The GeoLaB 1 exploration borehole in the Odenwald Crystalline Complex: constraints for the development of the geothermal research infrastructure GeoLaB

The 500 m deep, fully cored and logged GeoLaB1 exploration borehole was drilled in Q1/2025 into the Tromm ridge in the south-eastern Odenwald. The up to 578 m a.s.l. high Tromm ridge is a N-S-trending, morphologically distinct feature and the explored potential target for the first geothermal research underground laboratory in Germany. The granitic-monzonitic rocks are intensively faulted and fractured exhibiting predominant argillitic, minor propyllitic, alteration. Hydrotesting revealed transmissivityies and permeabilityies values common for crystalline rocks. Fault planes commonly outline normal faulting displacements, but also minor reverse faulting can be documented. At c. 410 m below surface deformation style and petrophysical properties and fracture density change significantly with the occurrence of subhorizontally foliated, partly mylonitic to ultramylonitic, granitic-granodioritic and metasedimentary rocks below the Tromm granites. We interpret these findings as a major tectonic contact between the Tromm pluton and the underlying metamorphic rocks. Regional structural data, a concave upward seismic reflector, and gravimetric data suggest a continuation of subhorizontal, possibly antiformal, metamorphic rocks of the Böllstein antiform (and/or the “Zwischenzone”) below the granites of the Tromm ridge. Further structural, geochemical, geochronological, and geophysical work will have to constrain this hypothesis.