D. Hindle et M. Burkhard, Strain, displacement and rotation associated with the formation of curvature in fold belts; the example of the Jura arc, J STRUC GEO, 21(8-9), 1999, pp. 1089-1101
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 Unpublished PhD thesis, Universite de Paris VI (France)], tectoni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 Unpublished PhD thesis, Universite de Chambery (France)].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.