Layer Parallel Shortening: Early-Stage Deformation, or an Ongoing Process throughout the Entire Deformation Sequence?

There is debate as to whether the process of layer parallel shortening (LPS) occurs only during the initial stages of a deformation sequence or whether occurs throughout that deformation sequence. Secondly, there is no consensus on whether LPS is a continuous, steady-state process, or a pulsed process, occurring before the development of each new set of major structures. Structures such as pressure-solution cleavage, stylolite development, reduction of porosity, and pressure solution changes to grain boundaries, which result from LPS, can be identified in the field and estimates of LPS amount can be obtained. Thus it is crucial to understand how these field-based estimates, taken from the end-point of the deformation sequence, relate to the entire deformation sequence.

Four analog models, run to different final bulk shortening (BS) amounts between 5% and 20%, were used to investigate the relative amounts of tectonic shortening and LPS, which together comprise the total BS. Once the model had been completed and sliced, representative cross-sections were digitized and restored using Move structural software. The amounts of LPS and tectonic shortening were then calculated from these restorations.

Results indicate that the proportion of LPS, relative to tectonic shortening, is high in the initial stages of deformation (the models with low BS) but then decreases to a lower proportion of the total BS that does not change even as the total BS is increased. A second result is that LPS is most significant in deeper stratigraphic levels, although the same overall pattern as previously is followed at each level. This suggests that LPS is a significant deformation mechanism in the early stages of deformation but becomes less significant as deformation proceeds. LPS estimates from field data therefore need to be placed in the correct tectonostratigraphic and deformation sequence context in order to be properly understood.