Crustal thickness and VP/VS variations in the Grenville orogen (Ontario, Canada) from analysis of teleseismic receiver functions

We have developed a simple semblance-weighted stacking technique to estimate crustal thickness and average V_P/V_S ratio using teleseismic receiver functions. We have applied our method to data from 32 broadband seismograph stations that cover a 700 × 400 km² region of the Grenville orogen, a 1.2–0.98 Ga Himalayan-scale collisional belt in eastern North America. Our seismograph network partly overlaps with Lithoprobe and other crustal refraction surveys. In 8 out of 9 cases where a crustal-refraction profile passes within 30 km of a seismograph station, the two independent crustal thickness estimates agree to within 7%. Our regional crustal-thickness model, constructed using both teleseismic and refraction observations, ranges between 34.0 and 52.4 km. Crustal-thickness trends show a strong correlation with geological belts, but do not correlate with surface topography and are far in excess of relief required to maintain local isostatic equilibrium. The thickest crust (52.4 ± 1.7 km) was found at a station located within the 1.1 Ga mid-continent (failed) rift. The Central Gneiss Belt, which contains rocks exhumed from deep levels of the crust, is characterized by V_P/V_S ranging from 1.78 to 1.85. In other parts of the Grenville orogen, V_P/V_S is found to be generally less than 1.80. The thinnest crust (34.5–37.0 km) occurs northeast of the 0.7 Ga Ottawa–Bonnechere graben and correlates with areas of high intraplate seismicity.     

Seismic velocities of granulite-facies xenoliths from Central Ireland: Implications for lower crustal composition and anisotropy

V_p and V_s values have been measured experimentally and calculated for granulite-facies lower crustal xenoliths from central Ireland close to the Caledonian Iapetus suture zone. The xenoliths are predominantly foliated and lineated metapelitic (garnet–sillimanite–K-feldspar) granulites. Their metapelitic composition is unusual compared with the mostly mafic composition of lower crustal xenoliths world-wide. Based on thermobarometry, the metapelitic xenoliths were entrained from depths of c. 20–25 ± 3.5 km and rare mafic granulites from depths of 31–33 ± 3.4 km. The xenoliths were emplaced during Lower Carboniferous volcanism and are considered to represent samples of the present day lower crust.V_p values for the metapelitic granulites range between 6.26 and 7.99 km s^{− 1} with a mean value of 7.09 ± 0.4 km s^{− 1}. Psammite and granitic orthogneiss samples have calculated V_p values of 6.51 and 6.23 km s^{− 1}, respectively. V_s values for the metapelites are between 3.86 and 4.34 km s^{− 1}, with a mean value of 4.1 ± 0.15 km s^{− 1}. The psammite and orthogneiss have calculated V_s values of 3.95 and 3.97 km s^{− 1}, respectively.The measured seismic velocities correlate with density and with modal mineralogy, especially the high content of sillimanite and garnet. V_p anisotropy is between 0.15% and 13.97%, and a clear compositional control is evident, mainly in relation to sillimanite abundance. Overall V_s anisotropy ranges from 1% to 11%. Poisson's ratio (σ) lies between 0.25 and 0.35 for the metapelitic granulites, mainly reflecting a high V_p value due to abundant sillimanite in the sample with the highest σ. Anisotropy is probably a function of deformation associated with the closure of the Iapetus ocean in the Silurian as well as later extension in the Devonian. The orientation of the bulk strain ellipsoid in the lower crust is difficult to constrain, but lineation is likely to be NE–SW, given the strike-slip nature of the late Caledonian and subsequent Acadian deformation.When corrected for present-day lower crustal temperature, the experimentally determined V_p values correspond well with velocities from the ICSSP, COOLE I and VARNET seismic refraction lines. Near the xenolith localities, the COOLE I line displays two lower crustal layers with in situ V_p values of 6.85–6.9 and 6.9–8.0 km s^{− 1}, respectively. The upper (lower velocity) layer corresponds well with the metapelitic granulite xenoliths while the lower (higher velocity) layer matches that of the basic granulite xenoliths, though their metamorphic pressures suggest derivation from depths corresponding to the present-day upper mantle.     

Crustal structure below the Polish Basin: Is it composed of proximal terranes derived from Baltica?

