Cenozoic volcanism and tectonics in the southern Tyrrhenian sea] … · 2014-11-22 · Cenozoic volcanism and tectonics in the southern Tyrrhenian sea] space!time distribution and

\PERGAMON Geodynamics 16 "0888# 398Ð321

9153Ð2696:88:, ! see front matter Þ 0888 Elsevier Science Ltd[ All rights reserved[PII] S S 9 1 5 3 ! 2 6 9 6 " 8 7 # 9 9 9 1 4 ! 7

Cenozoic volcanism and tectonics in the southernTyrrhenian sea] space!time distribution and geodynamic

signi_canceA[ Argnani�\ C[ Savelli

Istituto per la Geologia Marina!CNR\ Via Gobetti 090\ 39018 Bologna\ Italy

Accepted 16 June 0887

Abstract

Short!lived magmatic episodes separated in space and time are recognizable in the southern portion of theTyrrhenian sea and on the island of Sardinia[ Distinct episodes of subduction!related calcalkaline arcvolcanism can be identi_ed] on the island of Sardinia during the OligoÐMiocene "21Ð02Ma#\ in the centralpart of the Tyrrhenian basin during the Pliocene "4Ð1Ma#\ and in the active Aeolian arc\ the southeasternbasin margin\ from the middle Pleistocene to Holocene "0Ð9Ma#[

The composition of the orogenic magmatic products in these arcs indicates the existence of a geochemicalpolarity of interarc type above a NW!dipping subducted slab[

NWÐSE migration of calcalkaline volcanic activity was accompanied by the opening of the interarcVavilov and Marsili sub!basins in late Tortonian to early Pliocene and in the upper Pliocene\ respectively[During these time intervals\ volcanic activity was mainly con_ned within the sub!basins with emplacementof MORB!like basalts[

Rift!related intra!plate volcanism occurred in the Pliocene "4Ð1Ma#\ following the opening of the Vavilovbasin\ and resumed in the Quaternary "0[1Ð9[0Ma# following the opening of the Marsili basin[ This volcanismhas a large areal distribution covering the island of Sardinia\ the western Tyrrhenian margin\ the Vavilovbasin\ the supposed central Tyrrhenian arc and\ sporadically\ the Marsili volcanic seamount[

The identi_ed magmatic cycles allow constraints to be placed on the geodynamic evolution of theTyrrhenian backarc basin which appears to be controlled by a rolling back subducted slab that becomesprogressively steeper in time[ Þ 0888 Elsevier Science Ltd[ All rights reserved[

0[ Introduction

The main geological features of the Mariana!type convergent margins "Uyeda\ 0871# are exten!sion and magmatism in the upper plate\ up to the point where oceanic lithosphere is produced in

� Corresponding author[ Fax] 9928!40!528!7839^ e!mail] andreaÝboigm1[bo[cnr[it

A[ Argnani\ C[ Savelli : Geodynamics 16 "0888# 398Ð321309

a backarc basin "BAB# setting\ and a generally steep dip of the subducted slab[ The thermo!chemical perturbation induced by the subducted lithosphere strongly a}ects the overlying mantlewedge and the upper plate[ This process is accompanied by magmatism\ tectonism\ seismic activityand hydrothermal circulation of notable intensity which have attracted the attention of manyspecialist groups in the geological community "Jakes and White\ 0861^ Barberi et al[\ 0863^Dickinson\ 0864^ Isacks and Barazangi\ 0866^ Scott and Kroenke\ 0879^ Pearce\ 0871^ Tarney etal[\ 0871^ Uyeda\ 0871^ Kobayashi\ 0873^ Channell and Mareschal\ 0877^ Yilmaz\ 0889^ Szabo etal[\ 0881^ Innocenti et al[\ 0872^ Hole et al[\ 0884^ Clift et al[\ 0884#[

Heterogeneity of magmatism and tectonic complexity characterize this kind of margin[ Supra!subduction volcanism does not take place as a continuum but develops with discrete events^ studieson the age\ areal distribution and magmatic characters of the eruptive products provide aninvaluable insight into the processes of subduction and back!arc extension "Scott and Kroenke\0879^ Clift et al[\ 0884#[ Generally\ abundant volcanites of intraplate "IP# a.nity are erupted inthe late rifting stages\ subsequent to the calcalkaline\ subduction!related volcanic activity and tothe back!arc basalt volcanism of MORB a.nity "Tarney et al[\ 0871^ Innocenti et al[\ 0872^ Savelli\0873^ Yilmaz\ 0889^ Szabo et al[\ 0881^ Hole et al[\ 0884^ Pouclet et al[\ 0884#[

The oceanic basement of the BABs is characterized by strong tectonic subsidence"Kobayashi\ 0873# and its depth is greater than that of comparable age basement in {normal|oceanic ~oors[

The Tyrrhenian Sea is a deep Neogene to Recent intra!orogenic basin that opened in thewake of the contemporaneously growing ApennineÐMaghrebian fold!and!thrust belt[ During itsevolution it was a}ected by voluminous magmatic activity and by strong tectonic subsidence"Kastens et al[\ 0877^ Savelli\ 0877^ Argnani\ 0886#[ In order to explain the volcanic and tectonicprocesses related to the Tyrrhenian opening in internal position with respect to the Apennines andSicilian Maghrebids\ some authors proposed the marginal basin model derived from the westernPaci_c backarc basins "Barberi et al[\ 0863^ Boccaletti and Manetti\ 0867#[ In this frame\ theevolution of the Tyrrhenian Sea and of the adjacent Apenninic orogenic wedge is described asbeing due to roll!back of the northwest!dipping Ionian lithosphere "Malinverno and Ryan\ 0875^Doglioni\ 0880#[ Other authors\ however\ favoured geodynamic models which do not contemplatesubduction and are\ instead\ based on the diapiric uprise of hot asthenosphere "Locardi\ 0871^Wezel\ 0874^ Locardi and Nicolich\ 0881#\ on lithospheric scale extension over low!angle faults"Lavecchia\ 0877^ Wang et al[\ 0878^ Crisci et al[\ 0880#\ or on lateral extrusion due to convergencebetween the European and the African plates "Mantovani et al[\ 0881#[

