The lithosphere^asthenosphere boundary in the westernMediterranean from 3D joint gravity and geoid modeling:

tectonic implications

Conxi Ayala a;�, Montse Torne a, Jaume Pous b

a Department of Earth Sciences, Institute of Earth Sciences Jaume Almera-CSIC, Llu|¤s Sole¤ i Sabar|¤s s/n, 08028 Barcelona, Spainb Departament de Geodina'mica i Geof|¤sica, Universitat de Barcelona, Mart|¤ Franque¤s s/n, 08028 Barcelona, Spain

Received 19 September 2002; received in revised form 5 February 2003; accepted 13 February 2003

Abstract

A new integrated three-dimensional (3D) gravity and geoid forward modeling was used to investigate thelithospheric structure of the northwestern Mediterranean basins, namely the Gulf of Lyons, Liguro-Provenc1al Basin,Valencia Trough, and South Balearic Basin. Available geological and geophysical data were used to constructpreliminary maps of the thickness of the sediments and the base of the crust and lithosphere, which were furtherrefined by 3D modeling. Our model results show that the base of the crust thins from 22 km in the SW areas of theValencia Trough to less than 12 km at its NE transition to the oceanic domain of the Provenc1al Basin and from 28km in the shelf area of the Gulf of Lyons to about 12 km in the deep oceanic part of the basin. Upper mantle densitiesvary from 3240 kg/m3 beneath the Valencia Trough (where P-wave velocities are anomalously low, suggesting thepresence of an anomalous uppermost mantle), to 3320 kg/m3 beneath the Gulf of Lyons and the Provenc1al Basin,where ‘normal’ P-wave velocities are recorded. The base of the lithosphere rises from V75^80 km at the shoreline ofthe Valencia Trough and Gulf of Lyons to 60 km in the northern part of the Valencia Trough and 55^60 km at thecenter of the Provenc1al Basin. The structure of the crust and lithospheric mantle obtained from the 3D modelingindicates differential thinning. Moreover, other geophysical observations, e.g., the uneven distribution of heat flowvalues, suggest an asymmetric extensional model.B 2003 Elsevier Science B.V. All rights reserved.

Keywords: geoid; gravity; lithospheric structure; Western Mediterranean; 3D modeling

1. Introduction

The western Mediterranean Sea is formed by aseries of Neogene extensional basins that weredeveloped in a region of overall convergence be-tween the European and African plates [1,2]. LateCretaceous^Cenozoic convergence between thetwo plates gave rise to the present-day westernMediterranean con¢guration, characterized by a

0012-821X / 03 / $ ^ see front matter B 2003 Elsevier Science B.V. All rights reserved.doi:10.1016/S0012-821X(03)00093-1

* Corresponding author. Tel. : +34-409-54-10;Fax: +34-93-411-00-12.E-mail addresses: cayala@ija.csic.es (C. Ayala),

mtorne@ija.csic.es (M. Torne), jaume@natura.geo.ub.es(J. Pous).

EPSL 6585 7-4-03

Earth and Planetary Science Letters 209 (2003) 275^290

www.elsevier.com/locate/epsl

system of orogens which bound the extensionalbasins (Fig. 1). In this convergent scenario thereis no consensus on the mechanism capable of trig-gering the opening of the small Neogene basinssurrounded by the Alpine belts. Some of the ba-sins are of continental nature, e.g., Gulf of Lyons,Valencia Trough and Alboran Sea, whereasothers are thought to be partially formed by oce-anic crust, e.g., Provenc1al and South Balearic ba-sins (Fig. 1). Earlier studies carried out in the areahave resulted in di¡erent geodynamic hypotheses.Some authors support the view that intraconti-nental rifting is the mechanism triggering basinformation [1,2], while others believe that the ba-sins are a consequence of roll-back of the Tethyanlithosphere slab [3] or back-arc extension inducedby subduction (e.g., [4]) and largely in£uenced byseveral episodes of slab detachment [5]. Other hy-potheses like mantle delamination [6] or orogeniccollapse [7], which exerts its in£uence mainly at acrustal level [8] apply only to the Alboran^Beticarea. Moreover, there is no consensus on the de-tailed kinematics of the whole region despite thelarge amount of quality geophysical and geologi-cal data (see [9], [10] and references therein) cur-rently available from the area (seismic data issummarized in Fig. 2).

The wealth of geophysical data, including heat£ow, gravity, near-vertical re£ection and refrac-tion/wide-angle re£ection data have provided adetailed picture of the crustal con¢guration ofthe Gulf of Lyons (e.g., [11,12]) and the ValenciaTrough (see [9] for further details). However, littleis known of the crustal con¢guration of the Pro-venc1al and South Balearic basins and of the pre-cise location and crustal geometry of the ocean^continent boundary or the ocean^continent tran-sition zone. Furthermore, although a number ofstudies have been carried out to investigate thetopography of the base of the lithosphere (seefor instance [13,14]), they have concentrated on2D pro¢les cutting across di¡erent areas of theValencia Trough and the Gulf of Lyons. Todate no attempt has been made to study the 3Dlithospheric structure of the western Mediterra-nean.

In this paper we present an integrated three-dimensional gravity and geoid forward model de-rived for the northwestern Mediterranean basinsto obtain a 3D lithosphere image of the area andthe distribution at depth of the di¡erent crustaland lithosphere density boundaries. Gravity data,when constrained with seismic data, provide accu-rate information about the crustal structure,whereas geoid data can provide reasonably goodinformation on the topography of the litho-sphere^asthenosphere boundary. Therefore, theinherent uncertainties associated with the model-ing of each of these potential ¢elds alone are sig-ni¢cantly reduced. The 3D modeling has allowedus to construct a map of the depth to the Mohoand to the base of the lithosphere for the NWMediterranean region.

2. Input data

2.1. Elevation, gravity and geoid data

Elevation, gravity and geoid data used in themodeling are shown in Figs. 2, 3 and 4. Gravityand geoid are the two observables used to con-strain the density model, whereas elevation (to-pography and bathymetry) was used to de¢nethe geometry of the uppermost layer of the den-

Fig. 1. Tectonic summary map of the northwestern Mediter-ranean (modi¢ed from Roca, 2001 [10]). The study area isoutlined in black. EB, Ebro Basin; CCR, Catalan CoastalRanges; CI, Columbretes Islands; VT, Valencia Trough;SBB, South Balearic Basin; C, Corsica; S, Sardinia; Ka, Ka-bilias; O, Olot.

