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1.
Petrological, geochemical and radiogenic isotopic data on ophiolitic‐type rocks from the Marlborough terrane, the largest (～700 km2) ultramafic‐mafic rock association in eastern Australia, argue strongly for a sea‐floor spreading centre origin. Chromium spinel from partially serpentinised mantle harzburgite record average Cr/(Cr + Al) = 0.4 with associated mafic rocks displaying depleted MORB‐like trace‐element characteristics. A Sm/Nd isochron defined by whole‐rock mafic samples yields a crystallisation age of 562 ± 22 Ma (2σ). These rocks are thus amongst the oldest rocks so far identified in the New England Fold Belt and suggest the presence of a late Neoproterozoic ocean basin to the east of the Tasman Line. The next oldest ultramafic rock association dated from the New England Fold Belt is ca530 Ma and is interpreted as backarc in origin. These data suggest that the New England Fold Belt may have developed on oceanic crust, following an oceanward migration of the subduction zone at ca540 Ma as recorded by deformation and metamorphism in the Anakie Inlier. Fragments of late Neoproterozoic oceanic lithosphere were accreted during progressive cratonisation of the east Australian margin.  相似文献   

2.
Ultramafic‐intermediate rocks exposed on the South Island of the Percy Isles have been previously grouped into the ophiolitic Marlborough terrane of the northern New England Fold Belt. However, petrological, geochemical and geochronological data all suggest a different origin for the South Island rocks and a new terrane, the South Island terrane, is proposed. The South Island terrane rocks differ from ultramafic‐mafic rocks of the Marlborough terrane not only in lithological association, but also in geochemical features and age. These data demonstrate that the South Island terrane is genetically unrelated to the Marlborough terrane but developed in a supra‐subduction zone environment probably associated with an Early Permian oceanic arc. There is, however, a correlation between the South Island terrane rocks and intrusive units of the Marlborough ophiolite. This indicates that the two terranes were in relative proximity to one another during Early Permian times. A K/Ar age of 277 ± 7 Ma on a cumulative amphibole‐rich diorite from the South Island terrane suggests possible affinities with the Gympie and Berserker terranes of the northern New England Fold Belt.  相似文献   

3.
Advancements of zircon U-Pb dating techniques using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) provide an opportunity to precisely constrain the age of the Bay of Islands Ophiolite Complex (BOIC) in the Newfoundland Appalachians. New LA-ICP-MS U-Pb dates are presented for a large population of relatively simple magmatic zircon grains from trondhjemites using selective domain analyses and a 3% discordance filter in five samples collected from a single ophiolite massif, with four of the samples from a single pluton. All samples were derived from plagiogranite plutons intruded just below the gabbro-sheeted dike contact of the Blow Me Down Mountain massif, the massif sampled for three of the four prior single sample ID-TIMS age studies of the ophiolite. A highly consistent series of LA-ICP-MS concordant ages from the five samples collected yield a composite concordia age of 488.3 ± 1.5 Ma, which we propose for the revised igneous age of the ophiolite massif. This new age for the BOIC is based on 127 individual LA-ICP-MS zircon analyses and is slightly older and with lower uncertainty than the previous consensus age. We review previous pioneering ID-TIMS U-Pb dates, ages, intercept assumptions, and uncertainties for the BOIC in light of this new formation age. Legacy ages and uncertainties differ somewhat when data are reduced with modern recommended techniques and without geological interpretations that fix lower intercepts. The ca. 488.3 Ma BOIC age indicates its formation along a younger supra-subduction forearc trench-orthogonal spreading center that rifted the older Coastal Complex forearc ophiolitic assemblages reported to be formed at ca. 500-508 Ma. This rifted forearc is analogous to many modern ophiolitic forearcs. The BOIC age is highly correlated with similar ages of the peri-Laurentian Notre Dame arc-proximal Betts Cove and Point Rousse ophiolites, indicating temporal BOIC spreading center linkages within the Baie Verte Oceanic Tract.  相似文献   

