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Volcanism and mantle–crust evolution: The Etna case
Earth and Planetary Science Letters 241 (2006) 831 – 843 www.elsevier.com/locate/epsl
Volcanism and mantle–crust evolution: The Etna case Giuseppe Patane` a,1, Santo La Delfa a,b,*, Jean-Claude Tanguy a,c,2 a
b
Dipartimento di Scienze Geologiche, University of Catania, Italy Osservatorio Meteorologico Geodinamico ed Ambientale (O.Me.G.A.), Acireale, Italy c Institut de Physique du Globe de Paris, Observatoire de St. Maur, France
Received 28 April 2005; received in revised form 27 October 2005; accepted 28 October 2005 Available online 15 December 2005 Editor: R.D. van der Hilst
Abstract Mount Etna is located in a particular region of convergence of African and Eurasian plates where intense post-collisional tectonics caused considerable uplift. However we present arguments supporting the hypothesis that volcanism and associated seismic activity would result from a local mantle uprise leading to a bhorstQ, probably linked to a deep-rooted hot spot. It ensued deformation and fracturing of the overlying crust with emission of aphyric tholeiitic basalts directly from their mantle source, and subsequent development of a bdeep reservoirQ (or complex of intrusions) at the top of a mantle diapir near 30 km depth. This is advocated by the appearance of porphyritic alkaline lavas whose mineral equilibria and differentiation processes are consistent with an 8–10 kbar pressure, and by the development of central volcanoes. The horst itself appears to have begun in the SW sector of the present volcanic area. Its uplift was greater westward, as seen from the trend of the terraces along the Simeto river, and became later obvious toward the SE. These differential movements produced fractures and faults which are to day evident in the southern area of Mt Etna. The growth of the horst then proceeded in a NE direction, following the regional tectonic lines and with a greater intensity along the side facing SE, crossed by the regional NNW–SSE line (Aeolian–Maltese escarpment). The seismicity and ground deformation registered over the last twenty years support the proposed model. Earthquakes are unfrequent in the lower southern and western areas of the volcano, whereas they are numerous and stronger to the north-east, in the summit area above 1600 m a.s.l., and in the eastern sector along the NW–SE faults and fractures. Finally, a digital elevation model recently published reveals the existence of two tectonic domains. The first one is associated with the horst and contains prevalently NE–SW oriented faults, whereas the second is mainly linked to regional tectonics with NNW–SSE and NW–SE faults and fractures. D 2005 Elsevier B.V. All rights reserved. Keywords: Mt. Etna; morphology; volcanism; tectonic; earthquake
1. Introduction
* Corresponding author. Dipartimento di Scienze Geologiche, Universita` di Catania, Corso Italia 55, 95129 Catania, Italy. E-mail addresses:[email protected] (G. Patane`), [email protected] (S. La Delfa), [email protected] (J.-C. Tanguy). 1 Tel./fax: +39 0957195707. 2 Tel.: +33 145114180. 0012-821X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2005.10.039
Active volcanoes are geodynamically correlated to the behaviour of the underlying mantle and the geotectonic evolution of the crust. Conversely, the gradual development of sometimes huge volcanic edifices can significantly alter the geodynamics of the region. Thus, Mount Etna during its growth and through accumulation of volcanic products has modified the tectonics of eastern Sicily and has had continual repercussions
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Fig. 1. Geological sketch of eastern Sicily (modified after [1,15]): (1) main tectonic lineaments; (2) main faults. The volcanic areas of Etna and Hyblean Mountains are indicated.
which changed the morphology of the area [1–3]. In present times eruptions may result from gravitational sliding of the unbuttressed east flank towards the Ionian sea, in relation to forceful injection of dikes [4–6], as it occurred for the major event taking place in October 2002 [7–9]. These interactions between magmatism and tectonics are also evident from the unusually high level of seismicity recorded in the Etna area [10–12]. In this paper we analyse the morphological framework and the current tectonics and seismicity of Mount Etna in order to present a model of the volcano structure and evolution over time. For this purpose the volcanological and morphostructural data available in the literature [13] are reevaluatued, as well as the Digital Elevation Model from Favalli et al. [14], and dating results obtained through various methods. 2. Geological setting of Mount Etna Mount Etna is a relatively young volcano which developed during the last 500 kyr between the southern boundary of the Calabro–Peloritan Arc and the northern boundary of the Hyblean foreland [1,15,16]. This is an area of roughly south–north convergence of the African and Eurasian plates, although regions of local extension
oriented E–W may have developed, which are believed responsible for the tholeiitic and alkaline volcanism of the Hyblean Mountains and Etna itself [17]. Furthermore, an important system of faults is obvious on the eastern shore of Sicily, that extends practically uninterrupted from Malta to the Aeolian Islands, through the Hyblean and Etna volcanic regions (Fig. 1). This Maltese–Hyblean–Aeolian fault zone is to day considered as a major discontinuity between the African plate and the Ionian oceanic microplate [18] and could have contributed to the feeding of the Etnean volcanism through an asthenospheric window [19]. During an early stage (500–250 kyr circa) fissure emissions of tholeiitic basalts occurred. In a second stage, however, magma composition gradually shifted to alkaline trachybasalts, which are by far the most abundant Etnean products. Several stratovolcanoes grew and overlapped, sometimes with explosive eruptions of trachyandesites and trachytes. It must be pointed out that these lavas are also commonly called hawaiites–mugearites–benmoreites, although they were shown to be somewhat different [20]. Mount Etna presently reaches 3314 m elevation (last measured in 2002 by J.B. Murray, pers. comm.) and extends over an area of about 1200 km2. The height is somewhat misleading, however, as the sedimentary basement reaches as much as 1000 m above sea level (a.s.l.) on the north-western border of the volcano and about 700 m in the north-eastern sector. Beneath the volcanic pile, sedimentary materials could reach an even greater height (about 1300 m a.s.l.) as shown by Ogniben [21]. According to the same author, the maximum thickness of volcanic products should thus be in the order of only 2000 m, although the sedimentary base presents a general dip from the north-west to the south-east and lies below sea level on the south-east side. This has significant impact on the global volume of the volcano, estimated from these data to be only 350–375 km3 [22,23]. An explanation for a high sedimentary basement is provided by a general uplift of Sicily during recent geological times (Plio–Pleistocene), in association with intense post-collisional dynamics [15,24]. In fact the northern coastal chain (Peloritan Mountains, Nebrodes and Madonie) was already partly uplifted before the eruptions of Mt. Etna, which began at the end of Pleistocene as evidenced by volcanic products overlying the emergent Quaternary blue clays [25,26]. The uplift proceeded with decreasing intensity from the north to the south, as can be seen through the height of the marine terraces bordering the eastern shore of the island, being more marked to the north. Moreover, the covering of lavas protected the Quaternary sediments from erosion.
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Fig. 2. Structural framework of Mount Etna. A domain (a) with prevalent NE–SW structures, should be distinguished (white dots line) from a domain (b) where NW–SE and NNW–SSE structures are predominant.
