Rend Lake Fault Zone (New Name)

Old Name: Rend Lake Fault System

Location

From T7S, R2E, Franklin County, to T3S, R1E, Jefferson County (I-5 to J-6)

References

Keys and Nelson 1980, Nelson 1981

Description

Keys and Nelson (1980) referred to this structure as the Rend Lake Fault System, but the name is changed to Rend Lake Fault Zone because the faults are parallel and all the same type. In addition, the faults are confined to a zone that is nearly 50 times as long as it is wide.

The Rend Lake Fault Zone is known in considerable detail from exposures in underground coal mines. Its mapped length is about 24 miles (39 km) and its width varies from about 30 feet (10 m) to more than 0.5 mile (0.8 km). The southern end of the Rend Lake Fault Zone is just north of the Cottage Grove Fault System (fig. 31). The fault zone strikes due north for about 15 miles (24 km) then curves to a heading of N15°W. It apparently terminates along the flank of the northeast branch of the Du Quoin Monocline.

The Rend Lake Fault Zone is composed mainly of high angle normal faults that strike parallel to the overall trend of the zone. Individual faults overlap one another end-to-end. Displacements range from less than 1 inch to about 55 feet (17 m). Some faults are downthrown to the east, others to the west. The overall displacement is not consistent along the length of the fault zone. Horsts and grabens are common. Aside from minor drag, no folding or tilting of strata adjacent to faults is observed. Small reverse and oblique-slip faults occur in some areas in the Rend Lake Fault Zone (fig. 61). These may signify an episode of compression or they may simply reflect local wedging or rotation of fault slices during normal faulting.

The fault zone follows the west flank of a long narrow shallow syncline parallel to the Benton Anticline on the east. The syncline evidently began to form late in the Mississippian or early in the Pennsylvanian Period and it underwent further development after Pennsylvanian sedimentation. Faulting was interpreted as a product of late-stage differential uplift and subsidence along the syncline (Keys and Nelson 1980). The extent of the Rend Lake Fault Zone at depth is unknown.

Figure(s)

Figure 61   Field sketches showing small scale complexity of faults in the Rend Lake Fault Zone in an underground coal mine in Franklin County. Left sketch shows two parallel faults with a downdropped block between them. Striations on the surface of the east fault plunge 25° south, indicating a component of left-lateral movement. Right sketch demonstrates at least three periods of movement: (1) reverse movement on fault A with fracture B as a hinge, (2) horizontal movement on bedding plane fault C displacing fault A, and (3) normal movement down to the west on fault A, offsetting fault C.