Subsurface Structural Predictions and Section Construction in the French Subalpine Fold and Thrust Belt

Robert W. H. Butler

The prediction of subsurface structure in foreland thrust belts has been greatly aided by the construction of balanced cross sections which can test various geometric models for internal consistency. However, even with good well and seismic data, balancing rarely produces unique solutions; but possible options can be further limited by incorporating along-strike geology. This contribution illustrates the method by using examples from the northwestern French subalpine chains and southern Jura. Between tear faults or distributed zones of wrench shear, we can assume that thrust belts maintain horizontal shortening. So by establishing thrust linkages, estimating shortening on all folds and thrusts, and defining movement directions from fault-zone kinematic indicators, we can onstruct serial sections which maintain displacement compatibility. Between the Arve Valley and Diois, thrusts show consistent west-northwest-east-southeast movement axes, and sections constructed parallel to this axis show about 30 km shortening. There are, however, gradual changes in structural style within the thrust belt, with shortening accommodated on discrete faults and overturning fold limbs. These variations occur down individual thrust-shear zone profiles in the Mesozoic cover rocks and also along strike; they can be related to competence variations in the alternating limestones and shales and to prethrusting extensional faults. Variations in thrust-zone widths require formational area balancing rather than conventional methods which rely on the conservation of stratigraphic be length, cut-off lengths, or ramp angles. Serial sections will be presented to illustrate these techniques which offer new models for subalpine structural evolution.

AAPG Search and Discovery Article #91032©1988 Mediterranean Basins Conference and Exhibition, Nice, France, 25-28 September 1988.