Crustal and uppermost mantle structure in Italy from the inversion of P-wave arrival times: geodynamic implications

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1 Geophys. J. Int. (1999) 139, 483^498 Crustal and uppermost mantle structure in Italy from the inversion of P-wave arrival times: geodynamic implications R. Di Stefano, C. Chiarabba, F. Lucente and A. Amato Istituto Nazionale di Geo sica, Via di Vigna Murata 605, Rome 00143, Italy Accepted 1999 June 21. Received 1999 June 21; in original form 1998 March 5 SUMMARY In this paper we present a 3-D image of the P-wave velocity structure in the crust and uppermost mantle beneath Italy, obtained by inverting P-wave arrival times of crustal earthquakes. We used about arrival times at stations close to the epicentres (within 200 km) to mitigate the e ects of both reading errors and intrinsic complexity for long-distance ray paths due to the scale of heterogeneities in the Earth structure beneath Italy. Whereas two of the inverted layers (located in the lower crust and beneath the Moho) are well resolved by the data, the upper crustal layer has regions of relatively poor resolution beneath the central Apennines, due to the sparse station spacing. We made several attempts, varying the Moho geometry, to obtain reliable and stable results without introducing too complex and unconstrained aprioriinformation. One of the most important results is the discovery of low-velocity anomalies at 38 km depth beneath the northern Apenninic and Calabrian arcs, which we interpret as subducted crustal material of the Adriatic and Ionian lithospheres. In the lower crust beneath the entire Apennines, a pronounced low-velocity belt is present, suggesting high temperatures in front of the bent Adriatic lithosphere. Based on our results and on those previously obtained from seismic tomography, we propose a model for the geodynamic evolution of the Apenninic system in which the present-day scenario re ects lateral heterogeneities in the subducting Ionian (oceanic) and Adriatic (continental) lithosphere. Key words: crust, geodynamics, Italy, P waves, upper mantle. INTRODUCTION The Italian peninsula plays a key role in the geodynamic evolution of the Mediterranean area. In Italy, the convergence between the African and Eurasian plates, active since at least 65 Ma (Scandone 1979; Dercourt et al. 1986; Bally et al. 1986; Malinverno & Ryan 1986; Patacca & Scandone 1987; Dewey et al. 1989; Patacca et al. 1990), has produced an almost continuous belt of subducted material beneath the Alps, the Apennines and the Calabrian arc, where the subduction hinge developed at *90 0 from the main plate convergence direction. Based on geological and geophysical information (Royden et al. 1987; Patacca et al. 1990), the Tyrrhenian subduction zone is not continuous from the northern Apennines to Sicily, but is divided into two main arcs (the northern Apenninic and Calabrian arcs) and a complex area in between (the southern Apennines, see Patacca & Scandone 1987). This complexity is probably a consequence of lateral heterogeneities in the subducting lithosphere. In recent years, images of the subducting plate in the upper mantle of Italy have been provided by tomographic studies (Amato et al. 1993; Spakman et al. 1993; Selvaggi & Chiarabba 1995) and deep earthquake distributions (e.g. Anderson & Jackson 1987; Giardini & Velona 1991; Selvaggi & Amato 1992; Selvaggi & Chiarabba 1995; Frepoli et al. 1996). Among the tomographic models available for the area, those of Piromallo & Morelli (1997), Cimini & De Gori (1997) and Lucente et al. (1999) are the most recent and detailed descriptions of the velocity structure in the mantle beneath Italy. However, the crustal structure, not de ned by these models, and its relation with both the deep subducting and the overriding plates remain poorly known and di cult to interpret. Recently, Mele et al. (1997) found evidence for a broad low-velocity and high-attenuation anomaly of Pn phases in the uppermost mantle beneath the Apennines that they interpreted as crustal delamination. Chiarabba & Amato (1996) found a strong lowvelocity zone in the lower crust beneath the central-southern Apennines. Based on all the available results, it is not clear yet whether a subduction model or a crustal delamination model is applicable to the Apennines. The aim of this paper is to improve our knowledge of the crustal structure of Italy and to de ne better the properties of the material above the subducting slab. In this paper we focus on the velocity structure of the upper 40 km, in the ß1999RAS 483

