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1.
Locations of the Eger Rift, Cheb Basin, Quaternary volcanoes, crustal earthquake swarms and exhalation centers of CO2 and 3He of mantle origin correlate with the tectonic fabric of the mantle lithosphere modelled from seismic anisotropy. We suggest that positions of the seismic and volcanic phenomena, as well as of the Cenozoic sedimentary basins, correlate with a “triple junction” of three mantle lithospheres distinguished by different orientations of their tectonic fabric consistent within each unit. The three mantle domains most probably belong to the originally separated microcontinents – the Saxothuringian, Teplá-Barrandian and Moldanubian – assembled during the Variscan orogeny. Cenozoic extension reactivated the junction and locally thinned the crust and mantle lithosphere. The rigid part of the crust, characterized by the presence of earthquake foci, decoupled near the junction from the mantle probably during the Variscan. The boundaries (transitions) of three mantle domains provided open pathways for Quaternary volcanism and the ascent of 3He- and CO2-rich fluids released from the asthenosphere. The deepest earthquakes, interpreted as an upper limit of the brittle–ductile transition in the crust, are shallower above the junction of the mantle blocks (at about 12 km) than above the more stable Saxothuringian mantle lithosphere (at about 20 km), probably due to a higher heat flow and presence of fluids.  相似文献   

2.
Our objective is to look for deep paths of Cenozoic volcanism and migration routes of active mantle volatiles through the lithosphere of the western Bohemian Massif. We show that the rejuvenated junction of three mantle domains, delimited by different orientation of seismic anisotropy and belonging to originally separated microplates — the Saxothuringian (ST), Moldanubian (MD) and Teplá-Barrandian (TB) — can provide the easiest upward routes of fluids through the deep lithosphere. Geographic distribution of mantle-fluid escapes at the surface suggests fluid migration through the ductile lower crust and through partly open faults in the rigid upper crust, which is locally detached and shifted from its lower part and from the mantle lithosphere. Present-day escapes of mantle-derived helium and CO2 concentrate mainly in two tectonically different crust edifices — in the Cheb Basin (CHB) and in an allochtonous block called the Mariánské Lázně Complex (MLC). Crystalline basement of the CHB developed above the Variscan ‘triple junction’ of the mantle lithosphere domains. The basement was extended during the Cenozoic and dissected by systems of faults into small partly sunken blocks. Thanks to buoyancy the mantle fluids migrate upwards along the lithosphere junction into the faulted basement of the CHB. The highest CO2 flow and the highest 3He/4He ratios are observed at intersections of major normal faults and along the southern boundary of the Smrčiny (Fichtelgebirge) granite Pluton. The fluid escapes are separated from the earthquake swarm epicentres. Routes of the fluids to the MLC are longer and more complicated. Surface escapes tap the mantle fluids mainly from the Mariánské Lázně Fault (MLF) and from the tectonic boundaries along which the MLC block of the TB lower crust was thrust over the ST complexes. Hypocentres of earthquake swarms of the two major focal areas at Novy Kostel and Lazy, located mainly at depths of 6–13 km, reside either in granite or in underlying gneiss, while the escapes of mantle fluids follow major faults or boundaries of crystalline units outside the Smrčiny and Karlovy Vary granite Plutons. We suggest that primarily those parts of faults in the upper crust, which is strengthened by granite magmatism and rigid enough to selectively accumulate stresses, are seismoactive. On the other hand, other parts of the faults tapping ascending mantle volatiles are ‘lubricated’ by the fluids and secondary mineralogical changes, and thus they cannot accumulate sufficient stresses to be released by earthquakes. A comparison of the most probable paths of the mantle fluids with the space-time distribution of the Novy Kostel hypocentres does not seem to support the model of the earthquake swarms triggered by pressurized fluids of mantle origin.  相似文献   