EPSL 6585 7-4-03

C. Ayala et al. / Earth and Planetary Science Letters 209 (2003) 275^290276

sity model. Moreover, topography was used ingeoid calculations to compute the e¡ect of themasses above sea level. Topography was takenfrom the ETOPO5 data set [15] with a resolutionof 5U5 min. As shown in Fig. 2, topographyvalues range from V0^250 m at the central partsof the Ebro Basin to maximum values of 2000^2500 m at the Pyrenees. Note that gridding ofdata resulted in ¢ltering the short wavelengthcomponent of the topography corresponding tolocal peaks. However, this is not relevant to ourresults since they do not make signi¢cant contri-bution to the geoid.

Bathymetry was obtained from the Internation-al Bathymetric Chart of the Mediterranean(IBCM [16]). The bathymetric contours shownin Fig. 2 were computed from tracks whose spac-ing varies between 3 and 10 km in the westernMediterranean. The tracks were gridded to a con-stant interval of 5U5 km. The Valencia Trough is

characterized by a central channel, with depthsranging from about 1000 m at its SE end to2000 m at the NW. Its NW margin is character-ized by the presence of a 50 km wide continentalshelf that extends from its southern region to thenorth of the Ebro Delta (located at approximately41‡N, Fig. 2). Further north, the shelf narrows tovalues of less than 5 km. To the SE, the trough isbounded by the Balearic rise that extends alongthe Balearic Promontory towards the NE and it ischaracterized by a gentle slope, showing a clearasymmetry with the NW margin of the trough(see Fig. 2). The continental rise o¡ Menorca isapproximately 30 km wide in its northwesternpart, becoming narrower to the northeast. Tothe SE, a narrow shelf and a rather abrupt slope(the Emile Baudot Escarpment) mark the transi-tion from the Balearic Promontory to the £atabyssal plain of the South Balearic Basin, wherewater depths reach values of more than 2500 m.

Fig. 2. Bathymetry and topography of the study area (m, positive upwards). Contours every 250 m. A synthesis of the seismic ex-periments in the northwestern Mediterranean that are relevant for this paper are also shown. VT, Valencia Trough. Nums: ex-panded spread pro¢les (ESP). Dashed lines: Wide-angle seismics. Dotted-dashed line: common depth point (CDP), ESCI pro-gram (Estudios S|¤smicos de la Corteza Ibe¤rica). Continuous lines at the VT: COP+CDP, VALSIS project (COP stands forcommon o¡set point; VALSIS is the name of the project of long listening multichannel seismic pro¢les in the Valencia Trough).Dotted line: ANNA project. VT transect shown in Fig. 9A has the same location as PI.

EPSL 6585 7-4-03

C. Ayala et al. / Earth and Planetary Science Letters 209 (2003) 275^290 277

The Gulf of Lyons is characterized by a widecontinental shelf (more than 50 km) whose tran-sition to the abyssal plain of the Provenc1al Basinis marked by a fairly abrupt slope. The water

depths increase towards the center of the Pro-venc1al Basin, where bathymetry reaches valuesof more than 3000 m.

For the gravity modeling we used free air

Fig. 3. (A) Free air anomaly (mGal) at sea and Bouguer anomaly (mGal) on land, contours every 10 mGal. (B) Bouguer anom-aly, contours every 20 mGal.

EPSL 6585 7-4-03

C. Ayala et al. / Earth and Planetary Science Letters 209 (2003) 275^290278

anomalies at sea and Bouguer anomalies on land(Fig. 3A). Free air anomalies were obtained fromthe Defence Mapping Agency (DMA-AerospaceCenter) with coverage of at least one point every2 km2. The data are referred to the InternationalGravity Standardization Net 1971. Free airanomalies in the study area are characterized byan intermediate to long wavelength componentdominated by values of about 0 mGal in the axialand central parts of the basins, whereas their£anks are characterized by elliptical to circularanomalies of variable length and amplitude thatoscillate from 20 to 50 mGal. This pattern sug-gests that the regional features of the area are inlocal isostatic equilibrium, which is in accordancewith previous studies that show that the litho-sphere has acquired little or no strength since rift-ing (e.g., [17]). Bouguer anomalies on land weretaken from [18]. Bouguer gravity data show thatthe NE corner of the Iberian Peninsula is charac-terized by regional relative gravity lows of vari-able length and amplitude that delineate the Pyr-enees and Iberian chain, attaining minimumvalues of 3120 mGal and 390 mGal, respec-

tively. The Ebro Basin is also marked by aNNW^SSE relative gravity low, which rangesfrom 320 mGal nearby the shoreline to morethan 350 mGal in the NW corner of the basin.

Although the free air anomaly at sea was thedata used in the modeling, Fig. 3B shows theBouguer anomaly gravity map to facilitate an ini-tial qualitative approach of the crustal thicknessvariations of the region. The 3D gravity e¡ect ofthe water layer was computed assuming a con-stant density of 2670 kg/m3. Bouguer gravitydata show that the Gulf of Lyons and the Pro-venc1al Basin are associated with positive gravityvalues that range from 10 mGal at the shorelineof the Gulf of Lyons to more than 240 mGal inthe central areas of the Provenc1al Basin, whereasthe Valencia Trough is associated with an enlon-gated SW^NE trending regional anomaly high ofabout 100^150 mGal, which clearly delineates theaxial parts of the trough. The southern part of theBalearic Islands is characterized by strong gra-dients that rapidly lead to the South Balearic Ba-sin, where Bouguer anomalies range from 150 to220 mGal. The pattern of the Bouguer gravity

Fig. 4. Residual geoid anomaly (m), contours every 0.5 m. Heat £ow values (in mW/m2) are superimposed: circles: 6 80; dia-monds: 80^100; squares: s 100.

EPSL 6585 7-4-03

C. Ayala et al. / Earth and Planetary Science Letters 209 (2003) 275^290 279

anomalies can be explained by the combined ef-fect of mantle and crustal thickness variations. Inthe western Mediterranean, the regional gravityhighs correlate with areas of crustal thinning, assuggested by available seismic data.

