Rinaldo Nicolich

The Calabrian Arc and the Ionian Sea in the dynamic evolution of the Central Mediterranean

In the Central Mediterranean area it is possible to recognize three main realms: the Tyrrhenian Sea, the Pelagian block and the Ionian block. These domains are limited by important fracture zones that are easily recognizable on land and offshore. These fracture zones can be grouped into four main trends: (1) the NW-SE trend which is characterized by dextral horizontal movements; (2) the NE-SW trend which is generally characterized by sinistral lateral horizontal movements; (3) the E-W trend which also shows dextral shear movements; and (4) the N-S trend which is characterized by normal faults only. The Tyrrhenian Sea realm is characterized by a thinned crust of oceanic type and is connected to the Apenninic chain by means of listric faults. From a geophysical point of view, the area shows a high heat flow which follows the main structural trends, and positive gravimetric anomalies which are compatible with seismic refraction data and can be explained by the proposed model. The Pelagian block shows a continental crust, about 20 km thick, which progressively increases beneath the Sicilian chain. Here an abrupt interruption with the Tyrrhenian block takes place. In the Pelagian realm, rifting processes developed according to the NW-SE and N-S trend. The N-S trend characterizes the westernmost zone of the realm where volcanic processes and a relatively high heat flow can also be observed. The Ionian block is characterized by the following: a thinned crust (17–20 km) which increases (40 km) towards the Southern Calabrian chain; a thick sedimentary sequence; high positive Bouguer anomalies (+300 mGal); and very low heat flow (50 mW m−2). Geophysical and structural analyses allowed us to evaluate the post-Tortonian evolution of the whole area under consideration. This evolution resulted in continental crust formation according to a rigid—plastic deformation model. According to our model, the Ionian block can be considered as a domain, characterized by a thinned continental crust and affected by intrusive material derived from the upper mantle. Furthermore, this domain, of probably Jurassic age, can be considered as a pelagic basin bordered by the Pelagian block to the west and by the Apulian block to the east-northeast. This basin, according to its position with respect to the Apenninic and Sicilian chains, has not been affected by tectogenesis and still represents a site of continuous sedimentation. From a kinematic point of view, the Ionian realm can be linked with the Pelagian block shifting towards the east-northeast, whereas it is limited towards the north by an E-W fracture zone which regulates the opening of the Tyrrhenian Sea and the counterclockwise rotation of the Apenninic chain. According to the proposed model, the opening of the Tyrrhenian Sea can be considered as a megaextension-fracture which developed parallel to the maximum principal stress direction and evolved as a triangular-shaped structural feature. In preparing the model the authors have taken into account a large set of data collected both on land and offshore.

Roots of Etna volcano in faults of great earthquakes

Abstract Results from several seismic methods allow us to sketch the deep structure of Etna and its Ionian margin. Under Etna a volume of high velocity material is found in a structurally high position; the emplacement of this suggests spreading of the surrounding medium. Just offshore, down-to-the-east normal faults penetrate through the upper crust. The deeper crustal structure beneath appears upwarped from the basin towards Etna. Juxtaposed with the crust of Sicily, a thinner crust reaches from the Ionian Basin under Etna, and the mantle is upwarped. In such a structure, magma can then be viewed as a melted lens capping a mantle upwarp at shallow depth, rather than in an intracrustal chamber. This reduces the conflict between estimates of its volume from excess output of volatiles and short residence times. A link in time is indicated between volcanic and seismic activity at a large scale: over the millennium the reported ends of episodes of high output rates of magma are followed by the reported occurrences of magnitude 7 + earthquakes which caused destruction in southeastern Sicily. Several steep active normal fault have been imaged to a depth of 10 km the crust up to 30 km offshore of the cities of Catania and Augusta, which may be fault planes for such large earthquakes. They expand and prolongate the system of the Timpe faults on the eastern flank of Etna, thus linking large-scale tectonics offshore with the volcano. Etna developed together with normal faulting, upwarp, and spreading during the recent evolution of the former Ionian subduction. Activation of the material at depth at the lateral edge of the slab, by vertical motion with extension above, could produce the peculiar type of Etna magmatism.