A combination of these phenomena may explain the presence of sediments at a high level and their absence at corresponding heights in areas opposite the volcano, as well as the fact that coeval sediments located to the west and south of Etna are found at lower altitudes. However a local upwelling of the mantle, to which volcanism should be linked, might at least be partly responsible for the crustal uplift in the Etna region. Studies based on seismic refraction profiles [10], show evidence for two distinct low-velocity layers below 15 km depth. Sharp et al. [27] suggest a low-velocity layer at 16–24 km depth, interpreted as a partly molten portion of the uppermost mantle. Laigle et al. [28] by using seismic tomography suggest mantle upwarp and magma ponding under the crust. Chiarabba et al. [29] inverting P-wave arrival times from local and regional earthquakes, found a Vp anomaly at a depth of 22–38 km beneath Mt. Etna. According to a detailed petrological study [22,30], this region corresponds to accumulation of magma at about the same depth during the uprise of a mantle diapir. This view is advocated by evidence of differentiation from primary basalt to trachybasalt (or hawaiite), involving mainly clinopyroxene at an 8–10 kbar pressure (i.e., 25–30 km depth). In fact trachybasalts are by far the most common lavas on Etna and their abundance requires ex-
tensive fractionation (640%) of the primary melts, which almost never erupted (see details in [22]). Further geochemical investigations [31] show that as much as 150–300 km3 of magma could be present within a broader zone of nearly solid residual mantle, probably the elliptical-shaped body of NNE–SSW axis found by Sharp et al. [27], and interpreted in terms of the overall plumbing of the volcano [32]. We will show in the following sections that mantle upwelling formed a horst-shaped uplift responsible for fracturing of the crust, where magma penetrated first into the deeper levels and later at shallower depth. This horst would have mainly controlled volcanic and seismic activity in the Etna area. 3. Structural framework and volcanological evolution As shown on Fig. 1, Mount Etna is a region of convergence of regional faults or tectonic lineaments oriented SSW–NNE (Fiumefreddo–Messina line, Scicli–Ragusa–Giarratana–Mt Lauro line, Acate–Caltagirone–Pontebarca line) and SSE–NNW (Hyblean– Maltese scarp already mentioned, and its Giardini–Tindari prolongation). These fault systems cut across the Sicilian–Maghrebides thrust system, made up of
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Fig. 3. Sketch map showing the succession of eruptive centres identified in the Etna area (modified after [37]): MP=Tardaria-Monte Po; C=Calanna; SA=Sant’Alfio; T1=Trifoglietto 1; T2Z=Trifoglietto 2Zoccolaro; VB=Vavalaci-Belvedere; SGP=Serra Giannicola Piccola; MG=Mongibello. The projection on the surface (1) of the low velocity zone found by [27] is shown, as well as the contour (shaded area 2) of the presumed primitive shield volcano.
folds and faults oriented in an E–W direction [33], and the Gela fault. They are associated with faults oriented NW–SE and NE–SW which have considerable morphotectonic impact along the eastern coast of Sicily [1,34]. The south-western border of the volcanic edifice is cut by morphologically obvious faults oriented NE–SW (Biancavilla-S.M. Licodia and Ragalna faults, Fig. 2). However, there are also hidden NW–SE faults, less evident because they involve Quaternary clays covered by flood plain terraces or buried below lava flows. The NW–SE Fogliarino and the NE–SW Piedimonte faults form the northern border of the volcanic area. This sector includes volcano–tectonic structures known as bTimpe della NacaQ, oriented SW–NE, and the bNE riftQ which involves a number of eruptive fractures, vents and cinder cones. The NE rift is connected to the Fiumefreddo fault through the Pernicana fault, a major W–E dislocation cutting the horst to the north (Figs. 2 and 10). The lower eastern and south-eastern sectors are affected by faults with a NW–SE trend (Mascalucia, Trecastagni, Santa Tecla, Pozzillo faults), and a NNW–SSE trend (San Leonardello, Moscarello fault systems) which represent the northern end of the Hyblean–Maltese scarp (Figs. 1 and 2).
On the whole two tectonic domains are identified, probably with different geodynamic styles. Domain baQ is characterised by faults and fractures with a prevalent NE–SW direction, while in domain bbQ fractures are mostly oriented NW–SE. For simplicity reasons, Fig. 2 only contains those structural discontinuities which show evident morphological expression on the surface, and/or have frequently been involved in epicentres of recent earthquakes [11]. Regarding the volcanological evolution [4,35–37], it seems that a rather long bpre-etneanQ period (500 to 200 kyr circa, [16]) was characterised by scarce fissure eruptions from which tholeiitic basalts partly flooded the area, though at long intervals [22,30]. Exposures of these early volcanics occur on the lower south-eastern slopes (Acicastello, Aci Trezza) and to the south-west (Adrano Biancavilla cliffs) where they are not covered by younger lavas. Towards the end of the pre-etnean period, magma compositions gradually shifted to alkali basalts, and then to differentiated alkaline series, mostly trachybasalts. The differentiation processes were demonstrated to occur at 25–30 km depth (see above), showing evidence for the development of a deep magma reservoir (without excluding possible melt accumulation at shallower depth into a complex system of intrusions or even small chambers, see [22]). Volcanism then became more focused and a shield volcano might have been formed (Fig. 3, from [35]), or at least a volcanic edifice whose surface shape strikingly reflects the NE–SW orientation of Sharp’s magma reservoir [27]. From 200 kyr to the present, volcanic activity built different edifices which can be identified as the Tardaria-Monte Po (MP), the Calanna (C), the Trifoglietto 1 (T1), the Trifoglietto 2-Zoccolaro (T2Z), the Vavalaci-Belvedere (VB), the Serra Giannicola Piccola (SGP) and the Mongibello (MG) units [1,35]. These various volcanic structures resulted from central activity that moved roughly westward with time, although with some trends toward NE–SW alignments (Fig. 3). Thus there are in chronological order the MP (~220 to 100 kyr), C (100–80 kyr), T1 and T2Z (80–55 kyr), VB and SGP (55–30 kyr), and MG (~30 kyr to the present) [1,4,16,35,36]. An additional center, that of Sant’Alfio (SA, Fig. 3) was also evidenced [38] and could have been active between 170 and 80 kyr [34]. This volcano developed almost simultaneously with the TMP and C, and therefore may be linked to these. In conclusion to the volcanological evolution, volcanism in the Etna area suggests a general movement from ESE to WNW of various centers broadly aligned in NE–SW directions. These alignments may be con-
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Fig. 4. River terrace outline between Adrano and Misterbianco (see Fig. 2), modified after [41,42]. F=faults; LF=lava flows.
sidered as resulting from preferential fracture lines which made magma uprising easier. 4. River terraces in the Alcantara and Simeto valleys: interaction between volcanic and alluvial sequences The morphology of the two main valleys around Mt Etna, i.e., those of the Alcantara and Simeto rivers (Fig. 2), was thoroughly studied by Cristofolini [39], Kieffer [25,26], Duncan [40], and Chester and Duncan [41,42]). Stratigraphical interpretations by these authors show that the long time interval (about 280 kyr) which elapsed between early tholeiitic volcanism and the appearence of the first alkaline lavas is characterised by considerable uplift (see also [43]). The profile of Fig. 4 indicates, moreover, that the uplift of sedimentary layers within the various terraces is greater westwards, in the Adrano area. The continuity of the oldest terrace (7th order), on which tholeiitic lavas are emplaced, is interrupted between Biancavilla and Santa Maria di Licodia, and more frequently further to the south-east (Fig. 2). These interruptions suggest very active tectonic disturbances at least until the trachybasaltic stage of volcanism. From about 200 kyr onwards tectonic activity between Adrano and S.M. Licodia decreased, as the trend of the 4th order terrace (on which trachybasaltic lavas are emplaced) and the lower ones is much less frequently interrupted. Between S.M. Licodia and
Paterno`, however, a displacement of the 4th order terrace is observed, that did not affect the lower terraces formed over the last 20 kyr [42]. It may be concluded, therefore, that tectonic activity continued somewhat during the development of the ancient alkaline centres and a little later. Tectonic displacements are linked to NE–SW faults and lava flows from the younger Mongibello channelled in the same direction [13], suggesting that buried morphotectonic depressions may be present, similarly oriented. These might have originated during the paroxysmal phase where the uplift rate varied from 1.2 mm/yr in the Adrano area to less than 1 mm/yr toward tha south-east, between Motta Sant’Anastasia and Misterbianco [44]. In a study of the terraces to the north-west of Mt Etna, Chester and Duncan [42] conclude that they have a different origin and that the oldest terrace, dated about 20 kyr BP, was formed in a calm period in which alluvial deposits prevailed. Moreover, the beginning of recent Mongibello (14 kyr BP [45]) would have coincided with the diversion of the Flascio river, previously a tributary of Simeto, towards the present course of Alcantara. The formation of the upper part of the Alcantara course is therefore very recent and probably began as the result of an uplift in the area, the rate of which increased with creation of the present-day eruptive axis of the volcano. Further evidence for this increased uplift are the deep incisions in the lava sequence along the whole course of Alcantara.