The geoid used in the 3D modeling is the resid-ual geoid (Fig. 4) derived by subtracting from thegeoid of GEOMED (Geoid of the Mediterranean[19]) a regional geoid obtained from the coe⁄-cients of the EGM96 geopotential model [20] de-veloped in spherical harmonics up to degree andorder 12. This is a standard procedure that allowsto remove from the geoid data the contribution ofvery deep mantle density sources, which corre-spond to the very long wavelength anomalies(e.g., [14], [21] and references therein). As ob-served in Fig. 4, the Valencia Trough is associatedwith a wide SW^NE trending anomaly rangingfrom 2 m in its SW corner to 30.5 m in the north-east. The £anks of the trough are characterized byresidual anomalies of up to 2.5 m in the BalearicPromontory whereas they reach more than 3 m inthe Pyrenees and the Iberian chain. In the Gulf ofLyons the residual geoid ranges from 1 m at theshoreline to 0 m at the continental slope. A gentlegradient characterizes the transition to the Pro-venc1al Basin where values range from 31.5 m inits northern area to 0 m at its southern part. Ge-oid lows correspond to a mass de¢cit whereashighs are associated with a mass excess. In thewestern Mediterranean, earlier 2D gravity and ge-oid modeling [14] suggests that the residual geoidis largely in£uenced by variations in the litho-spheric structures due to the rifting episodes thatshaped the basins from at least Tertiary times.Thus, an initial qualitative interpretation suggeststhinning of the lithosphere from the mountainranges to the axial region of the Neogene basins,which is in agreement with available geophysicaldata, on land and at sea, and with the regionalpattern of the Bouger gravity anomalies. The re-sidual geoid will henceforth be referred to as thegeoid.

2.2. Sediment thickness

The thickness of the Plio-Quaternary sedimentswas digitized from the 1:1 000 000 scale IOC maps

(International Oceanic Commission [22]), con-toured every 160 m and used to de¢ne the geom-etry of the upper part of the sedimentary layer.The thickness of the Miocene sediments was com-piled from di¡erent publications. At the Gulf ofLyons and the Provenc1al Basin, data come from[11,12,23]. In the Valencia Trough, informationabout the thickness of the Miocene sedimentswas obtained from [24^26]. The thickness of theMiocene sediments was also re¢ned through mod-eling in areas where data coverage is poor or ab-sent (e.g., the southern parts of the Provenc1al Ba-sin).

As shown in plate 1 ([22], p. 248), the distribu-tion of Plio-Quaternary sediments is fairly un-even, particularly in the SW half of the studyarea (South Balearic Basin and Valencia Trough).In this area, the thickness of the Plio-Quaternarycover ranges from 0.2 to 0.4 km, except at theIberian margin of the Valencia Trough. The larg-est accumulation of Plio-Quaternary sediments isfound in front of the Ebro Delta (with thicknessup to 2.8 km), in front of the Rhone deep sea fanand to the NE of the Provenc1al Basin, whereelongated depocenters with thicknesses between0.8 and 2.2 km are recorded. Local accumulationsof Plio-Quaternary sediments are also observed inthe southern areas of the Provenc1al Basin, withthicknesses up to 1.4 km.

Miocene sediments are widespread all along thestudy area with highly variable thickness distribu-tion. Circular to elliptical trending depocentersare observed in the axial parts of the basins andin the continental shelves. This pattern is alsoobserved along the axis of the Valencia Troughwhere the thickness of the Miocene locally variesfrom 0.8 to 2.4 km. The isopach maps (Fig. 5),together with the bathymetric map (Fig. 2), indi-cate that the Valencia Trough is partly sedimentstarved (see also [9]). In the Gulf of Lyons, thethickness of Miocene sediments increases from theshelf to the axial zones of the basin, where localdepocenters reach values of more than 4 km.Larger accumulations occur in the northern areasof the Provenc1al Basin, where values of more than6 km are recorded, whereas the values oscillatearound 4 km or less in the central and southernregions of the basin.

EPSL 6585 7-4-03

C. Ayala et al. / Earth and Planetary Science Letters 209 (2003) 275^290280

2.3. Moho depth and lithospheric thickness

Seismic data, when available (see Fig. 2 for lo-cation of the seismic experiments), were used toconstrain the thickness of the crust (e.g.,[11,12,27,28]). The results of the seismic experi-ments show that the Valencia Trough is charac-terized by a strongly attenuated continental crustwhich is underlain by an anomalous low densityvelocity uppermost lithosphere mantle (V7.6 km/s), except in its northeasternmost region wheretransitional or even oceanic crust is probablypresent (see [27]). A thin continental crust isalso observed to the SW of the Gulf of Lyonsand in the northern part of the Provenc1al Basin,where the Moho is located at 12^14 km. Towardsthe center of the Provenc1al Basin, the interpreta-tion of seismic data suggests that the crust hasreached oceanic stage with a total thickness of2^4 km. The Moho deepens towards the coastof Iberia and France, reaching a depth of V20^24 km beneath the shoreline, and towards theBalearic Promontory, with depths of V23^28 km.

Information on the con¢guration of the litho-sphere^asthenosphere boundary comes primarily

from modeling results. 2D lithosphere modelingalong selected transects cutting across the Valen-cia Trough and the Provenc1al Basin (e.g., [13,14])shows that the base of the lithosphere remains atabout 65^60 km in the central regions of bothbasins and rises towards the emerged areas, whereit is found at V70^75 km at the shoreline of theIberian Peninsula and France. Moreover, otherregional works, such as surface-wave studies(e.g., [29]) also favor thinning of the lithospheretowards the central regions of the basins.

3. Three-dimensional modeling

The study area (Fig. 1) has an extension of650U600 km between 38.5 and 43.5‡N, and1‡W and 7‡E. The three-dimensional density dis-tribution was modeled as a set of layers formedby an assemblage of rectangular prisms, each witha constant density. The model consists of sixlayers: elevation (topography and bathymetry),Plio-Quaternary sediments, Tertiary (Miocene)sediments, crystalline crust, lithospheric mantleand the uppermost part of the asthenosphere.

Fig. 5. Thickness of Miocene sediments (km, positive downwards), contours every 0.4 km.

