Subduction Retreat Research Articles

3D subduction dynamics: A first-order parameter of the transition from copper- to gold-rich deposits in the eastern Mediterranean region

The natural variability of geometry and dynamics of subduction zones leads to a variety of mantle and crustal processes that may influence the genesis of ore deposits in the overriding plate. These complex interactions cannot be fully represented by two-dimensional (2D) models but require that the spatial and temporal evolution of ore deposits be examined in a detailed 3D tectonic framework. We compare and discuss the geodynamic and metallogenic evolution of the eastern Mediterranean subduction zone since the late Cretaceous by integrating a newly compiled metallogenic database with a recent kinematic reconstruction model. The resulting paleotectonic maps identify (1) a late Cretaceous Cu-rich metallogenic period with large deposits formed above a linear and stable subduction zone that produced large amounts of potentially fertile magmas in a narrow magmatic arc and (2) a late Eocene-Miocene Pb-Zn- followed by Au-rich metallogenic period with generally smaller deposits spread over a wide back-arc basin that was opened due to subduction retreat and lateral slab tearing. Supported by high-resolution numerical modeling, the proposed dynamic arc and back-arc metallogenic models emphasize (1) the influence of 3D slab dynamics and associated asthenospheric flow on ore distribution in space and time and (2) the importance of exhuming metamorphic domes in extensional back-arc setting to focus metal-bearing fluid circulations. These models differ from collision and post-collision models and may be valid on the western and eastern terminations of the whole Tethyan orogenic belt (i.e. Mediterranean and southeast Asia) where fast 3D subduction dynamics has influenced the geodynamic and metallogenic evolution of the overriding plate.

The dominant driving force for supercontinent breakup: Plume push or subduction retreat?

Understanding the dominant force responsible for supercontinent breakup is crucial for establishing Earth's geodynamic evolution that includes supercontinent cycles and plate tectonics. Conventionally, two forces have been considered: the push by mantle plumes from the sub-continental mantle which is called the active force for breakup, and the dragging force from oceanic subduction retreat which is called the passive force for breakup. However, the relative importance of these two forces is unclear. Here we model the supercontinent breakup coupled with global mantle convection in order to address this question. Our global model features a spherical harmonic degree-2 structure, which includes a major subduction girdle and two large upwelling (superplume) systems. Based on this global mantle structure, we examine the distribution of extensional stress applied to the supercontinent by both sub-supercontinent mantle upwellings and subduction retreat at the supercontinent peripheral. Our results show that: (1) at the center half of the supercontinent, plume push stress is ∼3 times larger than the stress induced by subduction retreat; (2) an average hot anomaly of no higher than 50 K beneath the supercontinent can produce a push force strong enough to cause the initialization of supercontinent breakup; (3) the extensional stress induced by subduction retreat concentrates on a ∼600 km wide zone on the boundary of the supercontinent, but has far less impact to the interior of the supercontinent. We therefore conclude that although circum-supercontinent subduction retreat assists supercontinent breakup, sub-supercontinent mantle upwelling is the essential force.

Petrogenesis of Cretaceous (133–84 Ma) intermediate dykes and host granites in southeastern China: Implications for lithospheric extension, continental crustal growth, and geodynamics of Palaeo-Pacific subduction

