Basaltic crust is present in the oceans and marginal seas. Oceanic accretion from inception to ending may be usefully recognized in small basin setting like the Tyrrhenian. Alternating episodes of strong and moderate extensional tectonics characterized the small Tyrrhenian opening. Hyperextension (drifting) of late-Miocene and latemost Pliocene age was followed by Pliocene and Late Quaternary moderate extension (rifting). Early hyperextension (~7.5–6.3 Ma) acted in the submerged margin of Hercynian Sardinia. Sardinia offshore, E-directed low-angle faults were accompanied by MORB-like volcanism of non linear shape in the shallow Vavilov plain - inherited segment of alpine-age orogen. Late hyperextension (~1.9–1.7 Ma) acted along the central N-S lineament of Vavilov plain, former metamorphic core complex. At the lineament northern side, E-dipping detachment faulting exposed serpentinized peridotite. At the other side, Vavilov volcano was faulted and its east flank tilted westwards. At the same time, volcanism with affinity to transitional MORB induced opening of Marsili basin. The drift episodes were characterized by absence or scarcity of volcanic activity on the conjugated emerged margins. The rift episodes (respectively ~5–1.9 Ma, and ~1/0.8 Ma-Recent) saw growth of major north-south trending volcanoes in bathyal area as intense volcanic activity developed on the continental margins.
Oceans are thought to originate following continental rupture. Oceanic crust with MORB affinity forms in oceans and marginal seas. Basaltic crust accretion from inception to ending may be investigated straightforwardly in small basins rather than in ocean. Two episodes of hyperextension and E-MORB volcanism characterized the Tyrrhenian intra-orogenic basin opening1,2. The small oceanic basins of Vavilov and Marsili are located in the central part of the sea (Fig. 1) overlying west-directed subduction of lithosphere belonging to Adria/Ionian microplate. In and around the bathyal area are present stretched remnants of orogenic accretion exhibiting Hercynian, Alpine and Apenninic age. Rifting of Hercynian Corsica-Sardinia and inherited orogen of Alpine age was initiated in early Oligocene3,4. Burdigalian-age spreading (~20–16 Ma) of Sardinia-Provence oceanic basin accompanied the counter-clockwise rotation of the adjacent continental blocks of hercynian and alpine age. The Tyrrhenian orogen was produced by SE-directed, flat-slab subduction of lithosphere located to the east of Corsica-Sardinia foreland5,6. Orogenic accretion was followed by rifting which was in turn supplanted by convergent tectonics reflecting inversion of subduction polarity3. The W-directed, steep-slab type of subduction was linked to E-verging Apenninic thrusting, back-arc opening and slab migration. The flat-slab subduction produced alpine double vergence3,5,6 (Fig. 2). Eastdipping front- and westdipping back-thrusts crop out on the western and eastern margins (NE Corsica and Calabrian arc), respectively. Geophysical investigations, deep drilling results of the Deep Sea Drilling Project (DSDP) Leg 42 and of the Ocean Drilling Program (ODP) Leg 107, and seafloor sampling indicate that Vavilov and Marsili bathyal plains are floored by basalts and exposed ultramafic rocks, and were affected by rapid subsidence4,6,7,8,9,10,11,12. Since the basins’ magnetic anomaly field (Fig. 3) shows significant intensity but does not have the linear symmetric patterns typical of the ocean floor, the intra-orogenic Tyrrhenian opening can not be recognized in a straightforward way on the basis of magnetic signature13,14. Thus, in spite of young age (< 8 Ma) the Tyrrhenian spreading process remains poorly documented. In order to contribute to the comprehension of such process we have examined the relationships between (i) extensional tectonics and volcanism in bathyal area, and between (ii) this volcanism and volcanism developed on the conjugated continental margins.
