Seismic investigations along the European Geotraverse and its surroundings in Central Europe

Since 1975 several high-resolution seismic-refraction and reflection surveys have been carried out in western Germany to investigate the structure of the Earth's crust and uppermost mantle. The investigation culminated in the seismic-refraction survey along the 825 km long central part of the European Geotraverse (EGT) in 1986. This contribution summarizes the main results of the more recent crustal investigations along and around the EGT. The internal crustal structure throughout the area of the Variscides is very complex and changes laterally considerably. Distinct crustal blocks differing in their internal structure can be assigned to geologically defined units of the Variscan and Caledonian orogeny. In spite of local deviations, in general a more or less transparent and low-velocity upper crust contrasts with a highly reflective lower crust. A subdivision of upper and lower crust by a well-defined boundary (Conrad discontinuity) is not always seen. Towards the Alps the average velocity of the lower crust is as low as 6.2 km s^?1, in contrast to the area north of the Swabian Jura where the velocities above Moho vary between 6.8 and 7.2 km s^?1. In Northern Germany, the Elbe line separates the lower crust into two regions with 6.4 km s^?1 average velocity in the south and 6.9 km s^?1 in the north. The total crustal thickness under the Variscan part of Germany is fairly constant between 28 and 30 km, except under the Rhine Graben area with 25–26 km and beneath the central part of the Rhenish Massif where an anomalous crustal thickening to 37 km is observed. Under northern Germany the Moho rises to about 26 km depth and the data indicate at least one fault-like step of 1 km before the crust thickens toward the Ringkobing-Fyn basement high. The synthesis of seismic velocity structure and petrological information from xenolith studies allows us to propose a mafic composition for the deeper levels of the crust and uppermost mantle which may be valid at least for the central part of the Variscan crust along the European Geotraverse in Central Europe.     

The crustal structure beneath the Central Andean forearc and magmatic arc as derived from seismic studies — the PISCO 94 experiment in northern Chile (21°–23°S)

In this study, we present an interpretation of seismic refraction profiles from the PISCO 94 experiment in northern Chile. As the PISCO experiment was a combined active and passive seismological study, we also discuss results of the passive part in the context of the seismic refraction model. Previous seismic refraction and gravimetric studies indicate a maximum crustal thickness of about 70 km beneath the Pre- and Western Cordillera. The new seismic refraction data lead to a differentiated image of the Andean crust which shows strong varying characteristics. The crustal discontinuities (up to five are detected) dip from W to E. The upper crust has a thickness of 18 km (Precordillera) to 23 km (magmatic arc) underlain by the recent middle crust down to 35–45 km where the velocity increases to about 7 km/s at its base. This crustal level is interpreted as old continental lower crust and its base as blurred continental (paleo) Moho. Beneath the Precordillera, a strong discontinuity at 70 km depth with a velocity increase to about 8 km/s was detected, interpreted as the recent geophysical Moho. For the magmatic arc, this deep discontinuity could not be found by active seismic measurements. The tomographic models of the seismological studies, in general, confirm the seismic refraction results. Anomalously high v_p/v_s ratios in the deeper part of the forearc indicate a hydrated mantle wedge consisting of serpentine and amphibole-bearing peridotite and the 70 km discontinuity is interpreted as the boundary between these two different stages of the hydrated mantle wedge. A zone of high attenuation (Qp) and high v_p/v_s ratios beneath the magmatic arc coincides with the low velocity zones and indicates partially molten rocks from a depth of 20 km down to the asthenospheric wedge.     

Estimation of crustal discontinuities from reflected seismic waves recorded at Shillong and Mikir Hills Plateau,Northeast India

In this study, an attempt is made to determine seismic velocity structure of the crust and upper mantle beneath the Shillong-Mikir Hills Plateau in northeast India region. The principle of the technique is to relate seismic travel times with crustal thickness above the Conrad and Moho discontinuities. Broadband digital waveforms of the local earthquakes make a precise detection of the seismic phases possible that are reflected at these discontinuities. The results show that the Conrad discontinuity is at 18–20 (±0.5) km beneath the Shillong-Mikir Hills Plateau and the Moho discontinuity is at 30 ± 1.0 km beneath the Shillong Plateau and at 35 ± 1.0 km beneath the Mikir Hills.     

