Deep Moho Research Articles

Topography of the Seismic Velocity Discontinuities beneath the Chugoku and Shikoku Districts, Southwest Japan

The first P-arrival-time data from 513 local earthquakes were analyzed to study lateral variation of the depth to the Conrad and Moho discontinuities beneath the Chugoku and Shikoku districts, southwest Japan, as well as to determine earthquake hypocenters and P-wave station corrections. The depth to the discontinuity was estimated by minimizing the travel-time residuals of more than 8700 first P arrivals observed at 55 seismic stations. The Conrad and Moho discontinuities are located within depth ranges of 15–25 km and 30–40 km, respectively. The Moho is deeper under the mountain area than under the Seto Inland Sea area, and especially deep under the Pacific Coast of the Shikoku district and the mountain area in the Chugoku district. The depth variation of the Moho is quite similar to the Bouguer gravity anomaly distribution and the lateral variations of the P-wave velocity. The deep Moho under the southern Shikoku is located at the portion in which the continental Moho under the island arc meets the oceanic Moho that is the boundary interface between the oceanic crust and the Philippine Sea (PHS) plate dipping toward the back arc. Although there are high mountains in the northern and middle Shikoku, the Moho is not so deep because subduction of the PHS plate prevents the Moho from getting deep, while the Moho is deep due to isostatic balance under the mountain area in the Chugoku district. In addition, we indicated the possibility that the upper boundary of the oceanic crust just above the high-velocity PHS plate is in contact with the deep Moho under the western Chugoku. The contact of the Moho with the oceanic crust can explain the markedly negative gravity anomaly observed in the western Chugoku and the later phase that appears just after the first P arrival from local earthquakes.

Teleseismic imaging of subducting lithosphere and Moho offsets beneath western Tibet

Teleseismic images suggest that the Tarim plate plunges ∼45°S, down to ∼300 km depth, beneath NW Tibet. The 410 km discontinuity shallows by ∼10 km under the plateau, implying ∼100°C cooler upper mantle. The deepest Moho on record (∼90 km) lies under W Qiangtang. It rises abruptly by ∼20 and ∼10 km beneath the Altyn Tagh Fault and Bangong Suture, respectively. V p/ V s ratios are normal, except in the Yecheng flexural basin and deep under the south Karakax volcanics (∼1.92). W Kunlun’s Neogene tectonics are simply accounted for by oblique subduction of lithospheric mantle beneath an upward-extruding thrust wedge of the Tarim crust.

Topography of the crust–mantle interface under the Western Superior craton from gravity data

The Moho undulations beneath the western part of the Archean Superior Province have been investigated with a three-dimensional gravity inversion algorithm for a single interface of constant density contrast. Inversion of the complete gravity data set produces unreal effects in the solution due to the ambiguity in the possible sources of some crustal gravity anomalies. To avoid these effects a censored gravity data set was used instead. The inversion results are consistent with reflection and refraction seismic data from the region and, therefore, provide a basis for the lateral correlation of the Moho topography between parallel seismic lines. The results indicate the existence of a major linear east–west-trending rise of the Moho below the metasedimentary English River subprovince, which is paralleled by crustal roots below the granite–greenstone Uchi and Wabigoon subprovinces. This correlation between the subprovincial structure at the surface and deep Moho undulations suggests that the topography of the crust–mantle boundary is related to the tectonic evolution of the Western Superior belts. Although certain features of the crust–mantle boundary are likely inherited from the accretionary and collisional stages of the Western Superior craton, gravity-driven processes triggered by subsequent magmatism and crustal softening may have played a role in both the preservation of those features, as well as in the development of new ones.

