TECTONICS, VOL. 18, NO. 1, PAGES 108-118, FEBRUARY 1999

Recent tectonic evolution and present stress in the Northern Apennines (Italy)

Maria Teresa Mariucci, Alessandro Amato, and Paola Montone

Istituto Nazionale di Geofisica, Rome

Abstract. The present-day stress field and its recent tectonic evolution in the Northern Apennines are reconstructed from borehole breakout analysis and focal mechanisms of crustal earthquakes and through the comparison with paleostress data. We have considered 86 wells for breakout analysis, with depths down to 6-7 km, 125 fault plane solutions of crustal earthquakes with M<5 that occurred between 1988 and 1995 in the Northern and Central Apennines, and data of stronger earthquakes (M_<6)reported in other studies. The Tyrrhenian coastal region and the Apenninic belt are characterized by S h ..... direction mainly trending NE-SW, with predominantly normal fault plane solutions. Along the outer front of the belt and the Adriatic offshore, Shin m is oriented NW-SE, and focal solutions are thrust or strike-slip, with maximum compression around NE-SW. Conversely, south of 43øN, breakouts evidence an orthogonal direction of horizontal compression (NW-SE), following the Southern Apennine trend, where a widespread NE-SW extension was recognized by previous investigations. Comparing these results to the recent tectonic evolution interred from structural geology, we argue that the extension- compression pair, characteristic of the post-Tortonian evolution of the mountain belt, has been migrating in time from late Miocene to Present only in the northern sector of the arc, whereas the southern sector underwent a generalized extension, at least since middle Pleistocene. The striking correspondence between the active compression front and the region with evidence of a remnant subducted slab suggests that the migrating extension-compression pair has been controlled by progressive retreat of the slab.

1. Introduction

The stress field in the brittle crust is due to the interaction of "large-scale" processes with "local sources." The former include relative movements of plates, active subduction and collision processes, and density anomalies in the lithosphere and in the asthenosphere due to remnant slabs or lithospheric roots. The local sources are mainly related to density anomalies in the crust, fault zones, and areas with different

Copyright 1999 by the American Geophysical Union.

Paper number 1998TC900019. 0278-7407/99/1998 TC900019512.00

108

strengths. When studying the state of stress in a specific region, it is important to take into account the different scales of observation of the stress field and try to discriminate between the "regional" stress and local sources [Rebai et al., 1992].

In order to understand the recent evolution of the Apennine belt and the forces acting in this region, we consider borehole breakouts and focal mechanisms of crustal earthquakes, which are considered the most important categories of active stress indicators [Zoback, 1992]. The knowledge of the stress field allows us to determine the behavior of the seismogenic structures, and therefore it is important in the seismotectonic zoning of a region. If active faults in a region are identified (from geological, geomorphologic, seismological, and geodetic analysis) and the stress field is well known, then we can determine which faults are favorably oriented with respect to the stress field and hence which are more likely to rupture in future earthquakes.

In the Northern Apennines the knowledge of the present- day stress field in the crust, compared to the recent geologic evolution of the region, is also needed to evaluate the influence of active processes, such as the motion of the Adfia plate, the hypothesized subduction of Adriatic lithosphere beneath the Apennine, or the asthenosphere upwelling beneath Tuscany, and thus help in constraining dynamic modeling of the region [see, e.g., Bassi and Sabadini, 1994; Bassi et al., 1997].

The development of the eastward verging Apennines fold and thrust belt [Bally et al., 1986; Mostardini and Merlini, 1986] is somehow related to the convergence between the African and the European plates, but it has also been strongly affected by other processes, like the eastward retreat of the Adriatic lithosphere below the belt [Patacca and Scandone, 1989]. The Northern Apennines history began in the Late Cretaceous with the closing of the Ligure-Piemøntese basin. After the consumption of oceanic lithosphere with the formation of an accretionary wedge [Carmignani and Kligfield, 1990], continental collision took place, with a general E-W trend, and the crust was thrusted toward the east on top of the westward dipping Adriatic slab. It is not clear whether subduction of continental lithosphere took place and how long it lasted.