2 484 R. Di Stefano et al. region where the Adriatic and Ionian lithospheres underlie the Apenninic belt. We have applied the tomographic technique of Zhaoet al. (1994) to the most complete P-wave arrival time data set available from 1975 to The use of such a large data set has allowed us to gain a reasonably detailed model de nition. DATA AND TECHNIQUES We inverted P-wave arrival times from 2968 selected earthquakes (Fig. 1) located by Chiarabba & Selvaggi (1997) using data contained in the ING instrumental catalogue (1975^1997). Earthquakes have been selected from an initial data set of more than events considering hypocentral formal errors of less than 2 km, azimuthal gaps less than or close to 180 0,andmorethan13P-wave arrivals at stations within 200 km of the hypocentres. Large-distance Pn phases have been excluded from the inversion due to the non-negligible reading uncertainties (see Chiarabba & Amato 1996). Since small-scale heterogeneities of the Moho (of the order of tens of kilometres) and the uppermost mantle have been observed by active (Ponziani et al. 1995; Pialli et al. 1998) and passive seismology (Amato et al. 1998), our choice should avoid the smearing of these heterogeneities in broad regions. Hypocentral depths (Fig. 2) are mostly con ned to the upper 18 km of the crust, but there are some regions such as the Apulian foreland and the Calabrian arc where the seismicity is located deeper in the crust. The inversion has been performed using the technique of Zhao et al. (1994), which allows one to invert for velocity perturbations at nodes of a 3-D grid, using sharp velocity discontinuities to represent the Moho and other seismic interfaces. The model has been discretized by three layers, two of which are in the crust, at 8 and 22 km depth, and one of which is beneath the Moho, at 38 km depth. In each layer, grid nodes are spaced in latitude and longitude. The velocity discontinuity of the Moho is kept xed at 34 km depth. Starting velocity values were averaged from those obtained by Chiarabba & Frepoli (1997) by the inversion for the minimum 1-D models in Italy (5.9 km s 1 between z~0 and 15 km; 6.2 km s 1 between z~15 and 34 km; 8.0 km s 1 below the Moho). We have also tried di erent velocity values for the two crustal layers (Vp between 5.6 and 5.9 km s 1 in the upper crust and between 6.2 and 6.5 km s 1 in the lower crust), nding similar results but a larger data mis t. Velocity parameters have been computed using the LSQR algorithm. We obtained a nal rms value of 0.38 s, with a variance improvement of 28 per cent. During the inversions, we observed that the mixed hypocentral and velocity parameters computation is strongly non-linear, due to both data errors and the complexity of the structure. Since the largest data variance reduction occurs at the rst iteration, while is negligible during subsequent iterations, we preferred to compute a one-step inversion for velocity parameters, enhancing only the gross structure resolvable by the data set. The improvement obtainable by more iterations would be signi cant only using higher-quality Figure 1. Seismic stations (triangles) and earthquakes (white dots) used in the inversion; the traces of the six cross-sections are shown.

3 Crustal and uppermost mantle structure in Italy 485 Figure 2. Depth distribution of the selected earthquakes along six pro les crossing (a) the western Alps, (b) from the northern Apennines to the southeastern Alps, (c) the northern Apennines, (d) the central Apennines, (e) the southern Apennines and (f) the Calabrian Arc. Note the sharp seismicity cut o around 18 km depth. data than those presently available (bulletin data). In our case, a noise level of about 0.3 s in the bulletin data has been estimated by Chiarabba & Amato (1996). Since the nal rms of the inversion is comparable to the noise level, subsequent iterations do not provide signi cant improvement in the images. Fig. 3 shows how much the earthquakes move after the relocation in the 3-D model. The hypocentral variations are mostly less than 4 km and 6^7 km in the horizontal and vertical directions respectively, with a small tendency of shallowing the hypocentral depths. The resolution of the images obtained has been veri ed with synthetic tests. Classical checkerboard and restore tests have been performed (see Zhao et al. 1994). Arrival times obtained in a known model (synthetic arrival times) were inverted, after summing random noise with variance equal to the nal variance of the real inversion, using a uniform velocity structure as the starting model, and the results were compared with the known model. The results of the checkerboard test (Fig. 4) show that the two deep layers, at 22 and 38 km depth, are well resolved in almost the whole modelled area. A poorly resolved region is evident at 8 km depth in the central part of the Apennines, due to the lower amount of criss-crossing of rays for the large spacing between seismic stations. In the eastern and western Alps and in Sicily and Calabria, the resolution is also su ciently good in the upper crust. In order to check whether the data can also resolve di erent velocity patterns (see Leveque et al. 1993), a restore test was also performed. In this test, theoretical arrival times are computed tracing the rays within the 3-D model obtained by the inversion of the real data (see next section). Then, after adding random noise (see above), these synthetic arrival times are inverted with a uniform starting model. Fig. 5 shows the results of the restore test; the main anomalies found in this study (Fig. 6) are su ciently well reproduced, making us con dent in the anomalies retrieved. Standard errors for velocity parameters (Fig. 7) are smaller than 0.4 per cent in the whole modelled area, less than 6 per cent of the computed perturbations. THE P-WAVE VELOCITY MODEL: RESULTS Fig. 6 shows the velocity perturbations in the three inverted layers. At 8 km depth, we observe high-velocity anomalies beneath the western and eastern Alps, the southern Apennines and the Calabrian arc, and beneath the Iblean foreland in Sicily. High-velocity anomalies are also found beneath the

