Timing and Types of Extension in the Westernmost Mediterranean from Onshore-Offshore Studies

Guillermo Booth-Rea, Cesar R. Ranero, José Miguel Martínez-Martínez, José Miguel Azañón, Flavio Giaconia, and Eulalia Gràcia

The mode and timing of extensional tectonics in the westernmost Mediterranean is mostly determined by the tectonic context within the Gibraltar arc, where extension occurred. Forearc domains like the western Alboran basin and the western Betics show important Early Miocene extension related to the collapse of the Paleogene Alboran orogen. These extensional structures that show ductile-brittle N-S extension have travelled hundred of km in a fore-arc position following the Middle to Late Miocene rollback of the Paleotethys oceanic lithosphere. Extension related to slab roll-back, with westward kinematics, further thinned the Western Alboran basin during the Middle to Late Miocene. Meanwhile, arc and back-arc oceanic domains of the Gibraltar arc, represented by the East Alboran and the Algero-Balearic basins, respectively, were mostly extended by magmatic accretion. Onshore in both the Betics and Rif, continental extension during the Middle to Late Miocene initiated in the uppermost crust, coeval to underplating of tectonic units derived from the subducted South-Iberian margin. This extension resulted in the development of large core-complex type domes that were exhumed by the sequential activity of several SW-directed extensional detachments. The late Miocene evolution of these extensional systems was probably fueled by lithospheric delamination and was accompanied by important lower crustal melting, Si-K rich magmatism in the eastern Betics and Rif and by regional topographic uplift. The boundaries between different types of extension occurring during the Middle to Late Miocene in the Westernmost Mediterranean are ENE to ESE striking transform faults like the Alpujarras, Carboneras, Yusuf, Mazarrón or Moreras faults. The Alpujarras dextral fault, for example, permitted differential extension between the Sierra Nevada elongated dome core-complex and a domain of extensional tilted blocks to the south. Meanwhile, the Carboneras sinistral fault separates a tilted block domain to the northwest from a domain extended by arc-type magmatic accretion that intruded thinned continental crust to the SE. The Yusuf and Mazarrón faults to the SW and NE of the Algero-balearic basin separate areas that underwent backarc oceanic accretion in the basin from thinned continental crust intruded by arc-magmatism at the Betic and Rif offshore margins. Late Miocene extension in the eastern Betics has normally been interpreted as related to local transtensional settings in a general context of continental transcurrent and contractive tectonics. Here we show the presence of low-angle brittle extensional detachments and associated listric faults produced during Middle Miocene to Tortonian large-scale upper-crustal extension. The normal faults cut Tortonian marine sediments, producing syndepositional progressive unconformities typical of synrift sedimentary sequences. Extensional faults at the western end of the Algero-Balearic back-arc basin show an eastward component of extension, observed on deep seismic reflection lines and share the activity timing with the onshore structures described above. Coeval extension in the adjacent oceanic and continental domains probably occurred by related mechanisms, slab retreat beneath the Algero- Balearic basin and edge delamination and lateral tearing of the subducted south Iberian continental slab under the continental margin. Late Miocene extension in the Betics was strongly heterogeneous with westward directed extension occurring in great part of the Betics and Alboran sea, whilst eastward-directed extension occurred to the east of the Alboran volcanic arc in the Algero-Balearic basin and the Easternmost Betics.

AAPG Search and Discovery Article #90161©2013 AAPG European Regional Conference, Barcelona, Spain, 8-10 April 2013