All the tectonic processes linked to the Apennines buildup migrated eastward toward the Adriatic foreland [Malinverno and Ryan, 1986; Carmignani and Kligfield, 1990; Lavecchia et al., 1994], and progressively younger foredeeps were therefore involved in the thrusting [Ricci Lucchi, 1986; Patacca and

MARIUCCI ET AL.: PRESENT STRESS IN NORTHERN APENNINES (ITALY) 109

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Scandone, 1989]. It was hypothesized that these migrating foredeeps are due to the slab pull, not only to the sediment load [Kruse and Royden, 1994].

Postorogenic extension began in Corsica, from Oligocene to lower Miocene [Jolivet et al., 1990], and it migrated eastward to reach the Tyrrhenian area (lower-middle Miocene) and Tuscany (lower Pliocene), as inferred from the timing of the volcanic activity and the graben structures [Serri, 1990; Barberi et al., 1994; Carmignani et al., 1994; Jolivet et al., 1998].

In this paper we face the problem of the present tectonic regime in the Northern Apennines and the relation with its Neogene evolution. We first describe the main characteristics of the present stress field as inferred from borehole breakout analysis along 86 wells (drilled for hydrocarbon exploration and geothermal research) and the focal mechanisms of crustal earthquakes, including those which struck the Umbria-Marche region in 1997 and 1998. Then we compare these results with the recent geologic data, and we propose an improved reconstruction of the tectonic evolution of the region.

2. Seismicity distribution

Seismicity in the Northern and Central Apennines is mainly clustered along the axis of the belt (Figure 1), where the strongest earthquakes are located, but a diffuse, weaker seismic

activity affects the outer front of the belt. These have maximum magnitude of M--6 in the northern arc, M--7 if we consider the Central Apennines. The hypocentral depths are confined to the upper 20 km of the crust, with subcrustal earthquakes down to 90 km that are probably related to a remnant westward dipping subduction [Selvaggi and Amato, 1992]. In the westernmost sector of the arc, the backarc region, a weaker (M<4) and shallower seismicity occurs (depth less than 7 km) in the regions of Quaternary volcanism.

The earthquakes with M=4-6, which occurred in the inner part of the Northern Apenninic arc between 1939 and 1987 (selected after Gasparini et al. [1985] and centroid moment tensor (CMT) solutions, after Dziewonski et al. [1983], and all subsequent reports in Physics of the Earth and Planetary Interior (PEPI), have normal faulting focal mechanisms with a small strike-slip component and with T axes mostly between NE-SW and E-W (Figure 2). Also, the numerous earthquakes which occurred at the Umbria-Marche boundary from September 1997 to April 1998 (six of which with M w between 5 and 6 in a 40 km NW-SE elongated belt) have normal faulting solutions with NE trending T axes [Amato et al., 1998; Ekstrom et al., 1998]. The seismicity along the Adriatic coast seems to occur on thrust and strike-slip faults with P axes from NE-SW to E-W. The information obtained by this data set is, however, sparse, owing to the few well-constrained solutions available.

110 MARIUCCI ET AL.: PRESENT STRESS IN NORTHERN APENNINES (ITALY)

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In this work we compare the 125 earthquake focal mechanisms analyzed by Frepoli and Amato [1997] with the borehole breakout data. These events have been recorded by the National Seismic Network of the lstituto Nazionale di

Geofisica in the period 1988-1995, and the magnitudes reported range between 2.6 and 4.8 (Md).

The fault plane solutions have been classified by Frepoli and Amaro [ 1997] in stress regime categories, according to the plunge ranges of T and P axes, following the World Stress Map criteria [Zoback, 1992]. The focal solutions were also used to get the stress tensor orientations in two regions of the Northern Apennines using the technique of Gephart and Forsyth [1984]. We do not show these results in any map of this paper, because the two areas in which the stress tensor was computed are very large, and therefore the results do not reflect the small-scale variations evident in our analysis (see Figures 2-4). The main finding of Frepoli and Amato is the clear separation between the external belt in which reverse and strike-slip solutions prevail and a broad internal region in which normal and strike-slip solutions dominate. Also, it appears that the thrust or strike-slip solutions are confined to the Northern Apenninic arc, and only normal (and strike-slip) faulting characterizes the Southern Apennines [see also Amato and Montone, 1997].

