A possible mechanism for the thermal asymmetryof the Ligurian basin

V. Pasquale,* M. Verdoya and P. ChiozziDipartimento per lo Studio del Territorio e delle sue Risorse, Settore di Geofisica, Universita di Genova, V.le Benedetto XV, 5, I-16132 Genova,

Italy

Introduction

The Ligurian basin is a crucial areafor understanding the kinematic evo-lution of the NW Mediterranean(Fig. 1). It is interpreted as a back-arc (marginal) basin, originated sinceUpper Oligocene times by relativelyrapid continental extension, as a resultof the westward subduction of theAdriatic plate beneath the Europeanplate. The eastward rotation and driftof the Corsican–Sardinian block, withthe formation of oceanic crust inter-posed between continental marginsand, on the other hand, of the Apen-nines range, are major consequencesof these processes that continued untilthe Early Miocene (Burrus et al.,1987; Pasquale et al., 1996).The thermal and subsidence history

of the basin is, to a first approxima-tion, accounted for by pure shearstretching (Pasquale et al., 1994, andreferences therein). However, there issubstantial asymmetry in the heat-fluxpattern and the style of extensionbetween the western (Alpine–Proven-cal) and eastern (Corsican) continen-tal margins (Fig. 1). This implies thata more complex mechanism should beevaluated to give a complete explana-tion of the basin evolution.In this paper, we argue for a ther-

mo-mechanical model, which includes

the removal of a portion of mantlelithosphere beneath the Corsican mar-gin, due to convective flow induced inthe asthenosphere by the subductingslab. The model is proposed anddiscussed in relation to the availablegeophysical evidence of the deepstructure.

Geophysical evidence

Figure 1 shows the terrestrial heat fluxof the Ligurian basin, derived from anumber of available thermal datasources, and treated for the main

terrain effects (Verdoya, 1992; Pasqu-ale et al., 1997, and references there-in). The contour lines show a NE–SWelongated pattern with steeper lateralvariation across the Alpine–Provencalmargin than across the Corsican mar-gin, and high heat-flux values (100–110 mW m)2) in the oceanic centralpart of the basin. Such a trend is alsoevident inland, in Provence where theheat flux is about 65 mW m)2 andin Corsica with values of about80 mW m)2. The heat flux is on aver-age about 75 mW m)2 on the westernmargin, and 95 mW m)2 on the eastern

ABSTRACT

We propose a thermo-mechanical model and a new interpret-ation of heat flux data for the Ligurian basin that may be a keyto understanding the evolution of the NW Mediterranean. Themodel incorporates the removal of a portion of mantlelithosphere to explain the heat-flux and subsidence anomalyof the eastern (Corsican) margin of the basin. This process isenvisaged as a result of eastward asthenosphere flow inducedby the Apennines subduction system. After a heating phase,

time-dependent conductive cooling and re-thickening of thelithosphere result in re-equilibrium of the thermal gradient toits initial value. Such a rifting mode can account for theasymmetric heat-flux and subsidence pattern observed acrossthe basin and the present-day lithospheric thickness.

Terra Nova, 14, 484–490, 2002

Fig. 1 Pattern of terrestrial heat flux in the north-western Mediterranean. 1, Provenceforeland; 2, Hercynian Corsica; 3, Alpine Corsica; 4, continental margin; 5, transi-tional crust; 6, oceanic crust.

*Correspondence: Prof. V. Pasquale, Di-

partimento per lo Studio del Territorio e

delle sue Risorse, Settore di Geofisica,

Universita di Genova, V.le Benedetto XV,

5, I-16132 Genova, Italy. Fax: +39 010

352169; e-mail: pasquale@dipteris.unige.it

484 � 2002 Blackwell Science Ltd

margin. This yields an asymmetricthermal pattern, which seems to ex-tend to a regional scale. In fact, higherheat flux is recognized along the entirewestern side of the Corsica–Sardiniablock, implying its origin might berelated to long-wavelength, deep-sea-ted processes (Pasquale et al., 1996).Information on the crustal structure

and nature of the Ligurian basinderives from seismic studies (Ginzburget al., 1986; Egger, 1992; Makris et al.,1999; De Franco et al., 2000; Cont-rucci et al., 2001) and tectonic

subsidence analyses (Pasquale et al.,1994). If we exclude the post-Miocenesediments, the oceanic crust betweenthe continental margins is on averageabout 6 km thick. Both margins areseparated from the oceanic domain bya belt of crust, whose nature is likelytransitional, being characterized byanomalous seismic velocity comparedto that of typical oceanic crust (Figs 1and 2).Figure 3 shows contours of the

