The décollement level of the North Alpine Foreland

 (Jura fold-and-thrust belt and Molasse Basin)

          

The project is concerned with the mechanical and tectonic relevance of the basal décollement zone beneath the North Alpine Molasse Basin and the Jura fold-and-thrust belt. Different aspect of the décollement zone are addressed such as :

• The distribution and thickness changes of the lithostratigraphic levels linked to the décollement zone (Muschelkalk and Keuper)
• Link of the thickness changes of the evaporites of the décollement zone and the evolution of the structural style;
• Changes in the basal friction and relevance for the detachment and the associated structures
• Links between topography and basal décollement in the frame of wedge mechanics

 An update of our present knowledge has been published in The Role of the Triassic Evaporites underneath the North Alpine Foreland , by Sommaruga, J. Mosar, M. Schori, M. Gruber, in  Permo-Triassic Salt Provinces of Europe, North Africa and the Atlantic Margins. http://dx.doi.org/10.1016/B978-0-12-809417-4.00021-5, © 2017 Elsevier Inc.

Background:

The northwestern Alpine foreland, mainly in Switzerland and France is located in front of the Alps and can be subdivided into two structurally different regions: to the N-NW the frontal Jura fold-and-thrust belt (FTB), and between the Alps and the Jura Mountains, the Molasse Basin. The Jura FTB evolved over a main décollement zone. The Molasse Basin initiated as a flexural foreland basin during the Alpine orogeny and subsequently evolved into a wedge-top basin when the Alpine foreland basal décollement became active and the Jura FTB formed. The foreland evolution is linked to the subduction towards the S of the European plate beneath the Adriatic plate and is largely driven by the geodynamics of the subducting slab and the subsequent collision.

 Ever since the pioneering work of Buxtorf (1907), the arc shaped geometry of the Jura FTB, and the trailing Molasse Basin have been related to the lateral distribution and thickness of the Triassic evaporite layers of the sedimentary cover. These evaporites find their origin in a paleogeographic setting during the Middle to Upper Triassic times when the European epicontinental platform evolved from continental sedimentation (Permian – Lower Triassic) to a shallow marine environment where wide spread precipitation of evaporites occurred. This platform was bordered to the South by the Alemannic land, which is a SE extension of the Vindelician High. The high was located on top of the External Crystalline Massifs (ECM) in the Alps (Aiguilles – Rouges Massif, Gastern-Aar Massifs), and opened to the N to the Germanic Basin. During the late Tertiary phase of Alpine orogeny these evaporites were the locus of strong deformation enabling the detachment, folding and thrusting of the overlying Mesozoic and Tertiary cover sequences. This evolution is coeval with the imbrication and exhumation of the ECM, and the foreland décollement roots under these basement massifs. This coeval evolution led Buxtorf to propose the theory of a distant push, Fernschub hypothesis to explain the formation of the Jura Mountains.

The tectonic development of the foreland has classically been considered to have occurred between 15-4 My. However, recent work suggests that the foreland is still actively deforming as also witnessed by continued present seismic activity.

Tectonic relevance and thickness changes of the décollement zone

The evaporites of the Middle and Upper Triassic play a key role in the development of the western Alpine foreland. They form the major décollement zone along which the Molasse Basin and Jura fold-and-thrust belt are deformed and displaced towards the foreland to the N-NW, above the mechanically rigid basement s.l.

Two main décollement horizons can be discriminated: the Muschelkalk unit and the overlying Keuper unit. The Muschelkalk layers act as main décollement zone in the East, and the Keuper layers in the West. The main thickness of the cumulated evaporite series is due to deformation and can be found under the central main range of the Jura fold-and-thrust belt. However, it can be shown that tectonic thickening of these series can also be correlated with individual structures, both in the Molasse Basin and the Jura Mountains. Based on our new distribution and thickness maps of the Triassic series, and especially the Muschelkalk and Keuper units, we suggest that the thickness in excess of >200 m in either of these units are due to tectonic processes. The original depositional thickness of these units appears to be rather constant and only minor thickness variations (<200 m) are due to the original paleogeography of the basin. One major exception can be seen in the Buntsandstein unit at the western edge of the Jura FTB, which is linked to a large half graben related to the Bresse Graben structure.

The main structural styles observed in relation with thrusting over the décollement zone range from fault-and ramp-related folds to evaporite-cored folds. This change in structural style is linked to sediment thickness above the décollement zone and the position within the critical wedge. Deformation directly associated with the décollement zone units of the Muschelkalk and Keuper are salt pillows and evaporite duplexes. Evaporites also core the major structures of the Central Haute Chaine of the Jura Mountains. They are the locus of important tectonic thickening of the evaporite formations. Multiple thrust levels can be observed and the weak evaporites facilitate stepping and forward propagation of the décollement over an inherited, possibly tectonic, basement topography. The Muschelkalk and Keuper units of the Middle and Upper Triassic act as a broad décollement zone with distributed deformation and multiple detachment levels.

Internal deformation of the evaporite layers in the décollement zone is highly heterogeneous and is strongly partitioned along several decollement layers. Deformation of anhydrite is controlled by temperature, strain rate, and grain size, as well as, the impurity content. The temperatures expected along the décollement zone increase from N to S from some 40° to 200°C. Thus the evaporites deform ductilly by intracrystalline deformation mechanisms, and locally mylonites may develop. With strain rates of 10-13-15 sec-1 and geothermal gradients up to 35°C the yield strength of salt is below 1MPa.

The foreland evolves from a flexural basin to a detached wedge-top basin with the activation of the décollement zone. The very weak décollement allows for a broad thrust belt and a narrow taper. The lateral variation in the décollement strength may explain the out-of-sequence thrusting as well as hinterland directed thrusting. The whole foreland behaves as a critical wedge that develops above the main décollement zone ramp in accordance with the Fernschub hypothesis and coeval with the imbrication of the External Crystalline Massifs in the Alps, with the Molasse Basin to the South and the Jura fold-and-thrust belt to the N-NW. Given the changes in topography and the décollement fault strength it appears that the foreland has not yet attained a state of equilibrium and is possibly still active, although under very low rates of deformation.