4 486 R. Di Stefano et al. Figure 3. Variations of hypocentral coordinates after relocation in the 3-D model; negative values for depth changes indicate the shallowing of earthquakes. central and northern parts of the Apennines (between and 43 0 ), but these are located in a poorly resolved area. Areas of low-velocity material are present beneath the eastern margin of both the western Alps and the northern Apennines (Padanic foredeep), in the Vesuvio area, in the Aeolian area and around the Mt Etna volcano, and in the western part of Sicily (Gela foredeep). High-velocity anomalies are present beneath Mt Etna volcano. In layer 2, at 22 km depth, an almost continuous narrow area of low-velocity anomalies is imaged beneath the Apenninic belt, from northern Tuscany to Calabria. The velocity perturbations are as large as 6^8 per cent. The low-velocity belt seems to be displaced in the central Apennines (around a latitude of 42 0^43 0 ) and it is narrower in the southern Apennines (25^50 km) than in the northern Apennines (50^75 km). To the east of this central belt of low velocities, a ring of high-velocity material is observed in the Padanic area and in the Apulian and Iblean forelands. Low-velocity anomalies are also found beneath the western and eastern Alps. Highvelocity zones are imaged beneath the Vesuvio and Mt Etna volcanoes. High-velocity anomalies are found beneath the peri-tyrrhenian margin of the Apennines from northern Tuscany to the Vesuvio. In layer 3, at 38 km depth, two main belts of low-velocity anomalies are foundösurrounding the northern Apenninic arc from the Padanic area to Mt Conero, and around the Calabrian arc. This latter anomaly is interrupted by a central high-velocity area in the Tyrrhenian margin of Calabria. Velocity perturbations are of the order of 4^6 per cent. Between the two arcs, an isolated low-velocity anomaly is present in the central Apennines (centred at 42 0^ latitude). To the west of the low velocity of the northern Apenninic arc, relatively high-velocity areas are observed beneath the belt. EFFECTS OF AN APRIORIMOHO ON THE TOMOGRAPHIC RESULTS The real geometry of the Moho in the study region is very complex and di erent Mohos have been observed (Tyrrhenian versus Adriatic Moho in the Apennines; Ponziani et al. 1995; Amato et al. 1998; Giudici & Gualteri 1999; three di erent Mohos in the western Alps; Kissling 1993; at least two Mohos in the eastern Alps; Scarascia & Cassinis 1997). In all the tomographic studies made so far in the region, a constant depth has been used for the Moho in the inversion. In this section, we investigate how the assumption of a Moho with constant depth throughout Italy or the adoption of a heterogeneous geometry in uences the tomographic results. In the rst trials, we inverted the data, xing the depth of the planar Moho at 30 and 38 km depth. We found that the main pattern of anomalies in the crust (8 and 22 km depth) remains similar in both models (Fig. 8), suggesting that anomalies in the crust are scarcely in uenced by the choice of the apriori Moho depth. The low-velocity anomaly at 22 km depth beneath peninsular Italy is still retrieved and it is wider in the northern than in the southern Apennines. Even if the lateral continuity of the anomaly slightly di ers between the two inversions, the most important pattern is substantially identical to that shown in Fig. 6. In the inversions with the Moho xed at 30 and 38 km depth, we observe a di use pattern of either negative or positive anomalies in the uppermost mantle, indicative of a reference Moho that is too shallow or too deep. This con rms that the apriorimoho at 34 km depth is the most appropriate assumption. Then, we tried to invert the data with an apriori3-d Moho, taken from the map of crustal thickness for the Mediterranean area proposed by Geiss (1987). This map was computed by an

5 Crustal and uppermost mantle structure in Italy 487 Figure 4. Checkerboard test in the three inverted layers. Note the good reproduction of synthetic anomalies in the two deep layers and in some regions of the upper layer. The contour line marked 60 delineates the modelled area. interpolation of data from seismic re ection and refraction studies, which, for the Italian region, are sparse, with many unsampled areas. Recent studies on the deep structure of Italy revealed some di erences from Geiss' map (Amato et al. 1998; Pialli et al. 1998; Giudici & Gualteri 1999). The use of the heterogeneous Moho introduces a severe complexity in the inversion, since the depth variations are huge, between 20 and more than 55 km (see also Nicolich 1989). With the earthquake arrival time data set presently available, the sampling of the volume does not allow one to resolve more than two layers in the crust and one in the mantle and nodes spaced at With this parametrization, it is not possible to take into account rapid variations of crustal thickness such as those found by Geiss (1987). Although the results for some regions are encouraging, the reliability of the velocity images is more di cult to assess, and we suspect that artefacts in the results exist related to both our sparse sampling and local inappropriate Moho depths. Since in seismic tomography the simplest solution is always preferable, we believe that, at present, the e ects on the results obtained by using a planar Moho can be better understood than those derived from a too complex heterogeneous Moho. Moreover, the model with a planar

