Oligo-Miocene thinning of the Beni Bousera peridotites and their Variscan crustal host rocks, Internal Rif, Morocco

Abstract

Deciphering Variscan versus Alpine history in the Internal Rif system is a key to constrain the tectonic evolution of the Alboran domain and hence the geodynamics of the western Mediterranean system during the Cenozoic. This study focuses on the evolution of the metamorphic envelope of the Beni Bousera massif and its relation to the underlying peridotites. Combining structural geology, metamorphic petrology, and laser ablation inductively coupled plasma mass spectrometry U-Th-Pb dating of monazite, this study contributes to the understanding of the tectonic history of the western Internal Rif. The regional foliation (S2) is characterized by low pressure-high temperature (LP-HT) mineral assemblages and obliterates a former foliation (S1) developed along a Barrovian (medium pressure-medium temperature, MP-MT) metamorphic gradient. The dating of some metamorphic monazite grains from a micaschist and a migmatitic gneiss demonstrates that the crustal envelope of the peridotite recorded two distinct tectonometamorphic episodes. Data from monazite inclusions in S1 garnet suggest that the first event, D1, is older than 250-170 Ma and likely related to the Variscan collision, in agreement with the Barrovian type of the metamorphic gradient. The second event, D2, is Alpine in age (at circa 21 Ma) and corresponds to a strong lithosphere thinning allowing subsequent subcontinental mantle exhumation. Such a tectonic context provides an explanation for the LP-HT metamorphic gradient that is recorded in the regional foliation of the western Betic-Rif system. This extension is probably related to a subduction slab rollback in the western end of the Mediterranean realm during the Oligo-Miocene times. No evidence for a Tertiary high pressure/low temperature metamorphism has been identified in the studied area.

Fast switch from extensional exhumation to thrusting of the Ronda Peridotites (South Spain)

“…The new age of c . 22.5 Ma for the rift‐related structures is compatible with previous geochronological data attesting to Oligo‐Miocene rifting in the Alboran Domain: U–Pb ages of 22.0 ± 0.3 Ma and 22.7 ± 0.3 Ma, obtained on zircon rims from granulites (Platt et al., ) in Ronda and Beni Bousera (Moroccan equivalent of the Ronda peridotites); LA‐ICP‐MS ages of 21.3 ± 0.3 Ma obtained on syn‐foliation monazites within the granulites, and an 40 Ar/ 39 Ar biotite age of 22.5 ± 0.5 Ma from granulites overlying the Beni Bousera Peridotites (Gueydan et al., ; Michard et al., ).…”

“…These first‐order geological features have yielded a diversity of tectonic models for the exhumation of the Ronda Peridotites from mantle depths to the surface. The three main models are: (1) exhumation during the Hercynian orogeny (collision/collapse) with only passive reworking during the Alpine orogeny (Massonne, ; Ruiz Cruz & Sanz de Galdeano, ); (2) exhumation during the Tethysian rifting, followed by the subduction of the Tethyan passive margin (and its lithospheric mantle) during the complex Alpine phase (Tubía et al., ; Van der Wal & Vissers, ) and (3) Oligo‐Miocene exhumation during westward Alboran slab roll‐back, followed by rift inversion during the lower Miocene (Garrido et al., ; Gueydan et al., ; Marchesi et al, ; Précigout et al., ).…”

Western Mediterranean Subcontinental Mantle Emplacement by Continental Margin Obduction

“…Slab tearing along African and Iberian continental margins is the primary mechanism that controls the formation of such arcuate system by the amplification of the trench curvature during slab roll‐back (Spakman & Wortel, ). The polyphase deformation history of Alboran rocks (Variscan, Tethyan, and Alpine deformation stages, see review and related references in Gueydan et al, ) and the overall reset of medium to low temperature ages during lower Miocene (Esteban et al, ) led to contrasting explanations of the relationships between Alboran slab evolution and the peridotites emplacement (see review in Précigout et al, , for Ronda and Gueydan et al, , for Beni Bousera and in Platt et al, ).…”

Neogene polyphase deformation related to the Alboran Basin evolution: new insights for the Beni Bousera massif (Internal Rif, Morocco)

“…The upper Sebtides are basically made up of Permian to Triassic series; part of these units exhibits mineral associations indicating highpressure, low-temperature (HP-LT) conditions of metamorphism typical of subduction zones (Bouybaouène, 1993;Bouybaouène et al, 1995). In the Beni Bousera massif, the lower Sebtides consist of crustal metamorphic units overlying ∼ 2 km thick unit of peridotites and affected from bottom to top by high to low-grade HT-LP metamorphism (Milliard, 1959;Kornprobst, 1974;Bouybaouène et al, 1998;El Maz and Guiraud, 2001;Gueydan et al, 2015;Homonnay et al, 2018) (Fig. 2).…”