Deep Lithospheric Structure Research Articles

Wawrzyniec Karol deTeisseyre: A pioneer in the study of “Cryptotectonics”

Contemporary studies of deep lithosphere structure in Europe focus on the so‐called Teisseyre‐Tornquist (or Tornquist‐Teisseyre) line, named for its discoverers, Polish geologist Wawrzyniec Teisseyre (1860–1939) and Swedish magnetologist A. J. H. Tornquist (1868–1944). Deep seismic soundings, first performed there by A. Guterch from the Institute of Geophysics, Polish Academy of Sciences; and later in cooperation with M. Grad from the Institute of Geophysics, University of Warsaw, supported by the physical interpretation, established that this line relates to a 50‐ to 100‐km‐wide deep rift zone which appears to be part of the broad deformation zone now called the Trans‐European Suture Zone (TESZ).

Modern Imaging Using Seismic Reflection Data

▪ Abstract Imaging using seismic reflection data has undergone tremendous advances over the past few years. The advances have been fostered in part by the availability of faster computers that have made more reliable algorithms for migration imaging feasible. The conventional approach to migration imaging, ray-based Kirchhoff migration, has been improved by the use of multiple-valued traveltime tables, ray amplitudes, and ray phases that can be calculated from various ray-tracing implementations. Wave-equation imaging, based on implementations of solutions of the wave equation, one-way wave equation, and approximations to the Lippmann-Schwinger equation, has become tractable. Wave-equation methods take account of wave phenomena such as focusing, defocusing, and diffraction that are important in many geological environments where imaging is used for petroleum exploration. There have also been applications of various types of migration imaging in basic studies of Earth structure. Such studies have been made to investigate deep Earth structure and large-scale lithospheric structure using waveforms from teleseisms as sources.

The application of Curie depth, effective elastic thickness, seismic tomography and seismicity in regional exploration area selection

When selecting geological provinces prospective for giant ore deposits knowledge of the location of deep lithospheric structures and craton margins, and an understanding of the temperature distribution of the Earth, are all important. Curie depth, effective elastic thickness, seismic tomography and seismicity are techniques with the potential to provide this information. Curie depth depends on the geothermal gradient and may, in principle, be derived from aeromagnetic data. However, it is concluded here that such Curie depth maps may have little value in many shield areas. Effective elastic thickness (TE) is a measure of the strength of the lithosphere. Methods for estimating TE are described and, as an example, a new TE map of Brazil is presented. TE gradients correlate well with province boundaries and seismicity, implying that they map major lithospheric structures. Seismic tomography uses the travel times of earthquake P- and S-waves to map velocity variations within the Earth. Many new tomographic velocity models are now becoming available, with resolution <100km in some cases. It appears that temperature is the major control on velocity, at least in the upper mantle. Global seismicity information may be analysed in terms of the distribution of earthquakes, their magnitudes and their focal depths. A correlation between seismicity and large mineral deposits has been observed in certain regions, which motivates the use of this data. Possible reasons for the correlation are discussed here. Since the different maps - TE, seismic velocity and seismicity - respond to different physical properties (or the same physical properties in different ways), it is most beneficial to interpret all three together.

