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1.
Long-lived subduction complexes, such as the Franciscan Complex of California, include tectonic contacts that represent exhumed megathrust horizons that collectively accommodated thousands of kilometres of slip. The chaotic nature of mélanges in subduction complexes has spawned proposals that these mélanges form as a result of megathrust displacement. Detailed field and petrographic relationships, however, show that most Franciscan mélanges with exotic blocks formed by submarine landsliding. Field relationships at El Cerrito Quarry in the eastern San Francisco Bay area suggest that subduction slip may have been accommodated between the blueschist facies metagreywacke of the Angel Island nappe above and the prehnite-pumpellyite facies metagreywacke of the Alcatraz nappe below. Although a 100–200 m-thick mélange zone separates the nappes, this mélange is a variably deformed, prehnite-pumpellyite facies sedimentary breccia and conglomerate deposited on the underlying coherent sandstone, so the mélange is part of the lower nappe. A 20–30 m-thick fault zone between the top of the mélange, and the base of the Angel Island nappe displays an inverted metamorphic gradient with jadeite-glaucophane-lawsonite above lawsonite-albite assemblages. This zone has a strong seaward (SW)-vergent shear fabric and hosts ultracataclasite and pseudotachylite. These relationships suggest that significant subduction megathrust displacement at depths of 15–30 km was accommodated within the 20–30 m-thick fault zone. Field studies elsewhere in the Franciscan Complex suggest similar localization of megathrust slip, with some examples lacking mélanges. The narrow megathrust zone at El Cerrito Quarry, its uniform sense-of-shear, and the localization of slip along the contact of, rather than within a mélange, contrast sharply with the predictions of numerical models for subduction channels.  相似文献   

2.
ABSTRACT

The Franciscan Complex comprises the largely sedimentary basement of the California Coast Ranges. This classic trench deposit has undergone a series of superimposed tectonic events since the end of Jurassic time, involving accretion, high-pressure (HP) recrystallization, buoyancy and wedge-driven exhumation, and transcurrent slip. Processes reflect plate convergence, transpressive-orthogonal subduction, and transpressive–transtensive offset. Besides stratigraphically intact strata, the Franciscan displays widespread mélanges of four main types: diapiric serpentinite intrusions, sedimentary olistostromes, broken formations, and tectonic block-in-matrix units. In the northern Coast Ranges, mélanges are especially prevalent in the Central Belt, but also occur in the Eastern and Coastal belts. Diapirs show upward, buoyant flow relative to wall rocks, but some also appear to have involved wedge-driven thrusting. Many serpentinite diapirs and tectonic mélanges contain exotic metamafic inclusions rimmed by actinolite–chlorite reaction rinds. Olistostromes include gravity slump blocks and conglomeratic lenses; petrologically similar to larger slide blocks, pebble layers document a surficial, sedimentary origin, as does the presence of volcanic arc clasts. Broken formations grade by degrees from intact stratal continuity to disrupted units; they only contain cognate boudins of rocks present in the ductile matrix. Some tectonic mélanges are simply intensely disaggregated broken formations, and include rock types of the stratigraphic host. Other tectonic mélanges carry exotic HP blocks of diverse lithologies, generally reflecting higher pressures than attended recrystallization of the low-density matrix. The four mélange types formed through diverse convergent plate-tectonic processes. Many were subjected to a multi-stage overprint; most are strongly deformed, obscuring original textures and structures. Broken formations are the most common disrupted units, accompanied by lesser amounts of tectonic mélanges, olistostromes, and ductile-matrix diapirs. In aggregate, these units reflect the operation of contrasting processes that attest to plate-tectonic evolution of the Franciscan Complex. Strong deformation accompanied oceanic plate underflow, but also took place during coeval HP metamorphism and surfaceward return of accretionary packets, then transitioned to long-sustained, chiefly dextral slip.  相似文献   