This paper presents U-Pb zircon geochronology, petrology, and major and trace element, Sr-Nd and zircon Hf isotopic geochemistry of Cretaceous granites and intermediate dykes in the Quanzhou and Xiamen regions of southeastern China. These data are used to investigate igneous petrogenesis and Cretaceous tectonic evolution, and interpret the geodynamics of Palaeo-Pacific slab subduction. Granites in Quanzhou and Xiamen range in age from 133Ma to 87Ma, have high SiO2 and K2O contents, low abundances in P2O5, and an A/CNK index that ranges from 0.97 to 1.09, indicating that they are high-K calc-alkaline metaluminous I-type rocks. Slightly negative ɛ Nd (t) values (−1.2 to −4.4), young Nd model ages (0.87Ga to 1.20Ga) and positive ɛ Hf (t) values (−0.5 to +9.9) of zircon grains indicate that the granites were derived from magmas that melted amphibolite in the middle-lower crust, and which may have assimilated country rocks during emplacement in shallow chambers. The intermediate dykes have no genetic link to the granites and magma mixing was negligible. Eight dyke samples have low SiO2 and high MgO, Ni and Cr contents. Negative ε Nd (t) values (−1.5 to −2.7) and positive ε Hf (t) values (2.7 to 7.6) suggest that the dykes were derived from residual basic lower crust after mafic-crystal accumulation. Two samples of adakite-like dykes are characterised by high Sr/Y ratios (89 to 100) and high SiO2, low K2O, Ni, Cr contents. In combination with slightly negative ε Nd (t) values (−1.7 to −1.8) and positive ε Hf (t) values (2.9 to 4.3), the adakite-like dykes were derived from cumulate basic lower crust which had a mixed source between depleted mantle- and crust-derived melts. Based on our data, combined with previously published work, we suggest that extension-induced middle-lower crustal melting and underplating of mantle-derived basaltic melts were the principal driving mechanisms for Cretaceous granitic magmatism in coastal Fujian Province. Extension was related to subduction retreat whereas steep slab subduction caused underplating of mantle-derived basaltic melts. These processes were coupled and mainly responsible for the tectonic transition during the Cretaceous from compression to extension in the coastal belt of the Cathaysia Plate.

Circum-Arctic Lithosphere Evolution (CALE) Transect C: displacement of the Arctic Alaska–Chukotka microplate towards the Pacific during opening of the Amerasia Basin of the Arctic

Abstract This paper synthesizes the framework and geological evolution of the Arctic Alaska–Chukotka microplate (AACM), from its origin as part of the continental platform fringing Baltica and Laurentia to its southward motion during the formation of the Amerasia Basin (Arctic Ocean) and its progressive modification as part of the dynamic northern palaeo-Pacific margin. A synthesis of the available data refines the crustal identity, limits and history of the AACM and, together with regional geological constraints, provides a tectonic framework to aid in its pre-Cretaceous restoration. Recently published seismic reflection data and interpretations, integrated with regional geological constraints, provide the basis for a new crustal transect (the Circum-Arctic Lithosphere Evolution (‘CALE’) Transect C) linking the Amerasia Basin and the Pacific margin along two paths that span 5100 km from the Lomonosov Ridge (near the North Pole), across the Amerasia Basin, Chukchi Sea and Bering Sea, and ending at the subducting Pacific plate margin in the Aleutian Islands. We propose a new plate tectonic model in which the AACM originated as part of a re-entrant in the palaeo-Pacific margin and moved to its present position during slab-related magmatism and the southward retreat of palaeo-Pacific subduction, largely coeval with the rifting and formation of the Amerasia Basin in its wake.

Neoproterozoic paleogeography of the Tarim Block: An extended or alternative “missing-link” model for Rodinia?

Recent reconstructions of the Rodinia supercontinent and its breakup incorporate South China as a “missing link” between Australia and Laurentia, and place the Tarim craton adjacent to northwestern Australia on the supercontinent's periphery. However, subsequent kinematic evolution toward Gondwana amalgamation requires complex geometric shuffling between South China and Tarim, which cannot be easily resolved with the stratigraphic records of those blocks. Here we present new paleomagnetic data from early Ediacaran strata of northwest Tarim, and document large-scale rotation at near-constant paleolatitudes during Cryogenian time. The rotation is coeval with Rodinia breakup, and Tarim's paleolatitudes are compatible with its placement between Australia and Laurentia, either by itself as an alternative “missing link” or joined with South China in that role. At the same time, indications of subduction-related magmatism in Tarim's Neoproterozoic record suggest that Rodinia breakup was dynamically linked to subduction retreat along its northern margin. Such a model is akin to early stages of Jurassic fragmentation within southern Gondwana, and implies more complicated subduction-related dynamics of supercontinent breakup than superplume impingement alone.

Preface: Earth system evolution of the Pacific and Indian oceans and the South China Sea

AbstractEarth system includes Earth's surface system and deep Earth system. This Special Issue tries to establish a link between these two systems. It contains 46 contributions on Earth system evolution of the Pacific and Indian oceans and the South China Sea. The themes cover the relationship of ocean to continent, tectonic jumping, morphotectonics in the continental margin, back‐arc basin formation, formation of transform faults, subduction retreat, hot and cold subduction, actions of typhoons, plume‐tail‐related hydrothermal activity, sediment transportation and dynamics and others. Copyright © 2016 John Wiley & Sons, Ltd.

