Early Jurassic K-Ar illite age data of fault gouge from a reverse fault of the Gernsbach-Neuenbürg flexure in crystalline basement rocks of the Nordschwarzwald (SW Germany)

We investigated a SSE-dipping reverse fault zone of the ENE-striking Gernsbach-Neuenbürg flexure within c. 325 Ma old coarse grained two-mica granite. The fault zone consists of a hematized, cataclastic 15-20 cm thick, damage zone, predominantly developed in the hanging wall, and a c. 1 cm thick greyish-greenish clay-rich fault core. We took two gouge samples, spaced c. 1 m apart from each other.

Both samples were fractionated into three grain sizes (<0.2 µm, <2 µm and 2-6 µm), all of which yielded ages between 198 Ma and 171 Ma. The fact that these ages are much younger than the age of the faulted host rock, and that both samples exhibit variations between the different fractions of a few tens of Myr at most, indicates efficient authigenic clay growth. This authigenic crystallization probably occurred between 200 °C and 300 °C, based on the illite “crystallinity” Index and the predominance of the 2M₁ illite polytype in the samples.

The Gernsbach-Neuenbürg flexure plunges to the ENE beneath deformed Triassic strata of the ENE-striking, c. 100 km long „Neckar-Jagst-Furche“, which is yet kinematically poorly understood. Its large strike length and apparent low bulk displacements are consistent with reactivation of the southern boundary fault zone of the Permocarboniferous Kraichgau basin as suggested by mapping of the Gernsbach-Neuenbürg flexure. The widespread Jurassic radiometric age data in southern Germany indicate major hydrothermal activity and a structurally complex, yet poorly resolved, deformation pattern, most likely associated with oblique rifting that developed into formation of the North Penninic ocean.