3.
Melanges play three principal, overlapping roles in the architecture of subduction accretionary complexes (SACs) during and after SAC formation. First, tectonic melanges serve as zones of concentrated deformation within and below the accreted rocks that are assembled during the subduction-accretion process. These melanges facilitate preservation of inter-melange, less deformed, accretionary units (AUs). Beneath the trench side of the SAC, the initial deformation zone along the decollement at the top of the down-going plate is marked by non-melange, tectonically dismembered formations or thinner units of scaly rock and breccia that separate accreted rocks above from the subducting rocks below. Olistostromal rocks may be incorporated into the decollement here. In the mid-arc to inner-arc areas of the SAC, exotic block-bearing mélanges develop in zones of tectonic fragmentation and mixing of accreting ocean plate stratigraphy, in mud diapirs, and along out-of-sequence faults, some of which facilitate uplift of high-pressure rocks and serpentinite-matrix mélanges. Second, after accretion, the sedimentary, tectonic, diapiric, and polygenetic mélange units serve as single block or sheet architectural AUs (or subunits within larger accreted AUs) of the SAC. Diapiric melanges and sedimentary olistostromal mélanges formed in and above SAC fault blocks, respectively, may become incorporated into the SAC during ongoing deformation and become architectural units, as well. Third, melanges serve as post-subduction stress guides that focus shear strain during continuing and post-accretion deformation of SACs, allowing ongoing modification of the SAC during progressive deformation of the orogen. Examples of each type of role reveal the importance of all three processes in the current architecture of outer orogenic belts.  相似文献   

4.
Accretionary complex histories are broadly understood. Sedimentation in seafloor and trench environments on drifting subducting plates and in associated trenches, followed by (1) deformation and metamorphism in the subduction zone and (2) subsequent uplift at the overriding plate edge, result in complicated stratigraphic and structural sequences in accretionary complexes. Recognizing, defining, and designating individual terranes in subduction complexes clarify some of these complicated relationships within the resulting continent-scale orogenic belts. Terrane designation does not substitute for detailed stratigraphic and structural mapping. Stratigraphic and structural mapping, combined with radiometric and palaeontologic dating, are necessary for delineation of coherent, broken, and dismembered formations, and various mélange units, and for clarification of the details of subduction complex architecture and history. The Franciscan Complex is a representative subduction complex that has evolved through sedimentation, faulting, folding, and low-temperature metamorphism, followed by uplift, associated deformation, and later overprinted deformation. Many belts of Franciscan rocks are offset by strike-slip faults associated with the dextral San Andreas Fault System. In the Franciscan Complex, among the terrane names applied widely, are the ‘Yolla Bolly Terrane’ and the ‘Central Terrane’. Where detailed mapping and detrital zircon ages exist, data reveal that the two names have been applied to rocks of similar general character and age. In the northeastern Diablo Range, Franciscan Complex rocks include coherent units, broken and dismembered formations, and various types of mélanges, all assigned at various times to the Yolla Bolly and other terranes. The details of stratigraphic and structural history revealed by large-scale mapping and radiometric dating prove to be more useful in clarifying the accretionary complex history than assigning a terrane name to the rocks. That history will assist in resolving terrane assignment issues and allow discrimination of subduction-associated and post-subduction events, essential for understanding the overall history of the orogen.  相似文献   

5.
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.  相似文献   

6.
The Franciscan Complex of California is better understood now than in 1972, when Berkland et al. defined it as a complex and divided it into three geographic belts. A re-evaluation is needed. Belts first served as major architectural units, but they have been abandoned by some and renamed as and subdivided into tectonostratigraphic terranes by others. The Franciscan Complex – considered to be the archetypical accretionary complex by many – is the folded, faulted, and stratally disrupted rock mass comprising the supramantle basement of the California-Southern Oregon Coast Ranges exposed east of the Salinian Block and west of and structurally below principal exposures of the Coast Range Fault, Coast Range Ophiolite, Great Valley Group, and Klamath Mountains. The Complex is dominated by sandstones and mudrocks, but contains mafic oceanic crustal fragments with chert, limestone, and other rock types, and zeolite, prehnite-pumpellyite, blueschist, and rare amphibolite and eclogite facies metamorphic rocks. Review of historical precedence, new data, available large-scale maps, and fundamental definitions suggest now (1) that the Belt terminology as applied to the entire Franciscan Complex conflicts with current knowledge of Franciscan rocks and architecture; and (2) that most named Franciscan terranes and nappes are inconsistent with basic definitions of those unit types. The major architectural units into which the Franciscan Complex can be divided are accretionary units – mélanges and underthrust sheets. Underthrust sheets can be subdivided into smaller units, e.g. broken formations and olistostromal mélanges, mappable using traditional lithostratigraphic and structural mapping techniques. Unresolved controversies in reconstruction of the nature and history of the accretionary complex relate to specific mélange origins; megathrust versus subduction channel mélange models; chert conundrums; delineation of the ages, subdivisions, and regional architecture of Franciscan units; palinspastic reconstruction of the pre-Late Cenozoic architecture; and reconstruction of the complete histories of accretionary units.  相似文献   

