Extrusion vs. accretion at the frictional–viscous décollement transition in experimental thrust wedges: the role of convergence velocity

In many cases, thrust wedges accreted at shallow crustal levels show an across‐strike rheological variability along the basal décollement, notably from brittle to ductile behaviour. In this paper, we illustrate the results of sandbox analogue modelling research devoted to studying the influence of convergence velocity on wedge architecture when laterally juxtaposed frictional and viscous materials occur along the basal décollement of accreting thrust wedges. Our results show that slow convergence favours a near symmetrical distribution of thrust vergence within wedge sectors accreted above viscous décollement material, whereas fast convergence favours vergence asymmetry. In particular, at fast convergence rates the hinterlandward extrusion of viscous décollement material at the toe of the frictional wedge is favoured and contributed to accommodate a significant amount of the total contraction. Terra Nova, 18, 241–247, 2006     

Growth of accretionary wedges and pulsed ophiolitic mélange formation by successive subduction of trench‐parallel volcanic elevations

Fault‐bounded coherent belts alternating with belts of mélanges are common in accretionary wedges and are usually interpreted as a result of imbrication along subduction zone megathrusts. Using the Neoproterozoic/early Cambrian Blovice accretionary complex (BAC), Bohemian Massif, as a case example, we present a new model for the origin of alternating belts through the repetition of several cycles of (1) offscraping and deformation of trench‐fill sediments to form the coherent units, interrupted by (2) arrival and subduction of linear, trench‐parallel volcanic elevations. The latter process leads to an increase in the wedge taper, triggering mass‐wasting and formation of olistostromes. At the same time, ophiolitic mélanges form by disruption of an upper part of the volcanic ridge and incorporation of the disrupted ocean‐floor succession into the olistostromes. Specifically, the BAC represents a complete section across an accretionary wedge and records three such major pulses of ophiolitic mélange formation through subduction of an outboard back‐arc basin.     

Numerical modeling of porosity waves in the Nankai accretionary wedge décollement,Japan: implications for aseismic slip

Seismic and hydrologic observations of the Nankai accretionary wedge décollement, Japan, show that overpressures at depths greater than ～2 km beneath the seafloor could have increased to near lithostatic values due to sediment compaction and diagenesis, clay dehydration, and shearing. The resultant high overpressures are hypothesized then to have migrated in rapid surges or pulses called ‘porosity waves’ up the dip of the décollement. Such high velocities—much higher than expected Darcy fluxes—are possible for porosity waves if the porous media through which the waves travel are deformable enough for porosity and permeability to increase strongly with increasing fluid pressure. The present study aimed to test the hypothesis that porosity waves can travel at rates (kilometers per day) fast enough to cause aseismic slip in the Nankai décollement. The hypothesis was tested using a one-dimensional numerical solution to the fluid mass conservation equation for elastic porous media. Results show that porosity waves generated at depths of ～2 km from overpressures in excess of lithostatic pressure can propagate at rates sufficient to account for aseismic slip along the décollement over a wide range of hydrogeological conditions. Sensitivity analysis showed porosity wave velocity to be strongly dependent on specific storage, fluid viscosity, and the permeability–depth gradient. Overpressure slightly less than lithostatic pressure could also produce porosity waves capable of traveling at velocities sufficient to cause aseismic slip, provided that hydrogeologic properties of the décollement are near the limits of their geologically reasonable ranges.     

