The late-Variscan high-temperature collisional episode in the southwestern Moldanubian Zone (Bohemian Massif)
Abstract
This paper presents new structural, anisotropy of magnetic susceptibility (AMS), and petrological data in tandem with existing geochronological data to determine a polyphase Variscan tectonometamorphic event in the southwestern Moldanubian Zone of the Bohemian Massif. The high-grade metamorphic rocks of this Zone underwent the successive tectonometamorphic evolution (from ca. 360 to 310 Ma) that portrays three main geodynamic episodes: (a) The collisional to exhumation episode (ca. 360-335 Ma) recorded by the relict, steeply dipping, an ~ N(NNE) to S(SSW) striking foliation planes (S1) developed at temperatures of 720-754 °C and pressures of ca. 790 MPa (M1) and superimposed flat-lying foliation planes (S2) under temperatures of 674 +27 °C and pressures of 680 +/- 110 MPa (M2). (b) The late-orogenic ~ N-S oriented shortening (ca. 335-325 Ma) controlled by convergence of a crustal segment derived from the Gondwana continent (newly named as the "Salzburgia Block) and the mostly consolidated Bohemian Massif.
These collisional processes resulted in superimposition of the foliation planes (S3) dipping steeply to moderately to the ~ N to ~ NNE(NE), re-heating and metamorphic overprint at temperatures of 770-830 °C and pressures of 450-530 MPa (M3) and syn-tectonic emplacement of the numerous late-orogenic granitoids. (c) Subsequently, the switch in the paleostress regime from ~ N-S shortening to that of late-orogenic ~ ENE-WSW extension (ca. 325-310 Ma) led to the activation of regional right-lateral transtension and exhumation of the deep-seated rocks. Associated re-equilibration proceeded under temperatures of 662-701 °C and pressures of 362-437 MPa (M-4 event).
In the latter stages of this episode, the synchronous reactivation of the regional ~ NW-SE striking right-lateral shear zones (Pfahl and Danube) and the almost perpendicular ~ NNE(NE)-SSW(SW) striking left-lateral shear zones took place.