7.
ABSTRACT

Sedimentary serpentinite and related siliciclastic-matrix mélanges in the latest Jurassic to Lower Cretaceous lower Great Valley Group (GVG) forearc basin strata of the California Coast Ranges reach thicknesses of over 1 km and include high-pressure (HP) metamorphic blocks. These units crop out over an area at least 300 km long by 50 km wide. The serpentinite also contains locally abundant blocks of antigorite mylonite. Antigorite mylonite and HP metamorphic blocks were exhumed from depth prior to deposition in the unmetamorphosed GVG, but the antigorite mylonite may be mistaken for metamorphosed serpentinite matrix in localities with limited exposure. These olistostrome horizons can be distinguished from intact slabs of serpentinized peridotite associated with the Coast Range Ophiolite (CRO) or serpentinite mélanges of the Franciscan subduction complex (FC) on the basis of internal sedimentary textures (absent in CRO), mixing/interbedding with unmetamorphosed siliciclastic matrix and blocks (differs from CRO and FC), and preserved basal sedimentary contacts over volcanic rocks of the CRO or shale, sandstone, and conglomerate of the GVG (differs from CRO and FC). Even in the relatively well-characterized Palaeo trench–forearc region of the California Coast Ranges the GVG deposits are difficult to distinguish from similar units in the FC and CRO. In typical orogenic belts that exhibit greater post-subduction disruption, distinguishing forearc basin olistostrome deposits, subduction complex, and opholite mantle sections is much more difficult. Forearc basin olistostromal deposits have probably been misidentified as one of the other trench–forearc lithologic associations. Such errors may lead to erroneous interpretations of the nature of large-scale material and fluid pathways in trench–forearc systems, as well as misinterpretations of tectonic processes associated with HP metamorphism and exhumation of the resultant rocks.  相似文献   

8.
Mélanges occur as discontinuous, mappable, units along an extensive N–S-trending, steeply dipping zone of distributed shear in metamorphic complexes along the coast of central Chile. Large mélange zones, from north to south, near Chañaral, Los Vilos, Pichilemu, and Chiloé Island, contain variations in lithologic and structural detail, but are consistent in exhibiting cross-cutting fabric features indicating a progressive transition from earlier ductile to more brittle deformation. In the Infiernillo mélange near Pichilemu, Permian to Early Triassic, sub-horizontal schistosity planes of the Western Series schist are disrupted, incorporated into, and uplifted along high-angle, N–S- to NNE–SSW-trending brittle–ductile shears. Mylonitic and cataclastic zones within the mélange matrix indicate active lateral shear during cumulative exhumation from depths exceeding 12 km in some areas. Exotic lithologies, such as Carboniferous mafic amphibolite and blueschist, formed during earlier Gondwanide subduction, match well with similar rocks in the Bahia Mansa to Los Pabilos region 750 km to the south, suggesting possible dextral offset. The development of the Middle to Late Triassic, N–S=trending, near-vertical shear zones formed weaknesses in the crust facilitating later fault localization, gravitational collapse, and subduction erosion along the continental margin. The length and linearity of this zone of lateral movement, coincident with a general hiatus of regional arc magmatism during the Middle to Late Triassic, is consistent with large-scale dextral transpression, or possible transform movement, during highly oblique NNE–SSW convergence along the pre-Andean (Gondwana) margin. The resultant margin parallel N–S-trending shear planes may be exploited by seismically active faults along the present coastal area of Chile. The palaeo-tectonic setting during the transitional period between earlier Gondwanide (Devonian to Permian) and later Andean (Late Jurassic to present) subduction may have had some similarity to the presently active San Andreas transform system of California.  相似文献   