Joint inversion of local, regional and teleseismic data for crustal thickness in the Eurasia-Africa plate boundary region

SUMMARY A new map for the Moho discontinuity (EAM02) in the Eurasia‐Africa plate boundary region is presented. Reliable results have also been obtained for the southern and eastern Mediterranean Basin, the northern African coasts and the eastern Atlantic Ocean, regions only occasionally considered in studies on the Mediterranean region. The Moho topography model is derived from two independent sets of constraints. Information contained in the fundamental and highermode Rayleigh waves obtained from waveform modelling is used to constrain the Moho depth between estimates of crustal thickness taken from published reflection and refraction surveys, gravity studies and receiver function analysis. Strong lateral variations in the Moho topography have been observed in the Mediterranean Sea, confirming the complex evolution of this plate boundary region. In the west, the Moho discontinuity has been found at 15‐20 km depth, suggesting extended and, at least in some locations, oceanic crust, while in the east the crust is on average 25‐30 km thick. There it is interpreted either as Mesozoic oceanic or thinned Precambrian continental crust covered by thick sedimentary deposits. Standard continental crust (30‐35 km) is observed along the eastern part of the northern African coast, while to the west a rapid change from a relatively deep Moho (down to 42 km) below the Atlas Mountain Range to the thin crust of the southwestern Mediterranean Sea has been found. The crust beneath the eastern North Atlantic Ocean can be up to 5 km thicker compared with standard oceanic crust (6 km). The crust has been interpreted to be heterogeneous as a consequence of irregular magma supply at the Mid-Atlantic ridge. In addition, serpentinization of the subMoho mantle could contribute to the imaging of apparently anomalous thick oceanic crust. In Europe, the presence of crustal roots (>45 km) beneath the major mountain belts has been confirmed, while thin crust (<25 km) has been found beneath extensional basins. Comparing the obtained Moho topography and Moho depth computed assuming isostatic compensation at 60 km depth shows that most of the Mediterranean and eastern Atlantic region appears to be in isostatic equilibrium. The large positive residuals observed for the eastern Mediterranean are likely to be due to overestimating crustal thickness, owing to the thick sediment deposits present.

Recent progress of deep seismic experiments and studies of crustal structure in northern South China Sea*

The South China Sea (SCS) is one of the largest marginal seas in the western Pacific. Its northern part has the features of a passive continental margin. The studies of deep crustal structure in this area are very important for understanding the tectonic nature, evolution history, basin formation of the northern margin, and the origin of the SCS. In the past decades, the deep seismic experiments of crustal studies in the northern SCS have gone through three stages, namely the sonobucy, two-ship Expanding Spread Profile (ESP), and Ocean Bottom Hydrophone/Seismometer (OBH/OBS). Along the continental slope, the sonobuoy experiments provided useful information about the velocity structure of the upper crust, while the ESP data recorded for the first time the seismic signals from deep crustal structure and Moho interface. And the OBH/OBS profiles revealed the crustal structure in much greater detail. This paper first gives a brief historical review of these deep seismic experiments and studies, then a summary of the latest progress and important research results. The remaining problems and suggestions for further research work are presented as conclusive remarks.

Crustal velocity structure of western Dharwar Craton, South India

A deep seismic sounding (DSS) experiment was carried out across the Indian shield in 1972–1975. Kaila et al. (Kaila, K.L., Roy Chowdhury, K., Reddy, P.R., Krishna, V.G., Hari Narain, Subbotin, S.I., Sollogub, V.B., Chekunov, A.V., Kharetchko, G.E., Lazarenko, M.A., Ilchenko T.V., 1979. Crustal structure along Kavali-Udipi profile in the Indian peninsular shield from deep seismic sounding. J. Geol. Soc. of Ind., 20, 307–333) presented a crustal depth section based on the interpretation of the analog seismic data. In this paper, we re-examine the crustal structure of the Western Dharwar Craton (WDC) by reprocessing the data of three major shot points. Kinematic 1-D inversion, followed by 2-D forward modeling of the first arrival refraction and a few persistent wide-angle reflection phases, was carried out to build, a first order two-dimensional velocity model of this segment of the profile. This model brings out a simple crustal velocity structure consisting of an upper and lower crust. The upper crust (velocity 6.0–6.2 km/s) is on average 23 km thick, which is underlain by a lower crust of velocity 6.8–7.0 km/s. The average Moho depth in this part is about 37–40 km, with higher-than-normal P n velocity of 8.4 km/s. A relatively deep Moho in this part of the Archean peninsular shield is associated with relatively low velocities in the lower crust, perhaps indicating absence of underplating in this region. Present results bring out the fact that the crust of WDC is not so typical as many other Archean crusts, in terms of both thickness and velocities.