From collisional to rifted basins: an example from the southern Calabrian arc (Italy)

Structural interpretation of available geological and geophysical data carried out along a regional transect extending across the southern Calabrian arc from the Tyrrhenian margins to the Ionian off-shore, demonstrates that a complex interplay between compressional and extensional processes has controlled the evolution of Upper Miocene-Pleistocene sedimentary basins developing in this region. Our data indicate that an Upper Miocene-Lower Pliocene succession outcropping in the southern Calabrian arc represents the infilling of perched basins developed between crystalline basement thrust sheets. A similar tectonic pattern characterises the Plio-Pleistocene basins occurring on the frontal part of the arc in the Ionian off-shore. Upper Pliocene-Pleistocene sediments outcropping along the Tyrrhenian side of the arc reveal that they in fact represent the infilling of extension-related basins. Time-space migration of compressional- and extensional-related sedimentary basins can be explained as the result of different tectonic processes developing on the upper plate as a result of the underplating of the Ionian domain beneath the Calabrian block. In this framework Upper Miocene-Lower Pliocene and Upper Pliocene-Pleistocene perched basins represent, therefore, fore-arc basins developed on the frontal accretionary wedge and/or on the hangingwall buttress of the arc during its southeastwards migration. Late Pliocene-Early Pleistocene extensional tectonics occurring along the Tyrrhenian side of the arc, may indeed be related to accommodation processes which characterised the rear of the wedge to maintain a stable geometry as a result of the underplating of the Ionian crust.

Mount Etna dense array local earthquake P and S tomography and implications for volcanic plumbing

Inversion for the three-dimensional velocity structure of Mount Etna is performed with a data set of arrival times of P and S waves of local earthquakes from temporary dense arrays of three-component seismographs. A high-V p body revealed by the original tomography without nearby stations is confirmed, and its image is sharpened using new velocity constraints provided by refraction data. Synthetic tests of V p and V p /V s , and comparison with an independent artificial source tomography with a fundamentally different geometry consistently calibrate the significance threshold of the resolution indicators. The trustworthy part of the image shows a high-V p body centered under the southern part of Valle del Bove above the 6 km below sea level deep basement, which extends towards sea level and may be rooted in or through the crust. It has a large contrast of over 1 km/s with the surrounding sediments and sharp lateral limits and can thus be regarded as made of intrusive material of magmatic origin. The massive high-V p body is heterogeneous in V p /V s . The regions inside it where V s is relatively low can then be suspected of containing a proportion of melt or be fractured and act as pressure links or transport zones. Such features may be structurally linked and appear to be activated in eruptive phenomena. By taking into account the heterogeneities in structure and physical state retrieved by seismic tomography a succession of seismic events, deformational episodes, and geochemical variation in lavas can be discussed with respect to the well-observed eruptions.

Geological model of a young volcano-plutonic system: The geothermal region of Monte Amiata (Tuscany, Italy)

