Detecting rift basins in the Evans Ice Stream region of West Antarctica using airborne gravity data

A total of 11,500 line km of aerogravity data have been used to construct an free-air gravity anomaly map for the Antarctic region that may contain the microplate boundary between the Haag Nunataks block and southern Antarctic Peninsula. Along-line free-air gravity anomaly data resolved wavelengths of 9 km or greater with better than 5 mGal accuracy. Coincident radio echo soundings provided data to construct a digital terrain model. The gravity effect of the terrain was calculated by Gauss–Legendre quadrature (GLQ) and spectrally correlated with the free-air gravity data. Terrain-correlated free-air anomalies related to possible isostatic imbalances of the crust were separated from terrain-decorrelated anomalies that may reflect intra-crustal density contrasts. Subtracting terrain-correlated free-air anomalies from the gravity effects of the terrain yielded compensated terrain gravity effects (CTGE) that were used to model the Moho by inversion. The results indicate moderate but significant crustal thinning below the Evans Ice Stream that is consistent with an extensional origin for the deep, wide, steep-sided trough that contains the ice stream as well as the continued elevation of the footwall flank of the basin. Changes along the axis of the rift, both in the gravity anomaly field and the distribution of Moho topography, can be explained by processes associated with continental lithospheric extension. Subsequently, many of the features produced by extension have been modified by glacial erosion and the sub-ice topography and gravity data reflect this.     

The crustal structure of the Eastern Alps from a combination of 3D gravity modelling and isostatic investigations

The transition between European and Adriatic crust is an important feature related to the plate collision that formed the European Alps. The diversity of seismic and geological information allows the construction of two alternative 3D density models, which both match the observed gravity field. Different seismic experiments suggest a thickness for the Adriatic crust between 30 and 40 km. The thick crust model requires an unusually dense lower crust (>3050 kg/m³) to reproduce the observed Bouguer anomaly. To evaluate the two alternative models, the isostatic implications of the geometry and density distribution within both 3D models are investigated, using local (Airy) and regional (Vening Meinesz) isostasy.Airy isostatic investigations show that the Eastern Alps are not isostatically compensated and the residuals correlate strongly with exposed geological formations. Subsequently, subsurface loading is an important factor controlling isostatic processes. The different geometry and densities in the two 3D models imply different loading at the crust–mantle boundary. The subsurface loads calculated from the 3D density models were used to estimate regional isostasy by a convolution method. In general, small rigidity values (D<10×10²¹ Nm) are determined for the Eastern Alpine lithosphere. In the model with a 40-km-thick Adriatic crust, high flexural rigidities are inferred for the Adriatic plate (>100×10²¹ Nm), but these values are unusual for an active orogenic region. The results point to the interfingering of European and Adriatic crust that results in the squeezing of European crust between Adriatic crust and mantle with additional contamination by mantle material.     

New gravity maps of the Eastern Alps and significance for the crustal structures

The deep seismic profile Transalp crosses, from north to south, Germany, Austria and Italy. The gravity measurements for each country were made by national agencies with different reference systems and data reduction methods. Within the frame of the Transalp-project a comprehensive database of the Eastern Alps was compiled covering an area of 3.5° by 4° in longitude and latitude (275 by 445 km), respectively. To increase the data coverage in the south Alpine area two gravity surveys were carried out, resulting in 469 areally distributed new stations, of which 215 have been measured with the intent to improve the geoid in the area of the planned Brenner Basistunnel (BBT). The resulting gravity database is the best in terms of resolution and data quality presently available for the Eastern Alps. Here the free air, Bouguer and isostatic gravity fields are critically discussed. The spatial density of existing gravity stations in the three countries is discussed. On the Italian side of the Alps the spatial density is rather sparse compared to the Austrian side. The Bouguer-gravity field varies between − 190 * 10^− 5 m/s² and + 25 * 10^− 5 m/s², with the minimum located along the Alpine high topographic chain, but with a small offset (a few tens of km) to the greatest topographic elevation, showing that the Airy-type local isostatic equilibrium does not fully apply here. The maximum of the Bouguer anomaly has an elongated shape of 100 by 50 km located between the towns of Verona and Vicenza and covers the Venetian Tertiary Volcanic Province (VTVP), a feature not directly related to the plate collision in the Eastern Alps. The gravity high is only partly explainable by high-density magmatic rocks and requires also a deeper source, like a shallowing of the Moho. The isostatic residual anomalies (Airy model) are in the range ± 50 * 10^− 5 m/s², with the greatest positive anomaly corresponding to the location of the VTVP, indicating here under-compensation of masses. At last a discussion of a 2D density model based on reflection seismic data and receiver functions is made.     

