The Carpathian Mountains are an almost semicircular shaped mountain belt that is 1500km long and 50-150km wide. They curve through northeastern Romania, western Ukraine, southern Poland, and Czechoslovakia. The Carpathians are geographically divided into the West, East, and South Carpathians (Fig. 1 from Burchfiel, 1976). The Carpathians are also divided along the length. This division recognizes a discontinuous inner belt and a continuous outer belt that is present only in the West and East Carpathians (Fig. 2-from Royden, et al., 1982). The inner belt, which formed first, consists of mainly crystalline rocks, and the outer belt consists of flysch sequences (Foldvary, 1988).
|Figure 1. The Carpathian mountains are divided into the West, East, and South Carpathians.|
|Figure 2. The Carpathian mountains are divided into a discontinuous inner belt and a continuous outer belt that is not present in the South Carpathians.|
The Carpathians formed during the Cretaceous to Miocene as the result of continental collision of Europe with small continental fragments. Figure 3 (from Foldvary, 1988) shows the names of stages in the Cretaceous that will be used to describe the timing of events in the Carpathians. The continental collision was the result of subduction of oceanic and some continental crust that closed part of the Tethys Sea (Burchfiel, 1980).
|Figure 3. The Mesozoic timescale.|
Although formed by continental collision like the high Himalayas, the Carpathians do not compare in elevation. The highest point, Gerlachovsky Peak, is only 2,665m (Foldvary, 1988). Low elevation is one characteristic of a retreating subduction boundary. A retreating subduction boundary, such as the Carpathians, is a subduction boundary in which the rate of subduction is greater than the rate of convergence (Royden, 1993).
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Four main fragments, the Apulian, Rhodopian, Moesian, and North Dobrogean fragments, were recognized during the late Jurassic (Fig. 4-from Burchfiel, 1980). Both continental and oceanic crust composed all of these fragments except the North Dobrogean fragment.
|Figure 4. The positions of the fragments in the Jurassic. E=Europe, R=Russia, A=Apulian fragment, R=Rhodopian fragment, M=Moesian Fragment, and ND=North Dobrogean fragment.|
|Figure 5. Positions of the fragments during Albian time.|
In the Cenomanian, the convergence rate between the Rhodopian and Moesian fragments decreased while it increased in the northern area of the Apulian fragment. As the Apulian fragment moved north, both A-type (continental) and B-type (oceanic) subduction occurred. However, in the West Carpathians, there is no evidence for B-type subduction.
During the Coniacian, the Apulian and Rhodopian fragments collided. B-type subduction occurred first in this area and later changed to A-type subduction. Rotations also occurred at this time. The Apulian fragment rotated counterclockwise, and the Rhodopian fragment rotated clockwise as it moved around the Moesian fragment (Burchfiel, 1980).
By the late Cretaceous, there was no more convergence in the South Carpathians. Convergence in the East and West Carpathians was off and on from the Cretaceous to Paleocene. In the outer Carpathians, the last thrusting events occurred in the Miocene (Royden, et al 1982). By this time, the four fragments were about in the same positions they are today (Fig. 6-from Burchfiel, 1980).
|Figure 6. The present positions of the fragments. OR=oceanic crust remanents.|
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The Precambrian crystalline rocks in the Getic terrane consist of schists and gneisses that were metamorphosed to amphibolite grade. Sandstones, conglomerates, shales, and coal of Carboniferous age overlie the crystalline rocks. The Permian sequence is similar to the Carboniferous, but it is thicker and contains volcanic tuff instead of coal. Mesozoic rocks occur as shales, limestones, and marly limestones.
Although these two terranes contain rocks of the same ages, the entire Getic terrane was thrust over the Danubian terrane during the late Mesozoic. Evidence from overlapping crystalline rocks revealed eastward movement of about 60km of the Getic terrane. The Severin nappe, which consists of upper Mesozoic sedimentary rocks, was caught up in between the two terranes (Fig. 7-from Burchfiel, 1976).
|Figure 7. These drawings show the thrusting of the Getic terrane over the Danubian terrane.|
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In Romania, near the southern end of the East Carpathians, is an area referred to as the Vrancea region. This region experiences many earthquakes which originate at intermediate depths. By studying the earthquakes' origin times, epicenters, and focal depths, a "picture" of a seismic body that was 60km long, 30km wide, and 160km deep began to develop. The highest activity was between 60km and 160km. Low activity occurred between 30km and 60km, and no earthquakes occurred below 160km.
