SEARCH

SEARCH BY CITATION

Abstract

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Tectonics Framework
  5. 3. Seismological Constraints
  6. 4. Geochemical Constraints
  7. 5. Discussion
  8. References

[1] Combining tectonics, with seismological and geochemical data, we reconstruct the deformation history of the presently narrow Calabrian slab and the path of mantle circulation during the last 10 Ma. We show that during the slab deformation the mantle laterally flowed inside the back arc region permitting its retrograde motion and giving a seismological and volcanological record after 1–2 myr.

1. Introduction

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Tectonics Framework
  5. 3. Seismological Constraints
  6. 4. Geochemical Constraints
  7. 5. Discussion
  8. References

[2] The possibility for mantle to flow around the edge or inside windows of subducting slabs has been proposed on the basis of geochemical and geophysical evidences and turns out to be a fundamental mechanism for subduction processes. Tectonic reconstructions [Faccenna et al., 2004], volcanological models [Gvirtzman and Nur, 1999; Trua et al., 2003] and shear wave splitting studies [Civello and Margheriti, 2004] are evidence that lateral mantle flow was also active around the Calabrian slab in the central Mediterranean. However, the amplitude and the role of this process are still poorly constrained. We combine tectonics, seismological and geochemical data, to reconstruct the deformation of the Calabrian slab and the mantle flow path during the last 10 Ma. Our findings show that slab deformation and lateral mantle circulation controlled the style and timing of the Tyrrhenian back arc extension.

2. Tectonics Framework

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Tectonics Framework
  5. 3. Seismological Constraints
  6. 4. Geochemical Constraints
  7. 5. Discussion
  8. References

[3] The Calabrian subduction zone retreated southeastward for about 400 km during the last 10 Ma, causing the opening of the Tyrrhenian back arc basins [Patacca et al., 1990]. During the course of its retreat, the Calabrian slab deformed, decreasing its width to the present-day shape (Figure 1). We identified three discrete episodes of slab deformation. The first (Figure 2a) occurred in northern Tunisia during the Tortonian producing the initial separation of the Calabrian slab from the inactive African (Tell) slab [Casero and Roure, 1994]. At the same time, rifting and subsidence in the western Tyrrhenian region had also initiated [Patacca et al., 1990]. On the site of the initial break (northern Tunisia) the calc-alkaline rhyolite suite of Nefza and Galite islands (14–8 Ma [Savelli, 2002]) were replaced by the Upper Miocene (about 8–6 Ma) with the (Na)-alkaline basalt of Nefza and Mogodos (Figure 2a). The second episode occurred in the late Messinian with the enlargement of the slab window (Figure 2b) shifting eastward about 300 km from the active portion of the foreland-thrust front [Argnani et al., 1987; Casero and Roure, 1994]. Contemporaneously, in the Tyrrhenian, seafloor spreading had initiated in the Vavilov basin and a rift zone was formed in the Sicily channel cutting the inactive thrust system (Figure 2b) [e.g., Argnani, 1990]. North of the newly formed window, the alkaline Aceste seamount emplaced just west of the calk-alkaline Anchise seamount [Savelli, 2002] (Figures 1 and 2b). Anorogenic alkaline magmatism also started in Sardinia [Savelli, 2002]. The present-day narrow configuration of the Calabrian slab (Figure 1) was achieved at about 1–0.8 Ma ago with further enlargement of the slab window (400 km) (Figure 2c). The southern Sicily thrust system (Gela nappe) [Argnani et al., 1987] and the southern Apennines thrust system [Patacca et al., 1990; Mattei et al., 2004] ceased to be active restricting the present-day active front to the Ionian area. In the back arc domain, the seafloor spreading jumped eastward into the Marsili basin, and Ustica volcano and Prometeo lava field, located just to the west of the Aeolian volcanic fields, erupted material with the same anorogenic affinities of Pantelleria (Figure 2c).

image

Figure 1. Tectonic map of the Southern Tyrrhenian Sea showing subduction related volcanoes (light grey) and alkaline intraplate volcanoes (dark grey). Dashed lines indicate isobath of the Wadati-Benioff zone.

