Holocene vertical deformation along the coastal sector of Mt. Etna volcano (eastern Sicily, Italy): Implications on the time–space constrains of the volcano lateral sliding

Journal of Geodynamics 82 (2014) 194–203

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Holocene vertical deformation along the coastal sector of Mt. Etna volcano (eastern Sicily, Italy): Implications on the time–space constrains of the volcano lateral sliding Stefano Branca a,∗ , Giorgio De Guidi b , Gianni Lanzafame a , Carmelo Monaco b a b

Istituto Nazionale di Geofisica e Vulcanologia-Osservatorio Etneo, P.zza Roma 2, Catania, Italy Dipartimento di Scienze Biologiche, Geologiche e Ambientali, Sezione di Scienze della Terra, Università degli Studi di Catania, Corso Italia 55, Catania, Italy

a r t i c l e

i n f o

Article history: Received 29 January 2014 Received in revised form 14 July 2014 Accepted 15 July 2014 Available online 29 July 2014 Keywords: Mt. Etna Holocene Paleo-shoreline Coastal uplift Flank instability

a b s t r a c t A detailed survey of morphological and biological markers of paleo-shorelines has been carried out along the coastal sector of Mt. Etna volcano (eastern Sicily, Italy), in order to better define causes and timing of vertical deformation. We have mapped markers of raised Holocene shorelines, which are represented by beach rocks, wave-cut platforms, balanid, vermetid and algal rims. The timing of coastal uplift has been determined by radiocarbon dating of shells collected from the raised paleo-shorelines and, to correctly assess the total amount of tectonic uplift of the coast during the Late Holocene, we have compared the elevation-age data of sampled shells to the local curve of Holocene sea-level rise. Taking into account the nominal elevation of the associated paleo-shorelines, an uplift rate of 2.5–3.0 mm/year has been estimated for the last 6–7 ka. This general process of uplifting is only locally interrupted by subsidence related to flank sliding of the volcanic edifice, measured at docks and other manmade structures, and by acceleration along the hinge of an active anticline and at the footwall of an active fault. Based on this new data we suggest more precise time–space constraints for the dynamics of the lower eastern flank of Mt. Etna volcano. © 2014 Elsevier Ltd. All rights reserved.

1. Introduction Mount Etna is located along the Ionian coast of Sicily (see inset in Fig. 1) on the footwall of a Late Quaternary oblique fault system that extends between the Aeolian Islands and the eastern Sicily offshore (Lanzafame and Bousquet, 1997; Palano et al., 2012 and reference therein). Many examples of sea-level indicators, namely marine notches and dated shells found above the present sea level, show that differential tectonic and volcano-tectonic uplift took place during the Holocene along the coasts of eastern Sicily (Firth et al., 1996; Stewart et al., 1997; Kershaw, 2000; Rust and Kershaw, 2000; Antonioli et al., 2003; De Guidi et al., 2003; Spampinato et al., 2012). Holocene uplifting has already been evidenced both in the northernmost and in the southernmost coastal sectors of Mt. Etna volcano (Fig. 2) by several raised paleoshorelines, whose elevation measurement and radiocarbon dating indicated average uplift rates of 1.8 and 2.6 mm/year, respectively (Firth et al., 1996; Stewart et al., 1997; Antonioli et al., 2003, 2006; Branca, 2003; Spampinato et al., 2012). This vertical deformation has been related

∗ Corresponding author. Tel.: +39 0957165820. E-mail address: [email protected] (S. Branca). http://dx.doi.org/10.1016/j.jog.2014.07.006 0264-3707/© 2014 Elsevier Ltd. All rights reserved.

to distinct sources: tectonic regional uplift, local deformation along faults and volcanic dome effect (De Guidi et al., 2012). Conversely, submerged manmade structures indicate historical collapse in the central coastal sector, which suffered a subsidence of 0.5–1.0 m in the last 60 years (Platania, 1905; Monaco et al., 2010; Carveni et al., 2011). In addition, for this coastal area, GPS and PSInSAR data show a more marked subsidence, interpreted as being related to flank sliding (Bonforte and Puglisi, 2006; Bonforte et al., 2011). To better define the magnitude and timing of vertical deformation along the coastal sector of Mt. Etna volcano and relate it to sliding, magmatic or tectonic processes, a detailed survey of morphologic and stratigraphic indicators of raised Holocene paleoshorelines has been carried out. In particular, the coastal stretch between Acireale and Catania was investigated (Fig. 1) and new data compared to published ones. The timing of coastal uplift has been determined by sampling and radiocarbon dating of bioconstructions associated with the raised paleo-shorelines. Since sea level is the general reference to detect vertical motion and assess short- and long-term tectonic activity in coastal areas (Lajoie, 1986), we compared the position of Late Holocene shorelines with a recently released sea level curve which takes into account eustatic and glacio-hydro-isostatic processes (see Lambeck et al., 2004).