3.
In view of an anomalous crust–mantle structure beneath the 2001 Bhuj earthquake region, double-difference relocations of 1402 aftershocks of the 2001 Bhuj earthquake were determined, using an improved 1D velocity model constructed from 3D velocity tomograms based on data from 10 to 58 three-component seismograph stations. This clearly delineated four major tectonic features: (i) south-dipping north Wagad fault (NWF), (ii and iii) south-dipping south Wagad faults 1 and 2 (SWF1, SWF2), and (iv) a northeast dipping transverse fault (ITF), which is a new find. The relocated aftershocks correlate satisfactorily with the geologically mapped and inferred faults in the epicentral region. The relocated focal depths delineate a marked variation to the tune of 12 km in the brittle–ductile transition depths beneath the central aftershock zone that could be attributed to a lateral variation in crustal composition (more or less mafic) or in the level of fracturing across the fault zone. A fault intersection between the NWF and ITF has been clearly mapped in the 10–20 km depth range beneath the central aftershock zone. It is inferred that large intraplate stresses associated with the fault intersection, deepening of the brittle–ductile transition to a depth of 34 km due to the presence of mafic/ultramafic material in the crust–mantle transition zone, and the presence of aqueous fluids (released during the metamorphic process of eclogitisation of lower crustal olivine-rich rocks) and volatile CO2 at the hypocentral depths, might have resulted in generating the 2001 Bhuj earthquake sequence covering the entire lower crust.  相似文献   

4.
Large (> 100 km) meteorite impact cratering events play important roles in surface and biosphere evolution, however, their potential for widespread ductile modification of the lithosphere has been difficult to assess, due partly to our inability to isotopically age-correlate deep mineral fabrics with surface records. We have integrated benchmark U–Pb zircon dating methods (ID-TIMS, SHRIMP) with new microstructural techniques (EBSD, µXRD) to demonstrate that crystal–plastic deformation can cause rapid out-diffusion of radiogenic Pb and accompanying trace element alteration in crystalline zircon. We have used this phenomenon to directly date fabric in Archean zircons and xenoliths of the lower crust of South Africa at 2023 ± 15 million years, coeval with the 2020 ± 3 million year old Vredefort cratering event at surface, with extent ≥ 20,000 km2. Our findings indicate that regional exogenic fabrics, similar to high-temperature tectonic fabrics, exist in ancient crust. Moreover, our results establish that crystal-plastic deformation in the lithosphere can now be directly dated and linked to planetary evolution by zircon U–Pb strain chronometry.  相似文献   

5.
The South Sandwich volcanic arc is sited on a young oceanic crust, erupts low-K tholeiitic rocks, is characterized by unexotic pelagic and volcanogenic sediments on the down-going slab, and simple tectonic setting, and is ideal for assessing element transport through subduction zones. As a means of quantifying processes attending transfer of subduction-related fluids from the slab to the mantle wedge, boron concentrations and isotopic compositions were determined for representative lavas from along the arc. The samples show variable fluid-mobile/fluid-immobile element ratios and high enrichments of B/Nb (2.7 to 55) and B/Zr (0.12 to 0.57), similar to those observed in western Pacific arcs. δ11B values are among the highest so far reported for mantle-derived lavas; these are highest in the central part of the arc (+ 15 to + 18‰) and decrease toward the southern and northern ends (+ 12 to + 14‰). δ11B is roughly positively correlated with B concentrations and with 87Sr/86Sr ratios, but poorly coupled with other fluid-mobile elements such as Rb, Ba, Sr and U. Peridotites dredged from the forearc trench also have high δ11B (ca. + 10‰) and elevated B contents (38–140 ppm). Incoming pelagic sediments sampled at ODP Site 701 display a wide range in δ11B (+ 5 to ? 13‰; average = ? 4.1‰), with negative values most common. The unusually high δ11B values inferred for the South Sandwich mantle wedge cannot easily be attributed to direct incorporation of subducting slab materials or fluids derived directly therefrom. Rather, the heavy B isotopic signature of the magma sources is more plausibly explained by ingress of fluids derived from subduction erosion of altered frontal arc mantle wedge materials similar to those in the Marianas forearc. We propose that multi-stage recycling of high-δ11B and high-B serpentinite (possibly embellished by arc crust and volcaniclastic sediments) can produce extremely 11B-rich fluids at slab depths beneath the volcanic arc. Infiltration of such fluids into the mantle wedge likely accounts for the unusual magma sources inferred for this arc.  相似文献   