EPSL 6585 7-4-03

C. Ayala et al. / Earth and Planetary Science Letters 209 (2003) 275^290 281

Throughout the process, the model is assumed tobe in isostatic equilibrium, a requirement neces-sary to avoid edge e¡ects in the geoid calculations[14,30]. Within each layer, the vertical prisms havea variable height and a horizontal cross-section of10U10 km, which allow us to resolve structurescorresponding to intermediate and long wave-lengths, such as the geometry of the Moho orthe topography of the base of the lithosphere.This represents a total of 66U61 prisms for eachone of the six layers. For a single prism of con-stant density, the analytical solution of the gravityanomaly can be found in [31]. The geoid contri-bution is calculated solving the integral of thegravity potential for a regular prism (Fig. 6) andsubstituting the potential in Brun’s formula(N=T/Q, where N is the geoid and T the poten-tial), as described as follows (Zeyen, personalcommunication):

NP ¼ Gb

Q

Z x2

x1

Z y2

y1

Z z2

z1

1R

dxdydz ¼����zy lnðzþ RÞ þ xz lnðyþ RÞ þ yz lnðxþ RÞ3����� x2

2arcsin

x2 þ y2 þ yR

ðyþ RÞffiffiffiffiffiffiffiffiffiffiffiffiffiffiffix2 þ y2

p !

þ y2

2arcsin

x2 þ y2 þ xR

ðxþ RÞffiffiffiffiffiffiffiffiffiffiffiffiffiffiffix2 þ y2

p !

þ"

�����x2y2z2

z2

2arcsin

x2 þ z2 þ xR

ðxþ RÞffiffiffiffiffiffiffiffiffiffiffiffiffiffiffix2 þ z2

p !#�����

x1y1z1

R ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffix2 þ y2 þ z2

p, where x= j3x, y=R3y,

j= h3z ; (j,R,h) are the coordinates of the obser-vation point and x, y, z de¢ne the prism coordi-nates; G is the gravitational constant, 6.67U10311

m3 kg31 s32 ; b is the density of a given prism; Q isthe gravity acceleration.

The geoidal contribution of all prisms will be

the sum N ¼Pn

1NP where N is the geoidal re-sponse of the model, NP is the geoidal contribu-tion of each prism and n is the number of prisms.

The response of the model was calculated andcompared with the observables. The model pa-rameters (densities and coordinates of the topand bottom of the prisms) were successively modi-¢ed until the best ¢tting was obtained. To avoidedge e¡ects in the calculations the model was ex-tended in local isostatic equilibrium far beyondthe limits of the study area.

Seismic velocities from available expandedspread pro¢les (ESP) in the Valencia Trough[27] and the Gulf of Lyons (e.g., [11]) were usedto assign densities to the sediments, the crust andto the lithospheric mantle using standard P-wavevelocity to density empirical relationships, e.g.,[32] for the sedimentary cover, [33] for the crys-talline crust and [34] for the lithospheric mantle.For the oceanic crust in the Provenc1al Basin, thedensity was taken from [35]. These density valuesare consistent with the ones used by other authors(e.g., [13,26,36]). Density for the asthenospherewas taken from [13]. In agreement with the avail-able geophysical data, lateral variations of densitywere only used for the sedimentary cover of theGulf of Lyons^Provenc1al Basin transition zone,and for the uppermost lithosphere mantle of theValencia Trough where anomalously low P-wavevelocities are recorded (Table 1).

4. Results

4.1. Sedimentary cover

The results obtained by the 3D modeling al-lowed us to quantify the gravity and geoid e¡ectof the sedimentary cover and in turn to validatethe regional trend of the thickness of the Miocenesediments in the study area. In areas of poor datacoverage or where modeling results showed largemis¢ts, the thickness of the Miocene sedimentswas slightly modi¢ed to account for these discrep-ancies mainly in the southern part of the Pro-venc1al Basin and in some areas of the South Ba-learic Basin. Fig. 5 shows the modi¢ed thicknessof the Miocene sediments.Fig. 6. Parameters used in the geoid formula for a prism.

EPSL 6585 7-4-03

C. Ayala et al. / Earth and Planetary Science Letters 209 (2003) 275^290282

4.2. Crustal structure

The obtained depth to Moho is shown in Fig.7A. The Moho boundary from our modeling is inagreement with the regional trend of the interpre-tation from the seismic data. Nonetheless, in someplaces, we had to slightly modify the thickness ofthe crust to ¢t the gravity and geoid observations.In the Valencia Trough, the Moho rises along theaxis of the basin from 22 km in the SW to 12 kmin the NE, at the transition to the Provenc1al Ba-sin. Across the trough, the crust thins towards thecenter of the basin showing an asymmetric gra-dient, which is steeper from the axis to the SE,towards the Balearic Promontory, than from theaxis to the NW, towards the Iberian Peninsula. Inthe Gulf of Lyons, the Moho rises progressivelyfrom V26^28 km along the shoreline to about 12km at its SE end, with a gradient that becomessteeper in the direction of the Provenc1al Basin. Inthe Provenc1al Basin, the Moho remains at an al-most constant depth of V11^12 km.

The thickness of the crystalline crust (withoutsediments) varies signi¢cantly between the basins.Along the axis of the Valencia Trough, the crustthickens from approximately 20 km in the SW toabout 6 km in the NE. In the Balearic Promon-tory, crustal thickness increases from approxi-mately 24 km beneath Ibiza to about 28 km belowMallorca, and decreases again towards Menorca,where thickness of V22 km is found. In the Gulfof Lyons, the crust thins gradually from 30^32 kmo¡shore in France to 4^8 km beneath the centralpart of the Provenc1al Basin. Locally, in the oce-anic domain in the Provenc1al Basin, the crust at-tains a thickness of approximately 2 km. Theseresults are in accordance with earlier studies(e.g., [37]).

Previous studies based on the interpretation ofBouguer anomalies [38] and the magnetic ¢eld[39] and on the analysis of the P-wave velocitydistribution [11] indicate that the domain of theoceanic crust is restricted only to the deeper partof the Provenc1al Basin. There is no agreement onthe precise location of the ocean^continentboundary and on the extension of the ocean^con-tinent transition zone. As part of the 3D modelingwe tested these interpretations and conclude thatthe best ¢t for the gravity and geoid anomalies isobtained using the boundary de¢ned by Morelli[38] (Fig. 7A), who suggests that the area of oce-anic crust is outlined by the bathymetric contoursof 2500^2700 m. In all the models, the error ¢t-ting gravity anomalies di¡er in general within arange of Y 15 mGal, but the best geoid anomalieserror ¢tting (in general Y 0.25 m) was obtainedfor the boundary proposed by Morelli [38] (Fig.8). For [10,23,40,41], the error ¢tting for the geoidis in places larger than 0.75 m.

