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1.
Isotopic characteristics of river sediments on the Tibetan Plateau   总被引：1，自引：0，他引：1  
We systematically collected 40 modern clastic sediment samples from rivers in different tectonic units of the Tibetan Plateau and measured their Sr–Nd isotopic compositions. The isotopic characteristics provide insight into the controversial paleo-tectonic affinity of terranes of the Tibetan Plateau and the provenance of Songpan–Ganzi flysch complex. The Qilian Terrane and Himalaya Terrane have more negative εNd(0) values (from ? 14.3 to ? 11.8 and from ? 20.64 to ? 13.26, respectively) and high 87Sr/86Sr values (from 0.719674 to 0.738818 and from 0.721020 to 0.824959, respectively), reflecting old and mature continental crust origin of these two terranes. The southern Lhasa Terrane is more radiogenic in εNd(0) values (from ? 8.82 to ? 3.8) and low in 87Sr/86Sr values (from 0.711504 to 0.719489), implying the combined impact of the Neo-Tethys mantle and Himalaya old continental crust. Sr–Nd isotopic compositions of the Qilian Terrane are similar to those in the Yangtze Craton, indicating that the Qilian Terrane was probably separated from the Yangtze Craton. Sr–Nd isotopic characteristics of the Songpan–Ganzi Terrane are similar to the Yangtze Craton and are remarkably different to those in the North China Craton, eastern Kunlun–Qaidam and the central Qiangtang metamorphic belt, implying that the widely distributed flysch complex of the Songpan–Ganzi Terrane was sourced from the Yangtze Craton.  相似文献   

2.
Following the 2008 Mw 7.9 Wenchuan and 2013 Ms 7.0 Lushan earthquakes, the Longmenshan thrust-fault belt and the Songpan–Ganzi terrane have been the focus of several investigations. Here we use the H–k stacking technique and neighborhood algorithm to investigate the seismic structure of this area. Based on the presence of felsic lower crust and the Mesozoic crustal architecture of the Songpan–Ganzi and Longmenshan area, we exclude the model on the eastward flow of the middle and lower crust assigned as the cause for the crustal thickening in previous studies. In contrast, the E–W trending cumulative compression induced by the continued northward motion of the Indian plate and India-Asian collision are identified as the dominant factors leading to the crustal thickening in the Longmenshan thrust-fault region as well as the Songpan–Ganzi terrane. Particularly, the 2008 Mw 7.9 Wenchuan and 2013 Ms 7.0 Lushan earthquakes likely indicate the cumulative offsets and the E–W continuing compression.  相似文献   

3.
In order to constrain the detrital provenance of the siliciclastic rocks, palaeogeographic variations, and crustal growth history of central China, we carried out simultaneously in situ U–Pb dating and trace element and Hf isotope analyses on 368 detrital zircons obtained from upper Permian–Triassic sandstones of the Songpan terrane, eastern Tibetan Plateau. Two groups of detrital zircons, i.e. magmatic and metamorphic in origin, have been identified based on cathodoluminescence images, zircon Ti-temperatures, and Th/U ratios. Our data suggest that the derivation of siliciclastic rocks in the Songpan terrane was mainly from the Qinling, Qilian, and Kunlun orogens, whereas the Yangtze and North China Cratons served as minor source areas during late Permian–Triassic times. The detrital zircons from Middle–Late Triassic siliciclastic rocks exhibit wide age spectra with two dominant populations of 230–600 Ma and >1600 Ma, peaking at ~1.8–1.9 Ga and ~2.4–2.5 Ga, suggestive of a derivation from the Qinling, Qilian, and Kunlun orogens and the Yangtze Craton being the minor source area. The proportions of detrital zircon populations from the northern Qinling, Qilian, and Kunlun orogens distinctly decreased during Middle–Late Triassic time, demonstrating that the initial uplift of the western Qinling occurred then and it could have blocked most of the detritus from the Qilian–northern Qinling orogens and North China Cratons into the main Songpan–Ganzi depositional basin. The relatively detrital zircon proportions of the Yangtze Craton source decreased during Early-Middle Late Triassic time, indicating that the Longmenshan orogen was probably being elevated, since the early Late Triassic and gradually formed a barrier between the Yangtze Craton and the Songpan terrane. In addition, our Lu–Hf isotopic results also reveal that the Phanerozoic magmatic rocks in central China had been the primary products of crustal reworking with insignificant formation of a juvenile crust.  相似文献   

