Hypothesis of plate tectonic and origin of the Carpathinas
E. Argand (1922) was the first who applied the Wegenerian mobilistic concept to the origin of the Mediterranean ranges. He regarded the Alps and other mountains of southern Europe as formed by collision of two continents, Eurasia and Africa. J. F. Dewey and J. M. Bird (1970) followed this line of thought with modifications implied by the plate tectonics hypothesis. To explain the complex Mediterranean mountainous system they introduced the concept of „microcontinents” and „island arcs” situated in the Tethys Ocean between the two continents. M. Bleahu and al. (1973) compare the present Carpathians with an island arc: the Sub-Carpathian foredeep corresponds to the outer trench, the Carpathians to the folded arc with an inner arc of calc-alkaline volcanic rocks, the Pannonian and Transylvanian basins are to represent retro-arc basins. This comparison is not complete: at present, there is no ocean on the outward side of the trench, its crust was entirely subducted and consumed. The calc-alkaline magma was produced by melting of this crust, deeply subducted beneath the Carpathians along the Benioff plane. The subduction is still active in the bend of Southern Carpathians, as shown by intermediate earthquakes in the Vrancea region (Roman, 1970). It should, however, be noted that the epicentral area is located on the very border of the Carpathians, the sinking slab is very steep, and neither the Tertiary folding nor volcanism can be attributed to the subduction of this slab. Some authors presume that the outer Carpathian flysch was deposited partly or totally on the crust of oceanic type. According to D. P. Radulescu and M. Sandulescu (1973), in the southern marginal region of the Eurasian iplate two rift zones formed: in the inner zone the ophiolite series of Late Jurassic and Early Cretaceous age was formed (the Metalliferous Mts.), while in the outer zone the Sinaia flysch was deposited. Most of the flysch zone 'in the Rumanian Carpathians was to be deposited on the sialic crust. The assumption of an oceanic crust beneath the Sinaia zone is based only on the rare occurrences of basic rocks in the Sinaia flysch. Moreover, from the inner zone are also reported (M. Andelkovic & M. Lupu, 1967, table) crystalline schists and presumably Palaeozoic rocks (1. c., p. 15), which seem to indicate the presence of a sialic basement in this zone. N. He r z and H. Sa vu (1974) assume that the crustal distension commenced in the Jurassic, and eventually a wide belt with oceanic crust, called „Siret Ocean” formed between the Moldavian platform and Pannonia. In this ocean the outer flysch was laid down. Basic rocks occurring in the Upper Jurassic — Neocomian „black flysch” and other inner regions of the flysch zone are regarded as indicators of the simatic crust beneath the Siret Ocean. The subduction and melting of this crust provoked the Neogene andesitic eruptivity on the inner side of the Carpathian range. According to W. Sikora (1976) the crust beneath the outer flysch basin at first was sialic, but during the Jurassic-Tertiary times it underwent both rifting and basification. Rifting started in the Early Cretaceous. It is marked by the pre-Albian emplacement of teschenites in the Western Carpathians. Owing to this rifting a few troughs were formed, each with the crust of „oceanic” type, separated by sialic blocks, which acted as Cordilleras, reconstructed on the basis of an ample palaeogeographical evidence (M. Książkiewicz, 1965). In the western sector of the flysch zone there existed at least four cordilleras. If the occurrence of the teschenites actually indicates an incipient rifting, further stages of distension were not marked by any extrusions of basic rocks. An occurrence of a rock found by the present .author in the Middle Eocene conglomerates at Osielec, determined by T. Wieser (1952) as „ophiolite”, is taken by Sikora as a proof that the Magura flysch was deposited on a simatic crust. However, later T. Wieser (1967, p. 18) regards this rock to be a gabbro-diorite „transformed to prasinitic amphibolites during the batholitic stage of probably Variscan diastrophism”. Several authors presume the beds of the Pieniny Klippen belt were deposited on the crust of oceanic type (J. F. Dewey et al., 1973, D. P. Radulescu & M. Sandulescu, 1973, R. Ney, 1976). According to Sandulescu (1974, Fig. 17) the sedimentation area of the Klippen belt was situated in the prolongation of the simatic crust, on which the beds of the Metalliferous Mts. and the Transylvanian nappes were deposited ( = inner intra-continental basin with ocean-type floor of D. P. Radulescu & M. Sandulescu, 1973). Also A. Grubić (1974) assumes an oceanic crust beneath the sediments of the Klippen belt and links it with the Sinaia zone, the northern Balkan and the Black Sea in the east, and with the Pennicum in the west. He calls „Mesoparatethys” the thus reconstructed „palaeomicrooceam”, separated from the „Mesotethys” by the Serbo-Macedonian massif and the Getic zone (Fig. 1). However, the composition of the Transylvanian nappes, with well developed Triassic beds and the Stramberg