The large-scale seismic refraction and wide-angle reflection experiment POLONAISE'97 together with LT-7 and TTZ profiles carried out with the most modern techniques gave a high resolution of crustal structure of the Trans-European Suture Zone (TESZ) in NW and central Poland. The results of seismic investigations show the presence of relatively low velocity rocks (V_p < 6.1 km/s) down to a depth of 20 km beneath the Polish Basin (PB), and a high velocity lower crust (V_p = 6.8–7.3 km/s). The crustal thickness in the TESZ is intermediate between that of the East European Craton (EEC) to the northeast (40–45 km) and that of the Variscan crust (VB) to the southwest ( 30 km). Velocities in the uppermost mantle are relatively high (V_p = 8.25–8.45 km/s). The crust is three-layered with substantial differences in the velocities and thickness of individual layers. The area of the TESZ in NW and central Poland can be divided into at least two crustal blocks (terranes), called here Pomeranian Unit (PU, in the northwest) and Kuiavian Unit (KU, in the southeast). The postulated boundary between KU and PU is rather sharp at particular levels of the crust. Velocity distribution in the middle and lower crystalline crust in the TESZ area resemble values recognized in the EEC area, the fundamental difference being the much smaller thickness of both these layers. Our hypothesis/speculation is that the attenuated lower and middle crust of the TESZ belong to proximal terranes built of the EEC crust detached in the southeast and re-accreted to the EEC due to the process of anti-clockwise rotation of the Baltica paleocontinent during the Ordovician–Early Silurian.     

Crustal structure in the Changbaishan volcanic area, China, determined by modeling receiver functions

During 1998–1999, we installed a temporary broadband seismic network in the Changbaishan volcanic region, NE China. We estimated crustal structure using teleseismic seismograms collected at the network. We detected a near surface region of strong anisotropy directly under the main volcanic edifice of the volcanic area. We modeled 109 receiver functions from 19 broadband stations using three techniques. First we used a “slant-stacking” method to model the principal crustal P reverberation phases to estimate crustal thickness and the average crustal P to S speed ratio (v_p/v_s), assuming an average P-wave velocity in the crust. We then estimated crustal S-wave velocity (v_s) and v_p/v_s profiles by modeling stacked receiver functions using a direct search. Finally, we inverted several receiver functions recorded at stations closest to the main volcanic edifice using least squares to estimate v_s velocity profiles, assuming a v_p/v_s value. The results from the three estimation techniques were consistent, and generally we found that the receiver functions constrained estimates of changes in wave speeds better than absolute values. We resolved that the crust is 30–39 km thick under the volcanic region and 28–32 km thick away from the volcanic region, with a midcrust velocity transition at about 10–15 km depth. We estimated that the average crust P-wave velocity is about 6.0–6.2 km/s surrounding the main volcanic region, while it is slightly lower in the vicinity of the main volcanic edifice. The estimates of v_p/v_s were more ambiguous, but we inferred that the bulk crustal Poisson's ratio (which is related to v_p/v_s) ranges between 0.20 and 0.30, with a suggestion that the Poisson's ratio is lower under the central volcanic region compared to the surrounding areas. We resolved low S-wave velocities (down to about 3 km/s) in the middle crust in the region of the main volcanic edifice. The low velocity anomaly extends from about 5–10 to 15–25 km below the surface, probably indicating a region of elevated temperatures. We were unable to determine if partial melt is present with the data we considered in this paper.     

江南造山带中段地壳结构与成矿启示：基于广昌-浏阳剖面接收函数的认识

江南造山带位于华南大陆扬子块体和华夏块体之间，其深部地壳结构与变形特征记录了扬子块体与华夏块体拼合与相互作用的痕迹，且在其内部与邻区发育了丰富的多金属矿床，并形成了巨型Cu-Au-Pb-Zn-Ag多金属成矿带，是深化认识华南大陆地壳演化、岩浆作用与成矿系统的关键地域。针对华南大陆地区的地壳结构与成矿过程，国家科技重点研发计划“华南陆内成矿系统的深部过程与物质响应”项目在该区实施了一条密集宽频带地震流动探测剖面，旨在探测其深部结构与物性变化特征和深部成矿背景。本文利用其中江西广昌-湖南浏阳段长320km的宽频带地震流动台站数据开展了远震P波接收函数研究，获得了剖面辖区深部地壳结构和V_p/V_s变化特征。研究结果表明：(1)剖面Moho界面深度在29～35km之间变化，呈近穹窿状分布，平均Moho界面深度为31km左右，低于全球大陆地壳平均值，且与地形高程在整体上呈镜像相关，均衡程度较好；(2)剖面沿线地壳V_p/V_s在1.64～1.83之间呈波浪状起伏变化，平均值为1.72左右，且华夏块体略高于江南造山带...     