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Fig. 1. Geological sketch map of Mt. Etna (modified from Branca et al., 2011a); major active tectonic structure and sampling sites are indicated. (a) Inset shows the tectonic model of the Central Mediterranean; lines represent the main Quaternary faults (arrows indicate oblique motion); lines with triangles represent the main thrusts.

2. Geological setting 2.1. Stratigraphic and structural framework Mount Etna is a 3330 m high composite basaltic stratovolcano built up over the past 500 ka on the eastern coast of Sicily in the complex geodynamic setting of the Neogene-Quaternary convergence between the African and European plates (Ben Avraham and Grasso, 1990; Branca et al., 2011a). The volcano lies at the front of the Sicilian-Maghrebian thrust belt and on Early-Middle Pleistocene foredeep clayey successions (Fig. 1) deposited on the flexured margin of the foreland Pelagian block (Branca et al., 2011a). This succession is currently deformed by detachment folds related to the recent frontal migration of the thrust belt, as a response to the approximately N–S compressive regional tectonic regime

(Bousquet et al., 1987; Labaume et al., 1990; Monaco et al., 1997; Bousquet and Lanzafame, 2004; De Guidi et al., 2013). Since the Middle Pleistocene, contractional structures of the orogen have been coupled with oblique extensional faults along the Ionian offshore. These faults form a lithospheric boundary extending between the Aeolian Islands and the eastern Sicily offshore, including the Malta Escarpment, characterized by strong seismicity and active volcanism (Lanzafame and Bousquet, 1997; Palano et al., 2012). After an earlier phase (named Basal Tholeiitic) of discontinuous and scattered volcanic activity occurring about 500 ka and 330 ka, the volcanism in Mt. Etna region was concentrated along the Ionian coast between 220 and 121 ka ago (Timpe phase; Branca et al., 2011a,b; De Beni et al., 2011). In this time span, the extensional tectonic of the Ionian margin of Sicily (Monaco et al., 1997,

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measurement of the altitude of the MIS 5.5 marker (Ferranti et al., 2006 and references therein). In particular, for the Mt. Etna area, long-term uplift rates of 1.3 mm/year have been estimated. Conversely, the Pleistocene foredeep clayey succession and the overlying coastal-alluvial deposits (locally named Terreforti), located along the southern margin of the volcano have been involved in regional tectonic uplift at a rate of 1.2 mm/year, forming in the last 240 ka a series of terraces, that, in turn, have been involved in the recent compressive deformation (Bousquet and Lanzafame, 1986; Ristuccia et al., 2013). This produced a roughly 10 km long and W–E trending asymmetric south-facing anticline at the front of the chain (Terreforti anticline, Fig. 1, Labaume et al., 1990; Monaco et al., 1997). Recent SAR and PSInSAR analyses have highlighted the current growth of another large W–E oriented anticline in the north-western outskirts of Catania. This is characterized by maximum uplift rate of about 10 mm/year along the hinge zone, and has been attributed either to gravitational spreading of the volcanic edifice (Catania anticline, Fig. 1, Lundgren et al., 2004; Bonforte et al., 2011) or to tectonic convergence (De Guidi et al., 2013). At the same time, InSar data also revealed that the west and north base of Mt. Etna is locally affected by uplift of 5 mm/year (Solaro et al., 2010). The folding and the related N–S compression that produced the Terreforti anticline are older than the building of the main Mt. Etna edifice, so contractional structures south of Mt. Etna can be interpreted as the detachment response of a shallow thrust migrating within the foredeep deposits at the chain front (Bousquet et al., 1987; Labaume et al., 1990; Monaco et al., 1997; Bousquet and Lanzafame, 2004; De Guidi et al., 2013).

Fig. 2. Digital elevation model map of Etna volcano showing the short-term vertical deformation rate from published sea-level markers data and the position of the LGM abrasion platform below the sea level.

