4.1. Hot-Cold spots determination and description using tomography velocity maps
As mentioned earlier, according to wave studies from within the earth, the earthquakes waves are originating in Earth’s crust or upper mantle, which traveling most rapidly through cold, dense regions, and more slowly through hotter rocks. Thus, in this study, we assumed that each low-velocity (slow) region with a dark red shade is a hot spot and each high-velocity (fast) region with dark blue-green-yellow shade is a cold spot. So, in order to identify and describe Hot-Cold spots inside the earth based on increasing and decreasing wave velocity anomalies, the obtained 2D tomographic velocity maps in Fig. S4 and Fig. 3 shows this property. Schematic diagram of Fig. S7 in Supplementary Materials, has depicted to understand better the hot-cold spots procedure inside the earths. It shows the physical processes within the Earth’s upper mantle that lead to the generation of magma in steps A to D for different plate tectonic settings. Tomographic maps with distinct velocities over short-periods of 5 to 25 seconds (equivalent to a depth of 6 to 53 km), are more sensitive to the structure of the upper to lower crust, and Moho. These periods represent sediments in the basins, chemical interactions of hydrocarbon resources, molten material and magma chambers beneath volcanoes and Moho discontinuity which these areas can be considered temporary and unstable hot and cold spots. These short-periods, which is also known as the crust, include soil, vegetation growth, construction, surface-groundwater, oil-gas resources, magma chambers, metallic, non-metallic mines, and chemical interactions. Although, slow velocities in crust/upper mantle under regions of active volcanism do not require "Hot Spot" (i.e, plume-related) magmatism. These could simply reflect decompression melting and/or crustal melting following slab-rollback, delamination, or breakoff. The lithosphere-aesthenosphere and upper mantle is the source of volcanic lava and the origin of some earthquakes in the mantle and remnants of the old tectonic plate. So, short-periods of 5 to 25 seconds can be considered temporary and unstable hot and cold spots. Dark red spots that are seen below the chain of volcanoes (e.g. Elbrus), upon some segments of faults, and basins in the study area indicate the hot spots and these hot spots, are located in an appropriate depth of shallow area of the earth's crust as geothermal energy resources. The rest of the areas on the tomography maps with various periods that has shown with shades dark blue-green-yellow, include colder and more rigid rocks and stones and remnants of an old tectonic plate and is calm (aseismic), which represent cold spot. Cold spots usually cover a wide area and can even cover tectonic plates and continents and even can include the mantle-core, which the mantle is the source of cold (old) lava volcanic and some deep earthquakes. The core is the Earth planet balance in the solar system and its magnetic property.
Dark red Low-velocity area which we assume as a hot spot number 1; there are 8 hot spots in period of 5 seconds that hot spot number 1 in our study (Fig. 3), there is a small area near Kars mountain in NE Turkey (Eastern Anatolia) known as the Erzurum-Kars Volcanic Plateau (EKVP)- (with depth ~6.6 to 13.33 km). According to Duru Olgun et al., (2020) study, this part of the plateau is known to have been formed by the eruptions during the Zanclean (~4.5 Ma) period, related an earlier continental collision event between Eurasian and Arabian continents ~15 Ma ago. Also, Eastern Anatolia is known for its thin lithosphere (Sengör, A. M. C. et al., 2003). The EKVP is composed mainly of andesitic and dacitic lavas and their trachytic equivalents intercalated with acidic ignimbrites and tuffs. In the northwest of Kars, an eroded stratovolcano is present which is possibly coeval with the plateau. It consists of a thick sequence of rhyolitic lavas, tuffs and perlitic-obsidian.
