The disrupted ophiolitic belt of the southwest Pacific: Evidence of an Eocene subduction zone

The Papua-New Guinea, Solomon, New Hebrides and New Caledonia ophiolitic massifs come from an Eocene intra-oceanic subduction occurring in the southwest Pacific. This hypothesis is suggested by the age of the ophiolite-related metamorphic soles which would be the result of a metamorphism arising at the expense of volcanic and sedimentary series of oceanic supracrustal origin when involved in a subduction zone. When this subduction also involves a continental crust portion, amphibolites and blue schists are formed, as observed in Papua-New Guinea and New Caledonia. When the subduction occurs in an intra-oceanic environment, as in the Solomon islands and New Hebrides, only amphibolites and green schists are to be found.The ophiolitic belt (basic-ultrabasic massifs and their related metamorphic soles) created by the Eocene subduction has been disrupted by later transcurrent faults, more recent spreading phenomena and two other subductions (Oligocene-Miocene and Recent).     

A geological transect between the Klamath Mountains and the Pacific Ocean (Southwestern Oregon): A model for paleosubductions

A geological cross-section between Vulcan Peak (Klamath Mountains) and Gold Beach (Pacific Ocean) shows several tectonic sheets thrust to the west. From the coast to the east, in tectonic superposition:

1. (1) The metamorphosed Franciscan Complex includes the Tithonian-Neocomian Otter Point and Dothan Formations, and, above them, a mélange unit and the Colebrooke Schist.

2. (2) Over them all lie the Red Flat, Game Lake and Snow Camp peridotite units, which are cut by granodioritic dykes of the Nevadan Orogeny and covered by Tithonian-Neocomian sediments (Myrtle Group). We have considered these granite-bearing ultramafic units as klippen thrust westward from the Klamath Mountains.

Several stages of deformation are superimposed:

1. (1) Disconformable upon the Otter Point Formation are the unmetamorphosed detrital Cape Sebastian and Hunters Cove Formations. Their Late Cretaceous age gives here an upper limit to the Franciscan metamorphism and tectonics.

2. (2) The first reworked fragments of Colebrooke Schist or ultramafic units are not found in the Campanian Cape Sebastian basal conglomerates (which are affected by some thrusting), but in the Middle Eocene Lookingglass Formation. Thus, major thrusting happened during Late Cretaceous and Early Tertiary times.

A geodynamic model is given: in southwestern Oregon, the Franciscan was probably deposited during Late Jurassic and earliest Cretaceous (Tithonian and Neocomian) times in a marginal basin, between an island arc to the west (Otter Point Formation in part) and the North American continent (Klamath Mountains) to the east. This Franciscan marginal basin was probably closed during the Early Cretaceous (i.e. Franciscan subduction). The present day structures (thrust plates, folds) are probably due mainly to the later collision between the continental margin and the arc and, to a much lesser degree, to early subduction, the marginal basin being consumed by subduction first, and then by collision.     

Coalification and graphitization in high-pressure schists in New Caledonia

The northern portion of the Tertiary high-pressure schist belt in New Caledonia contains, from west to east, a metamorphic progression from lawsonite-albite facies through glaucophanitic greenschists to eclogitic albite-epidote amphibolites. This belt is flanked to the west by Upper Cretaceous-Eocene metasediments, of prehnite-pumpellyite grade. Paraschists throughout this whole sequence contain abundant carbonaceous material which shows a progressive metamorphism from coal to graphite. Structural analysis of lithostatic load and oxygen isotope data have provided a PT profile for the carbon metamorphism. In the prehnite-pumpellyite metasediments, phytoclasts were progressively coalified to anthracite rank under PT conditions which extended up to 3 kb/255 ° C at the lawsonite isograd where graphite first appears. On the high grade side of the lawsonite isograd a transitional mixed zone of continued coalification and graphitization occurred within the PT range 3 kb/255 ° C to 5.5 kb/335 ° C which included the ferroglaucophane isograd. Immediately beyond this zone all phytoclasts were completely graphitized before the epidote isograd was reached at 6.3 kb/ 390 ° C. The prevailing metamorphic environment retarded coalification, but accelerated graphitization, under conditions of high pressure and a low temperature gradient (7 ° C/km) that had been generated within the sedimentary pile by rapid tectonic thickening and consequent deep burial.     

