TheTyrrhenian Basinis asedimentary basinlocated in the westernMediterranean Seaunder theTyrrhenian Sea.It covers a 231,000 km2area that is bounded bySardiniato the west,Corsicato the northwest,Sicilyto the southeast, and peninsularItalyto the northeast. The Tyrrhenian basin displays an irregular seafloor marked by severalseamountsand two distinct sub-basins - the Vavilov and Marsili basins. The Vavilov deep plain contains the deepest point of the Tyrrhenian basin at approximately 3785 meters.[1]The basin trends roughly northwest–southeast with the spreading axis trending northeast–southwest.
Regional geologic setting
edit
The Tyrrhenian basin is located in a geologically complex portion of the Mediterranean Sea. The basin is partially encircled by several orogenic belts, including theApenninesto the northeast, theAlpsto the north, and theAtlas Mountainsto the southwest. It is also bounded by aconvergent boundaryand associated trench to the southeast. Deep reflection seismic lines near the trench clearly showoceanic lithospherefrom the African plate flexing beneathCalabriato join a seismogenic slab that extends down to 500 km beneath the Tyrrhenian basin.[1][2]Between the upper slope of the trench and Calabria are a number of small fore-arc basins that developed through extensional listric faulting during theTortonian.[1]These basins subsequently subsided between the latePlioceneandQuaternaryand are now infilled withmélange.[1]
TheAeolian Islandsnorth of Calabria and Sicily represent thevolcanic arcof the convergent system. The volcanism associated with these islands occurs at the northeast closure of the downgoing slab and began approximately during the late Pliocene to early Pleistocene (1.5–1.7 Ma).[1]
The Tyrrhenian basin lies to the northwest of the Aeolian Islands and represents aback-arc basinformed from tensional forces associated with the northwest subduction of theAfrican Platebeneath theEurasian Plate.Similar to other back-arc basins, the Tyrrhenian basin displays a shallowing of theMoho discontinuitytowards the central part of the basin, aWadati Benioff zone,abnormally highheat flow(exceeding 200 mW/m2in some places), and an activevolcanic belton the arc-ward edge of the basin.[3][4]
Basement structure
editTyrrhenianbasement rocksconsist of latePaleozoicgranites derived from theHercynian orogeny.[5]Basement rocks in the southeastern portion of the basin were reactivated during theAlpine orogenywhile those to the northwest were undisturbed.[5]
Stratigraphy
editUpper Sardinia Margin
editThe Upper Sardinia Margin is a fault-boundedpassive marginlocated northwest of the Vavilov Basin.Seismic reflectionsurveys across the Upper Sardinia Margin exhibit a stratigraphic geometry suggestive of pre-rift, syn-rift, and post-rift sequences.[6]Drill cores penetrating to the base of the syn-rift sediments identified a transgressive sequence related tosubsidenceof thecontinental crustduring the rifting stage of the Tyrrhenian Basin opening.[6]At the base of the syn-rift sequence are 60 meters ofTortonianconglomeratewith subrounded clasts derived from metamorphosed carbonate and quartzitic basement.[6]It is inferred that this conglomerate formation was deposited in a high-energy subaerial environment like an alluvial fan. Overlying the conglomerate formation are oyster-bearing glauconitic sandstones deposited in a nearshore environment.[6]Late Tortonian to EarlyMessiniancalcareous ooze and claystone withbenthicforamassemblages overlays the sandstone formation; this indicates that water depth increased,[6]perhaps due to subsidence at the end of the syn-rift period. The boundary between the syn-rift and post-rift periods is believed to be located in a 50-meter section of late Messiniangypsumthat overlays the calcareous ooze and claystone formations. At the top of thestratigraphic sectionare 243 meters ofPliocenetoPleistocenepost-rift sediment containing calcareous mud with occasional terrigenous clastics andvolcanic ash.[6]
Lower Sardinia Margin
editThe Lower Sardinia Margin lies near the transition between continental andoceanic crust.This is the easternmost point on the margin where pre-rift, syn-rift, and post-rift sediments can be clearly discerned on seismic reflection profiles.[6]A drill core to the base of the syn-rift sediments revealed 533 meters of thinly-bedded, calcareous, siltstone and sandstone with dispersed detrital gypsum grains andanhydritenodules. The entire sequence is reversely magnetized, which when placed in the context of the adjacent stratigraphy, suggests that it was deposited during the reversed polarity event of the Gilbert epoch (between 4.79 and 5.41 Ma).[6]The environment of deposition for this section is uncertain; however, due to the presence of thin, well-graded layers and a lack of marine fossils, it is possible that the formation was deposited in a closedlacustrinesetting. Overlaying the syn-rift sediments are 200 meters of Pliocene to Pleistocenehemipelagicmarine sedimentwith intermittent volcanic glass. This sediment layer is believed to be deposited after the end of rifting due to its subhorizontal seismic reflection profile.[6]
Vavilov Sub-Basin
editThe basement rock in the Vavilov Basin consists of strongly serpentinizedperidotitewith both high and low-temperature phases of deformation.[6]The peridotite is overlain by 120 meters oftholeiiticpillow basaltcontaining carbonate-filled veins.[6]Nannofossils and planktonic foraminifera within the carbonate veins constrain the age of emplacement between 3.1 and 3.6 Ma.[6]Immediately above the basalt is a 100-meter-thick section of Pleistocene sedimentary cover, consisting primarily of nannofossil-rich mud with occasional reworkedvolcanogenicdebris.[6]