EUROBRIDGE'95: deep seismic profiling within the East European Craton

The EUROBRIDGE deep seismic sounding (DSS) profile is a key component of a EUROPROBE project to examine Palaeoproterozoic processes of continental collision and crustal accretion. Its purpose is to establish the deep lithospheric structure of the East European Craton between the exposed Proterozoic and Archaean complexes of the Baltic and Ukrainian Shields. In 1994 a DSS experiment was recorded across the Baltic Sea from Västervik (Sweden) to Shventoji (Lithuania). We report on EUROBRIDGE'95, the first onshore stage of the seismic profile. It is 280km long, recorded from NW to SE on the Lithuanian part of the East European Platform, traversing the Proterozoic West Lithuanian Granulite Domain (WLG) and East Lithuanian Belt (EL) terranes. Explosive shots of up to 1000kg TNT were detonated at 10 shotpoints (SP01–SP10) at intervals of about 30km. Arrivals were recorded at 76 3-component seismograph stations with an average station spacing of 3.5km, providing high quality records. A 11th shot of 3000kg (SP00) was fired in the Baltic Sea close to Gotland. Raytracing analysis of refracted and reflected P-waves has been used to determine a 2-dimensional seismic velocity model for the crust and uppermost mantle below EUROBRIDGE'95. The thickness of the Phanerozoic sedimentary cover decreases from 2.2km in the north–west near the Baltic Sea coast to 0.4km at the south–east end of the profile near the Lithuania/Belarus border. Crust in the north–west and central part of profile consists of two major layers with a thickness of about 44km, increasing to 50km and three layers in the south–east. Crystalline upper crust is about 20km thick, thinning in the south–east, with P-wave velocities of 6.0–6.3km/s. A very weak low velocity zone, with a velocity contrast of 0.1–0.2km/s, occurs at 8–13km depth below the north–west and central part of the profile only. Lower crust exhibits velocities of commonly 6.5–6.9km/s, and thickens to the south–east with P-wave velocities up to 7.0km/s in the deepest parts. Crystalline crust is characterised by low velocity gradients and small velocity contrasts at most seismic boundaries. Major lateral changes in crustal velocity structure at all depths can be spatially correlated with the WLG–EL boundary determined from near surface geological information. Very strong reflections from the Moho boundary are observed. The mantle P-wave velocity immediately below the crust is 8.2–8.35km/s. A reflector in the lower lithosphere at a depth of almost 70km was found below Lithuania. Reflectivity modelling of the Gotland shot data suggests that this interface is absent offshore, where mantle velocities are lower. The DSS model supports the interpretation of the WLG and EL as terranes of Proterozoic age forming part of Fennoscandia. Later modification of crustal structure may have occurred, possibly by a mantle-heating episode centred on the Baltic Sea area.

Lithosphere structure along the northern part of EUROBRIDGE in Lithuania; results from integrated interpretation of DSS and gravity data

In the summer of 1995, the first on-shore phase of the EUROBRIDGE DSS experiment took place in Lithuania. The quality of recordings was good and contained clear P-wave reflections from inter-crustal interfaces and the Moho boundary. Very clear reflections were also registered from the interface below the Moho boundary. The arrivals of both reflected and refracted P-waves from 10 shotpoints were processed using the seis83 ray-tracing program. As a result, a 2-D seismic model along the Lithuanian part of EUROBRIDGE has been obtained. The comparison of synthetic and experimental seismograms showed good agreement of the model with registered travel times. To visualise better the existing tectonic discontinuities, an integrated 2-D seismic and density model along the northern part of EUROBRIDGE was compiled. For this purpose, the technique of gravity data inversion based on a non-linear relationship between density and seismic P-wave velocity was used. This integrated velocity–density model helped to reveal differences in deep lithosphere structure beneath the northern part of EUROBRIDGE where the tectonic contact of two Proterozoic terrains is located.

Temporary Array Data for Studying Seismic Anisotropy of Variscan Massifs – The Armorican Massif, French Massif Central And Bohemian Massif

We present the first results of a comparison of deep lithosphere structure of three Variscan massifs - the Armorican Massif, French Massif Central and Bohemian Massif, as revealed by recent tomographic studies of seismic anisotropy. The data originate from several field measurements made in temporary arrays of stations equipped with both short-period and broadband seismometers with digital recording. The study is based on teleseismic body waves and a joint inversion of anisotropic data (P-residual spheres, the fast shear-wave polarizations and split times) and demonstrates that the three Variscan massifs appear to consist of at least two parts with different orientation of large-scale fabric derived from seismic anisotropy. The boundaries of anisotropic lithospheric domains are related to prominent tectonic features recognised on the surface as sutures, shear zones or transfer fault zones, as well as grabens, thus indicating that some of them extend deep through the entire lithosphere.

