The pre-Cenozoic structural structure of the Subhercynian Basin and its influence on the Cenozoic sedimentary deposits

The Central German realm is characterised by heterogeneous and complex structures within the pre-Cenozoic substratum. Many narrow fault zones separate domains that are very little deformed. Moreover, also wide large-wavelength and low-amplitude anticlines formed that are associated with the migration of thick evaporite and salt layers of the Zechstein (Late Permian). This complex structural setting is the result of multiple phases of tectonic deformation. During the Late Palaeozoic, the metamorphic basement deformed during the Variscan Orogeny became separated into local grabens and pull-apart basins. Subsequently, latest Palaeozoic (Upper Rotliegend and Zechstein) and Mesozoic strata were deposited in an intracontinental basin system, the so-called Central European Basin System of which the North German Basin is its central part. However, several tectonic events intermitted a general uniform sedimentation. First, Late Triassic extension localised along few fault zones (e.g. the Allertal Fault Zone), where large offsets are documented within the Mesozoic stratigraphic pile. Late Jurassic-Early Cretaceous extension affected large parts of Central Europe, where deformation became more distributed and multiple NW-SE trending fault zones with a uniform spacing evolved (e.g. the Halberstadt Fault and the Offleben-Oschersleben-Sta ss furt Fault). The last Mesozoic tectonic event in the Central German realm was characterised by Late Cretaceous shortening, which led to uplift of large basement-cored anticlines (e.g. the Harz Mountains and the Flechtingen High located south and north of the Subhercynian Basin, respectively) as well as to thin-skinned thrusts and anticlines in the forelands of basement uplifts. During all these events, a certain amount of halokinetic movements must be assumed for the Central German realm. The Subhercynian Basin in Saxony-Anhalt (Central Germany), located between the basement uplifts of the Harz Mountains in the south and the Flechtingen High in the north, is considered as a key area for understanding Mesozoic intraplate tectonics in Central Germany. In parts, these structures are well known due to intense geological and geophysical exploration during the second half of the last century, which enables us to analyse the structural setting of the Mesozoic and Cenozoic substratum of the Subhercynian Basin in detail. Therefore, we used modern geoscientific techniques, i.e. the interpolation of depth maps for the Cenozoic as well as the compilation of a homogeneous 3D structural model of the research area by use of an implicit modelling approach. A huge database of seismic reflection sections, boreholes and geological maps of the surface and the subsurface were digitised and prepared. The result of this modelling process shows a generalised overview of the structural situation of the deep subsurface (Late Palaeozoic to Mesozoic). Furthermore, investigation on the only locally preserved Cenozoic cover sequence for the Subhercynian Basin provides insights into the influence of deep structures on the sedimentary evolution during the Cenozoic. The main findings of our study are the following: Prominent NW-SE trending fault zones are observed throughout the Subhercynian Basin and dominate its structural setting. In parts, these fault zones are concomitant with salt intrusions (e.g. the Allertal salt structure or the Offleben-OscherslebenSta ss furt salt anticline) and, locally, even detached short-wavelength anticlines and thrusts (e.g. the Quedlinburg anticline). Long-wavelength, fault-related folds and deep synclines also occur. These fault zones can be used to subdivide the Subhercynian Basin into single segments with different structures and kinematics: (1) The Weferlingen-Schonebeck Block forms the northernmost part of the Subhercynian Basin between the Flechtingen High and the Allertal Fault Zone (with the associated Allertal salt structure). In this area only small normal and reverse faults with relatively low offsets do occur. The entire area mildly dips with less than 3 degrees towards the southwest, where the Allertal Fault zone forms the structural boundary towards the southerly adjacent Lappwald Block. Although fault kinematics and geometries of the Allertal Fault zone are still under debate, it is likely that this fault zone initially formed as a large-offset (>2.5 km), gently SW-dipping, detached normal fault during Late Triassic and Late Jurassic/Early Cretaceous times. A Late Cretaceous compressive reactivation of the Allertal Fault Zone, as proved in the case of its western continuation in Lower Saxony or other large fault zones in Central Germany, was only mild. The magnitudes of offset were much less than those related to earlier extension. (2) The Lappwald Block forms a mild, approximately 10 km wide and 60 km long (in NE-SW direction) syncline with gently dipping Mesozoic strata. Towards the south, it is separated from the Central Subhercynian Basin by the OfflebenOschersleben-Stassfurt salt anticline.