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Contribution of numeric dynamic modelling to the understanding of the seismotectonic regime of the northern Apennines
Tectonophysics 315 (1999) 15–30 www.elsevier.com/locate/tecto
Contribution of numeric dynamic modelling to the understanding of the seismotectonic regime of the northern Apennines A.M. Negredo a, *,1, S. Barba b,c, E. Carminati a, R. Sabadini a, C. Giunchi a, 2 a Department of Earth Sciences, Section of Geophysics, University of Milano, Via Cicognara 7, 20129 Milan, Italy b Istituto Nazionale di Geofisica, Via di Vigna Murata 605, 00143 Rome, Italy c Department of Earth Sciences, University ‘La Sapienza’, P. Aldo Moro 5, Rome, Italy
Abstract In this paper we investigate the forces possibly active in the area of the northern Apennines by means of twodimensional finite element modelling assuming a viscoelastic rheology. The forces included in the models are related to continental convergence between Africa and Eurasia, to negative buoyancy of the subducted Adriatic lithosphere and to positive buoyancy of anomalously hot mantle material underneath Tuscany. Model-predicted stress distribution is compared with seismotectonic data available for the area of the northern Apennines and with earthquakes distribution. Our results indicate that continental convergence cannot be the only mechanism acting in the study area since it causes insignificant subcrustal stresses, in disagreement with intermediate seismicity observed under the Apennines. Asthenospheric upwelling under Tuscany (back-arc region) is shown to play a crucial role in the presentday dynamics of the Tyrrhenian–Apennines. Positive buoyancy causes an upwards and eastwards flow that generates extensional bending stresses in Tuscany and at the top of the slab under the Apennines. Horizontal pushing of the slab is proposed as an alternative mechanism to slab pull to generate subcrustal stresses. Activation of slab pull does not result in a significant change in orientation of the principal stress axes at shallow depths with respect to the model in which only asthenospheric diapirism is active. Therefore, the existence of a continuous gravitationally sinking slab cannot be ruled out on the basis of comparison with seismotectonic observations. Both models successfully reproduce extension in Tuscany and compression in the outer margin of the Apenninic belt, but fail to reproduce extension along the inner portion of the chain. Our modelling highlights the importance of better constraining the lateral extent of the asthenospheric diapir in order to find out whether the extension and tectonic uplift in the Apennines is caused by asthenospheric upwelling as in the area of Tuscany. © 1999 Elsevier Science B.V. All rights reserved. Keywords: dynamic modelling; earthquakes; Northern Apennines; seismotectonics; stress
* Corresponding author. Tel.: +34-91-3945190; fax: +34-91-3944398. E-mail address: [email protected] (A.M. Negredo) 1 Present address: Departmento de Geofı´sica, Facultad CC Fı´sicas, Universidad Complutense de Madrid, 28040 Madrid, Spain. 2 Present address: Istituto Nazionale di Geofisica, Via di Vigna Murata 605, 00143 Rome, Italy. 0040-1951/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S0 0 4 0- 1 9 51 ( 9 9 ) 0 02 8 6 -3
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1. Introduction The post-Middle Miocene evolution of the Tyrrhenian sea–northern Apennines system displays a pattern of coupled compression and extension, with a wave of compressional deformation moving towards the foreland (to the east–northeast) followed by a wave of extension (e.g. Pialli and Alvarez, 1997). The tectonics of the Apenninic fold-and-thrust belt is dominated by an eastward propagation of the thrust fronts and of the related foredeep basins (Ricci Lucchi, 1986). Contemporaneously to the eastward migration of the Apenninic belt, extensional basins opened in a back-arc position ( Tyrrhenian basin and Tuscany; e.g. Bartole, 1995). Episodes of compression superimposed on the Tuscany extensional regime have, however, been reported (e.g. Boccaletti et al., 1997 and references cited therein). This pattern of coeval contraction in the foldand-thrust belt and extension in a back-arc posi-
tion (Fig. 1) is here considered to be produced by two major interacting dynamic processes: convergence between Africa and Eurasia and the rollback of subducting Adria–Ionian lithosphere. The Africa–Europe convergence, discontinuously active since the Cretaceous (e.g. plate rotation data by Mueller and Roest, 1992) and still continuing today (e.g. VLBI data by Lanotte et al., 1996), is believed to play a major role in the dynamics of the central Mediterranean area. Support for the importance of tectonic escape processes related to the Africa–Europe convergence is given by geological studies (Lavecchia, 1988; Mantovani et al., 1996) and by analogue and numerical modelling ( Faccenna et al., 1996; Negredo et al., 1997, 1999). The roll-back of the subduction zone is suggested by several geological studies (e.g. Boccaletti and Guazzone, 1972; Malinverno and Ryan, 1986; Patacca and Scandone, 1989); the consequences of this mechanism have been investigated in recent numerical simulations (Bassi et al., 1997; Giunchi
Fig. 1. Map view of the main tectonic features in the northern Apennines including the profile line of the modelled section. Circles represent earthquakes occurring from January 1988 to September 1997, as reported by the Istituto Nazionale di Geofisica (ING). The size of circles increases with magnitude. The two events with larger magnitude are those occurring on 09/26/97 near Colfiorito (Assisi). The solid and dashed lines, respectively, indicate the areas characterized by prevalent tensional and compressive stress field, following Frepoli and Amato (1997).
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et al., 1996; Negredo et al., 1997; Carminati et al., 1998, 1999) and analogue modelling ( Faccenna et al., 1996). The retreat of the slab likely induced lithospheric thinning in the Tyrrhenian and Tuscany areas and an asthenospheric upwelling (black arrow in Fig. 2) beneath these areas (e.g. Boccaletti et al., 1997). In this geodynamic scenario, the upper crust of the Adriatic slab is likely to be delaminated and accreted to the fold-andthrust belt (Fig. 2), as indicated by seismic studies providing evidence for a thick-skinned tectonic style of the northern Apennines (Ponziani et al., 1995; Barchi et al., 1998). Also geochemical studies of the back-arc magmatism of Tuscany (Serri et al., 1993) indicate that a process of delamination and subduction of the Adriatic lithosphere can be responsible for the incorporation of a large amount of crustal material within the upper mantle in Tuscany. The subduction style and depth change notice-
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ably from the Calabrian arc to the northern Apennines. A continuous NW-dipping Wadati– Benioff plane down to 500 km depth in the southern Tyrrhenian sea (Selvaggi and Chiarabba, 1995) is related to the consumption of oceanic Ionian lithosphere. In contrast, a maximum seismicity depth of 90 km has been reported in the northern Apennines (Selvaggi and Amato, 1992; Di Giovambattista and Barba, 1997) and is associated with the consumption of the continental Adriatic lithosphere (Pialli and Alvarez, 1997). The onset of continental collision in this area is likely to have occurred in the Late Oligocene (Carmignani and Kligfield, 1990). The present-day continuity of the subducting Adriatic slab has recently been discussed. On the basis of tomographic images showing an interruption between the sinking slab and the Adriatic plate, Wortel and Spakman (1992) suggest the Adriatic slab to be detached. Moreover, these
Fig. 2. Schematic cross-section through the northern Apennines, modified from Boccaletti et al. (1997) and showing the model geometry assumed in the present study. The earthquakes falling in a 40 km wide band across the modelled section are projected on the profile.
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authors suggest that the detachment tear propagated from the northern to the southern Apennines. Support for this lateral migration hypothesis is given by stratigraphic studies showing a southward migration of the Adriatic foredeep depocentre (van der Meulen et al., 1998). On the basis of numerical modelling studies, Carminati et al. (1999) relate a Pliocene slab detachment event to the peculiar Quaternary tectono-stratigraphic features of the northern Apennines also detected by other authors (e.g. Vai, 1987; Argnani et al., 1997; Bertotti et al., 1997). However, the most recent tomographic studies do not provide clear evidence for slab detachment under the northern Apennines (Amato et al., 1993; Cimini and De Gori, 1997; Piromallo and Morelli, 1997). This controversial point will be addressed in this study. The main goal of this paper is to investigate the effect of the different forces possibly active in the northern Apennines (continental convergence, asthenospheric upwelling under Tuscany and slab pull ) on the stress and strain regime of the area. In the first part of the paper we present the seismotectonic data available for the northern Apennines and discuss the model constraints provided by geophysical observations, in particular heat-flow, crustal thickness and seismic tomography images. We then describe the 2D dynamic models developed to study the present-day tectonic regime in the northern Apennines and surrounding areas, and finally compare the results with earthquake distribution and focal mechanisms.
