Terrane sutures in the Maine Appalachians,USA and adjacent areas

Terrane sutures in the Maine Appalachians and adjacent areas are recognized as melange dominated, deformed accretionary prisms of Ordovician age, and as a broad synmetamorphic transcurrent fault zone of probable Late Silurian-Early Devonian age. Although the accretionary prisms are associated with present day aeromagnetic and Bouguer gravity anomalies, they are probably not associated with present day crustal penetrating boundaries. The geology of the accretionary prisms indicates subduction-obduction dominated regimes during which (1) the Gander and Boundary Mountain (Dunnage) terranes amalgamated and (2) the composite Boundary Mountain-Gander terrane accreted to the Laurentian margin. The Penobscottian orogeny produced and deformed the older of the two accretionary prisms. This accretionary prism indicates that the Penobscottian was a continuous or perhaps diachronous event which spanned the late Cambrian to early Late Ordovician. The younger accretionary prism was produced and deformed during the Taconian orogeny during late Middle to early Late Ordovician. Initial deformation of this accretionary prism may have overlapped the waning stages of the Penobscottian. Portions of the Taconian arc locally overlie the Penobscottian accretionary prism. A synmetamorphic fault zone lies within Precambrian(?) to Ordovician(?) bimodal metavolcanic and metapelitic rocks assigned here to the Avalon terrane. This zone is several kilometres wide and is interpreted to be the postsubduction suture between the Avalon and Gander terranes, and may, in part, represent a fossil transform fault system. The fault zone contains phyllonites as well as shear zones which generally record dextral motion. The phyllonites were previously interpreted as a stratigraphic unit, whereas the shear zones span or are contained within mappable compositional units. Formation of and movement along these phyllonites and shear zones ceased before the intrusion of Early Devonian plutons. Not all faults in south-western Maine are related to the suture. Younger dip and/or strike-slip and thrust faults are approximately parallel to, or may lie within, the older shear zones and they complicate the recognition of the older faults.     

Diapir structure and its constraint on gas hydrate accumulation in the Makran accretionary prism,offshore Pakistan

The Makran accretionary prism is located at the junction of the Eurasian Plate, Arabian Plate and Indian Plate and is rich in natural gas hydrate (NGH) resources. It consists of a narrow continental shelf, a broad continental slope, and a deformation front. The continental slope can be further divided into the upper slope, middle slope, and lower slope. There are three types of diapir structure in the accretionary prism, namely mud diapir, mud volcano, and gas chimney. (1) The mud diapirs can be grouped into two types, namely the ones with low arching amplitude and weak-medium activity energy and the ones with high arching amplitude and medium-strong activity energy. The mud diapirs increase from offshore areas towards onshore areas in general, while the ones favorable for the formation of NGH are mainly distributed on the middle slope in the central and western parts of the accretionary prism. (2) The mud volcanoes are mainly concentrated along the anticline ridges in the southern part of the lower slope and the deformation front. (3) The gas chimneys can be grouped into three types, which are located in piggyback basins, active anticline ridges, and inactive anticline ridges, respectively. They are mainly distributed on the middle slope in the central and western parts of the accretionary prism and most of them are accompanied with thrust faults. The gas chimneys located at different tectonic locations started to be active at different time and pierced different horizons. The mud diapirs, mud volcanoes, and gas chimneys and thrust faults serve as the main pathways of gas migration, and thus are the important factors that control the formation, accumulation, and distribution of NGH in the Makran accretionary prism. Mud diapir/gas chimney type hydrate develop in the middle slope, mud volcano type hydrate develop in the southern lower slope and the deformation front, and stepped accretionary prism type hydrate develop on the central and northern lower slope. The middle slope, lower slope and deformation front in the central and western parts of the Makran accretionary prism jointly constitute the NGH prospect area.     