6.
Two-dimensional crustal velocity models are derived from passive seismic observations for the Archean Karelian bedrock of north-eastern Finland. In addition, an updated Moho depth map is constructed by integrating the results of this study with previous data sets. The structural models image a typical three-layer Archean crust, with thickness varying between 40 and 52 km. P wave velocities within the 12–20 km thick upper crust range from 6.1 to 6.4 km/s. The relatively high velocities are related to layered mafic intrusive and volcanic rocks. The middle crust is a fairly homogeneous layer associated with velocities of 6.5–6.8 km/s. The boundary between middle and lower crust is located at depths between 28 and 38 km. The thickness of the lower crust increases from 5–15 km in the Archean part to 15–22 km in the Archean–Proterozoic transition zone. In the lower crust and uppermost mantle, P wave velocities vary between 6.9–7.3 km/s and 7.9–8.2 km/s. The average Vp/Vs ratio increases from 1.71 in the upper crust to 1.76 in the lower crust.The crust attains its maximum thickness in the south-east, where the Archean crust is both over- and underthrust by the Proterozoic crust. A crustal depression bulging out from that zone to the N–NE towards Kuusamo is linked to a collision between major Archean blocks. Further north, crustal thickening under the Salla and Kittilä greenstone belts is tentatively associated with a NW–SE-oriented collision zone or major shear zone. Elevated Moho beneath the Pudasjärvi block is primarily explained with rift-related extension and crustal thinning at ∼2.4–2.1 Ga.The new crustal velocity models and synthetic waveform modelling are used to outline the thickness of the seismogenic layer beneath the temporary Kuusamo seismic network. Lack of seismic activity within the mafic high-velocity body in the uppermost 8 km of crust and relative abundance of mid-crustal, i.e., 14–30 km deep earthquakes are characteristic features of the Kuusamo seismicity. The upper limit of seismicity is attributed to the excess of strong mafic material in the uppermost crust. Comparison with the rheological profiles of the lithosphere, calculated at nearby locations, indicates that the base of the seismogenic layer correlates best with the onset of brittle to ductile transition at about 30 km depth.We found no evidence on microearthquake activity in the lower crust beneath the Archean Karelian craton. However, a data set of relatively well-constrained events extracted from the regional earthquake catalogue implies a deeper cut-off depth for earthquakes in the Norrbotten tectonic province of northern Sweden.  相似文献   

7.
We have studied the dependency between incoming plate structure, bending-related faulting, lithospheric hydration, and outer rise seismic activity offshore Maule, Chile. We derived a 2D Poisson's ratio distribution from P- and S-wave seismic wide angle data collected in the trench-outer rise. High values of Poisson's ratio in the uppermost mantle suggest that the oceanic lithosphere is highly hydrated due to the water infiltration through bending-related normal faults outcropping at the seafloor. This process is presumably facilitated by the presence of a seamount in the area. We conclude that water infiltrates deep into the lithosphere, when it approaches the Chile trench, producing a reduction of crustal and upper mantle velocities, supporting serpentinization of the upper mantle. Further, we observed a mantle Vp anisotropy of 8%, with the fast velocity axis running normal to the abyssal hill fabric and hence in spreading direction, indicating that outer rise processes have yet not affected anisotropy.The first weeks following the megatrust Mw = 8.8 Maule earthquake in 2010 were characterized by a sudden increase of the outer rise seismic activity, located between 34° S and 35°30′ S. We concluded that this phenomenon is a result of an intensification of the water infiltration process in the outer rise, presumably triggered by the main shock, whose epicenter was located some 100 km to the south east of the cluster.  相似文献   

