The Beni Bousera Peridotite (Rif Belt, Morocco): an Oblique-slip Low-angle Shear Zone Thinning the Subcontinental Mantle Lithosphere

Short wavelength lateral variability of lithospheric mantle beneath the Middle Atlas (Morocco) as recorded by mantle xenoliths

“…In addition to the inferred primary and final conditions, the P-T trajectory proposed in Fig. 10 for the Tafraoute lithospheric mantle is constrained by studies in orogenic peridotites providing evidence that decompression associated with extensional or transtensional lithospheric thinning follows a nearly isothermal trajectory, before final cooling (e.g., Fabriès et al, 1998;Frets et al, 2014). Primary equilibrium pressure is poorly constrained; the fact that pyroxene-spinel symplectites are present only in some xenoliths suggests an origin at moderate depth, in the transition field between spinel and garnet peridotites (~2-3 GPa pressure -O'Neill, 1981).…”

“…At medium to high temperature and lithospheric pressure, the [010]-fiber symmetry results from transpressional deformation (e.g., Tommasi et al, 1999). However several studies have stressed that such CPO symmetry may also develop through compaction of molten peridotites Tommasi, 2012, 2014), even when the deformation accommodates normal faulting (Frets et al, 2014). The Tafraoute olivine CPOs also show a clear correlation between grainsize and fabric strength (Fig.…”

Hyperextension of continental to oceanic-like lithosphere: The record of late gabbros in the shallow subcontinental lithospheric mantle of the westernmost Mediterranean

“…The geodynamic significance of the shallow, gabbro-forming igneous event that we have unveiled in the Betic peridotites relies on the interpretation of the age and nature of the processes that led to the internal tectono-magmatic zoning so characteristic of the Betic and Rif orogenic peridotites (Frets et al, 2014;Garrido and Bodinier, 1999;Garrido et al, 2011;Hidas et al, 2013a;Lenoir et al, 2001;Obata, 1980;Précigout et al, 2013;Van der Wal and Vissers, 1996). Previous studies have proposed either a Cenozoic-Mesozoic (Michard et al, 2002;van Hinsbergen et al, 2014) or an Alpine (Garrido et al, 2011;González-Jiménez et al, 2013;Platt et al, 2013) age for the late tectono-magmatic evolution and intra-crustal emplacement of Betic-Rif peridotites.…”

“…1c), that belong to the western part of the Betic Cordillera. Earlier studies provide a basis to distinguish several kilometer-scale petrologic, geochemical and structural domains in otherwise coherent subcontinental lithospheric mantle sections in the Ronda and Beni Bousera Peridotites (Frets et al, 2014;Garrido et al, 2011;Hidas et al, 2013a;and references therein). Based on the correlation and structural relationship to the crustal envelope, from top to the bottom of the section these tectonometamorphic domains include : (1) overlying garnet-spinel mylonite in contact with the metamorphic crustal units (missing in the Ojén massif); (2) spinel tectonite composed of highly foliated porphyroclastic spinel peridotites; (3) granular spinel peridotite that consists of coarse grained spinel peridotites, separated from the overlying unit across a narrow (c. 200-400 m wide) and continuous (c. 20 km long) transitional zone referred to as the recrystallization front (field occurrence is less patent in the Ojén massif and the Beni Bousera Peridotite); (4) underlying, youngest plagioclase tectonite (not cropping out in the Beni Bousera Peridotite) that overprints the granular spinel peridotite domain and attests for partial equilibrium in the plagioclase lherzolite facies.…”

“…Based on the correlation and structural relationship to the crustal envelope, from top to the bottom of the section these tectonometamorphic domains include : (1) overlying garnet-spinel mylonite in contact with the metamorphic crustal units (missing in the Ojén massif); (2) spinel tectonite composed of highly foliated porphyroclastic spinel peridotites; (3) granular spinel peridotite that consists of coarse grained spinel peridotites, separated from the overlying unit across a narrow (c. 200-400 m wide) and continuous (c. 20 km long) transitional zone referred to as the recrystallization front (field occurrence is less patent in the Ojén massif and the Beni Bousera Peridotite); (4) underlying, youngest plagioclase tectonite (not cropping out in the Beni Bousera Peridotite) that overprints the granular spinel peridotite domain and attests for partial equilibrium in the plagioclase lherzolite facies. Although various interpretation of these tectonometamorphic domains exist in the literature (for a recent summary see Frets et al, 2014), there is a consensus that they show a protracted record from the early exhumation at the roots of thick continental lithosphere in the diamond facies (N 140 km, Davies et al, 1993) to final emplacement in an extremely attenuated shallow continental lithosphere in Miocene times. Moreover, the latest ductile history of the massif, prior to emplacement in the crust, is preserved in the plagioclase tectonite domain (Hidas et al, 2013a and references therein).…”