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Fig. 5. Seismic array used in the present work: Catania University analogical (circles) and digital (stars), Italy; IRIGM-CNRS Grenoble, France (squares); Observatory of Acireale, Italy (triangles).
In the north-eastern sector and towards the Ionian coast, tectonic activity was still more intense and appears to have continued over a longer period of time. According to Monaco et al. [34], the uplift rate in the region of the Piedimonte fault (Fig. 2) is 1.7 and between 1.1–1.2 mm/yr for the last 35 and 500 kyr, respectively, and similar values were found for the period 1200–600 kyr [24]. However, these latter authors indicate considerably less uplift (0.4–0.6 mm/ yr) for the southern part of the Etna region. All these observations, therefore, lead us to conclude that the northern and the southern sectors of the volcano underwent substantially different dynamics. In the roughly south-half part, uplift proceeded from NW to SE and reached its maximum near the northwest. In the northhalf, uplift was greater in the NE. 5. Seismological and ground deformation data The seismic activity recently recorded is consistent with the overall tectonic setting described above [46–49]. Earthquakes are mostly associated with faults and fractures which form distinctive morphological features on the lower slopes, though sometimes buried by ancient lava flows, particularly on the western part of the volcano [13]. The faults more frequently involved are oriented in the directions NE–SW, NNW– SSE, and NW–SE. They usually become active several
months before the start of a summit (or central) eruption. Seismic swarms with hundreds or thousands of ML b 4 earthquakes may occur a few days or hours before opening of flank eruptive fissures, as examplified by the 1983 eruption [50] or those of 1989 [51], 1991–93 [52] 2001 and 2002 [53–55]. However some seismic crises may take place immediately after the end of a summit eruption, as for instance in the case of the 1984 SE Crater activity [56], and other earthquakes are best explained by considering a horst-shaped structure. Fig. 5 shows the distribution of our seismic stations that remained the same from 1983 to 1988. Earthquakes (recorded in at least 10 stations) that occurred in 1986 have a suitable location with respect to the geometry of the network used (ERH, ERZ b 2.5 km, RMS b 0.3 s, GAP b 1808) and are thought to be mainly associated to movements of the presumed horst. Most of the epicentres are illustrated in Fig. 6. They are essentially located on the area of the horst, and several of them took place on its edges facing SE and NW (black crosses in Fig. 6). They are distributed along the SW–NE tectonic line (inside the white line) which involves in particular the Ragalna fault (Figs. 2 and 5), the eastern edge of the Valle del Bove, the Piedimonte fault and finally the Fiumefreddo fault (Fig. 5). Macroseisms occurring in the period 1974–1991 [11] and in 1998 [48] are also reported on Fig. 6 with their epicentral areas. Some of them (1, 2, 3, 8) are located on
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Fig. 6. Epicentral distribution of seismic swarms in 1986 (see text) and mesoseismic areas of main earthquakes occurring between 1974 and 1998 [11,48]. 1 = 11/02/84 (Io = 7); 2 = 14/02/1974 (6); 3 = 21/02/1974 (5); 4 = 27/03/83 (4–5); 5 = 15/10/83 (4–5); 6 = 03/03/84 (4); 7 = 15/04/84 (7); 8 = 03/ 11/86 (6); 9 = 19/06/88 (5); 10 = 20/09/91 (5); 11 = 10/01/98 (4–5). Black crosses indicate earthquakes linked to the horst.
Fig. 7. Seismic swarms preceding the 2001 (red squares; courtesy of Domenico Patane`, INGV staff) and the 2002 eruptions (yellow circles [58]). Earthquakes from the period 2003–2005, are shown by crosses (Observatory of Acireale).
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Fig. 8. Map of the main Etna structures and epicentral distribution of the year 2001–2002 [57]. Focal plane solutions of earthquakes 1,3,4,5,6,8 are reported. Inset shows the INGV seismic array used.
the south-eastern and eastern border of the horst, others near its SW (6, 7, 10) or NE ends (9). The mesoseismic axes linked to the direction of the seismogenetic faults follow the tectonic lineaments shown on Fig. 2 and the same considerations may be applied to the mesoseismic areas of earthquakes n. 4, 5, 11 (these are aligned along the S.M. Licodia and Biancavilla fault). By contrast, Fig. 6 shows that the side of the horst facing NW reveals a high seismic level only in correspondence with the NE Rift and Naca Fault System. Fig. 7 is extracted from recent works by colleagues from the INGV, University of Catania and Omega observatory, which support our preceding observations. The seismic swarms preceding large eruptions in 2001 and 2002 [57,58] exhibit epicenters clustered along the NW border of the horst and near its NE end. These eruptions were fed by new basaltic magma coming from more than 30 km depth [59,60] and involved most of the tectonic structures in the lithosphere.
Fig. 7 also show the earthquakes occurring in 2003– 2005, which are for a large part located along the SE border of the horst and the NW–SE structural trend (e.g., S. Tecla fault, S. Leonardello, Pozzillo and Moscarello fault systems, see Fig. 2). Fig. 8 presents all the earthquakes occurring in 2001–2002 [57]. In this case also, the epicenters are mostly located along the structural lineaments and the limits of the horst precedently specified. Fig. 8 shows focal mechanisms for some of these earthquakes, and it may be observed that one of the focal planes is always oriented along the NE–SW trend which is particularly active in the northern and western sector of the volcano. Detailed ground deformation studies (Fig. 9) were carried out during 1992–1994 by Puglisi et al. [61]. According to these authors the results are well constrained by considering two magma bodies feeding the large 1991–93 eruption, the deepest of these was replenished during 1993–1994, causing uplift of large exten-
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Fig. 9. Contour lines of vertical displacements [from [61], modified]. Blu is for negative value, Yellow for positive ones; height variation values are in meters. Major inflation is measured inside our horst to the NE.
sion on most of the area of the horst as defined in the present work. The maximum uplift was observed in the NE part of the volcano, which is also in accordance with our own views. Both seismic and ground deformation data, therefore, lead to the conclusion that the NE part of the horst is presently more active that the SW one, which remains uncoupled because of the NW–SE structural discontinuity as explained in the present work. 6. Digital elevation model and volcano–tectonic evolution of Mount Etna The observations described above are supported by the 3D model of Mt Etna generated by digitising the contour lines of a 1 / 10 000 map produced by the Cartografia Tecnica della Regione Sicilia [14]. The Digital Elevation Model (DEM) obtained has altimetric and planimetric resolutions of 1 and 5 m, respectively, and covers an area of about 1200 km2. The large number of readings results in an optimal 3D reproduction of the structural features on medium and large scales (0.1 to several tens of km). With illumination of the DEM from the north at 458 elevation, many tectonic and volcanotectonic structural elements are brought into relief. The various ancient volcanic centres and calderas appear to overlay a large horst-like structure elongated from SW to NE, i.e., in the same direc-
tion as that of the presumed shield volcano [35] and of the body with low seismic velocity discovered by Sharp et al. [27]. The chrono-stratigraphy of the river terraces and of the sedimentary and volcanic layers support the hypothesis that formation of this morphostructural framework began some 500 kyr ago. Although most of the tectonic uplift in northern Sicily certainly resulted from large scale convergence between Africa and Europa [3,24], it cannot be precluded that the particular horst-like structure here evidenced was caused by uprising of the mantle beneath the Etna area. In fact, the evolution of the shape of this volcanic structure appears controlled by the inferred deep magma reservoir located at a depth of more than 20 km. From about 500 to 250 kyr BP, uplift of the horst created a series of fractures and faults mainly oriented NE–SW, from which tholeiitic magmas were erupted. These built a first shield volcano, however not associated with magma uprise along a single central axis. The early tholeiitic basalts are almost entirely aphyric, except for a few olivine crystals [20,37], indicating rapid ascent directly from the zone of generation. During the transition from tholeiitic to alkaline volcanism, however, large pyroxene phenocrysts reveal crystal fractionation within a probably large magma body near 30 km depth [22,30]. Further evidence for a deep reservoir is provided by the fact that central volcanoes
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Fig. 10. Present volcano–tectonic structure of Mount Etna, as explained in the text (conclusion).