2. Seismotectonic regime The Italian peninsula shows a strong seismicity (e.g. Pondrelli et al., 1995; Barba et al., 1995) which follows the trend of the Apenninic belt and mainly occurs in the crust. The structural setting of the Apenninic belt is described by a double arc-shaped bending of the major structures, forming the northern Apenninic and the Calabrian arcs. These structural arcs are separated in the central Apennines by the Olevano–Antrodoco N–S trending fault zone (Patacca and Scandone, 1989). Along the northern Apenninic arc, contemporaneous extension and compression is observed. Frepoli and Amato
(1997), inverting earthquake data to obtain the stress tensor, evidenced E–W extension in the periTyrrhenian area and along the inner margin of the northern Apenninic belt, and NE–SW compression along the outer eastern thrusts (Fig. 1). To better identify the stress pattern in the northern Apennines, we project P and T axes, named P∞ and T∞, onto the modelled profile, striking about 60° from North (Fig. 3a and b; P and T axis data from Frepoli and Amato, 1997). We have carried out an orthogonal projection of data of earthquakes falling in a 200 km wide band centred on the modelled section. To avoid subjective interpretation of such plots, the function f=p∞t∞ sin[ pl(T∞)−pl(P∞)] has been introduced, where pl is the plunge of the corresponding axis if its trend lies between 0 and 180°, and the supplementary angle to the plunge if the axis trend exceeds 180°; p∞ and t∞ are the lengths of the axes once projected onto the section. The function f ranges from −1 (ideal extension along the profile) to +1 (ideal compression), since it is the product of two numbers ranging, respectively, between +1 and −1 (sin function) and between 0 and 1 ( p∞t∞; P and T axis lengths are set to 1). Values of f around 0 are obtained for earthquakes with prevalent motion out of the section. In Fig. 3c we plot crosses for compression ( f>0), and circles for extension ( f<0), whose sizes are proportional to the magnitude of f, and therefore to the ‘weight’ of extension/compression along the modelled cross-section. Data represented by small symbols are, therefore, less significant for comparison with our plane strain modelling. Fig. 3c displays in the area of Tuscany (between 0 and 100 km NE of the Tyrrhenian coast) prevalent extensional shallow seismic activity, which is in correspondence with normal faulting and graben tectonics. Between 120 and 150 km, the extensional activity deepens towards the inner margin of the belt, where minor compressional activity also occurs. This observation is in agreement with centroid moment tensor (CMT ) solutions for the most recent earthquakes in the Umbria–Marche region (see Fig. 1 for location), which indicate extension at a depth of 15 km under the Apennines. Compressional activity prevails in the external portion of the belt, between 150 and 200 km. These
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Fig. 3. Plots (a, b) represent the projection of P and T axes (normalized to a length of 1), named P∞ and T∞, of about 100 earthquakes ( located at distances less than 100 km from the section) onto the modelled section. Projection characteristics are described in the text (P and T axis data from Frepoli and Amato, 1997). In panel (c), a circle is plotted for mainly distensional mechanisms, while a cross characterizes mainly compressional mechanisms. The size of circles and crosses is proportional to the amount of extension/compression in the modelled section. Vertical scale in (c) is three times the horizontal one.
three seismotectonic domains seem to be separated by seismicity gaps, some 15–20 km wide, probably corresponding to transitional areas of reduced bending stress. The earthquakes characterized by out-of-plane motion (mainly strike slip) are scattered throughout the section.
3. Geophysical constraints The geometry of the modelled cross-section is constrained by tomographic and seismic data. The results of seismic–gravimetric models of the NE– SW profile CROP-03 show that the crustal thickness increases from 30 km under the Adriatic sea to more than 40 km under the Apenninic chain and again decreases to about 20–25 km under the northern Tyrrhenian sea and Tuscany (Marson et al., 1998). The shallow Moho depth in the latter area is related to a noticeable thinning of the entire
lithosphere, as derived from surface-wave studies by Panza (1985), which indicate a lithospheric thickness of 50 km underneath the northern Tyrrhenian sea and Tuscany, and 70 km under the Adriatic plate. There is a correlation between the areas of crustal thinning and the positive anomaly of surface heat flow (Fig. 4), which reaches values higher than 120–160 mW m−2 in the areas with a crust thinner than 25 km (Mongelli et al., 1992). The presence of the so-called ‘Tuscan–Tyrrhenian anomaly’ is confirmed by recent studies of Pn and Sn phase attenuation (Mele et al., 1997). These authors mapped a high attenuation zone in the uppermost mantle beneath the Apennines and western margin of the Italian peninsula and southern Tyrrhenian sea ( Fig. 4). They suggested that the presence of hot asthenospheric material below the Moho is responsible for inefficient Pn and Sn transmission. This interpretation is consistent with present-day extension along the Tyrrhenian basin
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Fig. 4. Heat flow distribution in the central Mediterranean area after Mongelli et al. (1992). Sn and Pn attenuation zones after Mele et al. (1997) are represented. The profile corresponding to the modelled section is also shown.
and the Apennines, and with the extensive Neogene and Quaternary volcanic activity in these areas, but it is difficult to explain the low heat flow values recorded in the Apennines. Further evidence for the presence of an anomalously hot upper mantle beneath Tuscany comes from seismic tomography studies by Piromallo and Morelli (1997) ( Fig. 5). They mapped the lateral heterogeneity of P wave velocity in the Mediterranean region and obtained a shallow lowvelocity anomaly beneath the western Italian margin, in correspondence with the results of Mele et al. (1997), and a high-velocity anomaly at deeper levels beneath the northern Apennines, in agreement with previous seismic tomography studies (Spakman et al., 1993; Amato et al., 1993; Cimini and De Gori, 1997). We use these results to introduce in our modelling the geometry of the descending slab and of the anomalous uppermost mantle beneath Tuscany.
4. Model description Fig. 6 illustrates the model geometry and the boundary conditions. The modelled cross-section
Fig. 5. Tomographic image along the analysed section, obtained from the velocity model of Piromallo and Morelli (1997). Velocity perturbations of maximum ±2% are shown in shades of grey, and the sign of the anomaly is drawn to better discriminate between positive and negative anomalies. The horizontal and vertical resolution of the tomographic image is 20×50 km2. A continuous positive anomaly with respect to the initial layered velocity model is shown beneath Italy down to 200 km. Details about the initial model, data and inversion procedure can be found in Piromallo and Morelli (1997).
Fig. 6. Geometry and boundary conditions of the models. V c denotes the velocity applied to the lithospheric portion of the left boundary of the model to simulate the effects of Africa– Eurasia convergence. The springs represent the buoyant restoring force applied at the surface. The circles denote free slip condition. The barbed portion of the subduction fault is locked. The remaining portions are unlocked and slip with zero friction. Different grey shadings indicate different materials (see text for further details). Overprinted dots indicate regions with density anomalies: Dr =−100 kg m−3; Dr =80 kg m−3. a s
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has a horizontal and vertical extent of 1250 km and 670 km, respectively; the lateral boundaries are far enough from continental Italy in order to avoid border effects in the area of interest. We have assumed a linear viscoelastic Maxwell rheology with viscosities 1024 Pa s for the crust, 5×1022 Pa s for the lithospheric mantle and 1021 Pa s for the asthenospheric mantle ( Whittaker et al., 1992; Spada et al., 1992). The elastic structure is based on the PREM reference model (Dziewonski and Anderson, 1981). We have assumed a Young’s modulus of 9×1010 Pa for the crust and 1.75×1011 Pa for the remaining materials. We have chosen a viscoelastic rheology because it reproduces the long-term viscous behaviour of the lithosphere and accounts for elastic flexure at the short time-scale. In contrast to viscous flow models using a temperature-dependent rheology (Zhong and Gurnis, 1995; Marotta and Sabadini, 1995), we do not intend to model the initiation of subduction and progressive sinking of the slab, but the state of stress created by different subduction conditions. The boundary conditions are as follows. The bottom of the model is fixed in the vertical direction. The buoyant restoring force (denoted by the springs in Fig. 6) is applied to the surface of the section, and is assumed to be proportional to the density contrast and to the vertical displacement at the surface (Desai, 1979; Williams and Richardson, 1991). At the left lateral boundary, we introduce for the lithosphere a constant horizontal velocity, which reproduces the eastward tectonic escape of the Tyrrhenian block related to the indentation of the African plate. Following the results of the 3D study by Negredo et al. (1999) this effect causes a relative convergence rate of about 3.5 mm/yr in the modelled cross-section. This value is applied to the western boundary, whereas at the right boundary no motion of the lithosphere is allowed. The interaction between the overriding and subducting plate is assumed to occur via a subduction thrust fault, which is locked between 0 and 35 km and unlocked at deeper levels, from 35 to 200 km. This condition of locking–unlocking of the fault is justified by the studies of hypocentral locations and focal mechanisms in subduction
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zones (e.g. Davies and House, 1979; Yoshii, 1979), which show that along the main thrust zone the predominant mechanism is thrusting along the surface of the zone, whereas at greater depths, in the Benioff zone, the activity is not concentrated at the interface separating the top of the slab from the mantle wedge and the slip is not parallel to this Benioff interface. It can be deduced that interplate and intraplate earthquakes occur at shallow and deep levels, respectively, and therefore the interplate motion is accommodated by stick slip on the thrust zone and by continuous aseismic slip on the Benioff interface. During the phase of strain accumulation, the fault is supposed to slip at depth and to remain locked closer to the surface (Savage, 1983). As discussed by several authors (Giunchi et al., 1996; Carminati et al., 1999; Negredo et al., 1999), the pattern of surface vertical motion inferred from geologic observations is only reproduced when free slip along an unlocked subduction fault is permitted. Actually, in a geologic timescale the overriding and the subducting plates are decoupled by the occurrence of earthquakes and by aseismic creep that accommodate the slip on the thrust zone. Unlocking of the fault enables progressive retreat of the subduction hinge line ( Whittaker et al., 1992; Giunchi et al., 1996). Therefore, the condition of unlocked fault is more appropriate to study processes with a long characteristic time-scale, whereas the condition of partially locked fault better accounts for the phase of strain accumulation and, in consequence, for the present-day seismotectonic regime. For the sake of simplicity we have considered in our modelling a single thrust fault, which intersects the surface of the cross-section at the boundary between the Apenninic chain and the Adriatic foredeep, as shown in the map of neostructural domains of Italy (Ambrosetti et al., 1987). The present-day stress and strain distribution along the modelled cross-section are calculated using the finite element code MARC (1994). The mesh consists of 930 eight-node plane strain elements with an increased resolution in the vicinity of the slab. Density contrasts and convergence are activated at the initial time and maintained constant thereafter, following the same procedure as
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Whittaker et al. (1992). After a time interval of about 250 kyr since loading, the dynamic equilibrium between the buoyant restoring force and the forces arising from density contrasts and convergence is attained. As a consequence, the initial exaggerated values of stress and velocity related to instantaneous gravity loading have disappeared and reached steady-state values. Modelling results are valid for a time-scale of 105–106 yr, during which the geometric configuration does not change significantly; for longer integration times viscoelastic models overemphasise the plate-like behaviour of the lithosphere. We investigate the seismotectonic implications of three models, assuming the same geometric configuration and rheological properties and differing for the mechanisms which drive motion. In the model CONVER only convergence is active, whereas in model CONAS we add a negative density contrast of 100 kg m−3 to account for the positive buoyancy caused by the asthenospheric diapir underneath Tuscany. In a third model, CONASLAB, we also activate a positive density contrast of 80 kg m−3 in the subducting slab to evaluate the effects of slab pull. This homogeneous density contrast is an average of the real density distribution, characterized by density contrasts higher than 100 kg m−3 due to the eclogitization of the lower crust gabbroic rocks and by temperature-related density contrasts within the lithospheric mantle of the slab, which are of the order of 50 kg m−3 in the case of a slowly sinking slab, as inferred from thermal models (e.g. Davies and Stevenson, 1992).