Moho discontinuity depth togetherwith the main magnetic anomalies of

magnitude >50 and <) 50 nT,deduced from airborne surveys at3000 m a.s.l. (Wonik et al., 1992).Contouring of the Moho depth hasbeen adjusted by taking into accountthe compilation by Giese and Buness(1992) and the 3-D gravity inversionby Chamot-Rooke et al. (1997). Thecrust–mantle boundary is steeper inthe Provencal than in the Corsicanmargin, where a crustal thickness ofabout 25 km is observed at the coast-line. This value is slightly lower thantypical continental crust as a Meso-zoic episode of stretching has affectedthis area (Burrus and Bessis, 1986).Other evidence that the mechanism

of formation of the basin has causedstructural heterogeneities is given bystudies of SH wave propagation(Shapiro et al., 1996). Moreover, lat-eral variations of seismic velocity areobserved in the mantle west of Cor-sica, which may correspond to frag-ments of the Adriatic lithosphere(Egger, 1992). Since the Adriatic slabseems now to be clearly outlined onlybeneath the Apennines, a possiblemechanism could be the eastwardrecoil and roll-back of the subductingplate (e.g. Malinverno and Ryan,1986). An E–W polarization directionof the fast split – shear wave wasalso found east of Corsica, and itwas interpreted as a result of east-ward mantle flow (Margheriti et al.,1996). This region, which has beensubject to a relatively younger epi-sode of stretching (e.g. Pasqualeet al., 1997), is characterized by thehighest heat flux.Interpretation of the magnetic data

is quite complicated owing to thesedimentary cover and the possiblepresence of high-Ti titanomagnetiteseries and fairly low blocking temper-atures. Magnetic features typical ofoceanic crust are not evident. In thecentral part of the basin, anomaliesare very weak (< 50 nT) and attribu-table to the large depth of themagnetic source (Burrus, 1984).Except for the large positive anomaly(> 650 nT) related to the offshoreprolongation of the western Alpsophiolite body (Pasquale et al., 2001),major positive anomalies (120–240 nT)indicate scattered magmatic intrusionsrather than a typical ocean-floor pat-tern. Wide negative anomalies of lowmagnitude are located in the easternLigurian Sea.

Fig. 3 Moho depth and main magnetic anomalies. Contour lines in km; 1, negativeand 2, positive magnetic anomalies in nT; numbers indicate maximum values.

Fig. 2 Heat flux (continuous line), crustal stretching factor b (dashed line) and crustalstructure along profile AA¢ shown in Fig. 1. 1, Oceanic crust; 2, transitional crust;3, Miocene–Quaternary sediments; 4, Messinian evaporite sequence.

Terra Nova, Vol 14, No. 6, 484–490 V. Pasquale et al. • Thermal asymmetry of the Ligurian basin

.............................................................................................................................................................

� 2002 Blackwell Science Ltd 485

Thermo-mechanical model

Active and passive models have beenproposed to explain back-arc basinformation (see, e.g., Rodkin andRodmikov, 1996, for a recent review).Both models invoke a secondary con-vective cell caused by the interactionbetween the mantle wedge and thesubduction slab, which leads to back-arc spreading (e.g. Andrews andSleep, 1974; Bodri and Bodri, 1978;Hsui et al., 1983).In the Ligurian basin, we suggest

that eastward mantle convection, in-duced by the subducting Adriaticplate, may have generated lithosphericextension with mechanical removaland heating of a portion of the lowerlithosphere, which subsequently wasassimilated within the convectingmantle (Fig. 4). This process can bequantitatively evaluated by means ofan instantaneous pure shear stretchingcoupled with an additional reductionof the mantle lithosphere. In this case,the crustal extension factor b is theratio between the prerift and thepresent-day crustal thickness, and bris the stretching factor of the wholelithosphere which takes account of theremoved portion of the lithosphericmantle. Figure 5 shows a sketch of themodel while the mathematical formu-lation is described in the Appendix.Extension initially causes heating anda consequent uplift due to dilatationafter an initial subsidence. After theheating process and the consequentuplift terminate, a lithosphere coolingphase occurs. This leads to a decreasein heat flux while the tectonic subsi-dence increases with time.The average b of the Corsican

