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1.
Shinji Yoneshima Kimihiro Mochizuki Eiichiro Araki Ryota Hino Masanao Shinohara Kiyoshi Suyehiro 《Tectonophysics》2005,397(3-4):225-239
The Woodlark Basin, located south of the Solomon Islands arc region, is a young (5 Ma) oceanic basin that subducts beneath the New Britain Trench. This region is one of only a few subduction zones in the world where it is possible to study a young plate subduction of several Ma. To obtain the image of the subducting slab at the western side of the Woodlark Basin, a 40-day Ocean Bottom Seismometer (OBS) survey was conducted in 1998 to detect the micro-seismic activity. It was the first time such a survey had been performed in this location and over 600 hypocenters were located. The seismic activity is concentrated at the 10–60 km depth range along the plate boundary. The upper limit just about coincides with the leading edge of the accretionary wedge. The upper limit boundary was identified as the up-dip limit of the seismogenic zone, whereas the down-dip limit of the seismogenic zone was difficult to define. The dip angle of the plate at the high seismicity zone was found to average about 30°. Using the Cascadia subduction zone for comparison, which is a typical example of a young plate subduction, suggests that the subduction of the Woodlark Basin was differentiated by a high dip angle and rather landward location of the seismic front from the trench axis (30 km landward from the trench axis). Furthermore, as pointed out by previous researchers, the convergent margin of the Solomon Islands region is imposed with a high stress state, probably due to the collision of the Ontong Java Plateau and a rather rapid convergence rate (10 cm/year). The results of the high angle plate subduction and inner crust earthquakes beneath the Shortland Basin strongly support the high stress state. The collision of the Ontong Java Plateau, the relatively rapid convergence rate, and moderately cold slab as evidenced by low heat flow, rather than the plate age, may be dominantly responsible for the geometry of the seismogenic zone in the western part of the Woodlark Basin subduction zone. 相似文献
2.
The Solomon Sea region is an area of intense tectonic activity characterized by structural complexity, a high level of seismicity and volcanism, and rapid evolution of plate boundaries. There is little accretion in the eastern New Britain Trench. Accretion gradually increases westward with thick accretion in the western New Britain Trench and in the Trobriand Subduction System. The thick accretion in the western part of the New Britain Trench may be a result of collision from the north of Finisterre-Huon block with New Guinea mainland. The present boundary of the collision is along the Ram-Markham fault. Deformation structures and present day seismicity suggest that the northern block is under compression.
Accretion has occurred in the sediment filled trenches in the Solomon Sea. The scale of the accretionary wedge depends on the amount of trench-fill sediment available. It is unlikely that there is no sediment supply to the eastern part of the New Britain Trench where no accretion is observed and subduction erosion may be occurring. There are two possible mechanisms for subduction erosion of sediment; either a rapid rate of subduction relative to the supply of sediment inhibiting sediment accumulation in the trench; or horizontal tensional force superimposed on both the forearc and backarc regions of the arc. Seafloor spreading in both the Manus and Woodlark basins is fan-like with nearby poles in the western margins of the basins. This may be a reflection of a horizontally compressional field in the western part and a tensional field in the eastern part of the Solomon Sea. Therefore it is possible to conclude that the consumption of sediment in the eastern New Britain Trench is related to the horizontal tensional field superimposed on both the forearc and backarc regions of the subduction system.
Imbricated thrust and overthrust faults in the western New Britain Trench and Trobriand Trough are not linear over long distance, but form wavy patterns in blocks with unit distance of approximately 10 km. 相似文献
3.