4.
Since the pioneer wide-angle seismic profile along the Yadong–Gulu rift acquired in 1974 by the ex-Institute of Geophysics, Chinese Academy of Sciences (CAS), several research programs aimed to deep geophysics, performed thanks to the participation of Chinese national and international institutions, have been developed during last 35 years, including 23 wide-angle seismic profiles with total length of about 6000 km. These profiles are unevenly distributed, most of them in eastern Tibet and few profiles in western Tibet. In this paper, we make a summarized presentation of all these wide-angle seismic profiles and provide an overall view of the seismic velocity structure of the crust beneath the broad Tibetan plateau, which is the product of the continuous convergence and collision of the Indian and Eurasian plates since about 50 Ma ago. Different patterns of crustal thickness variation related to the tectonic blocks and along suture zones of the region are displayed. The crust thickness is confirmed to be about 70–75 km under southern Tibet, and 60–65 km under northern, northeastern and southeastern Tibet. The leading edge of the subducted lithosphere reaches the northern margin of the plateau and directly contacts with Tarim Basin. Westward of the 90°E boundary, the Indian crust is moving towards the northern edge of the plateau and collides with Tarim Basin at 80°E while reach the Bangong–Nujiang suture belt at 88°E; eastward of the 90°E boundary, the northern edge of the crust should be at 50–100 km south of Bangong–Nujiang suture. The results supply helpful constrains to understand the mechanism of the continent–continent collision and its consequences in the plateau and neighbouring areas.  相似文献   

5.
The Chaochou Fault, a major geological boundary in southern Taiwan is considered to be a part of the convergent plate boundary between the Eurasia Plate and the Philippine Sea Plate. We applied the Common Conversion Point stacking technique to teleseismic radial receiver functions and obtained Moho variation and crustal structure across the Chaochou Fault. In the Eurasia Plate to its west, the Moho depth is about 37 km and the crust is subducting to the east beneath the Philippine Sea Plate with a dip angle of about 30° between the Backbone Belt and the Tananao Schist. In the Philippine Sea Plate, the Moho depth is about 17 km. The Longitudinal Valley marks the collision boundary between the Eurasia Plate and the Philippine Sea Plate. The results suggest that the depth extent of the Chaochou Fault is about 30–35 km and the fault becomes a “shallow-angle” thrust fault at depth. The Common Conversion Point image also shows several bending interfaces of velocity contrast in the crust. We proposed a simple model to explain the Philippine Sea Plate and Eurasia Plate collision process and the observed crustal deformations.  相似文献   

6.
青藏高原东缘地壳上地幔结构及其动力学意义   总被引：3，自引：0，他引：3       下载免费PDF全文
张忠杰  陈赘  田小波 《地质科学》2009,44(4):1136-1150
本文综述了我们在青藏高原东缘实施的垂直切过龙门山断裂带宽频带地震探测的研究成果,揭示了研究区复杂的地壳上地幔结构,结果表明松潘-甘孜地块与四川盆地西缘莫霍面深度为58 km与40 km±,在龙门山断裂带下方存在约15 km的莫霍面错断; 松潘-甘孜与龙门山断裂带域地壳纵横波速度比Vp/Vs比值远大于173,预示着粘性下地壳流或基性/超基性物质的存在。探讨了研究区强烈的盆山之间以及深部不同层圈之间的相互作用,推断四川盆地对青藏高原东缘软流圈驱动的物质东向逃逸阻挡作用可能深达整个上地幔。  相似文献   

