V.G. Trifonov

Holocene-historical volcanism and active faults as natural risk factors for Armenia and adjacent countries

Abstract Examples of Holocene-historical volcanism in the territory of Armenia and adjacent areas of Eastern Anatolia and Western Iran are discussed. Holocene-historical activity is proved for the volcanoes of Tskhouk–Karckar, Porak, Vaiyots-Sar, Smbatassar and Ararat. Based on the analysis of remote sensing data, field work, and historical and archeological information, it is demonstrated that there was a considerable number of cases of volcanic activity in Armenia and adjacent regions of Turkey, Syria and Iran during the historical period. The Holocene volcanic centers are situated within pull-apart basin structures and controlled by active faults. Situated in an area prone to many types of natural hazards, Armenia and adjacent countries face high natural risk. The evidence presented shows that volcanic hazard also contributes to the natural risk for these countries.

Volcanic hazards in the region of the Armenian Nuclear Power Plant

Abstract We address volcanic hazards in the region of the Armenian Nuclear Power Plant and discuss the assessment of these hazards conducted in the framework of the International Atomic Energy Agency (IAEA) programs in 1994–1995. An important problem of volcanic hazard assessment is posed by assumptions that the apparent absence of recent volcanic activity in Armenia means that future eruptions in the vicinity of the site are impossible. We present new historical, archaeological, and field data, as well as records of the volcanic activity based on radiocarbon, fission-track, K/Ar and plateau-age determinations. This new evidence attests to volcanism in Armenia and adjacent areas during Holocene and historical time. Volcanic activity is demonstrated for Tskhouk-Karckar, Porak, Vaiyots-Sar, Smbatassar, Gegham Ridge and Ararat volcanoes. Volcanic eruptions occurred on Ararat at distances of 27 and 52 km from the plant site in 2500–2400 BC and in 1840 AD, respectively. New information permits a re-assessment of the volcanic hazards at a level higher than in the 1994–1995 studies.

World map of active faults (preliminary results of studies)

Abstract The Project ‘World map of major active faults’ was confirmed in 1989 as a part of the International Lithosphere Program. The objectives are to compile the World map in scale 1:10,000,000, the maps of continents in scale 1:5,000,000 and the maps of some seismic regions in scale of 1:1,000,000 or 1:5,000,000 with the Explanatory Notes and the Catalogue of major active faults of continents. The maps will show location, age, sense and rate of motion and reliability of identification of faults not older, than 100,000 years, as well as Middle Pleistocene faults, contemporary folds, flexures, volcanoes and epicentres of strong earthquakes differentiated by magnitude, depth and age. The first results of the Project are discussed. One of them is predominance of the strike-slip component of motion over the vertical one for the majority of continental active faults. It has occurred because the strike-slip motion is more efficient, than the thrust, the reverse and even the normal ones. Strike-slip faults are differentiated into faults of translation, rotation and squeezing. The contribution of seismicity to faulting depends on the inpulse, the creep and the inpulse-creep regimes of recent motion in the fault-zones. Different geological techniques for estimating the motion regime, the magnitudes and the recurrence interval of strong Holocene earthquakes are discussed.

Cenozoic tectonics and evolution of the Euphrates valley in Syria

Abstract Late Cenozoic tectonics affected the evolution of the Euphrates river valley in northern Syria. Data on the height and composition of terraces and new K–Ar dating of overlying basalts are presented for the area between the Assad Reservoir and the town of Abou Kamal. The presence of the Late Cenozoic Euphrates Fault, longitudinal with respect to the valley, is established by the lower height of the terraces on the NE side of the valley compared with the same terraces on the SW side. Geophysical profiling (dipole axial sounding; correlation refraction method and georadar) across the southern side of the valley (opposite the town of Ar Raqqa) confirms the offset on the fault as >25 m. Movements along the transverse Rasafeh–El Faid fault zone and the Halabiyeh–Zalabiyeh deformation zone have resulted in local uplift and the splitting of river terraces. During the Pliocene–Early Pleistocene, uplift and strong incision of the Euphrates valley propagated from near the Syrian–Turkish border to near the Iraq–Syrian border. The Euphrates began to deposit alluvium onto the pre-existing low-lying Mesopotamian Foredeep at c. 3.5 Ma. Intense incision began by late Late-Pliocene time to form terrace IV. Comparable incision further downstream began during the Early Pleistocene to form terrace III.

