USING PENETRATIVE STRAIN AND STRAIN PARTITIONING TO RESOLVE THE “MISSING SHORTENING” PROBLEM: INSIGHTS FROM ANALOG MODELS (Invited Presentation)

Commonly, a discrepancy exists between geologically-derived and geodetically-derived shortening rates for collisional plate margins, which is known as the “missing shortening” problem. One mechanism that may reconcile this discrepancy is the process of shortening accommodated by penetrative deformation. The present working definition is that penetrative strain constitutes the proportion of the total shortening across an orogen that is not accommodated by the development of macroscale structures. This strain is not typically considered in cross-section reconstruction because the variation in the distribution of penetrative strain during a deformation sequence is not well understood, neither is the partitioning between penetrative and macroscale strain well constrained.

Analog models are an ideal scenario for exploring this concept, since the starting conditions are pre-set by the experimenter; other variables in cross-section restoration such as original layer thickness are also established at the start of the experiment, and tracked through the run. The series of studies presented provides some first-order constraints on magnitude, and distribution of penetrative strain during deformation. Three series of contractional experiments, one with a brittle basal detachment, one with a ductile basal decollement and one with both a basal and an intermediate ductile decollement were systematically shortened. Within the limits of analog models, each initial configuration was geometrically similar, with mechanical variation deliberately introduced by the use of different materials.

Model results indicate that penetrative strain is variable with depth, and the style of variation depends on the mechanical stratigraphy. The proportion of the total shortening accommodated by penetrative strain decreases as deformation progresses. Models also contain a foreland zone of penetrative strain, in which penetrative strain decreases exponentially away from the deformation front. These results are consistent with available field data and cross-sections, indicating that model results can be used to predict the penetrative strain and thus true total shortening across a deformed region, as well as reconciling the “missing shortening” concerns in collisional margins.