2010; Azzaro et al., 2012) favoured the ascent of magma in Mt. Etna region, transforming the previous scattered fissural volcanism into an almost continuous volcanic activity that about 129 ka ago shifted westward (Branca et al., 2011b; De Beni et al., 2011). According to Branca and Ferrara (2013), about 80% of the volcanic products were erupted in the past 110 ka due to the stabilization of the magma source in the present Valle del Bove area (Fig. 1), that favoured the beginning of the central-type volcanism and the formation of several small polygenic centres active up to about 60 ka (De Beni et al., 2011). Subsequently, a further increase in erupted products helped build up the bulk of the present edifice, the Ellittico (60–15 ka) and Mongibello (15–0 ka) volcanoes, as a consequence of an efficient piling of volcanics close to the central vents (Branca et al., 2011b). The current morphostructural framework of the volcanic edifice is the result of a complex interaction of magmatic processes with regional tectonics and flank instability processes (Monaco et al., 2010; De Guidi et al., 2012; Azzaro et al., 2013). The lower eastern flank of Mt. Etna is characterized by several morphological scarps (locally known as Timpe) which are the result of Late Quaternary normal faulting (Timpe fault system, Fig. 1; Monaco et al., 1997, 2010; Azzaro et al., 2012). The Acireale master rightnormal fault morphologically controls a 10 km-long coastal stretch forming an up to 150 m high cliff and is characterized by a vertical slip rate of 4.3 mm/year in the past 35 ka (Azzaro et al., 2012). It represents one of the northernmost segments of the NNW–SSE oriented Malta escarpment fault system, a seismogenic lithospheric boundary responsible for the historical destructive earthquakes (Bianca et al., 1999; Argnani and Bonazzi, 2005; Palano et al., 2012). Accordingly, the coastal sector of eastern Sicily has been affected by a vigorous rift-driven tectonic uplift whose Upper Pleistocene rate, progressively decreasing southwards, was estimated by the

2.2. Existing constraints on vertical deformation along the coastal sector of Mt. Etna volcano Holocene uplift of the southern coast of Mt. Etna volcano (Fig. 2) is evidenced by carbonate reef deposits encrusting the basalt pinnacles of the Ciclopi Islands, offshore of Aci Trezza village, up to 6.5 m above sea level, which would indicate an average rate of 2.6 mm/year in the last 8000 years (Firth et al., 1996). Holocene uplifting has taken place with less intensity towards the south, where a rate of 0.5–1.0 mm/year has been calculated by analyses of bore-hole cores in a lagoon located on the Catania Plain be precise here, simple, say what it is (Monaco et al., 2004; Spampinato et al., 2011). Similarly, in the Taormina-Capo Schisò area, along the coast north of Mt. Etna (Fig. 2), raised notches on limestone cliffs and other biological and archaeological paleo-sea level markers indicate lower uplift rates (1.7–1.8 mm/year in the last 5 ka; Stewart et al., 1997; Antonioli et al., 2003, 2006; Branca, 2003; Spampinato et al., 2012). The general process of uplift, with similar rates, of this coastal area during the last 20 ka is also confirmed by the position of the continental shelf between Taormina-Capo Schisò and the mouth of the Simeto river, 10 km south of Catania (Fig. 2). In this area, high-resolution seismic profiles highlight the presence of welldeveloped abrasion platforms formed during the sea level stillstand of the LGM, unconformably overlaid by a weak debris cover, a dozen metres thick (Bousquet et al., 1998; Argnani et al., 2013). The abrasion platform was recognized at about 90 m below sea level at Capo Mulini (Bousquet et al., 1998), while it deepens to about 100 m between Riposto and Pozzillo (Argnani et al., 2013) and to about 120 m to the south, at the Simeto river mouth (see also Torelli et al., 1998). Taking into account that the erosional surface constituting the upward limit of the abrasion platforms formed between and/or just above the tide elevation range which in the Mediterranean is between 50 and 70 cm and thus negligible for the quantitative analysis of magnitude and rate of Late Pleistocene-Holocene vertical tectonic movements (Pepe et al., 2014) and that the sea stood about 130 m below the present sea level around the study area

