Crustal structure under the Sydney Basin and Lachlan Fold Belt,determined from explosion seismic studies

The Lachlan Fold Belt has the velocity‐depth structure of continental crust, with a thickness exceeding 50 km under the region of highest topography in Australia, and in the range 41–44 km under the central Fold Belt and Sydney Basin. There is no evidence of high upper crustal velocities normally associated with marginal or back‐arc basin crustal rocks. The velocities in the lower crust are consistent with an overall increase in metamorphic grade and/or mafic mineral content with depth. Continuing tectonic development throughout the region and the negligible seismicity at depths greater than 30 km indicate that the lower crust is undergoing ductile deformation.

The upper crustal velocities below the Sydney Basin are in the range 5.75–5.9 km/s to about 8 km, increasing to 6.35–6.5 km/s at about 15–17 km depth, where there is a high‐velocity (7.0 km/s) zone for about 9 km evident in results from one direction. The lower crust is characterised by a velocity gradient from about 6.7 km/s at 25 km, to 7.7 km/s at 40–42 km, and a transition to an upper mantle velocity of 8.03–8.12 km/s at 41.5–43.5 km depth.

Across the central Lachlan Fold Belt, velocities generally increase from 5.6 km/s at the surface to 6.0 km/s at 14.5 km depth, with a higher‐velocity zone (5.95 km/s) in the depth range 2.5–7.0 km. In the lower crust, velocities increase from 6.3 km/s at 16 km depth to 7.2 km/s at 40 km depth, then increase to 7.95 km/s at 43 km. A steeper gradient is evident at 26.5–28 km depth, where the velocity is about 6.6—6.8 km/s. Under part of the area an upper mantle low‐velocity zone in the depth range 50–64 km is interpreted from strong events recorded at distances greater than 320 km.

There is no substantial difference in the Moho depth across the boundary between the Sydney Basin and the Lachlan Fold Belt, consistent with the Basin overlying part of the Fold Belt. Pre‐Ordovician rocks within the crust suggest fragmented continental‐type crust existed E of the Precambrian craton and that these contribute to the thick crustal section in SE Australia.     

Anomalous lithospheric structure of Northern Juan de Fuca plate — a consequence of oceanic rift propagation?

Anomalous crustal and upper mantle structure of northern Juan de Fuca plate is revealed from wide-angle seismic and gravity modelling. A 2-D velocity model is produced for refraction line II of the 1980 Vancouver Island Seismic Project (VISP80). The refraction data were recorded on three ocean bottom seismometers (OBSs) deployed at the ends and middle of a 110 km line oriented parallel to the North American continental margin. The velocity model is constructed via ray tracing and conforms to first-arrival amplitude observations and travel time picks of direct, converted and reflected phases. Between sub-sediment depths of 3 to 11 km, depths normally associated with the lower crust and upper oceanic mantle, the final model shows that compressional-wave velocities decrease significantly from southeast to northwest along the profile. At sub-sediment depths of 11 km at the northwestern end of the profile, P-wave velocities are as low as 7.2 km/s. A complementary 2-D gravity model using the geometry of the velocity model and velocity–density relationships characteristic of oceanic crust is produced. The high densities required to match the gravity field indicate the presence of peridotites containing 25–30% serpentine by volume, rather than excess gabbroic crust, within the deep low velocity zone. Anomalous travel time delays and unusual reflection characteristics observed from proximal seismic refraction and reflection experiments suggest a broader zone of partially serpentinized peridotites coincident with the trace of a pseudofault. We propose that partial serpentinization of the upper mantle is a consequence of slow spreading at the tip of a propagating rift.     

