Recent and active tectonics in the Calabrian arc (Southern Italy)

E L S E V I E R Tectonophysics 243 (1995) 37-55

TECTONOPHYSICS

Recent and active tectonics in the Calabrian arc (Southern Italy)

Luigi Tortorici a, Carmelo Monaco a, Carlo Tansi b, Ornella Cocina c a Istituto di Geologia e Geofisica, Universitgt di Catania, Corso Italia 55, 95129 Catania, Italy

b C,N.R., Istituto di Ricerca per la Protezione Idrogeologica, Rende, Cosenza, Italy c C.N.R., Istituto Internazionale di Vuleanologia, Catania, Italy

Received 19 December 1993; revised version accepted 11 July 1994

Abstract

Normal faulting which has developed since the Middle Pleistocene, with an overall ESE-WNW extension, is the dominant mode of deformation which has characterized the Calabrian arc up to the present. Quaternary normal faults extend for a total length of about 180 km along the inner side of the arc. The different normal fault segments, which, during the Pleistocene, controlled the evolution of the main marine sedimentary basins, are varying in length from 10 to 20 km and have fault escarpments showing a very young morphology which define the fronts of the major mountain ranges of the arc. The morphological features of the fault escarpments suggest slip rates of 0.8-1.1 mm/yr for the last 700 k.y. and values of 0.6-0.9 mm/yr for the last 120 k.y., indicating a uniform rate of faulting since the Middle Pleistocene. Crustal seismicity and the mesoseismic areas of historical large events (6.5 < M ~< 7.1) which occurred in the Calabrian arc are located within a narrow belt along the hanging walls of the Quaternary normal fault segments. This suggests that the normal faults which dissect the inner side of the Calabrian arc may be seismically active.

1. Introduct ion

The Calabrian arc is a prominent arc-shaped structure of the Mediterranean orogenic belt. The arc connects the E - W and N W - S E trending branches of the belt, which are represented by the Maghrebian and the southern Apennines chains, respectively. The overall architecture of the arc is made up of a series of basement nappes and ophiolite-bearing tectonic units that are con- sidered to be a remnant of the Cretaceous- Paleogene Europe-verging Eo-alpine chain in- volved, during the Neogene, in the building of the Apennines orogenic belt (Haccard et al., 1972;

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Alvarez, 1976; Amodio-Morelli et al., 1976). Thrusting has been related to the subduction process involving the African paleomargin and was accompanied by a progressive southeastward migration of the Calabrian arc and by the opening of the Tyrrhenian Sea (Rehault et al., 1984; Malinverno and Ryan, 1986; Dewey et al., 1989).

The arc, whose recent geodynamic evolution has been closely related to the opening of the southern Tyrrhenian Sea, is one of the areas of the western Mediterranean where the effects of intense Quaternary tectonics are well represented. The most impressive tectonic feature of the arc is represented by a prominent normal

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L. Tortorici et al. / Tectonophysics 243 (1995) 37-55 39

fault belt (Fig. 1) that extends, more or less continuously, along the inner side of the arc. The individual fault segments separate the main Pliocene-Pleistocene basins from the uplifted mountain ranges (Aspromonte, Serre and Catena Costiera). The faults strongly deform Upper Pleistocene deposits and are morphologically defined by escarpments several hundred meters high, thus suggesting very recent activity.

From a seismic point of view, the Calabrian arc, as well as the southern Apennines and eastern Sicily, represent a very active area characterized by historic crustal events, the largest of which reached (in the last 6 centuries) an MSK intensity of X-XI (6 < M ~< 7.1), and by the occurrence of intermediate and deep focus earthquakes located along the inner side of the arc, beneath the southern Tyrrhenian Sea (McKenzie, 1972).

The occurrence in the Calabrian arc of both intense Quaternary faulting and active crustal seismicity suggests that these phenomena might be related to each other. At present, specific studies have been carried out only for a few fault segments located on the southern sector of the arc (Tortorici et al., 1986; Ghisetti, 1992; Valen- zise and Pantosti, 1992); a general scheme for the whole region is still lacking.