8.
Miocene to Quaternary large basaltic plateaus occur in the back-arc domain of the Andean chain in Patagonia. They are thought to result from the ascent of subslab asthenospheric magmas through slab windows generated from subducted segments of the South Chile Ridge (SCR). We have investigated three volcanic centres from the Lago General Carrera–Buenos Aires area (46–47°S) located above the inferred position of the slab window corresponding to a segment subducted 6 Ma ago. (1) The Quaternary Río Murta transitional basalts display major, trace elements, and Sr and Nd isotopic features similar to those of oceanic basalts from the SCR and from the Chile Triple Junction near Taitao Peninsula (e.g., (87Sr/86Sr)o = 0.70396–0.70346 and εNd = + 5.5 − + 3.0). We consider them as derived from the melting of a Chile Ridge asthenospheric mantle source containing a weak subduction component. (2) The Plio-Quaternary (< 3.3 Ma) post-plateau basanites from Meseta del Lago Buenos Aires (MLBA), Argentina, likely derive from small degrees of melting of OIB-type mantle sources involving the subslab asthenosphere and the enriched subcontinental lithospheric mantle. (3) The main plateau basaltic volcanism in this region is represented by the 12.4–3.3-Ma-old MLBA basalts and the 8.2–4.4-Ma-old basalts from Meseta Chile Chico (MCC), Chile. Two groups can be distinguished among these main plateau basalts. The first group includes alkali basalts and trachybasalts displaying typical OIB signatures and thought to derive from predominantly asthenospheric mantle sources similar to those of the post-plateau MLBA basalts, but through slightly larger degrees of melting. The second one, although still dominantly alkalic, displays incompatible element signatures intermediate between those of OIB and arc magmas (e.g., La/Nb > 1 and TiO2 < 2 wt.%). These intermediate basalts differ from their strictly alkalic equivalents by having lower High Field Strength Element (HFSE) and higher εNd (up to + 5.4). These features are consistent with their derivation from an enriched mantle source contaminated by ca. 10% rutile-bearing restite of altered oceanic crust. The petrogenesis of the studied Mio-Pliocene basalts from MLBA and MCC is consistent with contributions of the subslab asthenosphere, the South American subcontinental lithospheric mantle and the subducted Pacific oceanic crust to their sources. However, their chronology of emplacement is not consistent with an ascent through an asthenospheric window opened as a consequence of the subduction of segment SCR-1, which entered the trench at 6 Ma. Indeed, magmatic activity was already important between 12 and 8 Ma in MLBA and MCC as well as in southernmost plateaus, i.e., 6 Ma before the subduction of the SCR-1 segment. We propose a geodynamic model in which OIB and intermediate magmas derived from deep subslab asthenospheric mantle did uprise through a tear-in-the-slab, which formed when the southernmost segments of the SCR collided with the Chile Trench around 15 Ma. During their ascent, they interacted with the Patagonian supraslab mantle and, locally, with slivers of subducted Pacific oceanic crust that contributed to the geochemical signature of the intermediate basalts.  相似文献   

9.
It has long been recognized that the Kii Peninsula in the southwest Japan arc is peculiar in a non-volcanic region, indicated by the presence of high temperature hot springs, high terrestrial heat flow and high 3He content in hot spring gases. Geophysical and geochemical studies were carried out to understand the geotectonic environment in the southern part of the Kii Peninsula. Most of the measured 3He / 4He ratios are similar or higher than air, indicating wide spread incorporation of mantle-derived helium into meteoric water. A region with rather high 3He / 4He ratios (> 4 RA) on the west side of the Omine Mountains coincides with the occurrence of high temperature hot springs. A deep crustal resistivity structure across the Omine Mountains was imaged by wide-band magnetotelluric soundings. A 2-D inversion with N–S strike using both TM and TE modes reveals two conductors, one in the upper (3–7 km depth) and the other in lower crust (25–35 km depth) to the west of the Omine Mountains. The distribution of microearthquakes and low-frequency tremors, and the existence of seismic reflectors indicate that the large conductor in the lower crust is related to aqueous fluids derived from the Philippine Sea plate. The upper-crustal conductive zone may also reflect the aqueous fluids trapped in the upper crust, which are presumably derived from the subducting slab. Considering the occurrence of seismic events in the subducting slab beneath the southern Kii Peninsula, the aqueous fluids generated by dehydration of the slab mantle could plausibly include MORB-type helium derived from the residual lithospheric mantle. Therefore, the high temperature hot springs and high 3He emanations in hot spring gases and other geotectonic events in the southern Kii Peninsula may be due to heat flux and mantle-derived helium discharged from aqueous fluid in the upper crust.  相似文献   