7.
To study the crustal structure beneath the onshore–offshore transitional zone, a wide-angle onshore–offshore seismic experiment was carried out in northern South China Sea near Hong Kong, using large volume airgun sources at sea and seismic stations on land. The crustal velocity model constructed from traveltime fitting shows that the sedimentary thickness abruptly increases seaward of the Dangan Islands based on the characteristics of Pg and Multiple Pg, and the crustal structure beneath the sedimentary layer is relatively simple. The Moho depth is about 25–28 km along the profile and the P-wave velocity increases gradually with depth. The velocities in the upper crust range from 5.5 to 6.4 km/s, while that in the lower crust is 6.4–6.9 km/s. It also reveals a low velocity zone with a width of more than 10 km crossing the crust at about 75–90 km distance, which suggests that the Littoral Fault Zone (LFZ) exists beneath the onshore–offshore transitional zone. The magnetism anomalies, bouguer gravity anomalies and active seismic zone along the coastline imply the LFZ is a main tectonic fault in the onshore–offshore area. Combined with two previously published profiles in the continental South China (L–G profile) and in the northern margin of South China Sea (OBS1993) respectively, we constructed a land-sea super cross-section about 1000 km long. The results show the onshore–offshore transitional zone is a border separating the unstretched and the stretched continental crust. The low velocity layer (LVL) in the middle crust was imaged along L–G profile. However, the high velocity layer (HVL) in the lower crust was detected along OBS1993. By analyzing the mechanisms of the LVL in the middle crust and HVL in the base of crust, we believe the crustal structures had distinctly different attributes in the continental South China and in the northern SCS, which indicates that the LFZ could be the boundary fault between them.  相似文献   

8.
U–Pb analysis of zircons from igneous rocks in the Elashan Mountain, easternmost segment of the East Kunlun Orogen yielded 252–232 Ma. Geochemically, these rocks are mainly high in SiO_2, K_2O and K_2O+Na_2O contents, low in P_2O_5 and TiO_2 contents, depleted in Ba, Sr, P, Ti and enriched in U, Hf, Zr, showing features of I–type granite. The zircon εHf(t) values of the Early Triassic Jiamuge'er rhyolite porphyry(252±3 Ma) are positive(+1.6 to +12.1), suggesting a juvenile crustal source mixing with little old crustal component, and the zircon εHf(t) values of the Middle Triassic Manzhang'gang granodiorite(244±3 Ma) and Dehailong diorite(237±3 Ma) are predominately negative(-8.4 to +1.0), indicating an older crustal source. In comparison, the zircon εHf(t) values of the Late Triassic syenogranites from Suigen'ergang(234±2Ma), Ge'ermugang(233±2 Ma) and Yue'ergen(232±3 Ma) plutons vary from-3.8 to +5.0, suggesting a crust-mantle mixing source. From Early–Middle Triassic(252–237 Ma) to Late Triassic(234–232 Ma), the geochemical characteristics of these rocks show the change from a subduction–collision setting to a post-collision or within-plate setting. By comparing of these new age data with 77 zircon U–Pb ages of igneous rocks of the eastern part of East Kunlun orogen from published literatures, we conclude that the igneous rocks of Elashan Mountain and these of the eastern part of East Kunlun Orogen belong to one magmatic belt. All these data indicate that the Triassic magmatic events of the eastern part of East Kunlun Orogen can be divided into three stages: 252–238 Ma, 238–226 Ma and 226–212 Ma. Statistically, the average εHf(t) values of the threestage igneous rocks show a tendency, from the old to young, from-0.75±0.25 to lower-2.65±0.52 and then to-1.22±0.25, respectively, which reveal the change of their sources. These characteristics can be explained as a crust-mantle mixing source generated in a subductional stage, mainly crust source in a syn–collisional stage and a crust-mantle mixing source(lower crust with mantle-derived underplating magma) in a post-collisional stage. The identification of these three magmatic events in the Elashan Mountain, including all the eastern part of East Kunlun Orogen, provides new evidence for better understanding of the tectonic evolution of the northward subduction and closure of the Paleo-Tethyan(252–238 Ma), the collision of the Songpan–Ganzi block with the southern margin of Qaidam block(238–226 Ma), and the post–collisional setting(226–212 Ma) during the Early Mesozoic period.  相似文献   

