Fast Episodes of West-Mediterranean-Tyrrhenian Oceanic Opening and Revisited Relations with Tectonic Setting

Extension and calc-alkaline volcanism of the submerged orogen of alpine age (OAA) initiated in Early Oligocene (~33/32 Ma) and reached the stage of oceanic opening in Early-Miocene (Burdigalian), Late-Miocene and Late-Pliocene. In the Burdigalian (~20–16 Ma) period of widespread volcanism of calcalkaline type on the margins of oceanic domain, seafloor spreading originated the deep basins of north Algeria (western part of OAA) and Sardinia/Provence (European margin). Conversely, when conjugate margins’ volcanism has been absent or scarce seafloor spreading formed the plains Vavilov (7.5–6.3 Ma) and Marsili (1.87–1.67 Ma) within OAA eastern part (Tyrrhenian Sea). The contrast between occurrence and lack of margin’s igneous activity probably implies the diversity of the geotectonic setting at the times of oceanization. It appears that the Burdigalian calcalkaline volcanism on the continental margins developed in the absence of subduction. The WNW-directed subduction of African plate probably commenced at ~16/15 Ma (waning Burdigalian seafloor spreading) after ~18/16 Ma of rifting. Space-time features indicate that calcalkaline volcanism is not linked only to subduction. From this view, temporal gap would exist between the steep subduction beneath the Apennines and the previous, flat-type plunge of European plate with opposite direction producing the OAA accretion and double vergence.

Scientific RepoRts | 5:14271 | DOi: 10.1038/srep14271 convergence is probably at the origin of the tenet that melt uprising and metasomatism overlapped in time with subduction in the geological past, too. On the other hand, the geological record suggests that metasomatism has probably taken place earlier than the calc-alkaline eruptive activity [12][13][14] . This work, based on comparative examination of the distribution of continental margin volcanism at the times of oceanization, recognizes and tentatively quantifies the temporal gap between old and new (reversed) polarity of subduction. The gap may be useful to unravel the tectonic setting linked to calc-alkaline volcanism of the past 33 Ma. Igneous geochemistry data, from literature on the study area, is listed in the Supplementary Datasets S4, S6, it being a non-fundamental issue. Some of the examined geochronology data could be incorrect because of the diverse quality of the analysed material, and diversity of laboratories and analytical methods. However, dubious (few) ages can be tentatively pointed out, based on the large data set currently under examination and plausible temporal correlation.
Extension and magmatism commenced in Early Oligocene and reached the stage of oceanization by ~20-16 Ma (Burdigalian). Early Miocene sea-floor spreading formed the oceanic crust flooring of the north Algeria and of Sardinia-Provence deep plains. Overall, basin formation is linked to rotation of small continental blocks ( Fig. 1) [15][16][17] . The Tyrrhenian Sea's deep plain came into existence (Figs 1 and 2B) only after the Burdigalian age's counter-clockwise rotation of the eastern part of the OAA which, together with its Hercynian foreland of Corsica-Sardinia drifted away from the European margin (SE France). The Sardinia-Provence and North Algeria basins are separated by the major transcurrent structure known as the "north Balearic fracture zone" or the "Paul Fallot transform fault" which dates back to plate interactions of the Hercynian orogeny 4,16 . This structure, extending from the Catalan volcanic zone to the magmatic island of La Galite (Tunisian offshore) -hereafter called "Catalan-Tunisian fracture zone" (CTFZ) -separates the western segment of the Mediterranean OAA and the European margin from that  Oligocene-Aquitanian (between ~33/32 and ~20 Ma): intrusives (red circle), volcanics (red triangle) and allochthonous volcanoclastics (elongated red triangle without rim shows presumed