Experiments on Faulting in Zones of Oblique Shortening
Deformation of upper crustal rocks is generally distributed over zones up to several hundreds of kilometers in width and is controlled by the ductile flow of the underlying material. Analogue model experiments in normal gravity were performed to investigate the deformation of brittle sediments (modeled by sand and glass powder) above a salt or shale layer (modeled by viscous polydimethyl-siloxane) in zones of distributed oblique shortening. Models initially comprised horizontal, tabular layers. The 3-D fault shape and fault evolution of the brittle analogue materials were analysed by X-ray computerized tomography.
The imposed strain rate ratio of bulk simple shearing and bulk transverse shortening exerts an important control on fault style. In experiments with a relatively high strain rate ratio (>3.6), subvertical strike-slip faults develop initially, striking at angles of 25-40° to the bulk shear direction. Increasing strain results in several convergent strike-slip fault zones with characteristic positive flower structures. Gently dipping reverse faults develop between major fault zones and reflect local stress field modifications.
In experiments with lower strain rate ratios (< 2.7), gently dipping downward converging reverse faults (dipping at 30-45°) accommodate initial failure. They define pop-up structures which trend perpendicular to the direction of transverse shortening. Continuing deformation leads to a fault pattern dominated by oblique-slip reverse faults. At late stages of strain, subvertical strike-slip faults form either in between or within pop-up structures. Fault motion partitions locally into subparallel nearly pure strike-slip faults and nearly pure reverse faults.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California