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

Abstract

A new simpli®ed genetic classi®cation scheme for arcuate fold±thrust belts is proposed. Based on total strain patterns and displacement vector ®elds, we distinguish three extreme end-member models: (1)`Oroclines', pure bending of an initially straight belt, (2)`Piedmont glacier' with divergent transport directions and (3)`Primary arcs'. A simple geometric model set-up for the simulation of strain patterns in primary arcs with uniform transport direction demonstrates that divergent strain trajectories and rotations of passive marker lines do not require any divergence in displacement directions. These often quoted arguments are insu cient for the identi®cation of`Oroclinal bending' or`Piedmont glacier' type of arc formation. Only three-dimensional restorations of an arc provide the critical information about displacement directions. In their absence, arc parallel stretches and rotations in comparison with total strains provide the most useful criteria for the distinction of arc formation modes. As an example, the Jura fold±thrust belt of the external Alps is discussed. A large set of strain data includes 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 Unpublished PhD thesis, Universite de Paris VI (France)], tectonic stylolites and micro-strains from twinning in sparry calcite. Strain trajectories (maximum shortening direction) computed from these data de®ne a strongly divergent fan with a 908 opening. A complete displacement vector ®eld for the entire Jura has been determined from balanced cross-sections augmented with three-dimensional`block mosaic' restorations [Philippe, Y., 1995. Unpublished PhD thesis, Universite de Chambe ry (France)]. Displacement vectors diverge by about 408, markedly less than strain trajectories. The non-parallelism between strain trajectories and transport directions indicates that considerable wrenching deformation did occur in both limbs of the Jura arc. Paleomagnetically determined clockwise rotations of 0±138 from ten sites (Kempf, O., et al., Terra Nova 10, 6±10) behind the right-hand half of the Jura arc and two sites with a combined 238 anticlockwise rotation behind the left-hand half of the arc are and additional argument in favor of such a wrenching deformation. We conclude that the Jura arc formed as a`Primary arc' with a minor component of`Piedmont glacier' type divergence in transport directions.

Structural inheritance and cenozoic stress fields in the Jura fold-and-thrust belt (France)

“…As most of these normal faults occur near the Morez and the Vuache tear Faults (7 and 9 in Fig. 3), this local extension could reflect the thrustparallel stretching during the fold-and-thrust tectonics required by the divergence of the displacement vectors within the belt (Laubscher, 1972;Hindle and Burkhard, 1999). However, we find it unlikely as no normal slips coeval or postdating folding have been observed.…”

“…They occurred in 31 sites. It may indicate that small, undiscovered tear faults exist, as suggested by Hindle and Burkhard (1999).…”

“…PreNeogene palinspastic reconstructions show that the fan-shaped pattern cannot be the product of gravity sliding (Laubscher, 1973). It rather indicates that the Jura was indented by its stronger hinterland (Laubscher, 1972;Vialon et al, 1984;Burkhard, 1990;Hindle and Burkhard, 1999;Homberg et al, 1999). The regular NE -SW strikes of thrusts and folds in the external Alps behind the Jura suggest that the Adriatic lithospheric indenter did not produce significant stress deviation in the Jura Mountains.…”

A model of displacement and strain for arc-shaped mountain belts applied to the Jura arc

“…These follow from the work of Hindle and Burkhard (1999) which clari®es the fundamental dierences between many proposed arc formation models. We only produce models of thè Primary arc' type de®ned by Hindle and Burkhard (1999) and give more precise numerical data on the eects of deformation and its relationship to dierent displacement geometries. The aim is to try and identify which model is the most plausible for forming arcs.…”

Evolution and dynamics of a fold-thrust belt: the Sulaiman Range of Pakistan

“…Argand 1924;Carey 1955;Hindle & Burkhard 1999;Marshak 2004). Though poorly understood, the relationship between the size and shape of a mountain range, surface deformation, and continental rheology, is key to our understanding of mountain-building processes.…”