them juvenile and some with a long crustal residence. The variation in detrital zircon age patterns and εHf(t) values from different units within the Bambuí Group provided additional clues of provenance changes occurred during the evolution of the basin. The new data obtained for the Salinas Formation constrain its deposition between 548 and 500 Ma, which have an important implication on its tectonic significance. We propose that in both Brasília belt and Araçuaí orogen sides, the early foredeep deposits of the Bambuí basin should have been incorporated to the orogenic domains, which could explain the apparent lack of deposits recording the climax of the Brasilia belt uplift (c. 630 Ma) within the cratonic area. In this same direction, we consider that both Salinas Formation and Ibiá Group could represent remnants of these early foreland deposits related to the uplift of Brasília belt and Araçuaí orogen, respectively, incorporated to the orogenic wedges due to the advance of the deformational fronts. Therefore, what we know as Bambuí Group is in fact the remaining record of an advanced stage of the foreland system, when subsidence was already influenced by the two evolving orogens. Altogether, the analyses of the stratigraphic, structural and geochronological data converge towards an integrated tectonic model for the interaction between the Bambuí basin and the surrounding orogens during West Gondwana amalgamation.

Tectono‐Stratigraphic and Thermal Evolution of the Western Betic Flysch: Implications for the Geodynamics of South Iberian Margin and Alboran Domain

AbstractThe Betic‐Rif orogen is a key region to understand the evolution of the plate boundary between Africa and Iberia/Europe. This study focuses on the Flysch Complex, which is considered the sedimentary cover of a domain originally positioned between the Iberian and Alboran margins. Based on stratigraphic and depositional evolution constraints, evidence for salt tectonics, combined with new apatite fission‐tracks (AFT) and (U‐Th‐Sm)/He ages from the Flysch Complex and the Subbetic Zone, we propose a geodynamic interpretation for the formation of the Betic Cordillera, accounting for moderate N‐directed transport of the Flysch Complex and synchronous exhumation between External and Internal Zones of the Betic. Early contraction between Africa and Iberia/Europe is reflected in the Cretaceous Flysch basin by a prolonged period of residence in the partial annealing zone for AFT and onset of foreland subsidence at 50 Ma. This stage lasted until the Early to Middle Miocene (20–15 Ma), marked by the rapid succession, in less than 5 Ma, of the deposition of Cenozoic flysch and their rapid exhumation. This event is interpreted to reflect the W‐directed retreating mantle delamination between Africa and Iberia margins at the origin of the collapse of the proto‐Betic orogenic domain and formation of the Alboran domain.

Progressive Deformation Patterns from an Accretionary Prism (Helminthoid Flysch, Ligurian Alps, Italy)

This paper reports the results of a field-based structural investigation of a well-exposed paleo-accretionary prism, which experienced complex deformation in a low-grade metamorphic setting. Field analyses focused on the description of structural fabrics, with the main emphasis upon parameters like the orientation, style and kinematics of foliations, folds and shear zones. We address the research to the south-westernmost part of the Alpine chain, the Ligurian Alps, where, despite their origin as turbidite sequences deposited into the closing Alpine Tethys Ocean, the Helminthoid Flysch Nappes are presently distributed in the outer part of the chain, above the foreland. The new dataset highlights different deformation patterns related to the different spatial distribution of the flysch units. This regional-scale partitioning of strain is hence associated with progressive deformation within a two-stage geodynamic evolution. Correlations among the different orogenic domains allow the proposal of a kinematic model that describes the motion of the Helminthoid Flysch from the inner to the outer part of the orogen, encompassing the shift from subduction- to collision-related Alpine geodynamic phases.

Impact of Mafic Underplating and Mantle Depletion on Subsequent Rifting: A Numerical Modeling Study

AbstractOrogenesis in the Variscan belt of Western Europe was followed by a major magmatic event during the Permian that formed a mafic lower crust by crystallizing pyroxenite and gabbros from mantle‐derived melts at the base of the continental crust. Partial melting of the asthenosphere left a significantly depleted mantle that was progressively incorporated into the subcontinental lithospheric mantle as the orogenic domain cooled. The potential impact of such large‐scale thermal and lithologic layering has never been taken into account in the study of the Alpine Tethys and North Atlantic rift systems that developed subsequently in Western Europe. Here we investigate via numerical modeling how a mafic heterogeneity within the lowermost part of a quartzo‐feldspathic continental crust and/or a zone of depleted mantle within the lithospheric mantle beneath a former orogenic domain could have influenced subsequent rifting. Our numerical modeling results indicate that, in a thermally equilibrated lithosphere, a mafic body within the lower continental crust or a zone of shallow depleted mantle prevents rifting of overlying weaknesses (e.g., faults and suture zones). We propose that the regional mafic lower crust beneath the Variscan orogenic domain may explain why the Tethyan and southern North‐Atlantic rift systems did not localize at former suture zones of the Western European orogenic lithosphere.

