Faults in Virginia

faults (brown lines) mark the uplift of mountains, dropping of basins, and compression of geologic formations through tectonic thrusting
Source: ESRI, ArcGIS Online

The US Geological Survey defines a geologic fault as:¹

...a fracture or zone of fractures between two blocks of rock. Faults allow the blocks to move relative to each other. This movement may occur rapidly, in the form of an earthquake - or may occur slowly, in the form of creep. Faults may range in length from a few millimeters to thousands of kilometers. Most faults produce repeated displacements over geologic time. During an earthquake, the rock on one side of the fault suddenly slips with respect to the other. The fault surface can be horizontal or vertical or some arbitrary angle in between.

Earthquakes are associated with the existence of a fault or the creation of a new one. Though faults are zones where blocks of adjacent bedrock have moved differently, the potential for modern movement varies. There are active faults in California, including the famous San Andreas Fault, with regular earthquakes and differential movement along the fault lines. Sensationalist movies portray cracks at the surface swallowing buildings and actors during earthquakes, but the locations for those hypothetical events are not in Virginia.

Virginia lacks a surface expression of a fault as dramatic as the San Andreas Fault in California
Source: Doc Searls, San Andreas Fault, on the Carrizo Plain

Few faults are mapped in the Coastal Plain sedimentary formations. They have accumulated since the breakup of Pangea, and for that entire time the edge of Virginia has been a passive margin. In contrast, faults are common throughout the Piedmont. That region was compressed during multiple orogenies over 200 million years as terranes were accreted onto the edge of Laurentia.

The compression during assembling of Pangea affected bedrock of Virginia all the way to the Alleghenian Plateau. Blocks of crust under pressure were thrust westward, moving the Blue Ridge perhaps 40 miles westward during the Alleghenian Orogeny when African crushed into North America. Thrust faults cracked the limestone and sandstone sedimentary layers deposited between the Cambrian Period and the Alleghenian Orogeny when the edge of Virginia was a passive margin, thrusting blocks on top of other blocks and reducing the width of Virginia.²

mapped faults (brown lines) are common west of the Fall Line in the Piedmont, but relatively rare in the Coastal Plain
Source: ESRI, ArcGIS Online

In southwestern Virginia:³

The northern portion of Pine Mountain, north of the town of Louellan, has been thrust 4 miles (6.4 km) from its original position. The southern edge in Tennessee has been moved 11 miles (17.6 km).

black lines depict faults along Pine Mountain
Source: Kentucky Geological Survey, Kentucky Geologic Map Service

Geologic Faults and the North Anna Power Plant

Virginia Earthquakes

thrust faults displace layers of rock, stacking them on top of each other
Source: Wikipedia, Thrust fault (by Mike Norton)

Links

• Glossary of thrust tectonics terms
• US Geological Survey (USGS)
  • Quaternary Faults Database
  • What is a fault and what are the different types?
  • Where are the faults in the Central and Eastern U.S.?
• UVAToday
• An Earthquake History: Finding Faults In Virginia

faults in Prince William County show the boundaries of terranes accreted to Laurentia, effects of the Alleghenian orogeny and breakup of Pangea, and slumping in the Coastal Plain
Source: ESRI, ArcGIS Online

References