10.
We investigate the mantle dynamics beneath the North China Craton (NCC) and surrounding regions based on a synthesis of recent P-wave mantle tomographic data down to depths of 600–800 km and their correlation with the surface geological features, with particular reference to the Paleoproterozoic tectonic events associated with the incorporation of the NCC within the Columbia supercontinent amalgam. From the tomographic images, we identify a hot corridor in the mantle transition zone beneath the central region of the Western Block of the NCC sandwiched between two cold corridors. This scenario is similar to the donut-shaped high-velocity anomaly surrounding a region of low-velocity anomaly in the lowermost mantle under the Pacific and suggests that the cold regions might represent slab graveyards which provide the fuel for the plumes rising from the center. A tomographic transect along the collisional suture of the NCC with the Columbia supercontinent, covering the Yinshan-Ordos Blocks in the Western Block through the Central Orogenic Belt and into the Eastern Block of the NCC reveals a ca. 250 km thick lithospheric keel below the Ordos Block defined by a prominent high-velocity anomaly. We identify slab break-off and asthenospheric upwelling in this region and suggest that this process probably initiated the thermal and material erosion of the tectosphere beneath the Eastern Block from the Paleoproterozoic, which was further intensified during the Mesozoic when a substantial part of the sub-continental mantle lithosphere was lost. We visualize heat input from asthenosphere and interaction between asthenosphere and overlying carbonated tectosphere releasing CO2-rich fluids for the preservation of ultra-high temperature (ca. 1000 °C) metamorphic rocks enriched in CO2 as well as high-pressure mafic granulites as a paired suite in this region. We also identify a hot swell of the asthenosphere rooted to more than 200 km depth and reaching up to the shallow mantle in the tomographic section along 35°N latitude at a depth of 800 km. This zone represents a cross-section through the southern part of the NCC. The surface distribution of Paleoproterozoic Xiong’er lavas and mafic dykes in this region would indicate that this region might have evidenced similar upwellings in the past. Our study has important implications in understanding the evolution of the NCC and suggests that the extensive modification of the mantle architecture and lithospheric structure beneath one of the fundamental Precambrian nuclei of Asia had a prolonged history probably dating from the Paleoproterozoic suturing of the NCC within the Columbia supercontinent amalgam.  相似文献   

11.
In order to better understand the nature and formation of oceanic lithosphere beneath the Early Cretaceous Ontong Java Plateau, Re–Os isotopes have been analysed in a suite of peridotite xenoliths from Malaita, Solomon Islands. Geological, thermobarometric and petrological evidence from previous studies reveal that the xenoliths represent virtually the entire thickness of the southern part of subplateau lithospheric mantle (< 120 km). This study demonstrates that vertical Os isotopic variations correlate with compositional variations in a stratified lithosphere. The shallowest plateau lithosphere (< 85 km) is dominated by fertile lherzolites showing a restricted range of 187Os/188Os (0.1222 to 0.1288), consistent with an origin from ~ 160 Ma Pacific lithosphere. In contrast, the basal section of subplateau lithospheric mantle (~ 95–120 km) is enriched in refractory harzburgites with highly unradiogenic 187Os/188Os ratios ranging from 0.1152 to 0.1196, which yield Proterozoic model ages of 0.9–1.7 Ga. Although the whole range of Os isotope compositions of Malaita peridotites is within the variations seen in modern abyssal peridotites, the contrasting isotopic compositions of shallow and deep plateau lithosphere suggest their derivation from different mantle reservoirs. We propose that the subplateau lithosphere forms a genetically unrelated two-layered structure, comprising shallower, typical oceanic lithosphere underpinned by deeper impinged material, which included a component of recycled Proterozoic lithosphere. The impingement of residual but chemically heterogeneous mantle, mechanically coupled to the recently formed, thin lithosphere, may have a bearing on the anomalous initial uplift and late subsidence history of the seismically anomalous plateau root.  相似文献   

12.
We provide new petrological evidence for the strong influence of water on the formation of the oceanic lithospheric mantle, the subcontinental mantle above, and the continental lithosphere. Our analysis throws new light on the hypothesis that new continental lithosphere was formed by the passage of silicate-rich aqueous fluid through the sub-continental mantle. In order to investigate this hypothesis, we analyzed a representative collection of lherzolite and harzburgite xenoliths from the sample volcano known as “The Thumb”, located in the center of the Colorado Plateau, western United States. The studied sample collection exhibits multi-stage water enrichment processes along point, line and planar defect structures in nominally anhydrous minerals and the subsequent formation of the serpentine polymorph antigorite along grain boundaries and in totally embedded annealed cracks. Planar defect structures act like monomineralic and interphase grain boundaries in the oceanic lithosphere and the subcontinental mantle beneath the North American plate, which was hydrated by the ancient oceanic Farallon plate during the Cenozoic and Mesozoic eras. We used microspectroscopical, petrological, and seismological techniques to confirm multi-stage hydration from a depth of ∼150 km to just below the Moho depth. High-resolution mapping of the water distribution over homogeneous areas and fully embedded point, line and planar defects in olivine crystals of lherzolitic and harzburgitic origin by synchrotron infrared microspectroscopy enabled us to resolve local wet spots and thus reconstruct the hydration process occurring at a depth of ∼150 km (T ≈ 1225 °C). These lherzolites originated from the middle part of the Farallon mantle slab; they were released during the break up of the Farallon mantle slab, caused by the instability of the dipping slab. The background hydration levels in homogeneous olivines reached ∼138 ppm wt H2O, and the water concentration at the planar defects could reach up to ∼1000 ppm wt H2O. However, the formation of antigorite in grain boundaries was found to be the primary hydration mechanism for harzburgitic samples originating from the subcontinental mantle (for hydration, T ≈ 600 °C). Additionally, the formation of antigorite in lherzolites could be found in annealed cracks. From these observations, we conclude that hydration induces multi-stage water enrichment of the mantle wedge by a process that is dominated by the growth and movement of ubiquitous cracks, which acts as planar defects. Cracks in the mantle seem to be the an important feature in both the water cycle of the subduction zone and the formation of the continental lithosphere.  相似文献   