limestone in the Upper Malm, is hardly comparable with the Klippen belt, and that of the Metalliferous Mts. even less. The linking of the Sinaia zone with the Klippen zone is not justified not only because of a quite different composition but also because of their tectonic position: the Sinaia zone is situated on the outer side of the crystalline Marmaros zone, the Klippen belt on its inner side. If the beds of the discussed zone were to be deposited in a quasi-oceanic deep trough, they should exhibit more uniformity and a more accentuated deep-water character. There are several features of the Klippen belt which contradict, or, at any rate, do not support the concept of an oceanic crust beneath its sedimentary basin. Ophiolites are lacking, amid volcanic rocks are extremely rare. They occur in the most eastern part of the belt, tin the basin of the Teresva river, where small occurences of andesites, basaltic porphyrities and andesitic basalts of Upper Jurassic age are reported (M. O. Lomize, 1968). In the klippes of Poiana basic „cinerites” occur (M. Sandulescu, 1975). In the Polish part of the Klippen belt andesitic tuffs of Upper Cretaceous age were found (W. Sikora, 1972). The extrusion of basic magma does not necessarily indicate an oceanic crust below the basin. In the High-Tatra unit there occur Upper Jurassic limburgites which had to penetrate across the continental crust, at least 30 km thick. The crystalline Marmaros zone is also pierced by volcanic rocks. On either side of the basin, in which sediments of the Klippen belt were laid down, there existed zones composed of sialic rocks. The northern zone is indicated by pebbles of granite-gneisses, gneisses, porphyries and aplites occurring in the Bathonian of the Czorsztyn suite (K. Birkenmajer et al. 1960), while the southern rim of the basin supplied many pebbles of only sialic rocks to the Upper Cretaceous and Palaeogene sediments (D. Andrusov, 1938, T. Wieser, 1958). This, of course, does not preclude the existence of the oceanic crust between the two sialic zones (cordilleras). However, the character of sediments of the Klippen belt does not essentially differ from that of the High-Tatra and Sub-Tatra units, the sediments of which were laid down on the sialic basement. This suggests that the Klippen sediments were deposited at very much the same depths as the sediments of the inner region of the Western Carpathians, laid down on a submerged „microcontinent” linked with that of the Camic plate of J. F. Dewey and al. (1973), since the older views, that the inner Carpathian units correspond with the East-alpine nappes, are still maintained (A. Tollmann, 1969). The complex tectonics of the Pieniny Klippen belt may suggest that it formed above a subduction zone. For this reason E. Szadecky-Kardoss (1973) regards the Klippen belt as a crock mélange of a subduction zone. The use of the term „mélange” in this case is misleading, as it is currently being applied to the mixture of sedimentary and ultrabasic rocks. The last are absent in the Klippen belt. The complex imbricated structure of the belt might have been formed by underthrusting of a sialic block below another (M. Książkiewicz, 1972, Fig. 39, R. Ney , 1975, Fig. 5). This is the subduction sensu A. Amstutz , 1955, not the subduction of an oceanic crust below a continental block. South of the Slovakian platform there lies the Pannonian area, within which E. Szadecky-Kardoss (1973, 1976) reconstructed three eugeosynclinal belts, each with the presumably oceanic crust and each associated with an andesitic zone. Actually, only in the Mecsek region there occur basic rocks in some strength. If the discussed views are justified, north of the Dinaric accretion zone there would have been at least six belts with simatic crust (Fig. 1) and the same number of subduction zones (Fig. 2). If each subduction zone had its andesites, produced according to current views in the subduction zone at a depth of about 150 km, the width of the Tethys between the Mid-Tethysian Ridge and the edge of the Eurasian continent would have been at least 2000 km (about 900 km of the belts with oceanic crust and at least 1000 km of intervening microcontinents and sialic swells). M. Boccaletti and al. (1973) presume that the flysch basin was a marginal basin. W. Sikora (1976) also assumes that a part of the flysch basin was marginal. The presumed Carpathian marginal sea was bordered from one side by the continent, and on the other side by subsiding platforms („microplates” or „microcontinents”) which formed an extension of the Carnic plate of J. F. Dewey and al. (1973), i. e. the Slovakian platform in the west and the Traniscarpathian microplate in the east, neither of them comparable to the present-day island arcs. It appears that the actualistic model (marginal sea bordered, by an island arc on one side) cannot be applied to such a situation. One may envisage that the Silesian cordillera, with its extension eastward (which is not very clear, either the Marmaros zone, or the Coumanian cordillera, as presumed by M. Sandulescu, 1975) was an island arc, similarly as the Habkern massif in the Alps (K. J. Hsu, 1971b). If so, the