The structure of Mesta volcanic massif. Part I. Structural features of the volcanics in Mesta graben

Abstract

Mesta Paleogene volcanic massif (SW Bulgaria) is a complex synsedimentary structure. Its lava rocks form numerous endogenous domes and subvolcanic bodies that are components of two polygonal cauldrons and two linear volcano-tectonic zones. The related pyro- and epiclastics interager laterally with a 1500-2000 m thick terrigenous Upper Eocene-Oligocene section. The volcanics are silicic-rhyodacites and dacites (with very rare transitions to latites). According to Na2O+K2O/SiO2 they are transitional and normal, and according to K2O/SiO2 the rocks belong to the HKCA and SH series. Their structural anisotropy is defined by planar and linear flow structures of diverse morphology and petrography. Flow layers of different colour, texture and composition are widespread. Part of the structures (magmatic boudinage, magmatic folds and magmatic cleavage) resulted from synmagmatic deformations of the already layered but still not solidified melts. Magmatic folds are represented by mesoscopic folds and by crenulations on flow layers surfaces. Magmatic cleavage corresponds to parallel and convergent axial-plane cleavage, the cleavage planes being cemented during the cooling of the melts. Jointing in the magmatic bodies is controlled by their facies. Block, prismatic and platy jointing as well as various transitional types have been observed. A vertical zoning in the distribution of flow structures and jointing types was inferred. The air fall tuffs show parallel bedding while base surge and pyroclastics surge deposits are characterized by cross-bedding. The jointing in pyroclastics is a combination of regional jointing and jointing due to local syn- and post magmatic faulting.