4.3. Lithosphere^asthenosphere boundary

The most outstanding result of our study is the3D mapping of the lithosphere^asthenosphereboundary for the entire region. As observed inFig. 7B, in the Valencia Trough, the base of thelithosphere shallows from land (where it is foundat V75 km below the coastline), to the transitionin the Provenc1al Basin (where it is located at V60km). A certain asymmetry is observed across theaxis of the trough, with a steeper gradient fromthe Balearic Promontory to the center of thetrough than from the center of the basin to Iberia.This asymmetry is similar to the one observed atcrustal levels. In the Gulf of Lyons, the base ofthe lithosphere is located at about 80 km beneath

Table 1Densities used in the modeling

Layer Density Layer Density(kg/m3) (kg/m3)

Topography 2670 continental crust 2850Water 1030 oceanic crust 2870Plio-Quaternary sediments 2220 (VT) lithospheric mantle 3240 (VT*)

2400 (GL, PB) 3320Miocene sediments 2400^2550 asthenosphere 3245

EPSL 6585 7-4-03

C. Ayala et al. / Earth and Planetary Science Letters 209 (2003) 275^290 283

EPSL 6585 7-4-03

C. Ayala et al. / Earth and Planetary Science Letters 209 (2003) 275^290284

the coastline. At the transition from the Gulf ofLyons to the Provenc1al Basin, the lithospherethins towards the SE with a rather abrupt gra-dient in a N^S direction: the lithosphere^astheno-sphere boundary rises from 75 to 60 km in V50km of horizontal distance. One remarkable resultof our modeling is that the base of the lithosphereremains at a fairly constant depth of V60 km inthe Provenc1al Basin and in the northern part ofthe Gulf of Lyons and the Valencia Trough. Fig.7B also suggests that this depth extends locallyonland to the easternmost part of Iberia, in theOlot area (NE) and near Valencia and the Colum-bretes Islands (SE). The areas of thin lithospherecoincide with zones of recent (Neogene) volca-nism [42,43].

4.4. Error ¢tting

Fig. 8A and B show the error ¢tting (observeddata minus response of the ¢nal model) in thegravity and geoid anomalies. The results showthat, for most of the study area, the errors ob-tained with this model are small. These errors arein general less than Y 15 mGal for the gravitydata and mainly Y 0.25 m for the geoid data,respectively. Some mis¢ts of higher amplitudeand short wavelength component (like the oneslocated in the Valencia Trough and the Gulf ofLyons) are associated with lateral density varia-tions at shallow crustal levels and, therefore, un-related to variations in the lithospheric thickness.In this sense, at the Ebro platform, the positivegravity and geoid residual anomalies coincidewith the thickness of Plio-Quaternary sedimentslocally exceeding 3.5 km. Therefore, these residualanomalies can be associated with the fact that thelithosphere has not reached local isostatic equilib-rium in that area [17].

Sensitivity tests were carried out to ensure theaccuracy of the model. The tests consisted of

changing the thickness of the lithosphere andthe crust to evaluate the response of the modelto these changes.

Fig. 9 summarizes the results along two selectedpro¢les crossing the Valencia Trough (Fig. 9A)and Gulf of Lyons respectively (Fig. 9B) (seeFig. 2 for location). Changes of Y 2 km in thethickness of the lithosphere are within the limitof resolution for the calculated gravity and geoidanomalies (note the small di¡erence between theseresponses with respect to the solid line which cor-responds to our preferred model). Variations ofY 5 km are clearly detected: the gravimetric re-sponse changes only few mGal, still within theaccuracy of the data, whereas the geoid responseincreases, in general, more than 0.5 m.

Variations in the thickness of the crust (depthto Moho) causes more conspicuous changes in thegravity and geoid response. Fig. 9 shows the re-sponse of the gravity and geoid anomalies whenchanging the thickness of the crust by Y 2 kmwhich causes variations of more than 15 mGalin the calculated gravity and more than 1 m inthe computed geoid. We conclude that, when thegeometry of the sediments and the Moho is con-strained by seismic data, the base of the litho-sphere can be resolved by Y 2 km. In the southernpart of the Provenc1al and South Balearic basins,where fewer seismic constraints are available, weassumed that there are no lateral density varia-tions and used a constant value for each layer.In this area, we considered that the Moho depthis resolved with a resolution of Y 2 km and thelithosphere^asthenosphere boundary is deter-mined with a resolution of Y 5 km. Some testswere also computed to delimit the ocean^conti-nent boundary (OCB) in terms of lateral densityvariations. As we did not have enough constraintsto de¢ne a transitional zone between the oceanicand continental domains, we approximated theOCB as a line and introduced into the modeling

6

Fig. 7. (A) Depth to Moho (km, positive downwards), contours every 2 km, with the ocean^continent boundaries tested in themodeling according to: (1) Burru¤s, 1984 [47]; (2) Rehault et al., 1984 [23]; (3) Pasquale et al., 1996 [41] ; (4) Roca, 2001 [10];(5) present work, modi¢ed from Morelli, 1990 [38]. Grey numbers show the depth to Moho from expanded spread pro¢les (ESP)interpretation; circles: ESP from Torne et al., 1992 [27]; crosses: ESP from Pascal et al., 1993 [11]. (B) Depth to asthenosphere(km, positive downwards), contours every 5 km.

EPSL 6585 7-4-03

C. Ayala et al. / Earth and Planetary Science Letters 209 (2003) 275^290 285

the boundaries proposed by a number of authors:[10,23,38,40,41]. The best ¢t was obtained for theOCB from [38] (see outline in Fig. 7A).

5. Discussion and conclusions

The gravity and geoid modeling presented inthis paper allows us to obtain a detailed 3D ge-ometry of the crust and lithosphere in the north-western Mediterranean basins. Since available

geophysical constraints were taken into account,a fairly accurate 3D geometry of the base of thecrust and top of the asthenosphere was obtained.In this sense, we re¢ned the work done by otherauthors (e.g., [44,45]) who had only depicted thegross features of the European Mediterraneanlithosphere (for instance, Yegorova et al. [44]work with a grid of 1‡U1‡). In our study weused a grid interval of 10U10 km which allowedus to resolve the medium-to-long wavelengthcomponent of the topography of the base of thelithosphere. The results along the ValenciaTrough section (Fig. 9A) are in agreement withthe work from Zeyen and Ferna'ndez [13], thatmodel the lithosphere using as observables heat£ow, elevation and gravity.

