Strain, displacement and rotation associated with the formation of curvature in fold belts; the example of the Jura arc

A new simplified genetic classification scheme for arcuate fold-thrust belt s is proposed. Based on total strain patterns and displacement vector field s, we distinguish three extreme end-member models: (1) 'Oroclines', pure be nding of an initially straight belt, (2) 'Piedmont glacier' with divergent transport directions and (3) 'Primary arcs'. A simple geometric model set-u p for the simulation of strain patterns in primary arcs with uniform transp ort direction demonstrates that divergent strain trajectories and rotations of passive marker lines do not require any divergence in displacement dire ctions. These often quoted arguments are insufficient for the identificatio n of 'Oroclinal bending' or 'Piedmnont glacier' type of arc formation. Only three-dimensional restorations of an are provide the critical information about displacement directions. In their absence, are parallel stretches and rotations in comparison with total strains provide the most useful criteri a for the distinction of are formation modes. As an example, the Jura fold- thrust belt of the external Alps is discussed. A large set of strain data i ncludes total shortening estimates based on balanced cross-sections, local strain axes orientations from the inversion of fault populations [Homberg, C., 1996. Unpublished PhD thesis, Universite de Paris VI (France)], tectoni c stylolites and micro-strains from twinning in sparry calcite. Strain traj ectories (maximum shortening direction) computed from these data define a s trongly divergent fan with a 90 degrees opening. A complete displacement ve ctor field for the entire Jura has been determined from balanced cross-sect ions augmented with three-dimensional 'block mosaic' restorations [Philippe , Y., 1995. Unpublished PhD thesis, Universite de Chambery (France)]. Displ acement vectors diverge by about 40 degrees, markedly less than strain traj ectories. The non-parallelism between strain trajectories and transport dir ections indicates that considerable wrenching deformation did occur in both limbs of the Jura are. ;Paleomagnetically determined clockwise rotations o f 0-13 degrees from ten sites (Kempf, O., et al., Terra Nova 10, 6-10) behi nd the right-hand half of the Jura are and two sites with a combined 23 deg rees anticlockwise rotation behind the left-hand half of the are are and ad ditional argument in favor of such a wrenching deformation. We conclude tha t the Jura are formed as a 'Primary arc' with a minor component of 'Piedmon t glacier' type divergence in transport directions. (C) 1999 Elsevier Scien ce Ltd. All rights reserved.