Large‐scale morphotectonics of the ocean‐continent transition zone between the Western Pacific Ocean and the East Asian Continent: a link of deep process to the Earth' s surface system

AbstractAs an active margin of the Pacific Plate, the ocean‐continent transition zone records major tectonic events from the Late Mesozoic to the present‐day. The development of the transition zone between the Western Pacific Ocean and the East Asian Continental Margin occurred in two stages. In the first stage, the formation of the Songliao and Bohai Bay basins was initiated earlier in the Mesozoic and then the active tectonic events that moved southeastwards to develop the East China Sea and Japan Sea basins in the Cenozoic. We use the methods of tectonics and geophysical analysis for the tectonic reconstruction of the transition zone in the most significant periods of tectonic activity in this area. We propose that the deep processes have a direct influence on the Meso‐Cenozoic surface geomorphologic system in the transition zone. This paper tries to reveal the mechanism of continental margin dynamics by comparing the stratigraphy, sedimentary character and magmatism for each of the four basins, as well as the gravity anomalies, the results of mantle tomography, and thickness of the crust and lithosphere. Our results show that the Songliao and Bohai Bay basins and the East China Sea and Japan Sea basins have similarities in their geomorphological characteristics and sequential tectonic evolution. The Meso‐Cenozoic morphotectonic processes are dominated by the subduction retreat of the (Palaeo‐)Pacific Plate and the eastward tectonic jump. Copyright © 2016 John Wiley & Sons, Ltd.

Formation, tectonic evolution and dynamics of the East China Sea Shelf Basin

AbstractThe East China Sea Shelf Basin (ECSSB) is located on the southeastern continental margin of the Eurasian Plate. The basement of the ECSSB is an extension of the Cathaysian Block as well as an important part of the West Pacific or East Asian Continental Margin tectonic domain. By the analysis of global plate tectonic evolution, the ECSSB is situated in the eastern part of the West Pacific triangle region, as a huge convergence zone between the Indian‐Australian, Pacific and Eurasian plates, as well as a global convergence centre. The ECSSB is closely related to the evolution of the Tethyan and West Pacific tectonic domains. Overall, the ECSSB is a pull‐apart basin under the transtensional tectonic setting of a continental margin, which is contributed to by the combination of subduction retreat and back‐arc spreading between the Eurasian Plate and the Pacific Plate, and the far‐field effect of the collision and extrusion between the Indian‐Australian and Eurasian plates, and deep mantle dynamics. The formation mechanism of the ECSSB is under passive extension. The eastward mantle flow and the asthenospheric upwelling in the deep Earth are the main driving forces of the basin jumping and eastward tectonic migration. Copyright © 2016 John Wiley & Sons, Ltd.