Determination of 3-D Velocity Anomalies of the Nanbei Tectonic Zone of China Based on Local Earthquakes

In this paper, 3-D velocity images of the crust and upper mantle beneath the Nanbei tectonic zone of China are constructed using P-wave travel time residuals of earthquakes, with the data supplied by China's seismic networks and the International Seismic Centre.During the model parameterization in the tomographic inversion the interpolation function of grid node velocities is used as the space function of velocity, and the velocity function is allowed to be discontinuous. The difficulties caused by large memory demand and high computing cost in solving the system of equations are avoided by utilizing the least squares QR decomposition algorithm. Thus, the stability of the algorithm is ensured. Though resolution images are not derived in the inversion process based on the resolution theory of Backus and Gilbert, a covariance resolution method is put forward by the authors. The resolution analyses have proved the reliability of the image results. 3-D ray tracing is conducted to obtain the ray paths in a s     

Lower lithospheric structure beneath the Trans-European Suture Zone from POLONAISE''97 seismic profiles

The large-scale POLONAISE'97 seismic experiment investigated the velocity structure of the lithosphere in the Trans-European Suture Zone (TESZ) region between the Precambrian East European Craton (EEC) and Palaeozoic Platform (PP). In the area of the Polish Basin, the P-wave velocity is very low (Vp <6.1 km/s) down to depths of 15–20 km, and the consolidated basement (Vp5.7–5.8 km/s) is 5–12 km deep. The thickness of the crust is 30 km beneath the Palaeozoic Platform, 40–45 km beneath the TESZ, and 40–50 km beneath the EEC. The compressional wave velocity of the sub-Moho mantle is >8.25 km/s in the Palaeozoic Platform and 8.1 km/s in the Precambrian Platform. Good quality record sections were obtained to the longest offsets of about 600 km from the shot points, with clear first arrivals and later phases of waves reflected/refracted in the lower lithosphere. Two-dimensional interpretation of the reversed system of travel times constrains a series of reflectors in the depth range of 50–90 km. A seismic reflector appears as a general feature at around 10 km depth below Moho in the area, independent of the actual depth to the Moho and sub-Moho seismic velocity. “Ringing reflections” are explained by relatively small-scale heterogeneities beneath the depth interval from 90 to 110 km. Qualitative interpretation of the observed wave field shows a differentiation of the reflectivity in the lower lithosphere. The seismic reflectivity of the uppermost mantle is stronger beneath the Palaeozoic Platform and TESZ than the East European Platform. The deepest interpreted seismic reflector with zone of high reflectivity may mark a change in upper mantle structure from an upper zone characterised by seismic scatterers of small vertical dimension to a lower zone with vertically larger seismic scatterers, possible caused by inclusions of partial melt.     

Results of lithospheric studies from long-range profiles in Siberia

Several long-range seismic profiles, obtained during the last ten years in Siberia, show the complicated lithospheric structure of the Siberian platforms. The three component observations, conducted at distances up to 3000 km, made it possible to obtain information on P- and S-velocities in the crust, on P-velocity and Q-factor for the upper mantle, and on the seismic boundaries responsible for reflected, refracted and converted waves down to a depth of 400–700 km.The crustal models are typical of old platforms of Eurasia: the average thickness of 40 km, three layers with P-velocities 6.2, 6.5, 7.0 km/s and thicknesses of 10–15 km are distinguished. The depth to the M discontinuity varies from 45–50 km beneath the old Tunguss depression, to 35–40 km beneath the younger Vilyui basin. The most complicated Moho structure is observed in the boundary between the West Siberian and the Siberian platforms.A strong inhomogeneity of P-velocity models was revealed for the upper mantle. The horizontal inhomogeneities are more larger in the uppermost mantle to depths of 80–100 km, where P-velocities vary from 8.0–8.2 km/s beneath the young West Siberian plate to 8.4–8.6 km/s beneath some blocks of the Siberian craton. The fine vertical inhomogeneity was studied with reflections correlated after computer processing of seismograms. They outlined several low-velocity layers 20–50 km thick. The layers were characterized by low Q as well.Intensive waves were recorded from the transition zone between the upper and lower mantle. The top of the zone is nearly horizontal in the area; its depth is 400 ± 25 km. The bottom of the zone lies at about 700 km.     