Geochemical, isotopic, and geochronologic data for exhumed rocks in the Woodlark Rift of Papua New Guinea (PNG) allow a tectonic link to be established with the Late Cretaceous Whitsunday Volcanic Province (WVP) of northeastern Australia. Most of the metamorphic rocks in the Woodlark Rift have Nd isotopic compositions (εNd = + 1.7 to + 6.2) similar to the Nd isotopic compositions of rocks in the WVP (εNd = + 1.3 to + 6.6; Ewart et al., 1992), and contain inherited zircons with 90 to 100 Ma U–Pb ages that overlap the timing of magmatism in the WVP. None of the metamorphic rocks in the Woodlark Rift have the highly evolved Hf and Nd isotopic compositions expected of ancient continental crust. Magmas were erupted in the WVP during the middle Cretaceous as eastern Gondwana was rifted apart. The protoliths of felsic and intermediate metamorphic rocks in the Woodlark Rift are interpreted to be related to the magmatic products produced during this Cretaceous rifting event. Some mafic metamorphic rocks exposed in the western Woodlark Rift (eclogites and amphibolites) are not related to the WVP and instead could have originated as basaltic lavas crystallized from mantle melts at (U)HP depths in the Late Cenozoic, or as fragments of Mesozoic aged oceanic lithosphere.Isotopic and elemental comparisons between basement gneisses and Quaternary felsic volcanic rocks demonstrate that felsic lavas in the D'Entrecasteaux Islands did not form solely from partial melting of metamorphic rocks during exhumation. Instead, the isotopic compositions and geochemistry of Quaternary felsic volcanic rocks indicate a significant contribution from the partial melting of the mantle in this region. When combined with geophysical data for the western Woodlark Rift, this suggests that future seafloor spreading will commence south of Fergusson Island, and west of the present-day active seafloor spreading rift tip. 相似文献
4.
Georges Pascal 《Tectonophysics》1979,57(1):7-34
Earthquakes for the period 1964–1973 are relocated by the method of Joint Hypocenter Determination in order better to resolve the configuration and the structure of the New Guinea—New Britain—Solomon Islands region. Focal mechanism solutions are integrated with the seismicity and interpreted closely with it. A zone of subduction exists beneath New Britain and the Solomon Islands, a zone of left-lateral strike-slip movement extends from New Ireland to New Guinea. The zone of seismicity in northern New Guinea has developed as a result of a continent—island-arc collision in late Oligocene—Miocene times and does not exhibit a well-developed inclined seismic zone. It is proposed that plate tectonics theory does not apply rigorously, but slip-line field theory allows the presentation of a new geodynamic model for this region. 相似文献
5.
边缘海如何形成是地球科学的基本问题.本研究通过对南海区域深反射地震数据及钻井数据的综合解释,聚焦地壳深部结构和三维全变形机制,在南海陆缘张裂-海盆扩张的构造动力学研究中取得重要进展:(1)"大陆破裂非均一":拉张过程垂向上分层非均一,受拆离断层系统控制;裂离过程横向上高度变化,中-东侧受岩浆作用主导,西侧受构造作用主导... 相似文献
6.
J. J. Veevers 《Australian Journal of Earth Sciences》2013,60(4):451-462
The seafloor off the Otway/West Tasmanian Basins has an east‐west magnetic lineation attributable to seafloor spreading and notionally identified with the set of seafloor spreading anomalies A8‐A20. Anomaly A20 (45 Ma) lies immediately south of a magnetic quiet zone that extends northward past the continent‐ocean boundary (COB). The Southeast Indian Ocean has a constant angular width between the formerly conjugate margins of Australia and Antarctica, consistent with spreading that started along the entire margin about 96 Ma.The proximity of A20 to the Australian COB in some spreading ridge segments is therefore postulated as due to jumps of the spreading ridge to Australia with concomitant transfer of the older oceanic part of the Australian Plate to the Antarctic Plate. Accordingly, the age of the oldest seafloor at the COB in seven original ridge segments is estimated to step from about 96 to 82, 79, and 75 Ma. Break‐up marks a change in the subsidence of the margin from rapid, during rifting by continental extension, to slow during thermal subsidence of the seafloor. Subsequent ridge jumps to the COB are expected to cause uplift or at least still‐stand of the adjacent continental margin. The subsidence history of the Otway/West Tasmanian margin, as indicated by oil exploration wells, is sympathetic with the timing of the postulated ridge jumps in the adjacent seafloor, as is that of the Great Australian Bight Basin with adjacent seafloor to the west, and of the Bass and Gippsland Basins with the Tasman Sea adjacent to the east. The growth of structure at 80 Ma in the outer Gippsland Basin corresponds with a jump to Australia of the Tasman Sea ridge at 82 and 75 Ma, and at 65 Ma in the Great Australian Bight and Otway Basins to a ridge jump to Australia of the adjacent seafloor. The growth of structure at 60 Ma in the Bass Basin and at 55 Ma in the Gippsland Basin corresponds with the abandonment of the Tasman Sea ridge at A24 (55 Ma) during a re‐organization of spreading in the southwest Pacific. 相似文献
7.
