Western Mediterranean Subcontinental Mantle Emplacement by Continental Margin Obduction

Abstract

Obduction is the tectonic process that results from thrusting of an oceanic lithosphere section (ophiolite) onto a continent. In contrast, thrusting of subcontinental mantle, observed in several mountain belts, remains a major unknown of plate tectonics. In the western Mediterranean, the Ronda and Beni Bousera peridotites are the largest worldwide subcontinental mantle exposure. From a geological point of view, the Ronda peridotites are exhumed by hyperstretching of the continental lithosphere in a back‐arc immediately followed by thrusting, explaining their present‐day position inside the Alboran crust. Using 2‐D and 3‐D modeling of new gravimetric data combined with local seismic tomography, we show that the Ronda peridotites are rooted inside the Alboran mantle along the entire Gibraltar arc. On these bases, we propose that the emplacement of the Ronda peridotites occurred in a back‐arc setting and corresponds to the thrusting of an entire hyper‐stretched continental margin onto a continent, a process that we define as continental margin obduction. This results from two successive deformation events: continental upper plate extension driven by slab roll‐back, immediately followed by upper plate shortening, likely triggered when a buoyant continental domain enters the subduction. We propose that this process affected the entire western Alboran domain, with peridotite bodies embedded within the crust along the whole Gibraltar Arc. We suggest that other examples such as the Alps Ivrea mantle body could likely represent continental margin obduction at the onset of continental collision.

Evidence of Segmentation in the Iberia–Africa Plate Boundary: A Jurassic Heritage?

“…The RPM consists of tectonically stacked peridotite sheets or a mantle extrusion wedge emplaced by the retreating of a subduction slab at around 20 Ma [4,19,[29][30][31]. These mantle peridotite rocks could be either restricted to the upper-middle crust [32] or rooted into the lithospheric mantle [33,34]. The direction of emplacement and shortening undergone by these structures are uncertain and highly debated.…”

Lateral variations of pressure-temperature evolution in non-cylindrical orogens and 3-D subduction dynamics: the Betic-Rif Cordillera example

“…Lithospheric-scale E-W section through the Betic Cordillera. Deep crustal and lithospheric structures after de Lis Mancilla et al (2018) and Gueydan et al (2019). The Ronda peridotite massif is sandwiched between the Guadaiza and Blanca units underneath and the Jubrique Unit above.…”

The Beni Bousera marbles, record of a Triassic-Early Jurassic hyperextended margin in the Alpujarrides-Sebtides units (Rif belt, Morocco)

“…The kinzigites mainly consist of migmatitic metapelites characterized by the assemblage garnet-kyanite-rutile ± biotite-sillimanite and include metabasite lenses (Bouybaouene et al, 1998;Haissen et al, 2004;Álvarez-Valero et al, 2014). They are currently regarded as lower-crustal rocks separated from the ultramafics by a major extensional shear zone (Saddiqi et al, 1988;Afiri et al, 2011;Gueydan et al, 2015Gueydan et al, , 2019 hereafter labeled the "Kinzigite-Peridotite Shear Zone" (KPSZ, Figs. 3 to 5).…”

“…Like the kinzigites, the Filali Unit records a polyphase evolution, i.e., Barrovian metamorphism during the Hercynian orogeny (late anatectic phase at ~300 Ma; U-Pb zircon dating of the leptynites) and HT-LP during the Alpine orogeny (~22 Ma; U-Pb zircon/monazite and 40 Ar/ 39 Ar muscovite/biotite) (Rossetti et al, 2010; see also Gueydan et al, 2015). The Filali Unit corresponds to a thinned upper-crust section, currently regarded as separated from the underlying Beni Bousera Unit by an extensional shear zone parallel to the KPSZ (Saddiqi et al, 1988;Chalouan et al, 2008;Afiri et al, 2011;Álvarez-Valero et al, 2014;Gueydan et al, 2015Gueydan et al, , 2019. The occurrence of the marbles (BBMs) studied here between the Beni Bousera and Filali Units questions this classical interpretation, as soon as the age of their protoliths may be post-Paleozoic.…”

“…However, Rossetti et al (2010Rossetti et al ( , 2020 and Melchiorre et al (2017) demonstrated that the Beni Bousera peridotites were exhumed to lower-crust depth during the waning stage of the Hercynian orogeny. The final exhumation of the peridotite massifs is attributed to the Alpine orogeny, but is variably depicted in the recent literature (e.g., Mazzoli and Martín Algarra, 2011;Tubía et al, 2012;Gueydan et al, 2019;Rossetti et al, 2020).…”