Explosion seismic P and S velocity and attenuation constraints on the lower crust of the North–Central Tibetan Plateau, and comparison with the Tethyan Himalayas: Implications on composition, mineralogy, temperature, and tectonic evolution

P and S velocity and attenuation estimates in the lower crust are obtained from a set of wide angle reflection–refraction profiles in the region of active tectonics at the NE edge of the Tibetan Plateau and discussed together with respect to similar data at its Himalaya–south Tibet edge.The quality factor is estimated in the lower half of the crust by accounting for the differential effect on amplitude–frequency observed between waves of different penetrations, and both in P and S modes. Attenuation values allow to exclude a significant proportion of partial melt and to estimate the homologous temperature, ratio of in situ to solidus absolute temperatures. The latter depend on the physical conditions being of dry, wet or dehydration melting, which are found different among the regions of the northern Bayan Har and northern Qang Tang boundaries between blocks, as well as the Tethyan–Himalayas, south of the Indus–Tsangpo suture. Their in situ temperatures differ also as estimated from their different V_p for a similar felsic composition.Joint measurement of several parameters, V_p, V_s, Q_p and Q_s reveals the composition, mineralogy, temperature and hydration conditions of the lower half of the thickened crust of Tibet that may be discussed in terms of evolution. The material presently in the thickened crust, even its lower part, has a felsic composition, upper to middle crustal lithology, and the temperature conditions estimated suggest that basic material that could have underlain it could be eclogitized and not appear anymore above the seismic Moho.Under northern Qang Tang, the felsic material in the lower half of the crust appears as hot and dry. Its burial may have occurred earlier or may have been moderate in the postcollisional phase. This is consistent with a model of indentation of the Qang Tang crust by an originally thinner Bayan Har crust to bring part of its crust to greater depth, suggested from imaging the crustal architecture. Under northern Bayan Har, the material in the lower half of the crust appears as felsic, at low temperature and not dry conditions. This is evidence that it has been transported from a shallower depth, and this recently enough not to be yet dehydrated and temperature equilibrated in a conductive geotherm. It supports a model of recent overriding of the middle crust of the north Kun Lun block to the north independently suggested from the image of crustal architecture. The Tethyan Himalayas case appears bracketed by these two cases in northern Tibet for V_p and temperature conditions, but shows highest attenuation in the lower crust that is colder but less dry than under northern Qang Tang.     

Crustal transect from the North Atlantic Knipovich Ridge to the Svalbard Margin west of Hornsund

The crustal structure along a 312 km transect, stretching from the axial mountains of the North Atlantic Knipovich Ridge to the continental shelf of Svalbard, has been obtained using seismic reflection data and wide angle OBS data. The resulting seismic V_p and V_s models are further constrained by a 2-D-gravity model. The principal objective of this study is to describe and resolve the physical and compositional properties of the crust in order to understand the processes and creation of oceanic crust in this extremely slow-spreading counterpart of the North Atlantic Ridge Systems. V_p is estimated to be 3.50–6.05 km/s for the upper oceanic crust (oceanic layer 2), with a marked increase away from the ridge. The measured V_p of 6.55–6.95 km/s for oceanic layer 3A and 7.10–7.25 km/s for layer 3B, both with a V_p/V_s ratio of 1.81, except for slightly higher values at the ridge axis, does not allow a clear distinction between gabbro and mantle-derived peridotite (10–40% serpentized). The thickness of the oceanic crust varies a lot along the transect from the minimum of 5.6 km to a maximum of 8.1 km. The mean thickness of 6.7 km for the oceanic crust is well above the average thickness for slow-spreading ridges (<10 mm/year half-spreading rate). The areas of increased thickness could be explained by large magma production-rates found in the zones of axial highs at the ridge axis, which also have generated the off-axial highs adjacent the ridge. We suggest that these axial and off-axial highs along the ridge control the lithological composition of the oceanic crust. This approach suggests normal gabbroic oceanic crust to be found in the areas bound by the active magma segments (the axial and off-axial highs) and mantle-derived peridotite outside these zone.     