The nature and distribution of magmatism "Savelli\ 0877^ Argnani et al[\ 0884#\ the character ofdeep and intermediate seismicity "Anderson and Jackson\ 0876^ Giardini and Velona|\ 0880^Selvaggi and Chiarabba\ 0884#\ and the concomitant extension in the Tyrrhenian and contractionin the Apennines "Patacca et al[\ 0881# has led us to prefer subduction related models "Malinvernoand Ryan\ 0875^ Channell and Mareschal\ 0877#[

The purpose of this paper is to illustrate the time!space distribution\ the chemistry and thetectonic setting of the Cenozoic volcanism in the south Tyrrhenian basin and on the adjoiningIsland of Sardinia[ The episodicity of tectonics and volcanism provides useful constraints for abetter understanding of the subduction process and of backarc evolution in the SardoÐTyrrhenianregion "STR#[

A[ Argnani\ C[ Savelli : Geodynamics 16 "0888# 398Ð321 300

1[ Geological setting

The triangular!shaped Tyrrhenian sea is divided into a shallower northern part and a deepersouthern part[ In the deeper southern Tyrrhenian basin\ which is the object of this study "Fig[ 0#\mantle derived basic magmatism developed\ whereas acidic crustal derived magmatism prevailedin the northern Tyrrhenian basin[ The amount of extension in the southern basin\ where areas ofoceanic!like lithosphere are present\ was presumably greater than that in the northern basin[Estimates of migration rates of the extension!related magmatism are about 3[7 cm:a in the southerndeep basin\ but only 0[2 cm:a in the northern shallow area "Civetta et al[\ 0867^ Savelli\ 0877^Patacca and Scandone\ 0878^ Serri\ 0889#[

The southern Tyrrhenian basin is characterized by deep water "2599Ð2999 m#\ high heat ~ow"Mongelli et al[\ 0881# and a marked positive Bouguer gravimetric anomaly\ as well as highintensity "up to 0499 nT# and small amplitude magnetic patterns "Della Vedova et al[\ 0873^Faggioni et al[\ 0884#[ The Vavilov and Marsili sub!basins represent the deeper portions of thesouthern Tyrrenian sea and are characterized by relatively thin crust "Moho shallower than 09 km^Nicolich and Dal Piaz\ 0880#[ Large volcanic seamounts\ 19Ð34 km long and elongated roughlynorth!south\ are emplaced on the deep water sea ~oor[ The Vavilov and Marsili basins are separatedone from the other by the northÐsouth trending bathymetric high where the Glauco and the Isselseamounts are located[ Across this high an inter!basinal deepening of the Moho from 7Ð09 to04 km\ roughly extending from the island of Ustica to Issel seamount\ has been observed "Steinmetzet al[\ 0872#[ Within the sub!basins\ sediments are 599 m\ and locally up to 899 m\ thick and ODPdata indicate that sedimentation rates increased from late Pliocene to Pleistocene in concomitancewith a large input of volcaniclastics[

The results of the Deep Sea Drilling Project "DSDP# Legs 31 and Ocean Drilling Program"ODP# 096\ together with geophysical investigations\ indicate that accretion of oceanic!type crusttook place in the Vavilov and Marsili sub!basins[ Such oceanic!like areas are bounded by theAeolian volcanic arc to the east and by a well developed continental rise\ adjacent to the island ofSardinia\ to the west[

2[ Timing of tectonic and magmatic events

2[0[ Arc volcanism migration

Arc volcanism occurred in the Island of Sardinia\ the Tyrrhenian western border\ 21Ð02 Ma ago"Savelli et al[\ 0868# and migrated to the Islands and Seamounts of the Aeolian area wherecalcalkaline "CA# volcanics are 0Ð9 Ma old "Figs 0 and 1#[

A supposed arc of Pliocene age "4Ð1 Ma# is located in the central part of the Tyrrhenian sea\between the sub!basins of Vavilov and Marsili "Kastens et al[\ 0877^ Savelli\ 0877^ Ferrari andManetti\ 0882#[ When compared to those of the Aeolian and Sardinian arcs\ the CA volcanicactivity of the central arc appears to be less intensive\ and de_ned volcanic edi_ces are located onlyat the extremities of the structure[ The volcanic seamount Anchise\ the rhyolites of the island ofPonza and\ perhaps\ also the sequence of basalts and andesites recovered from deep drillings inthe Volturno river plain\ north of the presently active Phlegrean Fields\ belongs to the central arc


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Fig[ 1[ Scheme of the stratigraphic and geochronological correlation of magmatic products in the SardoÐTyrrhenianregion[ Horizontal bars represent number of radiometric datings[ The large time gap between OligoÐMiocene and MioÐPleistocene volcanism is related to the closure of the AlgeroÐProvencial backarc basin and the opening of the Tyrrhenianbasin[

volcanic sequence[ The Glauco and Ventotene!south volcanic seamounts are located in the centralportion of the arc^ although the geochemistry and age of their products have not been ascertained\we assume that these edi_ces are part of the central magmatic arc[ On the other hand\ the Isseland Poseidon seamounts\ consisting of metamorphic rocks\ may represent the continental basementof this arc Fig[ 0[

