Evolution and relationships between volcanism and tectonics in the central-eastern part of the Oligocene Borovitsa caldera (Eastern Rhodopes, Bulgaria)

Abstract

The nested Borovitsa caldera emplaced during the collision-related Paleogene volcanism in the Eastern Rhodopes. The pre-caldera succession consists in Priabonian to Early Oligocene sediments and lavas (absarokites, shoshonites, latites). The caldera filling corresponds to an acid volcanism Early Oligocene in age. The tectono-magmatic evolution of the caldera can be divided into six main stages. (1) Ignimbritic units (more than 1.5 km thick) with a trachydacitic to trachytic composition deposited. The K–Ar method yields an age of 34–33.5 Ma. The volcanic products are either strongly or not welded in the western and eastern parts of the caldera, respectively. (2) An initial Murga caldera, 7–10 km in diameter, collapsed. This event was accompanied by the intrusion of a circular body consisting of lenses-bearing rocks of trachyrhyodacitic to rhyolitic composition within the border faults. (3) The emission of pyroclastic rocks continued and a large sub-volcanic body (33 Ma) of trachydacitic to trachyrhyolitic composition intruded in the western part of the circular body. (4) The Borovitsa caldera (15 km × 34 km) collapsed. Rhyolitic and trachydacitic dykes dated at 32.5 Ma intruded along its border faults. (5) High-Si trachyrhyolitic–perlitic domes intruded in the eastern part of the Borovitsa caldera at 30–32 Ma and the Dushka caldera collapsed within the Borovitsa structure. (6) Dykes of various compositions (from shoshonite to rhyolite) and trachydacitic to rhyolitic sub-volcanic stocks finally intruded within the caldera and along its rims at 27.5–29.5 Ma. Observations on radar and optical satellite imagery allowed both a new mapping of the structural pattern in the Borovitsa caldera and the understanding of the relationships between faulting and volcanism in this area. Horse-tail features accommodating the right-lateral throw component at the termination of NW–SE and N–S right-lateral strike–slip faults are superimposed upon the Murga caldera and the eastern part of the Borovitsa caldera respectively. WNW–ESE to NW–SE extension along the horse-tail patterns was responsible for the ascent of pressurized magma through the crust via vertical open fractures leading to volcanic activity and subsequent emplacement of domes and crypto-domes during the fifth magmatic stage. This extension was followed by a compression with various orientations, which may be interpreted as a switch of the tensile stress to a compressive one at the onset of subsidence of the Dushka caldera. The last tectonic event in the area is characterized by a N–S extension that led to the reactivation of part of the previous structures, the formation of NW–SE to E–W normal faults and the emplacement of dykes and stocks during the sixth magmatic stage.