Episodes of magma-poor hyperextension of initial oceanization
Late-Tortonian/Early-Messinian (~7.5 – ~6.3 Ma)
Eastwards from the Sardinia island, ~180 km of stretched Hercynian crust form the Tyrrhenian passive margin. Strong asymmetric deformation was produced by east-dipping, low-angle detachment faults which were associated to late Miocene sediments6. Inspection of seismo-stratigraphic data (documented in Fig. 4 of ref.6) shows that N-S trending listric normal faults reduced from ~30 to 10 km the thickness of continental crust of submerged Sardinian margin, converging basinward with Vavilov plain serpentinized upper mantle (see Supplementary Information). At the plain’s eastern edge, DSDP Site 373 (Fig. 1) penetrated 190 m of basaltic crust; flows and breccias show MORB nature and K/Ar whole-rock age between ~7.5–6.3 Ma3,8,15. This dating indicates temporal overlap between Vavilov basin magmatism and hyperextension on the conjugated Sardinian margin without accompanying volcanic activity.
Such tectono-magmatic evidence indicates that late Miocene basalt crust formation of Vavilov basin preceded the Mediterranean salinity crisis (late Messinian; 5.96–5.33 Ma). The absence of evaporitic deposits in bathyal area is probably due to existing shallow water conditions3,8. Emplacement of MORB melts and exposure of serpentinized peridotite (ODP site 651) in the Vavilov basin, and hyperextension at the submerged Sardinian margin appear to recall the tectono-magmatic architecture linked to the Mesozoic continent–ocean transition at the Iberian margin6,16.
Latemost Pliocene (~1.9 – ~1.7 Ma; Olduvai subchron)
In the axial zone of Vavilov basin, inherited alpine orogen, the N-S elongated tectono-magmatic lineament linking Vavilov-volcano to exposed-peridotite8 is associated with lineated magnetic anomalies (Figs 3–5). The northern portion shows positive anomaly with intensity up to 70 nT (airborne survey)13. Intensity of 200 nT was obtained from shipborne data from the same area11. The southern portion of the lineament is characterized by alternating negative and positive values of magnetic field (Fig. 5b). Seismostratigraphic evidence suggests that this tectonic lineament is due to detachment faulting8. ODP Site 651 drilled through the faulted basement swell which consists of peridotites, dolerites and basalts. Such basement complexity is overlain by thin late Pliocene ooze (Globorotalia inflata zone; MPl-6; 2.6/2.4 – 1.9 Ma) which is below 340-m-thick Pleistocene sedimentary sequence (Fig. 6). Positive magnetization and foram assemblage of ooze directly overlying basement indicate that east-dipping detachment linked to mantle exposure occurred in the latemost Pliocene (LmP) - Olduvai subchron, ~1.9–~1.7 Ma. The classical scales of geological time contemplate that the Olduvai corresponds to the Tertiary/Quaternary boundary (~1.8 Ma) which separates the Pliocene from the Pleistocene portion of Matuyama chron. Based on seismostratigraphy evidence (Fig. 6), the peridotitic body was exposed through slip on east-dipping detachment fault acting in LmP time. Probably coinciding low-angle extensional faulting acted at Vavilov seamount too (Fig. 5c), without accompanying volcanic activity. Magmatism linked to the volcano-peridotite lineament is an issue of the next chapters.
At the same time of Vavilov plain hyperextension, oceanic opening was initiated in Marsili basin (Fig. 4) coinciding with volcanism absence on the emerged conjugate margins. At western edge of Marsili igneous basement (Fig. 4) ODP Site 650 drilled basaltic crust with MORB transitional to calcalkaline affinity. The basalt lava is linked to round shape, positive magnetic anomaly with intensity up to 200 nT11. The forams assemblage and the positive magnetization of ooze (Trubi) directly overlying the igneous basement indicate LmP age, as at ODP Site 651. The overlying Pleistocene sequence consists of 600-m thick, sediments of terrigenous and volcanoclastic nature8,9. Being strongly altered and vesicular, the early basalt flows of Marsili plain probably erupted in shallow marine environment.
Episodes of ocean-floor moderate tensional tectonics and seamount volcanism
Pliocene (~5 – 1.9 Ma)
The N-S trending Gortani ridge was drilled at ODP Site 655 (Fig. 4). 116 m-thick basalt flows beneath only 80 m of Pliocene-Quaternary sediments show MORB affinity and 40Ar/39Ar age of 4.3 Ma10. To the west the Gortani ridge, basaltic crust is flanked by N-S elongated sedimentary basin showing > 1 sec sediment thickness6. With respect to MORB volcanism from DSDP Site 373, Gortani’s location is more distant from the W-dipping front of Adria-Ionian subduction. Such remoteness is probably a consequence of (i) past shallow depth of subducted lithosphere with little slab pull towards the active margin and/or (ii) the hyperextension that affected bathyal area and Sardinia submerged margin in late Miocene allowed Pliocene MORB volcanism migration towards the passive margin. After the Gortani, such volcanism migrated only towards the hinge zone.