The composition of the magmatic products of the active Aeolian volcanic arc is more variable


than that of the old volcanites of the continental margins of Sardinia and of the central Tyrrhenian"Fig[ 2^ Table 0#[ The chemistry of the di}erent CA episodes "Argnani et al[\ 0884# indicates thatthere is a distinctive increment of the potassic alkalinity from the Sardinian island to the Aeolianarc[ Volcanites of shoshonitic a.nity are abundant among the basic rocks "basalts and basalticandesites# of the Aeolian area whereas they are absent among the basic rocks of the OligoÐMiocenemagmatic episode of Sardinia[

Following predicted relationships of magma source depth with the K content of volcanicproducts "Dickinson\ 0864#\ this interarc geochemical polarity suggests variation in the geometryof subducted lithosphere[

2[1[ The backarc spreading

The migration of arc volcanism was accompanied by two episodes of emplacement of oceanic!like crust in two sub!basins "Fig[ 3#[ The _rst episode occurred from late Miocene "7 Ma# to earlyPliocene "3 Ma# in the Vavilov sub!basin and the second took place during late Pliocene "1Ð0[5 Ma#in the Marsili sub!basin[ In each case\ BAB opening occurred prior to the onset of the correspondingarc volcanism to the east[ The chemical data of representative basalts of backarc setting are listedin Table 1 which shows that the deep!seated basalts from the DSDP Leg 262 consist of high! andlow!Ti lavas[

The TiÐZr discriminating plot of tectonic setting "Fig[ 4# shows that igneous basement rocks ofdrill hole DSDP 262 and drill hole 544 of the ODP Leg 096 fall in the _eld for ocean!~oor volcanites"Barberi et al[\ 0867^ Dietrich et al[\ 0867^ Beccaluva et al[\ 0889^ Bertrand et al[\ 0889#[ The lavasof two basalt units recovered in the drill hole 540 also belong to this _eld[ However\ the upper unitlavas of hole 540 are chemically di}erent from the lavas of the lower unit[ Their K contents arehigh in comparison to those of basalts from a MOR setting "Fig[ 5#\ and their Rb and Th valuesof 29Ð24 "Table 1# and 3Ð8[4 pm\ respectively "Beccaluva et al[\ 0889#\ indicate a more CA a.nity[Also the K\ Rb and Th values of the basalts of the site 549 in the Marsili basin are high\ and moreakin to those of basic products of the CA series[

The potassium and related LILE contents of the basalts in sites DSDP 262 and ODP 544 arelower than those of sites ODP 549 and 540[ The enrichment in LILE "K\ Rb and Th# of the rocksof sites 549 and 540 is likely to be a consequence of the enhanced chemical modi_cations inducedby ~uids derived from the subducted material on E!MORB!like parental magma[

2[2[ The intra!plate volcanism

Intra!plate "IP# rift!related volcanism occurred in two episodes and is distributed over a largearea\ extending from Sardinia to the southeastern Tyrrhenian Sea "Figures 0Ð1#[ In Sardinia thisvolcanism was initiated 4 Ma ago\ post!dating both the OligoÐMiocene CA episode and the OFvolcanism of the Vavilov basin\ and continued until 1 Ma ago[ This Pliocene activity is thereforecoeval with the CA volcanism of the supposed intermediate arc[ Volcanic activity stopped duringthe emplacement of oceanic lithosphere in the Marsili basin and then resumed again in con!comitance with the Aeolian arc CA volcanism "0Ð9 Ma#[

The chemical data of representative lavas of IP tectonic setting are listed in Table 1[The volcanites of di}erent magmatic and geodynamic signi_cance\ which a}ected the island of

Fig[ 2[ K1OÐSiO1 variation diagram of orogenic rocks of the Aeolian volcanic arc "A#\ and of the Central Tyrrhenianand Sardinia arc "B# on the basis of 252 rock analyses "Argnani et al[\ 0884#[ Boundary lines of the main volcanicassociations] "0# � boundary between low!K "tholeiitic# and medium!K "calcalkaline s[s[# series\ "1# � boundarybetween medium!K and high!K series "Le Maitre\ 0878#\ "2# � boundary between high!K and shoshonitic series"Peccerillo and Taylor\ 0865#[ Legend "A#] "Ž# � Aeolian volcanic seamounts^ "�# � island of Stromboli^ "x# � islandof Vulcano^ "¦# � island of Salina^ "r# � island of Panarea^ "R# � island of Lipari^ "�# � island of Filicudi^ "ž#� island of Alicudi^ "B#] thick line � _eld of the Aeolian volcanites^ dots � _elds of the Sardinian volcanites^ stripes� _elds of the volcanites of the central Tyrrhenian arc[


Table 0Representative chemical analyses of the calcalkaline episodes of arc volcanism in the SardoÐTyrrhenian region^ majorand trace elements in wt ) and in ppm\ respectively "refer[ in Argnani et al[\ 0884#

Sardinia Central Tyrrhenian arcSample a490 a603 a492 al0 ach42 a456 al2 nu260 co251 an011 napa2 fb0Ð4 po35Bser[ a}[ ca ca hk sho ca hk sho sho com hk hk hk hk