Crustal structure and seismicity distribution adjacent to the Pacific and North America plate boundary in southern California

New three‐dimensional (3‐D) VP and VP/VS models are determined for southern California using P and S‐P travel times from local earthquakes and controlled sources. These models confirm existing tectonic interpretations and provide new insights into the configuration of geological structures at the Pacific‐North America plate boundary. The models extend from the U.S.‐Mexico border in the south to the southernmost Coast Ranges and Sierra Nevada in the north and have a 15‐km horizontal grid spacing and an average vertical grid spacing of 4 km, down to 22 km depth. The heterogeneity of the crustal structure as imaged by VP and VP/VS models is larger within the Pacific plate than the North American plate. Similarly, the relocated seismicity deepens and shows more complex 3‐D distribution in areas of the Pacific plate exhibiting compressional tectonics. The models reflect mapped changes in the lithology across major geological terranes such as the Mojave Desert, the Peninsular Ranges, and the Transverse Ranges. The interface between the shallow Mono of the Continental Borderland and the deep Moho of onshore California forms a broad zone to the north beneath the western Transverse Ranges, Ventura basin, and the Los Angeles basin and a narrow zone to the south, along the Peninsular Ranges. The near‐surface increase in velocity, from the surface to up to 8 km depth, is rapid and has a logarithmic shape for stable blocks and mountain ranges but is slow with a linear shape for sedimentary basins. At midcrustal depths a rapid increase in VP is imaged beneath the sediments of the large sedimentary basins, while beneath the adjacent mountain ranges the increase is small or absent.

A 3D gravity model of crustal structure in the Central-Eastern Alpine sector

Assuming as a starting model the pattern of the Moho boundary as interpreted in a recent study on the basis of the available DSS profiles, a preliminary 3D gravity model of the crustal structures in the Central-Eastern Alpine sector is proposed. The aim of the present work is to confirm the seismic results concerning the Moho and to better shape the main discontinuities in the intermediate and upper crust, where the seismic data are too scattered to allow a reliable interpretation. The gravity field is calculated along twelve cross-sections oriented S-N and crossing the Alpine range from the Padan-Venetian plain to the Bavarian molasse and to the Austrian calcareous Alps. The westernmost section coincides with the European Geotraverse while the easternmost one is positioned at the longitude of about 14ºeast. The assumed density model is very simple (only 6 layers); for each unit the density is maintained constant. The model describes a European mantle dipping southwards underneath an overlapping, uplifted Adriatic mantle. As far as the lower crust is concerned, its top is found at depths between 18 and 28 km, the deepest values being reached in the south-eastern sector; the density appears higher in the Adriatic domain than in the European one and the Adriatic lower crust seems to be deeply indented northwards. The low density surface layers appear very thin in a large area of the northwestern sector, while in the south and southeast their thickness reaches about 10 km. This study must be considered as a complement to the seismic interpretation both as a validation of the model of the deep crust and Moho boundary and as an additional source of information on the upper crust.

Seismic structure and evidence for eclogitization during the Himalayan convergence