position; green numbers in italics; see Suppl. dataset S4). Site 13: buried andesite volcano of the early Oligocene. White arrows: 1 = pre-Oligocene (> 33 Ma), SE-directed subduction beneath the orogen of Alpine Age (OAA); 2 = Post-Burdigalian (< 16 Ma), WNW-directed subduction of African lithosphere; this reconstruction considers that subduction was absent in the Oligocene -Burdigalian time. (B) Burdigalian (~20-16 Ma): intrusives (black circle), volcanics (black triangle) and allochthonous volcanoclastics (elongated gray triangle); Langhian-Tortonian (between ~16 and 7.5 Ma): intrusives (yellow circle), volcanics (black rimmed yellow triangle) and allochthonous volcanoclastics (elongated yellow triangle). In the Burdigalian, tholeiitic lavas accompanied the calc-alkaline magmatism of andesitic and silicic type as the continental rifting reached the stage of oceanic spreading in the Sardinia-Provence and north Algerian basins respectively on the east and on the west of the Catalan-Tunisian fracture zone (CTFZ). The scheme shows the proposed position of the WNW subduction hinge zone at ~6.3 (Vavilov opening) and ~1.67 Ma (Marsili opening), the Apennine outcrops of the allochthonous volcanoclastic rocks, and the hypothetical sites of "lost" volcanic edifices and adjacent sedimentary lows which, according to the geodynamic interpretation (see Guerrera et al., 1998, andCibin et al., 2001 in the Suppl. info), were originally sited in the Tyrrhenian OAA (offshore Sardinia-Corsica). Dotted line: the lithosphere-asthenosphere section of Fig. 6A. The figure was created by the author with the use of plotmap software 41 .
to the east. In particular, the fracture zone divided Sardinia-Provence deep plain (Hercynian European margin) from the western segment of the OAA (north Algerian basin), and this from the eastern one. Another major lineament running along the 41° parallel 1,17 separates the northern Tyrrhenian's thinned continental crust from the oceanic crust to the south. The two areas are rimmed by the Apennines, which rotated counter-clockwise and clockwise, respectively to the north and south.
Volcanoclastic rocks showing calc-alkaline nature, are widespread among the allochthonous sediments of the Apennines ( Fig. 3; Supplementary Info S3 and Datasets S5, S6). If they have been produced from lost emission centres, which were originally sited in the Tyrrhenian OAA, their space-time distribution can also be meaningful for the reconstruction of the link between calc-alkaline volcanism and tectonic setting. For the first time, allochthonous and "in-situ" volcanics were jointly considered. Fast sea-floor spreading. Burdigalian (~20- 16 Ma). Between ~20 and 16 Ma, the oceanization of European lithosphere and submerged western part of OAA originated, respectively, the Sardinia-Provence and north Algeria deep basins (Figs 1 and 2). The Burdigalian timing of the two openings has been ascertained only from the geology of continental margin as deep drilling data from the basaltic crust are not available. The origin of Sardinia-Provence basin is linked to counter-clockwise rotation of Corsica-Sardinia; the age of rotation has been determined mainly from the combination of paleomagnetic and geochronological data of volcanic rocks from the magma-rich Corsica-Sardinian margin [18][19][20] . Temporal overlap between the Sardinia-Provence and north Algeria basin opening has been considered by various authors 1,3,16 . 17.8, 17.4 Ma are reported 21 for peridotite emplacement at the Edough granitoid Massif of Little Kabylia (eastern Maghrebides; Fig. 2; site 12). The authors propose temporal overlap between the onshore tectonics and seafloor spreading of the north Algeria basin. Along the coastal area of Algeria, granitoids, and andesites and dacites erupted at ~16-15 Ma 22 . The authors consider that such short-lived intense magmatism ought to be connected with slab break-off, and the last-stage of seafloor spreading in the Algerian offshore.