ODP-651 drilling is located ~20 km to the east of Gortani ridge, and 15 km to the north of Vavilov volcano along the volcano-peridotite tectonic line, at the axis of Vavilov basin. In the northern side of lineament, serpentinized harzburgites were drilled for 30 m at the base of the well. They rest below a 135-m-thick sequence of basalt flows separated by an intrusive dolerite body8 (Fig. 6). The Site’s lower basalts of MORB composition yielded 40Ar/39Ar dating of 3.0 Ma10. The upper basalts have calc-alkaline affinity and about 2.6 Ma of age. Such datings indicate the Gauss chron for the N-S elongated positive anomaly with maximum intensity of 200 nT at ODP Site 65111. Figure 6 shows that igneous basement is overlain by 40-m-thick sterile dolostone17 which rests directly beneath LmP ooze (see previous chapter). On the other side of lineament, the magnetic anomaly field associated with Vavilov volcano shows negative and positive values11 (Fig. 6). On the basis of shipborne measurement, the northern and southern negative magnetic anomalies exhibit values up to −430 and −360 nT, respectively. The intervening positive zone with maximum intensity of + 370 nT is close to the seamount’s top. It is W-E elongated whereas the “peak-values” of normally and inversely magnetized bodies are NNE-SSW aligned with the relief axis. Seabeam bathymetry and submersible dives (Fig. 5) reveal that the west flank of Vavilov is significantly steeper than that to the east18,19. Figure 5 shows that Vavilov volcanism is divided in lower, intermediate and higher units. The lower unit (I) at waterdepth between 3.600 and 1.500 m is linked to negative anomaly. It erupted in the Pliocene portion of Matuyama chron – between 2.6/2.4 and 1.9 Ma. In fact, “Trubi-type” ooze from the western flank at 2880 m depth contains foram assemblage showing late Pliocene age. The rock was thermally indurated on contact with hot lava18. Massive pillow and bolster flows form flow-fronts that are up to 50-m-high. On the eastern flank, the basalt scarps are next to sediment-covered shelves up to 250-m wide. Flows and shelves of this flank were manifestly tilted towards the west and stretched towards the east. Their inclination towards the volcano’s axial zone was likely produced by the same east-dipping detachment faulting that exposed the peridotite swell at ODP Site 651 in LmP time (~1.8 Ma). So, the intermediate unit (II) at waterdepth of 1500–1000 m probably erupted in post-Olduvai time (<1.7 Ma). It consists of pillow flows that have <10-m-thickness and equal dip at either flank of seamount. Scoriaceous lavas of alkaline (intra-plate) nature form the upper unit - discussed below.
Quaternary (~1/0.8 Ma to Recent)
In the volcano-peridotite lineament of Vavilov basin, volcanic activity developed also in the late Pleistocene after the Matuyama/Brunhes geomagnetic reversal (<0.8 Ma)20. Figures 3 and 5 show the N-S elongated Matuyama’s negative magnetic anomaly of Vavilov volcano which is interrupted by circa E-W elongated positive anomaly11. It is located near to the top of seamount and it widens eastwards. The NNE-SSW aligned “peak-values” of normally and inversely magnetized bodies parallel the relief axis. Basalts from the upper unit are most likely linked to the Brunhes chron15. They exhibit alkaline composition and K/Ar age of ~0.4 and <0.1 Ma18,21. Alkaline basalts form by partial melting of mantle sources which are compositionally distinct from sources which experienced metasomatic modifications from subducted lithosphere at the origin of calcalkaline melts. Despite the scrupulous dives of Cyana and Mir submersibles and numerous dredge hauls, the fresh pillow flows forming the bulk of Vavilov seamount did not yield any specimen. A reason for this may be that lava detritus is overlain by sediment. Given the diversity of morphology, age and magnetic field, the chemistry of lower unit and summit lavas is probably different.
