Fault relay zones evolving through distributed longitudinal fractures: The case of the Teruel graben system (Iberian Chain, Spain)

Abstract A new type of fault relay zone in extensional contexts, dominated by distributed along-strike or longitudinal fractures, is defined. It contrasts with the classical models reported in the literature, in which transverse connecting faults controlled by the own relay kinematics prevail. The new model is based on structural features of the Teruel graben system, as well as on analogue modelling. Relay zones between the NW-SE to NNW-SSE striking faults that delimit the eastern boundary of the Jiloca Graben (Calamocha, Sierra Palomera and Concud faults), together with the Teruel Fault, have been studied. All of these relay faults show recent (Neogene-Quaternary) ruptures at different scales, mostly parallel to the macrostructural trend and to the maximum horizontal stress (SHmax) trajectories (i.e., orthogonal to the ENE-WSW regional extension direction that characterises the nearly biaxial or radial stress regime active during Upper Pliocene-Quaternary times). Transverse ruptures are almost absent, with the exception of the northern relay zone (Calamocha-Sierra Palomera), where an incipient NE-SW striking connecting fault does exist. Analogue models have been run under a biaxial extension regime similar to the regional one. They allowed analysing the main factors controlling fracture propagation, depending on the ratio of extension velocities and the orientation of the master faults relative to extension directions. Laboratory fracture patterns, as in the natural studied examples, are mostly controlled by the inherited anisotropies and, in a greater extent, by the imposed extension trajectories, which results in a clear prevalence of longitudinal fractures. Such external controls, usually disregarded in numerical and analogue modelling, tend to induce fault coalescence through along-strike (parallel or at very-low-angle) propagation resulting in a final braided fault pattern.