The Oligocene-to-present tectonic history of the western Mediterranean region is characterized by the ESE-ward roll-back of Alpine and Neo Tethys oceanic slab fragments that determined the diachronous spreading of two back-arc basins: the Liguro-Provencal Basin between 30 and 15 Ma and the Tyrrhenian Sea between 10 and 2 Ma. Such geodynamic events induced the fragmentation and dispersal of the Alpine chain through the formation and migration of microplates and terranes, making the debate on the nature, origin, and evolution of such crustal blocks vivid since the 1970s. For instance, it is commonly accepted that the Corsica-Sardinia microplate rotated counterclockwise (CCW) by at least 50° during Oligo-Miocene and that the Calabro-Peloritan, Kabylies and Alboran blocks drifted hundreds of kms on top of nappe piles ESE-ward, SE-ward and SW-ward, respectively. These blocks, know all together as AlKaPeCa, presently form isolated and enigmatic igneous/metamorphic terranes stacked over the Meso-Cenozoic sedimentary successions of the Apennines and Maghrebides. Besides back-arc basins widths and ages, no other kinds of geologic/geophysical data from Corsica-Sardinia microplate or AlKaPeCa terranes constraining their drift magnitudes exist. On the other hand, drift timing may be properly documented by paleomagnetic vertical-axis rotations obtained from different age rocks, and such data usefully complement ages derived from back-arc basins.
Here we show the synthesis of paleomagnetic investigations carried out during the last few years on the Calabro-Peloritan terrane, and Sardinia, where a different pre-21 Ma rotation history is proposed. We paleomagnetically sampled the Meso-Cenozoic sedimentary cover of the Calabrian (Longobucco succession) and Peloritan (Longi-Taormina succession) terranes and the mid-late Eocene continental Cixerri Formation of SW Sardinia. In addition, we re-evaluated previous paleomagnetic results from the whole Corsica-Sardinia microplate and considered the robust Serravallian-Pleistocene dataset from the Calabrian block. Such data indicate a novel rotation and drift history in the western Mediterranean region (Siravo et al., 2022; 2023). The South Sardinia, Peloritan and Calabrian blocks belonged to the “Greater Iberia plate” before mid-Oligocene (<30 Ma) dispersal, as they all show its characteristic paleomagnetic fingerprint (middle Cretaceous 30°-40° CCW rotation). Rifting of the Liguro-Provencal between 30 and 21 Ma induced 30° CCW rotation of both South Sardinia and Calabria blocks, whereas the Peloritan block, located further south, was passively drifted SE ward at the non-rotation apex of a Paleo Appennine-Maghrebides orogenic salient. South Sardinia plus the adjacent Calabrian block and North Sardinia-Corsica blocks assembled in the early Miocene and rotated 60° CCW as a whole between 21 and 15 Ma. After 10 Ma the Calabrian block detached from south Sardinia following the opening of the Tyrrhenian Sea and rotated 20° clockwise (CW), at the apex of a Neo Appennine-Maghrebides Arc. On the other hand, the Peloritan terrane rotated 130° CW on top of the Sicilian Maghrebides, along the southern limb of the orogenic salient.
REFERENCES
Siravo, G., Speranza, F., & Macrì, P. (2022). https://doi.org/10.1029/2021TC007156
Siravo, G., Speranza, F., & Mattei, M. (2023). https://doi.org/10.1029/2022TC007705
How to cite: Siravo, G. and Speranza, F.: Paleomagnetic rotations and microplate-terrane dispersal during back-arc basin opening: from Greater Iberia rotation and fragmentation to Calabria and Peloritan terrane drift, 16th Emile Argand Conference on Alpine Geological Studies, Siena, Italy, 16–18 Sep 2024, alpshop2024-2, https://doi.org/10.5194/egusphere-alpshop2024-2, 2024.