9.
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.  相似文献   

10.
This paper compares features of unambiguous tectonic serpentinite mélanges (TSM) or serpentinite shear zones in the Coast Range ophiolite, Franciscan subduction complex, of coastal California and Sierra City Mélange of the northern Sierra Nevada of northeastern California with undisputed sedimentary serpentinite mélange (SSM) of the Great Valley Group (GVG) forearc basin deposits of coastal California, and with Franciscan serpentinite mélanges of disputed (sedimentary versus tectonic) origin. The GVG sedimentary serpentinite mélanges and disputed Franciscan serpentinite mélanges share strongly similar matrix textures and block-matrix relationships at scales from tens of meters or more to petrographic scale but differ significantly from serpentinite shear zones and TSM. This comparison suggests shared (non-diagnostic) and distinguishing features of TSM versus SSM. Internal bedding or foliation in blocks is oriented subparallel to mélange boundaries and matrix foliation for both TSM and SSM both may have strongly foliated matrix and both may feature localized shearing in matrix around block borders, especially if an SSM underwent significant post-depositional deformation. The same holds true for deformation and dismemberment of blocks, which is the block-forming and mixing mechanism in TSM but variably exhibited in SSM. In contrast only SSM have blocks or clasts whose internal foliation or bedding terminates abruptly along clast/block boundaries with a mismatch in mineralogy and/or lithology across such boundaries. Matrix foliation cuts blocks/clasts in TSM but not in SSM. SSM may show block/grain size grading but not TSM. SSM have exotic blocks and blocks may span a range of metamorphic grade, whereas TSM lack exotic blocks and blocks are isofacial.  相似文献   

11.
The origin of block-in-matrix mélanges has been the subject of intense speculation by structural and tectonic geologists working in accretionary complexes since their first recognition in the early twentieth century. Because of their enigmatic nature, a number of important international meetings and a large number of publications have been devoted to the problem of the origin of mélanges. As mélanges show the effects of the disruption of lithological units to form separate blocks, and also apparently show the effects shearing in the scaly fabric of the matrix, a tectonic origin has often been preferred. Then it was suggested that the disruption to form the blocks in mélanges could also occur in a sedimentary environment due to the collapse of submarine fault scarps to form olistostromes, upon which deformation could be superimposed tectonically. Subsequently it has proposed that some mélanges have originated by overpressured clays rising buoyantly towards the surface, incorporating blocks of the overlying rocks in mud or shale diapirs and mud volcanoes.Two well-known examples of mélanges from the Banda and Sunda arcs are described, to which tectonic and sedimentary origins were confidently ascribed, which proved on subsequent examination to have been formed due to mud diapirism, in a dynamically active environment, as the result of tectonism only indirectly. Evidence from the Australian continental Shelf to the south of Sumba shows that large quantities of diapiric mélange were generated before the diapirs were incorporated in the accretionary complex. Comparable diapirs can be recognised in Timor accreted at an earlier stage. Evidence from both Timor and Nias shows that diapiric mélange can be generated well after the initial accretion process was completed.The problem is: Why, when diapirism is so abundantly found in present convergent margins, is it so rarely reported from older orogenic belts? Many occurrences of mélanges throughout the world to which tectonic and/or sedimentary and origins have been ascribed, may in future investigations prove to have had a diapiric origin.It is emphasised that although the examples of diapiric mélange described here may contain ophiolitic blocks, they were developed in shelf or continental margin environments, and do not contain blocks of high grade metamorphic rocks in a serpentinous matrix; such mélanges originate diapirically during subduction in a mantle environment, as previous authors have suggested.  相似文献   

