Geodynamics, seismicity, and seismic hazards of the Caucasus

Abstract

Being a part of ongoing continental collision between the Arabian and Eurasian plates, the Caucasus region is a remarkable site of moderate to strong seismicity, where devastating earthquakes caused significant losses of lives and livelihood. In this article, we survey geology and geodynamics of the Caucasus and its surroundings; magmatism and heat flow; active tectonics and tectonic stresses caused by the collision and shortening; gravity and density models; and overview recent geodetic studies related to regional movements. The tectonic development of the Caucasus region in the Mesozoic-Cenozoic times as well as the underlying dynamics controlling its development are complicated processes. It is clear that the collision is responsible for a topographic uplift / inversion and for the formation of the fold-and-thrust belts of the Greater and Lesser Caucasus. Tectonic deformations in the region is influenced by the wedge-shaped rigid Arabian block indenting into the relatively mobile region and producing near N-S compressional stress and seismicity in the Caucasus. Regional seismicity is analysed with an attention to sub-crustal seismicity under the northern foothills of the Greater Caucasus, which origin is unclearwhether the seismicity associated with a descending oceanic crust or thinned continental crust. Recent seismic tomography studies are in favour of the detachment of a lithospheric root beneath the Lesser and Greater Caucasus. The knowledge of geodynamics, seismicity, and stress regime in the Caucasus region assists in an assessment of seismic hazard and risk. We look finally at existing gaps in the current knowledge and identify the problems, which may improve our understanding of the regional evolution, active tectonics, geodynamics, shallow and deeper seismicity, and surface manifestations of the lithosphere dynamics. Among the gaps are those related to uncertainties in regional geodynamic and tectonic evolution (e.g., continental collision and associated shortening and exhumation, lithosphere structure, deformation and strain-stress partitioning) and to the lack of comprehensive datasets (e.g., regional seismic catalogues, seismic, gravity and geodetic surveys).

Plume‐Induced Sinking of Intracontinental Lithospheric Mantle: An Overlooked Mechanism of Subduction Initiation?

“…Since the studies by Burov & Cloetingh (2009, 2010), no new attempts have been made to investigate plume‐lithosphere interaction as the driving mechanism for subduction‐like downward movements of the continental lithosphere. This is still the case despite a growing body of robust geophysical data, including observations from seismic tomography and magneto‐telluric sounding in areas such as the Caucasus (Ismail‐Zadeh et al., 2020; Koulakov et al., 2012; Zabelina et al., 2016); Central Asia (He & Santosh, 2018); North‐East China (Kuritani et al., 2019; Li et al., 2020; Wang et al., 2018; Zhang, 2012); Iberia and its margins (Civiero et al., 2019); the Carpathians (e.g., Wortel & Spakman, 2000; Koulakov et al., 2010; Ismail‐Zadeh et al., 2012; Ádám et al., 2017; Petrescu et al., 2019), and the Colorado Plateau (Levander et al., 2011) where upwelling of hot mantle material flanked by downgoing slabs of sinking mantle lithosphere has been recently documented. This makes plume‐induced intra‐continental mantle sinking/foundering a viable and testable mechanism, deserving detailed investigation by means of both modeling and critical analysis of pertinent observations.…”

“…Arrows indicate possible paths of feeding volcanic centers in the Lesser Caucasus and Greater Caucasus belts. Red colors mark areas of upper mantle upwelling rooted at the top of mantle transition zone (Ismail‐Zadeh et al., 2020). (b) Tomographic slice parallel to the 83 longitude from the Tibetan Plateau, through the Tianshan orogenic belt to the Junggar Basin (He & Santosh, 2018).…”

“…Delayed rupture of the lithosphere, combined with other possible factors as for example changes in far‐field forcing (i.e., tectonic inversion) and/or (re)activation of shear zones in the crust and the uppermost lithospheric mantle, might be important for the transition from initial plume‐induced deep‐mantle downthrusting/foundering into a fully established process of plate‐scale subduction. Present‐day observations (see Section 3) seem to detect an intermediate stage of this process when hot plume material already resides at relatively shallow depths—∼100 km in the Caucasus area (Ismail‐Zadeh et al., 2020; Koulakov et al., 2012), ∼80–100 km in North‐East China (S. Li et al., 2020), and ∼60–80 km in the Colorado Plateau (Levander et al., 2011) – but has not yet reached the crust to complete the rupture of the continental plate (Figure 1). This might indicate that a much longer time period is needed for mantle plumes to penetrate into shallowest lithospheric levels than usually expected in models for oceanic (<0.1 Myr; Figure 2a) and young continental (<5 Myr; Figure 2b1–3) lithosphere, for which plume‐induced break‐up is mandatory before or during subduction/downthrusting.…”

Structure and Density of Sedimentary Basins in the Southern Part of the East-European Platform and Surrounding Area

“…In the corrected model, it is shifted to the north close to the Apsheron Trough with the thickness up to 25 km. Several strong earthquakes are associated with the deepest part of the basin, e.g., [60], which confirms that its formation is directly related to the collision of the Arabian and Eurasian plates, e.g., [48]. In the southern part of the South Caspian basin, the sedimentary thickness is significantly reduced, likely following the inclined plate subducting to the north.…”

“…However, both models for this structure (the initial and the new ones) show similar thickness with its slight increase to the southeast while approaching the Caspian. The Greater Caucasus is clearly divided into two parts, which are characterized by remarkably different structure of the lithosphere, e.g., [60]. For the West Greater Caucasus we did not obtain any significant correction, which demonstrates that the initial model without sediments works well.…”