6 488 R. Di Stefano et al. Figure 5. Restore test in the two deep layers (22 and 38 km); note the good recovery of the real anomalies (see Fig. 6). Moho is directly comparable with those obtained by other tomographic studies with the same assumption (Amato et al. 1993; Spakman et al. 1993; Mele et al. 1997; Piromallo & Morelli 1997; Lucente et al. 1999). In any case, we are aware that di erences in the crustal thickness must be considered in the interpretation of velocity anomalies at the crust^mantle boundary. INTERPRETATION OF THE VELOCITY MODEL The interpretation of tomographic images is based on the available information for P-wave velocity of di erent rocks and a consideration of the e ects of the most important factors controlling seismic velocities in the crust and upper mantle (Christensen 1982; Christensen & Mooney 1995). Considering the current literature (see Eberhart-Phillips 1993 and references therein), the velocity anomalies in the upper crust (layer 1) are mainly related to lithological heterogeneities, and fast anomalies indicate the extent of metamorphic and limestone units in the Apennines (Amato et al. 1992; Chiarabba et al. 1995). In this work, the velocities computed at 8 km depth are averaged over the entire upper crust, and generally represent highs and lows in the basement topography. In the lower crust, the velocity anomalies are most probably due to thermal e ects. At 38 km depth (our deepest layer), the positive and negative anomalies re ect the Moho geometry, as previously explained. The Apennines The upper crustal structure of the Apennines consists of di erent units, each unit being about 5^6 km thick, mostly composed of marls and limestones (with average Vp values of about 4 and 4.5^5.2 km s 1, respectively), thrust over low-velocity ysch units (Vp 3^3.5 km s 1 ) and overlying crystalline basement or a deeper limestone platform (Apulian platform) in the southern Apennines (Vp higher than 5.9 km s 1 ). These velocity values are averages from seismic refraction and re ection studies (Mostardini & Merlini 1986; Bally et al. 1986; Amato et al. 1994; Scarascia et al. 1994; Ponziani et al. 1995). Information about the deep structure of the Apennines is sparse and less constrained. Active seismic studies show velocity inversions in the lower crust and a complex geometry for the Moho (Scarascia et al. 1994; Ponziani et al. 1995; Pialli et al. 1998). The image retrieved of layer 1 (Fig. 6) su ers from two distinct e ects that produce an aliased picture of the upper crustal structure. The rst e ect is produced by an uneven sampling of the real Earth structure, which consists of lateral lithological heterogeneities presumably smaller than the inverted grid spacing. The second e ect is related to the averaging in the same layer of both the very shallow anomalies (mostly underneath seismic stations) and the anomalies located around and below the reference level of 8 km depth. The parameter resolution does not permit us to resolve these ambiguities. Therefore, the velocity pattern retrieved at 8 km depth has to be considered as a rough approximation of the upper crustal structure. In the areas where the resolution is good, velocity anomalies at 8 km depth replicate the surface geology su ciently well. The high-velocity anomalies beneath the Apennines (Vp higher than 5.9 km s 1 ) may be interpreted as stacks of high-velocity units, thrust eastwards during the building up of the belt. Since the average velocity of the upper structural units is generally less than 5.0 km s 1, the high velocities retrieved probably

7 Crustal and uppermost mantle structure in Italy 489 Figure 6. P-wave velocity anomalies in the three layers. Contouring is every 2 per cent of velocity perturbations. indicate that the crystalline basement, or the Apulian platform in the southern Apennines, is involved in the thrust tectonics. This evidence is consistent with results obtained by local tomographic studies, where exceptionally high P-wave velocities (higher than 6.4 km s 1 ) were found at about 7^12 km depth (Chiarabba et al. 1995; Chiarabba & Amato 1997). A discontinuity in the velocity pattern is observed between the northern and southern Apennines at about 42 0 latitude, although the poor resolution in this area does not allow us to de ne this boundary well. This sharp transition is still visible in the two deeper layers and corresponds to a well-known transition between the northern and southern Apenninic arcs