4.
The ultramafic‐mafic complex in the Marlborough terrane of the northern New England Fold Belt is dominated by members of a Neoproterozoic (ca 560 Ma) ophiolite (V1). The ophiolite has been intruded by the products of three Palaeozoic tectonomagmatic episodes (V2, V3 and V4). The V2 magmatic episode is represented by tholeiitic and calc‐alkaline basalts and gabbros of island‐arc affinities. Sm/Nd isotopes give a whole‐rock isochron age of 380 ± 19 Ma (2σ) to this episode, some 180 million years younger than the V1 ophiolitic rocks. The V3 magmatic episode includes tholeiitic and alkali basalts with enriched geochemical signatures characteristic of intraplate volcanism. A whole‐rock Sm/Nd isochron age of 293 ± 35 Ma is obtained for this event. A fourth magmatic event (V4) is represented by basaltic andesites and siliceous intrusives with geochemical features similar to modern adakites. This event has its type locality in the Percy Isles. These data provide tectonic and geochronological constraints for the previously enigmatic Marlborough terrane and as such contribute to the ever‐evolving understanding of New England Fold Belt development.  相似文献   

5.
The ultramafic massif of Bulqiza, which belongs to the eastern ophiolitic belt of Albania, is a major source of metallurgical chromitite ore. The massif consists of a thick (> 4 km) sequence, composed from the base upward of tectonized harzburgite with minor dunite, a transitional zone of dunite, and a magmatic sequence of wehrlite, pyroxenite, troctolite and gabbro. Only sparse, refractory chromitites occur within the basal clinopyroxene-bearing harzburgites, whereas the upper and middle parts of the peridotite sequence contain abundant metallurgical chromitites. The transition zone dunites contain a few thin layers of metallurgical chromitite and sparse bodies are also present in the cumulate section. The Bulqiza Ophiolite shows major changes in thickness, like the 41–50 wt.% MgO composition similar with forearc peridotite as a result of its complex evolution in a suprasubduction zone (SSZ) environment. The peridotites show abundant evidence of mantle melt extraction at various scales as the orthopyroxene composition change from core to rim, and mineral compositions suggest formation in a forearc, as Fo values of olivine are in 91.1–93.0 harzburgite and 91.5–91.9 in dunite and 94.6–95.9 in massive chromitite. The composition of the melts passing through the peridotites changed gradually from tholeiite to boninite due to melt–rock reaction, leading to more High Cr# chromitites in the upper part of the body. Most of the massive and disseminated chromitites have high Cr# numbers (70–80), although there are systematic changes in olivine and magnesiochromite compositions from harzburgites, to dunite envelopes to massive chromitites, reflecting melt–rock reaction. Compositional zoning of orthopyroxene porphyroblasts in the harzburgite, incongruent melting of orthopyroxene and the presence of small, interstitial grains of spinel, olivine and pyroxene likewise attest to modification by migrating melts. All of the available evidence suggests that the Bulqiza Ophiolite formed in a suprasubduction zone mantle wedge.  相似文献   

6.
The Phuket-Slate Belt terrane can be traced for 1700 km from Phuket to Mandalay, and has a distinct stratigraphy and tectonic history. It is characterized by a very thick Carboniferous-Lower Permian succession which includes diamictites interpreted as glacio-marine rift-infill deposited when the Sibumasu block separated from Gondwana. It was emplaced in the Late Cretaceous-Palaeogene by dextral strike-slip movement on a fault system which includes the Khlong Marui and Panlaung Faults. Southwards the Khlong Marui bounding-fault and its close associate, the Ranong Fault, are postulated to extend to Sumatra where they align with the restored proto-Indian Ocean location of the India–Australia transform at the time that both were undergoing dextral displacement and Greater India was moving toward its collision with Eurasia. It is suggested that emplacement of the Phuket-Slate Belt terrane was the result of its coupling with the north-going India plate, resulting in up to about 450 km of dextral shift on the terrane's bounding fault system. Post-emplacement sinistral movement on the cross-cutting Mae Ping and Three Pagodas Faults offset the terrane boundary resulting in its present outline.  相似文献   

7.
The front of the Zoulang Nanshan Caledonian volcanic island arc zone in the northern Qilian Mountains is a forearc accretionary terrane, composed of multiple accretionary volcanic island arcs, flysch accretionary wedges,high-pressure metamorphosed detachment zones and remnants of ophiolites. It resulted from the northeastward subduction of the Early Palaeozoic Qilan oceanic crust beneath the Alxa block. High-pressure metamorphism, which occurred during the subduction, progressed through three stages: the initial stage of medium T-high P,the main stage of temperature decrease and pressure increase, and the lag stage of pressure decrease and temperature increase. Finally the paper presents a retrotrench subduction dynamic model indicative of northward subduction of the central Qilian block and southward accretion of the Alxa block during the period of 450-500 Ma.  相似文献   