Volcanism from large Marsili seamount appears to be not older than 0.8 Ma based on the Brunhes chron and the < 0.2 Ma age of rocks dredged from the summit zone11,15,22. The seamount magnetic anomaly field, on a small scale shows transition from a lateral spreading to a pure vertical growth which probably initiated at the Jaramillo subchron23. The recovered samples constantly show calc-alkaline affinity; high-potassium andesites are present at the summit, whereas basalts from the intermediate-deep zone show medium- to low-K composition24. Voluminous volcanism of calc-alkaline affinity of the Aeolian arc developed since ~1 Ma around Marsili plain1,3 from magma sources metasomatized from subducting lithosphere.
Volcanism on the conjugated continental margins of Sardinia and Campania; their relationships with bathyal volcanism
The Rifted Continental Margin from Sardinia
The orogenic (calcalkaline) cycle
Since early Oligocene the Hercynian island of Sardinia was affected by two distinct cycles of volcanic activity with an interruption lasting ~7 Ma (Fig. 7). The Oligo-Miocene cycle of calcalkaline affinity (~30–12 Ma) developed in the N-S trending trough of western Sardinia25,26. The “Fossa Sarda” was affected by rifting concomitantly with rifting of Provence conjugated margin until ~20 Ma. At this time, continental rift reached the stage of oceanic opening of Sardinia-Provence basin and rotation of the Corsica-Sardinia block (~20–16 Ma)3,27. Andesites and ignimbrites are the main manifestations of the Oligo-Miocene volcanism. Pillow-lavas with tholeiitic, high-Mg basalt composition are associated with early Burdigalian (~20–18 Ma) shallow marine sediments from central western Sardinia28. It appears that effusion of mafic volcanics in Sardinia (~20–18 Ma) was coincident with oceanic opening of Sardinia-Provence basin.
The anorogenic (alkaline) cycle of Pliocene-(late Quaternary) age
The subaerial anorogenic (within-plate) cycle initiated to develop only after opening of Vavilov plain. The Pliocene basalt volcanism with varying alkaline to subalkaline composition is found in both western and eastern Sardinia29 (~5/4–2 Ma; Fig. 7). It was related to tensional tectonics which involved also Tyrrhenian seafloor. Subaerial basalt flows bordering the Orosei Gulf have been argon dated between 3.6 and 2.0 Ma30. In the distant Gulf offshore (Fig. 1), by the edge of bathyal plain ~75 km west of Vavilov seamount is the location of Magnaghi seamount. Magnaghi’s alkaline basalt lava shows ~3.0–2.7 Ma31 which recalls the age of peak alkaline basalt volcanism from Sardinia (~2.8 Ma21,29). The Magnaghi rocks represent the only known manifestation of Pliocene volcanism from the Sardinia continental slope. The within-plate cycle from Sardinia ceased to develop between ~2–0.9/0.8 Ma. The early part of this period (~1.9–1,7 Ma) saw strong extensional tectonics and initial oceanic opening of Vavilov plain and Marsili plain, respectively. The anorogenic volcanism continued to develop in late Quaternary (~0.9/0.8–0.1 Ma). Yet, it is found only in the Logudoro region (i.e., NW Sardinia), and without accompanying manifestations with subalkaline characteristics21,32. The Logudoro “basalti delle valli” and the rocks from the summit area of Vavilov volcano were related to late Quaternary tensional tectonics.