Geochemical constraints on the petrogenesis of arc picrites and basalts,New Georgia Group,Solomon Islands 总被引：2，自引：0，他引：2
Island arc picrites are restricted to a few localities including the Lesser Antilles, Japan, Vanuatu and the Solomon Islands. The picrite occurrences appear to be linked to the subduction of young, hot oceanic crust and anomalous geotherms. At the Solomon arc, the Australian plate is presently subducted beneath the Pacific plate. A particular feature of the Solomon arc is the subduction of a spreading center (Woodlark Ridge). In the Solomon Islands, picrites only occur in the New Georgia archipelago, located above or close to the subducting Woodlark Ridge. These picrites contain between 12 and 30 wt% MgO, the associated primitive basalts show MgO contents from 11.5 to 13.6 wt%. Linear trends defined by Cr, Ni and other trace elements vs. MgO indicate that the picritic bulk compositions originate from mixing between a basaltic-picritic melt and a Mg- and Cr-rich endmember, rather than from fractional crystallization of extremely Mg-rich magmas. Major and trace element modeling identify mantle wedge peridotite as the most likely mixing endmember. Trace element abundances in the Solomon arc picrites indicate a mantle source enrichment by subduction components and a large depletion of Nb and Ta that is typical for island arc volcanic rocks. Most incompatible trace element patterns of the New Georgia picrites and basalts are parallel, supporting a cogenetic evolution of these rocks by mixing processes. 87Sr/86Sr and Nd values in the basalts and picrites range from 0.7033 to 0.7043 and +5.8 to +8.0, respectively. These values partially overlap with compositions of the Indian MORB field. Alternatively, subducted sediment and fluids from altered MORB may have displaced the Sr isotope composition to more radiogenic 87Sr/86Sr. Hf values range from +12.2 to +14.6 and show in combination with Nd that the picrites were most likely generated within the Indian mantle domain.This revised version was published online September 2004 with a correction to Table 2. 相似文献
8.
T. V. Gerya 《Petrology》2013,21(6):550-560
This work presents high-resolution 3D numerical model of transform fault initiation at rifted continental margins. Our petrological-thermomechanical visco-plastic model allows for spontaneous nucleation of oceanic spreading process in a continental rift zone and takes into account new oceanic crust growth driven by decompression melting of the asthenospheric mantle. Numerical model predicts that ridge-transform spreading pattern initiate in several subsequent stages: crustal rifting (0–1.5 Myr), spreading centers nucleation and propagation (1.5–3 Myr), proto-transform fault initiation and rotation (3–5 Myr) and mature ridge-transform spreading (> 5 Myr). Comparison of modeling results with the natural data from the Woodlark Basin suggests that the development of this region closely matches numerical predictions. Similarly to the model, the Moresby (proto-) Transform terminates in the oceanic rather than in the continental crust. This fault associates with a notable topographic depression and formed within 0.5–2 Myr while linking two offset overlapping spreading segments. Model reproduces well characteristic “rounded” contours of the spreading centers as well as the presence of a remnant of the broken continental crustal bridge observed in the Woodlark Basin. Proto-transform fault traces and truncated tip of one spreading center present in the model are also documented in nature. Numerical results are in good agreement with the concept of Taylor et al. (2009) which suggests that spreading segments nucleate en echelon in overlapping rift basins and that transform faults develop as or after spreading nucleates. Our experiments also allow to refine this concept in that (proto)-transform faults may also initiate as oblique rather than only spreading-parallel tectonic features. Subsequent rotation of these faults toward the extension-parallel direction is governed by space accommodation during continued oceanic crust growth within offset ridge-transform intersections. 相似文献
9.