North‐eastward subduction followed by slab detachment to explain ophiolite obduction and Early Miocene volcanism in Northland,New Zealand

Oligocene–Miocene models for northern New Zealand, involving south‐westward subduction to explain Early Miocene Northland volcanism, do not fit within the regional Southwest Pacific tectonic framework. A new model is proposed, which comprises a north‐east‐dipping South Loyalty basin slab that retreated south‐westward in the Eocene–earliest Miocene and was continuous with the north‐east‐dipping subduction zone of New Caledonia. In the latest Oligocene, the trench reached the Northland passive margin, which was pulled it into the mantle by the slab, resulting in obduction of the Northland allochthon. During and after obduction, the slab detached from the unsubductable continental lithosphere, inducing widespread calc‐alkaline volcanism in Northland. The new model further explains contemporaneous arc volcanism along the Northland Plateau Seamount Chain and sinking of the Northland basement, followed by uplift and extension in Northland.     

The Otago schist megaculmination: Its possible origins and tectonic significance in the Rangitata orogen of New Zealand

In South Island, New Zealand, the Otago schist, 30,000 square km in extent, consists mainly of greenschist facies quartzo-feldspathic metagreywacke and meta-argillite with minor metavolcanics and metacherts. Before metamorphism the sediments were probably Carboniferous to Jurassic; the flanking, steeply dipping greywackes are Triassic in the northeast and southeast, and Permian in the west and southwest.Regional metamorphism culminating in the Late Jurassic was accompanied by pervasive deformation generating a variety of interrelated folds on all scales. The scarcity of distinctive reference units makes recognition of macroscopic structures difficult, and much progress has depended on observations of vergence of mesoscopic folds interpreted as defining macroscopic folds having axial plane separation of between 2 km and 6 km.At least two phases of synmetamorphic deformation are distinguishable locally, but regionally have an overlapping multiphase relationship. The regional schistosity structure is an irregular flat-crested antiform trending S and SE. The internal megascopic structure defined by the mesoscopic folds, appears to consist of a stack of nappe-folds facing east and northeast, which pass into reclining isoclinal folds in the west, southwest and north-east, and is interpreted to be a megaculmination. Mineral and textural metamorphic zones were developed during deformation, and a relatively simple regional pattern established at a late stage by continuing recrystallisation.The Otago schist originated in a complex sequence of Paleozoic—Mesozoic plate interactions near the southwest Pacific margin of Gondwanaland. It included part of a volcaniclastic frontal arc-basin assemblage (Murihiku and Caples Terranes) lying north or northeast of an older crystalline foreland, and a quartzo-feldspathic assemblage of plutonic-metamorphic provenance lying further to the northeast (Torlesse Terrane). Parts of these terranes underwent mainly greenschist facies metamorphism during Late Jurassic subduction-collision to form the Haast Schist Terrane of which the Otago schist is a major part.The earliest Torlesse sediments are thought to have prograded as a vast fan complex onto oceanic crust from the southwesterly crystalline foreland in the Carboniferous, then in the Permian were separated from their source by a spreading zone which thereafter isolated them from the sedimentary province of the newly developing arc system. Tectonic recycling of these sediments at a Permian to Jurassic oceanic subduction zone is considered to have developed the westward progradation features and the products of limited vulcanism found in the present Torlesse Terrane. The New Zealand Geosyncline appears to have consisted of a spreading zone between two inwardly facing convergent zones, one flanked by a foreland to the southwest, the other wholly oceanic.The metamorphic climax of the Rangitata Orogeny was the result of the medial spreading zone passing into the westerly subduction zone, so permitting the convergent zones to collide, with the Torlesse sediments caught between them.The mantle system driving the spreading zone appears to have continued to function, and soon after the collisional climax caused Late Jurassic—Cretaceous rifting of the sialic edge of Gondwanaland, igneous activity, differential shear of the New Zealand region, and initiation of the Alpine Fault. It subsequently commenced sea-floor spreading in the Tasman Sea, and later in the southwest Pacific Ocean.     