12.
《Geodinamica Acta》2013,26(1-2):99-118
The Alpine Corsica (Corsica Island, France) is characterized by a stack of continent- and ocean-derived tectonic units, known as Schistes Lustrés complex. This complex is affected by deformation and metamorphic imprint achieved during Late Cretaceous – Early Tertiary subduction- related processes connected with the closure of the Ligure-Piemontese oceanic basin and subsequent continental collision. In the Schistes Lustrés complex, the Lento oceanic unit is characterized by four deformation phases, from D1 to D4 phase. The D1 phase, characterized by blueschist metamorphism, is regarded as related to coherent underplating in a subduction zone at a depth of about 25-30 km. The subsequent deformation phases can be referred to exhumation history, as suggested by the continuous decrease of metamorphic conditions. The transition from accretion to exhumation is represented by the D2 phase, achieved during the development of a duplex structure of accreted units. The D3 phase is in turn achieved by a further horizontal shortening, whereas the D4 phase is developed during an extensional event representing the final exhumation of the Lento unit.

On the whole, the data collected for the Lento unit suggest an history that include an accretion by coherent underplating followed by exhumation, more complex than previous described.  相似文献   

13.
How ophiolitic mèlanges can be defined as sutures is controversial with regard to accretionary orogenesis and continental growth.The Chinese Altay,East junggar,Tianshan,and Beishan belts of the southern Central Asian Orogenic Belt（CAOB） in Northwest China,offer a special natural laboratory to resolve this puzzle.In the Chinese Altay,the Erqis unit consists of ophiolitic melanges and coherent assemblages,forming a Paleozoic accretionary complex.At least two ophiolitic melanges（Armantai,and Kelameili） in East Junggar,characterized by imbricated ophiolitic melanges,Nb-enriched basalts,adakitic rocks and volcanic rocks,belong to a Devonian-Carboniferous intra-oceanic island arc with some Paleozoic ophiolites,superimposed by Permian arc volcanism.In the Tianshan,ophiolitic melanges like Kanggurtag,North Tianshan,and South Tianshan occur as part of some Paleozoic accretionary complexes related to amalgamation of arc terranes.In the Beishan there are also several ophiolitic melanges,including the Hongshishan,Xingxingxia-Shibangjing,Hongliuhe-Xichangjing,and Liuyuan ophiolitic units.Most ophiolitic melanges in the study area are characterized by ultramafic,mafic and other components,which are juxtaposed,or even emplaced as lenses and knockers in a matrix of some coherent units.The tectonic settings of various components are different,and some adjacent units in the same melange show contrasting different tectonic settings.The formation ages of these various components are in a wide spectrum,varying from Neoproterozoic to Permian.Therefore we cannot assume that these ophiolitic melanges always form in linear sutures as a result of the closure of specific oceans.Often the ophiolitic components formed either as the substrate of intra-oceanic arcs,or were accreted as lenses or knockers in subduction-accretion complexes.Using published age and paleogeographic constraints,we propose the presence of （1） a major early Paleozoic tectonic boundary that separates the Chinese Altay-East Junggar multiple subduction system  相似文献   

14.
Many concepts and interpretations on the formation of the Franciscan mélange have been proposed on the basis of exposures at San Simeon, California. In this paper, we show the distribution of chaotic rocks, their internal structures and textures, and the interrelationship between the chaotic rocks and the surrounding sandstones (turbidites). Mélange components, particularly blueschists, oceanic rocks, including greenstone, pillow lava, bedded chert, limestone, sandstone, and conglomerate, have all been brecciated by retrograde deformation. The Cambria Slab, long interpreted as a trench slope basin, is also strongly deformed by fluidization, brecciation, isoclinal folding, and thrusting, leading us to a new interpretation that turbiditic rocks (including the Cambria Slab) represent trench deposits rather than slope basin sediments. These rocks form an accretionary prism above mélanges that were diapirically emplaced into these rocks first along sinistral-thrust faults, and then along dextral-normal faults. Riedel shear systems are observed in several orders of scale in both stages. Although the exhumation of the blueschist blocks is still controversial, the common extensional fractures and brecciation in most of the blocks in the mélanges and further mixture of various lithologies into one block with mélange muddy matrix indicate that once deeply buried blocks were exhumed from considerable depths to the accretionary prism body, before being diapirically intruded with their host mélange along thrust and normal faults, during which retrograde deformation occurred together with retrograde metamorphism. Recent similar examples of high-pressure rock exhumation have been documented along the Sofugan Tectonic Line in the Izu forearc areas, in the Mineoka belt in the Boso Peninsula, and as part of accretionary prism development in the Nankai and Sagami troughs of Japan. These modern analogues provide actively forming examples of the lithological and deformational features that characterize the Franciscan mélange processes.  相似文献   

