Restoration of the Cretaceous uplift of the Harz Mountains, North Germany: evidence for the geometry of a thick-skinned thrust

Abstract

Reverse movement on the Harz Northern Boundary Fault was responsible for the Late Cretaceous uplift of the Harz Mountains in northern Germany. Using the known geometry of the surface position and dip of the fault, and a published cross section of the Base Permian horizon, we show that it is possible to predict the probable shape of the fault at depth, down to a detachment level. We use the ‘inclined-shear’ method with constant heave and argue that a shear angle of 30° was most likely. In this case, the detachment level is at a depth of ca. 25 km. Kinematic restoration of the Harz Mountains using this fault geometry does not produce a flat horizon, rather it results in a ca. 4 km depression. Airy–Heiskanen isostatic equilibrium adjustment of the Harz Mountains restores the Base Permian horizon to the horizontal, as well as raising the Moho to a depth of 32 km, a typical value for northern Germany. Restoration also causes a rotation of tectonic fabrics within the Harz Mountains of about 11° clockwise. We show that this model geometry is very good fit to the interpreted DEKORP BASIN 9601 deep seismic profile.

Appendix: The ‘inclined-shear’ method

Fig. 10

Inclined-shear method to predict the shape of the fault using the hanging-wall geometry, the footwall/hanging-wall cutoffs, and a specific shear angle, here 30°. Solid lines represent trace of Base Permian horizon, fine dashed thick line—known trace of the fault. Finely-dashed lines represent material vectors within the hanging wall. The angle of the material vector from the horizontal is given

‘Inclined-shear’ fault prediction relies on knowledge of the initial fault dip and position, as well as the cutoffs of a certain horizon (Fig. 10). The method assumes constant heave on the fault over the whole fault length. The angle at which the hanging wall underwent shear during fault movement (known as the shear angle) must be estimated; in Fig. 10, the shear angle is 30° from the vertical, i.e., it dips at 60° towards the fault. The hanging-wall cutoff (A $^\prime$) is projected at the shear angle to the regional, i.e., the level of the footwall (point P). The horizontal length from P to A is $\epsilon$. At regular intervals of $\epsilon$ from A, the points B, C, etc are marked (Fig. 10). At each of the points, a shear construction line is projected to the hanging wall at the shear angle (points B $^\prime$–G $^\prime$). The lengths t$_1$ to t$_n$ are measured. The depth to the fault is calculated by projecting the shear construction lines downwards from the regional, using vectors t$_1$ from A, t$_1 +$ t$_2$ from B, t$_1 +$ t$_2 +$ t$_3$ from C, etc. After Groshong (2006).