Opposite Symmetry in the Lithospheric

RESEARCH ARTICLE Opposite Symmetry in the Lithospheric Structure of the 10.1029/2020JB021388 Alboran and Algerian Basins and Their Margins (Western Key Points: Mediterranean): Geodynamic Implications • Oppositely symmetric lithosphere structure is observed in Alboran & Ajay Kumar1,2,3 , Manel Fernàndez1 , Jaume Vergés1 , Montserrat Torne1 , and Algerian basins, and their margins Ivone Jiménez-Munt1 with subduction related HP/LT rocks • Subducted Ligurian-Tethys slabs 1Group of Dynamics of the Lithosphere, Geosciences Barcelona, GEO3BCN-CSIC, Barcelona, Spain, 2Department of beneath the Betics & Kabylies are 3 400 °C colder and at least 30 kg/ Earth and Ocean Dynamics, University of Barcelona, Barcelona, Spain, Now at GFZ German Research Centre for m∼ 3 denser than upper mantle∼ Geosciences, Potsdam, Germany • Eastern Betics & Greater Kabylies show mantle delamination triggered by slab retreat in Mid-Late Miocene, Abstract The geodynamic evolution of the Western Mediterranean for the past 35 My is a matter of and uplift from slab tear/break-off debate. Present-day structure and composition of the lithosphere and sublithospheric mantle may help in constraining the geodynamic evolution of the region. We use an integrated geophysical-petrological Supporting Information: modeling to derive and compare the present-day thermal, density and compositional structure of the Supporting Information may be found lithosphere and sublithospheric mantle along two NNW-SSE oriented transects crossing the back-arc in the online version of this article. Alboran and Algerian basins, from onshore Iberia to the northern Africa margin. The crust is constrained using available seismic data and geological cross-sections, whereas seismic tomography and mantle Correspondence to: xenoliths constrain the upper mantle structure and composition. Results show a thick crust (37 and A. Kumar and I. Jiménez-Munt, [email protected]; 30 km) and a relative deep LAB (130 and 150 km) underneath the HP/LT metamorphic units of the [email protected] Internal Betics and Greater Kabylies, respectively, which contrast with the 16 km thick magmatic crust of the Alboran Basin and the 10 km thick oceanic crust of the Algerian Basin. The sharp change Citation: in lithosphere thickness, from the orogenic wedge to the back-arc basins, contrasts with the gentler Kumar, A., Fernàndez, M., Vergés, lithosphere thickening toward the respective opposed margins. Our results confirm the presence J., Torne, M., & Jiménez-Munt, I. of detached slabs 400 °C colder than upper mantle and a fertile composition than the continental (2021). Opposite symmetry in the lithospheric structure of the Alboran lithospheric mantle∼ beneath the External Betics and Saharan Atlas. Presence of detached quasi-vertical and Algerian basins and their sublithospheric slabs dipping toward the SSE in the Betics and toward the NNW in the Kabylies and the margins (Western Mediterranean): opposed symmetric lithospheric structure support an opposite dipping subduction and retreat of two Geodynamic implications. Journal of Geophysical Research: Solid Earth, adjacent segments of the Jurassic Ligurian-Tethys realm. 126, e2020JB021388. https://doi. org/10.1029/2020JB021388 Plain Language Summary Understanding the time evolution of geological processes is fundamental to understand the surface evolution of our planet. Subduction (tectonic plate sinking into the Received 19 NOV 2020 Accepted 5 JUN 2021 interior of Earth) is one such process known to produce the Alboran and Algerian basins in the Western Mediterranean in-between Africa and Spain. However, the time and space evolution of this process is not well understood in this area. Here, we integrate a wide range of observations (e.g., elevation, gravity, seismic tomography) to produce 400 km deep images of density and temperature along these basins and onshore margins of Africa and Spain. These images show two cold and dense part of the subducted plates sitting beneath north Algeria and southern Spain. These images suggest that the Alboran and Algerian Basins are produced by two separate subducting plates which were moving opposite to each other during last 35 million years. At present these plates are not attached to the outer solid layer of Earth (lithosphere) and are floating in the interior of Earth. When these plates broke-off from the lithosphere, Earth's surface uplifted by a few hundred meters. In southern Spain, this uplift could have disconnected the Mediterranean Sea from the Atlantic Ocean making it much saltier. 