Crust–upper mantle seismic velocity structure across Southeastern China

One in-line wide-angle seismic profile was conducted in 1990 in the course of the Southeastern China Continental Dynamics project aimed at the study of the contact between the Cathaysia block and the Yangtze block. This 380-km-long profile extended in NW–SE direction from Tunxi, Anhui Province, to Wenzhou, Zhejiang Province. Five in-line shots were fired and recorded at seismic stations with spacing of about 3 km along the recording line. We have used two-dimensional ray tracing to model P- and S-wave arrivals and provide constraints on the velocity structure of the upper crust, middle crust, lower crust, Moho discontinuity, and the top part of the lithospheric mantle. P-wave velocity, S-wave velocity and V_P/V_S ratio are mapped. The crust is 36-km thick on average, albeit it gradually thins from the northwest end to the southeast end (offshore) of the profile. The average crustal velocity is 6.26 km/s for P-waves but 3.6 km/s for S-waves. A relatively narrow low-velocity layer of about 4 km of thickness, with P- and S-wave velocities of 6.2 km/s and 3.5 km/s, respectively, marks the bottom of the middle crust at a depth of 23-km northwest and 17-km southeast. At the crust–mantle transition, the P- and S-wave velocity change quickly from 7.4 to 7.8 km/s (northwest) and 8.0 to 8.2 km/s (southeast) and from 3.9 to 4.2 km/s (northwest) and 3.9 to 4.5 km/s (southeast), respectively. This result implies a lateral contrast in the upper mantle velocity along the 140 km sampled by the profile approximately. The average V_P/V_S ratio ranges from 1.68–1.8 for the upper crust to 1.75 for the middle and 1.75–1.85 for lower crust. With the interpretation of the wide-angle seismic data, Jiangshan–Shaoxin fault is considered as the boundary between the Yangtze and the Cathaysia block.     

Vp/Vs ratio along the Vøring Margin, NE Atlantic, derived from OBS data: implications on lithology and stress field

A total of 13 regional Ocean Bottom Seismograph (OBS) profiles with an accumulated length of 2207 km acquired on the Vøring Margin, NE Atlantic have been travel time modelled with regards to S-waves. The V_p/V_s ratios are found to decrease with depth through the Tertiary layers, which is attributed to increased compaction and consolidation of the rocks. The V_p/V_s ratio in the intra-Campanian to mid-Campanian layer (1.75–1.8) in the central Vøring Basin is significantly lower than for the layers above and beneath, suggesting higher sand/shale ratio. This layer was confirmed by drilling to represent a layer of sandstone. This mid-Cretaceous ‘anomaly’ is also present in the northern Vøring Basin, as well as on the southern Lofoten Margin further north. The V_p/V_s ratio in the extrusive rocks on the Vøring Plateau is estimated to be 1.85, conformable with mafic (basaltic) rocks. Landward of the continent/ocean transition (COT), the V_p/V_s ratio in the layer beneath the volcanics is estimated to be 1.67–1.75. These low values suggest that this layer represents sedimentary rocks, and that the sand/shale ratio might be relatively high here. The V_p/V_s ratio in the crystalline basement is estimated to be 1.67–1.75 in the basin and on the landward part of the Vøring Plateau, indicating the presence of granitic/granodioritic continental crust. In the lower crust, the V_p/V_s ratio in the basin decreases uniformly from southwest to northeast, from 1.85–1.9 to 1.68–1.73, suggesting a gradual change from mafic (gabbroic) to felsic (granodioritic) lower crust. Significant (3–5%) azimuthal S-wave anisotropy is observed for several sedimentary layers, as well as in the lower crust. All these observations can be explained by invoking the presence of liquid-filled microcracks aligned vertically along the direction of the present day maximum compressive stress (NW–SE).     

Generalized geophysical model and dynamic properties of the continental crust

Detailed seismic investigations of the continental crust have produced evidence of definite regularities in the general layering of the consolidated crust despite its high degree of inhomogeneity. Three main layers may be resolved in the inner part of a continent: an upper layer with velocities of 5.8–6.4 km/s and a velocity gradient about 0.04–0.05 s⁻¹, an intermediate layer with velocities of 6.2–6.6 km/s and velocity gradient about zero, and a lower layer with velocities of 6.8–7.2 km/s and a high-velocity gradient of 0.05–0.1 s⁻¹. The intermediate layer is characteristically different not only because of its low average velocity gradient, but also because of its more pronounced horizontal layering, inversion zones, and its higher “transparency” and V_p/V_s ratio. The gravity and magnetic data have shown that basement inhomogeneities disappear at the top of the intermediate layer. Also there are few earthquakes in this layer. These pecularities may be interpreted as the result of partial melting (weakening) of rocks and their possible horizontal mobility inside this layer.Thus, dynamic models of tectonic processes must take into consideration the possible existence of a weak zone in the crust.     