Reflections on the deep Moho of the Tethyan Himalayas have been obtained from vertical seismics at the time predicted by wide-angle reflection interpretations, supporting the previous inference of a low average crustal velocity. A mostly felsic composition is then required through the 70 km of the thickened crust. The Moho appears as a heterogenous transition layer more than 10 km thick. The high velocity measured beneath the Moho from Pn refractions cannot be attributed any more to E-W-oriented olivine since SKS observations do not confirm such an anisotropy. Instead, eclogite just under the seismic Moho could account for the observations. These three results from independent parts of the seismic data set support a model of the Himalayan convergence in which the Indian crust has undergone phase transformation in the granulite-eclogite system upon burial. The intermediate and basic composition rocks of the middle and lower layers of an initial crust of average platform type can be transformed to such high velocities that they are lost across the Himalayas into the level of the Moho or just under the Moho defined by seismic velocity. Mostly upper-crustal-type material then remains to form the thickened crust above, consistent with the observed low velocity. The variation in Moho depth across the Himalayas as suggested from wide-angle seismics accounts for the first-order variation in teleseismic transit times. Variations in the velocity-depth structure of the mantle hence do not dominate, but a change in the orientation of SKS splitting anisotropy would suggest deformation within the lithosphere to change across the High Himalayas and the Indus-Tsangpo suture.

Färoe‐Iceland Ridge Experiment 2. Crustal structure of the Krafla central volcano

The seismic velocity structure of the Krafla central volcano is characterized by large variations in compressional velocity. A 40 km wide high‐velocity dome extends from the lower crust (11–14 km depth) beneath the volcano narrowing upward. A magma chamber sits at its top near 3 km depth. It is defined by both 0.2–0.3 s compressional wave delays and shear wave shadowing to be 2–3 km N‐S, 8–10 km E‐W, and 0.7–1.8 km thick. The near‐surface structure (uppermost 2.5 km) of the Krafla caldera is approximately flat‐lying, with only minor lateral heterogeneities. The crust beneath the magma chamber has low shear wave attenuation and anomalously high compressional and shear wave velocities. Shear waves, reflected from a 19 km deep Moho, are clearly visible for some paths through the crustal volume below the magma chamber, even though the more shallow diving S waves are severely attenuated. The midcrust beneath the shallow magma chamber cannot contain partial melt or even be at near‐solidus temperatures. The Krafla central volcano plays a major role in crustal genesis along the plate boundary. The high‐velocity dome, in our view, represents crust generated in and around the magma chamber, which has subsequently been advected to greater depths.

Crustal thickness variations beneath the peninsular ranges, southern California

We investigate the crustal thickness under the Peninsular Ranges using P‐to‐S converted phases of teleseismic body waves recorded on a temporary broadband seismometer array and isolated by the receiver function method. Ps minus P times at sites west of a compositional boundary that separates the Peninsular Ranges batholith into east and west zones indicate a relatively flat, deep Moho. Ps minus P times at sites east of the compositional boundary decrease eastward, Moho depth estimates (made from the Ps delays and crustal velocities from seismic tomography) indicate a relatively constant 36 to 41 km thick crust in the western zone. In the eastern zone the crust thins rapidly from 35 km thick at the compositional boundary to 25 km at the edge of the Salton trough, a lateral distance of 30 km. The lack of correlation between topography and Moho depths suggests compensation via lateral density variations in the lower crust or upper mantle. We propose that the compositional boundary decouples the eastern and western portions of the batholith, and that the eastern portion has thinned in response to regional Miocene extension, or Salton trough rifting, or both.

Three-dimensional crustal structure of the southern Sierra Nevada from seismic fan profiles and gravity modeling

Traveltime data from the 1993 Southern Sierra Nevada Continental Dynamics seismic refraction experiment reveal low crustal velocities in the southern Sierra Nevada and Basin and Range province of California (6.0 to 6.6 km/s), as well as low upper mantle velocities (7.6 to 7.8 km/s). The crust thickens from southeast to northwest along the axis of the Sierra Nevada from 27 km in the Mojave Desert to 43 km near Fresno, California. A crustal welt is present beneath the Sierra Nevada, but the deepest Moho is found under the western slopes, not beneath the highest topography. A density model directly derived from the crustal velocity model but with constant mantle density satisfies the pronounced negative Bouguer anomaly associated with the Sierra Nevada, but shows large discrepancies of >50 mgal in the Great Valley and in the Basin and Range province. Matching the observed gravity with anomalies in the crust alone is not possible with geologically reasonable densities; we require a contribution from the upper mantle, either by lateral density variations or by a thinning of the lithosphere under the Sierra Nevada and the Basin and Range province. Such a model is consistent with the interpretation that the uplift of the present Sierra Nevada is caused and dynamically supported by asthenospheric upwelling or lithospheric thinning under the Basin and Range province and eastern Sierra Nevada.

