Print this pageGeometric or Dynamic Restoration? Lessons from Analogue Models
KOYI, HEMIN
Restoration aims to achieve geometric consistency and kinematic compatibility between deformed and restored sections. Normal restoration do not take account of dynamics. Dynamic restoration requires achieving not only accurate geometric and kinematic restoration but also requires accounting for strain variations (e.g. layer parallel shortening) with depth and due to different rhelogical properties of the deformed units. Shortened sand models are used to illustrate the significance of layer parallel shortening in dynamic restoration.
Unlike natural examples, profiles of deformed stages of analogue models can be compared to profiles of their initial stages. This comparison allows accurate restoration of profiles of deformed stages and enables quantifying the amount of layer parallel shortening (layer-parallel thickening and area/volume loss) and its distinction from the other two main components of strain- folding and imbrication- which are relatively easier to measure. Restored profiles of sand models show that layer parallel shortening can be as much as 40 percent of the total amount of shortening. Individual model imbricate sheets undergo tectonic compaction (vertical and lateral) which result in an area loss of (10-13)% while the entire model shows a 17% area loss.
Model layers above decollement deform by different mechanisms with time and with their stratigraphic position within the sand wedge. At deep levels, layer parallel shortening dominates and is twice that at shallow levels where imbrication is the main component of deformation. Even within the same imbricate sheet, deep sand layers underwent 40-50% decrease in bed-length compared to no significant bed-length change of the shallow layers. In general, early penetrative strain and folding is largely replaced by imbrication at later stages of deformation.
Assuming no facies changes, in nature, comparing layer thicknesses between the deformed and undeformed areas in the same profile, porosity measurement of the units in the two areas and quantifying penetrative fabric and/or stylolites are first steps towards achieving dynamic restoration of contractional areas. Quantifying layer parallel shortening and any associated volume loss is of great significance for hydrocarbon exploration for they give an idea about porosity reduction due to tectonic compaction during deformation.