The aim of this paper is to test the relationships between the main Quaternary faults affecting the entire arc and the major crustal earthquakes and to discuss the style of Middle Pleis- tocene-Holocene deformation and its signifi- cance in the regional tectonic framework. This study is supported by field mapping, structural analysis and morphological observations of the main fault segments and by data from instrumental and historical regional seismicity which has occurred in the Calabrian arc.

2. Quaternary faults

In the Calabrian arc a normal fault belt separates Pleistocene sedimentary basins from the major mountain ranges of the region (Aspro- monte, Serre and Catena Costiera) and extends along the inner side of the arc for a total length of about 180 km (Fig. 1). The fault belt comprises

various fault systems which, from the north to the south, strike N-S along the eastern side of the Catena Costiera (Crati Valley fault system), NE- SW along the western side of the Serre and Aspromonte (Serre-Aspromonte fault system), ENE-WSW at the northern border of the As- promonte massif (S. Eufemia fault system) and again NE-SW in the Strait of Messina area (Re- ggio Calabria fault system).

2.2. Crati Valley fault system

This system (1 in Fig. 1) extends along a N-S direction for about 40 km and separates the Catena Costiera mountain range, with outcrops of metamorphic terranes from the alpine nappes and of Miocene-Lower Pliocene sediments, from the Late Pliocene-Pleistocene sedimentary basin of the Crati Valley.

The basin is filled by roughly 500 m of marine deposits composed of basal sands and conglomerates (Upper Pliocene-Lower Pleistocene), several hundred metres of marine clay (Lower-Middle Pleistocene) and a 100 m of beach sands and conglomerates (Middle Pleistocene). The upper- most deposits, mostly outcropping along the mountain front, are fanglomerates and sands (Middle Pleistocene) that unconformably overlie the marine sediments and represent the remnants of, formerly wider, alluvial fans.

The fault system (Fig. 2) is represented by three main normal fault segments which show an overall left-stepping, en 6chelon arrangement. Distinct fault segments, starting from the north, have lengths of 8 km (Fagnano Castello fault), 25 km (S. Marco Argentano-S. Fili fault) and 15 km (Montalto Uffugo-Rende fault). The fault planes dip steeply towards the east and develop at the base of the mountain range. They involve both highly fractured crystalline rocks and Lower- Middle Pleistocene sediments, which are severely tilted and deformed.

A structural analysis carried out along the master fault and along minor structures occurring either in crushed bedrock or in Quaternary sediments shows fault planes, striking N10-45°E, characterized by subvertical to oblique slickensides. The pitch, ranging between 60 ° and 90 ° ,

40 L. Tortorici et al. / Tectonophysics 243 (1995) 37-55

indicates a right component of motion related to a N125°E oriented extension (Fig. 3a).

Morphologically, the different fault segments

are represented by very sharp rectilinear escarpments bounding the uplifted footwall. The mountain front reaches a total height of 600-700 m,

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Fig. 3. Diagrams (Schmidt net, lower hemisphere) showing the at t i tude of the fault planes and slickensides (thin lines) measured along cataclastic belts of: (a) the Crati Valley fault system; (b) Cittanova fault, (c) S. Eufemia fault; (d) Reggio Calabria fault. Large arrows indicate azimuth of the mean extension direction derived from a qualitative and quanti tat ive analysis of the slickensid¢ data sets. Following the graphical approach of the right dihedrons method (Angelier and Mechler, 1977) the condit ioned square minima method proposed by Caputo and Caputo (1988) was applied.


from the bottom of the fault trace, and is characterized by the occurrence of three sets of triangular a n d / o r trapezoidal facets which have heights of 300-400 m, 250 m and 70-100 m. The drainage network flows almost perpendicular to the fault segments and is made up of deeply entrenched and flat valleys, occurring in uplifted and downthrown blocks, respectively. Fault escarpments are also marked by several still active alluvial fans. Along the southernmost fault segment (Montalto Uf fugo-Rende fault), a clearly recog- nizable 4 -6 m high surface break offsets the crests, which are displaced with a right-lateral component of motion (Fig. 4).