9.
J.D.A. Piper 《Tectonophysics》2009,463(1-4):185-207
The ~ 1100 Ma Sveconorwegian orogenic belt comprises allochthonous terranes juxtaposed by major fault zones and emplaced against, and onto, the south-western margin of the Fennoscandian Shield. To resolve the magnetic signature acquired during post-orogenic uplift and cooling and evaluate wider correlations with the contemporaneous Grenville belt of North America, this study reports a regional palaeomagnetic study on a range of rock types from sectors of the medium-high metamorphic grade Bamble terrane (48 sites and 390 cores) and the adjoining medium-low grade Telemark terrane (33 and 240 cores) juxtaposed by an orogen-parallel (Porsgrunn- Kristiansand) fault zone with major vertical displacement. Magnetite and ilmeno-hematites are magnetic carriers with the latter more significant in the higher metamorphic grades. Magnetic intensities are stronger in the higher-grade terrane presumably due to the growth of metamorphic ferromagnets, but are an order lower than values predicted for the lower continental crust and indicate that an additional mechanism is responsible for high magnetisations in deep crust. Anisotropy of magnetic susceptibility (AMS) largely reflects the NE–SW tectonic grain of the last stage of Sveconorwegian ductile deformation. The magnetisation record is filtered by excluding magnetisations possibly acquired during regional Mesozoic dyke emplacement, development of the Permo-Carboniferous Oslo Rift and Late Proterozoic magmatism. The remaining record is a dual polarity signature summarised by mean poles at 31.9°N, 50.9°E, (N = 191 components) in the Bamble terrane and at 34.2°N, 58.9°E (N = 151 components) in the Telemark terrane. However these means are non-Fisherian and embrace arcuate distributions of magnetic components acquired during protracted exhumation cooling of the orogen with the best-defined parts comprising clockwise trajectories correlating with each another but indicating that cooling in Telemark was more protracted; in each case directions of more shallow NW-direction tend to be derived from lower unblocking temperature components. The geochronological evidence indicates that regional temperatures had fallen to permit acquisition of magnetisation by ~ 950–900 Ma and the two swathes define the younger limb of a clockwise (Grenville-Sveconorwegian) APW loop embracing the approximate interval 940–850 Ma; the outward path of this loop (~ 1020–940 Ma) is probably at present recorded only in dyke swarms from the Finnish sector of the shield. Correlation of APW between Laurentia and Fennoscandia confirms that the two shields broke apart shortly after culmination of the Sveconorwegian orogeny when Fennoscandia rotated rapidly clockwise into a secondary configuration adjacent to the eastern margin of Laurentia; the Grenville and Sveconorwegian orogenic frontal zones formed in alignment were reoriented at a high angle to one another in a coupling that appears to have persisted during most of the remainder of Neoproterozoic times.  相似文献   

10.
《Gondwana Research》2013,23(3-4):1060-1067
Convergence between the Indian plate and the Eurasian plate has resulted in the uplift of the Tibetan Plateau, and understanding the associated dynamical processes requires investigation of the structures of the crust and the lithosphere of the Tibetan Plateau. Yunnan is located in the southwest edge of the plateau and adjacent to Myanmar to the west. Previous observations have confirmed that there is a sharp transition in mantle anisotropy in this area, as well as clockwise rotations of the surface velocity, surface strain, and fault orientation. We use S receiver functions from 54 permanent broad-band stations to investigate the structures of the crust and the lithosphere beneath Yunnan. The depth of the Moho is found to range from 36 to 40 km beneath southern Yunnan and from 55 to 60 km beneath northwestern Yunnan, with a dramatic variation across latitude 25–26°N. The depth of the lithosphere–asthenosphere boundary (LAB) ranges from 180 km to less than 70 km, also varying abruptly across latitude 25–26°N, which is consistent with the sudden change of the fast S-wave direction (from NW–SE to E–W across 26–28°N). In the north of the transition belt, the lithosphere is driven by asthenospheric flow from Tibet, and the crust and the upper mantle are mechanically coupled and moving southward. Because the northeastward movement of the crust in the Burma micro-plate is absorbed by the right-lateral Sagaing Fault, the crust in Yunnan keeps the original southward movement. However, in the south of the transition belt, the northeastward mantle flow from Myanmar and the southward mantle flow from Tibet interact and evolve into an eastward flow (by momentum conservation) as shown by the structure of the LAB. This resulting mantle flow has a direction different from that of the crustal movement. It is concluded that the Sagaing Fault causes the west boundary condition of the crust to be different from that of the lithospheric mantle, thus leading to crust–mantle decoupling in Yunnan.  相似文献   