The Central European, Tarim and Siberian Large Igneous Provinces, Late Palaeozoic orogeny and coeval metallogeny

The formation of the Central European and Tarim Large Igneous Provinces (LIPs) in the Early Permian coincided with the demise of the Variscan and the Southern Tianshan orogens, respectively. The Early Triassic Si- berian LIP was formed in the wake of the Western Altaid orogeny in the Late Permian. These processes coincided with the development of the majority of known Late Palaeozoic and Early Mesozoic hydrothermal and magmatic ore deposits in the corresponding orogenic domains. Nickel-copper(-PGE) deposits followed directly from the evo- lution of the (ultra-)mafi c melts which make up the LIPs. In Western Siberia, the diverse assemblage of associated noble and base metals in the Noril'sk-Talnakh Ni-Cu(-PGE) deposits suggests these metals also had their source in the mantle domain from which the (ultra-)mafi c melts were generated. The same metals variably found their way into the hydrothermal ore deposits in the defunct Variscan and Southern Tianshan orogenic domains. These ore de- posits have traditionally been viewed as a result of orogenic processes. However, their ages, together with the timing and nature of their by then intracontinental tectonic control cause uncertainty concerning the role of the orogens. In view of their mantle sources in their association with the Siberian LIP, the mantle contribution to these Late Palaeo- zoic hydrothermal ore deposits in the orogenic belts may have been more signifi cant than previously thought. An orogenic contribution to melting of mantle complexes and to mineralisation may have resided in the earlier modifi - cation of subcontinental mantle domains by subduction of oceanic lithosphere. In all three cases, the controlling tectonic setting of orogenic cessation, LIP formation and mineralisation was dominated by translithospheric strike- slip deformation, possibly in combination with orogenic collapse and lithosphere delamination. In view of their re- currence, the orogen-lip sequences were probably not fortuitous. The controlling strike-slip faults were principal elements of the lithosphere-scale dynamic framework that led to the amalgamation of Pangaea. At this scale, the exceptionally large volume of the Siberian LIP may, in addition to the strike-slip dissection of the lithosphere, have been related to extension in the continental lithosphere of the margin of the Supercontinent (cf. Gutierrez-Alonso et al. 2008). The peripheral extension was associated with compression in its centre. The explanation of these Large Igneous Provinces does not require the concept of an active mantle plume because deep-reaching strike-slip defor- mation, orogenic collapse and lithosphere delamination involved in the destruction of the orogenic edifi ces can have caused decompression melting in large domains of the subcontinental mantle.

A New Southern North Atlantic Isochron Map: Insights Into the Drift of the Iberian Plate Since the Late Cretaceous

AbstractThis paper presents a new southern North Atlantic plate model from Late Cretaceous to present, with the aim of constraining the kinematics of the Iberian plate during the last 83.5 Myr. This model is presented along with a detailed isochron map generated through the analysis of 3 aeromagnetic tracks and ~400 ship tracks from the National Centers for Environmental Information database. We present a new technique to obtain well‐constrained estimates of the Iberia‐North America plate motions from magnetic anomalies, overcoming the scarcity of large‐offset fracture zones and transform faults. We build an integrated kinematic model for NW Africa, Morocco, Iberia, Europe, and North America, which shows that the deformation is partitioned between Pyrenees and Betic‐Rif orogenic domain during the Late Cretaceous‐Oligocene time interval. In the Eastern Betics domain, the calculated amount of NW Africa‐Iberia convergence is ~80 km between 83.5 and 34 Ma, followed by ~150 km since the Oligocene. The motion of Iberia relative to Europe in the Central Pyrenees is characterized by overall NE directed transpressional motion during the Campanian and the Paleocene, followed by NW directed transpressional movement until the Lutetian and overall NNW directed convergence from Bartonian to Chattian. This motion occurs along the axis of the Bay of Biscay from the Santonian–Campanian boundary to the middle Priabonian, subsequently jumping to King's Trough at Anomaly 17 (36.62 Ma).

