Widespread extension in the Rhodope region from Late Eocene-Miocene times resulted in the development of a series of basins throughout the region. The basin infills provide a record of the nature of deformation in the area and allow the related tectono-magmatic-sedimentary evolution to be reconstructed.
The Mesta Basin in SW Bulgaria, located between the Pirin and Rhodope massifs, is bounded to the E by a N-S-trending detachment fault. Basin fill comprises an up to 3-km-thick succession including alluvial, volcaniclastic and volcanic units. The relationships between the various units, together with their relationship with the subsequently intruded acidic subvolcanic bodies, are controversial. Previous studies have suggested that the progressive lessening of stratigraphic dip from the base to the top of the succession is related to synsedimentary rotation (i.e. half-graben model), supporting the idea that crustal extension controlled the evolution of the basin.
Recent analysis of a key transverse regional section, together with supporting data from the entire Palaeogene succession, suggests a 3-stage development for the basin. An initial alluvial setting in an axial through-drainage half-graben was followed by a mixed alluvial-volcaniclastic environment. This latter setting commenced with the formation of volcanic centres (e.g. caldera structures in the S of the basin), related to a stage of rapid infilling of the previous basin topography, and was succeeded by a phase of waning volcanic activity, characterized by alternating alluvial and volcaniclastic depositional cycles. Facies architecture in the basin was influenced by the interaction between the interdigitating basin margin fans and the fluvial through-drainage system, compartmentalization by intrabasinal faults, and the relative distance to the volcanic centres.
The nature of the succession in the Mesta Basin indicates that a substantial amount of extension occurred prior to the onset of volcanic activity. Basin development comprised an initial phase of rapid extension and resultant creation of accommodation space, accompanied solely by alluvial sedimentation. This initial setting ended with the onset of volcanic activity in response to crustal thinning and coincided with the infilling of the basin with volcanogenic material. The tectono-sedimentary development of the Mesta Basin is a useful analogue for the evolution of related basins in the Aegean region.