Evolution of the petrological and seismic Moho-implications for the continental crust-mantle boundary

The chemical and petrological composition of mafic rocks from the lower continental crust are discussed by comparing mafic granulites and meta-gabbroic rocks from the Ivrea Zone and the Northern Hessian Depression (NHD) xenolith suite. Both regions contain contrasting types of meta-mafic lithologies (i) former basaltic rocks with trace element patterns ranging from MORB-Iike to subduction-related or intra-plate tholeütes and (ü) Ca-and Al-enriched, plagiodase-dominated gabbroic rocks showing positive Eu-anomalies generated by complex deep crustal magmatic processes such as fractionation, accumulation of plagiodase and pyroxene, and crustal contamination. The absence of typical garnet-omphadte parageneses in these rocks indicates that the eclogite stability field was not reached during Palaeozoic orogenic processes. A compilation of experimentally determined P-wave velocities and densities for mafic granulites, gabbroic rocks, eclogites and peridotites is used to evaluate key physical properties of lower crustal mafic rocks during crystal thickening caused by continent-continent collision. In a step-by-step scenario it is demonstrated that the position of the seismic Moho (defined as a first-order velocity discontinuity) and the petrological Moho (defined as the boundary between non-peridotitic crustal rocks and olivine-dominated rocks) is not identical for the case that mafic rocks are transformed into edogites at the base of orogenically thickened crust. P-wave velocities of eclogites largely overlap with those of peridotites, although their densities are significantly higher than common upper mantle rocks. As a consequence, refraction seismic field studies may not detect edogites as crustal rocks. This means that the seismic Moho detected by refraction seismic field studies appears at the upper boundary between edogites and overlying crustal units. Since edogites generally have higher densities than peridotites, they might be recycled into the deeper lithosphere thereby transferring excess Eu into the upper mantle. This process could be a due for understanding the negative Euanomaly in the upper continental crust which is apparently not balanced quantitatively by the abundance of common mafic crustal rocks.     

Lower crustal xenoliths from Junan,Shandong province and their bearing on the nature of the lower crust beneath the North China Craton

Geochronological, petrological and geochemical studies were performed on the granulite xenoliths from a Late Cretaceous basaltic breccia dike in Junan, Shandong province, eastern China. These xenoliths show close similarities to the Nushan granulite xenoliths from the southern margin of the North China Craton (NCC) and the Archean granulite terrains in terms of mineralogy and bulk rock compositions, but are quite different from the Hanuoba mafic granulite xenoliths from the northern NCC. In-situ zircon U–Pb age and Hf isotopic analyses, together with geochemical data reveal that the protolith of these xenoliths was formed around 2.3 Ga ago, through assimilation–fractional crystallization of a mafic magma. P–T conditions of these xenoliths suggest that the lower crust beneath the Junan region reaches to a depth of 35 km, which agree well with the result deduced from various geophysical methods. The consistent petrological and seismic Moho depths, the observed velocity structure and calculated velocity of these xenoliths imply the absence of underplating induced crust–mantle transition zone, which was well formed in the northern NCC. Compared to 40–50 km depth of the lower crust in Early Jurassic, the lower crust beneath Junan extended to a depth of 30 km in Late Cretaceous, suggesting that the lower crust of NCC was significantly thinned during Late Mesozoic.     

Crustal complexity in the Lachlan Orogen revealed from teleseismic receiver functions

