José Miguel Martínez-Martínez

To constrain the latest evolutionary stages and mechanisms of exhumation and emplacement of subcontinental peridotites in the westernmost Mediterranean, we present here a detailed structural study of the transition from granular spinel peridotite to plagioclase tectonite in the western Ronda Peridotite (Betic Cordillera, southern Spain). We show that the plagioclase tectonite foliation represents an axial surface particularly well developed in the reverse limb of a downward facing moderately plunging and moderately inclined synform at the base of the Ronda massif. The fold limbs are cut by several mylonitic and ultramylonitic shear zones with top-to-the-SW sense of shear. After restoring the middle to late Miocene vertical-axis palaeomagnetic rotation and the early Miocene tectonic tilting of the massif, these studied structures record southward-directed kinematics. We propose a geodynamic model in which folding and shearing of an attenuated mantle lithosphere occurred by backarc basin inversion during late Oligocene (23–25 Ma) southward collision of the Alborán Domain with the palaeo-Maghrebian passive margin, leading to the intracrustal emplacement of peridotites in the earliest Miocene (21–23 Ma).

Active tectonics in the central and eastern Betic Cordillera through morphotectonic analysis: the case of Sierra Nevada and Sierra Alhamilla

Terra Nova, 24, 505–512, 2012 The subducted North Maghrebian passive margin was exhumed in the Tortonian (11–7 Ma) by an upper‐crustal brittle‐ductile extensional detachment and brittle low‐angle normal faults in a continental subduction transform setting. The Temsamane detachment in the eastern Rif is defined by a ductile shear zone approximately 100‐m thick with a low‐angle ramp geometry that cuts down into the Temsamane fold‐nappe stack. The shear zone shows south‐westward kinematics and separates lower‐greenschist (≈400 °C) metapelites of the Temsamane units below from the anchizone and diagenetic rocks (<300–200 °C) of the Ketama‐Aknoul units above. To the east, the detachment becomes brittle, branching into a listric‐fan that cuts through 10–6 Ma sediments and volcanoclastics in the Tres Forcas cape. Both the North Maghrebian and the South Iberian subducted passive margins were exhumed in the Betic and Rif branches of the Gibraltar arc by SW‐directed brittle‐ductile detachments during the late Miocene in an oblique collisional setting.

Multichannel seismic reflection images across the transition between the east Alborán and the Algero‐Balearic basins show how crustal thickness decreases from about 5 s two‐way traveltime (TWTT, ∼15 km thick) in the west (east Alborán basin) to ∼2 s TWTT typical of oceanic crust (∼6 km thick) in the east (Algero‐Balearic basin). We have differentiated three different crustal domains in this transition, mainly on the basis of crustal thickness and seismic signature. Boundaries between the three crustal domains are transitional and lack evidence for major faults. Tilted blocks related to extension are very scarce and all sampled basement outcrops are volcanic, suggesting a strong relationship between magmatism and crustal structure. Stratigraphic correlation of lithoseismic units with sedimentary units of southeastern Betic basins indicates that sediments onlap igneous basement approximately at 12 Ma in the eastern area and at 8 Ma in the western area. Linking seismic crustal structure with magmatic geochemical evidence suggests that the three differentiated crustal domains may represent, from west to east, thin continental crust modified by arc magmatism, magmatic‐arc crust, and oceanic crust. Middle to late Miocene arc and oceanic crust formation in the east Alborán and Algero‐Balearic basins, respectively, occurred during westward migration of the Gibraltar accretionary wedge and shortening in the Betic‐Rif foreland basins. Arc magmatism and associated backarc oceanic crust formation were related to early to middle Miocene subduction and rollback of the Flysch Trough oceanic basement. Subduction of this narrow slab beneath the Alborán basin was coeval with collision of the Alborán domain with the Iberian and African passive margins and subsequent subcontinental‐lithosphere edge delamination along the Betic‐Rif margins.