Reactivation of basement faults: Interplay of ice-sheet advance, glacial lake formation and sediment loading

摘要

The Emme Delta is a small glacilacustrine delta, which developed on the southern flank of the Wesergebirge Mountains in NW Germany. Shallow shear-wave seismic surveys allow a detailed assessment of the structural style of the delta body. Two different fault systems are developed within the delta, both showing syn-sedimentary activity. The faults have planar to slightly listric geometries and show vertical offsets in a range of 2-15m. They form small graben and half-graben systems, which locally show roll-over structures. The fill of the half-grabens has a wedge-shaped geometry, with the greatest sediment thickness close to the fault. The fault system in the upper portion of the Emme Delta is restricted to the delta body and probably gravity induced. In the lower portion of the delta, normal faults occur that originate in the underlying Jurassic basement rocks and penetrate into the delta deposits. The grid of seismic lines shows that the normal faults are trending E-W. This fits to a late Triassic-early Jurassic deformation phase in the Central European Basin System. We hypothese that these faults were reactivated during the Pleistocene by the advancing ice-sheet, water and sediment loading. Based on the seismic data set, an overall model for the reactivation of the basement fault was developed. The advancing ice-sheet caused far field extension, which might have reactivated pre-existing normal faults. Later, the fault activity was enhanced due to sediment and water loading. In addition, high pore pressure due to lake formation might have supported the slip processes along the faults. After glacial unloading and lake drainage, the fault activity stopped.