THE FAULT WITH THE WHITE HOUSE: A STUDY OF LATE CENOZOIC TECTONISM IN WASHINGTON, D.C

In 1893, while mapping the geology in Washington, D.C., N. H. Darton encountered two reverse faults along Rock Creek, just south of today’s National Zoo. The smaller of these faults strikes N28^oE and the larger strikes N17^oW, and both vertically displace the contact between late Pliocene terrace gravels and Paleozoic crystalline rock. Darton recognized the larger fault again in an excavation at 18^th and California Streets, where it vertically displaces the Lower Cretaceous Potomac Group strata approximately 40 ft. These faults are herein referred to as the D.C. Fault Zone.

In the late 1960’s, drilling for the METRO subway system south of the 18^th and California Street locality, showed an irregularity at the base of the Cretaceous strata. This irregularity was interpreted as a large channel but is more likely the downthrown side of the D.C. Fault Zone. Further south at Lafayette Park, immediately north of the White House, the misinterpretation of this irregularity resulted in the abandonment of a METRO boring machine shield.

In 1976, the U.S. Geological Survey drilled 4 holes in Lafayette Park to evaluate whether the irregularity identified by the METRO drilling was an extension of the D.C. fault zone. These holes show that an abrupt 24 ft vertical displacement of the base of the Cretaceous strata occurs along the projection of the fault zone into Lafayette Park. Projecting the zone along strike across Pennsylvania Avenue places it near the east wing of the White House. Although the fault movement affects the Cretaceous and late Pliocene strata in the area, Quaternary strata show little to no displacement.

The D.C. fault zone qualifies as one of the youngest tectonic features in the eastern U.S. It is most likely a structural continuation of the Stafford Fault System in northern Virginia. Although the Stafford System is characterized by NE-trending en echelon reverse faults, these types of fault systems commonly have secondary faults at other orientations, which form because of variation in vertical displacement along the entire fault system. The configuration and longevity of these regional fault zones shows that east-west compression has dominated the Eastern U.S. for the last 100 Ma and will likely be responsible for future seismicity.