Crustal rheology and seismicity in the Gibraltar Arc (western Mediterranean)

Abstract

On the basis of the lithospheric structure of the Gibraltar Arc (western Mediterranean), we constrain depth distribution of crustal seismicity and active tectonics by means of rheological modeling. Crustal yield strength and depth of the brittle‐ductile transition zone (BDT) mimic the curvature of the arc with maximum depths of 12–9 km whereas in the Betics and Rif, BDT shallows eastward (to 6–5 km depth), oblique to crustal thickening. Most of the crustal seismicity (>60%) is placed within the brittle crust, decaying exponentially in the ductile crust. Active faults observed in surface geology, control the present topography of the Betics, connect in depth with scattered seismic swarms, and merge into the BDT. This horizon is interpreted as a decoupling horizon that conditions the mode of present‐day deformation partitioning. Below the BDT, low‐strength domains enable crustal flow under central Betics promoting stress rotation and topographic uplift.

Strength of the Iberian intraplate lithosphere: Cenozoic deformations and seismicity

“…The type of stress regime does not produce large variations in s T (Fig. 7c), as was also observed in a similar study in the Gibraltar Arc area (Fernández-Ibáñez and Soto 2008). Although the first transition can be situated locally at lower depths (~8-10 km), they correspond to a thin ductile sandwiched level (1 km thick) at the base of the upper crust under shear regime (mainly in Madrid Basin, Fig.…”

“…7b), and solve the problem of earthquakes within those previous ductile levels, which were attributed to uncertainties intrinsic to strength envelopes (Tejero and Ruiz, 2002). However, this discussion would remain open as the seismic layer may extend to greater depths than the brittle-ductile boundary (Scholz, 1998;Handy and Brun, 2004;Zang et al, 2007;Fernández-Ibáñez and Soto 2008;Chen et al, 2012). The cause of seismicity at ductile crustal levels is not well understood, and it has been explained by: high pore-fluid pressures that widens the brittle layer (Boncio, 2008;Brantut et al, 2011;Hirono and Tanikawa, 2011), increased strain rate in the co-seismic and early post-seismic phases (aftershocks) that would extend the brittle-ductile transition in depth (Boncio 2008;Chen et al, 2012), or alternative constitutive relationships such as brittle fracture mechanism (Zang et al, 2007), dislocation glide (Matysiak and Trepmann, 2012;White 2012), or mixed brittle-viscous region between the brittle upper crust and viscous lower crust where faults are able to penetrate (Scholz, 1998;Dempsey et al, 2012).…”

“…Moreover, the stress ratio of the elastic regions are low (<0.5), and large stresses must build up to produce faulting/ seismicity. Finally, the strength varies according to the strain rate, so that a strain rate decrease reduces s T and e T (Fernández-Ibáñez and Soto, 2008;Jiménez-Díaz et al, 2012 , Tesauro et al, 2007). Owing to the wide range of ε& and to check its effect, the integrated strength and the brittle-ductile transition has been modelled for 10 -16 s -1 and 10 -17 s -1 under shear and tensional regimes (Fig.…”

“…Several studies have addressed the estimate of lithospheric strength to explain these characteristics (Van Wees et al, 1996;Gómez-Ortiz 2001;Tejero and Ruiz 2002;Gómez-Ortiz et al, 2005b;Pérez-Gussinyé and Watts 2005;Jiménez-Díaz et al, 2012). Although strength strongly depends on the structure, temperature, composition, strain rate and stress regime of the lithosphere (Fernàndez and Ranalli 1997;Afonso and Ranalli 2004;Ruiz et al, 2006;Fernández-Ibáñez and Soto 2008;Burov 2011;Jiménez-Díaz et al, 2012), average crustal and mantle structures over large areas, diverse lithologies (hard/ soft) and stress regimes (compression/tension) were assumed in those previous studies. Consequently, these strength estimates are varied, and even shallow crustal ductile levels or brittle-ductile transitions are obtained that do not totally explain the intraplate seismicity.…”

Recent and active faults and folds in the central-eastern Internal Zones of the Betic Cordillera

“…The sector of the Betic Cordillera addressed in this paper is affected by distributed seismicity of low to moderate magnitude, which is limited to the upper crust (e.g., Stich et al, 2003Stich et al, , 2010Fernández-Ibáñez and Soto, 2008). Earthquakes larger than Mw 5 rarely occur (Table 1).…”