The isostatic state of the southern Urals crust

The Uralide orogen, in Central Russia, is the focus of intense geoscientific investigations during recent years. The international research is motivated by some unusual lithospheric features compared with other collisional belts including the preservation of (a) a collisional architecture with an orogenic root and a crustal thickness of 55–58 km, and (b) large volumes of very low-grade and non-metamorphic oceanic crust and island arc rocks in the upper crust of a low–relief mountain belt. The latter cause anomalous gravity highs along the thickened crust and the isostatic equilibrium inside the Uralides lithosphere as well as the overthrust high-metamorphic rocks. The integrated URSEIS '95 seismic experiment provides fundamentally new data revealing the lithospheric architecture of an intact Paleozoic collisional orogen that allows the construction of density models. In the Urals' lithosphere different velocity structures resolved by wide-angle seismic experiments along both the URSEIS '95- and the Troitsk profile. They can be used to constrain lithospheric density models: a first model consists of a deep subducted continental lower crust which has been highly eclogitized at depths of 60–90 km to a density of 3550 kg/m³. The second model shows a slightly eclogitized lower crust underlying the Uralide orogen with a crustal thickness of 60 km. The eclogitized lower crust causes a too-small impedance contrast to the lithospheric mantle resulting in a lack of reflectors in the area of the largest crustal thickness. Both models fit the measured gravity field. Analyzing the isostatic state of the southern Urals' lithosphere, both density models are in isostatic equilibrium.     

Flexural and gravity modelling of the Mérida Andes and Barinas–Apure Basin, Western Venezuela

The kinematic evolution of the Barinas–Apure Basin and the southern Mérida Andes from Lower Miocene to the Present is numerically modelled using flexural isostatic theory and geophysical and geological data. Two published regional transects are used to build up a reference section, which is then used to constrain important parameters (e.g. shortenings and sedimentary thicknesses) for the flexural modelling. To control the location of the main fault system in the flexural model earthquake information is also used. The estimated flexural elastic thickness of the South American lithosphere beneath the Barinas–Apure Basin and the Mérida Andes Range is 25 km. The value for the final total shortening is 60 km. The flexural isostatic model shows that the Andean uplift has caused the South American lithosphere subsidence and the development of the Barinas–Apure Basin.In addition, gravity modelling was used to understand deep crustal features that could not be predicted by flexural theory. Consequently, the best-fit flexural model is used to build a gravity model across the Mérida Andes and the Barinas–Apure Basin preserving the best-controlled structural features from the flexural modelling (e.g. basin wavelength and depth) and slightly changing the main bodies density values and deep crustal structures. The final gravity model is intended to be representative of the major features affecting the gravity field in the study area. The predicted morphology in the lower crustal level of the final gravity model favours the hypothesis of a present delamination or megathrust of the Maracaibo crust over the South American Shield. This process would use the Conrad discontinuity as a main detachment surface within an incipient NW dipping continental subduction.     

Gravity anomalies and dynamics of the Swiss Alps

An interpretation of the gravity observations in Switzerland is presented. The gravity anomalies are mainly caused by crustal effects. Taking 0.5 g/cm³ as the average density contrast between crust and upper mantle, the remaining positive residual field of + 50 mGal can be explained as the effect of the so-called “lithospheric root” underneath the Swiss Alps. This “root” must be considered as a relatively cold body with increased density. In order to calculate the thermally induced density distribution in the upper mantle, a kinematic and geothermal model has been constructed simulating the Alpine history for a time span covering the last 40 m.y.At the same time the moving lithospheric body (crustal uplift and subsidence of the lower lithosphere) is analysed from a dynamical point of view in order to test the kinematic conditions. The calculation leads to a mantle subsidence rate of 1 mm/y for the present period.     