margin deduced from the availableseismic data is 1.8. The heat fluxcontribution of the stretched litho-sphere can be calculated by assuminga radiogenic heat production in theuppermost mantle of 0.02 lW m)3

and a crustal contribution qc prior tostretching of 33 mW m)2. A basalheat flux from the asthenosphere of38 mW m)2 seems a reasonable valuefor the NWMediterranean extensionalbasins (Verdoya et al., 1998). Thethickness of the lithosphere in themainland zones varies from 75 to110 km and, on average, is 90 km(Suhadolc et al., 1990; Pasquale et al.,1994). Assuming that the lithosphereis in thermal equilibrium, the tempera-

ture at its base Ja is 1250 �C, i.e. 0.9times the mantle solidus, consideringthat ocean spreading took place afterrifting (cf. Pollack and Chapman,1977). Chosen values of coefficientsof thermal conductivity, diffusivity

and expansion are 2.9 W m)1 K)1,10)6 m2 s)1 and 3.3 · 10)5 �C)1,respectively.Although an age of about 20 Myr is

generally assumed for the basin, Pas-quale et al. (1996) found that the

Fig. 4 Evolutionary scheme of the Ligurian basin and Tyrrhenian Apennines systemfrom the Upper Oligocene to Present. 1, Sediments; 2, continental lithosphere;3, oceanic lithosphere; 4, removed portion of lithosphere.

Fig. 5 Sketch of the thermo-mechanical model. Dots indicate the removed portion oflithospheric mantle (see text).

Thermal asymmetry of the Ligurian basin • V. Pasquale et al. Terra Nova, Vol 14, No. 6, 484–490

.............................................................................................................................................................

486 � 2002 Blackwell Science Ltd

onset of the oceanic stage is 2–3 Myrolder in the south-western sector (Gulfof Lions – Sardinia traverse), whereasit appears slightly delayed in thenorth-eastern part (Provence–Corsicatraverse), post-dating previous esti-mates of the drifting stage for Corsica.Therefore, in our model we assumethat extensional processes of the basinstarted 18 Myr ago. This is in agree-ment with the most recent palaeomag-netic interpretations (Vigliotti andKent, 1996) and fission track datingboth on volcanic products along thewestern margin of Corsica (Rossiet al., 1998) and on different tectonicunits in northern Corsica (Cavazzaet al., 2001).Figure 6 depicts the thermal condi-

tions during the initial stage of theextension process. For times greaterthan 5–10 Myr, the temperature–depth curve differs from the quasi-stationary geotherm (� 50 Myr) byless than 25–50 �C. Since the uncer-tainties in the calculated lithospherictemperatures are estimated to be ofthe order of 50 �C, it can be reason-ably assumed that the warming phasebecomes insignificant after �10 Myr,whatever br is chosen.As a consequence of the finite

duration of the warming stage, theheat flux initially increases to values ofabout 90 mW m)2 for br ¼ 2.0 and110 mW m)2 for br ¼ 2.6, and then itdecreases with time, as shown inFig. 7. At t ¼ 18 Myr, a heat flux of95 mW m)2, the average value for theCorsican margin, is expected forbr ¼ 2.4, implying the removal of13 km of lower lithosphere.Figure 8 shows the expected tec-

tonic subsidence history curve for theCorsican margin. In the first stage theinitial subsidence associated withinstantaneous rifting decreases slightlywith time as a consequence of ther-mal expansion. The subsequent cool-ing leads to contraction, andsubsidence increases at a relativelyrapid rate. The observed subsidence,after correction for the sedimentload, is about 4 km in the centraloceanic part of the Ligurian basin,and it decreases towards the Corsicanmargin to an average value of 2.1 km(Pasquale et al., 1995). A substantialagreement between the calculatedsubsidence at 18 Myr and thatexperimentally inferred is obtainedfor br ¼ 2.4.

