Kordyliery Karpat Zachodnich w świetle tektoniki płyt litosfery

Wacław Józef Sikora



Cordilleres supplying large quantities of terrigenous material to the geosyncline of the external Carpathians appeared not before the Early Cretaceous. Their presence is reflected by a specific distribution of directions of transportation for the Flysch sea, occurrence of subaqueous slumps and exotics of different size. The area on which the external Carpathian geosyncline developed hi the Triassic retained its nature of polygenic craton consolidated during Baikalian, Caledonian and Hercynian tectonic epochs till the Triassic. The process of formation of the geosyncline of the external Carpathians lead to progressive deepening of the sea -which achieved maximum depth in the Carpian (Cenomanian - Early Eocene) times. As there is a fairly close correlation between the hypsometry and the type of Earth crust it may be regarded as proved that the Earth crust underneath the geosyncline became progressively thinner, gaining the character of subcontinental or suboceanic or, possibly, even oceanic crust. The mechanism of thinning out of the crust is still insufficiently known. The simplest model assumes tension ad formation of a few continental rifts more or less parallel to one another and subsequently undergoing transformation into troughs with crust (? of oceanic type) highly reduced in thickness. Areas separating the rifts give rise to elevations (cordilleras) separating the troughs. It follows from that model of formation of cordilleras that their width may be very larger, sometimes even larger than that of Flysch troughs. The predominance of compressional conditions, evidenced by the onset of acid magmatism in zones of subduction started in the geosyncline of the external Carpathians in the Cenomanian. The reduction of the width of the geosyncline also started at that moment. The mechanism of that reduction of width also remains not clear. Picotites found in the Flysch evidence the action obduction processes accompanied by formation of ophiolite melange (sensu Gansser). In the course of action of the obduction processes both the fragments of the upper mantle and the catazonal parts of granite layer of the crust were emerged above sea surfaces. The former include peridotites, and the latter - granulite exotics. The simple model of plate tectonics cannot be used for reconstruction of the tectogenesis of the external Carpathians. There were several zones of subduction in the geosyncline of the external Carpathians but their nature appears different from that of the zones known from the contact of the ocean and continent. Eventual consumption of (?) oceanic floor beneath the Flysch troughs would have taken place at the contact of that floor and cordilleras which should be treated as microcontinents in this model. When this model is accepted it appears neceplace at the contact of that floor and cordilleras ssary to assume that both the cordilleras and the geosyncline of FIysch Carpathians as a whole were very wide and that latter much more than 300-400 km wide.

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