Constraints on the Moho depth in the Iberian Chain.

Constraints on the Moho depth in the Iberian Chain. Restricciones a la profundidad de la discontinuidad de Moho en la Cadena Ibérica. L. Rivero1, J. Guimerà2, R. Salas1 y A. Casas1 1 2

Departament de Geoquímica, Petrologia i Prospecció Geològica, Facultat de Geologia, Universitat de Barcelona, Martí i Franqués s.n. 08028 Barcelona, Spain ([email protected], [email protected], [email protected]). Departament de Geodinàmica i Geofísica, Facultat de Geologia, Universitat de Barcelona, Martí i Franqués s.n. 08028 Barcelona, Spain ([email protected]).

Abstract: Using topographic and gravimetric data, several several models of Airy Moho depth are presented for the Iberian Chain. Four maps were obtained by using different values of Tc and ∆ρ . These maps show a similar shape of the Moho variations. Nevertheless, the maximum Moho depths obtained in the different maps range from 37 to 42 below the Iberian Chain and from 36 to 41 below the Spanish Central System. Key words: Moho depth, Bouguer anomaly, topography, Iberian Chain, Alpine orogeny.

Resumen: A partir de datos topográficos y gravimétricos se han obtenido varios modelos de profundidad del Moho, asumiendo isostasia local de Airy y diferentes valores de Tc y ∆ρ . Estos mapas muestran unos contornos similares del Moho. Sin embargo, la máxima profundidad del Moho obtenida en los diferentes mapas varía entre 37 y 42 km bajo la Cadena Ibérica y entre 36 y 41 bajo el Sistema Central. Palabras clave: Profundidad del Moho, anomalía de Bouguer, topografia, Cadena Ibérica, orogenia alpina.

INTRODUCTION

Pyrenean

i cyn Her an

Moho depth below the Iberian Chain has been a matter of discussion. Both shallow Moho (32 to 34 km: ie. Gómez-Ortiz et al., 2005 and De Vicente et al., 2007) and deeper Moho (35 to 43 km: i.e. Salas & Casas, 1993) have been proposed. Using topographic and gravimetric data, this paper presents several models, and discuss on their adequacy to the Iberian Chain.

Duero basin

Ebro basin em yst

al S Tajo n tr Ce basin

RELIEF AND STRUCTURE Intraplate contraction took place inside Iberia small plate during its Pyrenean collisional interaction with Europe and the early phases of the Betic orogeny. This

Iberian Chain

ma ss if

GEOLOGICAL SETTING The Iberian Chain (Fig. 1) developed by the Tertiary (Eocene to Early Miocene) inversion of the Iberian Basin Mesozoic rifts. During the Mesozoic the Iberian Basin underwent two main rifting stages. The first rift stage developed during the Triassic, when the Tethys and Arctic-North Atlantic rift systems propagated westwards and southwards, respectively. The second rift stage developed during the late Jurassic and early Cretaceous in relation to the opening of the Central and North Atlantic. During middle to late Jurassic and late Albian to Maastrichtian times the Iberian Basin subsided in response to post-rift thermal re-equilibration of the lithosphere (Salas et al., 2001).

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r uivi

sin ba

ic o Bet

en rog

Area of study 0

200 km

FIGURE 1. Main structural units of the Iberian Peninsula. Location of the area of study is shown.

controlled the late Eocene to Oligocene development of the Iberian Chain. The total Palaeogene crustal shortening in the Iberian Chain is estimated at about 75 km (~20%), resulting in the formation of a 43 km deep crustal keel (Salas & Casas, 1993; Salas et al., 2001). As a result of the Tertiary contraction, the Iberian Chain presents complex intraplate contractional structures: thrusts, folds and faults with a dextral strikeslip component (Guimerà, 1984, Salas et al., 2001) that strike NW-SE, NE-SW and E-W (Fig. 2), many of this faults are inherited from the Mesozoic rifting periods. Basement thrust-sheets with several tens of kilometres

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FIGURE 2. Topographic map of the study area obtaind from the GTOPO30 DEM, aplying a 30 km moving average search.