SiO1 49[47 41[60 42[92 47[30 47[75 48[12 51[98 69[06 60[56 40[67 43[02 63[67 64[09TiO1 0[92 9[63 9[85 9[67 9[50 9[54 9[68 9[32 9[39 0[13 9[79 9[23 9[08Al1O2 06[89 02[46 19[31 07[26 07[50 06[05 07[14 03[36 01[12 06[55 07[19 02[16 02[57Fe1O2 0[38 0[21 0[91 9[73 0[87 9[80 9[58 2[93 2[75 5[88 1[93 0[05 0[13FeO 7[85 6[81 5[00 4[94 3[78 4[37 3[04 9[15 9[68 1[07 4[10 9[87 9[46MnO 9[08 9[02 9[04 9[01 9[19 9[01 9[98 9[92 9[01 9[98 9[98 9[96 9[93MgO 4[41 8[80 7[99 0[72 1[11 2[21 9[32 9[30 9[23 3[14 3[48 9[21 9[09CaO 09[23 7[82 7[19 6[98 6[94 6[98 4[67 0[15 9[16 09[50 09[90 0[21 0[39Na1O 1[96 0[46 2[90 2[98 1[84 1[75 2[94 3[94 3[72 2[19 1[25 2[99 1[88K1O 9[79 0[92 0[68 2[62 0[73 1[09 3[15 4[03 3[73 0[61 1[15 3[23 3[55P1O4 9[29 9[07 9[14 9[17 9[07 9[12 9[16 9[97 9[93 9[22 n[a 9[93 9[92LOI 0[88 1[86 0[00 0[60 9[48 0[04 0[43 9[55 9[69 0[54 0[66 4[38 3[38Rb 08 22 34 026 70 69 88 080 146 30 63 066 172Sr 180 060 390 303 227 239 396 028 8 340 553 076 117Y 19 06 16 12 05 22 07 31 099 20 12 13 31Zr 47 42 090 060 098 032 050 331 0099 007 43 059 133Nb 4 4 8 01 5 8 00 20 024 05 6 01 12Ba 032 145 292 242 504 219 285 761 58 n[a 354 430 279La 6 8 06 15 19 12 16 50 000 n[a 13 24 46Ce 05 07 24 41 25 37 43 098 109 n[a 34 41 83Ni 04 119 09 04 03 03 00 n[a[ n[a[ n[a 07 4 n[aCr 37 690 03 25 4 15 10 8 5 n[a 49 5 21

Aeolian arcSample _l00 st0 alc20 st1 lp0 st033 lp22 st48 vu33 st30 lp28 pan97 lp8ser[a}[ ca sho ca ca ca ca sho hk le[teph hk sho hk hk

SiO1 38[36 38[66 49[44 41[55 42[22 42[82 44[52 45[63 47[59 59[91 59[14 54[23 60[99TiO1 9[63 9[88 9[56 9[52 9[56 9[66 9[79 9[60 9[59 9[67 0[99 9[45 9[04Al1O2 05[65 06[29 04[30 06[35 05[68 06[06 06[72 05[77 05[29 05[82 06[46 04[58 02[19Fe1O2 2[29 2[54 1[52 3[23 0[07 2[10 0[96 1[69 2[64 1[09 9[70 4[45 n[aFeO 5[39 4[19 4[39 3[73 6[94 3[99 5[31 3[04 1[34 2[36 3[73 n[a[ 1[69MnO 9[07 9[06 9[04 9[06 9[04 9[04 9[03 9[02 9[01 9[01 9[00 9[09 9[96MgO 5[88 4[51 8[50 4[12 4[77 5[02 2[08 3[70 1[59 2[03 1[34 1[04 0[09CaO 01[91 00[20 09[80 8[51 09[53 8[54 6[20 6[86 3[89 5[36 5[98 3[45 1[09Na1O 0[86 1[62 1[17 1[04 1[98 1[48 1[52 2[04 3[09 2[28 1[68 2[12 2[79K1O 9[85 1[04 0[22 9[85 0[22 0[22 2[93 0[67 4[59 1[72 2[36 1[57 3[79P1O4 9[14 9[42 9[14 9[06 9[11 9[07 9[18 9[14 9[23 9[14 9[13 9[02 9[95LOI 9[85 9[47 9[71 0[65 9[56 9[72 0[52 9[55 9[39 9[32 9[57 0[53 9[3Rb 12 61 23 08 31 28 67 36 124 72 011 86 179Sr 593 631 574 445 454 316 628 380 829 499 526 273 070Y 10 29 05 07 08 11 11 11 14 16 10 18 n[a[Zr 40 073 70 30 001 095 027 020 166 081 028 020 079


Table 0[*continued

Aeolian arcSample _l00 st0 alc20 st1 lp0 st033 lp22 st48 vu33 st30 lp28 pan97 lp8ser[a}[ ca sho ca ca ca ca sho hk le[teph hk sho hk hk

Nb 3 11 6 1 3 5 00 04 n[a[ 05 02 8 n[a[Ba 149 0983 289 209 178 217 641 499 860 714 600 456 199La 05 37 12 01 08 15 24 22 47 34 27 29 n[a[Ce 25 86 31 12 30 38 58 46 032 74 63 37 n[a[Ni 21 31 012 18 42 44 13 23 01 00 07 8 29Cr 88 52 385 58 046 115 56 012 12 05 27 04 19

ser[ a}[� serial a.nity^ ca�calcalkaline medium potassic^ hk�calcalkaline high!potassic^ sho�shoshonitic "com!prising the peralkaline rhyiolites#[ Com�comendites^ le[teph[� leucite tephrites[

