Evidence of extensional metamorphism associated to Cretaceous rifting of the North-Maghrebian passive margin: The Tanger-Ketama Unit (External Rif, northern Morocco) Vázquez Vílchez, María Mercedes Asebriy, L. Azdimousa, A. Jabaloy Sánchez, Antonio Booth Rea, Guillermo Barbero, Luis Mellini, M. González Lodeiro, Francisco Low-grade extensional metamorphism Illite crystallinity Kübler index Tanger-Ketama unit External Rif Northern Morocco 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. 2014-06-03T08:46:54Z 2014-06-03T08:46:54Z 2013 info:eu-repo/semantics/article Vázquez, M.; et al. Evidence of extensional metamorphism associated to Cretaceous rifting of the North-Maghrebian passive margin: The Tanger-Ketama Unit (External Rif, northern Morocco). Geologica Acta, 11(3): 277-293 (2013). [http://hdl.handle.net/10481/32090] 1695-6133 1696-5728 http://hdl.handle.net/10481/32090 10.1344/105.000001843 eng http://creativecommons.org/licenses/by-nc-nd/3.0/ info:eu-repo/semantics/openAccess Creative Commons Attribution-NonCommercial-NoDerivs 3.0 License Universidad de Barcelona; Instituto de Ciencias de la Tierra Jaume Almera