Multiply deformed foreland fold-thrust belt of the Balkan orogen The multiply deformed foreland fold-thrust belt of the Balkan orogen, northern Bulgaria B.C. Burchfi el1 and Radoslav Nakov2 1Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA 2Institute of Geology, Bulgarian Academy of Sciences, G. Bonchev str. Block 24, 1113 Sofi a, Bulgaria ABSTRACT opment of extensional basins and abundant ation to the east in the Intrapontide zone, which magmatism due to trench rollback. The time contains the remnants of a complex assemblage The generally east-west–trending Balkan of the fi nal foreland fold-thrust belt defor- of Mesozoic and early Cenozoic oceanic frag- orogen (eastern Europe) consists of a north- mation was late Eocene extending into Oli- ments (Fig. 2; e.g., Okay et al., 2001a, 2001b; ern belt of folded and thrusted Mesozoic gocene or early Miocene, contemporaneous Robertson et al., 2004). The main part of the and Cenozoic strata that forms the external with the extension to the south. The deforma- Balkan orogen within Bulgaria consists of a fold-thrust belt of late Mesozoic and early tion within the fold-thrust belt was caused northern belt of folded and thrusted Mesozoic Cenozoic age, and a southern belt that con- by a transfer of transpressional right shear and Cenozoic strata that forms the external fold- sists of deformed igneous and metamorphic within north Bulgaria and the Southern thrust belt of late Mesozoic and early Cenozoic rocks overprinted by Cenozoic extensional Carpathians as crustal units were translated age and a southern part that consists of deformed basins. Unlike most foreland fold-thrust northward west of the Moesian foreland igneous and metamorphic rocks overprinted by belts, wherein deformation commonly crust and moved northeast and eastward into Cenozoic extensional basins (Figs. 2 and 3). The migrates toward the foreland, the fold-thrust the eastern Carpathian west-dipping sub- fold-thrust belt is exposed within the Fore balkan, belt within the Balkan orogen is marginal to duction zone. During the third event of defor- Stara Planina, and the northern part of the Sredna the Moesian Platform to the north, but was mation crustal units were molded around Gora topographic and tectonic units and is the deformed in at least three events related the Moesian foreland crust. The shortening main focus of this paper (Fig. 3). Although to three different dynamic systems caused ceased by early Miocene time and the right Stara Planina means the “old mountains,” it is a by changes in plate interactions. The earli- shear west of Moesian foreland crust was misnomer, as it is fl anked on the south by the est event of late-Early to early-Late Creta- manifested by discrete right-slip faults to late Cenozoic to active extensional Sub-Balkan ceous deformed strata deposited within the the present. During this third event southern graben system, which is mostly responsible for Moesian continental margin and within a Bulgaria was in an extensional regime that its elevation (Tzankov et al., 1996; Roy et al., continental rifted belt containing deep-water dominated the south- to southwest-vergent 1996). Within the Sredna Gora to the south are fl ysch of Late Jurassic–Early Cretaceous age, Hellenide orogen throughout the Cenozoic, the remnants of a Late Cretaceous backarc or a probable eastward extension of oceanic thus dividing the Balkan orogen into two dif- intraarc basin that has also been deformed; the troughs from the Southern Carpathians. The ferent deformational regions. structures are well exposed in the west, but are shortening was a consequence of south or largely covered by younger strata to the east. southwest synthetic subduction within the INTRODUCTION Although these rocks are folded and thrusted, Vardar zone along the southern margin of this belt of structures is generally not considered the Balkan orogen. In Late Cretaceous time The Balkan orogen within Bulgaria and to be part of the external fold-thrust belt; how- a backarc and/or intraarc rift zone developed northern Greece is the eastern continuation of ever, we regard it as part of external fold-thrust along the southern margin of the fold belt, the European Mesozoic and Cenozoic Alpide belt (discussed herein). Farther south the rocks terminating shortening. The backarc and/or folded chain. Within Bulgaria, northern Greece, within the Rhodope Mountains and their contin- intraarc basin closed in Late Cretaceous– and parts of Serbia and Macedonia to the west, uation into Greece to the south consist of a com- early Paleocene time, deforming the fold- the orogen consists of topographic units that plex of Precambrian(?), Paleozoic, Mesozoic, thrust belt for a second time, but antitheti- are in part a refl ection of their underlying geol- and early Cenozoic rocks metamorphosed and cally to north or northeast subduction in the ogy (Figs. 1–3; e.g., see Boyanov et al., 1989; intruded