The Grenchenberg conundrum in the Swiss Jura: a case for the centenary of the thin-skin décollement nappe model (Buxtorf 1907)

Abstract

When Buxtorf in 1907 proposed his décollement hypothesis which visualized the Jura fold belt as a "folded décollement nappe" pushed by the Alps, he met with both fervent support of a few and skepticism by the many. As an illustration of recalcitrant problems within the Jura décollement nappe, that remain after 100 years, a model of the Grenchenberg complex is presented within the frame of 3D décollement kinematics, based on a set of rules gleaned from recurrent features in the Jura: (1) that generally progression of décollement was in sequence from south to north, (2) that in any given structure thrusting (with attendant ramp folding) preceded more generalized folding, (3) that progression of décollement was held up at "anchor points" (asperities) where the emerging thrusts and folds developed inflections with dextral transpression in the western and sinistral transpresssion in the eastern flank, (4) that at such asperities more southerly structures, riding piggyback on the moving décollement sheet, often collided with more northerly ones and even merged with them. The asperities occur on fault/flexure lines of Paleogene origin, the Pierre Pertuis anchor point on the Vicques and the Grenchenberg extended anchor domain on the "Schwarzwald" Line (the continuation of the eastern border of the Rhinegraben). These lines produced deformations of the décol-lement surface in the middle Triassic evaporites which acted as boundary conditions at the bottom boundary of the décollement nappe, which led to stress concentrations and the nucleation of faults. Although it is now widely recognized that these lines were reactivated during late Miocene Jura décollement, it ought to be stressed that this reactivation affected the décollement nappe only and there individual asperities or groups of asperities rather than the lines as a whole. Because in the course of nappe displacement the originally autochthonous asperities as expressed in the sedimentary cover moved into an allochthonous position, their original autochthonous location in the basement has to be found by retrodeformation enabled by a map-aspect kinematic model. The Grenchenberg structure developed by collision of the Montoz and Chasseral ranges and was affected by at least three different anchors. At one of these anchors a belt of brachyanticlines developed along which material was dextrally transferred from the Montoz-to the Chasseral fold, resulting in the disappearance of the former and the strengthening of the latter, which in turn merged with the rising Weissenstein fold. ZUSAMMENFASSUNGAls Buxtorf 1907 seine Abscherungshypothese vorstellte, in der der Faltenjura als eine von den Alpen her geschobene "gefaltete Abscherdecke" konzipiert war, stiess er bei Wenigen auf begeisterte Zustimmung; die Mehrheit blieb skeptisch. Um zu illustrieren, dass es auch heute noch, nach hundert Jahren, widerspenstige Probleme im abgescherten Jura gibt, wird unter Verwendung einer Anzahl von Regeln der Versuch unternommen, ein 3D-kinematisches Abschermodell des Grenchenberg-Komplexes...

Multiple detachments during thin-skinned deformation of the Swiss Central Jura: a kinematic model across the Chasseral

“…These differences stress the need for a separate examination of each domain and show that structural studies in the Eastern Jura may not apply to the Central Jura. Previous studies in the Eastern Jura are found in Laubscher (1986), Bitterli (1992), Noack (1995) and Laubscher (2008), and additionally in reports of the National Cooperative for the Disposal of Radioactive Waste (Nagra). The Central and Western Jura Mountains were extensively studied by Philippe (1995), Sommaruga (1997) and Affolter (2003).…”

“…The applied forward modelling technique grants a wide degree of freedom owing to its non-physical nature and therefore it is crucial to make use of a default structural style (Laubscher 2003(Laubscher , 2008, defined by assumptions and observations with a physical background. Here the default structural style is defined as follows: (1) the folds of the Haute-Chaîne were initiated with a detachment in the Triassic anhydrites and they developed into fault-propagation and fault-bend folds after the outbreak of thrusts (Sommaruga 1999).…”

Tectonics of the central Swiss Molasse Basin: Post‐Miocene transition to incipient thick‐skinned tectonics?

“…It presumably follows an inherited Paleozoic structure and has been repeatedly reactivated in Mesozoic times and during rifting of the Rhine Graben in the Oligocene [ Allenbach and Wetzel , ]. During the late Miocene main deformation phase in the Jura fold‐and‐thrust belt [ Becker , ], inherited structures related to the Rhenish Lineament have been reactivated in a left‐lateral strike‐slip manner, while at the same time they were transported 10–12 km to the NNW [ Laubscher , , ]. Seismicity shows present‐day activity of the SFZ‐Rhenish Lineament system (Figure ).…”