8.
Dun Mountain, Red Hills, and Red Mountain, are three of thelargest ultramafic bodies associated with Permian rocks in theSouth Island of New Zealand. In the three intrusions a centralcore of relatively unserpentinized dunite, harzburgite, andpyroxene peridotite, is surrounded by a margin of serpentinite.At the western margin of the Red Hills intrusion, the Permianvolcanics show high-grade thermal metamorphism with developmentof pyroxene hornfels at the contact. Layering between dunite and harzburgite is well developed atRed Hills, and on a larger scale at Dun Mountain. Layers atRed Hills are ? inch to 20 inch thick and can be traced overan entire outcrop. Harzburgites show poikilitic texture andeuhedral crystals. Dimensional orientation of platy olivines,which lie on the broad (010) face in the plane of layering,produces a strong fissility in some of the rocks. Petrofabricanalysis shows a strong X maximum normal to the plane of layeringand a Z maximum in the plane of layering. Size analysis of crystalsdemonstrates a size grading in the enstatites from large atthe bottom of a layer to small at the top. Olivines are alsosize-graded, but increased secondary enlargement of crystalsat the top of a layer has complicated this. Olivine (Fo93?8—89?4) is the main mineral in all the rocks.Enstatite (En93—88?5) contains numerous exsolution lamellaeof clinopyroxene on (100). Chrome diopside is nearly alwayspresent in small amounts and picotite is a constant accessory.Comparison of the textures and mineral chemistry of the ultramaficrocks with those of the ultramafic parts of layered intrusions,suggests that the New Zealand rocks have been derived by gravitationaldifferentiation from a tholeiitic magma. It is proposed thatthe intrusions represent the sub-volcanic differentiates ofa chain of Permian volcanoes on the margin of the New Zealandgeosyncline, and that the associated olivine-poor tholeiiticPermian basalts are the extrusive differentiates.  相似文献   

9.
The 92.5 Ma Fort Knox granodiorite stock, near the western end of the Fairbanks Belt in the Yukon–Tanana terrane (YTT) of central Alaska, hosts a world-class gold mine. The stock has been analysed paleomagnetically using thermal and alternating-field step demagnetization and isothermal remanence methods. This pluton retains a primary thermoremanent magnetization at 18 sites (232 specimens) that resides mainly in single-to pseudosingle-domain magnetite with a direction of D = 228.8°, I = 84.3° (N = 18, k = 130, α95 = 3.0°), giving a paleopole at 56.5°N, 197.1°E (dp = 5.9°, dm = 5.8°). The pluton's host rock, the Fairbanks schist, does not retain a stable coherent remanence. Relative to the North American craton, the stock's paleoinclination indicates that the Fairbanks Belt has undergone nonsignificant poleward (northwesterly) translation of 25 ± 750 km only. Analysed in concert with the few available paleoinclinations available for the YTT in Yukon, the paleoinclination suggests further that the YTT has undergone only  250 to 450 km of dextral displacement along the Tintina fault in the past  100 Ma and, therefore, is parautocthonous since the mid-Cretaceous. The stock's paleodeclination records 121 ± 35° of counterclockwise rotation relative to the North American craton. Consideration of models published for Alaska's tectonic evolution suggests that this paleodeclination discordance is caused by rotations associated with the opening of the Canada Basin, with dextral displacement on the Tintina fault, and with development of the western Alaskan orocline. Thus the paleomagnetic results for the Fort Knox stock support a thin-skin tectonic model for the accretion of the YTT and Intermontane Belt terranes to the northern Cordillera.  相似文献   

10.
Abstract The front of the Zoulang Nanshan Caledonian volcanic island arc zone in the northern Qilian Mountains is a forearc accretionary terrane, composed of multiple accretionary volcanic island arcs, flysch accretionary wedges, high - pressure metamorphosed detachment zones and remnants of ophiolites. It resulted from the northeastward subduction of the Early Palaeozoic Qilan oceanic crust beneath the Alxa block. High - pressure metamorphism, which occurred during the subduction, progressed through three stages: the initial stage of medium T - high P, the main stage of temperature decrease and pressure increase, and the lag stage of pressure decrease and temperature increase. Finally the paper presents a retrotrench subduction dynamic model indicative of northward subduction of the central Qilian block and southward accretion of the Alxa block during the period of 450-500 M a.  相似文献   

