An investigation of the significance of local site effects in Plovdiv, Bulgaria

Plovdiv is the second largest city in the Republic of Bulgaria. A large part of the city is located on Holocene alluvial sediments and the oldest neighborhoods are situated on syenitic rock outcrops. We believe that local site effects may be an important contributor to the destruction caused by earthquakes. The primary objective of this study was to estimate quantitatively the local site effects in the central area of Plovdiv in terms of fundamental site frequency and amplification factor. Another important objective was to see how these correlate with the geological structures underlying the city. Measurements of the seismic noise at more than two hundred regularly placed points were made in the central area of the city. The H/V spectra were then calculated and analyzed to determine the spatial distribution of the fundamental site frequency and the amplification factor. The results exhibit very good correlation with the local geology. They were also compared with an intensity map from the strong 1928 Plovdiv earthquake. The comparison clearly demonstrates that the local site effects were the main factor in the destruction of buildings—the zones where the most damage was observed are also the zones where we have low fundamental site frequencies and high amplifications. Similarly the areas with high fundamental site frequencies and low amplification factors cover the neighborhoods where less damage has been observed. This study may form a basis for a more comprehensive and systematic microzonation study in Plovdiv.     

Organic petrology, mineralogy and depositional environment of the Kipra lignite seam, Maritza-West basin, Bulgaria

The aim of the present study is to provide additional information about the properties and depositional environment of the Kipra lignite seam, which was deposited during the regressive stage of development of the Maritza-West basin. Petrographical and mineralogical data, along with ash yields and sulphur contents of 24 samples from a seam profile, have been used to study the vertical variation of the depositional settings during peat accumulation and subsequent coalification.The Kipra lignite is characterized by high ash yields and sulphur contents. It formed in a rheotrophic, low-lying mire with alkaline pH value. Vegetation with low preservation potential dominated within the palaeomire. During peat formation, frequent changes of the water level controlled the depositional environment. During the deposition of units 1 and 2, high water energy caused the transportation of high amounts of inorganic material into the mire, resulting in the formation of weakly gelified mineral-rich lignite. The organic matter from units 3 and 4 is characterized by enhanced gelification, which probably reflects the decreasing energy of the system. Good positive correlation between sulphur contents and the GI values was established in units 4, indicating that the gelification of the tissues was probably mainly controlled by the bacterial activity. In contrast, the gelification of the samples from unit 3 of the Kipra seam was probably governed by the redox conditions. The organic matter deposited under relatively wet conditions, in which the thermal and oxidative destruction of the tissues, was limited.A variety of major, minor and accessory minerals are present in Maritza-West lignite. The mineral composition is dominated mainly by pyrite, gypsum and calcite, and to a lesser extent limonite, quartz, kaolinite, montmorillonite, illite, chlorite and plagioclase. Jarosite, hematite, halloysite, mica, K-feldspar, aragonite, siderite, and dolomite were also determined in very low concentrations. These minerals formed syngenetically and epigenetically. The syngenetic stage is characterized mainly by the formation of pyrite, carbonates, silicates and sulphates, whereas the Fe-oxyhydroxides, partially the carbonates and almost all silicates are of detrital origin. During the epigenetic stage, carbonates, sulphates, clay minerals, pyrite, and Fe-oxyhydroxides were formed. Alteration products like gypsum, jarosite, limonite, chlorite, kaolinite, illite, mica, and calcite were generated due to the transformation of detrital and authigenic minerals.     

Peri-Gondwanan Ordovician crustal fragments in the high-grade basement of the Eastern Rhodope Massif,Bulgaria: evidence from U-Pb LA-ICP-MS zircon geochronology and geochemistry

Field, geochemical, and geochronologic data of high-grade basement metamafic and evolved rocks are used to identify the nature and timing of pre-Alpine crustal growth of the Rhodope Massif. These rocks occur intrusive into clastic-carbonate metasedimentary succession. Petrography and mineral chemistry show compositions consistent with Alpine amphibolite-facies metamorphism that obliterated the original igneous textures of the protoliths. Bulk-rock geochemistry identifies low-Ti tholeiitic to calc-alkaline gabbroic-basaltic and plagiogranite precursors, with MORB-IAT supra-subduction zone signature and trace elements comparable to modern back-arc basalts. The U-Pb zircon dating revealed a mean age of 455 Ma for the magmatic crystallization of the protoliths that contain inherited Cambrian (528–534 Ma) zircons. Carboniferous, Jurassic, and Eocene metamorphic events overprinted the Ordovician protoliths. The radiometric results of the metamorphic rocks demonstrate that Ordovician oceanic crust was involved in the build-up of the Rhodope high-grade basement. Dating of Eocene-Oligocene volcanic rocks overlying or cross-cutting the metamorphic rocks supplied Neoproterozoic, Ordovician and Permo-Carboniferous xenocrystic zircons that were sampled en route to the surface from the basement. The volcanic rocks thus confirm sub-regionally present Neoproterozoic and Paleozoic igneous and metamorphic basement. We interpret the origin of the Middle-Late Ordovician oceanic magmatism in a back-arc rift-spreading center propagating along peri-Gondwanan Cadomian basement terrane related to the Rheic Ocean widening. The results highlight the presence of elements of Cadomian northern Gondwana margin in the high-grade basement and record of Rheic Ocean evolution. The eastern Rhodope Massif high-grade basement compared to adjacent terranes with Neoproterozoic and Cambro-Ordovician evolution shares analogous tectono-magmatic record providing a linkage among basement terranes incorporated in the Alpine belt of the north Aegean region.     