Dark red Low-velocity area which we assume as a hot spot number 2 (Fig. 3) is approximately located on the northern slope of Aragats volcano (depth of ~7 to 13.66 km), which may be the reason for the existence of magma. Based on I.V. Chernyshev et al., 2002 and Vadim Milukov et al., 2018 studies, the Aragats center, one of the largest Quaternary volcanic centers in the Caucasus, is confined to the Aragats neovolcanic area located in the western part of Armenia, at the intersection of tectonic zones of a general Caucasian extension and the sublongitudinal Transcaucasus uplift. The development of the Pliocene-Quaternary volcanism of the Aragats area is defined by complex late collisional Geodynamics, which is related to global processes of the convergence of Eurasian and Arabian continental plates. The highest peak of the Lesser Caucasus, the polygene Aragats stratovolcano (altitude 4090 m, 70 km diameter), is situated in the western volcanic zone of the Aragats area which it occupies a special place in the neovolcanic evolution of Armenia in terms of diversity of the magmatic rocks, scales and duration of volcanism, variety of the eruption products, and intricacy of the geotectonic structure. The volcano represents a plan-convex asymmetric shield (40-42 km across) with a major crater in the northeastern part. The established duration of magmatic activity of the Aragats center is about 400 ka. Aragats volcano should presumably be ascribed to extinct volcanoes. Such large volcanoes (like Aragats), capable of generating many caldera collapse eruptions.
Dark red Low-velocity area which we assume as a hot spot number 3 (Fig. 3) in our study covers a wide area such as Garni, Shoraghbyur, Yerevan, Avan salt dome and Harazdan from oil and gas resources introduced by Jrbashyan et al. (2001). The Paleocene and Lower Eocene of the subthrust section yielded oil-saturated cuttings and oil-cut mud and oil-stained cuttings were reported as features of the upper Eocene section in these area (thermogenic chemical interactions). By Milanovski, E.E., (1962) about this hot spot, detailed explanation is given as Sevan and Central Troughs. Due to the chemical interactions of in-earth materials in oil-rich areas (contains hydrocarbons) and gas resources, the temperature inside the earth is high and spots relevant with gas plumes is anomalously hot compared to the surrounding. The depth of this hot zone varies from 6.6 to 13.66 km.
Dark red Low-velocity area which we assume as a hot spot number 4 (Fig. 3); is approximately located in the beneath of Ararat strato-volcanic structures (depth ~6.6-13.66 km, velocity 2 km/s) in the Julfa region, which could be due to the presence of a magma chamber beneath this volcanic complex. East-north foothills of this volcanic complex are affected by sediments of Aras river, which is limited by the uplifted basement of the Ararat volcanos to the south and by the Hrazdan Transverse Fault Zone to the west. Also, Ozgür Karaoğlu (2017) seismic tomography study indicates a magma reservoir at great depths (20-30 km) below the Ararat volcano. Geochemical constraints on some of the later-formed rocks suggest an interaction between a shallow chamber (8-10 km) and the deep reservoir approximately 0.5 Ma. This depth is consistent with the result of our study in period of 5 seconds (depth of 7-13 km; Fig. 3). Also, based on Jrbashyan et al., (2001) study, the Urts-Julfa Zone is limited by the uplifted basement of the Great Ararat to the south and by the Hrazdan Transverse Fault Zone to the west. Intersection with the Vedi Ophiolitic Belt is marked by a transition from alkali-basaltic volcanism to tholeiitic volcanism in the Mesozoic section. Relatively isolated strato-volcanic structures, such as Ararat and Aragats, occur at or near fault intersections. The largest volcanic edifice in this area is the Greater Ararat Stratovolcano, which is composed of intermediate lavas with andesitic-dacitic-trachyandesitic compositions, erupted ~3.5 Ma (i.e. Piacenzian).
Dark red Low-velocity area which we assume as a hot spot number 5 (Fig. 3) is approximately located in the northeast of the Lake Van includes Tendurek, Suphan and Nemrut Mountains. Study of Vural Oyan et al. (2018), shows collision related to Quaternary Mafic Volcanism to the north of Lake Van (Eastern Anatolia, Turkey) has been occurred by eruptions from both volcanic centers and extensional fissures trending approximately north-south. Also, the volcanic products in this area consist of mildly alkaline lavas and calculations based on crustal temperatures and Curie point depths indicate that the magma chamber might have been located at a depth of around 6-8 km, within the upper crust. We infer, that perhaps, the molten material beneath the Ararat volcano and the mountains around Lake Van are quite interconnected. In the period of 5 s in our study, this property has been shown at a depth of 6.6 to 13.66 km, which is consistent with Vural Oyan (2018) study. As well as, the pattern of concepts of hot spots 1, 4 and 5 is almost the same, as these regions are located in the Eastern Anatolia famous for its thin hot lithosphere structure.