The origin and P–T evolution of peridotites and serpentinites of NE New Caledonia: prograde interaction between continental margin and the mantle wedge

The Pouébo and Diahot terranes of NE New Caledonia mostly comprise eclogite to blueschist facies metabasite and metasedimentary rocks that experienced c. 40 Ma metamorphism. This Eocene high‐P event has been linked with the SW‐directed obduction of the New Caledonian Ophiolite, an extensive ultramafic nappe that dominates outcrop in the south of the island. In the north, ultramafic lithologies are found only as sheets or discrete lenticular masses interleaved with, but separated from, the eclogites and blueschists by foliated talc–chlorite–serpentine–carbonate‐bearing rocks. The base of the largest and best‐preserved ultramafic body at Yambé is marked by a distinctive (2 m thick) layer of high‐P mylonite that preserves evidence for early blueschist facies conditions (S1) as inclusions in eclogite facies minerals. Textural evidence preserved in olivine‐bearing serpentinites and their bounding mafic mylonites suggest that the ultramafic bodies were emplaced within the structurally highest levels of the high‐P terrane as serpentinite tectonites sourced from hydrated mantle, formerly in the hangingwall of the Eocene subduction zone. Serpentinite emplacement accompanied burial of the NE New Caledonian margin at T<500 °C and P<16 kbar. The ultramafic fragments were buried to depths of 50–60 km in the subduction zone, where olivine was stable and coarse‐grained garnet–omphacite‐rich assemblages developed in low strain domains within enclosing mylonites. Host metabasic and metasedimentary rocks from the structurally highest portions of the high‐P belt have a prograde record identical to that of the ultramafic tectonites. The early emplacement and similar P–T history of host rocks and ultramafic masses suggest that NE New Caledonia preserves a fossil slab/mantle–wedge boundary reactivated during exhumation.     

High-pressure schists in Northern New Caledonia

Regional high-pressure metamorphism in the Oligocene-lowermost Miocene (38-21 m.y. before present) produced a schist belt measuring 170×25 km adjacent to a thrust (melange) zone along the northeast margin of New Caledonia. The parent rocks were trench sediments with coeval basic and acid volcanics of Cretaceous-Eocene age in a sequence about 20 km thick. Tectonic events covering the same time span included westwards obduction of the great southern basalt-gabbro-peridotite massif which had no causative relationship with high-pressure metamorphism; eastwards-directed low-angle thrusts in the northern part of the Permian-Mesozoic sialic basement, originating along the melange zone, developed a thick stack of metagreywacke schuppen which capped the high-pressure metamorphic pile. Thus, while the site for sedimentation and metamorphism was an active trench in a northeasterly (oceanward) dipping convergence zone, the adjacent upper oceanic plate apparently was not directly involved in the metamorphic setting. Regional high-pressure assemblages are defined with reference to isograds for paraschist lawsonite, epidote and Ca-amphibole in a continuous progression from lawsonite-albite facies through glaucophanitic greenschists to eclogitic albite-epidote amphibolites.     

A Late Cretaceous and Cenozoic reconstruction of the Southwest Pacific region: Tectonics controlled by subduction and slab rollback processes