cordillera was bordered on either side by trenches with flysch sedimentation, and there are no indications that the cordillera was the site, of volcanic activity. The concept of the back-arc basin was applied by M. Boccaletti and al. (1974) to the Balkanids. According to their interpretation the Balkanid geosyncline formed on a continental crust. The accreting plate margin was situated in the Dinaric area (J. Dercourt , 1970), where probably an actively spreading ridge existed and the Tethys Ocean began to open. This is marked by extrusions of basic and ultrabasic magmas, which began in the Trias (Z. Besić, 1970) and continued well into the Jurassic. With the spreading was associated the subduction of the oceanic crust in the Vardar zone under the continental crust of the Rhodope massif and this caused warping of the continental crust north of the massif with the formation of troughs and swells. In the Upper Cretaceous the descending oceanic crust reached the depths at which it was partly melted. This produced intrusions and extrusions in the Rhodope and Sredna Gora, while further influence exerted by the descending crust caused folding and some thrusting north of the Rhodope, with polarity towards the Moesian platform (Fig. 3A). In this way the Balkanids were formed on the crust of continental type, as a back-arc belt. Absence of ophiolites and rare occurrence of radiolarites are given in support of this interpretation. It is tentatively presumed that the interpretation of Boccaletti and al. (1974), with considerable modifications, may be extended to the Carpathian area (Fig. 3B). The accretion and spreading in the Dinaric area (J. Dercourt, 1970, A. G. Smith, 1971, M. & M. Dimitrijevic, 1973) initiated an ocean with the simatic crust and rafted sialic blocks of the Drina-Pelagonian element and the „Horst-and-Graben” belt. This incipient ocean had probably a gulf indicated by the ophiolites of the Metalliferous Mts, which, however, did not extend to the north-west. The northward spreading drifted the continental crust away from the accretion zone. The drag exerted on the continental crust caused its warping. Several undulations were formed, which during the Jurassic and Cretaceous controlled the distribution of facies: on the highs more shallow-water deposits were laid down (High-Tatra suite, Czorsztyn suite etc.), while in troughs deep-water sediments formed (Pieniny suite, Sub-Tatra suite, Gemerides, Bakony). In the Balkanid area the subduction of the oceanic crust followed a fairly steep plane beneath the Rhodope and therefore produced the Upper Cretaceous calc-alkaline volcanism in the Balkanids. Such a voleanism of that age is only very weakly developed in the Carpathian area. Therefore one may presume that within the Carpathian area the subducted crust followed a nearly horizontal iplane and did not reach depths necessary for generation of magmas on any large scale. Possibly the Andine models of lithospheric underthrusting may serve for comparison: according to L. R. Sykes (1972) in the northern Andes the subducted oceanic plate was thrust down at a very low angle, while in the Chilean sector it followed a much steeper angle. The first case may be applied to the Carpathians, the second to the Balkan area. If the subduction was caused by convection currents, one may suppose that at some distance from the accretion zone the current pulled the crust down and formed an intensely subsiding basin, in which the outer flysch, at least 6000 m thick, was laid down. Beneath the southern flank of the basin the oceanic crust might have descended to such depths that some melting took place producing a few andesitic, admittedly submarine eruptions in the Pieniny and Magura zones (W. Sikora, 1972, 1976). The difference in plate motion in the Balkanid and Carpathian area may be explained with the presumption, that a fault or a fault zone was created between them. It is possible that the Mures fault represents a fragment otf this great fault rotated clockwise during the final folding of the Carpathians into the present position. The spreading movement probably stopped at the end of the Lower Cretaceous. It was followed by a compressional stage towards the end of the Cretaceous, which led to the formation of nappes in the inner zone of the Carpathians. This compression most probably was due to the movement of the African plate, which separated from America and counterclockwise rotated (J. F. Dewey et al., 1973) exerted pressure on the eastern part of the Mediterranean region. There are no indications that the folding of that time in the Carpathian area was due to a subduction. Neither there are calc-alkaline rocks which may be attributed to that phase. During the compressional phase the sialic crust of the outer flysch basin was probably broken into blocks, some of which were -uplifted above the sea-level and started to function as cordilleras, supplying immense amounts of material to the flysch basin. W. Sikora (1976) presumes that the crust below the outer flysch basin underwent basification. According to R. Trümpy (1971, 1975a), in the Alps replacement of the lower part of the crust by denser matter