13.
We report here the first detailed 2D tomographic image of the crust and upper mantle structure of a Cretaceous seamount that formed during the interaction of the Pacific plate and the Louisville hotspot. Results show that at ～ 1.5 km beneath the seamount summit, the core of the volcanic edifice appears to be dominantly intrusive, with velocities faster than 6.5 km/s. The edifice overlies both high lower crustal (> 7.2–7.6 km/s) and upper mantle (> 8.3 km/s) velocities, suggesting that ultramafic rocks have been intruded as sills rather than underplated beneath the crust. The results suggest that the ratio between the volume of intra-crustal magmatic intrusion and extrusive volcanism is as high as ～ 4.5. In addition, the inversion of Moho reflections shows that the Pacific oceanic crust has been flexed downward by up to ～ 2.5 km beneath the seamount. The flexure can be explained by an elastic plate model in which the seamount emplaced upon oceanic lithosphere that was ～ 10 Myr at the time of loading. Intra-crustal magmatic intrusion may be a feature of hotspot volcanism at young, hot, oceanic lithosphere, whereas, magmatic underplating below a pre-existing Moho may be more likely to occur where a hotspot interacts with oceanic lithosphere that is several tens of millions of years old.  相似文献   

14.
In this work we have modeled the thermal structure of the lithosphere of the Spanish Central System and the Tajo Basin, and their implications for lithospheric strength. For this, we have used refined heat-producing elements (HPE) values to obtain new estimates of heat production rates in the Spanish Central System and Tajo Basin areas, which have been used joined to the relation between topography and thermal structure of the lithosphere to calculate the best-fit surface heat flows in the study area. Moreover, we have implemented a temperature-dependent thermal conductivity (appropriate for olivine) for the lithospheric mantle to improve the calculations of temperature profiles in the mantle. The geotherms so obtained, together with the implementation of a new rheological law for the upper lithospheric mantle, have been used to calculate refined estimations of the strength and effective elastic thickness of the lithosphere. We have obtained surface heat flow values of 84 mW m−2 and ∼82 mW m−2 for the Spanish Central System and the Tajo Basin, respectively. The thermal state of the lithosphere affects mantle temperatures, and hence may be playing an important role in the uplift and maintenance of the Spanish Central System.  相似文献   

15.
Os–Hf–Sr–Nd isotopes and PGE were determined in peridotite xenoliths carried to the surface by Quaternary alkali basaltic magmas in the Tokinsky Stanovik Range on the Aldan shield. These data constrain the timing and nature of partial melting and metasomatism in the lithospheric mantle beneath SE Siberian craton. The xenoliths range from the rare fertile spinel lherzolites to the more abundant, strongly metasomatised olivine-rich (70–84%) rocks. Hf–Sr–Nd isotope compositions of the xenoliths are mainly within the fields of oceanic basalts. Most metasomatised xenoliths have lower 143Nd / 144Nd and 176Hf / 177Hf and higher 87Sr / 86Sr than the host basalts indicating that the metasomatism is older and has distinct sources. A few xenoliths have elevated 176Hf / 177Hf (up to 0.2838) and plot above the Hf–Nd mantle array defined by oceanic basalts.187Os / 188Os in the poorly metasomatised, fertile to moderately refractory (Al2O3 ≥ 1.6%) Tok peridotites range from 0.1156 to 0.1282, with oldest rhenium depletion ages being about 2 Ga. The 187Os / 188Os in these rocks show good correlations with partial melting indices (e.g. Al2O3, modal cpx); the intercept of the Al–187Os / 188Os correlation with lowest Al2O3 estimates for melting residues (∼0.3–0.5%) has a 187Os / 188Os of ∼0.109 suggesting that these peridotites may have experienced melt extraction as early as 2.8 Gy ago. 187Os / 188Os in the strongly metasomatised, olivine-rich xenoliths (0.6–1.3% Al2O3) ranges from 0.1164 to 0.1275 and shows no apparent links to modal or chemical compositions. Convex-upward REE patterns and high abundances of heavy to middle REE in these refractory rocks indicate equilibration with evolved silicate melts at high melt / rock ratios, which may have also variably elevated their 187Os / 188Os. This inference is supported by enrichments in Pd and Pt on chondrite-normalised PGE abundance patterns in some of the rocks. The melt extraction ages for the Tok suite of 2.0 to 2.8 Ga are younger than oldest Os ages reported for central Siberian craton, but they must be considered minimum estimates because of the extensive metasomatism of the most refractory Tok peridotites. This metasomatism could have occurred in the late Mesozoic to early Cenozoic when the Tok region was close to the subduction-related Pacific margin of Siberia and experienced large-scale tectonic and magmatic activity. This study indicates that metasomatic effects on the Re–Os system in the shallow lithospheric mantle can be dramatic.  相似文献   