Tectonic evolution of the Mid-Polish Trough: modelling implications and significance for central European geology

The Polish Basin forms the easternmost part of the Permian-Mesozoic northwest European basin. The depocentral axis of the Polish Basin, the Mid-Polish Trough (MPT), is superimposed on the boundary between the west European Phanerozoic and east European Proterozoic crustal domains, within the Trans-European Suture Zone. The presence of this fundamental crustal boundary may be paramount in structurally controlling the position of the MPT, concentrating stresses during post-Variscan wrench and extensional tectonics in central Europe. Tectonic subsidence analysis of the preserved and reconstructed stratigraphic record of the Polish Basin indicates the occurrence of an initial Late Permian-Early Triassic (255−241 Ma) ‘rifting’ phase that was followed by subsequent episodes of increased tectonic subsidence during the Oxfordian-Kimmeridgian (∼ 157−152 Ma) and beginning in the Cenomanian (∼97 Ma). The Oxfordian-Kimmeridgian episode is interpreted as corresponding to a second extensional event, which correlated with intersified rifting and wrench activity within the Arctic-North Atlantic rift system and along the northern Tethyan margin, while the Cenomanian may be considered a precursor of compressional deformations in the basin which culminated in basin inversion in the latest Cretaceous and Paleocene. Forward modelling results, in view of existing geophysical interpretations which show the presence of a deep Moho and a very velocity lower crustal layer beneath theMPT, suggest that Permo-Mesozoic basic development may be related at least in part to the intrusion of mantle material into and densification of the lower crust rather than exclusively to crustal extension and thinning.

The depth of Moho in the mainland of China

In the last decade, deep seismic sounding (DSS) of profiles several ten thousand kilometers in length has been conducted in the mainland of China. A lot of results published in Chinese were compiled and a map of Moho depth was presented in this paper. Coverage of DSS profile is not uniform, and is lacking in some areas, especially in the western part of China. In this case, the results from dispersions of seismic surface waves and other materials were considered. According to the Moho depth, the mainland of China can be divided into eight crustal blocks. The Moho depth is uniform and varies little inside the same block, while it changes abruptly from one block to another. The Tibetan Plateau is characterized by a thickest crust in the world. It may be due to the penetration of Indian crustal materials into the Tibetan crust or upper mantle. Deeper Moho in the neighbouring areas of Tibet may also be effected by the complicated collision processes between the two continents. Thin crust of the North China Plain is considered to be related to the magmatic activities in the lower crust and upper mantle. While the crust of the eastern part of South China may be due to the accretion of terrain from the ocean islands.

Implications of tectonic subsidence models for crustal structure beneath the Mid-Polish Trough

Tectonic subsidence analysis of the preserved and reconstructed stratigraphy of the Polish Basin indicates an initial Late Permian-Early Triassic (255-241 Ma) syn-rift phase of development with a subsequent extensional rejuvenation during the Late Jurassic (ca. 157-152 Ma). Forward modelling of the subsidence data, in view of existing geophysical interpretations which show the presence of a deep Moho and a very high seismic velocity lower crustal layer beneath the Mid-Polish Trough (MPT), suggest that Permo-Mesozoic basin development may be related at least in part to the intrusion of mantle material into and densification of the lower crust rather than exclusively to crustal extension and thinning.