Fig. 7. Surface vertical velocity (a), velocity field (b) and von Mises stress distribution (c) obtained for the shallow central part of model CONVER (only convergence is active). Panel (c) also shows the earthquakes located in a 40 km wide band centred in the modelled section.
5. Modelling results 5.1. Model CONVER Figs. 7 and 8 summarise the results of model CONVER. Fig. 7a shows the model predicted vertical surface velocity. Fig. 7b portrays the velocity field of the central part of the model. The velocity vectors indicate an eastward movement with rates decreasing from west to east. This rather uniform pattern, dominated by horizontal movements, results from the eastward velocity applied to the
lithosphere at the western boundary of the model (V =3.5 mm/yr). The negligible vertical motion c along the free surface of the model (Fig. 7a), resulting from the absence of vertical loads, contrasts with the recent noticeable uplift in Tuscany and the Apennines and subsidence in the Adriatic realm, as inferred from geological data (e.g. Ambrosetti et al., 1987; Carminati et al., 1999). A thorough discussion on the vertical motion at the surface is, however, beyond the scope of the paper. Our models, in fact, are not able to accu-
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Fig. 8. (a) Seismotectonic observations shown in Fig. 3c superimposed on the model geometry. (b) Stress distribution obtained for the shallow central part of model CONVER. Bars represent the major eigenvectors of the stress tensor, thin for tension (s ) and 3 thick for compression (s ). Eigenvector length is scaled to the eigenvalue. 1
rately reproduce the vertical crustal motions because of the locking condition of the shallow portion of the subduction fault assumed in the calculations, as discussed in the previous section. This condition, however, forces the continuity of the crustal motion from the Tyrrhenian to the Adriatic domain. This is in agreement with recent GPS results from southern Italy ( Tonti, 1997) showing the continuity of velocity vectors from the overriding to the subducting plate. Our modelling, therefore, is consistent with the coupling between upper and subducting plates at the small time-scale (see Negredo et al., 1999 for a more detailed discussion). Fig. 7c displays the equivalent von Mises stress distribution obtained for model CONVER. The von Mises stress (s: ) is calculated as an equivalent intensity of the deviatoric stress tensor:
S
s: =
1 S S , S =s − d s ij 3 ij kk 2 ij ij ij
3
where s and S are the stress and the deviatoric ij ij stress tensors, respectively (the Einstein convention for repeated tensorial indexes is used); d is the ij Kronecker symbol. The most noticeable feature of Fig. 7c is the uniform stress distribution in the crust, with values lower than 75 MPa. Open circles
indicate the hypocentres of earthquakes located in a 40 km wide band centred on the modelled crosssection (see Fig. 1 for epicentre location). The absence of significant subcrustal stresses predicted by this model is not consistent with intermediate seismicity beneath the northern Apennines. In Fig. 8a we have superimposed the geometry of the central and shallow part of the model on the earthquakes displayed in Fig. 3c in order to enable comparison with modelling results. Fig. 8b displays the principal stress axes s (compression, 1 thick lines) or s (extension, thin lines) calculated 3 for each element; for simplicity, only the axis with the major absolute eigenvalue is portrayed. The crust at depths lower than 30 km displays a quite uniform compressional regime (s horizontal ) 1 which contrasts with the seismotectonic observations summarized in Fig. 8a. The upper part of the slab is, on the contrary, characterized by tensional stresses (s parallel to the dip of the slab) caused 3 by the horizontal flow of material (Fig. 7b) which bends the subducting plate. From the considerations listed above it can be deduced that the eastward tectonic escape of the Tyrrhenian block caused by the convergence between Africa and Eurasia cannot be the only mechanism active in the area of the northern Apennines.
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5.2. Model CONAS With model CONAS we investigate the combined effects of the upwelling of buoyant mantle material beneath Tuscany (Dr =−100 kg m−3) a and of regional convergence (V =3.5 mm/yr). As c in the previous model, subduction is included as a rigidity contrast and no subduction-related negative buoyancy is modelled within the slab. Such an unloaded slab is conceptually equivalent to a detached slab. Nevertheless, we did not introduce a detached geometry in order to avoid the models becoming very speculative. Furthermore, this choice enables direct comparison with the other two models. The velocity field obtained with model CONAS (Fig. 9b) indicates an upwards and eastwards motion of the mantle wedge material. This motion results in surface uplift in Tuscany ( Fig. 9a) compatible with tectonic uplift recorded by PlioQuaternary marine sediments outcropping at high topographic elevation (up to 1000 m) in this area (see Carminati et al., 1998 for a discussion). At shallow depths corresponding to the locked portion of the fault, the velocity field is continuous, as obtained also for the previous model, while a major discontinuity in the vertical component of velocity occurs in correspondence with the unlocked portion of the subduction fault. At depths greater than 35 km, the overall eastward flow causes an active roll-back of the slab, that tends to become vertical. The slightly downwards component of the velocity in the subducting plate, which is due to this effect, results in subsidence in the eastern part of Italy; the locking condition of the uppermost portion of the subduction fault prevents overthrusting of the Apennines over the Adriatic plate and forces an unrealistic subsidence in the Apennines (Fig. 9a). The von Mises stress distribution (Fig. 9c) changes noticeably with respect to the previous model (Fig. 7c). Stress accumulates also at higher depths in correspondence with the slab bending under the Apennines, between 100 and 150 km from the Tyrrhenian coast, in agreement with the occurrence of intermediate seismicity. This model reveals a mechanism alternative to slab pull to generate subcrustal stresses: the bending caused by
Fig. 9. Surface vertical velocity (a), velocity field (b) and von Mises stress distribution (c) obtained for model CONAS, which is characterized by activation of convergence and a negative asthenospheric density anomaly under Tuscany.
horizontal pushing of the slab. The increased vertical deformation generates high bending stress values in Tuscany and in the Apennines. It is also interesting to note that the narrow areas of reduced seismicity (around 100 and 160 km) correlate with areas of low von Mises stress, thus favouring our interpretation that these seismic gaps are related
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Fig. 10. Stress distribution obtained for model CONAS. Same representation as Fig. 8b.
to reduced bending stresses. In order to better evaluate the effects of the upwelling asthenospheric material, we have carried out a test where plate convergence is not activated, and obtained the same general pattern of von Mises stress, except for the unrealistic low stresses predicted for the Adriatic domain. The major principal stress axes calculated with this model ( Fig. 10) show the typical tectonic style resulting from bending of the lithosphere: upwelling in Tuscany causes extension and compression in the upper and lower crust, respectively. This pattern is inverted in the Apennines, where the stress axes orientation is complicated at depth by the abrupt change from fault locking to unlocking and by the lateral density and rigidity contrasts. If we compare these results with the seismotectonic observations compiled in Fig. 8a, this model successfully reproduces compression in the outer margin of the Apenninic belt and extension in Tuscany. Extension at deep levels, at depths of about 10–35 km under the Apennines (120– 160 km away from the Tyrrhenian coast), is also compatible with the recent extensional seismic activity in the Umbria–Marche region, which occurred at depths of 15 km. However, the model fails to reproduce extension at shallower depth in the Apennines. It is worth noting that the seismic activity in Tuscany is concentrated in the mid and upper crust, in contrast to the high compressional stresses predicted by the model in the lower crust. This is probably related to the fact that in this area, due to the high temperatures, the lower crust deforms in a ductile manner when yield strength is exceeded (Pasquale et al., 1997).