Contrasting provenance and timing of metamorphism from paragneisses of the Araçuaí-Ribeira orogenic system, Brazil: Hints for Western Gondwana assembly

The Araçuaí orogen and the Ribeira belt make up a complex Neoproterozoic-Cambrian orogenic system, the Araçuaí-Ribeira orogenic system (AROS) located from the eastern to southeastern Brazil. Along the AROS, the Ediacaran Rio Doce magmatic arc represents a geotectonic connection between the Araçuaí and the Ribeira orogenic domains. Although the nature and evolution of the Rio Doce plutonic rocks is regionally well established, it lacks detailed studies on the paragneisses found along the western and central regions of this magmatic arc. Besides information on the nature and provenance of their sedimentary protoliths, the paragneisses provide data to unravel the palaeogeographic scenario from the precursor to arc-related basins. Six samples of Al-rich gneisses covering a large AROS region were selected for electron microprobe (EMP) mineral analyses in order to obtain geothermobarometric data and monazite ages, as well as for Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) isotopic analyses on zircon (U-Pb, Lu-Hf) and monazite (U-Pb). The different age spectra from detrital zircon grains and contrasting Hf isotopic signatures suggest a complex sedimentary history. Located in the western sector of the study region, the samples RC-02 and RC-34, with an 80% age peak of detrital zircon grains from 2158Ma to 1830Ma, εHf(t) from −2.2 to −22.7, and Hf TDM model ages from 3530Ma to 2440Ma, suggest sediment sources located in the São Francisco craton basement. The samples RC-03, also from the western sector, and RC-46 from the southern sector, have a more complex assemblage of detrital zircon grains with an 87% age peak from 987Ma to 592Ma, εHf(t) from +14.9 to −2.9, and Hf TDM model ages from 2220Ma to 720Ma, indicating provenance from mainly juvenile sources of distinct ages. Candidates to be juvenile sources for RC-03 and RC-46 sedimentary protoliths are the Rhyacian Juiz de Fora and Pocrane complexes in the basement of the Rio Doce arc, the Neoproterozoic Rio Negro arc system of the Ribeira belt, and AROS ophiolite complexes. Samples RC-30 and RC-38 from the eastern sector of the study region, with most detrital zircon ages between 650Ma and 552Ma and very negative εHf(t) (−25.3 to −16.5), suggest main sediment sources in the Rio Doce arc. By extending U-Pb analyses on metamorphic zircon and monazite, we have identified a complex timing of metamorphism, represented by metamorphic ages ranging from 621Ma to 480Ma, with the main collisional activity between 580Ma and 540Ma. Geothermobarometric studies on garnet porphyroblasts, syn-kinematic to the D2 regional foliation, show a retrograde metamorphic path typical of continental collision belts, starting with P-T conditions of Tmax=733°C and Pmax=6.43kbar. Our data also suggest: i) the studied paragneisses represent distinct Neoproterozoic basin stages, shifting from passive to active margin settings; ii) if the Rio Negro arc system really provided sediments for the basin stage represented by the RC-03 and RC-46 paragneisses, it would have amalgamated with the AROS before 614Ma; iii) the final amalgamation of Western Gondwana took place around 540Ma in the focused region; iv) an important re-heating period (520–480Ma) can be related to the AROS gravitational collapse, after Western Gondwana assembly.

The origin of contractional structures in extensional gneiss domes

Abstract The juxtaposition of domains of shortening and extension at different scales in orogens has fueled many debates about driving forces and tectonic interpretations, including timing of deformation. At the orogen scale, gravitational collapse and mass transfer from orogenic plateaux to forelands explain some of these juxtapositions. At a regional scale, structures in gneiss domes are commonly contractional yet are coeval with regional extension and denudation. Here we use three-dimensional numerical experiments to show that crustal flow in orogenic domains does not necessarily conform to plate motion. We document contractional crustal flow associated with the formation of a gneiss dome in an orogenic pull-apart setting where localized extension and crustal thinning focus the exhumation of deep crust. We show that the flow field results in a complex strain pattern in which an extensional strain regime that is collinear with the direction of plate motion is partitioned into the shallow crust, whereas contractional structures and fabrics at a high angle to the direction of imposed transport develop in the deep crust. Advective mass transfer across regions of contrasting yet coeval strain regimes leads to a polyphase tectonic history. We observe structural features remarkably similar to those documented in some natural gneiss domes such as the Montagne Noire, which developed in a dextral pull-apart domain at the southern margin of the French Massif Central.