Structural consequences of cohesion in gravitational instabilities triggered by fluid overpressure: Analytical derivation and experimental testing

The critical taper theory of Coulomb wedges has been classically applied to compressive regimes (accretionary prisms/fold-and-thrust belts), and more recently to gravitational instabilities. Following the initial hypothesis of the theory, we provide an alternative expression of the exact solution for a non-cohesive wedge by considering the balance of forces applied to the external surfaces. Then, we use this approach to derive a solution for the case of cohesive wedges. We show that cohesion has conspicuous structural effects, including a minimum length required for sliding and the formation of listric faults. The stabilizing effect of cohesion is accentuated in the foremost thin domain of the wedge, defining a required Minimum Failure Length (MFL), and producing sliding of a rigid mass above the detachment. This MFL decreases with less cohesion, a smaller coefficient of internal friction, larger fluid overpressure ratio, and steeper upper and basal surfaces for the wedge. Listricity of the normal faults depends on the fluid overpressure magnitude within the wedge. For moderate fluid overpressure, normal faults are curved close to the surface, and become straight at depth. In contrast, where fluid overpressure exceeds a critical value corresponding to the fluid pressure required to destabilize the surface of a noncohesive wedge, the state of stress changes and rotates at depth. The faults are straight close to the surface and listric at depth, becoming parallel to the upper surface if the wedge is thick enough. We tested some of these structural effects of a cohesive wedge on gravitational instabilities using analogue models where cohesive material was subjected to pore-fluid pressure. The shape of the faults obtained in the models is consistent with the predictions of the theory.     

Internal deformation and compaction of the Makran accretionary wedge

The Makran accretionary wedge is one of the largest on Earth. A 7-km-thick column of sands and quartzolithic turbidites are incorporated into this wedge in a series of deformed thrust sheets. We present the results of prestack depth migration and focusing-error analysis (migration velocity analysis) performed on a profile across the Makran wedge. The depth section shows the deformation style of the accreted sediments, and the migration velocities allow us to estimate porosity variations in the sediments. The thrust sheets show evidence of fault-propagation folding, with a long wavelength of deformation (≈ 12 km) and secondary thrusting in the kink bands of the folds, such that the central part of each thrust sheet is elevated to form an additional ridge. This deformation style and the 15° steep surface slope of the first ridge suggest a high degree of consolidation. Porosities were calculated from the seismic migration velocities and the ratio of fluid pressure to lithostatic pressure λ was estimated for 5 locations along the profile. Rather than being undercompacted and overpressured as in most accretionary wedges, the sedimentary input is normally compacted (exponential porosity decay) throughout almost the whole wedge. However, a slight increase in porosity and λ at depth, with respect to the normal compaction curve indicates, that the turbiditic sequence might be overpressured landward of the deformation front.     

Physical Modeling of Fold-and-Thrust Belt Evolution and Triangle Zone Development:Dabashan Foreland Belt(Northeast Sichuan basin,China) as an Example

Triangle zones,generally found in foreland fold-and-thrust belts,serve as favorable objects of petroleum exploration.Taking the Dabashan foreland belt as an example,we studied the formation and development of triangle zones,and investigated the effect of decollements and the mechanical contrast of lithology by employing the method of physical modeling.Four experimental models were conducted in the work.The results showed that ’sand wedges’ grew episodically,recorded by deformational length,height and slope angle.The height versus shortening rate presented an S-shape curve,and uplifting occurred successively in the direction of the foreland belt.During the formation of the triangle zone,layer-parallel shortening took place at the outset;deformation decoupling then occurred between the upper and lower brittle layers,divided by a middle-embedded silicone polymers layer.The upper brittle layers deformed mainly by folding,while the lower sand layers by thrusting. As shortening continued,the geometry of a triangle zone was altered.We consider that the triangle zone in the Dabashan foreland belt was modified from an early one based on available seismic profiles and the experimental results.In addition,decollements and mechanical contrast impose significant influence on structural development,which can directly give rise to structural discrepancies.More decollements and obvious mechanical contrast between brittle layers can promote the coupling between the upper and lower brittle layers.Basal decollement controls the whole deformation and decreases the slope angle of the wedge,while roof decollement determines whether a triangle zone can be formed.     