5.3. Model CONASLAB In contrast to the previous models, we now assume that the slab is continuous and sinks into the mantle, being loaded with a positive density anomaly of 80 kg m−3. The density anomaly reproducing the upwelling of buoyant mantle beneath Tuscany (Dr =−100 kg m−3) and the velocity a boundary condition reproducing regional convergence (V =3.5 mm/yr) are also active. As a c consequence of the slab gravity loading, the predicted surface subsidence in the Apennines and eastern margin of the Italian peninsula (Fig. 11a) is increased with respect to model CONAS ( Fig. 9a). The velocity field obtained with model CONASLAB ( Fig. 11b) is similar to that of Fig. 9b, the most pronounced difference being the increase in eastward component of motion. This is clearly induced by the retreat of the slab which tends to verticalize faster than in model CONAS, because of the combined effects of the eastward flow, which causes active roll-back, and of the density anomaly within the slab, which results in gravitational or passive roll-back. The observations made for model CONAS regarding the discontinuity in the vertical component of motion across the unlocked portion of the slab, and regarding the velocities at the surface, are also valid for this model. The von Mises stress distribution ( Fig. 11c) in the shallow portions of the model (crust) is quite similar to that obtained with model CONAS. In contrast, the stress distribution within the slab is quite different, being characterized by stress accumulation down to 200 km depth. This difference has to be ascribed to the pull exerted by the loaded
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contrasts with the absence of seismicity at depths larger than 90–100 km below the northern Apennines. This apparent misfit, however, is not sufficient to rule out the existence of a continuous slab exerting slab pull. In model CONASLAB, von Mises stresses mainly concentrate in the uppermost layer of the slab, which is assumed to be lower crust and is thus likely to behave in a ductile rather than in a brittle manner (Pasquale et al., 1997). Therefore, this ductile behaviour can explain the absence of seismic activity within the slab and also at deep crustal levels under Tuscany. Fig. 12 displays the major principal stress axes calculated with model CONASLAB. The stress pattern within the upper plate is almost identical to that displayed in Fig. 10. The addition of the slab pull mechanism, however, produces a decrease of 20 km in the area characterized by tensional stresses compared with model CONAS. Model CONAS, therefore, reproduces slightly better the seismotectonic observations compiled in Fig. 8a (compression in the outer margin of the Apenninic belt and extension in Tuscany). Within the subducting plate, the area characterized by tensional stresses is enlarged and the orientation of the tensional eigenvectors becomes clearly parallel to the slab. This indicates a predominance of slab pull on eastward mantle flow in the determination of the stress pattern in this part of the model. The obtained stress field is perfectly compatible with the extensional seismicity observed at deep levels (at depth of 10–35 km) under the Apennines, as already obtained for model CONAS. Fig. 11. Surface vertical velocity (a), velocity field (b) and von Mises stress distribution (c) obtained for model CONASLAB, which is characterized by the activation of convergence and the density anomalies under Tuscany and in the Adriatic subducted lithosphere.
portion of the slab. In correspondence with the higher slab bend, model CONASLAB displays von Mises stresses higher (up to 200–250 MPa) than model CONAS. This is due to the combined effects of the slab bending related to eastward mantle flow (also active in model CONAS ) and of slab pull. The accumulation of stress down to 200 km
6. Discussion and conclusions Our modelling shows that continental convergence cannot be the only mechanism acting on the study area since it causes insignificant subcrustal stresses and therefore is not consistent with intermediate-depth seismicity. On the other hand, we show that stresses at deep levels are not necessarily caused by bending related to slab pull. In fact, upwelling of hot asthenospheric material produces extensional bending stresses along the top of the Adriatic slab under the Apennines. These extensional stresses within the slab are enhanced
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Fig. 12. Stress distribution obtained for the shallow central part of model CONASLAB. Same representation as Fig. 8b.
when slab pull is activated. Our results favour the idea that not only subduction and collision, but also asthenospheric upwelling, plays a crucial role in the present-day dynamics of the Tyrrhenian– Apennines system. The upwelling of low density mantle material underneath the crust generates extension and tectonic uplift in Tuscany. Therefore, our results are compatible with a process of active rifting affecting this area. In order to better evaluate the relative roles of asthenospheric upwelling and slab pull, we have carried out additional tests where only slab pull is activated. We have obtained that gravitational sinking of the slab causes compressional stresses at shallow depth related to downbending of the overriding plate and generates subsidence in the area of Tuscany, in clear disagreement with the observations. A possible error source in our modelling resides in the assumed density distribution. A high negative density anomaly in the mantle under Tuscany had to be considered to obtain an overall surface uplift of this area, since uplift must overcome the subsidence caused by the crustal thinning during rifting (McKenzie, 1978). We have, however, carried out several tests with values for Dr lower a than 100 kg m−3, obtaining the same qualitative results. With model CONASLAB, stress concentrates in the upper layer of the slab, in contrast to the lack of deep seismicity (Benioff zone) beneath the northern Apennines. This apparent contradiction can be explained taking into account that in our modelling this layer is assumed to be lower crust, likely behaving in a ductile manner. At crustal levels, models CONAS and CONASLAB result in a similar pattern of principal
stress axis orientation. Both models correctly reproduce extension in Tuscany, and compression in the outer margin of the Apenninic belt. In consequence, no definitive answer on the role of slab pull can be given on the basis of comparison with seismotectonic observations. Our three models, however, fail to explain extension in the inner area of the Apennines. There are several possible mechanisms to generate this normal-faulting regime. The correlation between high topography and extension seems to favour a local source of stresses, not included in our models, likely related to orogenic collapse. Alternatively, Bassi et al. (1997) propose that extension is caused by a combination of trench retreat and a rotation of the Apennines, possibly induced by the rotation of the Adriatic plate. However, an eastward horizontal velocity applied to the Adriatic plate to simulate this rotation would generate, in our models, widespread extension also in the outer front of the belt. In order to evaluate whether hinge retreat can result in extension in the Apennines, we have carried out several tests assuming an unlocked fault. We have obtained that compressional stresses related to surface downbending caused by slab sinking clearly overcome extension associated with hinge retreat. Therefore, despite roll-back likely playing a fundamental role in the definition of the tectonic style in the backarc areas of the Apenninic system (Malinverno and Ryan, 1986), slab sinking is shown here to be an inefficient mechanism to produce extension in the northern Apennines in the present-day situation. The opposite occurs in the southern Tyrrhenian sea, where roll-back of a deep and heavy slab generates tensional stresses in the
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Calabrian arc if an unlocked fault condition is applied (Giunchi et al., 1996). Some authors (e.g. Wortel and Spakman, 1992; van der Meulen et al., 1998; Carminati et al., 1999) suggest the Adriatic slab to be detached. In the modelled section we do not have clear tomographic evidence for this process. Carminati et al. (1999), however, obtain by means of dynamic modelling that the replacement of the slab by hot buoyant asthenospheric material would enlarge the extensional area to include the inner part of the Apennines. Furthermore, on the basis of modelling results, these authors show that a geodynamic scenario with a detached slab better accounts for Quaternary vertical motions recorded in the northern Apennines and in the Adriatic foreland basins. Ongoing seismic tomography studies would help to better constrain the lateral extent of the anomalous asthenosphere, which is here shown to be a key factor in the generation of tensional stresses in the Apennines.
Acknowledgements The authors wish to thank F. Beekman, B. Sperner and G. Spadini for very constructive criticism of the manuscript. Claudia Piromallo and Andrea Morelli, from the Istituto Nazionale di Geofisica, are acknowledged for allowing us to use their tomographic results. This work is financially supported by the EU Project ‘Geodynamic Modelling of the Western Mediterranean’ No. CHRX-CT94-0607.