Decratonic gold deposits: a new concept and new opportunities

Early Cretaceous gold deposits in the North China Craton and the famous Carlin-type gold deposits in Nevada are given the name ‘decratonic gold deposits’ [1]. This is the first new type of ore deposit proposed in the last 20 years. This new type of deposit introduces a concept that may change perspectives and provide fresh opportunities for gold deposit research and exploration. Cratons are stable ancient crustal domains characterized by thick lithospheric mantle roots (>150 km), weak tectonism, and little or no magmatic activity within their boundaries. Since their stabilization, the formation of endogenic mineral deposits in such ancient cratonic blocks is very rare owing to a lack of significant magmatism, deformation, or generation of hydrothermal fluids. Several cratons, however, including the North China, the North American and the Brazilian cratons, have experienced destruction to differing degrees [1]. The eastern part of the North China Craton was extensively destructed in the Early Cretaceous [2]. Meanwhile, gold deposits with a total reserve of 4000– 5000 tons formed there within a few millionyears (130–120Ma), coevalwith craton destruction [1]. Similarly, the western part of the North American Craton was destabilized in theCenozoic and that destructionmay be responsible for the famous Nevada Carlin-type gold deposits that host a total of over 6000 tons of gold reserves [1,3]. It is suggested that all these deposits nowbe termed ‘decratonic type’ gold deposits, deposits formed by magma-derived fluids under extensional tectonics conditions associated with craton destruction [1]. The model further postulates that the westward subduction of the western Pacific Plate in the Early Cretaceous and the eastward subduction the eastern Pacific Plate in the Cenozoic were responsible for these twomajor gold mineralization events. Dehydration of the subducted and stagnant slabs led to continuous hydration andmetasomatism of the overlying mantle wedges and the enrichment of gold and other chalcophile elements. Subsequent partial melting of the mantle wedge produced ore-forming fluids (Fig. 1) [1]. Gold deposits in the North China Craton have been classified into two types: ‘altered-rock’ type and ‘quartzvein’ type. The altered-rock-type gold deposits are characterized by disseminated and stockwork goldmineralization, which is usually controlled by brittle or ductile to brittle fault zones. These types of deposits have commonly been intensively overprinted by hydrothermal alteration and suffered later brittle deformation. The quartz-vein-type gold deposits are hosted in the Archean amphibolite facies rocks or earlier granitoids and typically have large proportions of visible native gold. In contrast, the Carlin-type gold deposits in Nevada typically contain invisible gold hosted in fine-grained sulfides disseminated in Paleozoic marine sedimentary rocks.The North China Craton and Carlin gold deposits are seemingly entirely different, but both types are genetically related to craton destruction. Carlin-type gold deposits are abundant in Nevada but have not been found in the North China Craton. The decratonic gold deposit model ascribes the contrast in mineralization styles to host rocks, controlling structures, and ore-forming depths [1]. Decratonic gold deposits in both the North China Craton andNevada formed within a relatively short time interval (<10Ma). In both areas, the deposits become progressively younger toward the subduction zones. Such temporal trends may plausibly be explained by subduction retreat or rollback of the subducted slab at rates of 8.8 and 3.3 cm y−1 for the western and eastern Pacific plates, respectively [1]. Craton destruction starts deep in the mantle with mantle-derived melts and fluids, implying that decratonic gold deposits may extend to great depths. This suggests newopportunities for future discoveries. It is proposed that gold mineralization belts extending approximately NNE in the eastern and the western portions of the Eastern Block of the North China Craton are the most favorable areas for future exploration [1]. ‘Decratonic gold deposit’ is a new term. It will inevitably face reactions that will affirm or challenge it, and that may improve or refute it. Major questions are: the (paleo)-Pacific plate has subducted westward underneath the whole of east China followed by slab rollback during three periods since the Jurassic [4,5].Why did goldmineralization in the North China Craton take place mainly within a short period in the Early Cretaceous? Episodes of tectonic extensions in the North China Craton also have lasted for long periods. What made the Early Cretaceous so different? If craton destruction was the controlling factor, then what made craton destruction unique? Alternatively, are craton

2D thermomechanical modelling of continent–arc–continent collision

Arc–continent collision is a key process of continental growth through accretion of newly grown magmatic arc crust to older continental margin. We present 2D petrological–thermo-mechanical models of arc–continent collision and investigate geodynamic regimes of this process. The model includes spontaneous slab bending, dehydration of subducted crust, aqueous fluid transport, partial melting of the crustal and mantle rocks and magmatic crustal growth stemming from melt extraction processes. Results point to two end-member types of subsequent arc–continent collisional orogens: (I) orogens with remnants of accretion prism, detached fragments of the overriding plate and magmatic rocks formed from molten subducted sediments; and (II) orogens mainly consisting of the closed back-arc basin suture, detached fragments of the overriding plate with leftovers of the accretion prism and quasi insignificant amount of sediment-derived magmatic rocks. Transitional orogens between these two endmembers include both the suture of the collapsed back-arc basin and variable amounts of magmatic production. The orogenic variability mainly reflects the age of the subducting oceanic plate. Older, therefore colder and denser oceanic plates trigger subduction retreat, which in turn triggers necking of the overriding plate and opening of a backarc basin in which new oceanic lithosphere is formed from voluminous decompression melting of the rising hot asthenosphere. In this case, subducted sediments are not heated enough to melt and generate magmatic plumes. On the other hand, young and less dense slabs do not retreat, which hampers opening of a backarc basin in the overriding plate while subducted sediments may reach their melting temperature and develop trans-lithospheric plumes. We have also investigated the influences of convergence rate and volcanic/plutonic rocks' ratio in newly forming lithosphere. The predicted gross-scale orogenic structures find similarities with some natural orogens, in particular with deeply eroded orogens such as the Variscides in the Bohemian Massif.