Relicts of an intra-oceanic arc in the Sapi-Shergol mélange zone (Ladakh, NW Himalaya, India): implications for the closure of the Neo-Tethys Ocean

In the Ladakh–Zanskar area, relicts of both ophiolites and paleo-accretionary prism have been preserved in the Sapi-Shergol mélange zone. The paleo-accretionary prism, related to the northward subduction of the northern Neo-Tethys beneath the Ladakh Asian margin, mainly consists of tectonic intercalations of sedimentary and blueschist facies rocks. Whole rock chemical composition data provide new constraints on the origin of both the ophiolitic and the blueschist facies rocks. The ophiolitic rocks are interpreted as relicts of the south Ladakh intra-oceanic arc that were incorporated in the accretionary prism during imbrication of the arc. The blueschist facies rocks were previously interpreted as oceanic island basalts (OIB), but our new data suggest that the protolith of some of the blueschists is a calc-alkaline igneous rock that formed in an arc environment. These blueschists most likely originated from the south Ladakh intra-oceanic arc. This arc was accreted to the southern margin of Asia during the Late Cretaceous and the buried portion was metamorphosed under blueschist facies conditions. Following oceanic subduction, the external part of the arc was obducted to form the south Ladakh ophiolites or was incorporated into the Sapi-Shergol mélange zone. The incorporation of the south Ladakh arc into the accretionary prism implies that the complete closure of the Neo-Tethys likely occurred by Eocene time.     

Comparative characteristics of the Samarka (Sikhote-Alin) and Ultra-Tamba (Japan) terranes as grounds for correlating fragments of the Jurassic accretionary prism in two regions

Comparative data on tectono-stratigraphic complexes of the Ultra-Tamba terrane (Inner Zone of Japan) and upper structural level of the Samarka terrane in the Jurassic accretionary prism of Sikhote Alin are considered. Structural, lithological, petrographic data and age constraints characterizing rock associations of the terranes show that the latter are similar to a great extent, and consequently the Ultra-Tamba terrane can be regarded as an element of the Tamba-Mino-Ashio accretionary prism of the Jurassic but not Permian age, as it was thought earlier. The considered data substantiate confident structural correlation of both fragments of the Jurassic prism and of two regions in general.     

构造控制型天然气水合物矿藏及其特征

构造环境是天然气水合物富集成藏的重要控制因素,增生楔、断裂体系、褶皱、(泥)底辟、滑塌等特殊构造体是影响天然气水合物成藏的主要地质载体。通过对这些特殊构造体与天然气水合物成藏关系的研究,结合流体活动对水合物形成的影响,总结出陆缘地区有增生楔型、盆缘斜坡型、埋藏背斜型、断褶型、滑塌型及底辟型等六类构造控制型水合物矿藏。南海位于欧亚板块、太平洋板块及印澳板块的交汇处,早期为活动陆缘,晚期演化为被动陆缘,其构造活动具有早期张裂、后期挤压的特点,这既不同于被动陆缘,也有别于活动陆缘,可视为“复合型”大陆边缘,兼具了“被动陆缘沉积速率高、活动陆缘构造活跃”的优点,从而形成了“增生楔型、断褶型、底辟型、滑塌型、盆缘斜坡型”等多种构造控制型水合物矿藏,是“复合型”大陆边缘水合物成藏地质模式的典型代表。     

BSRs and Associated Reflections as an Indicator of Gas Hydrate and Free Gas Accumulation: An Example of Accretionary Prism and Forearc Basin System along the Nankai Trough, off Central Japan

Abstract. Multi-channel seismic data obtained from the Nankai accretionary prism and forearc basin system has been studied to elucidate the migration and accumulation process of gas to the BGHS and examine the distribution pattern of BSRs and characteristic reflections associated with them.
BSRs are distributed widely in the Nankai accretionary prism and associated forearc basins (33,000 km²) and 90 % of them have migration and recycling origins. The widest distribution of the BSRs can be seen at the prism. A correlation between the BSR distributions and prism size shows that the BSRs tend to be more well-developed in a prism of large size. This suggests that a large prism may produce much amount of gas-bearing fluids that migrate to the BGHS and form the BSRs (tectonic control), hi the forearc basins, the BSRs are identified at topographic highs, anticlines and basin margins (structural control).
The upward migration of gas-bearing fluids is carried out through permeable sand layers and as a result, the distribution of BSRs is confined to alternating beds of sand and mud facies (sedimentary control). However, if there is enough time for upward migration and accumulation of gas to the BGHS, the BSRs can be generated widely in low-permeable mud facies (time control).
Those results imply that structural, tectonic, sedimentary and time controls are primary factors to decide the distribution of BSRs in the Nankai Trough area.     

