Amphibolite facies metamorphic event within the Upper Sebtides tectonic units (Internal Rif, Morocco): a record of a hyperextended margin at the border of the western Tethys

Description

the Alpine belt that exhibits peridotite massifs associated with high-grade crustal metamorphic units. The timing and mechanisms of exhumation of these crust–mantle associations are still debated in the Rif in Morocco as well as in the Betic Cordilleras in Spain. A structural, petrographic, and geochronological study (40Ar–39Ar method) is performed in the Beni Bousera region (Internal Rif) on the Upper Sebtides units that consist of Paleozoic basement and Permo-Triassic metasedimentary rocks. We point out that, in the metagreywackes, an early assemblage of K-feldspar, plagioclase, quartz, muscovite, biotite, and garnet, which is diagnostic of amphibolite facies metamorphism. By dating white mica porphyroclasts from this metamorphic assemblage, we provide the first evidence of internal Rif metamorphism of Triassic age. We assume that these crustal units belong to the ancient continental margin of the Western Tethys, and thus, we interpret this metamorphic event in the framework of a hyperextended margin that is coeval with the exhumation of the subcontinental mantle during the Upper Triassic. Afterward, these units underwent greenschist facies metamorphism during the Lower Miocene due to a thermal event related to the back-arc opening of the Alboran Basin.

Abstract

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