AN INTEGRATED PETROFABRIC STUDY OF THE HIGH-PRESSURE ORLICA-SNIEZNIK COMPLEX, CZECH REPUBLIC AND POLAND

In the northeast corner of the Bohemian Massif (Sudeten Variscides of Poland and Czech Republic), an exhumed mosaic of tectonic massifs preserves UHP & HP assemblages that exhibit rapid cooling indicative of fast exhumation. New geochronometric data reveal that individual massifs were at different crustal levels at the same time with respect to each other. For instance, the Orlica-Snieznik complex was still punching its way through the lithosphere from ultra-deep depths as the nearby Gory Sowie Block was already exhumed for at least 20 m.y. The dominant subhorizontal macroscopic foliation in the Orlica-Snieznik complex is considered by some to be an extensional fabric; the metamorphic complex also contains a pervasive stretching lineation with more localized zones of sub-vertical foliations. However, short-lived, rapid two-stage exhumation is inconsistent with an extensional origin for this massif as crustal detachments are hitherto unrecognized. Structural field measurements, supplemented with AMS orientation data obtained from samples of migmatites and gneisses collected from sixty-four sites across the 400 km2 of the Orlica-Snieznik complex, yield remarkably consistent lineation attitudes which plunge 11º toward 192º in rocks containing both steep and subhorizontal planar fabrics. Rock magnetic data reveal that the magnetic fabrics are dominated by paramagnetic mineral phases, primarily biotite, amphiboles, feldspar. We note that the gentle lineations and more localized steep fabrics are sub-parallel to the curvature of the Variscan orogenic belt in this corner of Bohemia. Exhumation via extension is inconsistent with both the geochronologic and petrofabric database that now exists for this area. We suggest instead that transpressional extrusion during rapid stage-two decompression (from the lower crust) produced the dominant petrofabric geometry now preserved. Further detailed, integrative studies of this relatively small and narrow orogenic belt are needed to better understand lateral and vertical variations of lithospheric deformation and to address the P-T-t-D dynamics of lower and mid-crustal exhumation. Our study successfully demonstrates the viability of supplementing structural observations with AMS analyses in UHP & HP terranes.