Geological data and the young morphology of the fault scarps show that the Crati Valley fault system was active during the Pleistocene. In par- ticular, its activity is documented since the Mid- dle Pleistocene ( ~ 700 k.y.) when the uplift of the Catena Costiera started (Lanzafame and Tor-

torici, 1981) which gave rise to the development of the large alluvial fans. This implies that the total height of the mountain front (600-700 m) corresponds to the cumulative vertical throw of the fault, thus suggesting a long-term minimum slip rate of about 0.8-1.0 m m / y r during the last 700 k.y.

Further information about the slip rate of the fault system during the Middle-Late Pleistocene can be obtained by analyzing the geometry of the different sets of triangular facets. These features are the result of complex interactions between the tectonic movements occurring along the fault plane and the rates of the erosional processes which, in turn, are related to changes in climatic conditions (Briais et al., 1990; Armijo et al., 1992; Avouac et al., 1993). If we assume a uniform slip rate during the last 700 k.y., the evolution of a set of triangular facets necessarily coincides with a major erosional event occurring during each cli-

Fig. 4. View from the east of the fault break (thin arrows) affecting, with a light right-lateral component of motion, ridges and valleys at the southern edge of the S. Fili-S. Marco Argentano fault.


matic cycle. This indicates that the three distinct generations of triangular facets characterizing the fault escarpment were formed during the three last glaciations. Also, according to our observations showing that the V-shaped entrenching downcuts the entire fault scarp, we can infer that the major erosional events occurred when the glacial conditions ceased. Consequently, breaks between different sets of triangular facets indicate warmer and wetter periods, dated at ~ 240 and ~ 120 k.y. for the last two climatic cycles (Martinson et al., 1987). This implies that the slip rate of the Crati Valley fault system has values of 1.0 m m / y r and 0.6-0.9 m m / y r for the last 240 k.y. and 120 k.y., respectively.

2.2. Cittanova fault

This structure (2 in Fig. 1) is part of a large N E - S W trending normal fault system occurring in the Southern Calabria (Ser re-Aspromonte fault system) that runs more or less continuously for a total length of about 80 km along the boundary between the uplifted Serre and As- p romonte mounta in ranges and the Late Pl iocene-Pleis tocene basins of Mesima and Gioia Tauro.

The Cittanova fault (Fig. 5) is a 15 km long normal fault which, to the east bounds the Gioia Tauro half graben. It changes its trend from N E - S W (from Cittanova to S. Cristina d'Aspro- monte ) to E N E - W S W (from S. Cristina d 'Aspromonte to Delianuova). The basin is filled by roughly 600 m of marine deposits, consisting of about 70 m of cross-bedded sands and calcaren- ites (Upper Pl iocene-Lower Pleistocene), by several hundred metres of clay and silty clay contain- ing pumice-rich levels (Lower-Middle Pleis- tocene) and by 100 m of regressive sands and conglomerates (Middle Pleistocene). This sequence is unconformably overlain by alluvial fanglomerates and sands (Middle -Upper Pleis- tocene) capped by a Late Pleistocene planation surface. In the Cittanova area Upper Pleistocene deposits, belonging to a large, gently sloping alluvial fan, are banked against the fault scarp.

The uplifted block is characterized by basement rocks (gneiss and granites) directly overlain

by alluvial deposits, which form two main orders of terraces. The highest order extends onto the top of the Aspromonte massif, at elevations of 900 m to the northeast and up to 1200 m to the southwest. The second order occurs along the northeastern part of the mountain slope at an elevation of 700 m. Both terraces correspond to Early-Middle Pleistocene marine high stand tracts (Ghisetti, 1981; Dumas et al., 1982).

The fault plane dips steeply toward the west and involves both the youngest sediments of the Gioia Tauro basin, severely warped along the fault plane, and the crystalline rocks, which show a 50 m wide highly cataclastic zone. The fault plane also deforms the Late Pleistocene planation surface at the top of the downthrown block and is locally characterized by clay-rich gouge zones a few metres wide.

Structural analysis carried out within the cataclastic belt developing along the master fault show N25°-45°E trending minor fault planes characterized by subvertical slickensides (pitches ranging between 50 ° and 90°), indicating N140°E oriented extension (Fig. 3b).