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D.A., Mueller, S. ( Eds.), Second EGT Workshop: The Southern Segment. European Science Foundation, Strasbourg, pp. 47–51. Pasquale, V., Verdoya, M., Chiozzi, P., Ranalli, G., 1997. Rheology and seismotectonic regime in the northern central Mediterranean. Tectonophysics 270, 239–257. Patacca, E., Scandone, P., 1989. Post-Tortonian mountain building in the Apennines. The role of the passive sinking of a relic lithospheric slab. In: Boriani, A., Bonafede, M., Piccardo, G.V., Vai, G.B. ( Eds.), The Lithosphere in Italy, Accad. Naz. Lincei-Roma 80, 157–176. Pialli, G., Alvarez, W., 1997. Tectonic setting of the Miocene Northern Apennines: the problem of contemporaneous compression and extension. In: Montanari, A., Odin, G.S. Coccioni (Eds.), Miocene Stratigraphy: An Integrated Approach. Elsevier, Amsterdam, pp. 169–185. Piromallo, C., Morelli, A., 1997. Modelling Mediterranean upper mantle by travel time tomography: preliminary results. Ann. Geofis. XL, 963–979. Pondrelli, S., Morelli, A., Boschi, E., 1995. Seismic deformation in the Mediterranean area. Geophys. J. Int. 22, 938–952. Ponziani, F., De Franco, R., Minelli, G., Biella, G., Federico, C., Pialli, G., 1995. Crustal shortening and duplication of the Moho in the Northern Apennines: a view from seismic reflection data. Tectonophysics 252, 391–418. Ricci Lucchi, F., 1986. The Oligocene to Recent foreland basins of the Northern Apennines. Spec. Publ. Int. Assoc. Sedimental. 8, 105–139. Savage, J.C., 1983. A dislocation model of strain accumulation and release at a subduction zone. J. Geophys. Res. 88, 4984–4996. Selvaggi, G., Amato, A., 1992. Subcrustal earthquakes in the northern Apennines (Italy): evidence for a still active subduction? Geophys. Res. Lett. 19, 2127–2130. Selvaggi, G., Chiarabba, C., 1995. Seismicity and P-wave velocity image of the southern Tyrrhenian subduction zone. Geophys. J. Int. 121, 818–826. Serri, G., Innocenti, F., Manetti, P., 1993. Geochemical and petrological evidence of the subduction of delaminated Adriatic continental lithosphere in the genesis of the Neogene– Quaternary magmatism of central Italy. Tectonophysics 223, 117–147. Spada, G., Ricard, Y., Sabadini, R., 1992. Excitation of true polar wander by subduction. Nature 360, 452–454. Spakman, W., van der Lee, S., van der Hilst, R.D., 1993. Travel time tomography of the European–Mediterranean mantle down to 1400 km. Phys. Earth Planet. Inter. 79, 3–74. Tonti, G., 1997. Deformazioni crostali nel Mediterraneo Centrale misurate con GPS. M.Sc. Thesis, Universita` di Bologna, 160 pp. Vai, G.B., 1987. Migrazione complessa del sistema fronte deformativo–avanfossa–cercine periferico: il caso dell’Appennino Settentrionale. Mem. Soc. Geol. Ital. 38, 97–105. van der Meulen, M.J., Meulenkamp, J.E., Wortel, M.J.R., 1998. Lateral shifts of Apenninic foredeep depocentres reflecting detachment of subducted lithosphere. Earth Planet. Sci. Lett. 154, 203–219. Yoshii, T., 1979. A detailed cross-section of the deep seismic
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Contribution of numeric dynamic modelling to the understanding of the seismotectonic regime of the northern Apennines A.M. Negredo a, *,1, S. Barba b,c, E. Carminati a, R. Sabadini a, C. Giunchi a, 2 a Department of Earth Sciences, Section of Geophysics, University of Milano, Via Cicognara 7, 20129 Milan, Italy b Istituto Nazionale di Geofisica, Via di Vigna Murata 605, 00143 Rome, Italy c Department of Earth Sciences, University ‘La Sapienza’, P. Aldo Moro 5, Rome, Italy
Abstract In this paper we investigate the forces possibly active in the area of the northern Apennines by means of twodimensional finite element modelling assuming a viscoelastic rheology. The forces included in the models are related to continental convergence between Africa and Eurasia, to negative buoyancy of the subducted Adriatic lithosphere and to positive buoyancy of anomalously hot mantle material underneath Tuscany. Model-predicted stress distribution is compared with seismotectonic data available for the area of the northern Apennines and with earthquakes distribution. Our results indicate that continental convergence cannot be the only mechanism acting in the study area since it causes insignificant subcrustal stresses, in disagreement with intermediate seismicity observed under the Apennines. Asthenospheric upwelling under Tuscany (back-arc region) is shown to play a crucial role in the presentday dynamics of the Tyrrhenian–Apennines. Positive buoyancy causes an upwards and eastwards flow that generates extensional bending stresses in Tuscany and at the top of the slab under the Apennines. Horizontal pushing of the slab is proposed as an alternative mechanism to slab pull to generate subcrustal stresses. Activation of slab pull does not result in a significant change in orientation of the principal stress axes at shallow depths with respect to the model in which only asthenospheric diapirism is active. Therefore, the existence of a continuous gravitationally sinking slab cannot be ruled out on the basis of comparison with seismotectonic observations. Both models successfully reproduce extension in Tuscany and compression in the outer margin of the Apenninic belt, but fail to reproduce extension along the inner portion of the chain. Our modelling highlights the importance of better constraining the lateral extent of the asthenospheric diapir in order to find out whether the extension and tectonic uplift in the Apennines is caused by asthenospheric upwelling as in the area of Tuscany. © 1999 Elsevier Science B.V. All rights reserved. Keywords: dynamic modelling; earthquakes; Northern Apennines; seismotectonics; stress
* Corresponding author. Tel.: +34-91-3945190; fax: +34-91-3944398. E-mail address: [email protected] (A.M. Negredo) 1 Present address: Departmento de Geofı´sica, Facultad CC Fı´sicas, Universidad Complutense de Madrid, 28040 Madrid, Spain. 2 Present address: Istituto Nazionale di Geofisica, Via di Vigna Murata 605, 00143 Rome, Italy. 0040-1951/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S0 0 4 0- 1 9 51 ( 9 9 ) 0 02 8 6 -3
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1. Introduction The post-Middle Miocene evolution of the Tyrrhenian sea–northern Apennines system displays a pattern of coupled compression and extension, with a wave of compressional deformation moving towards the foreland (to the east–northeast) followed by a wave of extension (e.g. Pialli and Alvarez, 1997). The tectonics of the Apenninic fold-and-thrust belt is dominated by an eastward propagation of the thrust fronts and of the related foredeep basins (Ricci Lucchi, 1986). Contemporaneously to the eastward migration of the Apenninic belt, extensional basins opened in a back-arc position ( Tyrrhenian basin and Tuscany; e.g. Bartole, 1995). Episodes of compression superimposed on the Tuscany extensional regime have, however, been reported (e.g. Boccaletti et al., 1997 and references cited therein). This pattern of coeval contraction in the foldand-thrust belt and extension in a back-arc posi-
tion (Fig. 1) is here considered to be produced by two major interacting dynamic processes: convergence between Africa and Eurasia and the rollback of subducting Adria–Ionian lithosphere. The Africa–Europe convergence, discontinuously active since the Cretaceous (e.g. plate rotation data by Mueller and Roest, 1992) and still continuing today (e.g. VLBI data by Lanotte et al., 1996), is believed to play a major role in the dynamics of the central Mediterranean area. Support for the importance of tectonic escape processes related to the Africa–Europe convergence is given by geological studies (Lavecchia, 1988; Mantovani et al., 1996) and by analogue and numerical modelling ( Faccenna et al., 1996; Negredo et al., 1997, 1999). The roll-back of the subduction zone is suggested by several geological studies (e.g. Boccaletti and Guazzone, 1972; Malinverno and Ryan, 1986; Patacca and Scandone, 1989); the consequences of this mechanism have been investigated in recent numerical simulations (Bassi et al., 1997; Giunchi
Fig. 1. Map view of the main tectonic features in the northern Apennines including the profile line of the modelled section. Circles represent earthquakes occurring from January 1988 to September 1997, as reported by the Istituto Nazionale di Geofisica (ING). The size of circles increases with magnitude. The two events with larger magnitude are those occurring on 09/26/97 near Colfiorito (Assisi). The solid and dashed lines, respectively, indicate the areas characterized by prevalent tensional and compressive stress field, following Frepoli and Amato (1997).
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et al., 1996; Negredo et al., 1997; Carminati et al., 1998, 1999) and analogue modelling ( Faccenna et al., 1996). The retreat of the slab likely induced lithospheric thinning in the Tyrrhenian and Tuscany areas and an asthenospheric upwelling (black arrow in Fig. 2) beneath these areas (e.g. Boccaletti et al., 1997). In this geodynamic scenario, the upper crust of the Adriatic slab is likely to be delaminated and accreted to the fold-andthrust belt (Fig. 2), as indicated by seismic studies providing evidence for a thick-skinned tectonic style of the northern Apennines (Ponziani et al., 1995; Barchi et al., 1998). Also geochemical studies of the back-arc magmatism of Tuscany (Serri et al., 1993) indicate that a process of delamination and subduction of the Adriatic lithosphere can be responsible for the incorporation of a large amount of crustal material within the upper mantle in Tuscany. The subduction style and depth change notice-
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ably from the Calabrian arc to the northern Apennines. A continuous NW-dipping Wadati– Benioff plane down to 500 km depth in the southern Tyrrhenian sea (Selvaggi and Chiarabba, 1995) is related to the consumption of oceanic Ionian lithosphere. In contrast, a maximum seismicity depth of 90 km has been reported in the northern Apennines (Selvaggi and Amato, 1992; Di Giovambattista and Barba, 1997) and is associated with the consumption of the continental Adriatic lithosphere (Pialli and Alvarez, 1997). The onset of continental collision in this area is likely to have occurred in the Late Oligocene (Carmignani and Kligfield, 1990). The present-day continuity of the subducting Adriatic slab has recently been discussed. On the basis of tomographic images showing an interruption between the sinking slab and the Adriatic plate, Wortel and Spakman (1992) suggest the Adriatic slab to be detached. Moreover, these
Fig. 2. Schematic cross-section through the northern Apennines, modified from Boccaletti et al. (1997) and showing the model geometry assumed in the present study. The earthquakes falling in a 40 km wide band across the modelled section are projected on the profile.
