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VRANCEA SEISMIC ZONE - A NEW GEOPHYSICAL MODEL BASED ON WRENCH TECTONICS, VOLCANISM AND REGIONAL GEODYNAMICS
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The study dedicated to the Vrancea seismic zone is based on the interpretation of relevant geophysical data for regional, crustal and lithospheric geological structures: gravity, magnetics, refraction seismics, heat flow, and especially seismic tomography. New tectonic interpretations are offered at various scales, starting with the tectonic vortex determined by the three subduction zones in the Mediterranean Sea, Adriatic Sea and the East Carpathians, continuing with the wrench tectonics system developed between the Adriatic Sea and the Dniester river, and ending with the presence of the Romanian Trough, continuation of the Polish Trough along the western margin of the East European Platform. Reinterpretation of regional geophysical data, in the context of the newly proposed wrench tectonics system, and the tectonic events determined by horizontal displacement of large geological structures along the transcurrent faults, suggested that there is no need of another platform, such as the Scythian Platform, between the East European Platform and the Tisza-Dacia tectonic block, or of the Wallachian tectonic phase, postulated as being active during the Quaternary. Volcanism in Vrancea seismic zone is probably the most unexpected topic in an area long time considered to be devoid of magmatic processes-subduction related andesitic eruptions, followed by the intrusion of a large dioritic batholith, being interpreted on aeromagnetic, refraction seismics, seismic tomography and heat flow data, as well as on geological indirect evidences. The Vrancea zone high seismicity is interpreted to be associated at crustal levels with active normal faulting within the graben-like structure, in an extensional regime determined by the south-eastward regional drag, and to the strike-slip movements of the wrench tectonics southern transcurrent fault at lithospheric depths, in a transtensional regime.
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Book Chapter in Bucharest - European capital city with the most vulnerable response to a strong earthquake, Ionescu, C, Radulian, M., Bălă, A. (Eds.), Editura Cetatea de Scaun, Târgoviște, 2022.
Cite as: Stanciu, I.M., Ioane, D., Seghedi, A., 2022. Chapter 1. Geographical setting, Geomorphological and Geotectonic framework of Bucharest City. In: Ionescu, C., Radulian, M., Bălă, A. (Eds.) Bucharest – European capital city with the most vulnerable response to a strong earthquake, pp. 13-31, ISBN 978-606-537-601-4, ISBN ebook: 978-606-537-602-1, Editura Cetatea de Scaun, DOI: https://doi.org/10.5281/zenodo.7325800
A high magnitude earthquake (Mw = 6.4) occurred on November 26, 2019, at 2:54 AM, close to Durres city, Albania. Ten hours later, in Vrancea seismic zone occurred three seismic events (Mw = 1.7; 3.2; 2.6), with epicenters migrating eastward. During 26 and 27 November, a higher number of earthquakes occurred in Durres seismic zone, when comparing to its average seismicity. Almost simultaneously, significant seismic events have been recorded in Greece (N Peloponnesus ml = 3.8; W Crete ml = 6.0) and Bosnia-Herzegovina (ml = 5.9), on a regional NW–SE lineament. A recent study interpreted a wrench tectonics system across Romania, its southern NE–SW trending transcurrent fault being associated with the intermediate-depth, subcrustal seismicity in the Vrancea seismic zone. When extending south–westward the two transcurrent faults from Romania to Albania, resulted that most earthquake epicenters that occurred during 26 and 27 November 2019 in the Durres seismic area are located between them. A north–westward tectonic lineament, interpreted using seismic events occurred between W Crete and S Bosnia-Herzegovina during the same time interval, crosses the two transcurrent faults within the Durres–Shkoder seismic area, suggesting that the regional NE–SW and NW–SE fault systems have been activated during November 2019.