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at 20 ka (Lambeck et al., 2011), we assume that long-term (since the last 20 ka) uplift rates of about 2 mm/year can be estimated for the Capo Mulini-Aci Trezza offshore sector, decreasing to about 1.5 mm/year towards the north, between Pozzillo and Riposto and to 0.5 mm/year towards the south, at the mouth of the Simeto river (Fig. 2). Conversely, archaeological findings indicate a subsidence of the coastal stretch between Riposto and Acireale, at Torre Archirafi village, where the remains of a 14th century tower appeared partially submerged (Platania, 1905), and at the old dock of the Stazzo harbour (Fig. 2), built in 1949, which is now submerged, indicating a subsidence of 0.5–1.0 m in the last 60 years (0.8/1.6 cm/year; Carveni et al., 2011). In addition, for the same area, GPS (Bonforte and Puglisi, 2006) and PSInSAR data also indicate a strong subsidence due to flank sliding of the triangular coastal sector located between Stazzo and Riposto villages, that is bounded westwards by the San Leonardello normal fault (Giarre wedge of Bonforte et al., 2011). Aseismic slip with vertical rates of about 1.0 cm/year, measured in the last 20 years along the southern sector of this fault (Monaco et al., 2010; Carveni et al., 2011) is probably related to the subsidence and sliding of the coastal sector to the east. Researches done over the last decade (see Azzaro et al., 2013 for a complete review) have indicated that the eastern flank of Mt. Etna volcano has been sliding seaward for a long time, confirming earlier studies (Lo Giudice et al., 1982; Kieffer, 1983; Borgia et al., 1992; Lo Giudice and Rasà, 1992). The flank deformation is characterized by complex interaction with fault systems located in the Mt. Etna offshore (see also Chiocci et al., 2011). The sliding sector is confined to the west by the NE and S Rift zones passing through the Summit Craters, and to the north and to the south by the left-lateral Pernicana fault and by the right-lateral Tremestieri-Aci Trezza fault zone, respectively (Fig. 1), that transfer the extensions to the east (see Acocella et al., 2013 and references therein). According to the shallow sliding model (Lo Giudice and Rasà, 1992; Rasà et al., 1996; Bonaccorso et al., 2006), the eastern mobile sector would be dismembered into minor sub-blocks of volcanics slowly sliding eastwards under their own weight, accommodated by seaward dipping detachment surfaces within the sedimentary substratum (Bonforte and Puglisi, 2006). At Mt. Etna, slope movements have been considered active for a very long-time (14 ka, Tibaldi and Groppelli, 2002) and with velocities of several cm/year, but these very important motions do not match with clear morphological, tectonic and geological evidences, especially along the coastline or within the sliding area and at its southern release. In particular, the Tremestieri-Aci Trezza fault zone, even though clearly evidenced by SAR and PSInSAR data (Bonforte et al., 2011), is surprisingly only constituted by an alignment of discontinuous fractures that have been unable to define a discrete line of active dislocation (Azzaro et al., 2013). Finally, according to Bonforte et al. (2011) another accommodating lineament, located southwards and named Belpasso-Ognina line, has only been inferred by interferometry data, as it has no geological and morphological evidences.

3. Uplifted Holocene shorelines 3.1. Methods Methodology used to study the vertical deformation includes: (i) detailed mapping of morphological and biological markers of sea-level change, which are represented by beach rocks, wave-cut platforms, balanid, vermetid and algal rims, including their elevation and stratigraphic relations; (ii) AMS radiometric dating of material shells associated to the markers, and specifically of fossil shells; (iii) comparison of observed elevation/age data with the predicted sea-level rise curve of Lambeck et al. (2004), which accounts