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authors suggest that the detachment tear propagated from the northern to the southern Apennines. Support for this lateral migration hypothesis is given by stratigraphic studies showing a southward migration of the Adriatic foredeep depocentre (van der Meulen et al., 1998). On the basis of numerical modelling studies, Carminati et al. (1999) relate a Pliocene slab detachment event to the peculiar Quaternary tectono-stratigraphic features of the northern Apennines also detected by other authors (e.g. Vai, 1987; Argnani et al., 1997; Bertotti et al., 1997). However, the most recent tomographic studies do not provide clear evidence for slab detachment under the northern Apennines (Amato et al., 1993; Cimini and De Gori, 1997; Piromallo and Morelli, 1997). This controversial point will be addressed in this study. The main goal of this paper is to investigate the effect of the different forces possibly active in the northern Apennines (continental convergence, asthenospheric upwelling under Tuscany and slab pull ) on the stress and strain regime of the area. In the first part of the paper we present the seismotectonic data available for the northern Apennines and discuss the model constraints provided by geophysical observations, in particular heat-flow, crustal thickness and seismic tomography images. We then describe the 2D dynamic models developed to study the present-day tectonic regime in the northern Apennines and surrounding areas, and finally compare the results with earthquake distribution and focal mechanisms.
2. Seismotectonic regime The Italian peninsula shows a strong seismicity (e.g. Pondrelli et al., 1995; Barba et al., 1995) which follows the trend of the Apenninic belt and mainly occurs in the crust. The structural setting of the Apenninic belt is described by a double arc-shaped bending of the major structures, forming the northern Apenninic and the Calabrian arcs. These structural arcs are separated in the central Apennines by the Olevano–Antrodoco N–S trending fault zone (Patacca and Scandone, 1989). Along the northern Apenninic arc, contemporaneous extension and compression is observed. Frepoli and Amato
(1997), inverting earthquake data to obtain the stress tensor, evidenced E–W extension in the periTyrrhenian area and along the inner margin of the northern Apenninic belt, and NE–SW compression along the outer eastern thrusts (Fig. 1). To better identify the stress pattern in the northern Apennines, we project P and T axes, named P∞ and T∞, onto the modelled profile, striking about 60° from North (Fig. 3a and b; P and T axis data from Frepoli and Amato, 1997). We have carried out an orthogonal projection of data of earthquakes falling in a 200 km wide band centred on the modelled section. To avoid subjective interpretation of such plots, the function f=p∞t∞ sin[ pl(T∞)−pl(P∞)] has been introduced, where pl is the plunge of the corresponding axis if its trend lies between 0 and 180°, and the supplementary angle to the plunge if the axis trend exceeds 180°; p∞ and t∞ are the lengths of the axes once projected onto the section. The function f ranges from −1 (ideal extension along the profile) to +1 (ideal compression), since it is the product of two numbers ranging, respectively, between +1 and −1 (sin function) and between 0 and 1 ( p∞t∞; P and T axis lengths are set to 1). Values of f around 0 are obtained for earthquakes with prevalent motion out of the section. In Fig. 3c we plot crosses for compression ( f>0), and circles for extension ( f<0), whose sizes are proportional to the magnitude of f, and therefore to the ‘weight’ of extension/compression along the modelled cross-section. Data represented by small symbols are, therefore, less significant for comparison with our plane strain modelling. Fig. 3c displays in the area of Tuscany (between 0 and 100 km NE of the Tyrrhenian coast) prevalent extensional shallow seismic activity, which is in correspondence with normal faulting and graben tectonics. Between 120 and 150 km, the extensional activity deepens towards the inner margin of the belt, where minor compressional activity also occurs. This observation is in agreement with centroid moment tensor (CMT ) solutions for the most recent earthquakes in the Umbria–Marche region (see Fig. 1 for location), which indicate extension at a depth of 15 km under the Apennines. Compressional activity prevails in the external portion of the belt, between 150 and 200 km. These
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Fig. 3. Plots (a, b) represent the projection of P and T axes (normalized to a length of 1), named P∞ and T∞, of about 100 earthquakes ( located at distances less than 100 km from the section) onto the modelled section. Projection characteristics are described in the text (P and T axis data from Frepoli and Amato, 1997). In panel (c), a circle is plotted for mainly distensional mechanisms, while a cross characterizes mainly compressional mechanisms. The size of circles and crosses is proportional to the amount of extension/compression in the modelled section. Vertical scale in (c) is three times the horizontal one.
three seismotectonic domains seem to be separated by seismicity gaps, some 15–20 km wide, probably corresponding to transitional areas of reduced bending stress. The earthquakes characterized by out-of-plane motion (mainly strike slip) are scattered throughout the section.
3. Geophysical constraints The geometry of the modelled cross-section is constrained by tomographic and seismic data. The results of seismic–gravimetric models of the NE– SW profile CROP-03 show that the crustal thickness increases from 30 km under the Adriatic sea to more than 40 km under the Apenninic chain and again decreases to about 20–25 km under the northern Tyrrhenian sea and Tuscany (Marson et al., 1998). The shallow Moho depth in the latter area is related to a noticeable thinning of the entire
lithosphere, as derived from surface-wave studies by Panza (1985), which indicate a lithospheric thickness of 50 km underneath the northern Tyrrhenian sea and Tuscany, and 70 km under the Adriatic plate. There is a correlation between the areas of crustal thinning and the positive anomaly of surface heat flow (Fig. 4), which reaches values higher than 120–160 mW m−2 in the areas with a crust thinner than 25 km (Mongelli et al., 1992). The presence of the so-called ‘Tuscan–Tyrrhenian anomaly’ is confirmed by recent studies of Pn and Sn phase attenuation (Mele et al., 1997). These authors mapped a high attenuation zone in the uppermost mantle beneath the Apennines and western margin of the Italian peninsula and southern Tyrrhenian sea ( Fig. 4). They suggested that the presence of hot asthenospheric material below the Moho is responsible for inefficient Pn and Sn transmission. This interpretation is consistent with present-day extension along the Tyrrhenian basin
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Fig. 4. Heat flow distribution in the central Mediterranean area after Mongelli et al. (1992). Sn and Pn attenuation zones after Mele et al. (1997) are represented. The profile corresponding to the modelled section is also shown.
and the Apennines, and with the extensive Neogene and Quaternary volcanic activity in these areas, but it is difficult to explain the low heat flow values recorded in the Apennines. Further evidence for the presence of an anomalously hot upper mantle beneath Tuscany comes from seismic tomography studies by Piromallo and Morelli (1997) ( Fig. 5). They mapped the lateral heterogeneity of P wave velocity in the Mediterranean region and obtained a shallow lowvelocity anomaly beneath the western Italian margin, in correspondence with the results of Mele et al. (1997), and a high-velocity anomaly at deeper levels beneath the northern Apennines, in agreement with previous seismic tomography studies (Spakman et al., 1993; Amato et al., 1993; Cimini and De Gori, 1997). We use these results to introduce in our modelling the geometry of the descending slab and of the anomalous uppermost mantle beneath Tuscany.
4. Model description Fig. 6 illustrates the model geometry and the boundary conditions. The modelled cross-section
Fig. 5. Tomographic image along the analysed section, obtained from the velocity model of Piromallo and Morelli (1997). Velocity perturbations of maximum ±2% are shown in shades of grey, and the sign of the anomaly is drawn to better discriminate between positive and negative anomalies. The horizontal and vertical resolution of the tomographic image is 20×50 km2. A continuous positive anomaly with respect to the initial layered velocity model is shown beneath Italy down to 200 km. Details about the initial model, data and inversion procedure can be found in Piromallo and Morelli (1997).