15.
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.  相似文献   

16.
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17.
With the aim of better understanding the history of ocean closure and suturing between India and Asia, we conducted a geologic investigation of a siliciclastic matrix tectonic mélange within the western Yarlung suture zone of southern Tibet (Lopu Range region, ~ 50 km northwest of Saga). The siliciclastic matrix mélange includes abundant blocks of ocean plate stratigraphy and sparse blocks of sandstone. Metapelite and metabasite blocks in the mélange exhibit lower greenschist facies mineral assemblages, indicating that they were not deeply subducted. We obtained detrital zircon U-Pb geochronologic and sandstone petrographic data from sandstone blocks in the mélange and sandstone beds from Tethyan Himalayan strata exposed to the south of the suture. The sandstones from both units are all similar in U-Pb detrital zircon age spectra and petrography to the nearby Tethyan Cretaceous–Paleocene Sangdanlin section, which records the earliest appearance (at ~ 59 Ma) of arc-affinity strata deposited conformably on Indian-affinity strata. Two Paleocene sandstones, one of which is a schistose block incorporated in the siliciclastic matrix mélange, yielded indistinguishable maximum depositional ages of ~ 59 Ma. Mesozoic Asian-affinity sandstone blocks previously documented in the siliciclastic matrix mélange 200–500 km along strike to the east are notably absent in the Lopu Range region. We documented a gradational transition in structural style from the block-in-matrix mélange in the northeast to the south-vergent Tethyan thrust belt in the southwest. Blocks of Tethyan Himalayan strata increase in size and the volumetric proportion of matrix decreases from northeast to southwest. We conclude that no arc-affinity sandstone blocks were incorporated into the subduction complex until India-Asia collision at ~ 59 Ma when the Xigaze forearc basin became overfilled and Tethyan Himalayan strata entered the trench. As collision progressed, there was a gradual transition in structural style from block-in-matrix mélange formation to imbricate-style thrust belt formation.  相似文献   

18.
The West Junggar lies in the southwest part of the Central Asian Orogenic Belt (CAOB) and consists of Palaeozoic ophiolitic mélanges, island arcs, and accretionary complexes. The Barleik ophiolitic mélange comprises several serpentinite-matrix strips along a NE-striking fault at Barleik Mountain in the southern West Junggar. Several small late Cambrian (509–503 Ma) diorite-trondhjemite plutons cross-cut the ophiolitic mélange. These igneous bodies are deformed and display island arc calc-alkaline affinities. Both the mélange and island arc plutons are uncomfortably covered by Devonian shallow-marine and terrestrial volcano-sedimentary rocks and Carboniferous volcano-sedimentary rocks. Detrital zircons (n = 104) from the Devonian sandstone yield a single age population of 452–517 million years, with a peak age of 474 million years. The Devonian–Carboniferous strata are invaded by an early Carboniferous (327 Ma) granodiorite, late Carboniferous (315–311 Ma) granodiorites, and an early Permian (277 Ma) K-feldspar granite. The early Carboniferous pluton is coeval with subduction-related volcano-sedimentary strata in the central West Junggar, whereas the late Carboniferous–early Permian intrusives are contemporary with widespread post-collisional magmatism in the West Junggar and adjacent regions. They are typically undeformed or only slightly deformed.