At crustal levels we ¢nd attenuated continentalcrust in the deep waters of the basins. Our modelis compatible with the presence of oceanic crust inthe central part of the Provenc1al Basin. Nonethe-less, the modeled density structure of the crust inthat area could also be interpreted as a transition-al crust heavily intruded by magmatic material,which could also explain the observed highP-wave velocities. Moreover, the magnetic anom-alies [39], do not provide clear evidence of theextension of the oceanic crust in the area (theycan be mostly associated with volcanic bodies).Therefore, the characterization of the oceaniccrust in the northwestern Mediterranean is stillan open question.

The most striking result is that the base of thelithosphere in the northern part of the ValenciaTrough, the southern part of the Gulf of Lyonsand in the Provenc1al Basin is found at an almostconstant depth of V60 km. This lithosphericthinning reveals that the e¡ect of the Neogenerifting exerts an in£uence on all the basins andon land, towards the eastern Iberian Peninsulawhere it coincides with Tortonian to Recent alka-line volcanic centers (e.g., Olot, see Fig. 1 forlocation), indicating that the area could still beunder an extensional regime.

According to our 3D model results, di¡erentialstretching occurs between the crust and the man-tle. Relative stretching factors (L) were calculatedfor the crust and lithosphere, assuming an initialthickness of 32 km for the crust and 110 km for

Fig. 8. (A) Error ¢tting (observed3calculated) gravity anom-aly (mGal), contours every 10 mGal. (B) Error ¢tting (ob-served3calculated) geoid anomaly (m), contours every0.25 m.

EPSL 6585 7-4-03

C. Ayala et al. / Earth and Planetary Science Letters 209 (2003) 275^290286

Fig. 9. (A) Cross-section in the Valencia Trough, at the same location as PI from [27] (see Fig. 2 for location); VT, ValenciaTrough; MI, Mallorca Island; SBB, South Balearic Basin; PQ, Plio-Quaternary sediments; TS, Tertiary sediments. (B) Cross-sec-tion in the Gulf of Lyons^Provenc1al Basin coinciding with some ESP in the area (ESP 201^209, see Fig. 2 for location); PQ,Plio-Quaternary sediments; TS, Tertiary sediments. In panels (a) and (b), gray shade corresponds to the observed data Y 15mGal for the gravity data and Y 25 cm for the geoid data; these panels show the results from our ¢nal model and from the sen-sitivity tests for the lithosphere thinned and thicknened 2 and 5 km, and the crust thinned and thickened 2 km. Tk, thickness;Lth, lithosphere; C, crust. Panel (c), heat £ow data projected along the pro¢les.

EPSL 6585 7-4-03

C. Ayala et al. / Earth and Planetary Science Letters 209 (2003) 275^290 287

the lithosphere [46]. Except in the emerged areasand the southern Valencia Trough, maximum L

for the crust is 2.9 whereas maximum L for thelithosphere is 2, indicating that the crustal thin-ning is greater than the lithospheric thinning. Inthe southern part of the Valencia Trough, thebase of the lithosphere is located at 65^70 km(L= 1.6) in an area where the Moho lies between16 and 22 km (L= 1.4). This suggests that thelithosphere could have been in£uenced by inher-ited structures from the Mesozoic, since this zonewas a rift basin [10] prior the Pyrenean orogeny.In the Gulf of Lyons, the lithosphere thins with asteeper gradient than the crust, which can also beinterpreted as an indication of the decoupling be-tween the crust and the lithospheric mantle.Moreover, di¡erential stretching at crustal andmantle lithosphere levels across the ValenciaTrough and the Gulf of Lyons (Fig. 7) displaysasymmetry in the geometry of the extension, sug-gesting that the extensional mechanism in thewestern Mediterranean could have involved sim-ple shear deformation, in agreement with resultsobtained by other workers (e.g., Mau¡ret et al.[12]). Another geophysical parameter that indi-cates asymmetry in the lithospheric con¢gurationof the basins is the uneven distribution of the heat£ow values (see Figs. 4, 9A and B), as investigatedby several authors (e.g., [41,47]) who also favorthe hypothesis of simple shear.

As pointed out by [48], to the west of the Gulfof Lyons and to the NW of the Valencia Trough,there are two main directions of extension, main-ly deduced from extensional faults ([10,48]) : aNNE^SSW direction, Late Eocene^Oligocene inage, which can be related to the Cenozoic rift ofwestern Europe (e.g., [1]) and a NE^SW direction,Oligocene^Middle Miocene in age, which is asso-ciated to the eastward retreat of the Tethys sub-duction zone. As observed in Fig. 7A, the crustalthinning pattern in the Gulf of Lyons, the westernpart of the Valencia Trough and the Iberia adja-cent areas are fairly coincident with the regionalextensional fault pattern observed at upper crustallevels (NNE^SSW) which suggests the in£uence ofthe Cenozoic rift in these areas. By contrast, thecrustal and mantle lithosphere pattern along theBalearic margin of the Valencia Trough (NE^SW)

seems to correlate better with roll-back relatedextension. In the northernmost part of the Valen-cia Trough and the western part of the Gulf ofLyons, the thinning pattern of the lithosphericstructure (SSE) is compatible with the interferenceof the southward propagation of the Cenozoic riftof west Europe with the Tethyan back-arc exten-sion, whereas in the Provenc1al Basin, the litho-sphere structure could have resulted mainly fromroll-back of the Tethyan slab.

In summary three main hypotheses can be ad-vanced to account for the opening and evolutionof the Neogene basins of the NW Mediterranean:(a) Southern propagation of the Cenozoic Euro-pean rift system, (b) eastward retreat of a sub-ducting slab; and (c) a combination of both.Although our results alone are not su⁄cientlyconclusive to fully support any of these models,the compilation of the di¡erent data sets used inthis study combined with geological data seems toindicate that neither propagation of the Cenozoicrift nor the eastward retreating of a subductingslab can fully explain the current lithosphere con-¢guration of the study area, suggesting rather thatthe NW Mediterranean basins are the result ofboth processes.

Acknowledgements

We thank Prof. Dr. Hermann Zeyen for hishelp and constructive comments in the develop-ment of this study. We also thank R. Sabadini, J.Lorenzo and an anonymous referee for their use-ful comments to improve the manuscript.[BARD]

References

[1] P.A. Ziegler, European Cenozoic rift system, Tectono-physics 208 (1992) 91^111.