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for the eustatic and glacio-hydro-isostatic contribution. The field survey was based on direct measurement by stadia rod of the elevation of the paleo-sea level markers. Because of variable and rough sea conditions, the measurements have been referenced to the biological mean sea-level (b.m.s.l.), marked by a living organic rim encrusting the volcanic rocks all along the analyzed shores. The upper limit of coralline algal and vermetid constructions generally coincides with mean sea-level (Laborel and Laborel-Deguen, 1994). The altitude of raised shorelines (Table 1) has been estimated by direct measurement of difference between the upper limit of the elevated algal and/or vermetid reefs (where they are well preserved) and the upper limit of the present organic rim (see Laborel and Laborel-Deguen, 1994). Uncertainty on the sea-level in the past established by fossils depends on their specific environmental conditions, particularly on their degree of shelter or exposure to dominant swell directions. According to Peres and Picard (1964), well-constrained paleo-sea level positions are provided by morphological markers such as notches, and by intertidal organisms such as: (i) vermetids of the species Dendropoma petraeum, whose position relative to the sea level is limited within the local lower tide range (see also Antonioli et al., 1999); (ii) algal rim, made up of coralline algae of the species Lithophyllum byssoides (formerly known as Lithophyllum lichenoides), that marks with negligible uncertainty the lower mesolittoral environment between the mean and low tide; (iii) barnacles of the species Chthamalus depressus that commonly mark the upper mesolittoral biocenosis. The thickness of the barnacle band reflects the combined effect of the above-mean tidal range and of the locally variable wave splash zone. Compared with organisms living in sheltered parts of the studied area, the uncertainty in sea level positioning for this marker can be considered negligible (0.20 m), and thus no correction was adopted. Conversely, beach deposits and wave-cut platforms are less accurate sea-level markers because they formed below an unknown depth of the water column. As regards the beach deposits, based on the evidence of a primary position provided by the homogeneous sedimentary and faunal content and degree of preservation, we can exclude that it was reworked above the sea-level by storms, and hence a correction of +0.40 m was adopted. Based on the present-day gentle slope of the sea-bottom immediately below the strandline, we also adopted a correction of +0.40 m for the wave-cut platforms (Table 1). Radiocarbon analyses were carried at three distinct laboratories, CIRCE (Caserta, Italy), Beta Analytic Inc. (Miami, USA), CEDAD (Lecce, Italy). Radiocarbon dating results of aged fossil organisms were calibrated using the programme CALIB 6.0 (2, marine entry; Stuiver et al., 2011). 3.2. Shoreline features, elevation distribution and radiometric ages Along the footwall of the Acireale fault scarp (Fig. 1), at Acque Grandi locality, the stratigraphic succession is formed by transitional to Na-alkaline lava flows of the Timpe phase (154.9 ± 17.0 ka old; De Beni et al., 2011), moving upward to volcaniclastic deposits that are covered by lava flows of the Ellittico volcano (Branca et al., 2011a). At Acque Grandi locality (site T8 in Fig. 1), remnants of a 15 cm wide algal rim, encrusting the basal massive basalts, were found at an elevation of ∼2.9 m above b.m.s.l. (Fig. 3a). A sample of a balanid shell, collected from the algal rim, yielded an age of 1224.5 ± 93.5 cal BP (sample CT8 in Table 1). To the south, at the Capo Mulini promontory (Fig. 1), formed by a Mongibello volcano lava flow (Aciplatani lava flow, age: 15–3.9 ka, of Branca et al., 2011a), the Acireale fault ends. In this locality (site CT1 in Fig. 1), a bivalve shell was collected at an elevation of 3.0 m above b.m.s.l. (Fig. 3b) from beach deposits encrusting the basalts.

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Locality

Sample (laboratory number)

Coordinates

Marker

Measurement time (dd/mm/yyyy hh)

Measured marker elevation + data uncertainty (m)

Radiocarbon age (year BP)

Calibrated age (year BP)

Capo Mulini

CT1 (DSH4752) CT2 (DSH4759)

37◦ 34 30.39 N 15◦ 10 30.24 E 37◦ 33 41.48 N 15◦ 09 59.90 E

20/06/2012

3.0 + 0.4

4454 ± 24

−5.5

03/07/2009 11.30

6.0 ± 0.1

5831 ± 58

4799–4596 4697.5 ± 101.5 6447–6186 6316.5 ± 130.5

−9.3

1.8 + 0.13 −0.03 2.4 ± 0.1

CT3 (DSH4760) CT4 (LTL13387A)

37◦ 32 08.12 N 15◦ 07 44.54 E 37◦ 33 41.00 N 15◦ 10 01.00 E

Bivalve shell in beach deposit Gasteropod at the top of reef deposit Vermetid

29/05/2012 11.26

6.0 ± 0.1

3393 ± 48

−3.2

2.8 ± 0.1

03/07/2009 10.36

0.4 ± 0.1

104 ± 45

−0.1

3.0 ± 1.1

CT5 (Beta365949) CT7 (Beta365950) CT8 (DSH1221)

37◦ 33 14.10 N 15◦ 08 54.20 E

Lithophyllum at the top of algal rim Calcite on wave-cut platform Balanid

3447–3193 3320 ± 127 Before AD 1950

27/09/2013 12.16

2.0 + 0.4

6360 ± 30

6880–6620 6750 ± 130

−11.5

2.0 + 0.1 −0.04

27/09/2013 10.46

6.0 ± 0.2

3710 ± 30

3680–3390 3535 ± 145

−3.5

2.7 ± 0.2

Balanid

31/07/2008 10.55

2.9 ± 0.2

1609 ± 36

1318–1131 1224.5 ± 93.5

−0.8

3.0 ± 0.4

Isola Lachea

Cannizzaro Isola Lachea

Aci Castello

Cannizzaro

Timpa di Acireale

37◦ 32 32.80 N 15◦ 08 24.60 E 37◦ 35 36.10 N 15◦ 10 26.40 E

Predicted s.l. (m) (Lambeck et al., 2004)