The structure of the crust and upper mantle to depths of 320 km beneath the Kaapvaal craton, from P wave arrivals generated by regional earthquakes and mining-induced tremors

Travel times from earthquakes recorded at two seismic networks were used to derive an average P wavespeed model for the crust and upper mantle to depths of 320 km below southern Africa. The simplest model (BPI1) has a Moho depth of 34 km, and an uppermost mantle wavespeed of 8.04 km/s, below which the seismic wavespeeds have low positive gradients. Wavespeed gradients decrease slightly around 150 km depth to give a ‘knee’ in the wavespeed-depth model, and the wavespeed reaches 8.72 km/s at a depth of 320 km. Between the Moho and depths of 270 km, the seismic wavespeeds lie above those of reference model IASP91 of Kennett [Research School of Earth Sciences, Australian National University, Canberra, Australia (1991)] and below the southern African model of Zhao et al. [Journal of Geophysical Research 104 (1999) 4783]. At depths near 300 km all three models have similar wavespeeds. The mantle P wavespeeds for southern Africa of Qiu et al. [Geophysical Journal International 127 (1996) 563] lie close to BPI1 at depths between 40 and 140 km, but become lower at greater depths. The seismic wavespeeds in the upper mantle of model BPI1 agree satisfactorily with those estimated from peridotite xenoliths in kimberlites from within the Kaapvaal craton.The crustal thickness of 34 km of model BPI1 is systematically lower than the average thickness of 41 km computed over the same region from receiver functions. This discrepancy can be partly explained by an alternative model (BPI2) in which there is a crust–mantle transition zone between depths of 35 and 47 km, below which seismic wavespeed increases to 8.23 km/s. A low-wavespeed layer is then required at depths between 65 and 125 km.     

Three-dimensional P- and S-wave velocity structures beneath the Japan Islands obtained by high-density seismic stations by seismic tomography

We construct fine-scale 3D P- and S-wave velocity structures of the crust and upper mantle beneath the whole Japan Islands with a unified resolution, where the Pacific (PAC) and Philippine Sea (PHS) plates subduct beneath the Eurasian (EUR) plate. We can detect the low-velocity (low-V) oceanic crust of the PAC and PHS plates at their uppermost part beneath almost all the Japan Islands. The depth limit of the imaged oceanic crust varies with the regions. High-V_P/V_S zones are widely distributed in the lower crust especially beneath the volcanic front, and the high strain rate zones are located at the edge of the extremely high-V_P/V_S zone; however, V_P/V_S at the top of the mantle wedge is not so high. Beneath northern Japan, we can image the high-V subducting PAC plate using the tomographic method without any assumption of velocity discontinuities. We also imaged the heterogeneous structure in the PAC plate, such as the low-V zone considered as the old seamount or the highly seismic zone within the double seismic zone where the seismic fault ruptured by the earthquake connects the upper and lower layer of the double seismic zone. Beneath central Japan, thrust-type small repeating earthquakes occur at the boundary between the EUR and PHS plates and are located at the upper part of the low-V layer that is considered to be the oceanic crust of the PHS plate. In addition to the low-V oceanic crust, the subducting high-V PAC plate is clearly imaged to depths of approximately 250 km and the subducting high-V PHS zone to depths of approximately 180 km is considered to be the PHS plate. Beneath southwestern Japan, the iso-depth lines of the Moho discontinuity in the PHS plate derived by the receiver function method divide the upper low-V layer and lower high-V layer of our model at depths of 30–50 km. Beneath Kyushu, the steeply subducting PHS plate is clearly imaged to depths of approximately 250 km with high velocities. The high-V_P/V_S zone is considered as the lower crust of the EUR plate or the oceanic crust of the PHS plate at depths of 25–35 km and the partially serpentinized mantle wedge of the EUR plate at depths of 30–45 km beneath southwestern Japan. The deep low-frequency nonvolcanic tremors occur at all parts of the high-V_P/V_S zone—within the zone, the seaward side, and the landward side where the PHS plate encounters the mantle wedge of the EUR plate. We prove that we can objectively obtain the fine-scale 3D structure with simple constraints such as only 1D initial velocity model with no velocity discontinuity.     