Download figure to PowerPoint

image

Figure 2. Three-stage tectonic reconstruction based on estimation of the trench retreat of Faccenna et al. [2004]. Dashed lines indicate the lateral boundary between subduction-related magmatism and anorogenic. For each frame we show only the active volcanoes. Grey arrow indicates the possible mantle flow path [Civello and Margheriti, 2004].

Download figure to PowerPoint

3. Seismological Constraints

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Tectonics Framework
  5. 3. Seismological Constraints
  6. 4. Geochemical Constraints
  7. 5. Discussion
  8. References

[4] The Wadati-Benioff zone of Calabria represents a steep, 200 km narrow plane that can be followed from the Ionian foreland below the Calabrian Arc down to a depth of 500 km below the Tyrrhenian Sea [Selvaggi and Chiarabba, 1995] (Figure 1). The 3D contour of the 0.8% high seismic velocity anomaly (from the Piromallo and Morelli [2003] tomographic model) illustrates the shape of the slab (Figure 3). At 150 km depth (Figure 4), it appears as a narrow structure limited to the inner portion of the Calabrian arc, surrounded by negative anomalies in the area of western Sicily, the Sicily channel and southern Apennines. Below Tunisia and central-northern Apennines we find again traces of positive velocity anomalies. At a greater depth, below 200–300 km (Figure 3), the high velocity anomalies join forming a continuous belt from Apennines to the north to the Sicilian Maghrebides to the southwest [Wortel and Spakman, 2000; Piromallo and Morelli, 2003]. At 400–600 km depth (Figure 3), the Calabrian fast anomaly merges into a broader positive anomaly, spreading over the Central Mediterranean [Piromallo and Morelli, 2003]. The Calabrian slab mostly had a SE-NW profile, where the high velocity structure can be followed continuously from a shallow region on the Ionian side of the arc, down to the 660 km discontinuity [Lucente et al., 1999]. The 3D view and the longitudinal section (Figures 3 and 5) show that the Calabrian slab is presently reduced to a narrow, finger-like, structure.

image

Figure 3. Three-dimensional image of the upper mantle beneath Italy and the Tyrrhenian, from the tomographic model PM0.5 of Piromallo and Morelli [2003]. The green isosurface encloses the volume characterized by velocity anomalies larger than +0.8% relative to average mantle velocities. The layers at 250 and 650 km depth are shown in colored transparency: blue for regions of higher than average velocity (associated with cold material), and red for areas of lower than average velocity (hot material).

Download figure to PowerPoint

image

Figure 4. Compilation of SKS wave splitting measurements. A positive measurement is represented by a solid green line oriented parallel to the fast polarization, with length proportional to the delay time. Measurements are plotted over the tomographic image (layer at 150 km depth) at the surface projection of the 150 km depth SKS ray piercing point.

Download figure to PowerPoint

image

Figure 5. 87Sr/86Sr and Ba/Nb (data compiled from the literature) for key volcanoes of Southern Italy active during the last 800 ka (Pantelleria (P), Ustica (U), Prometeo (Pt), Aeolian islands (Ae1: Alicudi; Ae2: Filicudi+Salina+Vulcano+Lipari+Panarea; Ae3: Stromboli), Procida (Pr), Ischia (I), Campi Flegrei (CF), Vesuvius (Vs), Vulture (Vt). Data for Etna (E) and Tyrrhenian sea MORB (Ty) (Marsili) are also projected on the profile. Data are presented on a profile running approximately from SW to NE over the corresponding tomographic section.

Download figure to PowerPoint

[5] The pattern of SKS splitting measurements [Civello and Margheriti, 2004] in Southern Tyrrhenian Sea was plotted over the 150 km depth tomographic map (Figure 4). Anisotropy it is defined by two parameters, fast direction (Φ) and delay time (δt), related to lattice preferred orientation of olivine crystals induced by mantle deformation and/or flow. Shear wave splitting δt in the studied region ranges from 0.5 to 2.7 s; the average is about 1.6 s which corresponds to about a 180 km thick anisotropic body with about 5% anisotropy intensity. Fast directions in peninsular Italy are quite often parallel to the strike of the thrust belt, ranging from NW-SE in the Vesuvian area to NNE-SSW in eastern Sicily to EW in central Sicily, and could therefore be a result of the mantle deformation and the retrograde motion of the slab. Conversely, in the Tyrrhenian Sea, fast directions are mainly E-W oriented (Sardinia, Ventotene and Ustica Island) and can be interpreted as being a result of the trench-suction as they correspond with the back arc extensional direction. The Aeolian Islands have a complex pattern of polarization and is possibly a result of slab deformation. The NS oriented Φ in western Sicily have a high angle with respect to the thrust front and could be related to lateral return flow turning around the slab edge.