11.
High‐P/low‐T metamorphic rocks of the Hammondvale metamorphic suite (HMS) are exposed in an area of 10 km2 on the NW margin of the Caledonian (Avalon) terrane in southern New Brunswick, Canada. The HMS is in faulted contact on the SE with c. 560–550 Ma volcanic and sedimentary rocks and co‐magmatic plutonic units of the Caledonian terrane. The HMS consists of albite‐ and garnet‐porphyroblastic mica schist, with minor marble, calc‐silicate rocks and quartzite. Pressure and temperature estimates from metamorphic assemblages in the mica schist and calc‐silicate rocks using TWQ indicate that peak pressure conditions were 12.4 kbar at 430 °C. Peak temperature conditions were 580 °C at 9.0 kbar. 40Ar/39Ar muscovite ages from three samples range up to 618–615 Ma, a minimum age for high‐P/low‐T metamorphism in this unit. These ages indicate that the HMS is related to the c. 625–600 Ma subduction‐generated volcanic and plutonic units exposed to the SE in the Caledonian terrane. The ages are also similar to those obtained from detrital muscovite in a Neoproterozoic‐Cambrian sedimentary sequence in the Caledonian terrane, suggesting that the HMS was exposed by latest Neoproterozoic time and supplied detritus to the sedimentary units. The HMS is interpreted to represent a fragment of an accretionary complex, similar to the Sanbagawa Belt in Japan. It confirms the presence of a major cryptic suture between the Avalon terrane sensu stricto and the now‐adjacent Brookville terrane.  相似文献   

12.
New tectonic uplifts south of the Salt Range Thrust and Himalayan Front Thrust (HFT) represent an outward step of the plate boundary from the principal tectonic displacement zone into the Indo-Gangetic Plain. In Pakistan, the Lilla Anticline deforms fine-grained overbank deposits of the Jhelum River floodplain 15 km south of the Salt Range. The anticline is overpressured in Eocambrian non-marine strata. In northwest India south of Dehra Dun, the Piedmont Fault (PF) lies 15 km south of the HFT. Coalescing fans derived from the Himalaya form a piedmont (Old Piedmont Zone) 15–20 km wide east of the Yamuna River. This zone is uplifted as much as 15–20 m near the PF, and bedding is tilted 5–7° northeast. Holocene thermoluminescence-optically-stimulated luminescence dates for sediments in the Old Piedmont Zone suggest that the uplift rate might be as high as several mm/a. The Old Piedmont Zone is traced northwest 200 km and southeast another 200 km to the Nepal border. These structures, analogous to protothrusts in subduction zones, indicate that the Himalayan plate boundary is not a single structure but a series of structures across strike, including reactivated parts of the Main Boundary Thrust north of the range front, the HFT sensu stricto, and stepout structures on the Indo-Gangetic Plain. Displacement rates on all these structures must be added to determine the local India-Himalaya convergence rate.  相似文献   

13.
C.J. Adams  R.J. Beck  H.J. Campbell   《Lithos》2007,97(3-4):307-322
Nephrite jade occurs in three terranes (Dun Mountain–Maitai, Caples and Torlesse) in New Zealand, where it is associated with ultramafic and ophiolitic rocks in narrow metasomatic reaction zones at the margins of serpentinite (having harzburgite/gabbro/dolerite precursors) with silicic metasediments and metavolcanics. True nephrite fabrics are developed only locally where marginal shearing is intense, and late in the metamorphic history. 87Sr/86Sr values of these nephrites do not display the primitive values of their gabbro/dolerite precursor component i.e. 0.7030–0.7035, as expected if formed during serpentinisation. Rather, the nephrites have more evolved 87Sr/86Sr values inherited from the metasediment component at a later stage, and which fall within particular terrane groups: Dun Mountain–Maitai 0.7045–0.7060, Caples 0.7058–0.7075 and Torlesse 0.7085–0.7110. Rb–Sr ages and initial 87Sr/86Sr ratios of the metasediment component from in situ nephrite localities, when compared with their counterparts throughout the host terrane, show that nephrite Sr isotopic compositions are characteristic of the host terrane.  相似文献   