Abstract Geological, geophysical and petrologic data point to the presence of a granitic body below the geothermal region of Mt Amiata (central Italy). A broad area of about 900–1300 km 2 centered on Amiata volcano shows a regional uplift of the Pliocene beds to 950 m a.s.l. The uplift began during the lower Pliocene, with a regression of the Pliocene sea from an uplifted area centered in the volcano zone. The temperature distribution below the Piancastagnaio field shows an updoming of the isotherms, forming a thermal high, probably present since the earliest stages of interaction between geothermal fluids and country rocks. A re-evaluation of the petrologic data from the xenoliths included in the lava flows allows an estimate of the P-T conditions of the magma body; a minimum temperature of 575°C and pressures of 1550–2200 bars can be estimated for the confining rocks around the magma body. Magmatologic data show a temperature of 800–900°C and a P load - 1000 bar. Therefore the roof of the magma body should be present at about 6 km depth. Seismic reflection data reveal the continuous and widespread occurrence of a reflecting horizon ( K ) of all over the geothermal region. This horizon is present at a depth of 5–6 km. By analogy with Larderello, it is interpreted as a fracture interval filled with hot fluids, contact metamorphic minerals and hydrothermal minerals generated in the uppermost part of the granite and the basal levels of the wall rocks. By integrating geophysical and geological data, a two-dimensional gravimetric model of the volcano-plutonic system of Mt Amiata is proposed, with the following features: roof depth = 5–6 km, T = 820°C, d (magma) = 2.15 g cm −3 , d (wall rock) = 2.8 g cm −3 , shape of the intrusion is lens shaped or mushroom-like with possible thickening and roots just below Piancastagnaio. This model fits well with gravimetric data, which show a negative anomaly in correspondence with the uplifted area.

A geostatistical framework for incorporating seismic tomography auxiliary data into hydraulic conductivity estimation

Abstract A geostatistical approach is presented for the inclusion of seismic tomography data and sonic log data into the estimation of hydraulic conductivity. The procedure accounts for the errors in seismic tomography inversion and for the correlation of such errors. The proposed methodology consists of two steps: (1) the cross-variogram inference is carried out using only the data at the wellbore (both hydraulic and seismic); (2) a co-kriging procedure takes the cross-well data into account to interpolate between boreholes. No postulated a- priori relationship is needed. In order to illustrate the methodology a synthetic data set is generated on the basis of evidence from published case studies and simplified physical considerations. The numerical experiments show that the choice of the excitation frequency is critical. A trade-off exists between the need for a high-resolution survey (asking for higher frequencies) and the need for a good correlation between hydraulic conductivity and seismic properties (asking for frequencies below the squirt frequency of the medium). In the simulation using seismic data with the best excitation frequency (1 kHz in this case), the mean squared error of the hydraulic conductivity estimate is two-thirds lower than using hydraulic data alone. It is important to note also that only a part of the interwell region is adequately sampled by the tomographic experiment. Such a region can be readily identified by calculating the energy of the quasi-null space, through singular value decomposition of the tomographic matrix. In planning this type of experiments, it is necessary to carefully verify case by case whether the adopted range of high frequencies does not prevent the seismic energy from propagating effectively from sources to receivers.

New data and hypothesis on the development of the Tyrrhenian basin

Abstract Among those basins which developed during the Neogene and Quaternary at the rear of the perimenditerranean mountain chains, the Tyrrhenian basin is the most recently formed. Comparing the available geological and geophysical data, the Tyrrhenian domain and its neighbourhoods can be divided into more or less homogeneous sectors: namely Northern and Southern Tyrrhenian, Apenninic chain, Maghrebides chain, Apulian and Iblean-African foreland, Ionian foreland. For each sector the different phenomena involved and the crustal characteristics are discussed. The evolution of the domain has been explained by the application of a crustal stretching model developed for continental margins. This approach predicts the formation of basinal areas as a result of the Europe/Africa collisional movements within a plastic-rigid deformation model. Taking into account the mass redistribution after collision, the model explains both the distension developed within the collisional system and the penecontemporaneous development of compressive and distensive phases. As a consequence of the intraplate stresses, horizontal shearings affecting the whole lithosphere at different levels, occur, with delaminations, asthenosphere uprising and gradual collapse in the upper crust. E-W trending and strike-slip dextral faults played a primary role in the evolution of the Tyrrhenian basin inducing the largest tearings at the end of the major transcurrent systems. Such an evolution is complicated by the migrating motion of the shear zone from N to S and by the penecontemporaneous stop in the extension from the northern towards the southern domains. Of outstanding importance is the shear zone which borders the Southern Tyrrhenian enabling the opening of the basin and allowing the development of the major group of en-echelon sub-basins with a NW-SE trend.

