Young tectonic evolution of the Northern Alpine Foreland Basin, southern Germany, based on linking geomorphology and structural geology

Abstract

The Northern Alpine Foreland Basin (NAFB) formed as a result of alpine continental collision in Oligocene time. Tectonic shortening has been slow since the Miocene, implying that the NAFB subsidence has come to rest. The scope of this thesis is to find evidence that yield information regarding the recent vertical tectonic activity and a potentially ongoing basin inversion. The identification of such an active inversion can be accomplished by studying the landscape evolution. This is possible, because the earth’s surface contains information about tectonic and erosional processes. In order to understand and quantify the underlying mechanisms forming the landscape, both surface and subsurface information can be linked. Especially when geophysical or similar data are insufficient, geomorphological analyses provide new insights. The NAFB is a low-strain sedimentary basin, where historic subsurface data confirm subsidence, while the present-day erosional relief indicates uplift. The basin has a long prospection history and geometries of basement faults are well known, but their link to topography is ambiguous and the landscape is regarded as climatically controlled. In this context, dense population and energy exploration call for a fundamental understanding, if the basin is uplifting and whether or not faults could potentially be reactivated. This thesis aims at characterizing geomorphotectonic indicators of uplift, potentially indicating NAFB inversion. Related to this process, vertical motion of a few hundred meters to a few kilometers is expected to have been taking place during the Plio- Pleistocene. My study addresses the Quaternary geomorphology and distribution of sediments across the NAFB. To assess a potential link between asymmetric valleys and underlying basement faults in the NE portion of the basin (Tertiary Hills region), I analyzed high-resolution space-borne imagery and carried out geologic fieldwork. Results in distinct geomorphological response of the NAFB are (a) regional scale erosion of sediments, (b) fluvial incision, (c) reactivation of documented faults and (d) regional scale tilting of paleo-geodetic markers. 1In the NE portion of the NAFB, the geometry of a detected high-angle geologic subsurface contact associated with a surface escarpment, potentially originated from young surface faulting, while landscape morphology is intensely overprinted by climatic processes and anthropogenic land use. The investigation of mid-Miocene coastal features of the Swabian Alb cliff line addresses the landscape evolution along the marginal area of the NAFB. The particular questions here are, if coastal outcrops are part of the same coast, and which mechanisms have caused their different present-day elevations. I investigated marine features in a geologic field survey and compiled stratigraphic data to infer the regional response to uplift. The results show that two sites represent temporally and spatially different coastal sections, and both have been intensely modified by multiple erosional processes. The data can be explained by invoking a combination of surface erosion and possible lithospheric scale uplift. My study of well data and geomorphic markers, to infer NAFB Quaternary sediment budgets, yields a remarkable erosion of sediments along the central E-W basin axis and fluvial networks in disequilibrium. I examined erosion rates across different timescales, using sediment yields from NAFB catchments and basin sediment flux. On the Pleistocene timescale, geomorphic indicators of fluvial systems are investigated while for the Holocene, archeological data provide constraints on local erosion. The results of this study reveal a distinct oval-shaped erosion pattern across the NAFB, which can explain the modern erosional relief of the region, and further implies active inversion of the basin. Similar results are derived from drainage pattern and river steepness estimates. The results of this thesis imply that the Quaternary landscape in the NAFB has been significantly influenced by tectonic activity. Further, the timing for the NAFB transition from subsidence to uplift is likely to originate at the Pliocene – Quaternary boundary, and continues. Therefore, the possibility of future fault reactivation, controlled by lithospheric scale uplift, cannot be ruled out