There is an ongoing debate about the tectonic evolution of southeast Australia, particularly about the causes and nature of its accretion to a much older Precambrian core to the west. Seismic imaging of the crust can provide useful clues to address this issue. Seismic tomography imaging is a powerful tool often employed to map elastic properties of the Earth's lithosphere, but in most cases does not constrain well the depth of discontinuities such as the Mohorovi?i? (Moho). In this study, an alternative imaging technique known as receiver function (RF) has been employed for seismic stations near Canberra in the Lachlan Orogen to investigate: (i) the shear-wave-velocity profile in the crust and uppermost mantle, (ii) variations in the Moho depth beneath the Lachlan Orogen, and (iii) the nature of the transition between the crust and mantle. A number of styles of RF analyses were conducted: H-K stacking to obtain the best compressional–shear velocity (V _P /V _S) ratio and crustal thickness; nonlinear inversion for the shear-wave-velocity structure and inversion of the observed variations in RFs with back-azimuth to investigate potential dipping of the crustal layers and anisotropy. The thick crust (up to 48 km) and the mostly intermediate nature of the crust?mantle transition in the Lachlan Orogen could be due to the presence of underplating at the base of the crust, and possibly to the existing thick piles of Ordovician mafic rocks present in the mid and lower crust. Results from numerical modelling of RFs at three seismic stations (CAN, CNB and YNG) suggest that the observed variations with back-azimuth could be related to a complex structure beneath these stations with the likelihood of both a dipping Moho and crustal anisotropy. Our analysis reveals crustal thickening to the west beneath CAN station which could be due to slab convergence. The crustal thickening may also be related to the broad Macquarie volcanic arc, which is rooted to the Moho. The crustal anisotropy may arise from a strong N–S structural trend in the eastern Lachlan Orogen and to the preferred crystallographic orientation of seismically anisotropic minerals in the lower and middle crust related to the paleo-Pacific plate convergence.     

Structure of the crust and upper mantle beneath the central european rift system

The crustal structure of the central European rift system has been investigated by seismic methods with varying success. Only a few investigations deal with the upper-mantle structure. Beneath the Rhinegraben the Moho is elevated, with a minimum depth of 25 km. Below the flanks it is a first-order discontinuity, while within the graben it is replaced by a transition zone with the strongest velocity gradient at 20–22 km depth. An anomalously high velocity of up to 8.6 km/s seems to exist within the underlying upper mantle at 40–50 km depth. A similar structure is also found beneath the Limagnegraben and the young volcanic zones within the Massif Central of France, but the velocity within the upper mantle at 40–50 km depth seems to be slightly lower. Here, the total crustal thickness reaches only 25 km. The crystalline crust becomes extremely thin beneath the southern Rhônegraben, where the sediments reach a thickness of about 10 km while the Moho is found at 24 km depth. The pronounced crustal thinning does not continue along the entire graben system. North of the Rhinegraben in particular the typical graben structure is interrupted by the Rhenohercynian zone with a “normal” West-European crust of 30 km thickness evident beneath the north-trending Hessische Senke. A single-ended profile again indicates a graben-like crustal structure west of the Leinegraben north of the Rhenohercynian zone. No details are available for the North German Plain where the central European rift system disappears beneath a sedimentary sequence of more than 10 km thickness.     

Evidence of a partial melt zone beneath Stromboli volcano (Italy) from inversion of teleseismic receiver functions

Teleseismic body waves from broadband seismic stations are used to investigate the crustal and uppermost mantle structure of Stromboli volcano through inversion of the receiver functions (RFs). First, we computed RFs in the frequency domain using a multiple-taper spectral correlation technique. Then, the non-linear neighbourhood algorithm was applied to estimate the seismic shear wave velocity of the crust and uppermost mantle and to define the main seismic velocity discontinuities. The stability of the inversion solution was tested using a range of initial random seeds and model parameterizations. A shallow Moho, present at depth of 14.8 km, is evidence of a thinned crust beneath Stromboli volcano. However, the most intriguing and innovative result is a low S velocity layer in the uppermost mantle, below 32 km. The low S velocity layer suggests a possible partial melt region associated with the volcanism, as also recently supported by tomographic studies and petrological estimations.     