4. Geochemical Constraints

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Tectonics Framework
  5. 3. Seismological Constraints
  6. 4. Geochemical Constraints
  7. 5. Discussion
  8. References

[6] The Southern Tyrrhenian magmatism is characterized by the close association of alkaline and calc-alkaline products (Figures 1 and 2). Quaternary sodic alkaline basalts crop out at Pantelleria, Linosa and Ustica island, Etna, Iblei, Prometeo and Vulture (Figure 1). These volcanic suites are geochemically and isotopically akin to OIB [e.g., Cinque et al., 1988; Civetta et al., 1998; Beccaluva et al., 1998, 2002; Gasperini et al., 2002; Trua et al., 2003]. Calc-alkaline volcanic activity is represented by the Aeolian arc and other Tyrrhenian seamounts [e.g., Francalanci et al., 1993; Savelli, 2002]. The magmatism of the Neapolitan volcanoes (Campi Flegrei, Ischia, Procida, Vesuvius) has been also genetically related to the subduction process [e.g., Beccaluva et al., 1991; Tonarini et al., 2004].

[7] To evaluate the mantle composition (i.e., orogenic versus anorogenic), we utilized some geochemical and isotopic tracers (87Sr/86Sr; Ba/Nb) from the volcanic products erupted during the last 800 ka. The data is plotted along a section (Figure 5), incorporating from SW to NE: Pantelleria, Ustica, Prometeo, Aeolian islands (divided in three groups: Alicudi; Filicudi, Salina, Vulcano, Lipari, Panarea; Stromboli), Procida, Ischia, Campi Flegrei, Vesuvius, Vulture. We also projected on the profile data concerning Etna and the Tyrrhenian Sea MORB (Marsili, active during the last 2 Ma). To take into account the effects of fractionation, we filtered the chemical data to include rocks with MgO > 5 wt. For each volcano, we reported the lowest measured Sr-isotopic ratio and the average Ba/Nb ratio, to minimize effects of contamination and fractionation. The latter element/element ratio was chosen because arcs are characterized by high LILE/HFSE ratios, distinct from the trace element distribution of anorogenic magmas. The consensus is that this enrichment is a reflection of fluid/melt controlled partitioning of LILE relative to HFSE from the slab to the mantle wedge. The geochemical data are presented with the corresponding tomographic section, showing the along strike prospective of the narrow Calabrian slab. The Aeolian arc and the Neapolitan volcanoes are characterized by high Ba/Nb ratios, with respect to the Pantelleria, Ustica and Tyrrhenian Sea basalts, which are considered to be the most akin to the African asthenospheric mantle [D'Antonio et al., 1996; Civetta et al., 1998]. Prometeo and Etna are characterized by slightly higher Ba/Nb ratios. Vulture has a Ba/Nb ratio higher than those of the Pantelleria, Ustica and Tyrrhenian Sea basalts, but lower than those of the Aeolian Islands and of the Neapolitan volcanoes. The Sr-isotope ratio progressively increases from Pantelleria to the Neapolitan volcanoes; low value is found in the Tyrrhenian Sea basalts. The Vulture rocks display an isotopic composition less radiogenic than the Neapolitan volcanoes but significantly higher than the other volcanoes. Nd- and Pb-isotope ratios (data not shown) display similar relationships but with a reversed trend. Both Ba/Nb and Sr-isotope ratios patterns show rather distinct features that can be correlated with the distribution of velocity anomalies: low isotopic ratio in the Sicily channel region is placed on top of a low velocity anomaly, whereas the Aeolian Island high isotopic ratio is placed on top of a high velocity anomaly, the Calabrian slab. The Neapolitan area isotopic ratio instead is placed on top of the low-velocity anomaly below the Southern Apennines.

5. Discussion

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Tectonics Framework
  5. 3. Seismological Constraints
  6. 4. Geochemical Constraints
  7. 5. Discussion
  8. References

[8] The reconstruction and the data set presented here bear some implications, listed below.

