Simple shear or pure shear?

BMT is a reverse kinematic model. Another type of basin models is forward models (Wangen et al., 2008; Wangen & Faleide, 2008; Theissen & Rüpke, 2010). The reverse model reproduce the past by backstripping and decompaction routines; while the forward models start with the pre-rift configuration and resolve basin formation forward in time, and reproduce the observed present day basin stratigraphy (cf. figure below).

It is found that decompaction/backstripping-based methods may systematically underestimate sedimentation rates and total subsidence, because lack of sediment thinning during lithospheric extension, and thereby overestimating the palaeo heatflow (Theissen & Rüpke, 2010).

Wangen & Faleide (2008) and Theissen & Rüpke (2010) are right that there will be significant differences in predicted palaeo heatflow between models that do thin the sediments during extension and models which does not. The difference in estimated palaeo heatflow will also affect the estimated temperature history.

Illustration of the two types of thinning assumed by forward and reverse models (BMT). Forward models assume pure shear thinning of the crust and sediments; BMT assumes simple shear thinning of the sediments.  

There are reverse models that do not account for sediment thinning. However, BMT reconstruction involve sediment thinning, but BMT is thinning the sediments by simple shear and not by pure shear, as is the case with the forward models (cf. figure above). The question is therefore: what is most correct? Is a model with sediment thinning by pure shear during extension more realistic than a model with simple shear thinning? Or is a mix of both most realistic?

We strongly believe that the simple shear thinning of the sediments is the most realistic model. And we question the possibility of lithospheric thinning without significant sediment fault movements. The simple shear model is described in detail in Fjeldskaar et al. (2004).

Transect across the North Sea, and illustration of the typical basin development in an extensional basin with extensive faulting in the rift phase. Is it possible to reconstruct the temperature history in extensional settings without doing fault reconstruction?

It is, anyway, possible to do modelling in BMT assuming pure shear thinning of the sediments in addition to simple shear. This can be done after the lithosphere thinning is estimated. Below are shown examples of the pure shear thinning at a profile over Vøring area, offshore mid Norway.

Examples of combined pure shear and simple shear modelling on a profile over the Vøring area, offshore mid Norway. As shown the estimated lithosphere thinning is most pronounced in the west.

 

References

Fjeldskaar, W., ter Voorde, M., Johansen, H., Christiansson, H.P., Faleide, J.I. and Cloetingh, S.A.P.L., 2004.  Numerical  simulation  of  rifting  in  the  northern  Viking  Graben:  the  mutual effect of modelling parameters.  Tectonophysics 382, 189-212.

Theissen, S., and L. H. Rüpke (2010) Feedbacks of sedimentation on crustal heat flow: New insights from the Vøring Basin, Norwegian Sea. Basin Research 22, 976–990, doi: 10.1111/j.1365-2117.2009.00437.

Wangen, M. and Faleide, J.I., 2008. Estimation of crustal thinning by accounting for stretching and thinning of the sedimentary basin — An example from the Vøring margin, NE Atlantic. Tectonophysics (doi: 10.1016/j.tecto.2008.07.002).

Wangen, M., Fjeldskaar, W. Faleide, J.I. Wilson, J. Zweigel and J. Austegard, A. 2008. Forward modeling of stretching episodes and paleo heat flow of the Vøring margin, NE Atlantic. Journal of Geodynamics, Volume 45, Pages 83-98.