Are Cross Section Validation Techniques Valid?: A Test on an Experimental Half-Graben System

Richard H. Groshong Jr. and Baolong Chai

The constant-angle vertical simple shear, oblique simple shear, and flexural slip (constant bed length) kinematic models fail to provide a satisfactory relationship between the rollover geometry and the fault shape in an experimental half graben. The plane-strain sand model has a listric fault precut into a rigid footwall, comparable to many published examples. Extension is caused by the displacement of a rigid wall attached to a flexible mylar sheet at the base of the sand, producing constant displacement on the master-fault. The geometry of 9 hangingwall horizons is used to predict the shape of the master fault. The vertical simple shear and flexural slip models give detachments that are far too deep. Oblique simple shear with a different shear angle for each bed gives in excellent fit with shear angles that range from 69° to 103° to the horizontal. Simple shear requires layer-parallel extension that is proportional to the dip of bedding, very different from the experimental strain distribution. The best geometric fit is given by the modified Chevron model of Williams and Vann, designed for constant displacement on the master fault. The lost-area to depth relationship predicts the depth to detachment regardless of the kinematic model. The prediction is even more accurate when corrected for the dilation of the sand.

Vertical to oblique simple shear models are most suitable for extensional styles in which rollover beds are stretched in direct proportion to their dip. The shear angle is a function of the amount of extension. The modified Chevron model is appropriate for beds rigidly rotated above a listric fault. The area-depth relationship for depth to detachment works for both.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California