Some observations on the development of thrust faults in the Ultradauphinois Zone, French Alps

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Abstract
Summary: The Ultradauphinois Thrust Nappe involved a westerly movement of external autochthonous Jurassic and Eocene sedimentary sequences by décollement on the Trias. A complex thrust belt evolved at its southern end as the movement was hindered by the upfaulted Pelvoux basement massif. All of the Jurassic rocks exhibit strong, enetrative ductile strains—in the lowermost thrust slices (most westerly part of the sheet) forward spreading strains (NE-SW stretching) involving a combination of simple and pure shear, and in the uppermost thrust slices a NW-SE stretching and rearward compression as the main nappe continued to move past to the N of the hindrance. As the resistance to movement increased in the S, the completely distributed ductile strain became localized in numerous zones, characterized by the formation of many veins and strong local deformation of these veins. This marks the onset of stratigraphic imbrication and repetition. The progress from penetrative ductile to localized ductile to discrete brittle strain is recorded by the sequence of structures observed in the field. The main discrete, brittle thrust surfaces represent the last major stage in a continuous sequence of deformation.

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