Sardinia and its Tyrrhenian margin during the episodes of 21Ð02^ 4Ð1 and 0Ð9 Ma\ are plotted inthe Na1O¦K1O vs SiO1 discrimination diagram "Fig[ 6#[ Figure 6 shows that the OligoÐMiocenecalcalkaline rocks of Sardinia fall in the subalkaline _eld\ whereas most of the IP volcanites of thePliocene episode of Sardinia are of a more alkaline nature with the rocks of the Monti Ferruvolcanic complex characterised by marked potassic alkalinity "phonolites# "Table 1#[ However\ aconsistent portion of the eruptions of this episode have sub!alkaline composition "basaltic andes!ites#[ In fact\ the Monte Arci volcanic complex is made up of basalts\ andesites\ dacites andrhyolites of subalkaline a.nity as well as of a series of alkaline rocks\ consisting of hawaiites\mugearites and alkaline trachytes "Cioni et al[\ 0871#[ The ODP site 543 basalts of the late Pliocenein the west Tyrrhenian margin have subalkaline a.nity "Table 1#[

The IP volcanites of the Quaternary episode of NW Sardinia "the Logudoro district^ Macciottaand Savelli\ 0873# and of the west Tyrrhenian margin "Keller\ 0870# are entirely of alkaline nature[In general\ on the island of Sardinia and on its eastern margin the IP volcanites become morealkaline with time[ The Pliocene episode consists of subalkaline and alkaline rocks*in Sardiniathe basaltic andesites are topped locally by alkalic rocks*and the less voluminous Pleistoceneepisode is made up of alkaline lavas only[

In the TiO1×4ÐAl1O2ÐMgO diagram "Fig[ 7# for basic and intermediate rocks of Cabanis et al["0889# the products of the two IP volcanic episodes of Sardinia and of its eastern margin aredistinguished from those of the old CA episode[

The two IP volcanic episodes also took place in the large volcanic seamounts emplaced on theTyrrhenian basin ~oor "Table 1\ Figs 0 and 1#[ Not much is known about the chemistry of theseedi_ces\ however the bulk of Magnaghi volcano appears to be made up of IP basalts of Plioceneage "2[9Ð1[6 Ma#\ whereas rocks of this tectonic a.nity\ but of Quaternary age "9[3Ð³9[1 Ma#\occur locally on the ridges of the seamounts of Vavilov and Marsili "Selli et al[\ 0866^ Robin et al[\0876^ Faggioni et al[\ 0884#[ The bulk of Vavilov is made up of pillow lavas with age belonging tothe pre!Olduvay portion of the Matuyama polarity period "Fig[ 1#^ however\ their compositionhas not been ascertained[

Also the volcanic edi_ce of Ustica was constructed by IP volcanics "Table 1# belonging to theQuaternary episode "9[68 to ³9[02 Ma *post Tyrrhenian^ Calanchi et al[\ 0873^ Cinque et al[\


Fig[ 3[ Sketch maps showing the space!time distribution of Cenozoic volcanism in the SardoÐTyrrhenian region duringthe time intervals 21Ð02 Ma "OligoÐMiocene#\ 7[4Ð3[4 "upper Miocene#\ 3[4Ð1[9 Ma "Pliocene#\ 1[9Ð0[6 "uppermostPliocene# and 0[1Ð9 Ma "middle PleistoceneÐHolocene#[ Both the two volcanic episodes of spreading in the deep basinsof Vavilov and Marsili\ respectively of the upper Miocene and the uppermost Pliocene\ took place when the adjoiningarcs where quiescent[


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Fig[ 4[ Ti vs Zr discriminating plot of basalts from the igneous crust of the Vavilov and Marsili basins[ Fields "Pearceand Cann\ 0862#] A� island arc tholeiites\ C� calcalkaline basalts\ D� ocean ~oor basalts\ B� all three magma types[

Fig[ 5[ K1OÐSiO1 plot of basalts from the igneous crust of the Vavilov and Marsili basins] Symbols as in Fig[ 4[


Fig[ 6[ Alkalies vs SiO1 discrimination plot of the volcanites from Sardinia and the west Tyrrhenian margin "WTM#[Boundary line after Irvine and Baragar "0860#[

0877^ Savelli\ 0877^ Bousquet and Lanzafame\ 0884#[ This volcanic island is located between theseamount volcanoes of Anchise to the west and of Sisifo and Enarete to the east[ The former formsthe southern portion of the supposed intermediate arc of Pliocene age\ whereas the latter belongsto the western submerged sector of the Aeolian volcanic arc of the Quaternary episode[ The basaltand trachyte lavas and pyroclastites of Ustica have IP\ Na!alkalic a.nity with an arc signaturedue to their moderate Ti and Hf contents "Cinque et al[\ 0877#[ The TiÐZr plot "Fig[ 8# dis!criminating between IP and arc lavas "Pearce\ 0871# shows that the Ustica volcanites fall outsidethe _eld of the subduction!related rocks\ di}ering from the rocks of the adjoining Aeolian andremnant arcs[ The di}erent chemistry of the Ustica products is not problematic if one considersan inter!arc emplacement\ outside the present subduction zone\ and their age which is youngercompared to that of the Pliocene remnant arc[

The Aceste seamount\ located in the Tyrrhenian southern margin "Fig[ 0# yielded lavas ofhawaiitic\ mugearitic and alkali trachytic compositions and\ in addition\ rhyolite pyroclastites"Beccaluva et al[\ 0873#[ Its age is not younger than the Early Pliocene by biostratigraphic datingof Fe rich clays "Beccaluva et al[\ 0873#[ The transitional to weakly alkaline basic lavas of the