The deep structure of the Australian continent from surface wave tomography

We present a new model of 3-D variations of shear wave speed in the Australian upper mantle, obtained from the dispersion of fundamental and higher-mode surface waves. We used nearly 1600 Rayleigh wave data from the portable arrays of the Skippy project and from permanent stations (from Agso, Iris and Geoscope). Agso data have not been used before and provide better data coverage of the Archean cratons in western Australia. Compared to previous studies we improved the vertical parameterization, the weighting scheme that accounts for variations in data quality and reduced the influence of epicenter mislocation on velocity structure. The dense sampling by seismic waves provides for unprecedented resolution of continental structure, but the wave speed beneath westernmost Australia is not well constrained. Global compilations of geological and seismological data (using regionalizations based on tectonic behavior or crustal age) suggest a correlation between crustal age and the thickness and composition of the continental lithosphere. However, the age and the tectonic history of crustal elements vary on wavelengths much smaller than have been resolved with global seismological studies. Using our regional upper mantle model we investigate how the seismic signature of tectonic units changes with increasing depth. At large wavelengths, and to a depth of about 200 km, the inferred velocity anomalies corroborate the global pattern and display a progression of wave speed with crustal age: slow wave propagation prevails beneath the Paleozoic fold belts in eastern Australia and wave speeds increase westward across the Proterozoic and reach a maximum in the Archean cratons. The high wave speeds associated with Precambrian shields extend beyond the Tasman Line, which marks the eastern limit of Proterozoic outcrop. This suggests that parts of the Paleozoic fold belts are underlain by Proterozoic lithosphere. We also infer that the North Australia craton extends off-shore into Papua New Guinea and beneath the Indian Ocean. For depths in excess of 200 km a regionalization with smaller units reveals that some tectonic subregions of Proterozoic age are marked by pronounced velocity highs to depths exceeding 300 km, but others do not and, surprisingly, the Archean units do not seem to be marked by such a thick high wave speed structure either. The Precambrian cratons that lack a thick high wave speed “keel” are located near passive margins, suggesting that convective processes associated with continental break-up may have destroyed a once present tectosphere. Our study suggests that deep lithospheric structure varies as much within domains of similar crustal age as between units of different ages, which hampers attempts to find a unifying relationship between seismic signature and lithospheric age.

Deep structure of the continental lithosphere in an unextended orogen: An explosive‐source seismic reflection profile in the Urals (Urals Seismic Experiment and Integrated Studies (URSEIS 1995))

Explosive‐source near‐vertical seismic reflection data from the Urals Seismic Experiment and Integrated Studies (URSEIS) profile displays an image of orogenic lithosphere to depths of 225 km. The reflective crustal section is characterized by dipping, crustal‐scale shear zones, and a pronounced crustal root that show no evidence of overprinting by extension and are preserved since late Paleozoic collision. Toward the east, the Uralian crustal root is composed of a partially subducted volcanic arc, while to the west it has affinity with the East European platform. The reflection character of the Moho varies across tectonic strike and implies that the Moho (1) acted as a structural detachment beneath the Trans‐Uralian Zone, (2) forms a tectonic boundary between terranes beneath the East Uralian Zone, (3) consists of a transitional zone from eclogitized lower crust to peridotitic upper mantle in the crustal root, (4) is a 3–9 km thick reflective zone beneath the fold and thrust belt, and (5) is a boundary &lt;200 m thick in the west. Mantle reflections at 80–100 km depth (22–24 s) may represent a continuation of the Main Uralian fault into the subcrustal lithosphere. Alternatively, these reflectors may signify Paleozoic or younger mantle shear zones or the top of a zone of partial melting in the upper mantle. Deeper mantle reflections at 140–160 km (35–45 s) and 225 km (55 s) may image mafic intrusions at the base of the lithosphere or along localized shear zones in the upper mantle.

The role of deep lithospheric structure in the genesis of giant ore deposits

The role of deep lithospheric structure in the genesis of giant ore deposits

Geoid anomalies and the structure of continental and oceanic lithospheres

Geoid anomalies in the wavelength range corresponding to spherical harmonies 6 to 30 are used to bring new constraints on the deep structure of the continental lithosphere, on the nature of upper mantle seismic anomalies, and on the thermal evolution of the oceanic lithosphere. For the shallow lithospheric density anomalies considered here, the geoid anomaly is shown to follow approximately the moment law derived from isostasy. By a procedure of least squares inversion, the geoid over old cratons is found to be equal to that over relatively young (50 Myr) oceanic lithosphere. The oceanic geoid as a function of age follows the expected linear trend with flattening at old ages. These results are independent of the degree at which the spherical harmonics are truncated. If the tectosphere consisted only of a thick thermal boundary layer, marked geoid lows, with an amplitude larger than 20 m, should be present over cratons, in disagreement with the present observation. The predicted geoid agrees with the observations if the negative density anomalies associated with a depleted peridotitic layer are present to the bottom of the lithosphere. This analysis of the geoid confirms the petrological inferences that the deep roots observed seismically in the top upper mantle form a chemically differentiate reservoir. The density of these cold roots is similar to that of mantle material elsewhere, because the increase in density resulting from their lower temperature is largely balanced by the decrease in density caused by their depletion. Analysis in wavelength range corresponding to harmonics 6 to 30 in the oceans confirms that the geoid flattens at old ages, and thus supports the view that an important amount of heat is transferred at the base of the lithosphere by small‐scale convection.