The fault scarp is straight and marks the base of a 500-600 m high escarpment, which exhibits three sets of well developed triangular facets, having heights of about 400 m, 200-250 m and 80-90 m. Near S. Cristina d 'Aspromonte the youngest triangular facets are separated by narrow V-shaped and 'wine glass' valleys (Wallace, 1978). Between Oppido Mamertina and Molochio a continuous, 10 m high, fresh scarp is well ex- pressed, whereas south of Molochio a 2 m high fault break was found.

The total vertical throw of the Cittanova fault, inferred from the offset of the Upper Pl iocene- Lower Pleistocene (= 1.8 m.y.) beach deposits occurring beneath the highest planation surface on the uplifted wall and from the total thickness of the marine sequence (estimated to be 600 m) on the hanging wall, may reach values of about 1200 m. This suggests a minimum uplift rate of 0.6-0.7 m m / y r .

The morphological features of the Cittanova fault indicate very recent activity, which might be quantitatively estimated assuming that the two youngest sets of triangular facets have been


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Fig. 5. Morphotectonic map of the Cittanova fault (2 in Fig. 1). The fault runs at the base of the Aspromonte mountain range cutting the Pleistocene deposits of the Gioia Tauro basin. The fault escarpment exhibits three sets of well developed triangular facets that suggest high values of vertical slip rate. Elevation contour lines are from 1 : 25.000 topographic maps.

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formed during the two last climatic cycles. The average rate of vertical displacement is thus 0.8- 1.0 m m / y r for the last 240 k.y. and ~ 0.7 m m / y r for the last 120 k.y.

2.3. S. Eufemia fault

This 18 km long fault segment (3 in Fig. 1) is part of a right-stepping en 6chelon normal fault system (S. Eufemia fault system) which extends westward from the southern edge of the Cit- tanova fault to the Strait of Messina. This E N E - WSW trending fault (Fig. 6) steeply dips towards the north and, southward, bounds the Gioia Tauro basin. The fault plane, which affects the Pleis- tocene sediments, is marked by an approximately 100 m wide cataclastic belt characterized in places by slivers of clay-rich fault gouge.

Structural data collected in the crushed bedrock and within the fault gouge, show fault

planes trending N45-90°E characterized by oblique slickensides (pitches ranging between 35 ° and 70°), indicating a left-lateral component of motion related to a N135°E direction of extension (Fig. 3c).

The S. Eufemia fault is morphologically characterized by a 200-250 m high escarpment which exhibits well developed triangular facets separated by wine glass canyons (Fig. 7). The youngest order of triangular facets show heights of 70-80 m, suggesting an uplift rate of about 0.7 m m / y r during the last 120 k.y.

2.4. Reggio Calabria fault

The Reggio Calabria fault (4 in Fig. 1) is part of a major normal fault system (Reggio Calabria fault system) which, during the Early Pleistocene, controlled the evolution of the Reggio Calabria basin. This 15 km long, NNE-SSW trending nor-

Fig. 7. View from the north of the S. Eufemia fault escarpment showing well exposed triangular facets (T) separated by well developed 'wine glass' canyons (W).


mal fault (Fig. 8) extends throughout the Pliocene-Pleistocene basin, defining a half graben filled by Middle-Upper Pleistocene sediments (Ghisetti, 1981; Barrier, 1987). These deposits belong to a fault-controlled, marine, Gilbert-type, fan delta system prograding westwards. They con- sist of bathyal clay and marls (bottom set) 700-500

k.y. old (Cornette et al., 1987) and of coarse gravels and sands mostly dipping 30 ° westward (foreset) which, near the fault plane, contain lens-shaped bodies of angular breccias. The foreset deposits are capped by subhorizontal, littoral to continental, alluvial sands and conglomerates (topset) of Tyrrhenian age, extending both in the

'] Alluvial deposits ~ Late Pleistocene IT~T]] Early-Middle Pleistocene continental terraces continental terraces

~ Middle-.Late Pleistocene ~ Early-Middle Pleistocene J Fault trace sediments ~,':~'~.~.* ~..".'.'~ sediments

~ Crysfalline rocks and Miocene- ~ Alluvial fan ~ d A Triangular facets Pliocene .ledimentary cover

Fig. 8. Morphotectonic map of the Reggio Calabria fault (4 in Fig. 1). Note the en ~chelon fault segments that run between mounta in front and alluvial plain with several alluvial fans developing on the hanging wall.

