SPATIAL AND TEMPORAL VARIATION IN PENETRATIVE STRAIN DURING COMPRESSION: INSIGHTS FROM ANALOG MODELS (Invited Presentation)

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. The accommodation of shortening by penetrative strain is widely considered to be an important process during compression, but variation in the distribution of penetrative strain during a deformation sequence is not well understood. This study provides some first-order constraints on magnitude, timing and distribution of penetrative strain during deformation. A series of eight simple models, each with a geometrically and mechanically similar starting configuration, within the limits of sandbox models, were shortened to different amounts. Model results indicate firstly that penetrative strain increases with depth in any given model, and that the proportion of the total shortening accommodated by penetrative strain varies with time. As the deforming wedge approaches stability, penetrative strain is highest just before initiation of a new thrust fault, after which the penetrative strain component abruptly decreases. Each model also contains 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. Restoration of a seismic-scale cross-section indicates that model results can be used to predict the amount of penetrative strain and thus the true total shortening across a deformed region. Estimates of this type may be made for additional cross-sections and may provide answers to the problem of “missing shortening” across orogens and the total amount of shortening experienced at collisional plate margins.