Seismic structure of the uppermost mantle beneath the Kenya rift

A major goal of the Kenya Rift International Seismic Project (KRISP) 1990 experiment was the determination of deep lithospheric structure. In the refraction/wide-angle reflection part of the KRISP effort, the experiment was designed to obtain arrivals to distances in excess of 400 km. Phases from interfaces within the mantle were recorded from many shotpoints, and by design, the best data were obtained along the axial profile. Reflected arrivals from two thin (< 10 km), high-velocity layers were observed along this profile and a refracted arrival was observed from the upper high-velocity layer. These mantle phases were observed on record sections from four axial profile shotpoints so overlapping and reversed coverage was obtained. Both high-velocity layers are deepest beneath Lake Turkana and become more shallow southward as the apex of the Kenya dome is approached. The first layer has a velocity of 8.05–8.15 km/s, is at a depth of about 45 km beneath Lake Turkana, and is observed at depths of about 40 km to the south before it disappears near the base of the crust. The deeper layer has velocities ranging from 7.7 to 7.8 km/s in the south to about 8.3 km/s in the north, has a similar dip as the upper one, and is found at depths of 60–65 km. Mantle arrivals outside the rift valley appear to correlate with this layer. The large amounts of extrusive volcanics associated with the rift suggest compositional anomalies as an explanation for the observed velocity structure. However, the effects of the large heat anomaly associated with the rift indicate that composition alone cannot explain the high-velocity layers observed. These layers require some anisotropy probably due to the preferred orientation of olivine crystals. The seismic model is consistent with hot mantle material rising beneath the Kenya dome in the southern Kenya rift and north-dipping shearing along the rift axis near the base of the lithosphere beneath the northern Kenya rift. This implies lithosphere thickening towards the north and is consistent with a thermal thinning of the lithosphere from below in the south changing to thinning of the lithosphere due to stretching in the north.

Interpretation of Long-Period Magnetotelluric Soundings In Friuli (North-East Italy) and the Electrical Characteristic of the Lithosphere

SUMMARY One-year-long records of two magnetotelluric (MT) stations, located in the seismic Friuli region (north-east Italy), were analysed in the 60-28 800s period range. the aim was the investigation of the deep lithospheric electric structure. the computation of the impedances was made by an iterative signal-to-noise enhancing method. Impedances were analysed with complex singular value decomposition (SVD). Some tests were made trying to consider the distortion due to the shallow in-homogeneities. 1-D inversion of the apparent resistivity curves was carried out. the models obtained indicate that the lithosphere thickens from 90 to 120 km westward, in agreement with surface-wave inversion. the westernmost sounding, located in an area of thickened crust, indicates the presence of a ∼10 km thick intracrustal conductor, which may be associated with incipient melting of lower crustal rocks.

Quaternary kinematic evolution of the Southern Appennines. relationships between surface geological features and deep lithospheric structures

Quaternary kinematic evolution of the Southern Appennines. relationships between surface geological features and deep lithospheric structures

Deep lithospheric structure across the Central African Shear Zone in Cameroon

SUMMARY Teleseismic P- and PKP-arrival times recorded by a network of 40 seismic stations deployed along a 300 km profile across the Adamawa Plateau at the northern end of the Volcanic Line in central Cameroon provide constraints on lithospheric thickness and anisotropy within the subcrustal lithosphere. These data indicate a thinned lithosphere beneath the Central African Shear Zone, where seismologically defined asthenosphere upwells from a depth of about 190 km to about 120 km in a relatively narrow belt. Thus it has only a low-amplitude effect on the observed gravity anomalies; the Bouguer gravity high over the Garoua Rift is consistent with crustal thinning beneath it. An abrupt change of the lithospheric thickness beneath the Northern Boundary Fault correlates with both the topographic relief and a distinct change of the orientation of relatively high- and low-velocity directions, which we infer to be due to anisotropy within the subcrustal lithosphere. This fault may represent an important accretionary suture zone dividing lithospheric blocks that originated in different tectonic settings and acquired different, frozen-in anisotropy.