Dark red Low-velocity area which we assume as a hot spot number 6 and 7 (Fig. 3) are located in NW Iran near the north part of Sahand volcano and southeastern segment of the Tabriz fault. This fault is responsible destructive earthquakes in Tabriz (e.g. 7.7, 1721, Fig. 1). The epicenter of this earthquake, is located right into the hot zone number 6. It is clear that these hot spots are perhaps due to the interactions of the rocks of this famous active fault. Mehraj Aghazadeh et al. (2010) in a study has reported limited volcanic eruptions evidences in the South of the Tabriz fault (Sahand block) that are characterized by ages ranging from 11 Ma to present (era 4). The 11 Ma lavas have an alkaline potassic to ultrapotassic composition. Our results show distinct velocity anomalies for smaller periods along the North Tabriz Fault (NTF) and beneath the Sahand and Sabalan Volcanoes. In contrast, beneath the Sahand volcano a high-velocity zone is observed that could be due to the low temperature volcanic rocks or a deeper magma chamber at a depth of ~30.8 km.
Dark red Low-velocity area which we assume as a hot spot number 8 (Fig. 3) is approximately located near a segment of Salvard fault in the north east of Nakhichevan. According to several studies (e.g. Danelian et al., 2014; Sokolov, 1977), exposures of Jurassic sequences are located in Nakhichevan and in Iran, where a 500 m -thick Lower and Middle Jurassic sedimentary sequence overlies Upper Triassic strata. Lower Cretaceous deposits are absent on the south Armenian block and the Triassic-Jurassic deposits are unconformably overlain by Cenomanian reefal limestones that are covered by marls. Upper Devonian (the fourth period of the Paleozoic era) and Permian (the fifth period of the Paleozoic era) rocks could be petroleum source rocks (Sosson et al., 2010). Silurian and Lower and Middle Devonian marine clastic and carbonate rocks crop out in Nakhichevan and are presumed to be present in Armenia. Our study is shown this property in period of 5 s at a depth of 6 to 9.5 km (hot spot number 8).
Dark red Low-velocity areas which we assume as a hot spots number 9, 10, 11 and 12 (Fig. 3) in period of 10 s, in addition to the low-velocity anomalies during the period of 5 s, areas such as the eastern Black Sea basin and a segment of the Odishi fault in the Rioni basin (number 9), the Nalchik city in Russia and north and south western of Kazbek mount (number 10), the Chatma region in east of Georgia (number 11) and South Caspian Basin (number 12) are also covered by low-velocity anomaly and it follows the same pattern described for the hot spots 1 to 8. As mentioned, the low-velocity anomaly beneath the volcanoes in the depth associated with this period (14 to 28.66 km), reveals the presence of magma and the magmatic reservoir and sediments.
Dark red Low-velocity area which we assume as a hot spots number 13 (Fig. 3), the Elbrus volcanic complexes, Kazbek mount and Yanardag (natural gas fire on a hillside) in the great Caucasus have covered by the low-velocity anomaly. The low-velocity anomaly beneath the volcanoes in the depth associated (~30.8 km and 158.6 km) with these periods reveals the presence of magma and the magmatic reservoirs and mantle plume. In tomographic maps with long-period of 70 s (equivalent to a depth of 158.6 km); Azerbaijan, Kura and South Caspian basins and Talesh heights are covered with high-velocity, which we suggest cold lithosphere roots for deep areas. On the contrary, the low-velocity in the Greater Caucasus, eastern Black Sea basin and eastern Anatolia are resulting in very thin lithosphere and hot asthenosphere. The hot spots in period of 15 s follow the pattern described in periods of 5 and 10 seconds.