Marsili Sub-Basin
editThe basement rock of the Marsili Basin is vesicular basalt.[6]Due to the abundance (10–30% of rock volume) and size (up to 3 or 4 mm) of the vesicles, it is likely that the basalt was emplaced as a flow rather than asill.Overlaying the basement is 250 meters of calcareous mud and ooze with interbedded volcanoclastic layers. Benthic foraminifera and magnetic anomaly data from the base of this section constrain the terminus of rifting between 1.67 and 1.87 Ma.[6]At the top of the stratigraphic section are 350 meters of volcanoclasticturbidites.[6]
Basin tectonics and evolution
editTemporal onset of rifting
editIt is widely agreed that extension in the Tyrrhenian basin began in the lateMiocene,as shown by the recognition in seismic reflection profiles of pre-evaporitic (i.e. pre-Messinian) sediments in the western Tyrrhenian and by age estimates based on the overall lithospheric thickness, basement relief, and heat flow.[1][3][5][6]K-Ar datingof peripheral oceanic basalts, drilled at the southeast edge of the Vavilov plain, estimates the beginning of extension to be 7.3 ± 1.3 Ma.[7]Basalts recovered near the spreading center, in middle of the Vavilov plain, yielded ages of 3.4–3.6 Ma.[6]This means that back-arc extension in this part of the basin occurred approximately between the late Tortonian and the middle Pliocene. However, in the Marsili plain, the oldest basaltic crust was found to be 2.1 Ma.[3][6]This lower age limit, along with the presence of a saddle containing continental crust (the "Issel bridge" ) between the Vavilov and Marsili plains, implies that there were two distinct episodes of back-arc extension.
Mechanism of development
editBetween the Tortonian and middle Pliocene, W-E directed extension opened the Vavilov plain and Sardinia margin in the northwest portion of the present day basin. By the end of the Pliocene, extension rapidly turned to NW-SE and was limited to the southeastern Marsili plain. This rapid shift in the direction and spatial location of extension may be due to how the relative velocities of interactingtectonic plateschange over time. For example, if the horizontal velocity of the overriding Eurasian plate exceeds the velocity ofslab rollbackand trench retreat in the underthrusting African plate, then extension should not occur in the back-arc region.[8]However, if the velocity of slab rollback and trench retreat exceeds that of the overriding plate, then back-arc extension will occur.[8]
The Pliocene-Pleistocene changes in Tyrrhenian back-arc extension may have also been influenced by the adjacent Adriatic and Sicilian forelands. These sectors were not thinned by rifting and are characterized by normal continental lithosphere.[1]During its southeastward migration, the passively retreating oceanic slab had to adjust and deform in relation the large and buoyant continental sectors.[1]Post-Pliocene migration was eventually driven through the narrow corridor (250 km) represented by the present Ionian Sea and thus separated the Adriatic and Sicilian sectors.[1]Changes in lithospheric composition may have also contributed to differences in subduction geometry. For instance, during the first episode of subduction, thinned continental lithosphere underlying the Apenninic margin subducted beneath the Eurasian plate.[1][3][6]However, in the second episode of subduction, it was instead Ionian oceanic lithosphere that was involved in subduction.[1][3][6]A transition from continental to oceanic lithosphere subduction could explain the late onset of arc volcanism (2-1.5 Ma) as well as the lack of arc volcanism in the central Tyrrhenian basin.[1]
Rates of extension and subsidence
editThe full spreading rate of the Tyrrhenian basin has been estimated to be 3–5 cm/yr based on kinematic reconstructions and magnetostratigraphy.[1][6]This is a relatively slow spreading rate when compared to spreading rates in other back-arc basins like the East Scotia basin (5–7 cm/yr)[9]and the Manus basin (13 cm/yr).[10]The ratio between the initial crustal thickness and final crustal thickness, also known as the beta factor,[11]is estimated to be 3.3 for the Tyrrhenian basin.[6]
Petroleum resources
editAlthough there are numerous petroleum plays in mainland Italy and the northernIonian Sea,[12]the potential forhydrocarbonaccumulation in most of the Tyrrhenian basin is very poor. This is primarily due to the basin's highgeothermal gradientwhich pushes the oil window to very shallow depths where structural and stratigraphic traps are uncommon. There is, however, activehydrocarbon explorationon the south edge of the basin near theSicily Channel.Here, the small Narciso and Nilde fields produce sulphur-rich, light to medium-gradecrude oil(API gravityranging between 21 and 39).[12]The primary reservoirs in these fields are Miocenekarsticlimestonesthat are sealed by Plioceneclays.[12]The source of the hydrocarbons is unknown but is inferred to beMesozoiclimestone.[12]Traps are structural in nature and are primarilythrust faultsrelated to the Tunisian Atlas thrust belt.[12]
References
edit- ^abcdefghijklmSartori, R. (2003)."The Tyrrhenian back-arc basin and subduction of the Ionian lithosphere"(PDF).Episodes.26(3): 217–221.doi:10.18814/epiiugs/2003/v26i3/011(inactive 6 December 2024).Retrieved15 February2015.