According to plate tectonic theory and the history of the region, this seismic body is a lithospheric plate that moved from the southeast and started to subduct about 18 million years ago. Roman calculated an average slip rate of 1.6cm/yr for the lithosphere and assumed this rate has been constant over time. If the above information is correct, then about 300km of lithosphere has been subducted. 160km of this lithosphere is still rigid enough to produce earthquakes while the rest of it has most likely melted. The positions of the P and T axes of the focal mechanisms reveal that the gravitational forces of the lithosphere causes the slip.
Besides the seismicity evidence, gravity anomalies and the presence of andesites in the area validate the existence of the sinking lithosphere (Roman, 1970).
Recently, two models have been proposed to explain the processes involved with the sinking slab. One model involves delamination of lower lithosphere to cause the sinking of the oceanic lithosphere to continue after break-off of the slab(Fig. 8-from Linzer, et al., 1998). The other model involves a subducting oceanic slab (Fig 9-from Girbacea, et al., 1998). I apologize for the appearance of these figures. Half of figure 8 would not work and they did not look this bad in photoshop.
|Figure 8. Delamination of the lower lithosphere. The level of delamination in (C.) is 70km, which was the same level that of break-off of the slab.|
|Figure 9. Subduction of the oceanic lithosphere (no delamination). Top cross section is from the present and the bottom section is from 6 million years ago.|
Paleomagnetic data from rocks in the outer belt of the West Carpathians yielded pole positions which are numbered in Fig. 10 (figure 10 is from Bucha, et al., 1994). On the same stereographic projection, the apparent polar wandering path (APWP) for Europe is plotted. The West Carpathians pole positions do not match the APWP for Europe, but they are similar to African pole positions. Therefore, the outer belt of the West Carpathians were geographically similar to the African plate than the European plate.
|Figure 10. The apparent polar wandering path for Europe. West Carpathian pole positions are also shown and they are numbered 1-5.|
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Bucha, V. and Blizkovsky, M. Crustal Structure of the Bohemian Massif and the West Carpathians. Springer-Verlag, Berlin, 1994.
Burchfiel, B. C., 1976. Geology of Romania: Geological Society of America Special Paper 158, 82p.
Burchfiel, B. C., 1980. Eastern Alpine System and the Carpathian Orocline as an Example of Collision Tectonics: Tectonophysics, v. 63, p. 31-62.
Foldvary, G. Z. Geology of the Carpathian Region. World Scientific, Singapore, 1988.
Girbacea, R. and Frisch, W., 1998. Slab in the Wrong Place; Lower Lithospheric Mantle Delamination in the Last Stage of the Eastern Carpathian Subduction Retreat. Geology (Boulder), v. 26, p. 611-614.
Linzer, H. G., Frisch, W., Zweigel, P., Girbacea, R., Hann, H. P., and Moser, F., 1998. Kinematic Evolution of the Romanian Carpathians. In: PANCARDI; the Lithospheric Structure and Evolution of the Pannonian/Carpathian/Dinaride Region. (eds Decker, K., Lillie, R., and Tomek, C.). Tectonophysics, p. 133-156.
Roman, C., 1970. Seismicity in Romania-Evidence for the Sinking Lithosphere: Nature, v. 228, p. 1176-1178.
Royden, Leigh H., 1993. The Tectonic Expression Slab Pull at Continental Convergent Boundaries. Tectonics, v. 12, p. 303-325.
Royden, Leigh H., Horvath, F., Burchfiel, B. C., 1982. Transform Faulting, Extension, and Subduction in the Carpathian Pannonian Region. Geological Society of America Bulletin, v. 93, p. 717-725.
Websites: Picture on left taken by Radu Mot from http://www.dumitrescu.com/me/countries/romania/encyclopedia/romania/tourism/carpati.html. Picture on right from http://www.carpathians.org/photo.htm.
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