Deep Structure and Distribution of Earthquake in the Manzhouli-Suifenhe Geotransect Region

1.IntroductionTheManzhouli-SuifenheGeoscienceTransect(M-SGT)isinthenortheastChina,acrosstheprovincesofInnerMongoliaandHeilongiiang.Geologically,itissitllatedamongtheplatesofNorthChina,SiberiaandWesternPacific.ThewholeIengthoftheM-SGTisaboutl3Ookm,whichcrossesmanytectonicunits(Fig.l).ItisclearthatitstectonicsitUationisuniqueanditsgeologicstructUreiscomplex.Deepearthquakeshappenfrequentlya1ongthetransect.Therefore,itisarepresentativeprofileofnortheastChinaandtheNortheastAsia.TheM-S…     

Upper-mantle structure and nature of deep processes in island arcs and trench systems

Upper-mantle structure and heat flow and gravity data are considered. It is shown that crustal isostasy disturbance is compensated for by inhomogeneities in the density of the upper mantle; compaction beneath trenches and discompaction under island arcs. On the basis of the available data an upper-mantle density model is constructed which is represented by a seismoactive layer with a surplus density of from 0.05 to 0.1 g/cm³ and a discompacted zone overlying that layer with a density deficiency of 0.05 g/cm³ beneath the island arc and 0.01 g/cm³ in the remaining area. The available data are in poor agreement with the concept of “new global tectonics”, therefore the formation of density inhomogeneities is associated with gravity differentiation of the mantle material. A scheme of this process is proposed according to which heated differentiates of the mantle material rise along deep faults or weakened zones from depths of 700 km and more. Upper-mantle material differentiation occurs mainly in several levels, but mostly at depths down to 150–200 km within the interval of 350–450 km. Separation of heavy components as a result of differentiation and the “zone melting process” causes a high velocity and correspondingly a denser inclined layer. Discompaction of the upper mantle overlying the focal plane is associated with its being heated by high-temperature melts circulating over the inclined fault.     

Crustal structure beneath two seismic broadband stations revealed from teleseismic P-wave receiver function analysis in the Virunga volcanic area,Western Rift Valley of Africa

The shear velocity structure beneath the Virunga volcanic area was estimated by using an average solution in the time domain inversion of stacked teleseismic receiver functions provided by two seismic broadband stations KUNENE (KNN) and KIBUMBA (KBB). These two stations are 29 km apart and located at the eastern and western escarpment of the Western Rift Valley of Africa in the Virunga area, respectively. The velocity model was presented as P-wave velocity models. From these models, the crust mantle transition zone beneath the area sampled by KNN and KBB in the Virunga area was determined at depth from about 36 to 39 km and 30 to 41 km, respectively. A low velocity zone was observed below stations KNN and KBB at depths between 20–30 km and 18–28 km, respectively, and with average velocity 5.9 km/s and 6.0 km/s. This low velocity zone may probably related to a magma chamber or a melt-rich sill. The models show also high velocity material (6.8–7.4 km/s) lying beneath stations KNN and KBB at depths 3–20 km and 3–10 km, respectively, which is indicative of magma cumulates within the volcanic edifice. The result obtained in this study was applied to the determination of epicentres during the period prior to the 27 November 2006 Nyamuragira eruption. This eruption was preceded by a swarm of hybrid volcanic earthquakes with clear P-waves onset. Using the receiver function model was found to improve the location of events. The located events correlate well with the location of the eruptive site and data provided by the INSAR observations of surface deformation associated with eruption.     