[2] P.A. Ziegler, Cenozoic rift system of western and centralEurope: an overview, Geol. Mijnb. 73 (1994) 99^127.

[3] C. Doglioni, E. Gueguen, F. Sa'bat, M. Ferna'ndez, TheWestern Mediterranean extensional basins and the Alpineorogen, Terra Nova 9 (1997) 109^112.

[4] F. Horvath, H. Berckhemer, Mediterranean backarc ba-sins, in: H. Berckhemer, K. Hsu« (Eds.), Alpine Mediter-ranean Geodynamics, American Geophysical Union,Geodynamic vol. 7 (a), 1982, pp. 141^173.

EPSL 6585 7-4-03

C. Ayala et al. / Earth and Planetary Science Letters 209 (2003) 275^290288

[5] E. Carminati, M.J.R. Wortel, W. Spakman, R. Sabadini,The role of slab detachment processes in the opening ofthe western-central Mediterranean basins: some geologi-cal and geophysical evidence, Earth Planet. Sci. Lett. 160(1998) 651^665.

[6] J.E.T. Channel, J.C. Mareschal, Delamination and asym-metric lithosphere thickening in the development of theTyrrhenian Rift, in: M.P. Coward, D. Dietrich, R.G.Park (Eds.), Alpine Tetonics, Geol. Soc. London Spec.Publ. vol. 45, 1989, pp. 285^302.

[7] J.F. Dewey, M.L. Helman, E. Turco, D.H.W. Hutton,S.D. Knott, Kinematics of the western Mediterranean,in: M.P. Coward, D. Dietrich, R.G. Park (Eds.), AlpineTectonics, Geol. Soc. London, Spec. Publ. vol. 45, 1989,pp. 265^283.

[8] M.J.R. Wortel, W. Spakman, Subduction and slab de-tachment in the Mediterranean^Carpatian Region, Sci-ence 290 (2000) 1910^1917.

[9] M. Torne, E. Banda, M. Ferna'ndez, The ValenciaTrough: geological and geophysical constraints on basinformation models, in: P.A. Ziegler, F. Horva'th (Eds.),Peri-Tethys Memoir 2: Structure and Prospects of AlpineBasins and Forelands, Me'm. Mus. Natl. Hist. Nat. vol.170, 1996, pp. 47^40.

[10] E. Roca, The Northwest Mediterranean basin (ValenciaTrough, Gulf of Lyons and Liguro-Provenc1al basins):structure and geodynamic evolution, in: P.A. Ziegler,W. Cavazza, A.H.F. Robertson, S. Crasquin-Soleau(Eds.), Peri-Tethys Memoir 6: Peri-Tethyan Rift/WrenchBasins and Passive Margins, Me¤m. Mus. Natl. Hist. Nat.vol. 186, 2001, pp. 671^706.

[11] G.P. Pascal, A. Mau¡ret, P. Patriat, The ocean^continentboundary in the Gulf of Lyons from analysis of expand-ing spread pro¢les and gravity modelling, Geophys. J. Int.113 (1993) 701^726.

[12] A. Mau¡ret, G. Pascal, A. Maillard, C. Gorini, Tectonicsand deep structure of the Northwestern Mediterraneanbasin, Mar. Pet. Geol. 12 (1995) 645^666.

[13] H. Zeyen, M. Ferna'ndez, Integrated lithospheric model-ling combining thermal, gravity and local isostasy analy-sis: Application to the NE Spanish Geotransect, J. Geo-phys. Res. 99 (1994) 18089^18102.

[14] C. Ayala, J. Pous, M. Torne, The lithosphere^asthenosphere boundary of the Valencia Trough (westernMediterranean) deduced from 2D geoid and gravitymodelling, Geophys. Res. Lett. 23 (1996) 3131^3134.

[15] ETOPO5, Data Announcement 88-MGG-02, Digital re-lief of the Surface of the Earth, NOAA, National Geo-physical Data Center, Boulder, CO, 1988.

[16] C. Morelli, S. Val’chuk, The International BathymetricChart of the Mediterranean (I.B.C.M.) published underthe authority of the Intergovernmental OceanographicComission of UNESCO, IBCM Supporting Volume-Chapter 1, J.K. Hall (Ed.), 1997.

[17] A.B. Watts, M. Torne, Subsidence history, crustal struc-ture and thermal evolution of the Valencia Trough: A

Young Extensional Basin in the Western Mediterranean,J. Geophys. Res. 97 (1992) 20021^20041.

[18] A. Casas, M. Torne, E. Banda, Mapa Gravime'tric deCatalunya (1:500000), Servei Geolo'gic de Catalunya, De-partament de Pol|¤tica Territorial i Obres Pu¤bliques, 1986.

[19] M. Sevilla, Determinacio¤n del Geoide, Bolet|¤n ROA, No6/92, Real Instituto y Observatorio de La Armada, SanFernando (Ca¤diz), 1992.

[20] F.G. Lemoine, D.E. Smith, L. Kunz, R. Smith, E.C. Pav-lis, N.K. Pavlis, S.M. Klosko, D.S. Chinn, M.H. Tor-rence, R.G. Williamson, C.M. Cox, K.E. Rachlin, Y.M.Wang, S.C. Kenyon, R. Salman, R. Trimmer, R.H. Rapp,R.S. Nerem, The Development of the NASA GSFC andNIMA Joint Geopotential Model, Proceeding paper forthe International Symposium of Gravity, Geoid and Ma-rine Geodesy (GRAGEOMAR 1996), The University ofTokyo, Tokyo, September 30^October 5, 1996.

[21] E.C. Molina, N. Ussami, The geoid in southeastern Braziland adjacent regions: new constraints on density distribu-tion and thermal state of the lithosphere, J. Geodyn. 28(1999) 357^374.

[22] M. Gennesseaux, P. Burollet, E. Winnock, Thickness ofthe Plio-Quaternary sediments (IBCM-PQ), Boll. Geo¢s.Teor. Appl. 39 (1998) 243^284.

[23] J.P. Rehault, G. Boillot, A. Mau¡ret, The western Med-iterranean basin geological evolution, Mar. Geol. 55(1984) 447^477.