3. Present Stress Data

In this section we summarize the technique and results of breakout analysis in the Northern Apennines' then we compare these data with fault plane solutions of crustal earthquakes.

3.1. Method of Breakout Analysis

Breakouts are enlargements on opposite sides of the borehole wall, which are aligned along the direction of the minimum horizontal stress. It is generally accepted that breakouts derive t¾om conjugate shear fractures, which develop when a well is drilled in an anisotropic stress fieM [Bell and Gough, 1983; Zoback, 1992]. Therefore investigating breakouts and computing their average azimuth distribution allows us to get the orientation of the horizontal stress field around the well [Bell and Gough, 1983; Plumb and Hickmann, 1985; Zoback et al., 1985].

In Italy, the study of borehole breakouts began in the 1990s, and until now, about 200 wells have been analyzed [Montone et al., 1992, 1995; Cesaro, 1993; Amato et al.,

1995; Ragg et al., 1995]. In this paper we consider 86 boreholes as deep as 7 km. Most of the wells are located into

MARIUCCI ET AL.: PRESENT STRESS IN NORTHERN APENNINES (ITALY) 111

44 ø -

43 ø _

12 ø 16 ø

Figure 3. Results of the S•,mi,, directions inferred from breakout analysis. The lines represent average direction for each well. The boreholes discarded or without breakouts are not shown. The upper right corner shows the subcrustal and deep earthquakes from Selvaggi and Amaro [1992]. The legend refers to the following: 1, quality A-B; 2, quality C; 3, normal faults; 4, thrust faults; 5, strike-slip faults.

the Plio-Pleistocene Adriatic foredeep, two in the center of the Apenninic belt and 17 in the volcanic and geothermal areas of Larderello, Amiata, Vulsini, and Sabatini (Tyrrhenian coast).

The data used for breakout analysis are readings of four-arm calipers, mostly recorded on paper logs and some on digital logs. Usually, we do not examine the log between 0 and 500 m depth, in order to avoid possible influences of topographic features on the stress directions at shallow depth. The data were then processed with a FORTRAN code that allows us to select breakout zones on the basis of some criteria set a priori. All the breakout directions in the range +- 10 ø from the .hole azimuth at respective depth of breakout are not considered. This cutoff criterion was also applied to near-vertical wells, to avoid considering other enlargements of the well, such as those due to pipe rubbing (e.g., key seats), in the stress determination. In addition, in the analysis of both paper and digital logs, attention was paid to the discrimination of washouts and key seats versus real breakouts, looking at the proximity of the smaller caliper arm to the bit size. For each well the results are plotted into a rose diagram, scaled both by length and by number of breakout intervals.

A quality value is assigned to each well, on the basis of the breakout length, the standard deviation from the mean azimuth, the number of breakouts along the borehole, and the depth and lithology in which the breakouts are detected. In

general, breakouts recognized at greater depth (below -1500 m) and in more coherent rocks (e.g., limestones) are considered more reliable than those found at shallower depth and in clayey units.

The results of breakout analysis are shown in Figure 3. Only 59 out of the 86 analyzed wells yielded a reliable direction of Sh,,i n. Table 1 contains the main characteristics of the 47 useful boreholes located in the Apenninic belt and the Adriatic coast; the other 12 wells have already been described by Montone et al. [ 1995]. Analysis of some wells along the Larderello area (northern Tuscany) are in progress.