began to develop at this time, fed by a unique deep magma body. At roughly the same time (300–240 kyr BP), the sedimentary formation confining with the southern border of the volcanic area underwent a large E–W anticlinal fold which deformed a wide Quaternary terrace [44]. This is a further proof that upwarping of the crust led to tangent forces strong enough to bend the young sedimentary sequence cropping out along the southern edge of the volcano [62]. The beginning of alkaline magmatism at about 220 kyr BP coincided with the upper morphostructural uplift and with the genesis of various fault systems bordering and dislocating the horst (e.g., S.M. Licodia and Ragalna faults, oriented NE–SW). Then the ancient alkaline central volcanoes developed along a major NE–SW line, near the present eastern edge of the Valle del Bove. The uplift of the horst to the north-east probably occurred at the same time, together with a general westward displacement of the axis of volcanism. Along the Piedimonte fault a 60 m offset of the recent volcanics has been measured, corresponding to a slip rate of 1.7 mm/yr over the last 35 kyr [34], a value higher than the uplift rate of the whole area (1.1 mm/yr) estimated over 500 kyr. These data support the view that during the Mongibello development a significant variation occurred in the uplift of the northern part of the volcano [42], leading to diversion of the Flascio river and perhaps to the genesis of the NE Rift. A final consideration must be added. The Santa Tecla fault, oriented NW–SE, has a vertical slip rate
of 1.8 mm/yr [63], with an offset of the ancient lava flows of 190 m over the last 100 kyr [34]. This is a very deep-rooted structure [18]. The north-western spread of this fracture zone and its junction with faults of the horst may have favoured local extension and magma uprise. Its reactivation at the end of the ancient alkaline centres, which were active from 220 to 80 kyr, may have caused the westward migration of the successive eruptive axes (T1, T2Z, VB, Fig. 4), which all operated after 80 kyr BP [16,36]. In summary, the data presented here indicate that an upwarping of the crust at a local level, due to mantle uprise, overprinted a regional uplift involving the whole north-eastern Sicily. This deformation resulted in a morphostructural high very close to a horst, which was concomitantly or subsequently intersected by faults and fractures with varied orientations (Fig. 10). 7. Conclusions The present tectonic setting and seismic activity of Mount Etna would result from a local mantle uprise leading to a bhorstQ, very probably linked to a hot spot having produced a deeper-rooted diapir [64]. It ensued deformation and fracturing of the crust overlying the rising diapir, and emission of aphyric tholeiitic basalts directly from their mantle source (about 500–250 kyr BP), although a bdeep reservoirQ developed later near 30 km depth at the top of the mantle diapir [22]. This is advocated by the appearance of porphyritic lavas whose mineral equilibria and differentiation processes are con-
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sistent with an 8–10 kbar pressure, and by the subsequent development of central volcanoes. The horst itself appears to have begun in the SW sector of the present volcanic area. Its uplift was greater westward, as seen from the trend of the terraces along the Simeto river, and became later obvious toward the SE. These differential movements produced fractures and faults which are to day evident in the southern area of Mt Etna. The growth of the horst then proceeded in a NE direction, following the regional tectonic lines and probably with a greater intensity along the side facing SE. In fact the horst splits along fractures evolving into normal faults oriented in a NW–SE direction, associated with the regional NNW– SSE line (Hyblean–Maltese escarpment). Also, the first central volcanoes composing ancient Etna developed along the eastern edge of the horst in a northward direction, at the intersection of structures belonging to the two fault systems NE–SW and NW–SE. Subsequently, the horst uplift continued in a north-easterly direction, whereas the axis of volcanic activity migrated westward (Trifoglietto and Mongibello). The seismicity registered over the last thirty years is consistent with the proposed model. Earthquakes are unfrequent in the lower southern and western areas of the volcano, whereas they are numerous and stronger to the north-east, in the summit area above 1600 m a.s.l., and in the eastern sector along the NW–SE faults and fractures. The overall tensional tectonics that have operated in the region have been shown to change very quickly and control magma movements [56,58]. Finally, the Digital Elevation Model reveals the existence of two tectonic domains. The first one is associated with the horst and contains prevalently NE–SW oriented faults, whereas the second is mainly linked to regional tectonics with NW–SE faults and fractures. This distinction must be taken into account because of considerable influence on the methodology with which Etna’s seismicity should be studied. Acknowledgements We are indebted to Domenico Patane` who permitted to use the INGV data on the 2001 eruption, to Angus Duncan who made many helpful suggestions and corrections on an early draft of the manuscript, and to two anonymous reviewers for their constructive criticism. References [1] S. Branca, M. Coltelli, G. Groppelli, Geological evolution of Etna volcano, in Mt. Etna volcano laboratory, AGU Geophys. Monogr. 143 (2004) 49 – 63.
