ТЕКТОНОФИЗИЧЕСКАЯ ИНТЕРПРЕТАЦИЯ МЕХАНИЗМОВ ОЧАГОВ ЗЕМЛЕТРЯСЕНИЙ СИСТЕМЫ ЗАГРОС

 Structural-paragenetic and kinematic methods of tectonophysics are applied to study earthquake focal mechanisms of the Zagros system. Nodal planes of focal mechanisms are identified as L-, L′- and R-, R′-shears by the first method, whereby coordinates of principal stress axes P, T and N (i.e. in tectonophysics, σ1, σ3 and σ2, if σ1 ≥ σ2 ≥ σ3) are defined. ‘Working’ nodal planes corresponding to activated ruptures are revealed. Axes of the main normal stresses are combined into local groups on the basis of the kinematic identity of planes of seismogenic ruptures (Figure 2). The second method is applied to construct stereograms of the main axes P, T and N, to construct and interpret stereograms of vectors of seismogenic shifts (Figure 3), and to more clearly define coordinates of principal axes σ1, σ3 и σ2. As evidenced by their comparison, coordinates of the principal axes obtained by the two tectonophysical methods are well coincident (see Figure 2). Five groups of seismogenesis are distinguished; they differ in combination of deformation regimes and kinematic conditions. Locations are determined of the areas wherein earthquake foci of similar parameters are located. This means that seismogenic zones are distinguished; structural and kinematic characteristics of such zones are determined by parameters of stereographic models of corresponding types of seismogenesis (Figures 4 and 5). It is established that the region is dominated by shear and upthrust deformation regimes due to regional submeridional compression and SW-NE compression (see Figures 4 and 5). Submeridional subhorizontal compression is explained by the northward movement of the Arabian plate, and SW-NE compression is explained by divergent processes within the limits of the Red Sea rift. The time pattern of the seismogenic processes from 1979 to 2001 shows that submeridional compression and SW-NE compression are associated with different deep mechanisms. Processes of SE-NW compression, which are observed in the northern part of the Arabian plate, are caused by its interaction with the Eastern Black Sea microplate.   При интерпретации механизмов очагов землетрясений системы Загрос применены структурно-параге­нетический и кинематический методы тектонофизики. Первым методом нодальные плоскости механизмов очагов идентифицированы как L-, L′- и R-, R′-сколы, на основании чего уточнены координаты главных осей напряжений P, T и N (в тектонофизике σ1, σ3 и σ2, при σ1 ≥ σ2 ≥ σ3). Определены «рабочие» нодальные плоскости, соответствующие реальным разрывам. Оси главных нормальных напряжений объединены в локальные группы по признаку кинематической идентичности плоскостей сейсмогенных разрывов (рис. 2). Вторым методом построены стереограммы распределения главных осей P, T и N, построены и проинтерпретированы стереограммы векторов сейсмогенных подвижек (рис. 3) и уточнены координаты главных осей напряжений. Сопоставление координат главных осей, полученных двумя тектонофизическими методами, показало их хорошую сходимость (рис. 2). Обосновано пять типов сейсмогенеза, характеризуемых разными комбинаторными сочетаниями деформационных режимов и кинематических обстановок; локализованы участки размещения очагов со сходными параметрами, то есть выделены сейсмогенные зоны, структурно-кинематическая характеристика которых определяется параметрами стереографических моделей соответствующих типов сейсмогенеза (рис. 4, 5). Установлено, что доминирующими в регионе являются сдвиговый и взбросовый деформационные режимы, обусловленные обстановками субмеридионального и ЮЗ-СВ регионального сжатия (рис. 4, 5). Субмеридиональное субгоризонтальное сжатие объясняется движением Аравийской плиты на север, а ЮЗ-СВ сжатие – дивергентными процессами в пределах Красноморского рифта. Временная развертка сейсмогенных процессов за 1979–2001 гг. показывает, что субмеридиональное и ЮЗ-СВ сжатие связано с разными глубинными механизмами. Процессы ЮВ-СЗ сжатия, фиксируемые в северной части Аравийской плиты, обусловлены ее взаимодействием с Восточно-Черноморской микроплитой. 