The episodes
Late Miocene (~7. . Deep drilling data, data of basement lithology 23,24 and multibeam mapping 25 are essential for understanding the complex evolution of Tyrrhenian seafloor. However, the nonexistence of the typical lineated magnetic anomalies is at a disadvantage 26 28 , and ended at 5.33 Ma (start of the Pliocene). Evaporites are most likely not present in the lower Sardinian margin 23 . The authors, based on the evaporite occurrence in the upper Sardinian margin and on absence or scarcity in the lower one, consider that seafloor depth might have been diverse during the evaporitic episode. In this view, the shallow seafloor of lower Sardinian margin and adjacent Vavilov plain, too, might have impeded Atlantic-water-inflow in the sufficient amount to precipitate evapotitic gypsum.
The interaction among faulting and magmatism played a significant role in the development of Tyrrhenian's seafloor spreading. Seismic stratigraphy 24 indicates east-dipping low-angle detachment faults producing Late-Tortonian/Early-Messinian strong extensional deformation on the continental margin offshore Sardinia. The E-W oriented hyperextension of the southern part of Tyrrhenian Sea appears to be coeval with the punctiform MORB-type volcanism of DSDP well-373. At about the same time span, granitoids erupted in the northern part (~8/6 Ma; Figs 4 and 5). The intrusive rocks are distributed from the southern Vercelli seamount and Etruschi ridge to the subaerial outcrops of the islands of Montecristo and Elba to the north 17,29 . Overall, localized basalt volcanism, not-lineated and low-standing, combined with strong extensional deformation and mantle peridotite exposure (DSDP well-651) would characterize the Vavilov's seafloor speading, thus providing useful constraints for better understanding the early stage of the N-E Atlantic opening 30 .

Relationships between oceanization and peri-bathyal magmas. Age distribution and start of
WNW-directed subduction. An age histogram illustrates the geochronology data of magmatics from the Provence-Corsica-Sardinia-Tyrrhenian region and the peninsular Italy since the Oligocene (Fig. 4a). Figures 4b and 5 show that late-Miocene, along-strike magmatism consists of basalts and granitoids. These igneous rocks erupted respectively in the Vavilov plain (oceanic spreading OS2) and on the thinned continental crust of the north Tyrrhenian 37 . In the Burdigalian, seafloor spreading of the Sardinia-Provence basin (OS1) has been about concomitant with peak volcanism that accompanied the rotation of the Corsica-Sardinian block. The oceanization of the western part of OAA, too, has been accompanied by eruptive activity along the Algerian and Betic-Balearic conjugate continental margins (Fig. 2). On the other side, oceanic spreading OS2 and OS3 formed the Vavilov and Marsili plains (Fig. 4) while conjugate margin volcanism was absent or scarce.
Such space-time distribution indicates that the Burdigalian-age oceanization on one side, and those of the late Miocene and late Pliocene on the other, are probably linked to clearly distinct tectonic environment. This reconstruction considers that Burdigalian seafloor spreading, and Oligocene-Aquitanian rifting, and calc-alkaline volcanism developed in the absence of subduction (Fig. 6A)

Discussion
Mature and failed rift. Overall, rift tectonism is either of "failed" (aborted) or "mature" type: the former is linked to continental thinning typically characterized by horst-graben formation, and the latter reaches the eventual stage of oceanization. In various regions of the West Mediterranean and   It has here been assumed that the past lithosphere thickness of the Alpine-Betic orogen was comparable to that of nowadays Alps. Emplacement of the calc-alkaline magmas before start of WNW-directed subduction implies that the metasomatic modification of the corresponding igneous sources has been produced by lithosphere shortening of Hercynian or Alpine age. Because the orogenic accretion processes are repeated in time, Hercynian remnants are likely present in the metasomatized bodies of Alpine-age, and Alpine remnants in the bodies of Apennine-age. The figure was created by the author. surroundings (e.g., the Valencia basin, the Alboran Sea, the Sicily Channel, the Aegean Sea, the Rheintal Valley, the Rhone Valley, Limagne-Massif-Central-Bresse) Tertiary-Quaternary rifting of "failed" type produced only continental thinning and stretching. "Mature" rift can be distinguished from aborted rift spatially or temporally. A spatial distinction is found in the Tyrrhenian Sea as late Miocene rift activity of "failed" and "mature" type is found respectively in its northern and southern parts. Modest extension (incomplete rift) 37 accompanied the magmatism of granitoid nature of the north Tyrrhenian seafloor (Fig. 5). On the other hand, strong extensional deformation accompanied the Vavilov plain volcanism of MORB type to the south 29 . Granitoid magmas are widespread in the north Tyrrhenian, whereas basalt volcanism appears limited to the eastern rim of the Vavilov plain (DSDP well-373A; Fig. 3). Spatial distinction is also manifest in the Mediterranean OAA during the Burdigalian age. In fact, rifting of "failed" and "mature" type, in the absence of subduction, occur respectively to the east (future Tyrrhenian sea) and the west of the CTFZ (north Algerian basin). Further in the past, the Permo-Triassic "failed" rift of the southern Alps could have been associated with the distant opening of the Permo-Triassic Tethys. Regarding temporal distinction, rift/spreading transition of the Mediterranean OAA shows Early-Miocene, Late-Miocene and Late-Pliocene ages.