Uplift rate (mm/year)

S. Branca et al. / Journal of Geodynamics 82 (2014) 194–203

Table 1 Location, elevation and age of fossil organisms sampled from the Holocene paleo-shorelines along the coastal area of Mt. Etna. Radiocarbon analyses were carried at three different laboratories, CIRCE (Caserta, Italy), Beta Analytic Inc. (Miami, USA) and CEDAD (Lecce, Italy). Radiocarbon dating results of aged fossil organisms were calibrated using the programme CALIB 6.0 (2, marine entry; Stuiver et al., 2011). Uplift rates and relative errors are also indicated.

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Fig. 3. (a) Timpa di Acireale (Site CT8 in Fig. 1): algal and balanid rim at elevation of 2.9 m above b.m.s.l. (b) Capo Mulini (site CT1 in Fig. 1): beach deposits at 3 m above b.m.s.l. (c) Aci Trezza: reef deposit encrusting one of the Ciclopi islets (vertical bar length = 3.0 m); (d) Aci Trezza – eastern side of Isola Lachea (site CT2 in Fig. 1): lithophaga holes at the top (6 m a.s.l.) of reef encrustation containing corals, algae, gastropods and other molluscs; e) Aci Trezza – eastern side of Isola Lachea (site CT4 in Fig. 1): algal rim extending up to 0.4 m above b.m.s.l. (f) Aci Castello (site CT5 in Fig. 1): wave-cut platforms at 4.5 m and 2.0 m above m.b.s.l. Inset shows the sampled reef encrustation on the lower platform. (g) Cannizzaro (sample CT7 in Fig. 1): balanid incrustation at elevation of 6 m above m.b.s.l. (h) Cannizzaro (sample CT3 in Fig. 1): vermetid and balanid incrustation at elevation of 6 m a.s.l.

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Radiocarbon dating yielded an age of 4697.5 ± 101.5 cal BP (sample CT1 in Table 1). In the Aci Trezza-Aci Castello area (Fig. 1), the oldest volcanic products of Etna region outcrop. They are represented by pillow lavas and hyaloclastic pillow-breccia with a tholeiitic affinity (Tanguy et al., 1997; Corsaro and Cristofolini, 2000), interbedded in the Pleistocene marly clayey deposits and by the shallow subvolcanic body of the Ciclopi islets (Bousquet et al., 1988; Branca et al., 2011a). The islets are partially encrusted by a reef deposit (see also Firth et al., 1996) containing algae, corals, vermetids, Spondylus and other molluscs, bored by Lithophaga and extending from ∼6.0 m above b.m.s.l. down to the present coastline (Fig. 3c). It is worth noting that the reef encrustation does not show raised notches or other indicators of marine still-stand. In a deep and sheltered bay on the eastern side of the Lachea island (site CT2–CT4 in Fig. 1), a gasteropod was collected from the top of the reef, where it covers both the subvolcanic basalts and the marly clays (Fig. 3d). Radiocarbon dating yielded an age of 6316.5 ± 130.5 cal BP (sample CT2 in Table 1). In the same place, a lower paleo-shoreline is characterized by an uplifted discontinuous rim of calcareous algae (L. byssoides; Fig. 3e). From its top, that has been levelled at 0.4 m above b.m.s.l., a sample of algae was collected for age determination (radiocarbon age 104 ± 45 years BP; sample CT4 in Table 1). At the bottom of the impressive rock of Aci Castello (site CT5 in Fig. 1), two wave-cut platforms, carved in hyaloclastic pillowbreccia and pillow lavas, are exposed (Fig. 3f). A small exposure of the higher platform can be observed only in the southernmost side of the rock where it is surmounted, at an elevation of 4.5 m above b.m.s.l., by a Mongibello volcano lava flow (Cannizzaro lava flow, age: 15–3.9 ka, of Branca et al., 2011a). The lower platform is very large and surrounds the entire rock, extending from ∼2.0 m above b.m.s.l. down to 0.9 m above b.m.s.l. Remnants of calcareous reef deposits, bored by Lithophaga, encrust the southern portion of the lower platform. A sample of this deposit was sampled at an elevation of 1.9 m above b.m.s.l. and its radiocarbon dating yielded an age of 6750 ± 130 cal BP (sample CT5 in Table 1). The Cannizzaro lava flow outcrops along the southern coastal stretch, between Aci Castello and Catania (Fig. 1). There are several tunnels in this lava field in which ceramics of Early Bronze age (2200–1400 BC) were found (Branca et al., 2011a). The Cannizzaro lava flow is characterized by the occurrence of raised biological markers outcropping in isolated patches of eroded constructions or remnants of cemented beach deposits. Two sites have provided good clues for measuring Holocene uplift rates. In the northern one (site CT7 in Fig. 1), a balanid shell (Fig. 3g) was collected from a sheltered position at an elevation of 6.0 m above b.m.s.l. It encrusted the basalts at the top of a beach deposit rich in mollusc fragments, mostly outcropping at elevation of 4.0–5.0 m above b.m.s.l. The radiocarbon dating yielded an age of 3535 ± 145 cal BP (sample CT7 in Table 1). A vermetid shell was also collected at an elevation of 6.0 m above b.m.s.l. from the southern outcrop (site CT3 in Fig. 1), where isolated patches of vermetid rims occur (Fig. 3 h), and its radiocarbon dating yielded an age of 3320 ± 127 cal BP (sample CT3 in Table 1). 3.3. Uplift rates In order to correctly assess the total amount of tectonic uplift the coast underwent during the Late Holocene, we compared the elevation-age data of sampled shells to the local curve of Holocene sea-level rise (Lambeck et al., 2004, 2011), calibrated with field data at sites along the Italian coast unaffected by significant tectonic processes (see Spampinato et al., 2014). We preferred the model of Lambeck et al. (2004), as it can be considered the most suitable for the Mt. Etna offshore sector (Lambeck, personal communication). The total uplift of the paleo-shorelines, deriving from the corrected