8 490 R. Di Stefano et al. Figure 7. Standard errors of velocity parameters in per cent. Values are mostly lower than 0.4 per cent. (see Patacca & Scandone 1989). In the southern Apennines, zones of high velocity may indicate the extent of the Apulian carbonate platform at depth. Consistent with this interpretation, geological sections across the southern Apennines (Casero et al. 1988) show the presence ofthrustunits composed of limestones of the Apulian platform at about 8 km depth or even shallower. The same high-velocity units (Meso-Cenozoic platform limestones) may be responsible for the high velocities found in the Apulian and Iblean forelands, where they crop out or are found at shallow depth by seismic re ection studies (Mostardini & Merlini 1986; Casero et al. 1988). The lowvelocity anomalies located at 8 km depth beneath both the Padanic and Gela foredeeps may be explained by the thick terrigenous sediments present in the shallow crust, which are bent underneath the belt with a cover as thick as 8 km (Kruse & Royden 1994). In the lower crustal layer (22 km depth), the aliasing e ect present in the upper layer is strongly mitigated. The main anomaly is the low-velocity belt beneath the Apennines, which corresponds very clearly to areas of strong negative Bouguer anomalies (Fig. 9). The presence of the low-velocity belt beneath the Apennines is consistent with velocity inversions found at about 20 km depth by using wide-angle seismic pro- les across Italy (see Scarascia et al. 1994). Therefore, in the lower crust low-velocity and low-density materials are present beneath the belt. Considering the starting P-wave values used in the inversion for the crust (6.2 km s 1 at 22 km depth), the extreme velocity reduction observed beneath the belt is hardly explained by lithological heterogeneities. Since the Mesozoic limestones, which represent the upper part of the Adriatic lithosphere, are about 6 km thick and overlie pre-mesozoic metamorphic rocks, P-wave velocities higher than 6.0 km s 1 should be expected below *12 km depth. The velocity values lower than 6.0 km s 1 found in this study are incompatible with deep crustal basement rocks (see Christensen 1982), and require that the rocks sustain strong thermal e ects. We are

9 Crustal and uppermost mantle structure in Italy 491 Figure 8. Velocity perturbations found in the lower crust (top) and in the uppermost mantle (bottom) obtained using a xed Moho at 30 km depth (on the left) and 38 km depth (on the right). tempted to interpret the low velocities at 22 km depth as an area where lower crustal rocks are heated by the uplifted asthenosphere in front of the Adriatic slab. A pronounced thermal anomaly at depth is also consistent with the large attenuation of Pn phases observed by Mele et al. (1997). In the third and deepest layer, the low-velocity anomalies found along the northern Apennines (5^6 per cent lower than the starting Vp of 8.0 km s 1 ) may be interpreted in two alternative ways. The rst explanation is that the belt of low velocity represents hot asthenospheric material owing upwards in front of the underlying Adriatic slab. Such a hypothesis is consistent with the results obtained for the Pn velocity and attenuation beneath the Apennines by Mele et al. (1997). This anomaly may be the upper continuation of the

10 492 R. Di Stefano et al. The Calabrian arc and volcanic areas of Sicily In the Calabrian arc, high-velocity zones correlate with the metamorphic and intrusive units observed at the surface. In the southern Tyrrhenian subduction zone, de Voogd et al. (1992), based on crustal thickness (about 17 km in the Ionian area) and velocity pro les, proposed that the nature of lithosphere subducted beneath the Calabrian arc is oceanic, as already hypothesized by Barberi et al. (1973) based on volcanological data. In this area, the low-velocity anomaly found beneath the eastern (Ionian) part of the Calabrian arc at 38 km depth may indicate crustal material of the descending oceanic plate. In front of the slab (on the Tyrrhenian side), low velocities may be due to a pronounced thermal anomaly induced by the slab retreat and the subsequent rising of the asthenosphere in the Tyrrhenian basin. Beneath the Mt Etna volcano, high velocities are imaged at both 8 and 22 km depth, suggesting the presence of intrusive bodies and the absence (although at a large scale) of a wide magma chamber in the crust. A pronounced low-velocity anomaly is located beneath Mt Etna at 38 km depth, which may suggest a deep magmatic body in the uppermost mantle or at the base of the crust. Beneath the Aeolian volcanoes, lowvelocity materials, interpretable as crustal magmatic bodies, are imaged between 8 and 22 km depth, probably connected with thermal anomalies at depth, above the subducting slab. Figure 9. Bouguer anomalies for the Italian peninsula. low-velocity materials observed in front of the slab and deeper in the mantle (Piromallo & Morelli 1997; Lucente et al. 1999). Alternatively, the low-velocity anomaly can be interpreted as subducted continental crust attached to the subducting Adriatic lithosphere. Similar low-velocity anomalies dipping in the mantle have been related to the subduction of continental crust beneath Taiwan, the Hindu Kush and New Zealand (Roecker et al. 1987; Roecker 1993; Eberhart-Phillips & Reyners 1997). Roecker et al. (1987) suggested that apparent velocities of 7.7^7.8 km s 1 found at about 40 km depth require some 6^16 km of lower crust to lower the mantle velocity of 8.2 km s 1. Beneath the Pyrenees, a low-velocity anomaly extending between 30 and 90 km depth has been interpreted by Souriau & Granet (1995) as subducted lower crustal material. Low velocities similar to those found in our study were interpreted as crustal material attached to the subducting continental lithosphere. In our case, this hypothesis is consistent with the continental nature of the subduction in the entire Apennines, which is based on several pieces of geological evidence (Patacca et al and references therein), and with the results of DSS seismic refraction studies (Ponziani et al. 1995; Pialli et al. 1998). Previously available information and our results make both explanations equally possible. Since the low-velocity anomaly is con ned to the eastern part of the belt and does not spread beneath the entire area (as in Mele et al. 1997), we are more attracted by the hypothesis of subduction of continental material. At 38 km depth, the relatively high velocity observed to the west of the main low-velocity anomaly beneath the Apenninic region may be due to a Moho depth shallower than 34 km, in agreement with results from active seismic experiments (Pialli et al. 1998) and passive seismological studies (Amato et al. 1998). The Alps A striking similarity between velocity in the lower crust and gravity anomalies (Figs 7 and 9) is observed for the Alps, providing an independent constraint on the reliability of the tomographic images. Positive and negative velocity anomalies coincide with gravimetric highs and lows, respectively. In this area a variable Moho depth has been observed (Kissling 1993 and references therein) and crustal thickness is as large as 50 km. Thus, positive and negative anomalies in our deepest layer (38 km depth) primarily re ect di erences in Moho depth. In the western Alps, the `Ivrea^Verbano body' appears clearly as a high-velocity feature at 8, 22 and 38 km depth, in agreement with local studies by Kissling (1993) and references therein. The high-velocity Ivrea^Verbano body is a crustal megathrust uplifting deep crustal and uppermost mantle units (high-grade metamorphic basic rocks) to a very shallow depth. The high-velocity body is separated from low-velocity areas imaged beneath the Po plain by a sharp velocity gradient. To the north of the Ivrea^Verbano body, other high-velocity features can be related to the Penninic thrust units. In the lower crust (northwest of the Ivrea^Verbano high velocity), a low-velocity area is found. This anomaly may indicate the presence of upper crustal rocks at depth, with velocities lower than the Penninic thrust unit. Modelling gravity data, Bayer et al. (1989) found evidence for an approximately 14-km-thick low-density unit, possibly composed of upper crustal rock beneath about 20 km depth. Thus, our model supports the contention that the upper crustal rocks, overthrust by the Penninic unit and actually present at about 22 km depth beneath the western Alps, have not yet been completely metamorphosed. In the upper crust, high-velocity anomalies along the eastern Alps are evident, probably related to south-verging limestone

11 Crustal and uppermost mantle structure in Italy 493 thrusts. These anomalies are consistent with Mesozoic uplifted units of the south Alpine belt revealed by active seismic studies (Pieri & Groppi 1981). At 22 km depth, a broad low-velocity anomaly is found beneath the eastern margin of the southern Alps. Consistent with this, velocity inversions, with average P-wave velocities of about 5.9 km s 1, have been found by deep seismic refraction studies (Scarascia & Cassinis 1994). The velocity perturbations are smaller than those obtained beneath the Apennines and western Alps (lower than 4 per cent), making the interpretation more speculative. At 38 km depth, a small continuous area of low-velocity anomalies is present beneath the whole Alpine belt, in agreement with the results of Kissling (1993) and references therein. The deep low-gravity and low-velocity anomalies can be interpreted as the deep crustal roots of the Alps, which have P-wave velocities lower than the surrounding mantle lithosphere. A thicker crust beneath the Alps has already been delineated by active seismic experiments and gravity modelling (Hirn et al. 1989; Scarascia & Cassinis 1997). GEODYNAMIC EVOLUTION AND SEISMOTECTONICS OF THE APENNINIC SYSTEM Previous tomographic studies of Italy mainly focused on the mantle structure (Amato et al. 1993; 1998; Spakman et al. 1993; Selvaggi & Chiarabba 1995; Piromallo & Morelli 1997; Cimini & De Gori 1997), on the crust (Alessandrini et al. 1995; Chiarabba & Amato 1996) and on the Pn and Sn velocity at the Moho (Parolai et al. 1997; Mele et al. 1997). Results from these studies may seem somehow contradictory, promoting or not promoting the presence of detachments along the slab, visible as low-velocity areas interrupting the fast signature of the downgoing slab. Mele et al. (1997) presented