The Philippine Sea plate, located between the Pacific, Eurasian and Australian plates, is the world's largest marginal basin plate. The motion of the Philippine Sea plate through time is poorly understood as it is almost entirely surrounded by subduction zones and hence, previous studies have relied on palaeomagnetic analysis to constrain its rotation. We present a comprehensive analysis of geophysical data within the Parece Vela and Shikoku Basins—two Oligocene to Miocene back-arc basins—which provide independent constraints on the rotational history of the Philippine Sea plate by means of their seafloor spreading record. We have created a detailed plate model for the opening of the Parece Vela and Shikoku Basins based on an analysis of all available magnetic, gravity and bathymetric data in the region. Subduction along the Izu–Bonin–Mariana trench led to trench roll-back, arc rupture and back-arc rifting in the Parece Vela and Shikoku Basins at 30 Ma. Seafloor spreading in both basins developed by chron 9o (28 Ma), and possibly by chron 10o (29 Ma), as a northward and southward propagating rift, respectively. The spreading orientation in the Parece Vela Basin was E–W as opposed to ENE–WSW in the Shikoku Basin. The spreading ridges joined by chron 6By (23 Ma) and formed a R–R–R triple junction to accommodate the difference in spreading orientations in both basins. At chron 6No (20 Ma), the spreading direction in the Parece Vela Basin changed from E–W to NE–SW. At chron 5Ey (19 Ma), the spreading direction in the Shikoku Basin changed from ENE–WSW to NE–SW. This change was accompanied by a marked decrease in spreading rate. Cessation of back-arc opening occurred at 15 Ma, a time of regional plate reorganisation in SE Asia. We interpret the dramatic change in spreading rate and direction from E–W to NE–SW at 20±1.3 Ma as an expression of Philippine Sea plate rotation and is constrained by the spacing between our magnetic anomaly identifications and the curvature of the fracture zones. This rotation was previously thought to have begun at 25 Ma as a result of a global change in plate motions. Our results suggest that the Philippine Sea plate rotated clockwise by about 4° between 20 and 15 Ma about a pole located 35°N, 84°E. This implies that the majority of the 34° clockwise rotation inferred to have occurred between 25 and 5 Ma from paleomagnetic data may have in fact been confined to the period between 15 and 5 Ma. 相似文献
10.
Shaoru Yin Weiwei Ding Derek E. Sawyer Ziyin Wu Yong Tang 《International Geology Review》2020,62(7-8):887-907
ABSTRACT Based on approximately 11,000 km of seismic reflection data collected across the South China Sea oceanic basin, we describe the sedimentary filling characteristics of the basin since its Oligocene opening, as well as connections between this history and contemporaneous regional tectonic events. The seismic lines are spaced ~50 km apart, and the data are tied to International Ocean Discovery Program (IODP) Expedition 349 drilling data. Basin filling occurred in three phases, with basin-wide mean sedimentation rates increasing through time. During the Oligocene to middle Miocene, sediments accumulated primarily in the northern East and Northwest Sub-basins, with a mean basin-wide sedimentation rate of 8 m/m.y. The presence of these deposits over deep basement floor indicates that seafloor spreading initiated in these northern regions. During the late Miocene, deposition occurred primarily in the Northwest Sub-basin and partly in the southern East Sub-basin, with a mean basin-wide sedimentation rate of 30 m/m.y. Basin filling during this time seems to have been linked to slip reversal of the Red River Fault and collision of the North Palawan Block with the Luzon Arc. During the Pliocene and Pleistocene, sediments accumulated rapidly in the northeastern and southern East Sub-basin and the Southwest Sub-basin. The mean basin-wide sedimentation rate was 70 m/m.y. Basin filling during this phase seems to have been associated with the Taiwan and North Palawan collisions, SCS subduction along the Manila Trench, and Tibetan Plateau uplift. Gravity flow deposits predominate throughout the basin fill. 相似文献
11.
The eastern Coral Sea is a poorly explored area at the north-eastern corner of the Australian Tectonic Plate, where interaction between the Pacific and Australian plate boundaries, and accretion of the world's largest submarine plateau – the Ontong Java Plateau – has resulted in a complex assemblage of back-arc basins, island arcs, continental plateaus and volcanic products. This study combines new and existing magnetic anomaly profiles, seafloor fabric from swath bathymetry data, Ar–Ar dating of E-MORB basalts, palaeontological dating of carbonate sediments, and plate modelling from the eastern Coral Sea. Our results constrain commencement of the opening of the Santa Cruz Basin and South Rennell Trough to c. 48 Ma and termination at 25–28 Ma. Simultaneous opening of the Melanesian Basin/Solomon Sea further north suggests that a single > 2000 km long back-arc basin, with at least one triple junction existed landward of the Melanesian subduction zone from Eocene–Oligocene times. The cessation of spreading corresponds with a reorganisation of the plate boundaries in the area and the proposed initial soft collision of the Ontong Java Plateau. The correlation between back-arc basin cessation and a widespread plate reorganisation event suggests that back-arc basins may be used as markers for both local and global plate boundary changes. 相似文献
12.