Fig. 6. Geometry and boundary conditions of the models. V c denotes the velocity applied to the lithospheric portion of the left boundary of the model to simulate the effects of Africa– Eurasia convergence. The springs represent the buoyant restoring force applied at the surface. The circles denote free slip condition. The barbed portion of the subduction fault is locked. The remaining portions are unlocked and slip with zero friction. Different grey shadings indicate different materials (see text for further details). Overprinted dots indicate regions with density anomalies: Dr =−100 kg m−3; Dr =80 kg m−3. a s
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has a horizontal and vertical extent of 1250 km and 670 km, respectively; the lateral boundaries are far enough from continental Italy in order to avoid border effects in the area of interest. We have assumed a linear viscoelastic Maxwell rheology with viscosities 1024 Pa s for the crust, 5×1022 Pa s for the lithospheric mantle and 1021 Pa s for the asthenospheric mantle ( Whittaker et al., 1992; Spada et al., 1992). The elastic structure is based on the PREM reference model (Dziewonski and Anderson, 1981). We have assumed a Young’s modulus of 9×1010 Pa for the crust and 1.75×1011 Pa for the remaining materials. We have chosen a viscoelastic rheology because it reproduces the long-term viscous behaviour of the lithosphere and accounts for elastic flexure at the short time-scale. In contrast to viscous flow models using a temperature-dependent rheology (Zhong and Gurnis, 1995; Marotta and Sabadini, 1995), we do not intend to model the initiation of subduction and progressive sinking of the slab, but the state of stress created by different subduction conditions. The boundary conditions are as follows. The bottom of the model is fixed in the vertical direction. The buoyant restoring force (denoted by the springs in Fig. 6) is applied to the surface of the section, and is assumed to be proportional to the density contrast and to the vertical displacement at the surface (Desai, 1979; Williams and Richardson, 1991). At the left lateral boundary, we introduce for the lithosphere a constant horizontal velocity, which reproduces the eastward tectonic escape of the Tyrrhenian block related to the indentation of the African plate. Following the results of the 3D study by Negredo et al. (1999) this effect causes a relative convergence rate of about 3.5 mm/yr in the modelled cross-section. This value is applied to the western boundary, whereas at the right boundary no motion of the lithosphere is allowed. The interaction between the overriding and subducting plate is assumed to occur via a subduction thrust fault, which is locked between 0 and 35 km and unlocked at deeper levels, from 35 to 200 km. This condition of locking–unlocking of the fault is justified by the studies of hypocentral locations and focal mechanisms in subduction
21
zones (e.g. Davies and House, 1979; Yoshii, 1979), which show that along the main thrust zone the predominant mechanism is thrusting along the surface of the zone, whereas at greater depths, in the Benioff zone, the activity is not concentrated at the interface separating the top of the slab from the mantle wedge and the slip is not parallel to this Benioff interface. It can be deduced that interplate and intraplate earthquakes occur at shallow and deep levels, respectively, and therefore the interplate motion is accommodated by stick slip on the thrust zone and by continuous aseismic slip on the Benioff interface. During the phase of strain accumulation, the fault is supposed to slip at depth and to remain locked closer to the surface (Savage, 1983). As discussed by several authors (Giunchi et al., 1996; Carminati et al., 1999; Negredo et al., 1999), the pattern of surface vertical motion inferred from geologic observations is only reproduced when free slip along an unlocked subduction fault is permitted. Actually, in a geologic timescale the overriding and the subducting plates are decoupled by the occurrence of earthquakes and by aseismic creep that accommodate the slip on the thrust zone. Unlocking of the fault enables progressive retreat of the subduction hinge line ( Whittaker et al., 1992; Giunchi et al., 1996). Therefore, the condition of unlocked fault is more appropriate to study processes with a long characteristic time-scale, whereas the condition of partially locked fault better accounts for the phase of strain accumulation and, in consequence, for the present-day seismotectonic regime. For the sake of simplicity we have considered in our modelling a single thrust fault, which intersects the surface of the cross-section at the boundary between the Apenninic chain and the Adriatic foredeep, as shown in the map of neostructural domains of Italy (Ambrosetti et al., 1987). The present-day stress and strain distribution along the modelled cross-section are calculated using the finite element code MARC (1994). The mesh consists of 930 eight-node plane strain elements with an increased resolution in the vicinity of the slab. Density contrasts and convergence are activated at the initial time and maintained constant thereafter, following the same procedure as
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Whittaker et al. (1992). After a time interval of about 250 kyr since loading, the dynamic equilibrium between the buoyant restoring force and the forces arising from density contrasts and convergence is attained. As a consequence, the initial exaggerated values of stress and velocity related to instantaneous gravity loading have disappeared and reached steady-state values. Modelling results are valid for a time-scale of 105–106 yr, during which the geometric configuration does not change significantly; for longer integration times viscoelastic models overemphasise the plate-like behaviour of the lithosphere. We investigate the seismotectonic implications of three models, assuming the same geometric configuration and rheological properties and differing for the mechanisms which drive motion. In the model CONVER only convergence is active, whereas in model CONAS we add a negative density contrast of 100 kg m−3 to account for the positive buoyancy caused by the asthenospheric diapir underneath Tuscany. In a third model, CONASLAB, we also activate a positive density contrast of 80 kg m−3 in the subducting slab to evaluate the effects of slab pull. This homogeneous density contrast is an average of the real density distribution, characterized by density contrasts higher than 100 kg m−3 due to the eclogitization of the lower crust gabbroic rocks and by temperature-related density contrasts within the lithospheric mantle of the slab, which are of the order of 50 kg m−3 in the case of a slowly sinking slab, as inferred from thermal models (e.g. Davies and Stevenson, 1992).
Fig. 7. Surface vertical velocity (a), velocity field (b) and von Mises stress distribution (c) obtained for the shallow central part of model CONVER (only convergence is active). Panel (c) also shows the earthquakes located in a 40 km wide band centred in the modelled section.
5. Modelling results 5.1. Model CONVER Figs. 7 and 8 summarise the results of model CONVER. Fig. 7a shows the model predicted vertical surface velocity. Fig. 7b portrays the velocity field of the central part of the model. The velocity vectors indicate an eastward movement with rates decreasing from west to east. This rather uniform pattern, dominated by horizontal movements, results from the eastward velocity applied to the
lithosphere at the western boundary of the model (V =3.5 mm/yr). The negligible vertical motion c along the free surface of the model (Fig. 7a), resulting from the absence of vertical loads, contrasts with the recent noticeable uplift in Tuscany and the Apennines and subsidence in the Adriatic realm, as inferred from geological data (e.g. Ambrosetti et al., 1987; Carminati et al., 1999). A thorough discussion on the vertical motion at the surface is, however, beyond the scope of the paper. Our models, in fact, are not able to accu-
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Fig. 8. (a) Seismotectonic observations shown in Fig. 3c superimposed on the model geometry. (b) Stress distribution obtained for the shallow central part of model CONVER. Bars represent the major eigenvectors of the stress tensor, thin for tension (s ) and 3 thick for compression (s ). Eigenvector length is scaled to the eigenvalue. 1
rately reproduce the vertical crustal motions because of the locking condition of the shallow portion of the subduction fault assumed in the calculations, as discussed in the previous section. This condition, however, forces the continuity of the crustal motion from the Tyrrhenian to the Adriatic domain. This is in agreement with recent GPS results from southern Italy ( Tonti, 1997) showing the continuity of velocity vectors from the overriding to the subducting plate. Our modelling, therefore, is consistent with the coupling between upper and subducting plates at the small time-scale (see Negredo et al., 1999 for a more detailed discussion). Fig. 7c displays the equivalent von Mises stress distribution obtained for model CONVER. The von Mises stress (s: ) is calculated as an equivalent intensity of the deviatoric stress tensor:
S
s: =
1 S S , S =s − d s ij 3 ij kk 2 ij ij ij
3
where s and S are the stress and the deviatoric ij ij stress tensors, respectively (the Einstein convention for repeated tensorial indexes is used); d is the ij Kronecker symbol. The most noticeable feature of Fig. 7c is the uniform stress distribution in the crust, with values lower than 75 MPa. Open circles
indicate the hypocentres of earthquakes located in a 40 km wide band centred on the modelled crosssection (see Fig. 1 for epicentre location). The absence of significant subcrustal stresses predicted by this model is not consistent with intermediate seismicity beneath the northern Apennines. In Fig. 8a we have superimposed the geometry of the central and shallow part of the model on the earthquakes displayed in Fig. 3c in order to enable comparison with modelling results. Fig. 8b displays the principal stress axes s (compression, 1 thick lines) or s (extension, thin lines) calculated 3 for each element; for simplicity, only the axis with the major absolute eigenvalue is portrayed. The crust at depths lower than 30 km displays a quite uniform compressional regime (s horizontal ) 1 which contrasts with the seismotectonic observations summarized in Fig. 8a. The upper part of the slab is, on the contrary, characterized by tensional stresses (s parallel to the dip of the slab) caused 3 by the horizontal flow of material (Fig. 7b) which bends the subducting plate. From the considerations listed above it can be deduced that the eastward tectonic escape of the Tyrrhenian block caused by the convergence between Africa and Eurasia cannot be the only mechanism active in the area of the northern Apennines.
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5.2. Model CONAS With model CONAS we investigate the combined effects of the upwelling of buoyant mantle material beneath Tuscany (Dr =−100 kg m−3) a and of regional convergence (V =3.5 mm/yr). As c in the previous model, subduction is included as a rigidity contrast and no subduction-related negative buoyancy is modelled within the slab. Such an unloaded slab is conceptually equivalent to a detached slab. Nevertheless, we did not introduce a detached geometry in order to avoid the models becoming very speculative. Furthermore, this choice enables direct comparison with the other two models. The velocity field obtained with model CONAS (Fig. 9b) indicates an upwards and eastwards motion of the mantle wedge material. This motion results in surface uplift in Tuscany ( Fig. 9a) compatible with tectonic uplift recorded by PlioQuaternary marine sediments outcropping at high topographic elevation (up to 1000 m) in this area (see Carminati et al., 1998 for a discussion). At shallow depths corresponding to the locked portion of the fault, the velocity field is continuous, as obtained also for the previous model, while a major discontinuity in the vertical component of velocity occurs in correspondence with the unlocked portion of the subduction fault. At depths greater than 35 km, the overall eastward flow causes an active roll-back of the slab, that tends to become vertical. The slightly downwards component of the velocity in the subducting plate, which is due to this effect, results in subsidence in the eastern part of Italy; the locking condition of the uppermost portion of the subduction fault prevents overthrusting of the Apennines over the Adriatic plate and forces an unrealistic subsidence in the Apennines (Fig. 9a). The von Mises stress distribution (Fig. 9c) changes noticeably with respect to the previous model (Fig. 7c). Stress accumulates also at higher depths in correspondence with the slab bending under the Apennines, between 100 and 150 km from the Tyrrhenian coast, in agreement with the occurrence of intermediate seismicity. This model reveals a mechanism alternative to slab pull to generate subcrustal stresses: the bending caused by
Fig. 9. Surface vertical velocity (a), velocity field (b) and von Mises stress distribution (c) obtained for model CONAS, which is characterized by activation of convergence and a negative asthenospheric density anomaly under Tuscany.