evidence for delamination of the Adriatic lithosphere; that is, a pronounced low velocity and high attenuation of seismic phases in the uppermost mantle beneath the Apenninic area. This low velocity is located beneath the entire Italian peninsula, between the Tyrrhenian and Adriatic seas. Below this anomaly, a laterally continuous high-velocity slab is imaged, which appears to be segmented into two main arcs upwards (Ciaccio et al. 1998; Lucente et al. 1999). In this paper, we attempt to clarify the velocity heterogeneities in the crust and the uppermost mantle (the most critical depth for regional, teleseismic and Pn tomography) by using a robust inversion technique and a large data sampling. In the lower crust of the Apennines, we observe a continuous belt of low velocities. At 38 km depth, beneath the eastern margin of both the northern Apenninic and the Calabrian arcs two distinct low-velocity belts are found. Here, we propose a model for the Apennines based on seismicity distribution and velocity anomalies found by di erent authors and those retrieved in this study (Fig. 10). Our model is also constrained by several pieces of geological information available for the area (e.g. Reutter et al. 1980; Malinverno & Ryan 1986; Casero et al. 1988; Patacca & Scandone 1989; Serri et al. 1991; Royden 1993). In the northern Apennines (Fig. 10a), the Moho geometry is approximated considering the velocity pattern obtained in this work, the results from active seismic studies (Scarascia et al. 1994; Ponziani et al. 1995; Pialli et al. 1998) and the results from passive seismic experiments (Amato et al. 1998). In our model, we di erentiate an Adriatic Moho to the east (currently subducting or sinking beneath the belt) from a newly formed Tyrrhenian Moho to the west. The low-velocity anomalies at 38 km depth correspond to the Adriatic continental crustal material presently bent downwards beneath the belt. The low-velocity anomaly observed by Mele et al. (1997) in the uppermost mantle of the Apennines, located mostly below 40 km depth, may be the downward continuation of the lowvelocity material found in our study at 38 km depth on the eastern side of the belt. This anomaly is consistent with the presence of continental material belonging to the southwestdipping Adriatic plate. Intermediate-depth earthquakes are restricted to the upper 100 km, while the previously subducted lithosphere is mostly aseismic. The low velocity in the lower crust beneath the belt may indicate high temperatures related to the proximity of asthenospheric material advancing in front of the slab while the slab is retreating to the east (Royden 1993). The longer such a process develops, the more the asthenospheric substitution will spread to the Adriatic area. The subducting plate is con ned north of * and separated from the central Apennines, in agreement with the southernmost extension of the intermediate-depth earthquakes (Selvaggi & Amato 1992). In the central Apennines (at about 42 0 latitude), an isolated low-velocity anomaly is found at 38 km depth. The structure of this area is complex and probably re ects an intermediate stage of evolution between the subduction in the northern Apennines and the collision in the southern Apennines. Based on the spectral analysis of topography and geological data, D'Agostino et al. (1997) proposed that the observed topographic undulation and the widespread distribution of extensional features in this central part of the Apennines are related to thermally controlled boudinage of the lower crust. Negative anomalies modelled at 22 km depth are consistent with this interpretation. In the southern Apennines, the lack of low-velocity anomalies at 38 km depth suggests that the Adriatic continental lithosphere does not abruptly dip beneath the belt, as in the northern Apennines, but plunges at a relatively low angle. At greater depths, a `slabless' window was proposed by Amato et al. (1993), looking at teleseismic tomographic images and deep seismicity. Tomographic images from recent studies (Piromallo & Morelli 1997) and Lucente et al. (1999) at least partially con rm this hypothesis, which was based on a *300 km long interruption of the high-velocity zone, which runs along peninsular Italy in the upper 200 km. In fact, discontinuous patches of a weak (*2 per cent) high-velocity anomaly have been detected by tomographic studies in the southern Apennines (Lucente et al. 1999) that could represent the remnants of stretched continental lithosphere subducted in the past. In any case, both the deep upper mantle structure and the shallower structure depicted in this work reveal a lateral heterogeneity of the subducted lithosphere along the Italian peninsula, which separates the steep subduction of Calabria from that in the northern Apennines. While a steeper subducting and retreating plate has been related to a thinner crust for the northern Apennines (Royden 1993), the thicker continental Adriatic lithosphere in the southern Apennines (Panza 1984; Doglioni, et al. 1994) may subduct less easily. Here, the introduction into the trench of the thick continental promontory may have caused the termination of the subduction process.