[9] 1. We identify three discrete episodes of deformation along the southern Tyrrhenian Sea (10–8 Ma, 5–6 Ma, and 1–0.8 Ma), which progressively reduce the active portion of the slab. Geological, volcanological data and tomographic images all indicate that the locking of the trench is due to the formation and enlargement of slab window, both along the Sicilian Maghrebides and the southern Apennines. The age of the three episodes match the initiation of extension and the pulses of oceanic spreading in the Tyrrhenian (Vavilov, 4–3 Ma; Marsili <2 Ma) [Patacca et al., 1990]. This finding is also in agreement with the retreat of the slab being favored by the separation of the oceanic Ionian domain from the contiguous buoyant continental domains.

[10] 2. Through the newly formed tectonic windows, the hotter sub-slab materials entered inside the back arc domain. In the southern Tyrrhenian region the amount of mantle material flowing inside the back arc region is large enough to change the composition from predominantly calc-alkaline to alkaline. This explains the position of Nefza-Mogodos and Ustica-Prometeo located just north of the window in sites of previous calc-alkaline centers (Figure 2). This also suggests that the flow is not vigorous enough to contaminate the signature of the arc volcanoes (for example Aeolian Island or Anchise placed on top of the slab). The pulses of alkali magmatism recorded in Sardinia or further north in Provence would then require other explanations.

[11] 3. SKS splitting pattern in Sicily has been interpreted as related to lateral-return flow around the western slab edge (Figure 2c) [Civello and Margheriti, 2004]. The geochemical signature agrees with this model [Trua et al., 2003] indicating the way back arc mantle is contaminated by inflow of sub-slab (African) mantle material. Laboratory and numerical tests are needed to analyze the possible mantle flow trajectories.

[12] 4. The Neapolitan volcanic district is placed on top of the younger (<700 ka) slab window, but its geochemical and petrological data suggests that the mantle beneath the region is still contaminated by subducting material. This implies that the return flow was not efficient enough to produce a significant change of the mantle source; this is in accordance with fast polarization that appears to preserve the regional trend of the chain. Such an effect is possibly related to the time-scale of the process. Comparing the ages of the two slab windows, it is possible to deduce that a continuous flow for at least 2 million years (Central Mediterranean subduction rate 1–5 cm/yr) would be necessary to re-organize the mantle flow field to the point of being seismologically and geochemically detected. Mt. Vulture shows intermediate isotopic and geochemical features between Na-alkaline and K-alkaline, characterized by both subduction-related metasomatism and enrichment of anorogenic alkaline components [Beccaluva et al., 2002]. These contrasting features could be also related to the immature imprinting of the return-flow below the Apennines appearing first in a more external position.