Subduction zones with deep seismicity are believed to be associated with the descending branches of convective flows in the mantle and are subordinated to them. Therefore, the position of subduction zones can be considered as relatively fixed with respect to the steady-state system of convective flows. The lithospheric plate overhanging a subduction zone (as a rule of continental type) may:
- 1. (1) either move away from the subduction zone; or
- 2. (2) move onto it. In the first case extensional conditions originate behind the subduction zone and the new oceanic crust of back-arc basins forms. In the second case active Andean-type continental margins with thickening of the crust and lithosphere are observed.
13.
Gondwana breakup via double-saloon-door rifting and seafloor spreading in a backarc basin during subduction rollback 总被引：1，自引：0，他引：1
A model has been developed where two arc-parallel rifts propagate in opposite directions from an initial central location during backarc seafloor spreading and subduction rollback. The resultant geometry causes pairs of terranes to simultaneously rotate clockwise and counterclockwise like the motion of double-saloon-doors about their hinges. As movement proceeds and the two terranes rotate, a gap begins to extend between them, where a third rift initiates and propagates in the opposite direction to subduction rollback. Observations from the Oligocene to Recent Western Mediterranean, the Miocene to Recent Carpathians, the Miocene to Recent Aegean and the Oligocene to Recent Caribbean point to a two-stage process. Initially, pairs of terranes comprising a pre-existing retro-arc fold thrust belt and magmatic arc rotate about poles and accrete to adjacent continents. Terrane docking reduces the width of the subduction zone, leading to a second phase during which subduction to strike-slip transitions initiate. The clockwise rotated terrane is caught up in a dextral strike-slip zone, whereas the counterclockwise rotated terrane is entrained in a sinistral strike-slip fault system. The likely driving force is a pair of rotational torques caused by slab sinking and rollback of a curved subduction hingeline.By analogy with the above model, a revised five-stage Early Jurassic to Early Cretaceous Gondwana dispersal model is proposed in which three plates always separate about a single triple rift or triple junction in the Weddell Sea area. Seven features are considered diagnostic of double-saloon-door rifting and seafloor spreading:
- i) earliest movement involves clockwise and counterclockwise rotations of the Falkland Islands Block and the Ellsworth Whitmore Terrane respectively;
- ii) terranes comprise areas of a pre-existing retro-arc fold thrust belt (the Permo-Triassic Gondwanide Orogeny) attached to an accretionary wedge/magmatic arc; the Falklands Islands Block is initially attached to Southern Patagonia/West Antarctic Peninsula, while the Ellsworth Whitmore Terrane is combined with the Thurston Island Block;
- iii) paleogeographies demonstrate rifting and extension in a backarc environment relative to a Pacific margin subduction zone/accretionary wedge where simultaneous crustal shortening occurs;
- iv) a ridge jump towards the subduction zone from east of the Falkland Islands to the Rocas Verdes Basin evinces subduction rollback;
- v) this ridge jump combined with backarc extension isolated an area of thicker continental crust — The Falkland Islands Block;
- vi) well-documented EW oriented seafloor spreading anomalies in the Weddell Sea are perpendicular to the subduction zone and propagate in the opposite direction to rollback;
- vii) the dextral strike-slip Gastre and sub-parallel faults form one boundary of the Gondwana subduction rollback, whereas the other boundary may be formed by inferred sinistral strike-slip motion between a combined Thurston Island/Ellsworth Whitmore Terrane and Marie Byrd Land/East Antarctica.
Keywords: Gondwana breakup; Double-saloon-door seafloor spreading; Plate tectonics; Backarc basin; Subduction rollback; Opposite rotations of terranes 相似文献
14.
