THE SILER CITY HORST AND GRABEN NETWORK IN THE CAROLINA TERRANE, CHATHAM COUNTY, NORTH CAROLINA

Detailed geologic mapping in Chatham County, North Carolina by the NC Geological Survey, with partial support from STATEMAP, has identified a northwest-southeast oriented network of large-scale topographic lineaments that locally host diabase dikes, faulted rock, and massive to brecciated quartz. The lineaments are prominent on hillshade LiDAR images and, in some cases, extend for multiple miles. Detailed geologic mapping along the lineaments have identified offsets of geologic units and structures. Locally along faults the direction of relative movement can be interpreted from offset of geologic structures.

Locally along the same lineament, truncated and offset map patterns occur with continuous (unfaulted) map patterns. These seemingly contrasting map patterns are interpreted to represent normal faulting with associated development of subparallel stepping faults and production of relay ramp structures. Additionally, the fault network forms horst and graben structures within the crystalline rocks of the Carolina terrane.

Along some lineaments, occurrence of abundant massive and brecciated quartz may be a result of extension dilation with subsequent migration of silica-rich fluids and quartz precipitation. Prominent lineaments extend up to the edge of the Triassic basin, but do not appear to continue into the basin, perhaps indicating that the northwest-southeast oriented network of faults predate Triassic sediment deposition. However, diabase dike trends continue into the Triassic basin and may indicate that diabase utilized the older fracture system beneath the Triassic sediments as a preferred conduit.

A locally significant natural spring (Mount Vernon Springs Spring) is located on one of these lineaments. Locally, these lineaments are zones of increased groundwater permeability and flow and may have important ramifications to groundwater resources and pollutant transport.