PICASSO: Testing Models for Upper Mantle Processes beneath the Alboran Basin and the Gibraltar Arc (Western Mediterranean)

The Mediterranean hosts several arcuate thrust belts surrounding coeval extensional “back-arc” basins, some of which are associated with retreating subduction zones. The Gibraltar Arc and Alboran Basin have spawned competing hypotheses about upper mantle processes beneath the system, including a W-retreating slab of oceanic lithosphere, W-directed delamination, and outwardly propagating Rayleigh-Taylor drips of sub-continental lithospheric mantle. PICASSO is a multidisciplinary, international investigation using passive and active seismology, magnetotellurics, geochemistry, petrology, structural geology, and geodynamic modeling to help distinguish among these hypotheses.

The Alboran Basin formed on the site of an accretionary orogen created by Africa-Eurasia convergence during Paleogene time. This orogen underwent extensional collapse in the earliest Miocene (~24-18 Ma), attributed to the removal of its lithospheric mantle. Extension, accompanied by heating, resulted in exhumation of crustal rock from up to 50 km depth, together with underlying lithospheric (and possibly asthenospheric) mantle. Extension was accompanied by magmatism, which continued through the late Miocene. Magmatism has been attributed to subduction, to decompressional melting associated with extension, and to lithospheric melting associated with delamination.

The Alboran Domain moved west as it continued to extend during the middle to late Miocene; creating an arcuate thrust belt around its periphery due to shortening of the African and Iberian continental margins. The leading edge of the Iberian continental crust was subducted beneath the Alboran Domain, reaching eclogite facies at ~17 Ma, and was exhumed by 10-12 Ma. These rocks pierced the overlying Alboran Domain, forming an elongate dome that reaches the highest elevations in the Iberian Peninsula.

PICASSO will search for geophysical signatures of subducted ocean lithosphere, define the geometry of subducted, delaminated, or downwelling lithospheric mantle beneath the Alboran domain, determine lithospheric thickness variations throughout the region, and elucidate the history of exposed mantle rocks and their geochemical relationship to the magmatism.