48 L. Tortorici et al. /Tectonophysics 243 (1995) 37-55

downthrown and upthrown blocks. The deposition of these sediments was ended by the development of a large planation surface. The youngest deposits outcropping in the downthrown block are represented by continental alluvial red sands and conglomerates which, unconformably overly- ing the marine sequence, represent the remnants of large, gently sloping, alluvial fans of Wiirmian age (Dumas et al., 1978). These deposits progressively onlap the fault plane, thus suggesting faulting activity during that period (Fig. 9).

The fault plane dips steeply westward and deforms both crystalline rocks and Middle-Late Pleistocene sediments. Structural measurements, carried out on the main fault plane and within the crushed bedrock along its cataclastic belt, show N N E - S S W to N E - S W trending fault planes with subvertical and oblique slickensides (pitches between 60 ° and 90 °) related to a N l l2 °E direction of extension (Fig. 3d).

The fault trace is more or less continuous and runs near the coastline of the Strait of Messina as far as Reggio Calabria, from where it seems to continue offshore for about 10 km (Ambrosetti et al., 1987) Morphologically, it is defined by a 70- 100 m high escarpment, which extends discontin- uously along the boundary between the mountain front and the adjacent plain. Several proximal alluvial fans occur at the base of the fault escarpment, which is characterized by only one set of triangular facets, showing heights ranging from 50 m to 70 m.

All these observations indicate Quaternary activity of the Reggio Calabria fault. During the Middle-Late Pleistocene (between 500 and 120 k.y.), this structure controlled the deposition of the marine fan delta sediments which completely filled the tectonic depression. During the Wiirmian, the faulting activity generated gently sloping alluvial fans on the hanging wall which

Fig. 9. View from the south of the west-dipping Reggio Calabria normal fault (arrow indicates the fault trace) separating crystalline rocks, exposed in the footwall (right side), from the Late Pleistocene alluvial fan deposits (left side) which progressively onlap the fault plane.


progressively and partially onlap the fault escarpment. The vertical off-set of the Thyrrenian planation surfaces and the height of the triangular facets along the fault escarpment suggest a minimum slip rate of 0.6 mm/yr .

3. Seismicity

The seismicity of the Calabrian arc is defined by the occurrence of both crustal ( H < 35 km) and subcrustal (H > 35 km) earthquakes, the distribution of events with M > 5 is shown in Fig. 10. Deep-focus earthquakes are mainly located

beneath the southern Tyrrhenian Sea (data from Gasparini et al., 1982; Anderson and Jackson, 1987). They range in depth from 200 km to 350 km, depicting a slab dipping of 45 ° towards the NW and showing, for the magnitude range con- sidered here, a gap in depth distribution between 50 km and 200 km (Fig. 10). Only three intermediate depth events (50 < H < 100) occur beneath the east and west coast of the Calabria peninsula.

Crustal seismicity (Fig. 10) is defined by historical and instrumental earthquakes which have occurred since 1000 A.D. (data from Gasparini et al., 1982; Postpischl, 1985). The epicentral distribution of the earthquakes outlines a seismic belt

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running mainly along the downfaulted Pleis- tocene basins on the Tyrrhenian side of the Arc. Some smaller earthquakes (M < 6) occur in the frontal portion of the arc.

Fault plane solutions are available (Fig. 1) for crustal events regarding the 1908 Messina earthquakes ( M = 7.1) and two other small events which occurred in the Strait of Messina (January 6, 1975; M = 4.7) and in the Crati Valley (February 20, 1980; M = 4.4). According to the regional fault geometry we favour an east-dipping N N E - S S W trending fault plane for the Crati Valley event and a west-dipping N E - S W trending plane for the Strait of Messina events, all compatible with E S E - W N W (T axis directions ranging from N100°E to N127°E) extension (Cello et al., 1982).