12 494 R. Di Stefano et al. Figure 10. Geological sketches for subduction in the northern Apennines (a) and the Calabrian arc (b) based on velocity anomalies found in previous papers and in this study and earthquake hypocentres from Selvaggi & Amato (1992), Selvaggi & Chiarabba (1995) and Chiarabba & Selvaggi (1997). AMh is the Adriatic Moho, NMh is the `new' Tyrrhenian Moho, VdC is the Val di Chiana graben and Mu is the Mugello graben. In the southern Tyrrhenian subduction zone (Fig. 10b), two broad low-velocity zones are located beneath both the southeastern portion of the Calabrian arc and the Aeolian islands, separated by a central high-velocity anomaly above the high-velocity slab imaged by Selvaggi & Chiarabba (1995) and Lucente et al. (1999). The low velocities at 38 km depth beneath the eastern Calabrian coast may indicate the crustal portion of the Ionian lithosphere subducting beneath the Calabrian arc (Fig. 10d). This hypothesis is consistent with seismic re ection pro les showing lower crustal re ectors of the Ionian lithosphere underplating beneath the Calabrian coast (Monaco et al. 1996). To summarize, lateral heterogeneities, di erent thicknesses and a strongly irregular shape of the subducted Ionian^Adriatic lithosphere have produced a complex system of subduction in the Italian region (Fig. 11). The two main arcs are characterized by the subduction of oceanic lithosphere (Calabrian arc) and continental lithosphere (northern Apennines). In this latter zone, we speculate that only the lower crust remains attached to the descending lithosphere, appearing as low-velocity anomalies in the uppermost mantle. In the area between the two arcs, the thick continental lithosphere of the Adriatic promontory caused the cessation of subduction. The tomographic image retrieved of the lower crust allows us to make some inference on the broad-scale seismotectonics of the Italian region. Historical earthquakes, with magnitudes ranging between 5 and 7, that occurred in Italy in the past 1000 years (Boschi et al. 1995) are mostly located above regions of low-velocity anomalies in the lower crust (Fig. 12). This striking correspondence suggests a close cause/e ect relation between earthquakes in the upper crust and velocity anomalies in the lower crust. According to our interpretation, lower crustal regions are characterized by pronounced thermal anomalies. In the northern Apennines, where there is evidence for the

13 Crustal and uppermost mantle structure in Italy 495 Figure 11. Schematic evolution of the Apenninic system since the Late Miocene (modi ed from Malinverno & Ryan 1986 and Faccenna et al. 1996). For simplicity, in the present-day sketch no active faults are reportedöonly the direction of crustal extension is shown. subduction of continental lithosphere, the uprising and lateral ow of the asthenosphere in the backarc region is con rmed by deep seismic anisotropy (see Margheriti et al. 1996); the related heating and softening of the lower crust of the Apenninic belt may be the primary factors controlling the stretching of the lithosphere and the subsequent extensional tectonics. In the southern Apennines, where the cessation of subduction is at a more advanced stage, the asthenospheric substitution is more e ective and this could explain the order-of-magnitude higher deformation of this region than the northern Apennines (see Westaway 1992) and the widespread extensional stress regime (Montone et al. 1997). The observed regional uplift of the Apennines in the Pliocene and Quaternary (Ambrosetti et al. 1983), with normal faults accommodating deformation in the brittle upper crust, is consistent with this mechanism. The regions of the largest anomalies (perturbations larger than {4 per cent) are the sites where deformation in the upper crust mostly concentrates. In the Calabrian arc and eastern Sicily (where the correspondence between large earthquakes and low-velocity anomalies is poor) seismicity could be generated by a di erent geodynamic process, probably related to the still active subduction of the Ionian oceanic lithosphere. CONCLUSIONS In this study, we found evidence for the presence of low-velocity material at upper mantle depths in the outer margin of both the northern Apennines and the Calabrian arc, interpretable as subducted crustal material. A continuous low-velocity anomaly is observed in the lower crust of the Apenninic belt, possibly related to high temperatures caused by asthenospheric substitution in front of the descending slab. Lateral heterogeneities of the subducted Adriatic^Ionian lithosphere are responsible for the complex geodynamic evolution of the Apenninic system in the past 20 Myr. We also speculate that the thermal anomaly in the lower crust may be responsible for the extensional tectonics which a ect the upper crust of the Apenninic belt.

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