1. Introduction © 2021. The Authors. This is an open access article under The Western Mediterranean is characterized by the presence of several extensional basins floored by very the terms of the Creative Commons thin continental crust or newly formed oceanic crust that opened in a back-arc setting during the closure Attribution License, which permits use, of the Ligurian-Tethys Ocean (Figure 1a). The North Balearic Transform Zone is recognized as a major distribution and reproduction in any medium, provided the original work is transform fault separating the Liguro-Provençal Basin and the Tyrrhenian realm from the Alboran-Algerian properly cited realm (Figure 1a). While the Cenozoic evolution of the Liguro-Provençal, Tyrrhenian, and Aegean back-arc KUMAR ET AL. 1 of 33 Journal of Geophysical Research: Solid Earth 10.1029/2020JB021388 Figure 1. (a) Simplified geological map of the Western Mediterranean displaying the Alpine orogenic belts and foreland basins (Modified from Vergés and Sàbat, 1999). The Alpine-Himalayan collision zone is indicated as gray shaded area in the inset map. (b) Schematic illustration of the three-subduction models proposed for the geodynamic evolution of the Western Mediterranean also showing the paleogeography at 35 My (van Hinsbergen et al., 2014). Scenario 1 after van Hinsbergen et al. (2014) proposes a single short subduction zone initially dipping to NW starting near the Balearic Promontory and then retreats to the SE before it separates into two different segments. One of the segments continue retreating to SE before it collides with North Africa and the other continue retreating to the west and collides with Iberia resulting in 180° clock-wise rotation. Scenario 2 purposes a long initial subduction dipping to the N-NW along the entire Gibraltar-Balearic Promontory margin (e.g., Faccenna et al., 2004; Gueguen et al., 1998; Jolivet et al., 2009). Scenario 3 after Vergés & Fernàndez (2012) purposes two separate subduction segments with opposite subduction and retreating direction for the Alboran (dipping to the SE and retreating to the NW) and Algerian (dipping to the NW and retreating to the SE) basins. The yellow arrows show direction of rollback. Black dashed lines represent the proposed transform faults separating the different subduction segments (Figure modified after Chertova et al., 2014). NBTZ, North Balearic Transform Zone; EBF, Emile- Baudot Escarpment. KUMAR ET AL. 2 of 33 Journal of Geophysical Research: Solid Earth 10.1029/2020JB021388 basins are well understood (Faccenna et al., 2004), the evolution of the Alboran and Algerian basins in the Western Mediterranean is still debated (Casciello et al., 2015; Faccenna et al., 2004; Jolivet et al., 2009; Spa- kman & Wortel, 2004; van Hinsbergen et al., 2014; Vergés & Fernàndez, 2012). The opening of both basins for the last 35 My, is being explained by three different geodynamic scenarios, each considering slab roll- back as the driving mechanism (Figure 1b). There is consensus that the Algerian Basin is an oceanic basin that opened in the upper plate during the NNW-dipping Ligurian-Tethys slab retreat, in agreement with the SSE-polarity of the Kabylies-Tell-Atlas orogenic system; however, no agreement has been reached so far on the origin and evolution of the Alboran Basin and related Betic-Rif orogenic system. Main disagreements are on the original disposition of the tectono-sedimentary domains involved in the Betic-Rif subduction-related orogenic system and on the geodynamic interpretation used to build-up this orogenic system. The pros and cons of the three scenarios displayed in Figure 1b are discussed in detail by Chertova et al. (2014) using numerical modeling. Based on the present-day slab structure beneath the Betics and the high curvature of the Gibraltar slab, Chertova and co-authors concluded that both scenario 1 and scenario 3 are plausible but favoring scenario 1, whereas scenario two is incompatible with the numerical modeling. Recently, Romag- ny et al. (2020) based on GPlates reconstructions proposed a model involving two subduction zones with opposite directions similar to the scenario 3 but with an ENE dipping subduction of the Alboran segment at 35 Ma retreating to WSW and later twisting clockwise to form the highly curved Gibraltar slab. Irrespective of the geodynamic evolution model applicable to the Alboran and Algerian basins, subduction processes must have left their imprints on the present-day crust and mantle structure. A subduction system involves compression and commonly orogenic volcanism near the arc depending on the fertility degree of the lithospheric mantle. In settings dominated by slab roll-back, the upper plate undergoes ex- tension and thinning, producing anorogenic volcanism by mantle decompression which ultimately can lead to the formation of new oceanic lithosphere as in the Algerian Basin. The slab roll-back and further collision with the continental lithosphere leads to the stacking and exhumation of metamorphic

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