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[42] D.K. Chester, A.M. Duncan, The interaction of volcanic activity in quaternary times upon the evolution of the Alcantara and Simeto Rivers, Mt. Etna, Sicily, Catena 9 (1982) 319 – 342. [43] A. Rittmann, Vulkanismus und Tektonik des Aetna, Geol. Rundsch. 53 (1963) 788 – 800. [44] C. Monaco, Tettonica pleistocenica nell’area a Sud dell’Etna (Sicilia orientale), Il Quaternario 10 (1997) 393 – 398. [45] G. Kieffer, L’activite´ de l’Etna pendant les derniers 20 000 ans, C. R. Hebd. Se´ances Acad. Sci., Paris 288D (1979) 1023 – 1026. [46] G. Patane`, G. Lombardo, S. Gresta, R. Cristofolini, Modello di velocita` superficiale e determinazione dell’attenuazione dell’energia sismica nella zona di S. Alfio (versante orientale dell’Etna), in: Atti del 18 Convegno GNGTS, vol. 2, 1981, pp. 677 – 689. [47] G. Patane`, C. Centamore, S. La Delfa, Isochronal maps at Mt. Etna volcano (Italy): a simple and reliable tool for investigating large-scale heterogeneities, Ann. Geofis. 40 (1997) 1383 – 1393. [48] S. La Delfa, G. Patane`, R. Clocchiatti, J.L. Joron, J.C. Tanguy, Activity of Mount Etna preceding the February 1999 fissure eruption: inferred mechanism from seismological and geochemical data, J. Volcanol. Geotherm. Res. 105 (2001) 121 – 139. [49] D. Patane`, E. Privitera, Seismicity related to 1989 and 1991–93 Mt. Etna (Italy) eruptions: kinematic constraints by fault plane solution analysis, J. Volcanol. Geotherm. Res. 109 (2001) 77 – 98. [50] G. Patane`, S. Gresta, S. Imposa, Seismic activity preceding the 1983 eruption of Mt. Etna, Bull. Volcanol. 47 (1984) 941 – 952. [51] M. Castellano, F. Ferrucci, C. Godano, S. Imposa, G. Milano, Upwards migration of seismic foci: a forerunner of the 1989 eruption of Mt. Etna, Bull. Volcanol. 55 (1993) 346 – 351. [52] G. Patane`, A. Montalto, S. Vinciguerra, J.C. Tanguy, A model of the onset of the 1991–1993 eruption of Etna (Italy), Phys. Earth Planet. Inter. 97 (1996) 231 – 245. [53] D. Patane`, C. Chiarabba, O. Cocina, P. De Gori, M. Moretti, E. Boschi, Tomographic images and 3D earthquake locations of the seismic swarm preceding the 2001 Mt. Etna eruption: evidence for a dyke intrusion. Geophys. Res. Lett. 29 (2002) 135, 1–4. [54] S. Alparone, D. Andronico, S. Giammanco, L. Lodato, A multidisciplinary approach to detect active pathways for magma migration and eruption at Mt. Etna (Sicily, Italy) before the 2001 and 2002–2003 eruptions, J. Volcanol. Geotherm. Res. 136 (2004) 121 – 140. [55] G. Imme`, S. La Delfa, S. Lo Nigro, D. Morelli, G. Patane`, Gas radon emission related to geodynamic activity of Mt. Etna, Ann. Geophys. 48 (2005) 65 – 71. [56] S. La Delfa, G. Patane`, C. Centamore, The geodynamics of Mt. Etna volcano during and after the 1984 eruption, Ann. Geofis. 42 (1999) 545 – 563. [57] S. Gambino, A. Mostaccio, D. Patane`, L. Scarfı`, A. Ursino, High-precision locations of the microseismicity preceding the 2002–2003 Mt. Etna eruption, Geophys. Res. Lett. 31 (2004) L18604, doi:10.1029/2004GL020499. [58] C. Monaco, S. Catalano, O. Cocina, G. De Guidi, C. Ferlito, S. Gresta, C. Musumeci, L. Tortorici, Tectonic control on the eruptive dynamics at Mt. Etna volcano (Sicily) during the 2001 and 2002–2003 eruptions, J. Volcanol. Geotherm. Res. 144 (2005) 211 – 233. [59] R. Clocchiatti, M. Condomines, N. Guenot, J.C. Tanguy, Magma changes at Mt. Etna: the 2001 and 2002–2003 eruptions, Earth Planet. Sci. Lett. 226 (2004) 397 – 414. [60] D. Andronico, S. Branca, S. Calvari, M. Burton, T. Caltabiano, R.A. Corsaro, P. Del Carlo, G. Garfı`, L. Lodato, L. Miraglia, F. Mure`, M. Neri, E. Pecora, M. Pompilio, G. Salerno, L. Spampi-
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Volcanism and mantle–crust evolution: The Etna case Giuseppe Patane` a,1, Santo La Delfa a,b,*, Jean-Claude Tanguy a,c,2 a
b
Dipartimento di Scienze Geologiche, University of Catania, Italy Osservatorio Meteorologico Geodinamico ed Ambientale (O.Me.G.A.), Acireale, Italy c Institut de Physique du Globe de Paris, Observatoire de St. Maur, France
Received 28 April 2005; received in revised form 27 October 2005; accepted 28 October 2005 Available online 15 December 2005 Editor: R.D. van der Hilst
Abstract Mount Etna is located in a particular region of convergence of African and Eurasian plates where intense post-collisional tectonics caused considerable uplift. However we present arguments supporting the hypothesis that volcanism and associated seismic activity would result from a local mantle uprise leading to a bhorstQ, probably linked to a deep-rooted hot spot. It ensued deformation and fracturing of the overlying crust with emission of aphyric tholeiitic basalts directly from their mantle source, and subsequent development of a bdeep reservoirQ (or complex of intrusions) at the top of a mantle diapir near 30 km depth. This is advocated by the appearance of porphyritic alkaline lavas whose mineral equilibria and differentiation processes are consistent with an 8–10 kbar pressure, and by the development of central volcanoes. The horst itself appears to have begun in the SW sector of the present volcanic area. Its uplift was greater westward, as seen from the trend of the terraces along the Simeto river, and became later obvious toward the SE. These differential movements produced fractures and faults which are to day evident in the southern area of Mt Etna. The growth of the horst then proceeded in a NE direction, following the regional tectonic lines and with a greater intensity along the side facing SE, crossed by the regional NNW–SSE line (Aeolian–Maltese escarpment). The seismicity and ground deformation registered over the last twenty years support the proposed model. Earthquakes are unfrequent in the lower southern and western areas of the volcano, whereas they are numerous and stronger to the north-east, in the summit area above 1600 m a.s.l., and in the eastern sector along the NW–SE faults and fractures. Finally, a digital elevation model recently published reveals the existence of two tectonic domains. The first one is associated with the horst and contains prevalently NE–SW oriented faults, whereas the second is mainly linked to regional tectonics with NNW–SSE and NW–SE faults and fractures. D 2005 Elsevier B.V. All rights reserved. Keywords: Mt. Etna; morphology; volcanism; tectonic; earthquake
1. Introduction
* Corresponding author. Dipartimento di Scienze Geologiche, Universita` di Catania, Corso Italia 55, 95129 Catania, Italy. E-mail addresses:[email protected] (G. Patane`), [email protected] (S. La Delfa), [email protected] (J.-C. Tanguy). 1 Tel./fax: +39 0957195707. 2 Tel.: +33 145114180. 0012-821X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2005.10.039
Active volcanoes are geodynamically correlated to the behaviour of the underlying mantle and the geotectonic evolution of the crust. Conversely, the gradual development of sometimes huge volcanic edifices can significantly alter the geodynamics of the region. Thus, Mount Etna during its growth and through accumulation of volcanic products has modified the tectonics of eastern Sicily and has had continual repercussions
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Fig. 1. Geological sketch of eastern Sicily (modified after [1,15]): (1) main tectonic lineaments; (2) main faults. The volcanic areas of Etna and Hyblean Mountains are indicated.
which changed the morphology of the area [1–3]. In present times eruptions may result from gravitational sliding of the unbuttressed east flank towards the Ionian sea, in relation to forceful injection of dikes [4–6], as it occurred for the major event taking place in October 2002 [7–9]. These interactions between magmatism and tectonics are also evident from the unusually high level of seismicity recorded in the Etna area [10–12]. In this paper we analyse the morphological framework and the current tectonics and seismicity of Mount Etna in order to present a model of the volcano structure and evolution over time. For this purpose the volcanological and morphostructural data available in the literature [13] are reevaluatued, as well as the Digital Elevation Model from Favalli et al. [14], and dating results obtained through various methods. 2. Geological setting of Mount Etna Mount Etna is a relatively young volcano which developed during the last 500 kyr between the southern boundary of the Calabro–Peloritan Arc and the northern boundary of the Hyblean foreland [1,15,16]. This is an area of roughly south–north convergence of the African and Eurasian plates, although regions of local extension
oriented E–W may have developed, which are believed responsible for the tholeiitic and alkaline volcanism of the Hyblean Mountains and Etna itself [17]. Furthermore, an important system of faults is obvious on the eastern shore of Sicily, that extends practically uninterrupted from Malta to the Aeolian Islands, through the Hyblean and Etna volcanic regions (Fig. 1). This Maltese–Hyblean–Aeolian fault zone is to day considered as a major discontinuity between the African plate and the Ionian oceanic microplate [18] and could have contributed to the feeding of the Etnean volcanism through an asthenospheric window [19]. During an early stage (500–250 kyr circa) fissure emissions of tholeiitic basalts occurred. In a second stage, however, magma composition gradually shifted to alkaline trachybasalts, which are by far the most abundant Etnean products. Several stratovolcanoes grew and overlapped, sometimes with explosive eruptions of trachyandesites and trachytes. It must be pointed out that these lavas are also commonly called hawaiites–mugearites–benmoreites, although they were shown to be somewhat different [20]. Mount Etna presently reaches 3314 m elevation (last measured in 2002 by J.B. Murray, pers. comm.) and extends over an area of about 1200 km2. The height is somewhat misleading, however, as the sedimentary basement reaches as much as 1000 m above sea level (a.s.l.) on the north-western border of the volcano and about 700 m in the north-eastern sector. Beneath the volcanic pile, sedimentary materials could reach an even greater height (about 1300 m a.s.l.) as shown by Ogniben [21]. According to the same author, the maximum thickness of volcanic products should thus be in the order of only 2000 m, although the sedimentary base presents a general dip from the north-west to the south-east and lies below sea level on the south-east side. This has significant impact on the global volume of the volcano, estimated from these data to be only 350–375 km3 [22,23]. An explanation for a high sedimentary basement is provided by a general uplift of Sicily during recent geological times (Plio–Pleistocene), in association with intense post-collisional dynamics [15,24]. In fact the northern coastal chain (Peloritan Mountains, Nebrodes and Madonie) was already partly uplifted before the eruptions of Mt. Etna, which began at the end of Pleistocene as evidenced by volcanic products overlying the emergent Quaternary blue clays [25,26]. The uplift proceeded with decreasing intensity from the north to the south, as can be seen through the height of the marine terraces bordering the eastern shore of the island, being more marked to the north. Moreover, the covering of lavas protected the Quaternary sediments from erosion.