Origin of high-velocity anomalies beneath the Siberian craton: A fingerprint of multistage magma underplating since the Neoarchean

Despite the violent eruption of the Siberian Traps at ~ 250 Ma, the Siberian craton has an extremely low heat flow (18–25 mW/m²) and a very thick lithosphere (300–350 km), which makes it an ideal place to study the influence of mantle plumes on the long-term stability of cratons. Compared with seismic velocities of rocks, the lower crust of the Siberian craton is composed mainly of mafic granulites and could be rather heterogeneous in composition. The very high V_p (> 7.2 km/s) in the lowermost crust can be fit by a mixture of garnet granulites, two-pyroxene granulites, and garnet gabbro due to magma underplating. The high-velocity anomaly in the upper mantle (V_p = 8.3-8.6 km/s) can be interpreted by a mixture of eclogites and garnet peridotites. Combined with the study of lower crustal and mantle xenoliths, we recognized multistage magma underplating at the crust-mantle boundary beneath the Siberian craton, including the Neoarchean growth and Paleoproterozoic assembly of the Siberian craton beneath the Markha terrane, the Proterozoic collision along the Sayan-Taimyr suture zone, and the Triassic Siberian Trap event beneath the central Tunguska basin. The Moho becomes a metamorphism boundary of mafic rocks between granulite facies and eclogite facies rather than a chemical boundary that separates the mafic lower crust from the ultramafic upper mantle. Therefore, multistage magma underplating since the Neoarchean will result in a seismic Moho shallower than the petrologic Moho. Such magmatism-induced compositional change and dehydration will increase viscosity of the lithospheric mantle, and finally trigger lithospheric thickening after mantle plume activity. Hence, mantle plumes are not the key factor for craton destruction.     

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.     

Crustal density structure in the Spanish Central System derived from gravity data analysis (Central Spain)

Shallow and deep sources generate a gravity low in the central Iberian Peninsula. Long-wavelength shallow sources are two continental sedimentary basins, the Duero and the Tajo Basins, separated by a narrow mountainous chain called the Spanish Central System. To investigate the crustal density structure, a multitaper spectral analysis of gravity data was applied. To minimise biases due to misleading shallow and deep anomaly sources of similar wavelength, first an estimation of gravity anomaly due to Cenozoic sedimentary infill was made. Power spectral analysis indicates two crustal discontinuities at mean depths of 31.1 ± 3.6 and 11.6 ± 0.2 km, respectively. Comparisons with seismic data reveal that the shallow density discontinuity is related to the upper crust lower limit and the deeper source corresponds to the Moho discontinuity. A 3D-depth model for the Moho was obtained by inverse modelling of regional gravity anomalies in the Fourier domain. The Moho depth varies between a mean depth of 31 km and 34 km. Maximum depth is located in a NW–SE trough. Gravity modelling points to lateral density variations in the upper crust. The Central System structure is described as a crustal block uplifted by NE–SW reverse faults. The formation of the system involves displacement along an intracrustal detachment in the middle crust. This detachment would split into several high-angle reverse faults verging both NW and SE. The direction of transport is northwards, the detachment probably being rooted at the Moho.     

Role of lithosphere in intra-continental deformation: Central Australia

Since the Proterozoic, there has been a set of deformation cycles in central Australia culminating in the Alice Springs Orogeny around 400 Ma. These events occurred away from plate boundaries and involved extension as well as compression, although their precise history remains difficult to unravel from the geologic record. Much evidence of deformation is left in the central Australian crust, which features significant Moho topography and an associated gravity signal. In the past, several mechanical models invoked crustal thickening and considerable compression to explain these geophysical characteristics. However, it is hard to envisage extensive deformation affecting the crust alone, but leaving no deformation record in the sub-crustal lithosphere. In recent seismic tomography studies, there is continuous seismically-fast lithosphere in central Australia below depths of about 100 km. In this region, the uppermost lithospheric mantle is seismically slow, but exhibits no significant attenuation of seismic waves. These new constraints make simple crustal thickening unlikely to be the main mechanism to generate variations of the Moho depth in central Australia. Here we propose a mechanical model of deformation that involves the entire lithosphere. We make no strong assumptions about the history of deformation cycles. Our model does not require lithospheric thickening at any stage of the deformation cycle, and results in a present-day scenario compatible with shallow as well as deep constraints on the lithosphere structure.     