Satellite Gravity Anomalies and Their Correlation with the Major Tectonic Features in the South China Sea

The South China Sea (SCS) is a region of interaction among three major plates: the Pacific, Indo-Australian and Eurasian. The collision of the Indian subcontinent with the Eurasian plate in the northwest, back-arc spreading at the center, and subduction beneath the Philippine plate along Manila trench in the east and the collision along Palawan trough in the south have produced complex tectonic features within and along the SCS. This investigation examines the satellite-derived gravity anomalies of the SCS and compares them with major tectonic features of the area. A map of Bouguer gravity anomaly is derived in conjunction with available seafloor topography to investigate the crustal structure. The residual isostatic gravity anomaly is calculated assuming that the Cenozoic sedimentary load is isostatically compensated. The features in the gravity anomalies in general correlate remarkably well with the major geological features, including offsets in the seafloor spreading segments, major faults, basins, seamounts and other manifestations of magmatism and volcanism on the seafloor. They also correlate with the presumed location of continental-oceanic crust boundary. The region underlain by oceanic crust in the central part of the SCS is characterized by a large positive Bouguer gravity anomaly (220–330 mgal) as well as large free-air and residual isostatic anomalies. There are, however, important differences among spreading segments. For example, in terms of free-air gravity anomaly, the southwest section of mid-ocean has an approximately 50 km wide belt of gravity low superimposed on a broad high of 45 mgal running NW–SE, whereas there are no similar features in other spreading segments. There are indications that gravity anomalies may represent lateral variation in upper crustal density structure. For instance, free air and isostatic anomalies show large positive anomalies in the east of the Namconson basin, which coincide with areas of dense volcanic material known from seismic surveys. The Red River Fault system are clearly identified in the satellite gravity anomalies, including three major faults, Songchay Fault in the southwest, Songlo Fault in the Northeast and Central Fault in the center of the basin. They are elongated in NW–SE direction between 20±30'N and 17°N and reach to Vietnam Scarp Fault around 16°30'N. It is also defined that the crustal density in the south side of the Central Basin is denser than that in the north side of the Central Basin.     

Reviewing pre-TRANSALP DSS models

The interpretation of DSS (deep seismic soundings) profiles in Central and Eastern Alps is recalled in the paper and the models of the lower crust and Moho proposed several years ago are compared to the results of the TRANSALP seismic reflection profile. This evaluation highlights a good agreement as far as the geometry of the deep crustal structure is concerned. Therefore, the reliability of the interpretative models, previously exclusively based on DSS profiles, becomes improved. The deep structure beneath the whole Alpine range is examined reconsidering the map of the Moho boundary and the structural model already proposed for the central-eastern sector. Five main interpretative transects are put side by side, starting from the Western Alps and moving eastwards to the Swiss–Lombardian Central Alps (“European Geotraverse”), to the cross section from southern Bavaria to the Euganei Hills, to the TRANSALP profile, and finally to the easternmost profile available so far (southern Bavaria–Trieste). The comparison outlines lateral variations of the deep crustal structure as well as a sharp contrast between the Adria and the European lower crust and Moho. The transition from the Adria plate to the Dinaric domain remains, up to now, undefined.     

Temperature and density of the Tyrrhenian lithosphere and slab and new interpretation of gravity field in the Tyrrhenian Basin

The gravity anomaly field of the Tyrrhenian basin and surrounding regions reflects the complex series of geodynamic events active in this area since the Oligocene–Miocene. They can resume in lithospheric thinning and asthenospheric rising beneath the Tyrrhenian Basin, coexisting with the roll-back subduction of the African plate margin westward sinking beneath the Calabrian Arc. The geographic closeness between these processes implies an intense perturbation of the mantle thermal regime and an interference at regional scale between the related gravity effects.A model of the litho-asthenospheric structure of this region is suggested, showing a reasonable agreement with both the evidences in terms of regional gravity anomaly pattern and the results concerning thermal state and petro-physical features of the mantle. The first phase of this study consisted of the computation of the isotherms in the crust–mantle system beneath the Tyrrhenian Basin and, afterwards, of the density distribution within the partially melted upwelling asthenosphere. The second phase consisted of a temperature/density modelling of the slab subducting beneath the Calabrian Arc. Finally, a 2^1 / 2 interpretation of gravity data was carried out by including as constraints the results previously obtained. Thus, the final result depicts a model matching both gravity, thermal and petrographic data. They provide (a) a better definition of the thermal regime of the passive mantle rise beneath the Tyrrhenian basin by means of the estimation of the moderate asthenospheric heating and (b) a model of lithospheric slab subducting with rates that could be smaller than generally suggested in previous works.     

