Crustal density structure in the Spanish Central System derived from gravity data analysis (Central Spain)

Abstract

Shallow and deep sources generate a gravity low in the central Iberian Peninsula. Long-wavelength shallow sources are two continental sedimentary basins, the Duero and the Tajo Basins, separated by a narrow mountainous chain called the Spanish Central System. To investigate the crustal density structure, a multi taper spectral analysis of gravity data was applied. To minimise biases due to misleading shallow and deep anomaly sources of similar wavelength, first an estimation of gravity anomaly due to Cenozoic sedimentary infill was made. Power spectral analysis indicates two crustal discontinuities at mean depths of 31.1 ± 3.6 and 11.6 ± 0.2 km, respectively. Comparisons with seismic data reveal that the shallow density discontinuity is related to the upper crust lower limit and the deeper source corresponds to the Moho discontinuity. A 3D-depth model for the Moho was obtained by inverse modelling of regional gravity anomalies in the Fourier domain. The Moho depth varies between a mean depth of 31 km and 34 km. Maximum depth is located in a NW-SE trough. Gravity modelling points to lateral density variations in the upper crust. The Central System structure is described as a crustal block uplifted by NE-SW reverse faults. The formation of the system involves displacement along an intracrustal detachment in the middle crust. This detachment would split into several high-angle reverse faults verging both NW and SE. The direction of transport is northwards, the detachment probably being rooted at the Moho.

Strength of the Iberian intraplate lithosphere: Cenozoic deformations and seismicity

“…1b): an 11-14 km thick upper crust, a 7-12 km thick middle crust and a 7-9 km thick lower crust (Banda et al, 1981;Suriñach and Vegas, 1988;ILIHA DSS Group, 1993). The crustal thickness increases up to 34-36 km under the Central System (Suriñach and Vegas, 1988;ILIHA DSS Group 1993;Tejero et al, 1996;Gómez-Ortiz et al, 2005a;De Vicente et al, 2007) because of the buckling of the upper and lower crust by the Alpine compressive stresses (mainly Pyrenean) (Suriñach and Vegas 1988;Martín-Velázquez and De Vicente, 2012). The sedimentary cover has a thickness of ~2500 m in the Duero Basin (Gómez- Ortiz et al 2005a;De Vicente et al, 2007), while it reaches ~3400 m in the Madrid basin (Racero Baena 1988;Querol Müller 1989;Tejero et al 1996;Gómez-Ortiz et al 2005a;De Vicente et al, 2007;De Vicente and Muñoz,Martín, 2012).…”

“…The crustal thickness increases up to 34-36 km under the Central System (Suriñach and Vegas, 1988;ILIHA DSS Group 1993;Tejero et al, 1996;Gómez-Ortiz et al, 2005a;De Vicente et al, 2007) because of the buckling of the upper and lower crust by the Alpine compressive stresses (mainly Pyrenean) (Suriñach and Vegas 1988;Martín-Velázquez and De Vicente, 2012). The sedimentary cover has a thickness of ~2500 m in the Duero Basin (Gómez- Ortiz et al 2005a;De Vicente et al, 2007), while it reaches ~3400 m in the Madrid basin (Racero Baena 1988;Querol Müller 1989;Tejero et al 1996;Gómez-Ortiz et al 2005a;De Vicente et al, 2007;De Vicente and Muñoz,Martín, 2012). The lithosphere-asthenosphere boundary is located at a mean depth of ~100 km in the peninsular interior (Banda et al, 1981;Fernàndez et al, 1998;Tejero and Ruiz 2002;Martín-Velázquez and De Vicente, 2012).…”

“…Since the lithosphere in the peninsular centre varies laterally (Gómez-Ortiz et al, 2005a), its strength will be determined along a detailed cross section to the Central System and bordering basins, solving the thermomechanical equations with a finite element software (ANSYS, http:// www.ansys.com). Although, dry and wet dislocation creep were previously assumed for the upper crust (Tejero and Ruiz, 2002;Ruiz et al, 2006;Jiménez-Díaz et al, 2012), the numerical modelling of Cenozoic deformations discards a weak upper crust (Martín-Velázquez and De Vicente, 2012), and only dry rheology will be modelled in this layer.…”

Recent tectonic model for the Upper Tagus Basin (central Spain)

“…Models developed by these authors indicate a maximum thickness for the upper crust of 16 km within the Tagus Basin and of 11 km for the SCS. Moreover, two crustal discontinuities, at 11 and 31 km depth respectively, have been described from the spectral analysis of gravity data (Gómez-Ortiz et al, 2005). In consequence, in this work is assumed the interval between 11-16 km depth as the preferred crustal level for brittle deformation and seismogenic source, in order to evaluate the area for faulting rupture according to the main Quaternary faults traces described in the following sections.…”

“…This relatively high ground response is related to the relatively thick Cenozoic sedimentary filling of the basin (up to 3 km, i.e. Alonzo-Zarza et al, 2004;Gómez-Ortiz et al, 2005), but also the near-field effect has to be considererd (Carreño et al, 2008). The extensive ancient Late Neogene surface dominating the intrabasinal landscape, as well as the scarce evidence of recent earthquake-related deformations (Silva et al, 1997;De Vicente et al, 2007), make difficult to assign a deformation Quaternary tectonic slip-rate for the Upper Tagus Basin (UTB).…”