11.
Mafic granulite and spinel lherzolite xenoliths from Cenozoic alkaline basalts near Al-Ashkhara, eastern Oman, have been selected for a systematic mineralogical, geochemical and Sr–Nd–Pb isotopic study. This is the only place in E Arabia where samples of both lower crust and upper mantle can be examined. Lower crustal xenoliths consist of two mineralogically and chemically distinct groups: gabbronorite (subequal abundances of ortho- and clino-pyroxene and plagioclase) and plagioclase pyroxenite (dominant pyroxene and subordinate plagioclase). Temperature estimates for lower crustal xenoliths using the two pyroxene geothermometer (T-Wells) yield 810–865 °C. The mineral assemblage (spinel–pyroxene–plagioclase) and Al content in pyroxene indicate that plagioclase-bearing xenoliths equilibrated at 5–8 kbar (13 and 30 km depth) in the lower crust. εNd and 87Sr/86Sr calculated at 700 Ma for Al-Ashkhara lower crustal xenoliths (+ 6.4 to + 6.6; 87Sr/86Sr = 0.7028 to 0.7039) are consistent with the interpretation that juvenile, mafic melts were added to the lower crust during Neoproterozoic time and that there was no discernible contribution from pre-Neoproterozoic crust. Upper mantle xenoliths consist of both dry and hydrous (phlogopite-bearing) lherzolites. These peridotites are more Fe-rich than expected for primitive mantle or melt residues and probably formed by pervasive circulation of melts that have refertilized pre-existing mantle peridotites. Mineral equilibration temperatures range from 990 to 1070 °C. Isotopic compositions calculated at 700 Ma are εNd = + 6.8 to + 7.8 and 87Sr/86Sr = 0.7016 to 0.7025, indicating depleted upper mantle. Pb isotopic compositions indicate that the metasomatism was relatively recent, perhaps related to Paleogene tectonics and basanite igneous activity. Nd model ages for the spinel peridotite xenoliths range between 0.59 and 0.65 Ga. The xenolith data suggest that eastern Arabian lower crust is of hotspot origin, in contrast to western Arabian lower crust, which mostly formed at a convergent plate margin. Geochemical and isotopic differences between lower crust and upper mantle indicate that these are unrelated, possibly because delamination replaced the E Arabian mantle root in Neoproterozoic time.  相似文献   

12.
Northward indentation of the Indian Plate has brought about significant tectonic deformation into East Asia. A record of long-term tectonic deformation in this area for the past 50 M yr, particularly the vertical axis rotation, is available through paleomagnetic data. In order to depict rotational deformation in this area with respect to Eurasia, we compiled reliable paleomagnetic data sets from 79 localities distributed around eastern Himalayan syntaxis in East Asia. This record delineates that a zone affected by clockwise rotational deformation extends from the southern tip of the Chuan Dian Fragment to as far as the northwestern part of the Indochina Peninsula. A limited zone that experienced a significant amount of clockwise rotation after an initial India–Asia collision is now located at 23.5°N, 101°E, far away from an area (27.5°N, 95.5°E) where an intense rotational motion has been viewed by a snapshot of GPS measurements. This discrepancy in clockwise rotated positions is attributed to southeastward extrusion of the tectonic blocks within East Asia as a result of ongoing indentation of the Indian Plate. A quantitative comparison between the GPS and paleomagnetically determined clockwise rotation further suggests that following an initial India–Asia collision the crust at 30°N, 94°E paleoposition was subjected to southeastward displacement together with clockwise rotation, which eventually reached to present-day position of 23.5°N, 101°E, implying a crustal displacement of about 1000 km during the past 50 M yr.  相似文献   

13.
We present a new set of contour maps of the seismic structure of South America and the surrounding ocean basins. These maps include new data, helping to constrain crustal thickness, whole-crustal average P-wave and S-wave velocity, and the seismic velocity of the uppermost mantle (Pn and Sn). We find that: (1) The weighted average thickness of the crust under South America is 38.17 km (standard deviation, s.d. ±8.7 km), which is ∼1 km thinner than the global average of 39.2 km (s.d. ±8.5 km) for continental crust. (2) Histograms of whole-crustal P-wave velocities for the South American crust are bi-modal, with the lower peak occurring for crust that appears to be missing a high-velocity (6.9–7.3 km/s) lower crustal layer. (3) The average P-wave velocity of the crystalline crust (Pcc) is 6.47 km/s (s.d. ±0.25 km/s). This is essentially identical to the global average of 6.45 km/s. (4) The average Pn velocity beneath South America is 8.00 km/s (s.d. ±0.23 km/s), slightly lower than the global average of 8.07 km/s. (5) A region across northern Chile and northeast Argentina has anomalously low P- and S-wave velocities in the crust. Geographically, this corresponds to the shallowly-subducted portion of the Nazca plate (the Pampean flat slab first described by Isacks et al., 1968), which is also a region of crustal extension. (6) The thick crust of the Brazilian craton appears to extend into Venezuela and Colombia. (7) The crust in the Amazon basin and along the western edge of the Brazilian craton may be thinned by extension. (8) The average crustal P-wave velocity under the eastern Pacific seafloor is higher than under the western Atlantic seafloor, most likely due to the thicker sediment layer on the older Atlantic seafloor.  相似文献   