Local tomography and focal mechanisms in the south-western Alps: Comparison of methods and tectonic implications

We investigate how focal solutions and hypocenter locations may depend on the ray tracing algorithm and the strategy of velocity inversion. Using arrival times from a temporary seismological network in the south-western Alps, a local earthquake tomography has been performed by Paul et al. [Paul, A., Cattaneo, M., Thouvenot, F., Spallarossa, D., Béthoux, N., and Fréchet, J., 2001. A three-dimensional crustal velocity model of the south-western Alps from local earthquake tomography. J. Geophys. Res. 106, 19367–19390.] with the method developed by Thurber [Thurber, C.H., 1993. Local earthquake tomography: velocity and Vp/Vs-Theory, in Seismic Tomography: Theory and practice, Iyer, H.M., and Irahara eds., Chapman and Hall, New York, 563–583.]. Another inversion of the same data set is performed here using a different tomography code relying on a shooting paraxial method and cubic interpolation of velocities. The resulting images display the same main features, although Thurber's code appears to be more robust in regions with scarce ray coverage and strong velocity contrasts. Concerning hypocenter location in Piemont units, one major result is the concentration of hypocenters at the boundary between the mantle wedge of the Ivrea body and the European crust. Forty-six focal mechanisms are shown that were computed using both the take-off angles in the minimum 1-D model and in the 3-D velocity structures resulting from the two inversions. The sets of focal solutions are very similar, proving the reliability and the coherency of the focal solutions. The widespread extension in the core of the western Alps is confirmed whereas a few compressive solutions are found east of the Piemont units. These results constrain the sharp change of stress tensor and evidence a decoupling of strain beneath the east of Dora Maira massif up to beneath the north of Argentera massif. On a geodynamical point of view seismicity and focal mechanism distribution are compatible with the present day models published for the western Alps, where the major feature is the lithospheric thickening [Schmid, S.M., and Kissling, E., 2000. The arc of the western Alps in the light of geophysical data on deep crustal structure. Tectonics, 19, 62–85.], implying widespread extension in the core of the western Alps [Sue, C., Thouvenot, F., Fréchet, J., and Tricart, P., 1999. Widespread extension in the core of the western Alps revealed by earthquake analysis. J. Geophys. Res., 104, 25611–25622.]. However the existence of compressive events dealing at depth with the boundary of Ivrea body allows to postulate that this geological structure is still tectonically active. Even if field work has not shown this so far, the Insubric line appears to extend toward the south at depth, as a blind fault, and to play a key role in the dynamics of the south-western Alps.     

Lithospheric transition from the Variscan Iberian Massif to the Jurassic oceanic crust of the Central Atlantic

A 1000-km-long lithospheric transect running from the Variscan Iberian Massif (VIM) to the oceanic domain of the Northwest African margin is investigated. The main goal of the study is to image the lateral changes in crustal and lithospheric structure from a complete section of an old and stable orogenic belt—the Variscan Iberian Massif—to the adjacent Jurassic passive margin of SW Iberia, and across the transpressive and seismically active Africa–Eurasia plate boundary. The modelling approach incorporates available seismic data and integrates elevation, gravity, geoid and heat flow data under the assumptions of thermal steady state and local isostasy. The results show that the Variscan Iberian crust has a roughly constant thickness of 30 km, in opposition to previous works that propose a prominent thickening beneath the South Portuguese Zone (SPZ). The three layers forming the Variscan crust show noticeable thickness variations along the profile. The upper crust thins from central Iberia (about 20 km thick) to the Ossa Morena Zone (OMZ) and the NE region of the South Portuguese Zone where locally the thickness of the upper crust is <8 km. Conversely, there is a clear thickening of the middle crust (up to 17 km thick) under the Ossa Morena Zone, whereas the thickness of the lower crust remains quite constant (6 km). Under the margin, the thinning of the continental crust is quite gentle and occurs over distances of 200 km, resembling the crustal attitude observed further north along the West Iberian margins. In the oceanic domain, there is a 160-km-wide Ocean Transition Zone located between the thinned continental crust of the continental shelf and slope and the true oceanic crust of the Seine Abyssal Plain. The total lithospheric thickness varies from about 120 km at the ends of the model profile to less than 100 km below the Ossa Morena and the South Portuguese zones. An outstanding result is the mass deficit at deep lithospheric mantle levels required to fit the observed geoid, gravity and elevation over the Ossa Morena and South Portuguese zones. Such mass deficit can be interpreted either as a lithospheric thinning of 20–25 km or as an anomalous density reduction of 25 kg m⁻³ affecting the lower lithospheric levels. Whereas the first hypothesis is consistent with a possible thermal anomaly related to recent geodynamics affecting the nearby Betic–Rif arc, the second is consistent with mantle depletion related to ancient magmatic episodes that occurred during the Hercynian orogeny.     