Discussion

An alternative model to that proposedhere might be a simple shear along alow-angle detachment (cf. Wernicke,1981). This would require a large-scalefault system dipping south-east underan actively retreating Corsican–Sar-dinian block. However, there is noseismic evidence of a detachment zonewithin the continental lithosphere be-neath the two islands. The detachment

level such that Corsica and Sardiniacould undergo substantial rotationand drift must therefore be sought atthe base of the lithosphere (Blundellet al., 1992). Moreover, seismic activ-ity is concentrated only in the Alpine–Provencal margin and denotes a com-pressional regime, and the centralparts of the basin, Corsica andSardinia, are seismically quiescent(Bethoux et al., 1992; Pasquale et al.,1994).

Fig. 6 Lithosphere geotherms at different times during the warming phase for b¼ 1.8and br ¼ 2.2 and 2.6. Surface temperature J0 ¼ 15 �C.

Fig. 7 Heat flux as a function of time for different br.

Terra Nova, Vol 14, No. 6, 484–490 V. Pasquale et al. • Thermal asymmetry of the Ligurian basin

.............................................................................................................................................................

� 2002 Blackwell Science Ltd 487

The proposed thinning model forthe Corsican margin is consistent withthe eastward migration of the exten-sion–compression tectonic systemassociated with the westward subduc-tion of the Adriatic plate (Gueguenet al., 1997; Jolivet et al., 1998). Therecoil and roll-back of the subductedplate may have triggered convectiveremoval and thermal assimilation ofpart of the lower lithosphere. Theasthenosphere flow may have causedsubsequent splitting of the edge of theplate, leaving relics floating withinthe mantle (Egger, 1992; Pasqualeet al., 1996). We suggest that theinduced asthenosphere flow has

migrated eastward affecting the Tyr-rhenian area since the end of theLigurian basin rifting. Mongelli andZito (2000) demonstrate that the ob-served regional heat flux and tectonicsubsidence in the peri-Tyrrhenian areawell agree with the hypothesis of rift-ing with a removal of part of the lowerlithosphere. Moreover, geophysicalevidence is given by E-W seismicanisotropy observed in the region eastof Corsica, which is interpreted to berelated to recent asthenospheric flow,controlling, in turn, lithosphere exten-sion (Margheriti et al., 1996).A picture of the thermal history

predicted for the Corsican margin isshown in Fig. 9. Geotherms have beencalculated for b¼ 1.8 and br ¼ 2.4,which seem the most reliable values inthe light of thermal, structural andsubsidence evidence. At the end of theinitial warming stage (10 Myr), litho-spheric mantle re-thickening due tocooling is expected. After 18 Myr thethickness of the thermal lithosphere,defined by the isotherm which approa-ches the asthenosphere temperatureJa, will increase from about 40 to70 km. Again, this result is consistentwith information on the lithospherethickness beneath the margin. Duringthe cooling stage, the time necessaryfor the lithosphere to thicken from avalue hL ⁄br to a thickness equal to theprerift thickness is about 50 Myr. Thediscontinuity in the temperature gra-dient between the lithosphere and theasthenosphere is an artefact of themodel which could be removed by

considering the details of the convec-tive heat transport (Parsons andMcKenzie, 1978). However, the heatflux and subsidence would be littleaffected.

Conclusions

The proposed thermo-mechanicalmodel can explain deviations of theheat-flux and subsidence pattern alongthe Ligurian basin eastern margin withrespect to prediction of a uniformextension model. Such a model seemscompatible with the eastward recoilwith time of the subducting Adriaticplate, which is invoked as a possiblemechanism for the origin of the Apen-nines belt and the extensional back-arcbasins of the central–western Mediter-ranean. Provided that the crust hasbeen extended about twice, an addi-tional removal of 13 kmof lithosphericmantle material accounts for theobserved heat flux and the tectonicsubsidence on the Corsican margin.The expected lithosphere re-thickeningdue to thermal relaxation is consistentwith the present-day thickness.

References

Andrews, D.J. and Sleep, N.H., 1974.Numerical modelling of tectonic flowbehind island arcs. Geophys. J. R. Astr.Soc., 38, 237–251.

Bethoux, N., Frechet, J., Guyoton, F.,Thouvenot, F., Cattaneo, M., Eva, C.,Nicolas, M. and Granet, M., 1992. Aclosing Ligurian Sea. Pure Appl. Geoph.,139, 179–194.