The impact of analogue material properties on the geometry,kinematics, and dynamics of convergent sand wedges

Simulation of geodynamic processes in sandbox experiments requires analogue materials with a deformation behaviour that reproduces the deformation mechanisms of typical crustal rocks. We present data on the frictional strength of different sand types employing static and dynamic shear tests. The sand types analysed are characterised by an elastic/frictional plastic mechanical behaviour with a transient strain-hardening and strain-softening phase prior to transition to stable sliding. This is in conflict with the standard assumption of an ideal cohesionless Coulomb-material with constant frictional properties. The influence of the identified transient material properties on the kinematics, growth mechanisms, and internal deformation patterns of convergent sand wedges results in characteristic wedge segments which vary—depending on material compaction—between wedges with well defined segments (i.e. frontal-deformation zone, frontal-imbrication zone and internal-accumulation zone) with straight slopes and wedges with a continuous convex topographic profile. For most materials, only the frontal part of the wedge is critical during experimental runs. Taper and strength of the wedge segments can be shown to be controlled by the frictional properties of active faults. Wedge segmentation is controlled by a bulk-strength increase toward the rear of the wedge due to fault rotation in mechanically less-favourable orientations and plastic material hardening. The limit between the frontal critical parts of a wedge and internal stable parts is largely controlled by a critical state of stress upon which either renewed failure or fault inactivation occurs. On this basis, we suggest that critical-taper analysis of wedges must be restricted to specific kinematic segments. Comparison of the experimental results with the Nankai accretionary wedge suggests that our interpretation also applies to natural convergent wedges. Moreover, we provide constraints for the selection of adequate granular analogue materials to simulate typical crustal rocks in natural convergent wedges.     

Fragments of oceanic islands in accretion–collision areas of Gorny Altai and Salair, southern Siberia, Russia: early stages of continental crustal growth of the Siberian continent in Vendian–Early Cambrian time

The Altai-Salair area in southern Siberia is a Caledonian folded area containing fragments of Vendian–Early Cambrian island arcs. In the Vendian–Early Cambrian, an extended system of island arcs existed near the Paleo-Asian Ocean/Siberian continent boundary and was located in an open ocean realm. In the present-day structural pattern of southern Siberia, the fragments of Vendian–Early Cambrian ophiolites, island arcs and paleo-oceanic islands occur in the accretion–collision zones. We recognized that the accretion–collision zones were mainly composed of the rock units, which were formed within an island-arc system or were incorporated in it during the subduction of the Paleo-Asian Ocean under the island arc or the Siberian continent. This system consists of accretionary wedge, fore-arc basin, primitive island arc and normal island arc. The accretionary wedges contain the oceanic island fragments which consist of OIB basalts and siliceous—carbonate cover including top and slope facies sediments. Oceanic islands submerged into the subduction zone and, later were incorporated into an accretionary wedge. Collision of oceanic islands and island arcs in subduction zones resulted in reverse currents in the accretionary wedge and exhumation of high-pressure rocks. Our studies of the Gorny Altai and Salair accretionary wedges showed that the remnants of oceanic crust are mainly oceanic islands and ophiolites. Therefore, it is important to recognize paleo-islands in folded areas. The study of paleo- islands is important for understanding the evolution of accretionary wedges and exhumation of subducted high-pressure rocks.     

Submarine landslides caused by seamounts entering accretionary wedge systems

Seamounts entering active subduction zone trenches initially collide with the frontal sedimentary accretionary wedges resulting in severe deformation of the overriding plate. A typical feature of this deformation is the occurrence of submarine landslides due to gravitational instabilities. Such landslides have been reported from the Middle America and Hikurangi trenches and potentially generate tsunami waves. Yet, the dynamics of accretionary wedges during seamount indentation, and landsliding as a mechanical response in particular, have not been investigated qualitatively. Here, I use 3D high‐resolution numerical experiments to model the collision of conical and flat‐topped seamounts into accreting sedimentary sequences. Results show that the topographical evolution of an accretionary wedge mainly depends on the volume of the entering seamount and not on its height. Submarine landslides occur only if seamounts are not completely buried by the sedimentary sequence, and the volume of the avalanche is roughly correlated with the seamount volume overtopping the incoming sediments.     