Gravity anomalies and the Indian lithosphere: Review and analysis of existing gravity data

The upper part of the lithosphere has been actively involved in various exogenic and endogenic processes which have left their imprint on the gravity field on the Indian Peninsula and the Himalaya. Analysis of the gravity field over the Dharwar craton shows that the greenstone belts of this craton have been formed as a result of development of deep fractures in the earth's crust during Archaean times. Precambrian mountain ranges such as the Aravallies, Vindhyans, Satpura and Eastern Ghats are located peripheral to Archaean cratons. Most of these mountain belts are characterized by gravity highs suggesting that the underlying crust is of higher than normal density. These mountain ranges with the exception of the Eastern Ghats do not appear to be locally compensated. Regional compensation seems to prevail over all these areas. Eastern Ghats ranges are also underlain by a crust of higher than normal density relative to the Dharwar and Bastar cratons and exist with a sharp contact with the cratons in the West. Isostatic compensation in the Eastern Ghats appears to have been achieved by thickening of the underlying crust. The Himalaya has attained a fairly high degree of isostatic compensation.     

Gravity anomalies and associated tectonic features over the Indian Peninsular Shield and adjoining ocean basins

A combined gravity map over the Indian Peninsular Shield (IPS) and adjoining oceans brings out well the inter-relationships between the older tectonic features of the continent and the adjoining younger oceanic features. The NW–SE, NE–SW and N–S Precambrian trends of the IPS are reflected in the structural trends of the Arabian Sea and the Bay of Bengal suggesting their probable reactivation. The Simple Bouguer anomaly map shows consistent increase in gravity value from the continent to the deep ocean basins, which is attributed to isostatic compensation due to variations in the crustal thickness. A crustal density model computed along a profile across this region suggests a thick crust of 35–40 km under the continent, which reduces to 22/20–24 km under the Bay of Bengal with thick sediments of 8–10 km underlain by crustal layers of density 2720 and 2900/2840 kg/m³. Large crustal thickness and trends of the gravity anomalies may suggest a transitional crust in the Bay of Bengal up to 150–200 km from the east coast. The crustal thickness under the Laxmi ridge and east of it in the Arabian Sea is 20 and 14 km, respectively, with 5–6 km thick Tertiary and Mesozoic sediments separated by a thin layer of Deccan Trap. Crustal layers of densities 2750 and 2950 kg/m³ underlie sediments. The crustal density model in this part of the Arabian Sea (east of Laxmi ridge) and the structural trends similar to the Indian Peninsular Shield suggest a continent–ocean transitional crust (COTC). The COTC may represent down dropped and submerged parts of the Indian crust evolved at the time of break-up along the west coast of India and passage of Reunion hotspot over India during late Cretaceous. The crustal model under this part also shows an underplated lower crust and a low density upper mantle, extending over the continent across the west coast of India, which appears to be related to the Deccan volcanism. The crustal thickness under the western Arabian Sea (west of the Laxmi ridge) reduces to 8–9 km with crustal layers of densities 2650 and 2870 kg/m³ representing an oceanic crust.     

Lithospheric evolution of the Andean fold–thrust belt, Bolivia, and the origin of the central Andean plateau