Attenuation of crustal waves across the Alpine Range

We present observational evidence of an anomalous propagation of Lg waves across the south‐western part of the Alpine range, and we use numerical simulations to model these observations. We consider a set of 48 earthquakes which occurred in Switzerland, northern Italy, and southeastern France and were recorded by the French (Laboratoire de Detection et de Geophysique) and northwest Italian (Istituto Geofisico di Genova) seismic networks. While the amplitude of the Pn phase is stable throughout the region studied, Lg wave amplitude undergoes strong variations. We map this anomaly in Lg wave propagation by dividing the region into a grid and attributing to each cell a value equal to the mean value of the Lg/Pn amplitude ratios computed for all the paths which cross this cell. The image obtained shows that the extinction of Lg waves occurs in a limited region of the western Alps which corresponds to the zone of highest positive Bouguer anomaly. This zone located to the east of the high peaks of the Massifs Cristallins Externes does not correspond either to the region of the highest topographies or to the one of the deepest Moho. At a frequency of 2 Hz, the crustal waves that cross this anomalous region have amplitudes divided by more than 10 with respect to waves that propagate along other paths. In order to investigate the cause of the anomaly we perform numerical simulations of SH wave propagation through a model of the western Alps which includes the main characteristics of the geological structure. Our results indicate that the geometrical effect of the lateral variations of the medium does not account entirely for the actual vanishing of crustal waves. The simulation predicts a decay of the amplitude by only a factor between 2 and 3. The introduction of corrugation in the interface shapes further adds to the decay. However, other sources of attenuation such as anelasticity or severe heterogeneity must be invoked to explain fully the observations.

Anomalously deep crustal earthquakes in the Ventura Basin, southern California

Earthquakes at depths of 20–30 km occur beneath the Ventura Basin in southern California. The epicentral distribution of deep seismicity outlines an east trending ellipse which corresponds closely with the mapped Santa Clara Syncline, the main structural element of the Ventura basin with 15 km thickness of sediments. Clear Pn and Pg phases are seen for even the deepest earthquakes which demonstrate that these earthquakes are occurring within the crust. Travel time curves from shallow and deep earthquakes in the western Transverse Ranges require that the depth to the Moho under the Ventura basin must be depressed by 7–10 km relative to the surrounding area, indicating crustal thickening of 20–35%. Other researchers have shown that the Ventura basin has the lowest heat flow in western California. Thus, low heat flow, exceptionally thick (15 km) sediments, rapid shortening of the crust, the deep earthquakes, and a depressed Moho all coincide within a small area, strongly suggesting a common cause for all five phenomena. The confinement of the deep earthquakes and depressed Moho to the Ventura region suggests that the rapid shortening of the Ventura basin is anomalous and not representative of the rate of shortening across all of the western Transverse Ranges.

The influence of second-scale convection on the thickness of continental lithosphere and crust

The effect of sublithospheric convection on the thickness and structure of the continental lithosphere is studied in numerical models assuming different rheologies (Newtonian, non-Newtonian, and temperature and pressure dependent), heat fluxes, and heating modes (bottom versus internal heating). The stagnant regions near the top of the models are identified with the lithosphere. We distinguish a seismic and thermal lithosphere (controlled by temperature), an elastic lithosphere (controlled by viscosity), and a mechanical lithosphere (controlled by strain rate). Strong lateral thickness undulations with a thick lithosphere above the downwelling regions and a thin lithosphere above the upwelling regions develop. The thickness of the elastic lithosphere is about two-thirds of the mechanical lithosphere, while the ratio of the elastic to the thermal lithosphere varies between 0.4 and 0.8. The time dependence of some models (with internal heating and a surface heat flow of 20 mW/m 2) is characterized by long periods (of the order of 1 Ga) close to steady state and short periods (100 Ma) of changing cell patterns and lithospheric thicknesses. Models showing thick lithospheric roots suggest that the mantle beneath some old shields may be associated with cold, slowly downwelling convective flow rather than being firmly attached to the lithosphere. The missing gravity and topography signals from such regions can be explained by elastic bending of the lithosphere and by dynamic adjustment of the Moho. Observed lithospheric thickness variations in Europe and their minimum to maximum spacing agree well with the models, suggesting sublithospheric convection to be the cause. Compared to other observed features, thickness undulations of the lithosphere seem to provide the strongest indications for sublithospherical convection. Relatively low viscosities in the lower crust may lead to a long-term decoupling of surface topography from Moho topography and mantle dynamics. The Moho depth is capable of adjusting to mantle stresses within relaxation times of the order of 100 Ma, while surface topography may flatten out. Such a mechanism may be important in regions with a deep Moho such as Fennoscandia or with a shallow Moho such as the Pannonian Basin, neither region showing a pronounced surface topography.