Results of 1992 seismic reflection experiment in Lake Baikal

Lake Baikal, at more than 600 km long and 1632 m deep, covers the central third of the Baikal Rift (Figure 1). It is the world's most voluminous lake, containing 20% of the world's surficial freshwater, and it is probably also the oldest lake, at &gt;15 Ma. The Lake Baikal Rift occupies the boundary between the Precambrian Siberian craton and several microplates of south‐central Asia [Zonenshain and Savostin, 1981] (Figure 1). Topics of current geoscience research in Lake Baikal include the nature and history of extension and subsidence in the region, deep lithospheric structure, the paleoclimate record of central Asia, and the history of sedimentation and water level fluctuation in the lake. Another topic of recent debate is whether the rift formed actively via mantle doming [Logatchev and Florensov, 1978], or passively as a result of distant plate interactions [e.g., Tapponnier and Molnar, 1979].

THE DNIEPER‐DONETS PALEORIFT, UKRAINE: DEEP STRUCTURES AND HYDROCARBON ACCUMULATIONS

Numerous deep seismic and borehole studies have been made in the Dnieper‐Donets Paleorift (Ukraine), and, when combined with studies of oil‐ and gasfield distribution, indicate that there exists an interesting relationship between deep lithospheric structure and evolution, and the hydrocarbon potential of the Paleorift.It has been found that hydrocarbon accumulations tend to be associated with slopes in the Moho discontinuity, where most faults are located, and also where the basement and the lower layers of sedimentary cover have been fragmented by faults. These areas are the most promising for exploration.These facts could imply an abiogenic, mantle origin for oil and gas. Alternatively, the hot mantle ascending beneath the Dnieper‐Donets Paleorift may have created thermodynamic or fluid‐flow conditions within the sedimentary cover which were favourable for the transformation of biogenic material into oil and gas. In this case, mantle fluid‐flow along faults assisted the migration of hydrocarbons and its accumulation in traps.

Metallogeny and deep structure of the West Tien-Shan lithosphere

Metallogeny and deep structure of the West Tien-Shan lithosphere

The deep lithosphere in the Alps: a model inferred from P residuals

The tomographic studies of the Alps and their surroundings, based on the relative P residuals, indicated two lithosphere roots attaining depths over 200 km separated by an upwelling of the lithosphere-asthenosphere transition beneath the central part of this mountain range. The directional dependence of relative P velocities indicates an anisotropic wave propagation in the subcrustal lithosphere beneath the Alps and their surroundings. The presented model of the deep lithosphere structure suggests several phases of the tectonic development of this region: collision of a tip of the Adriatic plate with Europe in the eastern Alps and dextral strike-slip movement of the Adriatic ending with underthrusting of Europe beneath the western Alps and counterclockwise rotation. The latter contributes to a steep plunge of both plates beneath the eastern Alps and to stress accumulation in the Friuli region. A model with delamination of the rigid upper crust and its rotation resolves some of the inconsistencies between the near-surface and deep tectonics and also explains the distribution of seismicity.

Seismologically determined deep lithosphere structure in Fennoscandia

(1988). Seismologically determined deep lithosphere structure in Fennoscandia. Geologiska Foreningen i Stockholm Forhandlingar: Vol. 110, No. 4, pp. 380-382.

Compensation of Paleozoic orogens: a comparison of the Urals to the Appalachians

The Urals and Appalachians are two Paleozoic mountain belts with similar geologic and morphologic features. We combine geophysical (Bouguer gravity and deep seismic sounding) and geological observations from the Ural Mountains to test whether the surficial similarities of these Paleozoic orogens extend to include deep lithospheric structure and compensation mechanism. We find the combined data suggest support of the mountains by a stiff continental slab which is depressed by an effective subcrustal load in addition to the topography. As has been recently proposed for the Appalachians, our preferred model, which fits the geophysical data and predicts the presence and shape of the Ural foredeep to the west of the mountains requires eastward-directed underthrusting of the Russian platform behaving as an elastic plate more than 50-km thick. It is unlikely that an elastic plate this stiff could have resulted from conductive cooling of a thermal plate only 125-km thick. The weight of the existing mountains on such an elastic plate is insufficient to produce the deflection of the Moho inferred from seismic and gravity data. An additional load on the elastic plate may be provided by a buried mass of similar origin to surface outcrops of dense mafic and ultramafic rocks in the eastern Urals. Alternatively, the load may represent the effect of a shallowing of the Moho east of the mountains which is a remnant of a buried passive continental margin. Seismic data and Bouguer gravity anomalies suggest that the effect of the subsurface mass anomaly is at least as great as the present-day topography and that the Russian plate flexurally underthrusts the Urals for more than 100 km. We speculate that similar effective subsurface loads may exist at younger continental orogens but are obscured by the topographic load in less eroded mountain belts.