2D model of the crust and uppermost mantle along rift profile, Siberian craton

A two-dimensional model of the crust and uppermost mantle for the western Siberian craton and the adjoining areas of the Pur-Gedan basin to the north and Baikal Rift zone to the south is determined from travel time data from recordings of 30 chemical explosions and three nuclear explosions along the RIFT deep seismic sounding profile. This velocity model shows strong lateral variations in the crust and sub-Moho structure both within the craton and between the craton and the surrounding region. The Pur-Gedan basin has a 15-km thick, low-velocity sediment layer overlying a 25-km thick, high-velocity crystalline crustal layer. A paleo-rift zone with a graben-like structure in the basement and a high-velocity crustal intrusion or mantle upward exists beneath the southern part of the Pur-Gedan basin. The sedimentary layer is thin or non-existent and there is a velocity reversal in the upper crust beneath the Yenisey Zone. The Siberian craton has nearly uniform crustal thickness of 40–43 km but the average velocity in the lower crust in the north is higher (6.8–6.9 km/s) than in the south (6.6 km/s). The crust beneath the Baikal Rift zone is 35 km thick and has an average crustal velocity similar to that observed beneath the southern part of craton. The uppermost mantle velocity varies from 8.0 to 8.1 km/s beneath the young West Siberian platform and Baikal Rift zone to 8.1–8.5 km/s beneath the Siberian craton. Anomalous high Pn velocities (8.4–8.5 km/s) are observed beneath the western Tunguss basin in the northern part of the craton and beneath the southern part of the Siberian craton, but lower Pn velocities (8.1 km/s) are observed beneath the Low Angara basin in the central part of the craton. At about 100 km depth beneath the craton, there is a velocity inversion with a strong reflecting interface at its base. Some reflectors are also distinguished within the upper mantle at depth between 230 and 350 km.     

A first interpretation of crustal structure in the Adelaide Geosyncline in South Australia using quarry blasts

Explosions at two large open cut mines (Leigh Creek and Iron Baron) were used as sources of seismic energy to record along two linear profiles, parallel and approximately transverse to the axis of the Adelaide geosyncline in South Australia. Records at approximately 120 sites were obtained out to distances of the order of 350 km with Kinemetrics PS‐1A portable seismographs, using smoked paper and a recording speed of 4 mm/s. Times of blasting were determined from records at some of the permanent stations of the University of Adelaide seismograph network. Station spacing was normally 5 km, but at large distances from the source this increased to the order of 10 km.

The simplest model of the crust consistent with the observed travel times comprises two essentially homogeneous layers overlying the mantle. The average P wave velocities in the upper and lower crustal layers are 5.94 km/s and 6.46 km/s, with the boundary between the layers at approximately 18 km and possibly 8 km below Eyre Peninsula. Although such a division has been found in other parts of Australia, none of the earlier studies in S.A. found evidence for such a discontinuity or velocity gradient. The P wave velocity in the upper mantle is 7.97 km/s and the mean thickness of the crust is 39 km. Both the intermediate and Moho “discontinuities” may vary by up to 5 km from their mean depths. Shear waves have velocities of 3.43 and 4.45 km/s in the upper crustal layer and the upper mantle, respectively.     

Deep seismic investigations in the Baikal rift zone

The largest rift zone of Europe and Asia is located in the region of Lake Baikal. In 1968–1970 deep seismic measurements were carried out along a number of profiles with a total length of about 2000 km within the rift zone and in the adjacent parts of the Siberian platform and the region of the Baikal Mountains. These investigations were of a reconnaissance nature, and therefore the point sounding method was used.A low-velocity region for compressional waves (7.6–7.8 km/sec) has been found and could be traced over a large area in the upper parts of the mantle. The width of this anomalous zone is 200–400 km. The Baikal rift lies in its northwestern part. Within the studied part of the Siberian platform the thickness of the earth's crust is 37–39 km, while in the rift zone it is 36 km, and further to the southeast the crust-mantle boundary lies at a depth of 45–46 km. The Baikal rift proper is bounded in the northwest by a deep fracture zone and does not seem to be associated with any significant “root” or “antiroot” in the relief of the Mohorovi?i? discontinuity.The reduced compressional velocity in the upper parts of the mantle beneath the Baikal zone is considered to correspond to the same phenomena found under the mid-oceanic ridges and the extended rift system in the Basin and Range province of North America. The Baikal rift in the narrow sense of the word lies over the northwestern edge of the anomalous mantle region. This asymmetric position seems to be its main peculiarity.     

