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

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 décollement 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.