14.
《Gondwana Research》2013,24(4):1455-1483
The crust and upper mantle in mainland China were relatively densely probed with wide-angle seismic profiling since 1958, and the data have provided constraints on the amalgamation and lithosphere deformation of the continent. Based on the collection and digitization of crustal P-wave velocity models along related wide-angle seismic profiles, we construct several crustal transects across major tectonic units in mainland China. In our study, we analyzed the seismic activity, and seismic energy releases during 1970 and 2010 along them. We present seismogenic layer distribution and calculate the yield stress envelopes of the lithosphere along the transects, yielding a better understanding of the lithosphere rheology strength beneath mainland China. Our results demonstrate that the crustal thicknesses of different tectonic provinces are distinctively different in mainland China. The average crustal thickness is greater than 65 km beneath the Tibetan Plateau, about 35 km beneath South China, and about 36–38 km beneath North China and Northeastern China. For the basins, the thickness is ~ 55 km beneath Qaidam, ~ 50 km beneath Tarim, ~ 40 km beneath Sichuan and ~ 35 km beneath Songliao. Our study also shows that the average seismic P-wave velocity is usually slower than the global average, equivalent with a more felsic composition of crust beneath the four tectonic blocks of mainland China resulting from the complex process of lithospheric evolution during Triassic and Cenozoic continent–continent and Mesozoic ocean–continent collisions. We identify characteristically different patterns of seismic activity distribution in different tectonic blocks, with bi-, or even tri-peak distribution of seismic concentration in South Tibet, which may suggest that crustal architecture and composition exert important control role in lithosphere deformation. The calculated yield stress envelopes of lithosphere in mainland China can be divided into three groups. The results indicate that the lithosphere rheology structure can be described by jelly sandwich model in eastern China, and crème brulee models with weak and strong lower crust corresponding to lithosphere beneath the western China and Kunlun orogenic belts, respectively. The spatial distribution of lithospheric rheology structure may provide important constraints on understanding of intra- or inter-plate deformation mechanism, and more studies are needed to further understand the tectonic process(es) accompanying different lithosphere rheology structures.  相似文献   

15.
We have measured both P- and S-wave velocities (Vp and Vs) and Poisson's ratios (υ) of 60 typical ultrahigh pressure (UHP) metamorphic rock samples from the Chinese Continental Scientific Drilling (CCSD) main and pre-pilot holes and surface outcrops in the Sulu–Dabie orogenic belt at hydrostatic confining pressures up to 850 MPa. The experimental results, together with those compiled in Handbook of Seismic Properties of Minerals, Rocks and Ores [ Ji, S.C., Wang, Q., Xia, B., 2002. Handbook of Seismic Properties of Minerals, Rocks and Ores. Polytechnic International Press, Montreal, 630 pp.], reveal that except for monomineralic rocks such as quartzite, serpentinite, anorthosite, limestone, and marble the rest of the rock types have Poisson's ratios falling along an upward convex curve determined from the correlations between elastic moduli and density. Poisson's ratios increase with density as the lithology changes from granite, felsic gneiss and schist, through diorite–syenite, intermediate gneiss and metasediment, to gabbro–diabase, amphibolite, and mafic gneiss, and then decrease as the rocks become ultramafic in composition. Eclogite has a higher density but a lower Poisson's ratio than peridotite. The results were applied to constrain the crustal composition and tectonic evolution of the Chinese continental crust based on crustal thickness (H) and Poisson's ratio (υ) from 248 broadband seismic stations, measured using teleseismic receiver function techniques. The North China, Yangtze, South China and Northeast China blocks and Songpan–Ganzi Terrane are dominated by low (υ < 0.26) and moderate (0.26 ≤ υ < 0.28) υ values (> 70%), suggesting the dominance of felsic composition in the crust. The Lhasa terrane, Qiangtang terrane, and Indochina block are characterized by high proportions (33–42%) of measurements with very high υ values (≥ 0.30 and H is found for the South China block, Northeast block, Lhasa block, Qiangtang terrane and Indochina block, indicating either tectonic thickening of the felsic upper and middle crust by folding and thrusting or the removal of mafic layers from the lower crust into the upper mantle by delamination.  相似文献   