A Cenozoic tectonic reconstruction is presented for the Southwest Pacific region located east of Australia. The reconstruction is constrained by large geological and geophysical datasets and recalculated rotation parameters for Pacific–Australia and Lord Howe Rise–Pacific relative plate motion. The reconstruction is based on a conceptual tectonic model in which the large-scale structures of the region are manifestations of slab rollback and backarc extension processes. The current paradigm proclaims that the southwestern Pacific plate boundary was a west-dipping subduction boundary only since the Middle Eocene. The new reconstruction provides kinematic evidence that this configuration was already established in the Late Cretaceous and Early Paleogene. From ∼ 82 to ∼ 52 Ma, subduction was primarily accomplished by east and northeast-directed rollback of the Pacific slab, accommodating opening of the New Caledonia, South Loyalty, Coral Sea and Pocklington backarc basins and partly accommodating spreading in the Tasman Sea. The total amount of east-directed rollback of the Pacific slab that took place from ∼ 82 Ma to ∼ 52 Ma is estimated to be at least 1200 km. A large percentage of this rollback accommodated opening of the South Loyalty Basin, a north–south trending backarc basin. It is estimated from kinematic and geological constraints that the east–west width of the basin was at least ∼ 750 km. The South Loyalty and Pocklington backarc basins were subducted in the Eocene to earliest Miocene along the newly formed New Caledonia and Pocklington subduction zones. This culminated in southwestward and southward obduction of ophiolites in New Caledonia, Northland and New Guinea in the latest Eocene to earliest Miocene. It is suggested that the formation of these new subduction zones was triggered by a change in Pacific–Australia relative motion at ∼ 50 Ma. Two additional phases of eastward rollback of the Pacific slab followed, one during opening of the South Fiji Basin and Norfolk Basin in the Oligocene to Early Miocene (up to ∼ 650 km of rollback), and one during opening of the Lau Basin in the latest Miocene to Present (up to ∼ 400 km of rollback). Two new subduction zones formed in the Miocene, the south-dipping Trobriand subduction zone along which the Solomon Sea backarc Basin subducted and the north-dipping New Britain–San Cristobal–New Hebrides subduction zone, along which the Solomon Sea backarc Basin subducted in the west and the North Loyalty–South Fiji backarc Basin and remnants of the South Loyalty–Santa Cruz backarc Basin subducted in the east. Clockwise rollback of the New Hebrides section resulted in formation of the North Fiji Basin. The reconstruction provides explanations for the formation of new subduction zones and for the initiation and termination of opening of the marginal basins by either initiation of subduction of buoyant lithosphere, a change in plate kinematics or slab–mantle interaction.     

Orogenic evolution of the Hellenides: new aspects

This progress report is based on investigations of the tectonometamorphic development of crystalline complexes and is restricted to a few key problems of the Hellenides:

1. ‘Hinterland’. Rhodopia is strongly affected by Alpidic metamorphism, granitoid intrusions, orogenic deformation and intracrustal delamination. Therefore, close relations between the Balkanides and Hellenides have to be considered.
2. External zones. The Phyllite-Quartzite Series probably originated in a Late Palaeozoic rift within Apulia. In Middle Triassic times rifting stopped and the area became the basement on an extended carbonate platform (Late Triassic-Liassic). From the Dogger to Palaeocene, parts of that platform subsided, forming the Ionian pelagic basin. The Eocene orogenesis within the central Hellenides then caused an inversion of the buried Phyllite-Quartzite rift zone, whereas from the Late Oligocene onwards the previous rift zone underwent continental (A-) subduction. Finally, uplift of the Phyllite-Quartzite Series and nappe emplacement started in Miocene times.
3. Late orogenic intracrustal shearing. Structural analyses of crystalline complexes of Attica have shown that neotectonic extension and the large vertical displacement of the Aegean region were caused by low angle faulting and large-scale shearing within the deeper crust, probably along former overthrust planes.

These results reveal the mechanisms of intracratonic tectonics, remobilization of continental crust and intracrustal detachment throughout the evolution of the Hellenides.     