I. D. Lindley 《Australian Journal of Earth Sciences》2016,63(6):731-754
Woodlark Island (Muyuw) is located in a tectonically complex region, one of the few places on Earth where continental breakup is occurring ahead of seafloor spreading. Rifting commenced in the late Miocene (8.8–6 Ma) and is associated with the westward-propagating Woodlark Basin Spreading Centre. The island comprises approximately 850 km2 of raised Pleistocene coral reef and associated sediments with a central, moderately elevated range underlain by the middle Miocene calc-alkaline to shoshonitic Okiduse Volcanic Group (new name). It provides an exposure of upper Cenozoic geology in close proximity to the spreading centre. The Okiduse Volcanic Group is host to most of the island's historical gold and silver production and recently defined mineral resources totalling 1.75 Moz gold. This study uses facies analysis of pyroclastic deposits to develop a detailed geological map of the Okiduse Volcanic Group, with a revision and reinterpretation of the unit. Facies associations suggest that two major volcanic centres erupted synchronously during the middle Miocene (14–12 Ma), referred to as the Watou Mountain Eruptive Centre (new name) and the Uvarakoi Caldera (new name). The mafic–intermediate Watou Mountain Eruptive Centre formed during frequent small eruptions of widely varying style. Strombolian, subplinian, vulcanian and dome-related explosive eruptions occurred, alternating with extrusion of block and ash flow deposits and lava domes. Pyroclastic deposits were rapidly reworked from the steep cone, and were redeposited in a series of coalescing aprons surrounding the volcano. The felsic Uvarakoi Caldera formed during a series of violent explosive eruptions by rapid removal of magma from the underlying chamber, followed by collapse. Plinian and possibly phreatoplinian eruptions, as a result of magma–water mixing in the surface environment, resulted in widely dispersed, highly fragmented tuff deposits. The caldera was modified by widespread erosion following eruptions, resulting in fluvial, laharic and slope-wash deposits. This study highlights lithological controls (porosity and permeability) by various units within the Okiduse Volcanic Group on ore deposition. 相似文献
15.
This article reviews the electrical conductivity structures of the oceanic upper mantle, subduction zones, and the mantle transition zone beneath the northwestern Pacific, the Japanese Islands, and continental East Asia, which have particularly large potential of water circulation in the global upper mantle. The oceanic upper mantle consists of an electrically resistive lid and a conductive layer underlying the lid. The depth of the top of the conductive layer is related to lithospheric cooling in the older mantle, whereas it is attributable to the difference in water distribution beneath the vicinity of the seafloor spreading-axis. The location of a lower crustal conductor in a subduction zone changes according to the subduction type. The difference can be explained by the characteristic dehydration from the subducting slab in each subduction zone and by advection from the backarc spreading. The latest one-dimensional electrical conductivity model of the mantle transition zone beneath the Pacific Ocean predicts values of 0.1–1.0 S/m. These values support a considerably dry oceanic mantle transition zone. However, one-dimensional electrical profiles may not be representative of the mantle transition zone there, since there exists a three-dimensional structure caused by the stagnant slab. Three-dimensional electromagnetic modeling should be made in future studies. 相似文献
16.
《Gondwana Research》2010,17(3-4):545-562
This article reviews the electrical conductivity structures of the oceanic upper mantle, subduction zones, and the mantle transition zone beneath the northwestern Pacific, the Japanese Islands, and continental East Asia, which have particularly large potential of water circulation in the global upper mantle. The oceanic upper mantle consists of an electrically resistive lid and a conductive layer underlying the lid. The depth of the top of the conductive layer is related to lithospheric cooling in the older mantle, whereas it is attributable to the difference in water distribution beneath the vicinity of the seafloor spreading-axis. The location of a lower crustal conductor in a subduction zone changes according to the subduction type. The difference can be explained by the characteristic dehydration from the subducting slab in each subduction zone and by advection from the backarc spreading. The latest one-dimensional electrical conductivity model of the mantle transition zone beneath the Pacific Ocean predicts values of 0.1–1.0 S/m. These values support a considerably dry oceanic mantle transition zone. However, one-dimensional electrical profiles may not be representative of the mantle transition zone there, since there exists a three-dimensional structure caused by the stagnant slab. Three-dimensional electromagnetic modeling should be made in future studies. 相似文献
17.