16.
Inversion of local earthquake travel times and joint inversion of receiver functions and Rayleigh wave group velocity measurements were used to derive a simple model for the velocity crustal structure beneath the southern edge of the Central Alborz (Iran), including the seismically active area around the megacity of Tehran. The P and S travel times from 115 well-located earthquakes recorded by a dense local seismic network, operated from June to November 2006, were inverted to determine a 1D velocity model of the upper crust. The limited range of earthquake depths (between 2 km and 26 km) prevents us determining any velocity interfaces deeper than 25 km. The velocity of the lower crust and the depth of the Moho were found by joint inversion of receiver functions and Rayleigh wave group velocity data. The resulting P-wave velocity model comprises an upper crust with 3 km and 4 km thick sedimentary layers with P wave velocities (Vp) of ～5.4 and ～5.8 km s?1, respectively, above 9 km and 8 km thick layers of upper crystalline crust (Vp ～6.1 and ～6.25 km s?1 respectively). The lower crystalline crust is ～34 km thick (Vp ～ 6.40 km s?1). The total crustal thickness beneath this part of the Central Alborz is 58 ± 2 km.  相似文献   

17.
The Pannonian depression is an extensional back-arc basin in central Europe and is an integral part of the Alpine–Carpathian orogenic mountain belts. It can be characterized by thinned lower crust, shallow Moho discontinuity, high surface heat flow and Moho temperature, implying recent active tectonic processes. Imaging the velocity structure of the upper mantle may help us to better understand the structure and formation of the Pannonian region.In this paper, Pn traveltimes from regional earthquakes are used to tomographically image the lateral velocity variations in the uppermost mantle beneath the Pannonian basin. The set of linear tomographic equations, built up of the time term equation for each source–receiver pair, is solved by a truncated singular value decomposition algorithm. The explicit computation of the generalized inverse of the tomographic equations makes it possible to deduce both the resolution matrix and the model covariance matrix, allowing us to estimate the resolution and reliability of the solution.The mean compressional wave velocity in the uppermost mantle beneath the Pannonian basin is 7.9 km/s, substantially lower than the average continental Pn velocity of 8.1 km/s. It is mostly due to the high Moho temperature having values on average 400–500 °C more than those in the surrounding areas. The velocity anomalies range from −0.3 to 0.3 km/s relative to the mean velocity of 7.9 km/s. Due to high Moho temperature, below the North Hungarian range low (7.6–7.7 km/s) velocities can be found. High-velocity anomalies of around 8.1 km/s can be detected along the W-SW boundaries of Hungary and at the junction of the Pannonian basin and the Southern Carpathians. The Great Hungarian Plain shows average (7.9 km/s) Pn velocities.  相似文献   

