Contrasting fault reactivation patterns along the Betic foreland (Gibraltar arc, southern Spain)

During the Miocene formation of the Gibraltar Arc (western Mediterranean), the orogenic load on the SSE margin of the Betics (northern branch) foreland basin led to the formation of an ENE-WSW flexural relief on the foreland (forebulge), producing orthogonal extension accommodated by normal faults oriented parallel to the forebulge strike. However, structural and geomorphic results point to Quaternary relief rejuvenation of the Betics foreland that would account for buckling of the Iberian lithosphere, produced by mechanical coupling and strain transfer from the orogenic wedge to the under-thrusted foreland. This process has been attributed to the Africa-Eurasia convergence and/or to the westward migration of the arc.

In detail, relief rejuvenation of the Betic foreland is mostly accommodated through reactivation of inherited structures, although shows significant differences along strike. In the westernmost sector of the study area, most reactivated structures strike ca. NW-SE to WNW-ESE, which track the Variscan-Paleozoic structural pattern (folds and reverse and left-lateral tanspressional shear zones) of the foreland as well as transfer faults of the Triassic rifting event, and show a main reverse-lateral kinematics. By contrast, in the easternmost sector of the study area, former NE-SW to ENE-WSW extensional faults, likely inherited from the Triassic rifted margin, were reactivated with reverse kinematics.

Other differences between these two sectors are: (1) the eastern sector presents Triassic marly and evaporitic deposits, suggesting a more pronounced extension during the rifting event; (2) the boundary between the foreland basin and the foreland is NE-SW in the eastern sector and ENE-WSW in the western one; (3) the fold-and-thrust in front of the eastern sector defines a rough NE-SW striking secondary arc (the Cazorla arc) with orthogonal convergence kinematics, whereas in front of the western sector, it corresponds to a transitional zone (the Algodonales-Torcal zone) between two secondary arcs and shows dextral transpressional kinematics.

The transitional zone between these two sectors of the foreland shows a hybrid reactivation pattern. The fold-and-thrust in front of this intermediate segment is a ca. E-W striking secondary arc (the Central Betics) with orthogonal convergence kinematics.

The geometrical relationship between the two main sets of inherited structures of the foreland (WNW-ESE and NE-SW to ENE-WSW) and the tentative bulk convergence vector (WNW-ESE) in both sectors is very similar, thus it cannot account for the observed differences between them. Alternatively, in both sectors, the main reactivated structures seem to localize at former extensional faults regardless their age (e.g., the faults controlling the Miocene Bailén and Andújar basins in the eastern sector and the Permian Viar basin and other minor ones of the same age in the western sector). Ongoing research on the architecture of the reactivated faults and numerical modeling will contribute to constrain the main parameters responsible for the observed differences between the two studied sectors of the Betic foreland.