Fig. 4. Comparison of the predicted eustatic–isostatic sea level curve for the Catania coastal area (according to the model of Lambeck et al., 2004) with age and elevation above the modern sea level of aged fossil organisms (error bars are shown).

marker position above sea-level (nominal elevation) plus the corrected position for the sea-level rise, is illustrated in Fig. 4 and Table 1. Clearly, all the Holocene shorelines are displaced above the predicted position, and the amount of vertical displacement is given by the distance between the data points and the model sea level curve. Combining the amount of vertical displacement and age of the distinct markers surveyed in the southern coastal stretch of Mt. Etna, average uplift rates of 2.0–3.0 mm/year are obtained. A similar value of 2.6 mm/year was calculated by Firth et al. (1996) for the Aci Trezza area, based on radiocarbon dating of biological markers. In more detail, the older markers, located in the Aci Trezza-Aci Castello area (CTI, CT2, CT5), indicate lower uplift rates of 1.9–2.4 mm/year in the last 6750–4700 years for this area, whereas the younger ones (CT3, CT7, CT8) show an acceleration up to 3.0 mm/year in the last 3500 years both to the north, at the footwall of the Acireale fault, and to the south, along the hinge of the Catania anticline (Fig. 1). A similar acceleration in the last 160 years is also indicated for the Aci Trezza area by the CT4 marker. 4. Discussion Elevation measurements of the raised paleo-shorelines along the southern costal sector of Mt. Etna volcano, together with sampling and radiocarbon dating of shells collected from these biological markers, allowed confirming the major uplift of this area in the Late Holocene. In order to determine the total amount of tectonic uplift the coast underwent during the last 6–7 ka, we compared the elevation-age data of sampled shells to the local curve of Holocene sea-level rise (Lambeck et al., 2004). Taking into account the nominal elevation of the associated paleo-shorelines, a shortterm uplift rate of 2.5–3.0 mm/year has been estimated for the last 6–7 ka. It is worth noting that the morphological features and distribution of the measured markers prevented us from distinguishing between stick-slip (co-seismic uplift) and steady-state deformation (see Spampinato et al., 2014). For this reason, as an initial assumption we believe that the uplift would have operated slowly and constantly in this area.

