ALGERIAN Margin

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Tectonic inversion and geomorphic evolution of the Algerian margin since Messinian times

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Tectonic inversion refers to geological processes involving the reactivation of extensional basin structures in response to crustal shortening or the reactivation of reverse faults during crustal extension. Studied examples of passive margin inversion are numerous but most of the time, they correspond to advanced stages of inversion nowadays incorporated in the heart of mountain ranges. If several field examples are documenting the early stages of fossil margin inversion, the ongoing inversion of passive margins witnessing a process of subduction initiation is seldom found because it represents a short, transient phase before the onset of stable subduction. The central Algerian margin (Western Mediterranean Sea, Figure 1) represents one of the rare and best examples of present-day inversion of a passive margin in the early stages.

Figure 1: Three-dimensional shaded view toward the East of the Central Algerian margin (Strzerzynski et al., Tectonics, in press).

The geology of northern Algeria is marked by a succession of geodynamic events occurring within the framework of the Africa-Eurasia plate convergence, i.e. slab subduction and rollback, collision, and finally slab tearing. In Late Oligocene time, the decrease of the northward absolute motion of Africa is assumed to have triggered the rollback of the African (Tethyan) lithosphere, resulting in the southward migration of the internal (“ALKAPECA”) zones and the opening of a back-arc extensional domain. Slab breakoff of the Tethyan oceanic lithosphere occurred at ca. 17 Ma, shortly after the end of the Algerian back-arc basin spreading and the continental collision of the Kabylian blocks with Africa, inducing initial isostatic uplift, the exhumation of high-pressure metamorphic rocks, and thermal erosion of the continental mantle during Burdigalian times, followed by lateral slab tearing during Langhian-Serravalian times. The main mechanisms driving tectonic activity along the North Algerian margin since 30 Ma are (1) the pull linked to the Maghrebian slab retreat until the collision of ALKAPECA with Africa, (2) the delamination of the African sub-continental mantle in response to slab breakoff since then, and (3) the tectonic inversion resulting in a Tortonian exhumation of the upper margin and a widespread post-Messinian emersion of the Algerian Tell.

Figure 2: a) Synthetic cross-section of the Algerian margin at the longitude of Greater Kabylia (see Figure 3 for location) realized after b) line drawing of Maradja seismic line offshore (Strzerzynski et al., 2010), c) geologic cross-section onland (Raymond1976) and d) a velocity cross-section based on wide-angle seismic data (Aïdi et al., 2018), with the position of the 2003 Boumerdès-Zemmouri hypocenter and focal solution after Kherroubi et al. (2017). In Strzerzynski et al., 2021.

The study of continental margins faces several limiting factors related mainly to the difficulties to image their internal structures, to pass through the seawater barrier which strongly limits direct seafloor observations and rock sampling, and also to correlate geophysical and geological measurements acquired onland and at sea with different methods. To overcome part of these limitations, go deeper into the interpretation of geological and geophysical observations, and test the evolutionary scenario that emerged from these data, we implemented an experimental approach initially developed to study the interactions between Tectonics-Erosion-Sedimentation in intra-continental active tectonic settings (e.g. Graveleau and Dominguez, 2008; Graveleau et al., 2011, 2012, 2015). Previous experimental works have already investigated basin tectonic inversion but without integrating realistic surface processes (erosion, water transport, and sedimentation) as does our approach. This pioneering work represents, then, the first attempt to model experimentally the inversion of a passive continental margin by including onland and offshore domains together with realistic terrestrial and underwater geomorphogenetic processes. 

Figure 3: Top left: Experimental setup consisting of a deformation device, a rainfall system, and CCD cameras coupled to a laser interferometer. the MATIV analog material simulates the Meso-Cenozoic sedimentary units. The Silica powder simulates the more resistant Paleozoic basement and magmatic intrusions constituting the core of the Algerian margin. Center-left: Physical properties of the analog material. Bottom left: Shortening and sea-level evolution displayed on the timeline of the experiment. Top right: View of the model surface displaying a submerged domain to the left and an emerged domain to the right. Bottom right: Example of aerial and submarine model morphologies.