We present analogue models simulating subductions that occurred in the Western Mediterranean region, in order to understand how it impacted the regional tectonics. Models suggest that the tectonic evolution is largely controlled by slab roll-backs, that may be much faster than the Africa-Eurasia convergence. They reproduce the opening of the Western Mediterranean Basins and the dispersion of continental fragments that accompany slab roll-back. They show that oceanic subduction in the Western Mediterranean region favors the counterclockwise rotation of Adria. Some models reproduce the break-off of the oceanic slab that followed the beginning of continental subduction both beneath Northern Africa and Italia. The influence of subduction on the kinematics of Adria largely decreases following slab break-off. In models, the total counterclockwise rotation of Adria varies between 7° and more than 30°, depending on the timing of slab break-off. Since the process of subduction modifies the displacement of Adria, it also impacts the tectonic evolution of the regions that bound this plate, especially in the Alpine belt: in the Western Alps, an older Late Cretaceous to Eocene “Pyrenean-Provençal” tectonic phase accommodating N-S shortening is classically described resulting from the convergence between Africa and Eurasia. It is followed by the Neogene “Alpine phase” accommodating E-W shortening. Since this major tectonic change is not explained by a modification of the global Africa-Eurasia convergence, it should be explained instead by more local causes. Our models show that during slab-roll back and before slab break-off, the azimuth of convergence between Adria and Europe shifts from ~N-S to ~NE-SW, and that the oceanic subduction in the Western Mediterranean may explain both the post-Oligocene E-W shortening in the Western Alps and part of the Periadriatic right-lateral shear zones in the Central Alps. We compare the rotations observed in experiments with the post-Eocene rotations registered by paleomagnetic data in the Western Alpine realm, that are deduced from a synthesis of more than 55 paleomagnetic studies. We compare the counterclockwise rotations affecting internal units of the Alps evidenced by paleomagnetic data with the rotations observed in analogue experiments. The change from N-S to E-W shortening enhanced left-lateral motions in the Southern border of the Western Alps, which may explain the particularly large rotations registered in this sector. We conclude that the western Mediterranean region is a spectacular example showing how the tectonics of mountain ranges and plate boundaries may be controlled by distant subduction processes.
How to cite: Martinod, J., Maldonado, A., Crouzet, C., and Sue, C.: West Mediterranean subductions, puppeteers of the Alps: lessons from analog models and paleomagnetic data, 16th Emile Argand Conference on Alpine Geological Studies, Siena, Italy, 16–18 Sep 2024, alpshop2024-51, https://doi.org/10.5194/egusphere-alpshop2024-51, 2024.
The Apennines form part of the Africa-Eurasia convergent plate boundary and are cored by two diachronous back-arc basins formed in response to slab retreat, separated by the Sardinia-Corsica continental ribbon. Subduction of oceanic lithosphere in the central Apennines lasted until the Oligocene - early Miocene, coevally with the opening of the Liguro-Provencal back-arc basin and led to the development of a thin-skinned thrust pile. The subsequent transition from subduction to collision occurred when the distal portion of the Adria passive margin became involved in the orogenic system. The arrival of a progressively thicker continental crust of the Adria rifted margin imposed a deceleration of trench retreat, arresting the opening of the first back-arc basin. This stage is evidenced by the almost stable position of the thrust front and of the peripheral bulge until the late Miocene. During Tortonian, the forelandward migration of the thrust front and of the peripheral bulge re-accelerated, coevally with the opening of the Tyrrhenian back-arc basin. In our view, this occurred as a consequence of the relocalization of the subduction interface in the basement.
In this contribution, via cross-section balancing, we focus on the crustal structure of the central Apennines to investigate the relationship between shortening in the basement and in the sedimentary cover. We employ well-constrained surface geological data from available public maps, as well as tomography and seismological data, supplemented by thermochronological, biostratigraphic, and radiometric dating. Furthermore, the data are integrated into a coherent geodynamic framework supported by a geometrically balanced kinematic model, giving insight on the coupled forward migration of compressional and extensional domains due to the slab pull/trench retreat system.
How to cite: Maresca, A., Granado, P., Manatschal, G., Carminati, E., Cavinato, G., Muñoz, J. A., and Tavani, S.: A crustal balanced-cross section across the central Apennines and the relationship between thick- and thin-skinned tectonics, 16th Emile Argand Conference on Alpine Geological Studies, Siena, Italy, 16–18 Sep 2024, alpshop2024-41, https://doi.org/10.5194/egusphere-alpshop2024-41, 2024.