Due to increased precision in positioning and depth determination, recent seismicity data (January 2014-January 2018) from ROMPLUS Earthquake Catalogue of Romania [1] have been used to illustrate active faults at crustal and lithospheric depths in Vrancea, a seismogenic sector located in the East Carpathians Bend Zone (Romania). Multiple N-S, WE , NW-SE trending seismicity sections across the Vrancea area have been built and analyzed, as well as seismicity maps at various depths, or for depth intervals. The seismic events magnitude ranged between 1.2 Mw and 5.6 Mw, and their depth ranged from 1 km to 160 km. The recent crustal seismicity (1-60 km) revealed two lineaments trending NNE-SSW, presently interpreted as the reactivated tectonic contacts by extension processes of the graben-like Permo-Triassic structure, illustrated till 45-50 km by refraction seismics [2]. The lithospheric seismicity (90-160 km) shows a quite compact NE-SW seismic swarm elongated on cca 50 km, located between the two crustal lineaments. Seismicity sections, built for depths between 1 km and 160 km across the Vrancea zone, illustrate two main extensional tectonics stages: a) a Permo-Triassic one, developed NNE-SSW at crustal depths, its reactivated contacts contributing to the regional crustal seismicity; b) a more recent one, possibly Quaternary in age, developed beneath and between the crustal tectonic structure. They form presently a single seismo-tectonic structure, the active faults which represent their external boundary starting from a common sector at the highest lithospheric depths and showing the maximum extension immediately beneath the Carpathians. The evident change in orientation between the crustal and lithospheric structures suggests significant modifications in time of the regional driving forces involved in tectonic processes and may be an important cause of the associated high seismicity. The regional tectonic and geodynamic background for interpreting the recent seismicity in Vrancea is offered by the Bouguer gravity map of Romania [3] , which depicts at the East Carpathians Bend Zone anomalous lineaments trending NE-SW, associated with a system of normal faults crossing Vrancea and its adjacent regions [4]. Results of GPS geodynamic observations [5] , showing a horizontal displacement of the East Carpathians Bend Zone foreland towards SE, favored the interpretation of regional extension processes, in an area where compressional ones have been postulated in the past by most geodynamic models.
The Moesian Platform is considered to consist of two main compartments, with different geological age and petrographic features at the crystalline basement level, separated by the NW-SE trending Intramoesian Fault. Seismological data compiled from published local, regional and global earthquakes catalogues was used to illustrate and analyze the distribution of seismicity within the Moesian Platfom. A number of profiles across the Intramoesian Fault with the earthquake hypocenters are presented, aiming at detecting the tectonic contact between the Moesian Platform compartments.
Across the entire mantle we interpret 94 positive seismic wave-speed anomalies as subducted lithosphere and associate these slabs with their geological record. We document this as the Atlas of the Underworld, also accessible online at www.atlas-of-the-underworld.org, a compilation comprising subduction systems active in the past ~300 Myr. Deeper slabs are correlated to older geological records, assuming no relative horizontal motions between adjacent slabs following break-off, using knowledge of global plate circuits, but without assuming a mantle reference frame. The longest actively subducting slabs identified reach the depth of ~2500 km and some slabs have impinged on Large Low Shear Velocity Provinces in the deepest mantle. Anomously fast sinking of some slabs occurs in regions affected by long-term plume rising. We conclude that slab remnants eventually sink from the upper mantle to the core-mantle boundary. The range in subduction-age versus-depth in the lower mantle is largely inherited from the upper mantle history of subduction. We find a significant depth variation in average sinking speed of slabs. At the top of the lower mantle average slab sinking speeds are between 10-40 mm/yr, followed by a deceleration to 10-15 mm/yr down to depths around 1600-1700 km. In this interval, in situ time-stationary sinking rates suggest deceleration from 20-30 mm/yr to 4-8 mm/yr, increasing to 12-15 mm/yr below 2000 km. This corroborates the existence of a slab deceleration zone but we do not observe long-term (>60 My) slab stagnation, excluding long-term stagnation due to compositional effects. Conversion of slab sinking profiles to viscosity profiles shows the general trend that mantle viscosity increases in the slab deceleration zone below which viscosity slowly decreases in the deep mantle. This is at variance with most published viscosity profiles that are derived from different observations, but agrees qualitatively with recent viscosity profiles suggested from material experiments.
Amodel with a new type of delamination of the lower lithospheric mantle is proposed to explain the Pliocene to recent tectonic evolution of the Eastern Carpathians. We suggest that, after the continental collision in middle Miocene time, break-off of the west-dipping subducting slab occurred at a depth of 70 km. Slab break-off propagated horizontally toward the east, inducing lithospheric delamination and movement of the Vrancea slab into its present position. Delamination was followed by rapid asthenospheric rise, resulting in magma generation and the explosive alkaline basaltic magmatism of the PersOani Mountains. Contamination of the former subduction-related magmatic reservoirs with the alkalic basaltic material and differentiation processes produced the Harghita calc-alkaline and shoshonitic rocks. The asthenospheric rise induced crustal uplift, which is the triggering mechanism for extension and formation of the BrasOov-Gheorghieni hinterland basins. The extension was accommodated by shortening and folding in the foreland. The vertical Vrancea slab appears in our model as a segment of delaminated lower lithospheric mantle that is seismically active due to the ongoing pull of the eclogitized oceanic crust. The model explains the displaced and shallow position of the slab relative to the Eastern Carpathian suture zone.