Relationship between the fractal dimension anisotropy of the spatial faults distribution and the paleostress fields on a Variscan granitic massif (Central Spain): the F-parameter

Abstract

The spatial distribution of faults is usually described as a fractal set characterised by the fractal dimension. In this work, we have filtered fault patterns interpreted from digital elevation models, aerial photographs and field maps, by using structural geological parameters of the stress ellipsoid (stress tensor direction and stress ratio R 0 ) and age of deformation. From these filtered structural maps, we have obtained the fractal dimension associated with the fracture patterns developed during Permo-Triassic and Alpine tectonic events on a Variscan granitic massif located in the Spanish Central System. Oriented fractal dimensions were calculated on several transects crossing the fault-filtered maps. The fractal dimension (D), calculated by 1-D box-counting, describes an ellipse on a polar plot with the short axis as the minimum value (D Hmin ) and the long axis as the maximum value (D Hmax ) of the fractal dimensions measured. From these analyses, we have defined the F-parameter as a function of the maximum value, minimum value and vertical value of fractal dimension (D z ), FZ(D z KD Hmin )/(D Hmax K D Hmin ). Finally we have established, from a local scale analysis, a perpendicular relationship between the principal axes of the ellipse of the fractal spatial anisotropy of fractures and the principal axes of the stress tensor (s Hmax , s Hmin and s z ) that generates this dynamic pattern of fractures. Furthermore, the F-parameter and the stress ratio R 0 are equivalents and, applied in this area, both show a triaxial extension.

Intraplate stress state from finite element modelling: The southern border of the Spanish Central System

“…It is intensely fractured with a predominance of strike-slip and normal faults ( Fig. 2a and b) (Doblas, 1989;Campos et al, 1996;Pérez-López et al, 2005). Two principal paleostress fields, deduced from stress inversion of fault-slip data and fractal analysis of fault distribution, characterize the stress evolution of the massif (CSN, 2000;Pérez-López et al, 2005): a) a N-S extensional field probably related to a Late Permian-Early Triassic tectonic event (240 ± 10 Ma), which produced an important system of dikes and quartz veins and E-W normal faults, N60E right lateral strike-slip faults and N120E left lateral strike-slip faults and b) an Alpine field (Eocene to present?)…”

“…2a and b) (Doblas, 1989;Campos et al, 1996;Pérez-López et al, 2005). Two principal paleostress fields, deduced from stress inversion of fault-slip data and fractal analysis of fault distribution, characterize the stress evolution of the massif (CSN, 2000;Pérez-López et al, 2005): a) a N-S extensional field probably related to a Late Permian-Early Triassic tectonic event (240 ± 10 Ma), which produced an important system of dikes and quartz veins and E-W normal faults, N60E right lateral strike-slip faults and N120E left lateral strike-slip faults and b) an Alpine field (Eocene to present?) with a strike-slip regime and maximum horizontal stress orientated N160E, which basically shapes the present morphology of the massif and develops N160E normal faults, N120-140E right lateral strike-slip faults, N10-30E left lateral strike-slip faults and N60-70E thrust faults (Fig.…”

“…The Eocene-Present-day stress field in the studied area of the southern border of the Spanish Central System is characterized by a strike-slip regime with a maximum horizontal stress orientated NW-SE (CSN, 2000;Pérez-López et al, 2005). Although the stress models presented here are calculated under plane strain conditions (1) and taking into account that the principal stress tensors are in general orientated in the vertical/horizontal, the maximum stress tends to permute from the vertical to the horizontal axis when the tectonic load is added (cases 2-3; Figs.…”

“…The El Berrocal massif contains an important uranium mineralization system and several studies have been carried out in order to describe its structural, lithological, geochemical, hydrogeochemical and hydrogeological aspects, with the aim of understanding the natural processes of radionuclide migration in a fractured granitic environment (Campos et al, 1996;Pérez del Villar et al, 1996;CSN, 2000;Pérez-López et al, 2005;Gómez et al, 2006). The petrological, geophysical, structural and recent stress information of this intraplate mountainous region (De Vicente et al, 1996;CSN, 2000;Pérez-López et al, 2005) has been used to build a model geometry with well differentiated lithological bodies and to allow an adequate restriction on boundary conditions. A zone with a clear heterogeneity in the Variscan basement has been selected in order to assess the influence of such in the stress distribution.…”

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

“…Since the lithospheric folding implies large-scale deformation and large wavelength in vertical movements, the active tectonic slip-rate operating in the SCS has to be estimated from Pleistocene, at least. Pérez-López et al (2005) described the stress-field at the South Border of the SCS as strike-slip to uniaxial extension from Eocene to present. Martín-Velázquez et al (2009) modeled this stress-field by using finite elements and a punctual rheological model obeying the thermal model of Tejero and Ruiz (2002).…”