This seismic belt includes the largest earthquakes (6.5 ~<M~<7.1) which have occurred in the Calabrian arc, such as the 1783 earthquake sequence, the Cosenza earthquake of 1870 and the Messina earthquake of 1908, whose effects will be briefly discussed below.

The 1783 earthquake sequence includes four large events (6.0 <M~< 7.0) that occurred between February 5th and March 28th, which completely devastated all of southern Calabria, killing about 35,000-40,000 people (Hamilton, 1783; Vivenzio, 1783; Baratta, 1901). The first event, with a MKS intensity of XI, corresponding to M = 7.0 (Postpischl, 1985), was located under the Gioia Tauro Basin, near the northern foothills of the Aspromonte mountain range (Tortorici et al., 1986). The main shock was followed on February 6th by a second large earthquake with a MKS intensity of X (M = 6.0), located near the coastal area of the northern edge of the Strait of Messina, which was accompanied by a small tidal wave (Baratta, 1901). The third event, MKS intensity of XI ( M = 6.7 according to Postpischl, 1985), occurred on February 7th along the Mesima Valley at the western foot of the Serre mountain range. On March 28th the last important shock of the sequence (MKS intensity of X and M = 6.7, according to Postpischl, 1985) took place to the northeast, near the Ionian coast of the Catanzaro trough.

The mesoseismic area of the 1783 earthquakes

depicts a N E - S W trending elliptical zone which extends continuously for about 90 km along the downfaulted Pleistocene basin areas, following the piedmont mountain range boundary. This area was characterized by ground effects such as large landslides creating dammed lakes, rock slides, sand cones and craterlets and numerous cracks (Hamilton, 1783; Vivenzio, 1783; De Dolomieu, 1784; Baratta, 1901; Cotecchia et al., 1969). Dur- ing the first shock, between S. Giorgio Morgeto and S. Cristina d 'Aspromonte (see Fig. 5 for location), just along the basement rock-Pleisto- cene sediments boundary, a surface break extending for about 15 km developed (De Dolomieu, 1784). These observations suggest that the main events of the sequence occurred along the large, west-dipping normal faults separating the Pleis- tocene basins from the Aspromonte-Serre moun- tains (Tortorici et al., 1986; Westaway, 1992).

On October 4th, 1870, a large earthquake of MKS intensity X, corresponding to M = 6.6 (Postpischl, 1985), occurred in the Crati Valley near the town of Cosenza. This event ruined the town of Cosenza and several villages located to the south. The greatest damage (Conti, 1871) occurred within a 10 km long N N E - S S W trending elliptical mesoseimic area located in the downfaulted block of the Crati Valley fault system (Baratta, 1901), suggesting rupture along the southernmost east-dipping normal fault segment which bounds the Pleistocene trough.

On December 28th, 1908, one of the largest earthquakes of Southern Italy struck the Strait of Messina area. This event of MKS intensity XII and M = 7.1 (Postpischl, 1985) completely destroyed the towns of Reggio Calabria and Messina, killing about 70,000 people (Baratta, 1910). The most affected area, including all the villages that were completely ruined, lies on the Calabrian side of the strait. The damage zone extends from the coastline to the foothills of the Aspromonte mountain with a 25 km long and about a 10 km wide elliptical shape, trending in a N N E - S S W direction. It is worth noting that in the town of Reggio Calabria the greatest damage occurred along narrow belts extending along N N E-S S W trending step-like structures, which are characteristic of this town. Within these belts


all the buildings were completely destroyed whereas outside damage was less severe, as reported in the map of the damaged areas of Reg- gio Calabria by Baratta (1910).

Within the mesoseismic area several landslides, sand craterlets and a few surface breaks, extending in a N E - S W direction, from Reggio Calabria to Scilla, were developed; whereas along the coastline a permanent subsidence of 0.70 m was recognized (Baratta, 1910). On the Sicilian

coast many villages and the town of Messina were also destroyed. On this side of the strait the effects of the earthquake occurred only in a narrow belt along the coastline and were enhanced by the occurrence of tsunami waves reaching heights of up to 3 m, which occurred approximately 10 min after the main shock.