Crustal and upper mantle seismic structure of the Australian Plate, South Island, New Zealand

Seismic reflection and refraction data were collected west of New Zealand's South Island parallel to the Pacific–Australian Plate boundary. The obliquely convergent plate boundary is marked at the surface by the Alpine Fault, which juxtaposes continental crust of each plate. The data are used to study the crustal and uppermost mantle structure and provide a link between other seismic transects which cross the plate boundary. Arrival times of wide-angle reflected and refracted events from 13 recording stations are used to construct a 380-km long crustal velocity model. The model shows that, beneath a 2–4-km thick sedimentary veneer, the crust consists of two layers. The upper layer velocities increase from 5.4–5.9 km/s at the top of the layer to 6.3 km/s at the base of the layer. The base of the layer is mainly about 20 km deep but deepens to 25 km at its southern end. The lower layer velocities range from 6.3 to 7.1 km/s, and are commonly around 6.5 km/s at the top of the layer and 6.7 km/s at the base. Beneath the lower layer, the model has velocities of 8.2–8.5 km/s, typical of mantle material. The Mohorovicic discontinuity (Moho) therefore lies at the base of the second layer. It is at a depth of around 30 km but shallows over the south–central third of the profile to about 26 km, possibly associated with a southwest dipping detachment fault. The high, variable sub-Moho velocities of 8.2 km/s to 8.5 km/s are inferred to result from strong upper mantle anisotropy. Multichannel seismic reflection data cover about 220 km of the southern part of the modelled section. Beneath the well-layered Oligocene to recent sedimentary section, the crustal section is broadly divided into two zones, which correspond to the two layers of the velocity model. The upper layer (down to about 7–9 s two-way travel time) has few reflections. The lower layer (down to about 11 s two-way time) contains many strong, subparallel reflections. The base of this reflective zone is the Moho. Bi-vergent dipping reflective zones within this lower crustal layer are interpreted as interwedging structures common in areas of crustal shortening. These structures and the strong northeast dipping reflections beneath the Moho towards the north end of the (MCS) line are interpreted to be caused by Paleozoic north-dipping subduction and terrane collision at the margin of Gondwana. Deeper mantle reflections with variable dip are observed on the wide-angle gathers. Travel-time modelling of these events by ray-tracing through the established velocity model indicates depths of 50–110 km for these events. They show little coherence in dip and may be caused side-swipe from the adjacent crustal root under the Southern Alps or from the upper mantle density anomalies inferred from teleseismic data under the crustal root.     

Petrological-geophysical models of the internal structure of the lithospheric mantle of the Siberian Craton

Based on the simultaneous inversion of unique ultralong-range seismic profiles Craton, Kimberlite, Meteorite, and Rift, sourced by peaceful nuclear and chemical explosions, and petrological and geochemical data on the composition of xenoliths of garnet peridotite and fertile primitive mantle material, the first reconstruction was obtained for the thermal state and density of the lithospheric mantle of the Siberian craton at depths of 100–300 km accounting for the effects of phase transformation, anharmonicity, and anelasticity. The upper mantle beneath Siberia is characterized by significant variations in seismic velocities, relief of seismic boundaries, degree of layering, and distribution of temperature and density. The mapping of the present-day lateral and vertical variations in the thermal state of the mantle showed that temperatures in the central part of the craton at depths of 100–200 km are somewhat lower than those at the periphery and 300–400°C lower than the mean temperature of tectonically younger mantle surrounding the craton. The temperature profiles derived from the seismic models lie between the 32.5 and 35 mW/m² conductive geotherms, and the mantle heat flow was estimated as 11–17 mW/m². The depth of the base of the cratonic thermal lithosphere (thermal boundary layer) is close to the 1450 ± 100°C isotherm at 300 ± 30 km, which is consistent with published heat flow, thermobarometry, and seismic tomography data. It was shown that the density distribution in the Siberian cratonic mantle cannot be described by a single homogeneous composition, either depleted or enriched. In addition to thermal anomalies, the mantle density heterogeneities must be related to variations in chemical composition with depth. This implies significant fertilization at depths greater than 180–200 km and is compatible with the existence of chemical stratification in the lithospheric mantle of the craton. In the asthenosphere-lithosphere transition zone, the craton root material is not very different in chemical composition, thermal regime, and density from the underlying asthenosphere. It was shown that minor variations in the chemical composition of the cratonic mantle and position of chemical (petrological) boundaries and the lithosphere-asthenosphere boundary cannot be reliably determined from the interpretation of seismic velocity models only.     