Moho topography and lower crustal wide-angle reflectivity around the TESZ in southern Scandinavia and northeastern Europe

The Moho topography is strongly undulating in southern Scandinavia and northeastern Europe. A map of the depth to Moho shows similarities between the areas of the Teisseyre–Tornquist Zone (TTZ) in Poland and the Fennoscandian Border Zone (FBZ), which is partly coinciding with the Sorgenfrei–Tornquist Zone (STZ) in Denmark. The Moho is steeply dipping at these zones from a crustal thickness of approximately 32 km in the young Palaeozoic Platform and basin areas to approximately 45 km in the old Precambrian Platform and Baltic Shield. The Moho reflectivity (P_MP waveform) in the POLONAISE'97 refraction/wide-angle seismic data from Poland and Lithuania is variable, ranging from ‘sharp’ to strongly reverberating signals of up to 2 s duration. There is little or no lower crustal wide-angle reflectivity in the thick Precambrian Platform, whereas lower crustal reflectivity in the thin Palaeozoic Platform is strongly reverberating, suggesting that the reflective lower crust and upper mantle is a young phenomena. From stochastic reflectivity modelling, we conclude that alternating high- and low-velocity layers with average thicknesses of 50–300 m and P-wave velocity variations of ±3–4% of the background velocity can explain the lower crustal reflectivity. Sedimentary layering affects the reflectivity of deeper layers significantly and must be considered in reflectivity studies, although the reverberations from the deeper crust cannot be explained by the sedimentary layering only. The reflective lower crust and upper mantle may correspond to a zone that has been intruded by mafic melts from the mantle during crustal extension and volcanism.     

Crustal structure beneath the east side of Pearl River Estuary from onshore-offshore seismic experiment

ABSTRACT

The land-sea transition zone in the northern South China Sea (SCS) records important information from the continental rifting to the seafloor spreading. The crustal structure is the key to explore the deep tectonic environment and the evolution of the SCS. In 2015, the onshore-offshore 3D deep seismic experiment was carried out on the Pearl River Estuary (PRE). Explosions and air guns were used as sources on land and at sea respectively in this experiment.Onshore seismic stations and Ocean Bottom Seismographs (OBSs) synchronously recorded the seismic signals. We focus on an onshore-offshore seismic profile (L2, SE-trending) along the eastern side of the PRE. By modelling the seismic travel times, we constructed a P-wave velocity model along the profile. The model shows that the sediment on land is thin and has seismic velocities of 4.5–5.5 km/s. In contrast, thickness of the offshore sediment gradually increases to more than 4.0 km, and the velocities vary between 2.0 km/s and 4.5 km/s. The onshore and offshore crustal velocities are 5.8–6.8 km/s and 5.5–6.8 km/s, respectively. At depth between 15 km and 20 km, a low-velocity layer (LVL; only 5.9 km/s) is detected, pinching out under the Littoral Fault Zone (LFZ). The LVL has probably accommodated the crustal extension beneath the land area, resulting in low extent of the crustal thinning. A slightly uplifted Moho exists beneath the Dongguan fault depression zone, representing a place where hot mantle materials ascend. Localized thickening of the sediments and rapid thinning of the crust characterize the LFZ, and it can be regarded as a tectonic boundary between the South China (SC) with normal continental crust and the northern SCS margin with extended continental crust.     

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.     

P-wave velocity features of the lithosphere under the Eromanga Basin, Eastern Australia, including a prominent MID-crustal (Conrad?) discontinuity