The experimental set-up used in this study is constituted of a mechanical apparatus to deform the analog model, a rainfall system to erode its surface, and a digital monitoring device used to quantify model evolution during the experiment (Figure 3). The deformation device is made of a 1.5 m x 2.2 m aluminum structure supporting a 2 cm thick PVC plate bounded by 3 glass/PVC walls and a rigid backstop. The PVC plate is bent toward the backstop up to a dip of 10°. The rainfall system uses up to 8 sprinklers to diffuse water micro-droplets over the model surface at a rate of 25-30 mm/h. Water runoff enhances erosion of the emerged part of the model (coastal domain) triggering several natural processes shaping the topography such as; channel incision in the drainage network, slope diffusion, and gravity-controlled landslides. In the submarine part of the model, sedimentation processes dominate but gravity-driven instabilities also contribute to shaping the submarine morphology. The digital monitoring device is constituted by a laser interferometer coupled to CCD cameras to measure deformation kinematics and model topography during the experiment. To model jointly onshore and offshore fluvio-deltaic sedimentary systems, we used the analog material developed by Graveleau and Dominguez (2008), Graveleau et al. (2011), and modified by Strak et al. (2011). This material was initially designed to study tectonic-erosion interactions in active mountain forelands and along normal fault scarps. The analog material composition was only slightly modified mainly to adapt its cohesion to model scaling requirements (Figure 3). During the experiment, water run-off induces water over-saturation on the first millimeters of the model which drastically decreases the cohesion (Co < a few tens of Pa) favoring, then, model surface erosion. 

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Figure 4: Example of morphologic markers associated with the Low Stand Sea-level stage (LSSL), corresponding to Stage 2 of the MSC (5.6-5.5 Ma) on the upper part of the margin (left) and at its base (right). From Strzerzynski et al., (2021).

Our results (see Strzerzynski et al., 2021) highlight the key role played by the MSC sea-level oscillation on the ultra-fast building, destruction, and re-sedimentation of fans and deltas from the upper slope to the abyssal plain; (2) the development of a large popup structure sub-parallel to the coastline, with progressive strain migration from the backthrust onland toward a frontal thrust of opposite vergence at mid-slope and the margin toe, and (3) the importance of lateral changes in initial wedge shape and strain distribution for determining the non-cylindrical geometry of the margin and progradation of piggy-back basins during tectonic inversion. Our results support that the central Algerian margin is witnessing the early building of an accretionary wedge combining thin-skinned and thick-skinned styles.

Figure 5: Comparison of the tectonic and morphological features between the final stage of the model (bottom) and the central Algerian margin (top). The similar structures are labeled from offshore to onshore (north to south):(1) landward dipping thrust faults bounding perched (piggy-back) basins formed during the margin inversion stage 2; (2, 3, 4, 5, 6) landward dipping thrust faults and blind thrust faults (5) bounding perched basins on the continental slope (stage 1); (7) emerged part of the uplifted margin; (8) backthrust; (9) uplifted alluvial surface. Pink surfaces are the main zones of sedimentation in perched basins on the slope and deep basin. From Strzerzynski et al., (2021).

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Learn more:

-> Strzerzynski, P., Dominguez, S., Boudiaf, A., Déverchère, J., 2021. Tectonic inversion and geomorphic evolution of the Algerian margin since Messinian times: Insights from new onshore/offshore analog modeling experiments, Tectonics, 40, e2020TC006369. https://doi.org/10.1029/2020TC006369 -> PDF

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See also:

-> Graveleau F., Malavieille, J., Dominguez, S., 2012. Experimental modeling of orogenic wedges: A review, Tectonophysics, 538-540: 1-66, https://doi.org/10.1016/j.tecto.2012.01.027