Sequential kinetic modelling: A new tool decodes pulsed tectonic patterns in early hot orogens of Earth

Tectonic styles in an early hot Earth were different from the present-day situation governed by plate tectonics. Processes in such hot settings remain poorly understood because they often occur on timescales that are below the resolution of conventional isotopic clocks, the rock records are fragmentary, and these have been superposed by later high-temperature events. We have developed a tool based on diffusion kinetics to overcome these difficulties and reconstruct sequences of short-lived episodes. Application of the method to a rock from the ultra-hot c.1.6 Ga orogenic domain of the Central Indian Tectonic Zone, where additional data are available to verify the results, shows that pulses of approach and roll-back of colliding plates preceded the final closure and collision. We demonstrate that cooling from ultra-high temperature metamorphic conditions in the orogen took place in multiple pulses that occurred with a periodicity of about 10 Myr at rates that vary between 100's to 10's °C/Myr, and burial-/exhumation-rates that vary between 30 and 2 km/Myr, respectively. Such details of tectonic processes in the Precambrian, with quantification of variable heating-, cooling-, burial-, and exhumation-rates of individual stages, have not been accessible until now. Application of this method to other regions would provide a means of exploring the thermal viability of the inferred long durations (>100 Myr) for some ultra-high temperature orogenies.

Thermo‐kinematic evolution of theAnnapurna‐DhaulagiriHimalaya, centralNepal: TheCompositeOrogenicSystem

AbstractThe Himalayan orogen represents a “Composite Orogenic System” in which channel flow, wedge extrusion, and thrust stacking operate in separate “Orogenic Domains” with distinct rheologies and crustal positions. We analyze 104 samples from the metamorphic core (Greater Himalayan Sequence, GHS) and bounding units of the Annapurna‐Dhaulagiri Himalaya, central Nepal. Optical microscopy and electron backscatter diffraction (EBSD) analyses provide a record of deformation microstructures and an indication of active crystal slip systems, strain geometries, and deformation temperatures. These data, combined with existing thermobarometry and geochronology data are used to construct detailed deformation temperature profiles for the GHS. The profiles define a three‐stage thermokinematic evolution from midcrustal channel flow (Stage 1, >700°C to 550–650°C), to rigid wedge extrusion (Stage 2, 400–600°C) and duplexing (Stage 3, <280–400°C). These tectonic processes are not mutually exclusive, but are confined to separate rheologically distinct Orogenic Domains that form the modular components of a Composite Orogenic System. These Orogenic Domains may be active at the same time at different depths/positions within the orogen. The thermokinematic evolution of the Annapurna‐Dhaulagiri Himalaya describes the migration of the GHS through these Orogenic Domains and reflects the spatial and temporal variability in rheological boundary conditions that govern orogenic systems.

The Ediacaran Rio Doce magmatic arc revisited (Araçuaí-Ribeira orogenic system, SE Brazil)