3-D crustal structure of the extensional Granada Basin in the convergent boundary between the Eurasian and African plates

Three-dimensional P and S wave velocity models of the crust under the Granada Basin in Southern Spain are obtained with a spatial resolution of 5 km in the horizontal direction and 2 to 4 km in depth. We used a total of 15407 P and 13704 S wave high-quality arrival times from 2889 local earthquakes recorded by both permanent seismic networks and portable stations deployed in the area. The computed P and S wave velocities were used to obtain three-dimensional distributions of Poisson's ratio (σ) and the porosity parameter (V_p×V_s). The 3-D velocity images show strong lateral heterogeneities in the region. Significant velocity variations up to ±7% in P and S velocities are revealed in the crust below the Granada Basin. At shallow depth, high-velocity anomalies are generally associated with Mesozoic basement, while the low-velocity anomalies are related to the neogene sedimentary rocks. The south–southeastern part of the Granada Basin exhibits high σ values in the shallowest layers, which may be associated with saturated and unconsolidated sediments. In the same area, V_p×V_s is high outside the basin, indicating low porosity of the mesozoic basement. A low-velocity zone at 18-km depth is found and interpreted as a weak–ductile crust transition that is related to the cut-off depth of the seismic activity. In the lower crust, at 34-km depth, a clear slow V_p and V_s anomalous zone may indicate variations in lithology and/or with the rigidity of the lower crust rocks.     

Lithospheric structure of the Tornquist Zone resolved by nonlinear P and S teleseismic tomography along the TOR array

The main aim of the TOR project is to study the lithospheric–asthenospheric boundary structure under the Sorgenfrei–Tornquist Zone, across northern Germany, Denmark and southern Sweden. Relative arrival-time residuals of teleseismic P and S phases from 51 earthquakes, recorded by 150 seismic stations along the TOR array, were used to delineate the transition zone in the studied area. The effects of crustal structures were investigated by correcting the teleseismic residuals for travel-time variations in the crust based on a 3D crustal model derived from other data. The inversion was carried out for S phases. The results were then compared with the corresponding P-wave models. As expected, the derived models show that the relatively old and cold Baltic Shield has higher velocity at depth than the younger lithosphere farther South. The models show two sharp and distinct increases in depth to velocities which are low compared to our reference model, as we move from South to North. The location and sharpness of these boundaries suggests that the features resolved are, at least partially, compositional in origin, presumably related to mantle depletion. A sharp and steep subcrustal boundary is found roughly coincident with the southern edge of Sweden. This is below where the edge of the Baltic Shield is usually placed, based on surface geological evidence (the Sorgenfrei–Tornquist Zone). Another less significant transition is recognised more or less beneath the Elbe-lineament. Relatively high d(V_p / V_s) ratios under the central part of the profile (Denmark) indicate relatively low S-velocity in an area where a gravity high supports the hypothesis of extensive mafic intrusions.     

A composite geologic and seismic profile beneath the southern Rio Grande rift, New Mexico, based on xenolith mineralogy, temperature, and pressure