Alpine polyphase tectono-metamorphic evolution of the South Carpathians: A new overview

The main terrains involved in the Cretaceous–Tertiary tectonism in the South Carpathians segment of the European Alpine orogen are the Getic–Supragetic and Danubian continental crust fragments separated by the Severin oceanic crust-floored basin. During the Early–Middle Cretaceous times the Danubian microplate acted initially as a foreland unit strongly involved in the South Carpathians nappe stacking. Multistage folding/thrusting events, uplift/erosion and extensional stages and the development of associated sedimentary basins characterize the South Carpathians during Cretaceous to Tertiary convergence and collision events. The main Cretaceous tectogenetic events responsible for contraction and crustal thickening processes in the South Carpathians are Mid-Cretaceous (“Austrian phase”) and Latest Cretaceous (“Laramide” or “Getic phase”) in age. The architecture of the South Carpathians suggests polyphase tectonic evolution and mountain building and includes from top to bottom: the Getic–Supragetic basement/cover nappes, the Severin and Arjana cover nappes, and Danubian basement/cover nappes, all tectonically overriding the Moesian Platform. The Severin nappe complex (including Obarsia and Severin nappes) with Late Jurassic–Early Cretaceous ophiolites and turbidites is squeezed between the Danubian and Getic–Supragetic basement nappes as a result of successive thrusting of dismembered units during the inferred Mid- to Late Cretaceous subduction/collision followed by tectonic inversion processes.

Early Cretaceous thick-skinned tectonics was replaced by thin-skinned tectonics in Late Cretaceous. Thus, the former Middle Cretaceous “Austrian” nappe stack and its Albian–Lower Senonian cover got incorporated in the intra-Senonian “Laramide/Getic” stacking of the Getic–Supragetic/Severin/Arjana nappes onto the Danubian nappe duplex. The two contraction events are separated by an extensional tectonic phase in the upper plate recorded by the intrusion of the “Banatitic” magmas (84–73 Ma). The overthrusting of the entire South Carpathian Cretaceous nappe stack onto the fold/thrust foredeep units and to the Moesian Platform took place in the Late Miocene (intra-Sarmatian) times and was followed by extensional events and sedimentary basin formation.     

Jurassic strike slip versus subduction in the Eastern Alps

Late Jurassic formations of the Northern Calcareous Alps (NCA) contain ample evidence of synsedimentary tectonics in the form of elongate basins filled with turbidites, debris flows and slumps. Clasts are derived from the Mesozoic of the NCA; they commonly measure tens of metres in diameter and occasionally form kilometre-size bodies. These sedimentologic observations and the presumed evidence of Late Jurassic high-pressure metamorphism recently led to the hypothesis of a south-dipping Jurassic subduction zone with accretionary wedge in the southern parts of the NCA. We present new ⁴⁰Ar/³⁹Ar dates from the location of the postulated high-pressure metamorphism that bracket the age of this crystallization not earlier than 114–120 Ma. The event is therefore part of the well-documented mid-Cretaceous metamorphism of the Austro-alpine domain. Thus, there is currently no evidence of Late Jurassic high-pressure metamorphism to support the subduction hypothesis. The sediment record of the Late Jurassic deformation in the NCA, including the formation of local thrust sheets, is no conclusive evidence for subduction. All these phenomena are perfectly compatible with synsedimentary strike-slip tectonics. Large strike-slip fault zones with restraining and releasing bends and associated flower structures and pull-apart basins are a perfectly viable alternative to the subduction model for the Late Jurassic history of the NCA. However, in contrast to the Eastern Alps transect, where arguments for a Jurassic subduction are missing, a glaucophane bearing Jurassic high-pressure metamorphism in the Meliatic realm of the West Carpathians is well documented. There, the high-pressure/low-temperature slices occur between the Gemeric unit and the Silica nappe system (including the Aggtelek-Rudabanya units), which corresponds in facies with the Juvavic units in the southern part of the NCA. To solve the contrasting palaeogeographic reconstructions we propose that the upper Jurassic left lateral strike-slip system proposed here for the Eastern Alps continued eastwards and caused the eastward displacement of the Silica units into the Meliatic accretionary wedge.     

Sedimentological and Petrographical Aspects of Upper Miocene Evaporites in the Beypazari and Çankiri-Çorum Basins,Central Anatolia,Turkey

The Southeast Anatolian orogen is a part of the eastern Mediterranean-Himalayan orogenic belt. Development of the Southeast Anatolian orogen began with the first ophiolite obduction onto the Arabian platform during the Late Cretaceous, and it continued until the Miocene. Its lingering effects continue to be discernible at present. During the Late Cretaceous-Miocene interval, three major deformational phases occurred, related to Late Cretaceous, Eocene, and Miocene nappe emplacements. The Miocene nappes are composed of ophiolites and metamorphic massifs.

