Evidence of extensional metamorphism associated to Cretaceous rifting of the North-Maghrebian passive margin: The Tanger-Ketama Unit (External Rif, northern Morocco)

• Published in: Acta geologica hispanica Vol. 11; no. 3; pp. 277 - 294
• Main Authors: Vázquez, M., Asebriy, L., Azimousa, A., Jabaloy, A., Booth Rea, G., Barbero, L., Mellini, M., González Lodeiros, F.
• Format: Journal Article
• Language: Catalan; English
• Published: Barcelona Universitat de Barcelona 01.09.2013; Geologica Acta; Universitat de Barcelona Revistes Cientifiques de la Universitat de Barcelona; Institut de Ciències de la Terra "Jaume Almera"; Universitat de Barcelona (UB), Geociències Barcelona (Geo3BCN), Institut de Diagnosi Ambiental i Estudis de l'Aigua (IDAEA), Universitat Autònoma de Barcelona (UAB)
• Subjects: Cretaceous; Diagenesis; External Rif; Heat flow; Illite; Illite crystallinity; Kübler index; Low-grade extensional metamorphism; Mesozoic; Metamorphism; Metamorphism (Geology); Miocene; Northern Morocco; Rifting; Tanger-Ketama unit; Morocco; Kübler index; Low-grade extensional metamorphism; External Rif; Tanger-Ketama unit; Illite crystallinity; Northern Morocco
• ISSN: 1695-6133; 0567-7505; 1696-5728
• DOI: 10.1344/105.000001843

The distribution pattern of diagenetic conditions to very low-grade metamorphism in the eastern Rif has been determined based on a study of clay-mineral assemblages and illite crystallinity of Mesozoic metapelites. Low-grade conditions were reached in marbles and also in the Beni-Malek serpentinites, as suggested by the mineral assemblages present in the marbles and antigorite growth in serpentinites. Previous thermochronological data are based on i) 40Ar/39Ar in amphiboles from greenschists, ii) K/Ar in white micas from metasandstones, and iii) fission tracks in apatites and zircons from metasandstones. These data indicate a Late Cretaceous age (∼80 Ma) for the very low- to low-grade metamorphism. We propose an evolutionary model for the Tanger-Ketama Unit consisting of a Lower Cretaceous sequence deposited in half-graben basins over an exhumed serpentinized mantle in a setting similar to the West Galician non-volcanic margin. The sediments underwent diagenesis to very low-grade metamorphism under relatively high heat flow in this extensional setting. Miocene contractional deformation of the Tanger-Ketama Unit resulted in a penetrative crenulation cleavage associated to asymmetric inclined folds. This crenulation developed, mostly by solution-transfer processes, without significant mineral growth. Miocene metamorphism reset the apatite fission-tracks, but metamorphic conditions were not high enough to reset either the K/Ar ages or the zircon fission tracks.