The crustal structure of the central Eromanga Basin in the northern part of the Australian Tasman Geosyncline, revealed by coincident seismic reflection and refraction shooting, contrasts with some neighbouring regions of the continent. The depth to the crust-mantle boundary (Moho) of 36–41 km is much less than that under the North Australian Craton to the northwest (50–55 km) and the Lachlan Fold Belt to the southeast (43–51 km) but is similar to that under the Drummond and Bowen Basins to the east.The seismic velocity boundaries within the crust are sharp compared with the transitional nature of the boundaries under the North Australian and Lachlan provinces. In particular, there is a sharp velocity increase at mid-crustal depths (21–24 km) which has not been observed with such clarity elsewhere in Australia (the Conrad discontinuity?).In the lower crust, the many discontinuous sub-horizontal reflections are in marked contrast to lack of reflecting horizons in the upper crust, further emphasising the differences between the upper and lower crust. The crust-mantle boundary (Moho) is characterised by an increase in velocity from 7.1–7.7 km/s to a value of 8.15 + 0.04 km/s. The depth to the Moho under the Canaway Ridge, a prominent basement high, is shallower by about 5 km than the regional Moho depth; there is also no mid-crustal horizon under the Canaway Ridge but there is a very sharp velocity increase at the Moho depth of 34 km. The Ridge could be interpreted as a horst structure extending to at least Moho depths but it could also have a different intra-crustal structure from the surrounding area.The sub-crustal lithosphere has features which have been interpreted, from limited data, as being caused by a velocity gradient at 56–57 km depth with a low velocity zone above it.Because of the contrasting crustal thicknesses and velocity gradients, the lithosphere of the central Eromanga Basin cannot be considered as an extension of the exposed Lachlan Fold Belt or the North Australian Craton. The lack of seismic reflections from the upper crust indicates no coherent accoustic impedance pattern at wavelengths greater than 100 m, consistent with an upper crustal basement of tightly folded meta-sedimentary and meta-volcanic rocks. The crustal structure is consistent with a pericratonic or arc/back-arc basin being cratonised in an episode of convergent tectonics in the Early Palaeozoic. The seismic reflections from the lower crust indicate that it could have developed in a different tectonic environment.     

Seismic evidence of Caledonian deformed crust and uppermost mantle structures in the northern part of the Trans-European Suture Zone, SW Baltic Sea

Collisional structures from the closure of the Tornquist Ocean and subsequent amalgamation of Avalonia and Baltica during the Caledonian Orogeny in the northern part of the Trans-European Suture Zone (TESZ) in the SW Baltic Sea are investigated. A grid of marine reflection seismic lines was gathered in 1996 during the DEKORP-BASIN '96 campaign, shooting with an airgun array of 52 l total volume and recording with a digital streamer of up to 2.1 km length. The detailed reflection seismic analysis is mainly based on post-stack migrated sections of this survey, but one profile has also been processed by a pre-stack depth migration algorithm. The data provides well-constrained images of upper crustal reflectivity and lower crustal/uppermost mantle reflections. In the area of the Caledonian suture, a reflection pattern is observed with opposing dips in the upper crust and the uppermost mantle. Detailed analysis of dipping reflections in the upper crust provides evidence for two different sets of reflections, which are separated by the O-horizon, the main decollement of the Caledonian deformation complex. S-dipping reflections beneath the sub-Permian discontinuity and above the O-horizon are interpreted as Caledonian thrust structures. Beneath the O-horizon, SW-dipping reflections in the upper crust are interpreted as ductile shear zones and crustal deformation features that evolved during the Sveconorwegian Orogeny. The Caledonian deformation complex is subdivided into (1) S-dipping foreland thrusts in the north, (2) the S-dipping suture itself that shows increased reflectivity, and (3) apparently NE-dipping downfaulted sedimentary horizons south of the Avalonia–Baltica suture, which may have been reactivated during Mesozoic normal faulting. The reflection Moho at 28–35 km depth appears to truncate a N-dipping mantle structure, which may represent remnant structures from Tornquist Ocean closure or late-collisional compressional shear planes in the upper mantle. A contour map of these mantle reflections indicates a consistent northward dip, which is steepest where there is strong bending of the Caledonian deformation front. The thin-skinned character of the Caledonian deformation complex and the fact that N-dipping mantle reflections do not truncate the Moho indicate that the Baltica crust was not mechanically involved in the Caledonian collision and, therefore, escaped deformation in this area.     