For a decade, field studies in the region have shown that strike-slip tectonics played a role complementary to the major horizontal effects of the nappe movement, as indicated by: (1) fault systems active during the Eocene; (2) different Eocene rock units composed of coeval continental and deep-sea deposits and presently tectonically juxtaposed; and (3) other stratigraphic and structural data obtained across the present strike-slip fault zones.

These strike-slip faults possibly resulted from oblique subduction of the mid-oceanic ridge underneath the northerly situated Yuksekova ensimatic island-arc complex, causing a gradual cessation of the island-arc system. The subduction also led to the development of a back-arc pull-apart basin, i.e., the Maden basin, which opened on the upper plate. The geologic history in Southeast Anatolia resembles the development of the San Andreas fault system and subsequent tectonic evolution.     

Plate boundary evolution in the Halmahera region,Indonesia

Halmahera is situated in eastern Indonesia at the southwest corner of the Philippine Sea Plate. Active arc-arc collision is in process in the Molucca Sea to the west of Halmahera. New stratigraphic observations from Halmahera link this island and the east Philippines and record the history of subduction of the Molucca Sea lithosphere. The Halmahera Basement Complex and the basement of east Mindanao were part of an arc and forearc of Late Cretaceous-Early Tertiary age and have formed part of a single plate since the Late Eocene-Early Oligocene. There is no evidence that Halmahera formed part of an Oligo-Miocene arc but arc volcanism, associated with eastwards subduction of the Molucca Sea beneath Halmahera, began in the Pliocene and the Pliocene arc is built on a basement of the early Tertiary arc. Arc volcanism ceased briefly during the Pleistocene and the arc shifted westwards after an episode of deformation. The present active arc is built upon deformed rocks of the Pliocene arc. The combination of new stratigraphic information from the Halmahera islands and models of the present-day tectonic structure of the region deduced from seismic and other geophysical studies is used to constrain the tectonic evolution of the region since the Miocene. Diachronous collision at the western edge of the Philippine Sea Plate which began in Mindanao in the Late Miocene impeded the movement of the Philippine Sea Plate and further motion has been achieved by a combination of strike-slip motion along the Philippine Fault, subduction at the Philippine Trench and subduction of the Molucca Sea lithosphere beneath Halmahera.     

Morphotectonic evolution of the New Caledonia ridge (Pacific Southwest) from post-obduction tectonosedimentary record

The tectonostratigraphic and geomorphic study of two post-obduction fluvial sedimentary systems on mainland New Caledonia and imaged offshore on seismic reflection lines provides a new perspective on the post-orogenic evolution of the New Caledonia ridge. Relations between sedimentary sequence boundaries, erosion surfaces and faults, both on land and on offshore seismic reflection profiles indicate that an episode of extensional tectonics initiated in the Early Neogene led to the disruption and collapse of the island landsurface previously shaped during a Latest Oligocene phase of planation. Microtectonic analysis further suggests early slip on the normal faults was associated with ridge-normal extension. A later set of faults accompanied ridge-parallel to ridge-oblique extension that is interpreted to result from a shift toward a transtensional regime driven by the initiation of east-verging subduction of the Australian plate beneath the Pacific plate starting at least in the late Mid-Miocene.     

