GROWTH TRISHEAR MODEL AND ITS APPLICATION TO THE GILBERTOWN GRABEN SYSTEM, SOUTHWEST ALABAMA

The trishear kinematic model is the distributed shear in a triangular zone that is commonly associated with a propagating fault. Trishear modeling has been applied previously to a number of compressional and extensional structures with great success. However, trishear folding in syndepositional extensional settings, such as the Gulf of Mexico Basin, needs to be further explored.

We have proposed and investigated a new nonlinear 2-D numerical growth trishear model that is capable of modeling both symmetrical and asymmetrical trishear zones and is area-balanced. Velocity within the trishear zone decreases nonlinearly with respect to the distance from the reference point to the hanging-wall trishear boundary, and the velocity field is governed by a shape factor, r. Models indicate that the propagation-slip ratio (p/s) of the underlying fault plays an important role in governing the geometry of the resulting fold. Specifically, if p/s = 1, the hanging-wall active growth trishear boundary (AGTB-HW) and the footwall growth trishear boundary (AGTB-FW) are parallel to the propagating fault. If p/s<1, the AGTB-HW and the AGTB-FW are dipping away from the propagating fault. If p/s>1, the AGTB-HW and the AGTB-FW are convergent with the propagating fault. With greater p/s value, the trishear fold is more localized in the vicinity of the propagating fault.

The growth trishear model was applied to the extensional folding associated with an upward-propagating fault in the peripheral Gilbertown graben system of the Gulf of Mexico Basin. The fold is developed in Selma chalk, which has undergone significant compaction and fracturing and is an oil reservoir along the southern margin of the graben. Therefore, the graben was modeled using a compactional growth trishear model. The model can predict a geometry nearly identical to that of the extensional fault-propagation fold. The geometry of the propagating fault is controlled by the compacting chalk. The model also indicates an increased p/s value during deformation, suggesting that the fault propagated quickly during the late stages of deformation.