References

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Tectonics Framework
  5. 3. Seismological Constraints
  6. 4. Geochemical Constraints
  7. 5. Discussion
  8. References
  • Argnani, A. (1990), The strait of Sicily rift zone: Foreland deformation related to the evolution of a back-arc basin, J. Geodyn., 12, 311331.
  • Argnani, A., S. Cornini, L. Torelli, and N. Zitellini (1987), Dhiacronous foredeep-system in the neogene-quaternary of the Strait of Sicily, Mem. Soc. Geol. It., 38, 407417.
  • Beccaluva, L., P. Di Girolamo, and G. Serri (1991), Petrogenesis and tectonic setting of the Roman Volcanic Province, Italy, Lithos, 26, 191221.
  • Beccaluva, L., F. Siena, M. Coltorti, A. Di Grande, A. Lo Giudice, G. Macciotta, R. Tassinari, and C. Vaccaro (1998), Nephelinitic to tholeiitic magma generation in a transtensional tectonic setting: An integrated model for the Iblean volcanism, Sicily, J. Petrol., 39, 15471576.
  • Beccaluva, L., M. Coltorti, P. Di Girolamo, L. Melluso, L. Milani, L. Morra, and F. Siena (2002), Petrogenesis and evolution of Mt. Vulture alkaline volcanism (Southern Italy), Mineral. Petrol., 74, 277297.
  • Casero, P., and F. Roure (1994), Neogene deformations at the Sicilian-North African Plate boundary, in Pery Tethyan Platforms, edited by F. Roure, pp. 2750, Ed. Technip, Paris.
  • Cinque, A., L. Civetta, G. Orsi, and A. Peccerillo (1988), Geology and geochemistry of the island of Ustica (Southern Tyrrhenian Sea), Rend. Soc. It. Miner. Petrol., 43, 9871002.
  • Civello, S., and L. Margheriti (2004), Toroidal mantle flow around the Calabrian slab (Italy) from SKS splitting, Geophys. Res. Lett., 31, L10601, doi:10.1029/2004GL019607.
  • Civetta, L., M. D'Antonio, G. Orsi, and G. R. Tilton (1998), The geochemistry of volcanic rocks from Pantelleria Island, Sicily Channel: Petrogenesis and characteristics of the mantle source region, J. Petrol., 39, 14531491.
  • D'Antonio, M., G. R. Tilton, and L. Civetta (1996), Petrogenesis of Italian alkaline lavas deduced from Pb-Sr-Nd relationships, in Earth Processes: Reading the Isotopic Code, Geophys. Monogr. Ser., vol. 95, edited by A. Basu, and S. R. Hart, pp. 253267, AGU, Washington, D. C.
  • Faccenna, C., C. Piromallo, A. Crespo Blanc, L. Jolivet, and F. Rossetti (2004), Lateral slab deformation and the origin of the arcs of the western Mediterranean, Tectonics, 23, TC1012, doi:10.1029/2002TC001488.
  • Francalanci, L., S. R. Taylor, M. T. McCulloch, and J. D. Woodhead (1993), Geochemical and isotopic variations in the calc-alkaline rocks of Aeolian arc, southern Tyrrhenian Sea, Italy: Constraints on magma genesis, Contrib. Mineral. Petrol., 113, 300313.
  • Gasperini, D., J. Blichert–Toft, D. Bosch, A. Del Moro, P. Macera, and F. Albarède (2002), Upwelling of deep mantle material through a plate window: Evidence from the geochemistry of Italian basaltic volcanics, J. Geophys. Res., 107, 2367, doi:10.1029/2001JB000418.
  • Gvirtzman, Z., and A. Nur (1999), The formation of Mt. Etna as the consequence of slab rollback, Nature, 401, 782785.
  • Lucente, P. F., C. Chiarabba, G. B. Cimini, and D. Giardini (1999), Tomographic constraints on the geodynamic evolution of the Italian region, J. Geophys. Res., 104, 20,30720,327.
  • Mattei, M., V. Petrocelli, D. Lacava, and M. Schiattarella (2004), Geodynamic implications of Pleistocene ultra-rapid vertical-axis rotations in the Southern Apennine (Italy), Geology, 32, 789792, doi:10.1130/G20552.1.
  • Patacca, E., R. Sartori, and P. Scandone (1990), Tyrrhenian basin and Apenninic arcs: Kinematic relations since late Tortonian times, Mem. Soc. Geol. It., 45, 425451.
  • Piromallo, C., and A. Morelli (2003), P-wave tomography of the top 1000 km under the Alpine-Mediterranean area, J. Geophys. Res., 108(B2), 2065, doi:10.1029/2002JB001757.
  • Savelli, C. (2002), Time–space distribution of magmatic activity in the western Mediterranean and peripheral orogens during the past 30 Ma (a stimulus to geodynamic considerations), J. Geodyn., 34, 99126.
  • Selvaggi, G., and C. Chiarabba (1995), Seismicity and P-wave velocity image of the Southern Tyrrhenian subduction zone, Geophys. J. Int., 122, 818826.
  • Tonarini, S., W. P. Leeman, L. Civetta, M. D'Antonio, G. Ferrara, and A. Necco (2004), B/Nb and δ11B systematics in the Phlegrean Volcanic District (PVD), J. Volcanol. Geotherm. Res., 133, 123139.
  • Trua, T., G. Serri, and M. P. Marani (2003), Lateral flow of African mantle below the nearby Tyrrhenian plate: Geochemical evidence, Terra Nova, 5(6), 433440, doi:10.1046/j.1365-3121.2003.00509.x.
  • Wortel, M. J. R., and W. Spakman (2000), Subduction and slab detachment in the Mediterranean-Carpathian region, Science, 290, 19101917.