Progress in the Study of Deep Profiles of Tibet and the Himalayas (INDEPTH)

This paper introduces 8 major discoveries and new understandings with regard to the deep structure and tectonics of the Himalayas and Tibetan Plateau obtained in Project INDEPTH, They are mainly as follows. (1) The upper crust, lower crust and mantle lithosphere beneath the blocks of the plateau form a "sandwich" structure with a relatively rigid-brittle upper crust, a visco-plastic lower crust and a relatively rigid-ductile mantle lithosphere. This structure is completely different from that of monotonous, cold and more rigid oceanic plates. (2) In the process of north-directed collision-compression of the Indian subcontinent, the upper crust was attached to the foreland in the form of a gigantic foreland accretionary wedge. The interior of the accretionary wedge thickened in such tectonic manners as large-scale thrusting, backthrusting and folding, and magmatic masses and partially molten masses participated in the crustal thickening. Between the upper crust and lower crust lies a large detachment (e.g     

VRANCEA99—the crustal structure beneath the southeastern Carpathians and the Moesian Platform from a seismic refraction profile in Romania

The VRANCEA99 seismic refraction experiment is part of an international and multidisciplinary project to study the intermediate depth earthquakes of the Eastern Carpathians in Romania. As part of the seismic experiment, a 300-km-long refraction profile was recorded between the cities of Bacau and Bucharest, traversing the Vrancea epicentral region in NNE–SSW direction.

The results deduced using forward and inverse ray trace modelling indicate a multi-layered crust. The sedimentary succession comprises two to four seismic layers of variable thickness and with velocities ranging from 2.0 to 5.8 km/s. The seismic basement coincides with a velocity step up to 5.9 km/s. Velocities in the upper crystalline crust are 5.9–6.2 km/s. An intra-crustal discontinuity at 18–31 km divides the crust into an upper and a lower layer. Velocities within the lower crust are 6.7–7.0 km/s. Strong wide-angle PmP reflections indicate the existence of a first-order Moho at a depth of 30 km near the southern end of the line and 41 km near the centre. Constraints on upper mantle seismic velocities (7.9 km/s) are provided by Pn arrival times from two shot points only. Within the upper mantle a low velocity zone is interpreted. Travel times of a PLP reflection define the bottom of this low velocity layer at a depth of 55 km. The velocity beneath this interface must be at least 8.5 km/s.

Geologic interpretation of the seismic data suggests that the Neogene tectonic convergence of the Eastern Carpathians resulted in thin-skinned shortening of the sedimentary cover and in thick-skinned shortening in the crystalline crust. On the autochthonous cover of the Moesian platform several blocks can be recognised which are characterised by different lithological compositions. This could indicate a pre-structuring of the platform at Mesozoic and/or Palaeozoic times with a probable active involvement of the Intramoesian and the Capidava–Ovidiu faults. Especially the Intramoesian fault is clearly recognisable on the refraction line. No clear indications of the important Trotus fault in the north of the profile could be found. In the central part of the seismic line a thinned lower crust and the low velocity zone in the uppermost mantle point to the possibility of crustal delamination and partial melting in the upper mantle.     

Seismological features of island arc crust as inferred from recent seismic expeditions in Japan