Our data reveal that island arc calc-alkaline magmatism occurred at least from middle Cambrian to Late Ordovician time as constrained by igneous and detrital zircon ages. After accretion to another tectonic unit to the south, the ophiolitic mélange and island arc were exposed, eroded, and uncomfortably overlain by the Devonian shallow-marine and terrestrial volcano-sedimentary strata. The early Carboniferous arc-related magmatism might reflect subduction of the Junggar Ocean in the central Junggar. Before the late Carboniferous, the oceanic basins apparently closed in this area. These different tectonic units were stitched together by widespread post-collisional plutons in the West Junggar during the late Carboniferous–Permian. Our data from the southern West Junggar and those from the central and northern West Junggar and surroundings consistently indicate that the southwest part of the CAOB was finally amalgamated before the Permian.  相似文献   

19.
The Sierra del Convento and La Corea mélanges (eastern Cuba) are vestiges of a Cretaceous subduction channel in the Caribbean realm. Both mélanges contain blocks of oceanic crust and serpentinite subducted to high pressure within a serpentinite matrix. The bulk composition of serpentinite indicates spinel-harzburgite and -herzolite protoliths. The samples preserve fertile protolith signatures that suggest low melting degrees. High concentration of immobile elements Zr, Th, Nb, and REE contents (from ~0.1 to ~2 CI-chondrite) point to early melt–rock interaction processes before serpentinization took place. Major- and trace-element compositions suggest an oceanic fracture-zone–transform-fault setting. A mild negative Eu anomaly in most samples indicates low-temperature fluid–rock interaction as a likely consequence of seawater infiltration during oceanic serpentinization. A second, more important, serpentinization stage is related to enrichment in U, Pb, Cs, Ba, and Sr due to the infiltration of slab-derived fluids. The mineral assemblages are mainly formed by antigorite, lizardite, and chlorite, with local minor talc, tremolite, anthophyllite, dolomite, brucite, and relict orthopyroxene. The local presence of anthophyllite and the replacements of lizardite by antigorite indicate a metamorphic evolution from the cooling of peridotite/serpentinite at the oceanic context to mild heating and compression in a subduction setting. We propose that serpentinites formed at an oceanic transform-fault setting that was the locus of subduction initiation of the Proto-Caribbean basin below the Caribbean plate during early Cretaceous times. Onset of subduction at the fracture zone allowed the preservation of abyssal transform-fault serpentinites at the upper plate, whereas limited downward drag during mature subduction placed the rocks in the subduction channel where they tectonically mixed with the upward-migrating accreted block of the subducted Proto-Caribbean oceanic crust. Hence, we suggest that relatively fertile serpentinites of high-pressure mélanges were witness to the onset of subduction at an oceanic transform-fault setting.  相似文献   

20.
ABSTRACT

The Tarbagatay Complex, located in northwest Junggar, is situated tectonically between the Zharma–Saur arc to the north and the Tacheng terrane and the Boshchekol–Chingiz arc to the south. This Complex belt is variably composed of ophiolitic mélange, sedimentary mélange, and coherent units of turbidites and shallow water sediments. These rocks crop out in fault-bound slices with fault-parallel asymmetric folds. Both the lithologies and deformation features of the Tarbagatay Complex suggest an accretionary origin generally with a top-to-the-south tectonic vergence, suggesting N-dipping subduction beneath the Zharma–Saur arc. The presence of a former ocean is indicated by the Ordovician ophiolite mélanges and related marine fossils. The time duration of the Tarbagatay Complex can be bracketed by detrital zircon ages of turbidites and shallow water sediments with a lower limit of major peak ages of 350–370 Ma, and an upper limit of middle Permian indicated by detrital zircon ages of 262.3 Ma. Based on these data, we suggest that the subduction of the Tarbagatay Ocean likely started in the Late Devonian and lasted until the middle Permian. Taking into account the formation of the northern part of the Kazakhstan orocline, which has a similar temporal-spatial framework, we propose a tectonic model for the western CAOB that involves accretion and amalgamation from the Ordovician to the middle Permian.  相似文献   

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