European orogenic processes research transects the eastern Alps

The Alps—the youngest and most elevated mountain range in Europe—have inspired ideas about orogenic evolution for a long time. During the late 1980s, the western Alps were the site of intensive research using seismic profiling methods by Swiss, Italian, and French national programs [Rome et al., 1990; Pfiffner et al., 1997] .These investigations, some of which formed part of the European Traverse [Blundell et al., 1992], provided a great wealth of new data relevant to the Alpine orogeny. This orogeny is generally viewed in the context of the collision of the European and the Adriatic/African continental plates after the closure and subduction of the Penninic Ocean since about 40–50 Ma.

P and S reflection seismic profiling and well logging in the travale geothermal field

Abstract Two high resolution seismic reflection profiles, obtained with vibrators as sources of P and of horizontally polarized SH waves, were recorded and elaborated. In the well CH-3, 2704 m deep and located close to the profiles, several geophysical logs were carried out: temperature, SP and induction, density, natural radioactivity, and sonic logs with P and S wave velocity measurements. These data enabled the calculation of the elasticity moduli and of the mechanical properties of the rocks affected. The sonic log results were subsequently calibrated by means of a vertical seismic profile which permitted the computation of synthetic seismograms and a VSP log for comparison with the seismic sections and correlation between S and P seismic reflectors. The structural maps obtained from data interpretation describe with sufficient detail the fracture trend of the Travale field. By comparing V s and V p velocities for the different geological units, as defined by the interpretation, the change in the α = V p /V s ratio and in the Poisson coefficient has been tentatively utilized to correlate the seismic data with material properties. The influence of lithology on the elastic constants is discussed, as well as the influence of the saturating fluids and of the degree of saturation. The different attenuation of the P and S waves, depending on the above-mentioned characteristics, demonstrates the possibilities offered by seismic methods in the evaluation of the potential of geothermal reservoirs.

Point source moment tensor retrieval in volcanic, geothermal and orogenic areas by complete waveform inversion

Abstract Most of the seismicity characterising volcanic and geothermal areas is represented by relatively small events, that reflect the local answer to the global tectonic setting. Due to the strong seismic noise encountered in these areas, the source mechanisms of these events are usually very difficult to be studied by standard techniques based on first arrivals, unless a very dense network of seismometers is available. When studying induced seismicity, a well-known problem for geothermal areas, or volcanic events, it is highly desirable to treat a seismic source in a form not a priori restricted to a double couple because the mechanism may reflect the local conditions such as small-scale tectonics, fluid motion and man-made factors. The decomposition of the full (or unconstrained) moment tensor solution into a double-couple component (DC), a volumetric (V) component, and a “compensated linear vector dipole” (CLVD) component can indicate a measure of the relative influence of global tectonics and local effects. In orogenic areas seismic events can be very strong, but the largest part of the earthquakes, which very likely reflect small scale complexities of the tectonic structure, is characterized by very small magnitudes. Therefore, the study of weak events is very important for many different purposes and we apply here the inversion scheme of high-frequency seismograms, recorded by a local network, based upon the unconstrained moment tensor description. A good knowledge of the structural model is a definite prerequisite to obtain reliable source information (moment tensor and time history not biased by propagation effects, even though not unique). For this reason we have inverted signals recorded in the Pozzuoli (Italy) area, using as input structure the model determined from a set of independent high-accuracy measurements. The inversions shows a first episode with the main peak centred around 0.6 s, followed by another significant peak, with duration of 0.4 s, centred at 2 s, and by a final energy release extending for about 1.0 s. The procedure has also been applied to vertical component seismograms recorded in the Friuli (northeastern Italy) orogenic area in correspondence to the M = 2.9 event of December 27, 1987, and two events of February 01, 1988, with M = 3.2 and M = 3.6, respectively. The retrieved source mechanisms are generally in agreement with the distribution of the few first arrivals polarities available. The source time functions indicate possible multiple rupture processes also for relatively small magnitude events.