Petrology, geochemistry and geodynamic implications of Jurassic island arc magmatism as revealed by mafic volcanic rocks in the Mesozoic low-grade sequence, eastern Rhodope, Bulgaria

In the eastern Bulgarian Rhodope, mafic extrusive rocks and underlying greenschists are found in the Mesozoic low-grade unit, which represents the northern extension of similar sequences including the Evros ophiolites in Thrace (Greece). Both rock types define a suite of low-Ti tholeiitic basalts to transitional boninitic basaltic andesites and andesites and associated metapyroclastites (greenschists), intruded at its base by diorite dikes of a boninitic affinity. Mafic lavas and greenschists display large ion lithophile element (LILE) enrichment relative to high-field strength elements (HFSE), flat REE patterns of a slight light REE depletion, a strong island arc tholeiite (IAT) and weak MORB-like signature. All these rocks are characterized by negative Nb anomalies ascribed to arc lavas. They have positive Nd_i values in the range of + 4.87 to + 6.09, approaching the lower limit of MORB-like source, and relatively high (²⁰⁷Pb/²⁰⁴Pb)_i (15.57–15.663) at low (²⁰⁶Pb/²⁰⁴Pb)_i (18.13–18.54) ratios. The Nd isotopic compositions coupled with trace element data imply a dominantly depleted MORB-like mantle source and a contribution of subduction modified LILE-enriched component derived from the mantle wedge. The diorite dike has a low Nd_i value of − 2.61 and is slightly more Pb radiogenic (²⁰⁷Pb/²⁰⁴Pb)_i (15.64) and (²⁰⁶Pb/²⁰⁴Pb)_i (18.56), respectively, reflecting crustal contamination. Petrologic and geochemical data indicate that the greenschists and mafic extrusive rocks represent a magmatic assemblage formed in an island arc setting. The magmatic suite is interpreted as representing an island arc–accretionary complex related to the southward subduction of the Meliata–Maliac ocean under the supra-subduction back-arc Vardar ocean/island arc system. Magmatic activity appears to have initiated in the north during the inception of the island arc system by the Early–Middle Jurassic time in the eastern Rhodope that most likely graded to back-arc spreading southwards as represented by the Late Jurassic MORB-type Samothraki Island ophiolites. This tectonic scenario is further constrained by paleotectonic reconstructions. The arc–trench system collided with the Rhodope in the Late Jurassic times.     

Evidence from the Mesta half-graben, SW Bulgaria, for the Late Eocene beginning of Aegean extension in the Central Balkan Peninsula

The ENE-tilted Mesta half-graben contains a 3-km-thick section of Priabonian (Late Eocene) to Oligocene sedimentary and volcanic rocks that rest unconformably on basement metamorphic rocks along its west side. Basal strata dip 50–60° E and dip at progressively lower angles upward, indicating synrotational deposition. The southern part of the half-graben contains nested volcanic caldera complexes, formed during the deposition of the middle part of the sedimentary sequence, which have been rotated by about half the total rotation of the sedimentary succession. The half-graben is bounded on the east by a fault that steepens from more deeply exposed structural levels in the south (8–18° W) to shallower exposed structural levels in the north (70° W) and together with the rotation of Paleogene strata during deposition indicate the Mesta half-graben is underlain by a listric detachment fault, the Mesta detachment. Subhorizontal Middle Miocene strata that unconformably overlie tilted Paleogene strata yield an upper age limit to the extension. West and northwest of the Mesta half-graben are many other NNW-trending NE-tilted Paleogene half-grabens which we suggest are part of an important extended area in SW Bulgaria and eastern Macedonia that lies above one or more west-dipping detachment faults and date the beginning of Aegean extension in the southern Balkan region as at least as old as Priabonian. The Mesta detachment is oblique to the trend of a contemporaneous Paleogene magmatic arc in the southern Balkans and the origin of the detachment is probably related to gravitationally induced spreading of thickened hot arc crust and Hellenic trench roll back.     