A complete understanding of the processes of crustal growth and recycling in the earth remains elusive, in part because data on rock composition at depth is scarce. Seismic velocities can provide additional information about lithospheric composition and structure, however, the relationship between velocity and rock type is not unique. The diverse xenolith suite from the Potrillo volcanic field in the southern Rio Grande rift, together with velocity models derived from reflection and refraction data in the area, offers an opportunity to place constraints on the composition of the crust and upper mantle from the surface to depths of  60 km. In this work, we calculate seismic velocities of crustal and mantle xenoliths using modal mineralogy, mineral compositions, pressure and temperature estimates, and elasticity data. The pressure, temperature, and velocity estimates from xenoliths are then combined with sonic logs and stratigraphy estimated from drill cores and surface geology to produce a geologic and velocity profile through the crust and upper mantle. Lower crustal xenoliths include garnet ± sillimanite granulite, two-pyroxene granulite, charnokite, and anorthosite. Metagabbro and amphibolite account for only a small fraction of the lower crustal xenoliths, suggesting that a basaltic underplate at the crust–mantle boundary is not present beneath the southern Rio Grande rift. Abundant mid-crustal felsic to mafic igneous xenoliths, however, suggest that plutonic rocks are common in the middle crust and were intraplated rather than underplated during the Cenozoic. Calculated velocities for garnet granulite are between  6.9 and 8.0 km/s, depending on garnet content. Granulites are strongly foliated and lineated and should be seismically anisotropic. These results suggest that velocities > 7.0 km/s and a layered structure, which are often attributed to underplated mafic rocks, can also be characteristic of alternating garnet-rich and garnet-poor metasedimentary rocks. Because the lower crust appears to be composed largely of metasedimentary granulite, which requires deep burial of upper crustal materials, we suggest the initial construction of the continental crust beneath the Potrillo volcanic field occurred by thickening of supracrustal material in the absence of large scale magmatic accretion. Mantle xenoliths include spinel lherzolite and harzburgite, dunite, and clinopyroxenite. Calculated P-wave velocities for peridotites range from 7.75 km/s to 7.89 km/s, with an average of 7.82 km/s. This velocity is in good agreement with refraction and reflection studies that report P_n velocities of 7.6–7.8 km/s throughout most of the Rio Grande rift. These calculations suggest that the low P_n velocities compared to average uppermost mantle are the result of relatively high temperatures and low pressures due to thin crust, as well as a fertile, Fe-rich, bulk upper mantle composition. Partial melt or metasomatic hydration of the mantle lithosphere are not needed to produce the observed P_n velocities.     

Imaging of Vp, Vs, and Poisson’s ratio anomalies beneath Kyushu, southwest Japan: Implications for volcanism and forearc mantle wedge serpentinization

We determine detailed 3-D V_p and V_s structures of the crust and uppermost mantle beneath the Kyushu Island, southwest Japan, using a large number of arrival times from local earthquakes. From the obtained V_p and V_s models, we further calculate Poisson’s ratio images beneath the study area. By using this large data set, we successfully image the 3-D seismic velocity and Poisson’s ratio structures beneath Kyushu down to a depth of 150 km with a more reliable spatial resolution than previous studies. Our results show very clear low V_p and low V_s anomalies in the crust and uppermost mantle beneath the northern volcanoes, such as Abu, Kujyu and Unzen. Low-velocity anomalies are seen in the mantle beneath most other volcanoes. In contrast, there are no significant low-velocity anomalies in the crust or in the upper mantle between Aso and Kirishima. The subducting Philippine Sea slab is imaged generally as a high-velocity anomaly down to a depth of 150 km with some patches of normal to low seismic wave velocities. The Poisson’s ratio is almost normal beneath most volcanoes. The crustal seismicity is distributed in both the high- and low-velocity zones, but most distinctly in the low Poisson’s ratio zone. A high Poisson’s ratio region is found in the forearc crustal wedge above the slab in the junction area with Shikoku and Honshu; this high Poisson’s ratio could be caused by fluid-filled cracks induced by dehydration from the Philippine Sea slab. The Poisson’s ratio is normal to low in the forearc mantle in middle-south Kyushu. This is consistent with the absence of low-frequency tremors, and may indicate that dehydration from the subducting crust is not vigorous in this region.     

Lithosphere structure of the Ukrainian Shield and Pripyat Trough in the region of EUROBRIDGE-97 (Ukraine and Belarus) from gravity modelling

