EXHUMATION OF ECLOGITIZED CONTINENTAL BASEMENT DURING VARISCAN LITHOSPHERIC DELAMINATION AND GRAVITATIONAL COLLAPSE, SUDETY MOUNTAINS, POLAND

A Variscan, deep-crustal-level (eclogite-facies), continental basement massif in western Poland, the Snieznik complex, was tectonically exhu med. Crustal-penetrating mylonite zones record three main kinematic ev ents: early top-to-the-north-directed thrusting, right-slip transpress ion-tension, and late top-to-the-south and -east normal faulting. Thru sting resulted in extreme crustal thickening and associated eclogite-f acies metamorphism. Right-slip movements produced retrogressive crysta l-plastic simple-shear zones. Normal faults flank Carboniferous to Ear ly Permian terrigenous sedimentary basins, documenting tectonic and er osional denudation of the Snieznik complex during lithospheric extensi on. Sm/Nd isotopic dates previously reported for the in situ eclogite- facies metamorphic mineral assemblages are 341, 337, and 329 Ma (Bruec kner et al., 1991). Ar-40/Ar-39 isotopic dates for metamorphic hornble nde (338, 333, and 332 Ma), muscovite (329 and 329 Ma), and biotite (3 28 Ma) reflect times of cooling through the approximately 500, 350, an d 300-degrees-C isotherms, respectively. These nearly concordant miner al dates document rapid cooling from approximately 850-degrees-C (eclo gite-facies temperatures) to approximately 300-degrees-C. Rapid denuda tion of these deep-crustal rocks (approximately 19-22 kbar pressures, >70 km depth) is attributed to processes, similar to those of metamorp hic-core complexes, that operated during lithospheric delamination and gravitational collapse. The sequence of late Paleozoic (Alleghanian) crustal thickening followed by right-slip transpression-tension follow ed by normal faulting recognized in the U.S. Appalachians implies that this tectonic pattern may exist throughout the Alleghanian-Variscan b elt.