3.2. Results of Stress Analysis

The P and T axes of the fault plane solutions shown in Figure 4 (which includes recent focal solutions from Frepoli and Amato [ 1997], older polarity solutions from Gasparini et al. [1985], and CMT solutions of M>5 earthquakes which occurred between 1979 and 1997) reveal that the Northern

Apennine is characterized by a narrow area with prevailing thrust solutions and a broad region with normal fault solutions (Figure 4a). The former corresponds to the outer front of the belt, from the Po Plain to the southern Marche region, while the second one is a wide sector including the belt and the

112 MARIUCCI ET AL.: PRESENT STRESS IN NORTHERN APENNINES (ITALY)

Table 1. Summary of the Characteristics of the Analyzed Wells in the Apenninic Belt and the Adriatic Coast

Well Breakout Interval, m Length, m Azimuth +s.d. Quality

I 4125-4991 216 309ñ9 B 2 618-3993 277 77ñ23 C 3 500-5541 1431 11ñ29 D 4 500-2230 1166 336ñ31 C 5 1864-4178 154 322ñ21 C 6 2643-3152 191 302ñ8 B 7 1310-2306 45 80ñ20 C 8 1308-2673 102 300ñ30 D 9 3577-4621 163 324ñ24 C 10 559-1650 684 75ñ25 C 10 2945-3151 70 297ñ18 D 11 2790-4210 276 309ñ23 C 12 1627-3041 492 300-&-_28 D 13 1650-4185 1963 309ñ28 D 14 1963-2593 440 360-&-_15 B 15 1052-2250 204 82ñ28 D 16 827-2470 410 282ñ30 D 17 2350-3505 283 325ñ29 D 18 591-2273 473 89ñ22 D 19 1910-3700 686 353ñ29 D 20 1764-2998 657 24ñ27 D 21 2822-3438 255 11ñ19 D 22 1061-2882 135 30-&-_20 C 23 646-2419 258 15ñ26 D 24 2728-4300 391 26ñ20 C 25 1975-3955 900 37ñ19 D 26 500-3033 700 52ñ29 D 27 1250-2564 34 1ñ5 C 28 500-4402 895 358ñ30 D 29 4775-5775 437 34ñ7 A 30 633-1948 285 45ñ31 D 31 656-1973 170 54ñ4 B 32 1649-6310 1211 48ñ23 C 33 2564-3281 319 53ñ12 B 34 1222-3338 292 51ñ18 B 35 1750-2280 464 75ñ14 B 36 4246-4685 173 6_+9 D 37 568-895 150 48ñ4 B 38 1030-2049 257 70ñ16 B 39 500-1990 356 39ñ11 B 40 1830-3054 146 37ñ16 C 41 500-3054 392 70-&-_22 C 42 700-1914 734 320-&_19 C 43 500-1521 259 65ñ4 C 43 2336-3050 119 296ñ16 C 44 500-3777 76 21ñ18 D 45 1401-2737 339 8ñ20 C 46 825-1306 75 345ñ10 D 47 500-1499 608 58ñ27 D

For the boreholes in the Tyrrhenian area, see Montone et al. [1995].

Tyrrhenian coastal region. The strike-slip solutions are spread almost everywhere (Figure 4b). Along the Apenninic front the compression axes of thrust and strike-slip faults have directions perpendicular to the mountain belt, from about N-S to NE-SW [Frepoli and Amato, 1997].

A further confirmation of this stress pattern distribution comes /¾om the breakout analysis. In fact, the maximum horizontal stress directions Snm•x along the Adriatic border trend NW-SE north of latitude 43øN, indicating a maximum

compression in agreement with the earthquake data. Around latitude 43øN, the stress data are more scattered, especially on the Adriatic offshore. Many wells exhibit N-S oriented S h and some others yield a ENE-WSW direction of S h ..... . However, it appears that this region around 43øN represents a transition zone between the northern sector in which there is still active compression (or transpression) and the southern sector in which we observe extension perpendicular to the outer thrusts.

