Deformation and secondary faulting associated with basement-involved compressional and extensional structures

Abstract

Experimental clay modeling is used to study the geometry and evolution of deformation zones formed above basement faults. The nature of deformation is compared for reverse faults dipping 30, 45, and 60, vertical faults, and normal faults dipping 60 and 75. Basement faulting results in the formation of a sheared triangular deformation zone that widens upward in the cover units. Most of the deformation is focused within a central zone of secondary faults, which propagate upward and eventually break through the entire section. Basal units exhibit steeper dips and higher fault densities, whereas stratigraphically higher units exhibit gentler dips and lower fault densities. The width of the deformation zone is generally greater for reverse faults with low dips. The anticlinal axial surface typically dips in the same direction as the synclinal axial surface for low- to moderate-angle reverse faults; however, the axial surfaces dip in opposite directions for vertical faults and normal faults. Particle paths suggest a convex upward rotation within the deformation zone, suggesting transfer of material across the extension of the master fault. Reverse faults are associated with secondary faults with reverse and thrust separations. Vertical faults are associated with mostly vertical and reverse secondary faults, whereas normal faults are generally associated with normal separation of most secondary faults. Therefore, the nature of secondary faults varies with structural position, possibly within the same layer. These results provide some key insights that are useful in the interpretation of macroscopic surface and subsurface basement–involved structures.