ACCOMMODATION OF PENETRATIVE STRAIN DURING DEFORMATION ABOVE A DUCTILE DÉCOLLEMENT

The accommodation of shortening by penetrative strain is widely considered as an important process during contraction, but the distribution and magnitude of penetrative strain in a contractional system with a ductile décollement is not well understood. Penetrative strain constitutes the proportion of the total shortening across an orogen that is not accommodated by the development of macroscale structures, such as folds and thrusts. In order to create a framework for understanding penetrative strain in a brittle system above a ductile décollement, eight analog models, each with the same initial configuration, were shortened to different amounts in a deformation apparatus. Models consisted of a silicon polymer base layer, overlain by three fine-grained sand layers. A grid was imprinted on the surface, to track penetrative strain during shortening. As the model was shortened, a series of pop-up and pop-down structures developed, with a zone of penetrative strain in the foreland. Penetrative strain in the foreland decreases away from the fold belt. Restoration of the model layers to the horizontal indicates that penetrative strain accounts for 90.5% -30.8% of total shortening in a brittle system with a ductile décollement, compared to 45.2%-3.6% within a totally brittle system. Analog model geometries were consistent with the deformation styles observed in salt-floored systems, such as the Salt Ranges of Pakistan. Penetrative strain has not been accounted for in previous studies of saltfloored regions and estimates of this type could help resolve concerns of "missing shortening" highlighted by GPS data.