by abundant magmatic rocks of Meso- Vardar zone. North- and northwest-vergent Dabovski et al., 2002; Zagorchev et al., 2009). zoic and Cenozoic age (for a review see Burg, subduction within the Vardar zone caused The Balkan orogen is bounded on the north by 2012). They were deformed principally during magmatism, metamorphism, and deforma- the Moesian plain, which is underlain by a base- Mesozoic and early Cenozoic shortening events, tion within the Rhodope area of southern ment of Precambrian and Paleozoic rocks that and beginning in about middle to early-late Bulgaria south of the foreland thrust belt. In were largely undeformed during alpine time, Eocene time (Burchfi el et al., 2003), extensional Paleogene time the southern part of the Bal- and by the Vardar zone (sensu lato) on the west tec tonism became the dominant mode of defor- kan orogen became extensional with devel- and south (Schmid et al., 2008) and its continu- mation within central and southern Bulgaria Geosphere; April 2015; v. 11; no. 2; p. 463–490; doi:10.1130/GES01020.1; 17 fi gures. Received 24 December 2013 ♦ Revision received 6 November 2014 ♦ Accepted 23 December 2014 ♦ Published online 27 February 2015 For permissionGeosphere, to copy, contact April [email protected] 2015 463 © 2015 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/11/2/463/3336828/463.pdf by guest on 02 October 2021 Burchfi el and Nakov ROMANIA SERBIA Fore Black Moesian Plain Sea Forebalkan Stara SW Planina KOSOVO Black Sea Mts Sredna Gora Va Strandja Bulgarian BULGARIA rdar West Mountains Rhodopian East MACEDONIA Mountains Zone Ver GREECE Thrace TURKEY Mountainstiskos ALBANIA Zone 0 100 km Intrapontide Aegean Sea Figure 1. Topographic elements of the Balkan mountain chain in the southern Balkans. These topographic fea- tures are also an expression of the underlying geological units of the Balkan orogen shown in Figure 2. Dashed lines show the eastern and northern boundaries of the Vardar and Intrapontide zones, respectively. with the formation of numerous extensional the tectonics of Bulgaria (Yovchev, 1971; geo- FORELAND FOLD-THRUST BELT basins accompanied by abundant magmatic logical maps of Bulgaria at scales of 1:100,000 OF THE BALKAN OROGEN activity (Ivanov, 1988; Bonev and Beccaletto, [Cheshitev, 1990–1995] and 1:500,000 [Cheshi- 2007; Burchfi el et al., 2008). tev and Kancev, 1989]). Our reconstructions The Mesozoic and Cenozoic foreland fold- The fi rst description of the tectonics of Bul- concerning Bulgarian territory are based on thrust belt of the Balkan orogen is best exposed garia was by Cvijic (1904) in his studies on the these works as well as our own fi eld work and within the Forebalkan and Stara Planina units structure of the Balkan Peninsula. From south to fi eld trips led by numerous Bulgarian geologists and to a lesser degree within the western part north Cvijic (1904) distinguished the Rhodope to many parts of the orogen. of the Sredna Gora unit (Fig. 3). Rocks form- Massif, Transitional zone, Balkan system, and The main focus of this paper is the fold-thrust ing the fold-thrust belt are of Mesozoic and Balkan plate (Moesian Platform in modern belt that is exposed within the Forebalkan, Stara Cenozoic age and were deposited along the terminology). St. Bončev (1936) introduced Planina, and part of the Sredna Gora topographic southern part of the Moesian shelf, the southern the Balkanides as an orogenic system and sub- and structural units (Fig. 3). This fold-thrust belt margin of the European continental crust. The divided it into three main units, from north to has the unusual characteristic of having been fold-thrust belt also involves rocks deformed, south, the Forebalkan, Balkan, and Srednogorie; deformed in at least three superposed different metamorphosed, and intruded by plutons in he assigned the Moesian Platform to the north events of different ages, and deformation did not pre-Mesozoic time that formed the crust below to the foreland of the orogen. The Srednogorie migrate from its internal to external parts, as in the Mesozoic Moesian shelf strata. Triassic to was accepted as the oldest tectonic element many fold-thrust belts. In addition, it is necessary Middle Jurassic shallow-water deposits form and in the strictest sense did not belong to the to try to relate these three deformational events to the base of the Moesian Alpine succession and Balkanides. The boundaries between the units orogen-wide processes. For at least the two older unconformably overlie pre-Mesozoic basement within the orogen were traced and interpreted in deformations (late-Early Cretaceous to early- rocks; both sequences are exposed within the different ways based on different criteria (e.g., Late Cretaceous and latest Cretaceous–early foreland fold-thrust belt and to the south in cen- see Yovchev, 1971; Bonchev, 1986; Gocev, Paleogene) the dynamics are related to activity tral Bulgaria (Fig.