TECTONOPHYSICAL INTERPRETATION OF EARTHQUAKE FOCAL MECHANISMS OF THE ZAGROS SYSTEM

Structural-paragenetic and kinematic methods of tectonophysics are applied to study earthquake focal mechanisms of the Zagros system. Nodal planes of focal mechanisms are identified as L-, L′- and R-, R′-shears by the first method, whereby coordinates of principal stress axes P, T and N (i.e. in tectonophysics, σ1, σ3 and σ2, if σ1 ≥ σ2 ≥ σ3) are defined. ‘Working’ nodal planes corresponding to activated ruptures are revealed. Axes of the main normal stresses are combined into local groups on the basis of the kinematic identity of planes of seismogenic ruptures (Figure 2). The second method is applied to construct stereograms of the main axes P, T and N, to construct and interpret stereograms of vectors of seismogenic shifts (Figure 3), and to more clearly define coordinates of principal axes σ1, σ3 и σ2. As evidenced by their comparison, coordinates of the principal axes obtained by the two tectonophysical methods are well coincident (see Figure 2). Five groups of seismogenesis are distinguished; they differ in combination of deformation regimes and kinematic conditions. Locations are determined of the areas wherein earthquake foci of similar parameters are located. This means that seismogenic zones are distinguished; structural and kinematic characteristics of such zones are determined by parameters of stereographic models of corresponding types of seismogenesis (Figures 4 and 5). It is established that the region is dominated by shear and upthrust deformation regimes due to regional submeridional compression and SW-NE compression (see Figures 4 and 5). Submeridional subhorizontal compression is explained by the northward movement of the Arabian plate, and SW-NE compression is explained by divergent processes within the limits of the Red Sea rift. The time pattern of the seismogenic processes from 1979 to 2001 shows that submeridional compression and SW-NE compression are associated with different deep mechanisms. Processes of SE-NW compression, which are observed in the northern part of the Arabian plate, are caused by its interaction with the Eastern Black Sea microplate

Seismic model of the crust and upper mantle in the Scythian Platform: the DOBRE-5 profile across the north western Black Sea and the Crimean Peninsula

International audienceThe Scythian Platform (ScP) with a heterogeneous basement of Baikalian–Variscan–Cimmerian age is located between the East European Craton (EEC) on the north and the Crimean–Caucasus orogenic belt and the Black Sea (BS) Basin on the south. In order to get new constrains on the basin architecture and crustal structure of the ScP and a better understanding of the tectonic processes and evolution of the southern margin of the EEC during Mesozoic and Cenozoic time, a 630-km-long seismic wide-angle refraction and reflection (WARR) profile DOBRE-5 was acquired in 2011 October. It crosses in a W–E direction the Fore-Dobrudja Trough, the Odessa Shelf of the BS and the Crimean Plain. The field acquisition included eight chemical shot points located every 50 km and recorded by 215 stations placed every ∼2.0 km on the land. In addition, the offshore data from existing profile 26, placed in the Odessa Shelf, were used. The obtained seismic model shows clear lateral segmentation of the crust within the study region on four domains: the Fore-Dobrudja Domain (km 20–160), an offshore domain of the Karkinit Trough at the Odessa Shelf of the BS (km 160–360), an onshore domain of the Central Crimean Uplift (Crimean Plain, km 360–520) and the Indolo-Kuban Trough at the Kerch Peninsula (km 520–620) that is the easternmost part of the Crimea. Two contrasting domains of the ScP within the central part of the DOBRE-5 profile, the Karkinit Trough and the Central Crimean Uplift, may represent different stages of the ScP formation. A deep Karkinit Trough with an underlying high-velocity (>7.16 km s−1) lower crust body suggests its rifting-related origin during Early Cretaceous time. The Central Crimean Uplift represents a thick (up to 47 km) crustal domain consisting of three layers with velocities 5.8–6.4, 6.5–6.6 and 6.7–7.0 km s−1, which could be evidence of this part of the ScP originating on the crust of Precambrian craton (EEC). The thick heterogeneous basement of the Central Crimean Uplift shows inclusions of granitic bodies associated with magmatic activity related with Variscan orogeny within the ScP. General bending and crustal scale buckling of the Central Crimean Uplift with a wavelength of 230 km could be an effect of the Alpine compressional tectonics in the adjacent Crimean Mountains. The extended/rifted continental margin of the ScP (EEC) at the Odessa Shelf and buckling/uplifted domain of the Central Crimean Uplift affected by compressional tectonics, are separated by the N–S oriented Western Crimean Fault. The crust of the southern margin of the EEC is separated from the ScP, which originated on the EEC crust tectonised and reworked during the Palaeozoic–Mesozoic, by the crustal fault of ∼W–E orientation, which corresponds with the Golitsyn Fault observed at the surface between the EEC and the ScP. The Fore-Dobrudja Domain with a thick (>10 km) heterogeneous basement and two subhorizontal layers in the crystalline crust (with velocities 6.2–6.3 and 6.4–6.65 km s−1) differs from the ScP crust and its origin could be very similar to that of the Trans-European Suture Zone and Palaeozoic West European Platform