Dark red Low-velocity area which we assume as a hot spot number 14 (Fig. 3) in periods of 40 and 45 seconds, is observed a wide hot spots area with low-velocity in the South Caspian Basin (SCB), below the Sahand-Sabalan volcanoes, Astara region, Tabriz Fault and slightly in the southernmost mountain around Lake Van or Bitlis Massif in the border of Iraq, which, according to (Sugden et al., 2018) study, the mid-lithosphere magma source has a distinct composition compared to the base of the lithosphere, that is argued to be the result of the increased retention of metasomatic components in phases such as apatite and amphibole, that are stabilized by lower temperatures prior to magma generation. Also, partial melts of the deep lithosphere ~120 km (in our study 111 km) and mid-lithosphere sources to give a composition intermediate between magmas from the northern Lesser Caucasus and NW Iran could be the reason for this extensive hot spot.
Dark red Low-velocity area which we assume as a hot spot number 15 (Fig. 3) in the period of 45 and 50 seconds, is observed a wide very low-velocity anomaly in east and northeast of Lake Van and just east of the Ararat, Sahand, Sabalan, Bitlis, Nakhchivan, South Armenia, Astara, South Caspian Basin. According to some studies (e.g. Kearey, Philip et al. 2009; Condie, Kent C., 1997); this decrease of seismic wave velocity from lithosphere to asthenosphere, could be caused by the presence of a very small percentage of melt in the asthenosphere and seismic waves pass through the lithosphere-asthenosphere very slowly. Also, the lower boundary of the LVZ lies at a depth of 180-220 km (our study ~175 km), therefore, the hot wide area is not unexpected. We propose that perhaps it marks the depth of LAB and LVZ in Caucasus region and so the wave has penetrated to the asthenosphere layer and seismic waves pass through this area very slowly. The interactions and intrusion of very hot molten material from asthenosphere to lithosphere discontinuities for creating hot spot number 15 is not unexpected. Fig. S6 of the Supplementary Materials has been depicted the approximate depth of Moho, LAB and LVZ for this study. In the Lesser Caucasus, there is the link between the volcanic manifestations and low-velocity patterns, but it is not as clear as in the Great Caucasus. The Gegham volcanic group in Armenia also match with the location of the low velocity anomaly. Also, in Sugden et al. (2018) study, diagram of depth vs temperature of melting shows that after the depth of 150 km, the temperature has a significant increase in Gegham, Syunik, and Vardenis.
Long-period tomographic maps velocity structures of 55 to 70 (approximate depth of 200 km), indicate ultrahigh-velocity anomalies (5.04 km/s) and ultralow-velocity (1.4 km/s) areas. We infer the deep ultrahigh-velocity anomalies may be the broken off cold lithosphere generated slabs were sinking into the mantle transition zone and very-hot upper mantle with low mantle lid (cap). In contrast, for ultralow-velocity regions, is thought that the upper mantle has been rejuvenated by a phase of the upwelling hot mantle, and this metasomatic refertilization of the upper Cratonic mantle has increased its density and reduces seismic velocity and rocks experience temperatures above 1300-1600 \(℃\) at these depths. Also, according to depth-temperature diagrams (e.g. Sugden et al. 2018), at these depths, some interactions such as onset of dry melting in the convecting mantle and the Spinel out or hard-glassy mineral occurring as octahedral crystals of variable color and consisting chiefly of magnesium and aluminum oxides, cause an increase in temperature and density conflicts tension. So, we propose that at the depths common between the lithosphere-asthenosphere-upper mantle; anomalies accumulation, inhomogeneities and antagonistic behaviors are common in surface wave velocity variations. In these regions, due to continuous changes in temperature caused by the plate tectonic activity, the effect of active liquids penetrated by the asthenosphere, subsidence, uplifts, hot asthenospheric diapirs intrusion, the velocity of surface waves changes. Also, seismic waves slowly cross the lithosphere-asthenosphere boundary (LAB), which known as the low-velocity zone (LVZ), and then enter the upper mantle (Fig. S6). Poor coverage of ray paths in this part of the study area (periods of 60, 65 and 70 s) leads to stretching and smearing (butterfly-shaped areas) by this feature toward the northwest and southeast of the study area. In these periods, due to some reasons such as plates tectonic activities, hot asthenospheric diapirs intrusion, the effect of active liquids penetrated by the asthenosphere, subsidence, uplifts and mantle plumes the temperature changes constantly and therefore, the surface waves have variable behavior.