An active oblique-contractional belt at the transition between the Southern Apennines and Calabrian Arc: The Amendolara Ridge, Ionian Sea, Italy

High-resolution, single-channel seismic and multibeam bathymetry data collected at the Amendolara Ridge, a key submarine area marking the junction between the Apennine collision belt and the Calabrian subduction forearc, reveal active deformation in a supposedly stable crustal sector. New data, integrated with existing multichannel seismic profiles calibrated with oil-exploratory wells, show that middle to late Pleistocene sediments are deformed in growth folds above blind oblique-reverse faults that bound a regional pop-up. Data analysis indicates that ~10 to 20 km long banks that top the ~80 km long, NW-SE trending ridge are structural culminations above en echelon fault segments. Numeric modeling of bathymetry and stratigraphic markers suggests that three 45° dipping upper crustal (2–10 km) fault segments underlie the ridge, with slip rates up to ~0.5 mm/yr. Segments may be capable with M ~ 6.1–6.3 earthquakes, although an unknown fraction of aseismic slip undoubtedly contributes to deformation. The fault array that bounds the southern flank of the ridge (Amendolara Fault System) parallels a belt of Mw < 4.7 strike-slip and thrust earthquakes, which suggest current left-oblique reverse motion on the array. The eastern segment of the array shows apparent morphologic evidence of deformation and might be responsible for Mw ≤ 5.2 historic events. Late Pliocene-Quaternary growth of the oblique contractional belt is related to the combined effects of stalling of Adriatic slab retreat underneath the Apennines and subduction retreat of the Ionian slab underneath Calabria. Deformation localization was controlled by an inherited mechanical interface between the thick Apulian (Adriatic) platform crust and the attenuated Ionian Basin crust.

Observation of the spread of slow deformation in Greece following the breakup of the slab

AbstractOver the past two decades, geophysical observations have shown that earthquakes can trigger other earthquakes, raising the possibility that earthquake interaction plays an important role in the earth's deformation. We analyze here a “storm” of earthquakes in Greece and show that their interaction provides remarkable insight into the mechanics of one of the fastest deforming continental region in the world. A rupture of the African slab initiates a cascade of large earthquakes and a long episode of slow slip marking the downward plunge of the slab, the concomitant rollback of the subduction, and the subsequent detachment of southern Greece from the Eurasian plate. Intense crustal deformation, indicative of the resulting plate stretching, follows. This slow deformation which spreads in a few months over more than 500 km lasts ~3 years and triggers earthquakes. The observations also show that the retreat of the African subduction is the motor of the Aegean deformation.

Architecture and orogenic evolution of the northeastern Outer Carpathians from cross-section balancing and forward modeling

New balanced and restored cross-sections and a 2D kinematic model illustrate the present geometry of the northeastern Outer Carpathians and quantify their orogenic evolution between the late Eocene and the late Miocene (~35.3 to ~11.0Ma). The balanced cross-section is built on extensive surface and subsurface data and depicts an imbricate fan internally stacked along high-displacement out-of-sequence thrusts. Section restoration yielded 507km of minimum orogenic shortening — at least ~230km more than proposed in previous studies. Our shortening estimate relies on accurate thicknesses of lithostratigraphic units, in most cases thinner than applied before. The average convergence rate between ~35.3 and ~11.0Ma is estimated at 20.8km/My. The forward model, constrained by lower and upper ages of syn-orogenic deposits, traces the advance of the Outer Carpathian accretionary wedge and proves kinematic admissibility of the balanced and restored cross-sections. Progradation rates of the foreland basin depositional sequences, evolving from 9–11km/My between ~41.5 and ~21.1Ma to 22–39km/My between ~21.1 and ~11.0Ma, suggest a two-phase dynamics of the orogenic system. The acceleration of foreland migration at ~21.1Ma likely reflects the onset of northward subduction retreat, linked to the rollback of the European lower plate that gave rise to coeval back-arc extension in the Pannonian Basin.

Arc-parallel vs back-arc extension in the Western Gibraltar arc: Is the Gibraltar forearc still active?

