Lower Sebtides Neogene exhumation and uplift in Ceuta and Cabo Negro areas (Internal Rif, Northern Morocco) : insights from medium- and low-temperature thermochronology
Résumé
The Rif Chain (Northern Morocco) belongs, with the Betic Cordillera
(Southern Spain), to the western termination of the Alpine belt. Deepest
units of the Rif Internal Zones (i.e. Lower Sebtides) outcrop in three areas, from North to South : Ceuta, Cabo Negro and Beni Bousera.
The quick exhumation of these units during the Late Oligocene-Early
Miocene is related with a crustal thinning and the Alboran Sea rifting.
Despite Beni Bousera massif has been well investigated, no data are
available in Ceuta and Cabo Negro areas. This study aims to constrain
the last steps of cooling and exhumation in the Lower Sebtides using
medium- and low-temperature thermochronological data. Combination
of Ar-Ar on micas, fission tracks and (U-Th)/He on apatites (respectively
AFT and AHe) from high-grade rocks allows understanding thermal
history between 450-350°C and surface temperatures. These data
suggest a similar cooling evolution of the Ceuta and Cabo Negro units.
Ages obtained with all these methods are comprised in the same interval,
between 22 and 18 Ma. These data record an almost instantaneous
cooling (between 50 and 200°C/Ma) associated to final exhumation after
the Oligo-Miocene thermal event affecting these units at shallow
depths (10 km). Low-temperature data suggest that Ceuta and Cabo
Negro units cooled at surface temperatures before Beni Bousera ones.
Since the end of Burdigalian, the Rif internal zones vertical movements
did not exceed 1km, even during the Messinian Salinity. AFT and AHe
ages in the whole Sebtides are also slightly older than these obtained
in their Spanish equivalent (Alpujarrides). The Gibraltar arc is divided
into several independent crustal blocks separated by major fault zones
perpendicular to the coast that could have induced independent vertical
movements during the entire Neogene