L. RIVERO ET AL.

FIGURE 3. Bouguer anomaly map (in km) of the area studied. Random data were converted over an equally spaced grid using a moving average with a 30 km radius search. 4850000

of horizontal displacement and 4 to 5 km of vertical uplift produce the thrusting of the Iberian Chain over the Ebro basin (Casas Sainz, 1992 and Guimerà & Álvaro, 1990). Its overall structure is defined by two hectokilometric arches (major anticlines in Fig. 2) striking NW-SE with a wave-length ranging from 71 to 119 km; within them Tertiary contractional deformation involves the basement. The cores of both arches are located on thickened crust which mimics their shapes. The arches are neatly separated by the Almazán syncline (the major syncline in Fig. 2) and merge towards the Teruel area, where a wide gravimetric minimum can be observed, which indicates the area of thicker crust. Hence, both arches are crustal-scale structures (Salas et al., 2001).

Santander Bilbao

4800000

San Sebastian

4750000 Logroño Burgos

4700000

Huesca 4650000

Soria

Zaragoza

Valladolid 4600000 Sigüenza

4550000

4500000

4450000

Guadalajara Madrid

Teruel Cuenca

4400000 Valencia 4350000 Albacete 4300000

As a consequence of the contraction, the Hercynian basement reaches 2000 m above sea level in several places and Upper Cretaceous marine rocks (the last precontractional rocks) have been uplifted to about 1500 m above sea level in wide areas of the chain. The main relief of the Iberian Chain appears to be a consequence of the contraction. At the same time as the Iberian Basin was inverted, the basement block of the NE-SW trending Central System (Sierra Guadarrama) upthrust, leading to the isolation of the Duero Basin and the subsidence of the flexural Tajo Basin. To analyse the present topography of the Iberian Chain with a crustal significance (Tsuboi, 1983), a 30 km moving average grid was obtained from the GTOPO30 DEM (Fig. 2). The maximum heights obtained for the Iberian Chain and the Central System are 1400 m and 1300 m respectively. This averaged topography was used in the calculation of the Airy Moho depth maps.

Geo-Temas 10, 2008 (ISSN: 1567-5172)

4250000 250000 300000 350000 400000 450000 500000 550000 600000 650000 700000 750000 800000 850000

FIGURE 4. Airy Moho depth map (in km) obtained from the topography depicted in Fig. 2.

BOUGUER ANOMALY MAP A Bouguer Anomaly Map of the area studied was produced by the compilation of pre-existing data. All data were converted to International Gravity Standardization Net (IGSN 71). From the above compilation the most significant errors arise from elevation and topographic reductions, which yields a compounded error of not more than 5 mGal. Random data were converted over an equally spaced grid using a moving average with a 30 km radius search. This interpolation system confers a crustal significance to the data, and is very convenient for an isostatic study (Tsuboi, 1983). Bouguer map was machine contoured within an interval of 10 mGal (Fig. 3). The Bouguer anomalies depict a regional gravity low under the Iberian Range contrasting with the relative gravity high of the Ebro basin.

CONSTRAINTS ON THE MOHODEPHT IN THE IBERIAN CHAIN The gravity map also shows a rising trend towards the shoreline and reaches its maximum over the València trough. Bouguer gravity anomalies over the Iberian Chain are both large and negative, and therefore consistent with a crustal thickening beneath the mountain Range. According to Salas & Casas (1993) Moho depths reaches 43 km under the northern central part of the chain.

AIRY MOHO DEPTH MAP To calculate the isostatic Airy root (Moho depth), the 30 km moving average DEM (Fig 2) was used. After this grid, the Airy root was calculated using the formula (Simpson et al., 1983):

T( x ) = h( x ) where

ρt + Tc ∆ρ

T( x ) is the Airy root in km, h( x ) is the

topographic height in meters,

ρt

is the density of

topography,

∆ρ is the density contrast between lower crust and mantle and Tc is the normal crustal thickness in km (crustal thickness at the coast). Four maps were obtained by using a 3

,

ρt = 2.65 g cm-

Tc of 27 and 30 km, and ∆ρ of 0.3 and 0.5 g cm-3.