Crustal studies within the Japanese islands have provided important constraints on the physical properties and deformation styles of the island arc crust. The upper crust in the Japanese islands has a significant heterogeneity characterized by large velocity variation (5.5–6.1 km/s) and high seismic attenuation (Qp=100–400 for 5–15 Hz). The lateral velocity change sometimes occurs at major tectonic lines. In many cases of recent refraction/wide-angle reflection profiles, a “middle crust” with a velocity of 6.2–6.5 km/s is found in a depth range of 5–15 km. Most shallow microearthquakes are concentrated in the upper/middle crust. The velocity in the lower crust is estimated to be 6.6–7.0 km/s. The lower crust often involves a highly reflective zone with less seismicity, indicating its ductile rheology. The uppermost mantle is characterized by a low Pn velocity of 7.5–7.9 km/s. Several observations on PmP phase indicate that the Moho is not a sharp boundary with a distinct velocity contrast, but forms a transition zone from the upper mantle to the lower crust. Recent seismic reflection experiments revealed ongoing crustal deformations within the Japanese islands. A clear image of crustal delamination obtained for an arc–arc collision zone in central Hokkaido provides an important key for the evolution process from island arc to more felsic continental crust. In northern Honshu, a major fault system with listric geometry, which was formed by Miocene back arc spreading, was successfully mapped down to 12–15 km.     

Decompression and unmixing of crystals included in diamonds from the mantle transition zone

A suite of exceptional mineral inclusions in diamonds from the São Luiz river, Juina province, Brazil, shows a wide range of garnet/majorite mineral compositions co-existing with clinopyroxene; the overall bulk compositions are eclogitic. The inclusions have a wide variety of textural arrangements, but crystallographic data obtained by EBSD shows that each inclusion consists of a single garnet with constant crystallographic orientation whilst clinopyroxene grains have preferred orientation with relation to garnet {110} and <111>. This suggests that the inclusions were originally single phase majoritic garnets, and that they preserve various states of progressive unmixing (exsolution) into lower pressure garnet and clinopyroxene compositions during transport of the host diamonds towards the Earth’s surface. On the basis of high pressure–temperature experimental data some of the original majoritic garnets must have come from depths of 450 km or more, and therefore resided in the transition zone and asthenospheric upper mantle. Particularly extensive re-equilibration of many inclusions took place at depths of ca 180–200 km (probably close to the base of the continental lithosphere). The partially unmixed state of the inclusions provides a unique opportunity for using mineral diffusion data to roughly estimate the rate of transport through the asthenospheric upper mantle, and within error this rate is found to be broadly compatible with expected transport rates by upper mantle convection or plume flow.     

A three-dimensional crustal model of southwest Germany derived from seismic refraction data

From densely covered seismic refraction data obtained in 1978 (Urach experiment) and 1984 (“Schwarzer Zollern-Wald” experiment) and from seismic reflection data and results from previous refraction investigations, a three-dimensional crustal model of southwest Germany was derived. Travel-time and amplitude information of seismic refraction data were interpreted with two-dimensional forward modeling (ray tracing) to calculate two crustal cross sections in southwest Germany. These results fill a gap in the existing data and enabled the construction of a detailed three-dimensional crustal model.While seismically the upper crust is laterally homogeneous (5.9–6.0 km/s) throughout the area, the middle and lower crust show pronounced lateral variations in thickness, velocity, and reflectivity. The Moho is a flat surface at a relatively shallow depth (25–26 km). We classify the middle and lower crust of southwest Germany into two characteristic crustal types. Type I consists of a mid-crustal low-velocity zone (5.4–5.8 km/s) overlying a thick (> 10 km), high-velocity (6.6–6.8 km/s) lower crust. Type II has no prominent mid-crustal low-velocity zone, and a thin (< 10 km), low-velocity (6.3–6.4 km/s) lower crust. The crustal types correlate with the major geologic units exposed in the area: Type I is present beneath the Black Forest, forming the eastern flank of the Rhinegraben and beneath the Swabian Jura, while Type II is present beneath the intervening Triassic sediments. Beneath the South German Molasse Basin, a low-velocity zone is also present in the upper middle-crust. Seismic reflection investigations have shown that the lower crust in southwest Germany comprises a stack of layers of alternating high- and low-velocities. The lateral variation of the reflectivity of this laminated lower crust has been recognized even on refraction data. We found that high-reflectivity of the lower crust correlates to high average velocity (6.7–6.8 km/s) in the lower crust (Type I). Thus, the average velocity of the lower crust in southwest Germany seems to be an indicator of the intensity of its lamination. The uppermost mantle has a velocity of 8.3 km/s in the area and a strong, positive velocity gradient.