Lithospheric petrology beneath the northern part of the Arabian Plate in Syria: evidence from xenoliths in alkali basalts

A petrological model for the upper mantle and lower crust under the northern part of the Arabian Plate (Syria) has been derived on the basis of petrology of upper mantle and lower crustal xenoliths occurring in the Neogene to Quaternary alkali basalts of the Shamah volcanic fields. The xenolith suite has been classified by texture mineralogy and chemistry into the following groups: (1) Type I metasomatised and dry Cr diopside xenoliths with protogranular to porphyroclastic textures; (2) Type II Al augite spinal and garnet pyroxenite and websterite which have igneous and/or porphyroclastic textures and abundant phlogopite and/or amphibole; (3) Cr-poor megacrysts; and (4) mafic lower crustal xenoliths. Estimates of Type I xenolith temperatures are 990–1070°C with pressure between 13 and 19 kbar. Type II xenoliths yield temperatures of 930–1150°C and pressures in the range 12—13 kbar. The lower crustal xenolith mineral assemblages and geothermometry based on coexisting minerals suggest equilibration conditions between 6 and 8 kbar and 820–905°C. Mantle plumes, which may be the source of the volatile flux, have implications for melt generation in the Arabian basalt provinces. It is estimated that the lithosphere beneath the Arabian Plate is less than 80 km thick. Xenolith data and geophysical studies indicate that the Moho is located at a depth of 40–37 km and that the crust-mantle transition zone has a thickness of 8–5 km and occurs at a depth of 27–30 km. The boundary between an upper granitic crust and a lower mafic crust occurs at a depth of 19 km. Type I dry xenoliths show a low overall concentration of REE (La/Yb =1–2 and Sm = 0.7–1.1 times chondrite), whereas Type I hydrous xenoliths are LREE enriched (La/Yb=6–9 and Sm=1.1–1.3 times chondrite). Type II xenoliths show high overall LREE enrichment. Petrological and geochemical data for the lower crustal xenoliths indicate that these xenoliths represent basaltic cumulates crystallised at lower crustal pressures.     

Lower-crust S-wave velocity beneath western Yunnan Province from waveform inversion of dense seismic observations

We use teleseismic body waveforms to explore S-wave layered velocity structures beneath 30 portable digital seismic stations deployed around western Yunnan Province. Results show that the Moho depth in this region is ∼40 km and decreases in general from north to south, consistent with previous geophysical studies. Associated with this lateral variation of the Moho depth, the lower crust above the Moho discontinuity has a 15–25 km thick zone with an S-wave velocity lower than that of the upper crust. This lower velocity zone might be interpreted as a lower crust weak channel, which may mechanically partially decouple the upper-crust deformation from the underlying mantle. Thus, the inverted S-wave velocity structure could provide new evidence for the lateral flow of lower crust in the build-up of the south-eastern Tibetan plateau.     

Crustal structure of the Kaapvaal craton and its significance for early crustal evolution

High-quality seismic data obtained from a dense broadband array near Kimberley, South Africa, exhibit crustal reverberations of remarkable clarity that provide well-resolved constraints on the structure of the lowermost crust and Moho. Receiver function analysis of Moho conversions and crustal multiples beneath the Kimberley array shows that the crust is 35 km thick with an average Poisson's ratio of 0.25. The density contrast across the Moho is 15%, indicating a crustal density about 2.86 gm/cc just above the Moho, appropriate for felsic to intermediate rock compositions. Analysis of waveform broadening of the crustal reverberation phases suggests that the Moho transition can be no more than 0.5 km thick and the total variation in crustal thickness over the 2400 km² footprint of the array no more than 1 km. Waveform and travel time analysis of a large earthquake triggered by deep gold mining operations (the Welkom mine event) some 200 km away from the array yield an average crustal thickness of 35 km along the propagation path between the Kimberley array and the event. P- and S-wave velocities for the lowermost crust are modeled to be 6.75 and 3.90 km/s, respectively, with uppermost mantle velocities of 8.2 and 4.79 km/s, respectively. Seismograms from the Welkom event exhibit theoretically predicted but rarely observed crustal reverberation phases that involve reflection or conversion at the Moho. Correlation between observed and synthetic waveforms and phase amplitudes of the Moho reverberations suggests that the crust along the propagation path between source and receiver is highly uniform in both thickness and average seismic velocity and that the Moho transition zone is everywhere less than about 2 km thick. While the extremely flat Moho, sharp transition zone and low crustal densities beneath the region of study may date from the time of crustal formation, a more geologically plausible interpretation involves extensive crustal melting and ductile flow during the major craton-wide Ventersdorp tectonomagmatic event near the end of Archean time.