18.
Coupled 187Os/188Os and highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, and Re) abundance data are reported for pristine lunar crustal rocks 60025, 62255, 65315 (ferroan anorthosites, FAN) and 76535, 78235, 77215 and a norite clast in 15455 (magnesian-suite rocks, MGS). Osmium isotopes permit more refined discrimination than previously possible of samples that have been contaminated by meteoritic additions and the new results show that some rocks, previously identified as pristine, contain meteorite-derived HSE. Low HSE abundances in FAN and MGS rocks are consistent with derivation from a strongly HSE-depleted lunar mantle. At the time of formation, the lunar floatation crust, represented by FAN, had 1.4 ± 0.3 pg g? 1 Os, 1.5 ± 0.6 pg g? 1 Ir, 6.8 ± 2.7 pg g? 1 Ru, 16 ± 15 pg g? 1 Pt, 33 ± 30 pg g? 1 Pd and 0.29 ± 0.10 pg g? 1 Re (～ 0.00002 × CI) and Re/Os ratios that were modestly elevated (187Re/188Os = 0.6 to 1.7) relative to CI chondrites. MGS samples are, on average, characterised by more elevated HSE abundances (～ 0.00007 × CI) compared with FAN. This either reflects contrasting mantle-source HSE characteristics of FAN and MGS rocks, or different mantle–crust HSE fractionation behaviour during production of these lithologies. Previous studies of lunar impact-melt rocks have identified possible elevated Ru and Pd in lunar crustal target rocks. The new results provide no supporting evidence for such enrichments.If maximum estimates for HSE in the lunar mantle are compared with FAN and MGS averages, crust–mantle concentration ratios (D-values) must be ≤ 0.3. Such D-values are broadly similar to those estimated for partitioning between the terrestrial crust and upper mantle, with the notable exception of Re. Given the presumably completely different mode of origin for the primary lunar floatation crust and tertiary terrestrial continental crust, the potential similarities in crust–mantle HSE partitioning for the Earth and Moon are somewhat surprising. Low HSE abundances in the lunar crust, coupled with estimates of HSE concentrations in the lunar mantle implies there may be a ‘missing component’ of late-accreted materials (as much as 95%) to the Moon if the Earth/Moon mass-flux estimates are correct and terrestrial mantle HSE abundances were established by late accretion.  相似文献   

19.
Modeling of multimode surface wave group velocity dispersion data sampling the eastern and the western Ganga basins, reveals a three layer crust with an average Vs of 3.7 km s?1, draped by ～2.5 km foreland sediments. The Moho is at a depth of 43 ± 2 km and 41 ± 2 km beneath the eastern and the western Ganga basins respectively. Crustal Vp/Vs shows a felsic upper and middle crust beneath the eastern Ganga basin (1.70) compared to a more mafic western Ganga basin crust (1.77). Due to higher radiogenic heat production in felsic than mafic rocks, a lateral thermal heterogeneity will be present in the foreland basin crust. This heterogeneity had been previously observed in the north Indian Shield immediately south of the foreland basin and must also continue northward below the Himalaya. The high heat producing felsic crust, underthrust below the Himalayas could be an important cause for melting of midcrustal rocks and emplacement of leucogranites. This is a plausible explanation for abundance of leucogranites in the east-central Himalaya compared to the west. The uppermost mantle Vs is also significantly lower beneath the eastern Ganga basin (4.30 km s?1) compared to the west (4.44 km s?1).  相似文献   

20.
Common and radiogenic Pb isotopic compositions of plagioclase and anti-perthitic feldspars from granulite-facies lower crustal xenoliths from the Labait Volcano on the eastern margin of the Tanzanian Craton have been measured via laser ablation MC-ICP-MS. Common Pb in plagioclase and a single stage Pb evolution model indicate that the lower crust of the Tanzanian Craton was extracted from mantle having a 238U/204Pb of 8.1 ± 0.3 and a 232Th/238U of 4.3 ± 0.1 at 2.71 ± 0.09 Ga (all uncertainties are 2σ). Since 2.4 Ga, some orthoclase domains within anti-perthites have evolved with a maximum 238U/204Pb of 6 and 232Th/238U of 4.3. The spread in Pb isotopic composition in the anti-perthitic feldspars yields single crystal Pb–Pb isochrons of ～ 2.4 Ga, within uncertainty of U–Pb zircon ages from the same sample suite. The Pb isotopic heterogeneities imply that these granulites resided at temperatures < 600 °C in the lower crust of the Tanzanian Craton from ca. 2.4 Ga to the present. In concert with the chemistry of surface samples, mantle xenoliths, and lower crustal xenoliths, our data imply that the cratonic lithosphere in Tanzania formed ca. ～ 2.7 Ga, in a convergent margin setting, and has remained undisturbed since 2.7 Ga.  相似文献   

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