The Kinematics of a Soft-linked Thrust System; an example from the Haut Giffre, French Alps.

Complex structural styles can be developed within compressional mountain belts, with shortening achieved through a combination of distributed strain and thrusting. These variations in structural style will be controlled by rheology, pressure, temperature, strain rate, and the stratum thicknesses comprising the multilayer. Soft-linked faults can be defined as structures with discontinuous fault surfaces which operate with folds and distributed strain as a single kinematic entity.

The Haut Giffre region of the French sub-Alps, with its well-defined stratigraphy, provides an excellent opportunity to study the development of such a system of spatially discontinuous yet kinematically linked thrust fault surfaces, and their interaction with the surrounding rock, in three dimensions. The variation in strain localisation behaviour observed in the district covers the spectrum from large scale parallel buckle-folds through to discrete fault surfaces. This diversity of displacement localisation behaviour is largely a result of the rheological contrasts provided by the alternating stratigraphy of competent limestone and incompetent shale. Examples drawn from the Vogealle valley illustrate how kinematic ramps can cut up section with multiple changes in the width of the deforming zone which conserve values of displacement at different levels in the multilayer.

Changes in structural style are apparent not only with variations of depth in the stratigraphic pile but also laterally along strike. Kinematically equivalent along strike fold-thrust structures display differences in the level at which deformation is localised on to a thrust fault within a developing fold. These changes in structural style occur over along-strike distances comparable in scale to the dimensions of the fold-thrust complexes in profile. This raises problems of how the lateral transition in structural style is achieved in both the finite strain state and throughout the geometric evolution. To accommodate this variation, extensional faults exist which transfer displacement down from thrusts at a high structural level to those at a lower structural level within the fold.

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