Journal of the Virtual Explorer

The lower terrane is the Pangaion (also called Boz Dağ Unit) of the Greek literature [e.g. Kronberg et al., 1970; Kronberg and Raith, 1977]. It extends towards the NW, into Bulgaria, where it is called Pirin Unit. This terrane represents a microcontinent with a carbonate platform that cores the large antiform from the Rila Mountains to the island of Thassos through the Pirin and Pangaion mountains. Synmetamorphic thrusts, such as the pervasive and much studied Nestos (Meso-Rhodopean) Thrust Zone (Figs. 2 and 3) [Papanikolaou and Panagopoulos, 1981; Zachos and Dimadis, 1983; Gerdjikov and Milev, 2005], are responsible for regional metamorphic inversion, placing higher amphibolite-facies intermediate terranes (Mesta, Sideronero, Kerdylion) onto upper-greenschist to lower amphibolite-facies rocks of the structurally lower terrane [Papanikolaou and Panagopoulos, 1981; Mposkos, 1989]. The activity of such thrust zones is likely older than, and lasted until, the ca. 55 Ma, syn-folial yet late-deformation pegmatite veins, while a spread of younger ages, derived from a variety of geochronological methods, refers to lower-grade reactivation and/or fluid circulation [Bosse et al., 2009].

In this review, and in the light of protolith ages, we additionally attribute to the lower terrane the string of four separated domes. Those are, from northwest to southeast, the Chepinska, Arda, Kesebir and Biela Reka "units" in Bulgaria; the last two are called Kardamos and Kechros in Greece (Fig. 5). These domes expose monotonous, quartzo-feldspatic, strongly deformed gneiss of dioritic composition intruded by metagranitoids, some of which are presumably syntectonic. This interpretation would define the Nestos Thrust Zone (Fig. 2) as the ramp where the imbricates climb up the footwall sequence from the deeper orthogneissic levels, to the north, over the carbonate platform, to the south.

Lithological content - Metamorphism

The tectonostratigraphy of the lower terrane defined by the Thassos-Pangaion-Pirin half-window comprises from bottom to top i) a unit of schists underlying ii) pre-eminent marbles, iii) leucocratic orthogneiss and paragneisses, and iv) an upper unit of micaschists, amphibolites and thin intercalations of marbles [Meyer et al., 1963; Birk et al., 1970; Jacobshagen, 1986]. The lower-terrane is particularly well identified in the Pangaion area where marbles are involved in a hinterland-dipping thrust system [Kilias and Mountrakis, 1990]. Inverted, intermediate-pressure metamorphism evolves from greenschist-facies conditions in the Pangaion [Fig. 3, Zachos and Dimadis, 1983] to sillimanite-bearing migmatites to the north, against the Nestos Thrust [Mposkos et al., 1989]. Peak metamorphic conditions recorded in metapelites of the lower schists, in the Thassos Island, reached ca. 600-680°C for 0.6-0.8 GPa [Dimitriadis, 1989; Schulz, 1992].

The migmatite-gneiss sequence that cores the four northern domes is placed, in this review, below the tectonostratigraphy listed for the Thassos-Pangaion-Pirin antiform (Figs. 3 and 7). Consistent with this interpretation, minor marble and amphibolites occur at the top of this deeper migmatite-gneiss sequence. Evidence for high-pressure metamorphism [Mposkos and Liati, 1993; e.g. 450°C-1.3 GPa, Macheva, 1998] is rare. Leucosomes due to partial melting of the gneiss are common and more pervasive northwestward. In effect, the regional amphibolite-facies metamorphic grade is usually lower in the Biela-Reka--Kechros dome [lower amphibolite-facies, ca 550°C-0.6 GPa, Macheva, 1998] than in the central (Arda) and western (Chepinska) regions [higher amphibolite-facies and migmatites at ca. 650°C-0.7 GPa, Cherneva and Georgieva, 2007].

Figure 7. Cross section across the Pirin Mountains, located in Fig. 2.

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See also Moriceau [2000], Burchfiel et al. [2003] and Georgiev et al. [2010].