14.
D. Arcay  M.-P. Doin  E. Tric  R. Bousquet   《Tectonophysics》2007,441(1-4):27-45
At continental subduction initiation, the continental crust buoyancy may induce, first, a convergence slowdown, and second, a compressive stress increase that could lead to the forearc lithosphere rupture. Both processes could influence the slab surface P–T conditions, favoring on one side crust partial melting or on the opposite the formation of ultra-high pressure/low temperature (UHP-LT) mineral. We quantify these two effects by performing numerical simulations of subduction. Water transfers are computed as a function of slab dehydration/overlying mantle hydration reactions, and a strength decrease is imposed for hydrated mantle rocks. The model starts with an old oceanic plate ( 100 Ma) subducting for 145.5 Myr with a 5 cm/yr convergence rate. The arc lithosphere is thermally thinned between 100 km and 310 km away from the trench, due to small-scale convection occuring in the water-saturated mantle wedge. We test the influence of convergence slowdown by carrying on subduction with a decreased convergence rate (≤ 2 cm/yr). Surprisingly, the subduction slowdown yields not only a strong slab warming at great depth (> 80 km), but also a significant cooling of the forearc lithosphere at shallower depth. The convergence slowdown increases the subducted crust temperature at 90 km depth to 705 ± 62 °C, depending on the convergence rate reduction, and might thus favor the oceanic crust partial melting in presence of water. For subduction velocities ≤ 1 cm/yr, slab breakoff is triggered 20–32 Myr after slowdown onset, due to a drastic slab thermal weakening in the vicinity of the interplate plane base. At last, the rupture of the weakened forearc is simulated by imposing in the thinnest part of the overlying lithosphere a dipping weakness plane. For convergence with rates ≥ 1 cm/yr, the thinned forearc first shortens, then starts subducting along the slab surface. The forearc lithosphere subduction stops the slab surface warming by hot asthenosphere corner flow, and decreases in a first stage the slab surface temperature to 630 ± 20 °C at 80 km depth, in agreement with P–T range inferred from natural records of UHP-LT metamorphism. The subducted crust temperature is further reduced to 405 ± 10 °C for the crust directly buried below the subducting forearc. Such a cold thermal state at great depth has never been sampled in collision zones, suggesting that forearc subduction might not be always required to explain UHP-LT metamorphsim.  相似文献   

15.
吉林-黑龙江东部地区的中生代增生杂岩,主要由吉林-黑龙江高压变质带和那丹哈达增生杂岩(或那丹哈达地体)组成。它们将为古亚洲洋与环太平洋构造域的转换作用,大洋板块地层(OPS)层序重建,特别是古太平洋板块向欧亚大陆的俯冲历史提供重要的科学依据。吉林-黑龙江高压带分布在佳木斯-兴凯与松辽地块之间的具有高压变质带性质的缝合带,新的地质年代学研究表明其形成时代为210～180Ma,表明晚三叠-早侏罗世为南北向古亚洲洋关闭和西向俯冲增生开始的关键时期。那丹哈达增生杂岩则发育在佳木斯-兴凯地块东侧,并具体分为西部的跃进山杂岩和东部的饶河杂岩。新近发表的数据显示,跃进山杂岩就位时代为210～180Ma,这与佳木斯-兴凯地块西缘的吉黑高压带形成时代相似。而饶河杂岩就位时代为晚侏罗-早白垩世,最晚期就位的时代为早白垩世(137～130Ma)。因此,吉黑东部地区的中生代增生杂岩为古太平洋向欧亚大陆中生代的俯冲过程提供了关键的信息。  相似文献   