Described half a century ago, the Galiléia tonalite represents a milestone in the discovery of plate margin magmatic arcs in the Araçuaí-Ribeira orogenic system (southeastern Brazil). In the 1990's, analytical studies on the Galiléia tonalite finally revealed the existence of a Late Neoproterozoic calc-alkaline magmatic arc in the Araçuaí orogen. Meanwhile, the name Rio Doce magmatic arc was applied to calc-alkaline plutons found in the Araçuaí-Ribeira boundary. After those pioneer studies, the calc-alkaline plutons showing a pre-collisional volcanic arc signature and age between 630 Ma and 585 Ma have been grouped in the G1 supersuite, corresponding to the Rio Doce arc infrastructure. Here, we revisit the Rio Doce arc with our solid field knowledge of the region and a robust analytical database (277 lithochemical analyses, and 47 U–Pb, 53 Sm–Nd, 25 87Sr/86Sr and 7 Lu–Hf datasets). The G1 supersuite consists of regionally deformed, tonalitic to granodioritic batholiths and stocks, generally rich in melanocratic to mesocratic enclaves and minor gabbroic to dioritic plutons. Gabbroic to dioritic enclaves show evidence of magma mixing processes. The lithochemical and isotopic signatures clearly reveal a volcanic arc formed on a continental margin setting. Melts from a Rhyacian basement form the bulk of the magma produced, whilst gabbroic plutons and enclaves record involvement of mantle magmas in the arc development. Tonalitic stocks (U–Pb age: 618–575 Ma, εNd(t): −5.7 to −7.8, Nd TDM ages: 1.28–1.68 Ga, 87Sr/86Sr(t): 0.7059–0.7118, and εHf(t): −5.2 to −11.7) form the northernmost segment of the Rio Doce arc, which dies out in the ensialic sector of the Araçuaí orogen. At arc eastern and central zones, several batholiths (e.g., Alto Capim, Baixo Guandu, Galiléia, Muniz Freire, São Vítor) record a long-lasting magmatic history (632–580 Ma; εNd(t): −5.6 to −13.3; Nd TDM age: 1.35–1.80 Ga; 87Sr/86Sr(t): 0.7091–0.7123). At arc western border, the magmatic evolution started with gabbro-dioritic and tonalitic plutons (e.g., Chaves pluton, U–Pb age: 599 ± 15 Ma, εNd(t): −4.8 to −6.8, Nd TDM ages: 1.48–1.68 Ga, 87Sr/86Sr(t): 0.7062–0.7068, and εHf(t): −4.3 to −9.7; and Brasilândia pluton, U–Pb age: 581 ± 11 Ma, εNd(t): −8.2 to −10.2, Nd TDM ages: 1.63–1.68 Ga, 87Sr/86Sr(t): 0.7088–0.7112, εHf(t): −12.3 to −14.9), followed by late granodioritic intrusions (e.g., Guarataia pluton, U–Pb age: 576 ± 9 Ma, εNd(t): −12.52 to −13.11, Nd TDM age: 1.74–2.06 Ga, 87Sr/86Sr(t): 0.7104–0.7110, εHf(t): −12.9 to −21.6). The Muriaé batholith (U–Pb age: 620–592 Ma, εNd(t): −8.2 to −13.6, Nd TDM age: 1.41–1.88 Ga) and the Conceição da Boa Vista (586 ± 7 Ma) and Serra do Valentim (605 ± 8 Ma) stocks represent a segment of the Rio Doce arc correlated to the Serra da Bolívia and Marceleza complexes, making the link between the Araçuaí and Ribeira orogenic domains. We suggest three phases of arc development: i) eastward migration of arc front (632–605 Ma), ii) widespread magma production in the whole arc (605–585 Ma), and iii) late plutonism in the western arc region (585–575 Ma). Usual processes of volcanic arc development, like subduction of oceanic lithosphere under a continental margin, followed by asthenosphere ascent related to slab retreating and break-off may explain the Rio Doce arc evolution.

Tracking deep crust by zircon xenocrysts within igneous rocks from the northern Alxa, China: Constraints on the southern boundary of the Central Asian Orogenic Belt

The southern boundary of the central segment of the Central Asian Orogenic Belt (CAOB) with the Precambrian Alxa Block is not well constrained due to poor recognition of deep crust. Statistical analysis of zircon xenocrysts within igneous rocks from the northern Alxa and its adjacent regions was applied to resolve this problem. We compiled new and previously published geochronological zircon age data obtained by SHRIMP, SIMS, TIMS, and LA-ICP-MS for 316 igneous rock samples of which 61 samples contain zircon xenocrysts. New and previously published Hf isotopic compositions of these zircon xenocrysts are combined with zircon ages in this study. The zircon xenocrysts are mainly contained within Permian rocks from the Yabulai–Nurgong–Honggueryulin (YNH) zone and igneous rocks from the northwestern margin of the North China Craton (WNCC). A few xenocrysts were also found in Permian igneous rocks from the Zongnaishan–Shalazhashan (ZS) zone, Neo-proterozoic and Paleo-proterozoic intrusive rocks in the YNH zone, as well as Devonian to Carboniferous granitoids and volcanic rocks from southern Mongolia. Altogether we analyzed more than 270 zircon xenocrysts and considered only ages that are less than 10% discordant. Xenocryst ages within the Permian igneous rocks from the ZS zone are mainly around ca. 350Ma, ca. 600Ma and ca. 1400Ma. The oldest age of zircon xenocrysts in this zone is similar to those of zircon xenocrysts from the CAOB (∼1.1Ga). By contrast, abundant zircon xenocrysts within Permian igneous rocks from the YNH zone show highly variable age populations at ca. 2.6–2.1Ga, 1.8–1.6Ga, 930–750Ma and 460–300Ma. Zircon xenocrysts from the ZS zone have positive εHf(t) values of +6.3 to +13.9, whereas those from the YNH zone display highly variable εHf(t) values from −16.1 to +11.6. From the southern CAOB and the ZS zone to the YNH zone, the zircon xenocrysts show a significant shift from juvenile to crustal Hf isotopic compositions, suggesting that the area between the ZS and the YNH zones constitutes the southern boundary of the CAOB in the deep crust within this region. Our study indicates that statistical analysis (or isotopic mapping) of zircon xenocrysts is an effective method to trace the nature of the deep continental crust and to separate between newly-formed orogenic domain and ancient cratons.