Blundell, D. and Mueller, St. & Mengel,K., 1992. Geodynamics of Europe. In:A Continent Revealed, the EuropeanGeotraverse (D. Blundell, R. Freemanand St. Mueller, eds), pp. 215–231.Cambridge University Press, Cambridge.

Bodri, L. and Bodri, B., 1978. Numericalinvestigation of tectonic flow in island-arc areas. Tectonophysics, 50, 163–175.

Burrus, J., 1984. Contribution to a geody-namic synthesis of the Provencal Basin(North-Western Mediterranean). Mar.Geol., 55, 247–269.

Burrus, J. and Bessis, F., 1986. Thermalmodelling in the Provencal Basin (NWMediterranean). In: Thermal Modellingin Sedimentary Basins (J. Burrus, ed.),pp. 393–416. editions Technip, Paris.

Burrus, J., Bessis, F. and Doligez, B., 1987.Heat flow, subsidence and crustal struc-ture of the Gulf of Lions (NW Mediter-ranean): a quantitative discussion of theclassical passive margin model. In:Sedimentary Basins and Basin-Forming

Fig. 8 Total tectonic subsidence as a function of time.

Fig. 9 Lithosphere geotherms at theonset (t ¼ 0), at the end of the warmingphase (t ¼ 10 Myr) and at the present-day (t ¼ 18 Myr).

Thermal asymmetry of the Ligurian basin • V. Pasquale et al. Terra Nova, Vol 14, No. 6, 484–490

.............................................................................................................................................................

488 � 2002 Blackwell Science Ltd

Mechanisms (C. Beaumont and A.J.Tankard, eds). Mem. Can. Soc. Petrol.Geol., 12, 1–15.

Cavazza, W., Zattin, M., Ventura, B. andZuffa, G.G., 2001. Apatite fission-trackanalysis of Neogene exhumation in nor-thern Corsica (France). Terra Nova, 13,51–57.

Chamot-Rooke, N., Jestin, F. and Gaulier,J.M., 1997. Constraints on Moho depthand crustal thickness in the Liguro-Provencal basin from a 3D gravityinversion: geodynamic implications. Rev.Inst. Francais Petrol., 52, 557–583.

Contrucci, I., Nercessian, A., Bethoux, N.,Mauffret, A. and Pascal, G., 2001. ALigurian (Western Mediterranean Sea)geophysical transect revisited. Geophys.J. Int., 146, 74–97.

De Franco, R., Biella, G., Caielli, G.,Corsi, A., Mauffret, A., Contrucci, I.and Nercessian, A., 2000. Interpretationof a wide angle reflection refraction datain the Tuscan-Latium Peri-Tyrrhenianarea. Boll. Soc. Geol. It., 119, 171–188.

Egger, A., 1992. Lithospheric structurealong a traverse from the NorthernApennines to Tunisia derived from seismicrefraction data. PhD Thesis. Universityof Zurich.

Giese, P. and Buness, H., 1992. Mohodepth (Atlas Map 2). In: A ContinentRevealed: the European Geotraverse(D. Blundell, R. Freeman and St. Muel-ler, eds), pp. 11–13. Cambridge Univer-sity Press, Cambridge.

Ginzburg, A., Makris, J. and Nicolich, R.,1986. European Geotraverse: a seismicrefraction profile across the LigurianSea. Tectonophysics, 126, 85–97.

Gueguen, E., Doglioni, C. and Fernandez,M., 1997. Lithospheric boudinage in theWestern Mediterranean back-arc basin.Terra Nova, 9, 184–187.

Hsui, A., Marsh, B.D. and Toksoz, M.N.,1983. On melting of the subductedoceanic crust: effects of subductioninduced mantle flow. Tectonophysics, 99,207–220.

Jolivet, L., Faccenna, C., Goffe, B., Mattei,M., Rossetti, F., Brunet, C., Storti, F.,Funiciello, R., Cadet, J.P., D’Agostino,N. and Parra, T., 1998. Midcrustal shearzones in postorogenic extension: Exam-

ple from the northern Tyrrhenian Sea.J. Geophys. Res., 103, 12123–12160.

Makris, J., Egloff, F., Nicolich, R. andRihm, R., 1999. Crustal structure fromthe Ligurian Sea to the Northern Apen-nines – a wide angle seismic transect.Tectonophysics, 301, 305–319.

