--> 3-D Fault Geometries and Interactions Associated With Multiphase Extension

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3-D Fault Geometries and Interactions Associated With Multiphase Extension


Many rift basins have undergone multiple episodes of extension, commonly with differing extension directions. The resultant fault patterns are complex, potentially affecting both hydrocarbon migration and entrapment. We used experimental (analog) modeling to examine the 3D fault geometries and interactions that developed during multiphase extension. In the models, a homogeneous layer of wet clay underwent two phases of extension whose directions differed by 45°. Additional clay was added after each phase of extension. To examine the deformation within the models, we created closely spaced (1 mm apart) serial sections, interpreted them, and imported our interpretations into Petrel software. The serial sections and Petrel images showed that first-phase faults (striking subperpendicular to the first-phase extension direction) were most common at the base of the models, and second-phase faults (striking subperpendicular to the second-phase extension direction) were most common at shallow levels. The attitude of many faults varied with depth, striking subperpendicuar to the first-phase extension direction near the base of the model and oblique to both extension directions at shallower levels. Displacement profiles on these faults indicated that they formed at depth during the first phase of extension. As they propagated upward during the second phase of extension, their strike rotated, becoming more optimally oriented relative to the second-phase extension direction. Additionally, the dips of these faults varied along strike. Many second-phase faults nucleated at first-phase faults and propagated upward and outward, some terminated into first-phase faults, and others cut and offset first-phase faults. The linkage of the second-phase faults with the first-phase faults created composite faults with zig-zag geometries in both cross-sectional and map views. The 3D fault patterns in the models are similar to those documented in basins that have undergone multiple phases of extension (e.g., the North Malay basin, offshore Thailand; the Taranaki basin, offshore New Zealand; the Jeanne d'Arc basin, offshore Newfoundland, Canada).