Fig[ 7[ TiO1×4ÐAl1O2ÐMgO triangular plot "Cabanis et al[\ 0889# of the basic and intermediate volcanites fromSardinia and the west Tyrrhenian margin "WTM#[ OROG � calcalkaline volcanites\ ANOR� anorogenic "intra!plate#volcanites[ Symbols as in Fig[ 6[

seamount are of IP a.nity\ but also have the subduction!related\ geochemical signature of lowNb:Y "³9[7# and Ti:Y "³299# ratios which contrasts with the high Zr values "084 ppm#[

The geochemical signatures of both the Aceste and Ustica volcanites may be interpreted in termsof the co!existence of both the island arc and the IP "Na!alkaline# imprintings within the samemantle source[ In fact\ they have HFSE contents which are only slightly higher than those of theisland!arc series\ on one side\ and\ on the other\ they show relative depletion of K and Rb\ andNa!enrichment[ These signatures re~ect magma generation from a mantle which has not beenstrongly modi_ed by the uptake of slab!derived elements like the K and Rb[ The displacement ofthe front of lithosphere convergence and the spaced slab retreat may be responsible for the IPnature of the Aceste and Ustica volcanoes[ In fact\ their respective sources were located in a distalposition relative to the mantle wedges feeding the corresponding volcanic arcs of Pliocene andQuaternary age[ Generally\ the hybrid volcanism is located in intermediate areas between thewestern "relict# and the eastern "active# arcs[ Slab migration may be invoked to explain the


Fig[ 8[ Log Ti!log Zr binary discrimination diagram "Pearce\ 0871# of the volcanites from the 4Ð0[7 Ma volcanic episode"Pliocene# of the supposed central Tyrrhenian arc" _lled circles# and from 0[1Ð9 Ma volcanic episode "Pleistocene# of theisland of Ustica "empty circles#[ Stripes � representative volcanism of the Aeolian arc[ AL� arc volcanites^ IPL� post!subduction\ IP "alkaline# volcanites[

petrochemical di}erences existing between the Anchise "orogenic# and the Aceste "anorogenic totransitional# volcanic products during the Pliocene episode\ and between the Aeolian "orogenic#and the Ustica "anorogenic to transitional# volcanic products during the Quaternary episode[

The IP volcanism of the STR has a large areal distribution\ however its inception is not olderthan 4 Ma[ Typically\ anorogenic manifestations are widespread during the post!collision tectonicrejuvenation of the backarc opening "Tarney et al[\ 0871^ Szabo et al[\ 0881^ Pouclet et al[\ 0884#[The post!subduction volcanism of the island of Sardinia\ the west Tyrrhenian margin and theridges of volcanic seamounts\ in the Vavilov and Marsili basins\ was generated in mantle sourceswhich became slab distant because of the retreat of the convergent boundary[

2[3[ Tectonics of the western Tyrrhenian margin

The results obtained by ODP leg 096 have contributed greatly to the unravelling of the tectonicevolution of the Tyrrhenian BAB "Kastens et al[\ 0877#[ Stratigraphic data indicate that rifting in


the Tyrrheanian basin propagated from the western margin "Sardinia# to the southeast "Marsilisub!basin#[ The _rst documented episode of rifting occurred from late Tortonian to early Messinianon the upper Sardinia slope\ followed by a second episode "late MessinianÐearly Piocene# a}ectingthe lower Sardinia slope[ Particular emphasis has been put on this migration of rifting "Kastensand Mascle\ 0889^ Sartori\ 0889^ Spadini et al[\ 0884# although the overall crustal geometry of thewestern Tyrrhenian margin closely resembles that of a typical passive margin "Malinverno\ 0870#[In this paper\ the lithospheric stretching of the western Tyhhenian margin is considered as acontinuous process that lasted from late Tortonian to early Pliocene with the stratigraphy on theSardinia slope that records the progressive eastwards migration of extension "Kastens et al[\ 0877#[Seismic pro_les across the Sardinia margin show that extension was accomplished by block faultingwith formation of half grabens "Curzi et al[\ 0879# and accompanied locally by magmatic e}usions"Fabretti et al[\ 0886#[

The stratigraphy of ODP wells 543\ 541 and 549 drilled in the Tyrrhenian basin and on itswestern margin has been backstripped to derive tectonic subsidence curves "Argnani\ 0886#[ Therift related subsidence on the margin "upper MioceneÐmidÐlower Pliocene# can be seen on thecurves obtained from ODP wells 543 and 541\ but also a recent episode of subsidence has beendocumented at about 1 Ma "Fig[ 09#[ This episode of subsidence appears to be basinwide as it canbe observed also in the Marsili sub!basin "ODP 549# and occurred well after the end of blockfaulting on the margin[