Malinverno, A. and Ryan, W.B.F., 1986.Extension in the Tyrrhenian Sea andshortening in the Apennines as result ofArc migration driven by sinking of thelithosphere. Tectonics, 5, 227–245.

Margheriti, L., Nostro, C., Cocco, M. andAmato, A., 1996. Seismic anisotropybeneath the Northern Apennines (Italy)and its tectonic implications. Geophys.Res. Lett., 23, 2721–2724.

McKenzie, D., 1978. Some remarks on thedevelopment of the sedimentary basins.Earth Planet. Sci. Lett., 40, 25–32.

Mongelli, F. and Zito, G., 2000. Thermalfield in a basin after a sudden passivepure shear lithospheric extension andsublithospheric mechanical erosion:the case of the Tuscan Basin (Italy).Geophys. J. Int., 142, 142–150.

Parsons, B. and McKenzie, D., 1978.Mantle convection and the thermalstructure of the plates. J. Geophys. Res.,83, 4485–4496.

Pasquale, V., Verdoya, M. and Chiozzi, P.,1994. Types of crust beneath the Ligu-rian Sea. Terra Nova, 6, 255–266.

Pasquale, V., Verdoya, M. and Chiozzi, P.,1995. Rifting and thermal evolution ofthe Northwestern Mediterranean. Ann.Geofis., 38, 43–53.

Pasquale, V., Verdoya, M. and Chiozzi, P.,1996. Heat flux and timing of the driftingstage in the Ligurian-Provencal basin(Northwestern Mediterranean). J. Geo-dyn., 21, 205–222.

Pasquale, V., Verdoya, M. and Chiozzi, P.,2001. Radioactive heat generation andits thermal effects in the Alps–Apenninesboundary zone. Tectonophysics, 331,269–283.

Pasquale, V., Verdoya, M., Chiozzi, P. andRanalli, G., 1997. Rheology and seis-motectonic regime in the northern cen-tral Mediterranean. Tectonophysics, 270,239–257.

Pollack, H.N. and Chapman, D., 1977. Onthe regional variation of heat flow,

geotherms and lithosphere thickness.Tectonophysics, 38, 279–296.

Rodkin, M.V. and Rodmikov, A.G., 1996.Origin and structure of back-arc basins:new data and model discussion. Phys.Earth Planet. Int., 93, 123–131.

Rossi, P., Guennoc, P., Rehault, J.P.,Arnaud, N., Jakni, B., Popeau, G.,Tegyey, M., Ferrandini, J., Sosson, M.,Beslier, M.O., Rollet, N. and Gloaguen,R., 1998. Common occurrence of Mio-cene calc-alkaline volcanism along thewestern Corsican Margin (MARCOCruise). CR Acad. Sci., Serie IIa, 327,369–376.

Shapiro, N., Bethoux, N., Campillo, M.and Paul, A., 1996. Regional seismicphases across the Ligurian Sea. Lgblockage and oceanic propagation. Phys.Earth Planet. Int., 93, 257–268.

Suhadolc, P., Panza, G.F. and St. Mueller,1990. Physical properties of the litho-sphere-asthenosphere system in Europe.Tectonophysics, 176, 123–135.

Verdoya, M., 1992. Densita di flusso dicalore del segmento meridionale dellaGeotraversa Europea ed aree adiacenti.PhD Thesis. Dept. of Earth Sciences,University of Genoa.

Verdoya, M., Pasquale, V., Chiozzi, P. andKukkonen, I.T., 1998. Radiogenic heatproduction in the Variscan crust: newdeterminations and distribution modelsin Corsica (Northwestern Mediterra-nean). Tectonophysics, 291, 63–75.

Vigliotti, L. and Kent, D.V., 1996. Paleo-magnetic results of Tertiary sedimentsfrom Corsica: evidence of post Eocenerotation. Phys. Earth Plan. Int., 62,97–108.

Wernicke, B., 1981. Low-angle normalfaults in the Basin and Range Province:nappe tectonics in an extending orogen.Nature, 291, 645–647.

Wonik, T., Galdeano, A., Hahan, A. andMouge, P., 1992. Magnetic anomalies.In: A Continent Revealed, the EuropeanGeotraverse (R. Freeman and St.Mueller,eds), pp. 31–34. Cambridge UniversityPress, Cambridge.