The southern segment of the seismic profile EUROBRIDGE—EUROBRIDGE-97 (EB'97)—located in Belarus and Ukraine, crosses the suture zone between two main segments of the East European Craton—Fennoscandia and Sarmatia—as well as Sarmatia itself. At the initial stage of our study, a 3-D density model has been constructed for the crust of the study region, including the major part of the Osnitsa–Mikashevichi Igneous Belt (OMIB) superimposed by sediments of the Pripyat Trough (PT), and three domains in the Ukrainian Shield—the Volhyn Domain (VD) with the anorthosite–rapakivi Korosten Pluton (KP), the Podolian Domain (PD), and the Ros–Tikich Domain (RTD). The model comprises three layers—sediments with maximum thickness (6 km) in the PT and two heterogeneous layers in the crystalline crust separated at a depth of 15 km. 3-D calculations show the main features of the observed gravity field are caused by density heterogeneities in the upper crust. Allocation of density domains deeper than 15 km is influenced by Moho topography. Fitting the densities here reveals an increase (up to 2960 kg m⁻³) in the modelled bodies accompanied by a Moho deepening to 50 km. In contrast, a Moho uplift to a level of 35–37 km below the KP and major part of the PT is associated with domains of reduced densities. An important role for the deep Odessa–Gomel tectonic zone, dividing the crust into two regions one of basically Archean consolidation in the west (PD and RTD) and one of Proterozoic crust in the east (Kirovograd Domain)—was confirmed.2-D density modelling on the EB'97 profile shows that in the upper crust three main domains of different Precambrian evolution—the OMIB (with the superimposed PT), the VD with the KP, and the PD—can be distinguished. Deeper, in the middle and lower crust, layered structures having no connection to the surface geology are dominant features of the models. Least thickness of the crust was obtained below the KP. Greatest crustal thickness (more than 50 km) was found below the PD, characterised also by maximum deviation of velocity/density relation in the rocks from a standard one. The velocity and density models along the EB'97 profile have been interpreted together with inferred V_p/V_s ratios to estimate crustal composition in terms of SiO₂ content. In the course of the modelling, the status of the PD as a centre of Archean granulitic consolidation has been confirmed. The crustal structure of the anorthosite–rapakivi KP is complex. For the first time, a complicated structure for the lower crust and lower crust–upper mantle transition zone beneath the KP has been determined. The peculiarities of the crustal structure of the KP are quite well explained in terms of formation of rapakivi–anorthosite massifs as originating from melt chambers in the upper mantle and lower crust. An important role for the South Pripyat Fault (SPF), repeatedly activated during Proterozoic–Palaeozoic times, has been ascertained. At the subplatform stage of crustal evolution the SPF was, probably, a magma channel facilitating the granitic intrusions of the KP. In the Palaeozoic the fault was reactivated during rifting in the PT.     

Tectonic controls on metamorphism, partial melting, and intrusion: timing and duration of regional metamorphism and magmatism in the Ni de Massif, Turkey

Mafic high-pressure granulite, eclogite and pyroxenite xenoliths have been collected from a Mesozoic volcaniclastic diatreme in Xinyang, near south margin of the Sino-Korean Craton (SKC). The high-pressure granulite xenoliths are mainly composed of fine-grained granoblasts of Grt+Cpx+Pl+Hbl±Kfs±Q±Ilm with relict porphyritic mineral assemblage of Grt+Cpx±Pl±Rt. P–T estimation indicates that the granoblastic assemblage crystallized at 765–890 °C and 1.25–1.59 GPa, corresponding to crustal depths of ca. 41–52 km with a geotherm of 75–80 mW/m². Calculated seismic velocities (V_p) of high-pressure granulites range from 7.04 to 7.56 km/s and densities (D) from 3.05 to 3.30 g/cm³. These high-pressure granulite xenoliths have different petrographic and geochemical features from the Archean mafic granulites. Elevated geotherm and petrographic evidence imply that the lithosphere of this craton was thermally disturbed in the Mesozoic prior to eruption of the host diatreme. These samples have sub-alkaline basaltic compositions, equivalent to olivine– and quartz–tholeiite. REE patterns are flat to variably LREE-enriched (La_N/Yb_N=0.98–9.47) without Eu anomaly (Eu/Eu*=0.95–1.11). They possess 48–127 ppm Ni and 2–20 ppm Nb with Nb/U and La/Nb ratios of 13–54 and 0.93–4.75, respectively, suggesting that these high-pressure granulites may be products of mantle-derived magma underplated and contaminated at the base of the lower crust. This study also implies that up to 10 km Mesozoic lowermost crust was delaminated prior to eruption of the Cenozoic basalts on the craton.     

Seismic studies of the Brasília fold belt at the western border of the São Francisco Craton, Central Brazil, using receiver function, surface-wave dispersion and teleseismic tomography