{{cite journal}}:CS1 maint: DOI inactive as of December 2024 (link) - ^Anderson, H. and Jackson, J. (1987)."The deep seismicity of the Tyrrhenian Sea".Geophysical Journal International.91(3): 613–637.Bibcode:1987GeoJ...91..613A.doi:10.1111/j.1365-246x.1987.tb01661.x.Retrieved17 February2015.
{{cite journal}}:CS1 maint: multiple names: authors list (link) - ^abcdeMalinverno, A., and Ryan, W. (1986). "Extension in the Tyrrhenian Sea and shortening in the Apennines as result of arc migration driven by sinking of the lithosphere".Tectonics.5(2): 227–245.Bibcode:1986Tecto...5..227M.doi:10.1029/tc005i002p00227.
{{cite journal}}:CS1 maint: multiple names: authors list (link) - ^Della Vedova, B., Pellis, G., Foucher, J., and Rehault, J. (1984). "Geothermal structure of the Tyrrhenian Sea".Marine Geology.55(3–4): 271–289.Bibcode:1984MGeol..55..271D.doi:10.1016/0025-3227(84)90072-0.
{{cite journal}}:CS1 maint: multiple names: authors list (link) - ^abcSartori, R., Carrara, G., Torelli, L., and Zitellini, N. (2001). "Neogene evolution of the southwestern Tyrrhenian Sea (Sardinia Basin and western Bathyal plain)".Marine Geology.175(1–4): 47–66.Bibcode:2001MGeol.175...47S.doi:10.1016/S0025-3227(01)00116-5.
{{cite journal}}:CS1 maint: multiple names: authors list (link) - ^abcdefghijklmnopqrstuvwKastens, K. and Mascle, J. (1988). "ODP Leg 107 in the Tyrrhenian Sea: Insights into passive margin and back-arc basin evolution".Geological Society of America Bulletin.100(7): 1140–1156.Bibcode:1988GSAB..100.1140K.doi:10.1130/0016-7606(1988)100<1140:olitts>2.3.co;2.
{{cite journal}}:CS1 maint: multiple names: authors list (link) - ^Barberi, F., Bizouard, H., Capaldi, G., Ferrara, G., Gasparini, P., Innocenti, F., Jordon, J., Lambert, B., Treuil, M., and Allegre, C. (1978). "Age and nature of basalts from the Tyrrhenian abyssal plain".Deep Sea Drilling Project Initial Reports.42(1): 509–514.
{{cite journal}}:CS1 maint: multiple names: authors list (link) - ^abDewey, J. (1980)."Episodicity, sequence, and style at convergent plate boundaries".Geological Society of Canada Special Paper.20:555–573.Retrieved27 February2015.
- ^Barker, P. and Hill, I. (1981). "Back-arc extension in the Scotia Sea".Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences.300(1454): 249–261.Bibcode:1981RSPTA.300..249B.doi:10.1098/rsta.1981.0063.S2CID121280138.
{{cite journal}}:CS1 maint: multiple names: authors list (link) - ^Taylor, B. (1979). "Bismark Sea: Evolution of a back-arc basin".Geology.7(4): 171–174.Bibcode:1979Geo.....7..171T.doi:10.1130/0091-7613(1979)7<171:bseoab>2.0.co;2.
- ^McKenzie, D. (1978)."Some remarks on the development of sedimentary basins"(PDF).Earth and Planetary Science Letters.40(1): 25–32.Bibcode:1978E&PSL..40...25M.CiteSeerX10.1.1.459.4779.doi:10.1016/0012-821x(78)90071-7.Retrieved1 March2015.
- ^abcdeCasero, P. (2004)."Structural setting of petroleum exploration plays in Italy"(PDF).Special Volume of the Italian Geological Society.32:189–204.Retrieved23 February2015.