Received 17 July 2001; revised versionaccepted 1 August 2002

Appendix

After stretching, the crustal thicknesshc is reduced by a factor b and thetemperature at the lithosphere base isJb, i.e. the temperature at depth(hc + hm), where hm is the thicknessof the lithospheric mantle stripped ofits lowermost portion (Fig. 5). This isequivalent to assuming that, as aconsequence of stretching and upward

motion of the asthenosphere, thethinned lithosphere is warmed withtime until an equilibrium thermalprofile is reached between the tem-perature J0 at depth z ¼ 0 and Ja atthe asthenosphere top. For a litho-sphere thickness hL before extension,it follows that

hm ¼ bbr

hL � hc ðA1Þ

and the temperature at the thinnedlithosphere base is

#b ¼ #0 þ#a � #0

hLðhc þ hmÞ

¼ #0 þ ð#a � #0Þbbr

ðA2Þ

where, due to removal, the thinningfactor of the whole lithosphere br islarger than the crustal thinning

Terra Nova, Vol 14, No. 6, 484–490 V. Pasquale et al. • Thermal asymmetry of the Ligurian basin

.............................................................................................................................................................

� 2002 Blackwell Science Ltd 489

amount b. The temperature distribu-tion with time t in the thinned litho-sphere during the initial warmingphase is given by (Mongelli and Zito,2000)

#ðz; tÞ ¼ #0 þ#a�#0

hL=brz

þ 2ð#a�#0Þbr �b

br

� �

�X1n¼1

ð�1Þn

pnsin

npzðhL=brÞ

� exp � jn2p2t

ðhL=brÞ2

" #ðA3Þ

and during the subsequent cooling is(McKenzie, 1978)

#ðz; tÞ ¼ #a 1� zhL

þ 2

p

X1n¼1

ð�1Þnþ1

n

"

br

npsin

npbr

� �exp

�n2tjp2

h2L

� �

sinnpzhL

�þ #0

ðA4Þ

where j is the thermal diffusivity.The heat flux at the surface varies

with time according to

q0ðtÞ ¼ kd#dz

� �z¼0

þ ql þ qa ðA5Þ

where k is the thermal conductivity, qais the basal heat flux from the as-thenosphere and ql is the contributionfrom the radioactive isotopes of thelithosphere, which depends in turn onthe amount of stretching.The total tectonic subsidence S(t)

during the first stage is a combinationof the initial subsidence Si, in responseto the isostatic compensation of thelithosphere after the thinning event,and the uplift due dilatation of thethinned lithosphere

SðtÞ ¼ Si � av

Z hL=br

0

D#ðz; tÞdz ðA6Þ

where av is the volumetric coefficientof the thermal expansion and

D#ðz; tÞ ¼ #ðz; tÞ � #0 þ#b � #0

hL=brz

� �ðA7Þ

in which J(z,t) is given by eqn (A3).The expression for Si is of the form

Si ¼hc

q0c�q0

Lb þ qL � qc

þhL

q0L�qa

brþ qa � qL

q�qw

ðA8Þ

where qw is the density of water and qathat of the asthenosphere. Prior to the

stretching, qc and qL are the densitiesof the crust and the lithospheric man-tle, respectively, while their homolog-ous densities after the rifting processare q¢c and q¢L. Temperature andcompressibility effects on the densityvalues have to be taken into account.If (br ) b) < 4 ⁄5, eqn (A8) is ap-proximated, with uncertainty of lessthan 25 m, by (Pasquale et al., 1995)

Si ¼ 3:65 1� 1

b

� �: [km] ðA9Þ

Subsidence during the cooling phase,obtained from eqn (A4), is instead

S0ðtÞ ¼ Sf þðS1� Sf Þ

� 1� exp�tjp2

h2LÞ

� �� ðA10Þ

where S¥ is the total tectonic subsi-dence at infinite time given by

S1 ¼ 6:50 1� 1

br

� �: [km] ðA11Þ

and Sf is the subsidence calculatedfrom eqn (A6) at the end of thewarming phase.

Thermal asymmetry of the Ligurian basin • V. Pasquale et al. Terra Nova, Vol 14, No. 6, 484–490

.............................................................................................................................................................

490 � 2002 Blackwell Science Ltd