These observations suggest that the Messina earthquake may be closely related to the west- dipping N N E - S S W trending normal fault seg-

t I I

N

(Februar (Februarv 6)1783

0 40 km I

Fig. 11. Relationships between Quaternary faults and mesoseismal areas of the largest earthquakes (M/> 6.5) that have occurred in the Calabrian arc (after Conti, 1871; Baratta, 1901, 1910).


ments at the foot of the Aspromonte mountain which extend offshore (Shick, 1977; Ghisetti, 1992; Westaway, 1992).

4. Discussion and conclusion

The observations presented in this paper point out that different segments of the normal fault belt, extending for a total length of about 180 km along the inner side of the Calabrian arc, exhibit evidence for a Middle Pleistocene to Holocene faulting activity related to a mean E S E - W N W dominant extension. The distinct normal fault segments, which, during the Pleistocene, controlled the evolution of major marine sedimentary basins, have lengths ranging from 10 km to 20 km. They exhibit huge fault escarpments showing a youthful morphology which defines the fronts of the main mountain ranges of the arc (Aspromonte, Serre and Catena Costiera).

Morphological features of fault escarpments suggest long-term slip rates of 0.8-1.1 m m / y r for the last 700 k.y. and values of 0.7-0.9 m m / y r for the last 120 k.y., indicating a uniform rate of faulting activity since the Middle Pleistocene.

Crustal seismicity patterns show that most of the earthquakes, including the largest events which have occurred in the Calabrian arc (6.5 ~< M ~< 7.1), are located within a narrow belt which extends along the hanging walls of the Quater- nary normal fault belt. This suggests that earthquake activity occurs on these different fault segments.

This relationship is also supported by the analysis of historical documents that describe the largest earthquakes that occurred in the region. Mesoseismic areas of these events (1783 earthquake sequence, 1870 Cosenza earthquake and 1908 Messina earthquake) extend continuously along the downthrown blocks associated with normal fault segments, suggesting that they are seismically active (Fig. 11).

These data, together with structural and morphological observations carried out along the main fault segments, allow us to highlight earthquake-faul t relationships. According to Tortorici et al. (1986), the first three shocks of the 1783

earthquake sequence may be related to slip on large, west-dipping, N E - S W trending normal fault segments which separate the Aspromonte (Cittanova fault) and Serre (Mesima fault) moun- tains and the Palmi coastal high from the Pleis- tocene sediments of the Gioia and Mesima basins. In turn, the 1870 Cosenza earthquake may be related to the activity of the southernmost east- dipping N -S trending segment of the Crati Val- ley fault system (Montalto Uf fugo-Rende fault).

Structural, morphological and seismological data regarding the Strait of Messina area strongly suggest that the 1908 Messina earthquake may be related to the activity of the offshore segment of the steeply west-dipping N N E - S S W trending Reggio Calabria fault (Shick, 1977). On the basis of seismological and geodetic modelling this event has been conversely related to the activity of a blind, east-dipping, low-angle, N -S trending normal fault located along the Sicilian side of the strait (Capuano et al., 1988; De Natale and Pingue, 1991; Valenzise and Pantosti, 1992). This hypothesis strongly contrasts with our data and also with the well known structural pattern of the Strait of Messina (Cortese, 1895; Lembke, 1931; Bousquet et al., 1980; Ghisetti, 1980, 1981, 1984; Montenat et al., 1991; Raffy et al., 1981; Dumas et al., 1982; Atzori et al., 1983; Ott d'Estevou et al., 1987). Neither N-S trending normal faults nor low-angle normal faults have been reported in this area. Indeed, the Strait of Messina is characterized by mainly N E - S W trending high- angle normal faults. Since the Pliocene-Pleisto- cene these have shown the highest rate of deformation on the west-dipping fault segments extending along the Calabrian side, where they have controlled the uplift of the Aspromonte mountain (Ghisetti, 1992).

