No abstract
The territory of Bulgaria covers part of the active continental margin of the Eurasian plate. Several first-order tectonic units may be distmguished: Moesian platform, paraautochthonous margin of the platform, zone of Mid-Mesozoic collage units, Late Cretaceous island-arc system and a system of Cenozoic collisional and post-collisional grabens. The Alpine magmatism is genetically related to the evolution of the Eurasian margin and the Tethyan ocean south of it. The earliest Alpine magmatic activity is represented by Triassic, basic and intermediate, strongly altered volcanics (NW Bulgaria, in boreholes). They are related to initial, embryonal rifting of the Moesian platform. The Triassic and Jurassic-Lower Cretaceous small bodies of basic volcamcs in the SE collage units are interpreted as ensimatic. During the Upper Cretaceous an ensialic island-arc system originated. The related intensive magmatism formed the Srednogoric volcano-intrusive zone (SVIZ). The magmatic rocks are products of complex differentiation processes. Their formation was accompanied by deep-water sedimentation. All magmatic groups, according to SiO2 content, occur. By the K2O/SiO2 ratio they belong to the TH, CA, HKCA, SH, HKTR (high-K transitional) and BG (bulgaritic) series. Longitudinal and transversal zonahties may be traced. In a global aspect SVIZ is one of the most ancient segments of the Alpine-Himalayan volcano-intrusive belt. The bulgaritic petrochemical trend is related to the earliest generation of K-cnriched magmas in the Mediterranean region. The products of the collisional magmatism are exposed in the Macedonian-Rhodope-North Aegean volcanic zone (MRNAVZ) located south of the Late Cretaceous island arc. It originated during the Eocene-Oligocene (37-25 Ma) as a result of the collision between Eurasia and the Apulian promontory of Africa. The magmatic rocks belong dominantly to the intermediate and acid groups and to the CA, HKCA and SH series. The distribution of intermediate and acid rocks in this zone is controlled by the thickness of the crust. In the Eastern and Central Rhodopes the K-content increases from south to north. The volcanic activity in the zone occurred in conditions of intensive block orogeny and was accompanied by terrigenous molasse sedimentation. During the Neogene a zone of transversal faulting developed in the collisional orogen (remainding of Himalayan-type orogen) and parts of the Moesian platform. This zone is marked by small bodies of basic and ultrabasic, Na-alkalinc and subalkalinc rocks. The K-content increases in southward direction, toward the collisional front.
Abstract:The Istranca Crystalline Complex in NW Anatolia and SE Bulgaria includes structural units that differ in lithostratigraphy, metamorphism, age and structural position. They are collectively named as the "Istranca nappes" comprising from bottom to top the Sarpdere, Mahyadag and Doganköy Nappes. The Sarpdere Nappe consists of Lower Triassic arkosic metasandstones with slate interlayers, followed by Middle to Upper Triassic carbonates and an alternation of Upper Triassic clastics and carbonates. The Mahyadag Nappe comprises a low-grade metamorphic Late Paleozoic-Triassic carbonate-siliciclastic sedimentary succession. The Doganköy Nappe includes Precambrian?-Paleozoic metasediments, intruded by Late Carboniferous-Early Permian calc-alkaline granitoids. Its Triassic cover comprises metaclastics and metacarbonates. The Istranca nappes were juxtaposed at the end of the Triassic and transgressively covered by Lower Jurassic coarse clastics, followed above by Middle to Late Jurassic carbonates, black shales and carbonate-siliciclastic sedimentary succession. The phosphate concretions in black shales yielded radiolarian assemblages indicating Late Bajocian-Early Bathonian, Early Bathonian and Early Kimmeridgian ages. These nappes and their Jurassic cover are unconformably overlain by the Cenomanian-Santonian volcano-sedimentary successions intruded by Santonian-Campanian Dereköy-Demirköy intrusive suite. The preliminary data suggest that the Variscan basements of the Mahyadag and Sarpdere Nappes were juxtaposed prior to the Triassic and overridden by the Doganköy Nappe of possible Rhodopean origin from S to N during the Cimmerian compressional events.
Data newly obtained by the authors together with already published evidence allow for a new analysis of the Triassic stratigraphy in the Strandzha Mountains. The Triassic in the Strandzha Mountains is referred with certainty to the Tethyan type of evolution. Its zonal stratigraphy is being detailized on the basis of conodont fauna, and the section is reconstructed and correlated with the other sections in Bulgaria. The numerous lithostratigraphic units already introduced have been created mainly due to secondary features as different degree of metamorphism and of tectonic reworking. They are now generalized into four formal units (Formations), and namely (from bottom to top): the basal Stravnitsa Formation built up of metagravelites, metapsammites and interbedded fine-pebble conglomerates; the Zaberska Formation (with two members, Gramatikovo and Kaziyskidol Members, respectively), built up of flyschoid alternation of shales, phyllites, metasiltstones, metapsammites, and marmorized limestones as well as of basic volcanics and ore bodies, within the age range from upper parts of the Spathian to lowermost parts of the Aegean Substage of the Anisian Stage; Kondolovo Formation built up of limestones and marmorized limestones, with Anisian (Pelsonian and Illyrian Substages) and Ladinian (Fassanian and lower and middle parts of the Longobardian Substage) age; and (at the top) the Lipacka Formation built up of flyschoid interbedding of limestones, slates and sandstones within the range upper parts of the Ladinian (Upper Longobardian Substage) – Norian (Lacian and parts of the Alaunian Substage).
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