is more probable than the mechanism of ocean floor spreading of the Atlantic type. He terms the crust with reduced „granitic” layer ,,para-oceanic” crust, which certainly is better than the misleading term ,,sub- -oceanic” crust used by other authors. The formation of the para-oceanic crust in the Alpine area is indicated, according to Trümpy, by deep-sea sediments, in some instances resting directly on serpentinites, the presence of ophiolites and intensity of folding. Only the first argument may be used in the case of the Carpathian flysch basin, since it attained, although transiently, considerable depths (L. Koszarski & K. Żytko, 1965, M. Książkiewicz, 1975). Ophiolites are absent in the flysch zone, and the crystalline basement, unlike in the Alps, was not involved in the thrust movements: the flysch nappes were stripped off from it. Even if reduced in thickness by oceanization or subcrustal erosion, the crust beneath the flysch basin was of „granitic” type to the very end of the flysch sedimentation. This is indicated toy immense amount of detrital sialic material present in all members of the flysch. No doubt, some of this material might have been supplied by sources situated outside the flysch basin, but the palaeocurrent analysis points clearly to important intrageosynclinal sources (M. Książkiewicz, 1960, 1962, M. & C. Dumitriu, 1968). During the folding huge 'blocks of granitic rocks were torn away from the basement iby thrusting, as the case is in the outer Klippen zone (M. Książkiewicz 1935). The presence of calc-alkaline volcanic rocks on the inner side of the Carpathians constitutes the strongest argument in favour of the presumption that the flysch ibasin was underlain by an oceanic crust. Since the experiments of D. H. Green and A. E. Ringwood (1966) it has been believed that calc-alkaline magmas are generated by selective melting of eclogites, into which basaltic crust may be converted in deeply subducted zones. Also partial melting of (basalts, if they are sufficiently „wet” or covered by sediments, may produce these magmas. The mechanism of underthrusting of the continental crust from the north and its melting (H. Stille , 1953) presents several difficulties. It is doubtful whether the sialic crust might have reached sufficient depths for melting. In addition, the basement of the flysch zone consisted mostly, as shown by the composition of exotic pebbles, of dehydrated metamorphic rocks, which, when melted, were unable to produce explosive andesitic magmas. These difficulties may be left out, if we assume that the oceanic crust of the flysch basin was subducted beneath the inner part of the Carpathians and there partly melted. However, as shown above, it is extremely difficult to prove the existence of such a crust on the outer side of the volcanic arc. Therefore other solutions should be sought. Tentatively, the following interpretation is proposed. The underthrust of the continental crust which existed beneath the flysch basin, dislodged the oceanic crust which during the spreading (period had reached the southern flank of the flysch basin (Fig. 3B), broke it into slabs and pushed it below the „microplate” of the inner Carpathians, i.e, the Slovakian block with the Tatra region in the west and the Marmaros zone in the east. Under the push of the underthrusitiinig some slaibs reached the depths at which they might have been partly melted (Fig. 5). In this way the origin of the inner Carpathian volcanic arc may be connected with the idea of underthrusting. The concept of underthrusting has been developed by several authors beginning with L. Mrazec and I. Popescu-Voitesti (1914). Using the modern expression, one may call this movement „subduction”. The subduction of the European foreland is also presumed in the Alps, where the most intense gravity deficit is attributed to this subduction (R. Trümpy, 1975b). In the Western Carpathians the zone of the lowest gravity values is situated close to the Pieniny Klappen belt, where downbuckling of the crust has been found by deep seismic soundings (J. Uchman, 1973). Most probably, this downbuckling is due to the underthrustmg. The discussed underthrusting movement, in all probability was linked with the motion of the Eurasian plate, which split from the North America drifted south-east for a considerable distance (M. Talwani & W. C. Pitman, 1972). Owing to this motion and the counterclockwise rotation of Africa (which started in the Eocene, A. G. Smith , 1971) converging movements acted in the zone lying between these two lithospheric plates (J. Dewey et al., 1973). These movements caused the final compression in the Carpathian area. It follows from the preceding remarks that it is very difficult in a simple way to apply the hypothesis of plate tectonics to the problem of the origin of the Carpathians. No actualistic models, which, incidently, seem to be simplified in many respects, as underlined by M. Lemoine (1973), may be applied here without objections. It seems that the origin of the Carpathians did not invoke the disappearance of the oceanic crust. The Carpathians may be interpreted as formed on the site of ensialic basins.