3[ Discussion

The western Tyrrhenian margin\ with its tilted fault blocks and its thinned crust\ resembles atypical stretched passive margin[ ODP stratigraphy and stratal relationships within half grabenssuggest that stretching occurred from late Tortonian to early Pliocene[ Datings of basalts recoveredin the Vavilov sub!basin fall within the same time interval "7 to 3 Ma#\ indicating that emplacementof MOR!type basalts occurred during the extension of the western Sardinia margin[ This exten!sional tectonics was followed by some CA activity in the central Tyrrhenian arc and\ mainly\ byIP volcanism in Sardinia and in the Magnaghi seamount[ The second extensional episode\ thatbrought about the opening of the Marsili sub!basin\ is not recorded on the western Tyrrhenianmargin[ Although the structural elements of this last episode of extension are likely to be foundon the margins boarding the Marsili sub!basin on the south and east\ there is a lack of studiesfocussing on this aspect\ mainly because the large volcanic outpour in this area tends to concealthe structures[ However\ sediments from well ODP 549 and seismic pro_les within the Marsili sub!basin allow us to infer a late Pliocene age for the stretching episode[ The basalts emplaced on thebasin ~oor during this extension\ sampled only in well ODP 549\ have CA a.nity\ and probablyre~ect the close proximity to the subducted slab[ As in the case of the Vavilov sub!basin\ thestretching in the Marsili sub!basin was followed by major CA volcanism in the Aeolian arc andby IP activity in Sardinia\ in the Vavilov seamount\ in the relict central Tyrrhenian arc and in theupper part of the Marsili seamount[

Taking all the episodes that can be recognised during the evolution of the Tyrhenian basin\ twomajor cycles can be identi_ed\ each one composed of two episodes\ the _rst dominated by extensionand the second characterised by CA and IP volcanism[ The _rst cycle occurred from 7[4 to 1[4 Ma


Fig[ 09[ Subsidence curves for the Tyrrhenian basin and margins[ Data from ODP leg 096 wells[ The curves representthe tectonic subsidence "total subsidence minus sedimentary load# and the vertical bars the range of paleobathymetricestimates[ Gray pattern indicates the duration of syn!rift derived from seismic re~ection geometries "After Argnani\0886#[

"late Tortonian to mid Pliocene#\ and the second\ shorter\ lasted from 1[9 Ma "late Pliocene# to thePresent[

It is considered that this kind of cyclicity could re~ect some fundamental geodynamic processes[Times of major trench retreat can give rise to the extensional dominated episodes causing basaltsemplacement into the stretched and thinned crust[ The nature of the crust of the Vavilov andMarsili sub!basins represents an intriguing problem that\ so far\ geophysical and petrological datahave failed to solve[

The crust that underlies the Vavilov and Marsili sub!basins is quite thin "less than 7 km# butcrust of similar thickness has been reported also from highly extended continental basins "Tate etal[\ 0882#\ and although E!MORB basalts have been drilled in the Vavilov sub!basin the actualmode of emplacement may be quite di}erent from that operating at mid!ocean ridges[ In fact\ inthe southern Tyrrhenian basin there is no such a feature as a spreading ridge and the irregularmagnetic pattern bears little evidence of spreading[


Considering that extensional faulting was active on the western Tyrrhenian margin while theVavilov sub!basin opened\ the emplacement of basalts probably occurred as a dense network ofdykes and sills "di}use spreading^ Cochran\ 0870# rather than from an eruptive ridge[ During theseepisodes very little volcanism occurs outside the basin[

The slab subducted underneath the Tyrrhenian sea dips very steeply "Selvaggi and Chiarabba\0884# and is one of the steepest subduction zones known "Isacks and Barazangi\ 0866#[ It is hereassumed that the slope of the subducted slab was progressively increasing through time and thatthis process of slab sinking into the asthenosphere was dominating the episodes were CA and IPvolcanism occurred[ Slab sinking stirs the mantle "see for instance displacement patterns in Giunchiet al[\ 0885\ Figs 00 and 02# and promotes instability that can interrupt the separation of theconvective systems of the lower and upper mantle\ favouring the rise of lower mantle melts fertilisedby megaliths "Ringwood and Irifune\ 0877#[ The geochemical characters of the IP volcanics seemto be the result of magma generation from a mantle whose composition was not modi_ed byuptake of slab!derived elements[ The deep magmatic source is\ in fact\ in a distal position relativeto the mantle wedge overlying the present subduction front[ We hypothesize that an upwelling ofthe lower mantle\ induced by\ and following\ the slab roll!back\ can generate the IP imprinting\namely\ the elevated HFSE contents and the Na!alkalinity\ whereas the overlying mantle "therelict mantle wedge# because of its distal position from the active subduction\ does not receivesigni_cant geochemical inputs from subduction!derived ~uids[

As stretching did not occur or was limited during IP volcanism\ basalt emplacement was notdi}used into a swarm of _ssures[ The construction of the major volcanic seamounts\ that occuredduring these episodes\ may be due to the fact that basalts erupted from the few major fractures\favouring the build!up of large edi_ces[ Moreover\ the along!length linear propagation of the axesof the seamounts might have been impeded by the occurrence of thicker crust on the adjacentbasin margins\ as it seems to be the case for the Marsili seamount "Faggioni et al[\ 0884#[

The evolution envisaged for the Tyrrhenian backarc basin on the ground of tectonics andmagmatic evidence is presented in Fig[ 00 and shows alternating episodes of trench retreat andtrench stability within the general slab steepening trend[ The causes for the change from trenchretreat to trench stability are di.cult to asses[ Certainly\ a locking of the subduction margin canstop the trench retreat "Giunchi et al[\ 0885#\ but also\ considering the overall geometry of thebasin\ the presence of temporary obstacles along the subduction zone "e[g[ impingement of buoyantobjects like thick carbonate platforms# could produce the same e}ect[

