The significance of penetrative strain in the restoration of shortened layers - Insights from sand models and the Spanish pyrenees

Dynamic restoration is achieved when one accounts for the changes that occur in area or volume during deformation. In contractional areas, layer-parallel shortening (LPS) cannot always be easily estimated or measured, although it is a significant component of deformation, as is gravitational compaction. Five model analogs with known initial dimensions and boundary conditions were shortened from one end. Profiles of these models were used to (1) estimate the amount of layer-parallel compaction (LPC), the main modality of layer-parallel shortening in granular analog materials; (2) outline variation of LPC with depth, lateral location, and percentage shortening; and (3) estimate the effect of lithology on LPC. During progressive deformation, a modeled accretionary wedge, which formed during the shortening of the models, did not undergo homogeneous compaction; instead, loss of area varied in both space (with depth and laterally) and time. Balancing the area of sequential sections of one of the sand models, which was shortened above a high-friction basal décollement, shows that the layers experienced tectonic compaction during deformation and lost as much as 17% of their cross-sectional area during 50% bulk shortening. Restoration of two model profiles shows that LPC is three times greater in the model with high basal friction than in the model with low basal friction. In models where a sand layer was embedded within a viscous layer (a Newtonian material simulating rock salt), the layer accommodated all the shortening by folding and underwent no significant LPC. Examples from the Spanish Pyrenees are used to illustrate the significance of LPS in restoring profiles of contractional areas. In the eastern Spanish Pyrenees, on the basis of deformed raindrop marks and burrows, from 16 to 23% of total shortening is estimated to be by LPS, whereas only 6 to 10% of the total shortening is accommodated by folding. Model results illustrate the lateral and temporal variations of penetrative strain within shortened layers. Outlining this heterogeneous distribution of penetrative strain and any associated volume loss is important in distinguishing areas of reduced porosity, which are significant for hydrocarbon exploration.