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Fig. 2. Structural framework of Mount Etna. A domain (a) with prevalent NE–SW structures, should be distinguished (white dots line) from a domain (b) where NW–SE and NNW–SSE structures are predominant.
A combination of these phenomena may explain the presence of sediments at a high level and their absence at corresponding heights in areas opposite the volcano, as well as the fact that coeval sediments located to the west and south of Etna are found at lower altitudes. However a local upwelling of the mantle, to which volcanism should be linked, might at least be partly responsible for the crustal uplift in the Etna region. Studies based on seismic refraction profiles [10], show evidence for two distinct low-velocity layers below 15 km depth. Sharp et al. [27] suggest a low-velocity layer at 16–24 km depth, interpreted as a partly molten portion of the uppermost mantle. Laigle et al. [28] by using seismic tomography suggest mantle upwarp and magma ponding under the crust. Chiarabba et al. [29] inverting P-wave arrival times from local and regional earthquakes, found a Vp anomaly at a depth of 22–38 km beneath Mt. Etna. According to a detailed petrological study [22,30], this region corresponds to accumulation of magma at about the same depth during the uprise of a mantle diapir. This view is advocated by evidence of differentiation from primary basalt to trachybasalt (or hawaiite), involving mainly clinopyroxene at an 8–10 kbar pressure (i.e., 25–30 km depth). In fact trachybasalts are by far the most common lavas on Etna and their abundance requires ex-
tensive fractionation (640%) of the primary melts, which almost never erupted (see details in [22]). Further geochemical investigations [31] show that as much as 150–300 km3 of magma could be present within a broader zone of nearly solid residual mantle, probably the elliptical-shaped body of NNE–SSW axis found by Sharp et al. [27], and interpreted in terms of the overall plumbing of the volcano [32]. We will show in the following sections that mantle upwelling formed a horst-shaped uplift responsible for fracturing of the crust, where magma penetrated first into the deeper levels and later at shallower depth. This horst would have mainly controlled volcanic and seismic activity in the Etna area. 3. Structural framework and volcanological evolution As shown on Fig. 1, Mount Etna is a region of convergence of regional faults or tectonic lineaments oriented SSW–NNE (Fiumefreddo–Messina line, Scicli–Ragusa–Giarratana–Mt Lauro line, Acate–Caltagirone–Pontebarca line) and SSE–NNW (Hyblean– Maltese scarp already mentioned, and its Giardini–Tindari prolongation). These fault systems cut across the Sicilian–Maghrebides thrust system, made up of
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Fig. 3. Sketch map showing the succession of eruptive centres identified in the Etna area (modified after [37]): MP=Tardaria-Monte Po; C=Calanna; SA=Sant’Alfio; T1=Trifoglietto 1; T2Z=Trifoglietto 2Zoccolaro; VB=Vavalaci-Belvedere; SGP=Serra Giannicola Piccola; MG=Mongibello. The projection on the surface (1) of the low velocity zone found by [27] is shown, as well as the contour (shaded area 2) of the presumed primitive shield volcano.
folds and faults oriented in an E–W direction [33], and the Gela fault. They are associated with faults oriented NW–SE and NE–SW which have considerable morphotectonic impact along the eastern coast of Sicily [1,34]. The south-western border of the volcanic edifice is cut by morphologically obvious faults oriented NE–SW (Biancavilla-S.M. Licodia and Ragalna faults, Fig. 2). However, there are also hidden NW–SE faults, less evident because they involve Quaternary clays covered by flood plain terraces or buried below lava flows. The NW–SE Fogliarino and the NE–SW Piedimonte faults form the northern border of the volcanic area. This sector includes volcano–tectonic structures known as bTimpe della NacaQ, oriented SW–NE, and the bNE riftQ which involves a number of eruptive fractures, vents and cinder cones. The NE rift is connected to the Fiumefreddo fault through the Pernicana fault, a major W–E dislocation cutting the horst to the north (Figs. 2 and 10). The lower eastern and south-eastern sectors are affected by faults with a NW–SE trend (Mascalucia, Trecastagni, Santa Tecla, Pozzillo faults), and a NNW–SSE trend (San Leonardello, Moscarello fault systems) which represent the northern end of the Hyblean–Maltese scarp (Figs. 1 and 2).
On the whole two tectonic domains are identified, probably with different geodynamic styles. Domain baQ is characterised by faults and fractures with a prevalent NE–SW direction, while in domain bbQ fractures are mostly oriented NW–SE. For simplicity reasons, Fig. 2 only contains those structural discontinuities which show evident morphological expression on the surface, and/or have frequently been involved in epicentres of recent earthquakes [11]. Regarding the volcanological evolution [4,35–37], it seems that a rather long bpre-etneanQ period (500 to 200 kyr circa, [16]) was characterised by scarce fissure eruptions from which tholeiitic basalts partly flooded the area, though at long intervals [22,30]. Exposures of these early volcanics occur on the lower south-eastern slopes (Acicastello, Aci Trezza) and to the south-west (Adrano Biancavilla cliffs) where they are not covered by younger lavas. Towards the end of the pre-etnean period, magma compositions gradually shifted to alkali basalts, and then to differentiated alkaline series, mostly trachybasalts. The differentiation processes were demonstrated to occur at 25–30 km depth (see above), showing evidence for the development of a deep magma reservoir (without excluding possible melt accumulation at shallower depth into a complex system of intrusions or even small chambers, see [22]). Volcanism then became more focused and a shield volcano might have been formed (Fig. 3, from [35]), or at least a volcanic edifice whose surface shape strikingly reflects the NE–SW orientation of Sharp’s magma reservoir [27]. From 200 kyr to the present, volcanic activity built different edifices which can be identified as the Tardaria-Monte Po (MP), the Calanna (C), the Trifoglietto 1 (T1), the Trifoglietto 2-Zoccolaro (T2Z), the Vavalaci-Belvedere (VB), the Serra Giannicola Piccola (SGP) and the Mongibello (MG) units [1,35]. These various volcanic structures resulted from central activity that moved roughly westward with time, although with some trends toward NE–SW alignments (Fig. 3). Thus there are in chronological order the MP (~220 to 100 kyr), C (100–80 kyr), T1 and T2Z (80–55 kyr), VB and SGP (55–30 kyr), and MG (~30 kyr to the present) [1,4,16,35,36]. An additional center, that of Sant’Alfio (SA, Fig. 3) was also evidenced [38] and could have been active between 170 and 80 kyr [34]. This volcano developed almost simultaneously with the TMP and C, and therefore may be linked to these. In conclusion to the volcanological evolution, volcanism in the Etna area suggests a general movement from ESE to WNW of various centers broadly aligned in NE–SW directions. These alignments may be con-
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Fig. 4. River terrace outline between Adrano and Misterbianco (see Fig. 2), modified after [41,42]. F=faults; LF=lava flows.