16.
《Gondwana Research》2015,28(4):1487-1493
Receiver function imaging along a temporary seismic array (ANTILOPE-2) reveals detailed information of the underthrusting of the Indian crust in southern Tibet. The Moho dips northward from ~ 50 km to 80 km beneath the Himalaya terrane, and locally reaches ~ 85 km beneath the Indus–Yalung suture. It remains at ~ 80 km depth across the Lhasa terrane, and shallows to ~ 70 km depth under the Qiangtang terrane. An intra-crustal interface at ~ 60 km beneath the Lhasa terrane can be clearly followed southward through the Main Himalaya Thrust and connects the Main Boundary Thrust at the surface, which represents the border of the Indian crust that is underthrusting until south of the Bangong–Nujiang Suture. A mid-crustal low velocity zone is observed at depths of 14–30 km beneath the Lhasa and Himalaya terranes probably formed by partial melt and/or aqueous fluids.  相似文献   

17.
The Ganzi–Litang fault zone, an outstanding tectonic element in the eastern Tibetan Plateau has been intensively debated as an in-situ suture zone marking relict of a subducted Palaeo-Tethyan oceanic crust or a failed intracontinental rift. This paper reports the garnet amphibolites discovered along the Ganzi–Litang fault zone, eastern Tibetan Plateau. These garnet amphibolites are characterized by the garnet–hornblende–rutile–sphene–plagioclase–quartz assemblage. Conventional geothermobarometry figures out the metamorphic temperature and pressure conditions at 582–626°C and 1.61–1.82 GPa, respectively. Geochemical analysis (no Nb–Ta deletions and left-inclined to flat patterns of rare-earth elements) indicates that the garnet amphibolites could represent metamorphic product of the mid-ocean-ridge (MORB)-type mafic rocks that were contaminated by a mantle plume. The protolith of the garnet amphibolites was dated at 236 Ma using in-situ U–Pb zircon method, and the retrograde metamorphism was dated at 218 Ma using in-situ U–Pb sphene method. A comprehensive analysis combined with the development of the Palaeo-Tethys Ocean and the Yidun arc through geologic time indicates a Triassic to Early Jurassic age (236–195 Ma) for the metamorphism of the garnet amphibolites. The low geothermal gradient of 9.8ºC/km and the N-MORB nature of the garnet amphibolites suggest a subduction-zone environment for the high-pressure metamorphism. Therefore, the Ganzi–Litang fault zone is a Palaeo-Tethyan suture separating the Yidun arc and the Songpan–Ganzi terrane, representing the relics of a branch of the Palaeo-Tethys Ocean that was contaminated by a mantle plume.  相似文献   

18.
An integrated interpretation of multi-channel seismic reflection, gravity and magnetic datasets belonging to northern most part of the 85°E Ridge in the Mahanadi offshore is carried out to study the crustal structure and mode of its emplacement. The basement structure map of the ridge reveals that it is 130–150 km wide and is composed of an eastern high which appears as a continuous, broad and smooth topographyand the western high characterized by several steep isolated highs. The seismic velocities reported for the first time over the ridge indicate several sedimentary sequences ranging in velocities between 1.6 and 4.0 km/s above the acoustic basement top. The salient aspects of the sedimentary velocities are; a low velocity layer (2.6–3.2 km/s) within the Cretaceous sequence in the intervening depressions encompassing the flank region, and a regionally widespread higher velocity layer (3.5–3.8 km/s) belonging to the Eocene–Oligocene section overlying the ridge. A layer having a velocity of 4.2–4.7 km/s probably made of volcanoclastic rocks is observed immediately below the acoustic basement. The sediment isopach maps presented here for three major horizons are used to compute the 3-D sediment gravity effect to obtain a crustal Bouguer anomaly map of the region. Detailed analysis of the gravity and magnetic anomaly maps clearly demonstrates the continuity of ridge up to the Mahanadi coast at Chilka Lake. Seismically constrained gravity and magnetic models indicate that the ridge is composed of volcanic material that was emplaced on continental crust in the shelf-slope areas and over the oceanic crust in the deep offshore areas. The modeled crustal structure below the ridge further indicates volcanic emplacement of the ridge on a relatively younger lithosphere. We propose two alternative models for the emplacement of the ridge.  相似文献   

