THE EAST CARPATHIANS - INDICATIONS OF PHASE-TRANSITIONS, LITHOSPHERICFAILURE AND DECOUPLED EVOLUTION OF THRUST BELT AND ITS FORELAND

An analysis is presented of the tectonic development of the East Carpa thians since the Late Oligocene. The geological and geomorphological d ata show that five epochs of convergence and nappe emplacement took pl ace with the last one occurring 11-12 m.y. B.P. After these compressio nal events the nappe surface remained near sea level. The crustal upli ft which formed the present mountains began 3 m.y. B.P. and after 99% of the shortening of the Carpathians had been completed. The Pre-Carpa thian foreland basin, up to 3-5 km deep, formed on the margin of the E ast European Platform in the Middle Miocene. Analysis of the evolution of the basin shows that it deepened away from the nappe-stack and mos t of the subsidence occurred when there was no convergence. This precl udes lithospheric flexure due to convergence as a mechanism for basin formation. At 11-12 m.y. B.P. the inner part of the basin, similar to 50 km wide, was overridden by the Carpathian nappe, up to 12-14 km thi ck. At that time, a slight crustal uplift took place, rather than subs idence, in the shallow remnant-outer basin part (the present foredeep) which emerged above sea level. An additional 7-8 km of subsidence the n occurred in the foredeep of the southeastern Carpathians in the Late Miocene-Early Pleistocene when very little convergence took place. Th e evolution of the Carpathian foredeep shows that lithospheric flexure of 2-5 km occurred in regions, 20-40 km wide. Similar amounts of flex ure and large basement faults exist under the nappe. This style of def ormation indicates flexural rigidity of the lithosphere that is severa l orders of magnitude smaller than in stable cratonic regions. Under a low flexural rigidity, the crust should be close to local isostasy. T hen the isostatic response to superposition of a 10-13-km-thick nappe on the platform margin which was near sea level would form high mounta ins. The crustal surface, however, remained at a low altitude. We inte rpret this to be the result of contraction of gabbro in the lower crus t with the formation of dense garnet granulites during nappe emplaceme nt. A layer of these dense rocks, similar to 20 km thick, with P-wave velocities 7.5-7.6 km/s, exists in the lower crust of the East Carpath ians. According to the geophysical data, there is no cool high-velocit y slab now subducting towards the Intra-Carpathian area. An increase i n the mean height of the Carpathians by only similar to 1 km since the termination of convergence precludes a recent slab detachment.