The offshore Campania margin
Seaward from Campania three sedimentary basins border on the structural high of Ponza Palmarola and Zannone islands which form the western portion of Pontine Archipelago (Fig. 8)4,33,34. To the south, very steep slope directly connects Palmarola with the bathyal domain (Fig. 4). Conversely, the basins from Palmarola and Ventotene, positioned respectively NW and SE of the structural high, are parallel to Campania coast. Ponza Ventotene and Palmarola to the west, and Ventotene and S. Stefano to the east are entirely volcanic islands, whereas at Zannone crop out also metamorphic rocks (phyllites) and sediments showing Mesozoic - Tertiary age4,35. Pontine volcanism has episodic nature33,36,37. ~4.4 Ma old rhyolites of calcalkaline affinity and ~1.1 Ma K-rich trachytes extruded at Ponza. Ponza trachytes, and 0.8-Ma-old basalts from Ventotene and S. Stefano islands are early products of the late Quaternary Roman Magmatic Region38,39. More to the east, significant K-rich volcanism of the last 0.2 Ma developed in connection with tensional and transtensional faulting in and around Ischia and Procida islands (Campanian shelf), and Phlegrean Fields and Vesuvius (Naples bay)40. The extrusion dome of Palmarola ~1.75 Ma of age (LmP) is made of rhyolites of transitional composition between calcalkaline and alkaline33,41. Palmarola rocks appear to overlap temporally the hyperextension along the lineament linking Vavilov-volcano to exposed-peridotite. Early Pliocene calcalkaline rhyolitic volcanism, and normal and wrench extension faulting initiated to form the Palmarola and Ventotene basins34. Based on seismic reflection profiling and geological data, a widespread middle-Pliocene sedimentary unconformity was recognized on the Tyrrhenian margins, and it was considered to reflect overall basin-wide subsidence42. In Vavilov plain, Gortani ridge volcanism and adjacent basin formation developed in the early Pliocene (~4.3 Ma) shortly before large-scale unconformity occurrence6,10 (Supplementary Information). Gortani’s localized tectono-magmatic manifestation has probably put an end to late-Miocene seafloor shallowness of Vavilov basaltic crust. In the Campania shelf, landward from Ventotene basin a late-Miocene to Quaternary W-E trending submerged sedimentary sequence has been recognized34. The paleo-basin of Terracina, “presently masked by the continental shelf shows a marginal, terrigenous facies type surely lacking the evaporitic rocks so characteristic of some Tyrrhenian margins”34. With time, the late Miocene W-E distensive deformation from Gaeta gulf was supplanted by NW-SE extensional tectonics and volcanic activity compatible with the youngest extension direction of Tyrrhenian basin4,40.
The basalt - peridotite lineament of Vavilov plain
Regarding the nature of Tyrrhenian igneous basement, some authors12 suggest that the area floored by peridotite is larger than that showing basalt compostion6. From a geodynamic perspective, the former view may imply that Tyrrhenian opening was mainly forced by extensional tectonics and the latter by magmatic activity. The magmato-tectonic lineament linking Vavilov-volcano to peridotite might abut the volcanic island of Palmarola (Figs 4 and 8). LmP hyperextension along the Vavilov plain tectonic line accompanied by rhyolitic volcanism from Palmarola might announce inception of Vavilov-Marsili basaltic crust rapid subsidence. Such event was preceded by basin formation linked to high-angle faulting affecting basaltic plain and margins’ continental crust. Pliocene to Quaternary sedimentary basins are found only in submerged Sardinia margin, and in subaerial and submerged portions of Campanian margin, and may be also elsewhere6,34,43. On the Sardinia margin, ODP Site 654 drilled lava flow interbedded in Pliocene-Quaternary sedimentary sequence10 (Fig. 1). It is a basaltic andesite of LmP age, and is analogous to some subaerial Pliocene manifestations of intraplate affinity from Sardinia29. Figure 1 suggests that NW-SE trend of LmP oceanic opening supplanted the late Miocene E-W extension linked to basaltic crust formation of Vavilov plain. In the LmP episode, volcanism was localized only in marine areas. On the emersed margins of Sardinia and Campania the volcanic activity resumed to develop only about 1/0.9 Ma ago.
N-S elongated sedimentary basin showing > 1 s sediment thickness formed to the east of the LmP lineament6 (Supplementary info). Location of lineament is in the central basalt plain of Vavilov amid De Marchi and Flavio Gioia seamounts (Fig. 4). Crystalline-metamorphic basement is found at either relief. The Flavio Gioia is composed by late Hercynian granitoids and metamorphic rocks of various grade involved as tectonic units of the Alpine orogen; this crystalline-metamorphic association recalls the back-thrusts from Calabrian arc44. Conversely, the crystalline-metamorphic De Marchi relief includes tethyan ophiolites. They have been sampled in several other Tyrrhenian sites and appear to be similar to the front-thrust ophiolites from Alpine Corsica6,45. From a structural point of view, the tectonic line probably represents the collapsed geosuture linked to past double verging alpine-age crustal accretion (Fig. 2). Back-thrusted allochthons of metamorphic basement are present also in the internal side of Apennines46. The Apennines are characterized by clearly distinct modes of tectonic activity. Extensional and compressional deformation affect the thrust belt internal (Tyrrhenian) and external (Adriatic) side, respectively. Also the seismicity focal aspects of compressional and extensional type mirror the tectonic mode diversity at either Apennines side47. Extensional tectonics of the calcareous reliefs from Monti Sibillini and surroundings was at the origin of strong earthquakes and prolonged aftershock sequences from August 2016 to January 2017.