19.
The Weibao copper–lead–zinc deposit, located in the eastern part of the Qimantagh area, East Kunlun Orogenic Belt (EKOB), consists of three skarn ore blocks known as Weixi, Main and Weidong from west to east. The mineralization within the Weibao Cu–Pb–Zn deposit is hosted by the Mesoproterozoic Langyashan Formation. In this study, we describe for the first time basaltic lavas that intruded into this host sequence and chronological, isotopic, major and trace element data of these volcanic rocks are presented here to constrain their eruption age as well as the tectonic setting. Two basaltic lava samples yield sensitive, high-resolution ion-microprobe (SHRIMP) U–Pb zircon ages of 393.0 ± 5.5 Ma–392.0 ± 5.0 Ma, indicating that volcanic rocks in the Weibao deposit erupted in Middle Devonian. The majority of the volcanic rocks have compositions characterized by high potassium, light rare earth element (LREE)-enriched patterns in chondrite-normalized rare earth elements (REE) diagrams, and evident enrichment of Rb, Ba and K and depletion of Th, U, Nb and Ta contents in primitive mantle-normalized patterns, although the degrees of enrichment and depletion are variable. These characteristics of major and trace element data highlight a hornblende-dominated fractionation during ascent of magmas. The εHf(T) values of zircons are relatively scattered and slightly enriched, ranging from −2.6 to +7.5. Modelling the features of the major, trace and isotopic element data indicates a hybrid origin involving combined depleted mantle (and hence asthenospheric mantle) and melts and/or fluids inherited from an early subduction event. Besides, these geochronological and geochemical data, together with previously published data in the EKOB, suggest that the Weibao basaltic lavas formed in a post-collisional setting, and that the Qimantagh area underwent strong interactions between mantle and crust in Early Paleozoic–Middle Devonian.  相似文献   

20.
Sr-Nd-Pb-Hf isotope mapping combined with U-Pb zircon SHRIMP ages of granitoids from four sampling profiles across terrane boundaries in Uzbekistan reveal distinct reservoir types (cratonic and accretionary), witnessed by the diverse nature and origin of the predominant Paleozoic granitic magmatism that provided hosts for major ore-bodies. The study region comprises four major terranes, including 1) the Sultan-Uvais terrane, 2) the Kyzylkum-Nurata Segment and 3) the Gissar Segment of the South Tien Shan and 4) the Chatkal-Kurama terrane of the Middle Tien Shan. Sr-Nd isotope analyses show a wide range of εNdt (− 5 to + 7) and (87Sr/86Sr)t of 0.704–0.707, indicating involvement of both mantle-derived material and older crustal sources. A wide range of Hf-isotope compositions found in zircons of Chatkal-Kurama granites, Middle Tien Shan (εHf mainly ~ − 5 to + 5), could be due to recycling of older crustal protolith(s); in particular, the earliest (Silurian) granites may be directly derived from 1.5 to 1.7 Ga lower crust. In the Southern Tien Shan, some involvement of subducted oceanic crust is evidenced by strongly juvenile εHft values of up to + 14 and + 16 (Sultan-Uvais, Teskuduk-Kyzylkum). Permo-Carboniferous granitoids, which occur across all terranes also exhibit a wide range of isotope signatures, corresponding to Mesoproterozoic–Neoproterozoic crustal protoliths with a westward increase in juvenile contributions. Pb isotopes (whole-rock) imply the dominance of a crustal component and crust-mantle mixing processes. New age data confirmed: 1) old age of the Turkestan Ocean (505 Ma in Sultan-Uvais), 2) fragments of Silurian island arcs in the accretionary complex of the Chatkal-Kurama terrane (granites of 429–416 Ma) and in the upper allochthon of the South Tien Shan (gabbro 438 Ma in Tamdytau), and 3) a significant volume of granitoid magmatism of subduction or early-collisional stages (around 320–310 Ma) in the Chatkal-Kurama Segment and especially in the Gissar Segment. The westernmost part of the Tien Shan is characterized by multiple subduction processes responsible for 300 million years of geodynamic evolution history (accretionary collage, crustal growth) with the pre-Mesozoic crust formation concluded by Permian post-collisional extensional magmatism.  相似文献   

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