The Tocantins Province in Central Brazil is composed of a series of SSW–NNE trending terranes of mainly Proterozoic ages, which stabilized in the Neoproterozoic in the final collision between the Amazon and São Francisco cratons. No previous information on crustal seismic properties was available for this region. Several broadband stations were used to study the regional patterns of crustal and upper mantle structure, extending the results of a recent E–W seismic refraction profile. Receiver functions and surface wave dispersion showed a thin crust (33–37 km) in the Neoproterozoic Magmatic Arc terrane. High average crustal Vp/Vs ratios (1.74–1.76) were consistently observed in this unit. The foreland domain of the Brasília foldbelt, on the other hand, is characterized by thicker crust (42–43 km). Low Vp/Vs ratios (1.70–1.72) were observed in the low-grade foreland fold and thrust zone of the Brasília belt adjacent to the São Francisco craton. Teleseismic P-wave tomography shows that the lithospheric upper mantle has lower velocities beneath the Magmatic Arc and Goiás Massif compared with the foreland zone of the belt and São Francisco craton. The variations in crustal thickness and upper mantle velocities observed with the broadband stations correlate well with the measurements along the seismic refraction profile. The integration of all seismic observations and gravity data indicates a strong lithospheric contrast between the Goiás Massif and the foreland domain of the Brasília belt, whereas little variation was found across the foldbelt/craton surface boundary. These results support the hypothesis that the Brasília foreland domain and the São Francisco craton were part of a larger São Francisco-Congo continental plate in the final collision with the Amazon plate.     

Depths to density discontinuities beneath the Adamawa Plateau region, Central Africa, from spectral analyses of new and existing gravity data

New gravity data from the Adamawa Uplift region of Cameroon have been integrated with existing gravity data from central and western Africa to examine variations in crustal structure throughout the region. The new data reveal steep northeast-trending gradients in the Bouguer gravity anomalies that coincide with the Sanaga Fault Zone and the Foumban Shear Zone, both part of the Central African Shear Zone lying between the Adamawa Plateau and the Congo Craton. Four major density discontinuities in the lithosphere have been determined within the lithosphere beneath the Adamawa Uplift in central Cameroon using spectral analysis of gravity data: (1) 7–13 km; (2) 19–25 km; (3) 30–37 km; and (4) 75–149 km. The deepest density discontinuities determined at 75–149 km depth range agree with the presence of an anomalous low velocity upper mantle structure at these depths deduced from earlier teleseismic delay time studies and gravity forward modelling. The 30–37 km depths agree with the Moho depth of 33 km obtained from a seismic refraction experiment in the region. The intermediate depth of 20 km obtained within region D may correspond to shallower Moho depth beneath parts of the Benue and Yola Rifts where seismic refraction data indicate a crustal thickness of 23 km. The 19–20 km depths and 8–12 km depths estimated in boxes encompassing the Adamawa Plateau and Cameroon Volcanic Line may may correspond to mid-crustal density contrasts associated with volcanic intrusions, as these depths are less than depths of 25 and 13 km, respectively, in the stable Congo Craton to the south.     

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.     

Upper mantle structure of the Northern Eurasia from peaceful nuclear explosion data

Several long-range seismic profiles were carried out in Russia with Peaceful Nuclear Explosions (PNE). The data from 25 PNEs recorded along these profiles were used to compile a 3-D upper mantle velocity model for the central part of the Northern Eurasia. 2-D crust and upper mantle models were also constructed for all profiles using a common methodology for wavefield interpretation. Five basic boundaries were traced over the study area: N1 boundary (velocity level, V = 8.35 km/s; depth interval, D = 60–130 km), N2 (V = 8.4 km/s; D = 100–140 km), L (V = 8.5 km/s; D = 180–240 km) and H (V = 8.6 km/s; D = 300–330 km) and structural maps were compiled for each boundary. Together these boundaries describe a 3-D upper mantle model for northern Eurasia. A map characterised the velocity distribution in the uppermost mantle down to a depth of 60 km is also presented. Mostly horizontal inhomogeneity is observed in the uppermost mantle, and the velocities range from the average 8.0–8.1 km/s to 8.3–8.4 km/s in some blocks of the Siberian Craton. At a depth of 100–200 km, the local high velocity blocks disappear and only three large anomalies are observed: lower velocities in West Siberia and higher velocities in the East-European platform and in the central part of the Siberian Craton. In contrast, the depths to the H boundary are greater beneath the craton and lower beneath in the West Siberian Platform. A correlation between tectonics, geophysical fields and crustal structure is observed. In general, the old and cold cratons have higher velocities in the mantle than the young platforms with higher heat flows.Structural peculiarities of the upper mantle are difficult to describe in form of classical lithosphere–asthenosphere system. The asthenosphere cannot be traced from the seismic data; in contrary the lithosphere is suggested to be rheologically stratified. All the lithospheric boundaries are not simple discontinuities, they are heterogeneous (thin layering) zones which generate multiphase reflections. Many of them may be a result of fluids concentrated at some critical P–T conditions which produce rheologically weak zones. The most visible rheological variations are observed at depths of around 100 and 250 km.