New Insights into the Lateral‐Strength Variations and Depth to the Brittle‐Ductile Transition Zone in NW Iberia

Abstract

The mechanism of faulting is controlled by pressure, temperature, fluid pressure, and strain-rate leading to brittle fracture and frictional slip in the upper levels of the continental crust, and ductile flow at greater depths (Pollard & Fletcher, 2005; Tullis & Yund, 1977). The transition from one domain to the other is defined by the brittle-ductile transition zone (BDTZ), which also represents the transition between the rheological changes from cataclasis to crystal plasticity (e.g., Stewart et al., 2000). This boundary demarcates the limit of the seismogenic zone (with 95% of all seismicity, Magistrale, 2002). This sharp level correlates to the depth to which earthquakes occur in the continental interiors due to the rapid reduction of resistance to shearing (e.g., Long & Zelt, 1991; Sibson, 2007). This boundary is controlled by factors affecting the deformation mechanisms, such as temperature, pressure, strain rate, composition, pore fluid pressure, and tectonic stress (Stewart et al., 2000; Tullis & Yund, 1977). Variations in these factors are responsible for changes of the BDTZ depth and therefore the thickness of the seismogenic crust (Figure 1a). In most tectonic regions, earthquakes are occurring in the shallow part of the lithosphere (C. Scholz, 1988), where rock strength is mostly controlled by thermo-mechanical properties (i.e., rheology and thermal gradient), crustal thickness and rock composition (Maggi et al., 2000; Watts & Burov, 2003). Early observations of brittle-plastic transition provide evidence of increasing plastic flow with depth and the progressive change in the physical factors that control rock deformation (Sibson, 1977). The BDTZ represents a transition in which semi-brittle deformation dominates (C. H. Scholz, 2002). This shift constrains the maximum depth at which earthquakes can occur and represents a mixture of brittle and plastic processes at the microscale, while the rheology is macroscopically ductile (Karato, 2012; C. Scholz, 1988).