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Abstract: Controls of Upper Crustal Rheology and Faults on Sub-basin Stress and Strain: Results from Rheological and Finite Element Modeling

Wees, Jan-Diederik van; Fred Beekman; Matthias Gölke; and Sierd Cloetingh -Vrije Universiteit

Rheological predictions for Phanerozoic basins indicate a pronounced decoupling of upper crustal deformation from subcrustal levels. As a consequence the structural expression of basins is primarily controlled by the movements along weak upper crustal faults, interacting with the rheological behavior of the upper and lower crust and basin infill. Rheological and finite element modeling results show that basin deformation features require permanent weakening of the lithosphere in which major normal faults (border faults), most likely weakened by reduced friction angle, play an eminent role.

Finite element models, incorporating the forementioned large scale Theological building blocks allow to analyze in detail stress-strain interactions at smaller sub-basin scales. Figs. 1 and 2 show typical examples of such finite element model runs, using the Tertiary and Neo-tectonic evolution of the Roer Valley Graben in the Netherlands as a natural laboratory. The finite element models such as presented in Figs. 1 and 2, integrating intraplate tectonics concepts and basin deformation observations, highlight a number of essential features with relevance to hydrocarbon exploration and production.

Relative weakness of major crustal scale faults, compared to surrounding rocks, results in strain localization at the faults, and as such determine predicted basin shape. Of particular importance is the effect of variation of frictional properties for individual faults having a strong influence on predicted fault throw (compare Figs. 1 and 2). These results indicate that faults with high magnitude in observed displacements are weaker than the ones with small displacements.

Furthermore finite models such as in figures 1 and 2 reveal significant in depth variations of fault throw and footwall and hanging wall displacement patterns, which is lacking in most conventional kinematic and flexural models for faulting. In the presented models with weak faults not extending towards the surface, the displacements show normal drag at the surface grading into reverse drag at depth. On the contrary conventional models with weak planar faults extending to the surface, would yield reverse drag patterns at the surface and at depth. The normal drag predicted at the surface for the Roer Valley is in agreement with observations on Neo-tectonic faulting, and may well apply to other settings of growth faulting.

As a consequence of displacements along weak faults bending stresses occur, which can strongly effect stress and strain variations. In the models for normal faulting as presented in Figs. 1 and 2 large variations in stress occur close to and across faults. At intermediate depths of 1 to 5 km in the foot wall vertical, horizontal and deviatoric stresses increase considerably towards the fault up to ca 40%, whereas in the hanging wall these decrease slightly towards the faults.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil