Ivrea Body Research Articles

A crustal‐scale cross‐section of the south‐western Alps combining geophysical and geological imagery

AbstractA geotransect across the south‐western Alps from the Pelvoux Massif (external French Alps) to the Dora‐Maira massif (internal Italian Alps), through the Monviso ophiolitic complex, was investigated in the framework of the ‘Géo‐France 3D Alpes’ programme. A new interpretative crustal‐scale section across the south‐western Alps is proposed, combining geological and geophysical 2D/3D data. The Apulian mantle (i.e. the Ivrea body) might be divided into two rigid pieces separated by the downward prolongation of the Penninic frontal thrust. These mantle indenters drive the decoupling of the European crust. Beneath the high to ultra‐high pressure metamorphic nappes, the deep structure results from the stacking of crustal slices extracted from the European lithosphere. The proposed structural model provides a basis for discussing the evidence of the crustal‐scale partitioning of the current strain pattern as well as the location of the seismicity.

Lithosphere structure and tectonic evolution of the Alpine arc: new evidence from high-resolution teleseismic tomography

Abstract Several continental and oceanic plates and/or terranes amalgamated during the formation of the tectonically complex Alpine arc. Reliable knowledge of the present structure of the lithosphere-asthenosphere system throughout the Alpine arc from the Western through the Central to the Eastern Alps is crucial for understanding the evolution of this orogen and the current interaction of lithospheric blocks, and additionally, for assessing the amount and orientation of lithosphere subducted in the geological past. We have compiled results from earlier geophysical studies and reinterpretations of existing seismic and geological data for the Alpine crust and Moho. High-resolution teleseismic tomography was used to produce a detailed 3D seismic model of the lower lithosphere and asthenosphere. The combination of these techniques provides new images for the entire lithosphere-asthenosphere system, showing significant lateral variations to depths of 400 km. Over the years the crustal structure has been determined extensively by active seismic techniques (deep seismic sounding) with laterally variable coverage and resolution. For a closer view three international seismic campaigns, using mainly near-vertical reflection techniques in the Western, Central and Eastern Alps, were carried out to assess the crustal structure with the highest possible resolution. The synoptic reinterpretation of these data and an evaluation of existing interpretations have allowed us to construct four detailed deep crustal transects across the Alps along the ECORS-CROP, NFP-20/EGT and TRANSALP traverses. In addition, contour maps of the Moho for the wider Alpine region and of the top of the lower crust were compiled from existing seismic refraction, near-vertical and wide-angle reflection data. Substantial structural differences in the structure of the deep crust appear between the Western, Central and Eastern Alps: doubling of European lower crust in the west resulted from collision with the Ivrea body; indentation of lower Adriatic crust between European lower crust and Moho occurred in the Central Alps; and a narrow collision structure exists under the transitional area between the western and eastern subduction regime under the Tauern Window of the Eastern Alps, where the crustal structure resembles a large-scale flower structure. Most recently, high-resolution teleseismic tomography based on the a priori known 3D crustal structure and compilation of a high-quality teleseismic dataset was successfully developed and applied to derive reliable detailed images of the lower lithosphere. Along strike of the Alps a fast slab-like body is revealed which in the western part is subducted beneath the Adriatic microplate. In the Western Alps detachment of parts of the lower continental slab occurred, possibly induced by the Ivrea body, which acted as a buttress in the collision process of the European and Adriatic plates. The generally SE-directed subduction of the European continental lithosphere changes gradually from west to east to almost vertical under the westernmost part of the Eastern Alps (western Tauern Window and Giudicarie lineament). Unexpectedly, some 50 km further east the subducted continental lower lithosphere is now part of the Adriatic lithosphere and dips NE beneath the European plate. Our tomographic image documents clear bipolar slab geometries beneath the Alpine orogen. The depth extent of the subducted continental lithospheric slab agrees rather well with estimates of post-collisional crustal shortening for the Western and Central Alps. This kinematic control on amounts of lateral motion of the collision zone in the west also allows estimates of the subduction and collision process in the Eastern Alps. The new 3D lithospheric picture for the wider Alpine region to 400 km depth demonstrates the clear connection and interaction between the deep structure of the lithosphere-asthenosphere system and near-surface tectonic features as seen today. It provides new and unexpected evidence for the entire Alpine tectonic evolution, a process which obviously changes significantly from west to east.

High-resolution teleseismic tomography of upper-mantle structure using ana priorithree-dimensional crustal model

SUMMARY The effect of an a priori known 3-D crustal model in teleseismic tomography of upper-mantle structure is investigated. We developed a 3-D crustal P-wave velocity model for the greater Alpine region, encompassing the central and western Alps and the northern Apennines, to estimate the crustal contribution to teleseismic traveltimes. The model is constructed by comparative use of published information from active and passive seismic surveys. The model components are chosen to represent the present large-scale Alpine crustal structure and for their significant effect on the propagation of seismic wavefields. They are first-order structures such as the crust‐mantle boundary, sedimentary basins and the high-velocity Ivrea body. Teleseismic traveltime residuals are calculated for a realistic distribution of azimuths and distances by coupling a finite-difference technique to the IASP91 traveltime tables. Residuals are produced for a synthetic upper-mantle model featuring two slab structures and the 3-D crustal model on top of it. The crustal model produces traveltime residuals in the range between −0.7 and 1.5 s that vary strongly as a function of backazimuth and epicentral distance. We find that the non-linear inversion of the synthetic residuals without correcting for the 3-D crustal structure erroneously maps the crustal anomalies into the upper mantle. Correction of the residuals for crustal structure before inversion properly recovers the synthetic slab structures placed in the upper mantle. We conclude that with the increasing amount of high-quality seismic traveltime data, correction for near-surface structure is essential for increasing resolution in tomographic images of upper-mantle structure.

Sequential inversion of local earthquake traveltimes and gravity anomaly-the example of the western Alps

We present a joint analysis of gravity anomaly and seismic arrival time data recorded in the western Alps. Seismological data were collected by a network of 126 permanent and temporary stations implemented in 1996. A set of ∼550 local events has been recorded. Gravity data result from the addition of two new gravity surveys to an existing data base. A published velocity model obtained by local earthquake tomography (LET), was used to construct an initial 3-D gravity model, using a linear velocity-density relationship (Birch's law). While the synthetic Bouguer anomaly field calculated for this model has the same shape and wavelength as the observed anomaly, its amplitude is strongly underestimated. To derive a crustal velocity-density model that accounts for both types of observations, we performed a sequential inversion of seismological and gravity data. The variance reduction of the arrival time data for the final sequential model was comparable to the variance reduction obtained by simple LET. Moreover, the sequential model explained ∼90 per cent of the observed gravity anomaly. The main features of our model compared with the LET model are: (1) an important broadening of the high-velocity anomaly associated with the high-velocity high-density Ivrea Body, (2) a 10 km thick low-velocity zone beneath the nappes of Digne and Castellane and (3) a high-velocity zone at more than 25 km depth under the internal zone of the range.

The Swiss Alps and their peripheral foreland basin: Stratigraphic response to deep crustal processes

This paper gives a synoptic view of the Cenozoic evolution of the Swiss Alps and their northern foreland basin. In this orogen, deep crustal processes (subduction, nappe stacking, underplating, and exhumation) are intimately linked with surface processes (surface uplift, erosion, basin formation, and basin‐axis migration). Within the foreland basin the spatial pattern of subsidence and alluvial fan construction suggests that an increase in flexure of the foreland plate and the creation of relief in the orogen migrated from east to west in the course of collision. In the orogen itself, crustal thickening involved lower crust of the Adriatic margin in the east and the European margin in the west. Exhumation of upper crustal units occurred earlier in the east as compared with the west. An Adriatic mantle wedge (the Ivrea body) and its associated wedge of lower crustal material are identified as an extra lithospheric load which contributed to downward flexure of the European plate. As a result of enhanced subsidence of the foreland plate, relief was generated presumably in order to adjust to critical taper geometry. It appears, therefore, that the westward motion of the Adriatic wedge ultimately caused the contemporaneous westward propagation of the location of enhanced rates of alluvial fan construction. Coeval strike‐slip and N‐S convergence juxtaposed the Adriatic wedge sequentially to different European upper crustal units which resulted in different styles of crustal structure and evolution along strike within the orogen.

A three‐dimensional crustal velocity model of the southwestern Alps from local earthquake tomography

A temporary network of 65 short‐period seismological stations was installed in the southwestern Alps during the second half of 1996. It complemented the permanent monitoring networks, obtaining an average interstation distance of ∼10 km. Travel time data from 446 local earthquakes and 104 quarry blasts were inverted simultaneously for hypocenter parameters and three‐dimensional velocity structure. The P wave velocity model displays strong lateral contrasts both at shallow and deeper levels. A low‐velocity anomaly stands out at shallow depths beneath the Digne and Castellane nappes in the southwestern part of the investigated area. Farther east, the Monviso ophiolitic massif appears to have a much larger extension at depth than previously assumed. The largest and strongest anomaly is located under the Dora Maira massif and the westernmost Po plain. It correlates with the well‐known Ivrea body, which is classically interpreted as a wedge of Adriatic upper mantle. At the best resolved depths (10 and 15 km) it appears as a rather thin (10 to 15 km), north‐south elongated, high‐velocity (7.4 to 7.7 km s−1) anomaly with very sharp edges, extending to the south as far as 10 km north of the surface trace of the Frontal Penninic Thrust. Special care was taken with regard to the quantitative estimation of the resolution for the main anomalies using the inversion of synthetic travel time data.

Comparison of two “pseudo-bending” raytracers

Fast and accurate ray tracing in heterogeneous media is necessary in many seismological analyses. In this paper, we present some comparative results obtained from applying two different pseudo-bending raytracers to an actual 3-D velocity model of north-western Italy. This area is characterized by strong lateral velocity variations and major anomalies, related to the so-called Ivrea Body, lead to velocity perturbations greater than 10%. The ray tracing algorithm implemented in the Simulps12 Code (Thurber [J. Geophys. Res. 88 (1983) 8226]; and Um and Thurber [Bull. Seismol. Soc. Am. 77 (1987) 972]) and a ray tracing scheme based on that of Prothero et al. [Bull. Seismol. Soc. Am. 78 (1988) 1190] were compared, adopting the same velocity interpolation scheme and the same integration parameters. The data set was composed of more than 8700 P-phases and 7500 S-phases and the performances of the two methods were evaluated in terms of travel time difference and ray path geometry. The results showed that the scheme based on that of Prothero et al. [Bull. Seismol. Soc. Am. 78 (1988) 1190] nearly always produced smaller travel time values. In particular, the travel time differences were greater than 0.05 s for 33.2% of the whole S-phases data set whereas, in the case of P-phases, these differences corresponded to 18.9%. Considering rays relevant to hypocenters further than 75 km, we obtained travel time differences greater than 0.05 s for 75.9% of S-phases and 47.1% of P-phases. Furthermore, travel time differences appeared strongly related to the distribution of velocity heterogeneities. Indeed, for all the considered stations, the most significant travel time differences (in some cases greater than 0.15 s) were found to correspond to the azimuthal sector embracing the Ivrea Body. In the case of incomplete azimuthal coverage, these differences had a significant influence on the location procedure. Moreover, the path differences can induce a notable bias in the tomographic procedure. The results of this study had to be taken into account in its companion paper on the reliability of earthquake location procedures (Spallarossa et al. [Phys. Earth Plant. Interiors (2000)]).

Une nouvelle carte gravimétrique des Alpes occidentales et ses conséquences structurales et tectoniques

Two new gravity surveys performed in the western Alps allow the drawing of a high-resolution gravity map. Moreover, a well-constrained Moho depth model was used to correct the initial map from the Moho depth variation gravity effect and to interpret the residual effect in terms of crustal heterogeneities. The main structures of the western Alps are pointed out, specially the Ivrea body parallel to the Penninic front, the crystalline massifs (Simplon-Tessin, Pelvoux, Mercantour) and the sedimentary basins and nappes of the southern Alps (Digne, Castellane, Valensole).

Thermo‐mechanical approach to validation of deep crustal and lithospheric structures inferred from multidisciplinary data: application to the Western and Northern Alps

Current multidisciplinary studies confront the difficult problem of validation and verification of internal compatibility of geophysical and geological data of different nature, quality and origin. Our ‘geodynamic’ approach exploits the natural interdependence between the data related to different physical properties of geological structures (density, rheology, temperature, stress). Using the latter as input for a thermo‐mechanical finite‐element model, we can verify whether the inferred properties and geometries (i) are mechanically stable, (ii) require rheologically consistent stresses, strains, realistic thermal conditions, etc., and (3) will evolve in a geologically realistic way in the future. This ‘mechanically balanced’ approach is tested on the Alpine orogen (ECORS and NFP20 profiles) in the framework of the GeoFrance 3D programme. The results suggest a number of corrections and adjustments that can be made to existing seismically‐and gravity‐predicted geometries of structures such as the Ivrea body and those of the depleted subducted crust.

GDS (Geomagnetic Depth Sounding) in Italy: applications and perspectives

The analysis of geomagnetic field variations is a useful tool to detect electrical conductivity contrasts within the Earth. Lateral resolution of outlined patterns depends on the array dimensions and density of measurement sites over the investigated area. The inspection depth is constrained by the period of geomagnetic variations considered in data processing. Regions with significant geological features such as boundaries of continental plates, marginal areas of contact between tectonic units or other geodynamical processes, are of primary interest for the application of the MagnetoVariational (MV) method. In the last ten years, in the frame of the ElectroMagnetic (EM) sounding techniques in applied geophysics, this method has been applied in Italy by researchers of the Istituto Nazionale di Geofisica, Rome, the Dipartimento di Scienze della Terra, Universitá di Genova and the Czech Science Academy of Prague. The Ivrea body in the Northwestern Alps and their junction with the Apennine chain, the micro-plate of the Sardinian-Corsican system and, recently, the central part of the peninsula along Tyrrhenian-Adriatic lithospheric transects were investigated. Studies in time and frequency-domain used in the first investigations, have been followed by more refined analysis involving tests on the induced EM field dimension, computations of single site Transfer Functions (TFs) through Parkinson arrows' and Fourier maps in the Hypothetical Event technique (HE). It was possible to describe the electrical conductivity distribution in the inner part of the SW Alpine arc and to confirm the presence of lithospheric and asthenospheric anomalies obtained by other geophysical methods. For the Sardinia-Corsica system, 2D and 3D inversion models highlighted the existence of two major conducting bodies, one north of Corsica, and the other south of Sardinia. In Central Italy, the regional electrical conductivity distribution pointed out a deep conductive structure beneath the Apennines and a very resistive root for this part of the mountain chain.

Lateral variations of Pn wave velocity in northwestern Italy

The Pn arrival times recorded from seismic networks operating throughout northwestern Italy and surrounding regions were inverted to map the structural variations of the uppermost mantle over the area and to estimate the crustal static delays at each station. By means of careful data selection a quality data set was obtained removing statistical outliers, poorly recorded events, and scantily sampled stations. Moreover, synthetic data were used to evaluate the resolution power of the available data set and the adopted iterative inversion technique. The agreement between synthetic and calculated models is more satisfactory where the path coverage of the rays is quite complete. A low‐velocity zone was found beneath the western and northwestern side of the Alps; it may be related to an increase in depth of the Moho, as supported by other geophysical data. The high Pn velocities found in the eastern side of the western Alps indicate the presence of high velocity and density, lower crust rocks of the so‐called “Ivrea Body.” The high Pn velocity underlying the Ligurian Sea could be related to a high‐velocity structure existing in the upper mantle. The shape of the velocity anomalies matches not only the major tectonic features, but also the Bouguer gravity anomalies of the area.

Ivrea seismic array: a study of continental crust and upper mantle

SUMMARY A seismic-array study of the continental crust and upper mantle in the Ivrea-Yerbano and Strona-Ceneri zones (northwestern Italy) is presented. A short-period network is used to define crustal P- and S-wave velocity models from earthquakes. The analysis of the seismic-refraction profile LOND of the CROP-ECORS project provided independent information and control on the array-data interpretation. Apparent-velocity measurements from both local and regional earthquakes, and time-term analysis are used to estimate the velocity in the lower crust and in the upper mantle. The geometry of the upper-lower crust and Moho boundaries is determined from the station delay times. We have obtained a three-layer crustal seismic model. The P-wave velocity in the upper crust, lower crust and upper mantle is 6.1±0.2 km s−1, 6.5±0.3 km s−1 and 7.8±0.3 km s−1 respectively. Pronounced low-velocity zones in the upper and lower crust are not observed. A clear change in the velocity structure between the upper and lower crust is documented, constraining the petrological interpretation of the Ivrea-type reflective lower continental crust derived from small-scale petrophysical data. Moreover, we found a VP/VS ratio of 1.69±0.04 for the upper crust and 1.82±0.08 for the lower crust and upper mantle. This is consistent with the structural and petrophysical differences between a compositionally uniform and seismically transparent upper crust and a layered and reflective lower crust. The thickness of the lower crust ranges from about 8 km in front of the Ivrea body (ARVO, Arvonio station) in the northern part of the array to a maximum of about 15 km in the southern part of the array. The lower crust reaches a minimum depth of 5 km below the PROV (Provola) station.

Compilation of a recent seismicity data base of the greater Alpine region from several seismological networks and preliminary 3D tomographic results

Local earthquake data collected by seven national and regional seismic networks have been compiled into a travel time catalog of 32341 earthquakes for the period 1980 to 1995 in South-Central Europe. As a prerequisite, a complete and corrected station list (master station list) has been prepared according to updated information provided by every network. By simultaneous inversion of some 600 well-locatable events we obtained one-dimensional (1D) velocity propagation models for each network. Consequently, these velocity models with appropriate station corrections have been used to obtain high-quality hypocenter locations for events inside and among the station networks. For better control, merging of phase data from several networks was performed as an iterative process where at each iteration two data sets of neighbouring networks or groups of networks were merged. Particular care was taken to detect and correctly identify phase data from events common to data sets from two different networks. In case of reports of the same phase data from more than one network, the phase data from the network owning and servicing the station were used according to the master station list. The merging yielded a data set of 278007 P and 191074 S-wave travel time observations from 32341 events in the greater Alpine region. Restrictive selection (number of P-wave observations >7; gap <160 degrees) yielded a data set of about 10000 events with a total of more than 128000 P and 87000 S-wave observations well suited for local earthquake seismic tomography study. Preliminary tomographic results for South-Central Europe clearly show the topography of the crust-mantle boundary in the greater Alpine region and outline the 3D structure of the seismic Ivrea body.

The maritime alps arc in the ligurian and tyrrhenian systems

After Late Eocene meso-alpine collision between the European and Adria plates, the Maritime Alps underwent further deformations linked to the Apennine orogenesis and consistent with a stress field developed during anticlockwise rotation. The western limit of the rotated sector is east of the Argentera massif and, to the southwest, represents the northwestern margin of the post-Eocene Ligurian sphenochasm. The sinistral rotation of the Maritime Alps initially occurred contemporaneous with the opening of the Ligurian basin (about 30-19 Ma) and probably continued during the subsequent opening of the Tyrrhenian basin (about 10-0 Ma). The rotating block comprises the northwestern part of the Adria indenter and a part of the paleo-European plate (namely, the Maritime Alps, Corsica and Sardinia). If a convenient configuration is assumed for the Moho of the Adria margin and suitable poles are chosen for subsequent rotations, then a simplified model of rigid body can explain the rotation structures in the Maritime Alps, the Apennines and Po Plain subsurface compressional arcs, the late foreland-directed transport of nappes in the Western Alps, the presence of extensional and dextral transcurrent structures, and the general uplift of the lithospheric mantle between the Ligurian sea and the Ivrea body. As to the causes of rotation, two alternatives (“subduction” or “asthenosphere uplift”) are discussed. Chronological and mechanical arguments seem to favour the latter.

The suture between the Alps and Apennines in the Ligurian sector based on geological and geomagnetic data

A ground magnetic survey was carried out over a wide area of northwestern Italy, including western and central Liguria, the southeastern Piedmont and a part of the Ligurian Sea. Several cross-sections, cutting the main structural elements (the Ligurian Sea, the Ligurian Alps, the Monferrato Hills) have been performed and magnetic models have been developed. The interpretation of the most characteristic cross-section (A1-NNW), is presented in this paper. A model involving the presence of a major suture between the Ligurian Alps and the Monferrato Hills fits the positive magnetic anomaly running from the surroundings of Turin towards Asti. This model implies that slabs of upper mantle or lower continental crust were trapped in the suture, as in the internal zone of the Western Alps, where the Ivrea body comes in contact at depth with the Pennidic nappes. To the south, this suture line does not appear throughout the Ligurian Sea, probably owing to the recent crustal thinning, which is revealed by both the magnetic and the gravimetric features.

Results of NFP 20 seismic reflection profiling along the Alpine section of the European Geotraverse (EGT)

SUMMARY Recent deep-crustal seismic reflection profiling across the central Alps of eastern and southern Switzerland has provided a detailed image of this continental collision zone. As part of the Swiss National Science project NFP 20, a series of reflection profiles was recorded during two phases along N-S transects crossing the Alpine edifice. The eastern and southern profiles coincide with the Alpine segment of the European Geotraverse (EGT) and in part with the earlier Swiss Geotraverse. These data together with other geophysical and extensive geologic information form a unique and comprehensive volume of crustal information for this region of the Alps. On a larger scale, the new reflection data also add to a growing set of profiles crossing the Alpine fold belt which includes an additional traverse carried out by NFP 20 across the western Swiss Alps and the ECORS-CROP profile across the western Alps. The initial results of the reflection data across the central Alps outline a crustal framework involving northward indentation of the Adriatic hinterland into the subducting European foreland. This occurs beneath the collapsed oceanic basins of the Penninic allochthon which is defined by its highly reflective crystalline nappes. The present crustal thickness within the Alpine collision zone has doubled to about 60 km by both vertical and horizontal displacements along an inferred complex detachment system controlled by the Adriatic wedge. The south-plunging European foreland implies that portions of the thickened crust have been subducted into the upper mantle. This general framework of flake tectonics is laterally consistent with the ECORS-CROP results, from the western Alps. However, the western Alps show a higher degree of crustal imbrication which may be related to the presence of the rigid Ivrea body and an overall greater amount of crustal shortening compared to the central Alps.

Propagation anomalies in Northwestern Italy by inversion of teleseismic residuals

ABSTRACTTravel‐time residuals of teleseismic P waves were analysed in order to elucidate the crust–upper mantle structure in Northwestern Italy, and the Western Alpine Arc. Using digital data obtained from both fixed seismograph networks operating in NW Italy (notably Liguria–Piedmont) and temporary arrays with the aid of cross‐correlation techniques reliable travel‐time residuals were calculated which were then inverted to obtain models of propagation anomalies. The reliability of the inversion procedure was tested using synthetic data. The model thus obtained appears to be stable and shows strong lateral heterogeneities at a litho–asthenospheric level; in particular, it confirms the high velocity contrast caused by the ‘Ivrea Body’ in the shallower layers and the presence of Alpine ‘roots’ reaching down to at least 200 km. A statistical analysis performed on the propagation times of rays crossing the resulting four‐layered model reveals rms below 0.1 s.

The problem of the Moho in the Alps

The Tertiary Alps are the product of continental collision, and material balance estimates require the subduction of continental crust and the underlying Moho. The present Moho in the Alpine region consists of three parts: that of the European plate (ME), that of the Adriatic (African) plate (MA), and a new Moho (M2), believed to have developed in the subducted continental crust by high pressure metamorphism at 40–60 km depth. The fate of M2, however, is uncertain. Some of it is apparently destroyed subsequently, as some eclogite facies (and even highest- P coesite-bearing) rocks somehow find their way to the surface, where they form exotic bodies from microscopic to nappe size. On the other hand, the continental Moho exposed in the Ivrea body tells a different story. Here continental granulite facies rocks are intruded by ultramafic and mafic masses. This suggests an origin in an extensional environment, possibly in subduction-related gravitational collapse basins. Such successor basins succeed the basins of the original Tethys that had been largely consumed by the Late Cretaceous. They puncture the 1000–2000 km wide belt of Africa-Europa collisional interaction (e.g. the Aegean, the Tyrrhenian, and the Pannonian basins). Although in some ways similar to the back-arc basins of the Pacific, these mostly intracontinental basins differ in important aspects such as their small size. In these basins asthenospheric upwelling replaces M2 by a new Moho (M3) and the eclogitic rocks underlying M2 by the ultramafic-mafic intrusions of the Ivrea type. Material balance considerations suggest that most of the eclogues remain part of the mantle for some time. On the other hand, recent deep reflection seismic surveys in several places indicate delamination of a part of the lower crust and the uppermost mantle with the formation of imbrications and duplexes, some of which are wedged into the middle crust of the opposite plate. They may be the first stage in the formation of granulite and eclogite nappes.

Some geophysical implications of the deformation and metamorphism of the Ivrea zone, northern Italy

The Ivrea zone is commonly cited as an upthrust and steepened section through the lower continental crust, being contiguous, via the geophysical “Ivrea body”, with the present day lower crust beneath the Po basin. The rocks presently exposed were buried to lower crustal depths through the Variscan orogeny and were gradually exhumed to mid-crustal depths through Permian and early Mesozoic times as a result of crustal extension. They were emplaced into their present position during the Alpine orogeny. An important time marker in the deep-crustal evolution of the rocks was the development of a coarse-grained, equigranular texture, following the attainment of the amphibolite and granulite facies metamorphic peak. This effectively separates earlier, pervasive, synmetamorphic deformation from later, highly localized plastic deformation associated with crustal extension and progressive cooling. The geophysical properties of the section would have been markedly different during each of these two stages of development. In the later stage the granulite facies rocks remained relatively dry and, when buried, would have been characterized by the high seismic velocities that they display under pressure in the laboratory. During the prograde (orogenic) part of the history, seismic velocities would have been substantially depressed through the effects of elevated pore water pressure, particularly at high temperatures.

A three-dimensional model of the crust and upper mantle along the Alpine part of the European Geotraverse (EGT)

We investigated the structure of the crust and the upper mantle in the Alpine part of the European Geotraverse by integrating gravity and seismic refraction data into a three-dimensional (3-D) model of the crust. The gravity data came from Swiss, Italian and German sources. The study area included the Swiss Molasse Basin north of the Alps and the Po Plain in the south. To enhance the resolving power of this data for short wavelength anomalies in the Alps, 550 additional stations were surveyed on a 10 km wide swath from Lake Constance (bordering Switzerland and the Federal Republic of Germany) to Bergamo (Italy). The Bouguer anomalies were reduced with 3-D models for the effect of the sediments of the Swiss Molasse Basin, the Po Plain and other bodies of known geometry such as the Quaternary sediments of the Alpine valleys and the mafic and ultramafic rocks of the Ivrea body. This resulted in a map of stripped Bouguer anomalies. A 3-D model of the crust was built using seismic data. The densities were determined by a least-squares inversion technique. After subtraction of the model effect from the stripped Bouguer anomalies we found a residual gravity anomaly of more than 400 km length that can be explained with a positive density anomaly in the upper mantle which is in agreement with seismological results. These findings fit well into a tectonic model with subducted material of higher density and seismic velocity in the upper mantle beneath the boundary between the European plate and the Adriatic Promontory (African plate).