Seismic refraction surveys conducted in 1976 and 1979 over the broken ice surface of the Arctic Ocean, reveal distinctly different crustal structures for the Fram, Makarov and Canada basins. The Canada Basin, characterized by a 2–4 km thick sedimentary layer and a distinct oceanic layer 3B of 7.5 km/s velocity has the thickest crust and is undoubtedly the oldest of the three. The crust of the Makarov Basin has a thin sedimentary layer of less than 1 km and is about 9 km in total thickness. The Fram Basin has a similarly thin sedimentary layer but is 3–4 km thicker than the Makarov as it approaches the Lomonosov Ridge near the North Pole. The ridge itself is cored by material with a velocity of 6.6 km/s and may be a metagabbro similar to oceanic layer 3A. This ridge root material extends to a depth of about 27 km, where a change occurs to upper-mantle material with a velocity of 8.3 km/s. The core is overlain by up to 6 km of material with a velocity of about 4.7 km/s which could be oceanic layer 2A basalts or continental crystalline rocks with some sedimentary material.The Fram Basin probably began to open contemporaneously with the North Atlantic about 70 m.y. ago, by spreading along the Nansen-Gakkel Ridge. Although not yet dated, the Makarov Basin is probably no older than the initiation of the Fram Basin and may be much younger. The Alpha Ridge may once have been part of the Lomonosov Ridge, splitting off to form the Makarov Basin between 70 and 25 m.y. ago and possibly contributing to the Eurekan Orogeny of 25 m.y. ago, evident on Ellesmere Island. In contrast, the likely age of the Canada Basin lies in the 125–190 m.y. range and may have been formed by the counter-clockwise rotation of Alaska and the Northwind Ridge away from the Canadian Arctic Islands. The Lomonosov Ridge emerges from this scenario as a block resulting from a strike-slip shear zone on the European continental shelf, related to the opening of the Canada basin (180-120 my) and then becomes an entity broken from this shelf by the opening of the Eurasia Basin (70-0 m.y.). 相似文献
18.
The interaction of the Australian, South Bismarck and Solomon Sea Plates in Papua New Guinea is the source of frequent earthquakes that occur as a result of subduction and arc continent collision. Previous investigators have drawn attention to a discontinuity in the horizontal azimuth of slip vectors along the southern boundary of the South Bismarck Plate, with those to the west of 148°E being systematically rotated 20ndash;30° clockwise compared to those located east of 148°E. This has led to the suggestion that relative motion may be occurring between the Huon Peninsula and New Britain or that more than two plates are acting south of the South Bismarck Plate. Global positioning system (GPS) measurements since 1991 indicate that there is no internal deformation occurring within the South Bismark Plate and that at least two distinct plates are in contact with the southern edge of the South Bismarck Plate. We show from a study of a recent earthquake dataset that the change in slip vector azimuth can be modelled by the interaction of the overriding South Bismarck Plate with the underthrusting Australian and Solomon Sea Plates, consistent with the GPS observations, while maintaining the South Bismarck Plate as a rigid entity. We found that a transition zone exists between 147°E and 148°E where the underlying plate changes from the Australian Plate to the Solomon Sea Plate. There are insufficient data at present to indicate whether or not a third plate, the Woodlark Plate, is also interacting directly with the South Bismarck Plate in this transition zone. Slip vector azimuths were used to estimate an Euler pole (6.74°S, 144.64°E), which describes the relative motion of the South Bismarck and Solomon Sea Plates along the New Britain Trench. 相似文献
19.
