Structural style and neotectonic activity along the Harz Boundary Fault, northern Germany: a multimethod approach integrating geophysics, outcrop data and numerical simulations

Abstract

We present new evidence for neotectonic activity along the Harz Boundary Fault, a Cretaceous reverse fault that represents a key structure in northern Germany. For the fault analysis, we use a multimethod approach, integrating outcrop data, luminescene dating, shear wave seismics, electrical resistivity tomography (ERT) and numerical simulations. A recent sinkhole at the SSW-ward dipping and WNW-ESE striking Harz Boundary Fault exposes a NNE-ward dipping and WNW-ESE striking planar fault surface that cuts through unconsolidated debris-flow deposits thus pointing to young Lateglacial tectonic activity. The fault shows a polyphase evolution with initial normal fault movement and a later reactivation as an oblique fault with reverse and strike-slip components. A shear wave seismic profile was acquired to analyse the geometry of the fault and show that the Harz Boundary Fault is steeply dipping and likely has branches. Partly, these branches propagate into overlying alluvial-fan deposits that are probably Pleniglacial to Lateglacial in age. The outcrop data in combination with the seismic data give evidence for a splay fault system with steep back-thrusts. One of these back-thrusts is most likely the NNE-ward dipping fault that is exposed in the sinkhole. The lateral extent of the fault was mapped with electrical resistivity tomography (ERT) profiles. The timing of fault movement was estimated based on optically stimulated luminescence dating of the faulted debris-flow deposits using both quartz and feldspar minerals. Consistent feldspar and quartz ages indicate a good bleaching of the sediment prior to deposition. The results imply fault movements post-dating ~ 15 ka. Numerical simulations of glacio isostatic adjustment (GIA)-related changes in the Coulomb failure stress regime at the Harz Boundary Fault underpin the assumption that the fault was reactivated during the Lateglacial due to stress changes induced by the decay of the Late Pleistocene (Weichselian) Fennoscandian ice sheet.

An examination of the glaciomarginal fan of the Odranian glaciation at the Mokrzeszów site, Sudetic Foreland, SW Poland

“…The fault was developing synchronously with the SMF activity, which was revived in the Quaternary (see: Štěpančíková 2022). A comparable situation is described in Müller et al (2020) at the Harz Boundary Fault in northern Germany, where a large Mesozoic fault was reactivated due to GIA (glacial isostatic adjustment)-related stress field changes at the end of the Weischselian glaciation.…”

Reactivation of Non‐Optimally Orientated Faults Due to Glacially Induced Stresses

“…However, a detailed study of the stress changes induced by GIA with a realistic ice load model is required to identify the background stress states and draw conclusions on past, current and future seismicity. In addition, GIA stresses in the peripheral bulge have not been considered here, which could also have induced earthquakes during the last glacial cycle and which is indicated by potential GIFs in central to northern Germany (e.g., Brandes et al., 2012; Müller et al., 2020). Finally, we note that our modeling, which assumes a critically stressed crust, is able to explain the reactivation of pre‐existing, non‐optimally orientated faults due to a glacial cycle, that is, GIFs, independent if the background stress regime is of the thrust‐ or strike‐slip‐faulting type.…”

Open-source learning as a skill for geoelectrical data processing: the case of pyGIMLi

“…Geoelectrical data taken with measurement equipment such as the ABEM Terrameter LS, AGI SuperSting, GeoSys, GeoTom, IRIS Instrument Syscal Pro, Lippmann [17][18][19][20][21][22] have different output formats, and other formats, such as res2dinv [23], also exist. The pyBERT library available in the Anaconda distribution [24] uses the lines of code in Algorithm 1 to convert these different data formats and line-3 into UDF.…”