Failure in accretionary wedges with the maximum strength theorem: numerical algorithm and 2D validation

The objective is to capture the 3D spatial variation in the failure mode occurring in accretionary wedges and their analog experiments in the laboratory from the sole knowledge of the material strength and the structure geometry. The proposed methodology relies on the maximum strength theorem which is inherited from the kinematic approach of the classical limit analysis. It selects the optimum virtual velocity field which minimizes the tectonic force. These velocity fields are constructed by interpolation thanks to the spatial discretization conducted with ten-noded tetrahedra in 3D and six-noded triangles in 2D. The resulting, discrete optimization problem is first presented emphasizing the dual formalism found most appropriate in the presence of nonlinear strength criteria, such as the Drucker–Prager criterion used in all reported examples. The numerical scheme is first applied to a perfectly triangular 2D wedge. It is known that failure occurs to the back for topographic slope smaller than and to the front for slope larger than a critical slope, defining subcritical and supercritical slope stability conditions, respectively. The failure mode is characterized by the activation of a ramp, its conjugate back thrust, and the partial or complete activation of the décollement. It is shown that the critical slope is captured precisely by the proposed numerical scheme, the ramp, and the back thrust corresponding to regions of localized virtual strain. The influence of the back-wall friction on this critical slope is explored. It is found that the failure mechanism reduces to a thrust rooting at the base of the back wall and the absence of back thrust, for small enough values of the friction angle. This influence is well explained by the Mohr construction and further validated with experimental results with sand, considered as an analog material. 3D applications of the same methodology are presented in a companion paper.     

Late Oligocene–early Miocene olistostromes (sedimentary mélanges) as tectono-stratigraphic constraints to the geodynamic evolution of the exhumed Ligurian accretionary complex (Northern Apennines,NW Italy)

In the Northern Apennines of Italy, mud-rich olistostromes (sedimentary mélanges) occur at different stratigraphic levels within the late Oligocene–early Miocene sedimentary record of episutural/wedge-top basins. They are widely distributed along the exhumed outer part of the Ligurian accretionary complex, atop the outer Apenninic prowedge, over an area about 300 km long and 10–15 km wide. Olistostromes represent excellent examples of ancient submarine mass-transport complexes (MTCs), consisting of stacked cohesive debris flows that can be directly compared to some of those observed in modern accretionary wedges. We describe the internal arrangement of olistostrome occurrences in the sector between Voghera and the Monferrato area, analysing their relationships with mesoscale liquefaction features, which are commonly difficult to observe in modern MTCs. Slope failures occurred in isolated sectors along the wedge front, where out-of-sequence thrusting, seismicity, and different pulses of overpressured tectonically induced fluid flows acted concomitantly. Referring to the Northern Apennines regional geology, we also point out a gradual lateral rejuvenation (from late Oligocene to early Miocene) toward the SE and an increasing size and thickness of the olistostromes along the strike of the frontal Apenninic prowedge. This suggests that morphological reshaping of the outer prowedge via mass-transport processes balanced, with different pulses over a short time span, the southeastward migration and segmentation of accretionary processes. The latter were probably favoured by the occurrence in the northwestern part of the Northern Apennines of major, inherited palaeogeographic features controlling the northward propagation of the prowedge. Detailed knowledge of olistostromes, as ancient examples of MTCs related to syn-sedimentary tectonics and shale diapirism, and of their lateral variations in term of age and size, provides useful information in regard to better understanding of both the tectono-stratigraphic evolution of the Apenninic prowedge and the submarine slope failures in modern accretionary wedges.     