Isotopic constraints on crustal architecture and Permo‐Triassic tectonics in New Guinea: Possible links with eastern Australia

New U–Pb zircon ages and Sr–Nd isotopic data for Triassic igneous and metamorphic rocks from northern New Guinea help constrain models of the evolution of Australia's northern and eastern margin. These data provide further evidence for an Early to Late Triassic volcanic arc in northern New Guinea, interpreted to have been part of a continuous magmatic belt along the Gondwana margin, through South America, Antarctica, New Zealand, the New England Fold Belt, New Guinea and into southeast Asia. The Early to Late Triassic volcanic arc in northern New Guinea intrudes high‐grade metamorphic rocks probably resulting from Late Permian to Early Triassic (ca 260–240 Ma) orogenesis, as recorded in the New England Fold Belt. Late Triassic magmatism in New Guinea (ca 220 Ma) is related to coeval extension and rifting as a precursor to Jurassic breakup of the Gondwana margin. In general, mantle‐like Sr–Nd isotopic compositions of mafic Palaeozoic to Tertiary granitoids appear to rule out the presence of a North Australian‐type Proterozoic basement under the New Guinea Mobile Belt. Parts of northern New Guinea may have a continental or transitional basement whereas adjacent areas are underlain by oceanic crust. It is proposed that the post‐breakup margin comprised promontories of extended Proterozoic‐Palaeozoic continental crust separated by embayments of oceanic crust, analogous to Australia's North West Shelf. Inferred movement to the south of an accretionary prism through the Triassic is consistent with subduction to the south‐southwest beneath northeast Australia generating arc‐related magmatism in New Guinea and the New England Fold Belt.     

Earliest Eocene (53 Ma) convergence in the Southwest Pacific: evidence from pre‐obduction dikes in the ophiolite of New Caledonia

Uncertainty about the timing and location of the initiation of convergence in the western and south‐western Pacific greatly hinders accurate plate tectonic reconstructions of subduction systems in that area. The chemistry and age of dikes intruding mantle peridotite in the ophiolite of New Caledonia infer that subduction‐related magmatism began before 53 Ma. These new results infer that obduction in the south‐west Pacific is unrelated to the reorientation of the Pacific plate motion that occurred c. 43 Ma and confirm new interpretations showing that changes in mantle flow, hotspot and plate motion may have occurred as soon as late Paleocene or early Eocene.     

Cenozoic geodynamics of the Bering Sea region

In the Early Cenozoic before origination of the Aleutian subduction zone 50–47 Ma ago, the northwestern (Asian) and northeastern (North American) parts of the continental framework of the Pacific Ocean were active continental margins. In the northwestern part, the island-arc situation, which arose in the Coniacian, remained with retention of the normal lateral series: continent-marginal sea-island arc-ocean. In the northeastern part, consumption of the oceanic crust beneath the southern margin of the continental Bering shelf also continued from the Late Cretaceous with the formation of the suprasubduction volcanic belt. The northwestern and northeastern parts of the Paleopacific were probably separated by a continuation of the Kula-Pacific Transform Fracture Zone. Change of the movement of the Pacific oceanic plates from the NNW to NW in the middle Eocene (50–47 Ma ago) was a cause of the origin of the Aleutian subduction zone and related Aleutian island arc. In the captured part of the Paleopacific (proto-Bering Sea), the ongoing displacement of North America relative to Eurasia in the middle-late Eocene gave rise to the formation of internal structural elements of the marginal sea: the imbricate nappe structure of the Shirshov Ridge and the island arc of the Bowers Ridge. The Late Cenozoic evolution was controlled by subduction beneath the Kamchatka margin and its convergence with the Kronotsky Terrane in the south. A similar convergence of the Koryak margin with the Goven Terrane occurred in the north. The Komandorsky minor oceanic basin opened in the back zone of this terrane. Paleotectonic reconstructions for 68–60, 56–52, 50–38, 30–15, and 15–6 Ma are presented.     