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In particular, the Acireale and the San Leonardello master faults divide the coastal sector in two main domains characterized by different vertical movements: the northern coastal sector located in the hanging wall, corresponding to the Giarre wedge delimited northward by the termination of the Pernicana fault, and the southern coastal sector, developed on the footwall blocks (Fig. 5). The northern coastal sector shows historical and current subsidence, superimposed on the long-term (since 20 ka) uplift recorded by the LGM platform, that concurs with the seaward sliding dynamics of this block of the volcano edifice (Froger et al., 2001; Lundgren et al., 2004; Puglisi and Bonforte, 2004; Solaro et al., 2010; Bonforte et al., 2011). Here, the subsidence related to the sliding of the Giarre wedge is prevalent with respect to the long-term regional uplift. On the contrary, the southern coastal sector, including the LGM platform, is characterized by long- (since 20 ka) and short-term uplift that is in contrast with the general seaward sliding dynamics. In particular, the GPS and interferometry data (Bonforte and Puglisi, 2006; Bonforte et al., 2011) reveal a fairly stable behaviour of the coastal sector between S. Tecla and Capo Mulini, at the footwall of the Acireale fault (Fig. 5), providing evidence that the subsidence related to the sliding process compensates for the regional and fault-related uplift. Conversely, between Capo Mulini and Catania we observe a general uplift of the coast during the last 20 ka, highlighting an acceleration of the vertical motion at the hinge of the Catania anticline, in contrast with the seaward sliding (Fig. 5). Therefore, the different behaviour of distinct coastal sectors can only be explained if we assume that the flank sliding process has only recently superimposed to the long-term volcano-tectonic and regional uplifting. 5. Conclusive remarks

Fig. 5. Structural sketch map of Etna volcano showing the vertical velocities, expressed in mm/year obtained by interferometric data of Bonforte et al. (2011) in the period 1995–2000, and the last 20 ka uplift/subsidence rates observed both along the coast (PSL, paleo-shoreline; DHA, docks and other manmade structures) and in the offshore (LGM, last glacial maximum platform).

In particular, in the Aci Trezza-Aci Castello area (samples CTI, CT2, CT5), uplift rates of 1.9–2.4 mm/year have been determined for the last 6750–4700 years, with an acceleration (3 mm/year) in the last 160 years (sample CT4). These values are consistent with the long-term (20 ka) rates suggested by offshore data (see Section 2.2). Higher rates (sample CT8) at the footwall of the Acireale fault (Fig. 5) concur with the vertical slip rate of the past 35 ka (Azzaro et al., 2012), suggesting that this structure has been active during the last 1.2 ka. Interferometry data (Bonforte et al., 2011) also indicate that the higher uplift rate of 3.0 mm/year in the last 3500 years (samples CT3, CT7), determined for the coastal sector between Aci Castello and Catania, may be related to the growth of the Catania anticline (Fig. 5). Northwards, the general process of uplifting is interrupted by strong subsidence with rates of 0.8–1.6 cm/year in the last 60 years, measured at the docks and other manmade structures between Stazzo and Riposto villages, at the Giarre wedge (see Section 2.2). In addition, the GPS and PSInSAR data point to a subsidence rate of more than 13 mm/year for this area (Fig. 5) in the period 1995–2000 (Bonforte et al., 2011). In our opinion, this subsidence is a local effect of flank sliding, as it is replaced to the north by the uplifting (1.8 mm/year) of the Taormina-Capo Schisò area (Spampinato et al., 2012). These data highlight the main role played by the Timpe fault system in controlling the lower eastern flank dynamics of the volcano.

The predominant process of uplift measured along the coastal sector of Mt. Etna is inconsistent with the inferred long-time flank sliding of the entire eastern slope of the volcanic edifice down towards the Ionian offshore. In our opinion, the sliding-related subsidence clearly involving the northern sector of Etna (the Giarre wedge) has only recently affected the south-eastern sector of the slope, where it failed to counteract the effects of the volcanotectonic uplifting. Moreover, long-term sliding dynamics should have led to a set of pervasive, significant and well-defined deformation belts along the sliding boundaries (Fig. 5); on the contrary, they show only local structural evidence and often at the initial stages (Azzaro et al., 2013). Therefore, the time–space boundaries of the lateral sliding, which is probably much younger and shallower than is generally stated and differentially involves distinct blocks, remains to be constrained more accurately. Indeed, compared to the PSInSAR data (Bonforte et al., 2011), our results seem to indicate that the sliding process is progressively involving, from the north to the south, the lower Mt. Etna flank. In this context, the Timpe fault system contributes at shallow depth to the differential motion of distinct sliding blocks. In a context of long-term uplift of the Mt. Etna coastline, we suggest that the evolution of the more recent sliding processes affecting the eastern flank on-offshore must be better time–space constrained. Acknowledgments The authors are grateful to Alessandro Bonforte for the helpful contribution on the interferometric data, to Fabrizio Pepe for his useful discussion on offshore data and to Fabrizio Antonioli and Rossana Sanfilippo for their suggestions on biological markers. We wish to thank the anonymous reviewers for their constructive comments that significantly improved the paper. This work

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