OPS mélange: a new term for mélanges of convergent margins of the world

Ocean plate stratigraphy (OPS) is essential to understanding accretionary wedges and complexes along convergent plate margins. Mélanges within accretionary wedges and complexes are the products of fragmentation and mixing processes during and following OPS accretion. A new term, ‘OPS mélange’, is proposed here for mélanges composed mostly of blocks of OPS with an argillaceous matrix, and for a mixture of mélanges of multiple origins with either broken or coherent formations. An OPS mélange results from the fragmentation and disruption of OPS, without admixing of other components. Three major types of OPS mélange can be distinguished on the basis of their components: turbidite type, chert–turbidite type, and limestone–basalt type. These three types potentially form similar mélanges, but they are derived from different parts of the OPS, depending on the level of the decollement surface. The concept of ‘OPS mélange’ can be applied to most of the mélanges in accretionary prisms and complexes worldwide. In addition, this proposal recognizes a distinction between processes of fragmentation and mixing of OPS components, and mixing of ophiolite components, the latter of which results in serpentinite mélanges, not OPS mélanges. Mélanges composed of OPS sequences occur worldwide. The recognition of OPS mélanges is a key aspect of understanding tectonic processes at convergent margins, which result in mélange formation in orogenic belts globally.     

Structural architecture and low‐grade metamorphism of the Mikabu‐Northern Chichibu accretionary wedge,SW Japan

To understand tectono‐metamorphic processes within or close to the brittle–ductile transition of quartz‐rich crustal rocks in an accretionary wedge, an integrated field, petrological, geochronological and Raman spectroscopic study was conducted on the Mikabu‐Northern Chichibu belt in SW Japan. Field mapping in central Shikoku reveals that the Northern Chichibu belt is comprised of a pile of four tectono‐stratigraphic units, referred to as A, B, C and D units. The A unit (dominated by pelagic sedimentary rocks) represents the structurally lowest and youngest accretionary complex that forms a composite unit with the Mikabu ophiolitic suite. The B unit (consisting of chert‐clastic rock sequences) overlies the A unit and is overlain by the C and D units (mudstone‐matrix mélange units). Raman spectroscopy of carbonaceous material constrains the peak temperature of each unit to be ~290°C for the A unit, 270–290°C for the B unit, 230–250°C for the C unit and ~220°C for the D unit. Ductile deformation and pervasive metamorphism are limited to rocks in the Mikabu, A and B units. Alkali pyroxene and sodic amphibole occur in metabasite from the Mikabu, A and B units, and the widespread occurrence of prograde veins containing lawsonite+quartz pseudomorphs after laumontite was newly recognized from the C unit. Phase petrological data constrain the peak pressure of each unit to be ~0.65 GPa for the Mikabu‐A unit (aragonite stable), ~0.45–0.6 GPa for the B unit (jadeite+albite stable in the structurally lower part), and ~0.35 GPa for the C unit (prehnite+lawsonite stable). The peak metamorphic pressure increases towards structurally lower and younger accretionary complexes, but the thickness of the preserved strata is insufficient to account for the inferred pressure range. The structural–metamorphic relations imply thickening of the accretionary wedge by underplating was followed by a significant phase of thinning by both ductile and brittle processes.     

Fluid inclusion evidence for deep burial of the Tertiary accretionary wedge of the Carpathians

Hydrothermal quartz from mineralized joints of the Carpathian accretionary wedge contains immiscible aqueous, oil‐condensate, methane and carbon dioxide‐rich fluid inclusions. Distribution patterns of the inclusion trapping PT parameters point to a crack‐seal mechanism during upward and lateral migration of hot methane‐rich fluids from overpressured sediments at the base of the accretionary wedge. A simple equation is proposed to calculate depths from densities and trapping pressures of the buoyant inclusion fluids. In the Carpathian accretionary wedge, the paleofluid pressures of 52–306 MPa correspond to a 5‐ to 11‐km‐thick overburden. Prior to exhumation, thickness of the wedge must have attained 10–25 km, of which only c. 50% was preserved until recently. Anomalously high methane densities (up to 0.43 g cm^?3) recorded in the lowermost nappe sheets are provisionally interpreted as a result of supralithostatic overpressure due to thermal cracking of oil and kerogen to methane and pyrobitumen at temperatures above 200 °C.     

