--> Abstract: Depositional Wedge-Tops, a Challenge for the Critical Wedge Theory? by Mary Ford; #90039 (2005)

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Depositional Wedge-Tops, a Challenge for the Critical Wedge Theory?

Mary Ford
ENSG-CRPG, Nancy, France

Growth structures develop mainly across depositional wedge-tops where the top of the distal orogenic wedge lies below local base level within the flexural foreland basin. Documented examples indicate that optimum conditions for their development occur when the dip of the upper surface of the outer orogenic wedge is low (α =0-1°), regardless of the taper angle (α+ β). Low α angles are associated with the presence of low friction décollements, usually salt. Therefore, depositional wedge tops develop most commonly on low basal friction wedges. In natural examples such as the northern Apennines, the South Central Pyrenees and the Zagros, it is shown that, while α is consistently low, β can vary considerably (0 -9°), being principally controlled by flexure of the lower plate. The applicability of the critical wedge model to low basal friction external orogenic zones is questioned. As a weak detachment such as salt cannot support any significant topography, it follows that low basal friction wedges cannot attain a critical state and therefore the critical wedge model cannot be satisfactorily applied. Neither taper angles, nor the complex distribution of internal deformation, nor the often associated forward slip of the whole wedge, can be satisfactorily explained by the critical wedge model. In the analysis of orogenic wedges, separate analysis of the angles α and β proves more useful than the critical taper angle α+β. The role of β in controlling low basal friction wedge dynamics is not yet well understood and remains to be explored by analogue and numerical modelling.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005