Reloca Slide: an ~24 km3 submarine mass‐wasting event in response to over‐steepening and failure of the central Chilean continental slope

Reloca Slide is the relict of an ~24‐km³ submarine slope collapse at the base of the convergent continental margin of central Chile. Bathymetric and seismic data show that directly to the north and south of the slide the lower continental slope is steep (~10°), the deformation front is shifted landwards by 10–15 km, and the frontal accretionary prism is uplifted. In contrast, ~80 km to the north the lower continental margin presents a lower slope angle of about 4° and a wide frontal accretionary prism. We propose that high effective basal friction conditions at the base of the accretionary prism favoured basal accretion of sediment and over‐steepening of the continental slope, producing massive submarine mass wasting in the Reloca region. This area also spatially correlates with a zone of low coseismic slip of the 2010 Maule megathrust earthquake, which is consistent with high basal frictional coefficients.     

Provenance and Evolution of the Guarguariiz Complex, Cordillera Frontal, Argentina

The Guarguardz Complex, basement of the Cordillera Frontal, included in the proposed Chilenia Terrane, consists of metasedimentary rocks deposited in clastic and carbonatic platforms. Turbiditic sequences point out to slope or external platform environments. According to geochemical data, the sedimentary protoliths derived through erosion of a mature cratonic continental basement. Volcanic and subvolcanic rocks with N and E-MORB signature were interbeded in the metasedimentary rocks during basin development. A compressional stage, starting with progressive deformation and metamorphism, followed this extensional stage. Continuing deformation led to the emplacement of slices of oceanic crust, conforming an accretionary prism during Late Devonian. The Guarguardz Complex and equivalent units in western Precordillera and also in the Chilean Coastal Cordillera share common evolutional stages, widely represented along the western Gondwana margin. These evidences imply that Chilenia is not an allochthonous terrane to Gondwana, but a portion of its Early Paleozoic margin. Regional configuration indicates that the Guarguardz Complex and equivalent units represent the accretionary prism of the Famatinian arc (Middle Ordovician-Late Devonian).     

A geodynamic model of the Southern Apennines accretionary prism

The Apennines comprise a Neogen—Quaternary accretionary prism that shows several anomalies with respect to classic alpine-type mountain belts, namely (i) low elevation, (ii) a shallow new Moho below the core of the belt, (iii) high heat flow in the internal parts, (iv) mainly sedimentary cover involved in the prism, (v) a deep foredeep and (vi) a fully developed back-arc basin. The suction exerted by a relatively eastward migrating mantle can determine the eastward retreat of the subduction zone and an asthenospheric wedging at the retreating subduction hinge. Heat flow, geochemical and seismological data support the presence of a hot mantle wedge underlying the western side of the Apenninic accretionary prism. A thermal model of the belt with foreland dipping isotherms fits with deepening of the seismicity toward the east. Mantle volatiles signatures are also widespread in springs along the Apennines.     

Formation of Hengchun Accretionary Prism Turbidites and Implications for Deep‐water Transport Processes in the Northern South China Sea

