Horsetail splays, wing cracks, and splay faults

These funny faults go by sev­eral names, but traditionally refer to the secondary faults and fractures that develop near the tips of a primary fault. Al­though these can occur on both nor­mal and reverse faults, the terminolo­gy is commonly used when referring to strike-slip faulting. Horsetail splays often look like the hairs from a horse’s tail near the tip of a fault. However, they are often limited to just one side of the fault representing a tensional quadrant. The opposite side of the fault would represent a contractional quadrant (figure below).

Horsetail splays occur as displace­ment dissipates near the end of a fault and slip is distributed through sever­al smaller branching faults that curve away from the main fault – sub-paral­lel to the maximum horizontal stress, SHmax – in an imbricate pattern. They are great kinematic indicators for slip direction, and in the case of strike-slip faults, for determining left-lateral verse right-lateral slip. Horsetail splays oc­cur as the rock is pulled away from the fault tip, forming an acute angle with the master fault “pointing” in the slip direction. The opposite side of the fault would be under more contraction/compression as the rock pushes to­wards the fault tip. Often, we can find dissolution (stylolites) or grain crush­ing (compaction bands) features in this quadrant.

The image below shows horsetail splays at two different scales, one in an outcrop occurring in Jurassic aeolian sandstone, and one in 3D seismic enhanced by structural attributes. Larger horsetail splays can be resolva­ble in 3D seismic volumes due to the extensional component of slip that accommodates the strike-slip fault­ing in the tensional quadrant. These are helpful for determining slip sense along strike-slip faults in the subsur­face where piercing points may be dif­ficult to come by.

Flow conduits

Identifying and understanding the tensional regions around faults can be important for energy ventures. Horsetail splays often represent ten­sional deformation where the faults and fractures can act as conduits for fluid flow, making them critical for both mineral exploration and geo­thermal development. Additionally, horsetails splays may even host pro­spective sedimentary basins along the tips of major strike-slip faults, mak­ing them important for petroleum exploration.