We combine geological and geophysical data to develop a generalized model for the lithospheric evolution of the central Andean plateau between 18° and 20° S from Late Cretaceous to present. By integrating geophysical results of upper mantle structure, crustal thickness, and composition with recently published structural, stratigraphic, and thermochronologic data, we emphasize the importance of both the crust and upper mantle in the evolution of the central Andean plateau. Four key steps in the evolution of the Andean plateau are as follows. 1) Initiation of mountain building by 70 Ma suggested by the associated foreland basin depositional history. 2) Eastward jump of a narrow, early fold–thrust belt at 40 Ma through the eastward propagation of a 200–400-km-long basement thrust sheet. 3) Continued shortening within the Eastern Cordillera from 40 to 15 Ma, which thickened the crust and mantle and established the eastern boundary of the modern central Andean plateau. Removal of excess mantle through lithospheric delamination at the Eastern Cordillera–Altiplano boundary during the early Miocene appears necessary to accommodate underthrusting of the Brazilian shield. Replacement of mantle lithosphere by hot asthenosphere may have provided the heat source for a pulse of mafic volcanism in the Eastern Cordillera and Altiplano at 24–23 Ma, and further volcanism recorded by 12–7 Ma crustal ignimbrites. 4) After 20 Ma, deformation waned in the Eastern Cordillera and Interandean zone and began to be transferred into the Subandean zone. Long-term rates of shortening in the fold–thrust belt indicate that the average shortening rate has remained fairly constant (8–10 mm/year) through time with possible slowing (5–7 mm/year) in the last 15–20 myr. We suggest that Cenozoic deformation within the mantle lithosphere has been focused at the Eastern Cordillera–Altiplano boundary where the mantle most likely continues to be removed through piecemeal delamination.     

Late Permian–Middle Jurassic lithospheric thinning in Peru and Bolivia, and its bearing on Andean-age tectonics

Integrated studies and revisions of sedimentary basins and associated magmatism in Peru and Bolivia (8–22°S) show that this part of western Gondwana underwent rifting during the Late Permian–Middle Jurassic interval. Rifting started in central Peru in the Late Permian and propagated southwards into Bolivia until the Liassic/Dogger, along an axis that coincides with the present Eastern Cordillera. Southwest of this region, lithospheric thinning developed in the Early Jurassic and culminated in the Middle Jurassic, producing considerable subsidence in the Arequipa basin of southern Peru. This 110-Ma-long interval of lithospheric thinning ended 160 Ma with the onset of Malm–earliest Cretaceous partial rift inversion in the Eastern Cordillera area.The lithospheric heterogeneities inherited from these processes are likely to have largely influenced the distribution and features of younger compressional and/or transpressional deformations. In particular, the Altiplano plateau corresponds to a paleotectonic domain of “normal” lithospheric thickness that was bounded by two elongated areas underlain by thinned lithosphere. The high Eastern Cordillera of Peru and Bolivia results from Late Oligocene–Neogene intense inversion of the easternmost thinned area.     

Structure of the lithosphere beneath the Eastern Alps (southern sector of the TRANSALP transect)

The interpretation of the seismic Vibroseis and explosive TRANSALP profiles has examined the upper crustal structures according to the near-surface geological evidences and reconstructions which were extrapolated to depth. Only the southern sector of the TRANSALP transect has been discussed in details, but its relationship with the whole explored chain has been considered as well. The seismic images indicate that pre-collision and deep collision structures of the Alps are not easily recognizable. Conversely, good records of the Neo-Alpine to present architecture were provided by the seismic sections.Two general interpretation models (“Crocodile” and “Extrusion”) have been sketched by the TRANSALP Working Group [2002]. Both illustrate the continental collision producing strong mechanical interaction of the facing European and African margins, as documented by giant lithosphere wedging processes. Arguments consistent with the “Extrusion” model and with the indentation of Adriatic (Southalpine) lithosphere underneath the Tauern Window (TW) are:

– According to the previous DSS reconstructions, the Bouguer anomalies and the Receiver Functions seismological data, the European Moho descends regularly attaining a zone south of the Periadriatic Lineament (PL). The Moho boundary and its geometry appear to be rather convincing from images of the seismic profile;

– the Tauern Window intense uplift and exhumation is coherent with the strong compression regime, which acted at depth, thus originating the upward and lateral displacement of the mobile and ductile Penninic masses according to the “Extrusion” model;

– the indentation of the Penninic mobile masses within the colder and more rigid Adriatic crust cannot be easily sustained. Wedging of the Adriatic stiffened lower crust, under high stresses and into the weaker Penninic domain, can be a more suitable hypothesis. Furthermore, the intrusion of the European Penninic crustal wedge underneath the Dolomites upper crust is not supported by any significant uplifting of the Dolomites. The total average uplift of the Dolomites during the Neogene appears to be 6−7 times smaller than that recognized in the TW. Markedly the northward dip of the PL, reaching a depth of approximately 20 km, is proposed in our interpretation;

– finally, the Adriatic upper crustal evolution points to the late post-collision change in the tectonic grow-up of the Eastern Alps orogenic chain. The tectonic accretion of the northern frontal zone of the Eastern and Central Alps was interrupted from the Late Miocene (Serravallian–Tortonian) onward, as documented by the Molasse basin evolution. On the contrary, the structural nucleation along the S-vergent tectonic belt of the eastern Southern Alps (Montello–Friuli thrust belt) severely continued during the Messinian and the Plio–Pleistocene. This structural evolution can be considered to be consistent with the deep under-thrusting and wedge indentation of the Adriatic lithosphere underneath the southern side of the Eastern Alps thrust-and-fold belt.