sidered as resulting from preferential fracture lines which made magma uprising easier. 4. River terraces in the Alcantara and Simeto valleys: interaction between volcanic and alluvial sequences The morphology of the two main valleys around Mt Etna, i.e., those of the Alcantara and Simeto rivers (Fig. 2), was thoroughly studied by Cristofolini [39], Kieffer [25,26], Duncan [40], and Chester and Duncan [41,42]). Stratigraphical interpretations by these authors show that the long time interval (about 280 kyr) which elapsed between early tholeiitic volcanism and the appearence of the first alkaline lavas is characterised by considerable uplift (see also [43]). The profile of Fig. 4 indicates, moreover, that the uplift of sedimentary layers within the various terraces is greater westwards, in the Adrano area. The continuity of the oldest terrace (7th order), on which tholeiitic lavas are emplaced, is interrupted between Biancavilla and Santa Maria di Licodia, and more frequently further to the south-east (Fig. 2). These interruptions suggest very active tectonic disturbances at least until the trachybasaltic stage of volcanism. From about 200 kyr onwards tectonic activity between Adrano and S.M. Licodia decreased, as the trend of the 4th order terrace (on which trachybasaltic lavas are emplaced) and the lower ones is much less frequently interrupted. Between S.M. Licodia and
Paterno`, however, a displacement of the 4th order terrace is observed, that did not affect the lower terraces formed over the last 20 kyr [42]. It may be concluded, therefore, that tectonic activity continued somewhat during the development of the ancient alkaline centres and a little later. Tectonic displacements are linked to NE–SW faults and lava flows from the younger Mongibello channelled in the same direction [13], suggesting that buried morphotectonic depressions may be present, similarly oriented. These might have originated during the paroxysmal phase where the uplift rate varied from 1.2 mm/yr in the Adrano area to less than 1 mm/yr toward tha south-east, between Motta Sant’Anastasia and Misterbianco [44]. In a study of the terraces to the north-west of Mt Etna, Chester and Duncan [42] conclude that they have a different origin and that the oldest terrace, dated about 20 kyr BP, was formed in a calm period in which alluvial deposits prevailed. Moreover, the beginning of recent Mongibello (14 kyr BP [45]) would have coincided with the diversion of the Flascio river, previously a tributary of Simeto, towards the present course of Alcantara. The formation of the upper part of the Alcantara course is therefore very recent and probably began as the result of an uplift in the area, the rate of which increased with creation of the present-day eruptive axis of the volcano. Further evidence for this increased uplift are the deep incisions in the lava sequence along the whole course of Alcantara.
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Fig. 5. Seismic array used in the present work: Catania University analogical (circles) and digital (stars), Italy; IRIGM-CNRS Grenoble, France (squares); Observatory of Acireale, Italy (triangles).
In the north-eastern sector and towards the Ionian coast, tectonic activity was still more intense and appears to have continued over a longer period of time. According to Monaco et al. [34], the uplift rate in the region of the Piedimonte fault (Fig. 2) is 1.7 and between 1.1–1.2 mm/yr for the last 35 and 500 kyr, respectively, and similar values were found for the period 1200–600 kyr [24]. However, these latter authors indicate considerably less uplift (0.4–0.6 mm/ yr) for the southern part of the Etna region. All these observations, therefore, lead us to conclude that the northern and the southern sectors of the volcano underwent substantially different dynamics. In the roughly south-half part, uplift proceeded from NW to SE and reached its maximum near the northwest. In the northhalf, uplift was greater in the NE. 5. Seismological and ground deformation data The seismic activity recently recorded is consistent with the overall tectonic setting described above [46–49]. Earthquakes are mostly associated with faults and fractures which form distinctive morphological features on the lower slopes, though sometimes buried by ancient lava flows, particularly on the western part of the volcano [13]. The faults more frequently involved are oriented in the directions NE–SW, NNW– SSE, and NW–SE. They usually become active several
months before the start of a summit (or central) eruption. Seismic swarms with hundreds or thousands of ML b 4 earthquakes may occur a few days or hours before opening of flank eruptive fissures, as examplified by the 1983 eruption [50] or those of 1989 [51], 1991–93 [52] 2001 and 2002 [53–55]. However some seismic crises may take place immediately after the end of a summit eruption, as for instance in the case of the 1984 SE Crater activity [56], and other earthquakes are best explained by considering a horst-shaped structure. Fig. 5 shows the distribution of our seismic stations that remained the same from 1983 to 1988. Earthquakes (recorded in at least 10 stations) that occurred in 1986 have a suitable location with respect to the geometry of the network used (ERH, ERZ b 2.5 km, RMS b 0.3 s, GAP b 1808) and are thought to be mainly associated to movements of the presumed horst. Most of the epicentres are illustrated in Fig. 6. They are essentially located on the area of the horst, and several of them took place on its edges facing SE and NW (black crosses in Fig. 6). They are distributed along the SW–NE tectonic line (inside the white line) which involves in particular the Ragalna fault (Figs. 2 and 5), the eastern edge of the Valle del Bove, the Piedimonte fault and finally the Fiumefreddo fault (Fig. 5). Macroseisms occurring in the period 1974–1991 [11] and in 1998 [48] are also reported on Fig. 6 with their epicentral areas. Some of them (1, 2, 3, 8) are located on
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Fig. 6. Epicentral distribution of seismic swarms in 1986 (see text) and mesoseismic areas of main earthquakes occurring between 1974 and 1998 [11,48]. 1 = 11/02/84 (Io = 7); 2 = 14/02/1974 (6); 3 = 21/02/1974 (5); 4 = 27/03/83 (4–5); 5 = 15/10/83 (4–5); 6 = 03/03/84 (4); 7 = 15/04/84 (7); 8 = 03/ 11/86 (6); 9 = 19/06/88 (5); 10 = 20/09/91 (5); 11 = 10/01/98 (4–5). Black crosses indicate earthquakes linked to the horst.
Fig. 7. Seismic swarms preceding the 2001 (red squares; courtesy of Domenico Patane`, INGV staff) and the 2002 eruptions (yellow circles [58]). Earthquakes from the period 2003–2005, are shown by crosses (Observatory of Acireale).
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Fig. 8. Map of the main Etna structures and epicentral distribution of the year 2001–2002 [57]. Focal plane solutions of earthquakes 1,3,4,5,6,8 are reported. Inset shows the INGV seismic array used.
the south-eastern and eastern border of the horst, others near its SW (6, 7, 10) or NE ends (9). The mesoseismic axes linked to the direction of the seismogenetic faults follow the tectonic lineaments shown on Fig. 2 and the same considerations may be applied to the mesoseismic areas of earthquakes n. 4, 5, 11 (these are aligned along the S.M. Licodia and Biancavilla fault). By contrast, Fig. 6 shows that the side of the horst facing NW reveals a high seismic level only in correspondence with the NE Rift and Naca Fault System. Fig. 7 is extracted from recent works by colleagues from the INGV, University of Catania and Omega observatory, which support our preceding observations. The seismic swarms preceding large eruptions in 2001 and 2002 [57,58] exhibit epicenters clustered along the NW border of the horst and near its NE end. These eruptions were fed by new basaltic magma coming from more than 30 km depth [59,60] and involved most of the tectonic structures in the lithosphere.
