Technischer Bericht NTB 90-04

This report presents the methodology and results of a geological interpretation of Nagra's reflection seismics data (1982 -1984) for North-Central Switzerland. The area under investigation, which is covered by the entire network of reflection seismics lines, is outlined by the towns of Schaffhausen/Zürich in the east and Säckingen/Olten in the west.

Basement

The structure and geological development of the crystalline Basement complex of Northern Switzerland and the Black Forest must be seen in the context of the Central European Variscan orogeny which, according to plate tectonics models, is the result of a continental collision during the Upper Devonian and the Carboniferous. The regional model implies that the conditions observed in the Southern Black Forest (and the Vosges) also pertain to the subsurface of Northern Switzerland, where borehole data confirm similar structural complexities in the Basement.

The Variscan development of the crystalline Basement of Northern Switzerland can thus be divided into the following phases:

• Formation of the Prevariscan gneiss complex (Precambrian - Early Palaeozoic);
• Variscan convergence, synkinematic plutonites (Devonian - Carboniferous);
• Doming and extension of the variscan chain, postkinematic plutonism (Carboniferous), formation of basins in the Upper Carboniferous;
• Late orogenic shearing, acid volcanism (Permian, Saalian tectonics);
• Postorogenic subsidence and peneplanation of the Variscan mountain range (Permian).

While reflection seismics surveys are unable to provide sufficiently reliable information on the tectonics and lithology of the crystalline Basement, the structure, and particularly the tectonics, of Late Variscan sedimentary trough infills can be mapped seismically. These dovetail with the Basement of Northern Switzerland and available data, particularly from the Nagra boreholes, indicate an intense, deep-reaching (brittle) Basement deformation caused by Permian tectonic events. These Late Palaeozoic fault zones, most of which were rejuvenated during the Tertiary, dominate the structural pattern of the region.

Late Palaeozoic sedimentary troughs

At least two major Late Palaeozoic sedimentary troughs have been identified in the subsurface of Northern Switzerland: a northern trough (Permo-Carboniferous Trough of Northern Switzerland, NPT) beneath the Tabular Jura, and a southern trough (Olten­ – Lenzburg Trough). No attempts have been made to map the latter.

The overall form of the NPT is that of an ENE-WSW-trending asymmetric half-graben (trap-door basin), dextrally offset by WNW-ESE-trending convergent wrench faults (e.g. Eggberg and Vorwald fault zones). These appear to divide the Trough into an eastern and western segment which differ with respect to depth and direction of asymmetry. Thrusting and folding, evident mainly in the western section of the Trough, are the result of a phase of transpressive deformation dated as Lower/Middle Permian (Saalian phase). In the Late Permian, the decreasing Variscan tectonic activity over the whole Trough was characterised by extension and differential subsidence.

The tectonic events are reflected by the sedimentary Trough infill, which can be up to 5 km thick:

• An older Lower Trough Infill comprises Stephanian to Lower Permian coal measures and lacustrine deposits from an originally wide basin, overlain by a syntectonic suite characterised by detrital fan deposits, rapid lateral facies changes and synsedimentary deformations.
• A younger Upper Trough Infill consists of Upper Permian redbeds. This overlies the Lower Trough Infill unconformably and reaches onto the shoulders of the Trough, far beyond older depocentres. The depositional pattern is indicative of a synsedimentary extensional regime involving low angle normal faulting.

Mesozoic and Tertiary sedimentary cover

Marine Mesozoic shelf deposits were laid down during a period of tectonic inactivity and rest unconformably on Basement or Late Palaeozoic sediments. Their distinct vertical lithological differentiation provides a seismic signature which allows detailed mapping.

The Early Mesozoic marine Lower Muschelkalk was deposited on an almost perfect subhorizontal peneplain. The present-day structural configuration of this peneplain (Base Mesozoic marker), which has been mapped using reflection seismics over an extended area, records the vertical, mostly Tertiary deformations of the Basement since Triassic time. A general tilt of the entire area to the SSE (doming of the Southern Black Forest and subsidence of the Alpine foredeep) is noticeable; the Base Mesozoic is dissected by a fault pattern emerging in the west of the Palaeogene Upper Rhine Graben tectonics and in the east of the Miocene tilting and doming. The latter includes reactivated preexisting (Late Paleozoic) fault zones. In the central and western area Late Miocene thin-skinned detachment tectonics (Jura Folding) is superimposed on extensive Basement structures.

Tabular Jura

The structural pattern of the Tabular Jura, located between the Southern Black Forest and the Folded Jura, reveals a superposition of extensional Basement faulting and thin-skinned detachment in the sedimentary cover.

The area west of Frick is dominated by NNE-SSW-trending normal faulting related to (Palaeogene-Early Miocene) Rhine Graben subsidence. Towards the east, the influence of Early to Late Miocene, WSW-ENE and NW-SE-trending faults and flexures becomes increasingly important. These relate to subsidence in the Molasse Basin and the doming of the Black Forest and include reactivated Late Palaeozoic fault zones.

The structural response of the Late Miocene thin-skinned detachment to the pre-existing Basement fault pattern was varied:

• The sedimentary cover of the Tabular Jura west of the Aare was transported northwards (by approx. 500 m) as a rigid slab; the front of the detachment ends at the Mandach overthrust.
• East of the Aare, the northward translation is taken up in a series of small-scale imbrications. The imbricated zone becomes progressively narrower towards the east and terminates somewhere east of the Irchel anticline.
• The Tabular Jura of Schaffhausen is located beyond the Jura decollement. The fault pattern observed in the sedimentary cover is thought to reflect the structure at Base­ment surface.

Folded Jura

Seismic reflection profiles crossing the Folded Jura have contributed greatly to a new understanding of the tectonics of the eastern Folded Jura. The most important points are:

• It is now possible to map the position and form of the basal detachment horizon beneath the Folded Jura, and to relate the main thrust units known from surface outcrops to features in the Basement below the decollement surface.
• The Basement relief shows a pattern of flexures and normal faults analogous to that mapped in the Tabular Jura and which is also linked to the Oligocene-Miocene reactivation of pre-existing Late Palaeozoic structures. The latter have exerted a significant control on the geometry of imbrication and folding.
• Reflection seismics have allowed an improved estimate of total shortening in the detached sedimentary cover.

Northern rim of the Molasse Basin

The style and intensity of basement tectonics observed on seismic sections in the area of the northern rim of the Molasse Basin are comparable to those of the Tabular and Folded Jura.

Due to the synsedimentary regional tilt the Molasse sediments show increasing thicknesses towards the SE (basinwards). In the west they have also been affected by Mio­cene thrusting and folding. While disharmonic thrusting is predominant at deeper levels (Triassic), gentle folding is mainly observed at shallower levels (Jurassic and Tertiary).

In the east of the investigation area, the northern rim of the Molasse Basin borders directly on the Tabular Jura; the deformation of the latter generally continues to the SE below the Molasse. The coincidence of the northern rim of the Molasse and the distribution of the Permo-Carboniferous deposits of Northern Switzerland is noticeable. To the east of the Aare, the Molasse sediments, particularly the Lower Freshwater Molasse, appear to wedge out in the region of the northern rim of the NPT. It is therefore possible that there is a genetic link between the outer rim of the Molasse Basin and the distribution of Palaeozoic sediment troughs in the Basement.