Located at the end of the northern Manila Trench,the Hengchun Peninsula is the latest exposed part of Taiwan Island,and preserves a complete sequence of accretionary deep-sea turbidite sandstones.Combined with extensive field observations,a’source-to-sink’approach was employed to systematically analyze the formation and evolutionary process of the accretionary prism turbidites on the Hengchun Peninsula.Lying at the base of the Hengchun turbidites are abundant mafic normal oceanic crust gravels with a certain degree of roundness.The gravels with U-Pb ages ranging from 25.4 to23.6 Ma are underlain by hundreds-of-meters thickness of younger deep-sea sandstone turbidites with interbedded gravels.This indicates that large amounts of terrigenous materials from both the’Kontum-Ying-Qiong’River of Indochina and the Pearl River of South China were transported into the deep-water areas of the northern South China Sea during the late Miocene and further eastward in the form of turbidity currents.The turbidity flow drastically eroded and snatched mafic materials from the normal South China Sea oceanic crust along the way,and subsequently unloaded large bodies of basic gravel-bearing sandstones to form turbidites near the northern Manila Trench.With the Philippine Sea Plate drifting clockwise to the northwest,these turbidite successions eventually migrated and,since the Middle Pleistocene,were exposed as an accretionary prism on the Hengchun Peninsula.     

Ophiolites of the Kamchatsky Mys Peninsula, eastern Kamchatka: Structure, composition, and geodynamic setting

Ophiolites of the Afrika Mys Block of the Kamchatsky Mys Peninsula, eastern Kamchatka, are a fragment of an accretionary prism that formed in the Late Cretaceous-Eocene on the southern side of the Kronotsky island arc as a result of its collision with the Smagino volcanic uplift that arose at the post-Neocomian time on the subducting plate. On the basis of the geologic, geochemical, and paleomagnetic data available to date, it is established that ophiolites are heterogeneous in their origin and were formed in different geodynamic settings that changed progressively with time. The heterogeneous structure of ophiolites displays the evolution of a fragment of the oceanic lithosphere, which was not submerged into subduction zone, from its origination in the spreading center via transformation under conditions of the plume-related volcanic uplift to the involvement in the structure of the Kronotsky island arc, which is currently a constituent of the accretionary system of Kamchatka. The reconstruction of ophiolites tectonically fragmented in the accretionary prism allows recognition of (1) derivatives of an ocean ridge (ultramafic-gabbro-basaltic complex of the Mount Olen’ya Massif) conjugated with a transform fault and volcanosedimentary rocks of the Smagino volcanic uplift (cover of the oceanic crust) and (2) a fragment of the lithospheric mantle (ultramafic rocks of the Lake Stolbovoe Massif) exhumed in the process of collision and genetically related to the evolution of the volcanic uplift. In the course of evolution of the Kronotsky island arc, all these elements were overlapped by tephrogenic turbidites (Pikezh Formation) and quartz-feldspar graywackes (Pikezh Sandstone) that were involved in the accretionary prism as well. The paleotectonic reconstructions broadly support the petrologic conclusions about the complementary nature of different igneous complexes and ascertain the temporal sequence of events.     

Siliceous-clayey rocks of the Jurassic accretionary prism, Khekhtsir Range, Sikhote Alin: Stratigraphy and genesis

Lithologic-stratigraphic aspects of siliceous-clayey rocks forming the Khabarovsk terrane of the Jurassic accretionary prism were studied in western spurs of the Bol’shoi Khekhtsir Range on the left side of the Ussuri River (Ussuri-Khekhtsir section). Two defined types of the examined section differ in the composition, age, and origin of their constituting rocks. The northern segment of the section is composed of middle Bajocian red-brown siliceous-tuffaceous silty and olive-gray silty mudstones that accumulated in the hemipelagic domain under the influence of continental provenance. Its southern segment is represented by lower Bathonian olive-gray siliceous mudstones, mudstones barren of any admixtures, and yellowish brown tuffaceous mudstones deposited far away from the continent in waters with abundant radiolarians. It is shown that these rocks are elements of two tectono-stratigraphic complexes that reflect different stages in the accretionary prism formation.     

