Original paper

Microfabrics of crystalline rocks from the drilling projects Gross-Umstadt Heubach and Wiebelsbach (Böllsteiner Odenwald): Constraints on deformation mechanisms, metamorphic temperature and kinematics

Loeckle, Filip; Zulauf, Gernold; Nesbor, Heinz-Dieter


Crystalline rocks from three boreholes in the northern Böllsteiner Odenwald were investigated for microstructures, kinematic indicators and qtz-textures; grt shapes were investigated in 3D using μCT-imaging. The main objective is to constrain the tectono-metamorphic evolution of the sampled rocks. These constraints are then compared to published data in order to unravel the position of the poorly exposed northern part of the Böllsteiner Odenwald within the Mid-German Crystalline Rise (MGCR). The core from the 90 m pilot borehole in Heubach displays granodiorite-gneiss with crosscutting igneous veins, while two sections from the nearby main borehole (613.3–617.8 m and 772.8–774.9 m) sample different bt gneisses. The core from a 40 m drilling near Wiebelsbach, about 2 km S of the Heubach site, displays partially mylonitic grt-bt gneiss. Basal and prismatic subgrain boundaries (SGBs) in qtz and lobate phase boundaries between qtz and fsp suggest deformation at T >650 °C for the Heubach granodiorite-gneiss, the mylonitic parts of bt gneiss from Wiebelsbach and grt-bearing bt gneiss from the main borehole (774 m depth); asymmetric qtz textures and kinematic indicators reveal a top-to-the-E sense of shear. Non-mylonitic bt gneiss from Wiebelsbach and graphite-bt gneiss from the main borehole (617 m) only display qtz microstructures typical for deformation at T 300–400 °C and show a compositional layering. Accordingly, qtz textures of the Heubach granodiorite-gneiss and mylonitic bt gneiss from Wiebelsbach reveal high-T prism slip, while low-T basal slip is dominant in the non-mylonitic bt gneiss from Wiebelsbach. Reaction fabrics in all samples suggest pervasive retrograde metamorphism during uplift, with deformation reactivating the dominant foliation and forming ultramylonites with a top-to-the-E sense of shear in the Heubach granodiorite-gneiss. The latest deformation includes brittle normal faulting and fracturing of qtz during further uplift and extension. While hbl-bearing orthogneisses are also present in the Bergsträßer Odenwald, the Heubach granodiorite gneiss and bt gneiss from Wiebelsbach both reveal features indicating similar peak-metamorphic conditions as in the Spessart and Böllstein crystalline areas, which suggests an affiliation to the MGCR part E of the Otzberg shear zone. Considering the structure of the Spessart, the drilling of several types of gneiss with different peak metamorphic overprint is tentatively interpreted an indication for the penetration of a nappe stack.


crystalline odenwaldμct imagingquartz-texturesmid-german crystalline highkinematicsebsdmicrostructuresböllsteiner odenwaldmgcr