U–Pb zircon dating of post-obduction volcanic-arc granitoids and a granulite-facies xenolith from New Caledonia. Inference on Southwest Pacific geodynamic models

In New Caledonia, the occurrence of one of the World’s largest and best-exposed subduction/obduction complex is a key point for the understanding of the geodynamic evolution of the whole Southwest Pacific region. Within the ophiolite, pre-and post-obduction granitoids intrude the ultramafic allochthon and provide new time constraints for the understanding of obduction processes. At 27.4 Ma, a new East-dipping subduction generated the active margin magmatism along the western coast of the island (Saint-Louis massif). At 24.3 Ma, the eastward shift of the magma activity and slightly different geochemical features (Koum-Borindi massif) was either related to the older slab break-off; or alternatively, due to the eastward migration of the mantle wedge following the collision of the eastern margin of the Low Howe rise. Finally, the occurrence of a granulite-facies xenolith in the Koum-Borindi massif with comparable 24.5 Ma U–Pb zircon age and isotopic features (initial ε_Nd = 5.1) suggests that these evolved magmas were generated within the lithospheric mantle beneath a continental crust of normal thickness. Geochronological evidence for continuous convergence during the Oligocene infers an East-dipping Eocene-Oligocene subduction/obduction system to have existed in the Southwest Pacific from the d’Entrecasteaux zone to the North Island of New Zealand.     

东海盆地形成的区域地质背景与构造演化特征

东海盆地处于西太平洋俯冲带前缘,是发育在华南克拉通基底之上的,以晚白垩世-新生代沉积为主的新生代盆地.东海盆地性质是在活动大陆边缘减薄陆壳之上的,由于洋-陆俯冲消减所引起的张裂、拉伸作用而形成的弧后裂谷型盆地,是西太平洋众多“沟-弧-盆”体系的一部分.东海盆地陆架外缘隆起控制着东海盆地的演化过程,该地质单元形成于晚白垩世,是陆缘隆起和增生楔的复合体,中新世后由于菲律宾海板块的活动而解体为现今的钓鱼岛隆褶带和琉球隆起.结合对陆架外缘隆起的研究后认为,东海盆地晚白垩世以来的演化历程具有3大构造阶段,即:第一阶段,古新世-中始新世西部坳陷形成发展期;第二阶段,中始新世-渐新世东部坳陷形成发展期,其中,中晚始新世太平洋板块的转向是东、西部坳陷构造迁移的分界点;第三阶段,中新世-全新世,东海盆地进入到菲律宾板块影响时期,原先的构造格局开始分解.      

Alpine metamorphism of Hercynian hornblende granodiorite beneath the blueschist facies schistes lustrés nappe of NE Corsica

Abstract The Hercynian granitic basement which forms the Tenda Massif in NE Corsica represents part of the leading edge of the European Plate during middle-to-late Cretaceous (Eoalpine) high P metamorphism. The metamorphism of this basement, induced by the overthrusting of a blueschist facies (schistes lustrés) nappe, was confined to a major ductile shear zone (c. 1000m thick) within which deformation increases upwards towards the overlying nappe. Metamorphism within the basement mostly records lower blueschist facies conditions (crossite + epidote) except near the base of the shear zone where the greenschist facies assemblage albite + actinolitic amphibole has developed instead of crossite. Study of the primary mafic phase breakdown reactions within hornblende granodiorite reveals the following metamorphic zonation. Zone 1: biotite to chlorite. Towards zone 2: biotite to phengite. Zone 2: Hornblende to actinolitic Ca-amphibole + albite + sphene, and biotite to actinolitic Ca-amphibole + albite + phengite + Ti-ore + epidote. Zone 3: Hornblende to crossite + low Ti-biotite + phengite + sphene, and biotite to crossite + low Ti-biotite + phengite + Ti-ore + sphene ± epidote. P-T conditions at the base of the shear zone are estimated to have been 390-490°C at 600-900 M Pa (6-9kbar) and the Corsican basement is therefore deduced to have been buried to 20-30 km during metamorphism. This relatively shallow metamorphism contrasts with some other areas in the Western Alps where the Eoalpine event apparently buried the European continental crust to depths of 80 km or more. As there is no evidence for a long history of blueschist facies metamorphism prior to the involvement of the European continent, it is deduced that the Eoalpine blueschists were produced during the collision of the Insubric plate with Europe, rather than during Tethyan intraoceanic subduction. Coherent blueschist terrains such as the schistes lustres probably record buovant feature collision and obduction tectonics rather than any preceding oceanic subduction.