Development of New Zealand according to the fore-arc model of crustal evolution

Analysis of New Zealand geology using a fore-arc model (Crook, 1980a) leads to the recognition of four arc terrains. The west facing Tuhua volcanic arc was active from the Late Proterozoic until the Middle or Late Cambrian. Post-subduction sediments, neritic in the east and flysch in the west, accumulated on the Tuhua accretionary prism from the Late Cambrian until the Early Devonian. Thermal equilibration, metamorphism, granitoid plutonism and penetrative deformation occurred in the Middle to Late Devonian. A small area of Permian platform cover has escaped later erosion. The east-facing Rangitata Terrain records subduction from Early Permian to late Early Cretaceous. Much of its accretionary prism consists of a submarine fan complex derived from Western Antarctica and carried sideways into the trench. The accretionary prism is thick and completely kratonized in southern New Zealand, but the thickness is more variable northwards. There the overlying Upper Cretaceous to Upper Oligocene post-subduction sequence comprises shelf sediments (implying an intermediate-thickness prism) or flysch followed by shelf sediments (implying a thin prism). During the accumulation of this sequence the Rangitata Terrain was a passive continental margin. The south-facing Jurassic-late Oligocene Northland Terrain collided with this passive margin in northern New Zealand at the end of the Oligocene, forming the Northland Allochthon. Subduction then flipped and the oldest part of the Kaikoura Terrain volcanic arc formed on the outer part of the Northland Terrain. Originally this terrain faced northeast and consumed the southwestern part of the South Fiji Basin crust, but during the Miocene the arc migrated clockwise to assume its present northeastern orientation. The fore-arc model employed here satisfactorily explains most first-order and many second-order features of New Zealand geology without requiring modification, thus attesting to the model's versatility and robustness. New Zealand provides a basis for elaborating some aspects of the model, particularly the transition from the syn- to post-subduction phases of fore-arc evolution. Combination of this study with a similar study of the southeastern Australian Paleozoic yields insights into the Phanerozoic evolution of the Australian: Pacific Plates' active margin.     

The transition from an active to a passive margin (SW end of the South Shetland Trench, Antarctic Peninsula)

The lateral ending of the South Shetland Trench is analysed on the basis of swath bathymetry and multichannel seismic profiles in order to establish the tectonic and stratigraphic features of the transition from an northeastward active to a southwestward passive margin style. This trench is associated with a lithospheric-scale thrust accommodating the internal deformation in the Antarctic Plate and its lateral end represents the tip-line of this thrust. The evolutionary model deduced from the structures and the stratigraphic record includes a first stage with a compressional deformation, predating the end of the subduction in the southwestern part of the study area that produced reverse faults in the oceanic crust during the Tortonian. The second stage occurred during the Messinian and includes distributed compressional deformation around the tip-line of the basal detachment, originating a high at the base of the slope and the collapse of the now inactive accretionary prism of the passive margin. The initial subduction of the high at the base of the slope induced the deformation of the accretionary prism and the formation of another high in the shelf—the Shelf Transition High. The third stage, from the Early Pliocene to the present-day, includes the active compressional deformation of the shelf and the base-of-slope around the tip-line of the basal detachment, while extensional deformations are active in the outer swell of the trench.     

The mud volcanoes of Pakistan

Marine-geologic investigations on the Arabian Sea by Bundesanstalt für Geowissenschaften und Rohstoffe (BGR) in 1995 and 1998, and land expeditions in 1998 and 1999 to the coastal regions of the Makran Desert/Pakistan have extended the knowledge of the aerial distribution of mud volcanoes. These structures rise from under-compacted formations within the regional accretionary prism, which is built by the subduction of the oceanic crust of the Arabian Sea and its km-thick sedimentary load. The occurrence of mud volcanoes is limited to the abyssal plain near the accretionary front, to the coastal region of the Makran Desert and to a region in the interior of the Desert to the south to southeast of the so-called Hinglay Synform. The location of mud volcanoes in Pakistan is clearly tied to fault systems. Mud volcanoes are conspicuously absent on the lower slope of the accretionary prism, where thick gas hydrate layers have developed. The presence of large gas plumes emerging from the seafloor landward of the gas hydrate stability zone at water depths of less than 800 m points to a redirection of fluids from depth, which might explain the absence of mud volcanoes along the lower slope.     