The Alboran Sea is a Neogene basin formed by the extension of a continental crustal domain (Alboran Domain) during its westwards displacement. This process involved the collision of its margins with the African and Iberian palaeomargins, which led to the formation of the Betic and Rif cordilleras. The Alboran Sea is characterized by the presence of several highs that correspond to volcanic edifices and/or submarine ranges. The distribution of the magnetic anomalies on this basin shows some dipoles centered in these volcanic edifices. The most intense dipoles are aligned in two groups in the center of the basin: a NE-SW elongated group starts in the Ibn-Batouta bank, runs eastwards centered in the Alboran Channel along the northern boundary of the Alboran Ridge and continues eastwards, changing to a NW-SE group of aligned dipoles, along the Yusuf fault. Since these anomalies are not related to surface volcanic highs, several profiles have been modeled across these dipoles. The models show that the sources are probably crustal scale, basic igneous intrusions located at depths from 8 to 14 km. It is remarkable that these intrusions are northwards displaced with respect to the Alboran Ridge, which seems to be the main volcanic high of the Alboran Sea. The orientation of these groups of magnetic dipoles and the absence of a clear relation with the Neogene calc-alkaline volcanism of the main highs support the idea that these intrusions may be related to the rifting of meso-Mediterranean terrains that formed the Alboran Domain (like the AlKaPeCa Domain) during Oligocene-Early Miocene. This rifting process led to the spreading of the Algerian basin and the individualization of the Alboran Domain from other domains. Furthermore, that orientation shows a similar trend to the rift axis proposed for that period, so the intrusions could represent the western tip of that rift. Later, these intrusions could be affected by the STEP fault (Subduction Tear Edge Propagator fault) that accommodated the westward displacement of the Alboran Domain along its southern limit and that is related with the formation of the Yusuf fault. Since Tortonian, the tectonic inversion of the Alboran basin was characterized by the prevalence of the NNW-SSE compression over the ESE-WSW extension, which continues today. During Pliocene and Quaternary, this stress led to the formation of a tectonic indentation, whose front, the Alboran Ridge, is located next to the main intrusions. Thus, it is likely that the intrusions act as a backstop that have favored the folding and uplift of the Alboran Ridge in the front of the indenter. This constitutes an excellent example of how intrusions originated during the extensional, initial stages of a basin can condition and control the style of the later tectonic inversion of this basin in the context of the peri-Mediterranean chains.
How to cite: Tendero Salmerón, V., Galindo-Zaldívar, J., d'Acremont, E., Catalán, M., Martos, Y. M., Ammar, A., and Ercilla-Zarraga, G.: Alboran Sea igneous intrusions revealed by magnetic anomalies and related to extensional opening constrain the ongoing continental collision, 16th Emile Argand Conference on Alpine Geological Studies, Siena, Italy, 16–18 Sep 2024, alpshop2024-43, https://doi.org/10.5194/egusphere-alpshop2024-43, 2024.
As stated by Argand’s mentor, Maurice Lugeon, the title of Argand’s publication is misleading, because it is a book about the general processes of the solid Earth, and by no means restricted to Asia. In fact large part of this book is about the Alps and the Mediterranean region, and its conclusions about the latter area precede by 50 years what scientists of the early « Plate Tectonics generation » will eventually assess and consider applying new methods of study, not yet available in the 1920’.
Argand’s last significant publication before « La tectonique de l’Asie » is that of 1916, in which he presents a cross section of the northwestern Alps, covering a vertical depth of more than 20 km and revealing the complete nappe structure of the orogen. Whereas the most important Alpine geologists of that time promptly react by constructing Argand-type cross sections in all parts of the orogen, Argand himself does not apply his concepts and approach to any other area of the Alps. He steps back from refining the architecture of specific Alpine regions and eight years later he presents in « La tectonique de l’Asie » with eight orogen-scale sections including the Alps, the Apennines, the Betic Cordillera, Carpathians, Anatolia, and the Himalayas, all simplified to the very essential elements that allow Argand and his readers to understand the 1st-order orogenic processes that mould mountain chains.
Just like Wegener, and at the same time, Argand cogitates about continental displacements. Each of these scientists builds his arguments upon the ones of the other. Wegener bases his interpretations on the synthesis of most diverse data sets, Argand upon the deep and detailed understanding of how horizontal movements are accommodatedand and recorded in the structure of mountain chains.
How to cite: Rosenberg, C. and Molli, G.: Centennial of « Tectonics of Asia », by Emile Argand : a milestone for Alpine and Mediterranean tectonics, 16th Emile Argand Conference on Alpine Geological Studies, Siena, Italy, 16–18 Sep 2024, alpshop2024-25, https://doi.org/10.5194/egusphere-alpshop2024-25, 2024.