Evolution of volcanism and magnetic anomaly field
The anomalies associated with deep-seated basaltic crust have low intensity (up to 200 nT11) and shapes which are almost rounded or poorly elongated (diffusional spreading linked to strong extension and low-angle faults (the drillsites DSDP 373 and ODP 650; see inset -a- of Fig. 3). By contrast, the magnetic patterns linked to the high-standing basaltic crust (volcanoes) exhibit high intensity (up to 1500 nT) and elongated, better-organized configuration (quasi-linear spreading associated to moderate extension and high-angle faults). The elevation of elongated edifices strongly increased moving eastwards. Gortani ridge, Vavilov seamount and the over-fed Marsili seamount may be considered to be a “sui-generis” southeastward migrated sequence of N-S oriented axes of spreading.
–The Tyrrhenian opening was initiated in the late Miocene before Messinian salinity crisis and margins’ volcanism. Strong extensional deformation (drifting) was a consequence of E-directed low-angle faulting of the Sardinia offshore margin accompanied by ascent of MORB melts at the shallow eastern edge of Vavilov plain (DSDP-Site-373). Subsequent, E-directed low-angle faulting along the magmato-tectonic lineament linking Vavilov volcano to exposed peridotite acted in LmP time (~1.9–1.7 Ma). The short-lived hyperextension along Vavilov axial zone experienced concomitant opening of Marsili plain linked to emplacement of basalt melts (ODP-Site-650 at the plain western edge) without accompanying volcanism on the emerged margins.
–The post drift, Pliocene (~5/4 – 1.9 Ma) and late Quaternary (<1 Ma) basalt crust formation was characterized by moderate extension, high-angle faulting (rifting) and voluminous volcanism. Concomitantly, Pliocene volcanism developed in bathyal area and on conjugated continental margins. Early Pliocene (~4.4 Ma) MORB lava at ODP site 655 and basin formation from Vavilov plain6,10, and rhyolites from Ponza island and basin formation at Palmarola (submerged Campanian margin)33,34 probably indicate starting evolution from shallow to deep water conditions of the mafic crust.
–From a structural point of view, the volcano-peridotite lineament might represent a geosuture produced by alpine-age crustal accretion that probably implied obduction of large ultramafic bodies12. In this view of peridotitic lineament, the terms exposure/exposed can be be more appropriate than exhumation/exhumed. The lineament was affected by episodic volcanic activity from early Pliocene (ODP-Site-651; ~3.0 Ma) to late Quaternary. It likely abuts the structural high of Ponza archipelago. The extrusion of Palmarola rhyolites and hyperextension of Vavilov plain (~1.75 Ma) possibly triggered basin-scale extension change from W-E to NW-SE direction and inception of basaltic crust rapid subsidence4,33,40. Extensional tectonics and basalt volcanism of Tyrrhenian opening reflect crustal stretching of inherited alpine-age orogen, being itself related to rifting of the Apennines internal side.
Nevio Zitellini and CS produced the PDF files of article34 and articles21,24,25,29,30,33,37,38,44, respecticely. Such PDF files are found in the authors’accounts at www.researchgate.net - ResearchGate is probably the only way to obtain all this info.
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Research sponsored by PRIN2012 Programm (grant no. 20125JKANY_002). C. S. acknowledges V. Yastrebov and A. Schreider (Shirshov Institute of Oceanology, Russian Academy of Sciences) for the opportunity to participate in insightful geological-geophysical cruises in Tyrrhenian sea. In this article were used only published data which all are properly cited in the text. The quality of manuscript was improved by insightful examination of two anonymous reviewers.