The Ishkinino Co-Cu massive sulfide deposit hosted in ultramafic rocks of the Main Ural Fault Zone, the southern Urals

The results of study of the Ishkinino Co-Cu massive sulfide deposit hosted in ultramafic rocks of the Main Ural Fault Zone are discussed. The ore field is localized in a fragment of Early Devonian accretionary prism composed of oceanic and island-arc tectonic sheets. The antiform structure of the ore field was formed at the collision stage in the Late Devonian. The primary ore was deposited near the bottom in the environment of the accretionary prism at the island-arc stage of evolution, whereas the superimposed ore mineralization was related to the collision stage. The primary ore is composed of massive, stringer-disseminated, and clastic varieties with two mineral assemblages of sulfides and oxides. The superimposed stringer-disseminated ore mineralization is represented by Co-Ni-Fe arsenides and sulfoarsenides, native gold, Bi and Te minerals, and late sulfides and oxides. Loellingite, safflorite, rammelsbergite, and krutovite were identified in the massive sulfide ore for the first time in the Urals. The geochemical attributes of Co-Ni minerals serve as indicators of superimposed processes. Chromites contained in rocks and ore correspond to Cr-spinel of suprasubduction ultramafic rocks in chemical composition. It is suggested that sulfide ore may be found in the accretionary prisms of the presently active island arcs composed of ultramafic sheets.     

Bela oceanic lithosphere assemblage and its relation to the Réunion hotspot

The Bela ophiolite of Pakistan contains a complete ophiolite-accretionary wedge-trench sequence emplaced onto the Indian continental margin during the northward drift of India-Seychelles over the active Réunion hotspot. A structurally higher ophiolite overlies an accretionary prism, which is thrust over a foreland basin. Shear-sense determinations in peridotite mylonites in the ophiolite footwall and imbrication structures in the underlying accretionary wedge indicate an ESE emplacement. Sedimentary rocks in the accretionary wedge indicate Aptian-Albian pillow lavas, initially deep water conditions, and increasing influence from the continent until the Maastrichtian. The ophiolite emplacement was predated and accompanied by Fe-tholeiitic and alkaline magmatism related to the Réunion hotspot and continuous incorporation of trench sediments into the accretionary wedge. ³⁹Ar/⁴⁰Ar dating shows that the ophiolite formed around 70 Ma. Intraoceanic subduction initiated between 70 and 65 Ma, obduction onto the Indian passive margin occurred during the formation of the Deccan traps at ≈ 66 Ma, and final thrusting onto the continental margin ended in the early Eocene (≈ 50 Ma). The ophiolite emplacement occurred during the counterclockwise separation of Madagascar and India-Seychelles which caused shortening and consumption of oceanic lithosphere between the African-Arabian and the Indian-Seychelles plates.     

New interpretation of the Franciscan mélange at San Simeon coast,California: tectonic intrusion into an accretionary prism

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.     

Petrochemical characterization of mafic rocks from the Ligurian ophiolites,southern Apennines

New mineralogical and chemical data for ophiolitic rocks from the southwesternmost Liguride Units are presented in order to constrain their ocean-floor origin and subsequent emplacement in an accretionary wedge. Their complete petrochemical evolution is particularly well preserved in the southern Apennine metabasites. Metadolerites show amphibolite and greenschist facies mineral assemblages of ocean-floor metamorphism. Metabasalts display greenschist facies ocean-floor metamorphism and spilitic alteration. Veins cutting the mafic rocks show mineral assemblage of the prehnite–pumpellyite metamorphic facies. HP/LT orogenic metamorphism, reflecting underplating of the ophiolitic suite at the base of the Liguride accretionary wedge during subduction of the western Tethys oceanic lithosphere produced a mineral assemblage typical of the lawsonite–glaucophane facies. Bulk-rock chemistry suggests that the mafic protoliths had a MORB-type affinity, and were affected by ocean-floor rodingitic and/or spilitic alteration. Hydrothermal alteration-induced LREE mobility and LREE enrichment may be correlated with the ocean-floor metamorphism.