The recent "1 Ma# subsidence episode that has been recorded all over the Tyrrhenian basin doesnot _t easily into subsidence models applied to {classic| passive margins[ This late subsidence is oftennot considered "e[g[ Spadini et al[\ 0884# although it seems well constrained by paleobathymethicindicators "Hasegawa et al[\ 0889#[ Given the backarc setting of the Tyrrhenian basin\ it is likelythat processes related to subduction contribute to shape the observed tectonic subsidence as coldand dense subducted material is present at depth "Anderson\ 0878#[ The subduction dynamics canalso account for the deeper basement observed in backarc basins on a global scale when comparedto ocean ~oors of similar age "Kobayashi\ 0873#^ this observation holds also in the Tyrrhenianbasin"Fig[ 01#[ Recent dynamic models of subduction in the Tyrrhenian region seem to supportthis view that subsidence in a backarc basin is strongly in~uenced by subduction processes "Giunchiet al[\ 0885#[ However\ subduction processes should more or less immediately a}ect the subsidencewhile the large subsidence observed in the Tyrrhenian basin occurred only in the last 1 Ma and\


Fig[ 00[ Proposed geodynamic interpretation of the southern Tyrrhenian basin evolution and space!time distribution ofthe volcanic episodes in the SardoÐTyrrhenian region[ Sardinia detaches from Europe following the opening of theSardoÐBalearic basin[ Slab roll!back and progressive steepening of the subducted lithosphere control the opening of thebasinal domains and the nature of magmatic products[ Episodes of trench retreat\ during which the Vavilov and Marsilisub!basins opened\ alternate with episodes of trench stability\ during which WP and CA volcanism occur[ Also depictedis the general slab steepening trend from the initial shallow subduction "21Ð02 Ma# to the present steep subduction[CA� Calcalkaline volcanics^ MORB� Mid Ocean Ridge Basalts^ WP� Within Plate volcanics[


Fig[ 01[ Comparison between subsidence curves as a function of age for normal oceans and backarc basin "BAB#[Numbers refer to DSDP sites[ The _eld of the deep sea ~oor areas of the Tyrrhenian basin is indicated[ Stars and _lleddots indicate active and inactive backarc basins\ respectively[ The gray area represents the depth ranges in Lau basinand Okinawa trough "After Argnani\ 0886#[

furthermore\ it is contemporaneous with the occurrence of large uplift in Calabria "Ghisetti\ 0870#and\ to a lesser extent\ in Sardinia[ The Pleistocene uplift of Sardinia is suggested\ in the north!western part of the island\ by the higher elevation of Pliocene basaltic plateau with respect to thelate Pleistocene lava ~ows that occur within the adjacent valleys "Beccaluva et al[\ 0870#[ So far\the relationships between these observables and the geodynamic causes is still far from clear[

4[ Conclusions

The nature and timing of tectonic and magmatic events that can be recognised in the Tyrrhenianbasin have been reviewed in order to derive a geodynamic model for the evolution of the basin[


The volcanism of the STR may be divided into _ve episodes whose magmatic products areclearly distinguished based on their magmatic and geodynamic signi_cance\ geographic distributionand age "Figs 1 and 2#[

Their timings are as follows] i# 21Ð02 Ma "OligoÐMiocene#] CA\ subduction related volcanismin western Sardinia[ ii# 7[4Ð3[4 Ma "Late MioceneÐEarly Pliocene#] sediment covered\ ocean ~oor"OF# basalts of the DSDP Site 262 and ODP Site 544 related to the back!arc opening of theVavilov basin^ sediment covered igneous bodies of the Cornaglia Terrace "not younger thanLate TortonianÐMessinian^ Fabretti et al[\ 0886#[ iii# 4Ð1 Ma "Pliocene#] CA volcanism of theintermediate Tyrrhenian arc^ IP volcanism in Sardinia\ in the passive western Tyrrhenian margin"Site 543\ Quirra volcanic seamount#\ on Aceste seamount and on Magnaghi seamount "Vavilovbasin#^ sediment covered OF "Sites 544 and 540# and CA volcanism "Site 540# in the Vavilov basin[iv# 1Ð0[1 Ma "Latest Pliocene to Early Pleistocene#] sediment!covered\ CA to OF volcanism "Site549# related to the opening of the Marsili back!arc basin[ v# 0Ð9 Ma "Middle Pleistocene toHolocene#] CA volcanism of the Aeolian arc and of the Marsili seamount^ IP volcanism innorthwestern Sardinia and the passive margin of western Tyrrhenian Sea\ on the crest of Marsiliseamount\ on the crest of Vavilov seamount\ and in the island of Ustica\ to the east of the remnantintermediate arc[

The _ve episodes can be grouped into two cycles\ each composed of a stretching dominated phase\with magmatism concentrated mainly in the newly created basin ~oor\ followed by calcalkalinevolcanism in the arc and intra!plate volcanism oll over the basin[ These cycles re~ect the geodynamicevolution of the Southern Tyrrhenian backarc basin which appears to be controlled by a backrolling subducted slab that becomes progressively steeper in time Fig[ 00[ The process of roll backappears to be discontinuous in time with episodes of fast trench retreat\ and associated backarcbasin stretching\ alternating with episodes of steady trench position\ and perhaps slab sinking\with associated CA and IP volcanic activity[

Acknowledgements

Jeremy Preston is thanked for his constructive review of the manuscript[ Wolfgang Jacoby andtwo anonymous reviewers greatly contributed to improve the clarity of the paper[ L[ Casoni andG[ Zini carefully looked after the drafting[ IGM contribution n[ 0029[

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Cenozoic volcanism and tectonics in the southern Tyrrhenian sea] … · 2014-11-22 · Cenozoic volcanism and tectonics in the southern Tyrrhenian sea] space!time distribution and