Seismic stratigraphy and structural setting of the Adventure Plateau (Sicily Channel)

Within the central Mediterranean, the northwestern sector of the Sicily Channel is the unique area where two independent tectonic processes can be analyzed: the building of the Sicilian–Maghrebian Chain occurred in Late Miocene and the continental lithospheric rifting of the northern African margin occurred since Early Pliocene. These two geodynamic processes generated a peculiar structural style that is largely recognizable in the Adventure Plateau. This plateau is the shallowest part of the Sicily Channel, where water depths do not generally exceed 150 m. It hosts several areas of geomorphic relief, which in some cases rise up to less than 20 m beneath sea-level. A series of submarine magmatic manifestations occur in this area, mainly associated with the extensional phase which produced the rift-related depressions of Pantelleria, Malta and Linosa. Seismic-stratigraphic and structural analyses, based on a large set of multichannel seismic reflection profiles and well information acquired mostly for commercial purposes in the 1970s and 1980s, have allowed us to reconstruct the Triassic-Quaternary sedimentary succession of the Adventure Plateau and define its structural setting. A broad lithological distinction can be made between the successions ranging from Triassic to Paleogene, predominantly carbonate, and the successions ranging from Miocene to Quaternary, predominantly siliciclastic. Three main structural belts have been identified within the Adventure Plateau: (1) the northern belt, affected during Late Miocene time by ESE-verging thrusts belonging to the External Thrust System orogenic domain, which represents the lowermost structural level of the Sicilian–Maghrebian Orogen; (2) the Apenninic–Maghrebian domain of the Sicilian–Maghrebian Orogen, which occupies the northwestern sector of the Adventure Plateau, and that is overthrusted on the External Thrust System orogenic domain during the Late Miocene; (3) the extensional belt of the southwestern sector of the Adventure Plateau, affected by broad NW-trending, high-angle normal faults associated with the Early Pliocene continental rifting phase. The eastern boundary of the Adventure Plateau corresponds to a broadly N–S trending lithospheric transfer zone separating two sectors of the Sicily Channel characterized by a different tectonic evolution.

Pliocene to Quaternary deformation in the Var Basin (Nice, SE France) and its interpretation in terms of “slow-active” faulting

Seismic hazard assessment of active faults in slow orogenic domains is a challenging issue. In this paper we present a multi-disciplinary approach based on a Digital Elevation Model (DEM), 3D-geological modelling, fracture analysis, and strain analysis of pebbles in a Pliocene molasse basin. The basin is cross-cut by "slow-active" faults of the Donaréo and St Blaise-Aspremont fault system. The DEM shows a topographic disturbance emphasized by slope gradients and the drainage system, which is ascribed to the Plio-Quaternary fault trace. Fracturation analysis evidences two fault corridors oriented approximately N150°E and N20°E. Paleo-stress analysis provides orientations similar to those derived from the focal mechanisms of current regional seismicity, with the main stress σ1 oriented N20°E and a (σ2 − σ3)/(σ1 − σ3) ratio of 0.31. The σ2 versus σ3 permutations are in agreement with ongoing strike-slip deformation at least since the early Pliocene. Discontinuous fracturation and comparison with seismic monitoring on regional active fault zones suggest that shallow seismicity may be expressed by low-magnitude (Mw < 4) seismic swarms. Deformation of pebbles occurs mainly by pressure-dissolution processes. Pebble striation orientations show a bimodal distribution, parallel to the two fault strands. Pebble deformation and the paucity of striated surfaces along the main faults suggests rare seismic deformation and long-lasting aseismic creep processes. Geometrical 3D analysis shows the formation and migration of a Plio-Quaternary basin about 500 metres east of the main fault system, together with folding and tilting of the post-Messinian Pliocene molasse. These observations indicate that the fault remained active from the Pliocene to the Quaternary, and possibly up to the present time. However, the estimates of the minimum slip rate on the faults of about 0.02 mm a−1 vertical and 0.03 mm a−1 horizontal are unlikely to produce any significant high-magnitude earthquakes, but rather swarm-like low-magnitude seismicity with long temporal recurrence.