16.
The 1000-km-long Darlag–Lanzhou–Jingbian seismic refraction profile is located in the NE margin of the Tibetan plateau. This profile crosses the northern Songpan-Ganzi terrane, the Qinling-Qilian fold system, the Haiyuan arcuate tectonic region, and the stable Ordos basin. The P-wave and S-wave velocity structure and Poisson's ratios reveal many significant characteristics in the profile. The crustal thickness increases from northeast to southwest. The average crustal thickness observed increases from 42 km in the Ordos basin to 63 km in the Songpan-Ganzi terrane. The crust becomes obviously thicker south of the Haiyuan fault and beneath the West-Qinlin Shan. The crustal velocities have significant variations along the profile. The average P-wave velocities for the crystalline crust vary between 6.3 and 6.4 km/s. Beneath the Songpan-Ganzi terrane, West-Qinling Shan, and Haiyuan arcuate tectonic region P-wave velocities of 6.3 km/s are 0.15 km/s lower than the worldwide average of 6.45 km/s. North of the Kunlun fault, with exclusion of the Haiyuan arcuate tectonic region, the average P-wave velocity is 6.4 km/s and only 0.5 km/s lower than the worldwide average. A combination of the P-wave velocity and Poisson's ratio suggests that the crust is dominantly felsic in composition with an intermediate composition at the base. A mafic lower crust is absent in the NE margin of the Tibetan plateau from the Songpan-Ganzi terrane to the Ordos basin. There are low velocity zones in the West-Qinling Shan and the Haiyuan arcuate tectonic region. The low velocity zones have low S-wave velocities and high Poisson's ratios, so it is possible these zones are due to partial melting. The crust is divided into two layers, the upper and the lower crust, with crustal thickening mainly in the lower crust as the NE Tibetan plateau is approached. The results in the study show that the thickness of the lower crust increases from 22 to 38 km as the crustal thickness increases from 42 km in the Ordos basin to 63 km in the Songpan-Ganzi terrane south of the Kunlun fault. Both the Conrad discontinuity and Moho in the West-Qinling Shan and in the Haiyuan arcuate tectonic region are laminated interfaces, implying intense tectonic activity. The arcuate faults and large earthquakes in the Haiyuan arcuate tectonic region are the result of interaction between the Tibetan plateau and the Sino–Korean and Gobi Ala Shan platforms.  相似文献   

17.
在东南极大陆内部及边缘发育3条晚新元古代—早古生代造山带,即东非造山带（南延部分）、普里兹造山带和罗斯造山带。东非造山带的南延部分主要出露于吕措—霍尔姆湾—毛德王后地—沙克尔顿岭地区,其内发育蛇绿岩、榴辉岩相超镁铁岩及逆冲—推覆构造,因而被解释为东、西冈瓦纳陆块拼合的缝合线。罗斯造山带主要出露于横贯南极山脉地区,其内保存有大陆裂解、洋壳俯冲和地体增生的地质纪录,代表冈瓦纳超大陆的活动大陆边缘。普里兹造山带主要出露于普里兹湾和登曼冰川,因其位于从前假设的统一东冈瓦纳陆块的内部,加之缺少蛇绿混杂岩、岛弧增生杂岩和高压变质岩（如蓝片岩或榴辉岩）等与大洋板块俯冲作用密切相关的岩石,所以当前存在着碰撞造山成因和板内改造成因两种不同的认识。普里兹造山带构造性质的确定不仅决定了冈瓦纳超大陆的汇聚过程和方式,也制约了罗迪尼亚超大陆的形成和演化过程。因此,开展普里兹造山带的研究对于揭示新元古代—早古生代的全球构造演化具有重要的科学意义。  相似文献   

18.
Seismic reflection profiles from three different surveys of the Cascadia forearc are interpreted using P wave velocities and relocated hypocentres, which were both derived from the first arrival travel time inversion of wide-angle seismic data and local earthquakes. The subduction decollement, which is characterized beneath the continental shelf by a reflection of 0.5 s duration, can be traced landward into a large duplex structure in the lower forearc crust near southern Vancouver Island. Beneath Vancouver Island, the roof thrust of the duplex is revealed by a 5–12 km thick zone, identified previously as the E reflectors, and the floor thrust is defined by a short duration reflection from a < 2-km-thick interface at the top of the subducting plate. We show that another zone of reflectors exists east of Vancouver Island that is approximately 8 km thick, and identified as the D reflectors. These overlie the E reflectors; together the two zones define the landward part of the duplex. The combined zones reach depths as great as 50 km. The duplex structure extends for more than 120 km perpendicular to the margin, has an along-strike extent of 80 km, and at depths between 30 km and 50 km the duplex structure correlates with a region of anomalously deep seismicity, where velocities are less than 7000 m s− 1. We suggest that these relatively low velocities indicate the presence of either crustal rocks from the oceanic plate that have been underplated to the continent or crustal rocks from the forearc that have been transported downward by subduction erosion. The absence of seismicity from within the E reflectors implies that they are significantly weaker than the overlying crust, and the reflectors may be a zone of active ductile shear. In contrast, seismicity in parts of the D reflectors can be interpreted to mean that ductile shearing no longer occurs in the landward part of the duplex. Merging of the D and E reflectors at 42–46 km depth creates reflectivity in the uppermost mantle with a vertical thickness of at least 15 km. We suggest that pervasive reflectivity in the upper mantle elsewhere beneath Puget Sound and the Strait of Georgia arises from similar shear zones.  相似文献   

