Intracrustal tectonic evolution of large lithosphere mantle slabs in the western end of the Mediterranean orogen (Gibraltar arc)

M. Sanchez-Gomez, J. Carlos Balanya, V. Gracia-Duenas, and J.M. Azañón

The Ronda and Rif peridotites interlayered with crustal rocks in the western Alboran Domain are a good example of a large volume of very deep rocks exhumed in an unlikely tectonic context. We have studied the western Alboran Domain on both sides of the Gibraltar arc, where a large number of peridotite bodies and high-pressure crustal rocks crop out. We have attempted to correlate microstructures, PT history and large-scale structures of nearby units (from the Alpujarride Complex) from the analysis of different tectonic levels, rock ages and geographic positions with respect to the Gibraltar arc. In this sense, the peridotite bodies, which have a definite structural position, have been taken as an important marker layer.

We propose four main tectonic events in the region related with the exhumation of the deep crustal and mantle rocks. The first event is a pre-Miocene collision that produced a similar high-pressure metamorphic record in both Alpujarride units, above and below the peridotites. It is therefore not considered to be directly related with the peridotite emplacement. The second event is the thinning and uplift of the former Alpujarride rocks, which produced a penetrative subhorizontal foliation and the rise of the top of the peridotites up to 6 kbar (ca. 18 km). A steep isothermal decompression is observed associated with the foliation development. The third event involves the peridotite emplacement on the crust. Large kilometric recumbent folds occur below the peridotite slab coupled with the development of crenulation cleavage. No clear metamorphic record of this event is observed, except diffuse heating in the unit beneath the peridotites. The fourth event corresponds to the Miocene collapse of the Alboran Domain, which resulted in the dismembering of the former peridotite slab. A N-S early extensional stage of this event is associated with ductile shearing and granite generation at 22-18 Ma. The N-S extension broke off the peridotite slab, individualizing the current bodies. Serpentinized peridotites flowed plastically between the main bodies. Throughout the Miocene, the peridotite bodies were dispersed and exhumed as passive elements of the Alboran Domain during its collision with the south-Iberian and Magrebian margins.

Several models for the western Mediterranean have implied radial emplacement or dispersion of the peridotite bodies, which disagrees with our observations. We propose that all the Alboran peridotite bodies were emplaced as a single slab, probably farther east from their present position. Afterwards, displacement to the west of the Alboran terrain was concomitant with N-S extension. A total cumulative N-S extension has been estimated based on the reconstruction of the former peridotite slab, of at least 1.4 to 2. Thus paradoxically, N-S extension is one of the most patent features of the Alboran orogen in a N-S plate convergence setting.