horizontal pushing of the slab. The increased vertical deformation generates high bending stress values in Tuscany and in the Apennines. It is also interesting to note that the narrow areas of reduced seismicity (around 100 and 160 km) correlate with areas of low von Mises stress, thus favouring our interpretation that these seismic gaps are related
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Fig. 10. Stress distribution obtained for model CONAS. Same representation as Fig. 8b.
to reduced bending stresses. In order to better evaluate the effects of the upwelling asthenospheric material, we have carried out a test where plate convergence is not activated, and obtained the same general pattern of von Mises stress, except for the unrealistic low stresses predicted for the Adriatic domain. The major principal stress axes calculated with this model ( Fig. 10) show the typical tectonic style resulting from bending of the lithosphere: upwelling in Tuscany causes extension and compression in the upper and lower crust, respectively. This pattern is inverted in the Apennines, where the stress axes orientation is complicated at depth by the abrupt change from fault locking to unlocking and by the lateral density and rigidity contrasts. If we compare these results with the seismotectonic observations compiled in Fig. 8a, this model successfully reproduces compression in the outer margin of the Apenninic belt and extension in Tuscany. Extension at deep levels, at depths of about 10–35 km under the Apennines (120– 160 km away from the Tyrrhenian coast), is also compatible with the recent extensional seismic activity in the Umbria–Marche region, which occurred at depths of 15 km. However, the model fails to reproduce extension at shallower depth in the Apennines. It is worth noting that the seismic activity in Tuscany is concentrated in the mid and upper crust, in contrast to the high compressional stresses predicted by the model in the lower crust. This is probably related to the fact that in this area, due to the high temperatures, the lower crust deforms in a ductile manner when yield strength is exceeded (Pasquale et al., 1997).
5.3. Model CONASLAB In contrast to the previous models, we now assume that the slab is continuous and sinks into the mantle, being loaded with a positive density anomaly of 80 kg m−3. The density anomaly reproducing the upwelling of buoyant mantle beneath Tuscany (Dr =−100 kg m−3) and the velocity a boundary condition reproducing regional convergence (V =3.5 mm/yr) are also active. As a c consequence of the slab gravity loading, the predicted surface subsidence in the Apennines and eastern margin of the Italian peninsula (Fig. 11a) is increased with respect to model CONAS ( Fig. 9a). The velocity field obtained with model CONASLAB ( Fig. 11b) is similar to that of Fig. 9b, the most pronounced difference being the increase in eastward component of motion. This is clearly induced by the retreat of the slab which tends to verticalize faster than in model CONAS, because of the combined effects of the eastward flow, which causes active roll-back, and of the density anomaly within the slab, which results in gravitational or passive roll-back. The observations made for model CONAS regarding the discontinuity in the vertical component of motion across the unlocked portion of the slab, and regarding the velocities at the surface, are also valid for this model. The von Mises stress distribution ( Fig. 11c) in the shallow portions of the model (crust) is quite similar to that obtained with model CONAS. In contrast, the stress distribution within the slab is quite different, being characterized by stress accumulation down to 200 km depth. This difference has to be ascribed to the pull exerted by the loaded
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contrasts with the absence of seismicity at depths larger than 90–100 km below the northern Apennines. This apparent misfit, however, is not sufficient to rule out the existence of a continuous slab exerting slab pull. In model CONASLAB, von Mises stresses mainly concentrate in the uppermost layer of the slab, which is assumed to be lower crust and is thus likely to behave in a ductile rather than in a brittle manner (Pasquale et al., 1997). Therefore, this ductile behaviour can explain the absence of seismic activity within the slab and also at deep crustal levels under Tuscany. Fig. 12 displays the major principal stress axes calculated with model CONASLAB. The stress pattern within the upper plate is almost identical to that displayed in Fig. 10. The addition of the slab pull mechanism, however, produces a decrease of 20 km in the area characterized by tensional stresses compared with model CONAS. Model CONAS, therefore, reproduces slightly better the seismotectonic observations compiled in Fig. 8a (compression in the outer margin of the Apenninic belt and extension in Tuscany). Within the subducting plate, the area characterized by tensional stresses is enlarged and the orientation of the tensional eigenvectors becomes clearly parallel to the slab. This indicates a predominance of slab pull on eastward mantle flow in the determination of the stress pattern in this part of the model. The obtained stress field is perfectly compatible with the extensional seismicity observed at deep levels (at depth of 10–35 km) under the Apennines, as already obtained for model CONAS. Fig. 11. Surface vertical velocity (a), velocity field (b) and von Mises stress distribution (c) obtained for model CONASLAB, which is characterized by the activation of convergence and the density anomalies under Tuscany and in the Adriatic subducted lithosphere.
portion of the slab. In correspondence with the higher slab bend, model CONASLAB displays von Mises stresses higher (up to 200–250 MPa) than model CONAS. This is due to the combined effects of the slab bending related to eastward mantle flow (also active in model CONAS ) and of slab pull. The accumulation of stress down to 200 km
6. Discussion and conclusions Our modelling shows that continental convergence cannot be the only mechanism acting on the study area since it causes insignificant subcrustal stresses and therefore is not consistent with intermediate-depth seismicity. On the other hand, we show that stresses at deep levels are not necessarily caused by bending related to slab pull. In fact, upwelling of hot asthenospheric material produces extensional bending stresses along the top of the Adriatic slab under the Apennines. These extensional stresses within the slab are enhanced
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Fig. 12. Stress distribution obtained for the shallow central part of model CONASLAB. Same representation as Fig. 8b.
when slab pull is activated. Our results favour the idea that not only subduction and collision, but also asthenospheric upwelling, plays a crucial role in the present-day dynamics of the Tyrrhenian– Apennines system. The upwelling of low density mantle material underneath the crust generates extension and tectonic uplift in Tuscany. Therefore, our results are compatible with a process of active rifting affecting this area. In order to better evaluate the relative roles of asthenospheric upwelling and slab pull, we have carried out additional tests where only slab pull is activated. We have obtained that gravitational sinking of the slab causes compressional stresses at shallow depth related to downbending of the overriding plate and generates subsidence in the area of Tuscany, in clear disagreement with the observations. A possible error source in our modelling resides in the assumed density distribution. A high negative density anomaly in the mantle under Tuscany had to be considered to obtain an overall surface uplift of this area, since uplift must overcome the subsidence caused by the crustal thinning during rifting (McKenzie, 1978). We have, however, carried out several tests with values for Dr lower a than 100 kg m−3, obtaining the same qualitative results. With model CONASLAB, stress concentrates in the upper layer of the slab, in contrast to the lack of deep seismicity (Benioff zone) beneath the northern Apennines. This apparent contradiction can be explained taking into account that in our modelling this layer is assumed to be lower crust, likely behaving in a ductile manner. At crustal levels, models CONAS and CONASLAB result in a similar pattern of principal
stress axis orientation. Both models correctly reproduce extension in Tuscany, and compression in the outer margin of the Apenninic belt. In consequence, no definitive answer on the role of slab pull can be given on the basis of comparison with seismotectonic observations. Our three models, however, fail to explain extension in the inner area of the Apennines. There are several possible mechanisms to generate this normal-faulting regime. The correlation between high topography and extension seems to favour a local source of stresses, not included in our models, likely related to orogenic collapse. Alternatively, Bassi et al. (1997) propose that extension is caused by a combination of trench retreat and a rotation of the Apennines, possibly induced by the rotation of the Adriatic plate. However, an eastward horizontal velocity applied to the Adriatic plate to simulate this rotation would generate, in our models, widespread extension also in the outer front of the belt. In order to evaluate whether hinge retreat can result in extension in the Apennines, we have carried out several tests assuming an unlocked fault. We have obtained that compressional stresses related to surface downbending caused by slab sinking clearly overcome extension associated with hinge retreat. Therefore, despite roll-back likely playing a fundamental role in the definition of the tectonic style in the backarc areas of the Apenninic system (Malinverno and Ryan, 1986), slab sinking is shown here to be an inefficient mechanism to produce extension in the northern Apennines in the present-day situation. The opposite occurs in the southern Tyrrhenian sea, where roll-back of a deep and heavy slab generates tensional stresses in the
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Calabrian arc if an unlocked fault condition is applied (Giunchi et al., 1996). Some authors (e.g. Wortel and Spakman, 1992; van der Meulen et al., 1998; Carminati et al., 1999) suggest the Adriatic slab to be detached. In the modelled section we do not have clear tomographic evidence for this process. Carminati et al. (1999), however, obtain by means of dynamic modelling that the replacement of the slab by hot buoyant asthenospheric material would enlarge the extensional area to include the inner part of the Apennines. Furthermore, on the basis of modelling results, these authors show that a geodynamic scenario with a detached slab better accounts for Quaternary vertical motions recorded in the northern Apennines and in the Adriatic foreland basins. Ongoing seismic tomography studies would help to better constrain the lateral extent of the anomalous asthenosphere, which is here shown to be a key factor in the generation of tensional stresses in the Apennines.
Acknowledgements The authors wish to thank F. Beekman, B. Sperner and G. Spadini for very constructive criticism of the manuscript. Claudia Piromallo and Andrea Morelli, from the Istituto Nazionale di Geofisica, are acknowledged for allowing us to use their tomographic results. This work is financially supported by the EU Project ‘Geodynamic Modelling of the Western Mediterranean’ No. CHRX-CT94-0607.
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