[24] E. Clavell, X. Berastegui, Petroleum geology of the Gulfof Valencia, in: A.M. Spencer (Ed.), Generation, Accu-mulation and Production in Europe’s Hydrocarbons,Spec. Publ. EAPG, Oxford University, 1991, pp. 335^368.

[25] V. Diaz del Rio, J. Rey, R. Vegas, The Gulf of Valenciacontinental shelf. Extensional tectonics in Neogene andQuaternary sediments, Mar. Geol. 73 (1986) 169^179.

[26] A. Maillard, A. Mau¡ret, A.B. Watts, M. Torne, G. Pas-cal, P. Buhl, A. Pinet, Tertiary sedimentary history andstructure of the Valencia trough (western Mediterranean),Tectonophysics 203 (1992) 57^75.

[27] M. Torne, G. Pascal, P. Buhl, A.B. Watts, A. Mau¡ret,Crustal and velocity structure of the Valencia Through(western Mediterranean), Part I. A combined refraction/wide-angle re£ection and near-vertical re£ection study,Tectonophysics 203 (1992) 1^20.

[28] J.J. Dan‹obeitia, M. Arguedas, J. Gallart, E. Banda, J.Makris, Deep crustal con¢guration of the Valencia troughand its Iberian and Balearic borders from extensive re-fraction and wide-angle re£ection seismic pro¢ling, Tecto-nophysics 203 (1992) 37^55.

[29] F. Marillier, St. Mueller, The Western Mediterranean re-gion as and upper mantle transition zone between twolithospheric plates, Tectonophysics 118 (1985) 113^130.

[30] C. Ayala, M. Ferna'ndez, M. Torne, J. Pous, Two-dimen-sional geoid modelling: some remarks on Chapman’s al-gorithm, Geophys. J. Int. 127 (1996) 542^544.

[31] H. Zeyen, J. Pous, 3D joint inversion of magnetic andgravity data with ‘a priori’ information, Geophys. J. Int.112 (1994) 244^256.

EPSL 6585 7-4-03

C. Ayala et al. / Earth and Planetary Science Letters 209 (2003) 275^290 289

[32] S.E. Nafe, C.L. Drake, Physical properties of marine sedi-ments, in: M.N. Hill (Ed.), The Sea vol. 3, Interscience,New York, 1961, pp. 794^819.

[33] N.I. Christensen, G.H. Shaw, Elasticity of ma¢c rocksfrom the Mid-Atlantic Ridge, Geophys. J. R. Astron.Soc. London 20 (1970) 271^284.

[34] J.P. Barton, The relationship between seismic velocity anddensity in the continental crust ^ a useful constraint?,Geophys. J. R. Astron. Soc. London 87 (1986) 195^208.

[35] R.L. Carlson, C.N. Herrick, Densities and porosities inthe oceanic crust and their variations with depth and age,J. Geophys. Res. 95 (1990) 9153^9170.

[36] J. Burru¤s, F. Bessis, B. Doligez, Heat £ow, subsidence andcrustal structure of the Gulf of Lyons (NW Mediterra-nean); a quantitative discussion of the classic passive mar-gin model, in: C. Beaumont, A.J. Tankard (Eds.), Sedi-mentary Basins and Basin-Forming Mechanisms,Canadian Society of Petroleum Geologists, Memoir 12,1987, pp. 1^15.

[37] N. Chamot-Rooke, J.M. Gaulier, F. Jestin, Constraintson Moho depth and crustal thickness in the Liguro Pro-venc1al basin from a 3D gravity inversion: geodynamicimplications, in: B. Durand, L. Jolivet, F. Horva¤th, M.Se¤ranne (Eds.), The Mediterranean Basins: Tertiary Ex-tension within the Alpine Orogen, Geol. Soc. LondonSpec. Publ. vol. 156, 1999, pp. 37^61.

[38] C. Morelli, The regional meaning of the Bouguer gravityanomalies in the Mediterranean, J. Geodyn. 12 (1990)123^136.

[39] A. Galdeano, J.C. Rossignol, Contribution de l’ae¤romag-netisme a' l’e¤tude du Golfe de Valence (Me¤diterrane¤e oc-cidental), Earth Planet. Sci. Lett. 34 (1977) 85^99.

[40] J. Burru¤s, Contribution to a geodynamic synthesis of theProvenc1al Basin (Northwestern Mediterranean), Mar.Geol. 55 (1984) 247^270.

[41] V. Pasquale, M. Verdoya, P. Chiozzi, Heat £ux and tim-ing of the drifting stage in the Ligurian^Provenc1al basin(Northwestern Mediterranean), J. Geodyn. 21 (1996) 205^222.

[42] J. Mart|¤, J. Mitjavila, E. Roca, C. Aparicio, Cenozoicmagmatism of the Valencia trough (western Mediterra-nean): relationship between structural evolution and vol-canism, Tectonophysics 203 (1992) 145^165.

[43] M. Wilson, G. Bianchini, Tertiary^quaternary magmatismwithin the Mediterranean and surrounding regions, in: B.Durand, L. Jolivet, F. Horva¤th, M. Se¤ranne (Eds.), TheMediterranean Basins: Tertiary Extension within the Al-pine Orogene, Geol. Soc. London Spec. Publ. vol. 156,1999, pp. 141^168.

[44] T.P. Yegorova, V.I. Starostenko, V.G. Kozlenko, Large-scale 3-D gravity analysis of the inhomogeneities in theEuropean-Mediterranean upper mantle, Pure Appl. Geo-phys. 151 (1998) 549^561.

[45] G.F. Panza, P. Suhaldoc, Properties of the lithosphere incollisional belts in the Mediterranean ^ A review, Tecto-nophysics 182 (1990) 39^46.

[46] P. Morgan, M. Fernandez, Neogene vertical movementsand constraints on extension in the Catalan CoastalRanges, Iberian Peninsula and the Valencia Trough (west-ern Mediterranean), Tectonophysics 203 (1992) 185^201.

[47] J. Burru¤s, J.P. Foucher, Contribution to the thermal re-gime of the Provenc1al basin based on Flumed heat £owsurveys and previous investigations, Tectonophysics 128(1986) 303^354.

[48] A. Mau¡ret, C. Gorini, Structural style and geodynamicevolution of Camargue and Western Provenc1al Basin,southeastern France, Tectonics 15 (1996) 356^375.

EPSL 6585 7-4-03

C. Ayala et al. / Earth and Planetary Science Letters 209 (2003) 275^290290