Fig. 7 also show the earthquakes occurring in 2003– 2005, which are for a large part located along the SE border of the horst and the NW–SE structural trend (e.g., S. Tecla fault, S. Leonardello, Pozzillo and Moscarello fault systems, see Fig. 2). Fig. 8 presents all the earthquakes occurring in 2001–2002 [57]. In this case also, the epicenters are mostly located along the structural lineaments and the limits of the horst precedently specified. Fig. 8 shows focal mechanisms for some of these earthquakes, and it may be observed that one of the focal planes is always oriented along the NE–SW trend which is particularly active in the northern and western sector of the volcano. Detailed ground deformation studies (Fig. 9) were carried out during 1992–1994 by Puglisi et al. [61]. According to these authors the results are well constrained by considering two magma bodies feeding the large 1991–93 eruption, the deepest of these was replenished during 1993–1994, causing uplift of large exten-
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Fig. 9. Contour lines of vertical displacements [from [61], modified]. Blu is for negative value, Yellow for positive ones; height variation values are in meters. Major inflation is measured inside our horst to the NE.
sion on most of the area of the horst as defined in the present work. The maximum uplift was observed in the NE part of the volcano, which is also in accordance with our own views. Both seismic and ground deformation data, therefore, lead to the conclusion that the NE part of the horst is presently more active that the SW one, which remains uncoupled because of the NW–SE structural discontinuity as explained in the present work. 6. Digital elevation model and volcano–tectonic evolution of Mount Etna The observations described above are supported by the 3D model of Mt Etna generated by digitising the contour lines of a 1 / 10 000 map produced by the Cartografia Tecnica della Regione Sicilia [14]. The Digital Elevation Model (DEM) obtained has altimetric and planimetric resolutions of 1 and 5 m, respectively, and covers an area of about 1200 km2. The large number of readings results in an optimal 3D reproduction of the structural features on medium and large scales (0.1 to several tens of km). With illumination of the DEM from the north at 458 elevation, many tectonic and volcanotectonic structural elements are brought into relief. The various ancient volcanic centres and calderas appear to overlay a large horst-like structure elongated from SW to NE, i.e., in the same direc-
tion as that of the presumed shield volcano [35] and of the body with low seismic velocity discovered by Sharp et al. [27]. The chrono-stratigraphy of the river terraces and of the sedimentary and volcanic layers support the hypothesis that formation of this morphostructural framework began some 500 kyr ago. Although most of the tectonic uplift in northern Sicily certainly resulted from large scale convergence between Africa and Europa [3,24], it cannot be precluded that the particular horst-like structure here evidenced was caused by uprising of the mantle beneath the Etna area. In fact, the evolution of the shape of this volcanic structure appears controlled by the inferred deep magma reservoir located at a depth of more than 20 km. From about 500 to 250 kyr BP, uplift of the horst created a series of fractures and faults mainly oriented NE–SW, from which tholeiitic magmas were erupted. These built a first shield volcano, however not associated with magma uprise along a single central axis. The early tholeiitic basalts are almost entirely aphyric, except for a few olivine crystals [20,37], indicating rapid ascent directly from the zone of generation. During the transition from tholeiitic to alkaline volcanism, however, large pyroxene phenocrysts reveal crystal fractionation within a probably large magma body near 30 km depth [22,30]. Further evidence for a deep reservoir is provided by the fact that central volcanoes
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Fig. 10. Present volcano–tectonic structure of Mount Etna, as explained in the text (conclusion).
began to develop at this time, fed by a unique deep magma body. At roughly the same time (300–240 kyr BP), the sedimentary formation confining with the southern border of the volcanic area underwent a large E–W anticlinal fold which deformed a wide Quaternary terrace [44]. This is a further proof that upwarping of the crust led to tangent forces strong enough to bend the young sedimentary sequence cropping out along the southern edge of the volcano [62]. The beginning of alkaline magmatism at about 220 kyr BP coincided with the upper morphostructural uplift and with the genesis of various fault systems bordering and dislocating the horst (e.g., S.M. Licodia and Ragalna faults, oriented NE–SW). Then the ancient alkaline central volcanoes developed along a major NE–SW line, near the present eastern edge of the Valle del Bove. The uplift of the horst to the north-east probably occurred at the same time, together with a general westward displacement of the axis of volcanism. Along the Piedimonte fault a 60 m offset of the recent volcanics has been measured, corresponding to a slip rate of 1.7 mm/yr over the last 35 kyr [34], a value higher than the uplift rate of the whole area (1.1 mm/yr) estimated over 500 kyr. These data support the view that during the Mongibello development a significant variation occurred in the uplift of the northern part of the volcano [42], leading to diversion of the Flascio river and perhaps to the genesis of the NE Rift. A final consideration must be added. The Santa Tecla fault, oriented NW–SE, has a vertical slip rate
of 1.8 mm/yr [63], with an offset of the ancient lava flows of 190 m over the last 100 kyr [34]. This is a very deep-rooted structure [18]. The north-western spread of this fracture zone and its junction with faults of the horst may have favoured local extension and magma uprise. Its reactivation at the end of the ancient alkaline centres, which were active from 220 to 80 kyr, may have caused the westward migration of the successive eruptive axes (T1, T2Z, VB, Fig. 4), which all operated after 80 kyr BP [16,36]. In summary, the data presented here indicate that an upwarping of the crust at a local level, due to mantle uprise, overprinted a regional uplift involving the whole north-eastern Sicily. This deformation resulted in a morphostructural high very close to a horst, which was concomitantly or subsequently intersected by faults and fractures with varied orientations (Fig. 10). 7. Conclusions The present tectonic setting and seismic activity of Mount Etna would result from a local mantle uprise leading to a bhorstQ, very probably linked to a hot spot having produced a deeper-rooted diapir [64]. It ensued deformation and fracturing of the crust overlying the rising diapir, and emission of aphyric tholeiitic basalts directly from their mantle source (about 500–250 kyr BP), although a bdeep reservoirQ developed later near 30 km depth at the top of the mantle diapir [22]. This is advocated by the appearance of porphyritic lavas whose mineral equilibria and differentiation processes are con-
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sistent with an 8–10 kbar pressure, and by the subsequent development of central volcanoes. The horst itself appears to have begun in the SW sector of the present volcanic area. Its uplift was greater westward, as seen from the trend of the terraces along the Simeto river, and became later obvious toward the SE. These differential movements produced fractures and faults which are to day evident in the southern area of Mt Etna. The growth of the horst then proceeded in a NE direction, following the regional tectonic lines and probably with a greater intensity along the side facing SE. In fact the horst splits along fractures evolving into normal faults oriented in a NW–SE direction, associated with the regional NNW– SSE line (Hyblean–Maltese escarpment). Also, the first central volcanoes composing ancient Etna developed along the eastern edge of the horst in a northward direction, at the intersection of structures belonging to the two fault systems NE–SW and NW–SE. Subsequently, the horst uplift continued in a north-easterly direction, whereas the axis of volcanic activity migrated westward (Trifoglietto and Mongibello). The seismicity registered over the last thirty years is consistent with the proposed model. Earthquakes are unfrequent in the lower southern and western areas of the volcano, whereas they are numerous and stronger to the north-east, in the summit area above 1600 m a.s.l., and in the eastern sector along the NW–SE faults and fractures. The overall tensional tectonics that have operated in the region have been shown to change very quickly and control magma movements [56,58]. Finally, the Digital Elevation Model reveals the existence of two tectonic domains. The first one is associated with the horst and contains prevalently NE–SW oriented faults, whereas the second is mainly linked to regional tectonics with NW–SE faults and fractures. This distinction must be taken into account because of considerable influence on the methodology with which Etna’s seismicity should be studied. Acknowledgements We are indebted to Domenico Patane` who permitted to use the INGV data on the 2001 eruption, to Angus Duncan who made many helpful suggestions and corrections on an early draft of the manuscript, and to two anonymous reviewers for their constructive criticism. References [1] S. Branca, M. Coltelli, G. Groppelli, Geological evolution of Etna volcano, in Mt. Etna volcano laboratory, AGU Geophys. Monogr. 143 (2004) 49 – 63.
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