Similarly, the significance of the magmatic activity for the construction of the Southern Alps crust and for its mechanical and geological differentiation, which qualified the evolution of the thrust-and-fold belt, is highlighted, starting with the Permian–Triassic magmatism and progressing with the Paleogene occurrences along the Periadriatic Lineament and in the Venetian Magmatic Province (Lessini–Euganei Hills).     

The gravity field of the U.S. Atlantic continental margin

Approximately 39,000 km of marine gravity data collected during 1975 and 1976 have been integrated with U.S. Navy and other available data over the U.S. Atlantic continental margin between Florida and Maine to obtain a 10 mgal contour free-air gravity anomaly map. A maximum typically ranging from 0 to +70 mgal occurs along the edge of the shelf and Blake Plateau, while a minimum typically ranging from −20 to −80 mgal occurs along the base of the continental slope, except for a −140 mgal minimum at the base of the Blake Escarpment. Although the maximum and minimum free-air gravity values are strongly influenced by continental slope topography and by the abrupt change in crustal thickness across the margin, the peaks and troughs in the anomalies terminate abruptly at discrete transverse zones along the margin. These zones appear to mark major NW—SE fractures in the subsided continental margin and adjacent deep ocean basin, which separate the margin into a series of segmented basins and platforms. Rapid differential subsidence of crustal blocks on either side of these fractures during the early stages after separation of North America and Africa (Jurassic and Early Cretaceous) is inferred to be the cause of most of the gravity transitions along the length of margin. The major transverse zones are southeast of Charleston, east of Cape Hatteras, near Norfolk Canyon, off Delaware Bay, just south of Hudson Canyon and south of Cape Cod.Local Airy isostatic anomaly profiles (two-dimensional, without sediment corrections) were computed along eight multichannel seismic profiles. The isostatic anomaly values over major basins beneath the shelf and rise are generally between −10 and −30 mgal while those over the platform areas are typically 0 to +20 mgal. While a few isostatic anomaly profiles show local 10–20 mgal increases seaward of the East Coast Magnetic Anomaly (ECMA: inferred to mark the ocean-continent boundary), the lack of a consistent correlation indicates that the relationship of isostatic gravity anomalies to the magnetic anomalies and the ocean—continent transition is variable.Two-dimensional gravity models have been computed for two profiles off Cape Cod, Massachusetts and Cape May, New Jersey, where excellent reflection, refraction and magnetic control appear to define 10 and 12 km deep sedimentary basins beneath the shelf, respectively and 10 km deep basins beneath the rise. The basins are separated by a 6–8 km deep basement ridge which underlies the ECMA and appears to mark the landward edge of oceanic crust. The gravity models suggest that the oceanic crust is between 11 and 18 km thick beneath the ECMA, but decreases to a thickness of less than 8 km within the first 20–90 km to the southeast. In both profiles, the derived crustal thickness variations support the interpretation that the ECMA occurs over the ocean-continent boundary. The crust underlying the sedimentary cover appears to be 12 to 15 km thick on the landward side of the ECMA and gradually thickens to normal continental values of greater than 25 km within the first 60 to 110 km to the northwest. Multichannel seismic profiles across platform areas, such as Cape Hatteras and Cape Cod, indicate the ocean-continent transition zones there are much narrower than profiles across major sedimentary basins, such as the one off New Jersey.     