Role of the Asthenosphere in Transfer and Deformation of the Lithosphere: The Ethiopian-Afar Superplume and the Alpine-Himalayan Belt

Seismic tomographic data showing the mantle structure of the Ethiopian-Afar superplume and various segments of the Alpine-Himalayan Orogenic Belt and their relationships with the adjacent megastructures of the Earth are presented. These data and their correlation with the geological evidence lead to the conclusion that lateral flows of mantle material are crucial for the evolution of the Tethys and its closure in the Cenozoic with transformation into an orogenic belt. The lateral flow of hot upper mantle asthenospheric matter spreading from the stationary superplume extending in the meridional direction (in present-day coordinates) was responsible for the accretion of the fragments torn away from Gondwana to Eurasia and for the development of subduction at the northeastern flank of the Tethys. The characteristic upper mantle structure of cold slabs passing into nearly horizontal lenses with elevated seismic wave velocity in the lowermost upper mantle is currently retained in the Indonesian segment of the orogenic belt. In the northwestern segments of this belt, a hot asthenospheric flow reached its northern margin after closure of the Tethys and onset of collision, having reworked the former structure of the upper mantle and enriched it in aqueous fluids. The effect of this active asthenosphere on the lithosphere gave rise to intense Late Cenozoic deformation, magmatism, and eventually resulted in mountain building.

Collision and mountain building

The spatial, chronological, and genetic relationships of recent (Late Alpine) collisions to mountain building are considered at three levels of scale: (i) in separate zones of the Arabian–Caucasus segment of the Alpine–Himalayan Orogenic Belt, (ii) throughout the central segment of this belt from the Alps to the Himalalayas, and (iii) in Central Asia and other mountain belts of continents. Three stages of mountain building are distinguished at all three levels. The first stage starts with widespread collision and similar plate interactions from the end of the Eocene to the middle Miocene and is expressed in the formation of uplifts, commonly no higher than the moderately elevated level in regions that concentrate deformations of transverse shortening induced by compression. The second short stage, which embraces the Pliocene–Quaternary and occasionally the end of the Miocene, differs in general, though differentiated in the value and intensification of vertical movements, when the height of mountains increases by 2–3 times. Elevations are spread over certain platform territories and even frameworks of rift zones. This is related not so much to the intensity of compression and shortening as to the compositional transformation of the upper mantle and the lower crust, leading to their decompaction. Comparison with the Hercynian and Caledonian orogenic stages shows that the second phase, predetermined by widespread collision, reflects a more important geodynamic event expressed in a change of the global plate interaction system and its deep-seated sources.

Recent mountain building of the central Alpine-Himalayan Belt

From the end of the Eocene through the Pliocene, the Alpine-Himalayan Belt underwent collisional shortening induced by convergence of the Gondwana plates with the Eurasian Plate and varied in orientation from the north-northwestern to the northeastern directions. The collisional shortening was expressed in folding, thrusting of continental crustal tectonic sheets over one another, and closure of the residual basins of Neotethys and its backarc seas; it resulted in local thickening of the crust and its isostatic uplifting. As a rule, the uplifts were not higher than ∼1.5 km. In other words, before the Pliocene, the growth of local mountain edifices was caused by collisional shortening of the belt. Isostatic uplifting of the thickened crust was continued in the Pliocene and Quaternary even more intensely than before, but the general rise of the mountain systems was superposed on this process. The rise substantially exceeded in amplitude the contribution of the uplift caused by shortening and did not depend on the preceding Cenozoic history of either territory. Not only the mountain ridges but also most adjacent basins were involved in rising, which eventually led to the contemporary mountain topography of the belt. The spread of the hot and fluidenriched asthenosphere of the closed Tethys beneath the orogenic belt could have been a cause of such additional rising. The uplift was an isostatic reaction to decompaction of the lithospheric mantle partly replaced with asthenosphere and of the lower crust subject to retrograde metamorphism under the effect of cooled asthenospheric fluids. The deep transformations are also probably responsible for deepening of some basins in the Pliocene-Quaternary and more contrasting transverse segmentation of the belt.