Most of the wells in the Quaternary volcanic belt (Figure 3) reveal a direction of the minimum horizontal stress between NNE-SSW and ENE-WSW, in agreement with the results of stress inversion obtained from seismicity in the Amiata- Vulsini Mountains and Alban Hills by Montone et al. [1995]. The directions of extension in the Larderello geothermal area, inferred from focal mechanism, point out that normal faults have different orientation, perhaps because of a stress field characterized by radial extension.

Unibrtunately, we have very few well data in the Apenninic belt; the only two boreholes point out a S h ..... oriented NNE- SSW and ENE-WSW (Figure 3). These results come from scattered data along the logged intervals, which do not seem to be simply related to the different tectonic units drilled by the wells. This large scatter may be due to the high degree of heterogeneity of the rocks in this region, since a great number of faults were recognized in each well, which might reorient the stress directions [Bell et al., 1992; Barton and Zoback, 1994].

The directions of the extensional axes in the belt and in the back arc region show a relatively high scatter, although the NE-SW to ENE-WSW directions prevail (Figures 2 and 4). It must be reminded that the largest earthquakes of the region, including those that occurred in the Umbria-Marche Apennines in September 1997, have normal faulting mechanisms with NE trending extensional direction.

A group of T axes, near the Gran Sasso thrust, have a direction NNW-SSE probably related to the interaction with local sources (Figure 2). The great complexity of this region between 42øN and 43øN may suggest that a major, deep structural discontinuity is located in this area, as hypothesized by both geologic data [Patacca and Scandone, 1989] and seismic tomography [Amato eta]., 1993].

Toward the south, the NE-SW S h ...... direction trend becomes more evident, getting closer to the Gargano area, north of which some S h ...... orientations rotate to N-S or NW-SE (Figure 3). One well in this region clearly shows two directions of minimum horizontal stress: in the upper part (characterized by allochthonous units) it is ENE-WSW oriented, whereas in the lower section it is WNW-ESE (well 43 in Table 1), suggesting stress variation with depth. The general NE-SW S h ...... direction in the Central Apennines mirrors the stress regime which dominates in the Southern Apennine, where the same orientations are well known [Amato et al., 1995].

An important finding of this work is the evident change in the orientation of S h .... in the outer front of the arc around latitude 43 ø (Figure 3), which suggests the existence of some primary regional structure (perhaps to a deep crustal level or even deeper) between the northern and the southern sector of the Apenninic belt. There must be a disengagement between the two arcs that influences the stress field, causing the observed-90 ø rotation.

1.2. ø 1.6 ø

44 ø __

42 ø

12 ø 1.6 ø

%•1. %2 %3 %4 Figure 4. P and T axes oF the earthquake Focal mechanisms. including (a) normal and thrust Fault solutions and (b) strike-slip fault solutions. The legend refers to the following: 1-2, P and T axes after Frepoli and Amaro [1997]; 3-4, P and T axes from focal plane solutions after Gasparini et al. [1985], Riguzzi et al. [1989], and CMT solutions (see Figure 2).

114 MARIUCCI ET AL.: PRESENT STRESS IN NORTHERN APENNINES (ITALY)

4. Discussion and Conclusion

The joint analysis of borehole breakouts and focal mechanisms of earthquakes is a useful tool for constraining the actual stress field in the crust. In this paper we present an updated data set referred to the Northern Apennine region. Although more detailed studies are required to further improve the knowledge of the active stress regimes, some new results have emerged and can be qualitatively discussed. Borehole breakout data agree with focal solutions and remark the existence of transpression-compression, with SH .... trending NE-SW to N-S, perpendicular to the outer thrust of the belt, north of 43øN. The new data presented here and those already available [Montone et al., 1995, 1997] indicate an extensional stress field in the back arc region (Tuscany), except in its northwestern sector where strike-slip solutions prevail. Along the inner part of the mountain chain we found a normal faulting stress regime, with coherent NE-SW to E-W oriented extensional axes.