19.
We analyzed the short period Rayleigh waves from the first crustal-scale seismic refraction experiment in the Korean peninsula, KCRUST2002, to determine the shear wave velocity and attenuation structure of the uppermost 1 km of the crust in different tectonic zones of the Korean peninsula and to examine if this can be related to the surface geology of the study area. The experiment was conducted with two large explosive sources along a 300-km long profile in 2002. The seismic traces, recorded on 170 vertical-component, 2-Hz portable seismometers, show distinct Rayleigh waves in the period range between 0.2 s and 1.2 s, which are easily recognizable up to 30–60 km from the sources. The seismic profiles, which traverse three tectonic regions (Gyeonggi massif, Okcheon fold belt and Yeongnam massif), were divided into five subsections based on tectonic boundaries as well as lithology. Group and phase velocities for the five subsections obtained by a continuous wavelet transform method and a slant stack method, respectively, were inverted for the shear wave models. We obtained shear wave velocity models up to a depth of 1.0 km. Overall, the shear wave velocity of the Okcheon fold belt is lower than that of the Gyeonggi and Yeongnam massifs by  0.4 km/s in the shallowmost 0.2 km and by 0.2 km/s at depths below 0.2 km. Attenuation coefficients, determined from the decay of the fundamental mode Rayleigh waves, were used to obtain the shear wave attenuation structures for three subsections (one for each of the three different tectonic regions). We obtained an average value of Qβ− 1 in the upper 0.5 km for each subsection. Qβ− 1 for the Okcheon fold belt ( 0.026) is approximately three times larger than Qβ− 1 for the massif areas ( 0.008). The low shear wave velocity in the Okcheon fold belt is consistent with the high attenuation in this region.  相似文献   

20.
The Guleman ophiolite,one of the most important ophiolitic massifs of the Southeast Anatolian Ophiolitic Belt,consists of a core of serpentinized mantle rocks overlain by an ultramafic sequence,layered and isotropic gabbro,and sheeted dykes.The ophiolite structurally overlies the Lower Miocene Lice Formation and is overlain by young sandstones and shales of the Upper Maashtrichtian-Lower Eocene Hazar Complex and Middle Eocene Maden Complex.The Guleman ophiolite tectonically overlain by Precambrian to Upper Triassic Bitlis metamorphic massif.The mantle peridotites compose mainly of fresh and in place serpentinized harzburgite tectonite with local bands and lenses of dunites with large-sized chromitite pods.The Guleman peridotites commonly show porphyroclastic texture,high-temperature fabrics such as kink-bands in olivines.According to microprobe analyses,the harzburgite and dunite have low Ca O and Al2O3 abundance similar to Mariana forearc,and their average Cr-(=Cr/(Cr+Al)atomic)ratio of Cr-spinelsis surprisingly high(0.63)besides Fo content of olivine is between 90.9 to 92.3 in peridotites.According to Mg#(Mg/(Mg+Fe2+))versus Cr#in spinel diagram,the degree of partial melting is higher than 35%and spinel values plot in the forearc peridotites field.The Gulemanharzburgites have low Ca O,Al2O3 and Ti O2 contents in orthopyroxene and clinopyroxene lammelles,resembling those of depleted harzburgites from modern forearcs and different from moderately depleted abyssal peridotites.Consequently,we propose that the Guleman peridotites form in a forearc setting during the subduction initiation that developed as a result of northward subduction of the southern branch of the Neo-Tethys in response to the convergence between Arabian and Anatolian plates.  相似文献   

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