《Tectonophysics》1999,301(1-2):35-60
The Solomon Islands are a complex collage of crustal units or terrains (herein termed the `Solomon block') which have formed and accreted within an intra-oceanic environment since Cretaceous times. Predominantly Cretaceous basaltic basement sequences are divided into: (1) a plume-related Ontong Java Plateau terrain (OJPT) which includes Malaita, Ulawa, and northern Santa Isabel; (2) a `normal' ocean ridge related South Solomon MORB terrain (SSMT) which includes Choiseul and Guadalcanal; and (3) a hybrid `Makira terrain' which has both MORB and plume/plateau affinities. The OJPT formed as an integral part of the massive Ontong Java Plateau (OJP), at c. 122 Ma and 90 Ma, respectively, was subsequently affected by Eocene–Oligocene alkaline and alnoitic magmatism, and was unaffected by subsequent arc development. The SSMT initially formed within a `normal' ocean ridge environment which produced a MORB-like basaltic basement through which two stages of arc crustal growth subsequently developed from the Eocene onwards. The Makira terrain records the intermingling of basalts with plume/plateau and MORB affinities from c. 90 Ma to c. 30 Ma, and a contribution from Late Miocene–present-day arc growth. Two distinct stages of arc growth occurred within the Solomon block from the Eocene to the Early Miocene (stage 1) and from the Late Miocene to the present day (stage 2). Stage 1 arc growth created the basement of the central part of the Solomon block (the Central Solomon terrain, CST), which includes the Shortland, Florida and south Isabel islands. Stage 2 arc growth led to crustal growth in the west and south (the New Georgia terrain or NGT) which includes Savo, and the New Georgia and Russell islands. Both stages of arc growth also added new material to pre-existing crustal units within other terrains. The Solomon block terrane collage records the collision between the Alaska sized OJP and the Solomon arc. Initial contact possibly first occurred some 25–20 Ma but it is only since around 4 Ma that the OJP has more forcefully collided with the Solomon arc, and has been actively accreting since that time, continuing to the present day. We present a number of tectonic models in an attempt to understand the mechanism of plateau accretion. One model depicts the OJP as splitting in two with the upper 4–10 km forming an imbricate stack verging to the northeast, over which the Solomon arc is overthrust, whilst deeper portions of the OJP (beneath a critical detachment surface) are subducted. The subduction of young (<5 Ma), hot, oceanic lithosphere belonging to the Woodlark basin at the SSTS has resulted in a sequence of tectonic phenomena including: the production of unusual magma compositions (e.g. Na–Ti-rich basalts, and an abundance of picrites); an anomalously small arc–trench gap between the SSTS and the Quaternary–Recent arc front; calc-alkaline arc growth within the downgoing Woodlark basin lithospheric plate as a consequence of calc-alkaline magma transfer along leaky NE–SW-trending faults; rapid fore-arc uplift; and rapid infilling of intra-arc basins. The present-day highly oblique collision between the Pacific and Australian plates has resulted in the formation of rhombohedral intra- and back-arc basins. 相似文献
20.
Seiichi Miura Kiyoshi Suyehiro Masanao Shinohara Narumi Takahashi Eiichiro Araki Asahiko Taira 《Tectonophysics》2004,389(3-4):191
A seismic refraction–reflection experiment using ocean bottom seismometers and a tuned airgun array was conducted around the Solomon Island Arc to investigate the fate of an oceanic plateau adjacent to a subduction zone. Here, the Ontong Java Plateau is converging from north with the Solomon Island Arc as part of the Pacific Plate. According to our two-dimensional P-wave velocity structure modeling, the thickness of the Ontong Java Plateau is about 33 km including a thick (15 km) high-velocity layer (7.2 km/s). The thick crust of the Ontong Java Plateau still persists below the Malaita Accreted Province. We interpreted that the shallow part of the Ontong Java Plateau is accreted in front of the Solomon Island Arc as the Malaita Accreted Province and the North Solomon Trench are not a subduction zone but a deformation front of accreted materials. The subduction of the India–Australia Plate from the south at the San Cristobal Trench is confirmed to a depth of about 20 km below sea level. Seismicity around our survey area shows shallow (about 50 km) hypocenters from the San Cristobal Trench and deep (about 200 km) hypocenters from the other side of the Solomon Island Arc. No earthquakes occurred around the North Solomon Trench. The deep seismicity and our velocity model suggest that the lower part of the Ontong Java Plateau is subducting. After the oceanic plateau closes in on the arc, the upper part of the oceanic plateau is accreted with the arc and the lower part is subducted below the arc. The estimation of crustal bulk composition from the velocity model indicates that the upper portion and the total of the Solomon Island Arc are SiO2 58% and 53%, respectively, which is almost same as that of the Izu–Bonin Arc. This means that the Solomon Island Arc can be a contributor to growing continental crust. The bulk composition of the Ontong Java Plateau is SiO2 49–50%, which is meaningfully lower than those of continents. The accreted province in front of the arc is growing with the convergence of the two plates, and this accretion of the upper part of the oceanic plateau may be another process of crustal growth, although the proportion of such contribution is not clear. 相似文献