These maps show a similar shape of the Moho variations but maximum Moho depths range from 37 to 42 below the Iberian Chain and from 36 to 41 below the Spanish Central System (Fig. 4). Gómez Ortiz et al (2005,) present a similar Bouguer map of that of Fig. 3. These authors performed a correction of the sedimentary fill of the Tertiary basins Gómez Ortiz et al (2005, fig 6), where the Bouguer anomaly beneath the Iberian Chain has not underwent any significant change, so they are not influenced by the Tertiary sediments. A Moho depth map (not presented here) was derived from the Bougher anomaly map of Fig. 3, according to the formula by Wollard & Strange (1962). As discussed above, these results are not influenced by the Tertiary basins. The Moho depths provided in this model, below the Iberian Chain, reach -44 km. These Moho depths are more coincident with the Airy root map of Fig. 4 than to the other models calculated. The similarity of Moho depths obtained from these independent methods takes us to propose the result depicted in Fig 4 as the more plausible model.

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and by the Consolider-Ingenio 2010 programme, under CSD 2006-0004 “Topo-Iberia”. REFERENCES Casas Sainz, A.M. (1992): El frente norte de las Sierras de Cameros: estructuras cabalgantes y campos de esfuerzos. Zubía, monográfico nº 4, p. 1-220. Instituto de Estudios Riojanos, Logroño. De Vicente, G., Vegas, R., Muñoz Martín, A., Silva, P.G., Andriessen, P., Cloethingh, S, Gonzalez Casado, J.M., Van Vees, J.D., Álvarez, J., Carbó, A., Olaiz, A. (2007): Cenozoic thick-skinned deformation and topography evolution of the Spanish Central System. Global and Planetary Change 58 : 335–381. Gómez-Ortíz, D., Tejero, R., Babín-Vich, R., Rivas, A., 2005. Crustal density structure in the Spanish Central System derived from gravity data analysis (Central Spain). Tectonophysics, 403 (1–4), 131– 149. Guimerà, J., (1984): Palaeogene evolution of deformation in the north-eastern Iberian Peninsula. Geological Magazine, 121: 413-420. Guimerà, J. & Álvaro, M. 1990. Structure et évolution de la compression alpine dans la Chaîne Ibérique et la Chaîne cô tière catalane (Espagne). Bulletin de la Société Géologique de France, 6(2): 339–348. Salas, R. & Casas, A., (1993): Mesozoic extensional tectonics, stratigraphy, and crustal evolution during the Alpine cycle of the eastern Iberian basin. Tectonophysics, 228: 33-55. Salas, R., Guimerà, J., Mas, R., Martín-Closas, C., Meléndez, A. & Alonso, A., (2001): Evolution of the Mesozoic Central Iberian Rift System and its Cainozoic inversion (Iberian Chain). In: Ziegler, P.A., Cavazza, W., Robertson, A.H.F. & CrasquinSoleau, S. (eds) Peri-Tethys Memoir 6: Peri-Tethyan Rift/Wrench Basins and Passive Margins. Mémoires du Muséum National de l’Histoire Naturelle, 186, 145–185. Simpson, Robert W., Jachens, Robert C., and Blakely, Richard J., 1983, Airyroot: A Fortran Program for Calculating the Gravitational Attraction of an Airy Isostatic Root Out to 166.7 KM. U.S.G.S. Open-File Report 83-883, 66 p. Tsuboi, C. (1983): Gravity. George Allen & Unwin, London, 254 p. Wollard, G.P. & Strange, W.E., (1962): Gravity anomalies and crust of the earth in the Pacific basin. In: The Crust of the Pacific Basin. Geophysical Monograph 6, p. 60-80. American Geophysical Union. Washington.

ACKNOWELEDGEMENTS Funding for the research was provided by the I+D+I research project: CGL2005-07445-CO3- 01,

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