The Geological Structure and Tectonic Complexity of Northern Thessaly That Hosted the March 2021 Seismic Crisis

Knowing the rich presence of active faults in northern Thessaly and the lack of any significant seismic activity since at least the mid-1940s, the 2021 seismic sequence did not surprise us. What did surprise us was the fact that (i) despite the great knowledge of the neotectonic faults in the area, the causative faults were unknown, or almost unknown; (ii) the direction of the 2021 faulting was different than the expected, and given that the focal mechanisms showed almost pure normal dip-slip motion, the extensional main axis was also different than the one we thought we knew for this area; and (iii) besides the co-seismic ruptures that occurred within the Domeniko-Amouri basin and along the Titarissios River valley, there is evidence of rupturing in the alpine basement of Zarkos mountains. After thoroughly reviewing both the alpine and neotectonic structural setting and all the available literature concerning the seismotectonic data and interpretations of the 2021 sequence, including investigations of our own, we end up in a complex tectonic setting with older alpine structures now operating as inherited faults, and we also suggest the possible occurrence of a roughly N-dipping, low-angle, detachment-type fault. This fault runs below Mt Zarkos, reaching at least the Elassona Basin, with splay faults bifurcating upwards from the main fault zone. Following this complexity, rupture of the first mainshock must have chosen a split route reaching the surface through the gneiss rocks of Zarkos and almost (?) reaching the basinal sediments of the local tectonic depressions. This seismic sequence is a perfect case study to shed some light on the tectonic and rupture processes in the context of both geodynamics and seismic hazard assessment

Alpine architecture and kinematics of deformation of the northern Pelagonian nappe pile in the Hellenides

Geological mapping and detailed structural investigations combined with geochronological and stratigraphic data, as well as fissiontrack age dating carried out on the northern part of the Pelagonian basement and the adjacent Vardar/Axios sedimentary and metamorphic sequences in the Hellenic Alps (northwestern Greece and Former Yugoslavian Republic of Macedonia) allow us to reconstruct the geometry, kinematics and deformation history of the Pelagonian nappe pile during the Alpine orogeny. We distinguish seven deformational events (D and D to D ). Deformation started in Middle to Late Jurassic time and was associated with inneroceanic HP 1 6 thrusting, ophiolite obduction, and NW- to WNW-directed nappe stacking of the Lower and Upper Pelagonian unit (D ). The lower unit 1 was metamorphosed under greenschist to amphibolite facies conditions with relatively high pressures (T=450-620°C, P=8-12,5 kb). Blueschist-facies metamorphic assemblages (D , T=450-500°C, P>12,5 kb) are restricted to the boundary zone between both Pela- HP gonian units. Transgressive Late Jurassic to Early Cretaceous shallow-water limestones and clastic sediments on top of the obducted ophiolites are probably related to extension and basin formation simultaneously with nappe stacking and metamorphism in the Pelagonian nappes beneath. Contractional tectonics with the same kinematics as during D continued in Aptian-Albian time and was asso- 1 ciated with intense retrogression (D , T=280-380°C, P=4-5 kb). Low-angle mylonitic extensional shear zones of low-grade metamor- 2 phism with top-to-NE sense of movement (D ) developed simultaneously with basin formation and sedimentation of shallow-water 3 limestones and flysch-like sediments in Late Cretaceous to Paleocene times. Intense imbrication under semi-ductile to brittle conditions of all tectonic units occurred during Paleocene to Eocene time with SW-directed movement towards the foreland (D ). A large 4 Pelagonian antiformal structure formed during D shortening. In Oligocene to recent time, D and D created brittle low- and high- 4 5 6 angle normal faults, respectively.Im nördlichen Teil des Pelagonischen Kristallins und in den metamorphen und sedimentären Gesteinsfolgen der angrenzenden Vardar/Axios-Zone der Helleniden (NW-Griechenland und Frühere Jugoslawische Republik Mazedonien) wurde eine geologische Kartierung und ausführliche strukturgeologische Untersuchungen durchgeführt. Verbunden mit geochronologischen und stratigraphischen Daten erlauben unsere Ergebnisse, die Geometrie, Kinematik und Deformationsgeschichte des pelagonischen Deckenlandes während der alpidischen Gebirgsbildung zu rekonstruieren. Wir haben sieben Deformationsereignisse (D und D bis D ) HP 1 6 unterschieden. Die Deformationsgeschichte begann im Mittleren bis Späten Jura mit intra-ozeanischer Überschiebung, Ophiolith- Obduktion und NW- bis WNW-gerichteter Deckenstapelung der Unteren und Oberen Pelagonischen Einheit (D ). Die untere Einheit 1 wurde unter den Bedingungen der Grünschiefer- bis Amphibolitfazies bei relativ hohem Druck metamorph überprägt (T 450-620°C, P 8-12,5 kb). Blauschieferfazielle Paragenesen sind in der tektonischen Kontaktzone zwischen den beiden pelagonischen Einheiten erhalten (D : T 450-500°C, P >12,5 kb). Transgressiv auf obduzierten Ophiolithen abgelagerte oberjurassische bis unterkretazische HP Flachwasserkalke und klastische Sedimente sind wahrscheinlich mit Extension und Beckenbildung im obersten Krustenstockwerk und gleichzeitiger Deckenstapelung und Metamorphose in den unteren pelagonischen Decken verbunden. Im Aptium-Albium setzte sich die Einengungstektonik mit derselben Kinematik wie während D fort und ging mit intensiver retrograder Metamorphose einher 1 (D : T 280-380°C, P 4-5 kb). Flach einfallende mylonitische Abscherungszonen, die unter schwachen grünschieferfaziellen Metamor- 2 phosebedingungen NE-gerichteten Schersinn anzeigen (D ), entwickelten sich während der Oberen Kreide und des Paleozäns, 3 gleichzeitig mit Beckenbildung und Sedimentation von Flachwasserkalken und Flysch. Intensive Verkürzung und Imbrikation aller tektonischen Einheiten (D ) fand unter semiduktilen bis spröden Bedingungen und nach SW (gegen das Vorland) gerichteter Haupt- 4 bewegung während des Paleozäns bis Eozäns statt. Eine große Antiform bildete sich im Pelagonikum während dieser Einengungsphase. Vom Oligozän bis heute bildeten sich spröde, flach oder steil geneigte Abschiebungen (D und D ) 5 6 _______________________________________________________________________________