Intrinsic versus extrinsic variability of analogue sand-box experiments – Insights from statistical analysis of repeated accretionary sand wedge experiments

Analogue models are not perfectly reproducible even under controlled boundary conditions which make their interpretation and application not always straight forward. As any scientific experiment they include some random component which can be influenced both by intrinsic (inherent processes) and extrinsic (boundary conditions, material properties) sources. In order to help in the assessment of analogue model results, we discriminate and quantify the intrinsic versus extrinsic variability of results from “sandbox” models of accretionary wedges that were repeated in a controlled environment. The extrinsic source of variability, i.e. the parameter varied is the nature of the décollement (material, friction and thickness). Experiment observables include geometric properties of the faults (lifetime, spacing, dip) as well as wedge geometry (height, slope, length).For each variable we calculated the coefficient of variance (CV) and quantified the variability as a symmetric distribution (Normal, Laplacian) or asymmetric distribution (Gamma) using a Chi squared test (χ²). Observables like fault dip/back thrust dip (CV = 0.6–0.7/0.2–0.6) are less variable and decrease in magnitude with decreasing basal friction. Variables that are time dependent like fault lifetime (CV = 0.19–0.56) and fault spacing (CV = 0.12 – 0.36) have a higher CV consequently affecting the variability of wedge slope (CV = 0.12–0.33). These observables also increase in magnitude with increasing basal friction. As the mechanical complexity of the evolving wedge increases over time so does the CV and asymmetry of the distribution. In addition, we confirm the repeatability of experiments using an ANOVA test. Through the statistical analysis of results from repeated experiments we present a tool to quantify variability and an alternative method to gaining better insights into the dynamic mechanics of deformation in analogue sand wedges.     

Deterministic chaos in frictional wedges revealed by convergence analysis

A triangular wedge, composed of a frictional material such as sand, and accreting additional material at its front, is the classical prototype for accretionary wedges and fold‐and‐thrust belts. A simplified method is proposed to capture the internal deformation of this structure resulting from a large number of faulting events during compression. The method combines the application of the kinematic approach of limit analysis to predict the optimum thrust‐fold and a set of geometrical rules to update the geometry accordingly, at each increment of shortening. It is shown that the structure topography remains approximately planar with a slope predicted by the critical Coulomb wedge theory. Failure by faulting occurs anywhere within the wedge at criticality, and its exact position is sensitive to topographic perturbations resulting from the deformation history. The convergence analysis in terms of the shortening increments and of the topography discretization reveals that the timing and the position of a single faulting event cannot be predicted. The convergence is achieved nevertheless in terms of the statistics of the distribution of the faulting events throughout the structure and during the entire deformation history. It is these two convergence properties that are presented to justify the claim that these compressed frictional wedges are imperfection sensitive, chaotic systems. Copyright © 2013 John Wiley & Sons, Ltd.     

Stratigraphy and structure of the central East Sakhalin accretionary wedge (Eastern Russia)