Extremely tight arcs, framed within the Eurasia-Africa convergence region, developed during the Neogene on both sides of the western Mediterranean. A complex interplate deformation zone has been invoked to explain their structural trend-line patterns, the shortening directions and the development of back-arc basins. Updated structural and kinematic maps, combined with earthquake data covering the complete hinge zone of the western Gibraltar arc help us to explore the mode of strain partitioning from 25My ago to present. During the Miocene, the strain partitioning pattern showed arc-perpendicular shortening in the active orogenic wedge –assessed from the radial pattern of tectonic transport directions– accompained by subhorizontal stretching. Structures accommodating stretching fall into two categories on the basis of their space distribution and their relationships with the structural trend-line pattern: i) arc-parallel stretching structures in the external wedge (mainly normal faults and conjugate strike-slip faults); and ii) extensional faults developed in the hinterland zone in which transport directions are centripetal towards the Alboran back-arc basin. Pliocene to Recent deformational structures together with focal solutions from crustal earthquakes (n=167; 1.5<Mw<6.3) support that this strain partitioning pattern still occurs. By contrast, the eastern end zones of the western Gibraltar arc, especially during the last 5My, underwent intense transpression tectonics with a NW to NNW main shortening axis. These results agree with a still active Gibraltar forearc, governed by westward migrating subduction retreat or subcontinental mantle delamination.

Past forearc deformation in Nicaragua and coupling at the megathrust interface: Evidence for subduction retreat?

Deformation at the boundary between the Cocos and Caribbean plates in Nicaragua is currently focused in the volcanic arc in the form of arc‐parallel shearing. In the middle Miocene, however, there was widespread subsidence with local uplift in the outer‐forearc and inner‐forearc broad uplift. To understand this complex deformation, we use numerical modeling to investigate the effect of inhomogeneous friction at the megathrust fault on strain localization in the upper plate. A good fit is obtained when (1) the interface between the slab and upper plate is not situated at the current Wadati‐Benioff zone but rather is moved 50 km landward to where subduction may have been active in the early Tertiary and (2) there is high stress accumulation between 15 and 25 km depths and low stress accumulation updip and downdip of the thrust interface. The results are consistent with a high‐velocity block being the remnant of an oceanic plateau that subducted in the Eocene and was later incorporated into the upper plate. We suggest that this geometry persisted into the middle Miocene, before the slab broke off and the thrust interface jumped seaward.

Transtensional origin of the NE–SW Simitli basin along the Strouma (Strymon) Lineament, SW Bulgaria

Several depocentres have developed along the NNW–SSE-striking Strouma (Strymon) Lineament since the Miocene, forming the Strouma/Strymon graben system. Some of these basins, including the Dzherman and Simitli, strike at high angles to the lineament. The rhomboid-shaped Simitli basin is limited in the south by the NE–SW-striking, seismically active Kroupnik fault and contains terrestrial clastic sediments, which have been deposited since the Sarmatian. Its formation is attributed to ‘wrench-dominated transtension’ (D B event) which activated the basin's NE–SW-striking boundary faults as left-lateral strike-slip structures during the Early–Middle Miocene. This deformation was associated with NNE–SSW contraction and WNW–ESE extension and succeeded the NNE–SSW contraction caused by ‘pure shear-dominated transpression’ (D A event), which governed the region in Late Oligocene–Early Miocene times. During the Middle–Late Miocene, the D B event was followed by WNW–ESE ‘pure shear-dominated extension’ (D 1 event), which caused further widening of the basin. The transtensional origin of the Simitli basin is explained by lateral extrusion of the crustal mass squeezed between the Apulian–Adriatic microplate and the European foreland towards the North Aegean Sea. The subduction retreat of the Hellenic orogen possibly enhanced this lateral extrusion, and from the Late Miocene onwards the NE–SW-oriented back-arc extension balanced the continuing retreat.

Miocene tectonics of the Maramures area (Northern Romania): implications for the Mid-Hungarian fault zone

The interplay between the emplacement of crustal blocks (e.g. “ALCAPA”, “Tisza”, “Dacia”) and subduction retreat is a key issue for understanding the Miocene tectonic history of the Carpathians. Coeval thrusting and basin formation is linked by transfer zones, such as the Mid-Hungarian fault zone, which seperates ALCAPA from Tisza-Dacia. The presented study provides new kinematic data from this transfer zone. Early Burdigalian (20.5 to ∼18.5 Ma) SE-directed thrusting of the easternmost tip of ALCAPA (Pienides), over Tisza-Dacia is linked to movements along the Mid-Hungarian fault zone and the Periadriatic line, accommodating the lateral extrusion of ALCAPA. Minor Late Burdigalian (∼18.5 to 16 Ma) NE-SW extension is interpreted as related to back-arc extension. Post Burdigalian (post-16 Ma) NE–SW shortening and NW–SE extension correlate with “soft collision” of Tisza-Dacia with the European foreland coupled with southward migration of active subduction. During this stage the Bogdan-Voda and Dragos-Voda faults were kinematically linked to the Mid-Hungarian fault zone. Sinistral transpression (16 to 12 Ma) at the Bogdan-Voda fault was followed by sinistral transtension (12–10 Ma) along the coupled Bogdan-Dragos-Voda fault system. During the transtensional stage left-lateral offset was reduced eastwards by SW trending normal faults, the fault system finally terminating in an extensional horse-tail splay.