These results suggest that the normal fault belt which dissects the Calabrian arc is seismically active and that along the major fault segments occurring in the southern part (characterized by lengths of about 20 km and slip rates of about 1 m m / y r ) earthquakes of M ---- 6 -7 may occur. This implies that along the segments of the Crati Val- ley fault system (northern Calabrian arc), with similar lengths and slip rates (e .g.S. Marco Ar- gentano-S. Fil i -Montal to fault), earthquakes

L. Tortorici et al. /Tectonophysics 243 (1995) 37-55 53

with s imi lar m a g n i t u d e may also occur even if a large event has not b e e n r e c o r d e d in h is tor ica l t imes. The re fo re , fu r the r and more de t a i l ed s tud- ies, b a s e d on quan t i t a t ive morpho log i ca l and s t ruc tu ra l ana lyses of these faul t segments , mus t be ca r r i ed out to de f ine the poss ib le la rges t e a r t h q u a k e as soc ia t ed with each faul t s egmen t and its r e cu r r ence interval .

T h e r ecen t and act ive tec tonics o f the Cal- ab r i an arc show that , s ince the M i d d l e Pleis- tocene , this reg ion has b e e n a f fec ted by no rma l faul t ing, r e l a t ed to un i fo rm E S E - W N W extension, r e spons ib le for the crus ta l seismici ty occur- r ing in this area . A l o n g the d i f fe ren t N E - S W t r e n d i n g n o r m a l faul t s egmen t s (Serre , C i t t anova and Regg io C a l a b r i a faul ts) ex tens ion is accom- m o d a t e d by d ip-s l ip movemen t s , whe rea s sl ightly r igh t - l a t e ra l c o m p o n e n t s of m o t i o n cha rac t e r i ze the N - S t r end ing faul ts (Cra t i Val ley faul t system). L a r g e l e f t - l a te ra l c o m p o n e n t s of mo t ion occur a long the E N E - W S W t r end ing faul t seg- m e n t s (e.g., S. E u f e m i a faul t ) in the sou the rn pa r t of the arc, a long which the ex tens ion j u m p s west- wa rds f rom the S e r r e - A s p r o m o n t e moun ta in s to the St ra i t of Mess ina area , w h e r e faul t s egmen t s cut the o rogen ic be l t of the arc. This ev idence suggests tha t Q u a t e r n a r y n o r m a l faul t ing may be r e l a t ed to a new r i f t ing p rocess r a t h e r than to the s u b d u c t i o n - r e l a t e d ex tens ion c o n n e c t e d with the u n d e r p l a t i n g of the I o n i a n d o m a i n b e n e a t h the Ca l ab r i an are. This hypo thes i s is also s u p p o r t e d by the ev idence that , at p resen t , in the f ron ta l zones of the arc bo th compres s iona l d e f o r m a t i o n in the Q u a t e r n a r y sed imen t s and reverse faul t ing e a r t h q u a k e s a re unknown and by the occur rence , for e a r t h q u a k e s wi th M >/5, of a se ismici ty gap b e t w e e n 50 km and 250 km, sugges t ing de tach- m e n t o f the s u b d u c t e d slab (Wor t e l and Spak- man , 1993; Wes taway , 1993).

W e the r e fo re suggest tha t the R e c e n t no rma l faul t ing cha rac t e r i z ing the C a l a b r i a n arc is re- l a t ed to a rift zone (Tor tor ic i et al., 1986) which has d e v e l o p e d since the M i d d l e P le i s tocene , a f te r the end of the subduc t ion o f the Ion ian domain . Ac t ive rif t ing, r eac t iva t ing the o lde r subduc t ion- r e l a t ed ex tens iona l b o r d e r faul ts of the La t e P l i o c e n e - E a r l y P le i s tocene s e d i m e n t a r y bas ins of the inner side of the arc, ex tends sou thwards

a long the Ion i an side of the Sicily. I t might also be r e spons ib le for the i n t r ap l a t e vo lcanism of the e a s t e r n m o s t is lands (S t rombol i , Sal ina, L ipa r i and Vu lcano) of the A e o l i a n a r ch ipe l ago and of the Mt. E t n a volcano.

Acknowledgments

The au thors wish to t hank R. B t i rgmann and R. C a p u t o for the i r cr i t ical and cons t ruc t ive com- men t s which improved many aspec ts of the manuscr ip t . This s tudy was f inancia l ly s u p p o r t e d by the C N R (grant N. 93.01040.05).

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Recent and active tectonics in the Calabrian arc (Southern Italy)

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