Balanced sections and the propagation of décollement: A Jura perspective

Abstract

[1] The propagation of thrusting is an important problem in tectonics that is usually approached by forward (kinematical) modeling of balanced sections. Although modeling techniques are similar in most foreland fold-thrust belts, it turns out that in the Jura, there are modeling problems that require modifications of widely used techniques. In particular, attention is called to the role of model constraints that complement the set of observational constraints in order to fully define the model. In the eastern Jura, such model constraints may be inferred from the regional geology, which shows a peculiar noncoaxial relation between thrusts and subsequent folds. This relation implies changes in the direction of translation and the mode of deformation in the course of the propagation of décollement. These changes are conjectured to be the result of a change in partial decoupling between the thin-skinned fold-thrust system (nappe) and the obliquely subducted foreland. As a particularly instructive case in point, a cross section through the Weissenstein range is discussed. A two-step forward (kinematical) model is proposed that uses both local observational constraints as well as model constraints inferred from regional data. As a first step, a fault bend fold is generated in the hanging wall of a thrust of 1500 m shortening. As a second step, this structure is transferred by flexural slip into the actual fold observed at the surface. This requires an additional 1600 m of shortening and leads to folding of the original thrust. Thereafter, the footwall is deformed so as to respect the constraint that this deformation must fit into the space defined by the folded thrust as the upper boundary and the décollement surface as the lower boundary, and that, in addition, should be confined to the area immediately below the fold. In modeling the footwall deformation a mix of balancing methods is used: fault propagation folds for the competent intervals of the stratigraphic column and area balancing for the incompetent ones. Further propagation of décollement into the foreland is made possible by the folding process, which is dominated by a sort of kinking and which is the main contribution to structural elevation and hence to producing a sort of critical taper of the moving thin-skinned wedge.

Tectonic evolution around the Mont Terri rock laboratory, northwestern Swiss Jura: constraints from kinematic forward modelling

“…We assumed stratigraphic thicknesses to be constant for the units on top of the basement and on top of the postPermo-Carboniferous sediments, even though the presence of mechanically weaker units are expected to have formed local, second, or third order structural duplications and/or omissions. We also assigned a general pre-folding SSE regional tilt for the stratigraphic sequence with a 2.0°i nstead of a 2.5°dip (Laubscher 2003). This decision was motivated by analysis of the reflexion seismic lines 73-BE5 and 74-BE10 acquired in 1973 and 1974 by Jura Bernois Pétrole SA, partly published by Suter (1978).…”

“…Also, these structures trend NN at the surface and comprise part of the Caquerelle fault exhibiting a ''Rhenish'' trend. This suggests an (Laubscher, 2003) 2 degrees Fig. 8 Forward-modelling variables, range of considered options for each variable, and selected options for the two alternative scenarios that are considered in this study influence from basement faults in determining their location and geometry.…”

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

“…The base excess-area (BEA) construction lines are indicated with dash-dot lines. D total displacement, d displacement across the pin line, h u ramp height, h 1 height of horizon 1, h 2 height of horizon 2, S 1 excess area underneath horizon 1, S 2 excess area underneath horizon 2, DS 2 area which moved past the pin line general dip of *2.5°to the SSE (Laubscher 2003). (3) There are foreland-verging as well as hinterland-verging thrusts (Bitterli 1992;Sommaruga 1999).…”

“…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).…”

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

“…Modern computer-assisted techniques of section balancing offered the hope of further refinement of the model, particularly with a view to improving the geometry of the décollement horizon (e.g. Bitterli 1992, Philippe 1994, Philippe et al 1996, Pfiffner et al 1997, Wilkerson & Dicken 2001, Laubscher 2003b, 2008a, 2008b.…”