Toward postplate tectonics

Half a century ago, principles of the theory of lithosphere plate tectonics, or plate tectonics, were first formulated. Since then, the theory has become substantially more complicated and tectonic processes and phenomena have been identified that are not described by the theory. This relates to certain types of vertical movements, primarily to the newest uplifts that led to the formation of modern mountain systems. Comparison of geological processes (both those described by the theory of plate tectonics and those unexplained thus far) with data of the seismic tomography of the mantle has made it possible to outline a new tectonic model, according to which the source of observable tectonic manifestations is lateral flows of upper mantle matter, propagating from superplumes—flows of matter and energy rising from the mantle’s bottom. These lateral flows not only move lithospheric plates but also determine structural–substantial transformations of the lithosphere and the upper underlithospheric mantle, which lead to vertical movements and mounting building.

Cyclicity of late Holocene seismicity in the Alpine-Himalayan belt

It has been shown for particular seismic zones and the Alpine-Himalayan Orogenic Belt as a whole that in addition to Fedotov cycles, the long-period hypercycles of seismicity are distinguished. Long-period variations were revealed in Syria, in southern and central segments of the El-Ghab Fault Zone of the Dead Sea Transform (EG DST), and at the southwestern end of the East Anatolian Fault Zone (EAFZ). The EG DST demonstrates a ∼1800-year hypercycle with a maximum in the 3rd-7th and the 19th-20th centuries A.D. To reveal variations in seismicity in the entire central part of the orogenic belt, we have corrected evidence for historical earthquakes, taking into account the probability of missing events and the area of their regular recording domains. As a result, we displayed maximums of seismic energy release from the mid-17th to mid-20th century A.D.; from the mid-4th to the end of the 6th century; and in the 15th-13th centuries B.C. When interpreting hypercycles, it is important to keep in mind that variation of seismicity in EG DST correlates with variation of the rate of elastic deformation accumulation, probably reflecting variability of the stress-and-strain state in the region and of velocity of tectonic movements in active domains. After additional investigations, hypercycles could be taken into account for to refine the seismic hazard estimate.

Comparison of Tectonic Phases and Geomagnetic Reversals in the Late Mesozoic and in the Cenozoic

The authors consider the chronological relation of two groups of phenomena in the history of the Earth over the last 150 mln years. One of them is relatively short (a few million years) tectonic phases, or orogenic phases, identified by H.W. Stille in 1924 and characterized by an increase in compressive deformation in mobile belts of the Earth. Deformations that occur during such phases are quite explicable by collisional interactions of lithospheric plates. However, these interactions do not explain the synchronous occurrence of phases in different belts and on different continents. The other group is the frequency of magnetic reversals, i.e., changes such that the positions of magnetic north and magnetic south are interchanged. Tectonic phases are more frequent in epochs of frequent geomagnetic reversals. During the last 24 mln years, when geomagnetic reversals were especially numerous, tectonic phases came one after another in short intervals. An emerging trend for them is the lagging of phase peaks by one to two million years relative to the most frequent magnetic reversals. The chronological relations identified show that tectonic phases are determined not only by geodynamic processes in the lithosphere but also by the action of energy pulses that occur in the Earth’s core and at the boundary of the core with the mantle, where the Earth’s magnetic field is generated. On the geological time scale, this interaction takes place quickly, which excludes energy pulse convection and prompts the search for other mechanisms of this transfer. It is possible that it takes place because the lithosphere is affected by alternating body forces that occur under a change in currents in the core, which is followed by changes in the mode of the Earth’s rotation and the adaptation to it of lithospheric masses.