Nascence of WNW-directed subduction and the fate of volcanoclastic rocks. Pre-Oligocene (> 33/32 Ma) lithosphere thickening above the SE-subduction of the European lithosphere produced the fore-belt and retro-belt of the Mediterranean-Tyrrhenian OAA 3,6 . Various authors and this reconstruction, consider that the alpine-age retro-belt is present in the internal part of the Apennines, possessing similar vergence to that of the future external part 1,6 thus facilitating commencement of WNW-directed subduction of the African plate. Overall, intra-mountain lithosphere rupture and volcanism of calc-alkaline nature, produce fault-bounded horst, exposing crystalline-metamorphic and volcanic rocks, which alternate with grabens that contain thick deposits of volcanoclastic and siliciclastic nature 8,9,38 . The Supplementary Info S3, and Dataset S5, S6 describe the calc-alkaline volcanoclastic layers of Oligocene -Burdigalian age, which are found as allochthonous bodies in the Apennines in the absence of the emission centres. The enigmatic locality of the lost centres (Sardinia, Tyrrhenian Sea, Adriatic foreland) has been discussed by various authors (Supplementary Info S3). This reconstruction tentatively contributes to the discussion, considering that, at the nascence of the WNW subduction and of Apennine thrusting, the proposed upper plate source area of the allochthonous volcanoclastic rocks was probably affected by inversion tectonics, in which a compressional stage follows the extension-dominated stage. The fate of volcanoclastics might have been determined by the significant change of the tectonic mode affecting their sites of origin in the Tyrrhenian OAA. By the beginning of the WNW subduction, inversion tectonics would induce the initial down faulting of the original horst volcanoes (past topographic highs) and the upthrust of fault-bounded grabens bearing volcanoclastic deposits (former lows). In the more orthodox concept of West-Mediterranean-Tyrrhenian evolution, the persistent WNW subduction of the last 33/32 Ma, would exclude horst-graben inversion tectonics. The presence of rifting and calc-alkaline volcanism in the lack of ongoing subduction most probably implies that igneous sources have been metasomatized by crustal material brought downwards during previous period(s) of lithosphere shortening [11][12][13][14] .
The Oligocene of the Western Alps. The study area may have had important geological and temporal connections with the Oligocene Alps. The compression tectonics of the Western Alps has been supplanted by extension and lithosphere thinning that lasted from the start of the Oligocene until the late-early Miocene (~20 Ma) 11,39,40 . Rifting and increase in geothermal gradient produced HT/LP metamorphism, which was accompanied by generation and eruption of calc-alkaline melts between the Oligocene and early Miocene 40 . By the end of early Miocene, igneous activity and extension of the Alps ceased and the orogenic accretion resumed. The resumption of lithosphere shortening of the Alps appears to be temporally related to the onset of oceanic opening of the West Mediterranean. The reversal of subduction polarity and its geotectonic implications have been studied at the junction between the Western Alps and the Northern Apennines 4 . The authors recognize that polarity inversion started at the Eocene/ Oligocene boundary, concomitantly with Alps's extensional setting and Apennines's thrusting. However, the authors consider also that subduction of true Apennine type has taken place only beginning from the late Miocene accompanied by the Calabrian slab pull. In view of this, a long-lived process of subduction flipping could have taken about 20 Ma.