Mineral and inorganic chemical composition of the Pernik coal, Bulgaria

The mineral and inorganic chemical composition of five types of samples from the Pernik subbituminous coals and their products generated from the Pernik preparation plant were studied. They include feed coal, low-grade coal, high-grade coal, coal slime, and host rock. The mineral matter of the coals contains 44 species that belong mainly to silicates, carbonates, sulphates, sulphides, and oxides/hydroxides, and to a lesser extent, chlorides, biogenic minerals, and organic minerals. The detrital minerals are quartz, kaolinite, micas, feldspars, magnetite, cristobalite, spessartine, and amphibole. The authigenic minerals include various sulphides, silicates, oxihydroxides, sulphates, and carbonates. Several stages and substages of formation were identified during the syngenetic and epigenetic mineral precipitations of these coals. The authigenic minerals show the greatest diversity of mineral species as the epigenetic mineralization (mostly sulphides, carbonates, and sulphates) dominates qualitatively and quantitatively. The epigenetic mineralization was a result of complex processes occurring mostly during the late development of the Pernik basin. These processes indicate intensive tectonic, hydrothermal and volcanic activities accompanied by a change from fresh to marine sedimentation environment. Thermally altered organic matter due to some of the above processes was also identified in the basin. Most of the trace elements in the Pernik coals (Mo, Be, S, Zr, Y, Cl, Ba, Sc, Ga, Ag, V, P, Br, Ni, Co, Pb, Ca, and Ti) show an affinity to OM and phases intimately associated with OM. Some of the trace elements (Sr, Ti, Mn, Ba, Pb, Cu, Zn, Co, Cr, Ni, As, Ag, Yb, Sn, Ga, Ge, etc.) are impurities in authigenic and accessory minerals, while other trace elements (La, Ba, Cu, Ce, Sb, Bi, Zn, Pb, Cd, Nd, etc.) occur as discrete phases. Elements such as Sc, Be, Y, Ba, V, Zr, S, Mo, Ti, and Ga exceed Clarke concentrations in all of the coal types studied. It was also found that a number of elements in the Pernik coals (F, V, As, Pb, Mo, Li, Sr, Ti, Ga, Ni, Ge, Cr, Mn, etc.) reveal mobility in water and could have some environmental concerns.     

Environmental Data in Bulgaria: Problems and Prospects*

Research projects dealing with transition in the former centrally planned economies of Eastern and Central Europe face great difficulties of data availability. Much data are not available and much of what are available are poorly documented or unreliable. This paper describes some of the problems of data availability in the context of environmental research in Bulgaria. It describes current efforts to upgrade national level environmental monitoring capabilities, and argues for the need for the development of locally sensitive systems of data collection and analysis.     

Tectonic configuration of the Apuseni–Banat—Timok–Srednogorie belt, Balkans-South Carpathians, constrained by high precision Re–Os molybdenite ages

The Apuseni–Banat–Timok–Srednogorie magmatic–metallogenic belt (ABTS belt), forms a substantial metallogenic province in the Balkan-South Carpathian system in southeastern Europe. The belt hosts porphyry, skarn, and epithermal deposits mined since pre-Roman times. Generally, the deposits, prospects, and occurrences within the belt are linked to magmatic centers of calc-alkaline affinity. Fifty-one rhenium-osmium (Re–Os) ages and Re concentration data for molybdenites define systematic geochronologic trends and constrain the geochemical-metallogenic evolution of the belt in space and time. From these data and additional existing geologic-geochemical data, a general tectonic history for the belt is proposed. Mineralization ages in Apuseni-Banat, Timok, and Panagyurishte (the central district of the larger E–W Srednogorie Zone) range from 72–83, 81–88, and 87–92 Ma, respectively, and clearly document increasing age from the northwestern districts to the southeastern districts. Further, Re–Os ages suggest rapidly migrating pulses of Late Cretaceous magmatic–hydrothermal activity with construction of deposits in ~1 m.y., districts in ~10 m.y., and the entire 1,500 km belt in ~20 m.y. Ages in both Timok and Panagyurishte show systematic younging, while deposit ages in Banat and Apuseni are less systematic reflecting a restricted evolution of the tectonic system. Systematic differences are also observed for molybdenite Re concentrations on the belt scale. Re concentrations generally range from hundreds to thousands of parts per million, typical of subduction-related Cu–Au–Mo–(PGE) porphyry systems associated with the generation of juvenile crust. The geochronologic and geochemical trends are compatible with proposed steepening of subducting oceanic slab and relaxation of upper continental plate compression. Resulting influx of sub-continental mantle lithosphere (SCML) and asthenosphere provide a fertile metal source and heat, while the subducting slab contributes connate and mineral dehydration fluids, which facilitate partial melting and metal leaching of SCML and asthenosphere. Cu–Au–Mo–(PGE) porphyry deposits may develop where melts are trapped at shallow crustal levels, often with associated volcanism and epithermal-style deposits (South Banat, Timok, and Panagyurishte). Mo–Fe–Pb–Zn skarn deposits may develop where felsic melts are trapped adjacent to Mesozoic limestones at moderate crustal levels (North Banat and Apuseni). Systematic spatial variations in deposit style, commodity enrichment, Re–Os ages, and Re concentrations support specific tectonic processes that led to ore formation. In a post-collisional setting, subduction of Vardar oceanic crust may have stalled, causing slab steepening and rollback. The slab rollback relaxes compression, facilitating and enhancing orogenic collapse of previously thickened Balkan-South Carpathian crust. The progression of coupled rollback-orogenic collapse is evidenced by the width of Late Cretaceous extensional basins and northward younging of Re–Os ages, from Panagyurishte (~60 km; 92–87 Ma) to Timok (~20 km; 88–81 Ma) to Apuseni-Banat (~5 km; 83–72 Ma). Generation of a well-endowed mineral belt, such as the ABTS, requires a temporally and spatially restricted window of magmatic–hydrothermal activity. This window is quickly opened as upper plate compression relaxes, thereby inducing melt generation and ingress of melt to higher crustal levels. The window is just as quickly closed as upper plate compression is reinstated. The transient tectonic state responsible for economic mineralization in the ABTS belt may be a paleo-analogue to transient intervals in the present subduction tectonics of SE Asia where much mineral wealth has been created in the last few million years.     

