Bulgaria is in southeastern Europe between reasonably well-described areas of predominantly Alpine crustal deformation on the north and northwest (Carpathians in Romania, Hungary, and Czechoslovakia; Alps in Austria), on the east (Pontides and Taurides in Turkey), and on the west and south (Dinaric Alps in Yugoslavia; Pindos Mountains in Greece) but has not been well described in easily available literature.

There are three major east-west trending morphotectonic units. The low-relief Moesian platform of northern Bulgaria and southern Romania owes its heritage to Hercynian deformation. It is mantled by flat-lying, shelf-type Mesozoic and Tertiary sedimentary (mostly carbonate) rocks. Along its southern boundary a foredeep developed during Jurassic-Cretaceous time in which thick flysch-like sediments accumulated. There is small oil and gas production. The Lorn basin, North Bulgarian swell, and Varna trough are other major structural elements within the platform.

The Rhodope massif of southern Bulgaria and northern Greece is a rugged mountainous terrain of Precambrian and early Paleozoic crystalline rocks. It is a fragment of a once much larger crustal block that began to break up in the Paleozoic and which has experienced uplift of 2,000 m in the Pliocene-Pleistocene.

Between the Rhodope and Moesian crustal blocks the narrow (10-20 km) Balkan Mountains (Stara Planina zone) consist of tightly folded and metamorphosed Paleozoic sedimentary rocks that have been deformed recurrently during the Hercynian and Alpine orogenies, culminating in 3 to 4 km of vertical uplift since the Miocene. The sub-Balkan fault bounds the south side of the Balkans and is at many places a spectacular scarp.

The Balkanide zone has been compressively molded between the large, relatively stable Rhodope massif and Moesian platform. Locally, north-moving gravitational glide masses lie on the platform but there has been minimal crustal shortening.

A fourth morphotectonic unit, the Kraishtide zone, trends northwest through western Bulgaria and into adjacent Yugoslavia. This rugged area, 15-60 km wide, is a megashear zone within which both right-lateral strike-slip and vertical movements have been common since the Paleozoic. It parallels the better known Vardar zone of Yugoslavia.

In addition to the long-lived, deep-seated faults that parallel the trend—and which are the boundary for some—of these four crustal units, a northeast-southwest fault system (Tvarditsa and Etropole), further breaks the Bulgarian crust into a giant block mosaic. Predominantly vertical movement along the major faults alternately has elevated or depressed individual blocks of the mosaic and has influenced profoundly the location and the character of sedimentation, igneous activity, occurrence of mineral deposits, and erosion.

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