Tectonics and magmatism. After the pre-Oligocene alpine accretion linked to subduction with SE polarity, the tectonic and magmatic activity of Oligocene-Burdigalian age can be distiguished from that of the Langhian-Recent. In the absence of subduction, rifting and magmatism showing acidic calc-alkaline nature and Oligocene-Aquitanian age (~33/32-20 Ma) were followed by sea-floor spreading (~20-16 Ma) and coeval calc-alkaline volcanism with basic to acidic composition around the oceanic domain. The start of WNW-directed subduction by the Burdigalian/Langhian transition preceded seafloor spreading of the Vavilov plain. Supra-subduction extension and basalt volcanism of Tyrrhenian seafloor have been discontinuous. Strong extension and scarce low-standing volcanism alternated with seamount volcanism linked to weak extension (Supplementary Info S2). MORB-type lavas (ODP well-655) created the modest elevation of 4.3 Ma old Gortani ridge (Fig. 3), located NW from the low-standing volcano of DSDP well-373. Afterwards, MORB volcanism migrated only towards the hinge zone. In the course of ESE-directed migration, from Gortani ridge to the axial volcanoes of Vavilov (< 2.6/2.4 Ma; pre-Olduvai Matuyama 26 ) and Marsili (< 0.8 Ma; the Brunhes chron), the seamount elevation gradually increased. Eventually, large magma input formed the over-fed Marsili volcano, the last of the "sui generis" spreading axes of Tyrrhenian seafloor 25 . By the final stage of Vavilov plain oceanization (about < 0.5 Ma), weak horizontal deformation went with eruption of alkaline basalt flows on the summit of Vavilov volcano (Fig. 5, and Supplementary Info S2).

Conclusions
Pre-Oligocene, SE-directed flat subduction of the European plate produced the submerged orogen of the West-Mediterranean-Tyrrhenian region. Subsequently, WNW-directed steep subduction of the African plate accompanied oceanization of the Tyrrhenian basin, the segment of the submerged orogen to the east of CTFZ. Post-orogenic continental extension and calc-alkaline volcanism initiated in the Oligocene, in the Burdigalian (~20-16 Ma) reached the stage of an oceanic opening in the European plate (Sardinia-Provence basin) and the western segment of the submerged orogen (north Algeria basin). In this same time period, volcanism of calc-alkaline type was widespread on the margins of the oceanic domain. By contrast, the oceanic plains of late-Miocene Vavilov and late-Pliocene Marsili, originated when across-strike volcanism had been absent or scarce. The contrast between abundance and lack of conjugate margins' volcanism, at various times of the seafloor opening, would turn out to be due to the diversity of the geotectonic setting. If so, from Early Oligocene to the Burdigalian/Langhian boundary continental extension seafloor spreading and calc-alkaline volcanism developed in rift setting, in the absence of subduction. The WNW-directed, steep subduction under the submerged and stretched orogen of alpine age, probably took place only in the last ~16/15 Ma (after the waning of Burdigalian sea-floor spreading). This reconstruction indicates that calc-alkaline volcanism is not linked exclusively to subduction. It appears that only the Tyrrhenian oceanization occurred in supra-subduction setting, after ~18/16 Ma between the conclusion of the SE-directed flat subduction and the nascence of steep WNW descent, representing the Alpine and the Apenninic mode of lithosphere consumption, respectively.