Lithosphere structure of European sedimentary basins from regional three-dimensional gravity modelling

A three-dimensional (3D) density model, approximated by two regional layers—the sedimentary cover and the crystalline crust (offshore, a sea-water layer was added), has been constructed in 1° averaging for the whole European continent. The crustal model is based on simplified velocity model represented by structure maps for main seismic horizons—the “seismic” basement and the Moho boundary. Laterally varying average density is assumed inside the model layers. Residual gravity anomalies, obtained by subtraction of the crustal gravity effect from the observed field, characterize the density heterogeneities in the upper mantle. Mantle anomalies are shown to correlate with the upper mantle velocity inhomogeneities revealed from seismic tomography data and geothermal data. Considering the type of mantle anomaly, specific features of the evolution and type of isostatic compensation, the sedimentary basins in Europe may be related into some groups: deep sedimentary basins located in the East European Platform and its northern and eastern margins (Peri-Caspian, Dnieper–Donets, Barents Sea Basins, Fore–Ural Trough) with no significant mantle anomalies; basins located on the activated thin crust of Variscan Western Europe and Mediterranean area with negative mantle anomalies of −150 to −200×10⁻⁵ ms⁻² amplitude and the basins associated with suture zones at the western and southern margins of the East European Platform (Polish Trough, South Caspian Basin) characterized by positive mantle anomalies of 50–150×10⁻⁵ ms⁻² magnitude. An analysis of the main features of the lithosphere structure of the basins in Europe and type of the compensation has been carried out.     

Mapping upper mantle anisotropy beneath SE France by SKS splitting indicates Neogene asthenospheric flow induced by Apenninic slab roll-back and deflected by the deep Alpine roots

The presence of two regional seismic networks in southeastern France provides us high-quality data to investigate upper mantle flow by measuring the splitting of teleseismic shear waves induced by seismic anisotropy. The 10 three-component and broadband stations installed in Corsica, Provence, and western Alps efficiently complete the geographic coverage of anisotropy measurements performed in southern France using temporary experiments deployed on geodynamic targets such as the Pyrenees and the Massif Central. Teleseismic shear waves (mainly SKS and SKKS) are used to determine the splitting parameters: the fast polarization direction and the delay time. Delay times ranging between 1.0 and 1.5 s have been observed at most sites, but some larger delay times, above 2.0 s, have been observed at some stations, such as in northern Alps or Corsica, suggesting the presence of high strain zones in the upper mantle. The azimuths of the fast split shear waves define a simple and smooth pattern, trending homogeneously WNW–ESE in the Nice area and progressively rotating to NW–SE and to NS for stations located further North in the Alps. This pattern is in continuity with the measurements performed in the southern Massif Central and could be related to a large asthenospheric flow induced by the rotation of the Corsica–Sardinia lithospheric block and the retreat of the Apenninic slab. We show that seismic anisotropy nicely maps the route of the slab from the initial rifting phase along the Gulf of Lion (30–22 Ma) to the drifting of the Corsica–Sardinia lithospheric block accompanied by the creation of new oceanic lithosphere in the Liguro–Provençal basin (22–17 Ma). In the external and internal Alps, the pattern of the azimuth of the fast split waves follows the bend of the alpine arc. We propose that the mantle flow beneath this area could be influenced or perhaps controlled by the Alpine deep penetrative structures and that the Alpine lithospheric roots may have deflected part of the horizontal asthenospheric flow around its southernmost tip.     

Subcrustal density inhomogeneities of Northern Eurasia as derived from the gravity data and isostatic models of the lithosphere

For the territory of Northern Eurasia (6°E–165°W; 30–75°N) the distribution of anomalous masses in the lithosphere has been estimated in accordance with the lithosphere isostatic model. The method of model construction is based on the admittance technique. The experimental admittance presents a relation between the part of the outer load uncompensated by the Moho undulations and the residual gravity field and is used to select the best model. The 1 × 1° averaged values of topography elevations, basement and Moho depths, sedimentary cover density and gravity anomalies have been used as initial data. According to the correlation equation relating the outer load and Moho depths, the mean density contrast between the lower crust and the subcrustal lithosphere is 0.43 g/cm³, but the Moho undulation can not provide complete isostatic equilibrium. In some areas, the part of the outer load uncompensated by Moho undulations may be as large as 10⁷ kg/m² and the residual gravity field is as intensive as + 260 mGal. Assuming that for loads of wavelength > 200 km, local isostatic compensation is valid, in accordance with the admittance analysis, the anomalous masses compensating for the part of the outer load, which is not compensated by Moho undulations, have to be located partly in the lower crust and in the subcrustal layer. The regional trend of anomalous compensating masses is negative under Western Europe, the Mediterranean, Eastern Asia and adjacent marginal seas, and positive under the East European Platform and Western and Central Asia. The local compensating masses correspond to particular tectonic units. The isostatic gravity anomalies of Northern Eurasia have been determined and the long-wave component of the field reflecting anomalous masses under the isostatic compensation level has been evaluated.