North of 43øN, both borehole breakouts and earthquake focal mechanisms indicate a clear change in the stress regime between the back arc region and the belt compared to the Adriatic foreland: thrust fault solutions are found only in a narrow area [Frepoli and Amato, 1997], which also corresponds to a different orientation of breakout inferred S h ...... . South of 43øN, along the Adriatic coast, only a few earthquakes with reverse solutions are found (Figure 4a), whereas S, ..... from breakouts in the same region are oriented NE-SW (Figures 3 and 4a). This could be due to a stress field change in depth, with a zone in compression (5-15 km)underlying one, sampled by breakouts, in extension (0-5 km). However, the t•w data available in this region do not allow us to draw any conclusion about this variation. This area (around latitude 42øN-43øN) marks the transition to the extensional regime that characterizes the Southern Apennines [Amato and. Montone, 1997]. Consistently, we find Shm m directions roughly NE-SW south of this region (see Figure 3). Unfortunately, also in this case not many earthquakes are located in this area, so we cannot constrain precisely how stress changes.

Most authors agree that during the evolution of the Northern Apennines there were contemporaneous compression at the front of the belt and extension at its back [Vai, 1987]. The same process repeated through time but shifted to the east. Our data confirm that the present situation is similar to the late Miocene and Pliocene one but shifted to a more eastward location. One of the most likely explanations is that the eastward migration of the extension-compression pair has been controlled by the progressive retreat of the Adriatic lithosphere below the belt [Patacca and Scandone, 1989]. Evidence of a relic subduction in the Northern Apennines comes from seismicity [Selvaggi and Amato, 1992] and seismic tomography [Amato et al., 1993; Spakmart et al., 1993].

In a schematic section across the Northern Apennine and the related data from breakouts and focal mechanisms (Figure 5), we can see how the present stress in the crust changes, from the Tyrrhenian basin to the Adriatic foreland (west to east). Along western Tuscany all the analyzed wells, located in the geothermal areas, point out Shmin directions ranging from NNE

to ENE (Figure 5). Many of these boreholes have a good quality, and they do not show important changes in direction with depth. The focal mechanisms, inferred from weak crustal seismicity in the Quaternary volcanic belt, have predominant normal faulting solutions, even though the directions of T axes are highly scattered in some regions, like northern Tuscany. Also, the inversion of microearthquake focal mechanisms in the Quaternary volcanoes of Colli Albani, Vulsini Mountains, and Mount Amiata revealed a stress field with horizontal, NE-SW trending c53, and oblique % and {52 [Morttone et al., 1995]. Along the mountain belt the earthquakes are due to strike-slip or normal faults, as pointed out by focal mechanisms solutions: the T axes agree with the breakout directions around NNE-SSW. On the contrary, most wells in the Pliocene-Pleistocene toredeep, in the northern sector of the Adriatic coast (north of 43øN), yield S h ...... directions nearly NW-SE (Figure 5), accordingly with the reverse fault solutions of the focal mechanism (Figures 3 and 4).

Although these data sample different depth ranges (0-7 km breakouts and 3-15 km fault plane solutions), the two results seem to be in good agreement with each other, suggesting that no important changes exist in the stress field throughout the brittle crustal layer. The only exception might be the coastal region of Adriatic, between 42øN and 43øN, that as discussed betbre, shows evidence of stress variations with depth.

From all these data it seems that nowadays the Apenninic belt, north of 43øN, is characterized by an extensional stress regime on its western side and a compressional or transpressional regime on the external (eastern) side. The present situation is similar to that which has happened in the past, that is, compression at the front and contemporary extension in the inner and rear sectors of the chain. The new data presented here allow us to define the extent of the extension and compression region better than before [see Lavecchia et al., 1994], and therefore the comparison with the paleostress data is more effective in evaluating how the process has been migrating through time.

We can subdivide the study area into two main regions (extensional and compressional domain) and possibly a narrow sector in between, which can be considered as a

transition zone between the two former regions (Figure 6a). This regionalization comes from all the data that we have collected and can be seen as a representation of large-scale (regional) stress acting in this sector of Italy. On the other hand, in some regions we note the influence of local structures, which generate smaller-scale rotations of the stress axes superimposed to the main trends.