The East Sakhalin accretionary wedge is a part of the Cretaceous-Paleogene accretionary system, which developed on the eastern Asian margin in response to subduction of the Pacific oceanic plates. Its formation was related to the evolution of the Early Cretaceous Kem-Samarga island volcanic arc and Late Cretaceous-Paleogene East Sikhote Alin continental-margin volcanic belt. The structure, litho-, and biostratigraphy of the accretionary wedge were investigated in the central part of the East Sakhalin Mountains along two profiles approximately 40 km long crossing the Nabil and Rymnik zones. The general structure of the examined part of the accretionary wedge represents a system of numerous east-vergent tectonic slices. These tectonic slices. tens to hundreds of meters thick. are composed of various siliciclastic rocks, which were formed at the convergent plate boundary, and subordinate oceanic pelagic cherts and basalts, and hemipelagic siliceous and tuffaceous-siliceous mudstones. The siliciclastic deposits include trench-fill mudstones and turbidites and draping sediments. The structure of the accretionary wedge was presumably formed owing to off-scraping and tectonic underplating. The off-scraped and tectonically underplated fragments were probably tectonically juxtaposed along out-of-sequence thrusts with draping deposits. The radiolarian fauna was used to constrain the ages of rocks and time of the accretion episodes in different parts of the accretionary wedge. The defined radiolarian assemblages were correlated with the radiolarian scale for the Tethyan region using the method of unitary associations. In the Nabil zone, the age of pelagic sediments is estimated to have lasted from the Late Jurassic to Early Cretaceous (Barremian); that of hemipelagic sediments, from the early Aptian to middle Albian; and trench-fill and draping deposits of the accretionary complex date back to the middle-late Albian. In the Rymnik zone, the respective ages of cherts, hemipelagic sediments, and trench facies with draping deposits have been determined as Late Jurassic to Early Cretaceous (middle Albian), middle Aptian-middle Cenomanian, and middle-late Cenomanian. East of the rear toward the frontal parts of the accretionary wedge, stratigraphic boundaries between sediments of different lithology become successively younger. Timing of accretion episodes is based on the age of trench-fill and draping sediments of the accretionary wedge. The accretion occurred in a period lasting from the terminal Aptian to the middle Albian in the western part of the Nabil zone and in the middle Cenomanian in the eastern part of the Rymnik zone. The western part of the Nabil zone accreted synchronously with the Kiselevka-Manoma accretionary wedge located westerward on the continent. These accretionary wedges presumably formed along a single convergent plate margin, with the Sakhalin accretionary system located to the south of the Kiselevka-Manoma terrane in the Albian.     

班公湖—双湖—怒江—昌宁—孟连对接带时空结构——特提斯大洋地质及演化问题

班公湖—双湖—怒江（中北段）—昌宁—孟连对接带广泛出露特提斯大洋岩石圈俯冲消减过程中产生的不同时代、不同构造环境、不同变质程度、不同变形样式的洋板块构造地层系统、增生混杂的构造—岩石组合体,可识别出增生的远洋沉积岩、海沟浊积岩、古生代—中生代蛇绿岩、蛇绿混杂岩、洋岛-海山消减增生楔、洋底沉积增生杂岩,基底残块以及以蓝片岩、榴辉岩为代表的高压—超高压变质岩带,记录了青藏高原原古特提斯大洋形成演化的地质信息。班公湖—双湖—怒江—昌宁—孟连对接带是青藏高原中部一条重要的原古特提斯大洋自北向南后退式俯冲消亡的巨型增生杂岩带,构筑了冈瓦纳大陆与劳亚-泛华夏大陆分界带。     

Tectonic setting required for the preservation of sedimentary mélanges in Palaeozoic and Mesozoic accretionary complexes of southwest Japan

The Palaeozoic to Mesozoic accretionary complexes of southwest Japan include various types of mélange. Most mélanges are polygenetic in origin, being sedimentary or diapiric mélanges that were overprinted by tectonic deformation during subduction. Sedimentary mélanges, without a tectonic overprint, are present in the Permian accretionary complexes of the Akiyoshi and Kurosegawa belts and in the Early Cretaceous accretionary complex of the Chichibu Belt. These mélanges are characterized by dominant basalt and limestone clasts, within a mudstone matrix. The basalt and limestone clasts within the sedimentary mélanges were derived from ancient seamounts. Subduction of a seamount results in deformation of the pre-existing accretionary wedge, and it is difficult to incorporate a seamount into an accretionary wedge; therefore, preservation of seamount fragments requires a special tectonic setting. Oceanic plateau accretion might play an important role in interrupting the processes of subduction and accretion during the formation of accretionary complexes. Especially the Mikabu oceanic plateau might have caused the cessation of accretion during the Early Cretaceous. The subduction and accretion of volcanic arcs and oceanic plateaux helps to preserve sedimentary mélanges from tectonic overprinting by preventing further subduction.