Deep geodynamics of far field intercontinental back-arc extension: Formation of Cenozoic volcanoes in northeastern China

There are three cases of variation of trench location possible to occur during subduction: trench fixed, trench advancing, and trench retreating. Retreat of trench may lead to back-arc extension. The Pacific plate subducts at low angle beneath the Eurasia plate, tomographic results indicate that the subducted Pacific slab does not penetrate the 670 km discontinuity, instead, it is lying flat above the interface. The flattening occurred about 28 Ma ago. Geodynamic computation suggests: when the frontier of the subducted slab reaches the phase boundary of lower and upper mantle, it may be hindered and turn flat lying above the boundary, facilitates the retreat of trench and back-arc extension. Volcanism in northeastern China is likely a product of such retreat of subduction, far field back-arc extension, and melting due to reduce of pressure while mantle upwelling.

Rift structure, evolution, and earthquakes in the Gulf of Corinth, from reflection seismic images

Continental extension is forming the Gulf of Corinth across the strike of earlier Alpine evolution. Here, we present the first deep reflection sections with pre-stack depth-migration processing across the deep basin of the Corinth active rift, which image structures unpredicted by current models. Resolving the infill as a pile of layers that are broken by faults allows one to follow their subsidence and deformation history. Variation among the profiles suggests that southern normal faults control the rift in a time progression from the east towards its western tip. On the central, Derveni–Itea transect, a 3-km widening of the basin accrued since the initiation of this control that is marked by an unconformity between the two main sedimentary units. This is estimated to have occurred 0.5–0.6 Myr ago, assuming the glacio-eustatic sea-level changes have controlled the stratigraphy of sediments deposited as a succession of layers on the subsiding hangingwall, as they did on the southern footwall in forming the famous flight of marine terraces of Corinth. A roll-over anticline and crestal collapse graben are diagnostic of the control by a normal fault of dip varying with depth. The deeper low-angle part of this bi-planar fault is indeed imaged as a reflector in the basement. The occurrence of the collapse with a breakaway at the steep southern basin-bounding fault of the hangingwall slab can be estimated 0.12–0.2 Myr ago, with a marked increase in extension rate that brought it to its present fastest value over 10 mm/yr. The low-angle part of the active fault might also have controlled earlier evolution upslope and in the basin. When compared with inferences from earthquake studies, this low-angle active fault may not appear to be seismogenic but may participate to the seismic cycle, possibly in a conditionally stable regime. Active faults seen as sea-bottom scarps merely accommodate deformation of its subsiding hangingwall. The footwall of the low-angle faults, which current seismicity shows to be in extension, appears then as being pulled out from beneath the rift, in a motion towards the rolling-back slab that causes the Hellenic subduction retreat.

Apennines subduction‐related subsidence of Venice (Italy)

The foreland regional monocline related to the northern Apennines subduction can be followed from the Po Basin to the Friuli region, Italy, at least 240 km north‐eastward. In seismic reflection profiles of the Po Basin and of the northern Adriatic Sea, the dip of the regional monocline gradually decreases from about 22° to close to 0°. Pleistocene sediments onlap and pinchout to the northeast. Venice is located on a segment of the active monocline dipping about 1.8° to the southwest. In sediments around the Venice area, the 1.43 Ma limit is in the 960–1500 m depth range, indicating a long term subsidence rate of about 0.7–1.0 mm/yr. Therefore a significant part of the natural component of the town subsidence is related to the north‐eastward retreat of the Adriatic subduction. The subduction related downflexure affects the whole of the Po Plain basin and part of the Southern Alps, where it opposes to the general uplift related to orogenesis.