Heavy mineral characteristics of the Neogene–Quaternary sediments from the southern part of the Blagoevgrad Basin, SW Bulgaria

The Blagoevgrad Basin, a graben filled with Neogene–Quaternary continental sedimentary successions, is one of the most prospective placer-gold bearing regions in southwestern Bulgaria. The Quaternary sediments have been well explored and economically prospective areas have been identified. Previous studies indicated very limited occurrences of placer gold in the Neogene sediments, the genesis of which is connected with resedimentation of Paleogene sediments from the adjacent Bobov dol Basin to the north. From these previous studies, a detailed allo- and lithostratigraphic scheme, and paleogeographic models were developed.The present study aimed to evaluate the plausibility of the existing stratigraphic schemes and paleogeographic models for the area, through detailed study of the heavy mineral fraction of the Neogene and part of the Quaternary sediments from the southern part of the Blagoevgrad Basin. Additional objectives were to determine the mode of occurrence and the stratigraphic position of the placer gold in the Neogene sediments, and to investigate possible source areas. Special attention was paid to the mineral composition and the structure of the opaque detrital Fe–Ti oxide minerals, and the typologic features of the zircons. The results obtained could be used as a provenance indicator and for determination of the nature of the source area and the processes that operated therein.A very important finding is the discovery of placer gold in an additional three Neogene units. The morphologic features of the gold grains indicate short transport or proximity to the primary source. This interpretation is inconsistent with existing hypotheses on the origin and presence of the gold in the sediments of the basin. The characteristics of the magnetic heavy mineral fraction, and the morphologic features of zircon grains in auriferous samples, indicate the diorites and gabbro-diorites from the Strouma Diorite Group, which outcrop along the eastern edge of the graben, as a possible source.The stratigraphic distribution of the heavy minerals fully supports the plausibility of the existing detailed allo- and lithostratigraphic subdivision, and the paleogeographic model for the development of the southern part of the basin. Also, the results provide support for the soundness of an allostratigraphic approach for the study and exploration of placer deposits in basins similar to the Blagoevgrad.     

Analyse quantitative des faciès carbonatés et cycles T–R de haute fréquence dans le Barrémien du Prébalkan central (Bulgarie)

Within the Emerici zone–Barremense zone biostratigraphic interval, the Barremian deposits of Central Fore-Balkan (Lovech–Veliko Tarnovo shelf) consist of a succession of several formations where alternate terrigenous argillaceous/sandy-dominated facies (Kormjansko Fm., Balgarene Fm.) and carbonate-dominated (‘Urgonian’) facies (Krushevo Fm., Emen Fm.). The qualitative and, particularly, quantitative facies analysis of the carbonate successions observed along 13 detailed cross-sections and in one drill hole show the stacking of about 40 fifth-order T–R cycles induced by numerous eustatic jerks contributing to the progressive settlement of this shelf. These high-frequency cycles of about 100 000 years must be regarded as valuable correlation tools for the subsurface hydrocarbon research. To cite this article: V. Minkovska et al., C. R. Geoscience 336 (2004).