The comparison with the paleostress directions inferred from structural geology data (Figure 6a) reveals that the extension-compression pair is now in a more eastward position with respect to its location in late Pliocene- Pleistocene. This suggests that the mechanism which has determined the migration of the two regions is still acting today. If, as proposed by many previous authors [Malinverno and Ryan, 1986; Patacca and Scandone, 1989; Royden, 1993; Jolivet et al., 1998], the basic process driving the tectonics is the retreating of the subducting slab, we propose that the active process today is not very different from the past 10 or 15 Myr.

115

116

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km b Figure 6. (a) Kinematics of the Northern Apennine: schematic map of the past and present stress field inferred from borehole breakouts, focal mechanism, and bibliography data. E is the present extension front; C is the present compression front; •d l e, 2e, 3e, l c, 2c, •d 3c are the positions of the extension (e) and compression (c) fronts at Messinian-early Pliocene, early Pliocene, and late Pliocene-Pleistocene, respectively [redrawn after Lavecchia et al., 1994]. (b) Tentative evolutionary section across the Apennines (a- a') compares the age of tectonics and volcanism (upper plot) after Jolivet et al. [1998] with the deep structures [Selvaggi aMAmato, 1992; Amato et al., 1993]. %e present configuration of the subducted lithosphere (shaded) is projected back in time (Messinian-early Pliocene) according to the migration of the compression- extension pair. %e legend refers to the following: 1, 2, and 3, epicenters of thrust, normal, and strike-slip fault solution earthquakes; 4, age of volcanism; 5, age of extension in the Tyrrhenian area and Tuscany region after Jolivet et al. [1998]; 6, age of extension in the belt after Lavecchia et al. [1994]; 7, age of compression after Lavecchia et al. [1994]; 8, hypocenters of subcrustal earthquakes (in Figure 6b); 9, proposed past and present positions of the slab flexure controlling the surface tectonics (in Figure 6b).

MARIUCCI ET AL.: PRESENT STRESS IN NORTHERN APENNINES (ITALY) 117

In the sketch of Figure 6b, we have summarized the possible evolution of the region, comparing the migration of the extension-compression pair (upper plot in Figure 6b) with the deep structures. The present position of the Adriatic slab is constrained by the subcrustal earthquakes [Selvaggi and Amaro, 1992] and seismic tomography [Amaro et al., 1993; Ciaccio et al., 1996]. We speculate that the present extensional front (line E in Figure 6a) is controlled by the slab geometry. Then we tentatively hypothesize the slab position in late Miocene-early Pliocene from the position of the extensional t¾ont at that time (line l e in Figures 6a and 6b), as inferred from geologic data [Bigi et al., 1990; Lavecchia et al., 1994].

In this hypothesis the eastward migration of the extensional and compressional fronts is controlled by the passive sinking of the Adriatic slab beneath the Apennines. This speculative model applies to the Northern Apenninic arc,

where there is evidence of a "continuous" lithospheric slab t¾om earthquake locations and seismic tomography [Amaro et al., 1993; Piromallo and Morelli, 1997], although other tomographic models hypothesize a detached slab below this region [Spakmart et al., 1993]. Both the analysis of more data and geodynamic modeling procedures, either with numerical procedures or with laboratory experiments, are needed to test this hypothesis.

The results of this study, besides providing new constraints for understanding the recent evolution of the region, allow us to define the extent of the seismotectonic domains better than

was previously possible, and they can then be used to predict the behavior of seismogenic sources in this hazardous region.

Acknowledgments. We are grateful to M. Brudy and M.D. Zoback for their constructive reviews. We also thank E. Boschi for his

encouragement in this research.

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A. Amato, M.T. Mariucci, and P. Montone, Istituto Nazionale di Geofisica, Via di Vigna Murata, 605, 1-00143 Rome, Italy. (e-mail: amato@ingrrn.it; mariucci @ingrm.it; montone@ing750.ingrm.it)

(Received July 25, 1997; revised September 14, 1998; accepted October 1, 1998.)