(3) The Central Subhercynian Basin forms a 25 km wide structural unit with many E-W and NW-SE trending fault zones. In contrast to the general trend direction of the Subhercynian Basin and its major fault zones (NW-SE), strike directions of these fault zones are variable. The Asse-Muhlenberg-Heeseberg Fault and the Huy Anticline in the outer west strike approximately E-W while in the east of the Central Subhercynian Basin the Aschersleben Anticline strikes NW-SE. Additionally, several thick-skinned thrusts and long-wavelength basement anticlines can be observed. They show an uplift of up to 3 km relative to their surroundings. Towards the south, the Halberstadt Fault Zone forms the border of the Central Subhercynian Basin towards the Southern Subhercynian Basin. (4) The Southern Subhercynian Basin is characterised by two deep synclines filled with up to 1.5 km thick synkinematically deposited strata of Late Cretaceous age. The Quedlinburg Anticline, a narrow NW-SE-striking anticline with internal detached thrust sheets, separates these two synclines. In contrast to other areas in Central and Northern Germany, where Cenozoic (Tertiary) sediments widely cover the pre-Cenozoic substratum, strata of this age are only locally preserved in the Subhercynian Basin. Areas of distinct preservation of Palaeogene sediments are found in the vicinity of the Offleben-Oschersleben-Stassfurt Anticline (e.g. the Egeln Synclines) and in the area around Nachterstedt (Nachterstedt Basin), where evaporites directly occur below base Tertiary. Here, the influence of halokinetic movements and subrosion on the Cenozoic deposition and erosion processes are inferred from the observations of local and circular depocentres, e.g. located on top of the Allertal salt structure, and from the fact that Palaeogene to Eocene relicts only occur close to salt structures and evaporitic rocks. Deposits of Oligocene or younger Tertiary ages are not preserved. The structural observations and interpretations mentioned above show that the most important influence on today's structural setting of the Subhercynian Basin is the result of Late Cretaceous to Palaeogene compression. Based on additional, thermochronological and stratigraphic constraints from the literature this event must be assumed to have occurred at 90 to 70 Ma before present, the time when the surrounding basement uplifts (Harz and Flechtingen High) became uplifted and up to 6 km of Palaeozoic to Mesozoic rocks were eroded. In parts, the eroded rocks were deposited in the Southern Subhercynian Basin and subsequently tilted due to ongoing exhumation of the Harz. The end of compression-related exhumation (c. 70 Ma as proved by thermochronological data) and deposition in the foreland of these basement uplifts is not documented by stratigraphic contacts. The youngest preserved Late Cretaceous rocks are of Early Campanian age (c. 80 Ma), while younger strata were eroded due to large-scale doming of the European crust between 75 Ma (Late Campanian) and 55 Ma (Early Eocene). While the amount of doming-related uplift is assumed to reach up to 2 km on top of the Harz, it only affected the Southern Subhercynian Basin and parts of the Central Subhercynian Basin as proved by Palaeogene to Eocene strata in the north (in the vicinity of the Offleben-Oschersleben-Stassfurt Anticline). These observations suggest a continuous tilting (c. 3 to 4(degrees)) of the Subhercynian Basin towards the north until the Eocene, which is even proved by analysis of flow directions in Palaeogene strata. Together with preserved Late Cretaceous (Maastrichtian) to Early Palaeogene (Danian) fauna in Eocene and Oligocene deposits in the north of the Subhercynian Basin, these observations indicate that a significant amount of uplift and deformation occurred at least until Oligocene time (younger than 34 Ma). The results of this study show that the structural setting of the pre-Cenozoic substratum, together with the Mesozoic Cenozoic deformation history significantly affected deposition and preservation of Tertiary strata in Central Germany. Insights into the structural setting and kinematic evolution in three dimensions are gained for the Mesozoic-Cenozoic period based on the new uniform 3D structural model. The modelling results also form the basis for further observations, interpretations and research as well as for future modelling and planning efforts in the area of the Subhercynian Basin.