A unified view of craton evolution motivated by recent deep seismic reflection and refraction results

An attempt is made to integrate recent continental seismic reflection, refraction, and geologic observations into a unified scheme of craton evolution. Nascent continental crust of basaltic bulk composition is produced in island arcs. As with oceanic crust, Moho in this setting lies within the magmatic edifice and corresponds to a downward transition from dominantly mafic cumulate rocks above to dominantly ultramafic cumulates below. The petrologic crust/mantle boundary, lying ∼25 per cent deeper in the lithosphere, has no seismic expression, and corresponds to a depositional/magmatic contact between cumulate ultramafic rocks above and residual mantle (ultramafic tectonite) below. Both contacts are presumably extensively disrupted by magmatic injection. Island arcs are amalgamated into continents at collision zones. Delamination of the lower mafic/ultramafic portion of the crust along with the mantle lithosphere during ‘hard’ collisions acts as a mechanical refining process that pushes the bulk crust toward intermediate composition. Subsequent extensional collapse of the overthickened crust together with thermal re-equilibration of the lithosphere results in ‘youthful’ stable continental crust, which is ∼30 km thick with its top surface awash at sea level and has the bulk composition of andesite. This crust characteristically exhibits extensional features (half grabens) in the upper portion, and is prominently laminated in the lower portion due to the injection of modest amounts of basaltic magma in the form of sills during delamination. The Moho and petrologic base of the crust coincide in this setting and can correspond to either a ductile high-strain zone or intrusive contact separating mafic/intermediate composition material above from fertile mantle (lherzolite) below. Subsequent ‘cratonization’ is the cumulative effect of episodic injection of the crust by mafic magma at intervals of hundreds of Myr (as manifest by the ubiquitous overlapping dike swarms of the Precambrian shields). This results in net long-term thickening of the crust by underplating, shifts the bulk crust back toward mafic composition, periodically produces a new deeper Moho and petrologic crust/mantle boundary that do not coincide, and through repeated dike injection disrupts pre-existing laminated reflectors. The first magmatic injection event ‘sweats out’ the light melting fraction in the pre-existing andesitic crust, producing voluminous granitic magmatism typified by the great Proterozoic anorogenic granite/rhyolite provinces. Subsequent injection events into refractory crust produce uplift with attendant erosional denudation of supracrustal sequences. The cumulative result is shield-type crust in which intermediate crustal levels are exposed over wide areas, crustal thickness commonly exceeds 40 km, average crustal velocity is somewhat higher than in younger stable crust, and the crust is commonly complexly reflective/diffractive throughout. Two consequences of this model are that exposed ‘lower’ crustal sections in cratonic regions likely do not expose the lower crust, and continental growth rate estimates based on the ‘Freeboard Argument’ are likely to be grossly in error.

Comparison of crustal structure along DSS profiles in SE Fennoscandia

SUMMARY main features of the crustal structure of the Karelian and Svecofennian provinces are discussed on the basis of data obtained from deep seismic sounding (DSS) profiles across the SE Fennoscandian Shield. The data show that in the ancient Karelian province the crust is thinner (about 40 km), the average velocity is lower (about 6.4kms-l) and the Moho boundary is a sharp discontinuity which produces strong reflections. The high-velocity layer in the lower crust, if it exists at all, is thin. In the younger Svecofennian province, crustal thickness averages about 45 km with a velocity of 6.6-6.8 km s-'. The reflectivity of the Moho is usually weak and a high-velocity layer with velocities greater than 7 km s-' is ubiquitous in the lower crust. High velocities in the lower crust and upper mantle are found in deep Moho areas. Beneath the younger granitic intrusions, Gothian complex and Vyborg Rapakivi granites, the crust is thinner than on average.